Molecular Dynamics Investigation of Correlations in Ion Transport in MeTFSI/EMIM–TFSI (Me = Li, Na) Electrolytes

Electric field induced associations in the double layer of salt-in-ionic-liquid electrolytes

Ionic liquids (ILs) are an extremely exciting class of electrolytes for energy storage applications. Upon dissolving alkali metal salts, such as Li or Na based salts, with the same anion as the IL, an intrinsically asymmetric electrolyte can be created for use in batteries, known as a salt-in-ionic liquid (SiIL). These SiILs have been well studied in the bulk, where negative transference numbers of the alkali metal cation have been observed from the formation of small, negatively charged clusters. The properties of these SiILs at electrified interfaces, however, have received little to no attention. Here, we develop a theory for the electrical double layer (EDL) of SiILs where we consistently account for the thermoreversible association of ions into Cayley tree aggregates. The theory predicts that the IL cations first populate the EDL at negative voltages, as they are not strongly bound to the anions. However, at large negative voltages, which are strong enough to break the alkali metal cation-anion associations, these IL cations are exchanged for the alkali metal cation because of their higher charge density. At positive voltages, we find that the SiIL actually becomes more aggregated while screening the electrode charge from the formation of large, negatively charged aggregates. Therefore, in contrast to conventional intuition of associations in the EDL, SiILs appear to become more associated in certain electric fields. We present these theoretical predictions to be verified by molecular dynamics simulations and experimental measurements.

Exploring the capabilities and limitations of the Van Hove function to understand directional correlations in ion movements within Li-ion battery electrolytes

Understanding microscopic directional correlations in ion movements within lithium-ion battery (LIB) electrolytes is important because these correlations directly affect the ionic conductivity. Onsager transport coefficients are widely used to understand these correlations. On the other hand, the Van Hove function (VHF) is also capable of determining correlated motions. However, identifying various types of ion correlated motions in LIB electrolytes using VHF is not well explored. Here, we have conducted molecular dynamics simulations of a representative experimental LIB electrolyte system-lithium hexafluorophosphate (LiPF6)-at different concentrations in a (9:1 wt. %) mixture of ethyl methyl carbonate and fluoroethylene carbonate in order to explore the capabilities and limitations of using VHF to understand different types of ion correlations. We conclude that analysis of VHF can qualitatively describe both the positive correlation between cation-anion at different salt concentrations and the negative correlation between cation-cation and anion-anion present in high salt concentration, but it cannot foretell which correlation is dominating at any given electrolyte concentration. This type of quantitative information can be obtained only via Onsager's approach. This could be seen as a limitation of relying solely on VHF to fully understand ion correlation in electrolyte media.

Diffusion and structure of propylene carbonate-metal salt electrolyte solutions for post-lithium-ion batteries: From experiment to simulation

The diffusion of cations in organic solvent solutions is important for the performance of metal-ion batteries. In this article, pulsed field gradient nuclear magnetic resonance experiments and fully atomistic molecular dynamic simulations were employed to study the temperature-dependent diffusive behavior of various liquid electrolytes representing 1M propylene carbonate solutions of metal salts with bis(trifluoromethylsulfonyl)imide (TFSI-) or hexafluorophosphate (PF6-) anions commonly used in lithium-ion batteries and beyond. The experimental studies revealed the temperature dependence of the diffusion coefficients for the propylene carbonate (PC) solvent and for the anions following an Arrhenius type of behavior. It was observed that the PC molecules are the faster species. For the monovalent cations (Li+, Na+, K+), the PC solvent diffusion was enhanced as the cation size increased, while for the divalent cations (Mg2+, Ca2+, Sr2+, Ba2+), the opposite trend was observed, i.e., the diffusion coefficients decreased as the cation size increased. The anion diffusion in LiTFSI and NaTFSI solutions was found to be similar, while in electrolytes with divalent cations, a decrease in anion diffusion with increasing cation size was observed. It was shown that non-polarizable charge-scaled force fields could correspond perfectly to the experimental values of the anion and PC solvent diffusion coefficients in salt solutions of both monovalent (Li+, Na+, K+) and divalent (Mg2+, Ca2+, Sr2+, Ba2+) cations at a range of operational temperatures. Finally, after calculating the radial distribution functions between cations, anions, and solvent molecules, the increase in the PC diffusion coefficient established with the increase in cation size for monovalent cations was clearly explained by the large hydration shell of small Li+ cations, due to their strong interaction with the PC solvent. In solutions with larger monovalent cations, such as Na+, and with a smaller solvation shell of PC, the PC diffusion is faster due to more liberated solvent molecules. In the salt solutions with divalent cations, both the anion and the PC diffusion coefficients decreased as the cation size increased due to an enhanced cation-anion coordination, which was accompanied by an increase in the amount of PC in the cation solvation shell due to the presence of anions.

