Although lithium batteries have tremendous potential, the flammability of the organic electrolyte commonly used hinders their consideration for large-scale applications, where the battery is expected to operate over a wide temperature range without compromising safety. To combat flammability, highly stable ionic liquids (ILs), entirely composed of ions and having negligible vapour pressure, have been widely investigated as electrolytes [1].

Tailored, in-house synthesized new ILs, based on cyclic quaternary ammonium cations, having no double bonds or acidic protons reacting with lithium, and bis(trifluoromethanesulfonyl)imide (TFSI) anion, will be here presented. A comparison between two families of ionic liquids, with ether- or sulfur-functionalized aliphatic side chains in the cation moiety, will be described. Their thermal and electrochemical properties, including the definition of the stability window, interface characteristics towards carbon-based and lithium electrodes, will be discussed. Electrochemical performances in advanced lithium cell configurations, adopting electrolytes based on the proposed ILs, will be finally presented. Tests in Li/S cells have revealed good reversibility of the polysulfides redox process and appreciable capacity, combined with good retention and coulombic efficiency leveling 99 %.

[1] M. A. Navarra, MRS Bulletin, volume 38, year 2013, pages 548 - 553: “Ionic liquids as safe electrolyte components for Li-metal and Li-ion batteries”.