The converter is based on the electrolyte used in the sodium–sulfur battery, sodium beta″-alumina, a crystalline phase of somewhat variable composition containing aluminum oxide, Al 2 O 3, and sodium oxide, Na 2 O, in a nominal ratio of 5:1, and a small amount of the oxide of a small-cation metal, usually lithium or magnesium, which stabilizes the beta″ crystal structure. The sodium beta″-alumina solid electrolyte (BASE) ceramic is nearly insulating with respect to transport of electrons and is a thermodynamically stable phase in contact with both liquid sodium and sodium at low pressure.