Molecular Origin of Negative Lithium Transference in Electrolytes with Star-Shaped Multivalent Anions

Abstract

Large multivalent anions have gained increasing attention for their potential to improve lithium transference in electrolytes. We employ large-scale molecular dynamics simulations based on the Onsager transport framework to investigate ion transport in a lithium electrolyte with star-shaped multivalent anions. The simulations show that t_+^0, the cation transference number with respect to solvent velocity, is negative over a wide range of concentration. This is consistent with experimental data reported previously1. The simulation-based Onsager transport coefficients reveal that the magnitudes of the cation-cation, anion-anion, and cation-anion correlations are comparable, a signature of highly correlated motion in the electrolyte. Examination of the cation solvation environment indicates the presence of strong cation-anion association across the entire concentration range, which leads to negative t_+^0 on the order of -1. Both simulation and experiment also show that the maximum value of t_+^0 reaches 0 when the cation concentration is c_+=0.4 M. This is the concentration at which the anions begin to spatially overlap, and lithium ions serve as dynamic linkers to balance cation-cation and cation-anion correlations. Our results provide molecular-level insights into the origin of transference in multivalent electrolytes.