Research On Spray Cooling Characteristics Of Lithium-Ion Battery During Overheating Process

With the rapid growth of the energy density of lithium battery,the heat production of the battery rises sharply and the thermal stability of the battery decreases rapidly.As a result,the cooling capacity of the existing thermal management methods of the battery is insufficient in dealing with overheating conditions such as fast charging、aging decay and high temperature environment.It is difficult to cool down quickly and block the decomposition reaction,resulting in the thermal runaway of the lithium battery.The development process of lithium battery from overheating to thermal runaway will experience the following chain decomposition reactions as the temperature rises: SEI film decomposition reaction,negative electrode with electrolyte reaction,positive electrode with electrolyte reaction and electrolyte decomposition reaction.There is a period of development process and reaction buffer time from the beginning of overheating to the early stage of thermal runaway,which has the necessary conditions to realize the advance control by adopting thermal management emergency cooling.However,blocking the decomposition reaction requires a more efficient cooling method than conventional thermal management to prevent the cell from overheating to the thermal runaway.In this paper,the cooling characteristics of refrigerant spray during battery overheating are studied.The refrigerant has lower boiling point and higher latent heat of phase transition under atmospheric pressure,and can be vaporized directly on the heat exchange surface.Co MPared with oxygen suppression of inert gas and destructive strong cooling of liquid nitrogen during thermal runaway stage,this method is more suitable for overheating suppression before thermal runaway occurs.Therefore,this paper mainly focuses on the overheating process of power battery,analyzes the critical characteristics of battery overheating behavior and the characteristics of refrigerant spray vaporization cooling process,explores the control strategy of battery overheating reaction stage under the action of spray cooling and the suppression method of group battery overheating propagation.Firstly,the macroscopic electric and thermal behavior and microscopic material change characteristics of lithium battery overheating under different working conditions are studied,and the starting point and range of each decomposition reaction stage are defined.By co MParing the curves of temperature,heat production rate and overheated decomposition reactants,the influences of different heat transfer coefficients,ambient temperature,discharge depth and charge ratio on the critical characteristics of thermal runaway are investigated.The results show that the high convective heat transfer coefficient,low charge ratio,low ambient temperature and high discharge depth are easier to guarantee thermal safety of lithium battery.Secondly,the research on refrigerant emergency spray cooling in the overheating stage of lithium battery is carried out,and a three-dimensional spray cooling model was established to simulate the calculation process of refrigerant two-phase flow and heat transfer.The influences of different spray forms、spray distance、controlled temperature、types of refrigerant and battery materials are mainly studied.The average surface temperature,the whole average temperature,the thickness of liquid film,the coverage width of liquid film and the heat flux of heat transfer are analyzed.The results show that the spray form and the type of refrigerant have more significant effects on the cooling effect than the spray distance,initial battery temperature and battery material.Co MParing the three spray forms,it can be seen that the best cooling effect is the vertical spray in the middle of two sides of the battery.In the co MParative study of four refrigerant media found that R410 A has the highest latent heat of phase transition and the highest surface cooling dose,resulting in the optimal spray cooling capacity.Thirdly,the thermal safety control effect of spray cooling in different overheating stages of lithium battery is studied.The coupling calculation model of overheating behavior and spray cooling of lithium battery is established.The effects of three main controlling factors,spray pressure,spray time and number of spray orifice are studied.The results show that increasing spray pressure has the most significant efficiency in improving cooling capacity co MPared with other main controlling factors.In addition,the spray cooling effects of SEI film decomposition reaction,negative electrode and electrolyte reaction,positive electrode and electrolyte reaction and electrolyte decomposition reaction are studied.The results show that the cell decomposition reaction can be blocked at SEI film reaction stage by adopting the control strategy of two-sided and double-orifice spray、0.8MPa spray pressure and 3.6s spray time.In the reaction stage between the negative electrode and the electrolyte,the spray pressure should be increased to2.4MPa and the spray time should be extended to 5.4s to cool the battery to the safe temperature;In the reaction stage of positive electrode and electrolyte,the number of spray orifices should be increased to four double-sided spray orifice to achieve the emergency cooling goal.In the phase of electrolyte decomposition reaction,spray cooling can not effectively inhibit the rapid temperature rise of the battery,and finally thermal runaway occurs.Finally,the suppression method of refrigerant spray cooling for overheating propagation of batteries is studied.A small number of batteries in the battery pack are induced to enter the overheating stage by 7C high rate charging.The influences of the number of orifices,the location of orifices and the timing of spray are studied,as well as the characteristics of the controllable boundary of overheating propagation inside the battery pack.At the same time,the influence characteristics and heat propagation suppression effects of three typical trigger positions in the battery pack,namely inner edge trigger,middle trigger and superposition trigger,were studied.The temperature variation of each cell is recorded and analyzed to evaluate the effect of overheating propagation inhibition of battery spray cooling.The results show that adding spray orifices near adjacent cells can effectively improve the overheating propagation inhibition effect.The starting time of triggering the third stage reaction of the battery was defined as the controllable boundary of overheating propagation The initial design of the twohole spray strategy can effectively restrain the overheating propagation and thermal runaway in the cases of edge trigger,intermediate trigger and superimposed trigger of single and double cells.In the case of four-cell edge trigger,superheat propagation and thermal runaway can be effectively controlled by increasing to four spray orifices and spraying cooling 10 s in advance.