Crystalline Ionic Conductors

1 Crystalline ionic conductors. Superionic conductors have already been briefly introduced in Section 1.2.2.2. They have been known for quite a long time, and a major NATO Advanced Study Institute on such conductors was held as early as 1972 (van Gool 1973). Of course, all ionic crystals are to a greater or lesser extent ionically conducting - usually they are cationic conductors, because cations are smaller than anions. Superionic conductors typically have ionic conductivities lO times higher than do ordinary ionic crystals such as KCl or AgCl. 

Beta-alumina, mentioned in Section 1.2.2.2, is just the best known and most exploited of this family. They have been developed by intensive research over more than three decades since Yao and Kummer (1967) first reported the remarkably high ionic conductivity of sodium beta-alumina. Many other elements have been used in place of sodium, as well as different crystallographic variants, and various processing procedures developed, until this material is now poised at last to enter battery service in earnest (Sudworth et al. 2000). 

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Defect model. Analogue to crystalline ionic conductors, a transport via defects is discussed for glasses. 

Further indirect information about the topology of the grain boundary phases can be obtained from the temperature dependence of the quantities and r i,. In crystalline ionic conductors in the extrinsic region, the conductivity is thermally activated and described by ... 

The desire to realise technological goals has spurred the discovery of many new solid electrolytes and intercalation compounds based on crystalline and amorphous inorganic solids. In addition an entirely new class of ionic conductors has been discovered by P. V. Wright (1973) and M. B. Armand, J. M. Chabagno and M. Duclot (1978). These polymer electrolytes can be fabricated as soft films of only a few microns, and their flexibility permits interfaces with solid electrodes to be formed which remain intact when the cells are charged and discharged. This makes possible the development of all-solid-state electrochemical devices. 

Various theoretical attempts have been made to provide a quantitative interpretation of the dispersion region (Funke, 1986 Funke and Hoppe, 1990). While the situation is still not fully resolved, it is now clear that such a dispersion, which has been observed in a wide range of crystalline as well as glassy ionic conductors, is associated with ion-ion relaxation effects. The conductivity dispersion, ff(co), is usually linear in a plot of log a vs log CO, which means that it can be represented by a power law expression ... 

Liquid crystalline ionic liquids can be used as low dimensional ion conductors [6, 8, 26-29]. For this purpose the formation of oriented monodomains in the macroscopic scale is important because the boundary in the randomly oriented polydomains disturbs high and anisotropic ion conduction. 

The idea that ions can diffuse as rapidly in a solid as in an aqueous solution or in a molten salt may seem astonishing. However, since the 1960s, a variety of solids that include crystalline compounds, glasses, polymers, and composite materials with exceptionally high ionic conductivities have been discovered. Materials that conduct anions (e.g. and 0 ) and cations including monovalent (e.g. H+, Fi+, Na+, Cu+, Ag+), divalent, and even trivalent and tetravalent ions have been synthesized. A variety of names that have been used for these materials include solid electrolytes, superionic conductors, and fast-ionic conductors. Solid electrolytes arguably provides the least misleading and broadest description for this class of materials. 

Appreciable ionic conductivity is found in open framework or layered materials containing mobile cations (see Ionic Conductors). Several phosphates have been found to be good ionic conductors and are described above NASICON (Section 5.2.1), a-zirconium phosphates (Section 5.3.1), HUP (Section 5.3.3), and phosphate glasses (Section 5.4). Current interest in lithium ion-conducting electrolytes for battery apphcations has led to many lithium-containing phosphate glasses and crystalline solids such as NASICON type titanium phosphate being studied. ... 

Irvine, J. and West, A., Crystalline lithium ion conductors, in High Conductivity Solid Ionic Conductors, T. Takahashi, Ed., World Scientific, Singapore, 1989. 

As with VT, identifying conduction pathways with bond valence maps provides accurate predictions for ionic conductors (both crystalline and amorphous) with a percolation mechanism of conductivity, but is less successful when modeling proton conductors. 

In modelling crystalline solids, the MC technique is of particular value in three distinct fields. The first concerns studies of sorbed systems, e.g. micropo-rous solids loaded with organic molecules. MC techniques are particularly suitable for studying the variation with temperature of the distribution of sorbed molecules in such systems (Yashonath et al., 1988). Secondly, the method has been fruitfully applied to the study of atomic diffusion. In this case the moves are atomic jumps of defined frequencies. In complex solids (including, e.g., alloys and ionic conductors), use of the MC technique allows accurate sampling of all the different jump mechanisms contributing to the diffusion, as shown in several studies of Murch and coworkers (e.g. Murch, 1982). 

The second class of crystalline sensors is based on the easily fabricated, low-resistance, selectively permeable cast-disk and pressed-pellet membranes based on AgaS. Silver sulfide is an ionic conductor in which silver ions are the mobile species. By itself, it can be used to detect silver ions or to measure sulfide-ion levels. The potential-determining mechanism in an AggS electrode is due to the very low solubility product of AgaS [ATgp = 10 ]. The silver-ion activity... 

Recent studies have extended the structure-property relationships in polyphosphazenes to include polymers that show interesting electrical or optical behavior in the solid state. These fall into three categories - (1) polymers that are good solid solvents for salts and which function as solid ionic conductors, (2) species that bear electronically active side groups, and (3) polymers that bear rigid organic units that generate liquid crystalline or non-linear-optical behavior. 

The equations for the diffusion and charge transfer processes into the crystalline ionic-electronic conductors were obtained by Wagner [11] and Yokota [12]. They became the basis for study of transport properties of solid electrolytes, in particular, for determination of the electronic conductivity value [13]. However, these theories are no longer tme if the Faradaic process of electrochemical decomposition of the PFC occurs at the interfaces. The elementary theory for stationary process at such conditions [8] and some experimental examples [9] are considered below. 

In recent years, more complex types of transport processes have been investigated and, from the point of view of solid state science, considerable interest is attached to the study of transport in disordered materials. In glasses, for example, a distribution of jump distances and activation energies are expected for ionic transport. In crystalline materials, the best ionic conductors are those that exhibit considerable disorder of the mobile ion sublattice. At interfaces, minority carrier diffusion and discharge (for example electrons and holes) will take place in a random environment of mobile ions. In polycrystalline materials the lattice structure and transport processes are expected to be strongly perturbed near a grain boundary. 

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