How Individual Versus Shared Coordination Governs the Degree of Correlation in Rotational Versus Residence Times in a High-Viscosity Lithium Electrolyte

Mixtures of lithium bis(trifluoromethylsulfonyl)amide (LiTFSI) salt and glyme-based solvents are potential alternative candidates for commonly used electrolytes. We perform classical molecular dynamics simulations to study the effect of concentration and temperature on translational and rotational dynamics. The radial distribution function shows stronger coordination of Li+ ions with tetraglyme (G4), as shown in earlier studies, and forms a stable [Li(G4)]+ cation complex. The self-diffusion coefficients are lower than the values experimentally observed but show improvement over other classical force fields without charge scaling. An increase in the salt concentrations leads to a higher viscosity of the system and reduces the overall ionic mobility of Li+ ions. Diluting the system with a larger number of G4 molecules leads to shorter rotational relaxation times for both TFSI and G4. Ion-residence times show that Li+ ions form stable and long-lasting complexes with G4 molecules rather than TFSI anions. The residence time of the [Li(G4)]+ complex increases in the highly concentrated system due to the availability of fewer G4 molecules to coordinate with a Li+ ion. G4 is also seen to form polydentate complexes with Li+ ions without shared coordination, allowing rotation without breaking coordination, unlike TFSI, which requires coordination disruption for rotation. This distinction explains the poor correlation between rotation and residence time for G4 and the strong correlation for TFSI.