Degradation Law And Failure Mechanism Of Li-ion Batteries Under High Temperature Conditions

New-energy electric vehicle is a national key development of the emerging strategic industry,Li-ion battery performance reliability and safety issues have attracted much attention.In summer,the temperature inside the outdoor parking car can be as high as 70℃.High temperature has a serious impact on the Li-ion battery,resulting in accelerated degradation of Li-ion battery performance,and even premature failure.Therefore,this paper studied the performance degradation law and failure mechanism of Li-ion batteries under high temperature conditions,specifically as follows:Aiming at the high temperature working condition of Li-ion battery,a 70℃ high temperature degradation test was designed to simulate the outdoor parking scene of new energy vehicles.A multi-stage high temperature storage test was conducted on the Li-ion battery,and battery performance parameters,including capacity,direct current resistance and electrochemical impedance spectrum data,were tested at the end of each stage.Aiming at the degradation of Li-ion battery performance at 70℃,it was found that the direct current resistance and capacity of batteries at different state of charge(SOC)increased and decreased with the increase of storage time at high temperature,and the higher the SOC,the greater the degradation rate of batteries.Aiming at the problem of non-uniform representation of degradation law of Li-ion battery with single parameter,a comprehensive model of state of health-state of charging-cumulative storage time was proposed to characterize the degradation law of Li-ion battery by integrating direct current resistance,maximum discharge capacity and gas production with the beta distribution,which solved the problem of non-uniform degradation law of Li-ion battery.It provides a theoretical reference for the degradation assessment of power batteries under high temperature working condition.Based on the analysis of battery failure,computed tomography found that the battery internal open-circuit failure was induced by the high temperature of 70℃,which resulted in the production of a large amount of gas and made the internal pressure too high,triggering the Li-ion battery pressure relief plate to flip over,and thus the open-circuit failure.In order to clarify the changes of internal electrode damage before open-circuit failure,electrochemical impedance spectroscopy(EIS)was used to test the impedance of solid electrolyte interface film(SEI),which was found to have the most significant changes.The reason is that the coverage of SEI film on the cathode material surface of the battery increased during high temperature storage.Through the observation of electrode morphology,it is shown that there are crack propagation,secondary particle shedding and byproducts attached to the cathode active material on the electrode surface during high temperature storage.Aiming at the battery life evaluation during high temperature storage process,normal distribution,lognormal distribution,Weibull distribution and exponential distribution were selected to fit the life data,and the best-fitting Weibull distribution was selected,and K-S goodness of fit test was conducted to establish the mapping relationship between Weibull parameters and charging state.Then the reliability model of Li-ion battery at 70℃ was established.