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

Abstract The compounds Na 3 M 2 (PO 4 ) 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 Na 3 M 2 (PO 4 ) 2 F 3 materials were found to have a tetragonal structure with a space group, P4 2 /mnm , and were isostructural with β -Na 3 V 2 (PO 4 ) 2 F 3 by X-ray powder diffraction (XRD) measurements. Within this series, Na 3 V 2 (PO 4 ) 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 Na 3 V 2 (PO 4 ) 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.