Correlating Structure and Properties of Super‐Concentrated Electrolyte Solutions: 17 O NMR and Electrochemical Characterization

Lithium-Metal Batteries Using Sustainable Electrolyte Media and Various Cathode Chemistries

Lithium-metal batteries employing concentrated glyme-based electrolytes and two different cathode chemistries are herein evaluated in view of a safe use of the highly energetic alkali-metal anode. Indeed, diethylene-glycol dimethyl-ether (DEGDME) and triethylene-glycol dimethyl-ether (TREGDME) dissolving lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium nitrate (LiNO₃) in concentration approaching the solvents saturation limit are used in lithium batteries employing either a conversion sulfur-tin composite (S:Sn 80:20 w/w) or a Li⁺ (de)insertion LiFePO₄ cathode. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) clearly show the suitability of the concentrated electrolytes in terms of process reversibility and low interphase resistance, particularly upon a favorable activation. Galvanostatic measurements performed on lithium-sulfur (Li/S) batteries reveal promising capacities at room temperature (25 °C) and a value as high as 1300 mAh g_S ^-1 for the cell exploiting the DEGDME-based electrolyte at 35 °C. On the other hand, the lithium-LiFePO₄ (Li/LFP) cells exhibit satisfactory cycling behavior, in particular when employing an additional reduction step at low voltage cutoff (i.e., 1.2 V) during the first discharge to consolidate the solid electrolyte interphase (SEI). This procedure allows a Coulombic efficiency near 100%, a capacity approaching 160 mAh g^-1, and relevant retention particularly for the cell using the TREGDME-based electrolyte. Therefore, this work suggests the use of concentrated glyme-based electrolytes, the fine-tuning of the operative conditions, and the careful selection of active materials chemistry as significant steps to achieve practical and safe lithium-metal batteries.

Hopping in High Concentration Electrolytes - Long Time Bulk and Single-Particle Signatures, Free Energy Barriers, and Structural Insights

Although ion-hopping is believed to be a significant mode of transport for small ions in liquid high concentration electrolytes (HCE), its bulk signatures over sufficiently long time intervals are yet to be shown. We computationally establish the long and short time imprints of hopping in HCEs using LiBF₄-in-sulfolane mixtures as models. The high viscosity of this electrolyte leads to significant dynamic heterogeneity in Li-ion transport. Li-ions exhibit a preference to transit to previously occupied Li-ion-sites, bridged through anion or solvent molecules. Hopping in the liquid matrix was found to be an activated process, whose free energy barrier and transition state structure have been determined. Evidence for nanoscale compositional heterogeneity at high salt concentrations is also presented. The simulations shed light on the composition, stiffness, and lifetime of the solvation shell of Li ions. The understanding of HCEs gleaned from this study will spearhead the choice, engineering and applicability of this class of electrolytes.