Carbon coated Na₇Fe₇(PO₄) ₆F₃: A novel intercalation cathode for sodium-ion batteries

Electrode materials are being developed to realise sodium-ion batteries that can provide energy storage solutions. Here, we develop amorphous carbon coated Na₇Fe₇(PO₄)₆F₃, prepared by combining hydrothermal and solid state reaction methods, as an insertion electrode for sodium-ion batteries applications. Na₇Fe ₇(PO₄)₆F₃ particles are surrounded by a thin layer (∼1.5-2 nm) of amorphous carbon. The Na₇Fe ₇(PO₄)₆F₃/C composite cathode undergoes reversible sodium intercalation/de-intercalation with an average operational potential of ∼3.0 V (vs Na⁺/Na). This cathode has a capacity of 65 mA h g^-1 at 100 mA g^-1 current after 60 cycles and features twice higher capacity than that of an uncoated Na ₇Fe₇(PO₄)₆F₃ sample. Therefore, the carbon-coated Na₇Fe₇(PO₄) ₆F₃ composite presents feasible sodium intercalation/de-intercalation capacity, offering possibilities for developing a low cost, high performance sodium-ion battery positive electrode.