【作者】 王燕；

【导师】 王碧佳；

【作者基本信息】 东华大学 ， 纺织化学与染整工程， 2022， 硕士

【摘要】 日益严重的能源短缺和传统汽车造成的环境污染是困扰当代社会和政府的主要问题,电动汽车以及混合动力汽车的发展可以有效缓解这些日益增加的压力。作为常用电源之一,锂离子电池因其高能量密度、高能源效率以及长循环寿命被广泛应用于电动汽车行业,但频繁的火灾和爆炸限制了其进一步和更广泛的应用。锂离子电池出现安全事故的根本原因是其自身的高度热敏感性,因此需要对其进行热管理,以避免发生热失控并同时保证其工作性能。相变材料可以在其相变过程中可逆地存储和释放来自周围环境中的大量热能,已被广泛用于锂离子电池的外部热管理,合适的相变温度、高潜热和充足的热导率是对相变材料的基本选择要求。人们致力于解决不同相变材料的内在缺点,例如水合盐的腐蚀性、过冷、相分离,石蜡的高成本和高可燃性,以及所有固-液相变材料普遍存在的泄漏及低热导率问题,以使之适用于锂离子电池的热量管理。本文以水合盐和石蜡作为杂化相变材料,采取两种不同的形状稳定策略,即聚合物基质定形法和多孔材料定形法,并引入碳基材料进行导热增强,制备了两种新型高导热复合相变材料,在锂离子电池及其它高储能设备热管理领域中展现出可观潜力:（1）在第二章的研究中,选用具有合适熔点及高潜热的十二水合磷酸氢二钠（Na₂HPO₄·12H₂O,DHPD）和正二十二烷作为相变材料,分别向其中加入可聚合单体,通过油包水乳液模板法将两相结合,然后向此体系中加入不同比例的石墨烯纳米片（GNFs）,热引发聚合便制得导热相变凝胶（Emulgel@GNFs）。Emulgel@GNFs表现出优异的形状稳定性、较高的相变焓值（171.1-190.1 J/g）以及良好的热循环性能（在200次加热-冷却后焓值下降小于5%）。凝胶中石蜡对水合盐的包封明显提高了其热稳定性,并有效缓解了水合盐的过冷。与不含GNFs的Emulgel相比,Emulgel@GNFs的导热性能大大提升,在GNFs用量为15wt%时,导热率达到2.74 W m^-1 K^-1,提升率约为490%。得益于聚合物网络、相变材料及GNFs的协同作用,Emulgel@GNFs在光/热刺激下表现出形状记忆功能。Emulgel@GNFs还具有优异的阻燃性能,在至少100 s内可保持不燃。选用含10 wt%GNFs的导热相变凝胶进行了锂离子电池降温实验,结果表明在高倍率放电实验中电池仍能保持适宜的工作温度。（2）在第三章的研究中,用水合盐、石蜡和膨胀石墨（EG）制备了高储能密度、高导热、形状稳定且阻燃的复合相变材料,实现了低成本和快捷的锂离子电池热管理材料的制备。以3:1的水合盐-石蜡质量比,将DHPD、水溶性单体（丙烯酸钠）乳化进正二十二烷中,然后通过丙烯酸钠的聚合增强DHPD内相;通过共混法将获得的水合盐/石蜡杂化相变材料（HPC）吸附进多孔支撑材料EG中,并进行机械压缩。由此产生的复合相变材料（HPC@EG）表现出良好的形状稳定性和增强的热导率(1.53-4.35 W m^-1 K^-1)。HPC@EG还具有高潜热（196.6 J/g）,在至少500次加热/冷却循环中保持稳定的热存储性能。正二十二烷的油封作用有效地限制了DHPD中结晶水的流失,使得HPC@EG的热稳定性增强。此外,使用水合盐和EG的一个额外优势是其固有的阻燃性,这也被HPC@EG复合材料所继承。正如我们所证实的那样,HPC@EG可以有效防止锂离子电池在高倍率放电下发生过热,从价格、安全性和冷却性能方面来说是用于电池及其它高储能设备热管理的理想选择。更多还原

