Solvent-shared Ion Pairs Research Articles

Mixture of monoglyme-based solvent and lithium Bis(trifluoromethanesulfonyl)amide as electrolyte for lithium ion battery using silicon electrode

Abstract The electrochemical property of a silicon electrode in a half-cell was investigated using the mixture of monoglyme-based solvents and lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) with 2:1 stoichiometry. The solvents used in this study are monoglyme (G1), 1,2-diethoxyethane (DEE), 1,2-dibutoxyethane (DBE), and 1,2-dimethoxypropane (P-G1). When Li (DEE)2 or Li(P-G1)2 was used, the cell retained ca. 20% capacity at a rate of 3 C, while the other samples showed almost no capacity under the same conditions. Raman spectroscopy measurement revealed that Li+ exists as two forms in the mixture, that is, a solvent shared ion pair (SSIP) [ Li+ ··· solvent ··· TFSA– ] and a contact ion pair (CIP) [ Li+ ··· TFSA– ]. According to the spectroscopic analysis, SSIP is the dominant species for Li (DEE)2 and Li(P-G1)2, and CIP for Li (G1)2 and Li (DBE)2. The order of the rate property was Li(P-G1)2 ∼ Li (DEE)2 > Li (DBE)2 ∼ Li (G1)2, which is almost consistent with the stability of free energy levels of Li+ in the respective monoglymes against CIP Li+ ··· TFSA– evaluated from the Raman spectra.

Ionic Conduction and Solution Structure in LiPF6 and LiBF4 Propylene Carbonate Electrolytes

Expanding the performance limit of current Li-ion batteries requires ion–ion and ion–solvent interaction, which governs the ion transport behavior of the electrolytes, to be fully understood as a matter of crucial importance. We herein examine the ionic speciation and conduction behavior of propylene carbonate (PC) electrolytes of 0.1–3.0 M LiPF6 and LiBF4 using Raman spectroscopy, dielectric relaxation spectroscopy (DRS), and pulsed-field gradient NMR (PFG-NMR) spectroscopy. In both LiPF6–PC and LiBF4–PC, free ions and a solvent-shared ion pair (SIP) are dominant species at dilute salt concentrations (<0.8 M), and SIP becomes dominant at intermediate concentrations (0.8–1.5 M). At higher concentrations (1.5–3.0 M), the solvent-shared dimer (SSD) and contact dimer (CD) are dominant in LiPF6–PC, whereas the contact ion pair (CIP), CD, and agglomerate (AGG) prevail in LiBF4–PC. Ionic conduction in 0.1–1.5 M LiPF6–PC and LiBF4–PC is governed by the migration of free ions and SIP. Notably, above 1.5 M of the two PC electrolytes, SSD participates in ionic conduction via the migration mode as well. Furthermore, it is suggested that the large number of CIPs present in LiBF4–PC may contribute to ionic conduction via a Grotthuss-type mechanism.

Raman spectroscopy and ab initio quantum chemical calculations of ion association behavior in calcium nitrate solution

AbstractIn this work, microscopic Raman spectroscopy and component analysis were used to study a calcium nitrate solution with molar water‐to‐salt ratio (WSR) values of 4–320. The results showed that the symmetrical stretching vibration wavenumber of the nitrate ion (v1‐NO3−) in the solution blue shifted from 1,048 to 1,053 cm−1, and the full width at half maximum increased from 9.7 to 11 cm−1 when the WSR was reduced from 320 to 4. The component analysis of v1‐NO3− showed that the first concentration region was for WSR ≥ 16.3. In this region, solvent‐shared ion pairs (SIPs) were the main species, and contact ion pairs (CIPs) and free‐hydrated ions were the secondary species. For 8.4 ≤ WSR &lt; 16.3, the main species were CIPs; the amount of complex structures in this region increased rapidly, whereas the amount of SIPs decreased rapidly. For WSR &lt; 8.4, the main species were the complex structures, and the amount of CIPs and SIPs dropped rapidly. Ab initio quantum chemical calculations showed that the SIPs in the first concentration range were composed of 2 hexahydrate calcium ions and 1 nitrate ion that formed a shared ion pair, SIP‐SIP. The calcium ions were directly bonded with water molecules, and the nitrate ions formed hydrogen bonds with the water molecules. In the second concentration region, the CIPs were each formed by 2 hydrated calcium ions and 1 nitrate ion via monodentate or bidentate coordination with a CIP‐CIP structure. When the WSR was further reduced, a large number of the complex structures such as Comp5 and Comp6 formed in the solution, which were the complex structures [(H2O)nCa2+(NO3−)Ca2+(H2O)m] clustered by C5 or C6.

