Solid-state lithium batteries with solid electrolytes have the potential to achieve high energy density and safety, which promise to be used in the electric vehicles and wearable devices. The garnet-type Li₇La₃Zr₂O₁₂ (LLZO) has attracted a great deal of attention due to its high ionic conductivity and good chemical stability to lithium metal. Here in this paper, based on recent progress, this review provides a fundamental understanding of garnet-based electrolytes by evaluating the thermodynamic/kinetics stability and analyzing the Li⁺ conduction mechanism of ceramics and composite garnet electrolytes. To address the key factors influencing the cyclability and safety of solid-state batteries, the rational design of solid electrolyte/electrode interfaces is discussed in terms of interface matching, charge transfer, strain/stress, thermal stability, etc. Finally, the design guidelines of high-energy-density solid-state batteries are introduced by analyzing the fabrication of electrodes, solid electrolyte and bipolar current collectors. Through the above discussion, this review provides an insight into the physical parameters affecting the performance of garnet-based electrolytes and interfaces, to guide one in carrying on more target-oriented researches of developing high-performance solid-state batteries.