【Abstract】 Increasing energy shortage and environmental pollution caused by conventional vehicles are major issues plaguing contemporary society and governments,and the development of electric vehicles as well as hybrid vehicles can effectively alleviate these pressures.As one of the commonly used power sources,lithium-ion batteries are widely used in electric vehicles for their high energy density,high energy efficiency,and long cycle life.However,their susceptibility to fires and explosions have limited their further and wider application.The fundamental cause of safety issues in Li-ion batteries is their intrinsic high thermal sensitivity,so establishing a thermal management system is necessary to avoid thermal runaway and ensure their working performance.Phase change materials(PCMs),which can reversibly store and release large amounts of latent heat from the surrounding during their phase transitions,have been widely used for external thermal management of Li-ion batteries.Suitable phase change temperature,high latent heat and sufficient thermal conductivity are the basic requirements for PCMs applicable in Li-ion batteries.Efforts have been made to address the inherent drawbacks of different phase change materials,such as the corrosion,supercooling,and phase separation of hydrated salts,the high cost and high flammability of paraffins,and the leakage and low thermal conductivity problems common to all solid-liquid phase change materials,to make them suitable for the thermal management of lithium-ion batteries.In this paper,we prepared two composite PCMs with enhanced thermal conductivity by employing hydrated salts and paraffins as hybrid PCMs and using two different shape stabilization strategies,i.e.,polymer matrix shape stabilization and porous material shape stabilization strategy.The former strategy relied on adding carbonaceous nanofillers for thermal conductivity enhancement,the latter adopted a highly thermally conductive support.The resulting composite PCMs showed considerable potential in the field of thermal management of lithium-ion battery and other high energy storage devices.(1)In Chapter 2,disodium hydrogen phosphate dodecahydrate(Na2HPO4·12H2O,DHPD)and n-docosane were selected for their suitable melting point and high latent heat,to which polymerizable monomers were added separately.The two were combined by the water-in-oil emulsion templating method.Different proportions of graphene nanosheets(GNFs)were added to the emulsion,and the thermally conductive phase change gels(Emulgel@GNFs)were produced by thermally initiated polymerization.The resulting Emulgel@GNFs exhibited excellent shape stability,high enthalpy of phase change(171.1-190.1 J/g)and good thermal cycling performance(less than 5%decrease in enthalpy after 200 heating and cooling cycles).The encapsulation of the hydrated salt by paraffin significantly improved its thermal stability and effectively alleviated the supercooling of the hydrated salt.Compared with the control Emulgel free of GNFs,the thermal conductivity of Emulgel@GNFs was greatly enhanced by up to 490%at GNFs loading of 15 wt%,reaching up to 2.74 W m-1 K-1.Thanks to the synergistic effect of polymer network,PCMs and GNFs,Emulgel@GNFs showed shape memory properties initiated by photo/thermal stimulation.It also exhibited excellent fire-resistant properties and could remain noncombustible for at least 100s.Emulgel@GNFs-10 was used as an example to demonstrate the feasiblity in lithium-ion battery cooling.The battery maintained stable operation in the appropriate temperature range under the high rates discharge experiments.(2)In Chapter 3,a low-cost composite PCMs with high thermal conductivity,shape stability,flame retardancy and high energy storage density was prepared with hydrated salt,paraffin wax and expanded graphite(EG).DHPD and water-soluble monomer(sodium acrylate)were emulsified into n-docosane at a hydrated salt/paraffin weight ratio of 3:1,and then the internal phase of DHPD was enhanced by polymerization of sodium acrylate.Finally,the obtained hydrated salt/paraffin hybrid phase change material(HPC)was adsorbed into the porous supporting EG by a blending method and mechanically compressed.The resulting composite phase change material(HPC@EG)exhibited good shape stability and enhanced thermal conductivity(1.53-4.35 W m-1 K-1).HPC@EG also exhibited high latent heat(196.6 J/g)with sustained thermal storage performance over at least500 heating/cooling cycles.The oil-sealing effect of n-docosane effectively limited the loss of bound water in DHPD,resulting in enhanced thermal stability of HPC@EG.In the meanwhile,an additional advantage of using hydrated salts and EG was their inherent flame retardancy,which was also inherited by the HPC@EG composite.HPC@EG was effective in preventing overheating of lithium-ion batteries at high rates of discharge as we demonstrated,showing great promise in Li-ion battery thermal management in respect of price,safety and the cooling performance.更多还原