Tailoring Ca²⁺/Li⁺ Dual-Cation Gel Polymer Electrolytes via Cross-Linking for Reductive and Oxidative Stability in Ca-Metal Batteries

Calcium–metal batteries with polymer electrolytes are emerging as promising next-generation energy systems owing to their high energy density, natural abundance of Ca in the Earth’s crust, and inherent safety. However, developing suitable polymer electrolytes for Ca-metal batteries remains challenging owing to compatibility issues with Ca-metal anodes and high-voltage cathodes. Herein, we report a dual-cation (Ca 2+ /Li + ) gel polymer electrolyte (GPE), designed using calcium monocarborane (Ca(CB 11 H 12 ) 2 ) and adjusting the lithium borohydride (LiBH 4 ) concentration, which enhanced oxidation stability while maintaining compatibility with Ca metal. The incorporation of the CB 11 H 12 – anion improved the oxidation stability of the electrolyte, increasing the oxidation potential from 2.2 to 2.6 V (vs Ca 2+ /Ca). Cyclic voltammetry and galvanostatic tests demonstrated reversible plating/stripping properties and high stability against Ca-metal anodes. The reversible operation of the Ca-metal battery was confirmed using a titanium disulfide (TiS 2 ) cathode, which achieved an initial discharge capacity of 188.5 mA h g –1 . Exsitu X-ray diffraction analysis corroborated this finding by revealing crystallographic changes in TiS 2 . This Ca-GPE, with its enhanced oxidation stability, paves the way for developing high-voltage Ca metal–polymer batteries.