The distribution of lithium intercalated in V2O5 thin films studied by XPS and ToF-SIMS

The distribution of lithium in V2O5/V lower oxide duplex thin films prepared by thermal oxidation of V metal was analysed by XPS and ToF-SIMS after intercalation at 2.8 V versus Li/Li+ and de-intercalation at 3.8 V following cycling between 3.8 and 2.8 V in 1 M LiClO4-PC. XPS analysis of the intercalated thin film evidenced a partial reduction (43 at.% V4+) of the V2O5 surface, the modification of its electronic structure and the presence of Li, consistent with the formation of the δ-LixV2O5 (0.9 ≤ x ≤ 1) phase. The Li in-depth distribution measured by ToF-SIMS shows a maximum in the outer layer of V2O5, but Li is also found at the oxide film/metal substrate interface indicating its diffusion across the inner layer of V lower oxides. The analyses performed after de-intercalation on the samples cycled 12, 120 and 300 times reveal the effect of aging on the trapping of lithium. A significant reduction (17–22 at.% V4+) of the V2O5 surface was measured after 300 cycles. The Li in-depth distribution shows a maximum at the interface between the outer layer of V2O5 and the inner layer of lower oxides. Aging favours the accumulation of lithium at this interface with a resulting enlarged distribution enriching the sub-surface of the outer layer of V2O5 and the inner layer of lower oxides after 300 cycles. Lithium is also found, but in smaller quantities, at the oxide film/metal substrate interface. Measurements performed in the non-electrochemically treated surface areas of the de-intercalated samples revealed the same type of modifications, evidencing the diffusion of lithium along the interfaces where it is trapped.