The origin and elimination of separator wrinkles in lithium-ion batteries

doi:10.19799/j.cnki.2095-4239.2020.0322

Abstract

Abstract:

In the manufacturing of lithium-ion batteries (LIBs), a common phenomenon occurs where wrinkles containing residual air bubbles are formed between the separator and electrodes, usually observed on local areas of the separator when the liquid electrolyte is added. Wrinkles on the separator and defects at the interface between the separator and electrodes will result in a non-uniform internal resistance distribution in the batteries. In the low internal resistance area, over-discharge or overcharge may occur during cycling, which might affect the cycle performance of the batteries. In this manuscript, various kinds of separators were investigated to address this issue. When dimethyl carbonate (DMC) is used to wet the separator surface, wrinkles form on all types of separators, and the wrinkle spacing increases with the thickness of the separator. The wetting frontier of the separator by DMC was investigated, and the resulting separator wrinkle can be attributed to two factors. When DMC is flowing on and wetting the separator, capillary action leads to the uplift of the separator, and a small gap can form between the separator and electrode. The air driven out of micropores in the separator and electrode after wetting with DMC aggregates at the interface to form air bubbles, resulting in local deformation and wrinkles. To address the wrinkle problem in LIB manufacturing, adhering polyvinylidene difluoride (PVDF) coated composite separators to electrodes under a hot press can counteract the capillary action and mitigate the formation of wrinkles. It was found that when the peeling strength is smaller than 10 mN/cm, the adhesion force cannot counteract the capillary action. Although local wrinkles may still be present, the number of wrinkles was significantly decreased. When the peeling strength is larger than 15 mN/cm, separator wrinkles are completely eliminated, suggesting that an increase in the adhesion force between the separator and electrodes reduces the defects at the interface. Reducing defects such as separator wrinkles is a valuable strategy to promote the consistency and cycle stability of LIBs.

Key words: separator wrinkle, capillary force, PVDF composite separator, adhesion force

CLC Number:

TQ 150.4

Xiaoqing YAN, Zhiyu HU, Fengquan LIU, Lin LI, Chuanming GU, Xiying DAI, Yu XIAO, Zhaoliang XING, Jianjun ZHOU. The origin and elimination of separator wrinkles in lithium-ion batteries[J]. Energy Storage Science and Technology, 2021, 10(1): 156-162.

Figures/Tables 7

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