Ionic liquids containing carbonate solvent as electrolytes for lithium ion cells

A novel aliphatic quaternary ammonium type ionic liquid, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide (DEME–TFSI), is a practical and useful ionic liquid as an electrolyte for electrochemical devices. Its desirable properties include non-flammability, a quite wide potential window (5.4 V), and a high ionic conductivity (4.0 mS cm−1 at 30 °C). We attempted to utilize Li-binary ionic liquids composed of LiTFSI and DEME–TFSI as an electrolyte in a lithium deposition and dissolution media. However, although DEME-TFSI had a relatively large potential window among recently reported ionic liquid species, it decomposed on the negative electrode at a positive potential relative to that of the Li/Li+. To provide a solid electrolyte interface that would overcome the reductive decomposition of the electrolyte, we added two kinds of organic solvent to the Li-binary ionic liquids. The selected additives, vinylene carbonate (VC) and ethylene carbonate (EC) effectively prevented the decomposition and improved the reversible lithium deposition/dissolution. To evaluate the potential of these liquids with additives as electrolytes for lithium ion batteries, we prepared demonstration cells composed of a graphite/Li–DEME–TFSI containing 10 wt.% of VC/LiCoO2 and investigated its cell performance. This is the first report of our knowledge that a lithium ion cell with an ionic liquid electrolyte performed, in terms of cell performance and cycle durability, at a level of practical utility.