Optimizing the Comprehensive Ion Effects in Glass‐Forming Aqueous Inorganic Salt Electrolytes for Supercapacitor Applications at Extremely Low Temperatures

Abstract

ABSTRACT Inorganic salt electrolytes have been explored for developing cold‐resistant aqueous energy storage devices. Current research on anti‐freezing inorganic salt solutions mainly focuses on the H‐bond regulation effects of individual cations or anions. The overall ionic effects (e.g., ion type, concentrations, cation hydration numbers, ion interactions, and ionic associations, etc.) on the anti‐freezing properties lack a comprehensive understanding. In addition, crystallization of the salt electrolyte below the solidification point can lead to an abrupt performance failure of the energy storage device. In this work, we study the overall ionic effects of glass‐forming aqueous electrolytes for enhancing their anti‐freezing properties. It is found that ion pairs with more positive cationic potentials and less negative anionic potentials, cations with large coordination numbers, and anions with large ionic size and multiple H‐bond sites are crucial for achieving glass‐forming aqueous electrolytes with exceptional anti‐freezing performance. Notably, the Ca(ClO 4 ) 2 eutectic electrolyte exhibits a pure glass transition at −122°C and maintains a visible liquid state at −85°C. A supercapacitor cell with a Ca(ClO 4 ) 2 electrolyte is operational at temperatures as low as −80°C. This study provides insightful understandings for designing glass‐forming anti‐freezing electrolytes with improved adaptability of aqueous energy storage devices under extremely cold conditions.