Asymmetric Anions Research Articles

The Interactions between Ionic Liquids and Lithium Polysulfides in Lithium-Sulfur Batteries: A Systematic Density Functional Theory Study.

Ionic liquids (ILs) based on hybrid anions have recently garnered attention as beguiling alternative electrolytes for energy storage devices. This attention stems from the potential of these asymmetric anions to reduce the melting point of ILs and impede the crystallization of ILs. Furthermore, they uphold the advantages associated with their more conventional symmetric counterparts. In this study, we employed dispersion-corrected density functional theory (DFT-D) calculations to scrutinize the interplay between two hybrid anions found in ionic liquids [FTFSA]- and [MCTFSA]- and the [C4mpyr]+ cation, as well as in lithium polysulfides in lithium-sulfur batteries. For comparison, we also examined the corresponding ILs containing symmetric anions, [TFSA]- and [FSA]-. We found that the hybrid anion [MCTFSA]- and its ionic liquid exhibited exceptional stability and interaction strength. Additionally, our investigation unveiled a remarkably consistent interaction between ionic liquids (ILs) and anions with lithium polysulfides (and S8) during the transition from octathiocane (S8) to the liquid long-chain Li2Sn (4 ≤ n ≤ 8). This contrasts with the gradual alignment observed between cations and lithium polysulfides during the intermediate state from Li2S4 to the solid short-chain Li2S2 and Li2S1. We thoroughly analyzed the interaction mechanism of ionic liquids composed of different symmetry anions and their interactions with lithium polysulfides.

Influence of anion structure on Li+ solvation and diffusion in ionic liquid-based electrolyte, and decomposition mechanism of anion

Ionic liquids (ILs) are promising electrolytes for lithium batteries. Particularly, the structure of anions plays a vital role in electrochemical performances. In this work, we chose two asymmetric anions ((fluorosulfonyl)(perfluoroethylsulfonyl)imide (FETI-) and 2,2,2-trifluoro-N-(trifluoromethylsulfonyl)acetamide (TSAC-)) and compared with the symmetric one (bis(trifluoromethanesulfonyl)imide (TFSI-)) to investigate the microcosmic mechanism of the effect of structure on Li+ diffusion through classical molecular dynamics (MD) simulations. We investigated the Li+ solvation structure in electrolytes and found that the asymmetry of anions facilitates the formation of monodentate configuration, but they are not effective in accelerating the Li+ diffusion. On the other hand, we found that the faster Li+ diffusion comes from both the higher diffusion coefficient of anions and the smaller size of Li+ aggregates in electrolytes. In addition, the decomposition mechanism of these anions was investigated by ab initio molecular dynamics (AIMD) simulations. The F directly attached to sulfonyl is unstable, leading to the earlier breaking of the S-F bond compared to S-O and S-N bond, forming early protective components of LiF in solid electrolyte interphase (SEI). This work provides guidance for the design of anions, revealing that the influence of anion structure on Li+ solvation and diffusion in IL-based electrolytes, and decomposition mechanism of anions.

The influence of counterion structure identity on conductivity, dynamical correlations, and ion transport mechanisms in polymerized ionic liquids.

We used equilibrium and non-equilibrium atomistic simulations to probe the influence of anion chemistry on the true conductivity, dynamical correlations, and ion transport mechanisms in polymeric ionic liquids. An inverse correlation was found between anion self-diffusivities, ionic mobilities, and the anion size for spherical anions. While some larger asymmetric anions had higher diffusivities than smaller spherical anions, their diffusivities and mobilities did not exhibit a direct correlation to the anion volumes. The conductivity and anion dynamical correlations also followed the same trends as displayed by the diffusivity and mobility of anions. All the systems we examined displayed positively correlated motion among anions, suggesting a contribution that enhances the conductivity beyond the ideal Nernst-Einstein value. Analysis of ion transport mechanisms demonstrated very similar hopping characteristics among the spherical anions despite differences in their sizes.

