Study On Fire Characteristics And Water Mist Suppression Of Thermal Runaway Of Large Capacity Ternary Lithium-ion Battery

Lithium-ion batteries are widely used in the field of power batteries,with ternary lithium-ion batteries and lithium iron phosphate batteries accounting for the highest percentage.The market has increasing requirements for high energy density,lightweight and reliability of power systems,resulting in the development of large capacity ternary lithium-ion batteries above 25Ah and even 100Ah.However,the thermal runaway of lithium-ion battery can cause fire accidents in electric vehicles,and people pay attention to the thermal safety of lithium-ion battery.Previous studies on large capacity lithium-ion batteries,especially ternary lithium-ion batteries with 100Ah and above,are rare,and their thermal runaway characteristics and water mist suppression effect are still unknown.It’s urgent to carry out experimental research.This paper takes large capacity ternary lithium-ion battery and lithium iron phosphate battery as the research object,through behavior,temperature,temperature rise rate,mass loss,shell deformation and other macro characteristics,compares the thermal runaway differences of large capacity lithium-ion battery with different cathodes and capacity under different SOC and heating directions.The differences were analyzed by XRD,SEM,EDS and other microscopic methods and mechanism.Water mist was used to suppress the thermal runaway fire of large capacity ternary lithium-ion battery.The suppression effect of continuous release strategy and pulse release strategy were compared by fire extinguishing effect,cooling effect and utilization efficiency,and the following main conclusions were obtained:(1)The thermal runaway of ternary lithium-ion battery is more severe than that of lithium iron phosphate battery.The thermal runaway is higher when the SOC is high and the bottom surface is heated.The severity of thermal runaway of ternary lithium-ion battery is more sensitive to the change of SOC and heating direction.Lithium iron phosphate batteries produce white smoke,while ternary lithium-ion batteries produce jet flame and explosion.The surface temperature of the latter can reach more than 600℃.The difference of thermal runaway of ternary lithium-ion battery in different SOC and heating direction is more obvious.The mass loss ratio of thermal runaway of ternary lithium-ion battery of 100%SOC is 1.8 times that of 75%SOC,and the maximum temperature rise rate when the bottom is heated is 2.3 times that of side heating,while the characteristic parameters of lithium iron phosphate battery have little difference in the above scenarios.These differences are related to the structure of the battery,thermal conductivity anisotropy,positive electrode reaction,internal thermal runaway propagation speed and other factors.(2)The larger the capacity of ternary lithium-ion battery is,the more severe the thermal runaway is.The thermal runaway behavior and shell deformation will lead to lateral jet fire.The relationship between the maximum surface temperature and the capacity roughly agrees with the exponential function y=-a·e-x/b+c.As the capacity decreases,the temperature decreases steadily for a period and then decreases rapidly,and this change is more obvious at lower SOC.Jet fire and explosion will occur many times.A large area of the shell will melt,the core will be exposed and even the core will be blown out.The larger the capacity is,the faster the internal thermal runaway propagation is.When the 150Ah ternary lithium-ion battery is heated at the bottom,the thermal runaway occurs almost at the same time.The more serve the thermal runaway is,the microstructure of the cathode will change in the order of disappearance of adhesive,blurred boundary of particles,formation of flake structure,formation of aggregates and disappearance of flake structure.(3)Water mist has a good fire suppression effect on large capacity ternary lithium-ion battery,and the fire extinguishing effect,cooling effect and utilization efficiency of pulse release strategy are better than those of continuous release strategy.The longer the continuous release time,the better the fire extinguishing effect and cooling effect is,but there is still reburning and the utilization efficiency is low.There is no reburning in the pulse release strategy,and the smaller the period and duty cycle,the cooling effect and utilization efficiency show an upward trend.When the period and duty cycle is too small,it will lead to temperature rise and explosion may occur.In this paper,the pulse water mist with 10L water consumption and 10s cycle has the optimal value when the duty cycle is 0.3.Compared with the continuous release with the same consumption,the maximum surface temperature decreased by 176℃,the actual heat taken away by water increased by 160%,and the water utilization efficiency increased by 17.66 percentage points.This study can provide a reference for the safety and fire extinguishing design of vehicle power battery system.