From the Irreversible Transformation of VO₂ to V₂O₅ Electrochromic Films

Since its discovery, electrochromism, known as the modulation of optical properties under an applied voltage, has attracted strong interest from the scientific community and has proved to be of significant utility in various applications. Although vanadium dioxide (VO₂) has been a candidate for extensive research for its thermochromic properties, its intrinsic electrochromism has scarcely been reported so far. In this study, multi-electrochromism is described for VO₂ thick films. Indeed, a VO₂ opaque film, doctor bladed from homemade monoclinic VO₂ powder, shows a pronounced color modulation from orange to green and blue associated with an amorphization-recrystallization phenomenon upon cycling in a lithium-based electrolyte. The strong memory effect allows us to follow the coloration mechanism by combining various ex situ and in situ characterizations addressing both structural and electronic aspects. Upon cycling, the multichromism of VO₂ finds its origin in the transformation of VO₂ into orange V₂O₅ upon oxidation, while in reduction, the blue lithiated state illustrates a mixed vanadium oxidation state.