Sodium aluminate solid electrolytes

Table I. Objectives for Sodium Aluminate Solid Electrolytes...

In these cells readily available substances are used as active materials -molten sodium and sulfur working in contact with a solid electrolyte (sodium beta-aluminate). Sulfur-sodium storage cells show rather large values of specific electrical energy. Their working temperature is 350 C, i.e. before use they must be heated up to this temperature. Storage cells with electrodes from iron sulfide and lithium alloys with a melt of chlorides as electrolyte exhibit similar properties. The working temperature of these cells is about 400°C. 

The alkali metals—lithium, sodium, and potassium—are logical choices for anodes in a sulfur-based electrochemical cell. All three have been incorporated into cells, and lithium and sodium remain under serious consideration. The lithium-sulfur combination is the topic of another chapter in this volume and will not be discussed further. Two types of sodium-sulfur cells have been constructed. One type uses thin-walled glass capillaries as a cell divider, and the other uses various sorts of ionically conducting sodium aluminate for this purpose. Of the two, the latter seems to hold the most promise and certainly has generated the most interest and enthusiasm (1). Because of the unique properties of the solid electrolyte cell separator this battery is also probably the most interesting from a purely scientific point of view. 

Alumina is one of the best electrical insulators, hence its dielectric applications. Electric conduction depends primarily on impurities which act as acceptors (for example, Mg, Fe, Co, V or Ni) or as donors (for example, H, Ti, Si, Zr or Y), the balance between electronic and ionic contributions depending on the temperature and the partial pressure of oxygen [KRO 84]. Among the impurities, sodium deserves a particular mention because the synthesis of alumina by the Bayer process brings into play sodic mediums sodium aluminate (Na2011Al203), also called beta alumina, is an ionic conductor with very high conductivity, which is why this compound is envisaged for solid electrolyte applications [WES 90]. 

Good descriptions of the production of aluminum can be found in the literature (Grjotheim etal. [7], Grjotheim and Welch [8], Grjotheim and Kvande [9], Burkin [10], and Peterson and Miller [11]). Referring to Fig. 2 [12], the first step in the production of aluminum from its ore ( bauxite ) is the selective leaching of the aluminum content (present as oxides/hy dr oxides of aluminum) into hot concentrated NaOH solution to form sodium aluminate in solution. After solution purification, very pure aluminum hydroxide is precipitated from the cooled, diluted solution by addition of seed particles to nucleate the precipitation. After solid-liquid separation the alumina is dried and calcined. These operations are the heart of the Bayer process and the alumina produced is shipped to a smelter where the alumina, dissolved in a molten salt electrolyte, is electrolyt-ically reduced to liquid aluminum in Hall- Heroult cells. This liquid aluminum,... 

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