LAUSR

Abstract Owing to the accumulation of trapped ions in host structures, electrochromic devices (ECDs) suffer from diminished optical modulation and reduced device efficiency under long-term charge insertions and extractions. The accumulation of trapped ions also results in structural changes and mechanical degradation of the electrochromic film, for which very few studies are currently on hand. Here, we report on an explorative study about the mechanical degradation for a WO3-based monolithic ECD: glass/ITO/WO3/LiClO4(PEO)/TiO2/ITO/glass. For the WO3 EC film, both the electrochemical degradation from cyclic voltammetry and the volume expansion by in-situ AFM monitoring were studied. Under the collective effects of these two aspects, the mechanical degradation process of the ECD was recorded through surface morphology evolutions of the WO3 film during life cycling of the ECD to its final failure for up to 90 000 cycles. The surface morphology of the WO3 film underwent three stages: (I) it first became increasingly denser and denser when the cycles were low, approximately 10 000 times; (II) when the cycles went up to 30 000 times or more, the surface of the WO3 film had an uneven distribution, with some dense islands surrounded by loose areas; and (III) when the cycling continued, this situation got worse, and finally, the WO3 film started to crack. This exploration provides new insights into the degradation mechanism of WO3-based ECDs upon repetitive insertions and extractions of Li ions in practical use.