Mechanism of influence of separator microstructure on performance of lithium-ion battery based on electrochemical-thermal coupling model

Separator is an important component of lithium-ion battery, and the microstructure of separator has an important influence on the performance of lithium-ion battery. In the present paper, an electrochemical-thermal full coupling model is developed to accurately describe the complex physicalchemical phenomena in lithium-ion battery in charge and discharge process. The simulation results by the present model are closer to the experimental results than those by the previously published model. What is more, the present model is widely used to investigate the effects of the separator porosity and tortuosity on the performance of lithium-ion battery, respectively. The simulation results show that with separator porosity decreasing or separator tortuosity increasing, the output voltage, maximum discharge capacity and average output power of lithium-ion battery decrease, and the lithium-ion battery surface temperature and its rising rate increase. In the initial stage of discharge, when the separator porosity decreases or separator tortuosity increases to a certain degree, the output voltage of lithium-ion battery first decreases and then increases. The smaller the separator porosity or the higher the separator tortuosity, the larger the range and rate of reducing the output voltage of lithium-ion battery become and the longer the rise time needs in the initial stage of discharge. To ensure that the output voltage of lithium-ion battery is higher than the cut-off voltage, the separator tortuosity must be less than the critical tortuosity (It is equal to the separator tortuosity of the lithium-ion battery with the lowest output voltage, which is just equal to the cut-off voltage in the initial stage of discharge). Finally, a comprehensive analysis is conducted on the dynamic distribution of the electrochemical parameters and various heat productions in lithium-ion battery during charge and discharge. It can be clearly found that the electrochemical reactions in the end of discharge, the diffusion coefficients and the conduction coefficients of Li+ of electrolyte in the initial and middle stage of discharge are mainly influenced by the separator porosity and tortuosity. The research results in the present paper not only provide theoretical and technical support for the separator microstructure design and optimization, but also has important realistic meanings for improving or perfecting the preparation technology of the separator.