@article{35885, author = {Helen K Bergstrom and Kara D Fong and David M Halat and Carl A Karouta and Hasan C Celik and Jeffrey A Reimer and Bryan D McCloskey}, title = {Ion correlation and negative lithium transference in polyelectrolyte solutions}, abstract = {

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 {\textquotedblleft}wrong direction{\textquotedblright} 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{\textendash}anion correlation through the polymer backbone and cation{\textendash}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.

}, year = {2023}, booktitle = {Chemical Science}, journal = {Chemical Science}, series = {Chemical Science}, volume = {14}, pages = {6546 - 6557}, month = {06/2023}, issn = {2041-6520}, url = {http://xlink.rsc.org/?DOI=D3SC01224G}, doi = {10.1039/D3SC01224G}, language = {eng}, }