Garnet-type solid-state electrolytes: crystal structure, interfacial challenges and controlling strategies

All-solid-state batteries (ASSBs) hold great promise for next-generation energy storage technologies owing to their advantage in different aspects such as energy density, safety, and wide temperature tolerance. However, the use of solid-state electrolytes (SSEs) instead of liquid ones meanwhile brings serious concerns related to the point-to-point contact between SSEs and electrodes, which is known to result in high interface resistance and inhomogeneous distribution of charges during the Li+ plating/stripping process, eventually leading to a premature failure of ASSBs. This review focuses on the garnet-type SSEs in the formula of Li7La3Zr2O12 (LLZO), and discusses the structure-performance relationship of this ceramic electrolyte in detail to achieve a clear understanding of its Li+ transmission mechanism. Meanwhile, the challenges of cubic phase LLZO (c-LLZO) for their application in solid-state batteries (SSBs) are demonstrated by the Li/LLZO interface, which features the importance of Li metal wettability and dendrite suppression for sustainable performance. Furthermore, this review summarizes the recent research strategies to combat these contact issues at the Li/LLZO interface, highlighting the essential role played by surface modification of LLZO electrolytes. Following the obtained insights, perspectives for future research on LLZO to accelerate its potential development of SSBs in commercialized applications are also provided.