High performance electrochromic energy storage devices based on Mo-doped crystalline/amorphous WO3 core-shell structures

The crystal structure and architecture of electrochromic (EC) materials are the key factors for their performance. In this paper, Mo-doped crystalline/amorphous WO ₃ (c/a-WO ₃ ) are fabricated via facile hydrothermal and electrodeposition methods, which combine the advantages of excellent cycle stability (c-WO ₃ nanobars) and fast switching speed and high coloring efficiency (Mo-doped a-WO ₃ thin films) of different WO ₃ structure. By optimizing the hydrothermal and electrodeposition parameters, the core-shell c/a-WO ₃ EC material shows a significant optical modulation (67.8%) owing to the low energy barrier and rapid ion migration in Mo-doped a-WO ₃ shell. More importantly, the EC devices based on Mo-doped c/a-WO ₃ exhibit fast switching speeds and high coloration efficiency (104.98 cm ²/C) due to enhanced Li ⁺ diffusivity. These great electrochemical performances could be attributed to the amorphous shell and the proper structure distortion caused by doped atoms. Meanwhile, the EC devices exhibit good cycling stability as the transmittance modulation has no decrease after 23000 s. As an energy storage device, the EC supercapacitor delivers a high energy density of 10.8 Wh/kg at a power of 117.6 W/kg and long cycle life (72.8% capacitance retention over 1500 cycles). The metal-doped core-shell structure can provide a reliable solution to produce high-performance EC materials and devices such as energy-saving smart windows, outdoor static displays and other energy-efficient applications.