Proper Thermal Equilibration of Simulations with Drude Polarizable Models: Temperature-Grouped Dual-Nosé-Hoover Thermostat

KUTE: Green-Kubo Uncertainty-Based Transport Coefficient Estimator

An algorithm for the calculation of transport properties from molecular dynamics simulations, kute, is introduced. The method estimates the integrals from the Green-Kubo theorem, taking into account the uncertainties of the correlation functions in order to eliminate arbitrary cutoffs or external parameters whose values could alter the result. In this contribution, the performance of kute is tested against other popular methods for the case of a protic ionic liquid for a variety of transport properties. It is found that kute achieves the same degree of accuracy as the equivalent formulation of the Einstein relations while performing better than other methods to calculate transport properties using Green-Kubo methods.

Lessons Learned on Obtaining Reliable Dynamic Properties for Ionic Liquids

Ionic liquids are nowadays investigated with respect to their use as electrolytes for high-performance energy storage materials. In this study, we provide a tutorial on how to calculate dynamic properties such as self-diffusion coefficients, ionic conductivities, transference numbers, as well as ion pair and ion cage dynamics, that all play a role in judging the applicability of ionic liquids as electrolytes. For the case of the ionic liquid[ C 2 C 1 Im ] [ NTf 2 ] ${[{\rm{C}}_2 {\rm{C}}_1 {\rm{Im}}][{\rm{NTf}}_2 ]}$ , we investigate the performance of different force fields. Amongst them are non-polarizable models employing unity charges, a charge-scaled version of a non-polarizable model, a polarizable model and another non-polarizable model with refined Lennard-Jones parameters. We also study the influence of the system size on the dynamic properties. While all studied force field models capture qualitatively correct trends, only the polarizable force field and the non-polarizable force field with refined Lennard-Jones parameters provide quantitative agreement to reference data, making the latter model very attractive for the reason of lower computational costs.

Vehicular Motions Dominate the Ion Transport in Concentrated LiTFSI Aqueous Solutions?

Water-in-salt electrolytes (WiSEs) show great promise for applications in grid-scale energy storage. The design of high-performance WiSEs requires a comprehensive understanding of their microstructures and ion transport properties. In the present work, based on the CL&Pol force field, we have developed a polarizable force field (PFF) tailored for high-concentration LiTFSI aqueous solutions, which accurately reproduces the structural and dynamical properties. Unlike the literature, we do not observe the presence of bulk-like water in LiTFSI solutions exceeding 19 mol/kg. Furthermore, we find that the vast majority of Li(H20)n+ are short-lived, and thus, the structural motion rather than the vehicular motion is the main mode of ion transport. Our results have significant implications for understanding the ion dynamics in WiSEs. Additionally, further in-depth experimental analyses are imperative.

GAFF-Based Polarizable Force Field Development and Validation for Ionic Liquids

Ionic liquids (ILs) have been used in many applications, including gas separations, electrochemistry, lubrication, and catalysis. Understanding how the different properties of ILs are related to their chemical structure and composition is crucial for these applications. Experimental investigations often provide limited insights and can be tedious in exploring a range of state points. Therefore, molecular simulations have emerged as a powerful tool that not only offers a microscopic perspective but also enables rapid screening and prediction of physical properties. The accuracy of these predictions, however, depends on the quality of the intermolecular potentials (force fields) used. The widely used classical fixed charge models, such as GAFF, OPLS, and CL&P, are popular due to their simplicity and computational efficiency. However, it has been shown that the use of integer charges with these classical models leads to sluggish dynamics. The use of scaled charge models can improve the dynamics, but these mean-field approaches are unable to account for polarization effects explicitly. Several different approaches have been proposed to include polarizability in IL force fields. In this work, we follow the protocol of the CL&Pol model to develop a Drude oscillator model based on the GAFF force field (Goloviznina, K., et al. J. Chem. Theory Comput. 2019, 15, 5858). We compare the performance of the model for eight imidazolium- and pyrrolidinium-based ILs against that of other models. We find that the new model provides reasonable estimations of density, self-diffusivity, and structural properties for these ILs and suggests a relatively simple way of extending the general GAFF model to more ILs.

