Cathode Properties of Na₃FePO₄CO₃ Prepared by the Mechanical Ball Milling Method for Na-ion Batteries

The carbonophosphate Na₃FePO₄CO₃ was synthesized by the mechanical ball milling method for the first time. The composition of the obtained sample with a higher amount of Fe²⁺ was Na_2.66Fe²⁺_0.66Fe³⁺_0.34PO₄CO₃ as confirmed by Mössbauer analysis, owing to the good airtight properties of this method. The obtained samples in an organic electrolyte delivered an initial discharge capacity of 124 mAh/g at room temperature, and a larger discharge capacity of 159 mAh/g (1.66 Na⁺/mole) at 60 °C. With 17 m NaClO₄ aqueous electrolyte, a discharge capacity of 161 mAh/g (1.69 Na⁺/mole) was delivered because of the high ionic conductivity of the concentrated aqueous electrolyte. During the charge-discharge process, the formation of Fe⁴⁺ after charging up to 4.5 V and the return of Fe²⁺ after discharging down to 1.5 V were detected by ex-situ X-ray absorption near edge structure (XANES) analysis.