Research On Thermal Runaway Process And Hazard Of Electric Vehicle Battery Based On Different Anodes

The inventory of electric vehicles is continuous increase in recent years,meanwhile,growth number of fire and combustion accidents have arisen due to thermal runaway and thermal runaway propagation of lithium ion battery(LIBs).The origin of the accidents is usually a series of uncontrollable exothermic and gas-generating reactions caused by overheating,overcharging,water immersion,short-circuit,collision,deformation,and penetrated by foreign objects during the service of the LIBs system.Regarding to the in-depth investigation of the safety status of the electric vehicle industry and the research achievements of the thermal runaway of LIBs,three typical thermal runaway triggering methods such as heating,overcharge,nail penetration were selected(correspondingly thermal,electrical,mechanical abuse)to conducted comprehensive research and analysis on the thermal runaway process characteristics and hazard of 25 Ah NCM 811-Si O_x/graphite and NCM 811-graphite pouch batteries.Suggestions on the early warning,prevention and control of thermal runaway have been proposed based on the results.Firstly,heating was selected as a thermal runaway triggering method to quantitatively and qualitatively analyze the thermal runaway spreading behavior of the battery through thermal runaway experimental research on batteries with different SOC and by analysis of various data source like temperature,voltage,digital video,infrared video and others.The results showed that heating time before thermal runaway of the graphite system LIBs was longer,more energy was introduced by heating before thermal runaway occoured,and higher critical temperature was observed during thermal runaway.The ingredients of gas produced during the thermal runaway of the two systems of batteries were CO_x,H₂,and hydrocarbons.At last,the energy source and energy diffusion form of heating triggering thermal runaway were analyzed,the failure mechanism of different negative electrode cells was summarized.It also brought forward analysis of the cause of the accidents,reasonable suggestions on the early warning and prevention of thermal runaway triggered by overheating in reality.In the second part of this work,overcharge was selected to trigger thermal runaway in simulation of electrical abuse.Firstly,relying on the adiabatic environment provided by the accelerating rate calorimeter,study performed to investigate the influence of adiabatic and non-adiabatic environment on the thermal runaway characteristics of silicon-based batteries and graphite-based batteries.It was found that under adiabatic conditions,the surface temperature of cells was continuously higher than the temperature in the non-adiabatic environment before thermal runaway compared with that under the adiabatic environment whereas there was negligible difference between voltage.Secondly,the thermal runaway characteristics of silicon-based batteries and graphite-based batteries under 1 C(25 A)and 3 C(75 A)overcharge rates were studied.The results indicated that the 3 C(75 A)high current triggered thermal runaway in a shorter period of time,lower overcharge rate and resulted in lower critical temperature of thermal runaway,though no significant difference in cells damage compared with 1C charge rate.Finally,referring to the actual state of the cells in the LIBs pack,thermal runaway of batteries with or without fixture was compared and analyzed on silicon-based batteries.The results concluded that the charging time was longer when the batteries were fixed by jigs,the critical overcharge rate was higher,the total heat generation and the heat generation power at the moment of thermal runaway was higher.Cells that were not fixed by jigs became more fragmented after thermal runaway.Finally,nail penetration was selected as the thermal runaway mechanical trigger method,to investigate the thermal runaway process and harzard of silicon-based and graphite-based batteries at different SOCs.The results showed that both the silicon-based and graphite-based batteries with 100%SOC experienced violent thermal runaway combustion and explosion in the nail penetration test,but the silicon-based battery showed a fiercer thermal runaway phenomenon that spark jets still could be seen at 10%SOC when experimental phenomenon of graphite-based batteries with 25%SOC and 10%SOC were mainly smoke.The lower the SOC,the longer it took for the battery surface temperature to reach the maximum value.When the SOC was the same,the combustion time of the silicon-based battery was shorter.The maximum temperature of thermal runaway increased with the increase of SOC.The more severe the thermal runaway,the maximum temperature of the battery surface was higher,and the weight loss ratio of the battery after thermal runaway was higher.In addition,during the nail penetration test of the silicon-based system battery under 25%SOC,the ambient temperature was still higher than 200?,which could easily induce thermal runaway of the surrounding battery,jeopardized the whole battery system as well as the electric vehicle.The graphite-based battery only had a similar risk when it was 100%SOC.The comprehensive analysis of the thermal runaway process,critical characteristics and impact on the surrounding environment of two commercial soft pack power batteries triggered by heating,overcharging,nail penetration method shows that compared with the graphite-based batteries,the thermal runaway phenomenon of the silicon-based pouch battery was more severe,resulting in severe damage,and caused serious damage to the surrounding environment.This was closely related to the difference in materials thermal stability and battery energy density.This thesis conducted research and data analysis on the thermal runaway process of three different abuse conditions that the battery systems may encounter,and initially established the thermal runaway characteristic parameters database of batteries with different anode materials,especially for silicon-based batteries with broad prospective applications.With the deepening of quantitative research on silicon-based battery thermal runaway,it will effectively promote the development of silicon-based battery pack safety design,thermal runaway early warning and alarm system development,and improve the safety of electric vehicles and energy storage system.