Experimental Study On High Temperature, Overdischarge And Acupuncture Thermal Runaway Of Ternary Lithium-ion Battery

At present,the high energy density lithium-ion battery is not safe,and the spontaneous combustion events of new energy vehicles caused by the runaway battery heat occur frequently.The safety has become one of the main constraints of the application of ternary lithium-ion battery in the field of new energy vehicles.This paper mainly studies the thermal stability of high Ni ternary lithium-ion battery materials,the high temperature thermal runaway characteristics of the battery monomer,the over discharge and thermal runaway process of the battery monomer and the process of the thermal runaway of the battery acupuncture.The factors and trigger process of the thermal runaway of lithium battery are studied in depth,which provides a theoretical basis for further exploring the mechanism of thermal runaway and the thermal runaway warning.This paper is divided into the following parts:(1)The thermal stability of positive,negative and diaphragm of NCM811(Li(Ni_0.8Co_0.1Mn_0.1)O₂)battery was analyzed by using Sta8000 synchrocalorimeter.The temperature range of reaction of positive,negative and diaphragm materials at high temperature was observed through the heat weight and heat flow curves of the materials at high temperature.The results show that the pyrolytic process of the battery has occurred in the process of the battery thermal runaway,such as the pyrolysis of Li PF₆,the decomposition of SEI film,the reaction between lithium-filled negative electrode and electrolyte,PE diaphragm,PP membrane,positive decomposition and electrolyte reaction.(2)The heat transfer from internal chemical reaction source to the outer surface of NCM523(Li(Ni_0.5Co_0.2Mn_0.3)O₂)battery with mandrel was studied.The research on the process of heat transfer to the outside during the process of heat loss provides guidance for the battery to add flame retardant materials,to explore the spread process of thermal runaway and to prevent measures.(3)With the increase of energy density,the safety of the battery is reduced.The effect of Ni content on the thermal safety of NCM523 and NCM811 batteries was investigated.Compared with NCM523,the temperature of T_cis reduced by 55.08°C.Maximum heating rate is 38.06 higher°C/min.The thermal safety of NCM811 battery is significantly lower,and the thermal runaway reaction is more severe.(4)The harm of NCM811 battery over discharge to 0V under extreme heat abuse condition was investigated.The difference of over discharge process of different discharge ratio(0.2C,0.5C,1C and 2C)was compared;The process of over discharge at high temperature(55°C)and the influence of single over discharge to 2 and 1V.The over discharge did not cause fire explosion without contact heating,and NCM811 battery showed good safety under the condition of over discharge.(5)The effects of SOC,acupuncture depth and different materials(NCM523/LFP)on the runaway of acupuncture heat were investigated.The maximum temperature of out of control heat by Acupuncture and battery temperature after acupuncture lasts at 100/200°C.The process of thermal runaway of acupuncture was studied in the time above C,drum rate,weight loss ratio and the evolution of thermal runaway after acupuncture.The maximum temperature of NCM523 battery after acupuncture 368°C.The maximum temperature of LFP cell penetration through acupuncture of 100%SOC is 102.2°C.The smoke was released after the battery acupuncture,but no thermal runaway occurred,and LFP showed better safety.(6)The three ways of triggering the battery heat out of control are compared,such as high temperature,over discharge and acupuncture.The three trigger risk levels were acupuncture,high temperature and over discharge.