Long-Lived Weak Ion Pairs in Ionic Liquids: An Insight from All-Atom Molecular Dynamics Simulations

The microstructure and local dynamics of ions in room-temperature ionic liquids (RTILs) have drawn a lot of attention due to their extensive potential applications in numerous fields. It is well-known that the widely used definitions of ion pairs (IPs) cannot reflect the full picture of RTILs. In this study, we find a universal residence time (τ_MR), which is regardless of the number of counterions in the first solvation shell in RTILs. Inspired by this, we propose a weak IP (WIP) model from a spatiotemporal perspective and demonstrate that the WIPs are long-lived and that their lifetimes obey a log-normal distribution, which is different from the literature. In addition, the electrostatic interactions are the main factors in the formation of WIPs, and the reorientations of ions are vital to the ruptures of WIPs. This research provides a new perspective for understanding the microstructural and dynamical properties of RTILs.

Symmetrized Drude Oscillator Force Fields Improve Numerical Performance of Polarizable Molecular Dynamics

Drude oscillator potentials are a popular and computationally efficient class of polarizable models that represent each polarizable atom as a positively charged Drude core harmonically bound to a negatively charged Drude shell. We show that existing force fields that place all non-Coulomb forces on the Drude core and none on the shell inadvertently couple the dipole to non-Coulombic forces. This introduces errors where interactions with neutral particles can erroneously induce atomic polarization, leading to spurious polarizations in the absence of an electric field, exacerbating violations of equipartition in the employed Carr-Parinello scheme. A suitable symmetrization of the interaction potential that correctly splits the force between the Drude core and shell can correct this shortcoming, improving the stability and numerical performance of Drude oscillator-based simulations. The symmetrization procedure is straightforward and only requires the rescaling of a few force field parameters.

Solvation Environments in Porous Ionic Liquids Determine Selectivity in CO2 Conversion to Cyclic Carbonates

Porous ionic liquids, which are suspensions of nanoporous particles in ionic liquids that maintain permanent porosity, are effective and selective media for the conversion of styrene oxide into styrene carbonate, absorbing CO₂ [Zhou et al. Chem. Commun. 2021, 57, 7922-7925]. Here we elucidate the mechanism of selectivity using polarizable molecular dynamics simulations, which provide a detailed view on the structure of the porous ionic liquid and on the local solvation environments of the reacting species. The porous ionic liquids studied are composed of tetradecyltrihexylphosphonium chloride, or [P₆₆₆₁₄]Cl, and the ZIF-8 zinc-methylimidazolate metal-organic framework (MOF). The CL&Pol polarizable force field was extended to represent epoxide and cyclic carbonate functional groups, allowing the ionic liquid, the reactants, and the MOF to be all represented by fully flexible, polarizable force fields, providing a detailed description of interactions. The presence of reactant and product molecules leads to changes in the structure of the ionic liquid, revealed by domain analysis. The structure of local solvation environments, namely, the arrangement of charged moieties and CO₂ around the epoxide ring of the reactant molecules, clearly indicate ring-opening the reaction mechanism. The MOF acts as a reservoir of CO₂ through its free volume. The solute molecules are found in the accessible outer cavities of the MOF, which promotes reaction of the epoxide with CO₂ excluding other epoxide molecules, thereby preventing the formation of oligomers, which explains the selectivity toward conversion to cyclic carbonates.

Structural elucidation of polydopamine facilitated by ionic liquid solvation

Minimal understanding of the formation mechanism and structure of polydopamine (pDA) and its natural analogue, eumelanin, impedes the practical application of these versatile polymers and limits our knowledge of the origin of melanoma. The lack of conclusive structural evidence stems from the insolubility of these materials, which has spawned significantly diverse suggestions of pDA's structure in the literature. We discovered that pDA is soluble in certain ionic liquids. Using these ionic liquids (ILs) as solvents, we present an experimental methodology to solvate pDA, enabling us to identify pDA's chemical structure. The resolved pDA structure consists of self-assembled supramolecular aggregates that contribute to the increasing complexity of the polymer. The underlying molecular energetics of pDA solvation and a macroscopic picture of the disruption of the aggregates using IL solvents have been investigated, along with studies of the aggregation mechanism in water.

Protex-A Python utility for proton exchange in molecular dynamics simulations

Protex is an open-source program that enables proton exchanges of solvent molecules during molecular dynamics simulations. While conventional molecular dynamics simulations do not allow for bond breaking or formation, protex offers an easy-to-use interface to augment these simulations and define multiple proton sites for (de-)protonation using a single topology approach with two different λ-states. Protex was successfully applied to a protic ionic liquid system, where each molecule is prone to (de-)protonation. Transport properties were calculated and compared to experimental values and simulations without proton exchange.

