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

The compounds Na3M2(PO4)2F3 [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)2F3 materials were found to have a tetragonal structure with a space group, P42/mnm, and were isostructural with β-Na3V2(PO4)2F3 by X-ray powder diffraction (XRD) measurements. Within this series, Na3V2(PO4)2F3 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)2F3 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.

► This is the first report about the cathode properties of Na3M2(PO4)2F3 against Na or NaTi2(PO4)3 NASICON anode.
► Na3V2(PO4)2F3 has 4 V voltage plateau with 120 mAh g−1, which is a rare high voltage cathode against Na.
► Reversible c lattice expansion/contraction of Na3V2(PO4)2F3 electrodes was observed by ex-situ XRD during cycling.