A Comparative In Situ X-Ray Absorption Spectroscopy Study of Nanophase Vsub 2Osub 5 Aerogel and Ambigel Cathodes

We examined the evolution of the oxidation state and atomic structure of V in nanophase aerogel and ambigel cathodes under in situ conditions using X-ray absorption spectroscopy (XAS). We show that the oxidation state of V in 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 However, the local structure of V in aerogel and ambigel has a higher degree of local symmetry relative to orthorhombic and/or the average V-O bond length is greater than that of orthorhombic Furthermore, we show that during the first cycle of discharge, the degree of Li insertion (x in for the ambigel cathode is greater than that of the aerogel cathode for the ambigel cathode compared to 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., Initially, pentavalent V is reduced to tetravalent V in the intercalation range and then tetravalent V is reduced to trivalent V upon insertion of more Li 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. © 2003 The Electrochemical Society. All rights reserved.