| Lithium-ion battery featuring high energy density,high working voltage and long cycle life,is a main power source for the industry of new energy automobiles.However,it releases mass of heat during operation,which might result in a serious attenuation in the effective capacity of the battery with the sharp rise in the temperature of the battery and the exacerbated secondary reactions of the electrodes.What's worse,it might also affect the overall working performance of the battery pack due to widened temperature difference and the exacerbated temperature non-uniformity between batteries in the pack.Hence,reasonable thermal management means should be adopted to timely remove the heat generated during battery operation to ensure that the battery temperature and the temperature difference between batteries are maintained in an appropriate range.Phase change material(PCM),as a medium that can store thermal energy,can either absorb or release large amounts of latent heat to maintain nearly constant temperature during phase change.Thus,applying it to battery thermal management can implement effective temperature control.Moreover,PCM requiring no additional power source for its thermal management is a favorable thermal management means with good shape adaptability and low cost.This dissertation is designated to develop insulated shape-stabilized composite PCM with high thermal conductivity for applying in the battery thermal management systems,and study its temperature control characteristics to provide evidence for practical application,so as to promote the application of composite PCM in the field of battery thermal management.To begin with,paraffin wax(OP44E)with a phase change temperature of 44 ? was selected and adsorbed into the SEBS powder for evenly mixing.By adding different proportions of BN powder,shape-stabilized BN/OP44E/SEBS composite PCM was then prepared through hot pressing with a press vulcanizer.On this basis,its flexibility,thermal properties and electrical conductivity were explored by means of experiments.According to the findings,when the maximum adsorption capacity of paraffin wax in the composite PCM is about 65 wt%,and the maximum additive amount of BN is 20 wt%,the shape-stabilized composite PCM obtained has satisfactory thermal conductivity and thermal reliability without no liquid phase leakage.Besides,melting phase change temperatures and enthalpy values of the three prepared shapestabilized composite PCM(incl.65%OP44E/35%SEBS,15%BN/65%OP44E/20%SEBS,and20%BN/60%OP44E/20%SEBS)are 37.8 ?,38.1 ? and 41.1 ?,as well as 133.1 J/g,136.7J/g and 127.8 J/g,respectively.The infrared analysis and XRD test showed that the three materials are composited in physical processes rather than chemical reaction.And thermal conductivities of the three shape-stabilized composite PCMs are 0.8601 W/(m·K),2.0043W/(m·K)and 2.8365 W/(m·K),respectively.Apparently,the thermal conductivity of the shapestabilized composite PCM is increased with the increased BN content.Also,when the temperature is higher than the phase change temperature of paraffin wax,the composite PCM featuring favorable softening property can fit well with the battery surface.What's more,the three composite PCMs present satisfactory insulation property with volume resistivities of18593.75 ?·m,20,805 ?·m,and 20106 ?·m,respectively.In addition,a passive battery pack thermal management experimental system was also built to study the temperature control performance of the prepared shape-stabilized composite PCM in the battery thermal management system.Specifically,cylindrical and square batteries were selected to conduct charging and discharging at different rates with the battery charge and discharge device.In this way,the surface temperatures of the battery under different rates were tested to obtain the maximum temperature of the battery pack and calculate the maximum temperature difference of the battery pack,so that the temperature control performances of the shape-stabilized composite PCM in the passive thermal management of battery packs with different structures were studied.According to the results,when the cylindrical battery is discharged at 1 C,the shape-stabilized composite PCM containing 20 wt% BN will lower the highest temperature and the maximum temperature difference of the battery pack by 23% and90%,respectively;when the battery is discharged at 1.5 C,the highest temperature and the maximum temperature difference will be decreased by 30% and 55%,respectively.When a square battery is discharged at 2 C,the shape-stabilized composite PCM containing 20 wt% BN can lower the highest temperature and the maximum temperature difference of the battery pack by 20% and 30%,respectively,proving that the composite PCM possesses a favorable temperature control effect. |
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