Polarizable Molecular Dynamics and Experiments of 1,2-Dimethoxyethane Electrolytes with Lithium and Sodium Salts: Structure and Transport Properties.

The structure and transport properties of electrolyte solutions of 1,2-dimethoxyethane (DME) having salts of Li with bis(trifluoromethanesulfonyl)imide ([TFSI]) or Na with [TFSI] are investigated with polarizable molecular dynamics and experiments. Polarizable force fields for Li and Na with DME and [TFSI] were developed based on quantum chemistry calculations, ab initio molecular dynamics simulations, and thermodynamic liquid-state properties. Simulation results for density, viscosity, self-diffusion coefficient, and conductivity of the electrolytes all agree well with the trends and magnitudes of available experimental data for a wide range of salt concentrations. As the concentration of salt increases, the electrolytes become more viscous and molecular species become less mobile. Ionic conductivity does not change monotonically with salt concentration and exhibits a maximum between 0.5 and 1.0 M for both Li[TFSI] and Na[TFSI] electrolytes. Comparatively, both cations are solvated by 5-6 DME or [TFSI] oxygen atoms and exhibit similar diffusivities and conductivities. The solvation shell of Na is found to be more weakly bound and to have a lower binding residence time than that of Li. The transport of Li therefore is more vehicular, through the motion of the solvation shell, while the transport of Na is based more on exchange, through the replacement of solvating species. The atomistic insight provided by this work can be used as the basis for future rational design of improved electrolyte solvents for lithium-oxygen, sodium-oxygen, and lithium-sulfur batteries.