A dense .beta.''-alumina/zirconia composite solid electrolyte and process for fabrication are disclosed. The process allows fabrication at temperatures at or below 1600.degree. C. The solid electrolytes include a dense composite matrix of .beta.''-alumina and zirconia, and one or more transition metal oxides that aid the conversion and densification of precursor salts during sintering. The composite solid electrolytes find application in sodium energy storage devices and power-grid systems and devices for energy applications.
STATEMENT REGARDING RIGHTS TO INVENTION MADE UNDER FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT
 This invention was made with Government support under Contract DE-AC05-76RLO1830 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.