Formation Of Contact Ion Pairs Research Articles

The hydration of Li+ and Mg2+ in subnano carbon nanotubes using a multiscale theoretical approach.

The separation of brines with high Mg/Li mass ratios is a huge challenge. To provide a theoretical basis for the design of separation materials, the hydration of Li+ and Mg2+ in confinement using carbon nanotubes (CNTs) as the 1-D nanopore model was investigated using a multiscale theoretical approach. According to the analysis of the first coordination layer of cations, we determined that the coordination shells of two cations exist inside CNTs, while the second coordination shells of the cations are unstable. Moreover, the results of the structure analysis indicate that the hydration layer of Li+ is not complete in CNTs with diameters of 0.73, 0.87, and 1.00nm. However, this does not occur in the 0.60nm CNT, which is explained by the formation of contact ion pairs (CIP) between Li+ and Cl- that go through a unstable solvent-shared ion pair [Li(H2O)4]+, and this research was further extended by 400ns in the 0.60nm CNT to address the aforementioned results. However, the hydration layer of Mg2+ is complete and not sensitive to the diameter of CNTs using molecular dynamics simulation and an ab initio molecular dynamics (AIMD) method. Furthermore, the results of the orientation distribution of Li+ and Mg2+ indicate that the water molecules around Mg2+ are more ordered than water molecules around Li+ in the CNTs and are more analogous to the bulk solution. We conclude that it is energetically unfavorable to confine Li+ inside the 0.60-nm diameter CNT, while it is favorable for confining Li+ inside the other four CNTs and Mg2+ in all CNTs, which is driven by the strong electrostatic interaction between cations and Cl-. In addition, the interaction between cations and water molecules in the five CNTs was also analyzed from the non-covalent interaction (NCI) perspective by AIMD.

Fine analysis of the component effect on the microstructure of LiCl solution

Lithium resources are abundant in Chinese salt lakes, but the high Mg/Li ratio in such aqueous natural sources has brought difficulties to the separation and purification of lithium. Crystallization processes are highly affected by the solution microstructure, which, in turn, depends on its composition. Synchrotron X-ray scattering, Raman spectroscopy, and molecular dynamics simulation methods were used to study the microstructure of aqueous LiCl solutions with different concentrations. In the reduced structure function, F(Q), the double peak around the scattering vector Q = 2.5 Å−1 becomes a single peak, which indicates the addition of LiCl changes the tetrahedral hydrogen bond structure in the aqueous solution. The deconvolution fitting of the Raman spectra show that the DDAA-type (double donor-double acceptor) hydrogen bonds tend to be transformed into DA-type (single donor–single acceptor) hydrogen bonds, confirming that the addition of LiCl breaks the tetrahedral hydrogen bond structure. This hydrogen-bond transformation was confirmed by pair distribution function analysis from molecular dynamics simulations of aqueous KCl solutions. In addition, considering the large number of magnesium ions coexisting with LiCl in the Salt Lake, the microstructure of mixed LiCl-MgCl2 solution was also studied. According to the X-ray scattering results, the peak in the reduced pair distribution function G(r) near r = 3.25 Å shifted from 3.25 Å to 3.10 Å with increased Mg/Li ratio. This change is likely to result from the Cl-O and OO interactions in the mixed aqueous solution. The deconvolution fitting results of Raman spectra show that the DDAA type hydrogen bond increases with the Mg/Li ratio, while the DA + DDA type hydrogen bond decreases. The molecular dynamics results showed that chlorine ions have a higher tendency to form contact ion pairs with Mg2+ than Li+. We propose this to be one of the processes retarding the separation of pure LiCl crystal from mixed LiCl-MgCl2 systems.

Direct Observation of Contact Ion-Pair Formation in La3+ Methanol Solution.

