XPS and TOF-SIMS study of the distribution of Li ions in thin films of vanadium pentoxide after electrochemical intercalation

The intercalation of vanadium pentoxide by lithium ions leads to a change in optical properties, a process that is of value in thin-film electrochromic devices. The extent of intercalation can be measured, electrochemically, from the charge capacity of the film and, is in good agreement with that determined in the outermost layers by X-ray photoelectron spectroscopy (XPS), when intercalation occurs homogeneously through the film thickness. SIMS profiles of V2O5-deposited on ITO-glass coupons have allowed examination of the interface between these layers and in prior work showed a marked build up of Li in this interphase. Investigation of the distribution of lithium within the interphase showed it to be present in parts of the testpiece that were not immersed in the electrolyte used for Li insertion. In this work we have tested the suggestion that the lithium found in the interface can be introduced, during intercalation, via the exposed edge of the V 2O5/ITO interface at the periphery of the sample. A series of intercalation cycles using modified cells to expose either: a) the periphery; or b) the centre; of the thin film have been carried out. The distribution of lithium after intercalation has been examined using a combination of ToF-SIMS and XPS. XPS showed lithium in the film to be related only to the immersed regions of the testpiece whilst SIMS profiles confirmed the widespread distribution of Li through the interface, only when an edge was exposed. We work confirms an important influence of an unsealed edge when samples are produced by cutting from larger glass plates. Copyright © 2008 John Wiley & Sons, Ltd.