A Fast and Highly Stable Aqueous Calcium‐Ion Battery for Sustainable Energy Storage

Abstract Aqueous alkali‐ion batteries are gaining traction as a low‐cost, sustainable alternative to conventional organic lithium‐ion batteries. However, the rapid degradation of commonly used electrode materials, such as Prussian Blue Analogs and carbonyl‐based organic compounds, continues to challenge the economic viability of these devices. While stability issues can be addressed by employing highly concentrated water‐in‐salt electrolytes, this approach often requires expensive and, in many cases, fluorinated salts. Here, we show that replacing monovalent K + ions with divalent Ca 2+ ions in the electrolyte significantly enhances the stability of both a copper hexacyanoferrate cathode and a polyimide anode. These findings have direct implications for developing an optimized aqueous Ca‐ion battery that demonstrates exceptional fast‐charging capabilities and ultra‐long cycle life and points toward applying Ca‐based batteries for large‐scale energy storage.