Comprehensive Investigation of the Na₃V₂(PO₄)₂F₃–NaV₂(PO₄)₂F₃ System by Operando High Resolution Synchrotron X-ray Diffraction

Na3V2(PO4)2F3 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 toward 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 NaV2(PO4)2F3 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 among 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-length variations can be tracked, as well as polyhedral distortions and site occupancy factors for mobile sodium ions. For NaV2(PO4)2F3 a full crystal structure determination was also carried out for the first time directly from operando measurements, assigning it to the Cmc21 space group and revealing two vanadium environments: V3+ and V5+. 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.