Garnet solid electrolytes: Material design, microstructural engineering, and pathways to high-energy density solid-state lithium batteries

Garnet-type Li₇La₃Zr₂O₁₂ (LLZO) solid electrolytes have been identified as potential candidates for high-energy solid-state lithium batteries due to their distinguished ionic conductivity, wide-ranging electrochemical stability, and compatibility with lithium metal. This review provides a systematic examination of the structure-property relationships of LLZO, with a specific focus on the pivotal role of grain boundary engineering in improving ionic conductivity. Advanced doping strategies, encompassing single/multi-ion substitution and anion-cation co-doping, are evaluated for their impact on stabilizing the cubic phase and optimizing lithium vacancy distribution. Innovations in sintering techniques and LLZO film fabrication methods are emphasized for their contributions to achieving high ionic conductivity and ultrathin thickness. Moreover, the design of ultrathin flexible organic-inorganic composite electrolytes is discussed to tackle challenges associated with mechanical brittleness and industrialization. The critical evaluations of ionic conductivity enhancement, ultrathin electrolyte fabrication strategies, and scalability challenges provide insightful references for the advancement of LLZO-based solid-state batteries toward high energy density and industrial practicality.