Impact of Li, Na and Zn metal cation concentration in EMIM-TFSI ionic liquids on ion clustering, structure and dynamics

We use molecular dynamics calculations to investigate the behavior of metal cations (Li, Na and Zn) within ionic liquids (ILs), specifically EMIM-TFSI, and their impact on key properties, particularly focusing on ion-ion correlations and their influence on diffusion and conductivity. The study explores the competition between metal cations and EMIM ions for binding to TFSI and analyzes ion pair dynamics, revealing that metal cation-TFSI pairs exhibit significantly longer lifetimes compared to TFSI-EMIM pairs. This competitive interaction and the increased stability of metal cation-TFSI pairs at higher concentrations leads to reduced ion exchange, resulting in decreased diffusion and conductivity. The observations underscore the importance of ion size and charge in determining their behavior regarding IL dynamics. Overall, this work provides valuable insights for designing ILs with customized properties, particularly in the context of optimizing conductivity and addressing energy storage challenges.

Mechanistic Basis of Conductivity in Carbon Dioxide-Expanded Electrolytes: A Joint Experimental-Theoretical Study

Electrolyte conductivity contributes to the efficiency of devices for electrochemical conversion of carbon dioxide (CO2) into useful chemicals, but the effect of the dissolution of CO2 gas on conductivity has received little attention. Here, we report a joint experimental-theoretical study of the properties of acetonitrile-based CO2-expanded electrolytes (CXEs) that contain high concentrations of CO2 (up to 12 M), achieved by CO2 pressurization. Cyclic voltammetry data and paired simulations show that high concentrations of dissolved CO2 do not impede the kinetics of outer-sphere electron transfer but decrease the solution conductivity at higher pressures. In contrast with conventional behaviors, Jones reactor-based measurements of conductivity show a nonmonotonic dependence on CO2 pressure: a plateau region of constant conductivity up to ca. 4 M CO2 and a region showing reduced conductivity at higher [CO2]. Molecular dynamics simulations reveal that while the intrinsic ionic strength decreases as [CO2] increases, there is a concomitant increase in ionic mobility upon CO2 addition that contributes to stable solution conductivities up to 4 M CO2. Taken together, these results shed light on the mechanisms underpinning electrolyte conductivity in the presence of CO2 and reveal that the dissolution of CO2, although nonpolar by nature, can be leveraged to improve mass transport rates, a result of fundamental and practical significance that could impact the design of next-generation systems for CO2 conversion. Additionally, these results show that conditions in which ample CO2 is available at the electrode surface are achievable without sacrificing the conductivity needed to reach high electrocatalytic currents.

