Electrolyte Dependence of Li⁺ Transport Mechanisms in Small Molecule Solvents from Classical Molecular Dynamics

As demands on Li-ion battery performance increase, the need for electrolytes with high ionic conductivity and a high Li⁺ transference number (t_Li) becomes crucial to boost power density. Unfortunately, t_Li in liquid electrolytes is typically <0.5 due to Li⁺ migrating via a vehicular mechanism, whereby Li⁺ diffuses along with its solvation shell, making its diffusivity slower than the counteranion. Designing liquid electrolytes where the Li⁺ ion diffuses independently of its solvation shell is of significant interest to enhance the transference number. In this work, we elucidate how the properties of the solvent influence the Li⁺ transport mechanism. Using classical molecular dynamics simulations, we find that a vehicular mechanism can be increasingly preferred with a decreasing solvent viscosity and increasing interaction energy between the solvent and Li⁺. Thus, a weaker interaction energy can enhance t_Li through a solvent-exchange mechanism, ultimately improving Li-ion battery performance. Finally, metadynamics simulations show that in electrolytes where a solvent-exchange mechanism is preferable, the energy barrier to changing the coordination environment of Li⁺ is much lower than in electrolytes where a vehicular mechanism dominates.