Dynamic Processes in the Coloration of WO 3 by Lithium Insertion

Electrochemical impedance spectroscopy was used in the study of lithium insertion in a WO 3 film Spectra obtained at different potentials showed the characteristic shape of a diffusion process, with a Warburg regime at high frequencies and a capacitive branch at low frequencies arising from the charge storage associated with the intercalation of Li + . But the low frequency wing showed, as in many other reports, a pronounced dispersion which could not be accounted for solely by the diffusion capacitance. The possibility of an additional kinetic process was investigated. A number of models accounting for such effect were considered, including the constant phase element restricted diffusion model that differentiates diffusion from blocking effects and another model with fast and slow charging processes. The evolution of the impedance with charge-discharge cycles was considered as well It was found that a suitable interpretation of the experimental results is provided by a new model that considers the coupling of diffusion and the solid-state reaction whereby inserted ions are incorporated in fixed sites of the lattice. This model could be further elaborated on the basis of a diffusion and multiple trapping scheme, which provides an ordinary or Warburg diffusion region in the range of highest frequencies, and additional components at lower frequencies which reflect the kinetics of the ion immobilizing process and which in some cases show an anomalous diffusion pattern. This explanation was found consistent with the results of complementary techniques of measurement such as the electro-optical transfer function.