ELECTRODE ARCHITECTURES FOR EFFICIENT ......to 300 times more energy than electric batteries per...
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ELECTRODE ARCHITECTURES FOR EFFICIENT ELECTRONIC AND IONIC TRANSPORT PATHWAYS IN HIGH POWER LITHIUM ION BATTERIES A Dissertation Presented by ANKITA SHAH FAULKNER to The Department of ELECTRICAL AND COMPUTER ENGINEERING in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the field of ELECTRICAL ENGINEERING Northeastern University Boston, Massachusetts MARCH 2014
Transcript of ELECTRODE ARCHITECTURES FOR EFFICIENT ......to 300 times more energy than electric batteries per...
ELECTRODE ARCHITECTURES FOR EFFICIENT ELECTRONIC AND IONIC
TRANSPORT PATHWAYS IN HIGH POWER LITHIUM ION BATTERIES
A Dissertation Presented
by
ANKITA SHAH FAULKNER
to
The Department of ELECTRICAL AND COMPUTER ENGINEERING
in partial fulfillment of the requirements
for the degree of
Doctor of Philosophy
in the field of
ELECTRICAL ENGINEERING
Northeastern University
Boston, Massachusetts
MARCH 2014
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Abstract
As the demand for clean energy sources increases, large investments have
supported R&D programs aimed at developing high power lithium ion batteries for
electric vehicles, military, grid storage and space applications. State of the art lithium ion
technology cannot meet power demands for these applications due to high internal
resistances in the cell. These resistances are mainly comprised of ionic and electronic
resistance in the electrode and electrolyte. Recently, much attention has been focused on
the use of nanoscale lithium ion active materials on the premise that these materials
shorten the diffusion length of lithium ions and increase the surface area for
electrochemical charge transfer. While, nanomaterials have allowed significant
improvements in the power density of the cell, they are not a complete solution for
commercial batteries. Due to their large surface area, they introduce new challenges such
as a poor electrode packing densities, high electrolyte reactivity, and expensive synthesis
procedures. Since greater than 70% of the cost of the electric vehicle is due to the cost of
the battery, a cost-efficient battery design is most critical. To address the limitations of
nanomaterials, efficient transport pathways must be engineered in the bulk electrode.
As a part of nanomanufacturing research being conducted the Center for High-
rate Nanomanufacturing at Northeastern University, the first aim of the proposed work is
to develop electrode architectures that enhance electronic and ionic transport pathways in
large and small area lithium ion electrodes. These architectures will utilize the unique
electronic and mechanical properties of carbon nanotubes to create robust electrode
scaffolding that improves electrochemical charge transfer. Using extensive physical and
electrochemical characterization, the second aim is to investigate the effect of electrode
parameters on electrochemical performance and evaluate the performance against
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standard commercial electrodes. These parameters include surface morphology, electrode
composition, and electrode density, and operating temperature. Finally, the third aim is to
investigate commercial viability of the electrode architecture. This will be accomplished
by developing pouch cell prototypes using a high-rate and low cost scale-up process.
Through this work, we aim to realize a commercially viable high-power electrode
technology.
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Acknowledgements
First and foremost, I would like to thank my dissertation advisor, Prof. Ahmed Busnaina
for his guidance, patience, and support throughout these past five years. I would also like
to thank my committee members, Prof. Sun and Prof. Abraham for their guidance
through the final stages of my PhD. Especially to Prof Abraham, your mentorship and
expertise has been invaluable towards my goals.
My deepest gratitude goes to Prof. Siva Somu for his support and guidance from the first
day of my PhD. I think I finally learned what “nanotechnology” means.
It has been a great pleasure working with my brilliant colleagues, Sharon Kotz, Nurullah
Ates, and Gizem Yilmaz on the Battery Project. Thank you for all of your help, hard
work, and creativity.
To all of the graduate students and post docs, I am so thankful for our fruitful “post-group
meeting” discussions. You all are not only my colleagues and mentors, but also my good
friends. It was my great pleasure to work with you all.
I would also like to thank the Kostas staff, Scott McNamara, Dave McKee, and Rich
DeVito. You have helped me through countless equipment failures with (almost) always
a smile on your face. I would not have finished this dissertation without your help. I also
want to extend my thanks to the CHN staff, Matt Botti, Eric Howard, Jess Viator, and
Matt Rogers.
Thank you to my best friends, Mary-Kate Balaconis, Jaclyn Lautz, and Christina
Bourgeois for their constant encouragement, camaraderie, not only through graduate
school but for the past eight years.
Thank you to my parents, Hasmukh and Bharati Shah and my sister, Shreya, for their
endless love and support in every single one of my endeavors. Finally, to my husband
Dan, thank you for being my sanity check and the love of my life. Words cannot describe
how grateful I am for your support.
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Table of Contents Abstract ............................................................................................................................................ i
Acknowledgements ........................................................................................................................ ii
Table of Contents ........................................................................................................................... iv
1. Introduction .............................................................................................................................. 1
1.1. Renewable Energy Technology ......................................................................................... 1
1.2. Conventional Applications for Lithium Ion Batteries ........................................................ 2
1.3. Batteries for Electric Drive Vehicles ................................................................................. 3
1.4. Research Objectives ........................................................................................................... 5
1.5. Executive Summary ........................................................................................................... 6
1.6. References .......................................................................................................................... 7
2. Theoretical Background .......................................................................................................... 8
2.1. Structure and Working Principle of Lithium Ion Batteries ................................................ 8
2.2. Traditional Lithium Ion Technology ................................................................................ 10
2.2.1. Cathode Materials ................................................................................................... 10
2.2.2. Lithium Rich Layered Materials ............................................................................. 14
2.2.3. Anode Materials ...................................................................................................... 15
2.2.4. Traditional Electrodes and Manufacturing .............................................................. 17
2.2.5. Cell Configurations ................................................................................................. 18
2.3. Governing Equations for High Power .............................................................................. 19
2.4. Advanced Materials for High Power ................................................................................ 23
2.4.1. Nanoscale Active Material ...................................................................................... 23
2.4.2. Nanoporous Active Material ................................................................................... 25
2.4.3. Nanoscale Coatings and Conductive Meshes .......................................................... 26
2.5. Electrode Architectures .................................................................................................... 29
2.5.1. One Dimensional Architectures .............................................................................. 29
2.5.2. Two Dimensional Architectures .............................................................................. 30
2.5.3. Three Dimensional Architectures ............................................................................ 31
2.6. Carbon Nanotube in Lithium Ion Batteries ...................................................................... 32
2.6.1. Structure and Properties .......................................................................................... 32
2.6.2. Electronic Conductivity ........................................................................................... 33
2.6.3. Mechanical Robustness ........................................................................................... 33
2.6.4. Thermal and Chemical Stability .............................................................................. 34
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2.7. References ........................................................................................................................ 35
3. Layer-by-Layer Carbon Nanotube Electrode Architecture .............................................. 41
3.1. Why Layer-by-Layer? ...................................................................................................... 41
3.2. Development of Low-cost Fabrication Process for Electrode Architecture ..................... 41
3.2.1. Fabrication ............................................................................................................... 41
3.2.2. MWNT Suspension ................................................................................................. 43
3.2.3. Active Material Suspension .................................................................................... 43
3.3. Physical Characterization ................................................................................................. 45
3.3.1. SEM Characterization ............................................................................................. 45
3.3.2. Surface Morphology ................................................................................................ 47
3.3.3. DC Conductivity ..................................................................................................... 48
3.4. Electrochemical Characterization .................................................................................... 49
3.4.1. Galvanostatic/Potentiostatic Cycling ..................................................................... 49
3.4.2. Electrochemical Impedance Spectroscopy .............................................................. 48
3.5. Effect of Electrode Parameters on Electrochemical Performance ................................... 54
3.5.1. Effect of Active Material Composition ................................................................... 54
3.5.2. Effect of Active Material Thickness ....................................................................... 55
3.5.3. Effect of Changing Active Material ........................................................................ 57
3.6. Conclusions ...................................................................................................................... 61
3.7. References ........................................................................................................................ 63
4. High-Temperature Performance .......................................................................................... 65
4.1. Introduction: Energy Loss Due to Structural Degradation ............................................... 65
4.2. Capacity Fade of LiMn2O4 ............................................................................................... 65
4.2.1. Efforts to Mitigate Capacity Fade ........................................................................... 67
4.3. Voltage Fade of Layered Lithium-rich Materials ............................................................ 68
4.3.1. Efforts to Mitigate Voltage Fade ............................................................................. 70
4.4. Multi-layered Structure to Mitigate Structural Change .................................................... 70
4.5. Experimental Setup .......................................................................................................... 71
4.6. High Temperature Cycling of LiMn2O4 Cells .................................................................. 73
4.6.1. Impedance Analysis of LiMn2O4 Electrode ............................................................ 75
4.6.2. XRD Analysis of LiMn2O4 Cells ............................................................................ 77
4.7. High Temperature Cycling of Li-rich NMC Cells ........................................................... 82
4.7.1. Impedance Analysis of Li-rich NMC Electrodes .................................................... 82
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4.7.2. XRD Analysis of Li-rich NMC Cells ...................................................................... 83
4.8. Conclusions and Future Work .......................................................................................... 86
4.8.1. References ............................................................................................................... 87
5. Development of High-Power Full Cells ................................................................................ 89
5.1. Introduction ...................................................................................................................... 89
5.2. Capacity Balancing .......................................................................................................... 89
5.3. Full Cells with Silicon Anodes ........................................................................................ 93
5.4. Scale Up ........................................................................................................................... 97
5.4.1. Spray Coating: Experimental .................................................................................. 99
5.4.2. Morphology of Spray-Coated Electrodes .............................................................. 100
5.4.3. Performance of Spray-coated Electrodes .............................................................. 102
5.5. Pouch Cell Fabrication and Testing ............................................................................... 108
5.6. Doctor Blade Technique ................................................................................................ 109
5.7. Conclusions and Future Work ........................................................................................ 112
5.8. References ...................................................................................................................... 114
6. Application of Layered Architecture to Microbatteries ................................................... 115
6.1. Introduction .................................................................................................................... 115
6.2. Multi-layered Electrode Architecture for High-Power Lithium Ion Microbatteries ...... 118
6.3. Electrophoretic Assembly Mechanism .......................................................................... 119
6.3.1. Stabilizing the Lithium Ion Active Material Suspension ...................................... 121
6.4. Narrowing the Particle Size Distribution ....................................................................... 123
6.4.1. Ultrasonication ...................................................................................................... 124
6.4.2. Centrifugation ....................................................................................................... 124
6.5. Changing the Solvent System ........................................................................................ 125
6.6. Other Assembly Techniques .......................................................................................... 126
6.6.1. Electro-Fluidic Assembly ...................................................................................... 126
6.6.2. Dielectrophoretic Assembly .................................................................................. 127
6.7. Electrochemical Performance of Assembled Electrodes ............................................... 129
6.8. Conclusions and Future Work ........................................................................................ 135
6.9. References ...................................................................................................................... 136
7. Concluding Remarks and Future Directions..................................................................... 137
7.1. Conclusions .................................................................................................................... 137
7.2. Future Directions ............................................................................................................ 138
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Chapter 1: Introduction
1.1. Renewable Energy Technology
Since the 1970s, reducing the dependency of transportation systems on petroleum has been a key
area of focus in US energy research activities (2, 3). Major efforts have focused on the
development of fuel cells, super-capacitors, and batteries to create an electric transportation
infrastructure. However from an energy and power standpoint, they all lag far behind the
petroleum powered internal combustion engine (Figure 1.1) (4). Fuel cells technology, which
gained popularity in the 1990s, was a strong candidate as hydrogen is clean and highly abundant
fuel source, with the potential to provide an unlimited source of energy (5). Fuel cells, which
operate on the principal of reverse electrolysis, are twice as efficient as combustion (1).
However, petroleum has been very difficult to offset because of its fundamental advantage which
Figure 1.1 Ragone plot comparing the gravimetric power and energy of various power sources.
Modified from Ref. (1).
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is the very high energy to cost ratio, and ease of storage and transportation. Gasoline contains 60
to 300 times more energy than electric batteries per unit of mass and 3000 times more than
hydrogen gas per unit of volume (1). Furthermore, since hydrogen is bound to other substances,
unleashing the gas takes a substantial amount of energy, nearly as much energy to produce as it
delivers.
Other contenders in the energy storage race are supercapacitors, which store energy by
means of static charge (1). The supercapacitor is ideal for energy storage device that undergoes
frequent charge and discharge cycles at very high current. However, because supercapacitors are
limited by voltage and energy and have a very limited power spectrum, they cannot be used in a
standalone device (6). Supercapacitors if used conjunction with other energy storage devices,
they may add unnecessary weight and cost to the vehicle limiting the gravimetric energy density
and cost benefits. Therefore, due to their relatively high energy storage capacity, broad power
spectrum, and relatively low cost, batteries have emerged at the forefront of energy storage
technologies. Lithium ion cells, with their high energy density and long cycle life are the best
candidates thus far. However, significant challenges exist before lithium ion batteries can
transition from portable electronics to electric vehicles (6).
1.2.Conventional Applications for Lithium Ion Batteries
Lithium ion batteries are most popular in consumer electronics, with laptop batteries being the
largest segment (7-9). A typical lithium ion laptop battery pack is comprised of a 2-6 prismatic
or cylindrical cells. Major components of a battery pack consist of a temperature sensor, a
voltage converter and regulator circuit, and a battery charge state monitor (10). Typical charge
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cycles consist of a constant current charge up to 80% capacity which lasts for two hours and an
additional trickle charge at constant voltage for another two hours (1). If the battery pack gets too
hot during charging or use the computer shuts down the flow of power. If the cells are over-
discharged, the computer will also shut the battery down to protect from permanent damage to
the cell. The computer also keeps track of the number of charge and discharge cycles and sends
information on the health of the battery. Most lithium ion batteries in consumer electronics
operate on a similar principal in which a lithium ion cells are continuously monitored using a
small protection circuit. High-energy lithium ion chemistries are ideal for these devices which
require small amounts of current for several days to several weeks. Since, operating conditions
are well within the safety limits of lithium ion batteries, parameters such as high power
capability, extreme temperature stability, and cycle life are much less critical, and small
protection circuit is adequate for safe operation (1, 11). Additionally, as next generation devices
are released every two years on average, the lifetime of the battery pack is generally optimized
for less than five years.
1.3.Batteries for Electric Drive Vehicles
Recently, however, the demand for clean energy sources has surged and large investments have
supported R&D programs to develop advanced lithium ion batteries for electric drive vehicles,
grid storage and military applications (9, 12). Unlike portable electronics, these applications
operate in a wide temperature range (-30ºC to +60ºC), draw huge amounts of power, and require
power sources to operate for decades (13). While these demands are well met by fossil fuels,
electrification of these applications requires much larger energy storage devices. Unfortunately,
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since all of the performance parameters (energy density, power density, cycle life, temperature
stability, and cost) are all strongly coupled, improving the energy storage devices along all of
these performance axes presents significant engineering challenges(14).
So far, lithium ion batteries are the best candidates for energy storage in electric drive vehicles.
Currently, three types of electric drive vehicles exist: the all-electric vehicle (EV), the plug-in
hybrid (PHEV), and the hybrid electric (HEV). In EV and PHEV’s electricity is drawn directly
from the grid and stored in batteries, while HEVs use electric energy stored from regenerative
breaking technology to boost fuel efficiency. In all EV cars, since the drivetrain is only powered
by batteries, the size of battery packs is large, ranging from 100kWh-150kWh, allowing the EV
to drive 150 miles at best on a single charge. The size of the battery in HEV is 60-75kWh,
significantly smaller than the EV battery. However the power density of the HEV batteries is 3
times larger than EV batteries. For example, lithium ion batteries produced by Saft for EV
applications have a specific energy of 140Wh/kg and a specific power of 476W/kg while the
energy density and power density for HEV applications is 77Wh/kg at 1550W/kg respectively
(2). To capture energy stored in regenerative breaking, batteries for HEV applications must
accept rapid charge, requiring extremely high power densities (15). However, cells that are
optimized for high energy, as in the case of EV applications cannot simultaneously deliver high
power and vice versa due to internal resistances in a battery cell(16, 17). As a result, a well-
known trade off exists between the power of a battery and the energy of a battery. Therefore a
technology that can improve the power density of a battery while maintaining the energy density
is highly desirable, bridging the gap between EV and HEV batteries, allowing electric drive
vehicles to be cost competitive in a petroleum based economy (3).
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1.4. Research Objectives
As a part of nanomanufacturing research being conducted at the Center for High-rate
Nanomanufacturing at Northeastern University, the overall aim of this work is to develop a low
cost lithium ion electrode technology to enhance the power density of lithium ion cells without
sacrificing the energy density. The specific aims of this work are:
1. To develop lithium ion electrode architecture that enhances electronic and ionic transport
pathways in lithium ion electrodes. This architecture will utilize the unique electronic and
mechanical properties of carbon nanotubes to create robust scaffolding that improves
electrochemical charge transfer.
2. To investigate the effect of various electrode parameters such as surface morphology,
electrode composition, electrode thickness, and operating temperature on the
performance of the battery.
3. To investigate the commercial viability of the electrode architecture by developing
pouch cell prototypes using high rate, low-cost fabrication methods and testing their
electrical performance.
4. To develop a fabrication process for constructing small format lithium ion batteries
utilizing the electrode architecture for on-chip applications.
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1.5. Executive Summary
Following the scope and executive summary provided in chapter 1, chapter 2 provides a
thorough literature review on the state of the art in lithium ion batteries. Described here are
performance requirements for emerging Li-ion battery applications such as electric drive
vehicles and various research activities aimed at meeting performance requirements. Also
detailed are the working principle of a lithium ion battery, cell designs, and electrochemical
parameters used to gauge the performance of the battery. In chapter 3, a new carbon nanotube-
based lithium ion electrode architecture is introduced. The fabrication of the new electrode
architecture is described in detail. The performance of new electrodes is also evaluated against
commercially available electrodes using performance metrics described in chapter 2. At the end
of chapter 3, the effects of changing electrode parameters are studied in order to understand the
versatility of the electrode structure. In chapter 4 electrode performance and failure mechanisms
are studied at high temperatures using various lithium ion active materials in conjunction with
the electrode architecture. Chapter 5 describes the development of a scale-up process and
prototype development. Chapter 6 evaluates the use of the electrode architecture for small scale
on-chip applications which require extremely high energy and power per footprint area. Finally,
chapter 7 provides conclusions of the work and outlines future directions.
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1.6. References
1. Isidor, Buchmann, Batteries in a Portable World - A Handbook on Rechargeable Batteries for
Non-Engineerings Cadex Electronics (2011).
2. Burke, A., Proc. IEEE, 95, 806 (2007).
3. Canis, B., Battery manufacturing for Hybrid and Electric Vehicles: Policy Issues, in, p. 34,
Congressional Research Service (2013).
4. Simader, K. Kordesch and G. R., Chem. Rev. , 95, 191 (1995).
5. J.S.Cooper, V. Mehta and, J. Power Sources, 114, 32 (2003).
6. A.K. Shukla, A.S. Arico, V. Antonucci, Renewable and Sustainable Energy Reviews, 5, 137
(2001).
7. M. Armand and J.-M. Tarascon, Nature, 451, 6 (2008).
8. Scrosati, B., and Garche, J., Journal of Power Sources, 195, 12 (2010).
9. Whittingham, M. Stanley, MRS Bulletin, 33, 9 (2008).
10. Emadi, J. Cao and A., IEEE Industrial Electronics Magazine, 5, 27 (2011).
11. Y. Wang, Y. Li, P. He, E. Hosono and H. Zhou, Nanoscale, 2, 12 (2010).
12. Kassatly, Sherif, The Lithium-Ion Battery Industry For Electric Vehicles, in Mechanical
Engineering, p. 124, Massachusetts Institute of Technology (2010).
13. S. G. Chalk, J. F. Miller, J. Power Sources, 159, 73 (2006).
14. Eetacheri, V., Marom, R., Elazari, R., Salitra, G., and Aurbach, D., Energy Environ. Sci. , 4, 20
(2011).
15. Brodd, M. Winter and R., Chem. Rev., 104, 4245 (2004).
16. Abraham, K.M., ECS Trans. , 41, 9 (2012).
17. Park, M., Zhang, X., Chung, M., Less, G., Sastry, A.M., Journal of power Sources, 195, 7904
(2010).
7
Figure 2.1 Working principle of a lithium ion battery (2).
Chapter 2: Theoretical Background
2.1. Structure and Working Principle of Lithium Ion Batteries
A standard lithium ion battery cell is comprised of an anode, cathode, electrolyte, separator, and
the battery casing (figure 2.1)(2). In commercial lithium ion batteries, the negative electrode and
positive electrode are typically metallic current collectors coated with intercalation materials,
such as transition metal oxides on the cathode and graphitic carbons on the anode. The cathode
and anode are separated by a porous polymer membrane, composed of polyethylene or
polypropylene. The separator prevents an electrical short between the two electrodes but still
allows solvated lithium ions in the electrolyte to flow between the electrodes. The electrolyte,
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which serves as a conduit for lithium ions to flow between the anode and cathode, consists of a
lithium salt dissolved in an organic solvent. To prevent the loss of electrolyte from the system or
any hazardous material from leaking, all of the components in the cell are hermitically sealed in
a metallic case, which also provides contacts to the external circuit.
