Hollow-Sphere-Structured NaFe(PO)(PO)/C as a Cathode Material for Sodium-Ion Batteries.

The mixed polyanionic material NaFe(PO)(PO) combines the advantages of NaFePO and NaFePO in capacity, stability, and cost. Herein, we synthesized carbon-coated hollow-sphere-structured NaFe(PO)(PO) powders by a scalable spray drying route. The optimal sample can deliver a high discharge capacity of 107.7 mA h g at 0.2C. It also delivers a capacity of 88 mA h g at 10C and a capacity of retention of 92% after 1500 cycles. Ex situ X-ray diffraction analysis indicates a slight volume change (less than 3%) in the NaFe(PO)(PO) lattice cell. Therefore, such a spraying-derived carbon-coated NaFe(PO)(PO) powder is a very attractive cathode electrode for sodium-ion batteries.