Carbon-coated Na
2FePO
4F and Na
2Fe
0.5Mn
0.5PO
4F are successfully prepared by a simple solid-state method with ascorbic acid as carbon source. Crystal structure of Na
2Fe
0.5Mn
0.5PO
4F is found to be isostructural with Na
2MnPO
4F by an X-ray diffraction method, which has a three dimensional fluorophosphate framework. Scanning/transmission electron microscopy and Raman spectroscopy reveal that the addition of ascorbic acid effectively suppresses the particle growth of the samples, forming the nano-sized carbon coated materials. Electrode performance of Na
2FePO
4F is compared with that of Na
2Fe
0.5Mn
0.5PO
4F. The carbon-coated Na
2FePO
4F prepared with 2 wt% ascorbic acid delivers discharge capacity of 100–110 mAh g
−1 at a rate of 1/20 C (6.2 mA g
−1) with well-defined voltage plateaus at 3.06 and 2.91 V vs. Na metal. In contrast, Na
2Fe
0.5Mn
0.5PO
4F is less electrochemically active. The higher content of carbon by the adding ascorbic acid (6 wt%) and ball-milling treatment are necessary to achieve high reversible capacity. A well-optimized Na
2Fe
0.5Mn
0.5PO
4F sample delivers the discharge capacity of 110 mAh g
−1 at a rate of 1/20 C (6.2 mA g
−1), and it is first demonstrated that average operating voltage is higher than that of Na
2FePO
4F based on the possible Mn
2+/Mn
3+ redox couple centered at 3.53 V.
View full abstract