Molecular Environment and Enhanced Diffusivity of Li⁺ Ions in Lithium-Salt-Doped Ionic Liquid Electrolytes

Abstract

Lithium salts dissolved in ionic liquids (ILs) are interesting alternatives to the commonly used electrolytes for Li-ion batteries. In this study, the solution of Li [bis-(trifluoromethanesulfonyl)imide] (LiTFSI) in N-butyl-N-methylpyrrolidinium TFSI (PYR14TFSI) ionic liquid in the 0.1:0.9 molar ratio is studied by heteronuclear NOE and NMR diffusion measurements. The main purpose is to spot on the interions organization and mobility. NOE data support the existence of strongly coordinated Li+ species, whereas variable temperature measurements of the self-diffusion coefficients D show large, selective, and unexpected enhancement of Li+ mobility with T. The measured activation energy for Li+ diffusion is significantly larger than those of TFSI− and PYR14+. These findings can be related to the mechanism of Li+ diffusion in ILs based on disruption formation of the coordination shells of Li+ with TFSI anions rather than on the Brownian motion of the whole Li+ coordinated species.