Li₇La₃Zr₂O₁₂ (LLZO) is considered as a promising solid-state electrolyte due to its high ionic conductivity, wide electrochemical window, and excellent electrochemical stability. However, its application in solid-state lithium metal batteries (SSLMBs) is impeded by the growth of lithium dendrites in LLZO due to some reasons such as its high electronic conductivity. In this study, lithium fluoride (LiF) was introduced into Ta-doped LLZO (LLZTO) to modify its grain boundaries to enhance the performance of SSLMBs. A nanoscale LiF layer was uniformly coated on the LLZTO grains, creating a three-dimensional continuous electron-blocking network at the grain boundaries. Benefiting from the electronic insulator LiF and the special structure of the modified LLZTO, the symmetric cells based on LLZO achieved a high critical current density (CCD) of 1.1 mA·cm⁻² (in capacity-constant mode) and maintained stability over 2000 h at 0.3 mA·cm⁻². Moreover, the full cells combined with a LiFePO₄ (LFP) cathode, demonstrated excellent cycling performance, retaining 97.1% of capacity retention after 500 cycles at 0.5 C. Therefore, this work provides a facile and effective approach for preparing a modified electrolyte suitable for high-performance SSLMBs.

All-solid-state lithium batteries (ASSLBs), which use solid electrolytes instead of liquid ones, have become a hot research topic due to their high energy and power density, ability to solve battery safety issues, and capabilities to fulfill the increasing demand for energy storage in electric vehicles and smart grid applications. Garnet-type solid electrolytes have attracted considerable interest as they meet all the properties of an ideal solid electrolyte for ASSLBs. The garnet-type Li₇La₃Zr₂O₁₂ (LLZO) has excellent environmental stability; experiments and computational analyses showed that this solid electrolyte has a high lithium (Li) ionic conductivity (10^-4-10^-3 S·cm^-1), an electrochemical window as wide as 6 V, stability against Li metal anode, and compatibility with most of the cathode materials. In this review, we present the fundamentals of garnet-type solid electrolytes, preparation methods, air stability, some strategies for improving the conductivity based on experimental and computational results, interfacial issues, and finally applications and challenges for future developments of LLZO solid electrolytes for ASSLBs.