Investigation of the N -butyl- N -methyl pyrrolidinium trifluoromethanesulfonyl- N -cyanoamide (PYR 14 TFSAM) ionic liquid as electrolyte for Li-ion battery

Abstract A new asymmetrical anion, trifluoromethanesulfonyl- N -cyanoamide (TFSAM − ), was paired with N -butyl- N -methyl pyrrolidinium (PYR 14 + ) to prepare PYR 14 TFSAM. It has been investigated for Li-ion battery application and compared to its PYR 14 + analogs paired with either the dicyanamide anion (DCA − ) or other anions ( i.e. bis(trifluoromethanesulfonyl) imide (TFSI − ), bis(fluorosulfonly)imide (FSI − ), trifluoromethanesulfonyl-fluorosulfonyl imide (FTFSI − )). The conductivity of PYR 14 TFSAM is not only higher than that of PYR 14 TFSI, but also higher than that of PYR 14 FTFSI with 3.8 mS cm −1 at 20 °C and 12.6 mS cm −1 at 60 °C. In addition, the ionic liquid does not crystallize and exhibits a viscosity similar to that of PYR 14 FSI (and even lower above 30 °C, which also results in a higher conductivity at high temperature). Compared to PYR 14 DCA, PYR 14 TFSAM has a higher anodic stability, more compatible with state-of-the-art cathodes such as NCM, even though the PYR 14 DCA electrolyte also allowed surprisingly good cycling results of NCM cathode considering its low anodic stability. PYR 14 TFSAM also allows Li + (de-)/insertion into graphite, using vinylene carbonate as additive. When used in conventional Li-ion electrolyte solvents, it leads to moderate conductivity (as compared with LiFSI or LiTFSI), although much higher than LiDCA. Additionally, it is shown that, even in EC/DMC-based electrolyte, LiTFSAM does not induce Al corrosion at 4.2 V.