Unexpected discovery of low-cost maricite NaFePO4 as a high-performance electrode for Na-ion batteries

Battery chemistry based on earth-abundant elements has great potential for the development of cost-effective, large-scale energy storage systems. Herein, we report, for the first time, that maricite NaFePO4 can function as an excellent cathode material for Na ion batteries, an unexpected result since it has been regarded as an electrochemically inactive electrode for rechargeable batteries. Our investigation of the Na re-(de)intercalation mechanism reveals that all Na ions can be deintercalated from the nano-sized maricite NaFePO4 with simultaneous transformation into amorphous FePO4. Our quantum mechanics calculations show that the underlying reason for the remarkable electrochemical activity of NaFePO4 is the significantly enhanced Na mobility in the transformed phase, which is similar to one fourth of the hopping activation barrier. Maricite NaFePO4, fully sodiated amorphous FePO4, delivered a capacity of 142 mA h g(-1) (92% of the theoretical value) at the first cycle, and showed outstanding cyclability with a negligible capacity fade after 200 cycles (95% retention of the initial cycle).