高容量多孔氮掺杂碳包覆Li2FeSiO4@CxNy纳米复合材料的制备及储锂性能研究

As one of the lithium ion battery cathode materials, Li2FeSiO4 has high theoretical discharge capacity of 331 mAhg-1. However, the advantage of its high capacity is not fully showed up due to the poor electronic conductivity and slow migration of Li+. To solve these problems, in this term, porous N doped-carbon-coating Li2FeSiO4@CxNy nanocomposites are designed by using porous aromatic framework compounds as template. The reactive sites on materials surface will increase while the particle size of Li2FeSiO4 will reduce by using PAF templates. Combined with the effect of N doped carbon coating layer, which enhance the electronic conductivity of carbon layer, the electrochemical performances will be improved. The relationship between the monomer property of aromatic framework compounds and the porous structure of the final products will be investigated, and also the effect of monomer property and porous structure on electron conductivity and lithium-ion migration. In addition, the influence of nitrogen existing form and the N、C content on discharge capacity, cycle stability and rate performance also will be investigated. By seriously analysis the experimental results, the relationship between porous structures, N doped carbon coating and its electrochemical performance will be figured out. Our results will provide a new method for modification of other polyanion cathode materials.

Li2FeSiO4作为锂离子电池正极材料具有331 mAhg-1的高理论放电比容量，然而较低的电子/离子导电率使其高容量的优势难以充分发挥。本项目主要针对Li2FeSiO4导电性差且难以实现高放电比容量的问题，拟以芳香骨架化合物为模板制备多孔氮掺杂碳包覆的Li2FeSiO4@CxNy纳米复合材料。通过模板作用以实现“大孔-介孔-微孔”多级孔结构的设计，从而增加材料表面反应活性位点，控制产物粒径尺寸；同时以氮元素对包覆碳层进行掺杂，提高导电碳层的导电性，二者相结合，使材料电化学性能得以提高。探究芳香骨架单体性质与芳香骨架化合物模板、Li2FeSiO4的孔结构及导电性的关系。考察氮的存在形态及氮、碳含量对产物放电比容量、循环稳定性及倍率性能的影响。阐明多孔结构及氮掺杂与材料电化学性能之间的构效关系，为聚阴离子型正极材料的改性提供新的思路。

Li2FeSiO4作为锂离子电池正极材料具有331 mAhg-1的高理论放电比容量，然而较低的电子/离子导电率使其高容量的优势难以充分发挥。本项目主要针对Li2FeSiO4导电性差且难以实现高放电比容量的问题，分别以联苯和三苯胺制备了多孔芳香骨架PAF化合物模板，分别以尿素、双氰胺和缩二脲为氮源，酒石酸为碳源，制备了氮掺杂碳包覆Li2FeSiO4@CxNy纳米复合材料。考察了煅烧温度（500-1000℃）对多孔芳香骨架化合物孔性质的影响。探讨了碳含量及氮掺杂含量对产物物相及结构的影响，探究了氮掺杂对材料导电性的影响，并研究导电性能与氮源存在形态（吡啶氮、吡咯氮和石墨氮等）的关系。结果表明，煅烧温度升高，孔容和表面积均有所减小，说明煅烧使得芳香骨架有所收缩。在500-1000℃煅烧效得到的材料存在的孔主要是微孔（0.5-0.7 nm）。氮掺杂碳包覆实验结果表明，1-2 nm厚的碳包覆层均匀的包覆着Li2FeSiO4，掺杂的氮元素主要存在于碳包覆层中，Li2FeSiO4晶粒大小为3-5 nm，较高含量的氮掺杂（高于1.6 wt%）会引入Li2SiO3杂质相，适合含量（低于0.9 wt%）的氮掺杂碳包覆不影响Li2FeSiO4的物相结构，且能够有效提高Li2FeSiO4的电化学性能，0.5C循环100周后，Li2FeSiO4@CN复合材料放电比容量仍能够维持在220 mAhg-1。电化学性能的提高主要归因于氮掺杂产生的吡啶N和吡咯N产生了缺陷和空位，增强了锂离子迁移速率，氮掺杂后复合材料的电子导电性有所增强，此外，良好的包覆效果以及较小的颗粒粒径尺寸均使得氮掺杂碳包覆后的Li2FeSiO4@CxNy复合材料具有良好的电化学性能。氮掺杂碳包覆是一种有效的提高锂离子电池正极材料电化学性能的方法，为其他聚阴离子型电极材料的电化学改性提供了新的研究思路。