Mesoporous amorphous tungsten oxide electrochromic films: a Raman analysis of their good switching behavior

• Mesoporous films exhibit better electrochemical kinetics compared to the dense films.
• Mesoporous films exhibit better reversibility compared to the dense films.
• Li+cations disrupt WO3 network in a reversible way in the mesoporous film.
• Li+ irreversibly intercalate in the voids of crystallites in the dense film.

The intercalation and de-intercalation of lithium cations in electrochromic tungsten oxide thin films are significantly influenced by their structural and surface characteristics.In this study, we prepared two types of amorphous films via the sol-gel technique: one dense and one mesoporous in order to compare their response upon lithium intercalation and de-intercalation.According to chronoamperometric measurements, Li+ intercalates/de-intercalates faster in the mesoporous film (24s/6s) than in the dense film (48s/10s). The electrochemical measurements (cyclic voltammetry and chronoamperometry) also showed worse reversibility for the dense film compared to the mesoporous film, giving rise to important Li+ trapping and remaining coloration of the film. Raman analysis showed that the mesoporous film provides more accessible and various W-O surface bonds for Li+ intercalation. On the contrary, in the first electrochemical insertion and de-insertion in the dense film, Li+ selectively reacts with a few surface W-O bonds and preferentially intercalates into pre-existing crystallites to form stable irreversible LixWO3 bronze.

Figure optionsDownload as PowerPoint slide