How Does Electronic Polarizability or Scaled-Charge Affect the Interfacial Properties of Room Temperature Ionic Liquids?

The room temperature ionic liquid (RTIL) air-liquid interface plays an important role in many applications. Herein, we present molecular dynamics simulation results for the air-liquid interface of a common RTIL, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, [C4mim][NTf2]. To elucidate the effects of electronic polarizability and scaled-charge ions on the properties of the RTIL air-liquid interface, we employ three different kinds of force fields: a nonpolarizable force field (FF) with united ion charges (FixQ), a nonpolarizable FF with scaled-charge by 0.8 (ScaleQ), and a polarizable FF (Drude). To identify whether the ions reside at the interface or not, the method of identification of the truly interfacial molecules is used. The structural and dynamical properties in the interfacial, subinterfacial, and central layers are evaluated. In general for bulk liquids, the FixQ model predicts too-ordered structures and too-sluggish dynamics, while the ScaleQ model can serve as a simple cure. However, the ScaleQ model cannot reproduce the results of the Drude model at the interface, due to an inappropriate scaled-down charge near the interface.

Molecular Dynamics Simulation Study of the Far-Infrared Spectrum of a Deep Eutectic Solvent

Deep eutectic solvents (DESs) are similar to ionic liquids (IL) in terms of physicochemical properties and technical uses. In ILs, far-infrared (FIR) spectroscopy has been utilized to reveal ionic interactions and even to produce a signature of the strengthening of the cation-anion hydrogen bond. However, for the situation of the DES, where the mixing of a salt and a molecular species makes the interplay between multiple intermolecular interactions even more complex, a full investigation of FIR spectra is still absent. In this work, the FIR spectrum of the DES, often referred to as ethaline, which is a 1:2 mixture of choline chloride and ethylene glycol, is calculated using classical molecular dynamics (MD) simulations and compared to experimental data. To explore the induced dipole effect on the computed FIR spectrum, MD simulations were run with both nonpolarizable and polarizable models. The calculation satisfactorily reproduces the position of the peak at ∼110 cm^-1 and the bandwidth seen in the experimental FIR spectrum of ethaline. The MD simulations show that the charge current is the most important contributor to the FIR spectrum, but the cross-correlation between the charge current and dipole reorientation also plays a role in the polarizable model. The dynamics of the chloride-ethylene glycol correlation span a wide frequency range, with a maximum at ∼150 cm^-1, but it participates as a direct mechanism only in the charge current-dipole reorientation cross-term. Anion correlations, whose dynamics are regulated via correlation with both ethylene glycol and choline, make the most significant contribution to the charge current mechanism. The MD simulations were also utilized to investigate the effect on the FIR spectrum of adding water to the DES and switching to a 1:1 composition.

Polarizable molecular dynamics simulations on the conductivity of pure 1-methylimidazolium acetate systems

The protic ionic liquid 1-methylimidazolium acetate is in equilibrium with its neutral species 1-methylimidazole and acetic acid. Although several experimental data indicate that the equilibrium favors the neutral species, the system exhibits a significant conductivity. We developed a polarizable force field to describe the ionic liquid accurately and applied it to several mixtures of the neutral and charged species. In addition to comparing single values, such as density, diffusion coefficients, and conductivity, with experimental data, the complete frequency-dependent dielectric spectrum ranging from several MHz to THz can be used to determine the equilibrium composition of the reaction mentioned above.

Recent Developments in Polarizable Molecular Dynamics Simulations of Electrolyte Solutions

Polarizable molecular dynamics simulations are a fast progressing field in the scientific research of ionic liquids. The fundamentals of polarizable simulations, as well as their application to ionic liquids, were summarized in a review [Bedrov, D.; Piquemal, J.-P.; Borodin, O.; MacKerell, Jr., A. D.; Roux, B.; Schröder, C. Molecular Dynamics Simulations of Ionic Liquids and Electrolytes Using Polarizable Force Fields. Chem. Rev. 2019, 119, 7940–7995] in 2019. Since then, new methods to treat intermolecular interaction of induced dipoles in these highly charged systems were developed. This concerns the damping of these interactions and additional charge transfer as well as the prediction of ionic materials with ultrahigh refractive indices. In addition to the progress of the polarizable force fields, also thermostats and barostats for polarizable simulations evolved recently.

