Hexagonal WO₃ nanogranular thin films with oxygen-vacancy engineering for ultra-stable electrochromic energy storage devices

This work reports the rational design of oxygen-deficient hexagonal WO 3 (h-WO 3 ) nanogranular thin films for high-performance electrochromic energy storage devices (EESDs). The h-WO 3 nanoparticles were synthesized via a facile acid precipitation method, where pH control guided the hexagonal phase formation, followed by rapid microwave heating processing to introduce oxygen vacancies. Formulated into a blade-coatable ink, the resulting films feature a synergistic combination of a hexagonal tunnel structure, nanogranular morphology, and oxygen vacancies, which collectively enhance ion transport and electrochemical activity. When integrated into a Zn-based EESD with a Li + -Zn 2+ hybrid electrolyte, the device achieves an optical modulation of 63.86 % at 633 nm and exceptional long-term stability, retaining 90 % of its initial modulation after 15,000 cycles. It also delivers a high areal capacity of 84.94 mA h m −2 at 1 A m −2 with excellent rate capability. This work underscores the critical role of synergistic defect and structural engineering and provides a versatile strategy for developing advanced multifunctional materials.