Synthesis and Characterization of Guanidinium-Based Ionic Liquids as Possible Electrolytes in Lithium-Ion Batteries

Abstract

Two guanidinium-based ionic liquids (ILs), N-cyclohexyl-N,N′,N′,N″,N″-pentamethylguanidinium bis(trifluoromethylsulfonyl) imide (Me5cHexGua-TFSI) and N,N-bis(2-ethylhexyl)-N′,N′,N″,N″-tetramethylguanidinium bis(trifluoromethylsulfonyl)imide (Me4(2-EtHex)2Gua-TFSI), were synthesized and investigated for their applicability as electrolyte components for lithium-ion batteries. Various properties, such as conductivity, viscosity, potential window at a platinum electrode and eventually the compatibility with graphite, Li4Ti5O12 (LTO) and LiFePO4 (LFP) electrodes were determined for the neat ILs and for electrolytes from these ILs with 0.3 mol/kg LiTFSI as conductive salt. The viscosities of the neat guanidinium-based ILs are 385 mPas and 683 mPas at 23°C for Me5cHexGua-TFSI and Me4(2-EtHex)2Gua-TFSI, respectively, and the conductivities are 0.67 and 0.18 mS/cm at 25°C. In the presence of 0.3 mol/kg LiTFSI the conductivities decreased to 0.29 and 0.10 mS/cm, respectively. Both ILs reached anodic stabilities of 4.8 V vs. Li/Li+ and showed cathodic stabilities of 0.6 V vs. Li/Li+. The guanidinium-based electrolytes can be applied with LTO and LFP electrodes, but are not compatible with graphite electrodes. The results obtained for guanidinium-based ILs/electrolytes are compared with those of a carbonate-based electrolyte [1 M LiPF6 in EC:DMC (1:1, w/w)], and a pyrrolidinium-based IL/electrolyte [1-butyl-1-methylpyrrolidinium-TFSI (BMPL-TFSI) and 0.3 mol/kg LiTFSI in BMPL-TFSI].