Design Of Phase Change Materials With Ultra-high Stability For Battery Thermal Management And Their Application Investigation

The promotion of electric vehicle is a national strategy.However,as the core of electric vehicles,fire and explosion hazards of power battery module occur frequently.The main reason is that the working temperature of power battery cannot be effectively controlled:electrochemical reaction is accompanied by a large amount of heat during long-term and/or high-power charging/discharging process.If the heat is accumulated,it will cause a series of side reactions such as electrolyte and seperator decomposition.At the same time,more heat and gas will be released while performance is decayed,and a large amount of gas will be generated.The sudden rise of the pressure and temperature eventually leads to the thermal runaway of the battery.To solve this problem fundamentally,it is of great significance to design an advanced thermal management system for power battery,and realize the effective control of temperature rise and temperature distribution of power battery module.At present,the commonly used battery thermal management system mainly includes air cooling and liquid cooling strategy.Air cooling strategy possesses the defect of low heat dissipation efficiency.While a complicated layout of pipelines is necessary for liquid cooling strtegy,which leads to leakage risk of the conductive heat transfer medium.In recent years,phase change material(PCM)cooling technology is regarded as the next generation battery thermal management technology because of its simple and compact structure,good temperature control and temperature uniformity capability.The bottleneck of PCM cooling technology mainly lies in the poor thermal stability of PCM,including the leakage issue of phase change components,and the relatively low shape stability tending to collapse after endothermic liquefaction.In order to improve the thermal stability of PCM,the traditional strategy focused on physical modification methods,i.e.,adding shape-stabilizing or adsorption additives to restrict the flow of phase change molecular segments.Although the physical modification method of adding shape-stabilizing or adsorption additives can adsorb the liquefied phase change components to a certain extent,gradually migration of the liquid phase change ingredients is still inevitable under the influence of gravity and inertial force.The above physical modification methods can not perfectly solve the problems of phase change components migration/leakage.Therefore,based on the chemical modification method of binding phase change molecular chain segment through chemical bond,and combined with the thermal physical property requirements of PCM for battery thermal management,this paper successfully developed a kind of solid-solid PCM with high thermal stability for battery thermal management,which completely solved the shortcomings of PCM battery module,such as low stability,severe leakage/large volume expansion and so on.This paper will provide alternative possibility for the realization of industrial application for PCM battery thermal management technology.The main contents of this paper are as follows:(1)Phase change polymer(PoPCM)with three-dimensional cross-linked main chain+aliphatic alkyl side chain was designed and successfully prepared by free radical polymerization.The as-prepared PoPCM showed intrinsic solid-solid phase change behaviors with a phase transition temperature of around 50?,which was suitable for battery thermal management.The best crosslinking agent among divinyl benzene,1,6-hexanediol diacrylate and diallyl terephthalate was confirmed to be 1,6-hexanediol diacrylate;the optimum molar ratio of monomer:crosslinking agent was 1:40,under which the latent heat of PoPCM reached the maximum value(98.8 J g-1).All of the samples showed excellent shape stability at 150?,much higher than the phase transition point,and no phase change component leakage or even shape collapse occurs during the experiment.(2)Theoretically,it is also feasible to use PoPCM as the adsorption polymer skeleton of phase change paraffin(PA):the three-dimensional cross-linked thermosetting main chain will endow the prepared PCM excellent shape stability at high temperature,while the alkyl side-chain can ensure the high PA adsorption capacity between the skeletons through intermolecular dispersion force,so as to obtain products with excellent leakage resistance;Moreover,the aliphatic alkyl side chain can provide additional latent heat.Herein,PoPCM skeleton was grown in-situ in PA slurry through sol-gel method.The as-obtained product with 8wt%EG exhibited high latent heat of 120.3 J g-1 and thermal conductivity of 2.92 W m-1 k-1,and then was assembled into a 21 V 16 Ah battery module.Compared with PCM module based on traditional polymer skeleton,the novel module showed excellent reliability and temperature control performance under extremely harsh working conditions.In the 50 cycles of 3C-4C charging/discharging,the novel PCM module achieved thermal stability after the 6th cycle,and the maximum temperature(Tmax)and temperature difference(?Tmax)can be controlled below 50.9? and 5.0? respectively.By stark contrast,the Tmax and ?Tmax of PCM module based on traditional polymer skeleton increased gradually due to the serious PA leakage during the whole cycle.(4)The phase transition temperature of the material is about 50?,suitable for the application of battery temperature control.However,the narrow phase transition temperature range limits the application of such material in other fields.Therefore,we further proposed a phase transition temperature regulation strategy for PoPCM-x(where x refers to the number of C atoms in the aliphatic side chain)from 9.7 to 65.3? by altering the number of C atoms in the aliphatic side chain.Wider phase change temperature range broadens its applications in building energy conservation,battery thermal management and waste heat recovery.In addition,this kind of material has excellent mold properties,and can be molded into tubes,bricks or modules according to application purposes.Because of the controllable phase transition temperature range,sufficient latent heat and excellent thermal stability,the as-prepared materials present huge potential in various temperature control and thermal storage applications.