To design safe and electrochemically stable electrolytes for lithium-ion batteries, this study describes the synthesis and the utilization of new deep eutectic solvents (DESs) based on the mixture of 2,2,2-trifluoroacetamide (TFA) with a lithium salt (LiTFSI, lithium bis[(trifluoromethane)sulfonyl]imide). These prepared DESs were characterized in terms of thermal properties, ionic conductivity, viscosity, and electrochemical properties. Based on the appearance of the product and DSC measurements, it appears that this system is liquid at room temperature for LiTFSI mole fraction ranging from 0.25 to 0.5. At χ LiTFSI = 0.25, DESs exhibited favorable electrolyte properties, such as thermal stability (up to 148 °C), relatively low viscosity (42.2 mPa.s at 30 °C), high ionic conductivity (1.5 mS.cm –1 at 30 °C), and quite large electrochemical stability window up to 4.9–5.3 V. With these interesting properties, selected DES was diluted with slight amount of ethylene carbonate (EC). Different amounts of EC ( x = 0–30 %wt) were used to form hybrid electrolytes for battery testing with high voltage LiMn 2 O 4 cathode and Li anode. The addition of the EC solvent into DES expectedly aims at enhancing the battery cycling performance at room temperature due to reducing the viscosity. Preliminary results tests clearly show that LiTFSI-based DES can be successfully introduced as an electrolyte in the lithium-ion batteries cell with a LiMn 2 O 4 cathode material. Among all of the studied electrolytes, DES (LiTFSI: TFA = 4:1 + 10 %wt EC) is the most promising. The EC-based system exhibited a good specific capacity of 102 mAh.g –1 at C/10 with the theoretical capacity of 148 mAh.g –1 and a good cycling behavior maintaining at 84% after 50 cycles.

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 30oC) and relatively good anodic stability (5.2 V vs. Li+/Li).