Coloration and Ion Insertion Kinetics Study in Electrochromic WO3 Films by Chronoamperometry

Devices relying on reversible ion intercalation are used in sustainable energy technology, for example, in batteries, in rechargeable electrical and electrochromic (EC) glazing for energy efficient buildings. Degradation of electrochromic thin films under extended charge insertion/extraction is a technologically important issue for which no in-depth understanding is currently available. The influence of coloration depth on the performance characteristics and lifetime of electrochromic WO3 materials is investigated using chronoamperometry technology in this work. It is shown that depending on the cycling time, and the applied voltage, two different types of adsorption sites, viz. the shallow sites, surrounded by low-energy barriers and deep sites with high-energy barriers are created in the WO3 host structures. During the bleaching process, higher cycling time and applied voltage create a large coloration depth leading to trapped immobile Li+ ions hindering free ion transportation and subsequent degradation of electrochromic performance. This study offers a new understanding on the degradation mechanisms of the electrochromic materials and provides a general guideline suggesting the use of low voltage and cycling time for controlling the charge insertion and extraction in electrochromic materials by avoiding the large coloring depths.