Effect of the cation structure on the properties of homobaric imidazolium ionic liquids

In this work we investigate the structure-property relationships in a series of alkylimidazolium ionic liquids with almost identical molecular weight. Using a combination of theoretical calculations and experimental measurements, we have shown that re-arranging the alkyl side chain or adding functional groups results in quite distinct features in the resultant ILs. The synthesised ILs, although structurally very similar, cover a wide spectrum of properties ranging from highly fluid, glass forming liquids to high melting point crystalline salts. Theoretical ab initio calculations provide insight on minimum energy orientations for the cations, which then are compared to experimental X-ray crystallography measurements to extract information on hydrogen bonding and to verify our understanding of the studied structures. Molecular dynamics simulations of the simplest (core) ionic liquids are used in order to help us interpret our experimental results and understand better why methylation of C² position of the imidazolium ring results in ILs with such different properties compared to their non-methylated analogues.

Does Explicit Polarizability Improve Molecular Dynamics Predictions of Glass Transition Temperatures of Ionic Liquids?

Molecular dynamics simulations are used for predictions of the glass transition temperatures for a test set of five aprotic ionic liquids. Glass transitions are localized with the trend-shift method, analyzing volumetric and transport properties of bulk amorphous phases. A classical nonpolarizable all-atom OPLS force-field model developed by Canongia Lopes and Pádua (CL&P) is employed as a starting level of theory for all calculations. Alternative approaches of charge scaling and the Drude oscillator model, accounting for atomic polarizability either implicitly or explicitly, respectively, are used to investigate the sensitivity of the glass transition temperatures to induction effects. The former nonpolarizable model overestimates the glass transition temperature by tens of Kelvins (37 K on average). The charge-scaling technique yields a significant improvement, and the best estimations were achieved using polarizable simulations with the Drude model, which yielded an average deviation of 11 K. Although the volumetric data usually exhibit a lesser trend shift upon vitrification, their lower statistical uncertainty enables to predict the glass transition temperature with lower uncertainty than the ionic self-diffusivities, the temperature dependence of which is usually more scattered. Additional analyses of the simulated data were also performed, revealing that the Drude model predicts lower densities for most subcooled liquids but higher densities for the glasses than the original CL&P, and that the Drude model also invokes some longer-range organization of the subcooled liquid, greatly impacting the temperature trend of ionic self-diffusivities in the low-temperature region.

Charge transfer and polarisability in ionic liquids: a case study

The practical use of ionic liquids (ILs) is benefiting from a growing understanding of the underpinning structural and dynamic properties, facilitated through classical molecular dynamics (MD) simulations. The predictive and explanatory power of a classical MD simulation is inextricably linked to the underlying force field. A key aspect of the forcefield for ILs is the ability to recover charge based interactions. Our focus in this paper is on the description and recovery of charge transfer and polarisability effects, demonstrated through MD simulations of the widely used 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C₄C₁im][NTf₂] IL. We study the charge distributions generated by a range of ab initio methods, and present an interpolation method for determining atom-wise scaled partial charges. Two novel methods for determining the mean field (total) charge transfer from anion to cation are presented. The impact of using different charge models and different partial charge scaling (unscaled, uniformly scaled, atom-wise scaled) are compared to fully polarisable simulations. We study a range of Drude particle explicitly polarisable potentials and shed light on the performance of current approaches to counter known problems. It is demonstrated that small changes in the charge description and MD methodology can have a significant impact; biasing some properties, while leaving others unaffected within the structural and dynamic domains.

Fine-Tuning the Polarizable CL&Pol Force Field for the Deep Eutectic Solvent Ethaline

Polarizable force fields are gradually becoming a common choice for ionic soft matter, in particular, for molecular dynamics (MD) simulations of ionic liquids (ILs) and deep eutectic solvents (DESs). The CL&Pol force field introduced in 2019 is the first general, transferable, and polarizable force field for MD simulations of different types of DESs. The original formulation contains, however, some problems that appear in simulations of ethaline and may also have a broader impact. First, the originally proposed atomic diameter parameters are unbalanced, resulting in too weak interactions between the chlorides and the hydroxyl groups of the ethylene glycol molecules. This, in turn, causes an artificial phase separation in long simulations. Second, there is an overpolarization of chlorides due to strong induced dipoles that give rise to the presence of peaks and antipeaks at very low q-vector values (2.4 nm^-1) in the partial components of the structure factors. In physical terms, this is equivalent to overestimated spatial nanoscale heterogeneity. To correct these problems, we adjusted the chloride-hydroxyl radial distribution functions against ab initio data and then extended the use of the Tang-Toennis damping function for the chlorides' induced dipoles. These adjustments correct the problems without losing the robustness of the CL&Pol force field. The results were also compared with the nonpolarizable version, the CL&P force field. We expect that the corrections will facilitate reliable use of the CL&Pol force field for other types of DESs.

