Characterization and Modeling of Highly Aniosotropic Materials Behavior: Application to Battery Separators

Abstract

Separators are microporous films sandwiched between the anode and cathode to prevent physical contact and allow the exchange of ions between the two electrodes. Degradation in the separator occurs under charge and discharge cycles affecting its mechanical properties. Failure in this component can cause internal shorting of the battery or thermal runaway. Hence, an accurate characterization and modelling of the separator's mechanical behavior and induced degradation under different charge and discharge cycles is essential for optimal design and life cycle operation of commercial lithium-ion batteries. In this work, the effect of cyclic fast charging on the mechanical and physico-chemical properties of the polyethylene separator is investigated. Initially, the mechanical behavior of a pristine polyethylene separator was studied. An anisotropic continuum damage coupled elastic-hyperelastic-viscoplastic model is developed to capture the mechanical and fracture behavior of battery separators. The capability of the proposed model to predict the anisotropic mechanical behavior up to fracture for two different types of battery separators highlights the versatility of the proposed modeling approach. Then, six lithium-ion battery pouch cells were subjected to cyclic fast charging conditions at 4C (fast) charge rate and 0.5C discharge rate up to 400, 800, and 1600 cycles. Once the cyclic fast charging was completed, the battery pouch cells were fully discharged and carefully disassembled. The physical and chemical microstructural properties of pristine and cycled separators were investigated using scanning electron micrographs (SEM), Xray photoelectron spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FTIR). Additionally, the mechanical behavior of the pristine and cycled separators was also investigated using uniaxial tensile tests and biaxial punch tests. While the XPS and FTIR analysis of the cycled separator surfaces did not reveal the presence of chemical degradation, the mechanical properties presented a decrease in the toughness and ductility with increased number of charging cycles. Finally, the thermal and mechanical behavior of an NMC 622 pouch cell in a battery pack array is characterized and discussed under fast charging conditions. A novel battery cell model for prediction of stress distribution inside a battery pouch cell during fast charging is presented. A Multiphysics model is developed to predict the core temperature of the battery, the loads applied, and the displacement behavior in the battery pack array. The thermal and mechanical behavior of an NMC 622 pouch cell in a battery pack array is characterized and discussed under fast charging conditions. A robust parameter identification scheme is proposed to determine the model parameters based on experimental data. The first principal stress in the separator was shown to be around 70 MPa. Creep and fatigue tests were conducted on the separator to generate deformation damage. The results show that 70 MPa can cause significant damage in the separator after repeated fast charging. The conducted studies helped in expanding the knowledge about the degradation mechanisms in separators during charge and discharge cycles. Additionally, the continuum damage model and Multiphysics model can provide designers with predictive tools to improve the battery design, life span and safety.