Li⁺ Ionic Liquids Based on Macro-Anion Concepts as Liquid Electrolytes for Lithium Battery

Electrolytes with a high lithium (Li) transport number, whether liquid or polymeric, are desired for use in both lithium-ion and lithium metal rechargeable battery technologies. Liquid electrolytes pose safety problems, while polymer electrolytes suffer from poor ionic transport. Ionic liquid (IL) electrolytes were proposed as an alternative solution for their electrochemical stability and low flammability. Nevertheless, Li-salt-containing IL electrolytes still suffer from sluggish Li transport owing to the complicated Li solvation structure and dynamics. One promising route to increase Li transport number and simplify Li solvation structure and dynamics in IL electrolytes is limiting the anion mobility while having a Li salt in liquid form. In this paper, a series of macro Li salts that respond to ILs behavior were synthesized via an original and simple synthetic approach combining Michael addition and epoxidation reactions. Different backbone platforms such as polyhedral oligomeric silsesquioxane (POSS), tetraethylenepentaamine (TEPA), branched poly(ethylenimine) (BPEI), and Jeffamine® ED were used for grafting both Li sulfonyl(trifluoromethylsulfonyl) imide derivative and short poly(ethylene glycol) (PEG) chains. Benefiting from the presence of grafted PEG chains of 550 g mol–1, these Li salt electrolytes are under liquid form at room temperature and possess IL properties such as low Tg (<0 °C), low vapor pressure and volatility, and high thermal stability (decomposition temperature >300 °C). Among different IL electrolytes investigated, those based on TEPA or BPEI exhibit a good conductivity approaching 10–4 S cm–1 at 40 °C, combining a high Li+ transport number above 0.7, which is very promising for battery application.