锂离子电池用具有空腔结构的Si/C复合负极材料的制备及储能机理研究

.Silicon (Si) is attracting increasing attention as an alternative anode electrode material for the application in Lithium-ion batteries (replacing carbonaceous materials), due to its very high theoretical specific capacity and relatively low working potential. However, poor cycle retention, which occurs because of the volume changes in Si during Li alloying/dealloying, has delayed the development of Si as a next-generation anode material for Lithium-ion batteries. Carbon coating is one of the effective solutions. Studies indicate that the most important requirement for successful Si electrode development is the thin and even carbon coating layer ,and the accommodation of the huge volume expansion/contraction during the cycles. In this work, we therefore designed a new approach to prepare Si nanoparticels encapsulated elastic hollow carbon spheres (SNHCs) whose electrochemical performance was also studied. The synthetic procedure is described as following: Si was firstly oxidized to form SiO2 oxide layer on its surface, and then polymer was coated on the layer of SiO2 by emulsion polymerization. Finally SNHCs were got by carbonization and the following washing by hydrofluoric acid. In this work, the oxidation conditions of Si and the optimization of the preparation conditions of emulsion polymerization were studied. Moreover, the effect of void volume, the property and thickness of carbon coating layer on the electrochemical performance of SNHCs were also investigated. This project is expected to provide a new idea and reference on the development of Si/C composite anode materials. It will be breakthrough in the improvement of the capacity and cycle performance of Lithium-ion batteries, and the mechanism of energy storage will also be further riched.

硅材料具有理论容量高和嵌锂电压较低等优点，有望成为替代商业化石墨负极的材料。然而，巨大的体积效应导致容量迅速衰减的问题成为硅负极商业化应用中的最大障碍。研究表明：碳包覆是有效的解决措施之一，其中，碳层的性质、厚度、均匀性以及Si/C复合负极材料内部是否有容纳硅体积膨胀的空间是影响其性能的关键因素。因此，本项目提出具有空腔结构的Si/C复合负极材料的制备技术（即：通过氧化在纳米硅表面形成SiO2层，并采用乳液聚合法在SiO2层表面包覆聚合物，再经炭化、氢氟酸洗涤制备Si/C复合材料），并对其储能机理进行研究。具体研究内容包括：纳米硅的氧化，乳液聚合工艺条件的优化，所制备Si/C复合材料中包覆碳层性质、厚度和预留空腔体积对其电化学性能的影响。本项目的实施有望在提高锂离子电池容量和循环性能方面取得突破，为新型Si/C复合负极材料的开发提供新的思路和借鉴，并进一步丰富硅基负极材料的储能机理。

硅负极材料的理论容量高达4200mAh/g，远高于商业化的石墨负极材料。然而，硅负极材料低的导电率、严重的体积效应和循环稳定性差等缺点，严重制约了其发展。碳材料具有良好的导电特性，结构稳定，可以作为硅电极的“缓冲基体”。因此，硅/炭复合负极材料的制备及性能成为研究热点。.本项目分别采用聚苯胺和淀粉作为包覆聚合物或碳源，纳米硅为硅源制备硅/炭复合负极材料并对其作为锂离子电池负极材料的电化学性能进行研究。研究中采用扫描电镜、红外光谱和X-射线衍射等手段对材料的结构进行表征，采用恒流充放电和交流阻抗测试对材料的电化学性能进行表征。.研究表明：以淀粉为碳源所制备的硅/炭复合材料作为锂离子电池负极材料时，当硅/石墨比为4:8，球磨时间为10h，硅/炭复合材料：导电石墨：羧甲基纤维素钠（CMC）=85:5:10，pH=3柠檬酸/氢氧化钾缓冲液为调浆溶剂时，复合材料首次充放电比容量分别为1347和934mAh/g，经过20次循环后充放电比容量还高达985mAh/g。这表明：复合材料中硅对比容量做主要贡献时，材料具有较高的比容量；以缓冲溶液做调浆溶剂时，CMC和复合材料之间形成共价键，在充放电过程中可有效缓解硅的体积效应，使整个极片保持更好的整体性，从而显示出优良的电化学性能。.以苯胺为包覆聚合物所制备硅/聚苯胺复合材料作为锂离子电池负极材料时，当硅/苯胺质量比为1:0.5，复合材料/导电剂/CMC的比例为85:5:10时，复合材料经20次循环后可逆容量仍为1450mAh/g，显示了优异的电化学性能。