An approach combining molecular dynamics (MD) simulations and X-ray absorption spectroscopy (XAS) has been used to carry out a comparative study about the solvation properties of dilute La(NO3)3 solutions in water and methanol, with the aim of elucidating the still elusive coordination of the La3+ ion in the latter medium. The comparison between these two systems enlightened a different behavior of the nitrate counterions in the two environments: while in water the La(NO3)3 salt is fully dissociated and the La3+ ion is coordinated by water molecules only, the nitrate anions are able to enter the metal first solvation shell to form inner-sphere complexes in methanol solution. The speciation of the formed complexes showed that the 10-fold coordination is preferential in methanol solution, where the nitrate anions coordinate the La3+ cations in a monodentate fashion and the methanol molecules complete the solvation shell to form an overall bicapped square antiprism geometry. This is at variance with the aqueous solution where a more balanced situation is observed between the 9- and 10-fold coordination. An experimental confirmation of the MD results was obtained by La K-edge XAS measurements carried out on 0.1 M La(NO3)3 solutions in the two solvents, showing the distinct presence of the nitrate counterions in the La3+ ion first solvation sphere of the methanol solution. The analysis of the extended X-ray absorption fine structure (EXAFS) part of the absorption spectrum collected on the methanol solution was carried out starting from the MD results and confirmed the structural arrangement observed by the simulations.

(Invited) Pushing Lithium-Metal Batteries to the Limit: Fast Charging, Low Temperature, and Safety

The performance of lithium metal batteries has advanced significantly, thanks to continuous improvement in the lithium metal anode. Many chemical and mechanical control strategies have been employed to combat its degradation mechanisms such as parasitic reactions, dendritic growth, and formation of isolated lithium. Electrolytes with high salt concentrations, including those with non-solvating diluents (known as localized high concentration electrolytes, or LHCE), electrolyte additives, artificial coatings, 3D plating hosts Li, and applying pressures in the 100s-1000s of kPa range have all been found to be effect in yielding dense, high efficiency Li metal deposits. Despite these advancements, reported lithium metal batteries tend to be charged at low rates, operated under ambient conditions, and lacking sufficient information on their safety characteristics. In this talk, we will review our recent progress in pushing the lithium metal batteries to extreme operating conditions in term of temperature, charging rates, and shorting behavior.To enable fast charging, we have focused on developing a nucleation agent on the surface of current collector that can induce the formation of large, uniform nucleation sites. These nanoscopic sites enable dense lithium plating at 5 mA c-2 of current density, when a planar Cu electrode will fail catastrophically. This uniform nucleation method leads to a 45 um thick Li deposit that is nearly porosity free. A lithium metal cell with a metal oxide cathode is capable of 1C charging for extended cycles.To enable low temperature operation, we have focused on the development of new electrolyte compositions that uses weakly solvating solvents. These electrolytes, represented by monodentate ethers and LHCEs made of ethers, promote the formation of contact ion pairs after solvation over solvent separated ion pairs. These electrolytes have enabled the formation of dense lithium metal deposits at as low as -60oC, while strongly solvating electrolytes will promote the formation of dendrites and cell shorting.Finally, any practical implementation of lithium metal batteries operating under these extreme conditions have to feature safety designs that mitigate the impact of internal shorting. In this regard, we have focused on separator designs that can detect and intercept lithium dendrites.

Deciphering the Double-Layer Structure and Dynamics on a Model LixMoO3 Interface by Advanced Electrogravimetric Analysis.

