A hyper-viscoelastic model for battery separators based on inverse-stress-solution parametrical calibration method

As the most important component in a rechargeable battery, the mechanical behaviors of separator play a crucial role in the overall safety of the battery. However, there is still a lack of effective calibration procedure for the parameters in the finite-strain viscoelasticity model of separators. To address this issue, a comprehensive analytical-experimental-numerical study is performed to characterize the time-dependent mechanical behavior in large deformation for the wet-processed isotropic separator within the framework of a hyper-viscoelastic (HVE) constitutive model. Based on ASTM E328-13, a tension-relaxation test is designed under various loading strain-rates (from 10⁻⁴ –10⁻¹ s⁻¹), and it is found that the difference in stress response under various loading strain-rates is mainly caused by viscoelastic effect instead of strain-rate effect. Then an inverse-stress-solution (ISS) parametrical calibration method is proposed to determine the parameters in the HVE model. By comparing the experimental results with the numerical results based on HVE models determined by three different parametrical calibration methods, it is found that the proposed ISS method can give the most robust and accurate predictive results for the isotropic separator within a finite-strain of 0.10 under loading strain-rates from 10⁻⁴ to 10⁻¹ s⁻¹ in this study.