Na3V2(PO4)2F3 revisited : a high-resolution diffraction study

Na3V2(PO4)2F3 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 P42/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) Å, b = 9.04444(3) Å, c = 10.74666(6) Å 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)2F3 (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)2F3, and we propose that fluctuations in the O/F ratio are at the origin of discrepancies found in the literature