A major feature of the electrolyte/electrode interface (EEI) that affects charge storage in lithium-ion batteries is the electrical double layer (EDL), but most of the available experimental approaches for probing its structuration have limitations due to electrical field and redox reaction disturbances, hence explaining why it is frequently overlooked. Herein we show that this is no longer true by using an advanced electrochemical quartz crystal microbalance (EQCM)-based method in the form of ac-electrogravimetry. For proof of concept, we studied the effect of various solvent/salt combinations, differing in their dipole moment and size/weight, respectively, on the structure of the EDL forming at the EEI of LixMoO3. We show that a significant amount of solvated lithium ions and anions contribute to charge compensation at the interface, and by varying the nature of the solvents (cyclic vs noncyclic), we provide a solid experimental proof of the direct relationship between the ions' solvation and solvent polarity. Moreover, we demonstrated a disappearance of the anionic motion in the less polar solvent (DMC) most likely due to plausible formation of contact ion pairs and agglomerates at the EDL level. Altogether, ac-electrogravimetry, when combined with classical EQCM, stands as an elegant and powerful method to experimentally assess the chemical structure and dynamics of the electrical double layer. We hope that the community will start to adopt it to better engineer interfaces of electrochemical energy storage devices.

Activation Modes in Asymmetric Anion-Binding Catalysis

AbstractOver the past two decades, enantioselective anion-binding catalysis has emerged as a powerful strategy for the induction of chirality in organic transformations. The stereoselectivity is achieved in a range of different reactions by using non-covalent interactions between a chiral catalyst and an ionic substrate or intermediate, and subsequent formation of a chiral contact ion pair upon anion binding. This strategy offers vast possibilities in catalysis and the constant development of new reactions has led to various substrate activation approaches. This review provides an overview on the different activation modes in asymmetric anion-binding catalysis by looking at representative examples and recent advances made in this field.1 Introduction2 Electrophile Activation by Single Anion-Binding Catalysis2.1 Prior In Situ Charged Electrophiles2.2 Neutral Electrophile Activation via Anion Abstraction2.2.1 Anion Abstraction via an SN1 Mechanism2.2.2 Anion Abstraction via an SN2 Mechanism3 Nucleophile Activation and Delivery4 Bifunctional and Cooperative Co-catalysis Strategies4.1 Amine Groups for Bifunctional and Cooperative Catalysis4.2 Brønsted Acid Co-catalysis4.3 Lewis Acid Co-catalysis4.4 Lewis Base Co-catalysis4.5 Nucleophilic Co-catalysis for Activation of Electrophiles4.6 Cooperative Metal and Anion-Binding Catalysis4.7 Combination of Photoredox and Anion-Binding Catalysis5 Anion-π Catalysis6 Conclusion

Ion-pairing equilibria and kinetics of dimethyl phosphate: A model for counter-ion binding to the phosphate backbone of nucleic acids

Dimethyl phosphate (DMP–) is the simplest model ion to assess ion-pairing phenomena between metal ions and the phosphate backbone of nucleic acids. Yet, the equilibria and dynamics of ion binding to DMP– have not been fully uncovered. Here, we study the interaction of DMP– with different cations and water in aqueous solutions of NaDMP using dielectric relaxation and nuclear magnetic resonance spectroscopies. We find DMP– to be weakly hydrated and weakly associated with Na+ in the absence of added salt. Upon addition of NaCl, MgCl2, or CaCl2 to solutions of NaDMP, we detect the formation of solvent-shared (NaDMP0) and contact (MgDMP+, CaDMP+) ion-pairs; the degree of ion association is 20–27 % at 2:1 salt:DMP molar ratio for the bivalent ions. Comparison to literature results suggests the formation constant of MgDMP+ to be a good estimate for the binding of Mg2+ to RNA. From the concentration dependence of the rotational relaxation time of the ion-pairs, we find ion-pair dissociation rates to follow the order Mg2+≪Ca2+<Na+. Strikingly, our data suggest that the overall ion-pair dissociation dynamics are governed by an ion-pair metathesis reaction, which provides a different pathway for the binding of ions by DMP–.