Effect of Zwitterionic Additives on Solvation and Transport of Sodium and Potassium Cations in (Ethylene Oxide)10: A Molecular Dynamics Simulation Study

Sodium- (Na+) and potassium- (K+) ion batteries are cost-effective alternatives to lithium-ion (Li+) batteries due to the abundant sodium and potassium resources. Solid polymer electrolytes (SPEs) are essential for safer and more efficient Na+ and K+ batteries because they often exhibit low ionic conductivity at room temperature. While zwitterionic (ZW) materials enhance Li+ battery conductivity, their potential for Na+ and K+ transport in batteries remains unexplored. In this study, we investigated the effect of three ZW molecules (ChoPO4, i.e., 2-methacryloyloxyethyl phosphorylcholine, ImSO3, i.e., sulfobetaine ethylimidazole, and ImCO2, i.e., carboxybetaine ethylimidazole) on the dissociation of Na+ and K+ coordination with ethylene oxide (EO) chains in EO-based electrolytes through molecular dynamics simulations. Our results showed that ChoPO4 possessed the highest cation-EO10 dissociation ability, while ImSO3 exhibited the lowest. Such dissociation ability correlated with the cation-ZW molecule coordination strength: ChoPO4 and ImSO3 showed the strongest and the weakest coordination with cations. However, the cation-ZW molecule coordination could slow the cationic diffusion. The competition of these effects resulted in accelerating or decelerating cationic diffusion. Our simulated results showed that ImCO2 enhanced Na+ diffusion by 20%, while ChoPO4 and ImSO3 led to a 10% reduction. For K+, ChoPO4 reduced its diffusion by 40%, while ImCO2 and ImSO3 caused a similar decrease of 15%. These findings suggest that the ZW structure and the cationic size play an important role in the ionic dissociation effect of ZW materials.

Molecular Dynamics Simulation on the Charge Transport Properties in a Salt-in-Ionic Liquid Electrolyte

A clear picture of charge transport properties in salt-in-ionic liquid electrolyte (SILE) is indispensable for the applications in lithium-ion batteries. In this study, we applied molecular dynamics (MD) simulations on a typical SILE system, composed of lithium bis(fluorosulfonyl)imide (LiFSI) with a molar fraction of 0.3 doped in 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIMFSI). Based on the MD simulations, we calculated conductivity spectra from 108 Hz to 1014 Hz, charge current correlation functions, and charge mean square displacements, based on the center-of-mass (COM) velocities of the ions. The conductivity spectra show a bimodal feature between 1012 Hz and 1013 Hz, attributed to the interionic vibrations of the EMIM+-FSI- and Li+-FSI- contact ion pairs, respectively. Structural relaxation is observed between 109 Hz and 1012 Hz, and a flat plateau below 109 Hz, attributed to the direct current (DC) conductivity. For this SILE composed of three constituent ions, i.e., Li+, EMIM+, and FSI-, the above transport properties are further partitioned to the contributions of the individual constituent ions, including self, distinct contribution of the same constituent ions, and also the cross correlation between them. Detailed analyses on the individual contributions reveal strongly correlated motions in this complex ionic system.

Polymer Architecture-Induced Trade-off between Conductivities and Transference Numbers in Salt-Doped Polymeric Ionic Liquids

Recent experiments have demonstrated that polymeric ionic liquids that share the same cation and anion but possess different architectures can exhibit markedly different conductivity and transference number characteristics when doped with lithium salt. In this study, we used atomistic molecular simulations on polymer chemistries inspired by the experiments to probe the mechanistic origins underlying the competition between conductivity and transference numbers. Our results indicate that the architecture of the polycationic ionic liquid plays a subtle but crucial role in modulating the anion-cation interactions, especially their dynamical coordination characteristics. Chemistries leading to longer-lived anion-cation coordinations relative to lithium-anion coordinations lead to lower conductivities and higher transference numbers. Our results suggest that higher conductivities are accompanied by lower transference numbers and vice versa, revealing that alternative approaches may need to be considered to break this trade-off in salt-doped polyILs.

