The separator is a critical component to the durability and safety of lithium-ion (Li-ion) batteries. It prevents the physical contact between the positive and negative electrodes while enabling ionic transport. The integrity of the separator is vital to the performance and reliability of a battery. In a Li-ion battery, stresses arise as the results of mechanical loading and constraints, intercalation-induced deformation in the active materials in the electrodes, and thermal expansion mismatch between the battery components. Currently, there are no methods available to evaluate the in-situ stresses of the separator. This work appertains to the development of a modeling tool for the evaluation of stresses in the separator in a Li-ion battery. Towards this end, a multiphysics model for a basic Li-ion battery cell had been developed and implemented in the multiphysics finite element package COMSOL®. The model considered charge/species transport through diffusion and migration, charge balance, electrochemical reaction kinetics, heat generation and heat transfer, as well as the stress and deformation due to mechanical loading, Li intercalation and temperature variation in the battery. The multiphysics model was based on a validated one-dimensional (1D) battery model developed by Newman's group and its implementation in COMSOL® for a LixC6|LiPF6|LiyMn2O4 cell. For stress analysis, a 1D “Thermal” sub-model and a 2D meso-scale representative volume element (RVE) “Stress” sub-model were subsequently added. These sub-models were fully coupled with each other. The temperature dependence of the transport properties and reaction rate parameters was also included. The separator was modeled with a linear viscoelastic model based on experimental data. Its temperature dependence was established through the time-temperature superposition principle. The model was used to investigate the stress in the separator with battery cycles. The simulation results revealed that the stress in the separator varies in phase with the battery cycles. The effects of Li interaction and temperature rise in the battery could not be superimposed, requiring both to be considered concurrently. The stress state and magnitude depended upon the mechanical properties of separator, active particle size and packing. Besides addressing separator stresses, the model was also used to evaluate the influence of the active particle size and component thickness on the heat generation rate and battery performance. The simulations revealed the evolution of the rate limiting mechanisms with charge/discharge rates and a set of complex relationships between the battery design parameters and battery utilization. The 1D battery model was limited by the assumption that the electrodes were constructed with uniformly distributed spherical particles of equal size. To consider the influence of real microstrucutural effects, a 2D microstructure resolved model had also been developed. This model was used to investigate the stresses in the active particles and conductive binder. This work presents a feasible approach to understanding the relationships among different physical phenomena in a Li-ion battery, evaluating the stress state and magnitude in battery components such as the separator, and battery design optimization.
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