Charge–discharge behavior of a Na2FeP2O7 positive electrode in an ionic liquid electrolyte between 253 and 363 K

The electrochemical properties of a Na2FeP2O7 positive electrode were investigated in the ionic liquid sodium bis(fluorosulfonyl)amide (NaFSA)–N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)amide (C3C1pyrrFSA). A stable charge–discharge behavior was obtained over the temperature range 253–363 K. The cell offered nearly a one-electron theoretical capacity of about 90 mAh g−1 at 298–363 K. The rate capability showed considerable enhancement with increasing temperature, delivering 50 mAh g−1 at a very high rate of 4000 mA g−1 (ca. 41C) at 363 K. Furthermore, the Na2FeP2O7 positive electrode demonstrated excellent cyclability, exceeding 300 cycles, in terms of both capacity retention and coulombic efficiency at 298–363 K, as only negligible capacity fade (<1%) was observed. This good performance over a wide temperature range will meet the requirements of the versatile applications expected for Na secondary batteries.