Electrochemical energy is converted to electrical energy when lithium ions intercalate
into the crystal structure of the host electrode material. Unlike traditional redox reactions,
intercalation (and de-intercalation) allow the host crystal structure to remain virtually unchanged.
During charging, lithium ions are extracted from the cathode host material (de-intercalation),
usually a lithiated metal oxide (LiMO2), solvated by the electrolyte, and inserted into the anode
host material (intercalation), typically graphite. Simultaneously, electrons flow from cathode to
anode through an external circuit. During discharge, this process is reversed. The redox reactions
at each electrode of a graphite/ lithium metal oxide cell are described in eq. 2.1-2.3 (9).
Cathode:
↔ (2.1)
↔ (2.2)
↔ (2.3)
Because the shuttling of lithium ions back and forth from the anode to the cathode occurs with
up to 99.9% efficiency, cells can be charged and discharged over 300-500 cycles with minimal
loss in energy storage capacity (10, 11). The electrical energy stored by the cell is dependent on
the properties of the cathode and anode materials while the power generated by the cell is
dependent on ionic and electronic transport pathways within the materials, at the material
interfaces, and from the surface of the material to the metallic current collectors. Therefore, for
next generation lithium ion batteries, both the choice of materials and the design of transport
pathways for lithium ions and electrons are of critical importance.
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2.2. Traditional Lithium Ion Technology
2.2.1. Cathode Materials
Conventional lithium ion cathodes are transition metal oxides which reversibly intercalate with
lithium ions. These metal oxides are coated onto an electrochemically stable metallic current
collector such as aluminum. Generally, the capacity of lithium ion cells is limited by the capacity
of the cathode. Therefore, extensive research has been done to design cathode materials with
higher storage capacity. These transition metal oxides can be classified by crystal structure into
layered compounds, spinel compounds, and olivine compounds (2, 9, 12). Table 1 provides
electrochemical properties of the common transition metal oxide cathode materials. Lithium
cobalt oxide (LiCoO2) is the most commonly used transition metal oxide material in batteries for
portable electronic devices. The first commercial lithium-ion battery, introduced by Sony
Corporation in 1991, was based LiCoO2/graphite couple. The prevalence of LiCoO2 is largely
due to a relatively high energy density and a very long cycle life compared to nickel metal
hydride (NiMH), lead acid, and many other Li-ion chemistries.
In the layered LiCoO2 structure , lithium atoms form alternating planes into which
intercalation and de-intercalation can occur (figure 2.2a) (1). High surface reactivity and the
instability of delithiated LiCoO2 limit the practical capacity of LiCoO2 to approximately
140mAh/g, which is 50% of the theoretical value of 273mAh/g. Furthermore, at elevated
temperatures (>130⁰C), LiCoO2 decomposes to produce oxygen. Evolved oxygen reacts with
organic materials in the cell, leading to a highly exothermic reaction and thermal runway.
Thermal runway can also be induced in adjacent cells as well, rendering LiCoO2 unusable in
advanced li-ion battery applications.
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In a spinel structure, lithium atoms occupy the tetrahedral sites and transition metal atoms
occupy the octahedral sites (figure 2.2.b). This three dimensional organization of lithium ions
allows for facile pathways for intercalation and de-intercalation (1, 13). A common and popular
spinel cathode material is lithium manganese oxide (LiMn2O4), which is under development for
high power EV/HEV/PHEV applications. Unlike LiCoO2, LiMn2O4 demonstrates stable cycling
behavior at five times the rated current. However, during extended cycling LiMn2O4 suffers from
drastic capacity fade due to the dissolution of manganese in organic electrolytes. Additionally, in
the compositional range LixMn2O4 ( a structural distortion occurs, termed the Jahn-
Teller distortion, in which the cubic spinel transforms to a tetragonal structure (1, 9). Therefore,
similar to LiCoO2, the practical capacity of LiMn2O4 is limited to 120mAh/g, which is less than
50% of the theoretical value of 248mAh/g. Of the common cathode materials, LiMn2O4 also
has the lowest practical capacity. These limitations render LiMn2O4 impractical for use in electric
vehicle application. However, the LiMn2O4/layered oxide composite cathodes are under
investigation for EV applications.
Phosphates, which adopt the olivine structure, are the third class of popular cathode
materials, the most common of which is lithium iron phosphate (LiFePO4). In LiFePO4, The
phosphorous atom occupies tetrahedral sites, the transition metal occupies octahedral sites and
lithium forms one-dimensional chains along the 010 direction (figure 2.3c). Although LiFePO4
exhibits 14% less capacity than LiCoO2, LiFePO4 exhibits the best thermal stability and
overcharge stability as it does not evolve oxygen, allowing the cathode to be stable up to 250⁰C.
Furthermore, when LiFePO4 nanomaterials are used, LiFePO4 can be discharged to roughly 20-
30 times the rated current. However, the use of nanomaterials causes a decrease in the packing
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density of the electrode. Furthermore, the low operating voltage (3.2V vs. lithium) of LiFePO4
causes the specific energy to be roughly equivalent to that of LiMn2O4.
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Table 1 Properties of layered, spinel, and olivine cathode materials.
Working Potential V
vs. Li/Li+
Theoretical / Practical Capacity
(Ah/kg)
Theoretical /
Practical Energy Density (Wh/kg)
Max Discharge
Rate
Electrical Conductivity
(S/cm)
Diffusion Coefficient
(cm2
/s)
Cost ($/kg)
Cycle Life
Safety at High Discharge Rates and
Temperatures
LiCoO2
(14-16) 3.6 273/ 162 980/ 648 0.5C 10
-4
10-10
- 10-8
20-50 800 Poor
LiMn2O
4
(17-21) 4.10 148/ 120 620/ 504 5C 10-6
10-11
-10-9
10-15 500 Moderate
LiFePO4
(22, 23) 3.45 170/ 150 580/ 515 30C 10
-9
10-14
-10-5
20-25 2000 High
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2.2.2. Lithium Rich Layered Materials
Drawbacks of the traditional LiCoO2, LiMn2O4 and LiFePO4 cathodes render these materials less
desirable in next generation electric vehicle applications. Several derivatives of these materials,
which utilize anion and cation doping within the transition metal oxide structure offer improved
properties such as thermal stability, cyclability, rate capability, and capacity. For example,
LiNi0.08Co0.15Al0.05O2, a compositional variant of LiCoO2, provides a slightly higher practical
capacity of 160-180mAh/g due to the stability of the structure upon delithiation (24). Yet these
improvements still do not meet the requirements of EV applications. Recently, a new class of
lithium-rich layered transition metal oxides has emerged, providing significantly larger
capacities of 230-250mAh/g (25-27). These materials have the general formula xLi2MnO3·(1-
x)LiMO2 (M = Mn, Ni, Co). They use a (xLi2MnO3) structural unit to stabilize the
Figure 2.2 Crystal structure of popular transition metal oxides used as active materials for
lithium ion batteries (1). (a). Layered (b). Spinel. (c). Olivine.(1)
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electrochemically active layered structure ((1-x)LiMO2) upon lithiation and delithiation, so that
the layered structure is stable over a much larger compositional range, yielding a significantly
higher practical capacity. However, these materials still possess several drawbacks which
include: (i) A large irreversible capacity in the first cycle caused by the activation of LiMn2O3
accompanied by the evolution of oxygen (28). (ii) A continuous decrease in the average cell
voltage with each cycle, and (iii) poor rate behavior.
2.2.3. Anode Materials
In the 1980s, lithium ion batteries were composed of a transition metal oxide cathode and a
lithium anode. The use of the lithium anode simultaneously allowed for high voltage and high
capacity. However, temperature changes inside the cell caused violent chemical reactions, raising
many safety concerns. It was discovered that these reactions were due to dendritic plating of
lithium at the surface of the anode, which led to short-circuiting (29).
As a safer alternative, graphitic carbon materials coated on a copper current collector
were chosen as a replacement for lithium anodes(30). In addition to their excellent inherent
safety, and their low cost, natural abundance have made graphitic carbons the most attractive
materials for lithium ion anodes. Graphite consists of extended sheets of sp2
hybridized carbons.
The working potential of the graphite electrode is 0.1mV versus Li/Li+ redox couple, which
enables high cell voltage in Li-ion batteries. Under normal operation, lithium ions intercalate in
the Van der Waals gaps above and below the carbon rings according to equation 2.3 (Figure
2.3a). This reaction results in a theoretical capacity of 372mAh/g. The formation of fully
lithiated graphite occurs through several stages commonly referred to as stage I, stage II, stage
III, and stage IV. In the staging process, thermodynamic limitations force lithium ions fills a gap
15
in between two graphite planes entirely before opening another plane. The Roman numeral
indicating the stage number refers to the number of gaps in between neighboring planes of
lithium. Due to the staging phenomenon, the potential profile of lithium ion intercalation into
graphite demonstrates distinct potential plateaus as the chemical structure of graphite transitions
to a new phase (Figure 2.3b). Types of graphitic carbons include natural and artificial graphite,
mesocarbon microbeads, and disordered amorphous carbons. These carbons show a large degree
of structural variations from highly crystalline graphites to highly amorphous carbons. Therefore,
the extent of lithium intercalation and reversibility of the intercalation process depend on many
factors such as structure, morphology, texture, grain-size, shape.
Another critical feature of graphite is the stability of the solid electrolyte interface (SEI)
layer formed at the interface of the anode and electrolyte. The SEI is a stable film-like layer that
Figure 2.3 (a) Lithium intercalation into graphitic carbons. (b). Voltage profile of a
graphite half-cell during discharge. Presence of stages is indicated by Roman numerals.
16
persists throughout the lifetime of the cell, but is permeable to lithium ions. The SEI layer is
typically formed during the first 2-3 cycles and composed of electrolyte decomposition products
such as solvent and salt reduction products. The decomposition occurs during the first cycle at a
potential of about 0.7V vs. Li/Li+ when an electrolyte based on organic carbonates is used. The
stability of the SEI layer is essential to the longevity of the battery because it prevents further
reaction of the electrolyte and consumption of lithium ions from the system.
For next generation lithium ion batteries, silicon, tin, and germanium have attracted
significant attention as anode materials due to their exceedingly high capacities. Unlike graphite
which undergoes an intercalation reaction with lithium ions, these new materials form highly
lithiated alloys. Therefore, replacing graphite with silicon, which exhibits 10x the theoretical
capacity of graphite, can significantly improve the gravimetric energy density of the cell as the
weight of the anode could be reduced by a factor of 10. However, these new materials have yet
to be commercialized due to an extreme capacity fade caused by structural stresses and SEI
instability (31, 32).
2.2.4. Traditional Electrode and Manufacturing
In a traditional electrode manufacturing process, aluminum and copper are used for the cathode
and anode current collectors respectively. Using an organic solvent, anode and cathode materials
are thoroughly mixed , separately, with conductive additives and polymeric binders in automated
millers and mixers to form highly uniform ink-like slurry (33-36). Conductive additives are
required because most transition metal oxide cathode materials have very poor electronic
conductivity, exhibiting insulating or semi-conducting properties at best. The polymeric binders
17
Figure 2.4 Traditional lithium ion cell designs. (a).
Cylindrical cell. (b). Button cell. (c). Prismatic cell.
(d). Pouch cell.(1)
are used to ensure that the coated materials remain properly adhered to the current collector
during cycling. However, the amount of binder must be minimized so that the binder content is
not detrimental to the conductivity of the electrode. Once the slurry is properly mixed, it is
coated onto the aluminum and copper using automated coating machines. This process, which is
known as calendaring, allows for micron precision in thickness of the coated ink on the current
collectors over 10-20m2
areas(37). Furthermore, automated coaters and feedback sensors allow
10-20m2
areas to be coated within minutes. By controlling the thickness of the coated slurry, and
the cell design, the battery can be efficiently tuned for high power (HEV) or high energy (EV)
applications. Once the electrodes are fabricated, they are cut and assembled into an electrode
stack with the separator. In a typical high energy cell, the loading of the coated material is
between 20-30 mg/cm2, allowing the energy per unit area to 15-22mWh/cm
2. In a typical high
power cell, the loading of the coated material is 8-10mg/cm2, translating to an energy of 5-
10mWh/cm2
2.2.5. Cell Configurations
For commercial lithium ion cells,
common cell designs are
cylindrical, prismatic, pouch, and
button (Figure 2.5). The most
common lithium on cell is the
18650 cylindrical cell (1, 36).
Each configuration is designed to
optimize a specific set of
18
performance parameters. In a cylindrical design, aluminum and copper foils with thin electrode
coatings are wound and fitted tightly into the shaft, allowing for a high energy cell that is still
capable of delivering relatively high power due to the thin electrode coating. The cylindrical
design also offers a high degree of mechanical stability and can withstand high internal
pressures. For this reason, cylindrical cells are most commonly used in laptop computers, power
tools, medical instruments and some electric vehicles. The simplicity of the cell design also
allows for ease of manufacturing. A typical 18650 cell containing LiMn2O4 active material
delivers 1.2-1.5Ah capacity while a cell containing LiCoO2 delivers 2.4-3.0Ah capacity.
Relative to cylindrical cells, prismatic cells are much flatter. They also allow for the best
form factor as void space in a battery pack is minimized. Thermal management within the battery
pack is also more efficient than in cylindrical cells. Prismatic cells are typically used with
lithium-polymer chemistries in mobile phones and other portable devices. The button cell
enables small devices such as watches and small medical devices. Button cells can also be easily
stacked together to produce high voltage battery packs for small devices. However due to a lack
of safety vent, button cells can only be charged and discharged at very low currents. Pouch cells
are the lightest and most cost effective to manufacture. They also offer 90-95% packing
efficiency. However, if not constructed with extreme care, exposure to high humidity and hot
temperature can cause swelling of the cell, leading to safety hazards and device failure.
2.2. Governing Equations for High Power
The power delivered by a lithium ion battery is commonly expressed by equation 2.4 (38):
(2.4)
(2.5)
19
Where is the load voltage and is the current delivered by the battery. An equivalent
expression for the power delivered is given by equation 2.5, where is the internal resistance of
the battery. From this expression, it is clear that to maximize the power delivered from the
battery, it is necessary to minimize . The internal resistance of the battery can be expressed as
a sum of the electronic and ionic resistive pathways in the cell (39-41). The primary ionic and
electronic resistive pathways include the electric resistance of the electrode ( ), the ionic
resistance of the bulk electrolyte ( ), ionic resistance in the electrode pores ( ), and the
resistance to charge transfer during lithium ion intercalation and deintercalation ( .
+ + (2.6)
Figure 2.5 provides a schematic of these resistances. can be expressed by equation 2.7, (42)
(2.7)
where is the resistivity of the composite electrode, is the thickness of the electrode and A is
the geometric area of the electrode. The value of is determined by the composition and
morphology of the electrode coating. and can are expressed by similar expressions in
equations 2.8 and 2.9(42):
(2.8)
(2.9)
In these equations, is the distance between the positive and negative electrodes, κ is the ionic
conductivity in the electrolyte and is the effective ionic conductivity in the porous electrode
network. In equations 2.7 and 2.9, the thickness of the composite electrode increases
proportionally with, and . Therefore as the electrode thickness increases, and
increase, thereby increasing and diminishing the power output of the battery. The expression
20
for can be derived from the kinetic Butler Volmer equation, which describes the general
relation between the potential and the electrochemical current (43-46);
(
( )) ( (
( )) (2.10)
where is the exchange current, is the reaction order, is the number of electrons involved in
the reaction, F is Faraday’s constant, R is the universal gas constant, T is the temperature and
is the overpotential. When the overpotential is very small, equation 2.10 can be
simplified to (46):
(2.11)
Zheng et al. investigated impact of electrode thickness on the rate capability, energy, power, and
long term cycling behavior of the cell and demonstrated a power-law relation between the
maximum working C-rate and electrode loading according to:
(2.12)
Where is the Peukert coefficient, Q is the nominal capacity in ampere-hours, and t is the
nominal discharge time (in hours) for a specific C rate. (33). For lithium nickel manganese cobalt
oxide (NCM) and LiFePO4 active materials, Peukert coefficients were found in the range of
1.013-1.075. As a result, when the electrode thickness of NCM active material was increased by
a factor of 4, the rate capability diminished by an order of magnitude. Similarly, when the
electrode thickness of LiFePO4 active material is increased by a factor of 10, the rate capability
diminishes by two orders of magnitude. The energy and power density of the electrode laminates
followed the same inverse relationship to capacity and rate capability. For the case of NCM
active material, increasing the thickness of the laminate from 24µm to 108µm allowed the
energy density to increase from 830Wh/L to 1,200 Wh/L. Similarly, for LiFePO4 active material,
the electrode energy density improved from 550Wh/L at 25µm laminate thickness to 700 Wh/L
21
at 108µm laminate thickness. However, the power density of the NCM laminates diminished
from approximately 10,000 W/L to 2,000 W/L and from 10000 W/L to 800 W/L for the LiFePO4
laminates. As described by equations 2.7 and 2.9, the decline in power density is mainly due to
the increase in the electronic resistance and ionic resistance of the electrode. The impact on the
long term cycling of the various laminate coatings was also evaluated. For both materials the
cycle life diminished as a function of the electrode laminate thickness. The relationship between
laminate thickness and capacity fade rate is due to the increase in internal resistance which
accelerates material isolation from electrochemical reactions and decreases electrode capacity
during cycling (47-49).
Figure 2.5 Schematic representation of ionic and electronic resistances in a lithium ion cell.
22
2.4. Advanced Materials for High Power
2.4.1. Nanoscale Active Material
To increase the power density of the electrode without sacrificing the energy density, novel
synthesis methods have allowed lithium ion active material to be reduced from the micron-scale
to the nanoscale. The advantage of shrinking lithium ion materials to the nano-size can be
described by equation 2.12 where is the characteristic time for lithium-ion diffusion, is the
lithium ion diffusion length of the intercalation compound, and is the diffusion coefficient
of the material (8, 50-54):
(2.12)
Since α , reducing the characteristic diffusion length of lithium ions from the microscale to
the nanoscale has a tremendous impact on the rate at which lithium ions can be extracted and
inserted into the material, thereby increasing the power density of the cell. Furthermore, reducing
the particle size also increases the surface area available for Faradaic reactions. Figures 2.6a and
2.6b show SEM images of LiMn2O4 nanoscale and micron-scale particles respectively
Figure 2.6 (a). SEM Images of nanoscale LiMn2O
4 active material. (b). SEM images of micron-
scale LiMn2O
4 active material. (c). Comparing rate behavior of nanoscale and micron-scale
LiMn2O
4 active material. Modified from ref. (7).
23
synthesized by Chen et al using a resorcinol-formaldehyde sol-gel route (7). Figure 2.6c shows
galvanostatic cycling results, demonstrating a tremendous increase in rate capability by simply
reducing the characteristic diffusion length of lithium ions in LiMn2O4. Enhanced rate capability
using nanomaterials has been demonstrated using a variety of synthesis methods including co-
precipitation(55), sol-gel synthesis (56), and hydrothermal synthesis (7, 57-59).
Similarly, this concept has been extended to high aspect ratio structures such as
nanowires and nanorods (4, 60, 61). In figure 2.7, LiMn2O4 nanowires that are 10nm in diameter
were synthesized by Hosono et al. using a two-step template based method (4). There are three
significant advantages to the nanowire morphology: i. The nanoscale diameter provides a short
lithium ion transport distance to the surface of the active material. ii. The high aspect ratio of the
nanowire provides a greater surface area enhancement than a nanoparticle of equal radius. iii.
High aspect ratio nanowires prevent Nanoscale active material from agglomerating, a common
challenge in nanoparticles. Nanowires synthesized by Hosono et. al. demonstrated strong
capacity retention and highly stable cycling behavior at greater than 160 times the rated current
Figure 2.7 (a). SEM Images of LiMn2O
4nanowires. (b). Comparing rate behavior commercial
LiMn2O4 nanoparticles to LiMn
2O
4 nanowires. Modified from ref (4).
24
for LiMn2O4. However due to the inefficiency of the templated synthesis method, high-power
cycling experiments were carried out using very low active material loadings. Therefore, a
commercial scale electrode has not been verified using nanowire active material.
In a similar concept, researchers also explored the synthesis of hollow active materials for
high-power Li-ion batteries (5, 62, 63). In figure 2.8, Sun et al. synthesized hollow particles in
which the inner portion of the active material particle is etched away to leave the thin outer shell.
The advantage of this design is two-fold:(54) (i) The thin nanoscale active material shell allows a
short diffusion length of lithium ions. (ii) The structural strain caused by lithium ions
intercalating and deintercalating is buffered by the inner void inside the particle, relieving
mechanical stresses which accelerate capacity fade during extended cycling. Researchers have
successfully synthesized single-shelled, double-shelled and triple-shelled active materials for a
variety of materials including LiCoO2, LiMn2O4, and vanadium oxides (V2O5). However, these
materials also require a multi-step synthesis and etching fabrication procedure, increasing the
price of the material.
