Solid electrolyte molecular structure

Transition Region Considerations. The conductance of a binary system can be approached from the values of conductivity of the pure electrolyte one follows the variation of conductance as one adds water or other second component to the pure electrolyte. The same approach is useful for other electrochemical properties as well the e.m. f. and the anodic behaviour of light, active metals, for instance. The structure of water in this "transition region" (TR), and therefore its reactions, can be expected to be quite different from its structure and reactions, in dilute aqueous solutions. (The same is true in relation to other non-conducting solvents.) The molecular structure of any liquid can be assumed to be close to that of the crystals from which it is derived. The narrower is the temperature gap between the liquid and the solidus curve, the closer are the structures of liquid and solid. In the composition regions between the pure water and a eutectic point the structure of the liquid is basically like that of water between eutectic and the pure salt or its hydrates the structure is basically that of these compounds. At the eutectic point, the conductance-isotherm runs through a maximum and the viscosity-isotherm breaks. Examples are shown in (125). 

The electrochemical current and potential parameters are connected to the properties of the solid electrolyte interface and we will devote this section to the structure of the electrode-electrolyte solution interface at the molecular scale. 

Figure 17 Molecular structures of a solid-state electrolyte OMeTAD and gelators.

The adsorption of surfactants at the liquid-solid surface is affected by the nature of the solid surface (surface charge, polarity and non-polarity), the molecular structure of surfactant molecules (head group charge and characteristics of hydrophobic tail) and the characteristics of the liquid phase (pH, electrolyte concentration, presence of additives and temperature). 

Chapters I to III introduce the reader to the general problems of fuel cells. The nature and role of the electrode material which acts as a solid electrocatalyst for a specific reaction is considered in chapters IV to VI. Mechanisms of the anodic oxidation of different fuels and of the reduction of molecular oxygen are discussed in chapters VII to XII for the low-temperature fuel cells and the strong influence of chemisorhed species or oxide layers on the electrode reaction is outlined. Processes in molten carbonate fuel cells and solid electrolyte fuel cells are covered in chapters XIII and XIV. The important properties of porous electrodes and structures and models used in the mathematical analysis of the operation of these electrodes are discussed in chapters XV and XVI. 

Molecular electronics Electrical displays Chemical biochemical and thermal sensors Rechargeable batteries and solid electrolytes Drug release systems Optical computers Ion exchange membranes Electromechanical actuators Smart structures Switches... 

The recent example of the ab initio structure determination of the polymer electrolyte Poly (ethylene oxide)6 LiAsFe by Bruce et is a notable example of the complex structures that can be determined from powder diffraction on a pulsed neutron source. Polymer electrolytes consist of salts dissolved in solid high molecular weight polymers, and represent a unique class of solid coordination compounds. Their importance lies in their potential in the development of truly all-solid-state rechargeable batteries. The structure of the 6 1 complex is particularly important, as it is a region where the conductivity increases markedly. The structure of the complex is distinct from all known crystal structures of PEO salt complexes (see Figure 7). The Li-i- cations are arranged in rows, with each row located inside a cylindrical surface formed by two PEO chains, with the PEO chains adopting a previously unobserved conformation. Furthermore the anions are located outside the PEO cylinders and are not coordinated with the cations. 

Stoddart and co-workers have developed molecular switch tunnel junctions [172] based on a [2]rotaxane, sandwiched between silicon and metallic electrodes. The rotaxane bears a cyclophane that shuttles along the molecular string toward the electrode and back again driven by an electrochemical translation. They used electrochemical measurements at various temperatures [173] to quantify the switching process of molecules not only in solution, but also in self-assembled monolayers and in a polymer electrolyte gel. Independent of the environment (solution, self-assembled monolayer or solid-state polymer gel), but also of the molecular structure - rotaxane or catenane - a single and generic switching mechanism is observed for all bistable molecules [173]. 

Any inference concerning the effects of a possibly altered molecular structure of water near the solid surfaces in soil clays must proceed from an acquaintance with the structure of liquid water in bulk and in aqueous electrolyte solutions. In this section, the current picture of the molecular arrangement in bulk water is reviewed. In Sec. 2.2, the same is done for aqueous solutions of inorganic electrolytes. These summaries are followed by discussions of the structure of water near the surfaces of phyllosilicates and the effect of these surfaces on the solvent properties of the water molecule. 

Not long after Dahn s work, Shenoy et al. employed molecular dynamics simulations to study the formation and growth of solid electrolyte interphase for the case of EC, DMC, and mixtures of these two solvent on lithium metalhc electrode [61]. In their work, they investigated the constitutes and structures of SEI on lithium metal electrode with the dependence of electrolyte composition and temperature change. The results show that the SEI films grow faster in the case of EC compared to DMC, with EC+DMC mixtures falling in between, as shown in Fig. 5.22. 

Popular approaches to molecular self-assembly, which can give structures in the nanometer to millimeter range, are based on SAMs and LBL deposition of electrolytes. Self-assembly leads to equilibrium structures that are close to the thermodynamic minimum and result from multiple weak, reversible interactious betweeu subuuits which include hydrogen bonds, ionic bonds, and van der Waals forces. As information is already coded in the building blocks, this is a means to avoid defect formation in aggregate formation. SAMs are molecular assemblies of long chain alkanes that chemisorb on the patterned and unpat-temed surfaces of appropriate solid materials. The structures of SAMs, effectively 2D-crystals with controllable chemical functionality, make them a means to modify substrates to direct protein adsorption and cell attachment, surface passivation, ultrathin resists and masks and sensor development. 

When a compound has strong covalent character then we expect it to be molecular with all the typical properties associated with simple molecular structures, such as relatively low melting and boiling points and non-electrolyte behaviour in water. This means that the solution will contain molecules and be non-conducting. Simple molecular substances are also non-conducting in the solid and liquid states. However, some molecular substances react with water to release ions. This is known as hydrolysis. An example is anhydrous aluminium chloride, which reacts with water in a hydrolysis reaction to form aluminium hydroxide and hydrochloric acid. 

With these developments, new interest arose in PEC as a solid material, partly due to the similarity to PEM concerning the local molecular structure so that they could act as reference material. The structural similarity was shown by solid state NMR, where the interatomic distances of the complexed monomers were found to be identical in PEC and in PEM [17]. Furthermore, employing regions of stability in dependence on electrolyte conditions, the phase diagrams of PEC and PEM systems are comparable to each other [18, 19]. The newer work on PEC was recently reviewed [20]. 

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