A Computational and Spectroscopic Analysis of Solvate Ionic Liquids Containing Anions with Long and Short Perfluorinated Alkyl Chains

Anion-driven, nanoscale polar-apolar structural organization is investigated in a solvate ionic liquid (SIL) setting by comparing sulfonate-based anions with long and short perfluorinated alkyl chains. Representative SILs are created from 1,2-bis(2-methoxyethoxy)ethane ("triglyme" or "G3"), lithium nonafluoro-1-butanesulfonate, and lithium trifluoromethanesulfonate. Molecular dynamics simulations, density functional theory computations, and vibrational spectroscopy provide insight into the overall liquid structure, cation-solvent interactions, and cation-anion association. Significant competition between G3 and anions for cation-binding sites characterizes the G3-LiC4F9SO3 mixtures. Only 50% of coordinating G3 molecules form tetradentate complexes with Li+ in [(G3)1Li][C4F9SO3]. Moreover, the SIL is characterized by extensive amounts of ion pairing. Based on these observations, [(G3)1Li][C4F9SO3] is classified as a "poor" SIL, similar to the analogous [(G3)1Li][CF3SO3] system. Even though the comparable basicity of the CF3SO3- and C4F9SO3- anions leads to similar SIL classifications, the hydrophobic fluorobutyl groups support extensive apolar domain formation. These apolar moieties permeate throughout [(G3)1Li][C4F9SO3] and persist even at relatively low dilution ratios of [(G3)10Li][C4F9SO3]. By way of comparison, the CF3 group is far too short to sustain polar-apolar segregation. This demonstrates how chemically modifying the anions to include hydrophobic groups can impart unique nanoscale organization to a SIL. Moreover, tuning these nano-segregated fluorinated domains could, in principle, control the presence of dimensionally ordered states in these mixtures without changing the coordination of the lithium ions.

Ether-Based Electrolyte Chemistry Towards High-Voltage and Long-Life Na-Ion Full Batteries

Although ether-based electrolytes have been extensively applied in anode evaluation of batteries, anodic instability arising from solvent oxidability is always a tremendous obstacle to matching with high-voltage cathodes. Herein, by rational design for solvation configuration, the fully coordinated ether-based electrolyte with strong resistance against oxidation is reported, which remains anodically stable with high-voltage Na₃ V₂ (PO₄ )₂ O₂ F (NVPF) cathode under 4.5 V (versus Na⁺ /Na) protected by an effective interphase. The assembled graphite//NVPF full cells display superior rate performance and unprecedented cycling stability. Beyond that, the constructed full cells coupling the high-voltage NVPF cathode with hard carbon anode exhibit outstanding electrochemical performances in terms of high average output voltage up to 3.72 V, long-term cycle life (such as 95 % capacity retention after 700 cycles) and high energy density (247 Wh kg^-1 ). In short, the optimized ether-based electrolyte enriches systematic options, the ability to maintain oxidative stability and compatibility with various anodes, exhibiting attractive prospects for application.

Solvate electrolytes for Li and Na batteries: structures, transport properties, and electrochemistry

Polar solvents dissolve Li and Na salts at high concentrations and are used as electrolyte solutions for batteries. The solvents interact strongly with the alkali metal cations to form complexes in the solution. The activity (concentration) of the uncoordinated solvent decreases as the salt concentration is increased. At extremely high salt concentrations, all the solvent molecules are involved in the coordination of the ions and form the solvates of the salts. In this article, we review the structures, transport properties, and electrochemistry of Li/Na salt solvates. In molten solvates, the activity of the uncoordinated solvent is negligible; this is the main origin of their peculiar characteristics, such as high thermal stability, wide electrochemical window, and unique ion transport. In addition, the solvent activity greatly influences the electrochemical reactions in Li/Na batteries. We highlight the attractive features of molten solvates as promising electrolytes for next-generation batteries.

