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BaSnF4 is a prospective solid state electrolyte for fluoride ion batteries. However, the diffusion mechanism of the fluoride ions remains difficult to study, both in experiments and in simulations. In principle, ab initio molecular dynamics could allow to fill this gap, but this method remains very costly from the computational point of view. Using machine learning potentials is a promising method that can potentially address the accuracy issues of classical empirical potentials while maintaining high efficiency. In this work, we fitted a dipole polarizable ion model and trained machine learning potential for BaSnF4 and made comprehensive comparisons on the ease of training, accuracy and efficiency. We also compared the results with the case of a simpler ionic system (NaF). We show that contrarily to the latter, for BaSnF4 the machine learning potential offers much higher versatility. The current work lays foundations for the investigation of fluoride ion mobility in BaSnF4 and provides insight on the choice of methods for atomistic simulations. [ABSTRACT FROM AUTHOR]

Electrochemistry is central to many applications, ranging from biology to energy science. Studies now involve a wide range of techniques, both experimental and theoretical. Modeling and simulations methods, such as density functional theory or molecular dynamics, provide key information on the structural and dynamic properties of the systems. Of particular importance are polarization effects of the electrode/electrolyte interface, which are difficult to simulate accurately. Here, we show how these electrostatic interactions are taken into account in the framework of the Ewald summation method. We discuss, in particular, the formal setup for calculations that enforce periodic boundary conditions in two directions, a geometry that more closely reflects the characteristics of typical electrolyte/electrode systems and presents some differences with respect to the more common case of periodic boundary conditions in three dimensions. These formal developments are implemented and tested in MetalWalls, a molecular dynamics software that captures the polarization of the electrolyte and allows the simulation of electrodes maintained at a constant potential. We also discuss the technical aspects involved in the calculation of two sets of coupled degrees of freedom, namely the induced dipoles and the electrode charges. We validate the implementation, first on simple systems, then on the well-known interface between graphite electrodes and a room-temperature ionic liquid. We finally illustrate the capabilities of MetalWalls by studying the adsorption of a complex functionalized electrolyte on a graphite electrode. [ABSTRACT FROM AUTHOR]

Quinones are redox active organic molecules that have been proposed as an alternative choice to metal-based materials in electrochemical energy storage devices. Functionalization allows one to fine tune not only their chemical stability but also the redox potential and kinetics of the electron transfer reaction. However, the reaction rate constant is not only determined by the redox species but also impacted by solvent effects. In this work, we show how the functionalization of benzoquinone with different functional groups impacts the solvent reorganization free energies of electron transfer half-reactions in acetonitrile. The use of molecular density functional theory, whose computational cost for studying the electron transfer reaction is considerably reduced compared to the state-of-the-art molecular dynamics simulations, enables us to perform a systematic study. We validate the method by comparing the predictions of the solvation shell structure and the free energy profiles for electron transfer reaction to the reference classical molecular dynamics simulations in the case of anthraquinone solvated in acetonitrile. We show that all the studied electron transfer half-reactions follow the Marcus theory, regardless of functional groups. Consequently, the solvent reorganization free energy decreases as the molecular size increases. [ABSTRACT FROM AUTHOR]

The structure of the double-layer formed at the surface of carbon electrodes is governed by the interactions between the electrode and the electrolyte species. However, carbon is notoriously difficult to simulate accurately, even with well-established methods such as electronic density functional theory and molecular dynamics. Here, we focus on the important case of a lithium ion in contact with the surface of graphite, and we perform a series of reference quantum Monte Carlo calculations that allow us to benchmark various electronic density functional theory functionals. We then fit an accurate carbon–lithium pair potential, which is used in molecular density functional theory calculations to determine the free energy of the adsorption of the ion on the surface in the presence of water. The adsorption profile in aqueous solution differs markedly from the gas phase results, which emphasize the role of the solvent on the properties of the double-layer. [ABSTRACT FROM AUTHOR]

