Synthesis of uniform hierarchical Na3V1.95Mn0.05(PO4)(2)F-3@C hollow microspheres as a cathode material for sodium-ion batteries

Uniform hierarchical Na3V1.95Mn0.05(PO4)(2)F-3@C hollow microspheres composed of nanoplates and nanoparticles were successfully synthesized by a one-step polyol-assisted hydrothermal method combined with chemical vapour deposition (CVD). We also investigated the formation mechanism of the hierarchical hollow microspheres and it was found that both tetraethylene glycol and citric acid play important roles in the formation of the hierarchical hollow microspheres. Na3V1.95Mn0.05(PO4)(2)F-3@C showed an initial reversible capacity of 122.9 mA h g(-1) and the discharge capacity of 109 mA h g(-1) was maintained with a coulombic efficiency of 99.1% after 500 cycles. The unique architecture, enhanced electronic conductivity and improved diffusion coefficient of sodium ions for Na3V2(PO4)(2)F-3 by Mn2+ doping as well as carbon coating account for the enhanced electrochemical properties. Therefore, we present an approach to design a tunable structure for multi-element compounds with enhanced electrochemical performance as a promising cathode material for practical applications.