Structural evolution and chemical bonds in electrochromic WO3 films during electrochemical cycles.

Electrochromic WO3 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 WO3 films is also observed in electrochemical step potential measurements. X-ray absorption spectroscopy indicates that the absorption energy of the W LIII-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 W–O bonds is caused by the reduction in W ions. LixWO3 and Li2WO4 are formed at −0.5 V and −1.0 V, respectively, during electrochemical insertion, as observed by transmission electron microscopy. The Raman scattering analysis indicates that only two ionic states, W4+ and W6+, exist in the WO3 films, due to excess injection of electrons and Li+ ions. We suggest that the degradation of the electrochromic properties of the WO3 films during electrochemical cycles is related to the formation of different W ionic states. [ABSTRACT FROM AUTHOR]