(557b) Ion Transport and Ion-Correlation in Non-Aqueous Lithium-Ion Polyelectrolyte Solutions

A majority of ionic conductivity in current state-of-the-art liquid electrolytes results from the movement of the anion rather than the electrochemically active Li⁺ ion. Passing current through such an electrolyte can cause severe ionic concentration gradients to extend into porous electrodes, which at high currents can reduce active material utilization and result in high overpotentials that cause deleterious side reactions to occur (e.g., Li plating on graphite electrodes)¹. The severity of these concentration gradients is directly influenced by the transference number (t₊), which describes the ratio of current carried by the electrochemically active Li⁺ ion to the total current passed. Non-aqueous polyelectrolyte solutions (PESs) have been recently suggested as a promising route to high transference number electrolytes that demonstrate potential for both high ionic conductivity and t₊^2–4. This combination is achieved by anchoring the anion to a polymer backbone – allowing for high t₊ by slowing down the motion of the electrochemically inactive anion – while maintaining high ion conductivity through improved ion dissociation and solvent-mediated Li⁺ transport. Recent molecular dynamic simulations of PESs have highlighted the critical importance of correlated ion motion in these systems and have called into question oligomeric PESs as a feasible strategy to achieving high t₊ and conductivity electrolytes^5,6.

In this presentation we discuss our efforts to date synthesizing and experimentally studying ion transport in lithium-ion and lithium metal battery-relevant PESs- specifically lithium triflimide appended polystyrene (PS-LiTFSI) and polymethacrylate (PA-LiTFSI) dissolved in carbonate blends. Using a combination of pulsed-field gradient NMR, Raman spectroscopy, and electrochemical experiments, we characterized the transport properties, including the conductivity, diffusion coefficients, and t₊ of PS-LiTFSI and PA-LiTFSI PESs. We seek to understand the role of correlated ion motion in these systems and the impact that these correlations have on lithium-ion transport parameters relevant to battery performance. Comparing our experiments⁷ to insights from molecular dynamics modeling we demonstrate that despite selectively slowing anion motion using polyanions, anion-anion correlation through the polymer backbone likely contributes to reducing the t₊ in PESs. We will discuss future strategies to tune this effect by manipulating polyion molecular weight, backbone chemistry, and charge distribution on the polymer chain, particularly in the entangled high-molecular weight regime.

References
(1) Diederichsen et al. ACS Energy Lett. (2017)
(2) Diederichsen et al. Macromolecules (2018)
(3) Diederichsen et al. J. Phys. Chem. B (2019)
(4) Dewing et al. Chem. Mater. (2020)
(5) Fong et al. ACS Cent. Sci. (2019)
(6) Fong et al. Macromolecules (2021)
(7) Bergstrom et al. ECSarXiv (2021)