Tuning of Polyoxopalladate Macroanionic Hydration Shell via Countercation Interaction.

Three types of macroanion-countercation interactions in dilute solution, decided by the strength of electrostatic attraction and the change of hydration shells are reported: minor interaction between macroanions [MO8 Pd12 (SeO3 )8 ]6- (M=Zn2+ or Ni2+ ) and monovalent cations (Na+ , K+ , Rb+ , Cs+ ), leaving their hydration shells intact (solvent-separated ion-pairs); strong binding between macroanions and divalent cations (Sr2+ , Ba2+ ) to form solvent-shared ion-pairs with partial dehydration; very strong electrostatic attraction between macroanions and Y3+ ion with contact ion-pairs formation by severely breaking their original hydration shells and forming new ones. In addition, divalent cations can help the macroanions self-assemble into hollow spherical blackberry structures through counterion-mediated attraction, whereas macroanions with mono- or trivalent cations only stay as discrete ions due to either weak interaction or a small number of bound countercations.

Elucidation of hydrated metal ions using flocculation-surface enhanced Raman scattering

Hydrated metal ions such as Na+, Mg2+, and Cu2+ in addition to surface species like citrate, p-mercaptobenzoic acid (PMBA), and halide anions were identified by exploiting coupled localized surface plasmons of flocculated silver nanoparticles (AgNPs). Indeed, SERS spectra provided clear evidence for the formation of citrate complexes of Co2+ and Cu2+ as well as ion pairs of metal ions and PMBA− anions on AgNPs. For the first time, we have detected pronounced Raman bands from hydrated metal ions in flocculates of halide-covered AgNPs assigned to restricted rotation and translation at 600–200 cm−1 and OH stretching bands at 3500–3600 cm−1. Interestingly, the peak frequencies of OH stretching mode from hydrated metal ions are sensitive to metal and halide species, proving the observed water molecules are sandwiched between trapped metal ions and halide ions on adjacent AgNPs while forming a solvent shared ion pair.

Hydration and ion association of La3+ and Eu3+ salts in aqueous solution.

Aqueous solutions of five lanthanide salts: LaCl3, La(NO3)3, La2(SO4)3, Eu(NO3)3 and Eu2(SO4)3 have been studied at 25 °C by dielectric relaxation spectroscopy over the frequency range 0.05 ≤ ν/GHz ≤ 89. Detailed analysis of the solvent-related modes located at higher frequencies showed that both La3+ and Eu3+ are strongly hydrated, even including partial formation of a third hydration shell similar to that of Al3+(aq). Up to two solute-related modes could be detected at lower frequencies, due to the formation of various types of 1 : 1 ion pairs (IPs). All five salts showed modest levels of association in the order Cl- < NO3- ≪ SO42-, mostly in the form of double-solvent-separated IPs with small amounts of solvent-shared IPs. Overall association constants, , calculated from the stepwise IP formation constants were consistent with literature values.

Solvent-Shared Ion Pairs at the Air–Solution Interface of Magnesium Chloride and Sulfate Solutions Revealed by Sum Frequency Spectroscopy and Molecular Dynamics Simulations

The ion distribution and ion pairing properties of Mg2+, SO42-, NO3-, and Cl- in the interfacial region of MgSO4, Mg(NO3)2, and MgCl2 solutions were investigated using vibrational sum frequency generation (VSFG) spectroscopy and molecular dynamics (MD) simulations. An electric field reversal relative to Mg(NO3)2 and MgCl2 solutions is observed at the interface of a MgSO4 solution. We show that, although magnesium cations are expected to have preference for bulk solvation, solvent-shared ion pairs (SIPs) exist in the interfacial region in which Mg2+ cations are closer to the solution surface than sulfate anions. While interfacial SIPs are few, they dominate the electric field effect observed. Thus, SIPs play a significant role in determining the electric field direction and magnitude at the air-aqueous interface. In addition to impact on the fundamental understanding of aqueous surfaces and interfacial ion-ion interactions, these findings have implications for atmospheric aerosol chemistry and thundercloud electrification.