2.4.2 Nanoporous Active Materials
Another method of improving the power density is by enhancing electrolyte wetting of the
electrode by using an ordered nanoporous or mesoporous material. These structures are
composed of micron-sized particles which are synthesized to contain ordered arrays of nanopores
or mesopores. The high porosity material allows electrolyte to penetrate deep within the
electrode material, enhancing lithium ion transport to into the lithium ion active material (64-67).
The mechanism by which the power density can be improved is given by evaluating the
tortuosity in equations 2.13 and 2.14(68).
25
(2.13)
(2.14)
Where and are the effective and intrinsic conductivities and and are the effective
and intrinsic diffusivities respectively of the conductive phase or electrolyte,. is the volume
fraction of the conductive phase or void fraction of the solid. If all conductive pathways are
made of straight channels of uniform cross section, then = 1. Otherwise, for pathways that are
tortuous, >1. Therefore, the effective transport properties are lower than intrinsic transport
properties in the bulk electrolyte. Nanoporous channels have the effect of reducing T, allowing
the and to increase.
2.4.3. Nanoscale Coatings and Conductive Meshes
The previous methods for enhancing the power density of lithium ion cells focused on reducing
by shortening the lithium ion diffusion length through the use of nanomaterials. However,
electronic conduction is equally critical for efficient electrochemical charge transfer. Most
lithium ion active materials exhibit insulating or semi-conducting properties, requiring
conductive additives to improve electronic transport in the bulk electrode. Uniformly dispersed
Figure 2.8 (a). SEM Images of hollow LiMn2O
4nanowires. (b). Comparing rate behavior
commercial LiMn2O nanoparticles to hollow LiMn
2O
4. Modified from ref (5).
26
conductive additives can form a robust percolating electrical network through which electron
transport occurs. However, strong Van Der Waals attractions at the nanoscale cause the nano
active materials to agglomerate into clusters, allowing only a small percentage of nanoparticles
to contact conductive additives. As a result, the electronic transport length, Le, is significantly
greater than the size of the particle (Figure 2.9a) (8). Additionally, the nanoparticle
agglomeration creates a large interfacial resistance in between neighboring particles. To mitigate
effects of non-uniform dispersion of conductive additives, two methods have been employed.
These include (i) the use of a conductive carbon coating on the nanomaterials (69-71) and (ii)
dispersing nanoparticles in a highly conductive network of graphene or carbon nanotubes (72-
74). Both methods have demonstrated significant enhancements in the rate capability. In the
schematic in figure 2.9b, the nanoparticles have been coated with a layer of carbon, allowing
(a) (b)
Figure 2.9 (a). Schematic of effective electronic path length in traditional electrodes caused by
agglomeration of conductive additives. (b). Schematic of electronic path length of nanoparticles coated
with conductive layer. Modified from ref. (8).
27
contacting nanoparticles to form a percolating electrical network on the outer surface of the
nanoparticles. Therefore, Le is reduced to approximately the particle diameter. In figure 2.10,
Xia et al. synthesized an ultrafine LiMn2O4/carbon nanotube (CNT) nanocomposite material
using a one-step hydrothermal treatment synthesis method (3). In this composite, 10-20nm
LiMn2O4 nanoparticles make intimate contact with the conductive CNT’s, allowing for highly
efficient Li+ diffusion and charge transfer. Additionally, high aspect ratio CNT’s offer robust
electrical connectivity throughout the bulk electrode structure resulting in significantly enhanced
rate behavior. However, both methods introduce large amounts of carbon into the electrode
structure. There are several drawbacks to using high mass loadings of carbon in commercial
scale electrodes. (i). High amounts of carbon add “dead” weight and volume to the cell reducing
gravimetric and volumetric energy density of the cell. (ii) High carbon mass loadings can also
result in a poor electrode packing density due to the high surface area of the carbon
nanomaterials, further limiting the volumetric energy density of the cell. Poor packing densities
also result in poor laminate adhesion to the current collector, causing material to delaminate,
possibly leading short circuiting. (iii) High mass loadings of carbon also accelerate electrolyte
decomposition due to a high surface area, often resulting in poor columbic efficiencies.
Therefore, while the usage of carbon composites to enhance the power density of lithium ion
electrodes is feasible, the mass loading of carbon must be controlled. Also, conductive carbons
must be carefully organized within the electrode structure to maximize the capacity and lifetime
of the cell.
28
Figure 2.10 (a). SEM Images LiMn2O
4 / CNT composite. (b). Comparing rate behavior
commercial LiMn2O nanoparticles to LiMn
2O
4./ CNT composites.(3)
2.5. Electrode Architectures
To mitigate the drawbacks of nanoparticle systems and control electron and ion pathways in the
bulk electrode, efforts have been made to organize electrode materials into macroscale
architectures (6). These architectures can be structurally classified into one dimensional, two
dimensional, and three dimensional designs. Electronic and ionic transport pathways within these
structures are thereby directed via transport channels within the architectures.
2.5.1. One Dimensional Architectures
One dimensional (1D) designs consist of vertically aligned structures fabricated on current
collecting substrates (Figure 2.11a). These 1D designs include microfabricated pillars, vertically
aligned CNTs or nanowires, and hybrid coaxial metal-oxide core-shell structures. These
structures are fabricated using templated techniques such as electrodeposition and chemical
vapor deposition (CVD). Unlike percolating networks of traditional electrode architectures, 1D
design provide direct pathways for electron transport to the current collector. Additionally, the
large void space between neighboring pillars facilitates ionic transport along the shaft of the
29
pillar, significantly enhancing the power density of the electrode. For example, Liu et al. reported
the excellent performance of 60nm diameter SnO2 nanorod arrays for lithium ion anodes. The
arrays were prepared on flexible metallic substrates via a hydrothermal process. The 1D nanorod
array electrode demonstrated a reversible capacity of 580mAh/g after 100 cycles at C/10 and
350mAh/g at 5C (75). However, electrochemical charge transfer is heavily dependent on the
contact resistance between vertically aligned structures and the current collector. Obtaining
robust contacts for vertically aligned structures over large electrode areas has proven to be very
difficult.
2.5.2. Two Dimensional Architectures
Two dimensional designs consist of a scaffolding-type matrix in which lithium ion active
materials are imbedded (Figure 2.11b) (76-78). Each nanoparticle makes intimate contact with
the conductive electronic mesh, reducing the electronic transport distance to the diameter of the
nanoparticle. The porosity of the mesh also allows for facile ion transport perpendicular to the
current collector. Additionally, the horizontal structure offers a high packing density. Popular
materials for the conductive scaffolding are carbon materials such as carbon nanotubes, porous
Figure 2.12 (a). One dimensional electrode architecture consisting of vertically aligned
pillars (b). Two dimensional electrode architecture consisting of conductive mesh. (c). Three
dimensional architecture consisting of a continuous porous network. Modified from ref. (6)
30
carbon, and graphene due to their excellent electrical conductivity and mechanical and
electrochemical stability. When used with high-capacity electrodes such as silicon, mechanically
robust networks of graphene and carbon nanotubes can mitigate the effects of large volume
expansion, allowing electrodes to sustain over 300 cycles (79). In some cases, CNT networks
were so electronically conductive and mechanically robust that the structure could be
delaminated from the current collector and cycled independently. Since, current collectors
comprise 40-60% of the weight of the cell, a free-standing electrode offers a significant
advantage in terms of gravimetric and volumetric energy density. Furthermore, the CNT
electrodes were highly flexible and demonstrated potential applications in flexible battery
designs.
2.5.3. Three Dimensional Architecture
Three dimensional designs are continuous or quasi-continuous networks of nanoporous metals or
conducting polymer scaffolds (Figure 2.11c). This structure allows for control of critical electron
transport lengths while maintaining a concentric interpenetrating network for lithium ions. Zhang
Figure 2.12 Three-dimensional bicontinuous electrode architecture fabricated from an
inverse opal template.
31
et al. demonstrated a highly effective 3D electrode architecture consisting of a bicontinuous
porous nickel structure (39) (Figure 2.12). In this design a self-assembled opal template, which
consisted of interpenetrating void space was electrodeposited with nickel and active material. By
controlling the thickness of active material deposit, a highly organized porous network could be
formed. This structure simultaneously addresses all limiting resistances of a high power
electrode structure. (i). The thin electro-deposited film allowed for a short Li+ diffusion distance
while the interconnected porous network enabled extremely long ion transport lengths. The high
surface area also enabled rapid electrochemical charge transfer, and the highly structured current
collector enabled rapid electronic transport to the external circuit. This electrode structure
demonstrated discharge rates up to 400 and 1000C, surpassing the power density of
supercapacitors while maintaining the energy storage capacity of batteries.
2.6. Carbon Nanotubes in Lithium Ion Batteries
2.6.1. Structure and Properties
Due to their remarkable electrical, thermal, and mechanical properties, carbon nanotubes (CNT)
have gain significant attention for applications in electronics, sensors, composites, and energy
storage. While these properties are important for any application, these properties are critical to
the safety and longevity of the battery. Here we review the electrical, mechanical properties of
carbon nanotubes for use as lithium ion electrode materials.
Carbon nanotubes are seamless cylinders of one or more graphite sheets. Single-walled
carbon nanotubes (SWNT) consist of one sheet while multi-walled carbon nanotubes (MWNT)
consist of concentric layers that are spaced 0.34nm apart. According to the rolling angle of the
32
graphene sheet with respect to the tube axis, CNTs can assume three different chiralities:
armchair, zigzag, and chiral. These chiralities are defined by the chiral vector (80):
(2.15)
Where and are the number of steps along the unit vectors and of the hexagonal lattice.
2.6.2. Electronic Conductivity
The chirality has a significant impact on the electronic properties. For a given ( , ), if 2 +
is a multiple of 3, then the CNT exhibits metallic behavior, otherwise, the behavior is semi-
conductor like. Since MWNT contain multiple layers with different chiralities, the electronic
properties are more complex. However, most MWNT exhibit metallic behavior. For purified
materials the theoretical electrical conductivity is 5x104 -5x10
5 S/m, an order of magnitude
higher than traditional lithium ion conductive carbons (81, 82). Additionally, unlike carbons such
as graphite, diamond, and fullerenes, CNTs are a 1D material with an aspect ratio of 10,000. Due
to their long aspect ratios, CNT composites exhibit long-range connectivity that allows for lower
weight doping levels to achieve a comparable percolation threshold.
2.6.3. Mechanical Robustness
Chemical structure CNTs are composed entirely of sp2
hybridized carbon-carbon bonds, which is
stronger than sp3 bonds found in diamond. This sp
2 hybridization gives rise to the exceptional
mechanical strength of CNTs. The Young’s modulus for SWNT paper is reported to be 5-10GPa,
which indicates a layer of randomly orientated carbon nanotubes can withstand a large applied
force before plastic deformation occurs (81). Similarly a tensile strength of 80-100MPa has also
33
been reported. While these properties vary significantly based on synthesis steps, the reported
values suggest that CNTs are among the strongest and stiffest material on earth.
2.6.4. Thermal and Chemical Stability
Additionally, CNTs have exceptional thermal stability and thermal conductivity (83, 84).
Berber et al. found the thermal conductivity of SWNT at room temperature to be about 6600 W
m-1 K-1(85). Yang et. al reported that the thermal conductivity of multi-walled carbon
nanotubes MWNT is 200 W m-1K-1(86). Again, as a result of synthesis steps and measurement
technique, large variations of measured thermal conductivities exist. Nevertheless, is clear that
the thermal conductivity of CNT’s is comparable to excellent thermal conductors such as
graphite or diamond. Carbon nanotubes are also thermally stable up to 600-700ºC. Therefore, an
architecture which utilizes the remarkable properties of CNTs is highly desirable and has the
potential to alleviate many shortcomings of high-power lithium ion batteries.
34
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40
Chapter 3: Layer-by-Layer Carbon Nanotube Electrode Architecture
3.1. Why Layer-by-Layer?
In the present work, a two-dimensional electrode architecture is chosen to enhance the power
density of lithium ion batteries and address engineering challenges nanomaterials such as low
packing density and high fabrication cost. This structure modifies traditional electrode
architectures composed of mixtures of lithium ion active material, carbon black, and polymeric
binders by incorporating alternating layers of carbon nanotubes. Fabricated in a bottom-up
fashion, layers of MWNT and lithium ion active material are stacked together on top of a
metallic current collector to form a multi-layer structure. The alternating layers of carbon
nanotubes form a highly conductive and highly porous matrix that facilitates lithium ion
diffusion throughout the electrode. However, the porosity of the MWNT layers does not sacrifice
the packing density or volumetric energy density of the electrode. Additionally, the simple planar
design is highly robust, allowing the electrode stack to withstand high cell pressure from
aggressive cycling conditions. Furthermore, the electrode structure can be fabricated under room
temperature and atmospheric pressure, demonstrating full compatibility with current electrode
manufacturing techniques. Finally, we demonstrate the versatility of the multi-layer structure
when applied to different lithium ion materials, such as the high capacity lithium-rich lithium
nickel manganese cobalt oxide, 0.3Li2MnO3 0.7LiMn0.333Ni0.333Co0.333O2 system (1, 2).
3.2. Development of Low-cost Fabrication Process for Electrode Architecture
3.2.1. Fabrication
41
The fabrication of the layer-by-layer electrode architecture is accomplished using a spin-casting
process in which a suspension of the material of interest is applied to the surface of a substrate
(Figure 3.1). The substrate is spun at high revolutions per minute (RPM) under a set acceleration
to achieve a uniform coating of the material over the substrate. The substrate is a circular metal
current collector. Since we focus our study on the cathode, we use aluminum as the current
collector. The surface of the aluminum is roughened by abrading with ultrafine sand paper to
remove the native aluminum oxide and increase the surface wettability. After a concentrated
suspension of MWNT (NanoLab) in N-methyl-2-pyrollidone (NMP) solvent is applied to the
roughened aluminum surface, the aluminum electrode is spun at high revolutions per minute
(rpm) to achieve a uniform layer of MWNT over the aluminum surface. The MWNT suspension
is reapplied and spin-casted several times until the desired MWNT loading is obtained. The
electrode is then dried at 80⁰C to remove residual solvent. Next, the active material layer is spin-
cast over the MWNT layer. The active material layer is composed of a mixture of LiMn2O4,
carbon black, and a polymeric binder. After the active material layer is spin-casted, the electrode
Figure 3.1. Fabrication of multi-layered electrodes using spin-coating method.
42
is dried again at 80⁰C to remove residual solvent. The spin-casting of the MWNT and active
material layers is repeated as necessary to obtain the desired active material loading. After
adding N number of MWNT and active material layers, the multi-layer stack is capped with a
final layer of MWNT. All electrodes contain a final capping layer of MWNT unless otherwise
indicated.
3.2.2. MWNT Suspension
For preparing a stock electrode suspension, MWNT powder purchased from NanoLab (15-45nm
diameter, 5-20µm length). MWNT power is suspended in N-methyl-2-pyrollidone (NMP) and
sonicated using a Crest Ultrasonics benchtop sonicator for 2-3 hours after preparation. Even after
sonication, the MWNT suspension is very non homogeneous due to the presence large (5-10µm)
MWNT agglomerates. To preserve the conductivity of the MWNT, no additional treatment steps
are performed to homogenize the suspension. Prior to each use the suspension is sonicated for
15 minutes. Using this MWNT suspension composition, the effect of the concentration on the
MWNT loading was studied to achieve a MWNT loading of 0.2mg/cm2 on the current collecting
substrate. It was found that this loading could be achieved using a concentration of 0.8mg/cm2
after 6 spin cycles.
3.2.3. Active Material Suspension
43
In this work two different lithium ion active materials were used. Lithium manganese oxide
(LiMn2O4) was purchased commercially (EM Industries, Merck) while layered-layered lithium
nickel manganese oxide (NMC) 0.3Li2MnO3 [0.7LiMn0.333Ni0.333Co0.333O2] was synthesized in-
house(1, 2). The active material suspension was prepared using a mixture of polyvinylidene
fluoride (PVDF) (Kynar), carbon black (Super P), and lithium ion active material. The
concentration and composition of the active material layer affected not only the active material
loading but also the quality of the film. Poor quality films often lead to material flaking and
delaminating from the current collector leading to short circuiting of the cell. It was found that
for a composition of 77: 20: 3 active material: carbon black: PVDF, a concentration of 333mg/ml
Figure 3.2. (a). Top-view SEM image of CNT Layer. (b). Side-angle SEM image of CNT layer.
(c), (d). Low magnification and high magnification images of active material layer consisting of
micron-sized active material particles and 60nm carbon black particles.
44
provided the appropriate viscosity to obtain an active material loading of 2 mg/cm2 of active
material loading per layer.
A typical layer of MWNT spin-casted over the surface of an aluminum current collector
is shown in the SEM image in figures 3.2a and 3.2b. A typical carbon nanotube loading of 0.08
mg/cm2 obtained after 4-6 spin cycles. The SEM image demonstrates a random entangled
network of singular carbon nanotubes with inter-tube distances ranging from approximately 50-
300nm. Figures 3.2c and 3.2d show the morphology of a typical active material layer consisting
of irregularly shaped active material particles approximately 1µm in diameter dispersed with
spherical carbon black particles approximately 60nm in diameter. A typical active material
loading of 2mg/cm2 per layer is obtained after 2 spin cycles. In the active material layer, the
chains of carbon black particles form percolating chains for electron transport to the underlying
MWNT layer. In contrast to the close-packed morphology of the active material layers, the
morphology of the intermittent MWNT layers is highly porous. The MWNT layers, therefore,
provides not only a highly conductive network for electronic transport, but also a highly porous
structure to facilitate lithium ion diffusion throughout the electrode. Furthermore, high aspect
ratios of MWNT create strong nanotube entanglement and eliminate the need for binders to
promote adhesion to the current collector.
3. 3. Physical Characterization
3.3.1. SEM Characterization
45
Layer-by-layer fabrication of films enables a high degree of control over the film thickness and
density, thereby allowing control over properties such as conductivity and optical transparency
of the film (3). In this work, layer-by-layer fabrication is used to tune the film thickness, thereby
tuning electronic and ionic transport properties of the electrode. Figure 3.3a and 3.3b
respectively show schematic and cross-sectional SEM images of the multi-layered sandwich
structure consisting of five MWNT layers and four active material layers fabricated on top of an
aluminum current collector. The bottom MWNT layer, which is in contact with the current
Figure 3.3. (a). Schematic of multi-layered electrode structure consisting of four bi-layers and a
capping MWNT layer. One bi-layer consists of one MWNT layer and one active material layer.
(b). Scanning Electron Microscope (SEM) image of cross-section of multi-layered structure with
four bi-layers. Total electrode thickness: 13.1µm. Total active material loading: 4.5mg/cm2. (c).
Cross-sectional SEM image of four bi-layer electrode with active material loading of 9.1 mg/cm2.
(d). Cross-sectional SEM image of standard fabrication electrode with active material loading of
5.6 mg/cm2.
46
collector, is fabricated to be slightly thicker (1-2µm) than the other MWNT layers (0.3-0.7µm).
The greater thickness of the bottom MWNT layer prevents delamination the electrode material
from the current collector. All electrodes also contain an additional “capping” layer of MWNT at
the top of the bi-layer stack. The capping MWNT layer serves to electrically connect the upper
most active material surface to the current collector and protect the electrode structure from
electrolyte attack. If the MWNT were simply mixed with the active material, the resulting
structure would result in a poorly percolating network, low porosity, and poor adhesion to the
current collector without significantly higher binder content. Thus, it is often reported that the
use of MWNT as conductive additives result in only modest improvements in capacity
enhancement (4).
Although the MWNT layers are highly porous, the high porosity is not detrimental to the
packing density of the overall structure. In figure 3.3c and 3.3d, a multi-layered structure is
compared to a conventionally prepared electrode. In the conventionally prepared electrode, the
active material layer is cast onto the current collector. The thickness of the four bi-layer electrode
is 25-30µm while the thickness of the standard fabrication electrode is 35-45µm. Even though
the thickness of the standard fabrication electrode is 5-10µm thicker than the four bi-layer
electrode, the loading of the four bi-layer electrode is 40% greater. These values correspond to a
total electrode density between 3.0-3.6g/cm3
and 1.2-1.6g/cm3
for the four bi-layer and standard
fabrication electrodes respectively(5).
3.3.2. Surface Morphology
In the cross sectional view of the MWNT layer deposited on a thick aluminum current collector
(figure 3.2b), a highly non-uniform MWNT surface is apparent. The non-uniformity of the
47
MWNT surface is due to large MWNT agglomerates from the MWNT suspension depositing
non-uniformly during the spin-casting process. Though, Atomic Force Microscopy (AFM) large
1µm peaks and valleys can be observed on the MWNT surface. The surface roughness of the
MWNT 10 µm2 surface is 0.164nm. The surface area enhancement provided by the non-
uniformity further promotes the adhesion of the overlying layer of active material.
3.3.3. DC Conductivity
To minimize the inactive
components of the cell, it is
important to optimize the
conductivity of the MWNT layers.
