Tailoring solid‐state synthesis routes for high confidence production of phase pure, low impedance Al‐LLZO

Abstract

Lithium lanthanum zirconium oxide (LLZO) garnet is a solid-state lithium ion conducting electrolyte promising all-solid-state batteries (ASSB) with high charge rates and good energy density due to its chemical stability against lithium metal anodes. LLZO has a high room temperature Li ion conductivity of ∼0.1–1 mS/cm in its cubic phase, but the stability of the cubic phase and ionic conductivity are highly sensitive to lithium stoichiometry. Stabilizing agents such as aluminum oxide and excess lithium are needed to preserve the cubic phase and compensate for lithium volatility. With the range of the end LLZO products spanning powders, porous membranes to dense membranes combined with sintering/calcination that often exceeds 1000°C, it is challenging to maintain an ideal lithium content given its high volatility from a single base powder. This study was designed to elucidate the sensitivities of aluminum doped LLZO powder synthesis and processing along its path to being utilized in a ceramic-manufacturing optimized ASSB. By utilizing thermogravimetric analysis in conjunction with in situ X-ray diffraction analysis of solid-state LLZO synthesis, it was discovered that the sensitivity of the LLZO cubic phase to lithium volatility can be reduced via early incorporation of excess lithium carbonate during initial phase formation in direct combination with controlled surface-to-volume ratios of the powders. Isostatically pressed powders of our LLZO sintered at 1100°C for 2 h showed RT ionic conductivity of 0.3–0.4 mS/cm measured via electrochemical impedance spectroscopy, and an improvement in microstructural uniformity with lowered porosity. The improved suppression of lithium volatilization has important implications for the scalable production of LLZO powders and assembly of ASSBs.