Mesostructure-Dependent Ionogel Infiltration in Electrochromic Nanocrystal Thin Films

Ionogels are promising electrolytes for nanocrystal-based near-infrared electrochromic devices, owing to their high ionic conductivity and dimensional stability. However, the extent of ionogel infiltration into nanocrystal films and its influence on electrochromic modulation remains poorly understood. Here, we investigate how thin-film mesostructure governs ionogel penetration and the resulting electrochromic response. WO3-x nanorod films with distinct mesostructures, including low-porosity densely packed films and high-porosity randomly packed films, are fabricated and laminated at room temperature with freestanding ionogel electrolytes (PVDF-HFP/Pp13TFSI/LiTFSI). X-ray photoelectron spectroscopy depth profiling and time-of-flight secondary ion mass spectrometry analyses reveal substantial ionogel infiltration throughout randomly packed films, whereas penetration into densely packed films is largely restricted to the near surface region. As a result, randomly packed films exhibit a higher charge capacity and enhanced optical modulation compared to densely packed counterparts. These findings underscore the critical role of mesostructure engineering in promoting the effective ionogel incorporation for high-performance solid-state electrochromic devices.