Li-intercalation behaviour of vanadium oxide thin film prepared by thermal oxidation of vanadium metal

Abstract

In order to produce thin films of crystalline V 2O 5, vanadium metal was thermally oxidised at 500 °C under oxygen pressures between 250 and 1000 mbar for 1–5 min. The oxide films were characterised by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), X-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS). The lithium intercalation performance of the oxide films was investigated by cyclic voltammetry (CV), chronopotentiometry and electrochemical impedance spectroscopy (EIS). It was shown that the composition, the crystallinity and the related lithium intercalation properties of the thin oxide films were critically dependent on the oxidation conditions. The formation of crystalline V 2O 5 films was stimulated by higher oxygen pressure and longer oxidation time. Exposure for 5 min at 750 mbar O 2 at 500 °C resulted in a surface oxide film composed of V 2O 5, and consisting of crystallites up to 200 nm in lateral size. The thickness of the layer was about 100 nm. This V 2O 5 oxide film was found to have good cycling performance in a potential window between 3.8 and 2.8 V, with a stable capacity of 117 ± 10 mAh/g at an applied current density of 3.4 μA/cm 2. The diffusion coefficients corresponding to the two plateaus at 3.4 and 3.2 V were determined from the impedance measurements to (5.2 and 3.0) × 10 −13 cm 2 s −1, respectively. Beneath the V 2O 5 layer, lower oxides (mainly VO 2) were found close to the metal. At lower oxygen pressure and shorter exposure times, the oxide films were less crystalline and the amount of V 4+ increased in the surface oxide film, as revealed by XPS. At intermediate oxygen pressures and exposure times a mixture of crystalline V 2O 5 and V 6O 13 was found in the oxide film.