XPS study of the Li intercalation process in sol-gel-produced V2O5 thin film:: influence of substrate and film synthesis modification

We report on the process of lithium intercalation in V2O5 thin films deposited onto standard ITO-coated glass substrates. The films were deposited via a well-established sol-gel route, and the samples were examined as working electrodes in a range of potentials versus lithium reference electrode. This paper follows up issues arising from parallel spectroscopic characterizations of the films by X-ray photoelectron spectroscopy (XPS). Specifically, the XPS examination showed that not all of the Li-ion charge inserted was accounted for by the V(5) to V(4) reduction, but the stoichiometric balance could be maintained only by considering additional oxygens arising from the intercalation procedure, leading to Li2O formation. In this work, we have examined the possibility that the source of oxygen is the ITO substrate. TO this purpose, films of V2O5 deposited on silicon substrates have been prepared using the sol-gel process and examined by XPS after electrochemical intercalation/de-intercalation cycles. We show that in this case a perfect balance between electrochemical charge, inserted Li and reduced vanadium is obtained. A further indication of ITO-substrate effects was obtained from examination, by the same methods, of some unconventional V2O5 films that had been co-precipitated with a siloxane, designed to provide a template structure. The results obtained from this material imply that a barrier layer is formed at the ITO interface and, therefore, the formation of Li2O is avoided. The results are discussed in terms of the possible degradation of conventional V2O5 on ITO as a result of electrochemically induced interface reactions. Copyright (c) 2005 John Wiley & Sons, Ltd.