Data from: 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
0.5m 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.