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.

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