Low-Temperature Characterization of a Nonaqueous Liquid Electrolyte for Lithium Batteries

Abstract

Rechargeable batteries exhibit poor performance at low temperatures due to sluggish ion transport through the electrolytic phase. Ion transport is governed by three transport parameters, conductivity, diffusion coefficient, and the cation transference number with respect to the solvent velocity, and the thermodynamic factor. Understanding how these parameters change with temperature is necessary for designing improved electrolytes. In this work, we combine electrochemical techniques with electrophoretic nuclear magnetic resonance to determine the temperature dependence of these parameters for a liquid electrolyte between -20 and 45°C. At colder temperatures, all species in the electrolyte tend to move slower due to increasing viscosity, which translates to a monotonic decrease in conductivity and diffusion coefficient with decreasing temperature. Surprisingly, we find that the field-induced solvent velocity at a particular salt concentration is a non-monotonic function of temperature. The cation transference number with respect to the solvent velocity thus exhibits a complex dependence on temperature and salt concentration. The measured properties are used to predict concentration gradients that will form in a lithium-lithium symmetric cell under a constant applied potential at multiple temperatures using concentrated solution theory. The calculated steady current at -20°C is lower than that at 45°C by roughly two orders of magnitude.