LiTFSI-BEPyTFSI as an improved ionic liquid electrolyte for rechargeable lithium batteries

We report an electrochemical study of solutions of lithium bis(trifluoromethanesulfonyl)imide, LiTFSI, in a N-n-butyl-N-ethylpyrrolidinium bis(trifluoromethanesulfonyl)imide, BEPyTFSI. We show that these ionic liquid solutions have stability towards lithium metal electrode which allows various electrochemical tests, including impedance spectroscopy and voltammetry. The ionic conductivity and lithium transference number, of the order of 10−3 S cm−1 and 0.4, respectively, make these solutions suitable for application as electrolytes in advanced lithium batteries. A prototype of these batteries, having lithium iron phosphate as the cathode, showed good performance in terms of charge–discharge efficiency and rate capability. The results reported in this work, although preliminary, are encouraging in supporting the practical interest of this LiTFSI-BEPyTFSI class of lithium conducting ionic liquids.