Simulation Study On Thermal Characteristics Of Li Ion Battery Based On Multi Layer Structure And Multi Physical Field Coupling

With the national policy support for the application of electric vehicles,lithium-ion battery as a key driving component of electric vehicles has attracted extensive research.Recently,many brands of electric vehicles have spontaneous combustion accidents,the main reason is that the lithium battery heat dissipation is not uniform,the battery temperature rises rapidly,the thermal runaway phenomenon occurs,and eventually leads to the battery fire.Therefore,it is very important to fully study the thermal characteristics and thermal runaway mechanism of lithium-ion battery,as well as the key electrochemical and mechanical effects,to improve the safety of the battery,reasonably design the heat dissipation structure,and optimize the thermal management algorithm.Numerical simulation is one of the important ways to study battery performance,optimize battery structure and shorten battery design process.In the actual working process,the direct interaction between the thermal characteristics and the stress caused by the collision,extrusion and the heat generation of the battery due to the external working environment has not been fully studied,especially in the multi-layer structure.The purpose of this paper is to build the electrochemical-thermal-mechanical coupling model of lithium-ion battery by finite element method,and to accurately simulate its thermal characteristics and related electrochemical and mechanical characteristics based on the multi-layer structure of lithium-ion battery model.At the same time,based on the above-mentioned electrochemical-thermal-mechanical coupling model,the thermal runaway mechanism of lithium battery in extreme working environment is simulated.Finally,in order to eliminate the interaction between the parameters and accurately capture the key parameters of the thermal characteristics of lithium batteries,this paper decouples the parameters of the electrochemical thermal mechanical coupling model,and establishes the correlation analysis between the material / geometric parameters and the electrochemical / thermal characteristics,hoping to provide new ideas and help for the design and thermal performance optimization of lithium batteries.The main contents of this paper are as follows:1.Firstly,the electrochemical-thermal-mechanical coupling model of lithium battery was constructed.Based on the above model,the temperature evolution process of Li ion battery at different discharge rates was simulated,and the relevant voltage curves and temperature curves at different test points were compared with the experimental results,which verified the effectiveness of the above electrochemical thermal mechanical coupling model.In addition,the effect of the number of lithium-ion battery layers on the temperature distribution is discussed.The results show that the number of battery layers has an impact on the accuracy of temperature prediction.2.Secondly,the mechanism of the battery thermal runaway in extreme cases is studied.Based on the above multi-layer structure,the multi physical field coupling model of lithium-ion battery is combined with the thermal runaway model and the heat contribution of different stack components after the contact heating is out of control,so as to explore the mechanism of thermal runaway and the corresponding threshold under extreme high temperature.The results show that the overall effect of0.5C-2C discharge ratio on the overall heat loss is not significant,which means that the small power charge and discharge can be used in high temperature environment or the battery is not evenly distributed.The temperature and thermal stress of the battery increase with the increase of discharge ratio.Through the analysis of the influence on the environment temperature,the critical threshold of temperature runaway contact heating is 145 ℃-155 ℃.3.Finally,based on the idea of parameter decoupling,the effects of electrochemical and thermal parameters on the electrochemical,thermal and mechanical properties of the battery are mainly considered.The results show that the physical and chemical parameters(plate thickness and active material volume fraction)of the battery have a significant effect on the voltage characteristics,and the discharge capacity of the battery is also affected by changing the time of reaching the cut-off voltage.However,the change of internal electrochemical parameters has little effect on the temperature difference range of the battery,while the external environment has a great influence on the overall temperature rise of the battery.