Conductors with Large Alkali Ions

Four structural families of materials with an Na -ion conductivity in excess of 0.1 S cm at 300 °C are known presently (Sections [7.4.1-7.4.4] see also Table 7.2). 

Of these, the P- and P -aluminas have been studied most extensively. The isostructural sodium gallates, potassium, rubidium and cesium gallates, aluminates, and ferrites are also known. The ideal formulas for the p and P structures are AMuOn and A2M11O17, respectively, but all of the compounds are intrinsically nonstoichiometric and the actual alkali content is always greater than unity but less than 2. The most obvious charge compensation mechanism is alower-valence cation substitution for trivalent M , namely Ai, )Oi7 (M = Mg, Co, Ni, Zn) and 

Al + x(Mj jt. 2 Li 2)Oi7- Without these dopants, the ferrites are charge-compensated by Fe (giving rise to electronic conductivity), and sodium gallates, by sodium substitution for gallium. For aluminates, these variants are impossible. As a result. 

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Tlie aim of this chapter is to provide an overview of materials where fast transport of alkali metal cations and protons is observed. A general discussion of factors affecting conductivity and techniques used to study ion migration paths is followed by a review of the large number of cation conductors. Materials with large alkali ions (Na-Cs) are often isostructural and therefore examined as a group. Tire lithium conductors with unique crystal structure types and proton conductors with unique conduction mechanisms are also discussed. 

The best Na +, K +, Rb, and Cs ion conductors all belong to the five families described above. Some other high-conductivity materials are represented in Table 7.2 and Figure 7.17, but descriptions of their structures are omitted here. Only a very specific class of oxide structures with a 3 A translation (types I-XIV in Figure 7.19 [48,49,53,168-187]) will be reviewed briefly. This short distance precludes the location of any atom or ion between two large alkali ions, guaranteeing their free motion in that direction [62]. The freedom may, however, be restricted by impurities, crystal defects (e.g., anions on cation sites [188]) and grain-boundaries in ceramics. 

A number of other -alumina related phases have been prepared. In some of these the spinel blocks have an increased thickness, the so-called P, P" and P " phases, while in others, the Na or A1 components have been replaced with similar species. Related structures, such as BaMgAlnOiy doped with Eu +, are widely used as phosphors. Crystal-structure studies on such materials show that the defects present depend sensitively upon both temperature and the constituents of the phase. Large replacement ions, lanthanide or alkali metals, tend to occupy the interlayer regions as interstitial defects, but surprisingly, some also enter the spinel blocks as substitutional defects, in association with oxide ion vacancies. Smaller ions occupy the spinel blocks as substitutional point defects. The delicate balance between oxygen interlayer interstitials and spinel block cation vacancies varies with composition. These defect interactions can often be successfully explored by using simulation techniques. Ordering occurs at lower temperatures see Ionic Conductors). 

Initial measurements carried out on PEO-alkali metal salt complexes indicated that the observed conductivities were mostly ionic with little contribution from electrons. It should be noted that the ideal electrolyte for lithium rechargeable batteries is a purely ionic conductor and, furthermore, should only conduct lithium ions. Contributions to the conductivity from electrons reduces the battery performance and causes self-discharge on storage. Salts with large bulky anions are used in order to reduce ion mobility, since contributions to the conductivity from anions produces a concentration gradient that adds an additional component to the resistance of the electrolyte. 

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