Cathode properties of Na3M2(PO4)2F3 [M = Ti, Fe, V] for sodium-ion batteries

The compounds Na3M2(PO4)(2)F-3 [M = Ti, Fe, V] were obtained by two-step solid-state synthesis, and its electrochemical properties as cathode active materials of sodium-ion batteries were investigated. The obtained Na3M2(PO4)(2)F-3 materials were found to have a tetragonal structure with a space group, P4(2)/mnm, and were isostructural with beta-Na3V2(PO4)(2)F-3 by X-ray powder diffraction (XRD) measurements. Within this series, Na3V2(PO4)(2)F-3 in particular exhibited stability on cycling and a capacity of approximately 120 mAh g(-1) against sodium metal. Capacity retention for this compound was 98%, even after the 40th cycle of prolonged charge/discharge measurements. Ex-situ XRD data for Na3V2(PO4)(2)F-3 electrodes charged and discharged at selected voltages showed that the material structure expanded along the c axis during charging up to 4.3 V and returned to its original dimensions after cycling. This is attributed to the extraction/insertion of Na atoms layers along the (002) a-b plane in the tetragonal structure. (C) 2012 Elsevier B.V. All rights reserved.