Microwave-assisted hydrothermal synthesis, structure and electrochemical properties of the Na3V2-yFeyО2x(PO4)2F3-2x electrode materials for Na-ion batteries

Abstract The Na3V2-yFeyО2x(PO4)2F3-2x (x = 1–0.8, y = 0–0.8) solid solutions have been prepared using a microwave-assisted hydrothermal technique. The materials crystallize in the disordered I4/mmm crystal structure as evidenced with electron diffraction and Rietveld refinement from powder X-ray diffraction data (Na3V1.2Fe0.8O1.6(PO4)2F1.4: a = 6.38666(9)A, c = 10.6562(2)A, RF = 0.023, RP = 0.013). A combination of electron energy loss spectroscopy and 57Fe Mossbauer spectroscopy revealed that vanadium is in the mixed 4+/5 ​+ ​oxidation state, whereas iron is exclusively in the 3 ​+ ​state, thus pointing to the V4+ → Fe3+/V5+ co-substitution. The solid solutions demonstrate reversible Na+ (de)intercalation in the electrochemical cells with Na metal counter-electrode through two sloping potential plateaus at near 3.5 and 4.0 V vs Na+/Na. The electrochemical capacity decays from 105 mAh/g for y = 0 to ~50 mAh/g for y = 0.8 because of decreasing amount of electrochemically active V4+ upon increasing the Fe content.