Electrolyte-Regulated Ion Accommodation and Microstructural Stability in WO3 Electrochromic Thin Films.

Electrochromic tungsten oxide (WO3) films suffer from gradual performance degradation during repeated ion insertion/extraction processes, while the relationship between electrolyte-dependent ion accommodation and structural stability remains insufficiently understood. Herein, magnetron-sputtered WO3 thin films were systematically investigated in H2SO4, LiClO4, ZnSO4, and Al2(SO4)3 electrolytes to clarify the coupling relationship among ion accommodation, electronic structure evolution, microstructural retention, and electrochromic durability. Combined electrochemical measurements with UV-visible spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and reflected electron energy loss spectroscopy (REELS) reveal that electrolyte chemistry regulates the balance between electrochemical activation and structural tolerance. Highly mobile H+ and strongly interacting Al3+ ions promote rapid ion transport and enhanced W6+ reduction but may induce excessive structural perturbation, defect accumulation, and accelerated degradation during cycling. Zn2+ exhibits intermediate behavior, whereas Li+ enables balanced ion accommodation through reversible W6+/W5+ conversion while preserving the WO3 framework. Consequently, the LiClO4 electrolyte achieves superior electrochromic performance with an optical modulation of 80.97% and improved cycling stability. These results demonstrate that durable electrochromic behavior is governed not by maximizing ion transport or reduction degree, but by achieving a reversible ion accommodation regime compatible with the structural tolerance of the host framework. This work provides a microstructure-oriented strategy for designing stable WO3-based electrochromic devices through electrolyte regulation.