Na 3 V 2 (PO 4 ) 2 F 3 is a positive electrode material for Na-ion batteries which is attracting strong interest due to its high capacity, rate capability and long-term cycling stability. The sodium extraction mechanism from this material has been always described in the literature as a straightforward solid solution, but several hints point towards a more complicated phase diagram. In this work we performed high angular resolution synchrotron radiation diffraction measurements, realized operando on sodium batteries upon charge. We reveal an extremely interesting phase diagram, created by the successive crystallization of four intermediate phases before the end composition NaV 2 (PO 4 ) 2 F 3 is reached. Only one of these phases undergoes a solid solution reaction, in the interval between 1.8 and 1.3 Na per formula unit. The ability to resolve weak Bragg reflections allowed us to reveal differences in terms of symmetry amongst the phases, to determine their previously unknown space groups and to correlate them with sodium (dis)ordering in the structure. Rietveld refinements enabled us to follow fine structural modifications in great detail. Intermediate identified phases are not simply described by their unit cell parameters but bond-lengths variations can be tracked, as well as polyhedral distortions and site occupancy factors for mobile sodium ions. For NaV 2 (PO 4 ) 2 F 3 a full crystal structure determination was also carried out for the first time directly from operando measurements, assigning it to the Cmc2 1 space group and revealing two vanadium environments: V 3+ and V 5+ . Our study demonstrates that improved angular resolution and high intensity diffraction data are key parameters for direct observation of fine reaction pathways in electrode materials, and that the obtained insight is crucial for the understanding of (de)intercalation mechanisms in Na-ion batteries.

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.