Ab Initio Investigation of the Micro-species and Raman Spectra in Ca(NO3)2 Solution

In this work, the structural details and Raman spectra of the Ca(NO3)2(H2O) n=0–10 clusters were studied by using ab initio method. The results show that the main species in the cluster is the contact ion pair (CIP) when n = 1–7. When n = 8–10, the main species changes into solvent shared ion pair (SIP) CaNO3(H2O) n …NO3 − in the bidentate form. One of the r Ca–N distances remains unchanged at ~2.95 A, while the other one increases to more than 4.8 A. The hydration distance r Ca–O remains at 2.42 A. The contact between Ca2+ and NO3 leads to a red shift of the v 1–NO3 − band while the polarization of water by Ca2+ leads to a blue shift. The vibrational frequency of water molecules remains unchanged for the same types of water molecules. Hydrogen bonds are the main reason for the red shift of vibrational frequency of water molecules.

Is the Solution Activity Derivative Sufficient to Parametrize Ion-Ion Interactions? Ions for TIP5P Water.

Biomolecular processes involve hydrated ions, and thus molecular simulations of such processes require accurate force-field parameters for these ions. In the best force-fields, both ion-water and anion-cation interactions are explicitly parametrized. First, the ion Lennard-Jones parameters are optimized to reproduce, for example, single ion solvation free energies; then ion-pair interactions are adjusted to match experimental activity or activity derivatives. Here, we apply this approach to derive optimized parameters for concentrated NaCl, KCl, MgCl2, and CaCl2 salt solutions, to be used with the TIP5P water model. These parameters are of interest because of a number of desirable properties of the TIP5P water model, especially for the simulation of carbohydrates. The results show, that this state of the art approach is insufficient, because the activity derivative often reaches a plateau near the target experimental value, for a wide range of parameter values. The plateau emerges from the interconversion between different types of ion pairs, so parameters leading to equally good agreement with the target solution activity or activity derivative yield very different solution structures. To resolve this indetermination, a second target property, such as the experimentally determined ion-ion coordination number, is required to uniquely determine anion-cation interactions. Simulations show that combining activity derivatives and coordination number as experimental target properties to parametrize ion-ion interactions, is a powerful method for reliable ion-water force field parametrization, and gives insight into the concentration of contact or solvent shared ion pairs in a wide range of salt concentrations. For the alkali and halide ions Li+, Rb+, Cs+, F-, Br-, and I-, we present ion-water parameters appropriate at infinite dilution only.

Transport Phenomena of Nonaqueous Electrolyte Solutions at High Concentrations: A Comparison Between the Li- and Na-Systems

In order to understand the solution chemistry of electrolyte solutions at concentrations far from infinite dilution, the solution density, viscosity and ionic conductivity of Li- and Na-TFSI dissolved in GBL and PC were measured at 0.1 ≤ C/mol·dm−3 ≤ 2.0 and 278 ≤ T/K ≤ 328 (TFSI = bis(trifluoromethanesulfonyl)imide, GBL = γ-butyrolactone and PC = propylene carbonate). The partial molar volume of the solute, derived from the density, confirmed that the Na-systems occupy more volume in the solution than the Li-systems. On the other hand, the viscosity and ionic conductivity suggested that the Na-systems are more fluid and conductive than the Li-systems. The relative viscosity vs. the molarity follows a modified empirical Jones-Dole equation. The molar conductivity linearly decreased with respect to the cube-root of the molarity, which was analyzed by the pseudolattice model. The Raman spectroscopy revealed that, while the solvation number is comparable at 1-2 for either the Li- or Na-systems, Li+ is more tightly bound to the solvent molecules than Na+. The higher fluidity and conductivity of the Na-systems than those of the Li-systems result from the less occurrence of the solvent-shared ion pairs in the former than in the latter. Reference: K. Kuratani et al., J. Electrochem. Soc., 163 (6) H417-H425 (2016)

