Effect Of Low Temperature Conditions On Charging Polarization Performance Of Lithium Ion Batteries

As an important driving force source for new energy vehicles,lithium ion batteries have received extensive attention in the automotive industry in recent years.However,the charging performance of lithium ion batteries is greatly affected by the ambient temperature.The problem of increasing the charging polarization voltage and lowering the charging capacity under low temperature conditions has always limited the development of lithium ion batteries.Based on the electrochemical reaction mechanism,heat transfer mechanism and electrothermal coupling characteristics of lithium ion batteries,the study of charging polarization characteristics of lithium ion batteries under low temperature conditions is of great significance for the development of lithium ion batteries.Based on the theory of electrochemical reaction kinetics,mass conservation,charge conservation and energy conservation,the electrochemical-thermal coupling transient calculation model based on LiFePO₄/graphite lithium ion battery was established by using COMSOL Multiphysics multiphysics simulation software.Since the electrochemical behavior and heat transfer behavior of lithium ion battery interact with each other during charging,the model takes the heat generation rate and the temperature of the battery system as intermediate variables,fully taking into account the electrochemical-thermal coupling effect during charging.The accuracy of the model at three low temperature conditions of -5°C,-10°C,and -15°C was verified by comparing the voltage test value of the terminal voltage with the analog value during the charging process.At the same time,the polarization characteristics of the battery during 1C constant current charging and the factors affecting the internal electrochemical of polarization under the above three low temperature conditions was studied under the above three low temperature conditions.The results show that the absolute value of the negative polarization voltage is lower than that of the positive polarization voltage.The positive polarization is the main part of the polarization of the whole lithium ion battery system.At the same time,the positive polarization voltage has a certain dependence in value for the overall SOC of the lithium ion battery.The decrease in temperature does not change the overall distribution tendency of the positive polarization voltage,but the lower temperature,the smaller SOC of the entire battery when the charge is turned off,and the smaller peak value of the corresponding positive polarization voltage.During the charging process,the diffusion polarization of the positive electrode phase can be divided into three phases:initial phase,plateau phase and rising phase.As the ambient temperature decreases,the polarization voltage level in the plateau and rising phases increases continuously,but the existing SOC interval continues shortening.The positive liquid phase ohmic polarization is divided into two phases:stable phase and rising phase.The ambient temperature does not affect the polarization voltage level in the stable phase.However,the lower ambient temperature,the smaller SOC interval of the stable phase,the rising voltage of the rising phase and the rate of ascent will continue to increase.At the same time,the temporal and spatial accumulation of the electrolyte salt concentration of the positive electrode and the instantaneous distribution of the electrolyte current density explain the reasons for the change of the above two polarization.