With the rapid advancement of portable electronic devices, electric vehicles, and large-scale energy storage grids, next-generation rechargeable batteries with higher energy densities and enhanced safety have attracted much attention. Solid-state lithium metal batteries with high-safety solid-state electrolytes and lithium metal at the maximum specific capacity as an anode hold a tremendous potential for the next generation of rechargeable batteries. As a critical component of solid-state lithium metal batteries, solid-state electrolytes play a pivotal role in advancing their development. The existing solid-state electrolyte materials (i.e., organic polymer electrolytes, oxide electrolytes, sulfide electrolytes, and halide electrolytes) are systematically investigated, with the oxide solid-state electrolytes standing out due to their excellent overall performance. To achieve the practical application of oxide solid-state electrolytes, substantial research efforts are directed at modifying ceramic grain boundaries to improve ionic conductivity and mechanical properties and reduce electronic conductivity. Also, research on ceramic films is equally important to achieve a high energy density.The sintering process of ceramics is a critical factor affecting material properties, especially for the improvement of grain boundary. Relative density, grain boundary strength, and ionic/electronic conductivity are dominant factors impacting the long-term stability of the battery, with relative density as the most fundamental performance of ceramic electrolyte that enables other characteristics to be manifested effectively. The regulation of the preparation process, including powder preparation method, ceramic sintering temperature, and atmosphere, can effectively enhance the ceramic relative density. The powder preparation route, in particular, has a considerable effect on the final properties of LLZO ceramic, such as grain size and size distribution, relative density, microstructure, and ionic/electronic conductivity. In the sintering, some parameters such as sintering temperature, dwelling time, pressure, and the lithium oxide atmosphere play a crucial role in promoting the densification of LLZO ceramic.Atmospheric sintering is still the most common, simple, and low-cost method to prepare LLZO ceramics although it has a longer sintering time and a higher sintering temperature, compared to the pressurized sintering. The prolonged high-temperature process leads to lithium loss and the formation of impurity phases like La₂Zr₂O₇ in the electrolyte, which is detrimental to reducing the densification and ionic conduction behavior of the ceramics. To promote the densification and lower the sintering temperature, various kinds of sintering aids are adopted to assist the sintering process. A variety of low melting point additives, such as Li₃BO₃, LiO₂–B₂O₃–SiO₂–CaO–Al₂O₃, etc., are used as ceramic sintering aids. Subsequently, optimization of sintering processes and sintering aids are emerging, such as embedding-sintering, atmosphere-reversible control additives, lithium source-green body separation sintering, endogenous atmosphere sintering aids, etc.. LLZO ceramics sintered in the endogenous atmosphere with sintering additive of Li₆Zr₂O₇ can realize the densities of up to 97.21% in the absence of mother powder and under atmospheric pressure.In highly densified LLZO ceramics, dendrite growth and even piercing of the ceramic sheet are still inevitable, indicating that dendrite growth is related to ceramic relative density and ionic conductivity and is also closely related to the electronic conductivity and fracture toughness of ceramics. Multifunctional additives are reported to regulate the compositions, microstructures, as well as physical and electrochemical properties at the internal grain boundaries of LLZO ceramic in addition to improving the relative density and ionic conductivity of the ceramics. Among them, (Li₂O)_0.733(ZrO₂)_0.267, Li₆Zr₂O₇, Li₂WO₄, Li₂CuO₂, LaTiO₃, etc., are considered as effective sintering additives.To realize the high energy density of solid-state lithium metal batteries, thin-filming of ceramic electrolyte has attracted recent attention. Since the reduction of the thickness of LLZO ceramic leads to a decrease in the mechanical properties of the ceramics, a balance between the thickness and strength of the electrolyte is of great significance. Realizing the thin-film of LLZO ceramic through rational structural design is crucial to achieving a high energy density of solid-state batteries. In this case, ceramic films with different structures, such as single-layer dense ceramic, multilayer ceramic, and single-layer porous ceramic, are reported. A lithium-metal battery based on an interface-optimized 74 μm-thick single-layer dense LLZO ceramic has the excellent long-cycle stability of more than 800 cycles. A lithium-symmetric battery based on an optimized 115 μm-thick multilayer ceramic exhibits an ultra-high critical current density of 100 mA·cm^–2. A 12 μm-thick single-layer porous ceramic composited with a polymer electrolyte is expected to achieve energy densities of greater than 350 W·h·kg^–1. In addition, the quality of the green body (i.e., uniformity, flatness, and residual stress) and the sintering regime have a significant effect on the phase purity, ionic conductivity, surface state, and film flatness of LLZO after sintering due to the high sintering temperature of LLZO as well as the volatile properties of Li, La, Zr, and O in the composition. Therefore, some novel preparation processes to prepare ceramic films are developed in recent years. In particular, the combination of tape-casting and ultrafast-sintering has a promising application in the preparation of LLZO ceramic films.Summary and prospectsAlthough significant progress is made in the development of garnet-type solid-state electrolytes, their practical application in all-solid-state lithium batteries still faces challenges. Future research should focus on some key issues, i.e., the application of artificial intelligence and high-throughput computing in ceramic sintering additives, the development of new preparation methods for ceramic thin films, and the preparation, evaluation, and analysis of the full battery. Moreover, it is essential for the practical application of ceramic-based solid-state lithium metal batteries to simultaneously adapt testing standards, characterization techniques and failure analysis methods.