A Comparison of Crystal Structures and Electrode Performance between Na sub(2)FePO sub(4)F and Na sub(2)Fe sub(0.5)Mn sub(0.5)PO sub(4)F Synthesized by Solid-State Method for Rechargeable Na-Ion Batteries

• Published in: Denki kagaku oyobi kōgyō butsuri kagaku Vol. 80; no. 2; pp. 80 - 84
• Main Authors: Kawabe, Y, Yabuuchi, N, Kajiyama, M, Fukuhara, N, Inamasu, T, Okuyama, R, Nakai, I, Komaba, S
• Format: Journal Article
• Language: Japanese
• Published: 01.02.2012
• Subjects: Ascorbic acid; Carbon; Discharge; Electric potential; Electrodes; Nanostructure; Scanning electron microscopy; Voltage
• ISSN: 1344-3542

Carbon-coated Na sub(2)FePO sub(4)F and Na sub(2)Fe sub(0.5)Mn sub(0.5)PO sub(4)F are successfully prepared by a simple solid-state method with ascorbic acid as carbon source. Crystal structure of Na sub(2)Fe sub(0.5)Mn sub(0.5)PO sub(4)F is found to be isostructural with Na sub(2)MnPO sub(4)F 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 sub(2)FePO sub(4)F is compared with that of Na sub(2)Fe sub(0.5)Mn sub(0.5)PO sub(4)F. The carbon-coated Na sub(2)FePO sub(4)F prepared with 2wt% ascorbic acid delivers discharge capacity of 100-110 mAhg super(-1) at a rate of 1/20C (6.2 mAg super(-1)) with well-defined voltage plateaus at 3.06 and 2.91V vs. Na metal. In contrast, Na sub(2)Fe sub(0.5)Mn sub(0.5)PO sub(4) 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 sub(2)Fe sub(0.5)Mn sub(0.5)PO sub(4)F sample delivers the discharge capacity of 110 mAhg super(-1) at a rate of 1/20C (6.2 mAg super(-1)), and it is first demonstrated that average operating voltage is higher than that of Na sub(2)FePO sub(4)F based on the possible Mn super(2+)/Mn super(3+) redox couple centered at 3.53 V.