Impedance studies on Li insertion electrodes of Sn oxide and oxyfluoride

Films of Sn oxide and oxyfluoride were made by reactive rf magnetron sputtering onto ITO-coated glass. We analyzed the composition by Rutherford backscattering spectrometry, the structure by x-ray diffraction, and the surface topography by atomic force microscopy. Li intercalation from a liquid electrolyte was more facile in the oxide than in the oxyfluoride, as found from cyclic voltammetry. Impedance spectra were taken for a wide range of frequencies and polarizing voltages. Nyqvist diagrams were interpreted from a circuit model with elements representing Li insertion at the electrolyte/film interface and electron insertion at the film/ITO interface. The data were consistent with a fractal surface of the Sn oxide film, with a dimension in excellent agreement with measures obtained through several independent techniques. The chemical diffusion coefficient was ∼10−13 cm2/s and slightly decreasing with increasing potential for all films.