In figure 3.4, MWNT were loaded
on a silicon substrate at different
densities and the conductivity the
MWNT film was measured using 4-
point probe to obtain the sheet
resistance. The MWNT layer
exhibits percolation like behavior as the sheet resistance exponentially decreases as a function of
the loading (6, 7). Similarly, the standard deviation of the sheet resistance also decreases as the
carbon nanotube film becomes more uniform. The threshold from percolation to linear behavior
occurs at a carbon nanotube loading of 0.05mg/cm2. For multi-layer electrodes, we require a
carbon nanotube loading that is above the percolation threshold. Therefore, a typical carbon
nanotube loading of 0.08 – 0.10 mg/ cm2
is chosen for the MWNT layer. For this loading, the
Figure 3.4. Sheet resistance and thickness of a
MWNT layer as a function of carbon nanotube
loading.
48
sheet resistance corresponds to 10 Ω/□. The corresponding resistivity of the MWNT layer is 3.4
x 10-4 Ω·m, which is similar in magnitude as MWNT current collecting electrode films (7, 8).
Figure 3.4 also shows the thickness increases linearly as a function of MWNT loading. However,
unlike, the sheet resistance measurements, the standard deviation also increases as a function of
loading. This is due to an increase in the roughness of the MWNT surface. The thickness
measurements in figure 3.4 translate to a carbon nanotube density of 0.268g/cm3. According to
equation 3.1, the porosity of the MWNT layer is approximately 90%, where φ represents the
porosity of the MWNT layer, represents the density of the MWNT layer, and
, represents the true density of MWNT (9). The density of the MWNT layer is similar
to the density of MWNT powder reported by MWNT commercial vendors. Therefore, the
MWNT layers serve as highly porous, highly conductive current collecting scaffolding
throughout the electrode structure (9).
(3.1)
3.4. Electrochemical Characterization
3.4.1. Galvanostatic/Potentiostatic Cycling
Electrochemical characterization
was performed in two different
electrochemical cell types: CR2025
lithium ion button cell and a T-cell
shown in figures 3.5a and 3.5b
respectively. CR2025 cells, which Figure 3.5. Electrochemical cells used to test multi-
layered electrodes. (a). T-cell. (b). CR2025 button cell.
49
were 20mm in diameter and 2.5mm in thickness, consisted of a round stainless steel casing
which also provided contacts between the aluminum current collector and the external circuit.
The T-cell consisted of a polypropylene casing and stainless steel contacts. Both cell types were
spring-loaded to maximize the pressure on the cell stack. A significant advantage of the CR2025
cell is the automated sealing, which allows for consistent pressure on the cell stack, minimizing
performance variability. The pressure applied to the cell stack in the T-cell is variable. However,
the T-cell allows for facile cell disassembly for post-cycling characterization. Cells tested in a
half cell configuration were cycled using lithium foil as the counter electrode while cells tested
in a full cell were cycled versus copper current collectors coated with various anode materials.
Figures 3.6a and 3.6b show cyclic voltammetry and galvanostatic cycling measurements
of a LiMn2O4 standard fabrication cathode electrochemically tested in a half cell configuration.
A half cell here is defined as a cell composed of a Li metal anode and a test electrode as the
cathode. In figure 3.6a, the anodic and cathodic scans demonstrate prominent peaks at 3.9V and
4.1V. These peaks correspond to lithium ion intercalation into LiMn2O4. Similarly, in figure
3.6b, lithium ion intercalation is represented by voltage plateaus, occurring at 3.9V and 4.1V. In
figure 3.6b, the discharge capacity of the standard fabrication LiMn2O4 cathode was further
Figure 3.6. (a). Cyclic voltammetry measurements of an LiMn2O
4 cathode in a half cell
configuration. Scan rate: 70µV/s. (b). Galvanostatic cycling of a LiMn2O
4 cathode.
Current density: 12mA/g.
50
evaluated using galvanostatic cycling under higher discharge currents. The data in figure 3.6b
was used as a baseline used for comparison of multi-layered electrodes at high and low discharge
currents. Figure 3.7shows the rate comparison of multi-layered and the standard fabrication
electrode. In figure 3.7a, the rate of discharge is described as a “C-rate” where 1C is 120mA/g.
At lower discharge rates (C/10 – 1C) all electrodes, including the standard fabrication electrode,
exhibit greater than 90% of the theoretical capacity of LiMn2O4. At 2C, all electrodes begin to
exhibit slight capacity fade due to electrode polarization. This is consistent with reported data on
LiMn2O4 (10). Beyond 2C, the rate performance is strongly dependent on of the number of bi-
layers. As the number of bi-layers increases, both the lithium ion diffusion distance through the
electrode and electronic resistance of the electrode increase, causing the rate capability to
diminish. Accordingly, the one bi-layer electrode exhibits the highest rate capability, achieving a
discharge capacity of 60mAh/g at a discharge rate of greater than 60C while the four bi-layer
51
electrode achieves 60mAh/g at approximately 14C. However, even at a lower loading, the
capacity of the standard fabrication electrode rapidly diminishes at rates higher than 5C. At a rate
of 10C, the four bi-layer electrode exhibits a capacity per area that is 20x greater than the
standard fabrication electrode due to the higher loading. Since all cell materials and components,
for both the multi-layer and standard fabrication electrodes, are identical, the superior rate
capability of the multi-layer structure can only be attributed to the high conductivity and porosity
of the intermittent carbon nanotube layers in the multi-layer electrodes. Additionally, figure 3.7b
shows that the rate behavior of the four bi-layer electrode is highly stable over 100 cycles.
3.4.2. Electrochemical Impedance Spectroscopy
Electrochemical impedance spectroscopy (EIS) is an analytical technique in which the
impedance of an electrochemical cell is analyzed by measuring the current of the cell in response
Figure 3.7. (a). Rate capability of multi-layer electrodes and standard fabrication electrode.
1C = 120mA/g current applied. (b). Extended cycling of four bi-layer electrode during
discharge at 15.7C. Charging rate is held constant at C/10.
52
to a small applied AC potential. (11-13). This technique is used to investigate processes in an
electrochemical cell, which occur at different time constants, by their characteristic impedances.
These processes include electronic/ionic conduction in the electrode and electrolyte, formation of
an electric double-layer between charged ions at the electrode surface, electrochemical charge
transfer, and the diffusion of Li+ though electrode pores and within the active material. By fitting
EIS data to an equivalent electrical circuit model, electrochemical process can be quantified
using basic circuit elements. To compare of the multi-layered electrode architecture and the
standard fabrication electrode, EIS spectra of both electrodes were measured and fitted to the
Randles circuit model. In the Randles circuit, Rs, is the ohmic resistance, which represents the
sum of all DC resistances in the cell including the anode, electrolyte, separator, cathode, and cell
casing. Rs is represented by a resistor because it contributes only to the real component of the
total impedance of the cell. Cd represents the capacitance due to the formation of an electric
double layer. This double layer exists on the interface between the charged electrode and its
surrounding electrolyte ions, which are oppositely charged. Electrochemical charge transfer,
which is a process with a measureable kinetic rate, is also represented by a real resistance, Rct.
The kinetic rate is dependent on the type of reaction, temperature, concentration of the reaction
products and the DC potential of the cell. When the real and imaginary impedances in equation 3
are decomposed and plotted as a function of the frequency, these basic processes appear as a
high frequency semi-circle and a low frequency linear region on a Nyquist plot. Approximate
values of the ohmic resistance, Rs, charge transfer resistance Rp, double layer capacitance Cd,
can be extracted from the first and second x-intercepts of the Nyquist plot and the y-intercept at
the peak of the semi-circle respectively.
53
In figure 3.8, electrochemical
impedance measurements of the four
layer electrode and standard fabrication
electrode were taken at a DC cell
potential of 4.2V. Both electrodes
exhibit one high-frequency semi-circle
and a low frequency linear region.
Therefore, both impedance spectra can
be described by the Randles circuit
shown in the inset of figure 3.8. The
ohmic resistances, Rs, of the electrode cells, are represented by the first x-intercept of the high
frequency semi-circles. These values are 5.1 Ω/cm2 and 4.91 Ω/cm
2 for the four bi-layer electrode
and standard fabrication electrode respectively. The charge transfer resistances, Rct, indicated by
the difference of the first and second x-intercept, are 42.5 Ω/cm2 and 90 Ω/cm
2 for the four
bilayer and standard fabrication electrodes respectively. The impedance spectra in figure 3.8
indicates the reason for improved rate capability of the four bi-layer electrode versus the standard
fabrication electrode in figure 3.8 is the improved charge transfer resistance of the four bi-layer
electrode.
3.5 Effect of Electrode Parameters on Electrochemical Performance
3.5.1 Effect of Active Material Composition
Since the high conductivity of the MWNT layers may enable the reduction of other inactive
components, we investigate the effect of reducing carbon content to increase the specific energy
Figure 3.8. Electrochemical impedance spectra of
four bi-layer electrode (dark blue) versus the
standard fabrication electrode (black).
54
of the battery. Figure 3.9 shows the rate performance of four bi-layer electrodes with varying
active material composition relative to standard fabrication electrodes. Similar to the case in
figure 3.7, the four bi-layer electrode with 87% LiMn2O4, 10% carbon black, and 3% PVDF in
the active layer exhibits a raw aerial capacity that is 14x greater than the standard fabrication
electrode at 2C. The comparative rate behavior in figure 3.9 demonstrates that the multi-layer
structure can be used significantly reduce inactive components such as carbon black and PVDF
while maintaining the rate capability.
3.5.2. Effect of Active Material Thickness
Using an identical composition as electrodes in figure 3.7, we investigate the effect of
Figure 3.9. Comparing rate capabilities of standard fabrication and four bi-layer
electrodes fabricated using various compositions of LiMn2O
4, carbon black, and PVDF.
Red squares: 87:10:3 LiMn2O
4: carbon black: PVDF. Blue circles: 77:20:3.
0 2 4 6 8 10 12 14 16 18 20 22
0
20
40
60
80
100
120 S td , 1 0 % ca rb o n 1 .8 5 m g /cm
2
S td , 2 0 % ca rb o n 2 .7 6 m g /cm2
4 L a ye r, 2 0 % ca rb o n 6 .1 8 m g /cm2
4 L a ye r, 1 0 % ca rb o n 7 .1 9 m g /cm2
Dis
ch
arg
e C
ap
ac
ity
(m
Ah
/g)
C -ra te
55
active material layer thickness on the rate capability. In figure 3.10, active material loading of the
four bi-layer electrode has been increased by 47% relative to that in figure 3.7. The four bi-layer
electrode, which has an active material loading of 8.93mg/cm2, achieves a total raw aerial
capacity of 1.0mAh/cm2. For discharge rates up to 10C, the four bi-layer electrode in figure
3.10b demonstrates identical capacity retention to that in figure 3.7. However, at rates higher
than 10C, we observe that capacity fade is more rapid in figure 3.10b.
As observed in figures 3.6 and 3.7, the rate capability is heavily dependent on the active
material loading. In figure 3.10c, we demonstrate how the multi-layered structure can be
effectively used to mitigate the effect of capacity fade at high discharge rates while maintaining a
high active material loading. In figure 3.10c, we plot the rate capability of two bi-layer electrodes
with varying active material loadings against a four layer electrode with the highest loading. For
the case of two bi-layer electrodes, it can be seen that rate curves branch out as a function of the
active material loading. However, the four bi-layer electrode, which has the highest active
material loading (7.19 mg/cm2) demonstrates a higher rate capability relative to the two bi-layer
electrode with an active material loading of 6.57mg/cm2. Therefore, although there is an inherent
tradeoff between rate capability and active material loading, as demonstrated in figures 3.7a and
3.10b, by tuning the active material loading per layer, the rate capability can be maintained
without sacrificing gravimetric or volumetric energy density.
56
3.5.3. Effect of Changing Active Material
Due to the high rate of lithium ion diffusion within the spinel structure, LiMn2O4 is well-known
to exhibit high rate behavior, exhibiting 80% capacity retention at 5C (14). Therefore, it is highly
valuable to understand the behavior of a lower-rate lithium ion active material employed in the
multi-layered MWNT architecture. In figure 3.11, we investigate the rate capability of in-house
synthesized lithium rich layered 0.3Li2MnO3 [0.7LiMn0.333Ni0.333Co0.333O2] cathode material
(called Li-rich NMC) in the multi-layered electrode architecture. The material loading and raw
aerial capacities of multi-layer electrodes fabricated using 0.3Li2MnO3
[0.7LiMn0.333Ni0.333Co0.333O2] are shown in figure 3.11a. Similar to LiMn2O4 multi-layer
electrodes, the loading and the initial discharge capacities linearly increase with the number of
layers. However, since the theoretical capacity of 0.3Li2MnO3 [0.7LiMn0.333Ni0.333Co0.333O2]
(280mAh/g) is significantly greater than LiMn2O4 (120mAh/g), the raw aerial capacities are
significantly higher than those of the LiMn2O4 multi-layer electrodes. The four layer electrode,
which contains an active material loading of 9.1 mg/cm2, achieves an initial discharge capacity
Figure 3.10. (a). Active material loading and raw aerial capacity of multi-layered electrodes as a
function of the number of bi-layers. MWNT are included in the mass of the active material. (b). Rate
capability of multi-layer electrodes. (c). Rate capability of two bi-layer electrodes of various loadings
compared to a four bi-layer electrode. All active material layers are composed of 87:10:3.
57
of 2.4 mAh/cm2
at a discharge rate of C/20. The inset of figure 3.11a shows an SEM image of the
0.3Li2MnO3 [0.7LiMn0.333Ni0.333Co0.333O2] particle morphology, which is similar to that of
LiMn2O4. Particles ranging from 300-500nm in size agglomerate to form large micron sized
clusters. Because the average particle sizes for both the LiMn2O4 and 0.3Li2MnO3
[0.7LiMn0.333Ni0.333Co0.333O2] active materials are equivalent, the lithium ion diffusion length in
the solid state of both LiMn2O4 and 0.3Li2MnO3 [0.7LiMn0.333Ni0.333Co0.333O2] multi-layer
electrode architectures is constant. Therefore, the improvement of the rate capability of multi-
layer electrodes is independent of the lithium-ion diffusion length in the solid state.
58
The rate capability of multi-layer electrodes is shown in figure 3.11b relative to a standard
fabrication 0.3Li2MnO3 [0.7LiMn0.333Ni0.333Co0.333O2] electrode. For simplicity, only the one bi-
layer and four bi-layer, and standard fabrication electrodes are shown. The loadings of the one
bi-layer, four bi-layer and standard fabrication electrodes are 1.98mg/cm2, 9.1mg/cm
2, and
6.5mg/cm2
respectively. All three electrodes in figure 3.11b demonstrate greater than 80%
Figure 3.11. (a). Active material loading and raw aerial capacity of multi-layered electrodes
as a function of the number of bi-layers. Rate of discharge = C/20 where 1C = 280 mA/g. (b).
Rate capability of multi-layer electrodes and standard fabrication electrode. (c). Ragone plot
comparing power and energy of four bi-layer and standard fabrication electrodes. Values are
normalized per area. (d). Cycling of four bi-layer electrode during discharge at 1C. Charging
rate is held constant at C/20. For all subfigures of figure 3.11, active material layer
composition is 77:20:3 LLNMC: carbon black: PVDF.
59
capacity retention at a discharge rate of C/20. As the discharge rate is increased to above C/10,
strong electrode polarization causes the capacity of all electrodes to diminish rapidly. At 1C, all
electrodes exhibit 150mAh/g, or 53.5% of the theoretical capacity. Significant electrode
polarization in LiMn2O4 electrodes in figure 3.7a is not evident at discharge rates slower than
1C. Furthermore, although evident, the dependence of rate capability on active material loading
is not as strong as for the case of LiMn2O4 electrodes. These results suggest that lithium ion
diffusion within the active material is the dominant resistance.
At higher discharge rates of 10C and 20C, the one layer Li-rich NMC electrode achieves
the highest rate capability, exhibiting 85.4mAh/g and 58 mAh/g, respectively. The four layer
electrode achieves a discharge capacity of 59.8mAh/g and 26.2mAh/g at discharge rates of 5C
and 10C respectively. The standard fabrication electrode achieves discharge capacities of
73.8mAh/g and 26.2mAh/g at 5C and 10C respectively. The aerial power output versus the
energy output of the four bi-layer and standard fabrication electrodes is presented as a Ragone
plot in figure 3.11c. Although the four layer and standard fabrication electrode demonstrate a
similar rate capability in figure 3.11c, the higher material loading of the four-layer electrode
allows for a higher discharge capacity per area, and therefore discharge energy and discharge
power per area.
Figure 3.11d demonstrates the cycle life of the four layer electrode. After 100 cycles at
1C, the discharge rate was reduced to C/20 for two cycles, increased to 1C for an additional 100
cycles, and again reduced to C/20 for two cycles. The four-layer electrode recovers greater than
90% of the initial capacity, when the cycling rate is reduced to C/20 confirming that there is no
structural damage to the electrode after high rate cycling. When the rate is increased to 1C, the
four-layer electrode exhibits a capacity of approximately 160mAh/g with a capacity fade of
60
0.28mAh/g per cycle over the course of 100 cycles. After 100 cycles at 1C, the rate is again
reduced to C/20, at which, the electrode yields 77.5% of the theoretical capacity. Therefore, over
the course of 100 cycles, there is an increase in internal resistance of the cell leading to a 10%
decreasing in the total capacity of the cell.
3.6. Conclusions
In this chapter, a high-power carbon nanotube-based electrode architecture has demonstrated in
conjunction with LiMn2O4 active material to exhibit 14-20x enhancement in aerial capacity over
standard electrodes. The architecture also exhibits highly stable cycling behavior at 15C for
greater than 100 cycles. The high-rate behavior is due to the planes of highly conductive and
highly porous carbon nanotube layers which enhance electronic and ionic transport through the
electrode. Table 3.1 compares performance parameters of LiMn2O4 electrodes in the literature
and multi-layered LiMn2O4 electrodes prepared at the Center for High-rate Nanomanufacturing.
In addition to high energy and power density, the multi-layered structure also offers excellent
capacity retention, high volumetric energy density, and low cost of fabrication.
The carbon nanotube-based architecture has also been demonstrated in conjunction with
Li-rich layered NMC active materials to exhibit 70% higher capacity at C/2. Similarly, highly
stable cycling behavior at 1C for over 500 cycles is also observed. Using the CNT-based
electrode architecture, we demonstrate that the power density of an electrode can be enhanced
without sacrificing the volumetric or gravimetric energy density of the electrode. Finally, using
a room temperature and atmospheric fabrication process, we demonstrate that the electrode
architecture is fully compatible with commercial electrode manufacturing techniques.
61
Specific Energy
(mWh/cm2
)
Specific Power
(mW/cm2
)
Inactive Components (%)
Reported Cycle Life
Volumetric Energy Density
(mWh/cm3
)
Cost of Fabrication
1. J. Electrochem. Soc. 2000. (15)
1.92 28.8 22 No long term
study High Low
2. Nano Letters .2010. (16)
0.432 8.64 25
78 mAh/g maintained
for 100 cycles Low Very high
3. J. Mater. Chem. 2011. (17)
1.92 4.8 30
4.8% loss after 80 cycles
Low Moderate
4. J. Power Sources. 2011. (18)
1.44 40.8 20 No long term
study Low Low
5. J. Mater. Chem. 2013. (19)
2.02 14.3 20 8% loss after
100 cycles High High
6. J. Mater. Chem. 2011. (20)
0.96 80 44 4% loss after
100 cycles Low Moderate
7. CHN NEU 4.32 21.6 13-23 5% loss after
100 cycles Very high Low
Table 3.1. Performance parameters of LiMn2O4 electrodes found in literature compared to multi-layered LiMn2O4 electrodes
constructed at the Center for High-rate Nanomanufacturing.
62
3.7. References
1. W.C. West, J. Soler, M.C. Smart, B. V. Ratnakumar, S. Firdosy, V. Ravi, M. S.
Anderson, J. Hrbacek, E. S. Lee, and A. Manthiram, J. Electrochem. Soc., 158, 7 (2011).
2. M.M. Thackeray, S.H. Kang, C.S. Johnson, J.T. Vaughey, R. Benedek and S.A. Hackney,
J. Mater. Chem., 17, 15 (2007).
3. S.W. Lee, B.S. Kim, S. Chen, Y. Shao-Horn, and P. Hammond, J. Am. Chem. Soc., 131,
10 (2009).
4. Landi, B., Ganter, M., Cress, C., DiLeo, R., Raffaelle, R., Energy Environ. Sci., 2, 638
(2009).
5. Yongyao Xia, Yunhong Zhou, and Masaki Yoshio, Journal of the Electrochemical
Society, 144, 9 (1997).
6. L. Hu, D.S. Hecht and G. Gruner, Nano Lett. , 4, 5 (2004).
7. L. Hu, J.W. Choi, Y. Yang, S. Jeong, F. La Mantia, L.F. Cui and Y. Cui, Proc. Natl.
Acad. Sci. U. S. A., 106, 5 (2009).
8. S. W. Lee, N. Yabuuchi, B.M. Gallant, S. Chen, B.S. Kim, P.T. Hammond, and Y. Shao-
Horn, Nat. Nanotechnol., 5, 7 (2010).
9. S. Babinec, H. Tang, A. Talik, S. Hughes and G. Meyers, J. Power Sources, 174, 7
(2007).
10. Z. Jiang and K.M. Abraham, J. Electrochem. Soc., 143, 9 (1996).
11. A. J. Bard and L. Faulkner, Electrochemical Methods: Fundamentals and Applications,
John Wiley & Sons, Inc., Danvers, MA (2001).
