Full Utilization of Superior Charge-Discharge Characteristics of Na_1.56Fe_1.22P₂O₇Positive Electrode by Using Ionic Liquid Electrolyte

Na 1.56 Fe 1.22 P 2 O 7 was synthesized via a conventional solid-state method and evaluated as a positive electrode for Na secondary batteries using Na[FSA]–[C 3 C 1 pyrr][FSA] (C 3 C 1 pyrr = N -methyl- N -propylpyrrolidinium and FSA = bis(fluorosulfonyl)amide) ionic liquids (IL) as electrolytes, over the temperature range of 298–363 K. A reversible capacity as high as 108 mAh g –1 has been achieved for the first time with a thermally stable IL at 363 K. This value is close to the theoretically calculated capacity of 118 mAh g −1 and is significantly higher than the previously reported values of ca . 85 mAh g −1 in organic electrolytes at 298 K. The capacity of 108 mAh g −1 also exceeds the value obtained for Na 2 FeP 2 O 7 electrodes (94 mAh g −1 ) under the same experimental conditions. Moreover, excellent rate capability and superior cyclability exceeding 3000 cycles have been achieved by using the IL electrolyte over a wide temperature range of 298–363 K.