双碳包裹磷酸钒钠的合成及电化学性能研究

Synthesis of Dual Carbon Coated Na3V2（PO4）3 with Excellent Electrochemical Performance

摘要:

聚阴离子型磷酸钒钠正极具有原料储量丰富、成本低和对环境友好的特点，具有十分稳定的钠离子超导体结构，其NASICON结构骨架形成了稳定的容钠位，并且开放的三维离子迁移通道有利于改善钠离子的扩散，是钠离子电池理想的正极材料。然而磷酸钒钠较低的电子导电性以及循环过程中结构的变化会导致较大的电极极化和晶体结构的变形，对磷酸钒钠的循环性能不利。为了改善磷酸钒钠的电子导电性以及缓解其在循环过程中的体积/结构变化，通过结合溶胶-凝胶以及原位聚多巴胺包裹的方法，引入柠檬酸和聚多巴胺，原位转化为多孔碳和氮掺杂碳，形成基于双碳包裹的磷酸钒钠。研究结果表明，双碳包裹结构能够提供高电子导电性以及足够的缓冲介质，该双碳包裹的磷酸钒钠表现出良好的电化学性能，在0.1 A?g^-1的电流密度下可逆比容量可达到99.7 mAh?g^-1，在10.0 A?g^-1电流密度下循环2000周后可逆比容量依旧高达76.1 mAh?g^-1。因此，双碳包裹对于提升磷酸钒钠电极性能的有益作用，为改善聚阴离子型正极材料的电化学性能提供借鉴。

Abstract:

Due to the rich reserves， low-cost and environmental friendliness， the Na₃V₂（PO₄）₃ （NVP） with NASICON structure， possessing rapid ionic diffusion pathways， has attracted increasing attentions. The open framework， stable sodium storage sites and three-dimensional networks for ion migration make the Na₃V₂（PO₄）₃ to be one of the most promising cathode materials for sodium-ion batteries （SIBs）. Unfortunately， the poor electronic conductivity and structure change of NVP lead to the increased voltage polarization and degradation of crystal structure， which results in poor cycling performance. To address those issues mentioned above， a facile approach combining sol-gel and subsequent polydopamine coating was developed for the synthesis of dual carbon coated NVP， using citric acid and polydopamine as the carbon sources， which can be converted to porous carbon and nitrogen doped carbon simultaneously. Benefitting from the improved electronic conductivity and efficient buffering matrix offered by the dual carbon coating， the resultant NVP wrapped by porous carbon and nitrogen doped carbon （NVP/C/NC） showed superior electrochemical performance， delivering reversible capacities of 99.1 mAh?g^-1 after 100 cycles at 0.1 A?g^-1 and 76.1 mAh?g^-1 after 2000 cycles at 10.0 A?g^-1. More importantly， this work sheds some light for the design and fabrication of high-performance NVP based electrodes.