In this work, we prepared octa-hydroxyhexyl viologen-substituted polyhedral oligomeric silsesquioxane (OHHV-POSS) as an organic–inorganic composite material applicable for large-area flexible electrochromic devices (FECDs). The organic–inorganic hybrid structure supported by the POSS cage and the enhanced interaction between hydroxyl end-groups and lithium salts can effectively increase the ionic conductivity of the ion gel films, thereby improving the electrochromic performance capabilities. Homogeneous ion gel was obtained with polyvinyl butyral and a lithium salt, and outstanding electrochromic properties were observed during electrochromic device operations. The FECDs of various sizes were fabricated for systematic evaluation, and the device of up to 22.5 × 19.5 cm2 was successfully constructed using the simple doctor blade coating method. The prepared large-area FECDs achieved a large transmittance difference of 70.0% at 604 nm, fast switching times of 21.1 and 18.0 s, high coloration efficiency of 260.61 cm2/C, long-term cycling stability of up to 300 cycles, and low power consumption of about 250 μW/cm2. Furthermore, the device was maintained stably in various application tests for heat insulation and storage, UV resistance, and mechanical stability. High performance and low power consumption of OHHV-POSS prove suitable for large-area FECDs, suggesting promising potential for smart windows, wristbands, and eyewear products.