Synthesis and electrochemical properties of new dicationic ionic liquids

Abstract Three classes of dicationic ionic liquids (DILs) were synthesized, in which the cationic species [ N ‑methyl pyrrolidinium (MPy), N ‑butyl imidazolium (BIm) or diethylsulfide (DES)] were separated with oligoether moieties. The counter anions are either dicyanamide (DCA) or bis(trifluoromethane sulfonyl)imide (TFSI). The molecular structures of these compounds were identified by FT-IR, 1 H NMR and 13 C NMR analysis. The DILs were investigated for their potential as electrolyte solvents. Thermal behaviors were also studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) showing excellent thermal stability up to 150 °C for all DILs. Their physicochemical properties were comprehensively characterized regarding their densities, melting points, viscosities, and miscibility with polar and nonpolar solvents. To compensate high viscosity limitation of DILs, two low viscosity ionic liquids (thinners), [MImEt] [DCA] and [MImEt] [TFSI] were used. To further improve the lithium ion conduction ability of the electrolyte systems, a lithium salt (LiTFSI) was added. Then several ternary electrolyte systems, LiTFSI in DILs/thinner (the weight ratio of DILs/thinner was fixed equal to 1/1) were developed, and their ionic conductivity and electrochemical stability were analyzed.