Quantifying selective solvent transport under an electric field in mixed-solvent electrolytes

Electrolytes in lithium-ion batteries comprise solvent mixtures, but analysis of ion transport is always based on treating the solvents as a single-entity. We combine electrophoretic NMR (eNMR) measurements and molecular dynamics (MD) simulations to quantify electric-field-induced transport in a concentrated solution containing LiPF6 salt dissolved in an ethylene carbonate/ethyl methyl carbonate (EC/EMC) mixture. The selective transport of EC relative to EMC is reflected in the difference between two transference numbers, defined as the fraction of current carried by cations relative to the velocity of each solvent species. This difference arises from the preferential solvation of cations by EC and its dynamic consequences. The simulations reveal the presence of a large variety of transient solvent-containing clusters which migrate at different velocities. Rigorous averaging over different solvation environments is essential for comparing simulated and measured transference numbers. Our study emphasizes the necessity of acknowledging the presence of four species in mixed-solvent electrolytes.

Automated summary

In this study, we used electrophoretic NMR (eNMR) measurements and molecular dynamics (MD) simulations to examine the electric-field-induced transport in a concentrated solution of LiPF6 salt dissolved in an ethylene carbonate/ethyl methyl carbonate (EC/EMC) mixture. It was found that ethylene carbonate (EC) exhibited selective transport compared to ethyl methyl carbonate (EMC) due to its preferential solvation of cations and its dynamic consequences. Molecular simulations also revealed the presence of various transient solvent-containing clusters migrating at different velocities. This study highlights the importance of considering the presence of four species in mixed-solvent electrolytes.