Eliminating degradation and uncovering ion-trapping dynamics in electrochromic WO(3) thin films

Amorphous WO(3) thin films are of keen interest as cathodic electrodes in transmittance-modulating electrochromic devices. However, these films suffer from ion-trapping-induced degradation of optical modulation and reversibility upon extended Li(+)-ion exchange. Here, we demonstrate that ion-trapping-induced degradation, which is commonly believed to be irreversible, can be successfully eliminated by constant-current-driven de-trapping, i.e., WO(3) films can be rejuvenated and regain their initial highly reversible electrochromic performance. Pronounced ion-trapping occurs when x exceeds ~0.65 in Li(x)WO(3) during ion insertion. We find two main kinds of Li(+)-ion trapping sites (intermediate and deep) in WO(3), where the intermediate ones are most prevalent. Li(+)-ions can be completely removed from intermediate traps but are irreversibly bound in deep traps. Our results provide a general framework for developing and designing superior electrochromic materials and devices.