High Conductivity in a Fluorine-Free K-Ion Polymer Electrolyte

Solid polymer electrolytes (SPEs) could play a major role in the transition to safer and high-energy-density potassium-based batteries. However, most polymeric K-ion electrolytes are based on fluorine-containing anions and flammable organic solvents, whose safety is nowadays in question. Herein, we report a facile solvent-free synthesis of a series of several poly(ethylene oxide) (PEO)-based SPE solid solutions with KBPh4 salt as potassium-ion source, including the formation of two crystalline (PEO)n/KBPh4 complexes. The ionic conductivity of these novel K-ion SPEs above and below the melting point of PEO is rationalized in light of their glass-transition temperature and chemical composition. We highlight that below the melting point of PEO, the crystalline complexes may not be intrinsically ionically conducting, but they act as ion sinks preventing the polymer cross-linking and the formation of contact ion pairs and lower the glass-transition temperature leading to a conductivity of 1.1 × 10–4 S cm–1 at 55 °C. A high ionic conductivity of 1.8 × 10–3 S cm–1 is achieved at 80 °C for the optimum (PEO)30/KBPh4 composition (3.2 mol % KBPh4). Besides, the SPE compositions were found to be stable up to 4 V vs K0 metal electrode at 60 °C. The (PEO)30/KBPh4 composite electrolyte employed in an all-solid-state symmetric cell with Prussian Blue electrodes showed reversible K+-ion (de)intercalation, where a reversible capacity of 20 mAh g–1 and low voltage hysteresis were achieved for 20 cycles. Considering the material availability, ease of synthesis, and promising electrochemical properties, this work may encourage future research on fluorine-free polymer electrolytes for K-ion batteries.

Intriguing Chloride: Involvement of Chloride Ions in Proton Transfers.

The proton transfer from carbon to a chloride ion and the proton transfer to a molecule of water promoted by chloride ions in the acid-catalyzed formation of hydroxamic acids from aldehydes and substituted nitrosobenzenes in mixed solvents have been proposed based on experimental and theoretical investigations. The formation of uncommon contact ion pairs consisting of the nitrosocarbinolic cation intermediate and a chloride anion, followed by the proton transfer from a C-H moiety of the cation intermediate, has been proposed. The influence of chloride on the proton transfer to a water molecule of the solvent-separated nitrosocarbinolic-cation–chloride ion pair was investigated too. The insights are based on the obtained kinetic and other evidence with regard to (1) influences of chloride anions on the observed reaction rates and primary kinetic isotope effects (PKIE) in the reaction; (2) the observed variation of the PKIE-s and rates of the reaction when perchlorate anions are present along with the chloride ions; and (3) the consideration of a model of the nitrosocarbinolic-cation-intermediate—chloride ion pair and transition structure for the proposed proton transfers based on the ab initio calculations.

Contact Ion Pair Formation Is Not Necessarily Stronger than Solvent Shared Ion Pairing.

Vibrational sum frequency spectroscopy (VSFS) and pressure-area Langmuir trough measurements were used to investigate the binding of alkali metal cations to eicosyl sulfate (ESO4) surfactants in monolayers at the air/water interface. The number density of sulfate groups could be tuned by mixing the anionic surfactant with eicosanol. The equilibrium dissociation constant for K+ to the fatty sulfate interface showed 10 times greater affinity than for Li+ and approximately 3 times greater than for Na+. All three cations formed solvent shared ion pairs when the mole fraction of ESO4 was 0.33 or lower. Above this threshold charge density, Li+ formed contact ion pairs with the sulfate headgroups, presumably via bridging structures. By contrast, K+ only bound to the sulfate moieties in solvent shared ion pairing configurations. The behavior for Na+ was intermediate. These results demonstrate that there is not necessarily a correlation between contact ion pair formation and stronger binding affinity.

Ab Initio Molecular Dynamics Study of Aqueous Solutions of Magnesium and Calcium Nitrates: Hydration Shell Structure, Dynamics and Vibrational Echo Spectroscopy.

