| As the heart of new energy electric vehicles,thermal safety of lithium-ion battery pack has been one of the factors restricting the development of electric vehicles.With the continuous improvement of the energy density of lithium-ion batteries and the continuous expansion of the application scale of power batteries,various fire accidents caused by battery failure emerge in endlessly.Improving the safety and reliability of lithium-ion batteries has become a key problem to be solved urgently in the promotion of new energy electric vehicles.The direct cause of power battery fire is the thermal runaway of single battery under various abuse conditions,and the thermal runaway propagation caused by thermal runaway of single battery is the main cause of large-scale fire.Therefore,in order to further understand the influencing factors of battery thermal runaway and improve the safety performance of battery pack,the thermal runaway explosion characteristics of single battery,the gas production characteristics and spatial distribution of combustible gas,the propagation characteristics of thermal runaway and the suppression method are studied by the method of battery thermal runaway experiment.The work,content and conclusions of this paper are summarized as follows:For cylindrical lithium-ion batteries with different cathode materials,the thermal safety of the battery was tested by heating experiments.The effects of battery size,charge state and external thermal radiation intensity on the thermal runaway explosion characteristics(Explosion phenomenon,temperature,heat release rate and mass loss)of the battery were analyzed.The results show that with the increase of heating power and charge state,the thermal runaway time of the battery is advanced and the initial temperature of thermal runaway is reduced.By comparing the average mass loss rate of the battery in the process of thermal runaway,it can be found that the high-temperature material injection intensity of the battery with the size of 21700(70.3%)is more intense than that of 18650(43.4%)when thermal runaway occurs.Nickel cobalt aluminum(NCA),lithium cobalt(LCO)and nickel cobalt manganese(NMC)batteries are more dangerous than lithium iron phosphate(LFP)batteries.The concentration variation of the main combustible gas(CO/CH4)in the thermal runaway process of the single cell and its concentration distribution in the finite space were studied,and the effects of different heating power and charge state on the gas production of the thermal runaway combustible gas were analyzed.The results showed that the peak concentration of CO gas exceeded 1000 PPM during thermal runaway,and the maximum concentration of CH4 gas reached 5.3%.With the increase of heating power,CO gas shows an obvious increasing trend;With the increase of charge state,the maximum concentration of CO and CH4 gas increased significantly,but the increase of CO was more obvious.The sensors located at the horizontal plane of the battery safety valve and above the horizontal plane can detect the presence of CO and CH4 gas earlier,and the gas concentration detected at the distance of 0.6 m from the battery is the highest.An early thermal out-of-control suppression method for battery pack based on liquid pipes with film-sealed microporous(LPFSM)was proposed.In this method,the thermal out-ofcontrol of the battery was detected by using the sealing membrane with automatic damage of temperature sensitivity.When the thermal out-of-control occurred,it was automatically damaged,thereby inhibiting the thermal out-of-control propagation.The influence of the melting point of the sealing membrane and the diameter of the microporous on the suppression effect of thermal out-of-control propagation was analyzed.The experimental results show that the PE sealing film with a melting point of 123℃ can be damaged before the battery temperature reaches the initial temperature of thermal runaway,so as to avoid the occurrence of thermal runaway.Increasing the diameter of microporous from 3mm to 5mm can improve the probability of sealing film damage in thermal runaway monitoring points.The battery module using LPFSM can reduce the number of thermal runaway batteries from total thermal runaway to one or no thermal runaway batteries.In summary,this paper mainly studied the thermal runaway detonation characteristics of single battery,the gas production and spatial distribution of main combustible gases,and the early thermal runaway propagation suppression method through experimental research.The influencing factors of battery thermal runaway,the characteristics of thermal runaway propagation and the application performance of membrane-sealed microporous pipes were analyzed.The research results can provide guidance and reference for the safety of lithium-ion power battery pack and the design and application of fire control system. |
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