高电导率、高倍率性能LiVPO4F/C复合正极材料的结构调控及其电化学性能研究

The commonly used cathodes of lithium ion batteries possess their different shortcomings. The newly developed LiVPO4F contains not only three dimensional diffusion channels and potential platform around 4.2V, but also possesses excellent thermal stability. However, its conductivity are very low and it contains excessive carbon powders, which is prepared by the typical two step carbothermal reduction method. Therefore, this project will be carried out in order to improve its conductivity and electrochemical performances. The new style one step carbothermal reduction method will be adopted to prepare LiVPO4F and the influencing mechanisms of the factors on the microstructures will be explored. The oxide or fluoride of aluminium, chromium, titanium and yttrium will be selected as doping materials to stabilize the crystallite structure and enhance the ionic conductivity of lattice. The influences of the carbon sources of polyvinyl alcohol, phenolic resin, epoxy resin and epoxy novolac resin on the conductivity and electrochemical performances of cathode will be discussed. The accelerating agents, which can accelerate the pyrolysis of carbon sources, will be added in order to accelerate the growth of the graphite crystallite in the coated carbon and enhance the electronic conductivity and the electrochemical performances. The investigations in this project will realize the structural regulation of LiVPO4F/C. The relationships between the structure, electrochemical performances and synthesis technologies will be grasped. The cathode with high conductivity, low concentration of impurities and excellent performances will be synthesized. These will satisfy the demands of power lithium ion batteries.

常用锂离子电池正极材料各有不足之处，新发展起来的LiVPO4F不仅含有三维扩散通道和4.2V的电压平台，还具有优异的热稳定性。但其电导率较低，通常采用两步碳热还原法制备时还含有过量的碳粉。为此，本项目围绕改善其电导率和电化学性能开展工作。采用新型一步碳热还原法制备LiVPO4F，探索影响因素对其微观结构的作用机理；选择铝、铬、钛、钇等的氧化物或氟化物作为掺杂原料，用以稳定LiVPO4F的晶体结构、改善晶格的离子电导率；探讨聚乙烯醇、酚醛树脂、环氧树脂和环氧酚醛树脂等碳源对正极电导率和电化学性能的影响；添加能加速碳源热解的促进剂，加快包覆炭中石墨微晶的生长，改善活性材料的电子电导率和电化学性能。研究将实现对LiVPO4F/C的结构调控，掌握其结构、电化学性能与制备工艺之间的内在关系，制备电导率高、杂质含量低、性能优异的正极材料，从而满足动力锂离子电池的要求。

新型正极LiVPO4F不仅拥有高达4.2V的电位平台，还具有优异的热稳定性；但其存在磷酸钒锂杂相含量高、电导率低、两步碳热还原时能耗高等问题。.提出了能在60℃反应的新型一步碳热还原法，溶液颜色的变化和XPS分析表明草酸能在短时间内还原高价钒离子；采用热力学分析确定了烧结温度，化学动力学分析表明其属于复杂的三级反应。探讨了结构、形貌、组成、性能等随烧结温度的演变过程，800℃制备的样品LVPF800杂相含量低、结晶完善，在0.2C、5C的容量分别为139.3mAh/g、116.5mAh/g，在5C时50次循环后的保持率高达97.85%；晶格条纹和衍射点阵结果表明温度过高时再次出现杂相。.与Ti4+、Mn4+相比，能形成稳定的LiAlPO4F结构的AlF3掺杂更有效，不仅缩短了扩散路径、减少了F原子的逸出、抑制了杂相的形成，还将电导率提高至0.29S/cm。掺杂量为3%时材料在0.2C时的容量提高了17.8%、达到140.9mAh/g；5C时50次循环后的容量为112.9mAh/g、循环保持率高达97.3%。该材料不仅具有较高的锂离子扩散系数1.42×10(-10)cm2/s、还具有优异的高低温性能；XRD分析表明多次充放电循环后正极仍具有完整的结构。研究发现，较低的环境温度(10℃，20℃)能抑制氟的挥发、减少杂相、改善正极的电化学性能。.采用导电性一般的PVDF热解炭也能构建高性能的正极，其在0.1C、5C时的容量达到147.6mAh/g、135.8mAh/g，高于现有的文献结果。PVDF热解时能释放HF、抑制氟的挥发、补充氟的损失、减少杂相的生成。TG-MS分析表明，其他碳源热解时形成的氢自由基能与氟结合，反而加速了氟化物的分解，过量的杂相使材料性能变差。.首次采用一步水热法获得结晶完善的球形正极，该材料具有良好的性能，在0.1C、5C时的容量为135.1mAh/g、104.7mAh/g。.查明了碳化促进剂的作用机理，样品不仅拥有4.2V的电位平台，容量达到良好的水平。有望显著提升正极的性能、取得原创性的成果。.项目提出了新型方法，运用经典的理论指导研究工作，查明了PVDF和AlF3的作用机理；掌握了材料的结构、性能和制备工艺的内在联系，LiVPO4F的性能达到较高的水平，研究具有一定的科学意义。作为结构稳定、性能优异的正极，LiVPO4F具有广阔的市场前景。