Ab initio molecular dynamics simulations are performed to study the hydration shell structure, dynamics, and vibrational echo spectroscopy of aqueous Mg(NO3)2 and Ca(NO3)2 solutions. The hydration shell structure is probed through calculations of various ion-ion and ion-water radial and spatial distribution functions. On the dynamical side, calculations have been made for the hydrogen bond dynamics of hydration shells and also residence dynamics and lifetimes of water in different solvation environments. Subsequently, we looked at the dynamics of frequency fluctuations of OD modes of heavy water in different hydration environments. Specifically, the temporal decay of spectral observables of two-dimensional infrared (2DIR) spectroscopy, three pulse echo peak shift (3PEPS) measurements and also of time correlations of frequency fluctuations are calculated to investigate the dynamics of vibrational spectral diffusion of water in different hydration environments in these solutions. The OD stretch frequencies of water molecules in the vicinity of both divalent cations are found to be red-shifted and also fluctuating at a slower rate than other water molecules present in the solutions. The Mg2+ ions are found to be strongly hydrated which can be linked to their lower tendency to form contact ion-pairs and essentially no water exchange between the cationic hydration shells and bulk during the time scale of the current simulations. The stronger hydration of Mg2+ ions make their hydration shells structurally and dynamically more rigid and make the dynamics of hydrogen bonds and vibrational spectral diffusion, as revealed through spectral observables of 2DIR and 3PEPS slower than that for the Ca2+ ions. The structural and spectral dynamics of water molecules outside the cationic solvation shells in the Mg(NO3)2 solution are also found to be relatively slower than that of the Ca(NO3)2 solution and pure water which show the effects of stronger electric fields of Mg2+ ions extending beyond their first hydration shells. Also, water molecules in the hydration shells of the NO3- ions are found to relax at a slower rate in the Mg(NO3)2 solution which manifests the effect countercations have on anionic hydration shells for divalent metal nitrate solutions.

On the Parameters Affecting Calcium Plating and Stripping from Organic Electrolytes – Cases of Electrolyte Optimization

Among the different next-generation battery technologies, the ones based on divalent cations have recently gained particular attention due to their higher raw material availability and potentially higher energy density when compared with Li-ion. Such is the case of calcium, being the 5th most abundant element in the earth crust and having a standard redox potential only 170 mV above metallic lithium. However, the development of a secondary calcium-metal battery had been hampered by the lack of organic electrolytes allowing for reversible plating and stripping, which was only recently reported by our group and others employing different electrolyte formulations.The different electrolyte components – salt(s), solvent(s) and additive(s) – are expected to play a significant role in the operation of a calcium-metal battery. Here we will show how the solvation structure of the cation evolves in relation to the solvent employed and the concentration of the salt. Combining conductivity measurements with Raman spectroscopy data, a high tendency to form contact ion pairs is evidenced, which significantly affects the mobility of the ions in the liquid media. The use of different anions and their bonding strength with Ca2+ in solution will be discussed. Combined, the anion and solvent characteristics dictate the solvation of the cation, which in turn can affect the Ca plating and stripping kinetics.Additionally, the presence of electrolyte contaminants will be explored. The crucial yet very challenging drying of electrolyte containing Ca(BF4)2 in carbonate solvents will be presented. The difficulties to obtain anhydrous and contaminant free solutions from commercially available salts will be discussed and two alternative routes for anhydrous synthesis of the target salt will be presented.

Phosphate Vibrations Probe Electric Fields in Hydrated Biomolecules: Spectroscopy, Dynamics, and Interactions.

