From high‐pressure β‐V2O5 to κ‐NaxV2O5 (x = 0.4 − 0.55): A structural, chemical, and kinetic insight into a sodiated phase with a large interlayer space

Abstract

AbstractVanadium pentoxide polymorphs are attracting much interest due to their remarkable electrochemical performance as positive electrodes for rechargeable metal‐ion batteries. High‐pressure β‐V2O5 is an interesting positive electrode material for Na‐ion batteries able to intercalate 1 Na+/formula unit delivering a capacity of 147 mAh g−1. The sodium intercalation of 0.4 Na+ has been described in the literature as a two‐phase domain involving an important structural rearrangement yielding NaxV2O5, but details about the structural changes upon sodium intercalation and structure of intercalated Na0.4V2O5 have not been elucidated so far. In this work, we performed operando synchrotron X‐ray diffraction measurements on sodium batteries on discharge. A full crystal structure determination for such phase, which we have called κ‐Na0.4V2O5, was carried out for the first time directly from operando measurements in monoclinic space group C2/m. Structural analysis was completed with bond valence sum calculations revealing the sites of intercalated sodium in κ‐NaxV2O5 and the structure was further optimized by means of density functional theory calculations. Operando X‐ray absorption spectroscopy and ex situ X‐ray photoelectron spectroscopy allowed to unveil the redox changes upon the phase transformation from β‐V2O5 into κ‐NaxV2O5, providing information about the reduction of vanadium from V5+ to V4+. The phase transformation also produced enhanced charge transfer and sodium diffusion processes, as deduced from both impedance and kinetic study.