Microstructure and electrochemical studies on carbon … · the negative plates, ... Microstructure...

Microstructure and electrochemical studies on carbon nano material additives for positive active mass of industrial cells Varna│June 13th , 2017

• Introduction

• Experimental procedure

• Analytical and microstructural studies

• Electrochemical test results

• Teardown analysis of formed active material

• Conclusions

June 13, 2017 Exide Technologies Europe | Nanostructured PAM | COMPANY CONFIDENTIAL 2

Agenda

Introduction

3

• New developments on Nanomaterials may help to improve performance and cycle life

of lead-acid batteries.

• Recently it has been proposed the use of Carbon Nano-Tubes (CNT) as additives for

the negative plates, but the high cost of these materials and the use of cheaper

alternatives (Conductive Carbons) make difficult an industrial application.

• On the other hand, their possible use in the positive plate is not fully explored although

the potential benefits are even greater, which could justify the material cost.

• Multi walled CNT dispersions are being investigated, with the aim to modify the

internal structure of the PbO2 electrodes (reducing particle size, increasing surface

area and inter-particle conductivity).

• Preliminary studies in laboratory cells showed that very small amounts of conductive

Carbon Nano-Tubes (CNT) act as crystallization nuclei of active material, improving

the contact between the insulating particles of basic Lead Sulfates.

• The aim is to improve the energy efficiency, both during formation and along the

charge and discharge cycles, i.e. to increase the durability of lead acid batteries.

Exide Technologies Europe | Nanostructured PAM | COMPANY CONFIDENTIAL June 13, 2017

• Introduction





• Conclusions


Agenda

Experimental procedure

5

• Several batches of positive mass (leady oxide and sulfuric acid) were prepared in a

laboratory mixer and then pasted on PbSnCa grids (10 Ah), cured and formed.

• Multi-walled CNT water dispersions were added to leady oxide at two different

concentrations (0.025% and 0.05%) and then mixed with sulfuric acid (1.4 g/cc) to

prepare the positive paste.

• The paste was applied to PbSnCa grids, cured for 24h at two different temperatures

(60ºC & 80ºC) with 85% relative humidity and then dried 24h at 70ºC.

• The dry unformed positive plates were formed in diluted sulfuric acid (1.1 g/cc),

washed, dried and assembled as electrochemical cells (2 negative / one positive).

• Three initial performance tests were performed in sequence:

• Capacity test (C20) at low current (0.5A) until 1.75V

• Recharge at 0.5A for 24 hours with a voltage limitation of 2.67V

• Charge acceptance (CA) measurement was done after a rest period (24h) at 90%

State of Charge (SOC) with a maximum Voltage (2.42V/cell).

• Finally, a teardown analysis of formed and tested active material was performed.


• Introduction





• Conclusions


Agenda

TEM of CNT dispersion

Analytical and microstructural studies

7

• Multi Walled CNT particles were analyzed by Transmission Electron Microscopy (TEM)

to study their characteristics and stability during the dispersion process.

• The pictures show the CNT particles that were deposited and dried over the TEM grid.

• As expected, there is a range of different CNT particle sizes, being the most common

values around 12 nm diameter and 12 walls per particle.


EDX analysis


8

• The EDX analysis show certain particles coming from the production processes, like

the presence of metals such as Fe (more abundant), Zn and Cr.

• It is important to note that no drastic changes are observed after the mechanical

dispersion process: the CNT remain undamaged, apart from maybe a possible

shortening of the tubes (not quantified).


Active material composition and pore distribution


9

• The presence of CNT do not affect significantly the apparent density and phase

composition of the active material (mainly composed by α-PbO and 3BS).

• However, It has been found that the addition of CNT modify the pore structure of the

active material, showing a linear dependence of the pore size with the carbon content.


CNT

Content

(%)

MERCURY

POROSIMETRY

X-RAY DIFFRACTION

Porosity

(%)

Density

(g/cc)

Pore

size

(µm)

-P

bO

-P

bO

1B

S

3 B

S

4 B

S

0 49.4 4.29 0.44 85 - - 11 -

0.025 47.8 4.42 0.32 76 - - 8 -

0.05 47.2 4.36 0.25 74 1 - 11 -

y = -0,038x + 0,4317 R² = 0,9774

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0,45

0,50

0 1 2 3 4 5 6

Po

re S

ize

(µ

m)

CNT content (x 0.01%)

Average Pore size

SEM and surface area (BET)


10

• CNT particles can be clearly observed in the unformed active material.

