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

Abstract

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.