Na3V2(PO4)2F3 Revisited: A High-Resolution Diffraction Study

Na3V2(PO4)(2)F-3 is a material that has been attracting great interest as a potential positive electrode for Na-ion batteries. Its crystal structure was determined from single-crystal X-ray diffraction in 1999 by Le Meins et al. in the tetragonal space group P4(2)/mnm at 298 K. In this work, we show how the use of very high angular resolution synchrotron radiation diffraction reveals a subtle orthorhombic distortion with unit-cell parameters of a = 9.02847(3) angstrom, b = 9.04444(3) angstrom, c = 10.74666(6) angstrom in the Amam space group. Although this only slightly impacts the structural framework of the material, it reveals a significantly modified distribution of Na ions. Furthermore, the crystal structure of the high-temperature form of Na3V2(PO4)(2)F-3 (at 400 K) was determined for the first time. This allowed comparing the totally disordered distribution of Na ions in this case with the partially ordered one of the room-temperature phase. We report here on an original structure and on an original electrochemical signature for stoichiometric Na3V2(PO4)(2)F-3, and we propose that fluctuations in the O/F ratio are at the origin of discrepancies found in the literature.