Ionic liquid‐based materials for electrochemical biosensing

AbstractDue to their interesting features including negligible volatility, ease of designing in the construction, good chemical, and thermal stability, and excellent ionic conductivity, ionic liquids (ILs) have attracted a lot of attention to themselves in recent years. They are constructed from asymmetric anions and cations and are found in a liquid state lower than 100°C. Their unique features introduce them as candidates for the application in the structure of different types of electrochemical biosensors, in which they could act as electrolytes, or could be used as detecting agents. According to these features, this review aims to present an overview of the application of ILs in the structure of electrochemical sensors and biosensors. Based on this, after a brief description of the ILs and biosensors, the application of ILs in the structure of some of the recent electrochemical biosensors has been discussed and then, remarks and perspectives on these types of biosensors are explained.

Interfacial structures and decomposition reactions of hybrid anion-based ionic liquids at lithium metal surface from first-principles and ab initio molecular dynamics

Ionic liquids (ILs) based on hybrid anions have recently been investigated as appealing alternative electrolytes for energy storage devices, because such asymmetric anions have the potential to reduce melting point and inhibit crystallization of ILs without losing advantages found for the common symmetric ones. In this work, interfacial structures and decomposition reactions of the ILs consisting of two hybrid anions (FTFSI− and MCTFSI−) with the [BMP] cation at lithium metal surface were intensively studied using the combination of first-principles DFT calculations and ab initio molecular dynamics (AIMD) simulations. The corresponding ILs comprising the symmetric anions (TFSI− and FSI−) were also considered for comparison. The adsorption of the ILs on the surface is governed by the interactions between the anions and the substrate mainly via the Li−O bonds. For [BMP][FTFSI] rapid decomposition of the anion triggered by the S−F and S−N bond cleavage was observed, while the first reaction step for [BMP][MCTFSI] involves the breaking of the C−S and S−N bonds of the anion. Particularly, the differences of adsorption properties and reaction mechanisms for these two classes of ILs on the Li surface were demonstrated in detail.

Molecular Insights into the Effect of Asymmetric Anions on Lithium Coordination and Transport Properties in Salt-Doped Poly(ionic liquid) Electrolytes

Molecular Insights into the Effect of Asymmetric Anions on Lithium Coordination and Transport Properties in Salt-Doped Poly(ionic liquid) Electrolytes

Non-Flammable Sodium Asymmetric Imide Salt-Based Deep Eutectic Solvent for Supercapacitor Applications.

This study reports two deep eutectic solvents (DESs) based on alkaline imide salts with asymmetric anions as functional electrolytes for supercapacitor (SC) application. The eutectic mixture of sodium (fluorosulfonyl) (trifluoromethanesulfonyl) imide (NaFTFSI) or sodium cyano-trifluoromethanesulfonyl imide (NaTFSICN) with ethylene carbonate (EC) delivers a non-flammable and stable liquid. The eutectic diagrams of the electrolytes directed to an optimal composition (wsalt =0.25), hinging to that of conventional carbonate-based electrolytes, i. e., 1 mol L-1 . The volumetric properties of the DESs revealed a "stacking" effect, reflecting a strong coordination bond between the imide and EC anions without solvating the Na+ cations. The DES transport properties (i. e., viscosity, conductivity, and ionicity) and temperature variations designate a high organization, similar to ionic liquids. The DESs, when coupled with activated carbon electrodes in a two-electrode symmetric configuration, yield specific capacities of 150 F g-1 at a normalized current density of 0.5 A g-1 (and 120 F g-1 at 2 A g-1 ). The SC maintained 80 % of its initial capacity beyond 100 h of floating at an operating voltage of 2.4 V and showed a 150 mV per hour potential loss under self-discharge. The devised eutectic mixtures offer a promising new pathway for simple, safe, and effective electrolytes for SC applications.

Tunning CO2 Separation Performance of Ionic Liquids through Asymmetric Anions.

