Observation of Elemental Inhomogeneity and Its Impact on Ionic Conductivity in Li-Conducting Garnets Prepared with Different Synthesis Methods

Abstract

Tantalum-doped lithium lanthanum zirconate garnet (Li7−xLa3Zr2−xTaxO12 [LLZTO]) has received interest as a solid electrolyte for solid-state lithium batteries due to its good electrochemical properties and ionic conductivity. However, the source of discrepancies for reported values of ionic conductivity in nominally or nearly equivalent compositions of LLZTO is not completely clear. Herein, synthesis-related factors that may contribute to the differences in performance of garnet electrolytes are systematically characterized. The conductivity of samples with composition Li6.4La3Zr1.4Ta0.6O12 prepared by various methods including solid-state reaction (SSR), combustion, and molten salt synthesis is compared. Varying levels of elemental inhomogeneity, comprising a variation in Ta and Zr content on the level of individual LLZTO particles, are identified. The elemental inhomogeneity is found to be largely preserved even after high-temperature sintering and correlated with reduced ionic conductivity. It is shown that the various synthesis and processing-related variables in each of the preparation methods play a role in these compositional variations, and that even LLZTO synthesized via conventional, high-temperature SSR can exhibit substantial variability in local composition. However, by improving reagent mixing and using LLZTO powder with low agglomeration and small particle size distribution, the compositional uniformity, and hence, ionic conductivity, of sintered garnet electrolytes can be improved.