Exploring the Low-Temperature Synthesis Pathway of Lithium Ionic Conductor Garnet-Type Solid Electrolytes

Garnet-type lithium-ion conductor Li7La3Zr2O12 is a potential electrolyte for all-solid-state batteries because of fast ionic conductivities and compatibility with Li metal. In this study, two different starting materials were used to understand how the crystallinity and the stability of the starting materials affect the reaction mechanism in the formation of Li6.5La3Zr1.5Nb0.5O12 (LLZNO). Two starting materials were used: destabilized starting materials, which have low crystallinity and are highly reactive (D-LLZNO), and conventional starting materials (C-LLZNO). It was observed that LLZNO was formed from D-LLZNO at 400 °C, whereas the formation from C-LLZNO required 500 °C. This results in D-LLZNO undergoing a thermodynamically favorable reaction, as supported by a DFT-based total energy estimate. The difference in onset temperature was a result of the difference in reactivity of transition metals. Significant changes in the coordination environment of La and Zr, as observed by XANES, occurred at temperatures 200 °C and 100 °C lower for D-LLZNO than for C-LLZNO, respectively. In addition, different precursors lead to different reaction routes. D-LLZNO forms an amorphous matrix with dispersed La3NbO7 nanocrystals at 300 °C as an intermediate, which is not observed in C-LLZNO. These results show that the stability of starting materials affects the reaction pathways. Understanding these aspects enables the design of reaction pathways in ceramics, which can lead to significant industrial benefits in the development and manufacture of electrochemical technologies.