Unconventional Aluminum Ion Intercalation/Deintercalation for Fast Switching and Highly Stable Electrochromism

Electrochromic devices have many important commercial applications ranging from electronic paper like displays, antiglare rear‐view mirrors in cars, to energy‐saving smart windows in buildings. Monovalent ions such as H + , Li + , and Na + are widely used as insertion ions in electrochromic devices but have serious limitations such as instability, high‐cost, and hard handling. The utilization of trivalent ions as insertion ions has been largely overlooked probably because of the strong electrostatic interactions between ions and intercalation framework and the resulted difficulties of intercalation. It is demonstrated that the trivalent ion, Al 3+ , can be used as efficient insertion ion by using metal oxide hosts in nanostructured form, which brings the desired fast‐switch, high‐contrast, and high‐stability as well to electrochromic devices. Differing from the usual structure degradation by repeated guest intercalation/deintercalation, the Al 3+ insertion introduces strong electrostatic forces, which on some degree stabilize the crystal structure and consequently yield much enhanced performances.