Eliminating degradation and uncovering ion-trapping dynamics in electrochromic WO3 thin films

Although WO3 thin films are promising as electrodes for electrochromic devices, they suffer from degradation of optical modulation. Ion-trapping-induced degradation can now be successfully eliminated by constant-current-driven de-trapping. There is keen interest in the use of amorphous WO3 thin films as cathodic electrodes in transmittance-modulating electrochromic devices1,2,3,4. However, these films suffer from ion-trapping-induced degradation of optical modulation and reversibility on 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; that is, WO3 films can be rejuvenated and regain their initial highly reversible electrochromic performance. Pronounced ion trapping occurs when x exceeds ∼0.65 in LixWO3 during ion insertion. We find two main kinds of Li+-ion-trapping site (intermediate and deep) in WO3, 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.