Ionic conductivity enhacements and low temperature synthesis of Li7La3Zr2O12 garnets by Bi aliovalent substitutions

We report on a novel approach to synthesize cubic-phase fast ionic conducting garnet-type solid state electrolytes based on Bi doped Li7La3Zr2O12 (LLZO). Bi aliovalent substitution into LLZO utilizing the Pechini processing method is successfully employed to synthesize Li7-xLa3Zr2-xBixO12 compounds. Ionic conductivities up to 2.0 x 10-4 S/cm are achieved in structures not fully densified. Cubic phase Li6La3ZrBiO12 powders are generated in the temperature range from 650 °C to 900 °C in air. In contrast, in the absence of Bi and under identical synthesis conditions, the cubic garnet phase of Li7La3Zr2O12 is not formed below 700 °C while a transformation to the tetragonal phase is observed at 900 °C for the un-doped compound. The critical role of Bi in lowering the formation temperature of the garnet cubic phase and the improvements in ionic conductivity is investigated in this work through microstructural studies and AC impedance measurements. We ascribe the effect of Bi doping in achieving these remarkable improvements to significant enhancements at lower temperatures in the kinetics of the solid-state reaction resulting in explosive grain growth and densification of the garnet. Moreover, XAS is utilized to identify the specific atomic site where Bi is incorporated in the LLZO garnet crystalline structure.