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

Abstract 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 (≈1.7 mS cm⁻¹ at 25 °C) is obtained for coarse‐grained Li_6.25Ga_0.25La₃Zr₂O₁₂ (GLLZO) samples, whereas fine‐grained samples exhibit a relatively lower yet still substantial conductivity (≈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 (≈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.