12. Li, H., Huang, X., Chen, L., Journal of Power Sources, 81-82, 340 (1999).
63
13. Dees, D., Gunen, E., Abraham, D., Jansen, A., and Prakash, J., J. Electrochem Soc., 152,
A1409 (2005).
14. K.M. Abraham, ECS Trans., 41 (2012).
15. M. Lanz, C. Kormann, H. Steininger, G. Heil, O. Haas and P. Novak, J. Electrochem
Soc., 147, 4 (2000).
16. Hosono E, Kudo T, Honma I, Matsuda H, Zhou H, Nano Letters, 3 (2009).
17. Xin Zhao, Cary M. Hayner and Harold H. Kung, Journal of Materials Chemistry, 21
(2011).
18. Y. Chen, K. Xie, Y. Pan and C. Zhen, J. Power Sources, 196 (2011).
19. Y. Qiao, S.-R. Li, Y. Yu, C.-H. Chen, J. Mater. Chem. A, 1, 860 (2013).
20. Bak, S., Nam, K., Lee, C., Kim, K., Jung, H., Yang, X., and Kim, K.,, Journal of
Materials Chemistry, 21, 7 (2011).
64
Chapter 4: High-Temperature Performance
4.1. Introduction: Energy Loss Due to Structural Degradation
Two of the most promising cathode materials for next generation batteries are LiMn2O4 and
layered Li-rich NMC due to the high power capability of the former and high energy capability
of the latter as discussed in Chapter 3. Companies such as Samsung are developing of
composites of these cathode materials to harness the properties of both materials in
EV/PHEV/HEV applications. However both of these cathode materials and many others exhibit
slow energy loss due to the slow structural degradation of the material during continuous
cycling. The energy of the battery is calculated according to equation 4.1 as:
∫ ( ) (4.1)
Where is the total energy of the cell and is the total capacity of the cell. ( ) is the cell
voltage, which affects the measured average voltage. The specific energy and energy density of
the battery are defined as the energy of the battery divided by the weight and volume
respectively. Therefore, if either or decreases during continuous cycling, the cell suffers
from a loss of total energy. Although the mechanism of structural degradation in cathode
materials is not entirely elucidated, it is clear that the degradation accelerated when the cell is
operated above room temperature, even at moderate temperatures of 50-60ºC (3). Therefore in
their present state, these cathode materials are not suitable for EV applications which operate
over the temperatures range of -40ºC to +50ºC for lifetimes of greater than 10 years.
4.2. Capacity Fade of LiMn2O4
65
The prevalent cause of energy loss in LiMn2O4 cathode materials is a gradual fading in Q upon
continuous cycling, commonly referred to as capacity fade. This capacity fade is ascribed to
three main factors: (1) the dissolution of Mn2+
ions, (2) Jahn-Teller distortion of Mn3+
ions upon
discharge, and (3), the decomposition of electrolyte leading to an increase in the cell internal
resistance. In mechanism 1, the dissolution of Mn2+
ions occurs through a disproportionation
reaction. This reaction is believed to occur when the LiMn2O4 electrode is fully discharged as the
active electrode material is gradually converted to a defect spinel via the following mechanism:
4H++ 2LiMn
3+Mn
4+O4 3λ – MnO2 + Mn
2+ +2Li
+ +2H2O (4.2)
The protons arise from hydrofluoric acid (HF), which originates from the hydrolysis of lithium
hexafluorophosphate (LiPF6) salt in the electrolyte. The Mn3+
oxidation state reacts
spontaneously to form Mn2+
and Mn4+
. Mn2+
goes into solution and re-deposits as Mn(s) at the
negative electrode according to (4-6):
Mn2+
+2e- Mn(s) (4.3)
The reaction in equation is 4.2 is greatly accelerated with increasing temperature. The water
produced in eq. 4.2 can also generate more protons making manganese dissolution autocatalytic
in nature.
In the second capacity fade mechanism, Jahn-Teller distortion causes the LixMn2O4
structure to change from cubic to tetragonal symmetry at the end of discharge, taking away
available sites for lithium insertion. In the cubic spinel, lithium ions remain on tetrahedral sites
within the cubic structure for in range of . This reaction occurs at approximately 4V
vs. Li/Li+ and allows for highly reversible cycling behavior. However, the insertion of lithium
into Lix+1Mn2O4, occurring at 3V for causes a structural transition to tetragonal phase,
66
Li2Mn2O4 in which the tetrahedrally
coordinated lithium ions are displaced into
octahedral sites. The resulting increase in Mn3+
concentration reduces the symmetry of the
structure from cubic in LiMn2O4 to tetragonal in
LiMn2O4. As a result, drastic capacity fade is
observed within the first 50 cycles during
LixMn2O4 cycling in the range of .
In the third capacity fade mechanism,
the formation of surface films on the electrode
caused by the decomposition of electrolyte
causes a steady increase in cell resistance over
time. The increase in cell resistance causes
capacity fade due to electrode polarization. This
behavior is common with all lithium ion cells.
Xia et al. demonstrated the cycling behavior of a LiMn2O4 half-cells cycled at 50ºC,
25ºC, and 0ºC (7). At 50ºC, cells exhibited 20% capacity fade after 50 cycles. At 25ºC, cells
exhibit 7.4% capacity fade after 50 cycles. At 0ºC, cells exhibited only 4% capacity fade. In
figure 4.1, Ling et al. demonstrate the dependence of cells on cycling rate. As the cycling rate
increases, capacity fade is further accelerated. Increasing temperature therefore accelerates all
three mechanisms of capacity fade (2).
4.2.1. Efforts to Mitigate Capacity Fade
Figure 4.1. Cycling performance of 18650 spinel
LiMn2O4/graphite power batteries with various
discharge rates at 25 ºC (a) and 55 ºC (b) (2).
67
Efforts to mitigate capacity fading include doping the LiMn2O4 spinel structure with cations such
as Al, Mg and transition metal ions to enhance structural stability. However, due to the contact of
electrolyte with the LiMn2O4 particles, Mn dissolution was still induced as a result of HF acid
generation by LiPF6 salt and electrolyte reactions. To solve the issue of direct contact between
the electrolyte active materials, oxide coatings over LiMn2O4 were investigated. These coatings
include nano-SiO2(8), MgO (9, 10), ZnO (11), CeO2 (12), ZrO2(13), Al2O3(14), and Co-Al mixed
metal oxide (15, 16). Non-oxide coatings have also been demonstrated including metal
phosphates, metal, carbon, fluoride. Minimization of the contact area between LiMn2O4 and
electrolyte has been shown to effectively suppress the dissolution of Mn and enhance the cycling
stability of LiMn2O4.
4.3. Voltage Fade of Layered Lithium-rich Materials
Figure 4.2. Voltage profile of lithium rich layered material after cycling for 21 cycles at 30ºC.
68
Due to the surface reactivity of LiCoO2
and instability upon delithiation, the
practical capacity of layered LiCoO2 is
limited from the maximum of
273mAh/g to 140mAh/g corresponding
to the full Li. Similarly, for layered
LiMnO2, the delithiated structure
transforms into a more stable spinel-
type structure. These degradations in
the layered structure diminish its high –
potential (4V) electrochemical
performance. In principle, if the
layered structure can be kept intact over
the full range of chemical lithiation, the
material would yield a high capacity of
286mAh/g. Building on this concept, a
new class of layered materials are under
investigation for next generation battery devices. Categorized as lithium rich layered metal
oxides, these materials utilize an electrochemically stable component (Li2MnO3) to stabilize the
layered structure, offering exceedingly high capacities of greater than 280mAh/g at operating
voltages of 4.3-4.7V vs. Li/Li+. At present xLi2MnO3·(1- x)LiMO2 (M = Mn, Ni, Co) materials
demonstrate the greatest promise. However, a slow voltage fade is still apparent in these
materials, indicated by a gradual depression in the voltage profile upon discharge. While the
Figure 4.3. (a). Voltage profile of undoped Li-rich NMC
and (b). Li-rich NMC doped with 5% sodium. Cycling is
performed at 50ºC (1).
69
mechanism of the voltage fade is still under debate, it is well understood that like capacity fade,
voltage fade is significantly accelerated at high temperatures, whicht cause degradation reactions
to accelerate. In figure 4.2, Bettge et al. illustrate the effect of voltage fade on the energy density
of the cell (17). After, 20 cycles of cycling at 55ºC, it is reported that average voltage drops from
3.65V vs. Li/Li+ to 3.52V for a layered Li and manganese (Mn) metal oxide of composition
0.5Li2MnO3•0.5LiMn0.375Ni0.375Co0.25O2. As a consequence of the voltage fade phenomenon, 4%
loss in the specific energy of the cell is observed after 21 cycles. Additional losses due to
capacity fade result in a total energy loss of 5-6%. Therefore, losses due to voltage fade account
for greater than 65% of the total energy losses, rendering these materials unusable for system that
require long cycle lifetimes. Additionally, the voltage fade phenomenon creates challenges for
state of health measurements in battery management systems.
4.3.1. Efforts to Mitigate Voltage Fade
Similar to strategies to prevent capacity fade, efforts have been made to preserve the voltage
profile of lithium rich layered materials. In collaboration with the Center for High-rate
Nanomanufacturing (CHN), Ates et al. developed lithium rich materials doped with 5 weight %
sodium (Na) to mitigate the conversion from a layered structure to spinel structure (Figure
4.3)(1). X-ray diffraction analysis revealed that the Na ions are exchanged with Li from solution
upon contact with the electrolyte, which produces a volume expansion of the crystal lattices. This
triggers favorable metal migration to Li depleted regions. This mechanism has a stabilizing
effect on the crystal structure of the composite material and protects against conversion to spinel.
4.4. Multi-layered Structure to Mitigate Structural Changes
70
In this work, we utilize the multi-layered structure to mitigate structural changes in multiple
cathode materials. Since these structural changes are thermodynamically driven, it is highly
advantageous to utilize a material that with a high thermal conductivity. The thermal
conductivity of multi-walled carbon nanotubes is 200W/mK, which is higher than carbon blacks
by two orders of magnitude, and higher than most known materials, second only to diamond. The
alternating layers of MWNT create highly thermally conductive planes within the electrode
structure to shunt localized heat generated by intercalation reactions during cell cycling.
Therefore, reactions inducing structural changes and therefore energy loss can be significantly
retarded, extending the lifetime and energy of the cell.
4.5. Experimental Setup
High temperature testing was performed on multi-layered electrodes consisting of LiMn2O4 and
Li-rich layered NMC active material of the composition
0.3Li2MnO3·[0.7LiMn0.33Ni0.33Co0.33O2]. Two-layer electrodes consisting of three MWNT layers
and two active material layers were fabricated using the spin-casting method. Active material
layer compositions were composed of 85:10:5 active materials: Super-P conductive carbon: and
PVDF binder. Similarly, standard fabrication electrodes were constructed using LiMn2O4 and Li-
rich NMC active materials using the same electrode composition. Active material loadings
ranged from 4-7mg/cm2. The loadings of the multi-layered electrodes were consistently greater
than their respective standard fabrication electrodes. Therefore, internal resistances dependent on
material loadings are consistent for all electrodes. Electrochemical testing was performed using
a CR2025 coin cells using a lithium foil counter electrode (Sigma Aldrich). To prevent
electrochemical shorting, two porous polypropylene separators were used. Coin cells were
constructed in a dry argon glovebox (LC Technologies) at 0 ppm O2 and H2O. Higher
71
temperature cycling experiments
were performed in temperature-
controlled chamber (Espec) at 50ºC.
Pre-and post-cycling X-ray
diffraction analysis was performed
using Cu Kα radiation using a
PANanalytical Philips X-ray
Diffractometer XPert Pro. X-ray
diffraction samples consisted of the
cathode slurry coated onto an
aluminum current collector.
Galvanostatic cycling was
performed using an Arbin BTU 2000 battery cycler and analyzed using the MITS Pro software.
Impedance Spectroscopy was performed using a VoltaLab potentiostat in the frequency range of
100kHz to 100mHz. Prior to impedance testing samples were charged to 100% state of charge
and allowed to relax to a stable OCP. High temperature testing was performed on one multi-
layer sample and one standard fabrication sample for each type of cathode material. Standard
fabrication samples were also cycled at room temperature for comparison. Cycling regiments
consisted of C/10 charge and C/10 discharge for the LiMn2O4 samples. For the Li-rich NMC
samples, C/20 charging and discharging was performed for 2-3 cycles, followed by C/10 for 2-3
cycles, followed by C/5 for 25 cycles. The final cycle was performed at C/10 to fully evaluate
the voltage curve.
Figure 4.4. Cycling behavior of a 2-layer (black) and
standard fabrication (red) LiMn2O4 electrode at 50ºC.
Standard fabrication electrode at 25ºC shown in blue.
Cycling rate: at C/10.
72
4.6. High Temperature Cycling of LiMn2O4 Cells
Figure 4.4 shows the cycling behavior of a two-layer electrode and a standard fabrication
electrode at 50ºC. For comparison a standard cell cycling at room temperature is also shown. All
three electrodes demonstrate very rapid capacity fade due to Mn dissolution as described in eq.
4.1. However, it is clear the two layer electrode exhibits significantly improved cycling behavior
over the standard fabrication electrode at high temperatures. The first charge capacities for both
electrodes were approximately 148mAh/g. On the first discharge, the high temperature standard
fab electrode exhibits 90mAh/g while the two layer electrode exhibits 108mAh/g. Upon
continuous cycling, the standard fabrication electrode fades at a rate of 0.77mAh/g per cycle
while the two layer electrode fades at a rate of 0.66mAh/g per cycle. The standard fabrication
electrode cycling rate exhibits a capacity fade of 0.60mAh/g per cycle. It is interesting to note
that the capacity of the standard fabrication electrode at the high temperature fades rapidly,
diminishing to 70mAh/g within the first 5 cycles, while the standard fabrication at low
temperatures and the two layer at high temperatures demonstrate a low capacity fade rate.
73
Figure 4.5 shows the voltage profiles under galvanostatic cycling conditions of the two
layer and standard fabrication electrodes at high temperature. On the second cycle, the two layer
electrode exhibits the flattest voltage profile showing two distinct plateaus at 3.9V and 4.2V.
These plateaus are characteristic of lithium ion intercalation into LiMn2O4 and consistent with
figure 3.6. At the 15th
cycle, the voltage profile changing to slightly sloping, suggesting that the
chemical structure of LiMn2O4 is changing. However the voltage of the plateaus remains at the
constant. For the standard fabrication electrode, at the second cycle, the shape of the voltage
profile is significantly more sloping. At the 15th
cycle, the voltage profile of the standard
fabrication electrode becomes significantly “s” shaped. However, for all electrodes the positions
of the plateaus remain constant. Similarly, the dQ/dV plots in figure 4.5b confirm a similar
trend. The positions of the peaks which correspond to voltage plateaus in figure 4.4a are
identical, with the peak at 4.15V being significantly more prominent. Upon continuous cycling,
the intensities of the peaks diminish which suggests that the active material utilization is
(a) (b)
Figure 4.5. (a). Voltage profile of two layer and standard fabrication electrodes cycling at 50ºC. (b).
dQ/dV plot of voltage profiles in figure 4.5a.
74
diminishing. Additionally, ratio of the first and second peak intensities is increasing. Therefore,
for all electrodes, it appears the structure of the material is degrading, which is a likely
consequence of Mn2+
dissolution from LiMn2O4.
Although the two layer electrode demonstrates significantly higher capacity retention and
cycling stability, all electrodes exhibit an aggressive rate of capacity fade of greater than 50%
after 100 cycles. Therefore, these LiMn2O4-based electrodes are unusable for electric vehicle
applications which require long cycle life and strong capacity retention.
4.6.1. Impedance Analysis of LiMn2O4 Electrode
Figure 4.6 shows an impedance analysis of the two layer and standard fabrication electrodes
prior to high temperature cycling. As indicated by the first x-intercept of each curve, the ohmic
(a) (b)
Figure 4.6. (a). Impedance spectra of standard and two-layer electrode before cycling. (b). Impedance spectra of
standard and two layer electrode after cycling at 50ºC at C/10. 75
resistances of the standard fabrication and the two layer cell are identical. This suggests that the
contribution to the ohmic resistance from all components of the cell such as separators, casing,
and counter electrodes is identical for both electrodes. The charge transfer resistance however,
determined from the x-intercept of the projection of semi-circle onto the real axis, is vastly
different. Even though the two electrodes have very similar active material loadings, the charge
transfer resistance of the standard fabrication is three times larger than the two layer electrode. In
Chapter 3, we discussed that the reduction in charge transfer resistance of the layered electrode is
due to the highly conductive planes of carbon nanotubes within the electrode which enhance
electrical conductivity and porosity of the bulk electrode structure. Due to the high electrical
conductivity of the layered structure over the standard fabrication structure and the well-
established high thermal conductivity of carbon nanotubes, we can surmise that the layered
structure has significantly improved thermal conductivity over the standard according to the
Wiedemann-Franz Law in equation 4.3:
(4.3)
where κ is the thermal conductivity in W m-1 K-1 and σ is electrical conductivity in S m-1, L is
a constant of proportionality called the Lorenz number and T is the temperature. The high
thermal conductivity mitigates thermodynamically driven reactions leading to rapid capacity
fade.
Following high temperature cycling at 50ºC, the impedance spectra were measured again.
The shape of the impedance spectra for both electrodes remains the same for the standard
fabrication electrode while the spectrum of the two-layer electrode shifts higher on the imaginary
axis in the low frequency region. The reason for the shift is unclear. Additionally, the two layer
76
exhibits higher ohmic resistance after cycling. However, the characteristic high frequency
semicircle and low-frequency linear region are evident for both electrode after cycling. Analysis
of the ohmic and charge transfer regions in figure 6 reveal that the increase in charge transfer
resistance after cycling is 80Ω/cm2 and 60 Ω/cm
2 for the standard fabrication and two-layer
electrodes respectively.
4.6.2. XRD Analysis of LiMn2O4 Cells
Figure 4.7 shows the Cu Kκ x-ray diffraction (XRD) patterns of a bare aluminum substrate,
LiMn2O4 powder, and a fresh standard fabrication electrode consisting of LiMn2O4, carbon
black, and PVDF binder coated on an aluminum current collector. The spectrum of LiMn2O4
powder demonstrates the characteristic peaks of X-ray reflections from the crystal planes [111],
[311], [222], [331], [511], [400] and [531]. The standard fabrication electrode also demonstrates
the characteristic LiMn2O4 peaks in addition to the aluminum peaks indicated by black squares.
Figure 4.7. XRD spectra of a bare aluminum electrode (top, black), LiMn2O4 powder (middle, red),
and standard fabrication electrode coating on an aluminum substrate (bottom, green). Black squares
indicate the position of aluminum peaks. 77
The peaks of carbon black and PVDF are not evident. In figure 4.9, the XRD spectrum of a fresh
standard fabrication electrode is compared to that of an electrode after 10 cycles at 50ºC and an
electrode after 50 cycles at 50ºC. Three regions of interest are identified as A, B, and C. In figure
4.8a, region A is enlarged. It is evident that over the course of cycling, peaks [311] and [222]
shift to the right. Additionally, after 50 cycles, a new peak develops at 2ϴ = 38.8º as indicated by
the red hash. This peak is consistent with the formation of Li2MnO3, a compound containing
Mn4+
(4). Region B, enlarged in figure 4.8b, shows the [400] shifting to the right upon
continuous cycling. Finally, region C in figure 4.8c shows the formation of a peak at 2ϴ = 66.0º
in the spectrum of the standard fabrication electrode after 50 cycles. This peak is also consistent
formation of the Li2MnO3 product. It is not clear why peak [511] demonstrates a shift to the right
after 10 cycles and a shift to the left after 50 cycles. In figure 4.10, regions A, B, and C, are
compared for the fresh standard fabrication electrode, standard fabrication electrode after 50
cycles, and the two layer electrode after 50 cycles. In region A, the two-layer XRD spectrum
demonstrates no resolved peak at 2ϴ = 38.8º, indicating the absence of Li2MnO3. Additionally,
the shift of peaks [311] and [222] to the right is not as extreme as in the case of the standard
fabrication electrode after 50 cycles. Similarly, in figure 4.9b, the right-shift of peak [400] is also
not as extreme in the two-layer electrode spectrum. In figure 4.9c there is no resolved peak at 2ϴ
= 66.0º for the case of the two-layer electrode. From regions A, B, and C in figure 4.9, it is clear
that structural changes due to the disproportionation reaction are mitigated in the two-layer
structure.
78
Figure 4.8. XRD spectra of a fresh standard fabrication electrode (top, green),
standard fabrication electrode after 10 cycles (middle, blue), and standard fabrication
electrode after 50 cycles (bottom, pink). A, B, and C indicate regions of interest.
79
Figure 4.9. (a). Region A enlarged from figure 4.8. (b). Enlarged
B. (c). Enlarged C. Red hashes indicate the presence of Li2MnO3.
A
B
C
80
A
B
C
Figure 4.10. (a). Region A of XRD spectra of fresh standard fabrication electrode (green, top),
standard fabrication electrode after 50 cycles (pink, middle), and two layer electrode after 50 cycles
(bottom, purple). ( (b). Enlarged B. (c). Enlarged C. Red hashes indicate the presence of Li2MnO3.
