Ionic Liquid Electrolytes for Lithium–Sulfur Batteries

A variety of binary mixtures of aprotic ionic liquids (ILs) and lithium salts were thoroughly studied as electrolytes for rechargeable lithium–sulfur (Li–S) batteries. The saturation solubility of sulfur and lithium polysulfides (Li₂S_<i>m</i>), the active materials in the Li–S battery, in the electrolytes was quantitatively determined, and the performance of the Li–S battery using the electrolytes was also investigated. Although the solubility of nonionic sulfur was low in all of the electrolytes evaluated, the solubility of Li₂S_<i>m</i> in the IL-based electrolyte was strongly dependent on the anionic structure, and the difference in the solubility could be rationalized in terms of the donor ability of the IL solvent. Dissolution of Li₂S_<i>m</i> in the ILs with strong donor ability was comparable to that achieved with typical organic electrolytes; the strongly donating IL electrolyte did not prevent redox shuttle reaction in the Li–S cells. The battery performance was also influenced by unfavorable side reactions of the anions (such as tetrafluoroborate (BF₄^–) and bis­(fluorosulfonylamide) ([FSA]⁻)) with polysulfides and by slow mass transport in viscous ILs, even though the dissolution of Li₂S_<i>m</i> into the IL electrolyte was greatly suppressed. Among the IL-based electrolytes, the low-viscosity [TFSA]-based ILs facilitated stable charge/discharge of the Li–S batteries with high capacity and high Coulombic efficiency. The unique <i>solvent effect</i> of the ILs can thus be exploited in the Li–S battery by judicious selection of ILs that exhibit high lithium-ion-transport ability and electrochemical stability in the presence of Li₂S_<i>m</i>

Effect of Variation in Anion Type and Glyme Length on the Nanostructure of the Solvate Ionic Liquid/Graphite Interface as a Function of Potential

Atomic force microscope (AFM) force curves are used to probe the effect of anion species and glyme length on the nanostructure of the solvate ionic liquid (SIL)/highly ordered pyrolytic graphite (HOPG) interface as a function of applied potential. At all potentials, the lithium tetraglyme bis­(trifluoromethylsulfonyl)­imide (Li­(G4)­TFSI)/HOPG is more structured than lithium tetraglyme bis­(perfluoroethylsulfonyl)­imide (Li­(G4)­BETI)/HOPG because [BETI]⁻ has greater conformational flexibility. The Li­(G3) trifluoroacetate (TFA)/HOPG interface is even more disordered because [TFA]⁻ coordinates strongly to the lithium ion, leading to a high concentration of free glyme. The Li­(G3)­TFSI/HOPG interface is more structured than the Li­(G4)­TFSI/HOPG interface because the longer glyme increases the molecular flexibility of the complex cation. The Li­(G1)₂TFSI/HOPG interface has weak interfacial structure because monoglyme is poorly coordinating so the free glyme concentration is high. Despite Li­(G3)­TFSI, Li­(G4)­TFSI, and Li­(G4)­BETI being good SILs (meaning the free glyme concentration is low), application of a negative potential to the HOPG surface leads to the desolvation of Li⁺ from the glyme at the surface

Solubility of Poly(methyl methacrylate) in Ionic Liquids in Relation to Solvent Parameters

The solubility of poly­(methyl methacrylate) (PMMA) in 1-alkyl-3-methylimidazolium ionic liquids (ILs) with different anionic structures has been explored. Nearly monodisperse PMMA-grafted silica nanoparticles (PMMA-<i>g</i>-NPs) were used as a measurement probe for evaluating the PMMA solubility in ILs. The hydrodynamic radius (<i>R</i>_h) of PMMA-<i>g</i>-NPs was measured in the ILs by dynamic light scattering (DLS). Changes in <i>R</i>_h and colloidal stability, that is, the PMMA-solubility change in the ILs, were observed depending on the ionic structures of the ILs. The solubility was mainly affected by the anionic structures of the ILs rather than by the alkyl chain length of the cationic structure. Solvent parameters, including Lewis basicity, solubility parameters, and a hydrophobicity parameter, were used to discuss the change in the PMMA solubility in ILs with different ionic structures. By considering the PMMA solubility in the ILs using these parameters, it was found that there is a good correlation between the PMMA solubility and the hydrophobicity parameter of the anions. Although the change in the PMMA solubility with different cationic structures was not remarkable, the hydrophobicity of the cations also played a role in the solvation of PMMA by providing a low-polarity environment adequate to dissolve PMMA

