Predicting Properties for Highly Concentrated Electrolytes: Transport Metrics, Liquid Permeation, Lithium Desolvation Energies and Other Quantities of Interest for Li-Ion Cells
Authors: Gering Kevin L. 
Published: December 2020

Ever-increasing performance requirements of Li-ion cell creates an abiding need to know more about electrolyte property behavior at high salt concentrations. 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 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 highly concentrated electrolytes (HCEs) having 3-5 Molar salt instead of conventional systems (1-1.3 Molar). 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. AEM is accurate for HCE conditions.

General HCE Attributes:

- Decrease of solvation number per lithium,

- Corresponding drop in lithium desolvation energy requirements,

- Higher salt content helps moderate concentration polarization,

- Surface tension is greater at higher salt content, which can help mitigate Li metal deposition,

- Voltage stability is also improved through low solvent activity (nearly all solvent is coordinated with ions),

- Ion hopping is more likely as a transport mechanism (very efficient).

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. Lithium desolvation burden is shown to decrease at higher salt concentrations due to fewer solvators per cation. Behavior of battery electrolyte surface tension at higher salt concentration 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.

Examples of electrolyte conductivity at higher salt concentrations are seen in Fig. 1 (a-d), wherein comparisons are made between measured and modeled values. Good agreement is seen between lab data and model predictions over the entire data range, and in two cases (Figs. 1 a and b) the AEM predicts the possible onset of solid phase formation based on molecular packing and density considerations. The methodology for conductivity predictions has been published (Gering, 2017), demonstrating that several factors influence the observed conductivity profiles. Figure 2 shows activation energies for data predicted for the product of {conductivity*t+} for electrolytes DMC+LiFSI and EC-DMC (1:1, molar)+LiFSI. It is seen that activation energies can be both a function of temperature and salt concentration. However, the DMC+LiFSI system shows lesser Ea variance due to competing trends caused by the low-permittivity state of this electrolyte at lower salt concentrations. Other results will be shown for surface tension that demonstrate liquid permeation rates of HCE as a function of pore attributes (diameter, length, roughness) and aggregate behavior under an assemblage of pores. Lithium desolvation behavior over HCE conditions will also be discussed, as this impacts the effective “cost of passage” for lithium as it goes from the bulk electrolyte into the host material. New HCE systems have been derived through AEM that show feasibility for use in Li-ion cells, e.g., fast-charge (FC) conditions. Reduction of the lithium desolvation energy in HCEs allows a competitive advantage in some Li-ion cell operating conditions provided the transport rate of lithium through the electrolyte is not the rate limiting step.

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Figure 1