Research On Thermal Characteristics Of High Specific Energy NCM/Si-G Power Battery For Vehicles

As the demand for long range of new energy electric vehicles gradually increases,compared to lithium iron phosphate battery systems,ternary lithium ion power batteries have developed rapidly due to their high energy density and excellent comprehensive performance.Generally,the higher the specific energy of the battery,the higher the heat generated during operation（such as under high current conditions）.If the heat is not dissipated in a timely manner,it can easily cause deterioration in battery performance,and even lead to safety issues such as thermal runaway of the battery.Studying the thermal characteristics of high specific energy lithium ion power batteries is of great significance for improving the thermal safety performance of batteries.This article takes a commercial 18650 ternary lithium ion power battery(Li[Ni_0.6Co_0.2Mn_0.2]O₂/Si-G,capacity 3.5 Ah)as the research object（hereinafter referred to as NCM/Si-G）,and studies the thermal characteristics of the battery through experimental testing and simulation.The main contents are as follows:（1）Experimental Study on Thermal Characteristics of NCM/Si-G System Power BatteryBy studying the heat production and temperature rise characteristics of batteries in different health states（SOH）under different test conditions,the changes in the DC internal resistance and entropy change coefficient of the battery under different states were analyzed,and the reversible and irreversible heat production power of the battery was calculated,revealing the trend of the heat production of the battery with the aging of the battery.The results showed that with the decrease of battery health（SOH 100%to75%）,the surface temperature of the battery increased significantly during high rate discharge.The maximum temperature at 100%SOH and 75%SOH,4C discharge is 62.4℃and 71.2℃,respectively,with a temperature increase of 8.8℃;The irreversible heat generation has a small change,and the proportion of irreversible heat generation increases,resulting in an increase in the total heat production of the battery;（2）Study on the thermal characteristics of NCM/Si-G system power batteries based on the established electrochemical thermal coupling modelBased on experimental modifications to the model parameters such as Li⁺solid phase diffusion coefficient,positive and negative electrode reference potential of NCM/Si-G system batteries,an electrochemical thermal coupling model was established using COMSOL Multiphysics.According to the 100%SOH battery,the accuracy of the model is verified by the multiple discharge test parameters at an ambient temperature of25℃.The average absolute error（MAE）between the simulated temperature value and the test temperature value is 0.0364℃,0.0368℃,and 0.0370℃,respectively.The overall simulation results are in good agreement with the test results,It can be explained that the electrochemical thermal coupling model based on NCM/Si-G power batteries established in this paper has good reliability under normal operating conditions and temperatures.Based on the coupling model,the thermal characteristics of the battery are further analyzed.Through the analysis of the internal temperature field of the battery under different discharge rates,the maximum temperature inside the battery core is 43.25℃at the end of low rate 1C discharge,and at the end of high rate 4C discharge,the maximum temperature inside the battery core is 62.05℃,and the radial temperature difference of the battery is greater than the axial temperature difference.During the charging and discharging process of the battery,the temperature field distribution of the single battery has the characteristics that the temperature in the central area of the battery is significantly higher than the surrounding edge temperature,and the internal temperature field of the battery gradually decreases from the center of the battery core to the surrounding temperature;The heat generation inside the battery is mainly generated at the positive and negative electrodes;During the rate discharge process,the reaction heat of the positive electrode of the battery is the main heat source for the heat generation of the battery.As the rate increases,the heat generation power of each part of the electrode gradually increases;Based on the study of battery heat dissipation using a coupled model,the simulation results show that both increasing the convection coefficient of the coupled model and increasing the air flow rate can improve the heat dissipation effect of the battery,but continuously increasing the convection coefficient and air flow rate will gradually reduce the impact on the heat dissipation effect of the battery.