XPS study of Li ion intercalation in V2O5 thin films prepared by thermal oxidation of vanadium metal

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 (O2 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 O/V concentration ratio fitting that of V2O5. Four to seven percent of VO2 is also observed. After the 1st 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 mol of Li per V2O5 mol at each intercalation step. Intercalated lithium was observed at a binding energy of 56.1 eV for Li1s. 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 Li1s 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.