Research On The Thermal Runaway Propagation And Its Mitigation Mechanism Of Ternary Lithium Ion Battery

As an energy carrier that can realize the conversion between chemical energy and electrical energy,lithium-ion batteries have been widely used in portable electronic equipment,new energy vehicles,energy storage power grids,aerospace and other fields due to their high operating voltage,high specific energy and long cycle life.However,the fire and explosion accidents associated with the lithium-ion batteries were reported from time to time in recent years.Especially in the fields of new energy vehicles and energy storage,if a battery undergoes thermal runaway in the battery module,it may lead to the thermal runaway of the entire battery module,and finally result in the fire or explosion accident of the battery system.In response to thermal runaway propagation events in lithium-ion battery modules,the following research is carried out in this paper.(1)The mechanism of thermal runaway related to single ternary lithium-ion battery(NCM)was studied.The thermal runaway process of lithium-ion battery under thermal abuse conditions was investigated from the perspective of materials and cell.The triggering temperature and heat production law of the internal components was analyzed,including the cathode,anode,separator,electrolyte as well as the mixed system.Based on these,the thermal runaway process was divided into several stages and the dominant reactions that caused the temperature rise at each stage were discussed.It was found that the reaction between the intercalates lithium and the electrolyte plays a leading role in the temperature rise of the battery before thermal runaway event.The reaction between positive material and electrolyte can release more intense heat,but its trigger temperature is relatively high,which is more reflected in the dramatic temperature rise during the thermal runaway stage.(2)The mechanism and influencing factors of thermal runaway propagation in ternary lithium-ion battery(NCM)module were investigated.During the process of thermal runaway propagation event,the thermal shock caused by the battery in thermal runaway will rapidly increase the temperature of its adjacent battery to a higher value.For the battery module with 100%SOC,this value is above 250℃ in cylindrical battery module,and is approximately 400℃ in prismatic battery module.Then,the surface temperature of the adjacent battery enters the "heat balance" stage,in which the local temperature of the battery is always maintained at a high value.After a while,the thermal runaway was initiated at the jelly roll that near the high-temperature area.Then,thermal runaway quickly spreads from the local part to the whole battery and further lead to the violent jet smoke or jet flame behavior.In addition,the decreased SOC can not only improve the thermal stability of the battery,but also reduce the peak temperature of the battery in thermal runaway.Thus,it both contribute to the delay of propagation time and alleviation of the combustion behavior of the battery module.(3)The dynamic model for thermal runaway,propagation and prevention of ternary lithium-ion batteries(NCM)was constructed.For thermal runaway propagation issues in lithium-ion battery module,the heat production law of chemical reactions inside the single cell during different temperature range is clarified.Based on these,the dynamics model of single cell in thermal runaway under the thermal abuse condition is constructed and it was verified by experiments.Further,the heat transfer process between cells in the module is simulated by the extended thermal runaway propagation module in semi-open space,in which the module is by five parallel cells.Besides,the effect of battery capacity,structural form,environmental temperature on thermal runaway propagation process is analyzed,respectively.The results indicate that the cell spacing can change the heat transfer path between cells,which showing a strong linear relationship with the thermal runaway propagation time between cells.Based on this model,the effects of insulation layers with different thicknesses(δ)and thermal conductivity(k)on the thermal runaway propagation process are compared and analyzed.Further,the safety level of the battery pack is semi-quantitatively divided according to its ability to prevent the thermal runaway propagation process.