Study On Thermal Runaway Of Lithium-ion Power Battery Or Module For Electric Vehicle Through Experiment And Simulation

Electric vehicles have been widely used in various fields,with the rapid development of global economy and the deterioration of different environmental problems.However,the frequent accidents have made the safety of power lithium-ion batteries be a key point to hinder the development of electric vehicles.In this paper,ARC?Adiabatic Rate Calorimeter?is mainly used to study the thermal runaway process of single lithium-ion batteries?5Ah LiFePO₄ 32650 battery,20Ah LiFePO₄ square battery?under heating and adiabatic overcharge.Then a finite element model for thermal runaway of single cells and battery modules was built to study the heat generation and transfer process of batteries.Then an experimental study was also carried out to investigate the heat diffusion process inside 32650 battery module during over-charging and overheating.In addition,a finite element model for thermal runaway of single 32650 battery was built to study the relationship between material decomposition and temperature rise.Moreover a themal runaway model for 32650battery module and square battery module was also set up to study the relationship between the heat transfer and the size of the battery spacing in the module,which obtained an conclusion that the proper safe spacing between cells will not cause thermal runaway of the adjacent cells.In the first place,it is found that the open circuit voltage of the battery has a significant decline before thermal runaway in the heating experiments of single battery.Comparing with different SOC of batteries,it is obtained that batteries generate more heat in pace with the raise of SOC.On the other hand,it can be found that the heat generation of 32650 battery is higher than square battery through comparing with the thermal runaway process of them.In the overcharge process of 32650 battery under different currents,total heat production could be saw as two stages.The first part is electrochemical heat production and the other one is heat generation caused by the decomposition of internal materials.Above all,the first stage is incentive of thermal runaway.Moreover,the second kind of heat was caused by the chain reaction of battery materials decomposition,which actually promote and achieve the thermal runaway of batteries.In the overcharge experiment of single battery,the overcharging process of the sqaure battery is slightly different from that of the 32650 battery at different currents because of the difference of ultimate current density.It was also found that 3C currents overcharging is more dangerous for 32650 batteries and 1C overcharge is more dangerous for the square battery,but the voltage changes of them are regular during overcharge.Secondly,according to experiments of 32650 battery module,it is revealed that the change regulation of voltage in the module is approximately the same as the cell during the overcharging process,but overcharged battery and battery module did not cause entire thermal runaway.After overcharge,part of the electrolyte leaked which resulted in serious corrosion of the battery package.In the heating experiments of single battery inside 32650 battery moule,it found that the maximum temperature of the broken cell in the module is lower than temperature of the cell under heating condition as a result of the heat conduction.After thermal runaway,temperature of the single cell drops rapidly.Nevertheless,the temperature of module will continue increasing and eventually lead to thermal runaway of entire module in the adiabatic environment.It is indicated that local heat of single cells may cause thermal runaway of the whole module under adiabatic conditions.It was also found that the voltage changes significantly before the battery module thermal runaway,indicating that the thermal runaway of the series battery module can also be forecasted by voltage.In addition,the thermal runaway process of 32650 battery can be accurately calculated by the lumping chemical reaction kinetics-three-dimensional heat transfer coupling model.It was found that the internal temperature of the thermal runaway battery is higher than the surface temperature,so the heat generation calculated by the surface temperature change rate will be less than the actual battery heat production.In the process of heat exchange between the thermal runaway battery and the surrounding environment,the heat that transferred by the heat radiation is higher than the heat convection.Before the battery reached 230°C,the SEI films was nearly complete decomposed,and some of the active materials decomposed in a small amount.After reaching 230°C,all the active materials decomposed rapidly and a large amount of heat was generated,which resulted in thermal runaway of the battery.It is also found by simulation that the higher SOC of battery,the more heat generation during thermal runaway,and the higher final temperature of battery.Finally,it is found that the process of heat diffusion is slightly different under diverse distance inside the 32650 battery module or square battery module by simulation.The starting time of the thermal runaway will go up with the increase of adjacent distance between cells.When the battery distance of the square battery module reaches 1mm,the battery distance of the 32650 battery module reaches 3mm,thermal runaway of the center battery will not cause runaway of the adjacent cells.Comparing with the square battery module,the starting time of the battery out of control in the32650 battery module is longer,The 32650 battery produces more heat during thermal runaway,so the longer safety distance to ensure uncontrolled transmission does not spread in adiabatic environment.