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.

Ion trapping has been found to be responsible for the performance degradation in electrochromic oxide thin films, and a detrapping procedure was proved to be effective to rejuvenate the degraded films. Despite of the studies on ion trapping and detrapping, its dynamics remain largely unknown. Moreover, coloration mechanisms of electrochromic oxides are also far from clear, limiting the development of superior devices. Here, we visualize ion trapping and detrapping dynamics in a model electrochromic material, amorphous WO₃. Specifically, formation of orthorhombic Li₂WO₄ during long-term cycling accounts for the origin of shallow traps. Deep traps are multiple-step-determined, composed of mixed W⁴⁺-Li₂WO₄, amorphous Li₂WO₄ and W⁴⁺-Li₂O. The non-decomposable W⁴⁺-Li₂WO₄ couple is the origin of the irreversible traps. Furthermore, we demonstrate that, besides the typical small polaron hopping between W⁵⁺ ↔ W⁶⁺ sites, bipolaron hopping between W⁴⁺ ↔ W⁶⁺ sites gives rise to optical absorption in the short-wavelength region. Overall, we provide a general picture of electrochromism based on polaron hopping. Ion trapping and detrapping were demonstrated to also prevail in other cathodic electrochromic oxides. This work not only provides the ion trapping and detrapping dynamics of WO₃, but also open avenues to study other cathodic electrochromic oxides and develop superior electrochromic devices with great durability.