Performance And Structural Optimization Of Low-temperature Thermal Management System For Lithium-ion Batteries Based On Composite Phase Change Materials

With the rapid popularity of electric vehicles,lithium-ion batteries,the core component of electric vehicle power,have also been widely used.However,the high internal resistance and poor chemical activity of the battery caused by low temperature seriously affect the working performance and safety performance of the battery.From the perspective of thermal safety of lithium-ion battery,the research on battery thermal management system(BTMS)based on composite phase change material(CPCM)was carried out,including the low-temperature performance decay test of the lithium-ion battery,the preparation of high thermal conductivity and shape stability CPCM,and the application of CPCM to low-temperature battery thermal management.In the first part,to analyze the influence of environmental temperature and charge-discharge ratio on the battery performance,and elucidate the relationship between the battery operating conditions and the macroscopic electrochemical characteristics of the battery,a low temperature performance test platform for single lithium-ion battery was built in this paper.The results showed that as the ambient temperature decreased,the viscosity of the electrolyte inside the lithium-ion battery increased,the lithium-ion mobility and electrode reaction activity decreased,which led to a significant increase in the internal resistance and temperature of the battery during the discharge process,and the discharge capacity decreased.In addition,the experiments designed for long-term cycling of lithium-ion batteries at different charge/discharge rates in lowtemperature environments revealed that compared to small rate charging,large rate charging had a greater impact on the performance degradation of the battery during long-term cycling,while large rate discharging yielded the opposite conclusion.The disassembly and analysis of the battery at the end of the low-temperature cycle revealed that the negative electrode and separator surfaces were covered with a large amount of deposits,while the positive electrode showed no significant changes.By further analysis of the microstructural characteristics of the anode material,it was found that the large multiplier charging cycle in low temperature environment easily leads to the occurrence of lithium deposition.In the second part,to address the problems of low thermal conductivity and easy leakage of organic phase change materials,this paper prepared a highly thermally conductive and shapestable CPCM composed of paraffin(PA),olefin block copolymer(OBC)and expanded graphite(EG)with different particle sizes by melt blending method.The effects of different particle sizes and mass fractions of EG on the structure and properties of CPCM were analyzed by characterizing the microstructure,chemical structure,thermophysical properties,thermal stability,and heat transfer characteristics of the samples.The results showed that: the CPCM components were simply physically mixed and no chemical reaction occurred;the thermal conductivity of CPCM increased with the increase of EG content,while the thermal conductivity of CPCM increased significantly with the increase of EG particle size,which was attributed to the smaller interfacial thermal resistance of large size EG at the same EG content;due to the three-dimensional structure of OBC and the porous structure of EG limiting the PA flow,S4 with EG-50 content of 10wt% has the best stability,and the leakage rate does not exceed 1wt% under10 thermal cycles.In addition,the EG with large size particle size was found to have the best thermal conductivity through heat transfer characteristic experiments.In the third part,considering the operating characteristics of lithium-ion batteries in lowtemperature environments,this paper established a low-temperature BTMS heat transfer model based on CPCM according to heat transfer and computational fluid dynamics,and studied the influence of CPCM on the temperature change characteristics of battery under different lowtemperature conditions and different material thermal properties through simulation calculations.The results showed that CPCM can effectively prolong the time that the battery pack remains above 0℃ by releasing a large amount of latent heat through the phase change process at low temperatures;with the increase of convective heat transfer coefficient,the insulation effect provided by CPCM on the lithium-ion battery pack decreased,and at the same time aggravated the temperature inhomogeneity inside the low-temperature BTMS;the thermal conductivity of CPCM with the addition of 10wt% and 50 mesh EG was greatly improved to ensure that the maximum temperature difference of the battery pack during operation was kept within a safe range.In addition,the simulation calculation results showed that adding different thicknesses of insulation materials to the periphery of the low-temperature BTMS can effectively improve the insulation effect,but it was necessary to select the appropriate thickness of insulation materials in combination with the actual demand.