This work aims to explore the gas permeation performance of two newly-designed ionic liquids, [C2mim][CF3BF3] and [C2mim][CF3SO2C(CN)2], in supported ionic liquid membranes (SILM) configuration, as another effort to provide an overall insight on the gas permeation performance of functionalized-ionic liquids with the [C2mim]+ cation. [C2mim][CF3BF3] and [C2mim][CF3SO2C(CN)2] single gas separation performance towards CO2, N2, and CH4 at T = 293 K and T = 308 K were measured using the time-lag method. Assessing the CO2 permeation results, [C2mim][CF3BF3] showed an undermined value of 710 Barrer at 293.15 K and 1 bar of feed pressure when compared to [C2mim][BF4], whereas for the [C2mim][CF3SO2C(CN)2] IL an unexpected CO2 permeability of 1095 Barrer was attained at the same experimental conditions, overcoming the results for the remaining ILs used for comparison. The prepared membranes exhibited diverse permselectivities, varying from 16.9 to 22.2 for CO2/CH4 and 37.0 to 44.4 for CO2/N2 gas pairs. The thermophysical properties of the [C2mim][CF3BF3] and [C2mim][CF3SO2C(CN)2] ILs were also determined in the range of T = 293.15 K up to T = 353.15 K at atmospheric pressure and compared with those for other ILs with the same cation and anion’s with similar chemical moieties.

Energetics of Li+ Coordination with Asymmetric Anions in Ionic Liquids by Density Functional Theory

The energetics, coordination, and Raman vibrations of Li solvates in ionic liquid (IL) electrolytes are studied with density functional theory (DFT). Li+ coordination with asymmetric anions of cyano(trifluoromethanesulfonyl)imide ([CTFSI]) and (fluorosulfonyl)(trifluoro-methanesulfonyl)imide ([FTFSI]) is examined in contrast to their symmetric analogs of bis(trifluoromethanesulfonyl)imide ([TFSI]), bis(fluorosulfonyl)imide ([FSI]), and dicyanamide ([DCA]). The dissociation energies that can be used to describe the solvation strength of Li+ are calculated on the basis of the energetics of the individual components and the Li solvate. The calculated dissociation energies are found to be similar for Li+-[FTFSI], Li+-[TFSI], and Li+-[FSI] where only Li+-O coordination exists. Increase in asymmetry and anion size by fluorination on one side of the [TFSI] anion does not result in significant differences in the dissociation energies. On the other hand, with [CTFSI], both Li+-O and Li+-N coordination are present, and the Li solvate has smaller dissociation energy than the solvation by [DCA] alone, [TFSI] alone, or a 1:1 mixture of [DCA]/[TFSI] anions. This finding suggests that the Li+ solvation can be weakened by asymmetric anions that promote competing coordination environments through enthalpic effects. Among the possible Li solvates of (Li[CTFSI]n)−(n−1), where n = 1, 2, 3, or 4, (Li[CTFSI]2)−1 is found to be the most stable with both monodentate and bidentate bonding possibilities. Based on this study, we hypothesize that the partial solvation and weakened solvation energetics by asymmetric anions may increase structural heterogeneity and fluctuations in Li solvates in IL electrolytes. These effects may further promote the Li+ hopping transport mechanism in concentrated and multicomponent IL electrolytes that is relevant to Li-ion batteries.

Enhancing the properties of PBAT/PLA composites with novel phosphorus-based ionic liquid compatibilizers

