Phase Evolution and Thermodynamics of Al-Doped Cubic LLZO Studied by High-Temperature X-Ray Diffraction

Li₇La₃Zr₂O₁₂ (LLZO) is a promising solid-state electrolyte for advanced Li-ion batteries due to its high ionic conductivity, non-flammability and high electrochemical stability against Li anodes and high voltage cathodes. However, synthesis of phase pure LLZO is challenging due to easy formation of secondary phases, such as the pyrochlore La₂Zr₂O₇, that are detrimental to the performance of a LLZO electrolyte. Although many studies have been performed on the electrochemical performance of LLZO, the understanding of the thermodynamics of the synthesis of LLZO is still lacking.<br/><br/>Here we report on high-temperature X-ray diffraction (HTXRD) following the crystallization of a gel of Al-doped LLZO. Gels with 0, 10 and 20 mol% excess Li were prepared and calcined at 500 °C to remove the volatile components before HTXRD. The powders calcined at 500 °C consisted primarily of La₂Zr₂O₇ with trace amounts of Li₂CO₃ and La₂O₂CO₃ and were used as starting materials to study the formation of the cubic LLZO phase. During HTXRD the samples were heated from 500 °C to 700 °C in steps of 20 °C, and further to 1000 °C in steps of 50 °C. Upon heating the pyrochlore phase gradually transforms to cubic LLZO. In all samples, cubic LLZO first appears around 600 °C and the pyrochlore phase disappears around 750 °C in the samples with 0 % and 20 % excess Li. We performed two analyses of the sample with 10 % excess Li by HTXRD, one with normal deposition thickness and another with thinner deposition thickness. In the thin sample, cubic LLZO appears at a lower temperature of 560 °C compared to the other experiments. The pyrochlore phase does not fully disappear but instead reappears and crystallizes at the highest temperatures investigated, indicating extensive Li loss from the thin sample. These observations suggest that atmospheric exposure and the relative surface area of powder are important factors for LLZO formation.<br/><br/>To understand the thermodynamics of LLZO formation, we propose the following reaction to take place based on the identified reactants and products:<br/>6.25 Li₂CO₃ + 0.25 Al₂O₃ + 2 La₂Zr₂O₇ + La₂O₂CO₃ → 2 Li_6.25−xAl_0.25La₃Zr₂O_12−0.5x + 7.25 CO₂(g) + x Li₂O(g).<br/>The enthalpy, entropy and Gibbs energy of the reaction were calculated from literature values and DFT calculations and agree with the experimental observations. Finally, we discuss the significance of the highly disordered Li sublattice for the entropy of cubic LLZO. We believe our findings will contribute to improved understanding of LLZO formation and facilitate the synthesis of high-performance phase pure LLZO electrolytes.