Electrochemical properties of mixed-phosphates Nax+2Fex+1(PO4)x(P2O7) with different ratios of PO43-/P2O74-

Iron-based phosphates are low cost and high structural stability cathode materials for sodium ion batteries, which are considered to be the most promising power source for large-scale energy storage systems. Here, we report the electrochemical properties of Nax+2Fex+1(PO4)x(P2O7) (x = 1, 2, 3, 4) by adjusting the molar ratio of PO43- to P2O74-. When PO43-/P2O74- is one or two, there are single-phases with higher capacity and similar structure. When PO43-/P2O74- is three or four, there are mixed-phases with lower capacity containing Na4Fe3(PO4)2(P2O7) and maricite NaFePO4. Both single-phases and mixed-phases have good stability and reversibility. This study provides a new strategy to regulate PO43- and P2O74- in the mixed-polyanion system for enriching materials design strategies for new sodium ion battery cathode materials.