ELECTROCHROMISM IN AMORPHOUS LITHIUM TUNGSTEN-OXIDE FILMS

A small ac-signal impedance analysis was utilized to characterize the surface property of an ITO electrode and Li transport in an Li//xWO//3 electrode. The as-deposited ITO film has a surface layer with extremely high carrier density, which could be easily removed by mechanical polishing. Based on the kinetic model of Li//xWO//3, the diffusion coefficient of Li transport was obtained. It depends on the various deposition conditions of WO//3 films and the composition of the electrolyte used. The response of Li//xWO//3 film showed a gradual decrease and reached a certain equilibrium after repeated cycling (more than 10**4 times). The examination of such a degradation phenomenon leads to the conclusion that there are two active sites in the WO//3 structure, which are available for the Li ion; one for coloration, and the other for an ion exchange reaction. The mechanism of these phenomena are further discussed. Impedance analysis has been proved to be very sensitive and applicable enough to the quantitative characterization of ITO and Li//xWO//3 electrodes involved in the electrochromic device.