Vinylimidazole-Based Polymer Electrolytes with Superior Conductivity and Promising Electrochemical Performance for Calcium Batteries.

Calcium batteries are next-generation energy storage technologies with promising techno-economic benefits. However, performance bottlenecks associated with conventional electrolytes with oxygen-based coordination chemistries must be overcome to enable faster cation transport. Here, we report an imidazole-based polymer electrolyte with the highest reported conductivity and promising electrochemical properties. The polymerization of vinylimidazole in the presence of calcium bis(trifluoromethanesulfonyl)imide (Ca(TFSI) 2 ) salt creates a gel electrolyte comprising a polyvinyl imidazole (PVIm) host infused with vinylimidazole liquid. Calcium ions effectively coordinate with imidazole groups, and the electrolytes present room temperature conductivities of >1 mS/cm. Reversible redox activity in symmetric Ca cells is demonstrated at 2 V overpotentials, stable cycles at 0.1 mA/cm 2 , and areal capacities of 0.1 mAh/cm 2 . Softer coordination, polarizability, and closer coordinating site distances of the imidazole groups can explain the enhanced properties. Hence, imidazole is a suitable chemical benchmark for the future design and advancement of polymer electrolytes for calcium batteries.