Not Just an Unwanted Impurity Phase: Pyrochlores as Convenient Precursors to Li Conducting Garnets

Garnet-type solid-state electrolytes such as Ta-doped Li₇La₃Zr₂O₁₂(LLZTO) are promising ceramic electrolytes for all-solid-state batteries because of their high (electro)chemical stability and ionic conductivity. LLZTO is typically synthesized from oxide precursors via solid-state reaction, but use of other precursors can lead to improved phase purity and more homogenous dopant distribution. We recently demonstrated that pyrochlores, usually seen as an unwanted impurity phase in the synthesis of garnets, can actually serve as the starting precursor for LLZTO, a process we call “pyrochlore-to-garnet” (P2G). Nanocrystalline pyrochlore prepared using molten salt synthesis and a scalable sol-gel method are demonstrated to enable formation of dense LLZTO ceramic directly via reactive sintering of the pyrochlore and a lithium source. The LLZTO prepared via P2G displays high relative density and ionic conductivity comparable to LLZTO prepared by solid-state reaction, but with much shorter sintering times. Investigation of the synthesis parameters shows that liquid phase sintering is important for achieving high pellet densities and phase-purity. Additionally, while the LLZTO grain sizes obtained using this P2G method are smaller than those from conventional sintering of LLZTO particles, the P2G pellets can sustain higher critical current densities before shorting in lithium stripping/plating experiments compared to conventional LLZTO prepared by solid-state reaction. This work shows P2G is promising as an alternate method for the synthesis and processing of LLZTO materials.