Prominent Electrochromism Achieved Using Aluminum Ion Insertion/Extraction in Amorphous WO₃ Films

Although monovalent lithium has been successfully used as a coloring ion in electrochromic applications, it still faces the challenges of low safety, high cost, and limited reserves. Herein, we demonstrate that the amorphous WO₃ films intercalated with Al³⁺ ions could exhibit desired wide optical modulation (â=63.0%) and high coloration efficiency (â=72.0 cm² A^-1, which is >100% higher than that with Li⁺ or Na⁺), benefiting from the three-electron redox properties of aluminum. Due to the strong electrostatic force and large atomic weight, the charge exchange processes for Al³⁺ ions are limited only to the near-surface region and consequently bring about enhanced electrochromic stability. Our findings provide in-depth insights into the nature of electrochromism and also open up a new route toward scalable electrochromic devices using sputtering techniques and earth-Abundant materials.