Biodegradable polybutylene adipate terephthalate/polylactic acid (PBAT/PLA) composites with good interfacial adhesion, mechanical properties, flame retardancy and melt processability are demanded by the plastics sector. To simultaneously achieve these attributes, researchers are actively seeking novel compatibilizers for these polymers. Herein, two ionic liquids (denoted as IL1 and IL2), comprising phosphonium cations (PR4+) with different length alkyl substituents (R) and phosphorus containing asymmetric anions (RPO32−), were successfully synthesized and applied as compatibilizers to prepare PBAT/ILs/PLA composites by melt blending and injection molding. Owing to chain entanglement between the long alkyl chains of PR4+ and RPO32− with the PLA and PBAT polymer chains, as well as interactions between the anion -PO32− groups and ester groups on PLA and PBAT, the toughness, flame retardancy and processability of the composites were all enhanced. Adding the ILs resulted in the more uniform dispersion of PLA in the PBAT matrix. The elongation at break of composites containing 0.4 wt.% IL1 or IL2 increased by 329.9 % and 168.5 %, respectively, relative to pristine PBAT/PLA composites. At an IL content of 0.7 wt.%, both of PBAT/IL1/PLA composites and PBAT/IL2/PLA composites possessed excellent flame retardancy (a limiting oxygen index of 28.1 and 27.3, respectively). Furthermore, the melt fluidity of the PBAT/ILs/PLA composites increased with increasing IL content. This work conclusively demonstrates that ionic liquids can serve as excellent compatibilizers for high performance biodegradable PBAT/PLA composites.

Synthesis of CoOx from ethaline on a stainless steel mesh for supercapacitor applications

Room Temperature Ionic liquids (RTIL) are molten salts that are liquid at around room temperature and consist of asymmetric anions and cations. Low volatility and wide electrochemical potential behaviour of RTIL make them preferable in reaction synthesis and electrochemical applications. In this study, cobalt was electrochemically deposited on an inexpensive stainless steel mesh substrate. The resultant modified electrode was studied for supercapacitor applications by measuring its capacitance performance. Choline chloride and ethylene glycol-based ionic liquid called Ethaline was used as the deposition electrolyte, and the ionic liquid was contained a cobalt salt (CoCl2) for cobalt growth. The deposition conditions of cobalt in Ethaline ionic liquid were found by cyclic voltammogram study. The cobalt in an ionic liquid was obtained by applying − 1.50 V potential on the stainless steel mesh by potentiostatic method. The uncoated and cobalt-coated steel mesh electrodes were then tested in KOH electrolyte to find appropriate potential window of the modified electrode. The cobalt-based material coated mesh electrode was scanned in 1 M KOH electrolyte between − 0.20 and 0.60 V and reported to have high electrochemical activity than uncoated mesh. The surface morphology of coated and uncoated mesh electrode was examined. Electrochemical characterizations of modified mesh electrodes were investigated using cyclic voltammetry. As a result, the obtained cobalt-coated mesh electrode had an areal capacitance of 650 mF cm−2 at scan rate of 5 mV s−1. The cobalt-coated steel mesh electrode may have potential application in OH−-based electrolyte for energy storage devices.

Ionic Liquid in Li Salt Electrolyte: Modifying the Li+ Transport Mechanism by Coordination to an Asymmetric Anion

In binary ionic liquid/Li salt electrolytes for lithium‐ion batteries the use of asymmetric anions reduces the tendency for crystallization and enables liquid systems with high Li concentration. The ionic liquid composed of the N‐(methoxyethyl)‐N‐methylpyrrolidinium (Pyr12O1) cation and the (fluorosulfonyl)(trifluoromethanesulfonyl)imide (FTFSI) asymmetric anion at molar Li FTFSI fractions up to 0.6 are investigated by 19F and 7Li chemical shifts, 1H, 19F, and 7Li electrophoretic nuclear magnetic resonance (NMR) and density functional theory (DFT) calculations. Thereby, the local coordination environment of Li is elucidated and correlated with the Li+ ion transport properties. At low Li salt fraction, the preferred Li+ coordination is to the trifluoromethanesulfonyl side of the anion, resulting in vehicular Li+ transport in stable, net negatively charged Li‐anion clusters causing negative Li transference numbers. The Li coordination is, however, shifting to the fluorosulfonyl group at salt fractions >0.4, as consistently evidenced by DFT and 19F NMR. Herein, Li+ mobilities give evidence of an increasing relevance of structural Li+ ion transport, which is key toward developing efficient ionic liquid–based batteries. This knowledge will serve further tailored design of cations and anions, which reduces crystallization and promote structural transport in ionic liquids for safe and high‐power batteries.

