Amorphous Hydrated Tungsten Oxide Films with Electrochromic Cycling Stability Up to 40 000 Cycles Enabled by Regulating the Coloring Potential and Structural Water

Amorphous tungsten oxide is widely used as an electrochromic (EC) material in smart windows but suffers from poor cycling stability due to the ion trapping effects and structural deterioration during long-term repeated ion intercalation-deintercalation. Herein, we demonstrate that the EC cycling stability of amorphous hydrated tungsten oxide films can be dramatically enhanced to 40 000 cycles without noticeable performance degradation by regulating the structural water content of films and adjusting the coloring potentials of the films in an Al³⁺-based electrolyte. We found that the enhancement of cycling stability can be attributed to a relatively low concentration of trapped ions and stable microstructures enabled by decreasing the coloring potential in the range from -0.2 to -0.8 V versus Ag/AgCl and increasing the content of structural water to 0.9H₂O for WO₃. Lower coloring potentials effectively mitigate ion trapping effects and structural damage that typically occur during the ion diffusion-controlled electrochromic process. In addition, more structural water in amorphous hydrated tungsten oxide films facilitates ion diffusion, leading to enhanced ion diffusion kinetics that can alleviate the accumulation of irreversible ions, thereby further improving the cycling stability. This work offers a simple yet effective strategy to enhance the cycling stability of amorphous phase tungsten oxide-based EC materials to a level that meets the requirement of practical application.