2D Black Phosphorous as a Polysulfide Trapping Agent in Lithium...
Transcript of 2D Black Phosphorous as a Polysulfide Trapping Agent in Lithium...
2D Black Phosphorous as a Polysulfide Trapping Agent in Lithium-Sulfur Batteries
Nikhil KoratkarJohn A. Clark and Edward
T. Crossan ProfessorEditor CARBON (Elsevier)
Department of Materials Science and Engineering
Rensselaer Polytechnic Institute110 8uth Street, Troy,
New York, USA.
e− e−
Li+
Li+
+−
Lit
hiu
m Poly-sulfides
Li2S6 catholyte
Before we look at Li-S, lets review Li-ion Batteries
Liquid Electrolyte: Solution of lithium-salt electrolytes, such as LiPF6, LiBF4, or LiClO4, in an organic solvent such as alkene carbonates
Specific Capacity ~ 370 mAh/gSpecific Capacity ~ 250 mAh/g
Do Lithium-Ion Batteries (LIBs) need to be improved ?
Source: Chem. Rev. 2004, 104, 4245.
Li-S Battery Chemistry Shows Great Potential
Sulfur cathode:S8+16Li++16e-
8Li2SLithium anode:
Li++e- Li
Charge: 8Li2S S8+16LiDischarge:S8+16Li 8Li2S
a b
Theoretical specific capacity: 1675 mAh/g Energy density of ~2,600 Wh/kg
5X higher than Lithium-ion batteries (~387 Wh/kg).
S8 + 16Li ↔ 8Li2S 2 electron transfer per S atom !
Source: Chem. Soc. Rev. 2016, 45, 5605–5634.
The challenge with Li-S technology- very poor cycle stability
Intermediate reaction products (lithium polysulfides- LiPS) are soluble in the liquid electrolyte (~1.0 m lithium bistrifluoro-methane-sulfonylimide in 1,3-dioxolane & 1,2-dimethoxyethane)Soluble LiPS include: Li2S2, Li2S4, Li2S6, Li2S8
Source: Chem. Soc. Rev. 2016, 45, 5605–5634.
S8 + 16Li ↔ 8Li2S
Fast Capacity Fade with Charge/Discharge Cycling
N-G-SA-G-S
Rapid Capacity Fade (~0.25% per cycle) due to Loss of S (LiPS Dissolution)
One solution strategy: Add a polysulfide immobilizer (trapping agent) to S electrode
0 20 40 60 80 1000.00
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Deca
y ra
te/c
ycle
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Mass ratio of adsorbent/S or Li2S(%)
500 cycles(S loading: 3.3 mg cm-2)
300 cycles(S loading: ~1.5 mg cm-2)
200 cycles
400 cycles(Li2S loading: 1 mg cm-2)
300 cycles(S loading: 0.7 to 1.2 mg cm-2)
500 cycles(S loading: 1 mg cm-2)
500 cycles
1500 cycles(S loading: 1.5 mg cm-2)
Baseline capacity fade rate ~ 0.25% per cycle
Common Trapping AgentsMetal and Carbon basedOxides, Sulfides andNitrides
Our Objective: To investigate new classes of trapping agents.We studied how Phosphorene (or 2D Black Phosphorous) performs as a S trapping agent in Li-S batteries
Phosphorene- a monolayer of black phosphorous (BP)“also called 2D Black Phosphorous”
Red Phosphorous (RP) to BP Conversion by Chemical Vapor Transport (CVT) Method
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Inte
nsity
2θ (degrees)
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500 μm Energy (keV)
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Solution Phase Exfoliation of BP into Few Layer Phosphorene (FLP) Nanosheets
10 μmPuckered Honeycomb Structure of Monolayer BP
Characterization of Few-Layer Phosphorene (FLP) sheets
8 nm6 nm
100 nm
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500 nm
Inte
nsity
(a.u
.)
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250 nm
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Li-S Battery Assembly: FLP Pre-Dispersed in a Carbon Nanofiber (CNF) Electrode, Followed by S Impregnation
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e− e−
Li+
Li+
+−
Lith
ium Poly-
sulfides
CNF
FLP
FLP+Li2Sx
dLi2S6 catholyte
C mapping P mappinguFLP
Cross-Sectional Imaging of Electrode
5 μm
5 μm
80 μm
5 μm5 μm
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Electrochemical Performance
Incorporation of FLP (~10% of S weight), results in very significant improvement in Cycle Stability
cFLP-CNFCNF
FLP-CNF CNF
57% 41%
Utilization ratio of sulfur
Electrochemical Performance
FLP-CNFCNF
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FLP-CNF
CNF
E
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Effect of Charge/Discharge Rate
Typical Voltage Profile
Understanding the ability of FLP to bind with Lithium Polysulfides (LiPS)
Phosphorene+Li2S
Phosphorene+Li2S2
Phosphorene+Li2S3
Phosphorene+Li2S4
Phosphorene+Li2S6
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b
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Li2S2Li2S Li2S3 Li2S4 Li2S6
f Binding energy to phosphorene
Binding energy to carbon
Density Functional Theory with Corrections for van der Waals Forces (DFT-D)
S-P: BondingLi-P: Electrostatic Interactions
Post-Cycling Characterization
2 μm
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5 μm 5 μm
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Comparison with state-of-art S trapping agents
500 cycles(S loading: 3.3 mg cm-2)
300 cycles(S loading: ~1.5 mg cm-2)
200 cycles
400 cycles(Li2S loading: 1 mg cm-2)
300 cycles(S loading: 0.7 to 1.2 mg cm-2)
500 cycles(S loading: 1 mg cm-2)
500 cycles
1500 cycles(S loading: 1.5 mg cm-2)
N-doped carbon nanotubes
N-doped grahene
TiS2
CaO
TiN
Carbon Nitride
C3N4
FLP: Comparable fade rate at 3X lower loading !S loading in our battery is 2-3 X greater than other work
FLP: Comparable fade rate at 3X lower loading, Why ?
2D Nanosheet Geometry (maximizes surface atoms) when compared to 3D particles
FLP density is ~ 2.69 gm/cm3
Lighter than:Metal oxides (CaO: 3.35 g/cm³), Sulfides (TiS2: 3.22 g/cm3), & Nitrides (TiN: 5.4 g/cm³).
Publication: Advanced Materialsdoi:10.1002/adma. 201602734 (2017)
Takeaway Points• Lithium polysulfides bind strongly to phosphorene Strong bonds formed between S and P atoms Electrostatic interaction between Li and P atoms
• Phosphorene excels as a polysulfide trapping agent Phosphorene addition (~10wt% of S) into the electrode
reduces the capacity fade rate of the Li-S battery to ~0.05% per cycle (5X lower than the baseline).
• Phosphorene outperforms other competing polysulfide trapping agents such as nitrogen doped nano-carbons, as well as metal oxides, sulfides and nitrides.
Future Research Directions• Vary loading fraction of FLP
• Disperse thinner FLP (ideally monolayers)
• Defect engineering of FLP to enhance LiPSbinding
• Explore Red Phosphorous (RP) as a polysulfide trapping agent. RP is earth abundant and relatively inexpensive ($50/Kg: comparable to graphite). However nano-size particles of RP must be produced, which is non-trivial.
Acknowledgements
Funding Support: NYSERDA & NSF
My Students: Lu Li, Shravan Suresh & Tushar Gupta
THANK YOU FOR YOUR ATTENTION !
Collaborators: Hui-Ming Cheng (Shenyang, China)Chandra Veer Singh (Toronto, Canada)