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

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 Ca²⁺ 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.