Comparative study of Ta and Ga doping effects on Li₇La₃Zr₂O₁₂ garnet electrolytes for advanced thermal battery applications

The paradigm shift from conventional molten-salt electrolytes to solid-state garnet-type Li7La3Zr2O12 (LLZO) electrolytes in thermal batteries represents a critical advancement in high-temperature energy-storage systems. This study evaluated Ta- and Ga-doped LLZO electrolytes for FeS2/Li-Si thermal batteries, focusing on their structural stability and electrochemical performance at 500 °C. While Ga-doped LLZO exhibited superior ionic conductivity at 25 °C, Ta-doped LLZO demonstrated exceptional high-temperature stability. Ta-doped LLZO cells achieved longer discharge durations and higher energy densities than Ga-doped LLZO cells, which is attributed to the retained cubic phase and minimised interfacial degradation. Conversely, Ga-doped LLZO exhibited cubic-to-tetragonal phase transitions, Ga precipitation, and formation of impurities such as Ga2O3 and Li-Ga alloys, leading to 54% loss of ionic conductivity post-discharge. These results contribute valuable insights for the optimisation of solid-state electrolytes in thermal battery systems, suggesting that conventional room-temperature performance metrics may not translate directly to elevated-temperature operations.