Mass-zero constrained molecular dynamics for electrode charges in simulations of electrochemical systems.

Classical molecular dynamics simulations have recently become a standard tool for the study of electrochemical systems. State-of-the-art approaches represent the electrodes as perfect conductors, modeling their responses to the charge distribution of electrolytes via the so-called fluctuating charge model. These fluctuating charges are additional degrees of freedom that, in a Born-Oppenheimer spirit, adapt instantaneously to changes in the environment to keep each electrode at a constant potential. Here, we show that this model can be treated in the framework of constrained molecular dynamics, leading to a symplectic and time-reversible algorithm for the evolution of all the degrees of freedom of the system. The computational cost and the accuracy of the new method are similar to current alternative implementations of the model. The advantage lies in the accuracy and long term stability guaranteed by the formal properties of the algorithm and in the possibility to systematically introduce additional kinematic conditions of arbitrary number and form. We illustrate the performance of the constrained dynamics approach by enforcing the electroneutrality of the electrodes in a simple capacitor consisting of two graphite electrodes separated by a slab of liquid water.

Transport Properties of Li-TFSI Water-in-Salt Electrolytes.

Water-in-salts are a new family of electrolytes that may allow the development of aqueous Li-ion batteries. They have a structure that is reminiscent of ionic liquids, and they are characterized by a high concentration of ionic species. In this work, we study their transport properties and how they evolve with concentration by using molecular dynamic simulations. We first focus on the choice of the force field. By comparing the simulated viscosities and self-diffusion coefficients with experimental measurements, we select a set of parameters that reproduces well the transport properties. We then use the selected force field to study in detail the variations of the self and collective diffusivities of all the species as well as the transport number of the lithium ion. We show that correlations between ions and water play an important role over the whole concentration range. In the water-in-salt regime, the anions form a percolating network that reduces the cation-anion correlations and leads to rather large values for the transport number compared to other standard electrolytes.

Navigating at Will on the Water Phase Diagram.

Despite the simplicity of its molecular unit, water is a challenging system because of its uniquely rich polymorphism and predicted but yet unconfirmed features. Introducing a novel space of generalized coordinates that capture changes in the topology of the interatomic network, we are able to systematically track transitions among liquid, amorphous, and crystalline forms throughout the whole phase diagram of water, including the nucleation of crystals above and below the melting point. Our approach, based on molecular dynamics and enhanced sampling or free energy calculation techniques, is not specific to water and could be applied to very different structural phase transitions, paving the way towards the prediction of kinetic routes connecting polymorphic structures in a range of materials.

Probing ice VII crystallization from amorphous NaCl-D2O solutions at gigapascal pressures.

We probe the possible inclusion of salt (NaCl) in the ice VII lattice over the pressure range from 2 to 4 gigapascal. We combine data from neutron diffraction experiments under pressure and from computational structure searches based on density functional theory. We observe that the high density amorphous precursor (NaCl·10.2D2O) crystallises during annealing at high pressure in the vicinity of the phase boundary between pure ices VII and VIII. The structure formed is very similar to that of pure ice VII. Our simulations indicate that substituting water molecules in the ice VII lattice with Na+ and Cl- ions would lead to a significant expansion of the lattice parameter. Since this expansion was not observed in our experiments, the ice crystallised is likely to be pure D2O or contains only a small fraction of the ions from the salt solution.

Liquid B2O3 up to 1700 K: x-ray diffraction and boroxol ring dissolution.

Using high energy x-ray diffraction, the structure factors of glassy and molten B2O3 were measured with high signal-to-noise, up to a temperature of T = 1710(20) K. The observed systematic changes with T are shown to be consistent with the dissolution of hexagonal [B3O6] boroxol rings, which are abundant in the glass, whilst the high-T (>~1500 K) liquid can be more closely described as a random network structure based on [BO3] triangular building blocks. We therefore argue that diffraction data are in fact qualitatively sensitive to the presence of small rings, and support the existence of a continuous structural transition in molten B2O3, for which the temperature evolution of the 808 cm−1 Raman scattering band (boroxol breathing mode) has long stood as the most emphatic evidence. Our conclusions are supported by both first-principles and polarizable ion model molecular dynamics simulations which are capable of giving good account of the experimental data, so long as steps are taken to ensure a ring fraction similar to that expected from Raman spectroscopy. The mean thermal expansion of the B-O bond has been measured directly to be αBO = 3.7(2) × 10−6 K−1, which accounts for a few percent of the bulk expansion just above the glass transition temperature, but accounts for greater than one third of the bulk expansion at temperatures in excess of 1673 K.

