基于三维石墨烯的锂硫电池正极材料和器件研究

Rechargeable lithium batteries have become one of the dominant portable rechargeable power sources due to their high volume and gravimetric energy density. However, the lower speci?c capacities of cathode materials compared to those of the anode have been a limiting factor to the energy density. Elemental sulfur is a promising cathode material for the next generation of high-specific-energy rechargeable lithium batteries due to its high theoretical specific capacity. However, Li/S batteries su?er from poor cyclability, which is mainly attributed to the dissolution of intermediate lithium polysul?de products, volumetric expansion and the poor conductivity of sulfur and polysul?de species. In order to address these challenges, porous carbon-based materials, especially graphene, have been used as cathode materials and have shown promising performance due to their obvious advantages of high conductivity and large surface area. Thus, in this project, 3D chemically bonded graphene foams with high electrical conductivity, large surface area and abundant 3D channels will be prepared and used as cathode materials after loading S. Self-assembly and chemically bonded graphene into macroscopic materials can translate the intriguing properties of graphene into the resulted macrostructures required for practical applications. When used as cathode materials of Li-S battery, the high surface area and 3D pore structure can achieve high loading and dispersion of sulfur, also the latter can effectively confine polysulfides from dissolving. The combination of the porous structure and excellent intrinsic properties of 3D graphene materials thus could provide fast mass and electron transport. To further improve the cycling performance of the Li-S battery, solid electrolyte will be applied to hinder the dissolution of intermediate lithium polysul?de products.

锂硫电池具有很高的理论比容量和比能量，是最具应用前景的锂离子电池之一。但锂硫电池发展至今仍存在比容量远低于理论值、循环性能较差的问题，主要原因是硫及其放电产物的低导电性、聚硫化物在电解液中的大量溶解流失以及相应的"穿梭效应"。本课题的主要研究目标就是设计合成具有良好的导电性、高的比表面、以及丰富孔洞结构的三维石墨烯材料，并以此材料作为支撑体负载硫，制备出具有高活性和高循环稳定性的三维石墨烯基锂硫电池正极材料。三维石墨烯材料高的比表面积和丰富的孔洞结构可以实现硫的高负载和高分散，丰富的三维孔洞结构可以起着很好的限域效果，有效地减少聚硫化物的扩散、流失。另外，我们还可以通过在三维石墨烯材料的表面负载一层多孔碳，构建一种三维石墨烯的大孔嵌套多孔碳的小孔的新颖孔结构，发辉大孔和小孔的协同效应，进一步抑制聚硫化物的扩散、流失。

随着环境问题的日益严峻以及电子电动设备的迅猛发展，设计和开发高效的储能设备势在必行。锂硫电池因具有高能量密度、髙理论容量、硫资源丰富、价格低廉以及环境友好等特点，被认为是下一代最有前景的能量存储系统之一。然而，由于硫放电产物硫化锂充放电过程中的体积效应以及溶解等问题，致使硫正极利用率低、电池容量衰减快、循环性能差，严重阻碍了锂硫电池的商业化进程。　　针对这些问题，我们设计并合成了一系列具有良好的导电性、高的比表面、以及丰富孔洞结构的三维石墨烯基复合材料，研究了材料的结构和性能；以三维石墨烯基复合材料为支撑体负载硫，制备出具有高活性和高循环稳定性的三维石墨烯基锂硫电池正极材料。其中，基于C@GO@MoS2@S正极材料、碳化巴沙木@S正极材料、碳化泥炭藓@CoNi2S4@S正极材料的锂硫电池的比容量分别达到了1450 mAh/g 、1120 mAh/g和1623 mAh/g ；基于C@GO@MoS2@S硫正极材料的锂硫电池循环后500次容量仍能保持600 mA g-1；同时我们还研究了三维石墨烯基复合材料的结构与其电化学性能间的构效关系，以C@GO@MoS2@S硫正极材料为例：介孔碳材料提供了合适的孔径（2-5nm）和大的比表面积(3000m2/g)，为硫的均匀沉积和负载提供了条件，石墨烯作为导电骨架可以改善硫正极不导电的问题，同时原位水热的方法生成的MoS2可以对多硫化锂产生有效的化学吸附作用，从而改善锂硫电池的循环性能。上述研究结果为石墨烯材料在锂硫电池中的应用打下了基础。