Li-La-Zr-O Garnets with High Li-Ion Conductivity and Air-Stability by Microstructure-Engineering

Li7La3Zr2O12 (LLZO) solid electrolyte (SE) is a potential candidate for developing safe and economically all-solid-state batteries (ASSBs) owing to its high Li-ion conductivity and electrochemical stability against lithium anodes. However, poor stability and significant reduction in conductivity when exposed to air, limit its practical use. Herein, a unique two-step sintering approach is designed to tailor the microstructure of LLZO that can withstand extended air exposure. The record high Li-ion conductivity (approximate to 1.7 mS cm(-1) at 25 degrees C) is obtained for coarse-grained Li6.25Ga0.25La3Zr2O12 (GLLZO) samples, whereas fine-grained samples exhibit a relatively lower yet still substantial conductivity (approximate to 1.3 mS cm(-1)). However, coarse-grained samples are vulnerable to atmospheric attacks, forming larger Li2CO3 on the surface, leading to spontaneous cracking and significantly reduced conductivity (approximate to 4 order). Despite these limitations, coarse-grained samples can still be good SE for ASSBs under certain conditions. Interestingly, fine-grained samples maintain structural integrity and Li-ion conductivity even after prolonged exposure to air. The differing transport and stability behaviors are attributed to variations in the bulk compositions originating from distinct sintering mechanisms. These findings represent a significant step toward achieving air-stable, highly conductive solid electrolytes with normal grain growth that will reduce interfacial resistance and improve the power density and cyclability of next-generation ASSBs.