石墨烯-氟磷酸钒钠混合导电网络修饰LiMnPO4基正极材料的设计合成及储锂性能研究

LiMnPO4 has been a potential cathode material due to its high structural safety, high voltage, environmental benignity, and low cost. However, the low conductivity and unstable structure of it results in poor cycling performance and rate capabilities. For enhancement of electronic and ionic conductivity and stabilization of interfacial structure of LiMnPO4, the project plans to construct the hierarchical LiMnPO4·Na3V2(PO4)2F3@C/rGO hybrid with mixed conductive network. Based on the functional co-modification of facilitating electron conduction by wrapped N-doped rGO and expanding ion transport channels by Na3V2(PO4)2F3 in-situ composite, the rate capability and cycling stability of LiMnPO4 is to be enhanced, via comprehensive strategies of micro-dimension and composition regulation，surface decoration of nano LiMnPO4,cross-linking and encapsulation of N-doped rGO, and shell growth of vanadium compound. The project focuses on the construction mechanism of micro/nano LiMnPO4 heterogeneous structure, building principle of mixed conductive layer, as well as the synergistic effect of the architectural optimization and interface phase control on the electrochemical properties. And then the controlled preparation approach for high performance LiMnPO4 cathode can be developed. This project also aims at analysis the evolution of interfacial phase and micro-structure during electrochemical reaction, revealing the electron and Li+ diffusion behavior, and illustrating the enhancement mechanism of its lithium storage performance. The research in this project will lay a foundation for the design and preparation of cathode materials with high rate performance and long cycle life.

LiMnPO4具有安全、高电压、环境友好等优点，是很有潜力的锂离子电池正极材料。但其电导率很低和充放电过程中结构不稳定，导致倍率性能和循环性能较差。项目结合氮掺杂石墨烯纳米片高电子传导和氟磷酸钒钠快离子传输的协同功能修饰，设计具有三维混合导电网络的LiMnPO4基复合正极材料，改善LiMnPO4导电性能和界面结构稳定性。通过微观尺度和组分的调控、纳米晶表面修饰、钒化合物表面复合与共热解氮化技术，在活性颗粒中形成穿插包裹的氮掺杂石墨烯与氟磷酸钒钠和纳米碳层共修饰的多层次异质结构。重点研究微纳复合结构的调控规律，功能导电网络的形成过程及材料微观结构、表面结构和组分比例的优化对电化学特性的强化效应，找到高性能LiMnPO4材料的设计合成及改性策略；通过分析电化学反应过程中复合材料结构和界面性质的变化，揭示材料储锂能力的增强机理。项目可为高倍率、长寿命正极材料设计合成奠定基础。

LiMnPO4具有安全、高电压、环境友好等优点，是很有潜力的锂离子电池正极材料，但面临离子与电子导电性差以及高电压下电解液之间表面稳定性的挑战。项目提出表面复合改性构建多层次导电结构的策略，引入三维离子扩散通道的高电压活性材料与碳导电网络进行协同改性，在LiMnPO4基复合材料内部构筑了互连离子电子复合导电网络，降低电化学阻抗，增强正极与电解液界面稳定性，改善了电化学性能。得到主要结论如下：（1）采用溶剂热法在不同原料体系下合成了还原氧化石墨烯交联的纺锤状纳微结构LiMnPO4/C/rGO 与LiMn0.8Fe0.2PO4@C/rGO复合材料，分析了LiMnPO4/C/rGO制备过程中形貌与物相转化历程，提出了纳微分级结构的形成机理，发展了纳微结构的可控制备技术。（2）开发了LiMn0.8Fe0.2PO4/C/rGO复合材料高效的连续化制备技术，分析了前驱体与产物的结构与形貌，获得的多层次分级结构利于电荷传递与倍率稳定性，2C下放电比容量保持在150mAh g−1，具有优异的电化学性能，为LiMnPO4基复合材料规模化制造提供技术基础。（3）采用表面溶胶-凝胶法制备了穿插互连的氮掺杂纳米碳导电相与氟磷酸钒钠协同修饰LiMn0.8Fe0.2PO4多层次复合结构。构筑的三维混合导电网络有助于提高材料倍率性能与界面结构稳定性。通过对复合材料的结构和界面性质表征，分析了氟磷酸钠复合对LiMnPO4基材料充放电过程动力学、电化学性能的影响规律，确定了优化的多层次复合结构和组成，揭示了复相正极材料的离子传输特征以及循环过程中电化学特性的强化效应。其中LiMn0.8Fe0.2PO4·0.15Na3V2（PO4）2F3/C/rGO材料在1C放电比容量＞150 mAhg-1，10C放电比容量保持在140 mAhg-1，显示了优异的倍率循环性能，达到了预期指标。项目研究阐明了复相磷酸盐正极材料的组成与结构对正极材料储锂能力的增强机理，可为高倍率、长寿命正极材料设计合成提供新的思路和依据。