Correlated Ion Transport in High-Concentration Electrolytes

Ion transports in the organic solvents with low permittivity, which is often used as electrolytes in lithium ion batteries, are strongly correlated. Recent evidences suggested that the strong couplings between ions and solvents in the high-concentration regime (>1M) can result in a negative correlation between the molar conductivity and self-diffusivity of lithium ions, in sharp contrast to the expectation based on the Nernst-Einstein relation. We calculate the full Onsager transport coefficients using all-atom molecular dynamics simulation over a wide range of concentrations, and decouple the contributions from self-diffusivity and ion-ion correlation. The results show that, in the dilute regime, the oppositely charged species are more likely to move collectively, which causes negative contribution to conductivity; in the concentrated regime, clusters are more abundant, which facilitates ionic hopping between neighboring solvation shells. These two competing effects lead to the salient peak in molar conductivity observed in both experimental and simulation data. Finally, we show that the correlations of ion transport decrease with increasing concentration by examining the variation of the Haven ratio with ion concentration.