In situ synthesis and structural morphology analysis of 3D porous hierarchical V₂O₅ films for transmissive-to-black all-solid-state electrochromic devices

3D, porous, low-dimensional, and nanostructured V₂O₅ films have significant potential for application in ECDs, but their development is still in its nascency. This paper reports a facile strategy of using PEG as a structure-directing agent and short-time heat treatments to obtain 3D, porous, and hierarchical V₂O₅ films comprising interpenetrating stacked ultra-long nanowire surfaces and nanorod/nanosheet mixed interiors deposited on ITO-glass substrates. The mechanism of morphological transformation was studied in detail, and the analysis shows that the formation of such special structured films is a synergetic result of the guided intercalation of PEG, atomic rearrangement, Ostwald ripening, anisotropy of crystal growth, and orientation attachment mechanism. Among them, the preferred PEG addition amount of 25 % for the 350-3-V₂O₅ film was applied in two types of all-solid-state ECDs: one using a gel quasi-solid electrolyte and the other using a solid-film electrolyte. The 350-3-V₂O₅ film served as an ion storage coating and WO₃ was the electrochromic layer. These devices achieved ideal reversible switching of transmissive-to-black electrochromic characteristics. Notably, the ‘Integrated-Type’ ECD, composed of inorganic all-solid-state films, has a large optical modulation range ΔT = 72.0 %, rapid switching responses (colouration 25.4 s and bleaching 20.8 s), and sustained electrochromic performance.