Structural characterization of eutectic aqueous NaCl solutions under variable temperature and pressure conditions.

The structure of amorphous NaCl solutions produced by fast quenching is studied as a function of pressure, up to 4 GPa, by combined neutron diffraction experiments and classical molecular dynamics simulations. Similarly to LiCl solutions the system amorphizes at ambient pressure in a dense phase structurally similar to the e-HDA phase in pure water. The measurement of the static structure factor as a function of pressure allowed us to validate a new polarizable force field developed by Tazi et al., 2012, never tested under non-ambient conditions. We infer from simulations that the hydration shells of Na(+) cations form well defined octahedra composed of both H2O molecules and Cl(-) anions at low pressure. These octahedra are gradually broken by the seventh neighbour moving into the shell of first neighbours yielding an irregular geometry. In contrast to LiCl solutions and pure water, the system does not show a polyamorphic transition under pressure. This confirms that the existence of polyamorphism relies on the tetrahedral structure of water molecules, which is broken here.

Highly confined ions store charge more efficiently in supercapacitors.

Liquids exhibit specific properties when they are adsorbed in nanoporous structures. This is particularly true in the context of supercapacitors, for which an anomalous increase in performance has been observed for nanoporous electrodes. This enhancement has been traditionally attributed in experimental studies to the effect of confinement of the ions from the electrolyte inside sub-nanometre pores, which is accompanied by their partial desolvation. Here we perform molecular dynamics simulations of realistic supercapacitors and show that this picture is correct at the microscopic scale. We provide a detailed analysis of the various environments experienced by the ions. We pick out four different adsorption types, and we, respectively, label them as edge, planar, hollow and pocket sites upon increase of the coordination of the molecular species by carbon atoms from the electrode. We show that both the desolvation and the local charge stored on the electrode increase with the degree of confinement.

Density functional theory, molecular dynamics, and differential scanning calorimetry study of the RbF-CsF phase diagram.

A multiscale modeling approach is developed to compute the phase diagram of the RbF-CsF binary system. The mixing enthalpies of the (Rb,Cs)F solid and liquid solutions are evaluated using density functional theory and classical molecular dynamics calculations, respectively. For the solid solution, 18 different configurations are studied with density functional theory and the surrounded atom model is applied in order to compute the configurational partition function. We also measure the solidus and liquidus equilibria using differential scanning calorimetry. Finally the RbF-CsF phase diagram is constructed using the calculated excess free enthalpies of the solid and liquid solutions and a very good agreement with our experimental data is found.

Ion adsorption at a metallic electrode: an ab initio based simulation study.

A method for parametrizing, from first principles density functional theory calculations, a model of the interactions between the ions in an ionic liquid and a metallic (electrode) surface is described. The interaction model includes the induction of dipoles on the ions of the liquid by their mutual interaction and the interaction with the electrode surface as well as the polarization of the metal by the ionic charges and dipoles ('image' interactions). The method is used to obtain a suitable interaction model for a system consisting of a LiCl liquid electrolyte and a solid aluminium electrode. The model is then used in simulations of this system for various values of the electrical potential applied to the electrode. The evolution of the liquid structure at the electrochemical interface with applied potential is followed and the capacitance of the electrochemical interface is measured. The electrolyte is found to exhibit a potential-driven phase transition which involves the commensurate ordering of the electrolyte ions with the electrode surface; this leads to a maximum in the differential capacitance as a function of applied potential. Away from the phase transition the capacitance was found to be independent of the applied potential.

Molecular dynamics simulation of hydrogen fluoride mixtures with 1-ethyl-3-methylimidazolium fluoride: a simple model for the study of structural features.

Mixtures of hydrogen fluoride with ionic liquids show unique physicochemical properties, including their ability to form polyfluoride species (pointed out for the first time in this media by von Rosenvinge et al. J. Chem. Phys. 1997, 107, 8012). Among those systems the acidic 1-ethyl-3-methylimidazolium fluoride (EMIF.2.3HF) has been widely studied experimentally since it is the more promising for electrochemical applications. Recent studies (Hagiwara et al. J. Electrochem. Soc. 2002, 149, D1), while yielding many results, raised some questions about structural features of the liquid: absence of hydrogen bonds between the EMI+ ring hydrogen atoms and the fluoride anions, persistence of stacks and layers of cations similar to those existing in the crystal, and interpretation of the X-ray diffraction spectra. To address these questions, we have developed a simple molecular dynamics model. Our simulations are very consistent with experimental results and complete them, providing an atomic scale interpretation.