Determining Factor on the Polarization Behavior of Magnesium Deposition for Magnesium Battery Anode

To clarify the origin of the polarization of magnesium deposition/dissolution reactions, we combined electrochemical measurement, operando soft X-ray absorption spectroscopy (operando SXAS), Raman, and density functional theory (DFT) techniques to three different electrolytes: magnesium bis(trifluoromethanesulfonyl)amide (Mg(TFSA)₂)/triglyme, magnesium borohydride (Mg(BH₄)₂)/tetrahydrofuran (THF), and Mg(TFSA)₂/2-methyltetrahydrofuran (2-MeTHF). Cyclic voltammetry revealed that magnesium deposition/dissolution reactions occur in Mg(TFSA)₂/triglyme and Mg(BH₄)₂/THF, while the reactions do not occur in Mg(TFSA)₂/2-MeTHF. Raman spectroscopy shows that the [TFSA]^- in the Mg(TFSA)₂/triglyme electrolyte largely does not coordinate to the magnesium ions, while all of the [TFSA]^- in Mg(TFSA)₂/2-MeTHF and [BH₄]^- in Mg(BH₄)₂/THF coordinate to the magnesium ions. In operando SXAS measurements, the intermediate, such as the Mg⁺ ion, was not observed at potentials above the magnesium deposition potential, and the local structure distortion around the magnesium ions increases in all of the electrolytes at the magnesium electrode|electrolyte interface during the cathodic polarization. Our DFT calculation and X-ray photoelectron spectroscopy results indicate that the [TFSA]^-, strongly bound to the magnesium ion in the Mg(TFSA)₂/2-MeTHF electrolyte, undergoes reduction decomposition easily, instead of deposition of magnesium metal, which makes the electrolyte inactive electrochemically. In the Mg(BH₄)₂/THF electrolyte, because the [BH₄]^- coordinated to the magnesium ions is stable even under the potential of the magnesium deposition, the magnesium deposition is not inhibited by the decomposition of [BH₄]^-. Conversely, because [TFSA]^- is weakly bound to the magnesium ion in Mg(TFSA)₂/triglyme, the reduction decomposition occurs relatively slowly, which allows the magnesium deposition in the electrolyte.

Solvate Ionic Liquids for Li, Na, K, and Mg Batteries

From the viewpoint of element strategy, non-Li batteries with promising negative and positive electrodes have been widely studied to support a sustainable society. To develop non-Li batteries having high energy density, research on electrolyte materials is pivotal. Solvate ionic liquids (SILs) are an emerging class of electrolytes possessing somewhat superior properties for battery applications compared to conventional ionic liquid electrolytes. In this account, we describe our recent efforts regarding SIL-based electrolytes for Li, Na, K, and Mg batteries with respect to structural, physicochemical, and electrochemical characteristics. Systematic studies based on crystallography and Raman spectroscopy combined with thermal/electrochemical stability analysis showed that the balance of competitive cation-anion and cation-solvent interactions predominates the stability of the solvate cations. We also demonstrated battery applications of SILs as electrolytes for non-Li batteries, particularly for Na batteries.

Glyme–Li salt equimolar molten solvates with iodide/triiodide redox anions

Room-temperature-fused Li salt solvates that exhibit ionic liquid-like behaviour can be formed using particular combinations of multidentate glymes and lithium salts bearing weakly coordinating anions, and are now deemed a subset of ionic liquids, viz. solvate ionic liquids (SILs). Herein, we report redox-active glyme–Li salt molten solvates consisting of tetraethyleneglycol ethylmethyl ether (G4Et) and lithium iodide/triiodide, [Li(G4Et)]I and [Li(G4Et)]I₃. The coordination structure of the complex ions and the thermal, transport, and electrochemical properties of these molten Li salt solvates were investigated to diagnose whether they can be categorized as SILs. [Li(G4Et)]⁺ and I₃⁻ were found to remain stable as discrete ions and exist as well-dissociated forms in the liquid state, indicating that [Li(G4Et)]I₃ can be classified as a good SIL. This study also clarified that the I⁻ and I₃⁻ counter anions exhibit an electrochemical redox reaction in the highly concentrated molten Li salt solvates. The redox-active molten Li solvates were further studied as a highly concentrated catholyte for use in rechargeable semi-liquid lithium batteries. Although the cell constructed using [Li(G4Et)]I₃ failed to charge after the initial discharge step, the cell containing [Li(G4Et)]I demonstrates reversible charge–discharge behaviour with a high volumetric energy density of 180 W h L⁻¹ based on the catholyte volume.

