Beta alumina

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). 

Sudworth, J.L. et al. (2000) Toward commercialization of the beta-alumina family of ionic conductors, MRS Bull. 25(3), 22. 

Figure 6 XPS of Pt/beta alumina acquired under electrochemical bias.

Montanaro L., Negro A., Pijolat C., Lalauze R., Synthesis and forming of beta alumina to be used as gas sensor, Annales de Chimie [Science des Materiaux] 1995 20 399-402. 

Sodium ion conduction appears to be common because of the well-known properties of the beta-aluminas and, to a lesser extent, the NASICONs (see Section 2.12.1), Table 2.1. There are, however, relatively few other examples of high Na ion conductivity, especially at room temperature. In contrast to Ag, the usual coordination number of Na is high, often 7-9, and the sites may be distorted. The bonding of Na in such structures is much more ionic than that of Ag, therefore. 

The beta-alumina structures show a strong resemblance to the spinel structure. They are layered structures in which densely packed blocks with spinel-like structure alternate with open conduction planes containing the mobile Na ions. The and /S" structures differ in the detailed stacking arrangement of the spinel blocks and conduction planes. Fig. 2.9. 

The beta-alumina structures (j8,j8"-alumina and the analogous gallates) can be prepared as H30 or NH4 derivatives by ion exchange and some of these are good proton conductors at temperatures up to 200-400 °C, until they decompose by loss of H2O/NH3. 

Like the Li/FeSx system, which is presently the most advanced rechargeable battery system based on a molten salt electrolyte, the Na/S system is presently the most advanced rechargeable battery system based on a solid electrolyte (beta-alumina) It operates at about 300 C. 

The Prospects For Solid Electrolyte SBs. For reasons discussed above, the Agl-based cells, being useful for some special types of primary batteries, are not very promising for secondary ones. The beta-alumina cells, on the contrary, have already been developed to the pilot-plant stage and their prospects are fairly good to become commercialized. They are the most advanced among the candidate batteries for traction. The high operating temperature could be lowered if a solid... 

Fig. 7.17 Relation of the spinel structure (left) io the structure of sodium beta alumina (right). The sodium ions are free to move in the open spaces between spinel blocks, held apart by Al—O—Al pillars in the "parking garage structure. [In part from Wells. A. F. Structural Inorganic Chemistry, 5th ed. Oxford University Oxford, 1984. Reproduced with permission. ...

Another example of this type of intercalation compound is sodium beta alumina where the sodium ions are free to move between the spinel layers. The sodium ions can be replaced by almost any +1 cation such as Li. K, Rb+, Cs. NHJ, H 0 Tl+, Ga+, NO+, etc. The conductivity of these materials varies with the size of the ions moving between the fixed-distance (A)—0—Ai) layers. 

Another solid electrolyte that may lead to important practical applications is sodium beta alumina. Its unusual name comes from a misidertificalion and an uncer-... 

Fig. 7.18 Sodium/sulfur battery with a sodium beta alumina solid electrolyte.

Consider the battery in Fig. 7.18. The sodium beta alumina barrier allows sodium ions formed at the anode to Row across to the sulfur compartment, where, together with the reduction products of the sulfur, U forms a solution of sodium trisulfide in the sulfur. The latter is held at 300 CC to keep it molten. The sodium beta alumina also acts like an electronic insulator to prevent short circuits, and it is inert toward both sodium and sulfur. The reaction is reversible. At the present state of development, when compared with lead storage cells, batteries of this sort develop twice the power on a volume basis or four times the power on a weight basis. 

Sintering is a critical step with schedules extending over typically 24 h and a peak temperature of approximately 1600 °C. At this temperature there is an appreciable sodium vapour pressure over beta aluminas and special precautions are taken to maintain the required composition. The component may be enclosed in a sealed spinel (MgAl204) saggar which can be re-used. 

A.T. Hunt, CCVD Processing of Lanthanum Phosphate and Beta-Alumina Fiber Coatings for Mullite Matrix Composites, Contract DM19561712, NSF SBIR Phase 1 Grant, 1996. 

The so-called beta-aluminas are characterized by the existence of layered blocks of a trivalent element (most frequently AT, but also Fe " and Ge ) with the same spinel structure separated by mirror planes where large monovalent (Na or K), divalent (Ba, and sometimes Ca or Sr) and trivalent (La) cations, which do not enter the cubic close packed array of oxide ions of the spinel blocks, are located, together with oxide anions bridging between the blocks. In these planes the cations... 

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