The electrochromic behavior of indium tin oxide in propylene carbonate solutions

We report on a study of transparent conducting tin-doped indium oxide (ITO) electrodes in propylene carbonate solutions containing lithium ions. The system was studied using electrochemical methods in combination with in situ techniques: ultraviolet-visible spectroscopy, X-ray diffraction, and quartz crystal microbalance. The results show that the cathodic process at E greater than or similar to 1.0 V vs. Li/Li+ mainly involves the reduction of the electrolyte solution, leading to the formation of a thin, lithium-rich surface film. At potentials less than or similar to.0 V vs. Li/Li+, degradation of ITO and the formation of metallic indium take place. No evidence was obtained that lithium-ion intercalation into ITO, which has been suggested by several workers, occurs to a significant extent. We conclude that ITO probably cannot be used as a combined ion-storage layer and transparent conductor for all-solid-state and laminated electrochromic switching devices in view of long-term stability.