High-Ionic-Conductivity Sodium-Based Ionic Gel Polymer Electrolyte for High-Performance and Ultrastable Microsupercapacitors

Due to the lower cost and greater natural abundance of the sodium element on the earth than those of the lithium element, sodium-based ionic gel polymer electrolytes (IGPEs) are becoming a more cost-effective and popular material choice for portable and stationary energy solutions. The sodium-based IGPEs, however, appeared relatively inferior to their lithium-based counterparts for use in high-performance microsupercapacitors in terms of ionic conductivity and electrochemical stability. To tackle these issues, poly(ethylene glycol) diacrylate (PEGDA) with fast polymerization to build a polymer matrix and sodium perchlorate (NaClO₄) with high chemical stability and high thermal stability are employed to generate free ions for an ionic conducting phase with the support of tetramethylene glycol ether (G4) and 1-ethyl-3-methylimidazolium bis(triflouromethylsulfonyl)imide (EMIM-TFSI). It was found that the ionic conductivity (σ_dc) of this sodium-based IGPE reaches up to 0.54 mS/cm at room temperature. To manifest a high-conductivity sodium-based IGPE (SIGPE), a microsupercapacitor (MSC) with an area of 5 mm² is designed and fabricated on an interdigital reduced graphene oxide electrode. This MSC demonstrates prominent performance with a high power density of ∼2500 W/kg and a maximum energy density of ∼0.7 Wh/kg. Furthermore, after 20,000 cycles at an operating potential window from 0.0 to 1.0 V, it retains approximately 98.9% capacitance. An MSC array in 3 series × 3 parallels (3S × 3P) was successfully designed as a power source for a basic circuit with an LED. Therefore, we believe that our sodium-based IGPE microsupercapacitor holds its promising role as a solid-state energy source for high-performance and high-stability energy solutions.

Theoretical Investigation of the Structure and Physicochemical Properties of Alkaline and Alkaline Earth Metal Perchlorate Solutions in Sulfolane

To assess the possibility of using solutions of perchlorates of alkali and alkaline earth metals in sulfolane as electrolytes for electrochemical energy storage devices with metal negative electrodes, the physicochemical properties of 0.5 M solutions of Me(ClO₄)_n (Me = Li, Na, K, Mg, and Ca) in sulfolane were simulated by the method of molecular dynamics. The density, viscosity, conductivity, self-diffusion coefficients, and transport numbers are calculated. Satisfactory agreement between the calculated and experimentally measured properties of 0.5 M solutions of LiClO₄ and NaClO₄ in sulfolane suggests that the calculated values of the physicochemical properties of solutions of K, Mg, and Ca perchlorates are also close to real values. The study of the structure of solvate complexes of salts of alkali and alkaline earth metals with sulfolane by quantum chemical and molecular dynamics modeling showed that the first solvate shell of metal cations consists of sulfolane molecules. Regardless of the nature of the cation, sulfolane is coordinated to the metal cation by only one oxygen atom. Based on the analysis of the calculated values of the physicochemical properties of solutions of metal perchlorates in sulfolane, it can be concluded that they can be used as electrolyte systems of electrochemical energy storage devices with negative electrodes made of alkali and alkaline earth metals.

Applying Classical, Ab Initio, and Machine-Learning Molecular Dynamics Simulations to the Liquid Electrolyte for Rechargeable Batteries

Rechargeable batteries have become indispensable implements in our daily life and are considered a promising technology to construct sustainable energy systems in the future. The liquid electrolyte is one of the most important parts of a battery and is extremely critical in stabilizing the electrode-electrolyte interfaces and constructing safe and long-life-span batteries. Tremendous efforts have been devoted to developing new electrolyte solvents, salts, additives, and recipes, where molecular dynamics (MD) simulations play an increasingly important role in exploring electrolyte structures, physicochemical properties such as ionic conductivity, and interfacial reaction mechanisms. This review affords an overview of applying MD simulations in the study of liquid electrolytes for rechargeable batteries. First, the fundamentals and recent theoretical progress in three-class MD simulations are summarized, including classical, ab initio, and machine-learning MD simulations (section 2). Next, the application of MD simulations to the exploration of liquid electrolytes, including probing bulk and interfacial structures (section 3), deriving macroscopic properties such as ionic conductivity and dielectric constant of electrolytes (section 4), and revealing the electrode-electrolyte interfacial reaction mechanisms (section 5), are sequentially presented. Finally, a general conclusion and an insightful perspective on current challenges and future directions in applying MD simulations to liquid electrolytes are provided. Machine-learning technologies are highlighted to figure out these challenging issues facing MD simulations and electrolyte research and promote the rational design of advanced electrolytes for next-generation rechargeable batteries.

