A comparative in situ x-ray absorption spectroscopy study of nanophase V₂O₅ aerogel and ambigel cathodes

We examined the evolution of the oxidation state and atomic of V in nanophase V₂O₅ aerogel and ambigel cathodes under in situ conditions using X-ray absorption spectroscopy (XAS). We show that the oxidation state of V in V₂O₅ aerogel and ambigel cathodes heated under vacuum (100 μTorr) at 220°C for 16 h is similar to that of V in a commercially obtained sample of orthorhombic V₂O₅. However, the local structure of V in aerogel and ambigel V₂O₅ has a higher degree of local symmetry relative to orthorhombic V₂O₅ and/or the average V-O bond length is greater than that of orthorhombic V₂O₅. Furthermore, we show that during the first cycle of discharge, the degree of Li insertion (x in Li_xV₂O₅) for the ambigel cathode is greater than that of the aerogel cathode (x = 3.06 for the ambigel cathode compared to x = 2.68 for the aerogel cathode) for the same cut-off voltage. Accordingly, the observed V K-edge energy shift for the fully discharged ambigel cathode is greater than that observed for the fully discharged aerogel cathode, and hence, the extent of reduction of pentavalent V is greater for the ambigel cathode. In both cases, the extent of reduction of the V is consistent with the amount of Li inserted (i.e., Li_xV₂O₅). Initially, pentavalent V is reduced to tetravalent V in the intercalation range 0 < x < 2 and then tetravalent V is reduced to trivalent V upon insertion of more Li (x > 2). Analyses of Fourier transform data reveal that V has a higher degree of local symmetry in the discharged state relative to that of the charged state.