The Intriguing Properties of 1-Ethyl-3-methylimidazolium bis(fluorosulfonyl)imide Ionic Liquid

Among all ionic liquids (ILs) which can be used as electrolyte solvents in Li-ion batteries (LIB), 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide, [C2mim][FSI], is certainly the most noteworthy. One reason for that is driven by its exceptionally low viscosity, which enhances its intrinsic conductivity and makes it a good solvent for the formulation of safer electrolytes in LIB in comparison with classical organic solvents. The solubility of lithium salts in [C2mim][FSI] can be very high (more than 4 mol·L−1 for LiFSI), which opens the field to the use highly concentrated electrolyte solutions containing only ions. With the aim of comparing the physico-chemical properties of different FSI based ILs and the electrolytes obtained by addition of a doping Li-salt, three [FSI] based ILs were selected: [C2mim][FSI], N-propyl-N-methyl pyrrolidinium bis(fluorosulfonyl)imide [C3mpyrr][FSI] and N-butyl-N-methyl pyrrolidinium bis(fluorosulfonyl)imide [C4mpyrr][FSI]. For comparison, a TFSI based IL ([C4mpyrr][TFSI]) and a reference alkylcarbonate mixture (ethylcarbonate (EC)/propylcarbonate (PC)/dimethylcarbonate (DMC) (in the proportion 1/1/3 by weight) are also studied. LiFSI or lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) were used as doping lithium salts. The following physico-chemical properties have been investigated: density, viscosity, ionic conductivity, ionicity and transference number. From these results, it can be concluded that all [FSI] based ILs considered in this work exhibit both a relatively low viscosity, a high conductivity, a high ionicity and Li-ion transference number. All these properties make them good candidates for use as electrolyte in Li-ion batteries. Moreover their great thermal stability and low flammability give them an advantage over classical organic solvents.