Research On Thermal Runaway Of Lithium-ion Batteries And Its Propagation Characteristics And Protection Strategie

Lithium-ion batteries are widely used in electric vehicles and energy storage as excellent energy carriers,but they are prone to safety accidents in the form of thermal runaway(TR)and thermal runaway propagation(TRP).In view of the above issue,this paper innovatively conducts research in two aspects: using a 64 Ah lithium iron phosphate(LFP)battery as the object to study its thermal runaway and thermal runaway propagation characteristics under different triggering modes,cooling conditions and SOCs;taking a 12 Ah ternary lithium(NCM)battery as an example,investigate the thermal runaway propagation mechanism and protection strategies among adjacent batteries under different heat transfer conditions.Firstly,the TR and TRP characteristics of LFP battery/module under different heat dissipation conditions are investigated in the overcharge trigger mode.For the LFP single battery,the heat dissipation conditions are changed by wrapping thermal insulation cotton on its surface;for the LFP module,the heat dissipation conditions are changed by adjusting the ambient temperature.It is found that the heat dissipation level determines whether TR occurs in LFP single battery: the battery wrapped with thermal insulation cotton undergoes TR,with a maximum temperature of 435.7 ℃;while TR does not occur in the battery not wrapped with thermal insulation cotton,and the maximum temperature is only 114.6 ℃.LFP module has high thermal safety under overcharging,as shown by the fact that it does not undergo TR and TRP even in an environment of 80 ℃,with a maximum temperature of 134.5 ℃.However,overcharging can cause structural damage and capacity loss of the battery.Then,the TR and TRP characteristics of LFP battery/module under different heat dissipation conditions are investigated in the overheating trigger mode.For the LFP single battery,the heat dissipation conditions are changed by wrapping thermal insulation cotton on its surface.For the LFP module,the effect of SOC on TRP is first studied,and then the TRP model of 100% SOC LFP module is established,and the heat dissipation condition of the module is changed by adjusting the heat convection coefficient.The results show that the LFP single battery wrapped with thermal insulation cotton exhibits lower TR onset temperature(158.9 ℃ vs 204.9 ℃)and higher TR maximum temperature(526.3 ℃vs 376 ℃)compared with that without thermal insulation cotton,and whether improving the heat dissipation level can prolong the TRP time is influenced by the thermal resistance value between batteries.As the battery SOC decreases from 100% to 80%,the maximum temperature of the module decreases from 480.6 ℃ to 447.6 ℃ and the TRP time increases from 140 s to 644 s,and the TRP is blocked when the battery SOC is 50%.Finally,a three-dimensional TRP model of NCM module is established and its accuracy is verified by experiments.The effects of heat dissipation and insulation on the TRP mechanism,including TRP type,TR trigger location,and heat flow variation,are investigated based on this model.The results show that the TRP type between adjacent batteries can be divided into two types with different heat dissipation and insulation conditions.The biggest difference between the two is that the direction of heat flow between batteries is opposite in the later stage of TRP progress.The analysis of heat flow shows that increasing the heat convection coefficient(h)and decreasing the heat conductivity(λ)of the insulation material can not only increase the heat dissipation rate of the module and reduce the heat transfer rate between adjacent batteries,but also play the role of "temperature equalization" and increase the heat storage threshold of the batteries.At the end of this study,we consider the coupling effect of heat dissipation and insulation on TRP,and propose a three-dimensional thermal safety interval: the functional relationship between h and λ that is required to completely block TRP or to make TRP time exceed 300 s,which provides theoretical guidance for the thermal safety protection design of battery modules.