Study On Compound Lithium-ion Battery Thermal Management Technology Based On C₆F₁₂O

Lithium-ion batteries(LIBs)have been widely used in electric vehicles,energy storage power stations,and other fields for their advantages of cleanliness,high efficiency,and high energy density.The cycle life and safety of LIBs are closely related to the temperature.The low temperature will lead to the deterioration of the charge-discharge performance of batteries.In severe cases,it may cause structural damage to batteries by forming lithium dendrites.At high temperatures,the self-discharge of batteries is aggravated,and the thermal runaway(TR)may occur due to the internal side reaction,leading to serious fire and explosion accidents.Therefore,it is important to ensure the proper operating temperature of the battery(system)and prevent potential TR propagation for improving the safety of the battery system.In this study,a composite battery thermal management system(BTMS)based on liquid cooling technology of dodecafluoro-2-methylpentan-3-one(C6F12O)coupled with the "heat pipe-heat insulation plate-heat pipe" structure is developed.The BTMS has the function of temperature control and thermal runaway propagation suppression,and the switch between the functions is realized by a single-chip microcomputer(SCM).At the same time,the performance of the composite BTMS is systematically studied through the combination of experiment and simulation.Firstly,the heat generation model and heat transfer model of the battery is established,and the thermal performance of the battery and the heat pipe is tested experimentally.The internal resistance of the battery is basically unchanged in the discharge process with a constant current.When SOC is less than 10%,the internal resistance will increase significantly,resulting in an increase in heat generation during discharging.The rotation angle and length of the heat pipe have little influence on its heat dissipation performance,while adding forced convection cooling on the condenser can effectively improve it.On this basis,a new thermal management technology coupling heat pipes and liquid cooling is designed,and the thermal management efficiency of the combined thermal management technology and the traditional technology is compared and analyzed.During the charge-discharge cycles at a 3C rate,the maximum temperature(Tmax)and maximum temperature difference(⊿Tmax)of the battery pack under the condition without thermal management exceeded 70℃ and 11℃,respectively.After using the composite thermal management technology,Tmax and ⊿Tmax are both kept below 43℃ and 5℃ respectively at the discharge rate of 1C,2C and 3C,indicating that the composite thermal management technology has excellent heat dissipation performance.Next,the simulation model of the composite BTMS is established in COMSOL.The influence of the type and flow rates of the coolant on the heat dissipation performance of the system and the preheating performance of the system at low temperatures is studied.C6F12O has a small difference in heat dissipation performance from the traditional coolants,and it can be used in a wider range,not only to inhibit thermal runaway propagation as an extinguishing agent,but also to preheat the battery system at low temperatures.The increase in the flow rate has a limited effect on the cooling performance of the system.The greater the flow rate,the smaller the impact of further increase of the flow rate on the thermal performance.The composite BTMS can preheat batteries quickly at low temperatures.When the ambient temperature is-20 ℃,it only takes 200 s to heat the battery to-10℃ to ensure the capacity of the battery for low rate discharge,and the time required to heat the battery to 0℃ to meet the high rate discharge is less than 610 s.Finally,the TR propagation suppression performance of the heat insulation materials and the spray of C6F12O in the composite BTMS is studied experimentally.The passive mitigation performance of different thermal insulation materials on TR propagation is compared.The results show that the average time of TR propagation between adjacent cells without insulation is only 99.33 s.By inserting a 3 mm aerogel insulation plate between the cells,the time can be extended to more than 1200 s while maintaining the energy density of the battery pack.The influences of spraying height and flow rate of C6F12O on TR propagation suppression are analyzed.The height and flow rate of the spray are set at 5 cm and 1.6 g·s-1 respectively,which could effectively restrain the temperature of downstream batteries and prevent the waste caused by excessive flow rate.Based on the above,the combination of the temperature control function and the TR propagation suppression function of the BTMS is realized by SCM.The functions of temperature control during charge-discharge cycles,propagation mitigation and suppression during TR processes are realized.