The competition between cation-anion and cation-triglyme interaction in solvate ionic liquids probed by far infrared spectroscopy and molecular dynamics simulations.

Abstract

Glyme-based electrolyte solutions provide new concepts for developing suitable lithium-ion batteries. The so-called solvate ionic liquids (SILs) are promising electrolytes. They are most efficient in equimolar mixtures of lithium bis(trifluoromethanesulfonyl)imide ([Li][NTf2]) and glyme, wherein the [Li]+ cation is supposedly fully solvated by glyme molecules. Here, we performed far (FIR) and mid (MIR) infrared spectroscopy for probing the solvation and local structures around the [Li]+ ions. In particular, we studied the competition between the triglyme molecule and the salt anions for the coordination to the lithium cations with increasing [Li][NTf2] concentration. The formation of nano structures in the [Li][NTf2]:G3 mixtures is discussed in terms of contact (CIP) and solvent-separated (SIP) ion pairs in solution. At low salt concentrations, the [Li]+ cations are solvated by two triglyme molecules resulting in SIPs only. With increasing salt concentration, [Li]+ is predominantly solvated by one triglyme molecule as [Li(glyme)1]+ but still remains in contact to one of the four oxygen atoms of the [NTf2]- anion. Molecular dynamics (MD) simulations provide a molecular picture of the [Li][NTf2]:G3 mixtures that supports the conclusions drawn from the experimental findings.