Structural evolution and chemical bonds in electrochromic WO 3 films during electrochemical cycles

Electrochromic WO ₃ films are prepared by the electron-beam evaporation method. The as-deposited films are amorphous in structure. The optical transmittance spectra show 13.4% decay of optical modulation after 20 electrochemical cycles. The degradation behaviour of the WO ₃ films is also observed in electrochemical step potential measurements. X-ray absorption spectroscopy indicates that the absorption energy of the W L _III-edge shifts towards lower energy with the insertion of Li ⁺ ions and electrons. The results of radial distribution functions reveal that the increase in the distance of the WO bonds is caused by the reduction in W ions. Li _xWO ₃ and Li ₂WO ₄ are formed at 0.5V and 1.0V, respectively, during electrochemical insertion, as observed by transmission electron microscopy. The Raman scattering analysis indicates that only two ionic states, W ⁴⁺ and W ⁶⁺, exist in the WO ₃ films, due to excess injection of electrons and Li ⁺ ions. We suggest that the degradation of the electrochromic properties of the WO ₃ films during electrochemical cycles is related to the formation of different W ionic states.