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 Na2.66Fe2+ 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.
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