新型纳米硅/硅金属合金/金属/石墨烯气凝胶锂离子电池复合负极材料的微结构调控与电化学研究

The main challenges for the practical implementation of Si anodes in lithium-ion batteries are the huge volume variation (>≈300%) and low electronic conductivity (≈10-3 S cm-1). Several strategies were utilized to resolve these issues, including 1) the nanocrystallization of Si, 2) alloying with conducted metal, 3) combining with graphene aerogel and so on. In this project, these three strategies will be presented in one novel system to enhance the performance of silicon anode by synergistic effect. Nano-Si/Si-metal alloy/metal/graphene aerogel composite (n-Si/MSi/M/GA), will be synthesized by solvothermal method combining with a high temperature reduction process. Firstly, the surface of Si nanoparticles will be modified by metal oxide to make sure the particles can disperse uniformly on the surface of graphene oxide through electrostatic interactions. Then, in situ reaction occuring between the metal oxide and graphene oxide (or graphene) during solvothermal processs may facilitate attractive interfacial interactions between particles and graphene,resulting in the well wrapping and homogeneous distribution of Si-based nanoparticles into the three dimensional graphene matrix. Finally, a high thermal reduction process will be carried to reduce metal oxide into metal and form Si-metal alloy phase, which may play the role of inert matrix and conductive adhesive between Si and graphene. Microstructure controllability of nano-Si/Si-metal alloy/metal/graphene aerogel and the structure-performance relationship, especially the effects of interfacial and synergistic effect on electrochemical performance will be investigated. Furthermore, the mechanism on reversible lithium storage will be studied as well. The implementation of this project will further promote the development of silicon based anode materials for lithium ion batteries.

硅体积效应严重和电导率低等问题是制约其商业化应用的主要瓶颈。硅纳米化，在硅负极添加金属并形成硅金属合金以及将硅引入石墨烯气凝胶等都是抑制其体积变化、提高电导率的有效途径。本项目拟将上述几种方式集结于一体，合成一种新材料：纳米硅/硅金属合金/金属/石墨烯气凝胶复合材料（n-Si/MSi/M/GA），有望通过各组分间协同效应，使硅负极瓶颈有所突破，得到理想的电化学性能。为了获得结构特性优异的复合材料，本项目以金属氧化物对纳米硅进行包覆修饰，在硅基颗粒和氧化石墨烯以及石墨烯间建立有效相互作用，提高硅基颗粒在石墨烯中的分散性并形成紧密的石墨烯包覆结构，而后经高温处理使金属基转变为更利于结构稳定性和导电性的金属及硅金属合金相，制得目标产物。优化制备条件，调控微观结构，研究结构和电化学性能之间的“构-效”关系，研究可逆储锂机理，总结影响电化学性能的因素和规律，为进一步开发高性能硅基负极提供依据。

将硅基颗粒引入石墨烯中，利用石墨烯的导电性和柔性包覆结构，可有效改善硅基材料体积效应严重和电导率低等缺陷。但是，硅基颗粒存在表面能高，易团聚、难分散，与石墨烯结合力弱等问题，为制备硅基/石墨烯复合材料增加了难度。为了克服这些问题，获得高分散且具有石墨烯包覆结构的复合材料，本项目提出多种解决方案，并对不同方案下所制得的硅基/石墨烯复合材料的微观结构和电化学性能进行了研究：1）利用冷冻干燥与溶剂热相结合方法制得“豌豆状”复合材料，纳米硅颗粒被包覆于相互交联的石墨烯“豌豆荚”中，使其导电性和体积效应都得以改善，因此在高倍率下获得稳定的循环性能。2）以自制纳米SiO2为原料，前期混入镁粉后，利用冷冻干燥与镁热还原相结合的方式制得均一性好、分散性高，且具有多孔结构的纳米硅基/石墨烯气凝胶复合材料，在电流密度为1A/g循环100次后，仍可获得高于1000mAh/g的比容量。3）利用金属氧化物对SiO2表面进行正电性修饰后，再将其引入石墨烯气凝胶，制得SiO2@金属基/石墨烯气凝胶复合材料。金属氧化物修饰对提高材料分散性，并且在硅基和石墨烯间建立紧密联系起到了关键作用。所制备材料展现了远优于纯SiO2的电化学性能，这主要得益于其独特的双层包覆结构，即内层金属基材料形成环状包覆结构为硅基颗粒的体积效应提供足够的缓冲空间，外层石墨烯三维包覆结构则提供了多维、快速的电荷传输通道。4）在喷雾干燥所制Si@GO前驱体表面，通过聚合和碳化反应包覆碳层后，制得核-壳-壳硅碳复合材料Si@G@PDA-C。其外层致密、保形的氮参杂碳，不仅可隔绝电解液与硅的直接接触，提高SEI膜稳定性，而且可以进一步提高整体碳保护层的机械稳定性。因此，双碳壳保护下的Si@G@PDA-C电极展现出更高的循环稳定性。综上可知，本项目制备方法简单、可操作性强，所制备材料具有一定的应用前景。另外，本项目深入探讨了材料结构与性能之间的关系，为进一步制备高性能复合材料提供了数据支持和理论支撑。