Ion Correlation and Negative Transference in Lithium-bearing Non-Aqueous Oligomeric Electrolyte Solutions

21 February 2023, Version 2
This content is a preprint and has not undergone peer review at the time of posting.


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.


Li-ion battery
transference number
electrolyte engineering
electrophoretic NMR

Supplementary materials

Supplementary Information : Ion Correlation and Negative Transference in Lithium-bearing Non-Aqueous Oligomeric Electrolyte Solutions
Supplementary information including synthetic details, NMR parameters, thermodynamic factor measurements and comparison with molecular dynamics.


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