Abstract
Lithium-ion batteries face low temperature performance issues, limiting the adoption of technologies ranging from electric vehicles to stationary grid storage. This problem is thought to be exacerbated by slow transport within the electrolyte, which in turn may be influenced by ion association, solvent viscosity, and cation transference number. How these factors collectively impact low temperature transport phenomena, however, remains poorly understood. Here we show using all-atom classical molecular dynamics (MD) simulations that the dominant factor influencing low temperature transport in LP57 (1 M LiPF6 in 3:7 ethylene carbonate (EC)/ethyl methyl carbonate (EMC)) is solvent viscosity, rather than ion aggregation or cation transference number. We find that ion association decreases with decreasing temperature, while the cation transference number is positive and roughly independent of temperature. In an effort to improve low temperature performance, we introduce γ-butyrolactone (GBL) as a low viscosity co-solvent to explore two alternative formulations: 1 M LiPF6 in 15:15:70 EC/GBL/EMC and 3:7 GBL/EMC. While GBL reduces solution viscosity, its low dielectric constant results in increased ion pairing, yielding neither improved bulk ionic conductivity nor appreciably altered ion transport mechanisms. We expect that these results will enhance understanding of low temperature transport and inform the development of superior electrolytes.
Highlights
Ion pairing may decrease in response to changes in the solvent dielectric constant, which is expected to increase at low temperature due to reduced thermal disruption of solvent dipole alignment.[11,62,63]
We conclude that despite contradictory evidence presented in the literature, solvent viscosity, or by proxy solvent selfdiffusivity, exerts a stronger effect on low temperature ionic conductivity than ion association
We found that the cation transference number, in contrast to the negative transference numbers previously reported for the given system, remains positive and roughly constant from −20 °C to 25 °C
Summary
Soc. 168 080501 View the article online for updates and enhancements. Despite the importance of slow low temperature electrolyte transport, 4,11–13 fundamental understanding of the behavior is limited. Previous studies of ion transport as a function of temperature have primarily relied upon experimental characterizations of ionic conductivity and electrochemical performance,[3,11,14,15,16,17] and less commonly, cation transference number[12] and ion association.[18,19] Previous experimental work provides a useful framework for understanding the issue, but in general sheds little light on the molecular origins of low temperature transport behavior.
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