Combinatorial study of the Li-La-Zr-O system

Abstract

Lithium-stuffed garnets based on Li7La3Zr2O12 (LLZO) have attracted a lot of attention as candidate electrolytes for all-solid lithium batteries. Doping studies have shown that substitutions will induce lithium vacancies and convert the structure from tetragonal to cubic and greatly increase ionic conductivity. However, the impacts of structure and composition have not been fully decoupled. Herein, we systematically study over 700 samples from the Li-La-Zr-O composition space to produce phase stabilities over the entire pseudoternary for the first time. The results obtained for samples made at 900 °C show that the pyrochlore phase La2Zr2O7, previously only known to exist at its formal composition in the Li-La-Zr-O system, is in fact part of a large lithium containing solid solution region extending well into the pseudoternary. Furthermore, the LLZO phase of highest interest is also a solid solution with the cubic phase representing a very small region on the lithium-deficient side of Li7La3Zr2O12 such that it is nearly impossible to synthesize the cubic phase in the Li-La-Zr-O system without the presence of the detrimental pyrochlore phase. We also show that the cubic phase here has only marginally higher bulk conductivity than the tetragonal phase, indicating that the structural change to cubic, though necessary, is not sufficient to yield the large improvements in conductivity seen in doping studies in the literature. We also demonstrate that secondary phases can help improve the grain boundary conductivity, as we reported previously in perovskite La-Li-Ti-O materials.