The structure of a concentrated solution of NaCl in D2O was investigated by in situ high-pressure neutron diffraction with chlorine isotope substitution to give site-specific information on the coordination environment of the chloride ion. A broad range of densities was explored by first increasing the temperature from 323 to 423 K at 0.1 kbar and then increasing the pressure from 0.1 to 33.8 kbar at 423 K, thus mapping a cyclic variation in the static dielectric constant of the pure solvent. The experimental work was complemented by molecular dynamics simulations using the TIP4P/2005 model for water, which were validated against the measured equation of state and diffraction results. Pressure-induced anion ordering is observed, which is accompanied by a dramatic increase in the Cl–O and O–O coordination numbers. With the aid of bond-distance resolved bond-angle maps, it is found that the increased coordination numbers do not originate from a sizable alteration to the number of either Cl⋯D–O or O⋯D–O hydrogen bonds but from the appearance of non-hydrogen-bonded configurations. Increased pressure leads to a marked decrease in the self-diffusion coefficients but has only a moderate effect on the ion–water residence times. Contact ion pairs are observed under all conditions, mostly in the form of charge-neutral NaCl0 units, and coexist with solvent-separated Na+–Na+ and Cl−–Cl− ion pairs. The exchange of water molecules with Na+ adopts a concerted mechanism under ambient conditions but becomes non-concerted as the state conditions are changed. Our findings are important for understanding the role of extreme conditions in geochemical processes. [ABSTRACT FROM AUTHOR]

Redox-active molecules are of interest in many fields, such as medicine, catalysis, or energy storage. In particular, in supercapacitor applications, they can be grafted to ionic liquids to form so-called biredox ionic liquids. To completely understand the structural and transport properties of such systems, an insight at the molecular scale is often required, but few force fields are developed ad hoc for these molecules. Moreover, they do not include polarization effects, which can lead to inaccurate solvation and dynamical properties. In this work, we developed polarizable force fields for redox-active species anthraquinone (AQ) and 2,2,6,6-tetra-methylpiperidinyl-1-oxyl (TEMPO) in their oxidized and reduced states as well as for acetonitrile. We validate the structural properties of AQ, AQ•−, AQ2−, TEMPO•, and TEMPO+ in acetonitrile against density functional theory-based molecular dynamics simulations and we study the solvation of these redox molecules in acetonitrile. This work is a first step toward the characterization of the role played by AQ and TEMPO in electrochemical and catalytic devices. [ABSTRACT FROM AUTHOR]

Electrochemistry experiments have established that the capacitance of electrode–electrolyte interfaces is much larger for good metals, such as gold and platinum, than for carbon-based materials. Despite the development of elaborate electrode interaction potentials, to date molecular dynamics simulations are not able to capture this effect. Here, we show that changing the width of the Gaussian charge distribution used to represent the atomic charges in gold is an effective way to tune its metallicity. Larger Gaussian widths lead to a capacitance of aqueous solutions (pure water and 1 M NaCl) in good agreement with recent ab initio molecular dynamics results. For pure water, the increase in the capacitance is not accompanied by structural changes, while in the presence of salt, the Na+ cations tend to adsorb significantly on the surface. For a strongly metallic gold electrode, these ions can even form inner sphere complexes on hollow sites of the surface. [ABSTRACT FROM AUTHOR]

Soda–lime–silica is a glassy system of strong industrial interest. In order to characterize its liquid state properties, we performed molecular dynamics simulations employing an aspherical ion model that includes atomic polarization and deformation effects. They allowed us to study the structure and diffusion properties of the system at temperatures ranging from 1400 K to 3000 K. We show that Na+ and Ca2+ ions adopt a different structural organization within the silica network, with Ca2+ ions having a greater affinity for non-bridging oxygens than Na+. We further link this structural behavior to their different diffusivities, suggesting that escaping from the first oxygen coordination shell is the limiting step for the diffusion. Na+ diffuses faster than Ca2+ because it is bonded to a smaller number of non-bridging oxygens. The formed ionic bonds are also less strong in the case of Na+. [ABSTRACT FROM AUTHOR]

Spurred by the increasing needs in electrochemical energy storage devices, the electrode/electrolyte interface has received a lot of interest in recent years. Molecular dynamics simulations play a prominent role in this field since they provide a microscopic picture of the mechanisms involved. The current state-of-the-art consists of treating the electrode as a perfect conductor, precluding the possibility to analyze the effect of its metallicity on the interfacial properties. Here, we show that the Thomas–Fermi model provides a very convenient framework to account for the screening of the electric field at the interface and differentiating good metals such as gold from imperfect conductors such as graphite. All the interfacial properties are modified by screening within the metal: the capacitance decreases significantly and both the structure and dynamics of the adsorbed electrolyte are affected. The proposed model opens the door for quantitative predictions of the capacitive properties of materials for energy storage. [ABSTRACT FROM AUTHOR]

