Structural Transformations within the Solvate Ionic Liquid [Li(Triglyme)][NTf2]: Implications for Self-Diffusion, Viscosity, and Ionic Conductivity

Solvate ionic liquids (SILs) are a promising new class of electrolytes for lithium-ion batteries. Prominent SIL candidates are equimolar mixtures of lithium salts with weakly interacting anions and glyme. In particular, the equimolar mixture of lithium ([Li]+)(bis(trifluoromethanesulfonyl)imide ([NTf2]-) and triglyme (G3) is of great interest. It has been suggested that this mixture exhibits a behavior similar to ionic liquids due to the formation of stable 1:1 complexes of [Li]+ with G3 molecules. We use up to multi-mocrosecond molecular dynamics (MD) simulations to better understand the structure and dynamics of the mixtures for varying mixing ratios and temperatures and to characterize the typical coordination patterns of the [Li]+ complexes. We find that at low [Li][NTf2] content, each [Li]+ cation is, on average, coordinated by two G3 molecules. For nearly equimolar mixtures, the complex changes to a one-fold G3-coordinated cation plus one additional anion. For higher than equimolar salt concentrations, cations are increasingly surrounded by their counterions, forming lithium bridges between adjacent anions. We observe that the structure primarily depends on the mixture composition while it is remarkably temperature-insensitive. The latter suggests that cluster equilibria in the SIL are subject to only small entropy differences, retaining the SIL-like structural features up to more than 200 degrees Celsius. We demonstrate that the structural changes have a major impact on the transport properties of the system: For the investigated temperatures, the self-diffusion coefficients of all components decrease by several orders of magnitude with increasing [Li][NTf2] content, while the viscosity strongly increases. For mole fractions between 0.4 and 0.5, both [Li]+ and G3 move concertedly and exhibit similar self-diffusion coefficients, indicating the formation of stable 1:1 complexes. We conclude that these mixtures can be categorized as highly temperature-stable SILs with possible implications for battery technology.