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 (Mg²⁺, Ca²⁺, Zn²⁺, and Al³⁺) 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, Mg²⁺, Zn²⁺, and Al³⁺-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.