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.
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