Beta-Alumina Ceramic Electrolytes

The cell is based upon a liquid sodium anode and liquid sulphur cathode, separated by a beta alumina ceramic-type electrolyte which is an electronic insulator, but through which sodium ions diffuse rapidly at 300-400 C. During discharge the reaction 2Na + 5S —> Na S. leads to an open circuit voltage (GCV) of 2.08V. Continued reaction beyond Na S results in the formation of lower polysulphides in the range Na - Na S (OCV 1.78V), after which solid separates out. The polysulphides... 

Sodium/sulfur and sodium/metal chloride technologies are similar in that sodium is the negative electrode material and beta-alumina ceramic is the electrolyte. The solid electrolyte serves as the separator and produces 100% coulombic efficiency. Applications are needed in which the battery is operated regularly. Sodium/nickel chloride cells have a higher open-circuit voltage, can operate at lower temperatures, and contain a less corrosive positive electrode than sodium/sulfur cells. Nevertheless, sodium/nickel chloride cells are projected to be more expensive and have lower power density than sodium/sulfur cells. 

An alternative, well-studied approach to develop Na metal anode rechargeable cells is to isolate the Na electrode from the cathode via a ceramic Na ion conductor, i.e., a solid electrolyte. Such batteries are operated above the melting point of Na, thereby enhancing electrode kinetics and solid state diffusion of Na ions through the special alumina ceramic. The most commonly employed material is beta double prime alumina, which has channels permitting the facile diffusion of Na ions. 

There is some interest in iron-doped beta alumina for use as an ionic cathode in conjunction with a beta alumina electrolyte. The obvious attraction of this system is that interfacial problems are largely eliminated and the ionic diffusivity in the cathode should approximate to that in the electrolyte. The a g j tages of an all-ceramic battery were recognised some while ago, but the observed power density was too low for practical battery applications, although it was suggested that the device might find application as a capacitor. 

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