A Ca‐Ion Electrochromic Battery via a Water‐in‐Salt Electrolyte

Abstract Multivalent‐ion batteries with electrochromic functionality are an emerging green technology for development of low‐carbon society. Compared to Mg 2+ , Zn 2+ and Al 3+ , Ca 2+ has a low polarization strength similar to that of Li + , therefore Ca 2+ for electrochromism and battery can avoid kinetic issues caused by other multivalent‐ions with high polarization strength. Here, by exploiting Ca‐ion carriers for electrochromism and a water‐in‐salt (WIS) Ca(OTF) 2 electrolyte for the first time, a new and safe aqueous Ca‐ion electrochromic battery (CIEB) has been demonstrated. The WIS Ca(OTF) 2 electrolyte demonstrates enhanced anion‐cation interactions and decreased water activity. Vanadium oxide (VO x ) and indium hexacyanoferrate (InHCF) films are respectively developed as anode and cathode because of their stable and high‐rate Ca 2+ insertion/extraction, as well as matched electrochromism. The CIEB demonstrates a stable and high‐rate capability, a high energy density of 51.4 mWh m −2 at a power density of 1737.3 mW m −2 , and a greenish yellow‐to‐black electrochromism. The presented results are beneficial for understanding redox kinetics in WIS electrolytes, and inspire researches on batteries and electrochromism with multivalent‐ions.