Cathode Properties of Na(3)FePO(4)CO(3) Prepared by the Mechanical Ball Milling Method for Na-ion Batteries

The carbonophosphate Na(3)FePO(4)CO(3) was synthesized by the mechanical ball milling method for the first time. The composition of the obtained sample with a higher amount of Fe(2+) was Na(2.66)Fe(2+)(0.66)Fe(3+)(0.34)PO(4)CO(3) 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 NaClO(4) 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 Fe(4+) after charging up to 4.5 V and the return of Fe(2+) after discharging down to 1.5 V were detected by ex-situ X-ray absorption near edge structure (XANES) analysis.