Study On Fire Hazards And Control Methods Of Typical Lithium-Ion Traction Battery For Electric Vehicle

Since the beginning of the 21st century,the global energy and environmental situation has become increasingly severe.With the increasingly stringent exhaust emission regulations,traditional vehicles will not be able to meet today's environmental protection requirements,and the transformation of the automotive industry has become a general trend.Electric vehicles(EV)are essentially zero-emission,which are in line with the country's strategic needs for achieving clean and sustainable development.Due to relevant incentive policies,the electric vehicle industry has developed rapidly.However,the continued growth of the EV production is accompanied by frequent fire accidents.As the core component of EVs,the safety problems of lithium-ion batteries(LIBs)have aroused widespread concern in the society,hindering the further development of the electric vehicle industry.Therefore,it is of great theoretical and practical value to study the fire hazard of lithium-ion power battery and its effective control method.This paper focuses on the fire behavior,thermal and gas toxicity hazards,thermal runaway propagation characteristics and effective prevention and control measures of lithium-ion batteries.Combining experimental observation with theoretical analysis,the fire risk and control methods of lithium-ion batteries are studied,including the following four aspects:First,the fire behaviors of large format pouch LIBs with no safety valve are evaluated using the combustion chamber of early fire characteristics.Several typical characteristic parameters,including ignition time,surface temperature,heat release rate(HRR),total heat release,mass loss rate and flame size are systematically analyzed.The effects of incident heat flux and state of charge(SOC)are also investigated.Secondly,the thermal and toxic hazards resulting from the thermally-induced failure of large format LIBs are systematically investigated by means of the Fourier transform infrared spectroscopy(FTIR)and the modified combustion chamber of early fire characteristics.It is found that the jet flame reaches a length of 55 cm for the fully charged battery and the normalized HRR value of the fully charged LIB is almost equivalent to that of gasoline.The thermal hazards have been quantitatively evaluated by combining the effects of convective and radiative heat and the batteries with higher SOCs are found to have greater thermal risks.For batteries with 50%or more SOC,the value of FEDtherm is equal to or greater than the tolerance limit 1.It is considered that people will suffer serious skin pain and burns,and half of them exposed to such fire scenarios would fail to escape due to the thermal hazards.The effects of 0%SOC battery can be negligible.The major toxic gases analyzed in the experiments are CO,HF,SO2,NO2,NO and HCI.The production of all these gases increases with SOC.FED and FEC models are used to perform a quantitative evaluation of the combined effects of toxic gases.According to the results,the effects of irritant gases are much more significant than those of asphyxiant gases with the difference being more than one order of magnitude.The maximum FEC value is 0.8,which is very close to the critical threshold,indicating that nearly half of the exposed people will lose the evacuation capability in such a fire scenario.The influence of HF and SO2 accounts for more than 85%of the FEC value,showing a much higher toxic threat than other gases,which should be paid more attention in fire rescue.Thirdly,the TR propagation characteristics of the prismatic lithium iron phosphate battery and its mitigation methods are investigated.The influence of state of charge,combustion flame and parallel condition on TR propagation characteristics are analyzed by taking the actual situation into consideration.The results show that the TR propagation speed of 100%SOC LIB module is 0.1 min-1.The TR propagation speed of the 50%SOC LIB module is reduced by 39.5%compared with that of 100%SOC.For the 100%SOC battery module with flaming combustion,the TR propagation speed only increases by 1.9%.It can be seen that the failure propagation is primarily driven by heat conduction between the LIBs,and the thermal radiation of flame has little effect on it.However,the TR propagation speed increases by 69.6%in parallel,indicating that there is a higher risk of TR chain reaction in parallel condition.Based on the experimental findings,the TR propagation mechanism in a parallel module is analyzed.In addition,the TR propagation speed between 1#and 2#battery is reduced by 78.2%,80.6%and 88.7%by adding mica,ceramic fiber paper and aerogel plate,respectively.Finally,it is verified through quantitative analysis that the TR propagation speed between the lithium iron phosphate batteries can be effectively delayed by the insulation and the continuous spread of TR in the battery modules can be prevented.Fourthly,the effectiveness of water mist in suppression of 18650 LIB fires as well as prevention of TR propagation in LIB arrays is investigated.Using water mist at a concentration of 11.1 and 14.1 wt.%prevents failure propagation in 50%and 40%of all conducted experiments,respectively.In addition,applying 11.1 wt.%of water mist and adding ceramic fiber board can prevent TR propagation in 75%of tests.Even in the tests that undergo complete failure propagation,the introduction of water mist at concentrations of 11.1 wt.%and 14.1 wt.%reduces the TR propagation speeds during the late failure stages in the test.Addition of water mist is found to effectively suppress flaming combustion of materials ejected from the LIBs during TR.The introduction of water mist at concentrations of 11.1 wt.%and 14.1 wt.%reduces the peak HRR by 53%and 30%,respectively,and also reduces the energy released of each failed LIB by 43%and 40%compared with air tests.The water mist is capable of reducing the combustion efficiency below 36%.Besides,the application of water mist can reduce the maximum temperature at the bottom of the battery by 100-200?,showing a good cooling effect.The results of this study are helpful to understand the fire risks of LIBs,provide constructive guidance for the development of LIBs fire prevention and control strategies,and also have certain guiding significance for the safety design of LIB power system.