• Surface area of the active material was estimated from N2 adsorption analysis (BET)

with up to 13 experimental points.

• The figure below shows that CNT addition increases the specific surface area.

• Results also suggest that low (LT) curing yields slightly higher surface areas than high

temperature (HT) curing process (although no 4BS was detected by XRD).


• Introduction





• Conclusions


Agenda

Influence of CNT on plate formation

Electrochemical test results

12 Exide Technologies Europe | Nanostructured PAM | COMPANY CONFIDENTIAL June 13, 2017

• Dry unformed plates were formed in diluted sulfuric acid (1.1 g/cc) at constant current.

• The figure shows the electrode potential average (4 plates each) along the formation.

• CNT addition increase the positive potential but also delay the onset of gassing

(oxygen evolution) thus potentially improving formation efficiency.

Influence of CNT on initial performance


13

• After formation, the plates were washed for two hours and dried at 80°C for 24h, then

assembled (as 1+/2- cells) and immersed in sulfuric acid (1.28 g/cc) for testing.

• Initial performance is slightly improved with 0.05% CNT content, however, the capacity

remain stable whereas with 0.025% CNT and the control samples actually increase.


0.025%

0.025% 0.025%

0.05% 0.05%

0.05%

0

20

40

60

80

100

120

140

FIRST CAPACITY SECOND CAPACITY THIRD CAPACITY

Cap

acit

y (

mA

h/g

)

Specific Capacity of Positive Electrode

Influence of CNT on charge acceptance (90% SoC)


14

• After initial performance tests, the cells were fully charged at 2.67V for 16 hours, then

discharged for 2h at 0.5A (90% SoC), rest for 24h and charged at 2.42V for 60s.

• CNT addition reduce in fact charge acceptance, results that confirm the previous

finding during formation (probably due to the increase of the oxygen overvoltage).


• Introduction





• Conclusions


Agenda

Phase composition and surface area

Teardown analysis of formed active material

16

• Samples of dry formed active material were also analyzed by XRD, BET, SEM and

Raman spectroscopy.

• The main component observed is β-PbO2, together with some α-PbO and α-PbO2.

• As expected, the BET measurements showed higher surface areas than unformed

plates (2-3 times), but the effect of CNT on surface is not relevant after formation.


SEM and Raman spectroscopy

Teardown analysis of formed active material

17

• It is worth noting that no CNT have been observed in the charged plates by SEM or

Raman spectroscopy, so apparently they could have been oxidized during the

formation process or remain unseen in the skeleton of the active material.


• Introduction





• Conclusions


Agenda

Conclusions

19

• Multi-Walled Carbon Nano Tubes (CNT) dispersions were characterized by TEM

showing 12 walls with average diameter of 12 nm and variable length.

• The addition of CNT reduces the pore size and increase surface area of the unformed

active material showing a linear dependence with the Nano-material dosage.

• No influence of CNT was observed in the phase composition of the active material.

• CNT increase positive potential and delay the onset of gassing during formation.

• There is an increase of the initial capacity that remain stable afterwards, whereas the

control and lower CNT dosage actually increase capacity in the initial cycling.

• Charge acceptance is reduced with the addition of CNT to positive active material,

possibly due to the increase of the oxygen over-potential.

• After formation, surface area is not affected by CNT neither it has been observed any

CNT particle in the positive formed active material by SEM observations.

• Raman spectroscopy detected the presence of CNT before formation but the

characteristic peaks disappear after formation, either because they are not stable or

remain inside the skeleton of the positive active material.


Thank you for your attention!

June 13, 2017 Exide Technologies Europe 20

Contact

Name: Francisco Trinidad

Position Title: Director Basic Research

Division: Europe

Phone: +34949277544

Mobile: +34616927664

[email protected]

Exide Technologies Europe | COMPANY CONFIDENTIAL 21 June 13, 2017

Microstructure and electrochemical studies on carbon … · the negative plates, ... Microstructure...

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