Inhibiting the phase transition of WO3 for highly stable aqueous electrochromic battery

Aqueous electrochromic battery (ECB) has shown intense potential for achieving energy storage and saving simultaneously. While tungsten oxide (WO3) is the most promising EC material for commercialization, the cycling stability of WO3-based aqueous ECBs is currently unsatisfactory due to the repeated phase transition during the redox process and the corrosion by acidic electrolytes. Herein, we present a titanium-tungsten oxide alloy (Ti-WO3) with controllable morphology and crystal phase synthesized by a facile hot injection method to overcome the challenges. In contrast to conventional monoclinic WO3, the Ti-WO3 nanorods can stably maintain their cubic crystal phase during the redox reaction in an acidic electrolyte, thus leading to dramatically enhanced response speed and cycling stability. Specifically, when working in a well-matched hybrid Al3+/Zn2+ aqueous electrolyte, our phase-transition-free cubic Ti-WO3 exhibits an ultra-high cycling stability (>20000 cycles), fast response speed (3.95 s/4.65 s for bleaching/coloring), as well as excellent discharge areal capacity of 214.5 mA h m−2. We further fabricate a fully complementary aqueous electrochromic device, for the first time, using a Ti-WO3/Prussian blue device architecture. Remarkably, the complementary ECB shows >10000 stable operation cycles, attesting to the feasibility of our Ti-WO3 for practical applications. Our work validates the significance of inhibiting the phase transitions of WO3 during the electrochromic process for realizing highly cyclable aqueous ECB, which can possibly provide a generalized design guidance for other high-quality metallic oxides for electrochemical applications.