This study describes the utilization of deep eutectic solvents (DESs) based on the mixture of the N-methylacetamide (MAc) with a lithium salt (LiX, with X = bis[(trifluoromethyl)sulfonyl]imide, TFSI; hexafluorophosphate, PF 6 ; or nitrate, NO 3 ) as electrolytes for carbon-based supercapacitors at 80°C. The investigated DESs were formulated by mixing a LiX with the MAc (at x Li = 0.25). All DESs show the typical eutectic characteristic with eutectic points localized in the temperature range from −85 to −52°C. Using thermal properties measured by differential scanning calorimetry (DSC), solid−liquid equilibrium phase diagrams of investigated LiX−MAc mixtures were then depicted and also compared with those predicted by using the COSMOThermX software. However, the transport properties of selected DESs (such as the conductivity (σ) and the fluidity (η −1 )) are not very interesting at ambient temperature, while by increasing the temperature up to 80°C, these properties become more favorable for electrochemical applications, as shown by the calculated Walden products: w = ση −1 (mS cm −1 Pa −1 s −1 ) (7 < w < 16 at 25°C and 513 < w < 649 at 80°C). This "superionicity" behavior of selected DESs used as electrolytes explains their good cycling ability, which was determined herein by cyclic voltammetry and galvanostic charge−discharge methods, with high capacities up to 380 F g −1 at elevated voltage and temperature, i.e., ΔE = 2.8 V and 80°C for the LiTFSI−MAc mixture at x Li = 0.25, for example. The electrochemical resistances ESR (equivalent series resistance) and EDR (equivalent diffusion resistance) evaluated using electrochemical impedance spectroscopy (EIS) measurements clearly demonstrate that according to the nature of anion, the mechanism of ions adsorption can be described by pure double-layer adsorption at the specific surface or by the insertion of desolvated ions into the ultramicropores of the activated carbon material. The insertion of lithium ions is observed by the presence of two reversible peaks in the CVs when the operating voltage exceeds 2 V. Finally, the efficiency and capacitance of symmetric AC/AC systems were then evaluated to show the imbalance carbon electrodes caused by important lithium insertion at the negative and by the saturation of the positive by anions, both mechanisms prevent in fact the system to be operational. Considering the promising properties, especially their cost, hazard, and risks of these DESs series, their introduction as safer electrolytes could represent an important challenge for the realization of environmentally friendly EDLCs operating at high temperature.

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This study proposes a new deep eutectic solvent based on sodium nitrate and N-methylacetamide as an electrolyte for carbon-based supercapacitors at 80°C.

This work proposes two deep eutectic solvents (DESs) based on lithium bis(fluorosulfonyl)imide and sodium bis(fluorosulfonyl)imide together with N-methylacetamide and formamide as electrolytes for activated carbon (AC) electrochemical double-layer capacitors (EDLCs) at 25 °C. The formulated DESs exhibit a large electrochemical window (ΔE &gt; 2.5 V), good thermal stability (∼150 °C) and ionic conductivity (3–4 mS cm−1), and moderate viscosity (11.3 mPa s). Through the Vogel–Tamman–Vulcher fitting equation, the evolution of pseudo-energy activation was delineated with respect to the nature of the H-bond donor or alkali salt. These electrolytes present a superionic character gleaned from the Walden classification, and their ionicity exceeds that of standard organic electrolytes based on similar alkali salts. The performance of the AC-based EDLC was assessed by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge/discharge, yielding 140 F g−1 with an 8% capacity retention during 200 h of floating. Based on the physicochemical properties and electrochemical performance of these DESs, they represent a promising green-alternative electrolyte for supercapacitor applications.