二次碳源调控制备纯相LiVPO4F及其储能机理与电学改性研究

It is of vital strategic importance and wide market perspective to develop high safety, high performance and low cost power lithium ion batteries for electric vehicles under the increasing global energy crisis and worldwide environment pollution. The emerging polyanion LiVPO4F could be used as an upgraded cathode material of the currently well-known LiFePO4 for power lithium ion batteries, to meet the stringent safety and energy requirements of next-generation power batteries, because of its superior thermal stability, high discharge voltage and large specific energy density. The key bottleneck issue presently hindering its research and development is the reliable synthesis of phase-pure LiVPO4F with a simple and inexpensive procedure, so as to study the intrinsic energy storage mechanism and to perform performance amelioration accordingly. We have recently achieved great progress on synthesis of phase-pure LiVPO4F by suppression of Li3V2(PO4)3 impurity via introducing a secondary carbon source during conventional two-steps carbothermal reduction (CTR) route. In this project, we are going to further optimize the synthetic process and double check the phase structure of the sample by combined utilization of instrumental analysis especially spherical aberration corrected high resolution transmission electron microscopy (HRTEM) and electrochemical characterization, therefore, to successfully producing phase-pure LiVPO4F with a simple, reliable, low cost and eco-friendly method. The underneath synthesis mechanism and practical battery performance will be thoroughly discussed. Moreover, based on this pure LiVPO4F, the intrinsic energy storage mechanism of LiVPO4F could be comprehensively studied without the interference of Li3V2(PO4)3 impurity. The fundamental issues on lithium (de)intercalation mechanism, electron transportation and lithium diffusion process, detailed structure-performance relationship and electrode thermodynamic/kinetic analysis will be fully investigated by both in-situ and ex-situ techniques, thus the key factors influencing its electrochemical performance will finally be exposed. According to that, novel, specific and effective strategies on performance improvement of LiVPO4F will be proposed and carried out under rational theoretical direction and scientific materials design. Consequently, high safety, high efficiency, high energy, high power, long lifetime, wide-temperature operation and low cost LiVPO4F cathode materials will be practically obtained, and the fundamental theory on secondary-carbon regulated synthesis and modification of LiVPO4F will be established. The achievements of this project are expected to enrich the “novel synthesis-structural property-advanced performance” theory and technique systems of battery materials, to promote the industrial application of LiVPO4F cathodes, and to expedite the development of power lithium ion batteries as well as that of electric vehicles.

新型聚阴离子LiVPO4F具有结构超稳、电压较高和比能量大等优点，可作为优秀正极材料LiFePO4的升级换代，以满足下一代动力锂离子电池更高的安全和储能需求。项目针对当前制约LiVPO4F发展的瓶颈问题，在碳热还原法中引入二次碳源调控抑制杂相产生，优选原料、优化工艺，仪器分析（特别是球差电镜）与电化学验证相结合，简单廉价地制备纯相LiVPO4F并探讨相关合成机制。以此关键突破，联合原位测试与离位表征，系统深入研究其充放电中的储能机理、结构性能关系以及热、动力学过程等基础问题，揭示影响电化性能的关键因素。进而有的放矢地进行材料科学设计，实施个性化的高效改性，获得高安全、高效率、高能量、高功率、长寿命、宽工况、低成本的实用化LiVPO4F，建立二次碳源调控制备和改性LiVPO4F的理论基础。项目成果有望丰富和发展电池材料理论与技术体系，促进LiVPO4F产业化应用，助推动力电池和电动汽车发展。

全球能源危机和环境污染使先进能源存储材料与器件研发具有重要的战略意义和广阔的市场前景。钒基磷酸盐结构稳定、比容量大、电压高、价格低，是一类具有重要理论和应用价值的锂（钠）电池正极材料。本项目聚焦制约该类材料发展的关键科学问题，系统深入研究了系列材料的结构性能关系，综合利用二次碳源调控、F结构调控、异质多效掺杂和复合界面构筑等技术设计开发了一系列高安全、高性能电池材料。. LiVPO4F被视为动力锂电池正极LiFePO4的升级换代材料。项目采用二次碳源调控方法有效抑制了其在固相烧结合成过程中其它杂相的产生，成功制备了纯相LiVPO4F正极材料。基于纯相LiVPO4F，详细分析了材料的元素组成、微观结构和电化学热、动力学过程，验证了LiVPO4F在充放电过程中Li+沿着一维通道迁移，明确了材料的离子传输和电子电导是制约其电学性能的关键因素。进而采用多种技术手段，有的放矢地对LiVPO4F进行改性研究，获得了一系列兼具高安全、高能量、大功率、长寿命的LiVPO4F电池材料，性能指标领先文献报道。同时，与电池企业紧密合作，率先在国内外开展LiVPO4F的中试生产研究，初步实现了部分高性能LiVPO4F的公斤级、百公斤级中试生产和客户试用。. 钒基磷酸钠是一系列组分宽广、性能各异的钠电池正极家族材料。与LiVPO4F相对应，项目系统研究了Na3V2(PO4)3、NaVPO4F、Na3V2(PO4)2F3等系列材料的结构演化规律，重点探讨了F调控对系列家族材料的结构性能影响，分析了F调控对材料结构稳定性、Na+迁移传输、电压平台、比容量和电极动力学过程的改善作用。将二次碳源调控与F调控有机结合，发现了F调控有利于抑制表面包覆碳层中C-O键的形成，可显著提升复合电极材料的电导率与实际电化性能。最后，多管齐下，综合设计制备了一系列比容量高、充放电速率快、循环稳定的含F钒基磷酸盐钠电池正极材料。相关成果奠定了二次碳源调控与F调控技术理论的普适性，对设计和改性其它先进电池材料具有重要的借鉴和推广价值。