Molecular Modeling of Electrolytes

Abstract Recent advances in molecular modeling provide significant insight into electrolyte electrochemical and transport properties. The first part of the chapter discusses applications of quantum chemistry methods to determine electrolyte oxidative stability and oxidation-induced decomposition reactions. A link between the oxidation stability of model electrolyte clusters and the kinetics of oxidation reactions is established and compared with the results of linear sweep voltammetry measurements. The second part of the chapter focuses on applying molecular dynamics (MD) simulations and density functional theory to predict the structural and transport properties of liquid electrolytes and solid elecfiolyte interphase (SEI) model compounds the free energy profiles for Uthium desolvation from electrolytes and the behavior of electrolytes at charged electrodes and the electrolyte-SEl interface. 

Electrochemistry Branch, Sensors Electron Devices Directorate, U. S. Army Research Latxrratory, 2800 Powder Mill Rd., Adelphi, MD 20783, USA e-mail oleg.a.borodin.civ mail.mil 

Jow et al. (eds.). Electrolytes for Lithium and Lithium-Ion Batteries, Modem Aspects of Electrochemistry 58, DOl 10.1007/978-l-4939-0302-3 8, Springer Sdence+Business Media New York 2014 

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The characteristic features of parameter estimation in a molecular model of adsorption are illustrated in Table 9.9, taking the simple example of the constant-capacitance model as applied to the acid-base reactions on a hydroxylated mineral surface. (It is instructive to work out the correspondence between equation (9.2) and the two reactions in Table 9.9.) Given the assumption of an average surface hydroxyl, there are just two chemical reactions involved (the background electrolyte is not considered). The constraint equations prescribe mass and charge balance (in terms of mole fractions, x) and two complex stability constants. Parameter estimation then requires the determination of the two equilibrium constants and the capacitance density simultaneously from experimental data on the species mole fractions as functions of pH. 

To move beyond the primitive model, we must include a molecular model of the solvent. A simple model of the solvent is the dipolar hard sphere model, Eq. (16). A mixture of dipolar and charged hard spheres has been called the civilized model of an electrolyte. This is, perhaps, an overstatement as dipolar hard spheres are only partially satisfactory as a model of most solvents, especially water still it is an improvement. 

In this section we consider the possibility of applying the ion association concept to the description of the properties of electrolyte solutions in the ion-molecular or Born-Oppenheimer level approach. The simplest ion-molecular model for electrolyte solution can be represented by the mixture of charged hard spheres and hard spheres with embedded dipoles, the so-called ion-dipolar model. For simplification we consider that ions and solvent molecules are characterized by diameters R and Rs, correspondingly. The model is given by the pair potentials,... 

In summary, diffraction techniques provide a powerful means of investigating the structure of electrolyte solutions. They give information about the pair correlation functions which can be directly related to modern theoretical techniques such as molecular dynamics calculations. This information can also be used to improve the statistical thermodynamic models of electrolyte solutions discussed in chapter 3. 

Molecular Model of Counterion Dissociation Equilibrium. The following molecular concept is supported, or suggested, by both these spectroscopic observations and past ultrasonic investigations of simple aqueous electrolytes. In particular, a four-state model reminiscent of the multistep ionic dissociation mechanism of Eigen et al., (22, 23) was adopted (24). With regard to Figure 3, tFie states are classified as 1) completely dissociated hydrated ion pairs, 2) ion pairs at the contact of undisturbed primary hydration shells, and 3) outer and 4) inner sphere complexes. The relative populations of these states, (P ... 

The. study of aqueous electrolyte solutions with the methods listed in Table 2.1 has as its objective the development of a molecular model of the... 

In addition to explicitly accounting for the molecular nature of electrolyte solutions near the EDL, computer simulations can also be used towards realistic modeling of the metal part of the electrochemical... 

Balbuena, P. B. Lamas, E. J. Wang, Y. Molecular modeling of pol5mer electrolytes for power sources. Electrochim. Acta 2005, 50, 3788-3795. 

A supercapacitor stores energy in electrical double layers at electrode/electrolyte interfaces. In molecular modeling of supercapacitors, the... 

There are a variety of methods for use in modeling of electrolytes in Li-air batteries, which have already been widely used in modeling of electrolytes and related SEl formation in Li-ion batteries [11-18], The methods that have been used for electrolytes in Li-ion batteries largely have utilized electronic structure or molecular dynamics methods. Since Li-air modeling reported so far has largely been based on electronic strncmre methods, a brief review of different levels of theory is given in this section. 

The Metropolis MC method in the canonical ensemble is the most frequently used simulation approach to solve the primitive model of electrolytes. Averages of static properties are taken from a large set of Boltzmann-weighted configurations. Molecular dynamics and Brownian dynamics constitute two other methods to determine static and dynamic properties of molecular systems. Their implementations are, however, comphcated for systems possessing impulsive forces in combination with other forces. Hence, a soft-sphere repulsion is frequently used instead of the hard-sphere one when simulating such systems with these methods. 

Khorasani, A.N. Molecular modeling of proton and water distribution in catalyst layer pores of polymer electrolyte fuel cells. Abstract MA2012-011059. 

Ire boundary element method of Kashin is similar in spirit to the polarisable continuum model, lut the surface of the cavity is taken to be the molecular surface of the solute [Kashin and lamboodiri 1987 Kashin 1990]. This cavity surface is divided into small boimdary elements, he solute is modelled as a set of atoms with point polarisabilities. The electric field induces 1 dipole proportional to its polarisability. The electric field at an atom has contributions from lipoles on other atoms in the molecule, from polarisation charges on the boundary, and where appropriate) from the charges of electrolytes in the solution. The charge density is issumed to be constant within each boundary element but is not reduced to a single )oint as in the PCM model. A set of linear equations can be set up to describe the electrostatic nteractions within the system. The solutions to these equations give the boundary element harge distribution and the induced dipoles, from which thermodynamic quantities can be letermined. 

The behavior of simple and molecular ions at the electrolyte/electrode interface is at the core of many electrochemical processes. The complexity of the interactions demands the introduction of simplifying assumptions. In the classical double layer models due to Helmholtz [120], Gouy and Chapman [121,122], and Stern [123], and in most analytic studies, the molecular nature of the solvent has been neglected altogether, or it has been described in a very approximate way, e.g. as a simple dipolar fluid. Computer simulations... 

Theories and simulation of the operation of AFM in liquid have been attempted [102-104], In principle, molecular dynamics or NEMD may be a suitable method to mimic the operation of a scanning tip. The time scale, however, precludes simulating a long-enough scan to see a complete atom. Most studies, therefore, were made with equilibrium conditions and a fixed position of the AFM tip. Explicit consideration of electrolytes and electrostatic effects has not been modeled. 

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