Enhancing Li–S Battery Performance with Limiting Li[N(SO₂F)₂] Content in a Sulfolane-Based Sparingly Solvating Electrolyte

By enhancing the stability of the lithium metal anode and mitigating the formation of lithium dendrites through electrolyte design, it becomes feasible to extend the lifespan of lithium–sulfur (Li–S) batteries. One widely accepted approach involves the utilization of Li[N(SO2F)2] (Li[FSA]), which holds promise in stabilizing the lithium anode by facilitating the formation of an inorganic-dominant solid electrolyte interface (SEI) film. However, the use of Li[FSA] encounters limitations due to inevitable side reactions between lithium polysulfides (LiPSs) and [FSA] anions. In this study, our focus lies in precisely controlling the composition of the SEI film and the morphology of the deposited lithium, as these two critical factors profoundly influence lithium reversibility. Specifically, by subjecting an initial charging process to an elevated temperature, we have achieved a significant enhancement in lithium reversibility. This improvement is accomplished through the employment of a LiPS sparingly solvating electrolyte with a restricted Li[FSA] content. Notably, these optimized conditions have resulted in an enhanced cycling performance in practical Li–S pouch cells. Our findings underscore the potential for improving the cycling performance of Li–S batteries, even when confronted with challenging constraints in electrolyte design

Glyme–Lithium Salt Equimolar Molten Mixtures: Concentrated Solutions or Solvate Ionic Liquids?

To demonstrate a new family of ionic liquids (ILs), i.e., “solvate” ionic liquids, the properties (thermal, transport, and electrochemical properties, Lewis basicity, and ionicity) of equimolar molten mixtures of glymes (triglyme (G3) and tetraglyme (G4)) and nine different lithium salts (LiX) were investigated. By exploring the anion-dependent properties and comparing them with the reported data on common aprotic ILs, two different classes of liquid regimes, i.e., ordinary concentrated solutions and “solvate” ILs, were found in the glyme–Li salt equimolar mixtures ([Li­(glyme)]­X) depending on the anionic structures. The class a given [Li­(glyme)]­X belonged to was governed by competitive interactions between the glymes and Li cations and between the counteranions (X) and Li cations. [Li­(glyme)]­X with weakly Lewis basic anions can form long-lived [Li­(glyme)]⁺ complex cations. Thus, they behaved as typical ionic liquids. The lithium “solvate” ILs based on [Li­(glyme)]­X have many desirable properties for lithium-conducting electrolytes, including high ionicity, a high lithium transference number, high Li cation concentration, and high oxidative stability, in addition to the common properties of ionic liquids. The concept of “solvate” ionic liquids can be utilized in an unlimited number of combinations of other metal salts and ligands, and will thus open a new field of research on ionic liquids