Unraveling the Mechanism of Enhanced Lithium Salt Solubility in Water-in-Salt Electrolytes Containing Ionic Liquids

The narrow electrochemical stability window of aqueous electrolytes of ~1.5 V can be extended to >2.5 V employing high salt concentration.1-3 At a molality of, e.g., 21 moles of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) per kilogram of water, the water:lithium ratio equals 2.65, causing strong cation-water interactions and the alignment of TFSI anions in a water-blocking electrochemical double-layer at the cathode, resulting in particularly high oxidative stability.1,2 As these electrolytes are typically saturated solutions at room temperature, they tend to crystallize below room temperature. We recently showed that employing salts with asymmetric anions such as lithium (pentafluoroethanesulfonyl)(trifluoromethanesulfonyl)imide (LiPTFSI) is a very effective strategy to suppress crystallization in such water-in-salt electrolytes to temperatures below −10 °C.4,5 LiPTFSI has further been combined with LiTFSI to form a solution with an overall molality of 56 mol kg−1, lowering the water:lithium ratio to 1.6 Reducing the water:lithium ratio is a key challenge to enhance the reductive stability of water-in-salt electrolytes. However, increasing the salt concentration to such high levels leads to a low ionic conductivity of only 0.1 mS cm−1 at 30 °C.6 Recently it was demonstrated that the addition of quaternary ammonium-based ionic liquids to a water-in-salt electrolyte increases LiTFSI solubility, while maintaining high ionic conductivity.7 Yet it is unclear, where the enhanced LiTFSI solubility originates from.In this contribution, we unravel the mechanism enabling very high lithium salt solubility in the presence of ionic liquids in water-in-salt electrolytes via nuclear magnetic resonance and vibrational spectroscopy.8 We investigate hybrid electrolytes with a water:lithium ratio of only 0.93 and a LiTFSI molality of up to 60 mol kg−1, almost three times as high as in water. Generally, these ternary water-ionic liquid-lithium salt electrolytes have much better transport properties at high salt concentration reaching 1.4 mS cm−1 at 25 °C at a molality of 40 mol kg−1 LiTFSI and 20 mol kg−1 ionic liquid. In addition, the enhanced reductive stability enables stable cycling of a Li4Ti5O12 anode in combination with a nickel-rich lithium nickel manganese cobalt oxide (NMC811) cathode with high Coulombic efficiency demonstrating the promising potential of water-in-salt electrolytes.

Asymmetric effects of anions in magnetic ionic liquids

Phase behaviors and magnetic properties of magnetic ionic liquids (mILs) were investigated at low temperature. The mILs were [C4mim][FeCl4], [C4mim][FeCl3Br], [C4mim][FeCl2Br2], [C4mim][FeClBr3], and [C4mim] [FeBr4], where [C4mim]+ is 1-butyl-3-methylimidazolium. The formation of symmetric and asymmetric anions was identified using Raman spectroscopy. In addition, [C4mim][FeCl4], [C4mim][FeCl3Br], and [C4mim][FeCl2Br2] indicated a paramagnetic behavior of magnetic susceptibility, which obeyed the Curie-Weiss law. In [FeClBr3]−-, and [FeBr4]−-based mILs, crystallization was induced at a low temperature upon cooling. Moreover, nonparamagnetic magnetic susceptibility was observed in [C4mim][FeClBr3], and [C4mim][FeBr4].

Suppressing Crystallization of Water-in-Salt Electrolytes by Asymmetric Anions Enables Low-Temperature Operation of High-Voltage Aqueous Batteries

The discovery of enhanced electrochemical stability for aqueous electrolytes with very high salt concentration has stimulated the development of high-voltage aqueous batteries. We show that a key f...

