Tuning solvent–salt interactions in ionic liquid–organic blends for enhanced supercapacitor performance

Advanced supercapacitors require electrolytes that extend the electrochemical stability window while preserving fast ion transport. Here, we investigate hybrid electrolytes based on mixtures of an ammonium-based ionic liquid (IL), N-butyl- N -trimethylammonium bis(trifluoromethylsulfonyl)imide ([N 1114 ][NTf 2 ]), and 1.0 M TEABF 4 in propylene carbonate, with IL contents ranging from 10 to 90 vol%. A comprehensive electrochemical and operando spectroscopic approach was employed to elucidate the interplay between ion–solvent interactions, nanostructuring, and charge storage mechanisms. Our results demonstrate that IL incorporation induces the formation of nanosegregated domains and voltage-dependent interfacial structures that regulate ion mobility and the organization of the electric double layer. At low IL content, the system achieved an extended voltage of 3.4 V, enhanced capacitance, and reduced internal resistance, attributed to improved dielectric environment and efficient pore accessibility. In contrast, higher IL fractions promote strong ion–ion correlations, limiting ion transport, despite improved electrochemical stability and safety. Operando spectroscopy reveals the formation of a dynamic, protective interfacial layer derived from preferential IL decomposition, which stabilizes the electrode–electrolyte interface and suppresses solvent degradation at high voltages. Overall, this work provides a rational design strategy for hybrid electrolytes, highlighting their potential to enable safer and high-performance supercapacitors for advanced energy storage applications. By validating performance in both coin and pouch cells, we demonstrate that the improved interfacial behavior and expanded voltage window are maintained in a practical device format, bridging fundamental electrochemical insights with scalable implementation for compact, high-voltage applications that require fast charge/discharge and long cycle life.