RF-sputtered vanadium oxide films effect of film thickness on structural and electrochemical properties

Structural and electrochemical properties of radio frequency (rf)-sputtered vanadium oxide films with different thicknesses (0.175, 0.35, 0.8, 1.6, and 3.2 μm) have been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), impedance spectroscopy, and a galvanostatic charge/discharge system. XRD and SEM studies show that change in the film thickness by deposition time has a significant influence on the crystalline phase and orientation of vanadium oxide films: c axis oriented V₆O₁₃ phase is Converted to the V₂O₅ phase with the ab plane perpendicular to the substrate with increasing film thickness. Similar discharge profiles are observed regardless of film thickness, whereas the first discharge capacities are higher for the thin films (∼100 μAh/cm² μm for 0.175, 0.35, and 0.8 μm) than those for the thicker films (∼82 μAh/cm² μm for 1.6 and 3.2 μm) at constant current of 40 μA/cm². Moreover the films with thickness of 1.6 and 3.2 μm show unstable cycle characteristics. From a long-term cycling performance for the samples with the thickness less than 0.8 μm, a large decrease in discharge capacity is observed after 1st cycle followed by almost constant value retained up to 1000th cycle. Comparison of impedance spectra for the as-prepared and the cycled 175 nm thick film reveals that there is no significant change in the charge-transfer resistance before and after cycling, indicating that capacity loss during cycling is mainly attributed to an irreversible phase transformation. However, with increasing film thickness, charge-transfer resistance at the electrolyte/electrode is found to have an influence, gradually, on the degree of capacity loss.