Solvation structures and dynamics of alkaline earth metal halides in supercritical water: A molecular dynamics study

Constrained molecular dynamics simulations of alkaline earth metal halides have been carried out to investigate their structural and dynamical properties in supercritical water. Potentials of mean force (PMFs) for all the alkaline earth metal halides in supercritical water have been computed. Contact ion pairs (CIPs) are found to be more stable than all other configurations of the ion pairs except for MgI2 where solvent shared ion pair (SShIP) is more stable than the CIP. There is hardly any difference in the PMFs between the M2+ (M=Mg, Ca, Sr, Ba) and the X− (X=F, Cl, Br, I) ions whether the second X− ion is present in the first coordination shell of the M2+ ion or not. The solvent molecules in the solvation shells diffuse at a much slower rate compared to the bulk. Orientational distribution functions of solvent molecules are sharper for smaller ions.

Spectral investigation of the effect of anion on the stability of non covalent assemblies of 2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecine (benzo-15-crown-5) with sodium halides

A series of complexes of 2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecine (benzo-15-crown-5) with sodium halides was synthesized in acetonitrile. The effect of anion on the stability and spectral properties of complexes of benzo-15-crown-5 with sodium halides was investigated. The synthesis of complexes of sodium fluoride and sodium chloride are reported for the first time. Chloroform was used as solvent to study the assembly in solution state by 1H and 13C NMR techniques. Single crystal diffraction studies on the easily crystallizable bromide complex confirmed 1:1 stoichiometry of the complex. IR and Raman studies provided valuable evidence for a water molecule shared between the crown encapsulated cation and the counter ion to give a solvent shared ion pair (SSIP). The fluorescence spectra of the complexes were obtained in chloroform by excitation at 270nm to study the effect of complexation on the fluorescent properties of benzo-15-crown-5.

Solvation structure of sodium chloride (Na+–Cl-) ion pair in acetonitrile (AN)–dimethyl formamide (DMF) mixtures

Solvation structures of Na+–Cl− ion pair are investigated in acetonitrile (AN)–dimethylformamide (DMF) isodielectric mixtures. The potentials of mean force of Na+–Cl− in the five compositions of mixtures show minima corresponding to a contact ion pair (CIP) and a solvent-shared ion pair (SShIP). The solvent-separated ion pair minima are present in lower mole fractions of AN (xAN ≤ 0.50). CIPs are found to be more stable than the SShIPs. From a thermodynamic decomposition of the potentials of mean force, we find that the formation of the ion pair is entropically driven in these compositions. The most stable CIP is in pure AN. The local solvation structures around the ion pair are analysed through the running coordination numbers, excess coordination numbers, solvent orientational distributions and density profiles. We find that both Na+ and Cl− are preferentially solvated by DMF.

Ion association in lithium metaborate solution: a Raman and ab initio insight

ABSTRACTIon association and hydration clusters in aqueous lithium borate solution are extremely important to understand some extraordinary properties of lithium borates. In the present work, polyborate distribution in aqueous LiBO2 solution was investigated through Raman and thermodynamics equilibrium analysis. Geometry and stability of hydrated clusters LiB(OH)4(H2O)n up to n = 8 were calculated at the B3LYP/aug-cc-pVDZ level. Three different types of ion association, namely, contact ion pairs (CIP), solvent-shared ion pairs (SIP) and solvent separated ion pairs (SSIP) were obtained; characteristics of all of these stable configurations were determined, and the most stable hydrated clusters were chosen. Then the mechanisms of ion aggregation and crystal nuclei formation in the LiB(OH)4 solution were proposed. The tight four-hydrated sphere of Li+ makes it difficult for the dehydrated form of its first hydration sphere to from a CIP, which is the passible reason that lithium borate always has a large supe...

