Beta-alumina solid electrolyte
Beta-alumina solid electrolyte (BASE) is a fast ion conductor material used as a membrane in several types of molten salt electrochemical cell. Currently there is no known substitute available.
β''-Alumina (beta prime-prime alumina) is an isomorphic form of aluminium oxide (Al2O3), a hard polycrystalline ceramic, which, when prepared as an electrolyte, is complexed with a mobile ion, such as Na+, K+, Li+, Ag+, H+, Pb2+, Sr2+ or Ba2+ depending on the application. Beta-alumina is a good conductor of its mobile ion yet allows no non-ionic (i.e., electronic) conductivity.
Sodium beta alumina is a non-stoichiometric sodium aluminate known for its rapid transport of Na+ ions. This material selectively passes sodium ions while blocking other species, including liquid sodium and liquid sulfur. It is a ceramic which can be formed and sintered by commercially available techniques and its conductivity at operating temperatures – 250 to 300 degrees Celsius – compares favorably with electrolytes used in conventional battery systems such as sulfuric acid and potassium hydroxide. The crystal structure of the Na-Al2O3 provides an essential rigid framework with channels along which the ionic species of the solid can migrate. Ion transport involves hopping from site to site along these channels.
BASE was first developed by researchers at the Ford Motor Company, in the search for a storage device for electric vehicles while developing the sodium-sulfur battery. The NaS battery consists of sulfur at the positive electrode and sodium at the negative electrode as active materials, and sodium-conducting beta alumina ceramic as the electrolyte separating both electrodes. This sealed battery is kept at approximately 300 degrees Celsius and is operated under the condition that the active materials at both electrodes are liquid and its electrolyte is solid. It is only at temperatures around 300 degrees Celsius or more, that it is possible for the negative sodium electrode to completely coat, or "wet," the ceramic electrolyte. At lower temperatures, molten sodium has issues in covering the surface of the beta alumina ceramic. This causes sodium to curl up akin to a drop of oil in water, reducing surface area contact and making the battery less efficient. At such high temperatures, since both active materials have high surface area contact and internal resistance becomes low enough, the NaS battery shows excellent performance. As a secondary battery, which allows reversible charging and discharging, the NaS battery can be continuously used. Several commercial installations use this type of battery for load leveling.
The sodium sulfur battery was a topic of intense worldwide interest during the 1970s and 1980s, but interest in the technology for vehicle use diminished for a variety of technical and economic reasons. In contrast, its "successor", the sodium nickel chloride battery, is now entering the commercialization phase. The sodium nickel chloride battery (or ZEBRA battery, so-called for the Zeolite Battery Research Africa Project) has been under development for almost 20 years.
When BASE is used in a sodium nickel chloride (ZEBRA) cell, several requirements must be met. It must have a low resistivity, typically 4 Ω/cm at 350 °C, and a strength in excess of 200 MPa. It must be produced in the form of a thin-walled (1.25 mm), convoluted tube by low-cost production methods, and it must maintain a stable cell resistance for up to 10 years. These requirements have mostly been met by a variation of the sol-gel process.
BASE is also used in alkali-metal thermal to electric converters (AMTECs). An AMTEC is a high efficiency device for directly converting heat to electricity. An AMTEC operates as a thermally regenerative electrochemical cell by expanding sodium through the pressure differential across the (BASE) membrane. BASE electrolytes have been used in some molten-carbonate fuel cells, as well as other liquid electrode/solid electrolyte fuel cell designs.
- Y.F.Y. Yao and J.T. Kummer, J. Inorg. Nucl. Chem. 29 (1967) p. 2453
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