Full Utilization of Superior Charge-Discharge Characteristics of Na1.56Fe1.22P2O7 Positive Electrode by Using Ionic Liquid Electrolyte

Na1.56Fe1.22P2O7 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 Na2FeP2O7 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. (C) The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org. All rights reserved.