Experimental Study On Thermal Performance And Thermal Safety Of Retired Lithium Ion Power Battery

New energy vehicles have been recognized and promoted all over the world.Considering environmental protection factors and lithium resource constraints,the decommissioning period of power batteries is coming,and the recycling and reuse of decommissioned batteries is a necessary work.Taking the retired ternary NCM batteries as the research object,this paper explores the charge and discharge characteristics of retired single batteries with different attenuation degrees from different angles,and studies the electrothermal characteristics of batteries with similar consistency after grouping;The influence of attenuation degree on thermal safety is explored under different abuse modes,which lays a theoretical foundation for the cascade utilization and recycling of retired batteries.Firstly,a series of pre-treatment is carried out for retired lithium-ion batteries to lay the foundation for thermal characteristics and thermal runaway experiment.The pre-treatment mainly includes five parts: appearance inspection and initial voltage detection,initial capacity detection,HPPC test,entropy heat coefficient test and capacity increment curve test,In order to explore the thermal characteristics and thermal safety change characteristics of retired power lithium-ion batteries with different attenuation degrees,a comprehensive experimental design was carried out.In terms of capacity increment,the performance attenuation of retired power lithium-ion batteries is analyzed.The capacity increment curve is composed of four obvious peaks,corresponding to four voltage ranges respectively.Within the voltage range of 3.60V～3.70 V,the battery capacity attenuation accounts for the largest proportion of the total capacity loss,about 49.3%.Calculate enthalpy change and entropy change,and analyze the heat production during operation.For batteries with low attenuation,the heat production power at the initial stage of discharge is smaller than that at the end of discharge.The heat production power at the end of discharge(80% ～ 100% Do D)is significantly higher than that in other DOD states.The heat production power of retired batteries with high attenuation is almost greater than that of batteries with low attenuation at the end of discharge,which is the most obvious at100% DOD.The variation law of characteristic parameters of decommissioned power batteries is explored and analyzed from the aspects of magnification characteristics,environmental temperature adaptability and state of charge characteristics.It is found that the retired single battery with higher attenuation is more obviously affected by charge discharge ratio,state of charge and ambient temperature,and the terminal voltage,temperature,capacity and energy characteristics are worse,especially on the battery temperature.With the increase of attenuation degree,the temperature rise during charge and discharge under different working conditions increases continuously.In addition,in order to further explore the thermal characteristics of retired batteries when used in groups of steps,three retired batteries with strong consistency of electrothermal performance were selected.The results show that the consistency of retired batteries No.1,No.2 and No.3 is the strongest.In terms of magnification characteristics and environmental temperature adaptability,the variation law of characteristic parameters of series module and parallel module of retired power battery is explored and analyzed.Analyze the charging and discharging performance of series module and parallel module respectively,explore the performance characteristics of retired power battery series module and parallel module,and judge the application scope of the module in the process of echelon utilization.Considering the changes of battery terminal voltage,temperature,capacity and energy,the optimal charging rate of the retired battery module is 0.5～1C.Considering the temperature rise of the battery during charging at different ambient temperatures,the temperature change trend of the central battery of the series and parallel modules of the retired battery is roughly the same as that of the monomer.The lower the ambient temperature,the smaller the temperature rise and the smaller the temperature change rate.Without any thermal management measures,the most suitable charging ambient temperature range of the retired battery module is 25～30 ℃.If thermal management measures are supplemented,the ambient temperature range during charging can be appropriately expanded.In addition,the retired battery module is not suitable for use at low temperature.In terms of thermal safety law,the safety of retired batteries used or recovered in echelon has not been known in the process of utilization or treatment.In order to understand the thermal safety law of retired batteries,analyze the thermal runaway parameters of retired batteries under different abuse conditions,and explore the thermal safety law of retired single batteries with different attenuation degrees,so as to provide a certain theoretical basis for cascade utilization and recovery.It is found that the higher the attenuation degree of retired batteries,the lower the self exothermic temperature point,the lower the initial thermal runaway temperature point,and the higher the maximum temperature of thermal runaway.For batteries with voltage failure,the self exothermic temperature point,initial thermal runaway temperature point and maximum thermal runaway temperature point are obviously lower than those of normal batteries.Attention should also be paid to safety in the recycling process.Compared with the retired batteries that can be reused,the higher the attenuation degree,the more heat produced by the retired single battery in the three abuse processes,and the lower the thermal safety.For retired batteries with voltage failure,the recovery process inevitably has the phenomenon of heat out of control,and the heat production can not be underestimated.It is also necessary to pay attention to safety issues.