Site Occupancy and Auger Parameter in Lithium Ion Intercalation of Vanadium Pentoxide

Abstract: X-ray photoelectron spectroscopy (XPS) is used in two distinct ways for the analysis of thin films, such as those used as the basis for electrochromic devices. Firstly it is used for quantification of the stoichiometry and for measurement of the amount of intercalated material, and secondly, for gaining information on the chemical states present. Chemical shifts in XPS are sensitive to changes in the dielectric properties of the material under observation. A well established method for the correlation of these changes is based on the direct relationship between the Auger parameter (AP), derived from the spectrum, and the electronic polarisability of the ionic lattice. Electrochromic materials, such as V2O5, undergo phase changes during the course of intercalation of lithium ions. The Auger parameter is only sensitive to crystalline phase when this is associated with a large change in lattice polarisability. The phase changes on intercalation of V2O5 arise because intercalation occurs in stages, corresponding to the sequential filling of specific sites. The lithium ions have a strong polarising influence and thus AP should show a distinct dependence on phase and degree of intercalation. In this paper we show that this effect can be observed as a result of electrochemical intercalation of thin films and also that it is not masked by changes that might occur during movement of samples between cell and spectrometer as long as this is done under a protective atmosphere. However, the change in AP on intercalation is not fully recovered on de-intercalation. The implications of this are discussed in the paper