Physical and electrochemical properties of DES solvents based on 2,2,2-trifluorocetamide and LiTFSI salt for Li-ion batteries

Abstract

The liquid electrolyte transports lithium ions from anode to cathode during charging, and vice versa. The choice of electrolyte is also important since high ionic conductivity between electrodes is essential for high-performance batteries. Liquid electrolytes with lithium salt dissolved in an organic solvent have been widely used since the 1970s when lithium primary batteries were first developed. Most lithium secondary batteries available today use organic electrolytes. Ionic liquids consist of organic cations and inorganic anions, due to the absence of a combustible and flammable organic solvent, they are known to produce safer batteries. Furthermore, they have a high polarity that allows dissolution of inorganic and organic metal compounds, and they can exist in a liquid state over a wide temperature range. Another type of solvent with similar physical properties and phase behavior to ILs is deep eutectic solvents (DESs) about which the first paper was recently published in 2001. These solvents are mixtures that have a much lower melting point than that of any of their individual components, mainly due to the charge delocalization occurring through hydrogen bonds between them. DESs are generally favored over ILs because they are cheaper and easier to prepare with high purity. In this work, Deep Eutectic Solvents (DESs) were prepared by simple mixing Lithium bis[(trifluoromethane)sulfonyl] imide (LiTFSI) salt and 2,2,2-trifluoroacetamide TFA at various ratios ranging from 1:1.5 to 1:4, respectively. The formation of DESs was characterized by Infrared Spectroscopy (IR) and Thermogravimetric analysis (TGA). Their physical and electrochemical properties were also evaluated based on their viscosity, conductivity, and oxidation stability window. Amongst our systems of interest, DES with LiTFSI: FAc ratio of 1:4 is the most promising as the electrolyte for Li-ion batteries, because it exhibited the lowest viscosity (42.2 mPa.s), the highest ionic conductivity (1.53 mS.cm^-1 at 30^oC) and relatively good anodic stability (5.2 V vs. Li⁺/Li).