Solid electrolyte interfaces thermodynamics

As already mentioned, salt-containing liquid solvents are typically used as electrolytes. The most prominent example is LiPF6 as a conductive salt, dissolved in a 1 1 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) as 1 molar solution. It should be mentioned that this electrolyte is not thermodynamically stable in contact with lithium or, for example, LiC6. Its success comes from the fact that it forms an extremely stable passivation layer on top of the electrode, the so-called solid-electrolyte interface (SEI) [35], Key properties of such SEI layers are high Li+ and very low e conductivity - that is, they act as additional electrolyte films, where the electrode potential drops to a level the liquid electrolyte can withstand [36],... 

While thermodynamics provides a starting point, kinetics is essential for providing any corrosion model of practical utility. The term electrode kinetics is often used as, in the electrochemical paradigm, the oxidation and reduction occur at independent sites, which can be considered as separate electrodes marking a solid/electrolyte interface at which the half-ceU reactions take place. In the case of chemical corrosion, these half-cell reactions can take place at the same location, in which case there is no external current flow between the half-reaction centers, but instead, direct charge transfer between the reactants via electronic contact at the same metal site. 

The equations of electrocapillarity become complicated in the case of the solid metal-electrolyte interface. The problem is that the work spent in a differential stretching of the interface is not equal to that in forming an infinitesimal amount of new surface, if the surface is under elastic strain. Couchman and co-workers [142, 143] and Mobliner and Beck [144] have, among others, discussed the thermodynamics of the situation, including some of the problems of terminology. 

The life-limiting increase of resistance and decrease of capacity of cycled cells is usually attributed to the deteriorating effects of corrosion of the positive current collector— the cell container in the case of sodium core cells or a rod in the case of sulfur core cells. Apart from consuming the active material, corrosion may lead to the deposition of poorly conductive layers at the current collector surface, thus interrupting the contact of the inert electrode fibers with the current collector. Corrosion products may also deposit and block both the solid electrolyte and the electrode surface. The thermodynamic instability of metals in polysulfide melts severely limits the choice of materials interfacing the sulfur electrode.A fully satisfactory solution has not yet been reported. 

When a metal (M) is immersed in a solution containing its ions (M ), several reactions may occur. The metal may lose an electron (corrosion) to form metal ions or the metal ions in solution gain electrons (reduction) and enter the solid metal state. The equilibrium across the metal-solution interface controls which reaction, if any, will occur at the metal-electrolyte interface. Because the equilibrium is determined by the equality of the partial Gibbs free-energy or chemical potentials (//) on either side of the electrode interface (i.e., Absolution=A dectrode). when any metal is immersed in the electrolyte, thermodynamics... 

Chebotin s scientific interests were characterized by a variety of topics and covered nearly all aspects of solid electrolytes electrochemistry. He made a significant contribution to the theory of electron conductivity of ionic crystals in equilibrium with a gas phase and solved a number of important problems related to the statistical-thermodynamic description of defect formation in solid electrolytes and mixed ionic-electronic conductors. Vital results were obtained in the theory of ion transport in solid electrolytes (chemical diffusion and interdiffusion, correlation effects, thermo-EMF of ionic crystals, and others). Chebotin paid great attention to the solution of actual electrochemical problem—first of all to the theory of the double layer and issues related to the nature of the polarization at the interface of the solid electrol34e and gas electrode. 

PPY is a very well known conducting polymer used in numerous works as the electroactive component of an all-solid-state ISE. Most of the papers dealt with a PPY-coated PVC electrode where PPY is doped with different anions, inorganic such as chloride or organic such as dodecyl sulphate. Several techniques were used to characterize these devices. Potentiometric measurements represent a method allowing thermodynamic characterization. AC electrogravimetry was also used to characterize ion and solvent motions at the PVC/electrolyte interface to imderstand how the electroactive film (Prussian blue, conducting polymer, etc.) ensures the mediation between the membrane and the electrode. ... 

M = Na or Li) is applied as a sensor component, the M2O oxide formed at the interface readily reacts with permeated CO2 and/or H2O gas species to form M2CO3 and/or MOH, respectively, and the formation of such compoimds results in deterioration of the sensing performance. It is therefore essential that a thermodynamically stable oxide forms at the interface to ensure reliable sensor output. When the Sc " cation-conducting solid electrolyte is used, the oxide formed at the interface is SC2O3, a compound which is extraordinarily stable. Sc2(W04)3 also exhibits high trivalent ion conductivity (6.5 x 10 S cm at 600 °C), which allows quick sensor response based on the electrochemical reactions described below. 

For liquid electrodes thermodynamics offers a precise way to determine the surface charge and the surface excesses of a species. This is one of the reasons why much of the early work in electrochemistry was performed on liquid electrodes, particularly on mercury - another reason is that it is easier to generate clean liquid surfaces than clean solid surfaces. With some caveats and modifications, thermodynamic relations can also be applied to solid surfaces. We will first consider the interface between a liquid electrode and an electrolyte solution, and turn to solid electrodes later. 

Interface and colloid science has a very wide scope and depends on many branches of the physical sciences, including thermodynamics, kinetics, electrolyte and electrochemistry, and solid state chemistry. Throughout, this book explores one fundamental mechanism, the interaction of solutes with solid surfaces (adsorption and desorption). This interaction is characterized in terms of the chemical and physical properties of water, the solute, and the sorbent. Two basic processes in the reaction of solutes with natural surfaces are 1) the formation of coordinative bonds (surface complexation), and 2) hydrophobic adsorption, driven by the incompatibility of the nonpolar compounds with water (and not by the attraction of the compounds to the particulate surface). Both processes need to be understood to explain many processes in natural systems and to derive rate laws for geochemical processes. 

In the first part of this century, electrochemical research was mainly devoted to the mercury electrode in an aqueous electrolyte solution. A mercury electrode has a number of advantageous properties for electrochemical research its surface can be kept clean, it has a large overpotential for hydrogen evolution and both the interfacial tension and capacitance can be measured. In his famous review [1], D. C. Grahame made the firm statement that Nearly everything one desires to know about the electrical double layer is ascertainable with mercury surfaces if it is ascertainable at all. At that time, electrochemistry was a self-contained field with a natural basis in thermodynamics and chemical kinetics. Meanwhile, the development of quantum mechanics led to considerable progress in solid-state physics and, later, to the understanding of electrostatic and electrodynamic phenomena at metal and semiconductor interfaces. 

In the present chapter, the properties of interfaces involving liquids and solutions are described from the point of view of thermodynamics, and also at a molecular level. Examples are given for the liquid gas, liquid liquid and liquid I solid interfaces. The electrical properties of interfaces are also considered and their relevance to processes such as the extraction of an ion from an electrolyte solution is discussed. As will be seen, a knowledge of interfacial properties and... 

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