Solid ion conductors

Anion Interstitials The other mechanism by which a cation of higher charge may substitute for one of lower charge creates interstitial anions. This mechanism appears to be favored by the fluorite structure in certain cases. For example, calcium fluoride can dissolve small amounts of yttrium fluoride. The total number of cations remains constant with Ca +, ions disordered over the calcium sites. To retain electroneutrality, fluoride interstitials are created to give the solid solution formula 

Double Substitution In such processes, two substitutions take place simultaneously. For example, in perovskite oxides, La may be replaced by Sr at the same time as Co is replaced by Fe to give solid solutions Lai Sr Coi yFey03 5. These materials exhibit mixed ionic and electronic conduction at high temperatures and have been used in a number of applications, including solid oxide fuel cells and oxygen separation. 

FIGURE 25.3 Schematic representation of ionic motion by (a) a vacancy mechanism and (b) an interstitial mechanism. (From Smart and Moore, 1996, Fig. 5.4, with permission from Routledge/Taylor Francis Group.) 

Notice that the energy of the ion is the same at the beginning and the end of the jump the energy required to make the jump, E, is known as the activation energy for the jump. This means that the temperature dependence of the mobility of the ions can be expressed by an Arrhenius equation  

The term Oq now contains n and ze as well as the information on attempt frequency and jump distance. This expression accounts for the fact that ionic conductivity increases with temperature. 

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The electrolyte used in lithium cells, i.e., for aU hthium couples, must be completely anhydrous (< 20 ppm H2O) alkali metals in general are compatible with neutral salt solutions in aprotic solvents or neutral molten salts or solid ion-conductors. 

O. Yamamoto, et ah, "Zirconia Based Solid Ion Conductors," The International Fuel Cell Conference Proceedings, NEDO/MITI, Tokyo, Japan, 1992. 

Fischer, W. (1989) in High Conductivity Solid Ion Conductors Recent Trends and Applications, Ed. T. Takahashi, World Scientific, Singapore, p. 595. 

For the sake of discussion, we have divided the separators into six types—microporous films, non-wovens, ion exchange membranes, supported liquid membranes, solid polymer electrolytes, and solid ion conductors. A brief description of each type of separator and their application in batteries are discussed below. 

Ionic conductors have many practical applications. For example, solid ion conductors are used as solid electrolytes and electrode materials in -> batteries, fuel cells, - electrochromic devices and - gas sensors. 

Under usual conditions at least one sublattice is very rigid and—in the case of interest (in particular when dealing with solid ion conductors)—one sublattice exhibits a significant atomic mobility. The selectivity of the conductivity (cf. also the selective solubility of foreign species) is indeed a characteristic feature of solids. 

In NMR work, spin-lattice relaxation measurements indicated a non-exponential nature of the ionic relaxation.10,11 While this conclusion is in harmony with results from electrical and mechanical relaxation studies, the latter techniques yielded larger activation energies for the ion dynamics than spin-lattice relaxation analysis. Possible origins of these deviations were discussed in detail.10,193 196 The crucial point of spin-lattice relaxation studies is the choice of an appropriate correlation function of the fluctuating local fields, which in turn reflect ion dynamics. Here, we refrain from further reviewing NMR relaxation studies, but focus on recent applications of multidimensional NMR on solid-ion conductors, where well defined correlation functions can be directly measured. 

In multidimensional NMR studies of organic compounds, 2H, 13C and 31P are suitable probe nuclei.3,4,6 For these nuclei, the time evolution of the spin system is simple due to 7 1 and the strengths of the quadrupolar or chemical shift interactions exceed the dipole-dipole couplings so that single-particle correlation functions can be measured. On the other hand, the situation is less favorable for applications on solid-ion conductors. Here, the nuclei associated with the mobile ions often exhibit I> 1 and, hence, a complicated evolution of the spin system requires elaborate pulse sequences.197 199 Further, strong dipolar interactions often hamper straightforward analysis of the data. Nevertheless, it was shown that 6Li, 7Li and 9Be are useful to characterize ion dynamics in crystalline ion conductors by means of 2D NMR in frequency and time domain.200 204 For example, small translational diffusion coefficients D 1 O-20 m2/s became accessible in 7Li NMR stimulated-echo studies.201... 

The local structure of the fast solid ion-conductor Cu2P3I2, i.e. (CuI)8Pi2, was investigated by 1-D and 2-D 31P and 65Cu MAS-NMR spectroscopy.55131P NMR studies on SmFe4P12 show that the system remains in a paramagnetic state above the Curie temperature.552 The silica-supported complex =Si-ORh(P Pr3)2(H)2 was characterised by 31P MAS-NMR.553... 

Solid-state cells for operation at ambient temperatures are mentioned for comparison with the wet cells. The low conductivities of fast solid ion conductors at ambient temperatures, cf. LijN and Lil (Table X) limit their use to fields where low discharge currents can be tolerated, e.g. batteries for cardiac pacemakers. 

Reversible Electrodeposition of a silver iodide complex from a solution of 0.3 M Agl and KI or Rbl, and Ij in DMSO or diethyl malonate is an example for the third class of wet non-emissive electro-optic displays . As long as the silver content of the solution is high enough the solvent did not deteriorate when pulses of 50 V were passed through. The addition of AljOj, for preventing TiOj from agglomeration, and the use of RbAg4lj in DMSO as the solid ion-conductor established a cell which survive more than 10 cycles when operated at <2V drive, the realized response times were < 10 ms. 

Preparation of solid-state electrochemical devices require that conventional fluid solution electrolytes be replaced with a solid ion conductor. Solid electrolytes have been widely studied, primarily for the development of high energy-density batteries. Classes of solid state electrolytes include classical solids such as the p-aluminas, polyelectrolytes such as Nation, gel electrolytes and polymer electrolytes. For the purpose of developing solid-state electrochemical devices, polymer electrolytes are promising, because they are easily confined to microelectrochemical arrays/and are gas permeable. 

Solid electrolytes. These correspond to soHd materials in which the ionic mobility is insured by various intrinsic and extrinsic defects and are called solid ion conductors. Common examples are ion-conducting solids with rock salt or halite-type solids with a Bl structure (e.g., a-AgI), oxygen-conducting solids with a fluorite-type Cl structure (A"02), for instance CaF and yttria-stabilized zirconia (YSZ, ZrO with 8 mol.% Y O,), a pyro-chlore structure (A BjO ), perovskite-type oxides (A"B" 03), La Mo O, or solids with the spinel-type structure such as beta-aluminas (NaAl 0 ) for which the ionic conduction is ensured by Na mobility. 

History of solid oxide electrolytes and of SOFC development in early years has been nicely summarized by Mobius [1], Interest in solid ion conductors first arose in coimection with the... 

NASICON (Na Super Ionic Conductor)-type solid ion conductors were discovered in 1976 by Goodenough and co-workers [29]. The general formula of this stmcture... 

As for thermodynamic measurements using liquid electrolytes, galvanic cells with solid ion conductors are widely applied to study thermodynamic properties of solids and melts. These measurements are based on the determination of galvanic cell emf (Chap. 1) when the reference electrode potential is known. In a simplest case, when A " cation-conducting electrolyte is employed and the RE comprises metal A, the cells... 

The solid ion conductors act both as electrolytes and separators. These materials aUow ions to migrate through their lattice when a chemical or electrical gradient is applied. 

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