Pyrophosphate Na2FeP2O7 as a low-cost and high-performance positive electrode material for sodium secondary batteries utilizing an inorganic ionic liquid

• The performance of Na2FeP2O7 positive electrode was evaluated at 363 K.
• NaFSA–KFSA inorganic ionic liquid was used as an electrolyte.
• The reversible capacity of Na2FeP2O7 reaches 91 mA h g−1.
• Capacity retention of 91% after 1000 cycles was demonstrated.
• Variations of Fe valence states and Fe–O bonds upon electrochemical cycling were studied by XAS.

The electrochemical performance of a Na2FeP2O7 positive electrode has been evaluated in an inorganic ionic liquid NaFSA–KFSA (FSA = bis(fluorosulfonyl)amide) at 363 K. The electrode delivers a reversible capacity of 91 mAh g−1 with excellent rate capability (59 mAh g−1 at 2000 mA g−1) and a capacity retention of 91% over 1000 cycles, which facilitates the development of low-cost and high-safety sodium secondary batteries for large-scale energy storage applications. The average oxidation state of iron increases upon sodium extraction, as evidenced by the edge shift of an X-ray absorption spectroscopy analysis. According to an extended X-ray absorption fine structure analysis, the sodium extraction is accompanied by a shortening of Fe–O bonds.