Redox-active glyme–Li salt equimolar molten solvates based on a I⁻/I₃⁻ couple could be employed as a highly concentrated catholyte for semi-liquid rechargeable lithium batteries.

Liquid Structures and Transport Properties of Lithium Bis(fluorosulfonyl)amide/Glyme Solvate Ionic Liquids for Lithium Batteries

The liquid structures and transport properties of electrolytes composed of lithium bis(fluorosulfonyl)amide (Li[FSA]) and glyme (triglyme (G3) or tetraglyme (G4)) were investigated. Raman spectroscopy indicated that the 1:1 mixtures of Li[FSA] and glyme (G3 or G4) are solvate ionic liquids (SILs) comprising a cationic [Li(glyme)]+ complex and the [FSA]− anion. In Li[FSA]-excess liquids with Li[FSA]/glyme molar ratios greater than 1, anionic Lix[FSA]y(y–x)– complexes were formed in addition to the cationic [Li(glyme)]+ complex. Pulsed field gradient NMR measurements revealed that the self-diffusion coefficients of Li+ (DLi) and glyme (Dglyme) are identical in the Li[FSA]/glyme=1 liquid, suggesting that Li+ and glyme diffuse together and that a long-lived cationic [Li(glyme)]+ complex is formed in the SIL. The ratio of the self-diffusion coefficients of [FSA]− and Li+, DFSA/DLi, was essentially constant at ~1.1–1.3 in the Li[FSA]/glyme<1 liquid. However, DFSA/DLi increased rapidly as the amount of Li[FSA] increased in the Li[FSA]/glyme>1 liquid, indicating that the ion transport mechanism in the electrolyte changed at the composition of Li[FSA]/glyme=1. The oxidative stability of the electrolytes was enhanced as the Li[FSA] concentration increased. Furthermore, Al corrosion was suppressed in the electrolytes for which Li[FSA]/glyme>1. A battery consisting of a Li metal anode, a LiNi1/3Mn1/3Co1/3O2 cathode, and Li[FSA]/G3=2 electrolyte exhibited a discharge capacity of 105mAhg−1 at a current density of 1.3mAcm−2, regardless of its low ionic conductivity of 0.2mScm−1.

Modifications in coordination structure of Mg[TFSA]2-based supporting salts for high-voltage magnesium rechargeable batteries

Systematic structural and electrochemical studies on the Mg[TFSA]₂-based electrolytes revealed that the coordination state of [TFSA]⁻ predominates the electrochemical magnesium deposition/dissolution activity.