Electric interactions have a strong impact on the structure and dynamics of biomolecules in their native water environment. Given the variety of water arrangements in hydration shells and the femto- to subnanosecond time range of structural fluctuations, there is a strong quest for sensitive noninvasive probes of local electric fields. The stretching vibrations of phosphate groups, in particular the asymmetric (PO2)− stretching vibration νAS(PO2)−, allow for a quantitative mapping of dynamic electric fields in aqueous environments via a field-induced redshift of their transition frequencies and concomitant changes of vibrational line shapes. We present a systematic study of νAS(PO2)− excitations in molecular systems of increasing complexity, including dimethyl phosphate (DMP), short DNA and RNA duplex structures, and transfer RNA (tRNA) in water. A combination of linear infrared absorption, two-dimensional infrared (2D-IR) spectroscopy, and molecular dynamics (MD) simulations gives quantitative insight in electric-field tuning rates of vibrational frequencies, electric field and fluctuation amplitudes, and molecular interaction geometries. Beyond neat water environments, the formation of contact ion pairs of phosphate groups with Mg2+ ions is demonstrated via frequency upshifts of the νAS(PO2)− vibration, resulting in a distinct vibrational band. The frequency positions of contact geometries are determined by an interplay of attractive electric and repulsive exchange interactions.

Contact Ion Pairs in the Bulk Affect Anion Interactions with Poly(N-isopropylacrylamide).

Salt effects on the solubility of uncharged polymers in aqueous solutions are usually dominated by anions, while the role of the cation with which they are paired is often ignored. In this study, we examine the influence of three aqueous metal iodide salt solutions (LiI, NaI, and CsI) on the phase transition temperature of poly(N-isopropylacrylamide) (PNIPAM) by measuring the turbidity change of the solutions. Weakly hydrated anions, such as iodide, are known to interact with the polymer and thereby lead to salting-in behavior at low salt concentration followed by salting-out behavior at higher salt concentration. When varying the cation type, an unexpected salting-out trend is observed at higher salt concentrations, Cs+ > Na+ > Li+. Using molecular dynamics simulations, it is demonstrated that this originates from contact ion pair formation in the bulk solution, which introduces a competition for iodide ions between the polymer and cations. The weakly hydrated cation, Cs+, forms contact ion pairs with I- in the bulk solution, leading to depletion of CsI from the polymer-water interface. Microscopically, this is correlated with the repulsion of iodide ions from the amide moiety.

Ion association in hydrothermal aqueous NaCl solutions: implications for the microscopic structure of supercritical water.

Knowledge of the microscopic structure of fluids and changes thereof with pressure and temperature is important for the understanding of chemistry and geochemical processes. In this work we investigate the influence of sodium chloride on the hydrogen-bond network in aqueous solution up to supercritical conditions. A combination of in situ X-ray Raman scattering and ab initio molecular dynamics simulations is used to probe the oxygen K-edge of the alkali halide aqueous solution in order to obtain unique information about the oxygen's local coordination around the ions, e.g. solvation-shell structure and the influence of ion pairing. The measured spectra exhibit systematic temperature dependent changes, which are entirely reproduced by calculations on the basis of structural snapshots obtained via ab initio molecular dynamics simulations. Analysis of the simulated trajectories allowed us to extract detailed structural information. This combined analysis reveals a net destabilizing effect of the dissolved ions which is reduced with rising temperature. The observed increased formation of contact ion pairs and occurrence of larger polyatomic clusters at higher temperatures can be identified as a driving force behind the increasing structural similarity between the salt solution and pure water at elevated temperatures and pressures with drawback on the role of hydrogen bonding in the hot fluid. We discuss our findings in view of recent results on hot NaOH and HCl aqueous fluids and emphasize the importance of ion pairing in the interpretation of the microscopic structure of water.

Infrared spectroscopic and computational studies of Co(ClO4)2 dissolved in N,N-dimethylformamide (DMF). Vibrations of DMF influenced by Co2+ or ClO4− or both

