Orthotropic Thermo-Viscoelastic Model for Polymeric Battery Separators with Electrolyte Effect

Abstract

The separator plays a critical role in the safety of lithium-ion batteries. To predict the structural integrity of polymeric separators in abuse scenarios, understanding the thermo-mechanical behaviors of the separators is very important. This paper presents an orthotropic linear viscoelastic material model for a polymeric separator that accounts for temperature and electrolyte effects. In this model, the temperature effect was introduced through the time-temperature superposition principle (TTSP). A time-temperature-solvent superposition method (TTSSM) was developed to model the behavior of the separator in electrolyte solutions based on the viscoelastic framework established in air. The developed model has been implemented in LS-DYNA® finite element (FE) package as a user-defined subroutine, which enables simulations with the thermal expansion/shrinkage behavior. The model parameters have been established for a polypropylene (PP) separator. The model predictions of the material anisotropy, temperature dependence, and solvent effect agree reasonably well with the experimental data. The results also demonstrated that the non-isothermal simulations without considering the thermal expansion/shrinkage behavior of the separator resulted in large errors.