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 (Ca2+/Li+) gel polymer electrolyte (GPE), designed using calcium monocarborane (Ca(CB11H12)2) and adjusting the lithium borohydride (LiBH4) concentration, which enhanced oxidation stability while maintaining compatibility with Ca metal. The incorporation of the CB11H12– anion improved the oxidation stability of the electrolyte, increasing the oxidation potential from 2.2 to 2.6 V (vs Ca2+/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 (TiS2) 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 TiS2. This Ca-GPE, with its enhanced oxidation stability, paves the way for developing high-voltage Ca metal–polymer batteries.