Electrochromism via reversible electrodeposition of solid iodine

Electrochromic materials were discovered in the 1960s when scientists observed reversible changes between the light and dark states in WO 3 thin films under different voltages. Since then, researchers have identified various electrochromic material systems, including transition metal oxides, polymer materials, and small molecules. However, the electrochromic phenomenon has rarely been observed in non-metallic elemental substances. Herein, we propose the development of non-metallic iodine electrodeposition-based electrochromic dynamic windows using a water-in-salt electrolyte containing iodine ions. The unique electrolyte environment and solvation structure of the water-in-salt electrolyte suppress the dissolution and shuttle effect of iodine, thereby achieving a different reaction pathway compared to traditional electrolytes. This pathway involves a reversible solid-liquid transition between solid iodine and solvated iodide ions. The iodine electrodeposition-based electrochromic dynamic window demonstrates a high optical contrast of 76.0% with near colour neutrality and excellent cycling stability. A practical 400 cm 2 complementary dynamic window is fabricated to demonstrate good electrochromic performance, including high optical contrast, a near colour-neutral opaque state, fast response time, uniform modulation, and polarity-switchable functionality.