锂离子电池电极材料表面石墨烯包覆网络的原位可控生长及对电化学性能影响规律的研究

Carbon coating is one of the key techniques to improve the electrochemical performance and material utilization of the lithium-ion battery materials. Typically,an uniform, ultra-thin and highly-graphitic carbon coating layer throughout the electrode surface is ideal for low conductive materials to achieve better performance. Graphenen thin-layer coating is therefore regarded as the most promising coating manner. However,in-situ growth of graphene coating layer on the electrode surface is very challenging given the optimum synthesizing temperatures of most electrode materials are not high enough for carbon to graphitize effectively. Therefore, it is necessary to develop an effective low-temperature catalytic graphitizaiton approach to in-situ grow graphene decorations. Inspired by the growth mechanism of typical techniques reported for graphene mass-production, we propose two novel bottom-up synthesis strategies based on "dissolution-precipitation" mechanism and "binary metal alloy catalysts", respectively. By precisely controlling the content and ratio of solid carbon sources and catalyst precursors, a thin layer of "nano-islands" composed of metal catalysts can be in-situ formed. Under the catalysis of such catalyst thin-film, a special graphene network can be in-situ grown on the electrode surface through the realignment of the carbon atoms arousing from the pyrolysis of carbon sources. The graphene sheets not only form a compact and uniform coating layer along the electrode surface,but also stretch out and cross-link into a conducting network around the electrodes.Meanwhile,the thickness, graphitization degree and doping degree of the graphene coatings can also be precisely tuned by the optimization of carbon sources and catalyst precursors. In addition, the effects of graphene coating quality on the electrochemical properties of electrodes will be further investigated and discussed detailedly through electrochemical measurements to optimize the parameters of the graphene coatings.What's more, the growth mechanism of the graphene coatings will also be detailedly studied using the spectrographic techniques and microscopic methods.

碳包覆技术是锂离子电池电极材料电化学性能发挥的关键支撑技术之一。电极材料要求碳包覆层需同时具备完整连续、高度石墨化及超薄的特点，因此石墨烯薄层被认为是最理想的包覆方式。然而在电极材料表面原位生长石墨烯包覆薄层具有极大的难度，只有通过发展有效的低温催化石墨化途径来实现。本项目借鉴目前石墨烯主流制备技术的生长机制并加以合理"微缩化"，提出"溶解-析出"法以及"二元合金催化剂"法两种生长策略。通过严格控制固态碳源和催化剂前驱体的种类、含量和比例，借助原位形成的金属催化剂"纳米岛"薄层在电极材料表面催化生长由紧密石墨烯包覆薄层以及向空间延伸的石墨烯片构成的特殊网络结构，同时实现对石墨烯层数、结晶度和掺杂度的精确调控。借助电化学测试和光谱、显微手段，我们将详细探讨电极材料电化学性能与石墨烯包覆网络结构及组成间的关联规律，优化石墨烯的最佳层数、结晶度和掺杂度，并对石墨烯包覆网络的生长机理进行深入研究。

碳包覆技术是锂离子电池材料电子导电性和电化学性能提升的关键支撑技术。电极材料要求碳包覆层需同时具备完整连续、高度石墨化及超薄的特点，因此石墨烯薄层被认为是最理想的包覆方式。然而在微/纳电极材料表面原位生长层数和结晶度精确可控的石墨烯包覆薄层具有极大的难度，需通过生长方法的原始创新和优化得以实现。.项目借鉴当前石墨烯主流制备技术的生长机制并加以合理“微缩化”，将石墨烯的原位可控生长从二维宏观平面基底拓展至微/纳尺度电极材料，实现电极材料表面完整石墨烯包覆层层数和结晶度的精确调控，并成功应用于锂/钠离子电池，揭示了石墨烯包覆层结构和组成与电极材料电化学性能间的关联规律，取得了以下两方面重要研究成果：1）提出基于“溶解-析出”生长机理的低温催化石墨化技术，通过严格控制固态碳源和催化剂前驱体的种类、含量和比例，借助原位形成的金属催化剂“纳米岛”薄层在电极材料表面催化生长完整石墨烯包覆薄层。项目成功制备出具有优异循环和倍率性能的石墨烯空心球包覆纳米硅复合负极材料。同时，我们深入研究了石墨烯在磷酸铁锂电极材料表面的生长过程，揭示了石墨烯在催化剂纳米岛上的六个基元步骤以及在不同碳源和催化剂前驱体比例下的生长模式。2）将化学气相沉积 (CVD) 技术成功引入到微/纳电极材料表面石墨烯包覆层的制备。通过碳源气体浓度和生长时间等参数控制，可在MnO纳米线表面获得自单层、1-2层至少层可控的石墨烯包覆层，并可精确调整其缺陷程度；在此基础上，我们深入探讨了石墨烯包覆层厚度和结晶度对电极材料电化学性能的影响规律。此外，我们通过等离子增强CVD技术，在Nb2O5纳米线表面均匀生长厚度在8-9层的石墨烯包覆层，并作为负极与活性炭正极组配得到了性能优异的钠离子混合电容。通过PECVD技术，我们还在泡沫铜表面生长出垂直石墨烯阵列结构，作为金属锂沉积的优良基底，可显著克服锂枝晶生长，制备出循环性能优异的三元正极/金属锂全电池。.项目已在Chem. Soc. Rev., Energy Environ. Sci., Adv. Mater., Adv. Energy Mater., ACS Nano, Adv. Funct. Mater., Energy Storage materials, Carbon等杂志以通讯作者发表和接收论文19篇；申请专利2项，授权3项；获2015年度江苏省科学技术二等奖1项。