Onsager Transport Coefficients and Transference Numbers in Polyelectrolyte Solutions and Polymerized Ionic Liquids

KD Fong and J Self and BD McCloskey and KA Persson, MACROMOLECULES, 53, 9503-9512 (2020).

DOI: 10.1021/acs.macromol.0c02001

Electrolytes featuring negatively charged polymers, such as nonaqueous polyelectrolyte solutions and polymerized 0.4 ionic liquids, are currently under investigation as potential high cation transference number (t(+)) electrolytes for lithium-ion batteries. Herein, we use coarse-grained molecular dynamics simulations to characterize the Onsager transport coefficients of polyelectrolyte solutions as a function of chain length and concentration. For all the systems studied, we find that the rigorously computed transference number is substantially lower than that approximated by the ideal solution (Nernst-Einstein) equations typically used to characterize these systems due to the presence of strong anion-anion and cation-anion correlations. None of the polyelectrolyte solutions achieve t(+) greater than that of the conventional binary salt electrolyte, with some solutions having negative t(+). This work demonstrates that the Nernst-Einstein assumption does not provide a physically meaningful estimate of the transference number in these solutions and calls into question the expectation of polyelectrolytes to exhibit a high cation transference number.

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