Infrared (IR) spectroscopy for N,N-dimethylformamide (DMF) shows that the OCN bend (δOCN) and the CO stretch (νCO) vibrations undergo an upshift and a downshift, respectively, on the dissolution of Co(ClO4)2. Quantum chemical calculations are performed for optimizing the structures and predicting the IR spectra of model complexes for solute species. The calculations reveal that Co2+ exerts a much larger influence than ClO4− on the vibrations of DMF. For Co2+(DMF)6, in which each DMF molecule is coordinated to Co2+ via the O atom, the Co2+⋯DMF interaction upshifts the δOCN frequencies (+24 cm−1 on average) while the dipole coupling gives rise to splitting (12 cm−1) of the modes. On the other hand, the Co2+⋯DMF interaction downshifts the νCO frequencies (−15 cm−1 on average) while the splitting of the modes amounts to 37 cm−1. As a result, one of the νCO modes is located at an upshifted position (+13 cm−1) despite the O-atom coordination. For six-coordinated isomers of Co2+(DMF)7, the δOCN and νCO frequencies of the second-sphere DMF are close to those of bulk DMF in neat liquid. The calculations indicate that it is difficult to prove or exclude the formation of contact ion pairs [Co(DMF)5ClO4]+ and solvent-shared ion pairs [Co(DMF)6ClO4]+ by IR spectroscopy in the δOCN and νCO regions. However, asymmetric ClO stretches of the ClO4− moiety suggest that conceivable is the coexistence of solvent-shared ion pairs only.

Ultrahigh coulombic efficiency electrolyte enables Li||SPAN batteries with superior cycling performance

Raising the coulombic efficiency of lithium metal anode cycling is the deciding step in realizing long-life rechargeable lithium batteries. Here, we designed a highly concentrated salt/ether electrolyte diluted in a fluorinated ether: 1.8 M LiFSI in DEE/BTFE (diethyl ether/bis(2,2,2-trifluoroethyl)ether), which realized an average coulombic efficiency of 99.37% at 0.5 mA cm−2 and 1 mAh cm−2 for more than 900 cycles. This electrolyte also maintained a record coulombic efficiency of 98.7% at 10 mA cm−2, indicative of its ability to provide fast-charging with high cathode loadings. Morphological studies reveal dense, dendrite free Li depositions after prolonged cycling, while surface analyses confirmed the formation of a robust LiF-rich SEI layer on the cycled Li surface. Moreover, we discovered that this ether-based electrolyte is highly compatible with the low-cost, high-capacity SPAN (Sulfurized polyacrylonitrile) cathode, where the constructed Li||SPAN cell exhibited reversible cathode capacity of 579 mAh g−1 and no capacity decay after 1200 cycles. A cell where a high areal loading SPAN electrode (>3.5 mAh cm−2) is paired with only onefold excess Li was constructed and cycled at 1.75 mA cm−2, maintaining a coulombic efficiency of 99.30% for the lithium metal. Computational simulations revealed that at saturation, the Li-FSI complex forms contact ion pairs, with a first solvation shell comprising DEE molecules, and a second solvation shell with a mix of DEE/BTFE. This study provides a path to enable high energy density Li||SPAN batteries with stable cycling.

Structure of aqueous sodium acetate solutions by X-Ray scattering and density functional theory

Abstract The structure of aq. sodium acetate solution (CH3COONa, NaOAc) was studied by X-ray scattering and density function theory (DFT). For the first hydrated layer of Na+, coordination number (CN) between Na+ and O(W, I) decreases from 5.02 ± 0.85 at 0.976 mol/L to 3.62 ± 1.21 at 4.453 mol/L. The hydration of carbonyl oxygen (OC) and hydroxyl oxygen (OOC) of CH3COO− were investigated separately and the OC shows a stronger hydration bonds comparing with OOC. With concentrations increasing, the hydration shell structures of CH3COO− are not affected by the presence of large number of ions, each CH3COO− group binds about 6.23 ± 2.01 to 7.35 ± 1.73 water molecules, which indicates a relatively strong interaction between CH3COO− and water molecules. The larger uncertainty of the CN of Na+ and OC(OOC) reflects the relative looseness of Na-OC and Na-OOC ion pairs in aq. NaOAc solutions, even at the highest concentration (4.453 mol/L), suggesting the lack of contact ion pair (CIP) formation. In aq. NaOAc solutions, the so called “structure breaking” property of Na+ and CH3COO− become effective only for the second hydration sphere of bulk water. The DFT calculations of CH3COONa (H2O)n=5–7 clusters suggest that the solvent-shared ion pair (SIP) structures appear at n = 6 and become dominant at n = 7, which is well consistent with the result from X-ray scattering.