Does Explicit Polarizability Improve Simulations of Phase Behavior of Ionic Liquids?

Molecular dynamics simulations are performed for a test set of 20 aprotic ionic liquids to investigate whether including an explicit polarizability model in the force field leads to higher accuracy and reliability of the calculated phase behavior properties, especially the enthalpy of fusion. A classical nonpolarizable all-atom optimized potentials for liquid simulations (OPLS) force-field model developed by Canongia Lopes and Pádua (CL&P) serves as a reference level of theory. Polarizability is included either in the form of Drude oscillators, resulting in the CL&P-D models, or in the framework of the atomic multipole optimized energetics for biomolecular application (AMOEBA) force field with polarizable atomic sites. Benchmarking of the calculated fusion enthalpy values against the experimental data reveals that overall the nonpolarizable CL&P model and polarizable CL&P-D models perform similarly with average deviations of about 30%. However, fusion enthalpies from the CL&P-D models exhibit a stronger correlation with their experimental counterparts. The least successful predictions are interestingly obtained from AMOEBA (deviation ca. 60%), which may indicate that a reparametrization of this force-field model is needed to achieve improved predictions of the fusion enthalpy. In general, all FF models tend to underestimate the fusion enthalpies. In addition, quantum chemical calculations are used to compute the electronic cohesive energies of the crystalline phases of the ionic liquids and of the interaction energies within the ion pair. Significant positive correlations are found between the fusion enthalpy and the cohesive energies. The character of the present anions predetermines the magnitude of individual mechanistic components of the interaction energy and related enthalpic and cohesive properties.

Deep Eutectic Solvents: Molecular Simulations with a First-Principles Polarizable Force Field

The unique properties of deep eutectic solvents make them useful in a variety of applications. In this work we develop a first-principles force field for reline, which is composed of choline chloride and urea in the molar ratio 1:2. We start with the symmetry adapted perturbation theory (SAPT) protocol and then make adjustments to better reproduce the structure and dynamics of the liquid when compared to first-principles molecular dynamics (FPMD) simulations. The resulting force field is in good agreement with experiments in addition to being consistent with the FPMD simulations. The simulations show that primitive molecular clusters are preferentially formed with choline-chloride ionic pairs bound with a hydrogen bond in the hydroxyl group and that urea molecules coordinate the chloride mainly via the trans-H chelating hydrogen bonds. Incorporating polarizability qualitatively influences the radial distributions and lifetimes of hydrogen bonds and affects long-range structural order and dynamics. The polarizable force field predicts a diffusion constant about an order of magnitude larger than the nonpolarizable force field and is therefore less computationally intensive. We hope this study paves the way for studying complex hydrogen-bonding liquids from a first-principles approach.

Effect of side chain modifications in imidazolium ionic liquids on the properties of the electrical double layer at a molybdenum disulfide electrode

Knowledge of how the molecular structures of ionic liquids (ILs) affect their properties at electrified interfaces is key to the rational design of ILs for electric applications. Polarizable molecular dynamics simulations were performed to investigate the structural, electrical, and dynamic properties of electric double layers (EDLs) formed by imidazolium dicyanamide ([ImX1][DCA]) at the interface with the molybdenum disulfide electrode. The effect of side chain of imidazolium on the properties of EDLs was analyzed by using 1-ethyl-3-methylimidazolium ([Im21]), 1-octyl-3-methylimidazolium ([Im81]), 1-benzyl-3-methylimidazolium ([ImB1]), and 1-(2-hydroxyethyl)-3-methylimidazolium ([ImO1]) as cations. Using [Im21] as reference, we find that the introduction of octyl or benzyl groups significantly alters the interfacial structures near the cathode because of the reorientation of cations. For [Im81], the positive charge on the cathode induces pronounced polar and non-polar domain separation. In contrast, the hydroxyl group has a minor effect on the interfacial structures. [ImB1] is shown to deliver slightly larger capacitance than other ILs even though it has larger molecular volume than [Im21]. This is attributed to the limiting factor for capacitance being the strong association between counter-ions, instead of the free space available to ions at the interface. For [Im81], the charging mechanism is mainly the exchange between anions and octyl tails, while for the other ILs, the mechanism is mainly the exchange of counter-ions. Analysis on the charging process shows that the charging speed does not correlate strongly with macroscopic bulk dynamics like viscosity. Instead, it is dominated by local displacement and reorientation of ions.