Controlling Li+ transport in ionic liquid electrolytes through salt content and anion asymmetry: a mechanistic understanding gained from molecular dynamics simulations

In this work, we report the results from molecular dynamics simulations of lithium salt-ionic liquid electrolytes (ILEs) based either on the symmetric bis[(trifluoromethyl)sulfonyl]imide (TFSI^-) anion or its asymmetric analogue 2,2,2-(trifluoromethyl)sulfonyl-N-cyanoamide (TFSAM^-). Relating lithium's coordination environment to anion mean residence times and diffusion constants confirms the remarkable transport behaviour of the TFSAM^--based ILEs that has been observed in recent experiments: for increased salt doping, the lithium ions must compete for the more attractive cyano over oxygen coordination and a fragmented landscape of solvation geometries emerges, in which lithium appears to be less strongly bound. We present a novel, yet statistically straightforward methodology to quantify the extent to which lithium and its solvation shell are dynamically coupled. By means of a Lithium Coupling Factor (LCF) we demonstrate that the shell anions do not constitute a stable lithium vehicle, which suggests for this electrolyte material the commonly termed "vehicular" lithium transport mechanism could be more aptly pictured as a correlated, flow-like motion of lithium and its neighbourhood. Our analysis elucidates two separate causes why lithium and shell dynamics progressively decouple with higher salt content: on the one hand, an increased sharing of anions between lithium limits the achievable LCF of individual lithium-anion pairs. On the other hand, weaker binding configurations naturally entail a lower dynamic stability of the lithium-anion complex, which is particularly relevant for the TFSAM^--containing ILEs.

Insights into the intermolecular interactions and temperature-concentration dependence of transport in ionic liquid-based EMI–TFSI/LiTFSI electrolytes

DFT calculations and MD simulations show that the EMI–TFSI/LiTFSI system is stabilized by strong nonbonded attractions, and the lithium-ion conductivity depends on the LiTFSI concentration and system temperature.

Salt-in-Ionic-Liquid Electrolytes: Ion Network Formation and Negative Effective Charges of Alkali Metal Cations

Salt-in-ionic liquid electrolytes have attracted significant attention as potential electrolytes for next generation batteries largely due to their safety enhancements over typical organic electrolytes. However, recent experimental and computational studies have shown that under certain conditions alkali cations can migrate in electric fields as if they carried a net negative effective charge. In particular, alkali cations were observed to have negative transference numbers at small mole fractions of alkali-metal salt that revert to the expected net positive transference numbers at large mole fractions. Simulations have provided some insights into these observations, where the formation of asymmetric ionic clusters, as well as a percolating ion network, could largely explain the anomalous transport of alkali cations. However, a thermodynamic theory that captures such phenomena has not been developed, as ionic associations were typically treated via the formation of ion pairs. The theory presented herein, based on the classical polymer theories, describes thermoreversible associations between alkali cations and anions, where the formation of large, asymmetric ionic clusters and a percolating ionic network are a natural result of the theory. Furthermore, we present several general methods to calculate the effective charge of alkali cations in ionic liquids. We note that the negative effective charge is a robust prediction with respect to the parameters of the theory and that the formation of a percolating ionic network leads to the restoration of net positive charges of the cations at large mole fractions of alkali metal salt. Overall, we find excellent qualitative agreement between our theory and molecular simulations in terms of ionic cluster statistics and the effective charges of the alkali cations.

