Metastable properties of a garnet type Li5La3Bi2O12 solid electrolyte towards low temperature pressure driven densification

Solid state electrolytes represent an attractive alternative to liquid electrolytes for rechargeable batteries. However, the fabrication of batteries with ceramic materials requires high temperature that could be detrimental to their electrochemical performance. In this work, we show that it's possible to densify a garnet-type Li5La3Bi2O12 solid electrolyte at low temperature (600 °C) with respect to standard high sintering temperature (T > 1000 °C) used for zirconium-based Li7La3Zr2O12 doped garnet. Li5La3Bi2O12 showed a high conductivity (1.2 × 10-4 S cm-1) after hot pressing at 600 °C. The synthesis conditions have been optimized: at 700 °C we observed the presence of the LiLa2O3.5 phase as a consequence of LLBO metastability, and the formation mechanism has been described by density functional theory (DFT) and density functional perturbation theory (DFPT) calculations. Moreover, we have reported the application of small amounts of LLBO as a sintering aid (5-10%) in the densification of LLZTO. Our investigation successfully demonstrated that LLBO introduction positively affects the densification process and global performances of LLZTO garnet, allowing us to obtain an ionic conductivity higher than 10-4 S cm-1 after annealing at 600 °C.