On The Flammability Limit Of Thermal Runaway Vent Gas Of Lithium Ion Batteries

In recent years,the low pollution,renewable and environment-friendly energy has received more and more attention.Therefore,the new energy vehicle market has developed rapidly.However,battery abuse or mechanical damage can lead to electric vehicle catch fire and explosion.There were 124 electric vehicle burning incidents reported in 2020,which seriously restricted the popularization and development of new energy vehicles.The flammable battery thermal runaway vent gas is the key factor that causes electric vehicles to catch fire.In order to explore the technology of suppressing the fire of electric vehicles,this paper aims to explore the flammability limit boundary of the battery vent gas,and systematically studies the flammability limit of vent gas under different initial conditions and inert gas dilution ratio.The conclusion can provide a reference for new energy vehicle fire safety prevention and fire suppression measures.Firstly,based on the PREMIX program of Chemkin software package,the influence of radiation heat loss of CO₂,H₂O and CO on the flammability limit was studied by adding the optical thin model and statistical narrow band model.The lean and rich flammability limit were solved by using one-point temperature-controlling continuation method.The simulation model was established to accurately predict the flammability limit of battery vent gas.Secondly,the effect of the initial temperature and pressure on the flammability limit of battery vent gas was studied systematically.The controlling chemical reactions on the flammability limit was investigated.The result shown that the lean flammability limit decreases linearly with the increase of initial temperature,while a reverse trend was shown for the rich limit.Both the lean and rich limits increase with the increase of the initial pressure,and the increasing trend gradually slows down in high pressure conditions.Based on the above results,a calculation formulas of the flammability limit under different temperature and pressure conditions are proposed.Meanwhile,the key elementary reactions affecting the flammability limit and the main free radicals were identified.Finally,the effects of different inert gases（CO₂,H₂O and N₂）on the flammability limits of different cathode materials（NCA,LCO and LFP）batteries’vent gas were studied.It is found that CO₂ can significantly reduce the flammable range of the vent gas,followed by H₂O,and N₂ have the smallest effect.For these three types of batteries,the vent gas of NCA and LCO battery is the most difficult to be inerted,and LFP battery is the easiest to be inerted.The addition amount of inert gas was quantified when the inerting point was reached.In addition,it was found that when the mole fraction of inert gas exceeded a certain value,the deviation between the flammability limit calculated by L-C formula and the value predicted by the simulation model increases gradually.At the same time,the influences of CO₂ and H₂O on the flammability limit with thermal effect,third-body reaction and direct chemical reaction were quantified.It was found that when the dilution gas is CO₂,the thermal effect is the main factor affecting the flammability limit.Especially for the rich flammability,it accounts for about 95%.Moreover,the effect of direct chemical reaction is almost negligible for the chemical aspect,which is completely caused by the third-body reactions.When the dilution gas is H₂O,for the lean flammability limit,the influence of chemical reaction is greater than that of the thermal effect,and the dominant effect is still the third-body reaction.For the rich flammability limit,the effect of the third-body reaction is similar compared to that of the direct chemical reaction.