(526f) Molecular Dynamics Characterization of Onsager Transport Coefficients and Transference Number in Polyelectrolyte Solutions

The energy density and rate capability of conventional lithium ion batteries are limited by the formation of concentration gradients within the electrolyte. The magnitude of these gradients is largely dictated by the electrolyte’s cation transference number (t₊), which describes the relative mobility of the lithium cation over the anion. Traditional Li-ion battery liquid electrolytes possess transference numbers over a narrow range, making it difficult to systematically study the effect of changing t₊ on battery performance and to design optimized systems. Nonaqueous polyelectrolyte solutions, in which a lithium-neutralized polyanion is dissolved in nonaqueous solvent, have been proposed as a unique strategy for tuning the mobility of the anion relative to that of the cation and thus controlling t₊. The battery-relevant transport behavior of these polyelectrolytes, however, is complex and often counterintuitive. It has recently been shown, for example, that while ideal (Nernst-Einstein) approximations suggest that these polyelectrolytes have very high t₊, their true transference number can be substantially lower due to the strong ion correlations in these systems.¹

In this work, we aim to understand the emergence of polyelectrolytes’ unique transport properties as we transition from monomeric to oligo- and polymeric anions using coarse-grained molecular dynamics simulations. We perform rigorous calculation of the Onsager transport coefficients, from which we can understand the ion correlations dictating transference number, conductivity, and electrophoretic mobility. We find that cation transference number decreases monotonically with increasing chain length, i.e. in all polyelectrolyte solutions studied, the transference number is lower than that of the monomeric electrolyte. Furthermore, at low concentrations and high chain lengths we observe negative transference number (t₊ < 0) due to the presence of long-lived, negatively charged aggregates. These results suggest that while polyelectrolyte solutions may not be promising for use as high transference number electrolytes, they nevertheless offer a unique means of systematically tuning t₊ over a very wide range, thereby enabling fundamental studies on the influence of electrolyte transport on battery performance.

1. Fong, K. D. et al. Ion Transport and the True Transference Number in Nonaqueous Polyelectrolyte Solutions for Lithium Ion Batteries. ACS Cent. Sci. (2019). doi:10.1021/acscentsci.9b00406