Synthesis of Fine Cubic Li₇La₃Zr₂O₁₂ Powders in Molten LiCl-KCl Eutectic and Facile Densification by Reversal of Li⁺/H⁺ Exchange

Recently, solid-state electrolytes have been a highly active area of research for future Li-ion batteries due to the potential for drastically improved energy density and safety. Among these materials, garnet structured lithium lanthanum zirconate (Li₇La₃Zr₂O₁₂, LLZO) shows particular promise owing to the high ionic conductivity of its cubic polymorph, inertness, and electrochemical stability against metallic lithium. Herein we report the facile preparation of phase-pure, cubic LLZO via molten salt synthesis in a eutectic mixture of LiCl-KCl at 900 °C. Fine powders of Al- and Ga-doped LLZO were obtained with primary particle sizes ranging from 0.3 to 3 μm. Depending on the consolidation conditions, pellets with up to 86% relative density could be obtained, with Li⁺ conductivity values ranging from 0.230 to 0.371 mS cm^-1. It is also observed that while the effect of hydration has a profoundly deleterious effect on sintering and densification, this effect can be mitigated by the simple addition of LiOH before sintering to reverse hydration and aid densification. Qualitative discussions on the mechanisms of LLZO formation in the molten salt medium are discussed, in addition to implications for scalable processing of LLZO electrolytes.