Aqueous Contact Ion Pairs of Phosphate Groups with Na+, Ca2+ and Mg2+ – Structural Discrimination by Femtosecond Infrared Spectroscopy and Molecular Dynamics Simulations

Abstract The extent of contact and solvent shared ion pairs of phosphate groups with Na+, Ca2+ and Mg2+ ions in aqueous environment and their relevance for the stability of polyanionic DNA and RNA structures is highly debated. Employing the asymmetric phosphate stretching vibration of dimethyl phosphate (DMP), a model system of the sugar-phosphate backbone of DNA and RNA, we present linear infrared, femtosecond infrared pump-probe and absorptive 2D-IR spectra that report on contact ion pair formation via the presence of blue shifted spectral signatures. Compared to the linear infrared spectra, the nonlinear spectra reveal contact ion pairs with increased sensitivity because the spectra accentuate differences in peak frequency, transition dipole moment strength, and excited state lifetime. The experimental results are corroborated by long time scale MD simulations, benchmarked by density functional simulations on phosphate-ion-water clusters. The microscopic interpretation reveals subtle structural differences of ion pairs formed by the phosphate group and the ions Na+, Ca2+ and Mg2+. Intricate properties of the solvation shell around the phosphate group and the ion are essential to explain the experimental observations. The present work addresses a challenging to probe topic with the help of a model system and establishes new experimental data of contact ion pair formation, thereby underlining the potential of nonlinear 2D-IR spectroscopy as an analytical probe of phosphate-ion interactions in complex biological systems.

Highly concentrated LiN(SO2CF3)2/dinitrile electrolytes: Liquid structures, transport properties, and electrochemistry.

Liquid structures, transport properties, and electrochemical properties of binary mixtures of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and dinitrile solvents [succinonitrile (SN), glutaronitrile (GN), and adiponitrile (ADN)] were investigated. In the LiTFSA/SN and LiTFSA/ADN systems, the stable crystalline solvates of LiTFSA-(SN)1.5 [melting point (Tm): 59 °C] and LiTFSA-(ADN)1.5 (Tm: 50 °C) were formed, respectively. In contrast, the LiTFSA/GN mixtures of a wide range of compositions were found to be glass-forming liquids at room temperature. Raman spectroscopy of LiTFSA/GN liquid mixtures revealed that increasing the LiTFSA concentration results in the formation of the solvent-bridged network structure Li+-GN-Li+. In addition, the considerable formation of contact ion pairs and ionic aggregates was observed in highly concentrated electrolytes. In the liquids, the Li+ ion dynamically exchanged ligands (GN and TFSA) and higher LiTFSA concentrations led to an increase in the ratio of the self-diffusion coefficients of Li+ and TFSA-, DLi/DTFSA, as determined by pulsed field gradient NMR spectroscopy. The Li+ transference number (tLi+ ) of the [LiTFSA]/[GN] = 1/1.5 electrolyte in an electrochemical cell under anion-blocking conditions was estimated to be as high as 0.74. Furthermore, electrochemical measurements revealed that the reductive stability of the LiTFSA/GN electrolyte increases with increasing LiTFSA concentration. A [LiTFSA]/[GN] = 1/1.5 electrolyte is stable against the Li metal electrode, provided that the polarization is relatively small. Owing to high tLi+ , a Li-S battery with the [LiTFSA]/[GN] = 1/1.5 electrolyte showed a high rate discharge capability despite its low ionic conductivity (0.21 mS cm-1) at room temperature.