A Combined Operando Synchrotron X-ray Absorption Spectroscopy and First-Principles Density Functional Theory Study to Unravel the Vanadium Redox Paradox in the Na₃V₂(PO₄)₂F₃–Na₃V₂(PO₄)₂FO₂ Compositions

The redox processes involved in the Na + deintercalation from Na 3 V 2 (PO 4 ) 2 F 3, Na 3 V 2 (PO 4 ) 2 F 2 O, and Na 3 V 2 (PO 4 ) 2 F 1.5 O 1.5 are investigated operando by synchrotron X-ray absorption spectroscopy at the vanadium K-edge. A continuous evolution in the pre-edge and edge regions is observed for the three compositions, which is in good agreement with a progressive oxidation of vanadium. In the oxygen-substituted compositions, Na 3 V 2 (PO 4 ) 2 F 2 O and Na 3 V 2 (PO 4 ) 2 F 1.5 O 1.5, the F–V 3+ O 4 –F and F–V 4+ O 4 ═O redox centers coexist and can be simultaneously activated owing to the reverse inductive effect promoted by the presence of highly covalent vanadyl bonds on V 4+ sites. Principal component analysis of the dataset collected operando for the three compositions is performed. It reveals that three independent spectra, corresponding to different charge orderings on vanadium sites at different states of charge, are required to describe these three systems. Furthermore, the extended X-ray absorption fine structure analysis of the principal components is performed in order to gain insightful information on the evolution of the vanadium local environment upon cycling. By the use of first-principles density functional theory calculations, we demonstrate that the potential step observed in all Na 2 V 2 (PO 4 ) 2 F 3– y O y compositions is related to a charge ordering on vanadium sites with a preferential oxidation of vanadium ions in the same bioctahedra.