A Comparison of Crystal Structures and Electrode Performance between Na2FePO4F and Na2Fe0.5Mn0.5PO4F Synthesized by Solid-State Method for Rechargeable Na-Ion Batteries

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Carbon-coated Na2FePO4F and Na2Fe0.5Mn0.5PO4F are successfully prepared by a simple solid-state method with ascorbic acid as carbon source. Crystal structure of Na2Fe0.5Mn0.5PO4F is found to be isostructural with Na2MnPO4F 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 Na2FePO4F is compared with that of Na2Fe0.5Mn0.5PO4F. The carbon-coated Na2FePO4F 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, Na2Fe0.5Mn0.5PO4F 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 Na2Fe0.5Mn0.5PO4F 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 Na2FePO4F based on the possible Mn2+/Mn3+ redox couple centered at 3.53 V.
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