Electrolyte Properties at High Concentrations: Conductivity, Viscosity, Diffusivity, Surface Tension and Other Quantities of Interest for Li-Ion Cells
Authors: Gering Kevin L. 
Published: May 2020

There is an abiding need to know more about electrolyte property behavior at high salt concentrations. This is driven by the consequences of concentration polarization (CP) in battery cells wherein salt enrichment occurs near one of the electrodes, and by the advantages that are foreseen in moving to electrolytes that are significantly more concentrated (3-5 Molar salt) than conventional systems (1-1.3 Molar). Since most published data for non-aqueous battery electrolytes falls within the 0-2 Molar range, there is a profound need for data that will explore the range of 2-5 Molar conditions (or greater in some cases). This work was achieved through computational modeling at INL based on the Advanced Electrolyte Model (AEM), a chemical physics-based tool that covers molecular to macroscale properties.

Property predictions up to 3-5 Molar help reveal how local transport behavior is affected under polarization conditions. Well-designed electrolytes with lower viscosity can greatly increase conductivity at high salt concentrations, thereby reducing CP effects. Lithium desolvation burden is shown to decrease at higher salt concentrations due to fewer solvators per cation. A platform for assessing rheological performance of electrolytes will also be discussed. Behavior of battery electrolyte surface tension at higher salt concentrations becomes non-linear, an artifact not well explored in published literature. This has meaningful consequences for design of wetting and formation protocols as well as understanding dynamic cell behavior under cycling conditions.

In designing new electrolytes we seek systems that will alleviate the consequences of CP and reduce the lithium desolvation burden in terms of energy and kinetics. For example, lowering viscosity has a central influence on increasing conductivity and ionic diffusivity, and expanding their acceptable operating range to higher salt concentrations. This will lead to lower voltage drops (and related impedance) as related to CP and expand the time window for power delivery. Since CP is more pronounced at lower temperatures, such redesigned electrolytes can improve power performance therein.

Examples of electrolyte conductivity at higher salt concentrations are seen in Fig. 1 (a,b,c), wherein comparisons are made between measured and modeled values. Good agreement is seen between lab data and model predictions out to at least 4 Molar, which in some cases would equate to 5-6 molal. The methodology for conductivity predictions has been published (Gering, 2017), demonstrating that several factors influence the observed conductivity profiles. Figure 2 shows results for prediction of surface tension out to concentrations that are typically outside the boundaries of published data. Surface tension impacts the liquid permeation rate into electrode porous structures and directly affects capillary pressure. Other results will be shown that demonstrate liquid permeation rates of such electrolytes as a function of pore atributes (diameter, length, roughness) and aggregate behavior under an assemblage of pores. Another aspect of concentrated electrolytes that will be examined is the lithium desolvation behavior over salt concentration and temperature, as this impacts the effective “cost of passage” for lithium as it goes from the bulk electrolyte into the host material.

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Figure 1