81
4.7. High Temperature Cycling of Li-rich NMC Cells
Figure 4.11a shows the cycling behavior two layer and standard fabrication Li-rich NMC cells at
50ºC. At low rates, electrodes exhibit similar capacity retention of 250mAh/g at rates of C/10
and lower. At a rate of C/5, the two layer electrode exhibits 20% higher capacity retention than
the standard fabrication electrode. This behavior is consistent with the higher rate behavior of the
layered electrode compared to standard fabrication Li-rich NMC electrodes discussed in Chapter
3. However, at a rate of C/5, the two-layer electrode fades at a rate of 1.52mAh/g per cycle while
the standard fabrication electrode fades at a rate of 1.1 mAh/g per cycle. In figure 4.11a, the
voltage curves of the standard fabrication and two layer electrodes are plotted for the 1st, 5
th, 19
th,
and 29th
cycles after cycling at 50ºC. The 1st cycles for both electrodes are shown in black. The
voltage profile for both electrodes on the first cycle is nearly identical. The first charge capacity
is approximately 330mAh/g for both electrodes due to the evolution of Li2O from the Li-rich
material. Upon continuous cycling, it is evident that the voltage profile of the standard electrode
decays more rapidly than the two layer electrode. In figure 4.11b, the voltage curves are plotted
as voltage versus depth of discharge (DOD) by normalizing by the curve to the discharge
capacity obtained at a voltage of 2.0V vs. Li/Li+. Therefore, the voltage fade phenomenon can be
observed independently of losses due to capacity fade. Again, the voltage of the standard
fabrication electrode degrades more rapidly than that of the two-layer electrode. Here it is
interesting to note that the loading of the two-layer electrode is slightly greater than the standard
fabrication electrode, suggesting that the internal temperature of the cell should be greater than
that of the two-layer, possibly accelerating structural degradation. However, this is not the case.
4.7.1. Impedance Analysis of Li-rich NMC Electrodes
82
Figure 4.13 shows the impedance spectrum of the two-layer and standard fabrication Li-rich
NMC electrodes prior to cycling at 50ºC. Similar to the impedance spectra of LiMn2O4
electrodes, the characteristic high-frequency semi-circle and low frequency linear region can be
observed for both electrodes. Both electrodes also exhibit the formation of a second “mid-
frequency” semi-circle. It is not clear what mechanism is the cause of the second semi-circle.
For the case of Li-rich NMC electrodes, the charge transfer resistance of the standard
fabrication electrode is almost one order of magnitude larger than the two-layer electrode.
Therefore, the electrical conductivity and correspondingly, the thermal conductivity of the
layered structure is also expected to be significantly higher than the standard fabrication structure
according to equation 4.4..
4.7.2. XRD Analysis of Li-rich NMC Cells
The Cu Kα XRD spectra of a bare aluminum electrode, fresh standard fabrication electrode, and
fresh two layer electrode are shown in figure 4.14a. The peaks from the standard fabrication and
two layer electrode are identical with aluminum peaks clearly identified in both spectra. After
cycling of 0.3Li2MnO3.0.7LiMn0.33Ni0.33Co0.33O2 material at 50ºC for 30 cycles, Ates et al. report
the splitting of the XRD peak at 2ϴ = 19.25º. This behavior is consistent with a conversion from
a layered material structure to a spinel structure. However, in figure 4.13b, although the peak of
the two-layer electrode shifts from 2ϴ = 19.25º to 2ϴ = 18.75º, there is no resolved splitting of
the peak at 2ϴ = 18.75º, under the same cycling regiment as that used by Ates et al (1).
Therefore, the XRD spectra, in conjunction with the voltage fade behavior of the standard
fabrication and two layer electrodes demonstrate the mitigation of a conversion from a layered
material structure to a spinel structure.
83
Figure 4.11. Cycling behavior of a 2-layer (blue) and standard fabrication (red)
electrode at 50ºC. Standard fabrication electrode at 25ºC shown in blue. Cycling rate: at
C/10.
(a) (b)
Figure 4. 12. (a). Voltage profile of two layer (blue) and standard fabrication (red) Li-rich NMC
electrodes cycling at 50ºC. Curves show 1st (black), 5
th, 19
th, and 29
th cycles (b). Voltage vs. Depth of
Discharge for curves in fig. 4.12a.
84
Figure 4.13. Impedance spectrum of two-layer (red) and standard fabrication
electrodes (black) before cycling.
(a) (b)
Figure 14. (a). XRD spectra of a bare aluminum electrode (top, green), fresh standard fabrication electrode (middle,
green), and fresh two-layer electrode (bottom, blue). (b). XRD spectra of a bare aluminum electrode (top, green),
fresh standard fabrication electrode (middle, green), and two-layer electrode after 30 cycles at 50ºC (bottom, blue). 85
4.8. Conclusions and Future Work
Materials such as LiMn2O4 and lithium-rich layered materials exhibit the slow loss of the energy
upon continuous cycling due to structural degradation leading to capacity fade and voltage fade.
Structural degradation in both materials, as well as many others, is accelerated at higher
temperatures, rendering these materials unusable in applications requiring long lifetimes. The
multi-layered structure has been effectively used to mitigate these structural conversions in these
materials. Planes of MWNT in the layered structure possess exceedingly high thermal
conductivities, improving the thermal conductivity of the bulk electrode. Mitigation of structural
conversion through increased thermal conductivity has been demonstrated using galvanostatic
cycling and post-cycling XRD analysis.
In future work, the effect of the morphology, loading, and geometrical organization of
MWNT within the layered structure on the structural conversion will be studied.
86
4.9. References
1. M.-N. Ates, Q. Jia, A. Shah, A. Busnaina, S. Mukerjee, and K.M. Abraham, J.
Electrochem. Soc, 161, A290 (2014).
2. G.-W. Ling, X. Zhu, Y.-B. He, Q.-S. Song, B. Li, Y-J. Li, Q.-H. Yang, Z-Y. Tang, Int. J.
Electrochem. Sci., , 7, 2455 (2012).
3. O. K. Park, Y. Cho, S. Lee, H.-C. Yoo, H.-K. Song, and J. Cho, Energy Environ. Sci., 4
(2011).
4. Thackeray, J. Cho and M.M., J. Electrochem Soc., 146, 3577 (1999).
5. G.C. Amatucci, C. N. Schmutz, A .Blyr, C. Sigala, A. S. Gozdz, D. Larcher, J. M.
Tarascon, J. Power Sources, 69, 11 (1997).
6. M. Wohlfahrt-Mehrens, C. Vogler, J. Garche, J. Power Sources, 127, 58 (2004).
7. Y. Xia, Y. Zhou, and M. Yoshio, J. Electrochem. Soc, 144, 2593 (1997).
8. D. Arumugam, G. P. Kalaignan, J. Electroanal. Chem, 624, 197 (2008).
9. Cho, S. Lim and J., Chem. Comm., 4472 (2008).
10. J.S. Gnanaraj, V.G. Pol, A. Gedanken, D. Aurbach, Electrochem. Commun., 5, 940
(2003).
11. H. Liu, C. Cheng, Zongqiuhu, K. Zhang, Mater. Chem. Phys. , 101, 276 (2007).
12. H.-W. Ha, N.-J. Yun, K. Kim, Electrochim. Acta, 52, 3236 (2007).
13. Y.-M. Lin, H.-C. Wu, Y.-C. Yen, Z.-Z. Guo, M.-H. Yang, H.-M. Chen, H.-S. Sheu, and
N.-L. Wu, J. Electrochem Soc., 152, A1526 (2005).
14. J.-S. Kim, C.-S. Johnson, J. T. Vaughey, S. A. Hackney, K. A. Walz, W. A. Zeltner, M.
A. Anderson, and M. M. Thackeray, J. Electrochem Soc., 151, A1755 (2004).
15. Z. Yang, W. Yang, D. Evans, Y. zhao, X. Wei, J. Power Sources, 189, 1147 (2009).
87
16. J. Cho, T.-J. Kim, Y. J. Kim, B. Park, Chem. Comm., 1074 (2001).
17. M. Bettge, Y. Li, K. Gallagher, Y. Zhu, Q. Wu, W. Lu, I. Bloom, D. Abraham, J.
Electrochem. Soc, 160, A2046 (2013).
88
Chapter 5: Development of High-Power Full Cells
5.1. Introduction
This chapter discusses the development of high-power full cells in pouch cell prototypes. In a
full cell configuration, the lithium foil counter electrode is replaced with a graphite anode. To
maintain high coulombic efficiency and excellent reversibility, the balancing of the cathode and
anode capacities is first discussed. Optimum mass balance ratios for full cells with graphite and
silicon anodes are then calculated and the performance of full cells in a coin cell format is
described using the calculated ratios. Next, a scale up process to develop large area multi-layered
electrodes is discussed using a spray-coating process. Finally, the development and performance
of pouch cell prototypes using large-area multi-layered electrodes and graphite anodes is
discussed.
5.2. Capacity Balancing
Figure 5.1. Schematic diagram of a lithium-ion cell on discharge (3).
89
Lithium-ion cells operate by shuttling lithium ions between two insertion electrodes. In a half-
cell configuration, a lithium counter electrode provides a large reservoir of lithium ions with
which to study electrochemical processes in a single electrode. However, lithium foil is not
suitable for commercial applications due to many safety concerns. Graphite and various graphitic
carbons are excellent replacements for lithium as negative electrode materials as they reversibly
intercalate with lithium ions at a potential of 0.1V versus lithium, allowing the cell to maintain
its operating voltage. Figure 5.1 shows the mechanism of lithium ion insertion into a full cell
consisting of a transition metal oxide cathode and a graphite anode. In the ideal case, the number
of lithium ions de-inserting from the cathode (xLi+) is equivalent to the number of lithium ions
inserting into the anode (yLi+). If a lithium ion cell were to operate for over 1000 cycles, as is
typical for commercial cells, electrodes must reversibly intercalate with 99.999% efficiency, also
known as columbic efficiency. Therefore, since the total number of lithium ions in a full cell
system is limited, irreversible reactions in which lithium ions are consumed must be minimized.
High columbic efficiency can be accomplished by precisely balancing the ratio of lithium
ion capacities of the anode and cathode material. Balanced capacity is achieved by balancing the
mass of the active materials on the positive and negative electrodes according to the ratio(3):
5.1)
Where C is the gravimetric capacity of each electrode in units of mAh/g and and refers to
the amount of lithium from the material that is reversibly cycled. If is smaller than optimal
ratio, the negative electrode will not be fully utilized. Likewise, if is too large, the high
concentration of lithium ions at the anode during charging may cause lithium plating at the anode
and consequently a short circuit may occur.
90
However, in real commercial cells, is varied from the ideal value. The primary reason
for this perturbation is due side reactions and secondary processes, which consumes a portion of
the cyclable lithium ions. The primary side reaction that occurs is the formation of the SEI layer
during the first cycle, which forms a passivating film on the surface of the anode and cathode.
Therefore, cells need to be precisely balanced to account for small side reactions.
Figure 5.2 shows galvanostatic cycling data for half cells constructed from LiMn2O4
cathode, Li-rich NMC cathode, and a graphite anode. The half-cell cycling data was used to
Figure 5.2. (a). First cycle capacities of Li-rich NMC half-cell. (b). First cycle capacities of graphite half-cell.
(c).First cycle capacities of LiMn2O4 half cell.
Figure 5.3. Full cell testing of (a). Li-rich NMC cathode and graphite anode. (c). LiMn2O
4 cathode and
graphite anode using various mass balance ratios.
91
determine the optimal mass balance ratio based on the capacities of the first cycle. For the
LiMn2O4 and Li-rich NMC cathode half cells, the charge capacity exhibits higher capacity than
the discharge capacity. For graphite anode half-cell, the discharge capacity is higher than the
charge capacity. The mismatch in charge and discharge capacities in the first cycle is due to the
decomposition of the electrolyte to form a passivating layer (SEI) on the surface the anode. The
Li-rich NMC half-cell demonstrates an exceedingly high charge capacity of 330mAh/g due to
the evolution of oxygen in the form of Li2O in the first charge in addition to SEI formation. The
LiMn2O4 and graphite half cells exhibit irreversible capacities of 8.7% and 15.6% respectively.
According to these half cells, the optimum mass balance ratios for a Li-rich NMC full cell with
graphite and a LiMn2O4 full cell with graphite are 1.27and 3.3respectively. Figures 5.3a and 5.3b
show full cells using Li-rich NMC and graphite and LiMn2O4 and graphite respectively
composed of various mass balance ratios. In figure 5.3a, when the mass balance ratio is too low
( = 0.77), the cell exhibits overcharging behavior as the cell exhibits an excessively large
charge capacity. It is not entirely clear why overcharging is observed at low . However, several
groups have attributed this to oxygen evolution due to structural damage at the positive
electrode. As the mass balance ratio is increased, the full cell demonstrates reversible cycling.
However, the gravimetric discharge capacity is approximately half of Li-rich half-cell capacity.
Similarly, in figure 5.3b, as the mass balance ratio of the LiMn2O4 full cell is increased, the
discharge capacity increases and demonstrates improved reversibility. However, similar to figure
5.3a, the discharge capacity is approximately half of the LiMn2O4 half-cell even at the optimum
mass balance ratio. These results indicate that the poor cyclability is due to additional side
reactions that result in the consumption of lithium ions from the system.
92
5.3. Full Cells with Silicon Anodes
To increase the energy
density of lithium ion batteries
beyond the state of the art
commercial cells, high capacity
anode materials such as silicon,
tin, and germanium have been
heavily researched. Instead of
intercalation, these materials
form alloys with lithium. Table
5.1 shows properties of common alloy materials. In the Li-Si system, Li22Si5 is the most Li-rich
phase, allowing the theoretical maximum specific capacity upon lithiation to be 4200 mAh/g,
greater than 10x the capacity of graphite (5, 6). However, due to this alloy formation, silicon and
most other alloy anodes undergo a significant volume expansion of 200-300% during cycling.
This volume expansion causes severe mechanical stresses in the alloy structure which lead to
rapid capacity fade severely impacting the cycle life of the electrode. Specifically, these losses
include: (i). Loss of active material caused by pulverization of active particles and disconnection
from surrounding electrical networks. (ii). Formation of unstable dynamic SEI films. This film is
formed, broken, and reformed, which rapidly consumes lithium ion in the system. (iii). Reaction
of lithium ions with surface oxide layers to form Li2O. (iv). Aggregation of alloy particles upon
cycling. Aggregation causes particles to become disconnected from the percolative networks
formed by carbon black particles. (v). Trapping of Li+ in the host alloy due to slow Li release
kinetics.
Figure 5.4. Charge-discharge voltage profiles of a pure
silicon anode with an average powder size of 10µm(4).
93
To buffer mechanical stresses due to silicon cycling, composite structures with silicon
have been utilized. For example, Lui et al demonstrated a yolk-shell structure in which silicon
nanoparticles are encapsulated in a carbon sphere can limit the expansion into the sphere void
(7). Yolk-shell nanoparticles demonstrated 2800mAh/g for at least 1000 cycles. However,
expensive synthesis and fabrication procedures, as well as poor capacity retention at high
loadings have inhibited
commercialization. Due to their
excellent mechanical and electrical
properties, carbon nanotubes can be
used in composite electrode materials
to buffer mechanical stresses due to
silicon cycling. As an alternative, the
multi-layered CNT electrode
architecture is adapted in conjunction
with a silicon anode in a parallel
research project. However, the use of
CNT’s in anode materials also present
several challenges that must be
Table 5.1. Common alloy anode materials(1).
Figure 5.5. Half-cell cycling behavior of various
graphitic carbons (2).
94
addressed before the multi-layered silicon structure can be effectively utilized in a full lithium
ion cell. In Figure 5.5, Landi et al. show the half-cell cycling data of various graphitic carbons
(2). It is important to note that the chemical structure of carbon nanotubes is very similar to the
structure of graphite, consisting of sp2 hybridized hexagonal carbon rings. In graphite planes of
carbon rings are stacked together, whereas in carbon nanotubes planes of carbon rings are rolled
up into a tube. However, the electrochemical performance of CNT is very different from
graphite. All graphitic carbons exhibit a plateau at 0.2V vs. lithium corresponding to lithium ion
intercalation. However, above 0.2V, carbon nanotubes exhibit a gently sloping voltage curve
with a small plateau at 1V. This region corresponds to irreversible capacity due to electrolyte
decomposition. In Figure 5.6 shows the electrochemical cycling behavior of a MWNT half-cell
constructed in-house. The first cycle exhibits the same large irreversible capacity described in
figure 5.5. On the second and third cycles, the reversible capacity is significantly reduced to less
than 25% of the irreversible capacity. Therefore, to utilize the layered carbon nanotube/silicon
anode in a full cell, the large irreversible capacity must be accounted for through capacity
balancing. According to equation 5.1, the mass balance ratio is 11.5, which is significantly larger
than mass balance ratios using graphite anodes and maybe detrimental to the gravimetric energy
density of the full lithium ion cells.
Figure 5.7 shows full cells constructed with a multi-layered cathode and multi-layered
anode. The cathode consists of a four layer Li-rich NMC cathode (4L Li-rich NMC) and a one
layer silicon anode (1L Si). Also shown in figure 5.7 is a full cell consisting of standard
fabrication cathode and 1L Si anode for comparison. The discharge capacity is normalized to the
mass of the cathode only. In the first cycle both full cells exhibit greater than 85% of the
theoretical capacity of the cathode. The multi-layered cathode exhibits slightly higher capacity
95
retention than the standard fabrication cathode. However, upon continuous cycling, the capacity
fades steadily, reducing to 53% of the theoretical capacity for both electrodes. This behavior is
consistent with capacity fade due to the silicon anode caused by volume expansion. In an effort
to increase silicon cyclability, carbon nanotubes were added to the silicon structure to increase
mechanical robustness. The percentage of carbon nanotubes was increased to 70% in the active
material layer in
Figure 5.6. (a). Half-cell testing of MWNT anode..
96
a
Figure 5.8. Full cell testing of Li-rich NMC cathode and MWNT/Si
anode.
Figure 5.7. Full cell testing of Li-rich NMC cathode and MWNT/Si
anode.
97
ddition to the MWNT layers. Due to the increase in percent of MWNT, the 1L Si half-cell shows
highly stable cycling behavior. However, when the experiment was repeated, as shown in figure
5.8, both electrodes exhibit characteristics of capacity misbalance. The layered electrode
exhibited overcharge behavior while the standard electrode demonstrated extremely low capacity
retention. This behavior is due to the large irreversible capacity of CNT’s in the first discharge
which consumes a large portion of the cyclable lithium ions. Similar to figures 5.5 and 5.6, a
large irreversible capacity is observed in the first discharge cycle. Therefore, although CNTs
have a strong potential to enhance the cyclability of alloying anode materials, the mass ratio of
CNTs must be minimized.
5.4. Scale Up
In previous studies, electrochemical tests were performed using CR2025 button cells which were
20mm in diameter and 2.5mm in thickness. By studying the impact of the active material loading
(in mg/cm2) on the electrochemical performance, coin cells can provide good insight on the
behavior of larger commercial cells. However, changing the format and geometry of the cell
changes electrochemical performance of the battery. Therefore, it is important to validate these
findings with larger cells. To validate the high performance behavior of the multi-layered
structure, we design a fabrication process to produce electrodes for pouch cells, which require
electrodes to have an area of 20cm2.
In commercial cells, electrodes can be coated over an area of 10m2 in under 1 minute.
The spin- coating technique cannot achieve commercial manufacturing scales due to several
drawbacks. (i). Due to the rotating substrate, the coating forms a circular pattern and does not
uniformly coat rectangular substrates. (ii). Due to the lengthy spin-cycle each layer requires a
98
total fabrication time of 10-20 minutes. (iii) Electrodes cannot be coated using a continuous,
processes. To fabricate large-area multi-layered electrodes compatible with commercial
manufacturing scales, two fabrication methods were explored: spray-coating and doctoral blade
coating. Using these techniques, the fabrication time per layer can be reduced by a factor of 2-3.
Also, the fabrication technique can be implemented on a continuous roll-to-toll process and is
independent of the size and shape of the substrate.
5.4.1. Spray-Coating: Experimental
In the spray coating method, a siphon-feed airbrush is used to convert the electrode materials,
suspended in solvent, into a fine aerosol spray which is used to coat a current collector (Figure
5.9.) The conical nozzle and tapered needle produces a uniform conical spray pattern. By moving
the airbrush in a raster pattern across the surface of the substrate, a uniformly thick coating can
be produced. The diameter of the nozzle and tapering on the needle has a significant impact on
the electrochemical performance of the coated electrode.
The suspension of electrode materials consists of active material, carbon black, and
PVDF in a typical ratio of 85:10:5 respectively. Another suspension is prepared using only
(a) (b)
Figure 5.9. Optical (a) and schematic (b) images of the Badger Crescendo 175 airbrush used to
develop the spray-coating process.
