Density Functional Theory-Assisted ³¹P and ²³Na Magic-Angle Spinning Nuclear Magnetic Resonance Study of the Na₃V₂(PO₄)₂F₃–Na₃V₂(PO₄)₂FO₂ Solid Solution: Unraveling Its Local and Electronic Structures

The local and electronic structures of Na3V2(PO4)2F3–Na3V2(PO4)2FO2 electrode materials have been investigated by a combination of 23Na and 31P magic-angle spinning NMR spectroscopy and density functional theory calculations. The spin distributions and the 31P NMR Fermi contact shifts in these materials are calculated based on the projector augmented wave approach implemented in the VASP code. Upon oxygen substitution, V4+ ions are formed and involved in highly covalent vanadyl bonds. We show that they exhibit a very specific electronic structure with a single electron on the 3dxy orbital perpendicular to the bi-octahedra axis. The V3+ ions, on the other hand, exhibit a partial occupation of the t2g orbitals by two electrons. The peculiar electronic structure of the V ions is at the origin of the complex spin transfer mechanisms observed in the Na3V2(PO4)2F3–Na3V2(PO4)2FO2 materials and results in the existence of several 23Na and 31P MAS NMR resonances. Owing to the proper signal assignment achieved using DFT calculations, we could estimate the degree of oxygen substitution for fluorine in the materials and discuss the local distribution of V3+/V4+ ions. Furthermore, through the 31P NMR study on the Na3V2(PO4)2FO2 composition, we here demonstrate that a 31P NMR resonance close to 0 ppm can also be observed in paramagnetic materials if there is no proper orbital overlap for the electron spin transfer to occur.