Study On Thermal Runaway Caused By Internal Short Circuit In Lithium-ion Batteries

Lithium ion batteries are widely used in the field of electric vehicles due to their advantages such as high energy density and good cycle life.However,the safety issues of lithium-ion batteries have gradually become a key factor restricting their rapid promotion.This thesis adopts ternary material system lithium-ion batteries as the research object to investigate the thermal,electrical,and electrochemical reaction processes during the thermal runaway process of high specific energy lithium-ion batteries.For this purpose,closed environment thermal runaway testing and open environment needle testing are used to trigger the thermal runaway of the battery,respectively.The work concerning the mechanism of thermal runaway process in high specific energy lithium-ion batteries,is of great significance for understanding the battery failure law,optimizing battery design,improving battery quality,and reducing the risk of thermal runaway of battery.It also provides guidance for the design of single battery application process and system design.A 63 Ah 6-series ternary soft pack lithium-ion battery was heated in a sealed chamber and then its five charged states(namely 100%SOC,75%SOC,50%SOC,25%SOC,and 0%SOC)were studied.The temperature,pressure,and voltage changes were monitored,and the gas components and content were also measured using a gas chromatograph to investigate the thermal runaway gas production process and key parameter varying law.The results indicate that batteries with 100%SOC,75%SOC,and 50%SOC experienced thermal runaway phenomena.During the process of battery thermal runaway,the rates of voltage drop and temperature rise,temperature peak,and gas production rate increase with the increase of SOC,while the starting time and triggering temperature of battery thermal runaway show opposite trends.The triggering temperatures for thermal runaway of 100%SOC,75%SOC,and 50%SOC batteries are 186 ℃,190℃,and 218 ℃,respectively.The peak temperatures after runaway are 732.7℃,605.5℃,and 501.5℃,respectively.The time corresponding to the highest temperature of thermal runaway is 126 min,149 min,and 324 min,respectively,and the peak gas production rate is 1.54 × 104 L·min-1、1.09 ×104 L·min-1、4.51 × 102 L·min-1,respectively.The high-gas-content components after thermal runaway in different SOC batteries are CO2,CO,and H2,with the content of 19.3%,17.1%,and 8.5%,respectively in 100%SOC batteries.Note that the severe thermal runaway did not occur in low-charge-state batteries of 25%SOC and 0%SOC.All these results suggest that the thermal runaway of the battery is closely related to its own state of charge,and the high energy is the vital factor triggering the thermal runaway of the battery.A step-by-step needling study was conducted on lithium-ion batteries with four states of charge,namely 100%SOC,75%SOC,50%SOC,and 25%SOC,at the same temperature of 25℃ and 55℃ by using a new high-precision and slow needling method,to explore the needle retention time during the needling process and its impact on the thermal runaway process of the battery.The main influencing factors of thermal runaway temperature was analyzed by controlling the state of charge,puncture depth,and puncture position.Furthermore,a study on the internal heat change caused by acupuncture in the battery was also performed based on the heat production after the experiment.The results show that the thermal runaway does not found in the batteries with 50%SOC and 25%SOC and only occurs in batteries with 100%SOC and 75%SOC.As the battery SOC increases,the flame time significantly shortens,and the existence time of the jet containing high-temperature gas and molten battery material sprayed from the fixture pores also shortens.The combustion process duration in high-temperature and room-temperature environments is significantly shortened by the 75%SOC batteries,with a combustion runaway time of 9 seconds at 25℃ and 5 seconds at 55℃,resulting in a 44.5%reduction in reaction time.Research on the mechanism of thermal runaway caused by needle piercing in lithiumion batteries shows that higher environmental temperature is more prone to trigger the thermal runaway of the high-state batteries.For batteries with 75%SOC,thermal runaway occurs at a needle piercing depth of 12 mm at 25℃.The pressure drop and temperature rise rate increase with the increase of SOC.The temperature rise rate during the needle retention stage is lower than that during the needle insertion process.In the longer duration,the total heat accumulation is relatively large,and the total voltage drop of the battery is greater.Thus,it is more likely to occur the thermal runaway during the needle retention stage.The heat generated during the acupuncture process is mainly concentrated in the contact area between the needle and the electrode,and no sudden temperature rise is observed in other areas of the battery,indicating that the temperature rise phenomenon is mainly caused by heat conduction.