Molecular Dynamics Investigation of Transport Mechanisms in Low Temperature Lithium-Ion Battery Electrolytes

Development of lithium-ion battery (LIB) electrolytes with better low temperature properties will increase the viability of a vast array of technologies requiring energy storage, including electric vehicles and stationary grid storage for intermittent renewable power generation. In cold climates, battery performance is significantly limited, preventing widespread adoption of the technology. The choice of a liquid electrolyte in conventional LIBs dramatically alters bulk transport processes and battery performance. The current state-of-the-art electrolyte blend of 1 M LiPF₆ in 3:7 ethylene carbonate-ethylmethyl carbonate exhibits substantially diminished bulk ionic conductivity at low temperatures. However, the molecular origins of the observed behavior in low-temperature electrolytes remain poorly understood. Here we show using all-atom classical molecular dynamics (MD) simulations that low-temperature bulk transport in the industry standard electrolyte is limited most strongly by the reduction of charge carrier mobility due to an increase in solution viscosity. In contrast to the prevailing opinion of ion association and low cation transference numbers as the essential determinants of low temperature battery performance, the results of this study suggest that neither factor contributes as strongly as solution viscosity. Ion speciation studies from MD demonstrate that the number of free charge carriers in solution actually increases at low temperatures. Further, in direct contrast to the negative transference numbers reported for this system at low temperatures in the literature,¹ we observe that the cation transference number is positive and roughly independent of temperature. Our results offer insight about molecular structure and dynamics within the industry standard electrolyte and imply that low temperature properties may be improved by the reduction of solution viscosity, for example using low-viscosity co-solvents. We expect that these results will help inform the development of superior low temperature electrolytes and provide a novel conceptual framework for further exploration of low temperature LIB performance.

References

(1) Landesfeind, J.; Gasteiger, H. A. Temperature and Concentration Dependence of the Ionic Transport Properties of Lithium-Ion Battery Electrolytes. J. Electrochem. Soc. 2019, 166 (14), A3079–A3097. https://doi.org/10.1149/2.0571912jes.