Optimization of Pore Structure of Cathodic Carbon Supports for Solvate Ionic Liquid Electrolytes Based Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries are a promising energy-storage technology owing to their high theoretical capacity and energy density. However, their practical application remains a challenge because of the serve shuttle effect caused by the dissolution of polysulfides in common organic electrolytes. Polysulfide-insoluble electrolytes, such as solvate ionic liquids (ILs), have recently emerged as alternative candidates and shown great potential in suppressing the shuttle effect and improving the cycle stability of Li–S batteries. Redox electrochemical reactions in polysulfide-insoluble electrolytes occur via a solid-state process at the interphase between the electrolyte and the composite cathode; therefore, creating an appropriate interface between sulfur and a carbon support is of great importance. Nevertheless, the porous carbon supports established for conventional organic electrolytes may not be suitable for polysulfide-insoluble electrolytes. In this work, we investigated the effect of the porous structure of carbon materials on the Li–S battery performance in polysulfide-insoluble electrolytes using solvate ILs as a model electrolyte. We determined that the pore volume (rather than the surface area) exerts a major influence on the discharge capacity of S composite cathodes. In particular, inverse opal carbons with three-dimensionally ordered interconnected macropores and a large pore volume deliver the highest discharge capacity. The battery performance in both polysulfide-soluble electrolytes and solvate ILs was used to study the effect of electrolytes. We propose a plausible mechanism to explain the different porous structure requirements in polysulfide-soluble and polysulfide-insoluble electrolytes

Long-Range Ion-Ordering in Salt-Concentrated Lithium-Ion Battery Electrolytes: A Combined High-Energy X‑ray Total Scattering and Molecular Dynamics Simulation Study

Herein, we report on a structural study for characterizing unique solution structures in the salt-concentrated electrolytes, which are promising new lithium (Li)-ion battery electrolytes. A combination of high-energy X-ray total scattering (HEXTS) experiments with all-atom molecular dynamics (MD) simulations was performed on the salt-concentrated electrolytes that were based on Li bis­(trifluoromethanesulfonyl)­amide (LiTFSA) and <i>N</i>,<i>N</i>-dimethylformamide (DMF). The radial distribution functions obtained from the HEXTS and MD approaches were in good agreement in the current LiTFSA/DMF solutions. We found that in the local structure: (1) the Li-ions were coordinated with both the DMF molecules and the TFSA anions in the concentrated solutions and (2) specific Li⁺···Li⁺ correlations were present in the radial distribution function over the <i>r</i> range of 3 Å–15 Å. The Li⁺···Li⁺ correlations originated from the extended multiple Li-ion complexes, that is, polymerized [Li⁺···TFSA^–···Li⁺]_<i>n</i> complexes so that they were highly structurally ordered. We concluded that this type of an ion-ordered structure plays a crucial role in the electrochemical stability and the ion-conducting mechanism, resulting in a unique LIB performance employing these salt-concentrated electrolytes

Glyme–Sodium Bis(fluorosulfonyl)amide Complex Electrolytes for Sodium Ion Batteries

The physicochemical and electrochemical properties of an equimolar complex of pentaglyme (G5) and sodium bis­(fluorosulfonyl)­amide (NaFSA), [Na­(G5)]­[FSA], mixed with a hydrofluoroether (HFE) were investigated. <i>Ab initio</i> calculations and Raman spectroscopy showed that the coordination structure of [Na­(G5)]­[FSA] was similar to that of [Na­(G5)]­[TFSA] (TFSA: bis­(trifluoromethanesulfonyl)­amide). The ligand G5 remained coordinated to Na⁺ and was not liberated from the cationic [Na­(G5)]⁺ complex even in the presence of HFE. The charge transport property was greater in [Na­(G5)]­[FSA]/HFE than in [Na­(G5)]­[TFSA]/HFE. A prominent difference was found in the Na metal deposition/dissolution behavior. Highly reversible Na deposition/dissolution with a Coulombic efficiency (95%) could be achieved in [Na­(G5)]­[FSA]/HFE; however, the reversibility in [Na­(G5)]­[TFSA]/HFE was very low. X-ray photoelectron spectroscopy (XPS) of the deposited Na metal in each electrolyte revealed that a thin and compact layer of electrolyte decomposition products was formed on the Na deposits in [Na­(G5)]­[FSA]/HFE. The FSA-derived thin layer can effectively inhibit the further decomposition of the electrolyte. By contrast, a thick electrolyte decomposition product found for [Na­(G5)]­[TFSA]/HFE suggested continuous decomposition of the electrolyte during Na deposition-dissolution. Highly stable charge and discharge of a hard carbon (HC) electrode was accomplished in [Na­(G5)]­[FSA]/HFE, with high Coulombic efficiency over 99.9% and negligible capacity decrease over 300 cycles. Electrochemical impedance measurements of a symmetrical cell for Na, HC, and Na_0.44MnO₂ electrodes with the above electrolytes verified that a stable electrode | electrolyte interface was formed on the HC and Na_0.44MnO₂ electrodes in [Na­(G5)]­[FSA]/HFE

