Multi-scale Numerical Simulation Of Thermal Management Of Lithium Batteries

In recent years,the transportation industry’s reliance on traditional energy sources has increased year by year.New energy vehicles based on secondary energy have been the most concerned energy-saving solutions in the world in recent years,among which electrified transportation solutions have developed the fastest.Lithium batteries are considered to be the most viable power source for electric vehicles due to their high energy density,long cycle life,and low self-discharge rate.its disadvantage is that the charging and discharging performance is greatly affected by temperature,and even thermal runaway may occur due to high temperature environments.In order to improve the performance,life and thermal safety of the battery pack,this article draws on the Tesla battery thermal management system（BTMS）,and uses finite element methods and molecular dynamics methods to simulate and analyze the temperature distribution in the area managed by the nano-modified coolant battery thermal management system.Then,analyze the performance difference of BTMS before and after nano-modification.First,the lithium-ion batteries electrochemical model and electrochemical-thermal model are studied.The process of lithium-ion battery packs from discharging,generating heat to cooling is a multi-physics problem in which multiple physical fields affect each other and are coupled with each other.Most literature simplifies the thermal problem of lithium-ion battery packs to a single constant heat source for processing.Starting from the electrochemical model,this article discusses the coupling relationship between the macro-scale temperature and the electrochemical reaction inside the battery,and provides accurate heat generation calculations for BTMS under different discharge rates.Secondly,based on the coupling of multi-physics,the electrochemical-thermal problems and non-isothermal flow problems in the process of electrochemistry,fluid heat transfer,and solid heat transfer are analyzed.With reference to the Tesla automobile BTMS structure,the simulation analyzes the maximum temperature of the lithium-ion battery module,the maximum temperature difference between battery cells,and the temperature distribution of the area managed by BTMS at 6 different flow speeds under continuous discharge rates of 1C,2C,3C,and 4C.Then,Analyze the causes of excessively high temperature and high temperature difference.Finally,nano-particles are used to construct water-based Al₂O₃coolant in order to improve the heat transfer performance of the fluid.The macroscopic thermal conductivity changes depend on the interaction between micro-scale nanoparticles and water molecules.Molecular dynamics methods are used to analyze the changes in thermal conductivity and viscosity of the water-based Al₂O₃ fluid after nano-modification,both of them obtained by simulation.Physical property parameters are used as new physical property parameters for multi-field coupling simulation.The numerical simulation results of BTMS marked by temperature distribution are obtained,and the thermal management performance changes of BTMS before and after nano-modification are compared.The results show that the use of a base fluid with a flow rate of 0.4m/s for BTMS cooling can reduce the temperature difference between battery cells from 9.84K to 6.86K,increase the volume fraction of nanoparticles from 0%to10%,when flow rate is 0.4m/s,The water-based Al₂O₃ fluid to cool the BTMS can further reduce the temperature between the battery cells to 5.97K.