石墨/纳米硅/石墨烯多层次复合负极材料构筑与电化学性能研究

A graphite/nano-silicon/graphene hierarchical composite anode material is designed to overcome the drawbacks such as the poor cycle and rate performances of silicon in application of lithium ion batteries. A synthesis approach involving mainly a low-cost electrolysis method in molten salts is also proposed in this proposal. While taking advantage of the high specific capacity of the silicon, we intend to solve the problems of the silicon through the following designs: (1) Electroreduction of the SiO2 in molten salts will be used to produce nano silicon to alleviate the stress and volume change effects of the silicon; (2) Graphene surface coating will be utilized to enhance the structure stability of the silicon and protect the silicon from direct and excessive contact with the electrolyte; (3) Meanwhile, the voids between the nano silicon will be intentionally adjusted by controlling the electrolysis process to alleviate the agglomeration of the nano silicon during the cycling; (4) A second graphene encapsulation layer (several layers ) will synthesized to construct a stable and robust 3-dimensinal conductive network with the graphite skeleton so as to decrease the detaching probability of the silicon and the current collector. Our research interests will first focus on the electrochemical and chemical reaction mechanisms and the heterogeneous nucleation processes at the reaction interfaces during electro-reduction of the SiO2 in molten salts to prepare the silicon-based composite materials. Then, we will concentrate on investing the effects of the graphene surface coating and encapsulation on the electrochemical performances of the composite material.

项目设计一种石墨/纳米硅/石墨烯多层次复合材料，旨在克服硅作为锂离子电池负极材料循环性能较差和倍率性能不良的问题；并拟采用低成本的熔盐电解法结合其他方法实现对这种复合材料的制备。在充分发挥硅比容量高的优势的同时，首先，拟利用石墨作为导电载体并通过电解还原二氧化硅制备出纳米硅，以缓解硅的应力和体积效应；其次，利用石墨烯对硅进行表面包覆进一步提升硅的自身结构稳定性，并减少硅与电解液的直接接触提升其表面稳定性；第三，对硅与硅之间的间隙进行控制，并结合石墨烯表面包覆的方法减少硅在体积变化中的团聚；最后，利用石墨烯优良的柔韧性和导电性，与石墨导电骨架相结合建立稳固的三维导电网络，降低硅与集流体失去电连接的几率；从而提升材料的循环和倍率性能。项目主要研究内容包括固态氧化物熔盐电解制备硅基复合材料过程中异相界面处的电化学和化学反应机制和异相形核过程，以及石墨烯包覆和包裹对材料电化学性能的影响。

硅负极材料具有比容量高、放电电位低等优势，对于发展高比能量锂离子电池具有重要意义；然而，充放电过程中巨大的体积变化导致的粉化、开裂、电失活、固态电解质膜不稳定等问题限制了其商业化应用。本项目设计并开发了一种石墨/纳米硅/碳包覆复合负极材料，研究了以熔盐电解为主的低成本制备方法，通过构建稳固的内部导电网络和固液界面提高了硅基负极材料的循环性能，并探索了材料在电池中应用技术。首次发现了熔盐电解过程中硅在石墨表面的形核与石墨的本征特性和表面状态有关；通过空气氧化、球磨、等离子轰击等石墨表面处理技术和对电解电流、电压、电量等电解工艺条件的控制，一定程度上实现了对硅在石墨表面异相形核和生长的控制；电解得到的石墨/纳米硅中间产物首次容量达到930.5mAh/g、扣式电池100周循环保持率≥55.7%。研究了基于沥青原料的低成本的无定型碳包覆技术，实现了对目标结构材料的制备，提高了材料与电解液界面的稳定性，碳包覆后最终产物比容量达到650mAh/g，在商业化2Ah电池制造工艺条件下循环性能达到250周容量保持率83.4%。研究了材料的电化学性能、石墨/硅界面结构特性和锂离子输运、以及相关的电解液配方、粒度分布等电池应用技术，为解决硅基负极材料的循环问题提供了一些研究思路。开展了技术放大验证研究，建立了年产1000kg级硅碳复合材料熔盐电解试制试验线，并进行了为期1年的运行生产。