Electrokinetic properties of NaCl solution via molecular dynamics simulations with scaled-charge electrolytes

Scaling ionic charges has become an alternative to polarizable force fields for representing indirect charge transfer effects in molecular simulations. In our work, we apply molecular dynamics simulations to investigate the properties of NaCl aqueous solutions in homogeneous and confined media. We compare classical integer- and scaled-charge force fields for the ions. In the bulk, we validate the force fields by computing equilibrium and transport properties and comparing them with experimental data. Integer-charge ions overestimate dielectric saturation and ionic association. Both force fields present an excess in ion-ion correlation, which leads to a deviation in the ionic conductivity at higher ionic strengths. Negatively charged quartz is used to simulate the confinement effect. Electrostatic interactions dominate counter-ion adsorption. Full-charge ions have stronger and more defined adsorption planes. We obtain the electroosmotic mobility of the solution by combining the shear plane location from non-equilibrium simulations with the ionic distribution from equilibrium simulations. From the Helmholtz-Smoluchowski equation, the zeta potential and the streaming potential coupling coefficient are computed. From an atomic-scale perspective, our molecular dynamics simulations corroborate the hypothesis of maximum packing of the Stern layer, which results in a stable and non-zero zeta potential at high salinity. The scaled-charge model representation of both properties is in excellent qualitative and quantitative agreement with experimental data. With our work, we demonstrate how useful and precise simple scaled-charge models for electrolytes can be to represent complex systems, such as the electrical double layer.

Electrolyte Dependence of Li+ Transport Mechanisms in Small Molecule Solvents from Classical Molecular Dynamics

As demands on Li-ion battery performance increase, the need for electrolytes with high ionic conductivity and a high Li+ transference number (tLi) becomes crucial to boost power density. Unfortunately, tLi in liquid electrolytes is typically <0.5 due to Li+ migrating via a vehicular mechanism, whereby Li+ diffuses along with its solvation shell, making its diffusivity slower than the counteranion. Designing liquid electrolytes where the Li+ ion diffuses independently of its solvation shell is of significant interest to enhance the transference number. In this work, we elucidate how the properties of the solvent influence the Li+ transport mechanism. Using classical molecular dynamics simulations, we find that a vehicular mechanism can be increasingly preferred with a decreasing solvent viscosity and increasing interaction energy between the solvent and Li+. Thus, a weaker interaction energy can enhance tLi through a solvent-exchange mechanism, ultimately improving Li-ion battery performance. Finally, metadynamics simulations show that in electrolytes where a solvent-exchange mechanism is preferable, the energy barrier to changing the coordination environment of Li+ is much lower than in electrolytes where a vehicular mechanism dominates.

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.

Molecular Dynamics and Emerging Network Graphs of Interactions in Dinitrile-Based Li-Ion Battery Electrolytes

Advancements in battery research have shown interesting formulations of battery electrolytes that have helped improve the efficiency of Li-ion batteries over the decades. However, the quest for a safer and affordable battery electrolyte still proceeds with more unique formulations reported in the literature regularly. The dinitriles, especially adiponitrile and glutaronitrile, have caught the attention of the research community as part of this quest. In this work, we performed molecular dynamics simulations of dinitrile electrolytes with lithium bistrifluorosulfonimide (LiTFSI) as the electrolyte salt at varying concentrations and temperatures. On analysis of our simulations, we find that the densities of the mixtures follow the same trend as that of experimental values. The solvation properties were explored using the radial distribution functions. The connectivity of the Li⁺ with the dinitrile molecules and anions is established for all of the electrolyte concentrations using network graphs. We observe that the electrolytes form highly networked structures as the concentration increases without being affected by the rise in temperature. The networking of ionic interactions was quantified by calculating the average degree of each graph. Ionic conductivity calculations were computed using three methods: Nernst-Einstein relation, correlated method, and current autocorrelation function. We report the importance of accounting for the correlated motion of ions while estimating the ionic conductivity. The correlated conductivity and current autocorrelation function calculations provide a satisfactory estimation of the ionic conductivity compared to the experimental values.

Operando Synchrotron XRD of Bromide Mediated Li-O2 Battery

Li-O₂ battery technology offers large theoretical energy density, considered a promising alternative energy storage technology for a variety of applications. One of the main advances made in recent years is the use of soluble catalysts, known as redox mediators (RM), decreasing the charge overpotential and improving cyclability. Despite its potential, much is still unknown regarding its dynamic, especially over higher loading electrodes, where mass transport may be an issue and the interplay with common impurities in the electrolyte, like residual water. Here we perform for the first time an operando XRD characterization of a DMSO-based LiBr mediated Li-O₂ battery with a high loading electrode based on CNTs aiming to reveal these dynamics and track chemical changes in the electrode. Our results show that, depending on the electrode architecture, the system's issue can move from catalytic to a mass transfer. We also assess the effect of residual water in the system to better understand the reaction routes. As a result, we observed that with DMSO, the system is even more sensitive to water contamination compared to glyme-based studies reported in the literature. Despite the activity of LiBr on the Li-peroxide oxidation and its contribution to cyclability, with the system and electrode configuration used in this study, we verified that a mass transfer limitation caused a cell "sudden death" caused by clogging after cycling.

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.

Ion Solvation Engineering: How to Manipulate the Multiplicity of the Coordination Environment of Multivalent Ions

Free energy analysis of solvation structures of free divalent cations, their ion pairs, and neutral aggregates in low dielectric solvents reveals the multiplicity of thermodynamically stable cation solvation configurations and identifies the micro- and macroscopic factors responsible for this phenomenon. Specifically, we show the role of ion-solvent interactions and solvent mixtures in determining the cation solvation free energy landscapes. We show that it is the entropic contribution of solvent degrees of freedom that is responsible for the solvation multiplicity, and the mutual balance between enthalpic and entropic forces or their concerted contributions is what ultimately defines the most stable ion solvation configuration and creates new ones. We show general consequences of ion solvation multiplicity on thermodynamics of complex electrolytes, specifically in the context of homogeneous or interfacial charge transfer. Identified factors and their interplay provide a pathway to formulation of solvation design rules that can be used to control bulk solvation, interfacial chemistry, and charge transfer. Our findings also suggest experimentally testable predictions.