硅/碳复合材料电极微结构设计及离子嵌脱过程结构演变规律研究

The development of electric vehicles and new energy technologies raises higher demands for the high-capacity lithium-ion batteries. Silicon has been considered as one of the most potential new-generation anode materials of the high-capacity lithium-ion batteries, however the lithiation and delithiation of lithium ions during the charge-discharge process of the silicon electrode would give rise to large volume change, resulting in the damage problem of the structure of the electrode material, and this problem has become a bottleneck problem of the commercial application of the electrode material. Currently, nanocrystallization of silicon and silicon/carbon composites have been mostly used for improving the electrochemical properties of the silicon-based electrodes. The influences of carbon type, carbon layer thickness and size of silicon nano-particles on the electrode properties of silicon/carbon composites need to be studied deeply. In order to tackle some of these problems, the following studies will be conducted: (1) Multiscale research methods including first principles method and molecular dynamics simulation will be used to investigate the effects of the lithiation-delithiation process of ions on the structural evolution and the damage mechanism of nanosilicon materials. (2) The structural evolution and the damage mechanism of the silicon/carbon interface in the nanocomposite materials during the lithiation-delithiation process of ions will be investigated. The effect of the presence of carbon-coated materials on the lattice structure and the mechanical response of silicon nanoparticles will be also studied. (3) Carbon/silicon composite materials will be synthesized. The relationships between the structural parameters, the ratio of carbon to silicon and the electrochemical properties of the electrodes will be investigated. Then, theoretical guides could be provided for the development and design of high-capacity and long-longevity lithium-ion batteries.

电动汽车和新能源技术的发展对大容量锂离子电池储能系统提出了更高要求。硅被认为是最有潜力的新一代大容量锂离子电池负极材料，但由于嵌锂过程体积膨胀过大、导电性差等问题，尚未实现商业化应用。将硅材料纳米化或进行硅/碳复合可以显著改善其性能。然而目前硅基材料电极微结构设计缺乏必要的理论支撑，硅/碳复合时碳相的类型、碳层的厚度、硅纳米粒子的尺寸等对硅基材料电极性能的影响缺乏细致深入的研究。针对上述问题，本项目开展如下研究：（1）采用多尺度方法研究锂离子在硅纳米材料中的扩散过程，分析锂离子的嵌入与脱出对硅纳米材料力学特性的影响规律；（2）研究离子嵌脱过程硅/碳复合材料内硅/碳界面结构演变过程，以及碳包覆材料的存在对硅纳米颗粒嵌锂后的晶格结构和力学响应特性的影响规律；（3）合成硅/碳复合材料，明确纳米活性粒子结构参数、硅/碳含量比等与其电化学性能的对应关系，为大容量锂离子电池的设计开发提供参考依据。

电动汽车和新能源技术的发展对大容量锂离子电池储能系统提出了更高要求。硅被认为是最有潜力的新一代大容量锂离子电池负极材料，但由于嵌锂过程体积膨胀过大、导电性差等问题，尚未实现商业化应用。将硅材料纳米化或进行硅/碳复合可以显著改善其性能。然而目前硅基材料电极微结构设计缺乏必要的理论支撑。针对上述问题，本项目研究了锂离子在硅负极材料中的扩散过程，分析了锂离子的扩散路径以及离子扩散对材料晶格结构的影响，计算了锂离子在硅纳米材料中的扩散势垒，分析不同嵌锂位点的结合能，锂离子的稳定嵌入位点以及材料晶体结构、键长等变化；分析了锂离子的嵌入与脱出对硅材料力学特性的影响规律；研究了离子嵌脱过程硅/碳复合材料内硅/碳界面结构演变过程，以及纳米活性粒子结构参数、硅/碳含量比等与其电化学性能的对应关系。. 结果发现：在较低的扩散浓度下，嵌入的锂离子位于Td位点，体系的能量最低，最稳定；在较低的嵌锂浓度下，锂离子以孤立杂质的形式存在，嵌入体系均匀分布；在较高的嵌锂浓度下，体硅材料原有的金刚石立方晶体结构被打破，硅晶体体系由原来的金刚石立方结构分解为很多小的团簇结构；在实际体硅材料中，锂离子沿着<111>晶向从一个Td位点，扩散至下一个Td位点；对于硅纳米材料，由于表面效应的影响，极大便利了锂离子的嵌入；对于硅碳复合材料电极，碳的引入使电子可以自由地穿过活性粒子表层，减小活性粒子间的界面阻抗，提高活性材料的导电性；同时又可以作为弹性层缓解活性粒子在嵌锂过程中的体积膨胀，避免电极碎裂的发生。上述研究结果为大容量锂离子电池的设计开发提供参考依据。