on phosphate nanosheets with high rate capability and ultra-long cycle life for sodium ion batteries †

In our previous work, we reported the formation and mechanism of mono/bi-layer phosphate-based materials and their high performance as cathode materials for Li-ion batteries. In this work, we report that 2D amorphous nanosheets can be used as cathode materials to achieve outstanding performance for sodium ion batteries (SIBs) e.g. a high initial discharge capacity of 168.9 mA h g 1 at 0.1C, ultra-long life (92.3% capacity retention over 1000 cycles), and high rate capability (77 mA h g 1 at 10C) for Na-ion storage, whose electrochemical performance is also much superior to the reported amorphous FePO4 or olivine NaFePO4 with advantages of short paths and larger implantation surface areas for fast Na-ion diffusion and large specific surfaces with more interfacial capacitance. Interestingly, NaFePO4 nanocrystals with about 10 nm sizes are self-nucleated from amorphous 2D nanosheets in the charge/ discharge process, which was verified by transmission electron microscopy (TEM) and in situ electrochemical impedance spectroscopy (EIS).