Solvation Structure and Dynamics in Ionic Liquid Electrolytes with Hydrofluoroether for Li-Ion Batteries

Abstract

The addition of hydrofluoroethers (HFEs) to Li-ion battery electrolytes reduces flammability and enhances the cycling behavior at a wider temperature range. In the case of ionic liquid electrolytes, where the ion concentration is high and Li+ solvation by anions govern the transport properties, HFEs facilitate the separation of ion aggregates. In this study, the addition of HFE to n-methyl-n-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide [PYR14][TFSI] with Li-salts of lithium (nonafluorobutane)(trifluoromethanesulfonyl)imide [Li][IM14] and [Li][TFSI] (0 ≤ x Li ≤ 0.3), were studied to understand the compositional changes in phase, the liquid structure and the transport properties. A suppression in the melting point with the addition of HFE was accompanied with improved conductivities at lower temperatures. Li+ was found to primarily solvated by [TFSI] with some contribution by [IM14], as determined from Raman spectroscopy and molecular dynamics (MD) simulations. Further, HFE encouraged florous domain formations as determined by HOESY NMR and even short contacts between HFE and Li+ as a result of tighter anion-solvated Li+ without aggregation. Li+ diffusivity of 1.2610-12 m2/s at 0 °C and x Li = 0.2 was achieved with a high Li+ transference of 0.16, relative to ionic liquid electrolytes at 0 °C. The developed electrolytes demonstrated cyclable Si-Li half cells. This study demonstrates the tunability of ionic liquid electrolytes by asymmetric fluorinated anions and HFE co-solvents for low temperature Li-ion battery applications.