The Progress and Outlook of Multivalent-Ion-Based Electrochromism.

Electrochromic technology has witnessed numerous achievements in recent years both in research and commercialization. Electrochromic devices (ECD) based on different electrochemical mechanism have been developed for various applications, ranging from smart windows, thermal management, rear views, display, camouflage, etc. Compared to conventional ECDs based on monovalent charge carriers (e.g., H, Li), incorporating multivalent ions with rich electrochemistry, high charge density, and small ionic radius has opened new possibilities in novel ECDs. The merits of multivalent ions are harvested in ECDs activated by ion intercalation/deintercalation (e.g., Zn, Al, Ca, Mg), reversible metal electrodeposition (e.g., Cu, Bi, Zn), and dynamic metal-ligand interactions (e.g., Cu, Fe). Herein, the working mechanism, characteristics, and up-to-date achievements in multivalent ECDs are summarized and classified accordingly. The applications of multivalent ECDs for smart windows, energy storage, thermal management, multicolor displays, etc., are exemplified. The issues and challenges encountered by multivalent ECDs are emphasized, and the future directions for developing multivalent ECDs are also summarized. The aim of this review is to inspire more efforts in the exploration and the proliferation of multivalent ECDs.