Ion correlation and negative lithium transference in polyelectrolyte solutions

Polyelectrolyte solutions (PESs) recently have been proposed as high conductivity, high lithium transference number (t₊) electrolytes where the majority of the ionic current is carried by the electrochemically active Li-ion. While PESs are intuitively appealing because anchoring the anion to a polymer backbone selectively slows down anionic motion and therefore increases t₊, increasing the anion charge will act as a competing effect, decreasing t₊. In this work we directly measure ion mobilities in a model non-aqueous polyelectrolyte solution using electrophoretic Nuclear Magnetic Resonance Spectroscopy (eNMR) to probe these competing effects. While previous studies that rely on ideal assumptions predict that PESs will have higher t₊ than monomeric solutions, we demonstrate that below the entanglement limit, both conductivity and t₊ decrease with increasing degree of polymerization. For polyanions of 10 or more repeat units, at 0.5 m Li⁺ we directly observe Li⁺ move in the "wrong direction" in an electric field, evidence of a negative transference number due to correlated motion through ion clustering. This is the first experimental observation of negative transference in a non-aqueous polyelectrolyte solution. We also demonstrate that t₊ increases with increasing Li⁺ concentration. Using Onsager transport coefficients calculated from experimental data, and insights from previously published molecular dynamics studies we demonstrate that despite selectively slowing anion motion using polyanions, distinct anion-anion correlation through the polymer backbone and cation-anion correlation through ion aggregates reduce the t₊ in non-entangled PESs. This leads us to conclude that short-chained polyelectrolyte solutions are not viable high transference number electrolytes. These results emphasize the importance of understanding the effects of ion-correlations when designing new concentrated electrolytes for improved battery performance.