Ion association in lithium metaborate solution: a Raman and ab initio insight

ABSTRACTIon association and hydration clusters in aqueous lithium borate solution are extremely important to understand some extraordinary properties of lithium borates. In the present work, polyborate distribution in aqueous LiBO2 solution was investigated through Raman and thermodynamics equilibrium analysis. Geometry and stability of hydrated clusters LiB(OH)4(H2O)n up to n = 8 were calculated at the B3LYP/aug-cc-pVDZ level. Three different types of ion association, namely, contact ion pairs (CIP), solvent-shared ion pairs (SIP) and solvent separated ion pairs (SSIP) were obtained; characteristics of all of these stable configurations were determined, and the most stable hydrated clusters were chosen. Then the mechanisms of ion aggregation and crystal nuclei formation in the LiB(OH)4 solution were proposed. The tight four-hydrated sphere of Li+ makes it difficult for the dehydrated form of its first hydration sphere to from a CIP, which is the passible reason that lithium borate always has a large super-saturation degree.

Water-Mediated Ion Pairing: Occurrence and Relevance

We present an overview of the studies of ion pairing in aqueous media of the past decade. In these studies, interactions between ions, and between ions and water, are investigated with relatively novel approaches, including dielectric relaxation spectroscopy, far-infrared (terahertz) absorption spectroscopy, femtosecond mid-infrared spectroscopy, and X-ray spectroscopy and scattering, as well as molecular dynamics simulation methods. With these methods, it is found that ion pairing is not a rare phenomenon only occurring for very particular, strongly interacting cations and anions. Instead, for many salt solutions and their interfaces, the measured and calculated structure and dynamics reveal the presence of a distinct concentration of contact ion pairs (CIPs), solvent shared ion pairs (SIPs), and solvent-separated ion pairs (2SIPs). We discuss the importance of specific ion-pairing interactions between cations like Li(+) and Na(+) and anionic carboxylate and phosphate groups for the structure and functioning of large (bio)molecular systems.

Kinetic inhibition of dolomite precipitation: Insights from Raman spectroscopy of Mg2 +–SO42 − ion pairing in MgSO4/MgCl2/NaCl solutions at temperatures of 25 to 200 °C

The origin of dolomite has been an issue for hundreds of years, and its kinetic inhibition is a critical aspect of this issue. Dissolved sulfate is regarded as an inhibitor for dolomite formation because it can bind Mg2+ to form tight ion pairs and thus prevent the incorporation of Mg2+ into dolomite. Using Raman spectroscopy, we investigated the Mg2+–SO42− association in vapor-saturated aqueous MgSO4/MgCl2/NaCl solutions at temperatures of 25 to 200°C. The Mg2+–SO42− association is highly temperature and concentration dependent: the fractions of contact ion pairs (CIPs) and triple ion pairs (TIs) increase with increasing temperature and MgSO4 concentration. The presence of MgCl2 increases the Mg2+/SO42− ratio and favors Mg2+–SO42− interactions to produce CIPs and TIs, whereas the presence of NaCl exerts a negative effect on Mg2+–SO42− interactions, particularly at high temperatures (i.e., ≥150°C). The primary sulfate species in concentrated MgSO4 solutions at high temperatures (i.e., ≥2mol/kg, 200°C) are various contact ion pairs, whereas those in diluted solutions at Earth surface temperature appear to be unassociated SO42− and weakly associated solvent-separated and solvent-shared ion pairs. We propose that dissolved sulfate can inhibit the incorporation of Mg2+ into dolomite crystals by attracting Mg2+ to form tight contact ion pairs under hydrothermal conditions. However, thermochemical sulfate reduction (TSR) can effectively remove sulfate and free Mg2+ to enhance the precipitation of hydrothermal dolomite from sulfate-bearing fluids. The inhibiting effect of dissolved sulfate on the formation of massive low-temperature dolomite appears to have been overestimated. Removal of sulfate by anaerobic bacterial sulfate reduction (BSR) may not be responsible for the formation of microbial dolomite at surface temperatures. These new understandings also have implications for the study of thermochemical sulfate reduction because the formation of CIPs can increase the activity of sulfate in reactions with hydrocarbons.

Solvent-shared pairs of densely charged ions induce intense but short-range supra-additive slowdown of water rotation.

