"Metal-air" batteries have garnered much attention in recent years due to their high intrinsic specific energy and use of inexhaustible and storage-free oxygen source -air- for the "metal-oxygen" reaction. In this study, we report the performance of a new type of all solid-state "iron-air" battery operated at 550 degrees C. The results show that CeO2 nanoparticles incorporated into the Fe-Fe3O4 redox-couple can improve the specific energy (Wh/kg) and round trip efficiency by 15% and 29%, respectively, over the baseline Fe-Fe3O4 battery. Use of supported Fe-Fe3O4 nanoparticles as the redox couple can increase the specific energy and round-trip efficiency by 13% and 48% over the baseline battery, respectively. However, the nanoparticle redox-couple is susceptible to thermal coarsening under operating conditions, causing cycle stability problem. Future research needs to focus on improving battery's performance and stability by utilizing thin-film electrolyte and high-performance and stable electrodes, and preventing thermal growth of active redox couple nanoparticles. A most recent testing on an optimized battery component has shown promising results: discharge specific energy can reach 91.0% of the maximum theoretical specific energy with a round-trip efficiency of 82.5%. (C) 2013 The Electrochemical Society.