The intercalation and deintercalation mechanisms of lithium into V2O5 thin films prepared by thermal oxidation of vanadium metal have been studied by X-ray photoelectron spectroscopy (XPS) using a direct anaerobic and anhydrous transfer from the glove box (O-2 and H2O < 1ppm), where the samples were electrochemically treated, to the XPS analysis chamber. Vanadium in the as-prepared oxide films is mostly (from 93 to 96% depending on samples) in a pentavalent state (V5+) with a stoichiometric ON concentration ratio fitting that of V2O5. Four to seven percent of VO2 is also observed. After the I st and the 2nd intercalation steps at E= 3.3 and 2.8 V versus Li/Li+, respectively, the V2p core level spectra evidence a partial reduction to V4+ states with a remaining concentration of 73 and 56% of V5+ in agreement with the intercalation of about 1/2 mot of Li per V2O5 mot at each intercalation step. Intercalated lithium was observed at a binding energy of 56.1 eV for Li 1 s. Changes of the electronic structure of the V2O5 thin film after intercalation are evidenced by the observation, at a binding energy of 1.3 eV, of occupied V3d states (V4+) originally empty in the pristine film (V5+). The V2p and Li Is core level spectra show that the process of Li intercalation is partially irreversible. In the first cycle, 34 and 14% of the vanadium ions remain in the V4+ state after deintercalation at E= 3.4 and 3.8 V versus Li/Li+, respectively, indicating a partially irreversible process already after the 1st deintercalation. The analyses of C1s and O1s XP spectra show the formation of a solid-electrolyte interface (SEI). The analyzed surface layer includes lithium carbonate and Li-alkoxides. (c) 2007 Elsevier Ltd. All rights reserved.