Acetamide-based hydrated eutectic electrolytes for supercapacitors with high voltage and low self-discharge

Hydrated eutectic electrolytes (HEEs) hold promise as green, safe and affordable electrolytes for high-voltage electrochemical energy storage. The water molecules in HEEs participate in the solvation structure of cations, which is critical to the electrolyte performance. We hereby report HEEs consisting of NaClO₄, N-methyl acetamide (NMAc) and water. By adjusting the ratio of the ingredients, the optimized HEEs-5 shows a wide electrochemical potential window, lowered viscosity and non-flammability. Spectroscopic analysis and molecular dynamics simulations reveal that NMAc and H₂O form the inner solvation shell of Na⁺, while the free ClO₄^– result in enhanced ionic conductivity. Moreover, NMAc molecules form a protective barrier on the surface of carbon upon charging, inhibiting the activation-controlled faradaic self-discharge reactions caused by free water molecules. The supercapacitor with HEEs-5 electrolyte shows a high voltage of 2.3 V, ultra-long cycle life and low self-discharge. This work reveals the crucial roles of NMAc, but also shed light on the design and optimization of other HEEs.