Physicochemical and electrochemical investigations of the ionic liquid N-butyl -N-methyl-pyrrolidinium 4,5-dicyano-2-(trifluoromethyl)imidazole

Abstract

A new ionic liquid formed by coupling 4,5-dicyano-2-(trifluoromethyl)imidazole (TDI−) anion with N-butyl -N-methyl-pyrrolidinium (Pyr14+) cation is successfully synthesized and characterized by Raman spectroscopy, thermal and rheological analyses, as well as electrochemical techniques. The Pyr14TDI-LiTDI mixture, melting at 49°C, shows remarkable stability within the 50–250°C range, as well as suitable ionic conductivity, lithium ion transport, and electrochemical stability window. Thus, it is proposed for application at 60°C in a lithium cell with stable LiFePO4 cathode. At this temperature, the electrolyte has viscosity of 65.8mPa s, ionic conductivity of the order of 5mScm−1, and limiting current density of 10−2mAcm−2. Lithium metal/LiFePO4 cells with such an electrolyte offer promising results in terms of stable LiFePO4/electrolyte interface, investigated by impedance spectroscopy, as well as delivered capacity above 160mAhg−1 with 81% of retention after 80 galvanostatic cycles.