The Competition Between Cation-Anion and Cation-Triglyme Interaction in Solvate Ionic Liquids Probed by Far Infrared Spectroscopy and Molecular Dynamics Simulations

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][NTf]) 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 (G3) and the salt anions for the coordination to the lithium cations with increasing [Li][NTf] concentration. The formation of nano structures in the [Li][NTf]: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(triglyme)] but still remains in contact to one of the four oxygen atoms of the [NTf] anion. Molecular dynamics (MD) simulations provide a molecular picture of the [Li][NTf]:G3 mixtures that supports the conclusions drawn from the experimental findings.