In situ XAS investigation of the oxidation state and local structure of vanadium in discharged and charged V₂O₅ aerogel cathodes

We have examined the evolution of the oxidation state and atomic structure of vanadium(V) in discharged and charged nanophase vanadium pentoxide (V₂O₅) aerogel cathodes under in situ conditions using X-ray absorption spectroscopy (XAS). We show that the oxidation state of V in V₂O₅ aerogel cathode heated under vacuum (100 μTorr) at 220 °C for 20.5 h is similar to that of V in a commercially obtained sample of orthorhombic V₂O₅. In addition, lithium (Li) insertion during the first cycle of discharging leads to the reduction of V(V) to V(IV) and V(IV) to V(III) in a manner consistent with the stoichiometry of the sample (i.e. Li_xV₂O₅). Li extraction during charging leads to oxidation of V(III) to V(IV) and then V(IV) to V(V). Furthermore, the oxidation state of V in fully charged cathodes remains unchanged with cycling (upto at least the 16th cycle) from that of V in the control V₂O₅ aerogel cathode. However, the average oxidation state of V in discharged V₂O₅ cathodes increased with cycling. Moreover, the local structure of V in the discharged state has a higher degree of symmetry than that of the fully charged state. A significant change in the structure of the V-V correlation of discharged cathodes is observed with cycling indicating the formation of electrochemically irreversible phases.