Capturing ion trapping and detrapping dynamics in electrochromic thin films

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.