99
MWNT. Instead of N-methyl-2-pyrollidone (NMP) which is used in spin-coating, the spray-
coating solvent is isopropyl alcohol which has a higher rapid evaporation rate and is significantly
less toxic. The concentration of the active material suspensions are 6.67mg/ml and 0.8mg/ml
respectively. The substrate is a rectangular piece of aluminum foil (25cm2). The surface of the
current collector is abraded using ultra-fine sand paper. During spraying, the substrate is attached
to a vertical glass plate using kapton tape at the corners. Attached to the back of the glass plate is
a resistive heater, which is controlled and monitored by a autotransformer, temperature
controller, and thermocouple. The temperature of the glass plate is set to 60ºC. For each MWNT
layer, 7.5 ml is sprayed onto the current collector while each active material layer requires 30-
45ml. This spraying process requires about 30min per electrode and results in an active material
loading of approximately 4mg/cm2.
5.4.2. Morphology of Spray-Coated Electrodes
Figures 5.10a and 5.10b
show optical images of
multi-layered electrodes
fabricated using the spin-
coating and the spray-coating
technique respectively. In
figure 5.10b, the spray-
coating technique allows both the CNT and active material layers to be coated uniformly over
the whole electrode. In the spin-coating technique, the CNT layers only deposit in the center of
the electrode due the non-uniform nature of the CNT suspension. SEM images demonstrate very
Figure 5.10. (a). Four layer electrode fabricated using spin-
coating process. (b). Four layer electrode using spray-coating
process.
100
similar surface morphologies of the two electrodes, showing sub-micron sized lithium ion active
material surrounded by percolating networks of 60nm carbon black particles. The SEM cross-
sectional electrode images in figure 5.11 demonstrate the density of the electrodes using the
spray-and spin-coating process is very different. Figure 5.11a shows a one layer electrode
constructed using the spray-coating process while the figure on the right shows a four layer
electrode using the spin-coating process. The loading and thickness measurements indicate the
density electrodes are 0.233g/cm3 and 3.6g/cm
3 for the spray and spin methods respectively.
Therefore, the density of electrodes and, consequently, the volumetric energy density using the
spin-method is one order of magnitude greater than electrodes fabricated using the spray method.
Furthermore, the SEM cross sectional image of the spray-coated electrode in figure 5.12 further
reveals that the morphology of the electrode is less robust as large and smaller pores can be seen
scattered throughout the electrode profile. These large (>5µm) and small (500nm - 5µm) pores
not only decrease the volumetric energy density of the electrode but could also disconnect
(a) (b)
Figure 5.11. Optical (a) and schematic (b) images of the Badger Crescendo 175 airbrush used to develop
the spray-coating process.
101
percolating electrical networks within the structure. It is important to note, however, that large
particles greater than 5µm are not evident as they are filtered by the narrow opening of the
airbrush nozzle.
5.4.3. Performance of Spray-Coated Electrodes
SEM images of the spray-and spin-coated structure indicate that while the materials and
composition of the electrodes is identical, the density and morphology are vastly different. The
porous nature of the spray-coated electrodes damages percolating electrical networks in the
structure. When multi-layered electrodes were constructed using the spray-coating method and
LiMn2O4 active material, all electrodes demonstrated poor capacity retention (Figure 5.13a).
Although the layered electrodes demonstrated better capacity retention than the standard
Figure 5.12. Cross sectional SEM image of one layer spray-coated electrode of density
0.233mg/cm2 and loading 0.7mg/cm
2. Insets provide enlarged images of active and MWNT
layers.
102
fabrication electrodes, all electrodes demonstrated less than 70% capacity retention overall. For
the case of multi-layered electrodes, as additional active material layers are added to the
electrode stack, poor electronic connectivity in the underlying layers subsequently cause the
upper layers to be disconnected. Therefore, in figure 5.13a, we observe that the electrode
performance diminishes as a function of the number of layers. Further investigations revealed
that the cycling results were significantly affected by the size of the airbrush opening, which is
controlled by the airbrush nozzle and needle (figure 5.13b.) As the diameter of the nozzle
increases increased and the taper of the needle increased from a blunt to a fine tip, the size of the
(a) (b)
Figure 5.13. (a). Multi-layered electrodes fabricated using spray-coating process. (b). Standard fabrication
electrodes fabricated using various airbrush nozzle and needle sizes given in the caption.
103
airbrush opening increases. This process likely causes larger particles to pass, increasing the
packing density of the electrode and therefore the discharge capacity is improved. In fact, mixing
smaller and larger particles is a commonly used technique in the lithium ion battery industry to
improve the packing density of the electrode. Nevertheless, the capacity retention at the optimal
airbrush tip size is still less than 90%, indicating that percolating electrical networks are not as
robust as required to obtain greater than 90% capacity retention, as observed using the spin-
coating method.
Figure 5.14. (a), (b). SEM images of spray-coated multi-layered electrode containing 1% CNT
in active material layer. (c), (d). 5% CNT in active layer.
A B
C D
104
In an effort to strengthen percolating electrical networks in the spray-coated electrode, a
small percentage of carbon nanotubes were added to the active material layers. Figure 5.14
shows SEMs image of two layer electrodes in which 1% and 5% CNTs were added to the active
material layers. LiMn2O4 is used as the active material. Since it is important to minimize the
percentage of inactive carbon in the electrodes, small CNT percentages of 1% and 5% are
chosen. SEM images show clusters of large 5-10µm particles surrounded by smaller clusters of
carbon black particles through which electron transfer normally occurs. In addition, occasional
high aspect ratio carbon nanotubes are also observed. The length of a single MWNT can be seen
connecting many active material particles and carbon black particles. However, in the figure
5.14c, the wide-view image of 5% CNT, large agglomerates of carbon nanotubes can also be
observed.
105
In figure 5.15a and b, the electrochemical performance of electrodes in figure 5.14 is
demonstrated. Spray-coated electrodes with a small percentage of CNTs in the active material
demonstrate significantly enhanced performance over all electrodes in figure 5.13. However, it is
interesting to note that the electrode with 1% CNT in the active layer demonstrates higher
capacity retention than the electrode with 5% CNT in the active layer. It is possible that the
agglomerates of CNTs observed in the electrodes 5% CNTs in figure 5.14c and 5.14d are
A B
C D
Figure 5.15. (a). Voltage profile of two-layer electrodes containing 1% (black) and 5% (red)
CNT in the active material layers. (b). Cycling stability of electrodes in figure 5.15a. (c).
Voltage profile of four layer electrodes with LiMn2O4 active material and (d) Li-rich NMC
active material containing 1% CNT in active material layer.
106
detrimental to the electrical networks. Therefore, we can assume that there is a critical
concentration of CNTs in the active material layer, beyond which capacity retention begins to
diminish. In figure 5.15b, both the 1% and 5% electrodes demonstrate stable cycling behavior,
with the 1% electrode demonstrating consistently higher capacity retention than the 5%
electrode.
In figure 5.15c and 5.15d, four layer electrodes were constructed using LiMn2O4 and Li-
rich NMC active materials with 1% CNT in the active material layers. Figures 5.15c and 5.18d
show two four-layer samples for each type of active material. Electrodes fabricated with both the
LiMn2O4 and Li-rich NMC active materials show consistent cycling behavior exhibiting greater
than 90% capacity retention for LiMn2O4 samples and 85% capacity retention for Li-rich active
materials. Figure 5.16 demonstrates that electrodes in figure 15c and d demonstrate highly stable
rate behavior, indicating that the electrical networks are robust enough to withstand higher
currents. Therefore,
by using 1% CNT
in the active
material layers, a
high-volume, high-
rate fabrication
process for
fabricating multi-
layer electrodes has
been demonstrated.
Figure 5.16. Rate behavior of electrodes in figures 5.15c and d.
107
5.5. Pouch Cell Fabrication and Testing
To validate the performance of spray-
coated multi-layered electrodes, CHN in
collaboration with MaxPower Inc. has
developed full lithium ion pouch cells
25cm2
in area. Pouch cells are rectangular
shaped. Contact to the negative and
positive electrodes is made through two metal leads. Cathodes, fabricated at CHN, are
composed of spray-coated four layer electrodes consisting of 1% CNT in active material layers.
Active material loading of the cathodes are approximately 4mg/cm2. Anodes consisting of
mesoporous micro beads (MCMB) cast on copper foil are fabricated by MaxPower Inc.
Capacity matching and pouch cell construction were completed at MaxPower Inc. Figure 5.18a
and b, show electrochemical testing of four layer Li-rich NMC/graphite full pouch cell during
formation cycling at a rate of C/5. Formation cycling demonstrates excellent capacity retention
and stable cycling. The capacity, when normalized to the weight of the cathode, the discharge
capacity exhibits greater than 90% of the of the theoretical capacity of the cathode. The Li-Rich
NMC cells demonstrate a slight voltage depression upon continuous due to the effect of voltage
fade described in Chapter 4. Similarly, other LiMn2O4 and Li-rich NMC full pouch cells with 1%
CNT in the active layer demonstrate excellent capacity retention (85-90%) and high columbic
efficiency (greater than 94-96%) during the formation cycles. However, upon continuous
cycling, both of the Li-rich NMC and LiMn2O4cells exhibit continuous capacity fade (Figure
5.19). This behavior is not evident in the corresponding cathode half cells. Since formation
cycling indicates that the capacity of the anode and cathode are well matched, the capacity fade
Figure 5.17. Image of pouch cell prototype.
108
effect might be attributed to delamination of the active material during continuous cycling. It is
likely that the porous nature of the spray-coated electrodes causes material to delaminate when
material expand and contract during continuous cycling. Therefore, the spray-coating fabrication
process requires further development to inhibit the delamination of active material .
(a) (b)
Figure 5.18. (a). Voltage profile of a Max Power pouch cell prototype consisting of a four layer Li-rich
NMC electrode during formation cycling. (b). Voltage profile in (a). repotted as a function of raw
capacity..
109
5.6. Doctor Blade Technique
To improve the density of the active
material layers and allow materials to
remain laminated to the current collector,
the doctor blade technique is explored
for the fabrication of the active material
layers (figure 5.19). The CNT layers are
deposited using spray-coating. In the
doctor blade technique, highly uniform
slurry of active material, conductive
carbon, and PVDF is applied to the surface of the current collector. A blade of a fixed height is
set onto the flat current collector and passed over the surface, allowing the slurry to be uniformly
casted over the surface of the current collector (Figure 5.19). If the composition of the slurry is
Figure 5.19. Fabrication sequence of the active
material layer using the doctor blade technique.
(a) (b)
Figure 5.18. (a). Voltage profile of a Max Power pouch cell prototype consisting of a four layer Li-rich
NMC electrode during formation cycling. (b). Voltage profile in (a). repotted as a function of raw
capacity..
110
homogenized, the doctor blade technique will allow for very price control over the active
material loading and thickness over a large area. Furthermore, due to the highly viscous nature of
the active material slurry, the electrode casting is significantly denser than in the spray-coating
method. This process is similar to the calendaring process used in the lithium ion battery
industry.
The viscosity of the active material suspensions is of critical importance to achieve
uniform coating over a large area. In our studies, the desired viscosity is achieved by ball-milling
all of the electrode components together in powder form and adding the solvent to the powder
mixture to adjust the viscosity of the suspension. To coat the active material layers, the height of
the doctor blade is first adjusted to a specific distance from the aluminum substrate. Next a thick-
paint like coating ink is applied to the surface of a current collector to form a tooth-like notch.
The doctor blade is then slowly passed over the current collector surface for even application,
and the coated substrate is dried to remove the solvent. Figure 5.20 shows cycling results after
the fabrication of a one layer electrode. Because the doctor blade technique significantly
improves the density of the active material layers, the loading, discharge capacity, and cycling
efficiency are significantly improved without the use of CNT in the active material layer.
However, the discharge capacity of the two layer electrode was significantly lower.
To add additional active material and carbon nanotube layers on the electrode stack, the
distance between the blade and the surface of the current collector must be precisely controlled.
As the number of active material layers increase, the surface roughness also increases, making it
increasingly difficult to control this distance. Therefore, casting subsequently layers leads to non-
uniform layer thickness and also removal of underlying layers of the electrode stack, negatively
impacting the discharge capacity.
111
5.7. Conclusions and Future Work
Chapter 5 focused on the development of high power lithium ion prototype cells to evaluate commercial
viability multi-layered electrodes. In this chapter, the multi-layer electrode structure was first tested in a
full cell configuration in-house, using both conventional graphite and new silicon anode material. Full
cells using graphite anodes demonstrated poor capacity retention due to irreversible losses of lithium ions
in the system. Full cells using silicon anodes demonstrated full capacity retention. However, drastic
capacity fade was observed upon continuous cycling due to the volume expansion of silicon.
Two scale up process were also investigated to increase the area of multi-layer electrodes for
applications in pouch cells. The best electrochemical performance was observed using the spray-coating
process in which a small amount of CNTs were incorporated into the active material layer in addition to
the MWNT layers. Electrodes fabricated using the spray-coating process were constructed into 22cm2
pouch cell prototypes, fabricated at MaxPower Inc. Electrodes demonstrated excellent capacity retention
during formation cycling. However, drastic capacity fade was observed during continuous cycling, due to
Figure 5.20. Multi-layered electrode fabricated using doctor blade technique
112
the delamination of active material from the electrode surface. To improve the robustness of the structure,
the doctor blade technique was also investigated which allowed for significantly more dense active
material layers.
In future studies, (i) the irreversible losses in lithium ion full cells with graphite will be studied to
improve capacity retention. (ii). Once the issue of the volume expansion in a silicon anode half-cell is
address, a full cell consisting of multi-layered cathodes and anodes will be constructed. (iii). Finally, the
scale up process using the doctor blade technique will be tuned further to improve the capacity retention.
113
5.8. References
1. Zhang, W.-J., J. Power Sources, 196, 13 (2011).
2. Landi, B., Ganter, M., Cress, C., DiLeo, R., Raffaelle, R., Energy Environ. Sci., 2, 638
(2009).
3. Arora, P., and White, R., J. Electrochem. Soc., 145, 3647 (1998).
4. J. W. Kim, J. H. Ryu, K. T. Lee, S. M. Oh, J. Power Sources, 147, 227 (2005).
5. M. T. McDowell, S. W. Lee, W. D. Nix, and Y. Cui, Adv. Mater., 25, 4966 (2013).
6. C. J. Wen, R. A. Huggins, J. Solid State Chem., 37, 271 (1981).
7. N. Liu, H. Wu, M.-T. McDowell, Y. Yao, C. Wang, and Y. Cui, Nano Lett., 12, 3315
(2012).
114
Chapter 6. Application of Layered Architecture to Microbatteries
6.1. Introduction
In recent years, lithium ion batteries
have received significant attention
for on-board power supplies for
autonomous devices, delivering the
highest energy and power per
footprint area. Due to the instability
of current provided by on-board power supplies such as photovoltaic cells, fuel cells, and energy
harvesters, these devices require onboard energy storage which can supply peak currents on
demand. Due to their superior storage capabilities, lithium ion microbatteries are ideal candidates
for on-board energy storage.
While microbatteries and common bulk scale rechargeable batteries are very similar in
working principle, microbatteries have several distinguishing features. As onchip energy storage
devices, the dimensions of a microbattery are often less than 1mm2. Common batteries typically
cannot be made smaller than the size of a coin cell. Additionally, since liquid electrolytes used in
common batteries present an inherent risk of leakage, microbatteries typically require a solid
state electrolyte. The separator is also excluded in a microbattery as the thickness the separator is
approximately 20µm. In a microbattery, the entire electrolyte layer is only approximately 1µm,
which serves as the separator also. The rigidity of the solid electrolyte sufficiently separates the
negative and positive electrodes, preventing electrical shorting.
To facilitate miniaturization, various microfabrication techniques have been explored.
These techniques, which are used to fabricate all of the layers of the battery stack, allow for
significant flexibility in the design. Basic microbattery designs consist of planar thin film battery
Figure 6.1. Basic planar microbattery design (1).
115
layers based on solid-state electrolytes and electrode active materials. (Figure 6.1) (5).
Microbatteries are typically deposited on any solid substrate which can withstand battery
fabrication temperatures. On top of the substrate the current collector, anode, electrolyte,
cathode, and second current collector layers are deposited. Deposition techniques for these layers
include RF sputtering, sol-gel deposition, chemical vapor deposition, and pulsed laser deposition.
To further increase energy and power per
footprint area, templated microfabrication
techniques have enabled the fabrication of three-
dimensional electrode designs. These designs allow
for a large increase in the electrode surface area
available for Faradaic reactions and therefore the
energy storage and power capability of the
microbattery. Figure 6.2 demonstrates a three
dimensional microbattery concept in which a
template consisting of arrays of pores is fabricated
onto a substrate. The pores are filled using an
electrode active material. The template is then
removed to form three dimensional arrays of rods.
The consecutive layers of the battery stack are then
deposited on top of the first layer. Perre et. al. calculated the surface area, , for this structure to
be:
( ) (6.1)
Figure 6.2. Three-dimensional microbattery
fabricated using template microfabrication
techniques (1).
116
Where and are the diameter and height of the deposited columns, is the spacing measured
between the centers of the rods, and is the angle of the pattern (6). Using a height and diameter
of the 3D rods of 10um and 200nm, and a spacing of 500nm, a surface area enhancement of 30
can be obtained(1). More advanced concepts include (i) integrating the 3D structure inside the
substrate by using microchannels or a perforated substrate. (ii) Creating a three dimensional
trench structure through an anisotropic etch of a silicon substrate. These structures offer similar
surface area enhancement as the array of rods if similar height, diameters, and spacing are used.
Advantages of the structures in figure 6.3 over the structure in figure 6.2 is higher structural
stability during electrochemical cycling.
Although, these designs allow for a large increase in electrode surface area over planar
electrode designs, several disadvantages limit their feasibility. For electrode structures that
protrude out of the plane of the substrate, mechanical stress due to volume expansion during
cycling causes these high-aspect ratio structures to crack and break apart, limiting the cycle life
of the microbattery. For high aspect ratio structures within the substrate, uniformly coating all
(a) (b)
Figure 6.3. (a). Schematic of 3D microbattery based on microchannel plates(3). (b). Microbattery
integrated on a 3D substrate (4).
117
layers of the battery stack becomes increasingly difficult as the aspect ratio increases. For these
reasons, a high surface area planar electrode design is highly desirable.
6.2. Multi-layered Electrode Architecture for High-Power Lithium Ion Microbatteries
In this work, the 2D multi-layered electrode architecture is applied to a microbattery
application to increase energy and power per footprint area over planar 2D electrode designs.
Each new MWNT layer provides a new electronically conductive and highly porous surface on
which the active material particles are assembled, significantly enhancing the electrode surface
area. Figure 6.4 shows the theoretical surface area enhancement which can be accomplished by
the 2D multi-layered electrode architecture versus 3D vertically aligned carbon nanotube
architecture at various percentages of active material loading. While the 3D structure offers a
larger theoretical surface area enhancement, the 2D designs are more practical as high
percentages of active material loadings are easier to achieve. Additionally, due to the mechanical
strength and flexibility of MWNT layers, the 2D multi-layer structure is more robust than
Figure 6.4. (a). Surface area enhancement allowed by 2D layer-by-layer structure as a function of the
number of layers. (b). Surface area enhancement allowed by vertically aligned CNT structure as a
function of the length of CNTs. Legend indicates the percentage of active material loading.
118
microfabricated 3D structures, buffering stresses due to volume expansion during cycling. The
2D structure also offers ease of fabrication as each consecutive layer of the battery stack sits on
top of the last and does not have to be impregnated within the structure. Finally, if all the MWNT
layers can be electronically connected, conductive additives which add to the “dead” weight and
volume of the battery can also be eliminated. Since the real surface area enhancement offered by
the two dimensional structure is heavily dependent on the precise placement of particles on the
carbon nanotube surface, in the next sections the electrophoretic assembly method is discussed to
uniformly assemble the active material layers onto the MWNT surface.
6.3. Electrophoretic Assembly Mechanism
Electrophoretic assembly (EPA) a technique in which charged particles in a colloidal suspension
are assembled onto a templated surface using an externally applied electric field. EPA is gaining
popularity in the fabrication of nanoscale devices, in which spatial control of nanoparticles is
critical. A high degree of spatial control can be achieved by tuning assembly parameters such as
the applied voltage, assembly time, the speed at which the substrate is removed from the solvent,
and the zeta potential of particles in suspension, which is a measure particle surface charge.
Unlike CVD, PVD, and electroplating, lithium ion active materials are pre-formed prior to
assembly, requiring no additional high-temperature and high or low pressure processing steps.
Overall the EPA technique enables high-rate and low cost fabrication of nanoscale structures.
119
In our setup, we utilize EPA in the
fabrication of the multi-layer architecture.
EPA enables precise assembly of lithium ion
active material over each MWNT surface to
create the multi-layer structure. The
substrate is the aluminum current collector.
The first carbon nanotube layer is deposited
on the aluminum current collector using
spin-coating. Then the aluminum electrode
and a gold counter electrode are dipped into
a suspension of charged lithium ion active
nanoparticles in ethanol (figure 6.5). A
voltage is applied between the two electrodes
to induce an electric field, which forces the
charged nanoparticles in the suspension to the desired aluminum electrode surface. The
deposition of nanoparticles is heavily influenced by the charge of the particles in the suspension.
The charge, , is governed by the equation (7):
( (
)) (6.2)
Where D is the diameter of the colloidal particle, is the permittivity of the material, is the
permittivity of free space, is the inverse Debye length, and is the zeta potential of the
particle. In this work, is the most crucial parameter that governs the morphology of the active
material layer.
