An air-stable iron/manganese-based phosphate cathode for high performance sodium-ion batteries

Iron-based phosphates as a low cost and high structural stability cathode materials for sodium ion batteries (SIBs) have been widely studied. However, the working potential basing on Fe3+/Fe2+ redox is very low (less than 3.05 V vs. Na/Na+), which has obviously affect on the energy/power density. In this work, we choose the non-precious metal manganese (Mn) to improve the working potential of Na3Fe2(PO4)(P2O7) (NFFPP). A series of mixed-phosphate Na3Fe2-xMnx(PO4)(P2O7) (x = 0.0, 0.5, 1.0, 1.5 and 2.0) compounds are firstly proposed as cathode materials for SIBs. After optimization, the as-prepared Na3FeMn(PO4)(P2O7) (NFMPP) could deliver a higher average working potential of 3.27 V (vs. Na/Na+) with a theoretical capacity of 120 mAh g−1. Furthermore, the carbon coating NFMPP cathode shows a reversible initial specific capacity of 105 mAh g−1 at 0.1C with a corresponding coulombic efficiency of 99%. The in-situ XRD and ex-situ XPS results indicate that this cathode material goes through the reversible redox Fe3+/Fe2+ and Mn3+/Mn2+ with a reversible two Na-ion insertion/deinsertion process.