Thermal Runaway Criticality And Fire Dynamics Of The 18650-type Ternary Lithium-ion Battery

Lithium ion batteries(LIBs)have been the dominant energy sources on the portable electronics due to their high energy density and good cycle performance.They also expand to large-scale applications such as energy storage system and electric vehicles.However,due to the high energy density and flammable components,the fire accidents caused by LIB thermal runaway(TR)are also reported frequently which lead to catastropic damage to life and property.Therefore,it is meaningful to investigate the self-heating reaction,TR criticality,combustion features,gas generation and fire dynamics model of LIB,so as to understand the TR mechanism and make proper safety precautions for LIB system.The Accelerating Rate Calorimeter is used to conduct the self-heating and TR tests on the commercial LIBs.The TR mechanism is revealed,and the reaction kinetics parameters of different SOC(state of charge)cells are calculated to help derive the critical ambient temperature of TR,which can provide guidance for safe storage and transportion of LIB.The TR process induced by self-heating reaction can be divided into six stages.The onset temperature of self-heating reaction,temperature of voltage drop,opening temperature of safety valve are measured as well as the trigger temperature of TR and the peak surface temperature.The predicted critical ambient temperature of TR is validated by oven tests.According to the model,the fully charged 18650-type Li(NixCoyMnz)O2/graphite LIB would self-ignite when the ambient temperature exceeds 149.6?.The standard high-pressure canister is used to conduct TR tests on the LIB,and the pressure variation is measured to establish the gas generation dynamics model.Thanks to the adiabatic condition,the battery and canister system could be regarded as a uniform-temperature system and described by a lumped-parameter model before TR.When the safety valve opens,the gas released from the cell is about 10mmol,and ascends with the decrease of SOC.The eruption index of LIB TR is about 1000 times smaller than the explosion index of common gas fuels or electrolyte solvents.The gas generation dynamics can be simulated by Arrhenius law and have multiple stages.During the TR process,the gas generation rate is approximately proportional to the heat generation rate.And the activiation energies of the above two processes are similar,implying the inherent link between the gas and heat generation processes.The combustion tests on different SOC cells are conducted in open space and a combustion chamber,respectively.The flame shape and combustion_process are recorded as well as the heat release rate and mass loss.A non-destructive method is proposed to estimate the combustion heat of LIB and assess the fire hazard.The height of the LIB jet flame is in the range of 0.095?0.217m,and increases with the SOC ascending.The flame height measurement could provide experimental validation for the following fire dynamics model.The peak heat release rate of the 100%SOC cell is 1.118MW m-2 with high fire risk that is comparable to the fuel oil.Based on the ratio of the flammable components in the cell,the mass loss caused by combustion could be predicted agreeing with experiments.Based on the specific combustion heat of each components,the total heat release of LIB could be estimated and validated by experiments.After dividing the cell's initial mass,the specific combustion heat is 2.2±0.2kJ g-1.The fire dynamics model of the 18650-type LIB is established based on the mentioned gas generation model and combustion test results.The whole process of TR is simulated including the gas generation,internal pressure increase,venting,jet flow and flaming combustion.The simulated internal pressure and flame height(trend and peak value)are validated by experiments.The gas generation process inside the cell follows the Arrhenius law,and the jet flow could be described by the isentropic flow equation.When the safety valve opens,the jet flow is choked and the velocity at the orifice is equal to the local speed of sound.Afterwards,the jet flow returns to subsonic level,and the peak ejection speed when TR occurs is also equal to the local sonic speed.The framework about the gas generation and fire dynamics model of LIB TR is established in this paper,and could be extended to other battery chemistries and different abuse conditions.