©2015 celgard, llc all rights reserved. an overview of mechanical, thermal, and chemical stability...

©2015 Celgard, LLC ALL RIGHTS RESERVED.

An Overview of Mechanical, Thermal, and Chemical Stability of

Separator Materials for Li-ion Batteries

INTERBAT 2015Prepared by Lie Shi

CTO / Separator Technology

Presented by Salvatore Cardillo - Sales Director And Dmitriy Tarasov – Azelis Rus

March, 2015

Cardillo, Salvatore

©2015 Celgard, LLC ALL RIGHTS RESERVED. 2

Mechanical, Thermal & Chemical Stability

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PERFORMANCE



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PERFORMANCE

• CHEMICAL• Polyolefin is stable

• THERMAL• Shrinkage• Highest Temperature stability

• MECHANICAL• Tensile Strength• Puncture Strength• Dimensional Stability

FILM TESTING = STATIC EVALUATION



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PERFORMANCE

IN-BATTERY ANALYSIS = DYNAMIC STUDY

- Results have been reported by multiple research groups

Strength Retention After AgingBy P. Ramadass

High Temperature StorageBy M. Ohashi, et al [1]

Stability in High Voltage SystemBy Y. Obana, et al [5]

Oxidation ResistanceBy J. Zhang, et al [3]

Hot Box TestBy R. Smith, et al [2]

Stability in High Voltage SystemsBy S. Nomura, et al [6]

Electrochemical StabilityBy D. Thurman, et al [4]

Property Change After CyclingBy Ron Smith, et al [2]


Thermal Stability: Hot Box Test

R. Smith, etc. AABC-EU 2010

PP-based separator is more stable.


Thermal and Mechanical Stability: PP based Separators

R. Smith, etc. AABC-EU 2010

0%

30%

60%

90%

120%Before Cycling

After Cycling

ZD Strength TD Strength

MD Strength

Puncture Strength

Shrinkage

PP-based Separators showed excellent thermal and mechanical stability. (after 500 cycles : 3V - 4.2V)


Mechanical Stability: Strength Retention After Aging

• Tests simulated aggressive aging of battery cells by maintaining cells at high voltage and high temperature (4.2V, 60C)

• 20 um PE and 20 um PP/PE/PP Trilayer separators were used in the tests

• Film mechanical strength was measured at different stages of aging

PP-based separator is more mechanically stable.


Electrochemical Stability:

• During a high temperature trickle charge test (4.25V, 60C), battery (18650 cell) could experience current leak.

J. Zhang, etc. AABC 2009



Electrochemical Stability (mAh) =

• Derek Thursman, et al. [4] , used the term “Electrochemical Stability” to measure separator’s resistance to oxidation by quantifying the current leak which occurred in a trickle charge test (4.3V, 60C, in 21 days).



Derek Thursman, et al. [4] concluded the following:

1. An electrochemical stability value up to 100mAh is acceptable for automotive Li-batteries.

2. PE separators showed an electrochemical stability ≥100 mAh.

3. By incorporating PP non-woven web on top of PE to a desirable proportion, the stability level can be enhanced and be kept ≤100mAh.

Not Acceptable ≥ 100 mAh

Acceptable


Chemical Stability: Oxidation Resistance

Wet PE Separator (This started out like the center image. Oxidation turns it dark and makes it brittle)

Celgard® Dry-process Separator

Wet PE Separator (This also started out like the center image. Oxidation turns it dark and makes it brittle)

18650 Cells After Trickle-charging (at 4.25V at 60degC ) for 15 Days


High Temperature Storage

• M. Ohashi, et al. [1] studied film stability by measuring High Temperature Storage (HTS) which was defined as the Capacity Retention ratio after battery is stored at 60ºC for 7 days

• It was determined that the presence of PE in the polyolefin membrane separator has some unusual consequences

• It generates a varying degree of blackening on the surface of the separator.

• It affects High Temperature Storage (HTS).


High Temperature Storage

The more PE, the more blackened the separator became.

The more PE, the more capacity loss the battery experienced.

0 10 20 30 40 50 60 70 80 90 10062

64

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68

70

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-5

0

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15

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% HTS and % Blackening as Function of PE in Surface of Film

% PE in Surface

% H

igh

Tem

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ture

Sto

rage

% B

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g


High Temperature & High Voltage

S. Nomura, etc. [6] studied OCV of cells charged at high voltages and reported the following:

• Cells charged at higher voltages dropped their voltage faster;

• Autopsy revealed that PE separator surfaces facing cathode have turned black;

• Coating TiO2 onto cathode seemed to reduce voltage drop.


What is the Black Material?

Y. Obana and H. Akashi [5] Also observed black spots on PE separator surface after high voltage cycling Attributed coloring to PE oxidation

Change from -CH2-CH2- to -CH=CH- saturated unsaturated


Cycling Performance (45oC)

J. Zhang, et al. AABC 2009


Summary

THERMAL

MECHANICAL

CHEM

ICAL

BATTERY

PERFORMANCE

Electrochemical stability of separators is critical.

Good Electrochemical stability provides mechanical and thermal stability and better battery performance.

PP-based separators demonstrated superior electrochemical stability over PE-based separators.

Dynamic study of the in-battery analysis by multiple groups revealed the following key points:


Acknowledgement

• Dr. Prem Ramadass carried out “Strength Retention After Aging” experiment.

• References:

1. M. Ohashi, T. Kondo, U.S. Pat. 7,618,743.

2. R. Smith, J. Zhang, P. Ramadass, S. Santhanagopalan, AABC EU (2010).

3. J. Zhang, P. Ramadass, S. Santhanagopalan, AABC (2009)

4. D. Thursman, P. Brant, K. Yamada, K. Kono, U.S. Pat. Appl. US 2012/0052362.

5. Y. Obana, H. Akashi, Meet Abstr. 2006, Volume MA 2006-01, Issue 3, Page 105 (209th ESC meeting, Abstract #105).

6. S. Nomura, N. Imachi, T. Sato, H. Nakamura, The 53rd Battery Symposium in Japan (2013), 2B03.


Celgard® Coated Separators

• Celgard has developed a variety of coated separators to serve both the EDV and CE markets

• The total thickness of coated films ranges from 16um to 30um• Celgard coated separators offer the following attributes:

Super low thermal shrinkage

Low moisture level

Low curling for one side coated film

High voltage resistance

Excellent coating uniformity

Wide selection of base films


Q-Series Ceramic Coated Data Sheet


Ceramic Coated Data Sheet


Celgard, LLC

Pioneered dry-process, multilayer and multi-functional separator technology to serve broad needs of Li-ion batteries

Celgard is the only company in the world possessing both wet and dry microporous membrane processes

Invented and developed ceramic coated separators to enhance safety and performance of Li-ion batteries

Established strong intellectual property position (patents, expertise, trade secrets)

ISO/TS 16949 Certified

Celgard is a recognized leader among Li-ion battery material suppliers


Thanks

©2015 celgard, llc all rights reserved. an overview of mechanical, thermal, and chemical stability...

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