氧空位二氧化钛与石墨烯双层包覆纳米硅负极材料的制备、性能调控及作用机理研究

The high-capacity Si anode material has several problems including poor electrical conductivity, huge volume change during charge/discharge process, unstable SEI formation in the electrode-electrolyte interface, which hinder its practical applications seriously. To solve these problems, the aims of this project is to prepare the yolk-shell oxygen-deficient TiO2@graphene encapsulated Si nanoparticles by means of a co-precipitation process and electrostatic self-assembly combined with sol-gel process and subsequent heat-treatment in argon/hydrogen atmosphere, followed by diluted HCl treatment. It is supposed that the inner graphene coating could prevent the Si nanoparticles from the electrolyte ingress, improve the electrical conductivity and enhance Li+ diffusion; The outer oxygen-deficient TiO2 coating could ensure SEI stability at the electrolyte–electrode interface and preserve integrity of the overall electrode; the synergy of the inner and outer coating could effectively accommodate the volume changes of Si nanoparticles and enhance the stability of electrolyte–electrode interface. In addition, the synergistic effect of the microstructure and thickness of each coating layer on the electrochemical properties of Si nanoparticles will be investigated by in situ TEM technique and the control of their electrochemical performance will be realized. As a result, the mechanism of the oxygen-deficient TiO2@graphene double-layer coating on the improvement of interfacial stability and the electrode structure together with the enhancement of the electron transport and the lithium ion diffusion of the composite electrode can be obtained, which will deliver important experimental basis for designing superior anode materials with high capacity、high rate and long cycle life.

高容量硅负极材料存在导电性差、充放电过程中体积变化大、界面SEI不稳定等问题，极大地限制了其商业化应用。针对这些问题，本项目拟通过共沉淀-静电吸附层层自组装结合溶胶凝胶-氢氩气氛热处理-稀盐酸刻蚀的方法构筑一种介孔氧空位二氧化钛与石墨烯双层包覆纳米硅中空核壳结构。利用内层石墨烯包覆防止纳米硅与电解液直接接触、改善其导电性和提高锂离子扩散速率；利用表层介孔氧空位二氧化钛包覆提高电极与电解液界面SEI稳定性和保持电极整体结构完整性；利用内外层的协同作用有效缓冲纳米硅的体积膨胀，提高复合电极界面稳定性。利用原位TEM技术，探明各包覆层微观结构和厚度对纳米硅电化学性能的协同作用机理，实现复合材料的性能调控。通过本项目的研究，揭示氧空位二氧化钛@石墨烯双包覆层对提高复合电极界面及电极结构稳定性和改善电子传导及离子传输的作用机理，从而为设计高容量、高倍率以及长寿命的高性能锂电负极材料提供科学依据。

硅(Si)负极因其超高理论比容量(4200 mA h g-1), 被认为是新一代锂离子电池负极材料的理想选择之一。然而高比容量Si负极存在电子导电性差，且在脱/嵌锂过程体积变化剧烈等突出瓶颈问题，导致其倍率性能不佳和循环过程中容量快速衰减，是Si负极研发过程中亟待解决的关键科学问题。对Si负极进行合理的结构设计和包覆改性，尤其是构筑合理的核壳复合结构，并对其微观结构、物相等基本物理化学性质与电化学性能间的构效关系进行精确解析，是解决上述核心问题的关键。此外，监控Si负极在充/放电过程中的动态结构演化过程对理解其失效机理并加以优化改进具有重要的指导意义。. 项目针对高比容量Si负极的上述关键科学问题，采用双层碳包覆策略并结合非电化学活性的铜-硅合金相(Cu3Si)的引入，成功制备出了具有优异循环和倍率性能的石墨烯-碳双层包覆Si纳米颗粒/Cu3Si复合负极材料。利用原位透射电镜结合有限元相场模拟，揭示了石墨烯-碳双层包覆及Cu3Si的存在对提高Si基复合电极结构稳定性的微观作用机理。在此基础上，课题组创新性的构筑了一维中空Cu线核外包覆Si和Ge双层壳结构来缓解Si负极体积膨胀并提高其导电性，显著提升了复合材料的倍率和循环性能。利用原位TEM技术结合有限元模拟发现了缓解Si体积膨胀的微观新机制, 揭示了其获得优异电化学性能的本质原因。3）进一步，课题组与武汉科技大学团队合作构筑了类蚁巢状多孔微米硅/碳核壳结构,有效解决了嵌脱/锂过程中Si体积膨胀和导电性差的问题。借助原位透射电镜技术揭示了其通过自身结构来容纳Si体积膨胀的“自体积适应”新机理，建立了多孔硅/碳负极材料微观结构与宏观电化学性能间的构效关系。此外，我们还开展了其他高比容量核/壳和蛋黄/壳结构负极材料储锂/钠/钾性能优化及机理方面的探索工作，对开发高性能碱金属二次电池负极材料有一定参考价值。. 项目已在Chem. Soc. Rev., Energy Environ. Sci., Mater. Today, Nat. Commun., Angew Chem Int Edit., Adv. Energy. Mater., ACS Nano, Adv. Funct. Mater., Nano Lett 等杂志以第一或通讯作者发表论文33篇，申请专利2项。项目负责人获2020 年中国新锐科技人物卓越影响奖。