Multivalent‐Ion Electrochromic Energy Saving and Storage Devices

Electrochromic devices (ECDs) show promising applications in various fields including energy‐saving smart windows, energy‐recycling batteries/supercapacitors, displays, thermal management, etc. Compared to monovalent cations (H + , Li + , Na + , and K + ), multivalent‐ion carriers (Mg 2+ , Ca 2+ , Zn 2+ , and Al 3+ ) can enable the ECDs with high optical contrast, high energy‐recycling capability, and attractive long‐term stability because of the multiple‐electron transfer redox. Additionally, Mg 2+ , Zn 2+ , and Al 3+ ‐based ECDs assembled with metal anodes are exploited for applications in EC electronics, EC mirrors, flexible devices, etc. Attempts to develop multivalent‐ion ECDs can be traced to 2013. However, since 2017, the research activity in this field has surged in the world. Despite the fascinating achievements, there is still a long way from their maturity due to challenges related to the limited electrode materials and electrolytes, as well as the obscure multivalent‐ion redox mechanisms. This review aims to discuss 1) the EC mechanisms of electrode materials with multivalent ions, 2) the advantageous functionalities of multivalent‐ion ECDs, and 3) strategies developed for exploring electrode materials, electrolytes, and ECD structures. Additionally, future perspectives for remaining challenges and corresponding strategies for developing multivalent‐ion ECDs with designed functionalities are discussed.