For the safety design and integration of battery modules in vehicles, a thermo-electro-mechanical battery model for vehicle crash simulations is under development. The current research focuses on thermomechanical modeling of the battery separators. A separator is a porous membrane with a thickness of several dozen microns. It prevents physical contact between the positive and negative electrodes while enabling ionic transport. The integrity of the separator is critical to the safety of the batteries. Separator failure can lead to a thermal event. Commonly used polymeric separators are manufactured by dry process and have anisotropic microstructures. During thermal ramp, they first expand and then shrink before reaching the melting temperature. The amount of shrinkage can be over 10%. In a constrained condition, the shrinkage induces tensile stress in the separator. To model the material behavior in this range, quantitative measurements of thermal and mechanical properties are needed. The models for this application need to consider the anisotropy, rate and temperature dependence of the material properties, and thermal shrinkage. Currently, the experimental data are incomplete for the development of such models. This is largely due to lack of characterization methods for thin polymeric membranes. In this research, experimental methods have been developed to measure the thermal and mechanical properties. For the thermal property characterization, an experimental method has been developed to measure the thermal expansion/shrinkage and the coefficient of thermal expansion (CTE) using a dynamic mechanical analyzer (DMA). The measurements were carried out for three common types of polymer based separators. The CTE as a function of temperature from ambient to near melting point was determined. The DMA offers continuous measurements in an automatic fashion, which is an efficient and convenient method to characterize the thermal expansion/shrinkage behavior of thin polymer films.The mechanical behavior of an orthotropic material is described by the stress-strain relationships in the principal material directions and shear, and the Poisson's ratios. The measurements of the shear property and the Poisson's ratio for polymer films with a thickness of tens micrometers have not been well established. In this work, these measurements were attempted with a DMA. Digital image correlation (DIC) was used for strain measurements. The shear property was measured using the off-axis tensile experiment. Based on the analogy for anisotropy between the elastic and linear viscoelastic domains, the shear creep response was also measured. The creep compliances in shear and in the principal material directions were determined. Due to its thin thickness, compression experiments with a single/a few layers of separator are difficult to perform. In this work, a capacitance based displacement set-up has been developed for the measurement of the through thickness direction compression stress-strain behavior of the separator and the investigation of its interaction with the electrode. The experiments were performed for a stack of two layers of separator, and separator/cathode/separator stack. The thermomechanical model is developed on an orthotropic viscoelastic framework. A discretization algorithm has been proposed for the evaluation of a stiffness-based hereditary integral with a kernel of Prony series. The model has been implemented in commercial FE package LS-DYNA® as a user defined material model. The implemented model has been verified with analytical solutions and validated with experiments under uniaxial loading conditions. The model validation for biaxial loading cases is ongoing.
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