Abstract
M-ion batteries (M = Li, Na, K ...) positive electrode materials most often operate through the reversible oxidation of transition-metal ions. In complex materials involving many transition metals or many redox centers, understanding the sequence in which they participate to the reaction is not trivial but is often necessary to explain the electrochemical properties. Mixed anion vanadium phosphates, such as KVPO4F0.5O0.5, are known to contain two different redox entities that are V3+O4F2 and V3+O5F “ionic” entities on the one hand and {V4+═O}O5 and {V4+═O}O4F “covalent” vanadyl-type units on the other hand. However, their participation to the redox mechanism occurring during the charge of this material has never been studied. Here, we use V K-edge X-ray absorption spectroscopy to unveil the redox mechanism of KVPO4F1–yOy (y = 0, 0.5, 1), performing data analysis via a chemometric approach. With XAS being very sensitive to the oxidation state and bond length, it was found that the ionic V3+–F units oxidize at a lower potential than the covalent {V4+═O} ones, which is surprising considering the high electronegativity of fluoride anions but is consistent with the redox potential observed for KVPO4F and KVOPO4. Further, ab initio calculations and ex situ X-ray diffraction analyses allowed an atomistic description of the redox mechanism with the sequential oxidation of the cis V site before the trans V site in KVPO4F upon charge. Finally, the complete atomically resolved redox mechanism of KVPO4F0.5O0.5 is proposed.