Figure 6.5. Fabrication schematic of
electrophoretic assembly of lithium ion
active material on MWNT /aluminum
substrate.
120
6.3.1. Stabilizing the Lithium Ion Active Material Suspension
In this study, lithium
manganese oxide (LiMn2O4)
particles suspended in ethanol
are used for electrophoretic
assembly. To stabilize the
particle suspension or increase
the zeta potential of LiMn2O4
particles, two chelating agents were studied: gallic acid and benzoic acid (figure 6.6) (8). Small
concentrations of each chelating agent were added to active nanoparticle suspensions in ethanol.
Although the two agents are structurally similar, the behavior of the suspension is different after
sitting for 24 hours (figure 6.7). Nanoparticles in the gallic acid suspension remain well
suspended while particles in the benzoic acid suspension precipitate due a poor surface charge.
While the mechanism of gallic acid binding is unclear, it is believed that the hydroxide groups on
the benzene ring are used to anchor the molecule to the surface of the particle. Using the gallic
acid suspension, zeta potential and particle size analysis via dynamic light scattering (DLS)
reveal that gallic acid increases the magnitude of from = 0 mV to = 55mV (figure 6.8).
Similarly, due to electrostatic repulsion between charged particles, decreases from 600nm to
238nm. The influence of and D on the morphology of the assembled active material layer can
be seen in figure 6.9a and b. Without gallic acid, the active material layer consists of occasional
large (10-20µm) particles and smaller (200-500nm) particles scattered over the MWNT surface,
demonstrating a very poorly utilized surface. With the use of gallic acid, the LiMn2O4 particles
assembly covers the entire surface, leaving no MWNT area exposed. Again, large (10-20µm)
(a) (b) Figure 6.6. Chemical structure of experimental chelating
agents (2). (a). Gallic acid. (b). Benzoic acid.
121
particles and smaller (200-500nm) are observed; however, smaller particles are much more
prevalent.
Figure 6.7. 6.67mg/ml of LiMn2O
4 in ethanol using benzoic acid and gallic acid
chelating agents. (a). Benzoic acid, 1mg/ml. (b) 0.5mg/ml. (c). 0.05mg/ml. (d). Gallic
acid, 1mg/ml (b). 0.5mg/ml. (c). 0.05mg/ml.
Figure 6.8. Particle size (right) and zeta potential (left) measurements via DLS for LiMn2O4
suspensions in ethanol with and without gallic acid.
122
6.4. Narrowing the Particle Size Distribution
6.4.1 Ultrasonication
While the gallic acid chelating agent significantly improves the stability of the particle
suspension, the binding interaction is specific to one type of lithium ion active material particle
in a specific solvent. Therefore other methods were explored to increase the stability of the
particle suspension. These methods were independent of the type of particle. The simplest
method is through the use of ultrasonication. In this technique, high frequency (>20 kHz) sound
energy is applied to agitate and break apart large agglomerated particles in a suspension. In our
experimental setup, a 10-gallon industrial ultrasonic tank (Crest Ultrasonics) was used. The tank
is filled with water. The ultrasonic frequency was fixed to 132 kHz and the power of the
ultrasonic energy is fixed to 500W. The LiMn2O4 particles suspended in ethanol (15 ml total
volume) were transferred to a 50ml gradated cylinder. The cylinder was clamped near the surface
of the tank so the whole volume of the particle suspension is immersed below the surface of the
water. The ultrasonics wer then applied for 1 hour. After 1 hour, electrophoretic assembly was
repeated onto the aluminum/MWNT substrate using the ultrasonicated particle solution. It was
necessary to complete the assembly within 1 minute after completing the sonication step to
prevent particles from re-agglomerating. Figure 6.9 demonstrates the morphology of the particle
assembly onto the aluminum/MWNT surface with and without ultrasonication. In the sonicated
solution, the particle size distribution is significantly narrower, consisting 100-300nm size
particles. The MWNT surface is also more effectively utilized as the smaller sized particles
closely follow the contour of the surface. Furthermore, all the particles remain electrically
connected. However, the mass loading is very low resulting in a low cell capacity. Repeatability
using the ultrasonication technique is also very poor.
123
6.4.2. Centrifugation
Repeated centrifugation was another technique
used to narrow the particle size distribution so
that the active material layer was uniformly
assembled onto the aluminum/MWNT surface.
In this technique, large particles were
precipitated from suspension by spinning at a
constant speed. In the first step, the particle
suspension was divided into 1ml Eppendorf
tubes which were inserted into a bench-top
centrifuge. The tubes were spun for 1 minute at 500rpm to pelletize large particles. In step two,
the supernatant liquid, in which smaller particles remain suspended was removed and collected
(a) (b)
Figure 6.9. (a). Assembly of LiMn2O4 particles before ultrasonication. (b). After ultrasonication. Inset
shows magnified image
Figure 6.10. Assembly of LiMn2O4 after
centrifugation.
124
in a separate beaker while the pelletized larger particles were discarded. In the third step, new
Eppendorf tubes were filled with the supernatant liquid collected in step 2. The tubes were again
inserted into the centrifuge and spun at 5000rpm for 3 minutes to pelletize the smaller particles.
The supernatant liquid, which now contained no particles, was discarded and 300µl of fresh
ethanol was added to tubes to re-suspend the particles. The new particle suspension was
collected from each tube. Figure 6.10 demonstrates the electrophoretic assembly results due to
centrifugation. Although large 20µm particles are evident, most of the surface is covered small
200nm particles, indicating that most of the large particles were effectively removed during the
centrifugation process. Also, the aluminum/MWNT surface was effectively utilized by particle.
However, the centrifugation process was extremely inefficient and the volume of small particles
isolated was not enough for electrode areas larger than 1cm2.
6.5. Changing the Solvent System
In addition to narrowing the particle size
distribution, the behavior of particles in
various solvents was also investigated to
stabilize the particle suspension. In figure
6.8, the zeta potential measurements of the
particle suspension without chelating agent
indicate that the distribution is centered
around 0mV. By changing the ethanol solvent, we aim to shift the zeta potential distribution to a
more negative or more positive value. In this experiment, 7 different solvents were investigated;
H2O at pH 2, H2O at pH 11, H2O at pH 7, ethanol, N-methyl-2-pyrollidone (NMP), toluene, and
Figure 6.12. Assembly of LiMn2O4 after in
NMP. Inset shows magnified image.
125
hexane. Figure 6.12 shows
optical images of particles
suspended in each of the
solvents 1 minute and 3
days after solution
preparation. After
preparation, particles
immediately precipitate out
of the toluene and hexane
suspensions, while the
other solvents remain well
suspended. After 3 days, only the NMP suspension demonstrates stability. However
electrophoretic assembly using the NMP/LiMn2O4 particle suspension demonstrated very poor
results as only occasional small particles could be seen across the Al/MWNT surface. It is likely
that the poor assembly results are due to incompatibility of the solvent with the hydrophobic
Al/MWNT surface.
6.6. Other Assembly Techniques
6.6.1. Electro-Fluidic Assembly
To account for the poor surface charge and large particle size distribution of the LiMn2O4
particles in suspension, other assembly techniques were explored in which the size and zeta
potential parameters are less critical for assembly. Fluidic assembly is a technique similar to dip
coating, which is widely used in industrial applications for depositing thin films on planar or
Figure 6.11. Optical images of suspensions of LiMn2O
4 in various
solvents 1 minute after solution preparation (top) and three days
after mixing (bottom). 1-3: H2O at pH 2, 7, and 11 respectively. 4:
ethanol. 5: NMP. 6: toluene. 7: hexane.
126
cylindrical surfaces. In
this technique, the
substrate is immersed
into a solution
containing the coating
material. The substrate
is slowly withdrawn
from the solution at a
constant speed. The
slow pulling action
introduces an
additional capillary force which causes particles to attach to the substrate. In our setup, fluidic
assembly is enhanced by using an applied voltage, which forces a large concentration of particles
to the aluminum/MWNT surface. Figure 6.13 shows the electro-fluidic assembly results using
an electrode pulling speed of 0.1mm/minute and an applied voltage of 200V. The morphology of
the assembled layer demonstrates 200-500nm particles assembled with high density. However
particles are concentrated only in highly contoured areas of the MWNT surface. Additionally,
due to the very slow pulling speed, the process of electrofludic assembly is too inefficient for the
fabrication of multi-layered electrodes which require many layers.
6.6.2. Dielectrophoretic Assembly.
Dielectrophoretic assembly is an assembly mechanism based on the principle of
dielectrophoresis. Dielectrophoresis occurs when a polarizable particle is placed into a non-
Figure 6.13. Electrofluidic assembly of LiMn2O
4 after in NMP. Inset
shows magnified image.
127
uniform electric field. Since all particles exhibit some type of dipole activity in the presence of
electric field, the particle is not required to be charged. Therefore, dielectrophoretic assembly is
used for LiMn2O4 particles suspended in ethanol as the zeta potential distribution is centered
around 0mV. When the particle is polarized by the electric field, the poles experience a force
along the field lines. Since the field is non-uniform the pole experiencing the greatest field will
dominate causing the particle to move towards one electrode or another. The average
dielectrophoretic force on a spherical particle is given by:
⟨ ⟩ {
} | ⃗ | (6.3)
Where is the radius of the particle, is the complex permittivity of the particle, and
is the
complex permittivity of the medium. In our setup, the non-uniform electric field is generated by
an applied AC electric field with frequency 1 kHz and 20V peak-to-peak amplitude. 20V peak-
to-peak is the maximum amplitude of the system. The AC electric field is applied for 100
seconds. All other assembly parameters are identical to those used in electrophoretic assembly.
Figure 6.14 demonstrates the morphology
of the LiMn2O4 layer after dielectrophoretic
assembly. Although the assembly is highly
inefficient, it is likely that 20V peak-to-
peak is not sufficient to impart enough
dielectrphoretic force for assembly. In
electrophoretic assembly, the applied DC
voltage is one order of magnitude higher at
200V. Therefore, decreasing the distance
Figure 6.14. Dielectrophoretic assembly of
LiMn2O
4 in NMP. Inset shows magnified image.
128
between the working and counter electrodes may increase the magnitude of the non-uniform
electric field.
6.7. Electrochemical Performance of Assembled Electrodes
From the assembly and suspension stabilization techniques mentioned above, the use of gallic
acid in conjunction with electrophoretic assembly yielded the highest assembly efficiency.
Therefore, these electrodes were tested for electrochemical performance. Figure 6.15 shows
cyclic voltammetry results of electrode assembled using electrophoretic assembly with and
without the use of gallic acid. Both electrodes demonstrate characteristic intercalation peaks of
LiMn2O4, as described in Chapter 3. These results suggest that the high voltages used in the
Figure 6.15. Cyclic voltammetry of aluminum/MWNT electrodes
assembled with LiMn2O
4 with (red) and without (green) gallic acid.
129
electrophoretic mechanism do not damage the crystal structure or intercalation capacity. The
capacity of each electrode on charge and discharge can be calculated as the area under the
forward and reverse scans according to equation:
∫
(6.4)
∫
(6.5)
From these values, and the mass loading of LiMn2O4, we can calculate an electrode utilization
efficiency (UE) as:
(
)
(6.6)
Table 6.1 provides the results of the calculations. Although the gallic acid electrode demonstrates
two orders of magnitude greater charge and discharge capacities over the electrode assembled
without gallic acid, the utilization efficiency is very low for both electrodes. For the case of
assembly without gallic acid, the assembled layer consists mostly of large (10-20µm) particles
scattered across the Al/MWNT surface. High magnification SEM images demonstrate that these
large particles consist of highly agglomerated smaller particles. Therefore, only the smaller
particles on the bottom surface of the large particles maintain electrical connectivity with the
electrode, causing very poor utilization efficiency. For electrodes assembled with gallic acid, the
prevalence smaller particles allows for significantly higher utilization efficiency. However, only
particles on the bottom layer in direct contact with the Al/MWNT surface maintain electrical
connectivity with the electrode. A significant portion of the assembled LiMn2O4 particles are
inactive, serving as “dead” weight on the electrode. Two and three layer electrodes were also
constructed using the gallic acid electrophoretic assembly technique (Figure 6.16). As the
130
number of active material layers increased, the capacity of the electrodes increased by
0.04mAh/cm2 per LiMn2O4 layer. These electrodes demonstrated excellent cycling stability up to
100 cycles and rate capability at 1 and 10C. However, the utilization efficiency decreased with
each LiMn2O4 layer. At three layers the utilization efficiency is 2.3%, indicating at almost all of
the assembled LiMn2O4 is inactive. Therefore, to construct high efficiency multi-layered
electrodes using the gallic acid/electrophoretic assembly technique, the morphology of the
assembled LiMn2O4 layer must be tuned so that all particles are in direct contact with the
Al/MWNT surface.
Table 6.1. Capacity and utilization efficiency of assembled electrodes calculated from
equations 6.4, 6.5, and 6.6.
131
To tune the morphology of LiMn2O4 particles across the aluminum/MWNT surface, the gallic
acid/electrophoretic assembly technique was modified further to remove large (10-20µm)
particles from the assembly suspension (Figure 6.17). After the LiMn2O4/ethanol suspension is
prepared with gallic acid, the solution is shaken vigorously and allowed to sit undisturbed for
24hours. After 24 hours, large particles precipitate out of the suspension creating a black film
layer at the bottom of the vial. The supernatant liquid, in which smaller 200-500nm particles
remain suspended, is removed from the vial, transferred to a new vial, and homogenized using a
bench-top sonicator. The vial containing the film with large precipitated particles is discarded.
Figure 6.16. Galvanostatic cycling of multi-layered electrodes composed of two (red) and three
(blue) stacked MWNT and LiMn2O4 layers
132
Figure 6.17. Schematic of LiMn2O
4 suspension in
ethanol using gallic acid as prepared (left) and after
sitting for 24hours (right).
The remaining solution is
then immediately used for
electrophoretic assembly
onto the aluminum/MWNT
electrode surface.
The morphology of
the assembled layer is
shown in figure 6.18. The
assembly is highly uniform as only 200-500nm LiMn2O4 particles are observed. Furthermore, the
particle layer closely follows the contour of the surface, fully utilizing the surface area
enhancement provided by the roughened MWNT surface. Finally, as the underlying MWNT
layer is partially exposed, the use of a capping MWNT layer ensures that all particles remain
electrically connected.
Figures 6.19a and b shows the electrochemical cycling results of a one layer electrode
assembled using the modified assembly technique. Figure 6.18a shows the voltage profile on the
second charge and discharge cycle. The normalized discharge capacity is 103mAh/g indicating
that the utilization efficiency of 86%, significantly higher than the values in table 6.1. However,
at the end of charge and discharge, the voltage profile is slightly sloping. The sloping nature
indicates some ohmic losses from the insulating nature of the assembled LiMn2O4 layer. In
figure 6.19, the capacity fades rapidly in the first 7 cycles, cycling at a rate of C/7. This behavior
may also be due to the ohmic losses. However upon continued cycling at higher rates, the one
layer electrode demonstrates excellent rate capability and capacity retention.
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(a) (b) Figure 6.18. (a). Electrophoretic assembly of LiMn
2O
4 particles using gallic acid/ethanol suspension. (b).
Electrophoretic assembly of LiMn2O
4 particles using modifed gallic acid/ethanol suspension described in
figure 6.17. Inset shows magnified image.
(a) (b) Figure 6.18. (a). Voltage profile of electrode shown in figure 6.17b at a rate of C/10. (b). Rate
capability of electrode in figure 6.18b. 1C = 120 mA/g
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6.8. Conclusions and Future Work
In this chapter, the multi-layered architecture was evaluated for enhancing the energy and power
per footprint area of lithium ion microbatteries. Theoretical calculations demonstrate that the
surface area enhancement allowed by the multi-layer two dimensional design is comparable to
that of a high aspect ratio three dimensional design. Yet, the 2D design allows for a robust
structure and ease of fabrication. To achieve precise placement of lithium ion nanoparticles over
the aluminum/MWNT surface, various directed assembly techniques were investigated. Results
demonstrate that electrophoretic assembly combined with a highly stabilized particle suspension
enables a highly uniform layer of lithium ion active material particles over the
aluminum/MWNT surface. Therefore, all inactive components such as carbon black and PVDF
binder can be eliminated. Electrochemical performances studies show that the electrode
demonstrates greater than 86% utilization efficiency and highly stable rate behavior up to 10C.
In future studies the electrophoretic assembly parameters will be tuned to construct
highly uniform multi-layer electrodes using LiMn2O4 and Li-rich NMC active materials.
Electrophoretic assembly will also be investigated to construct the additional layers of the battery
stack to develop a fully solid state microbattery.
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6.9. References
1. Oudenhoven, J. F., Baggetto, L., Notten, P. H. L., Adv. Energy Mater., 1, 10 (2011).
2. K. Wu, Y. Wang, I. Zhitomirsky, J. Colloid Interface Sci., 352, 371 (2010).
3. M. Nathan, D. Golodnitsky, V. Yufit, E. Strauss, T. Ripenbein, I. Shechtman, S. Menkin,
E. Peled, J. Microelectromech. Syst. , 14, 879 (2005).
4. L. Baggetto, R. A. H. Niessen, F. Roozeboom, and P. H. L. Notten, Adv. Funct. Mater. ,
18, 1057 (2008).
5. Jiang, J., Li, Y., Liu, J., Huang, X., Yuan, C, Lou, X. W., Adv. Mater., 24, 5166 (2012).
6. E. Perre, L. Nyholm, T. Gustafsson, P.-L. Taberna, P. Simon, K. Edstrom,
Electrochemistry Communications, 10, 1467 (2008).
7. Besra, L., and Liu, M., Progress in Materials Science, 52, 61 (2007).
8. Wu, K., Want, I., Zhitomirsky, I., Journal of Colloid and Interface Science, 352, 371
(2010).
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Chapter 7: Concluding Remarks and Future Directions
7.1. Conclusions
Developing energy storage devices that meet performance demands of next generation
technologies is one of the greatest challenges of modern engineering. Recently, advances in
lithium ion materials and chemistries have shown potential in meeting these demands. However,
engineering these materials into feasible devices so that their remarkable properties can be
harnessed is equally important. In Chapter 3, we describe the fabrication of a carbon-nanotube
(CNT) based lithium ion electrode architecture to significantly increase the power density and
energy density of lithium ion battery cathodes. The CNT-based architecture aims to address
engineering limitations of nanoscale active materials such as poor packing density, electrolyte
reactivity, and costly fabrication. The electrode architecture consists of alternating layers of
multi-walled carbon nanotubes (MWNT) and lithium ion cathode active material. These MWNT
layers create a highly porous and highly conductive scaffolding to enhance ionic and electronic
transport pathways within the electrode. However, the CNT layers are not detrimental to the
volumetric energy density of the electrode. Using a room temperature and atmospheric
fabrication process, we demonstrate that the electrode architecture is fully compatible with
commercial electrode manufacturing techniques.
In Chapter 4, the high temperature performance of the electrode architecture is evaluated
in conjunction with various lithium ion cathode active materials. We demonstrate that the planes
of carbon nanotubes present a significant advantage over traditional electrodes. The high thermal
conductivities of the carbon nanotube layers mitigate thermodynamically driven structural
changes, which ultimately degrade the energy of the battery. In LiMn2O4, the architecture
mitigates the disproportionation reaction which causes the material to change from a cubic
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structure to a tetragonal structure, taking away available intercalation sites, ultimately leading to
capacity fade. In Li-rich NMC, the architecture mitigates the conversion from a layered structure
to a spinel structure, which degrades the working potential of the cell.
In Chapter 5, a high-rate and low-cost spray-coating process for developing commercial
scale electrodes is developed, enabling the properties of the multi-layer electrode to be evaluated
in a prototype pouch cell. Results of pouch cell cycling demonstrated excellent capacity retention
during formation cycling. However, pouch cells suffered from capacity fade due to material
delamination upon continued cycling.
Finally, in Chapter 6, the multi-layer architecture is evaluated for use in lithium ion
microbatteries to enhance the power and energy per footprint area of on-chip energy storage
devices. Using electrophoretic assembly in conjunction with a highly stabilized active material
suspension, we demonstrate that highly uniform layers of lithium ion active nanoparticles can be
assembled over the aluminum/MWNT surface. By using this technique to build the number of
layers in the electrode stack, we demonstrate that a significant surface area can be achieved over
traditional two dimensional and three-dimensional designs.
7.2. Future Directions
Through this work, a commercially viable next generation lithium ion electrode technology has
been developed and demonstrated for use in a lithium ion cathode. In a parallel research study at
the Center for High-rate Nanomanufacturing, a multi-layer silicon anode is also under
development for next generation lithium ion batteries. Once the electrochemical performance of
the multi-layer silicon anode matches that of multi-layer Li-rich cathode in a half cell
configuration, the two electrodes will be assembled together to produce full cell. Theoretical
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calculations demonstrate the full-cell will demonstrate higher energy density over commercial
cells by a factor of three and higher power density by a factor of 10. Using high-rate and low cost
scale-up processes, the multi-layered lithium ion full cell will address many, if not all, of the
limitations of next generation lithium ion batteries including, cost, energy density, power density,
and safety.
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