Preparation Of Quasi-thermoplastic Phase Change Polymer By Copolymerization And Its Application In Battery Thermal Management

As the key component of electric vehicle,the thermal safety of power battery pack has been bothering the academic and industrial fields.If the battery temperature out of the optimal range,or the temperature difference inside the module（ΔT）is over-large,a series of negative effects appear.Therefore,battery thermal management（BTM）system has been developed to control the temperature rise and temperature difference of battery modules.Among various BTM systems,the passive temperature control technology based on phase change material（PCM）cooling has been widely investigated because of its simple installation and excellent cooling performance.For engineering applications,PCM is usually blended with thermal conductivity additives and shaped-stabilized components in the form of polymer skeleton.However,in such physical-blended solid-liquid PCM,after long-term use,phase change components such as paraffin（PA）migrate gradually,resulting in the degradation of the cooling performance.Therefore,in order to solve the problems of PCM leakage and molding,a quasi-thermoplastic phase change polymer（CoPCP）is prepared by copolymerization and then applied to BTM systems.The obtained material possesses high latent heat,and the phase-changable alkyl side chains are firmly fixed by the rigid main chain through chemical bonds,which solves the problem of leakage.Furthermore,the linear polymer main chain endows the CoPCP with excellent flexible molding and secondary processing characteristics.The main research contents and conclusions of this thesis are as follows:（1）Three groups of quasi-thermoplastic phase change polymers（CoPCP/EG）are prepared by copolymerization of acrylates with different ester chain lengths with methyl methacrylate（MMA）.The experimental results show that CoPCP/EG prepared by copolymerization of octadecyl acrylate（OA）and MMA possesses a phase change temperature range of 46.52～53.15℃,suitable for the application of BTM systems.In addition,when the molar mass ratio of OA to MMA is 10:1,the latent heat and thermal conductivity of CoPCP reach the highest values.At the same time,the sample possesses excellent thermal stability and anti-leakage property,and maintains its shape stability even at a high temperature of 90℃.When the temperature is increased above 110℃,CoPCP turn soft and presents certain fluidity,and thus can be secondarily molded.（2）Through the investigation on the addition amount of EG in CoPCP/EG,it is found that when the EG proportion reaches 6%,the thermal conductivity can be increased by more than 5 times without significantly reducing the latent heat.Then,CoPCP/EG boards suitable for BTM specification are prepared with this formula,and used to assemble CoPCP-based battery module for BTM performance tests by comparing with the battery module with natural air cooling,SLPCM cooling and Po PCM cooling.The experimental results show that the CoPCP module has excellent temperature control performance and stability:during the 3C-3C charge-discharge tests for 15 cycles under a high temperature environment of 35℃,the battery module based on CoPCP cooling shows excellent anti-leakage performance,and its maximum temperature(T_max)can be stably controlled below 52.5℃.The T_maxof thermosetting Po PCM battery module and classical SLPCM battery module demonstrates a gradual increasing trend.（3）In order to solve the problem of heat storage saturation of CoPCP under extreme harsh working conditions,we assemble a CoPCP-L battery module by coupling the CoPCP module with an aluminum alloy liquid cooling plate.By investigating the temperature characteristics of the module during the charge-discharge test process and comparing them with those of the air cooling module and CoPCP module,it is found that the air cooling module is only suitable for low ambient temperature and low charge discharge rate.Although the battery module cooled by CoPCP has excellent temperature-control performance,the overall temperature couldn’t be always controlled within the optimal operating temperature of the battery.Enhancing the secondary heat dissipation capability of CoPCP by coupling with liquid cooling is effective to avoid heat absorption saturation of CoPCP,keeping the T_maxandΔT_maxbelow 50℃and 5℃,respectively.The T_maxandΔT_maxtends to be stable in the 7th cycle,which proves that the water cooling plates can not only allievate the temperature rise,but also efficiently transfer the heat away.