Investigation On Flow Boiling-based LiFePO₄ Battery Cooling And Thermal Safety

Lithium-ion battery is the key component of electric vehicle and battery-based energy storage system.The operating performance of electric vehicle and mechano-electronic device are closely related to the state of lithium-ion battery.Higher heat generation rate during discharging process results in overheat which affects the performance and lifetime of battery significantly.Also,thermal runaway may be triggered under abuse conditions and lead to serious accidents eventually.Based on battery thermal management system(BTMS),performance improvement,electrochemical-thermal coupled model and thermal runaway behavior of lithium-ion battery were focused comprehensively from the view of thermal safety in this study.Firstly,the thermal and electrochemical behaviors of battery under different C-rates and operating temperatures were experimentally researched.Then,a battery cooling system based on flow boiling heat transfer of low-boiling point dielectric under atmosphere was built and the performance was analyzed.It was found this cooling method could effectively control the temperature of battery around at 40? when boiling occurred and it was also of efficiency on improving the temperature uniformity on battery surface.However,there was an interplay between temperature and voltage:the intensification of cooling performance would decrease output voltage in battery module.Heat generation rate was unsteady in discharging process.Therefore,there existed an appropriate coolant mass flow rate range during which the mode of heat transfer was boiling heat transfer to ensured the highest temperature of battery within the safe range and temperature difference could also be decreased simultaneously.The three dimensional BTMS model conjugating heat transfer and fluid flow was simplified to two dimensional steady model with convectional boundary condition based on the thermal conductivity anisotropy in battery.Then,the effects of aspect ratio,mass flow rate,width of tube and contact resistance between battery and tubes on performance of BTMS were analyzed.Meanwhile,the relationship between cooling performance and energy density of battery module was studied.A scheme to improve the performance of BTMS was presented from the views of cooling structure design of cell and module.The research on electrochemical reaction mechanism,heat generation behavior and electrochemical-thermal coupled characteristic in lithium-ion battery is the key to BTMS design.One dimentional electrochemical-thermal coupled model was developed and the effects of C-rate and operating temperature on performance of cell were analyzed:the higher C-rate or lower temperature would aggravate the distribution uniformity of electrochemical reaction rate in the electrode,meanwhile,the heat generation rate and the proportion of different mechanism heat were also affected.The higher thermal response,dramatic Li+intercalation/de-intercalation intensity and uneven SOC distribution in the thicker electrode would accelerate the fading rate of output voltage and capacity of the battery with thicker electrode compared with the battery with thinner electrode.The effect of outside thermal management condition on the electrochemical reaction in the battery was also analyzed:there was a higher temperature gradient in the battery when the battery cooled outside due to the thermal conductivity anisotropy in battery,which would result in the unbalance discharging behavior between the different cells in the battery.This temperature gradient would increase when outside cooling effect was enhanced.Then,a two dimensional electrochemical-thermal coupled model was also developed for a prismatic battery considering the tab effect and the layer to layer structure in the cross-plane direction.It was demonstrated that the potential and current density distribution in the cross-plane were uneven and this was dominated by the geometrical factor.There was a hotspot in the electrode material region close to tab because of the current accumulation in the tab.At the same time,this thermal effect would aggravate in the applications due to the higher contact electric resistance and small contact area between the tab and connector.An analytical thermal runaway model was developed considering the safety of battery under extreme condition based on MATLAB code.The effect of thermal transport characteristic within battery on thermal runaway behavior was analyzed and the result showed that the thermal safety was determined not only by the utside cooling condition,and the thermal resistance in cross plane was also a crucial factors determining the thermal safety of battery.The cooling method based on boiling heat transfer in mini-channels was viable to prevent the propagation of thermal runaway in the pack.However,it depended on the thermal transport characteristic in battery whether thermal runaway can be prevented within a single battery.