Lithium intercalation in tin oxide films : Physics and electrochemistry

Abstract: Tin oxide films were made by reactive rf magnetron sputtering under conditions that led to both electronic and ionic conductivity. It was found that a high sputter pressure promoted porosity and a high charge capacity. The film structure was studied by X-ray diffraction, atomic force microscopy , spectroscopic light scattering, and Rutherford back scattering . The effect of lithium intercalation on the optical constants was studied with spectrophotometry and ellipsometry, and the distribution of the lithium ions was investigated with nuclear reaction analysis, The concentration of lithium was highest at the surface, and decreased further into the film. Lithium intercalation produced electrochromism with coloration efficiency peaked in the infrared. Fluorine doping, or mixtures with cerium, resulted in a decreased charge capacity.Chronopotentiometry, chronoamperometry and cyclic voltammetry were used to investigate the electrochemical properties of the tin oxide electrodes. Charge/discharge curves as well as potential step techniques were used to interpret phase formation and transformations and to study the dynamics of these reactions. The diffusion coefficient was ~10-11 cm2/s, slightly decreasing with decreasing potential. Cyclic voltammograms were interpreted in terms of a fractal dimension of a self-similar surface relief. Excellent agreement was found with the fractal dimension obtained with atomic force microscopy. Impedance spectra were taken for a wide range of frequencies and polarising 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/substrate interface.Mössbauer spectroscopy was used to determine the valence state of the Sn-atoms; a change from Sn4+ to Sn2+ was detected after electrochemical intercalation of Li+. Three different effects of charging were identified: double-layer charging at low levels, valence change at medium levels, and deposition on the surface at high levels. A novel in situ light scattering experiment was developed to study the latter phenomenom.