Tunable Nb-Doped WO₃ for High-Performance Bifunctional Aqueous Zn²⁺-Based Electrochromic Batteries

Aqueous Zn²⁺-based electrochromic energy storage devices (ZEESDs) integrating electrochromism and energy storage functions are considered promising candidates in next-generation advanced energy-saving smart windows or displays. However, their practical applications are severely hindered by the unsatisfactory performances. Herein, we report a high-performance aqueous ZEESD utilizing tunable Nb-doped WO₃ as the electrochromic material/cathode, a metal Zn sheet as the anode, and 1 M ZnSO₄ aqueous solution as the electrolyte. The electrochromic performances of the Nb-doped WO₃ thin film with different Ar/O₂ flow rates, doping ratios, and film thicknesses fabricated by magnetron sputtering were systematically investigated. The results show that optimal Nb-doped WO₃ exhibits outstanding electrochromic performances including a large optical modulation (93.10% at 633 nm), a fast spectral response time (4/5 s at 633 nm), a high coloration efficiency of 75.02 cm² C¹, and superior cycling stability (remaining 80% of the initial optical modulation after 2000 cycles). Furthermore, it also achieves a high discharge areal capacity of 100 mAh m^-2, presenting a good energy storage capability. The assembled aqueous ZEESD based on Nb-doped WO₃ displays a fascinating practical application prospect. This work provides a simple and effective design strategy for ZEESDs, which plays an important role for boosting the practical development in the field of energy savings and energy storage.