The structural properties of glassy diboron trioxide, g-B2O3, are investigated from ambient to high pressure conditions using two types of atomic force-field models that account for many-body effects. These models are parameterized by a dipole- and force-fitting procedure of reference datasets created via first-principles calculations on a series of configurations. The predictions of the models are tested against experimental data, where particular attention is paid to the structural transitions in g-B2O3 that involve changes to both the short- and medium-range order. The models outperform those previously devised, where improvement originates from the incorporation of two key physical ingredients, namely, (i) the polarizability of the oxide ion and (ii) the ability of an oxide ion to change both size and shape in response to its coordination environment. The results highlight the importance of many-body effects for accurately modeling this challenging system. [ABSTRACT FROM AUTHOR]

Reliable operation of Na‐ion batteries requires a mastering of the interfacial processes governing intercalation kinetics. Ion desolvation, amongst others, is a crucial phenomenon as far as power applications are envisioned. Studying the intricacies of interfacial processes faces experimental challenges. Here the complementary use of electrochemical quartz crystal microbalance (EQCM) and operando infrared fiber evanescent wave spectroscopy (IR‐FEWS) combined with molecular dynamics (MD) simulations is reported to investigate the electrode/electrolyte interface of polyanionic Na‐insertion compounds Na3V2(PO4)3 (NVP) and Na3V2(PO4)2F3 (NVPF). Through screening of various electrolytes that differ in their solvation energy and viscosity, the existence of an unusual mass uptake/depletion signal is demonstrated at the NVP interfaces only, indicative of a dissimilarity in migration kinetics of solvent molecules between NVP and NVPF interfaces as supported by IR‐FEWS. This finding is rationalized by positing that sodium depletion is caused by the unusually rapid ion diffusion within NVP mediated by an intermediate metastable phase, which leads to opposite Na+ concentration and free solvent molecule gradients at the interface that causes the unusual and anomalous feature observed on EQCM frequency profiles. This feature does not appear with NVPF showing lower ionic diffusion. Altogether, this fundamental study provides valuable fundamental insights into the interface structuring of insertion compounds depending upon their Na+ mobility. [ABSTRACT FROM AUTHOR]

Most of the performances of electrochemical devices are governed by molecular processes taking place at the solution-electrode interfaces, and molecular simulation is the main way to study these processes. Aqueous electrochemical systems have often been studied using classical density functional theory (DFT) but with too crude approximations to consider the system description to be realistic. We study the interface between graphene electrodes and liquid water at different applied voltages using molecular DFT, improving the state of the art by the following key points: (1) electrodes have a realistic atomic resolution, (2) classical DFT calculations are carried out at a fixed imposed potential difference, and (3) water is described by a molecular model. This allows us to reveal the structural modification of water adsorbed at the graphene interface and the evolution of water dielectric permittivity when a voltage is applied. The computed capacitance of this device is in agreement with molecular dynamics simulations. This demonstrates the relevance of molecular DFT to study electrochemical systems at the molecular level. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations are a powerful tool for predicting the properties of molten salts at conditions sometimes challenging to achieve experimentally. Here, they are applied within the framework of the polarizable ion model (PIM) for the investigation of the NaCl-KCl-UCl 3 molten salt at two different compositions and for a temperature range between 1000 and 1400 K. This allowed us to understand the impact of sodium and potassium ions on the structure of the solvation shell of uranium ions, as well as predicting their density, viscosity, self-diffusion coefficients and heat capacity at different conditions. [ABSTRACT FROM AUTHOR]

When an ionic liquid adsorbs onto a porous electrode, its ionic arrangement is deeply modified due to a screening of the Coulombic interactions by the metallic surface and by the confinement imposed upon it by the electrode’s morphology. In particular, ions of the same charge can approach at close contact, leading to the formation of a superionic state. The impact of an electrified surface placed between two liquid phases is much less understood. Here we simulate a full supercapacitor made of the 1-butyl-3-methylimidazolium hexafluorophosphate and nanoporous graphene electrodes, with varying distances between the graphene sheets. The electrodes are held at constant potential by allowing the carbon charges to fluctuate. Under strong confinement conditions, we show that ions of the same charge tend to adsorb in front of each other across the graphene plane. These correlations are allowed by the formation of a highly localized image charge on the carbon atoms between the ions. They are suppressed in larger pores, when the liquid adopts a bilayer structure between the graphene sheets. These effects are qualitatively similar to the recent templating effects which have been reported during the growth of nanocrystals on a graphene substrate. [ABSTRACT FROM AUTHOR]