Research On Thermal Safety Management And Critical Overheating Behavior For Power Batteries

The demands of dynamic performance,fast charge,light weight,compact,energy efficiency for electric vehicles and the application of large capacity and high specific energy batteries make the safety of power batteries receive more attention.The critical identification and diagnosis become the key technology for battery thermal safety control.The research focuses on the thermal and gas generation characteristics,as well as analyzes the characteristics of the critical control.By seeking powerful diagnosis,accurate prediction and sensor identification of overheating,the thermal safety prevention and control system will be more accurate,which can lay the theoretical foundation for the future thermal safety control and overheating prevention technology.When exploring analytical methods for overheating diagnostic prediction,the temperature rise and venting events are mainly considered.By analyzing the identification of thermal safety control and the ability of thermal management system,the criticality of thermal safety prevention and control diagnosis is defined.Through analyzing the thermal and gas generation characteristics,the overheating controllable criticality is subdivided into three stages,including the initial,medium and late stage.The thermal management and thermal safety management system is designed,which includes conventional liquid cooling,direct cooling of refrigerant and emergency injection cooling system.In the whole process of thermal management,a new mechanism of overheating safety prevention and control is constructed to cope with the overheating safety prevention and control.By taking the temperature variation and gas generation into consideration,a framework for judging the critical characteristics of step-by-step thermal safety control is established,which includes sensing,measurement identification and parameter identification.During the research of heat generation characteristics,the species of side reactions,the thermal reaction capacity and the temperature range during overheating are analyzed.The staged upward trend of heat generation in battery is found.Research shows that,by taking SEI film decomposition as the effective boundary,battery thermal safety management can be achieved.After separator melts,the strong cooling capacity is needed to suppress the temperature rise of battery.When the battery enters the third stage,emergency cooling measures are required to avoid battery thermal safety accidents.Generally,it is necessary to identify the typical characteristics in the early stage of overheating.Combined with the cooling capacity of thermal management system,a dynamic control boundary can be formed to prevent battery overheating.At the same time,the dynamic thermal control threshold is subdivided to construct the thermal basis for judgment.In the process of researching gas generation characteristics,by analyzing the mechanism,the variation of gas species and content,the staged increasing trend of gases is pointed out.Research shows that,there will be two significant venting events during the overheating.The first venting event is due to safety vent opening,the second venting event owing to the reaction between cathode and electrolyte.According to the venting events and the variation of gas generation,the overheating process can be subdivided.In addition,through the battery overheating monitoring experiment,the basic rules of gas generation,venting events and temperature variation during the overheating process are verified.After safety vent opening,battery will exhaust a large amount of CO₂ and other gases,the obvious signal can be detected.Gas monitoring will not only assist in overheating diagnosis for thermal runaway suppression and fire safety protection,but also provide the basis for implementing emergency cooling.The research work provides assistance for analyzing the common character of battery overheating,which also offers theoretical basis for feature recognition to protect battery from overheating,even prevent and suppress thermal runaway.Finally,the overheating process of battery is further analyzed by calculation examples.Based on engineering application,an effective and simplified calculation method is provided for the determination of battery overheating.The effects of typical reactions on the battery temperature rise are quantitatively analyzed,which includes SEI film decomposition,internal short circuit and the reaction between cathode and electrolyte.By superposing the heat generation from conventional reactions and side reactions,an analytical mechanism is established,which have been verified by existing research.Furthermore,the heat generation characteristics of SEI film decomposition under different charge and discharge rates,the temperature changes of battery under different short-circuit area and charge and discharge rates,as well as the temperature changes of battery caused by positive-solvent reaction are analyzed.The research indicates that the heat generation capacity of the side reactions are gradually increased during overheating,it is helpful to understand the process of battery overheating and design the enhanced cooling system.