High performance cathode material based on Na3V2(PO4)2F3 and Na3V2(PO4)3 for sodium-ion batteries

Na3V2(PO4)2F3 attracts lots of attention due to its high plateau, three-dimensional ion diffusion channel and small volume deformation. However, it suffers from low intrinsic electrical conductivity. To improve the electrochemical performances, carbon incorporation is a common way. Interestingly, an unknown plateau at 3.4 V vs. Na/Na+ usually appears in the charge and discharge profile of Na3V2(PO4)2F3/C using carbon thermal reduction method. To understand this phenomenon, a mechanism involving fluorine loss and structure transformation during heat treatment of Na3V2(PO4)2F3/C is proposed. In the existence of carbon, fluorine in Na3V2(PO4)2F3 is not stable and Na3V2(PO4)2F3 would transform to Na3V2(PO4)3, V2O3 and Na3VF6. Based on this phenomenon, we fabricated a high performance cathode material based on Na3V2(PO4)2F3 and Na3V2(PO4)3 for sodium-ion batteries. High energy, high rate and long cycle are achieved by integration of high energy Na3V2(PO4)2F3, stable Na3V2(PO4)3, highly conductive V2O3, and sodium ion active Na3VF6. These findings can enrich the understanding of vanadium-based polyanion cathode materials for energy storage and may arouse interest in studies of hybrid cathode materials for batteries.