Understanding the Anomalous Diffusion of Water in Aqueous Electrolytes Using Machine Learned Potentials.

The diffusivity of water in aqueous cesium iodide solutions is larger than that in neat liquid water and vice versa for sodium chloride solutions. Such peculiar ion-specific behavior, called anomalous diffusion, is not reproduced in typical force field based molecular dynamics (MD) simulations due to inadequate treatment of ion-water interactions. Herein, this hurdle is tackled by using machine learned atomic potentials (MLPs) trained on data from density functional theory calculations. MLP based atomistic MD simulations of aqueous salt solutions reproduce experimentally determined thermodynamic, structural, dynamical, and transport properties, including their varied trends in water diffusivities across salt concentration. This enables an examination of their intermolecular structure to unravel the microscopic underpinnings of the differences in their transport properties. While both ions in CsI solutions contribute to the faster diffusion of water molecules, the competition between the heavy retardation by Na ions and the slight acceleration by Cl ions in NaCl solutions reduces their water diffusivity.

Efficient Parametrization of Force Field for the Quantitative Prediction of the Physical Properties of Ionic Liquid Electrolytes.

The prediction of transport properties of room-temperature ionic liquids from nonpolarizable force field-based simulations has long been a challenge. The uniform charge scaling method has been widely used to improve the agreement with the experiment by incorporating the polarizability and charge transfer effects in an effective manner. While this method improves the performance of the force fields, this prescription is ad hoc in character; further, a quantitative prediction is still not guaranteed. In such cases, the nonbonded interaction parameters too need to be refined, which requires significant effort. In this work, we propose a three-step semiautomated refinement procedure based on (1) atomic site charges obtained from quantum calculations of the bulk condensed phase; (2) quenched Monte Carlo optimizer to shortlist suitable force field candidates, which are then tested using pilot simulations; and (3) manual refinement to further improve the accuracy of the force field. The strategy is designed in a sequential manner with each step improving the accuracy over the previous step, allowing the users to invest the effort commensurate with the desired accuracy of the refined force field. The refinement procedure is applied on N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI), a front-runner as an electrolyte for electric double-layer capacitors and single-molecule-based devices. The transferability of the refined force field is tested on N,N-dimethyl-N-ethyl-N-methoxyethoxyethylammonium bis(trifluoromethanesulfonyl)imide (N112,2O2O1-TFSI). The refined force field is found to be better at predicting both structural and transport properties compared to the uniform charge scaling procedure, which showed a discrepancy in the X-ray structure factor. The refined force field showed quantitative agreement with structural (density and X-ray structure factor) and transport properties-diffusion coefficients, ionic conductivity, and shear viscosity over a wide temperature range, building a case for the wide adoption of the procedure.

Refined Force Field for Liquid Sulfolane with Particular Emphasis to Its Transport Characteristics.

An all-atom force field dedicated to capturing the properties of multifunctional sulfolane is necessary. In addition to being an excellent solvent and extractor, sulfolane is also a frequently investigated component for battery electrolytes in recent times. Given this, theoretically capturing its transport properties is essential. However, given the rather high shear viscosity of liquid sulfolane and its polar aprotic nature, formulating an appropriate non-polarizable force field for this compound remains a challenge. Starting from a generic force field, we report a refined force field for sulfolane which quantitatively captures its bulk properties, resulting in significantly improved estimates for self-diffusion constant and shear viscosity of sulfolane in comparison to force fields reported hitherto. Density, self-diffusion constant, and shear viscosity were determined between temperatures (303 and 398) K and at 1 bar pressure. All properties determined from the refined force field are in good agreement with experiments. The refined model employs atomic site charges obtained from the density-derived electrostatic and chemical (DDEC6) method for liquid sulfolane modeled using quantum density functional theory. Lennard-Jones parameters were refined using quantum potential energy scans. Despite possessing a large dipole moment, the large molecular size of sulfolane partially disrupts intermolecular dipolar ordering in liquid sulfolane. Molecular dipoles of near neighbor sulfolane, however, retain a partial preference for antiparallel orientation even at the highest temperatures investigated here.

Hopping in High Concentration Electrolytes - Long Time Bulk and Single-Particle Signatures, Free Energy Barriers, and Structural Insights.

Although ion-hopping is believed to be a significant mode of transport for small ions in liquid high concentration electrolytes (HCE), its bulk signatures over sufficiently long time intervals are yet to be shown. We computationally establish the long and short time imprints of hopping in HCEs using LiBF4-in-sulfolane mixtures as models. The high viscosity of this electrolyte leads to significant dynamic heterogeneity in Li-ion transport. Li-ions exhibit a preference to transit to previously occupied Li-ion-sites, bridged through anion or solvent molecules. Hopping in the liquid matrix was found to be an activated process, whose free energy barrier and transition state structure have been determined. Evidence for nanoscale compositional heterogeneity at high salt concentrations is also presented. The simulations shed light on the composition, stiffness, and lifetime of the solvation shell of Li ions. The understanding of HCEs gleaned from this study will spearhead the choice, engineering and applicability of this class of electrolytes.

Insights into the Stabilization of Fluoride Ions in Ionic Liquids: Pointers to Better Fluorinating Agents.

The fluorination efficiency of a fluorinating agent depends on the free availability of the fluoride ions, which in turn depends on its interaction with its solvation shell. A stable fluoride-based poor solvate ionic liquid (SIL) comprising 1-ethyl-3-methylimidazolium (EMIM) cation and ethylene glycol (EG) was recently reported and demonstrated as a fluorinating agent. Herein, we performed ab initio calculations and ab initio molecular dynamics simulations to gain a microscopic understanding of the intermolecular interactions in this SIL in gas, liquid, and crystalline phases. Ethylene glycol (EG), being capable of forming hydrogen bond(s) with the fluoride ion, prevents the latter from reacting with the EMIM cation. Fluoride forms hydrogen bonds with both the cation and the EG molecule, but it was found to have more affinity toward EG, forming a stronger hydrogen bond with its hydroxyl proton than with the acidic proton of the cation. An optimal concentration of EG in the SIL balances its contribution to stabilizing the fluoride ion and yet making fluoride available for fluorination.

Charge Environment and Hydrogen Bond Dynamics in Binary Ionic Liquid Mixtures: A Computational Study.

The extent of charge transfer between the cation and the anion in a room-temperature ionic liquid depends on the basicity of the anion. Ion charges determined in the condensed state via density functional theory calculations capture this effect rather well, and charges derived in such a manner have been employed in force field-based molecular dynamics simulations to quantitatively reproduce several physical properties of the liquids. However, the issue of transferability of cation charges in mixtures of ionic liquids, say with one type of cation and two different anion types needs to be addressed. Herein, we demonstrate that the cation charge in such a mixture varies linearly with anion composition, a result that ties in rather well with X-ray photoelectron spectroscopic experiments. The variation in cation charge with bulk anion composition is shown to be a result of changes in its coordination environment. Cations surrounded by a higher proportion of more basic anions possess lower charges than those surrounded by less basic anions. Time scales for the exchange of anion types for the occupation of hydrogen bonding sites around the cation have been determined and are seen to be constituted by three processes-breakage of existing hydrogen bond, diffusion to the hydrogen bonding site and displacement of the incumbent anion from its site in the cation coordination shell. These time scales explain the differences observed between infrared and NMR spectroscopic experiments in ionic liquid mixtures rather well.