New deep eutectic solvents based on ethylene glycol - LiTFSI and their application as an electrolyte in electrochemical double layer capacitor (EDLC)

Abstract

Deep eutectic solvents (DESs) have been considered the existing alternatives that are analog and cheap to ionic liquids for their favorable properties (e.g., chemical stability, low flammability, and low vapor pressure). In the present study, the physical and electrochemical properties of ethylene glycol (EG) and lithium bis[(trifluoromethyl)sulfonyl] imide (LiTFSI)-based DESs were comprehensively analyzed. According to the results of the differential scanning calorimetry (DSC) measurement, the formation of DESs was confirmed with a low melting point from −23 to −26 °C. Besides, the DESs were confirmed to be formed by the variations in the intermolecular interactions of two solid precursors observed under the Infrared spectroscopy (IR). By Vogel-Tamman-Vulcher (VTF) fitting equation, the temperature dependencies of ionic conductivity and the viscosity of these DESs were correctly expressed. The synthesized DESs exhibited superior properties (e.g., high electrochemical window (~4.5 V vs Li+/Li), high thermal stability (~200 °C), good flame resistance, as well as relatively high ionic conductivity about 3 mS cm−1 at 30 °C). Lastly, DES-based electrolytes were added to the activated carbon electrochemical double-layer capacitor (EDLC). The performance of the EDLC was assessed by Cyclic Voltammetry and Galvanostatic charge/discharge tests at ambient temperature. As revealed from the preliminary results, the overall performance of the DESs could address the main challenges in supercapacitor applications, i.e., limited operating voltage range and safety problems.