Thermal Analysis Of Ternary Soft-packed Lithium-Ion Battery Based On Electrochemical-Thermal Coupling Model

In this paper,a 36Ah ternary soft-packed lithium-ion battery was seen as the research object.A series of thermal characteristics were studied by combining theoretical research,scientific experiment and simulation.The main results are as follows:The internal resistance test,entropy heat coefficient test,and charge and discharge temperature rises test was carried out for the battery.The effects of temperature and depth of discharge?DOD?on the internal resistance of the battery were discussed.The effects of DOD on the entropy heat coefficient and charge and discharge rate on the temperature rise of the battery were studied.The results showed that the internal resistance of lithium-ion batteries was small at high temperatures,while the internal resistance was very high at low temperatures.The internal resistance of discharge was greater than the internal resistance of charging at the same temperature.The internal resistance of the charge and discharge remained basically the same during the whole discharge process?DOD=0⁰.8?,and increased sharply only in the late stage of the charge and discharge?DOD=0.8¹?.The entropy heat coefficient was negative in the DOD interval of 0⁰.85,and an exothermic reaction occurs.The DOD interval in0.85¹ was positive,and the endothermic reaction occurs.The entropy heat coefficient varied from-0.4 to 0.3mV·K^-1.The temperature rises from lithium-ion battery increases with the increase of charge-discharge rate.At low magnification,the temperature rises the curve showed a nonlinear trend of“rise-lower-rise”,while it showed a linear trend at high rate.The direct measurement method of the entropy heat coefficient of lithium-ion battery has problems such as long test period and slightly self-discharge phenomenon affecting open circuit voltage test.Using the Bernardi heat generation rate formula,the relevant parameters or related items in the formula under room temperature and adiabatic conditions were obtained by experimental tests.The entropy heat coefficient of the battery under two conditions was obtained by subtracting the irreversible heat.The results showed that the indirect heat coefficient obtained by indirect calculation under adiabatic and room temperature conditions were basically consistent with the change trend of DOD,and it was basically consistent with the entropy heat coefficient obtained by direct test,which verifies the accuracy of the indirect algorithm of entropy heat coefficient.A second-order RC equivalent circuit model was established,and the polarization internal resistance of the battery was identified.Polarization internal resistance obtained by dividing the amount of voltage change by the current was compared.It was found that the polarization internal resistance identified by the second-order RC model is larger than the latter.In order to compare the applicability of the polarization internal resistance calculated by the two methods in the process of continuous charge and discharge of the battery,the polarization internal resistance calculated by the two methods was brought into the Bernardi heat generation rate formula to obtain the heating power of the battery.The battery was subjected to thermal simulation analysis in the STAR-CCM+simulation platform,and the difference between the battery temperature rise curve and the experimental temperature rise curve based on the simulation of the two polarization internal resistances was compared.The results showed that the simulation results of the polarization internal resistance identified by the second-order RC model were closer to the experimental results,indicating that the polarization internal resistance identified by the second-order RC model is more suitable for thermal analysis during continuous discharge of the battery.Based on the COMSOL Multiphysics 5.3 simulation platforms,an one-dimensional electrochemical-thermal coupling model combining mass conservation,charge conservation,electrode process dynamics and energy conservation was established.The correlation between electrochemical parameters and temperature was considered comprehensively.The heat generation of various parts of the battery?positive electrode,negative electrode,separator?and the temperature rise of the battery surfaces under different discharge rate and the working temperature was studied.The results showed that the different mechanisms heat of the positive and negative electrodes all increased with discharge rate increases.The impact of the discharge rate on the ohmic heat was the largest,followed by that on the polarization heat and that on the electrochemical reaction heat.The change in total heats generation was subject to the electrochemical reaction,and most of the heat at low discharge rates was generated by the electrochemical reaction,and the proportion of the total heat generation by the Joule heat and the polarization heat increased and that by the electrochemical reaction decreased with discharge rate increases.The impact of the operating temperature on the different mechanisms heat of the positive electrode was similar to that of the negative electrode,and the impact of the operating temperature on the Joule heat was larger than that on the polarization heat.The highest temperature of the battery surface was located at below the geometric center.The larger discharge rate and the lower operating temperature resulted in larger battery surface temperature rise and worse temperature distribution uniformity.