An approach to flexible Na-ion batteries with exceptional rate capability and long lifespan using Na₂FeP₂O₇ nanoparticles on porous carbon cloth

As post-Li-ion batteries (LIBs), rechargeable Na-ion batteries (NIBs) are considered as one of the potential candidates for large-scale energy storage systems because of the abundance and low cost of sodium resources, and similar electrochemical behavior of Na ions to Li ions for intercalation in cathodes. While there exist many challenges in the fabrication of cathodes, a polyanionic compound, Na₂FeP₂O₇, has been in the spotlight as a potential cathode material for NIBs because of its rate capability, cyclability, and thermal stability. In this study, Na₂FeP₂O₇ nanoparticles (NFP-NPs) embedded in carbon were prepared via a citric acid-assisted sol-gel method, followed by a post-heat treatment process. For the first time, NFP-NPs exhibited a reversible capacity close to the theoretical value (95 mA h g⁻¹) over the voltage range of 2.0-4.0 V (vs. Na/Na⁺). Moreover, they displayed a superior rate capability of 77, 70, 66 and 65 mA h g⁻¹ even at high rates of 10, 20, 30 and 60C, respectively. Equally notable is their exceptional long-term cyclability at high rates. At the rate of 10 and 60C, the capacity retention after 10 000 cycles is 83 and 84%, respectively. In addition, NFP-NPs uniformly loaded on the surface of flexible porous carbon cloth (NFP-NPs@PCC) electrodes without any conductive agents and polymeric binders also exhibit excellent rate capability and long-term cyclability at a high rate of 10C (56 mA h g⁻¹ after 2000 cycles). We show high-performance free-standing NFP-NPs@PCC electrodes for possible application in flexible NIBs.