Physical properties and compatibility with graphite and lithium metal anodes of non-flammable deep eutectic solvent as a safe electrolyte for high temperature Li-ion batteries

Abstract

In this study we present the original formulation of a deep eutectic mixture based on lithium imide salt as an electrolyte for Li-ion batteries. The non-flammable liquid mixture obtained by mixing asymmetric lithium (fluorosulfonyl) (trifluoromethanesulfonyl) imide (LiFTFSI) with ethylene carbonate (EC) demonstrates its applicability as a safer electrolyte for Li-ion batteries. By comparison with conventional electrolytes containing the same salt, in 3EC/7EMC and EC/PC/3DMC, the volumetric properties of (EC-LiTFSI, 3/1, w), abbreviated EC-LiTFSI31, reveal the strong nature of the FTFSI−—EC coordination and the non-solvation of Li+, explaining the "stacking" effect observed. Transport properties (viscosity, conductivity and ionicity) are discussed and compared to those of conventional electrolytes, demonstrating that eutectic EC-LiTFSI31 has the typical ionicity of organized media such as ionic liquids or DESs, even if its concentration in salt is similar to that of the conventional electrolyte of 1 mol L−1.The electrochemical evaluation of the compatibility of this original electrolyte with lithium metal and with graphite in Li//Li and Gr//Li half-cells shows that EC-LiTFSI31 does not cause any erratic or asymmetric polarization over time in lithium metal and that it demonstrates very good intercalation and stability during the charge/discharge of graphite. The thermal and electrochemical stability of EC-LiTFSI31 gives a good performance and an outstanding stability of Gr//Li cell cycling at C/20 or C/10 up to 80 °C, during 5 months (100 cycles) without any damage to the battery. This original formulation could prove to be a promising electrolyte for Li-ion battery safety, which remains the main concern of industrialists in the field.