Extension of the CL&Pol Polarizable Force Field to Electrolytes, Protic Ionic Liquids, and Deep Eutectic Solvents

The polarizable CL&Pol force field presented in our previous study, Transferable, Polarizable Force Field for Ionic Liquids (J. Chem. Theory Comput. 2019, 15, 5858, DOI: http://doi.org/10.1021/acs.jctc.9b0068910.1021/acs.jctc.9b00689), is extended to electrolytes, protic ionic liquids (PIL), deep eutectic solvents (DES), and glycols. These systems are problematic in polarizable simulations because they contain either small, highly charged ions or strong hydrogen bonds, which cause trajectory instabilities due to the pull exerted on the induced dipoles. We use a Tang-Toennies (TT) function to dampen, or smear, the interactions between charges and induced dipole at a short range involving small, highly charged atoms (such as hydrogen or lithium), thus preventing the "polarization catastrophe". The new force field gives stable trajectories and is validated through comparison with experimental data on density, viscosity, and ion diffusion coefficients of liquid systems of the above-mentioned classes. The results also shed light on the hydrogen-bonding pattern in ethylammonium nitrate, a PIL, for which the literature contains conflicting views. We describe the implementation of the TT damping function, of the temperature-grouped Nosé-Hoover thermostat for polarizable molecular dynamics (MD) and of the periodic perturbation method for viscosity evaluation from non-equilibrium trajectories in the LAMMPS MD code. The main result of this work is the wider applicability of the CL&Pol polarizable force field to new, important classes of fluids, achieving robust trajectories and a good description of equilibrium and transport properties in challenging systems. The fragment-based approach of CL&Pol will allow ready extension to a wide variety of PILs, DES, and electrolytes.

Statistical mechanics of polarizable force fields based on classical Drude oscillators with dynamical propagation by the dual-thermostat extended Lagrangian

Polarizable force fields based on classical Drude oscillators offer a practical and computationally efficient avenue to carry out molecular dynamics (MD) simulations of large biomolecular systems. To treat the polarizable electronic degrees of freedom, the Drude model introduces a virtual charged particle that is attached to its parent nucleus via a harmonic spring. Traditionally, the need to relax the electronic degrees of freedom for each fixed set of nuclear coordinates is achieved by performing an iterative self-consistent field (SCF) calculation to satisfy a selected tolerance. This is a computationally demanding procedure that can increase the computational cost of MD simulations by nearly one order of magnitude. To avoid the costly SCF procedure, a small mass is assigned to the Drude particles, which are then propagated as dynamic variables during the simulations via a dual-thermostat extended Lagrangian algorithm. To help clarify the significance of the dual-thermostat extended Lagrangian propagation in the context of the polarizable force field based on classical Drude oscillators, the statistical mechanics of a dual-temperature canonical ensemble is formulated. The conditions for dynamically maintaining the dual-temperature properties in the case of the classical Drude oscillator are analyzed using the generalized Langevin equation.

Ion transport in small-molecule and polymer electrolytes

Solid-state polymer electrolytes and high-concentration liquid electrolytes, such as water-in-salt electrolytes and ionic liquids, are emerging materials to replace the flammable organic electrolytes widely used in industrial lithium-ion batteries. Extensive efforts have been made to understand the ion transport mechanisms and optimize the ion transport properties. This perspective reviews the current understanding of the ion transport and polymer dynamics in liquid and polymer electrolytes, comparing the similarities and differences in the two types of electrolytes. Combining recent experimental and theoretical findings, we attempt to connect and explain ion transport mechanisms in different types of small-molecule and polymer electrolytes from a theoretical perspective, linking the macroscopic transport coefficients to the microscopic, molecular properties such as the solvation environment of the ions, salt concentration, solvent/polymer molecular weight, ion pairing, and correlated ion motion. We emphasize universal features in the ion transport and polymer dynamics by highlighting the relevant time and length scales. Several outstanding questions and anticipated developments for electrolyte design are discussed, including the negative transference number, control of ion transport through precision synthesis, and development of predictive multiscale modeling approaches.

Sodium diffusion in ionic liquid-based electrolytes for Na-ion batteries: the effect of polarizable force fields

Understanding the transport of sodium ions in ionic liquids is the key to design novel electrolyte materials for sodium-ion batteries.