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

Abstract

AbstractMultivalent‐ion batteries with electrochromic functionality are an emerging green technology for development of low‐carbon society. Compared to Mg2+, Zn2+ and Al3+, Ca2+ has a low polarization strength similar to that of Li+, therefore Ca2+ 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 (VOx) and indium hexacyanoferrate (InHCF) films are respectively developed as anode and cathode because of their stable and high‐rate Ca2+ 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.