Stress Development Under Fast Charging and Its Effect on Degradation of Separator

Abstract

This paper discusses the predicted stress when a pouch-type lithium-ion (Li-ion) cell undergoes cyclic fast charging and how the stress is related to the observed degradation of the separator. Previous experiments showed a significant degradation in the mechanical integrity of the separator after repeated fast charging. In this paper, an integrated multiphysics model is developed for prediction of stresses when the cell, under compression as in a pack assembly, is charged at a high rate. Creep and fatigue tests of the separator demonstrate a significant damage can be accumulated under the predicted stress level. The separator is one of the most crucial safety components inside the lithium-ion battery preventing contact between the positive and negative electrodes and internal short circuits. Therefore, the separator should be mechanically robust to withstand the mechanical stresses encountered during the battery assembly, battery operation, and under abuse conditions. Both high tensile and high puncture strengths are required to avoid rupture and penetration of electrode material which can cause short circuits in the battery. We reported a gradual and upto 50% decrease in ductility of the separator after the battery cell underwent 400, 800, and 1600 charge-discharge cycles involving 4C-rate charging [1]. Scanning electron micrographs (SEM) analysis of the cycled separators revealed accumulated damage in the separator in the form of pore closure along the thickness direction and fiber fracture and cracks along the machine direction. The degradation was attributed to the damage caused by mechanical creep which can accumulate over the cycles. This paper therefore studies the stress distribution in the separator when the battery cell under mechanical loading inside a battery pack is fast charged. For that purpose, a multiphysics model is developed consisting of three main components: (1) a one-dimensional (1D) electrochemical model, (2) a lumped-parameter model, and (3) a two-dimensional (2D) solid mechanics model. The 1D electrochemical model is used to predict the heat generated and lithium-concentration in the battery cell. The heat generated is used as an input for a lumped-parameter model which simulates the thermo-mechanical behavior of the cell inside a pack array. The predicted lithium-concentration and temperature are used in the 2D solid mechanics model to predict the expansions and stresses inside the battery cell. Therefore, the proposed multiphysics model allows predicting the stress distribution in the separator under various charging rates, thermal management conditions, and pack assembly conditions such as pre-compressions.A cycling test is conducted on a lithium-ion battery cell inside a custom fixture that mimics the constrained condition inside a pack assembly. The setup is equipped with temperature, load, and displacement measurements. The parameters of the lumped-parameter model are optimized based on the experimental data using a non-linear regression technique. The model predictions show good agreements with the experimental data.The developed multiphysics model is simulated to estimate the stress distribution in the separator under the 4C charging condition (see Figure 1). The maximum first principal stress is found to be 70 MPa which is considerably lower than the yield strength of the separator. Creep and fatigue tests are conducted on the pristine separators to test their behaviors under the 70 MPa stress. Both test results show a significant amount of damage can be accumulated in the separator under a repeated loading of 70 MPa as in charge-discharge cycling. These test results corroborate our theory of mechanical creep being the main degradation mechanism of the separator while simultaneously validating our multiphysics model for prediction of stress. Reference [1] Makki M, Ayoub G, Lee CW, Bae C, Colin X. Effect of battery fast cyclic charging on the mechanical and fracture behavior of lithium-ion battery separator. In Preparation.Figure 1. Predicted two-dimensional (2D) distribution of the first principal stress in the separator near the center of Li-ion battery cell. Figure 1