Mallory Gobet

Influence of solvent on ion aggregation and transport in PY15TFSI ionic liquid-aprotic solvent mixtures.

Molecular dynamics (MD) simulations using a many-body polarizable APPLE&P force field have been performed on mixtures of the N-methyl-N-pentylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PY15TFSI) ionic liquid (IL) with three molecular solvents: propylene carbonate (PC), dimethyl carbonate (DMC), and acetonitrile (AN). The MD simulations predict density, viscosity, and ionic conductivity values that agree well with the experimental results. In the solvent-rich regime, the ionic conductivity of the PY15TFSI-AN mixtures was found to be significantly higher than the conductivity of the corresponding -PC and -DMC mixtures, despite the similar viscosity values obtained from both the MD simulations and experiments for the -DMC and -AN mixtures. The significantly lower conductivity of the PY15TFSI-DMC mixtures, as compared to those for PY15TFSI-AN, in the solvent-rich regime was attributed to the more extensive ion aggregation observed for the -DMC mixtures. The PY15TFSI-DMC mixtures present an interesting case where the addition of the organic solvent to the IL results in an increase in the cation-anion correlations, in contrast to what is found for the mixtures with PC and AN, where ion motion became increasingly uncorrelated with addition of solvent. A combination of pfg-NMR and conductivity measurements confirmed the MD simulation predictions. Further insight into the molecular interactions and properties was also obtained using the MD simulations by examining the solvent distribution in the IL-solvent mixtures and the mixture excess properties.

Structure and dynamics in yttrium-based molten rare earth alkali fluorides.

The transport properties of molten LiF-YF3 mixtures have been studied by pulsed field gradient nuclear magnetic resonance spectroscopy, potentiometric experiments, and molecular dynamics simulations. The calculated diffusion coefficients and electric conductivities compare very well with the measurements across a wide composition range. We then extract static (radial distribution functions, coordination numbers distributions) and dynamic (cage correlation functions) quantities from the simulations. Then, we discuss the interplay between the microscopic structure of the molten salts and their dynamic properties. It is often considered that variations in the diffusion coefficient of the anions are mainly driven by the evolution of its coordination with the metallic ion (Y(3+) here). We compare this system with fluorozirconate melts and demonstrate that the coordination number is a poor indicator of the evolution of the diffusion coefficient. Instead, we propose to use the ionic bonds lifetime. We show that the weak Y-F ionic bonds in LiF-YF3 do not induce the expected tendency of the fluoride diffusion coefficient to converge toward one of the yttrium cation when the content in YF3 increases. Implications on the validity of the Nernst-Einstein relation for estimating the electrical conductivity are discussed.

Mechanism of Conductivity Relaxation in Liquid and Polymeric Electrolytes: Direct Link between Conductivity and Diffusivity

Combining broadband impedance spectroscopy, differential scanning calorimetry, and nuclear magnetic resonance we analyzed charge and mass transport in two polymerized ionic liquids and one of their monomeric precursors. In order to establish a general procedure for extracting single-particle diffusivity from their conductivity spectra, we critically assessed several approaches previously employed to describe the onset of diffusive charge dynamics and of the electrode polarization in ion conducting materials. Based on the analysis of the permittivity spectra, we demonstrate that the conductivity relaxation process provides information on ion diffusion and the magnitude of cross-correlation effects between ionic motions. A new approach is introduced which is able to estimate ionic diffusivities from the characteristic times of conductivity relaxation and ion concentration without any adjustable parameters. This opens the venue for a deeper understanding of charge transport in concentrated and diluted electrolyte solutions.

First-principles molecular dynamics simulation and conductivity measurements of a molten xLi2O-(1- x)B2O3 system.

The electronic properties and atomic structure of a molten xLi2O-(1 - x)B2O3 system were investigated by measuring conductivity and using first-principles molecular dynamics (MD) simulations. The conductivities obtained were converted to a Li self-diffusion coefficient Dσ, using the Nernst-Einstein equation to assess charge transfer mechanisms. Dσ was compared with a Li self-diffusion coefficient, DNMR, which we measured in a previous study using high-temperature pulsed field gradient NMR. The DNMR/Dσ of xLi2O-(1 - x)B2O3 (0.2 ≤ x ≤ 0.5) at 1250 K ranged from 2.5 to 3.2, following the same trend as room temperature ionic liquids. First-principles MD simulations were performed using our own finite element density functional theory code, FEMTECK (finite element method-based total energy calculation) for molten xLi2O-(1 - x)B2O3 systems at 1250 K. We found that the O-B-O angle distribution functions were characterized by a peak at approximately 120°. Although the electron number from the electronic radial distribution function was arbitrary with regard to the cutoff distance, the net Li charge calculated from the integrated electron number surrounding Li was approximately 0.9 at 0.085 nm. The mean square displacement (MSD) of Li as a function of time was evaluated from the atomic configuration. Li self-diffusion coefficients calculated from the MSD were in better agreement with experimental results than they were using classical MD.

Transport Properties in Cryolitic Melts: NMR Measurements and Molecular Dynamics Calculations of Self-Diffusion Coefficients

Owing to their industrial interest, cryolite-based melts have been extensively studied. It seems however difficult to obtain a structural model consistent with all the experimental data available. The structure of molten NaFAlF3 mixtures have been investigated by Raman spectroscopy, using a captive liquid windowless cell [1] and NMR spectroscopy at high temperature [2] confirming the dissociation scheme proposed by Gilbert et al. for NaF-AlF3 based on the dissociation of AlF6 into AlF5 and AlF. The structural information given by high temperature Nuclear Magnetic Resonance by means of chemical shifts modifications with temperature can be now combined with the measurement of self-diffusion coefficients. This is possible thanks to the development of a new setup based on Pulsed Field Gradient NMR combined with laser heating [3],[4]. This new device makes possible in situ self-diffusion coefficients measurements up to 1500K. In several corrosive molten fluorides in a wide range of compositions and temperature, we have evidenced the different key parameters of their motion along with their structural characteristics. [4],[5]

Investigation of Fluoroacidity in Molten Fluorides by the Combination of High Temperature NMR and Molecular Dynamics

The control of acidity is a well-known requirement of any chemistry experiment performed in an aqueous solvent. In such liquids, acidity is given by a unique quantity, the pH. When dealing with non-aqueous solvents, another definition, which is due to Lewis, has to be used for the acidity concept: it is based on electron pair donors/acceptors. In molten salts, the exchanged species that defines the acid-base systems is the anion, for example O in molten oxides, or F in fluorides. Acidity scales are then built up by classifying the strengths with which different acids react with the different bases for a wide variety of molten salts except in molten oxides and fluorides. This is due to their strong reactivity and high melting temperature, which makes them difficult to handle properly in the laboratory scale. Nevertheless, they are widely used in important industrial applications: oxides in the glass and ceramics industry, fluorides in the electrowinning of aluminium. Our objective is to build quantitative scales of fluoroand oxo-acidities within a combined experimental and theoretical approach. The acidity of a molten salt will act directly on the formation of complexes involving the cations of the melt. The determination of the speciation with varying overall composition therefore provides an indirect measure of the acidity. This opens a route for building up the acidity scale through the use of some specific in situ spectroscopic approaches. To determine the speciation in the liquid state, Nuclear Magnetic Resonance (NMR) appears as the ideal candidate, as it allows estimating the relative concentration of the various complexes formed in the melt. In parallel, theoretical calculations are the unavoidable complement to this experimental technique. As the systems dealt with are liquids at high temperature, the Molecular Dynamics (MD) simulations provide an adequate level of description.