Phase Diagrams and Solvate Structures of Binary Mixtures of Glymes and Na Salts

We prepared a series of binary mixtures composed of selected Na salts and glymes (tetraglyme, G4, and pentaglyme, G5) with different salt concentrations and anionic species ([X]⁻: [N­(SO₂CF₃)₂]⁻ = [TFSA]⁻, [N­(SO₂F)₂]⁻ = [FSA]^–, ClO₄^–, PF₆^–) and studied the effects of concentration, anionic structure, and glyme chain length on their phase diagrams and solvate structures. The phase diagrams clearly illustrate that all the mixtures form 1:1 complexes, [Na­(G4 or G5)₁]­[X]. The thermal stability of the equimolar mixtures was drastically improved in comparison with those of diluted systems, indicating that all the glyme molecules coordinate to Na⁺ cations to form equimolar complexes. Single-crystal X-ray crystallography revealed that [Na­(G5)₁]­[X] forms characteristic solvate structures in the crystalline state irrespective of the paired anion species. A comparison of the solvate structures of the glyme–Na complexes with those of the glyme–Li complexes suggests that the ionic radii of the coordinated alkali-metal cations have substantial effects on the resulting solvate structures. The Raman bands of the complex cations were assigned by quantum chemical calculations. Concentration dependencies of cationic and anionic Raman spectra show good agreement with the corresponding phase diagrams. In addition, the Raman spectra of the 1:1 complexes strongly suggest that the glymes coordinate to Na⁺ cation in the same way in both liquid and crystalline states. However, the aggregated structure in the crystalline state is broken by melting, which is accompanied by a change in the anion coordination

Phase Diagrams and Solvate Structures of Binary Mixtures of Glymes and Na Salts

We prepared a series of binary mixtures composed of selected Na salts and glymes (tetraglyme, G4, and pentaglyme, G5) with different salt concentrations and anionic species ([X]⁻: [N­(SO₂CF₃)₂]⁻ = [TFSA]⁻, [N­(SO₂F)₂]⁻ = [FSA]^–, ClO₄^–, PF₆^–) and studied the effects of concentration, anionic structure, and glyme chain length on their phase diagrams and solvate structures. The phase diagrams clearly illustrate that all the mixtures form 1:1 complexes, [Na­(G4 or G5)₁]­[X]. The thermal stability of the equimolar mixtures was drastically improved in comparison with those of diluted systems, indicating that all the glyme molecules coordinate to Na⁺ cations to form equimolar complexes. Single-crystal X-ray crystallography revealed that [Na­(G5)₁]­[X] forms characteristic solvate structures in the crystalline state irrespective of the paired anion species. A comparison of the solvate structures of the glyme–Na complexes with those of the glyme–Li complexes suggests that the ionic radii of the coordinated alkali-metal cations have substantial effects on the resulting solvate structures. The Raman bands of the complex cations were assigned by quantum chemical calculations. Concentration dependencies of cationic and anionic Raman spectra show good agreement with the corresponding phase diagrams. In addition, the Raman spectra of the 1:1 complexes strongly suggest that the glymes coordinate to Na⁺ cation in the same way in both liquid and crystalline states. However, the aggregated structure in the crystalline state is broken by melting, which is accompanied by a change in the anion coordination