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 to 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 >1 mS/cm. Reversible redox activity in symmetric Ca cells is demonstrated at 2 V overpotentials, stably cycling at 0.1 mA/cm2 and areal capacities of 0.1 mAh/cm2. Softer coordination, polarizability, and closer coordinating site distances of the imidazole groups can explain the enhanced properties. Hence, imidazole is a suitable benchmark chemistry for future design and advancement of polymer electrolytes for calcium batteries.