Iron oxidation to amplify the Na and Li storage capacities of nano-sized maricite NaFePO4

This study reports an effective approach to improve dramatically the electrochemical performance of nanosized NaFePO₄ with a maricite structure, which is commonly considered as electrochemically inactive due to the absence of structural channels for alkaline ion mobility. The approach is based on the complete oxidation under mild conditions (i.e. at low temperatures around 280 °C and traces of oxygen) of the nanosized maricite phase. It is prepared by the phosphate-formate precursor method and is additionally ball-milled with a carbon additive. The oxidation of Fe²⁺ proceeds at the nanoscale level within the maricite nanoparticles and causes a massive structural transformation of the maricite phase into a monoclinic NASICON phase Na₃Fe₂(PO₄)₃ with the preservation of the crystallinity. The oxidized maricite phase exhibits high specific capacities, cycling stability and rate capability when it is used as an electrode in both Na and Li half-cells. The effect of different sodium and lithium electrolytes on the storage performance is investigated as well. It is found that the highest specific capacity (of about 150 mA h g^-1) is achieved in Li half-cells using the LiPF₆ electrolyte, while in Na half-cells the electrolyte NaFSI/EC:DMC achieves a specific capacity of around 100 mA h g^-1. The rate capability is better in Na half-cells than that in Li half-cells. The mechanism of the reversible intercalation/deintercalation of Na⁺ and Li⁺ ions is studied by ex situ XRD and TEM analyses. The results show that the maricite is an electrochemically inactive phase, but through manipulation including oxidation or amorphization it becomes an active electrode material.