A Highly Conductive and Stable Ionic Liquid Gel Electrolyte for Calcium Metal Batteries

Calcium batteries are promising alternatives to lithium batteries due to their high energy density, comparable reduction potential, and mineral abundance. Calcium is the fifth most abundant metal in the Earth's crust (4.1%), surpassing Na, K, Mg, and Li, and ranks third after aluminum and iron. Calcium batteries can achieve volumetric capacities comparable to incumbent Li-ion and emerging Li metal systems. Additionally, calcium-ion batteries generally exhibit improved safety characteristics compared to lithium-ion batteries, being less susceptible to thermal runaway and reducing the risk of overheating, combustion, or explosion.<br/><br/>However, to meet practical demands in high-performance applications, suitable electrolytes must be developed. Polymer gel electrolytes are emerging as a promising system for calcium metal batteries. Polymer gel electrolytes consist of polymer hosts swollen in liquid electrolyte and offer a combination of mechanical and electrochemical robustness from the polymer host, high ion conductivity and electrochemical reactivity from the liquid electrolytes, excellent wetting contact with the electrodes, more uniform electrode reactions (specifically, metal deposition), and have proven attractive for Li, Na, K, Zn, and Mg metal systems. Moreover, they provide a decent level of mitigation against potential leakage, electrolyte volatility, and flammability, thus offering the desired safety benefits similar to quasi-solid-state batteries.<br/><br/>In this study, polymer gel electrolytes (GPEs) for calcium-ion conduction were synthesized and characterized by photo-cross-linking poly(ethylene glycol) diacrylate (PEGDA) in the presence of calcium salt Ca(BF₄)₂ and an ionic liquid EMIM triflate. The ionic conductivity of the GPEs was observed to increase with salt concentration and temperature, reaching a maximum conductivity of 2.16 S/cm. The Ca stripping/plating behavior was analyzed on a Ca substrate with a current density of 0.2 mA cm⁻² and a total discharge areal capacity of 0.4 mA h cm⁻². The overpotential at the first cycle was 0.4 mV and increased with subsequent cycles. Furthermore, the PEGDA-based GPEs exhibited an electrochemical stability window of 4 volts and a thermal stability window exceeding 200 degrees Celsius.<br/><br/>Overall, these results demonstrate the potential of PEGDA-based GPEs as high-performance electrolytes for calcium batteries and provide valuable insights into the development of advanced GPEs for next-generation energy storage devices.