Studies On Combustion Characteristics Of Typical Lithium Ion Battery And Its Electrolyte As Well As Influence Mechanism Of Air Transport Environment On Them

With lithium ion batteries widely used and the rapid development of the China civil aviation industry,the number of the lithium ion batteries transported by air is increasing.Lithium ion batteries are currently classified as Class 9 hazardous materials,which have the risks of flammable and explosive.The specific environments of air transportation such as high temperature,extrusion and collision make their hazards more exposed.However,due to the lack of corresponding risk control technology,the lithium ion batteries fires and explosions have occurred frequently in the air transportation,resulting in catastrophic loss on casualties and property.Therefore,in order to guarantee the safety of the lithium ion batteries during the air transportation,it is necessary to analyze the factors that induce the unsafe safety of the lithium ion batteries and reveal their inherent influence mechanisms as well as take targeted measures to prevent the occurrence of the lithium ion batteries fires.Aim to reduce the risks of the lithium ion batteries during the air transportation,the combustion characteristics of lithium ion battery and its electrolyte as well as influence mechanism of air transport environment on them are studied.The main contents of this paper are as follows:Combustion characteristics of electrolyte pool fires under different external heat fluxs are investigated both theoretically and experimentally.Simultaneously,the thermal decomposition mechanism of electrolyte in high temperature is explored using the TG and FTIR combined technique.Combustion behaviours of electrolyte pool fire in different burning phases depend on the relative evaporation of the organic solvents of electrolyte,which is different from the conventional hydrocarbons.The linear carbonates are mainly consumed at the early stage while the cyclic carbonates are consumed at the later stage.Comparing to hydrocarbons,the electrolyte has relatively low effective heat of combustion and high heat of gasification resulting in lower HRR and MLR.The peak HRR and MLR per unit area are proportional to external heat fluxes and inversely proportional to pan diameters.An empirical model is developed to predict the peak HRR of the electrolyte pool fire using pan diameter and external heat flux.Besides,Heskestad's correlation is improved to predict the average flame height of the electrolyte pool fire by considering the effective heat of combustion,pan diameter and mass stoichiometric ratio of air to fuel.Peak CO and CO2 production rates increase with a bigger pan diameter or external heat flux,but these two factors show no influence on the mole ratio of CO and CO2.Evaporation of the organic solvents and hydrolysis reaction of LiPF6 is the first step decomposition mechanism.Then,thermal decomposition of LiPF6 is about 190?,releasing the strong Lewis acid PF5 and HF.The PF5 initiates ring-opening polymerization of EC and PF5 also reacts with EMC and DMC,releasing CO2,C2H5F and HF.Fire plume characteristics of electrolyte pool fires under different low pressures are investigated both theoretically and experimentally.At the same time,fire plume theoretical models modified by the pressure which are applicable to electrolyte pool fires are validated.The MLR is proportional to the atmospheric pressure.Because of the low carbon mass fraction of the organic solvents of electrolyte,the exponent is lower than that of the conventional hydrocarbons.The pressure modeling which is suitable to predict the MLR of electrolyte pool fire for convective domination stage is verified by fitting m"D and P2D3.The average flame height increases with decreasing pressure.The dimensionless HRR is modified by the pressure and then Heskestad's flame height correction equation which is applicable to the electrolyte pool fire is verified by fitting L/D and m"2/P?2D.The average flame temperature increases with decreasing pressure.The characteristic length is modified by the pressure and then Heskestad's flame temperature equation which is applicable to the electrolyte pool fire is verified by fitting ln(?T0/T?)and ln((z-z0)(P/Q)2/5).It is found by fitting the flame plume temperature and the modified characteristic length that the fire plume of electrolyte pool fire can be divided into two regions:the continuous flame zone and the buoyancy plume zone,which does not conform to the three regions of the McMaffrey model.18650 type lithium ion batteries under different SOCs and external heat fluxs are tested using a cone calorimeter to study the combustion characteristics and the intrinsic effect mechanisms of SOC and heat fluxs on these parameters are found.The trend of the HRR curves measured by oxygen depletion method and calculated by mass loss method is same.Although small quantities of oxygen are released from the lithium ion battery during burning,it is estimated that the energy consuming oxygen released from the lithium ion battery and the Joule heat by released by the internal short circuit,account for less than 13%of total energy released by a fully charged lithium ion battery.Meanwhile,the lithium ion battery surface temperature of the cone calorimeter experiments is basically consistent with those obtained by other calorimetric methodologies,and thus it is obtained that applying the cone calorimeter to evaluate the thermal hazards of lithium ion battery is a reasonable technique.The SOCs of the lithium ion batteries which are high than 50%,there are two peaks in the HRR curves,and the peak HRR and concentration of toxic gases as well as the maximum surface temperature rise with the increasing the SOCs,whereas the time to ignition and explosion as well as the onset temperatures of deflation and thermal runaway decrease.However,The SOCs of the lithium ion batteries which are less than 50%,there are only one peak in the HRR curves and weak combustion occurs.The peak HRR and concentration of toxic gases as well as the maximum surface temperature are significantly lower than those SOCs which are high than 50%,whereas the time to ignition is longer,fire hazard is significantly reduced.In addition,flashing hazards of the lithium ion batteries at less than 50%SOC is significantly lower than high than 50%SOC.Thus,restricting the SOC of the lithium ion batteries to 50%is suggested in the transportation.The peak HRR and the maximum surface temperature increase as the incident heat flux rises whereas the time to ignition and time to explosion as well as the temperature of deflation and thermal runaway onset temperature decrease,fire hazard increases.Time to ignition prediction formula of the lithium ion battery and the critical ignition heat flow are obtained by fitting the experimental data.Combustion characteristics of the lithium ion batteries under different low pressures are experimentally tested and quantitatively analyzed,and the intrinsic effect mechanism of pressure on these parameters is revealed.The burning intensity decreases with the decreasing pressure.When the pressure reduces to 30kPa,the burning does not occur.This shows that reducing pressure can suppress the flame.It is obtained by experimental data that the MLR is proportional to the ambient pressure.By comparing the surface temperature rise under different pressures,it is found that the pressure has a significant effect on the pyrolysis reaction rate of lithium ion battery after the surface temperature above 155?.The maximum surface temperature and ambient pressure is a positive exponential relationship when the pressure is higher than 50kPa,and the exponent is 0.4.However,the maximum surface temperature is linearly proportional to the pressure when the pressure is less than 50kPa.Time to deflation,ignition,and effective fire control increase with the decreasing pressure.In short,when the pressure reduce to less than 50 kPa,the fire hazards significantly decreases.