掺杂Na3V2(PO4)3/C复合多孔纳米纤维的制备及其电化学嵌入/脱嵌钠离子行为的研究

Sodium-ion batteries have been considered as promising candidates for large-scale applications because of their advantages such as abundance of sodium resources, low cost and high safety, etc. The development of cathodes with excellent performance is the key to develop sodium-ion batteries. The NASICON type of Na3V2(PO4)3 displays merits such as high operating voltage , excellent cyclability and thermal stability, which has been recognized as a potential cathode material for sodium-ion batteries. However, the poor electronic conductivity severely degrades the high-rate charge/discharge performance of Na3V2(PO4)3. In order to solve this scientific problem, the doped Na3V2(PO4)3/C composite porous nanofibers consisting of carbon nanofibers and doped Na3V2(PO4)3 nanofibers will be prepared by electrospinning method in this proposal. In this composite, Na3V2(PO4)3 will be doped by cations in V and/or Na sites, and the suitable dopant in V site will improve the intrinsic electronic conductivity, while the suitable dopant in Na site will enhance the ionic conductivity. On the other hand, the carbon nanofibers exhibit high electronic conductivity, which are potentially used to significantly improve electronic conductivity of composite cathode material. Additionally, the nanofibers facilitate rapid transportation of electrons and sodium cations, which is favorable to improve the kinetics performance of Na+ insertion/extraction within electroactive particles. As the result, the doped Na3V2(PO4)3/C composite porous nanofibers will be of high electronic and ionic conductivities and hence exhibit excellent electrochemical performance. In summary, this proposal focus on the study of the effects of preparation conditions on the morphology, structure, performance and electrochemical reaction mechanism of the doped Na3V2(PO4)3/C composite porous nanofibers and further elucidates the relationship between performance and structure of composite nanofibers. Moreover, this proposal will provide a new strategy for the development of cathode materials for sodium-ion batteries and the efficient utilization of vanadium resources in Guangxi.

钠离子电池因其钠来源丰富、成本低、安全性高等优点而具有大规模应用的前景。高性能正极材料是钠离子电池发展的关键。钠快离子导体Na3V2(PO4)3因具有放电平台高、循环和热稳定性能优异等优点，成为最具潜力的钠离子电池正极材料，但极低的电子电导率严重影响其高倍率充放电性能。针对该科学问题，本项目拟通过静电纺丝法合成掺杂Na3V2(PO4)3/C复合多孔纳米纤维，将碳纳米纤维的高电子电导率、纳米纤维能快速传输电子和离子、钒位掺杂改善材料的本征电子电导率、钠位掺杂提高材料的离子电导率等优点融合为一，使掺杂Na3V2(PO4)3/C复合多孔纳米纤维兼备高的电子和离子电导率，从而具有优异的电化学性能。研究制备条件对复合多孔纳米纤维的形貌、结构、性能及电化学机理的影响规律，阐明结构与性能之间的关系。为钠离子电池正极材料的发展及广西钒资源的高效利用提供新的思路和方法。

钠快离子导体Na3V2(PO4)3 因具有放电平台高、循环和热稳定性能优异等优点，成为最具潜力的钠离子电池正极材料，但极低的电子电导率严重影响其高倍率充放电性能。针对该科学问题，本项目采用制备兼具高电子电导率和离子电导率的纳米纤维复合材料的策略，利用纳米纤维显著缩短钠离子的扩散路径、较大的比表面积显著降低电极材料的真实电流密度的优点，从而显著降低其电化学极化，显著提升材料的电化学性能。碳纳米纤维的存在，可以明显提高材料的电子电导率；通过钒位的掺杂，改变材料的导带隙，提高材料的本征电子电导率并稳定材料的结构；通过钠位的掺杂，改变活性材料的晶胞体积，为钠离子的扩散提供更大的通道，提升材料的离子扩散速率；钠位、钒位的共掺杂，提升材料的本征电子电导率和离子电导率，从而使复合材料具有优异的电化学性能。.本项目首先通过静电纺丝法制备了Na3V2(PO4)3/C 复合多孔纳米纤维，在此基础上，改变静电纺丝液的成分，分别制备钒位掺杂Na3V2(PO4)3/C复合纳米纤维、钠位掺杂Na3V2(PO4)3/C复合纳米纤维及钠位、钒位共掺杂Na3V2(PO4)3/C复合纳米纤维。研究结果表明，静电纺丝条件及高温焙烧条件决定复合纤维的直径大小，在本项目最优条件下获得直径小于150 nm的复合纳米纤维；掺杂离子影响复合材料的容量及循环性能，最佳的钒位掺杂复合材料为Na3V1.93Mg0.07(PO4)3/C复合多孔纳米纤维、Na3V1.95Mn0.05(PO4)3/C复合多孔纳米纤维、Na2.9K0.1V2(PO4)3/C复合多孔纳米纤维、Na2.9K0.1V1.93Mg0.07(PO4)3/C复合多孔纳米纤维，这些掺杂材料的高倍率性能及循环性能都明显优于Na3V2(PO4)3/C复合纳米纤维。本项目所获得的研究结果将为磷酸钒钠纳米纤维产业化提供理论指导，并为制备其它高性能电极材料提供新的思路。