Competition between Cation–Solvent and Cation–Anion Interactions in Imidazolium Ionic Liquids with Polar Aprotic Solvents

The subtle interplay between ion solvation and association was analyzed in mixtures of imidazolium-based ionic liquids (ILs) with polar aprotic solvents. A site-specific pattern of cation-solvent and cation-anion interactions was disclosed by a careful analysis of the 1 H and 13 C NMR chemical shift dependence of the mixture composition. It was established that the less polar but more donating γ-butyrolactone is more prone to establish H-bonds with the imidazolium-ring hydrogen atoms of the IL cations than propylene carbonate, particularly at the H2 site and at high dilutions xIL <0.1. The H2 site was found to be more sensitive to intermolecular interactions compared to H4, 5 in the case of ILs with asymmetric anions like trifluoromethanesulfonate (TfO- ) or bis(trifluoromethylsulfonyl)amide (TFSA- ).

A guest-templated (6,3)-sheet constructed using asymmetric hydrogen-bonding anions

A hydrogen-bonded (6,3)-sheet similar to that of trimesic acid, containing asymmetric anions as the three-connecting nodes joined by NH⋯nitrile interactions, is templated by a cationic Cu(II) guest species.

Analysis of glycosaminoglycans and their oligosaccharide fragments by capillary electrophoresis.

Glycosaminoglycans are high molecular mass polymers found in living tissues. They are degraded by site specific enzymes to oligosaccharide fragments. Polymers and oligomers all carry negative charge and are found as salts soluble in water. Consequently, capillary electrophoresis has come to the fore as an analytical technique in this field. This review discusses application of the technique to the chondroitin-dermatan, heparin and hyaluronan groups of polymer and the derived oligosaccharides. Direct and indirect UV detection are discussed as well as detection methods involving precolumn derivatization. Capillary electrophoresis has found use in the characterisation of heparin and in following the changes in chondroitin and hyaluronan during the onset of arthritic disease in humans. Some glycosaminoglycans have been used as asymmetric anions in the separation of enantiomers of low molecular mass compounds by capillary electrophoresis.

Analysis of the Bulk and Surface-Induced Structure of Electrolyte Solutions Using Integral Equation Theories

We have developed robust and efficient numerical methods for solving integral equation theories for electrolyte solutions. These methods are hybrids of Newton–Raphson and Picard iterations and have been obtained as extended versions of the previous methods for pure solvents by solving nontrivial problems posed by the inclusion of ions. Bulk electrolytes and electrolytes near both inert and metallic surfaces are considered. The basic equations previously derived for a one-component fluid near a planar wall are extended to a multicomponent fluid. Analytical expressions for elements of the Jacobian matrices are arranged in compact form. A striking feature of the method for surface problems is that the Jacobian is determined only from bulk properties. A discussion of some special treatments that need to be considered for asymmetric anions and cations is included. These methods have been demonstrated using the full reference hypernetted-chain theory for various sizes of ions in a wide range of ionic concentrations.

The crystal structure of 1,1′-dimethylferricenium tri-iodide

The crystal structure of 1,1′-dimethylferricenium tri-iodide has been determined from three-dimensional X-ray data collected by counter methods, and refined by least-squares techniques to a final conventional R factor, based on F, of 3.5%, for 1278 independent reflections. The material crystallizes in a triclinic cell, space group P 1 , with dimensions a = 14.70(1), b = 7.440(6), c = 7.97(1) Å, α = 70.50(3)°, β = 81.49(3)° γ = 97.33 (4)°, Z = 2. Although the two independent tri-iodide anions in the structure lie at crystallographic centres, refinement of a structural model with a 50:50 disorder of asymmetric anions about the centres of symmetry gave a significantly better fit to the data than refinement of a model based on centrosymmetric anions. The II distances in the asymmetric anions are 2.85(2) 2.97(2) Å for one, and 2.86(3) and 3.00(3) Å for the other. The cyclopentadienyl rings in the cation are almost exactly eclipsed with the methyl groups adjacent. The rings are inclined at 6.6° to each other. Mean distances are Fe-ring (perpendicular) = 1.695(1), FeC(ring) = 2.073(5), CC(ring) = 1.401(5), CCH 3 = 1.49(1) Å.