How Temperature, Pressure, and Salt Concentration Affect Correlations in LiTFSI/EMIM-TFSI Electrolytes: A Molecular Dynamics Study

Classical polarizable molecular dynamics simulations have been performed for LiTFSI solutions in the EMIM-TFSI ionic liquid. Different temperature or pressure values and salt concentrations have been examined. The structure and dynamics of the solvation shell of Li+ cations, diffusion coefficients of ions, conductivities of the electrolytes, and correlations between motions of ions have been analyzed. The results indicated that regardless of the conditions, significant correlations are present in all systems. The degree of correlations depends mainly on the salt fraction in the electrolyte and is much less affected by temperature and pressure changes. A positive correlation between motions of Li+ cations and TFSI anions, leading to the occurrence of negative Li+ transference numbers, exists for all conditions, although temperature and pressure changes affect the speed of anion exchange in Li+ solvation shells.

Spatial-Decomposition Analysis of Electrical Conductivity in Mixtures of Ionic Liquid and Sodium Salt for Sodium-Ion Battery Electrolytes

Ionic liquid (IL)-based electrolytes are a promising material for the development of sodium-ion batteries, and their performance can be quantified by electrical conductivity. In this highly concentrated ionic system, the correlated motions of ion pairs are influential on the ionic transport properties. Herein, all-atom analyses are conducted through molecular dynamics simulations to bridge the macroscopically observable electrical conductivity with the molecular pictures of correlated motion of ion pairs. The analysis is applied to three mixtures of IL with sodium salt that are relevant to the electrolyte for a sodium-ion battery: [1-ethyl-3-methylimidazolium, Na][bis(fluorosulfonyl)amide] ([C₂C₁im, Na][FSA]), [N-methyl-N-propylpyrrolidinium, Na][FSA] ([C₃C₁pyrr, Na][FSA]), and [K, Na][FSA]. The computational results on electrical conductivities are in agreement with the experimental reports, and their dependency on temperature and sodium-ion composition is reproduced well. The overall contributions from cross-correlated motions are found to be negative in all the IL mixtures; thus, the total conductivities are less than their Nernst-Einstein estimates. The spatial view of cross-correlated motions is further obtained by decomposing the time correlation functions of velocities according to the distances between ion pairs. It is observed that ion pairs are moving in the same direction for ∼0.3 ps when they were initially within the first coordination shell, followed by motions toward opposite directions. The cross-correlation terms are also dissected into local and nonlocal components, and it is commonly seen for all the ion pairs that the local component is negative for cation-anion pairs and is positive for cation-cation and anion-anion pairs. The motions of ion pairs are accompanied by a "backflow" that manifests in the form of the nonlocal component whose sign is opposite to the corresponding local component. In fact, the contributions of the correlated motions of ions to the electrical conductivity are not localized to contact pairs and extend spatially beyond the first coordination shell of the cation-anion pairs.

Estimates of Electrical Conductivity from Molecular Dynamics Simulations: How to Invest the Computational Effort

Although the electrical conductivity of an electrolyte can be estimated from the molecular dynamics trajectory, it is often a challenging task because of the need to obtain a substantial amount of data to ensure sufficient averaging. Here, we present an analysis on the convergence of results with the number of simulated trajectories. A series of molecular dynamics simulations have been performed for a model electrolyte (NaCl in water) and the Einstein relation has been used to calculate the electrical conductivity. The standard deviation of the conductivity estimates is relatively large compared to the mean value, and it has been shown that the off-diagonal contributions to the collective displacement of ions are responsible for large deviations between systems. It has been found that about 40 independent MD simulations may be required to reduce the errors. A procedure to improve the final estimate of the conductivity has been proposed.

Attenuated total reflectance far-ultraviolet and deep-ultraviolet spectroscopy analysis of the electronic structure of a dicyanamide-based ionic liquid with Li+

Elucidating the unique electronic structure of ionic liquid molecules around Li⁺ using electronic absorption spectroscopy, theoretical calculations, and chemometric analyses.