Reliable energy systems with high energy density capable of operating at high temperatures, pressures and radiation levels are needed for certain NASA missions. Batteries based on solid-state ceramic oxide composite electrolytes provide the benefit of operation at temperatures up to 550o C. The current high-temperature Li battery technologies use carbonate, chloride and other molten compounds based electrolytes within porous separators. The operating temperatures of these systems are limited to below 450o C due to reactivity of these electrolytes with electrode materials and issues associated with sealing and packaging such batteries systems. Solid state electrolyte based batteries can be packaged to be leak proof under extreme conditions as there are no liquid components involved. Ceramatec Inc. is proposing to develop LiAl-solid electrolyte-FeS2 battery system based on co-pressed and laminated electrode-electrolyte structures. The Li ion conductivity of the solid electrolytes developed at Ceramatec approaches 10-1 S/cm at 400o C, and are chemically compatible with the selected electrode materials. These batteries can be developed as either primary or secondary battery systems for use at temperatures above 400o C. In our Phase I SBIR program, single cells will be fabricated and their specific energy, cyclability and rate capability will be evaluated.