Impedance spectroscopy on lithiated Ti oxide and Ti oxyfluoride thin films

Films of Ti oxide and Ti oxyfluoride were produced by reactive magnetron sputtering of Ti in Ar+O₂(+CF₄). Compositional and structural analyses were accomplished by Rutherford backscattering spectrometry, x-ray diffraction (XRD), infrared absorption spectrometry, and atomic force microscopy (AFM). Electrochemical characterization of films immersed in a Li conducting electrolyte was performed with cyclic voltammetry and coulometric titration. Detailed impedance spectra were recorded for the 2×10⁵-1×10^-3 Hz range. The impedance responses of pure and fluorinated Ti oxide films in the lithium containing electrolyte differed significantly even if their structures, according to AFM and XRD, were very similar. One main difference was the size of the charge transfer resistance, presumably connected to the Li ion injection from the electrolyte into the film. A modest fluorination lowered this resistance by about two orders of magnitude. The voltammetric and the impedance responses, as well as the magnitude of the chemical diffusion coefficient, of the fluorinated Ti oxide film were strikingly similar to the response of WO₃ films. This similarity does not occur for the pure Ti oxide films, where a process, believed to be the Li ion injection, could be identified with the main features of the frequency-dependent impedance. Underlying this charge transfer mechanism, however, a process represented by a constant phase element seems to be operating. This latter process may have its origin in Li diffusion into the film.