(Invited) Modeling of the Bulk and Electrical Double-Layer Structures of Localized High-Concentration Electrolytes

A new class of electrolytes, localized high-concentration electrolytes (LHCE), have shown many benefits to high-capacity electrodes (lithium (Li)-metal, silicon, sodium, zinc, and potassium).[1] These electrolytes combine high-concentration electrolytes (HCE) with salt and solvent with low-viscosity diluents. The dilutes were added to increase ionic conductivity while the locally highly concentrated salt-solvent clusters will facilitate stable solid-electrolyte interphase (SEI) formation while preventing metallic-dendrite formation. For example, it was demonstrated that an LHCE based on lithium bis(fluorosulfonyl)imide (LiFSI) salt, Dimethoxyethane (DME) solvent, and tris(2,2,2-trifluoroethyl)orthoformate (TFEO) diluent (e.g., LiFSI-1.2DME-2TFEO by mol) forms inorganic-rich, monolithic SEI and shows excellent cycling performance for Li-metal batteries under practical conditions.[2] Here, the interactions among cations, anions, solvent molecules, and diluent molecules create a complex design space, making the trial-and-error empirical approach less efficient. Using density functional theory and classical molecular dynamics simulations, we reveal several important design principles: a) The dielectric constant of the pure solvent or diluent is not a good descriptor to describe the solubility of the salt in solvent and diluent. b) The single ion–solvent (diluent) binding energy is insufficient to predict the solubility, which also depends on the molecular structures and the corresponding solvation shell structure. c) The salt-solvent, salt-diluent, and diluent-solvent solubilities jointly determine the solvation structures in LHCE. For example, at certain concentrations, the Li + solvation shells contain both anions and solvent molecules with minimal participation of the diluent molecules. The local salt concentration can be even higher than its counterpart in the HCE. These simulated solvation structures in bulk electrolytes compare well with Raman spectrum and SAXS data. d) Moreover, the solvation structures are temperature-sensitive. We revealed why the local salt concentration is higher at a certain temperature by MD simulations, which is further validated through Raman deconvolution analyses. e) The SEI formation is sensitive to the electrolyte structure near a charged surface. Therefore, the electrical double-layer (EDL) structure of LHCE needs to be resolved. Here we give a statistical representation of the LHCE in the EDL and show how it differs from the bulk structure and its impact on SEI formation. Reference: [1] J. Electrochem. Soc., 2021, 168, 010522 [2] Nat. Energy 2019, 4, 796-805