A separator is a porous membrane that prevents the physical contact between the positive and negative electrodes while enabling ionic transport. The integrity of the separator is vital to the performance and reliability of a battery. Presently, there is no method to evaluate the stress in a separator in situ in a battery. In this research, a numerical model is developed to address this need. The stress in a separator is mainly caused by thermal expansion differential between battery components and lithium diffusion induced deformation in the electrodes. To compute the lithium concentration distribution and temperature change during battery operation, multi-physics models were developed in COMSOL, and then mapped to a macro-scale prismatic cell model in ANSYS. In this macro-scale model, the porous battery components were treated as homogenized media and represented with the effective properties estimated using the rule of mixtures for a composite material. The stress analysis showed that the maximum stress in the separator always emerged in the area around the inner corner where the separator wrapped around the edge of an anode and when the lithium ion battery was fully charged. Numerical simulations were also conducted to investigate the influences of three groups of design adjustable parameters on the locations and magnitudes of the maximum strain and stress of the separator. The predicted results provide the reference conditions for the improvement of separator materials and for the design of lithium ion batteries.
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