高比容量Li-Co-O体系的制备及其物相研究

LiCoO2 remains one of the most attractive positiveelectrode materials for secondary lithium batteries.It is wellestablished that the practical capacity of LiCoO2 is limited to about 140 mAh/ g, around half of its theoretical capacity (273 mAh/g). During its charge/discharge process, as the ratios of Li and Co change, the structures of Li-Co-O systems become complex during the long-term cycling and spinel phase may be formed, which will influence the electrochemical performance..This project intends to prepare crystallized micro-nano LiCoO2 cathode material using solvent-thermal and solid state synthesis in autoclave. Besides coating carbon on the surface of LiCoO2 crystals, some lithium rich compounds, such as Li3PO4 and Li3P will be used to obtain LiCoO2 cathode material coated with lithium rich layer of different crystalline state. On the basis of research in relatively stable layered-rhomohedral phase and its change of electrochemical properties, we will further research on the phase change of rhomohedral or mixed phases coexisting with spinel phase during battery cycling, and analyze the phase and composition when the battery become inactive. By virtue of understanding the internal crystal chemical reasons, it gives an alternative way to discuss the reason why the specific capacity of LiCoO2 system is low on these phase investigations of Li-Co-O system, we aim at achieving LiCoO2 cathode material with improved charge/discharge capacity and cyclic stability..Based on these investigations of LiCoO2 system , we will further develop to study other Li-M-O cathode material ,such as Li-Fe-O and Li-Mn-O with improved charge/discharge capacity and cyclic stability . We will further to explore the possibility to generate other cathode materials such as Li-Fe-O and Li-Mn-O.

商用的LiCoO2实际容量仅为理论的50%，即使在三方相LiCoO2充放电过程中，Li和Co的比例会减小而物相也会有尖晶石相出现，由于Li离子的易流动性，致使其无序度增大，这显然会影响其容量及循环性。.本项目拟用溶剂热、高压釜中固相反应技术制备结晶的微纳LiCoO2正极材料，除用掺碳提高其导电性外，包覆富锂层或复合富锂层，得到不同结晶度、富锂的LiCoO2正极材料。在研究相对稳定层型三方相及其电化学性质变化的基础上，对三方相、尖晶石混合物相在电池循环过程中的物相变化进行跟踪研究；对电池失活时的物相和成分进行剖析；对LiCoO2体系比容量不高的结晶化学理解予以探讨，以期找出高比容量、相对稳定的LiCoO2起始物相体系。.在LiCoO2体系研究的基础上对LiFeO，LiMnO体系等新型正极材料提高比容量的途径进行探索。

电极材料物相及循环过程中物相的变化，将极大的影响电池的比容量和循环稳定性等电化学性能。本项目以LiCoO2为正极材料，寻找容量提升的且相匹配的负极材料，研究组装后全电池的性能: 合成的锗微米材料与钴酸锂组装成全电池，输出电压约为3.5 V，循环100圈后容量保持为原来的91%（398 Wh Kg-1）(Chem. Mater. 2015, 27 (11), 4156-4164)。制备了多种新型微晶正极材料，通过X-射线衍射、拉曼光谱、高分辨率电镜和电化学测试等手段，研究其物相及物相对电池材料性能的影响：在高温合成LiNi0.5Mn1.5O4时，通过高分辨率研究发现800°C直接煅烧的样品中存在P4332和Fd3m相的共生现象，而煅烧后热处理的样品全为P4332 相，仅煅烧的样品电化学性能明显好于热处理后的样品(Electrochim. Acta)。选择与Li-C-O体系相近的层状化合物，采用非原位高分辨透射电镜分析以及X射线衍射等手段，研究其作为锂离子电池材料电化学机理及循环过程中物相变化：将FeCl3-石墨夹层化合物用于锂离子电池负极材料，发现其电化学过程包含了与石墨相似的锂离子的脱嵌以及铁的氧化还原反应（Chem. Sus.Chem.）。丰富了层状化合物用于锂离子电池的种类及其性质研究，为发现新材料打下基础；通过材料物相和循环过程中物相分析，寻找相关层状化合物的物相对电化学循环和容量等性能影响，通过控制合成条件，得到电化学性能提升的电极材料。