Digging into Electrolyte Interactions—Challenging the Conventional Wisdom

The utilization of electrolytes for lithium batteries is linked with a significant body of conventional wisdom about electrolytes based largely upon assumptions—unfortunately, many of these assumptions are inaccurate. The misconstrual of the analysis of experimental data often leads to highly erroneous interpretations of electrolyte interactions. The resulting poor understanding of the molecular- and mesoscale-level solution structure of electrolytes is highly restrictive for the tailoring of new electrolyte formulations for advanced battery chemistries.

     This talk will discuss the utility (or lack thereof) of solvent polarization parameters (e.g., dielectric constant (ε), Gutmann donor number (DN), etc.) for determining the strength of solvation interactions with Li⁺ cations, the challenges associated with using Raman spectroscopy to obtain accurate Li⁺cation solvation numbers and determine the solvate species present in solution, how the strength of the ionic association interactions is uncorrelated with salt solubility and how Walden plots are poorly suited for characterizing the ionic association characteristics of electrolytes. The invaluable information which may be gleaned from the fusion of experimental and computational (Fig. 1) methods will be emphasized to provide a rigorous discourse on both the phase behavior and properties of dilute electrolytes, as well as highly concentrated (> 5M ) electrolytes.

Figure 1. Snapshot of a MD simulation of a (AN)_n-LiBF₄mixture (n = 10 or 1.7M): (top) original MD simulation box and (bottom) same MD simulation box with the uncoordinated acetonitrile (AN) molecules colored gray and the solvates grown across the periodic boundaries (Li-purple, B-tan, F-light green).

ACKNOWLEDGEMENT

The authors wish to express their gratitude to the U.S. Department of Energy (DOE) Office of Basic Energy Science-Division of Materials Sciences and Engineering and the Vehicle Technologies Program which supported the experimental research under awards DE-SC0002169 and DE-AC02-05-CH11231.