One key challenge in the deployment of future e-mobility systems is to ensure the safe operating condition of high-energy density batteries. Therefore, understanding battery failure mechanisms and reducing safety risks are critical in the design of electrified systems. Although the response of battery materials and systems under various conditions has been extensively explored in recent years, there are still a lot of challenges with developing models for predicting failures. One such challenge is the development of accurate thermomechanical models to predict battery failure caused by combined thermal and mechanical loadings. Such thermomechanical models aim to identify the thermomechanical failure condition of batteries through battery materials such as the separator. The structural integrity of battery separators plays a critical role in battery safety. This is because the deformation and failure of the separator can lead to an internal short circuit which can cause thermal runaway. In thermal runaway scenarios, the separator first expands and then shrinks before reaching its melting temperature. Furthermore, this shrinkage induces tensile stresses in the separator. Hence, developing a thermomechanical model that can predict the response of separators in their entire range of deformation is necessary.Commonly used battery separators are dry-processed polymeric membranes with anisotropic microstructures and deformation modes that involve various physical processes that are difficult to quantify. These complexities introduce challenges in their characterization and modeling as their properties and structural integrity depend on multiple factors such as loading rate, loading direction, temperature, and the presence of an electrolyte. To predict the structural integrity of polymeric separators in abuse scenarios an understanding of the thermal and mechanical behavior of the separator is needed. Due to the multiple factors influencing the structural integrity of polymeric battery separators, developing models for the prediction of their thermomechanical response has always been challenging. Furthermore, computational models in the form of user-defined material models are needed to account for these factors since existing material models in commercial software do not have that capability. In this study, thermomechanical models are developed to predict the response of polymeric battery separators in thermal ramp scenarios. The time-dependent response of polymeric battery separators is taken into account and the material is modeled as viscoelastic in the deformation region before yielding and as viscoplastic under large deformations post-yield. As a first step, a linear thermoviscoelastic model developed on an orthotropic framework was extended to account for the temperature effect and the plasticization effect of electrolyte solutions to predict the thermomechanical response of separators within the linear range of its deformation. In the developed linear orthotropic thermomechanical model, the temperature effect was introduced through the time-temperature superposition principle (TTSP). To account for the plasticization effect of electrolyte solutions on the thermomechanical response of the separator, 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. Furthermore, an orthotropic nonlinear thermoviscoelastic was developed to predict the material response under large deformations before the onset of yielding. The model was developed based on the Schapery nonlinear viscoelastic model and a discretization algorithm was employed to evaluate the nonlinear viscoelastic hereditary integral with a kernel of Prony series based on a generalized Maxwell model with nonlinear springs and dashpots. Temperature dependence was introduced into the model through the TTSP. Subsequently, the developed nonlinear viscoelastic model was coupled with a viscoplastic model developed on the basis of a rheological framework that considers the mechanisms involved in the initial yielding, change in viscosity, strain softening and strain hardening in the stress-strain response of polymeric battery separators. The coupled viscoelastic – viscoplastic model was developed to predict the thermomechanical response of separators in their entire range of deformation before the onset of failure. The material investigated in this work is Celgard®2400, a porous polypropylene (PP) separator. Experimental procedures were carried out under different loading and environmental conditions, using a dynamic mechanical analyzer (DMA), to characterize the material response, calibrate and validate the developed models. The developed thermomechanical models were implemented as user-defined subroutines in LS-DYNA® finite element (FE) package, which enables simulations with the thermal expansion/shrinkage behavior. Furthermore, analytical solutions were developed to verify the implementation and predictions of the viscoelastic models. The results from this study show that the model predictions of the material anisotropy, rate dependence, temperature dependence, and plasticization effect of electrolyte solutions agree reasonably well with the experimental data. The results also demonstrate that the non-isothermal simulations without considering the thermal expansion/shrinkage behavior of the separator resulted in large errors.
Read
