(232a) Ion Transport in Li+ Polyelectrolytes and Concentrated Binary Salt Electrolytes

Conventional Li-ion battery electrolytes have been designed to optimize numerous desirable properties, including interfacial and thermal stability, conductivity, and low flammability. However, all Li⁺-bearing electrolytes still possess low Li⁺ transference (t₊) numbers, where current passed through them is primarily carried by the counteranion, resulting in large concentration gradients that limit battery performance, particularly at high discharge and charging rates. The development of high t₊ electrolytes—those in which most (or all) current is carried by the Li⁺ ion—could enable safer battery cycling, faster charging rates, and thicker, more energy-dense cathode designs in Li-ion batteries.

In recent years, both polyelectrolyte solutions (charged polymers dissolved in solvent) and highly concentrated binary salt electrolytes (or so-called solvent-in-salt electrolytes) have been reported to provide high Li⁺ transference numbers, as measured using the popular Bruce-Vincent method.¹ In this presentation, we will discuss the complete transport characterization of model systems in both electrolyte classes. We show that, for modest-to-highly conductive (>1 mS/cm) electrolytes, voltage polarization methods in Li|Li symmetric cells are hindered by large Li interfacial resistances,² thereby limiting the utility of the Bruce-Vincent method that relies on this measurement. We therefore implement electrophoretic nuclear magnetic resonance to direct quantify ion velocities through an applied electric field, allowing ion transference numbers to be measured without interfacial artefacts.³ Ultimately, in polyelectrolytes and solvent-in-salt systems, ion correlations result in limited t₊ well below those reported in the literature. Furthermore, for polyelectrolytes with roughly 10 repeat units and greater, we observe that the average velocity of Li ions in an electric field is in the direction opposite than expected, evidence of a negative t₊ due to correlated motion through Li ion condensation on the polyelectrolyte chain. While this result is a negative outcome in our pursuit of high t₊ electrolytes, we anticipate many of the capabilities and insights gleaned throughout our studies to be of general use to those studying novel, concentrated electrolyte solutions where strong, non-ideal ion-ion correlations are observed.

References

1. Evans, J.; Vincent, C. A.; Bruce, P. G. Electrochemical measurement of transference numbers in polymer electrolytes. Polymer 1987, 28, 2324-2328. http://dx.doi.org/10.1016/0032-3861(87)90394-6
2. Bergstrom, H. K.; Fong, K. D.; McCloskey, B. D. Interfacial effects on transport coefficient measurements in Li-ion battery electrolytes. Journal of the Electrochemical Society 2021, 168, 060543. 10.1149/1945-7111/ac0994
3. Bergstrom, H. K.; Fong, K. D.; Halat, D. M.; Karouta, C. A.; Celik, H. C.; Reimer, J. A.; McCloskey, B. D. Ion correlation and negative lithium transference in polyelectrolyte solutions. Chemical Science 2023, 14, 6546-6557. 10.1039/D3SC01224G