A New Polyanion Na₃Fe₂(PO₄)P₂O₇ Cathode with High Electrochemical Performance for Sodium-Ion Batteries

Iron-based phosphate materials have been employed as cathodes for sodium-ion batteries (SIBs) because of their low cost and environmental friendliness, but the electrochemical performance of this kind cathode is far from that of a practical application. On one hand, the huge volume expansion during sodium-ion insertion/extraction restricts its cycle life. On the other hand, the relativity low capacity and working potential result in a low energy density. Herein, we report a new NASCION-phase iron-based phosphate material with a formula of Na₃Fe₂(PO₄)­(P₂O₇), which possesses a theoretical capacity of 119 mAh g^–1 and an average working potential of 3.1 V versus Na/Na⁺, along with a small volume change during Na-ion insertion/extraction in SIBs. Moreover, the reduced graphene oxide has been introduced during synthesis to enhance the electrochemical performance. The composite cathode exhibits an initial reversible specific capacity of 110.2 mAh g^–1 at 0.1 C and maintains a specific capacity of 89.7% after 6400 cycles at 20 C.