Thermal Runaway Study And Fire Risk Assessment Of Gel Electrolyte Lithium-ion Battery

With the gradual dwindling of fossil fuel supplies and the increasing public concern regarding reducing environmental pollution,lithium ion batteries(LIBs)have attracted widespread attention owing to their high energy density,reduced pollution and novel electrical performance.In order to further improve the characteristics of LIBs,and to tackle safety problems,gel electrolyte(GPE)lithium ion batteries have been introduced into the market.On the one hand,trapped in the pores of the gel polymer,the organic carbonate solvent is immobilized in a gel electrolyte.This eliminates the risk of flammable liquid leakage.On the other hand,the gel electrolyte has an adjustable shape and high elasticity,thereby inhibiting the growth of lithium dendrite,and reducing the risk of internal short circuits.Nevertheless,GPEs contain a variety of flammable solvents,and remain dangerous in cases of thermal abuse.This problem contradicts its safety propaganda,and the public is still absent of its risk.Therefore,studying the hazard of gel electrolyte lithium ion batteries helps improve our understanding of their thermal runaway mechanism,and provides guidance for improving battery safety.Moreover,our studies also quantify the risk of gel electrolyte batteries,and helps the public to establish a correct understanding of the risks of lithium batteries.In this study,a typical gel electrolyte battery was taken as an example,and accelerating calorimeter experiments and cone calorimeter experiments were conducted.The mechanism of thermal runaway,the fire risks and the effects of gel electrolyte on lithium ion batteries were also comprehensively studied.In our accelerating calorimeter experiments,the effects of states of charge on gel electrolyte lithium ion batteries thermal runaway behaviors were revealed.In this paper,the thermal runaway behaviors of gel electrolyte lithium ion batteries was divided into several progresses,and its evolution rules were revealed.Under different SOCs,critical parameters like the onset temperature,thermal runaway temperature,maximum temperature,and peak temperature rise rate were derived.The influence of membrane on thermal runaway of gelatinous lithium-ion battery was analyzed according to the voltage response law.Adopting the self-generated heat theory model,the thermal kinetic parameters such as apparent activation energy and pre-exponential factor were calculated,under different SOCs.Under the assumption that a battery do not undergo mass exchanges before self-heating,the heat capacity of the gel batteries were calculated.It was found that the capacity is independent of SOC,and the mean value of that is 1.144±0.060 J/g·K.On this basis,critical parameters like battery enthalpy,chemical reaction power,self-generated heat power,total self-generated heat and chemical reaction heat generation were calculated.In our cone calorimeter experiments,the effects of external heat fluxes and states of charge(SOC)on gel electrolyte lithium ion batteries fire behaviors were revealed.By analysis of fire behaviors,it was observed that 0%SOC gel electrolyte batteries do not ignite,and jet fires were observed for batteries with SOCs higher than 50%.It can be concluded that higher external heat fluxes or higher SOCs bring more violent combustion behaviors.Through linear fitting and numerical analysis,it was observed that the inverse of the TTI has an exponential relationship with the SOC and a linear relationship with the incident heat flux.Further analysis of these quantitative relations lead us to a critical heat flux of 2.16 kW/m2 and a critical SOC of 3.57%.In cases that a battery has a SOC lower than 3.57%,or that the external heat flux is lower than 2.16 kW/m2,the battery does not ignite.It was also observed that mass loss rate,and peak heat release rate,are positively associated with SOC or external heat flux.What is more,critical parameters like the gasification heat,total heat release,fire growth index,x parameter of gel electrolyte battery were derived.Petrella’s way of risk assessment was also introduced,through which we found a relatively high flashover risk for gel batteries with SOCs higher than 25%,and a medium heat contribution risk for gel batteries at heat fluxes higher than 15 kW/m2.Finally,combining the experimental results of accelerating calorimeter and cone calorimeter,the total thermal power and total heat production of gel lithium ion battery fire were obtained,and the fire energy balance equation of lithium ion battery was further improved.