Investigation On Thermally Induced Flexible Composite Phase Change Materials In Battery Thermal Management System

With the increasingly serious shortage of fossil energy and the realization of the"dual carbon"goal,the technology of electric vehicles（EVs）is booming.However,thermal abuse accidents of EVs occur frequently.Therefore,it is of great significance to explore an effective thermal management system for battery module.In this thesis,a novel thermally flexible composite phase change material（TF-CPCM）based on styrene-butadiene-styrene block copolymer（SBS）and paraffin（PA）has been proposed to solve the thermal safety and vibration safety problems faced by electric vehicle battery system in practical application.Meanwhile,the properties of CPCM are optimized by using expanded graphite（EG）,Thermoplastic Ester Elastomer（TPEE）and Ethylene-Propylene-Diene Monomer（EPDM）considering the drawbacks of composite phase change materials such as poor thermal conductivity,insufficient flexibility and easy leakage,respectively.The mechanism of different additive components in TF-CPCM has been analyzed by combining the macroscopic morphological changes and microstructure.In addition,the battery thermal management system based on TF-CPCM was successfully built,and compared with the traditional rigid PCM battery thermal management system under vibration condition.The temperature control,temperature equalizing ability and shock mitigation performance of thermally flexible composite phase change material in battery module are also analyzed.The main research contents and conclusions of this thesis are shown as follows:1.The SBS@PA/EG CPCM is prepared by dissolving and volatilizing organic solvent（CCl₄）,and comprehensively analyzed by macroscopic mechanical properties and microstructure.The experimental results show that:SBS can coat PA/EG powder well and effectively limit the leakage of liquid PA.The mass retention rate of SBS@PA/EG remains at99%after 5 h continuous heating.With the increase of mass proportion of SBS,the flexibility of CPCM is improved.The tensile strength of 2:1 CPCM is between 0.1 MPa and 0.25 MPa,and the bending strength is between 0.2 MPa and 0.3 MPa.2.EG with porous structure,is used as the heat conduction enhancing filler.It not only has a positive influence on the shape stability of the composite phase change material,but also significantly improves the thermal conductivity of the phase change material.The experimental results show that the thermal conductivity of SBS@PA/EG（4%）reaches 0.88W·m^-1·K^-1.At the same time,SBS@PA/EG based battery thermal management system can effectively control the temperature rise inside the battery module.At the discharge rate of 5 C,the maximum temperature of the battery is only 46℃,while the temperature difference between the battery modules can be kept below 4℃.3.TPEE has longer continuous phase and is more flexible than SBS at room temperature,effectively improving the flexibility of CPCM.When the mass fraction of TPEE was 5 wt%,the quality maintenance rate of CPCM increased to 96.8%,and the tensile fracture value increased from 130%to 175%.At the same time,the flexible TPEE can be wound between SBS and PA to form a more stable structure and inhibit PA leakage.Experimental and numerical simulation results show that the maximum temperature of TPEE-SBS/EG/PA battery module is only 66.4℃at the discharge rate of 3 C,and it can still remain at about 66℃in 10 charge-discharge cycles without heat accumulation phenomenon.4.EPDM can effectively increase the cross-linking of SBS,forming a flexible and compact support skeleton,and effectively improve the leakage resistance of CPCM.SBS/EPDM/PA/EG prepared with 3 wt%EPDM and 5 wt%EG has a distortion angle up to900° at 60℃and still does not break.EPDM/SBS support skeleton of TF-CPCM can effectively absorb external shocks,and the maximum temperature fluctuation of TF-CPCM battery module is less than 2℃under the vibration condition of 20 Hz frequency.