V^IV Disproportionation Upon Sodium Extraction From Na₃V₂(PO₄)₂F₃ Observed by Operando X-ray Absorption Spectroscopy and Solid-State NMR

Abstract

Among the series of polyanionic positive electrodes for sodium-ion batteries having the general formula Na3V2(PO4)2F3–yOy (0 ≤ y ≤ 2), the composition Na3V2(PO4)2F3 (y = 0) has the highest theoretical energy that offers competitive electrochemical performances compared to sodium transition metal oxides. Recently, the structural phase diagram from Na3V2(PO4)2F3 to Na1V2(PO4)2F3 has been thoroughly investigated by operando synchrotron X-ray diffraction revealing an unexpected structural feature for the end member composition. In fact, the crystal structure of Na1V2(PO4)2F3 has two very different vanadium environments within each bioctahedron that suggests a charge disproportionation of two VIV into VIII and VV. This work shows an operando X-ray absorption spectroscopy at vanadium K edge during the electrochemical extraction of Na+ in order to monitor the redox processes involved in this compound. The large data set provided by this experiment has been processed by the principal component analysis combined with multivariate curve resolution. The results suggest that the bioctahedra have to be considered as the basic structural unit. The peculiar geometry of this material combined with the mixed vanadium valence, directly investigated here along the reaction, seems to allow original electronic configurations. In particular, the two vanadium sites into the basic bioctahedra unit evolve from VIII–VIII to VIII–VIV and to a final VIII–VIV configuration. These observations are completed with 51V NMR sensitive to diamagnetic VV.