The question "Can ions exert supra-additive effects on water dynamics?" has had several opposing answers from both simulation and experiment. We address this ongoing controversy by investigating water reorientation in aqueous solutions of two salts with large (magnesium sulfate) and small (cesium chloride) effects on water dynamics using molecular dynamics simulations and classical, polarizable models. The salt models are reparameterized to reproduce properties of both dilute and concentrated solutions. We demonstrate that water rotation in concentrated MgSO4 solutions is unexpectedly slow, in agreement with experiment, and that the slowdown is supra-additive: the observed slowdown is larger than that predicted by assuming that the resultant of the extra forces induced by the ions on the rotating water molecules tilts the free energy landscape associated with water rotation. Supra-additive slow down is very intense but short-range, and is strongly ion-specific: in contrast to the long-range picture initially proposed based on experiment, we find that intense supra-additivity is limited to water molecules directly bridging two ions in solvent-shared ion pair configuration; in contrast to a non-ion-specific origin to supra-additive effects proposed from simulations, we find that the magnitude of supra-additive slowdown strongly depends on the identity of the cations and anions. Supra-additive slowdown of water dynamics requires long-lived solvent-shared ion pairs; long-lived ion pairs should be typical for salts of multivalent ions. We discuss the origin of the apparent disagreement between the various studies on this topic and show that the short-range cooperative slowdown scenario proposed here resolves the existing controversy.

Transport Phenomena of Nonaqueous Electrolyte Solutions at High Concentrations: A Comparison between the Li- and Na-Systems

In order to understand the solution chemistry of electrolyte solutions at concentrations far from infinite dilution, the solution density, viscosity and ionic conductivity of Li- and Na-TFSI dissolved in GBL and PC were measured at 0.1 ⩽ C/mol · dm− 3 ⩽ 2.0 and 278 ⩽ T/K ⩽ 328, where TFSI = bis(trifluoromethanesulfonyl)imide, GBL = γ-butyrolactone and PC = propylene carbonate. The results are compared to those of previously determined perchlorate salts. The partial molar volume of the solute, derived from the density, confirmed that the Na-systems occupy more volume in the solution than the Li-systems. On the other hand, the viscosity and ionic conductivity suggested that the Na-systems are more fluid and conductive than the Li-systems. The relative viscosity vs. the molarity follows a modified empirical Jones-Dole equation. The molar conductivity linearly decreased with respect to the cube-root of the molarity, which was analyzed by the pseudolattice model. The Raman spectroscopy revealed that, while the solvation number is comparable at 1-2 for either the Li- or Na-systems, Li+ is more tightly bound to the solvent molecules than Na+. The higher fluidity and conductivity of the Na-systems than those of the Li-systems result from the less occurrence of the solvent-shared ion pairs in the former than in the latter.

Molecular Dynamics Simulation of Na+–Cl– Ion-Pair in Water–Methanol Mixtures under Supercritical and Ambient Conditions

Constrained molecular dynamics simulations have been performed to investigate the structure and thermodynamics of Na(+)-Cl(-) ion-pair association in water-methanol mixtures under supercritical and ambient conditions in dilute solutions. From the computed potentials of mean force (PMFs) we find that contact ion pairs (CIPs) are more stable than all other associated states of the ion pairs in both ambient and supercritical conditions. Stabilities of CIPs increase with increase in the mole fraction of methanol. In supercritical conditions, major changes in PMFs occur as we go from x(methanol) = 0.00 to x(methanol) = 0.50. The stable solvent shared ion pair (SShIP) which occurs in x(methanol) = 0.00 and 0.25, vanishes when x(methanol) is 0.50 or greater. The stabilities of these ion pairs increase with increasing temperature. Local structures around the ions are studied using the radial distribution functions, density profiles, angular distribution functions, running coordination numbers and excess coordination numbers. Preferential solvation analysis shows that both Na(+) and Cl(-) ions are preferentially solvated by water. From the calculation of enthalpies and entropies, we find that Na(+)-Cl(-) ion-pair association in water-methanol binary mixtures is endothermic and driven by entropy both in ambient as well as under supercritical conditions.