Study On Electrochemical And Thermal Characteristics Of LiFePO₄ Battery During Charging And Discharging Process

Lithium-ion batteries are becoming preferred power source electric vehicles.However,the performance and safety problems caused by the high heat generation rate and uneven electrochemical reactions during charging and discharging are significant challenges faced by automakers.The key points of the battery thermal management system study are the electrochemical and thermal characteristics of the battery.Exploring the electrochemical reaction mechanism and heat generation mechanism of power battery during charging and discharging process is of great significant for the use and safety of the battery.A charging and discharging test bench of the battery was established and a lithium iron phosphate battery was as research object.The charging experiments were carried out at different charging rates(0.5 C,1 C,and 2 C)and the discharging experiments were carried out at different time rates(1 h,1/2 h,and 1/3)h)under the discharging mode of constant current and constant power.According to the experimental results,the experimental data for the later electrochemical-thermal coupling modeling and verification and the basic performance of the battery were obtained.The results show that the surface temperature of the battery is higher under constant power discharging,while the working voltage of the battery is lower.With the time rate decreasing or the ambient temperature decreasing,the difference of surface temperature and working voltage between the constant power discharging and constant current discharging increases.In order to study the heat generation characteristics of the battery under constant power discharging,an electrochemical-thermal coupling model of the battery was established and validated with experimental results.The heat generation characteristics each layer in of the battery were studied.And the effects of the positive electrode on the heat generation of each layer in the battery cell were discussed.The results show that the change of the heat generation rate of the negative electrode is the largest during the discharging process,the change of the heat generation rate of the positive electrode is second,the change of the heat generation rate of the separator and the positive and negative electrode current collectors is the least.The discharging power has little effect on the ratio of the heat generation of each layer to the total heat generation.The ratio of the heat generation of the positive electrode is the largest(above 70 %).The ratio of the heat generation of the negative electrode is second(above 24 %).The ratio of the heat generation of the positive and negative current collectors is the smallest(tend to 0).Increasing the thickness of the positive electrode can reduce the heat generation of the battery,but the effect will decrease as the thickness of the electrode increases.In order to study the electrochemical and thermal characteristics of the battery under constant current charging,an electrochemical-thermal coupling model of the battery was established and validated with experimental results.The electrochemical and thermal characteristics of the battery charged at 1 C charging rate at different initial temperatures were studied.And the charging energy efficiency of the battery at different charging rates(0.5 C,1 C,and 2 C)was analyzed.The results show that the initial temperature mainly effects the spatial and temporal distribution of local current density and heat generation rate of the battery.As the initial temperature decreases,the unevenness of the local current density increases.The charging energy efficiency of the battery depends on its electrochemical and thermal characteristics.The charging energy efficiency of the battery increases with the increasing of initial temperature at 0.5 C charging rate,while it will decrease after the initial temperature exceeds 40°C and 25°C at 1 C and 2 C charging rate,respectively.So in order to obtain higher charging energy efficiency,it should use reasonable charging rates at different initial temperatures.