Stabilization of SF5- with Glyme-Coordinated Alkali Metal Cations

The stabilization of complex fluoroanions derived from weakly acidic parent fluorides is a significant and ongoing challenge. The [SF₅]^- anion is recognized as one such case, and only a limited number of [SF₅]^- salts are known to be stable at room temperature. In the present study, glyme-coordinated alkali metal cations (K⁺, Rb⁺, and Cs⁺) are employed to stabilize [SF₅]^-, which provides a simple synthetic route to a [SF₅]^- salt. The reactivities of KF and RbF with SF₄ are significantly enhanced by complexation with G4, based on Raman spectroscopic analyses. A new room-temperature stable salt, [Cs(G4)₂][SF₅] (G4 = tetraglyme), was synthesized by stoichiometric reaction of CsF, G4, and SF₄. The vibrational frequencies of [SF₅]^- were assigned based on quantum chemical calculations, and the shift of the G4 breathing mode accompanying coordination to metal cations was confirmed by Raman spectroscopy. Single-crystal X-ray diffraction revealed that Cs⁺ is completely isolated from [SF₅]^- by two G4 ligands and [SF₅]^- is disordered along the crystallographic two-fold axis. Hirshfeld surface analysis reveals that the H···H interaction between two neighboring [Cs(G4)₂]⁺ moieties is more dominant on the Hirshfeld surface than the interaction between the H atom in glyme molecules and the F atom in [SF₅]^-, providing a CsCl-type structural model where the large and spherical [Cs(G4)₂]⁺ cations contact each other and the [SF₅]^- anions occupy interstitial spaces in the crystal lattice. The [SF₅]^- anion, combined with [Cs(G4)₂]⁺, exhibits a very limited deoxofluorinating ability toward hydroxyl groups in both neat conditions and THF solutions.

Binary Mixtures of Highly Concentrated Tetraglyme and Hydrofluoroether as a Stable and Nonflammable Electrolyte for Li-O2 Batteries

Developing a long-term stable electrolyte is one of the most enormous challenges for Li-O₂ batteries. Equally, the high flammability of frequently used solvents seriously weakens the electrolyte safety in Li-O₂ batteries, which inevitably restricts their commercial applications. Here, a binary mixture of highly concentrated tetraglyme electrolyte (HCG4) and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) was used for a novel electrolyte (HCG4/TTE) in Li-O₂ batteries, which exhibit good wettability, enhanced ionic conductivity, considerable nonflammability, and high electrochemical stability. Being a co-solvent, TTE can contribute to increasing ionic conductivity and to improving flame retardance of the as-prepared electrolyte. The cell with this novel electrolyte displays an enhanced cycling stability, resulting from the high electrochemical stability during cycling and the formation of electrochemically stable interfaces prevents parasitic reactions occurring on the Li anode. These results presented here demonstrate a novel electrolyte with a high electrochemical stability and considerable safety for Li-O₂ batteries.

Creating Lithium-Ion Electrolytes with Biomimetic Ionic Channels in Metal-Organic Frameworks

Solid-state electrolytes are the key to the development of lithium-based batteries with dramatically improved energy density and safety. Inspired by ionic channels in biological systems, a novel class of pseudo solid-state electrolytes with biomimetic ionic channels is reported herein. This is achieved by complexing the anions of an electrolyte to the open metal sites of metal-organic frameworks (MOFs), which transforms the MOF scaffolds into ionic-channel analogs with lithium-ion conduction and low activation energy. This work suggests the emergence of a new class of pseudo solid-state lithium-ion conducting electrolytes.

Pentaglyme–K salt binary mixtures: phase behavior, solvate structures, and physicochemical properties

Cationic Lewis acidity, anionic Lewis basicity, and their mutual interactions have a significant impact on the ionic nature of equimolar complexes composed of glymes and different alkali–metal salts.

Li+ solvation in glyme–Li salt solvate ionic liquids

Raman spectra and electrode potentials corroborated that glyme–Li salt solvate ionic liquids consist of crown-ether like complex cations and counter anions with a few uncoordinated glyme molecules in the liquid state.

Lithium solvation in bis(trifluoromethanesulfonyl)imide-based ionic liquids

The lithium solvation in (1 -x)(EMI-TFSI), xLiTFSI ionic liquids where EMI(+) is the 1-ethyl-3-methylimidazolium cation and TFSI(-) the bis(trifluoromethanesulfonyl)imide anion, is shown by Raman spectroscopy to involve essentially [Li(TFSI)(2)](-) anionic clusters for 0 < x < 0.4, but addition of stoichiometric amounts of solvents S such as oligoethers changes the lithium solvation into [Li(S)(m)](+) cationic clusters; the lithium transference number in TFSI-based ionic liquid electrolytes for lithium batteries should thus be strongly improved.