Study On Thermal Runaway Behavior And Warning Method Of Lithium-ion Battery For Electric Vehicle

Recent years,lithium-ion batteries have become the first choice for power batteries used in electric vehicle due to their great output performance and high energy density.However,the frequent fire accidents of batteries within electric vehicle have seriously threatened the personal and property safety of the occupants,and also hindered the development of electric vehicles vastly.Therefore,it is urgent to study the mechanism and behavior of thermal runaway reaction for the lithium ion battery and find methods to improve the safety performance of power battery system.Aiming at this research direction,the article studies the thermal runaway characteristics of lithium-ion batteries under different abuse conditions via simulation and experiment,thus determines the thermal runaway critical parameters under each abuse condition and proposes an early warning method that could identify faults location and abused type within battery module.In order to study the abuse behavior of power lithium-ion batteries,the research starts with the thermal runaway process of the battery under different abuse conditions,and experimentally analyzes the runaway characteristics under the four conditions including nail penetration,external short circuit,oven-heating and overcharge.In the adiabatic thermal runaway experiment for the pouch battery,it finds key characteristics of obvious gas venting and internal short-circuit during thermal runaway,and the safety performance of the battery decreases with the increasing state of charge significantly.When the battery is overcharged at various rates in the adiabatic environment,thermal runaway is triggered,and the initial temperature of thermal runaway decreases with the increasing rate gradually.Especially,the battery happens thermal runaway when its temperature reaches 61.865 ? during 1.50 C rate overcharging.However,under nail penetration and external short-circuit,thermal runaway has not been triggered for a single battery,but happens in the battery module.Especially in the case of needling a battery module,all batteries occur thermal runaway within the module,and the maximum temperature could reach 910.64 ?.However,only the center battery catches fire when the battery module is short-circuited,without thermal runaway phenomenon in other batteries.Based on the experimental results,a finite element model of the battery is established at each abused condition,so as to analyze the reaction mechanism of thermal runaway and the heat transfer process within the battery module in more detail.In the model establishment,based on the chemical reaction kinetics and threedimensional heat transfer coupling model,the equations described gas venting,internal short circuit and electrolyte evaporation are added to improve the accuracy of the model.The research results show that a single battery will not result in thermal runaway under the heating condition of 1500 W/m2,but the thermal runaway and transfer process of the battery will be triggered by the heating of a single cell or the whole module inside the battery module under this condition.During the process of overcharging and external short-circuit of the battery,the temperature of the tab position is always relatively high,indicating that by installing a positive temperature-related protection component at the tab,the two forms of abuse could be avoided.Under the triggering effect of various abuse conditions,it is found that the risk of thermal runaway of the module is much higher than that of a single battery.Based on the thermal runaway experiment and simulation work for the battery,the article summarizes the voltage and temperature character of battery under various abuse conditions.It finds that the voltage of the battery has the most obvious behavior at different abused conditions,and combined with the evolution of other characteristic parameters,a thermal runaway threshold model and an abused type judgment model are built.According to the heat transfer basic theory,a lumped heat transfer model is constructed using the thermal resistance method,and its accuracy has been verified by comparing the calculation results between the finite element model and the lumped model.On the basis of the lumped thermal runaway transfer model,the input source term is modified,and the estimation calculation model of the battery module temperature distribution based on three measurement points is established.It is found that if only the temperature term is input,the result of the temperature distribution on the node interval without the heat source term is relatively accurate,but the temperature distribution in the node interval with the heat source term are quite different.After considering the correction term of the temperature gradient,the accuracy of the model prediction gets a great improvement,which could realize the accurate calculation of the temperature distribution.