1e:T50a,Abstract Rechargeable lithium batteries (RLBs) have revolutionized energy storage technology. However, short lifetime and safety issues have hampered their further commercialization, which is mainly attributable to the unstable solid‐electrolyte interphase (SEI) and uncontrolled lithium dendrite growth. In recent years, research on SEI has been pursued with determination worldwide. However, the structure and composition of the SEI have long been debated. Especially, the role of the main component, LiF, remains elusive. In this review, the structure and composition of SEIs are focused upon and the role of LiF in SEI is further analyzed. To this end, first, the development history of the SEI model is recounted. Second, the fundamental understanding of SEI is recalled. Third, the anode materials that can generate LiF in the SEI are categorized and discussed. Fourth, the characterization techniques of SEI layers are introduced. Fifth, the transport mechanism of Li + ions within the SEI is discussed. Sixth, the physical properties of LiF are revisited. Seventh, the source of LiF is deeply analyzed. Finally, general conclusions and a perspective on the future research directions for SEI that may promote the large‐scale applications of lithium metal batteries is discussed.1f:T4bc,Ceramic oxide electrolytes are outstanding due to their excellent thermostability, wide electrochemical stable windows, superior Li-ion conductivity, and high elastic modulus compared to other electrolytes. To achieve high energy density, all-solid-state batteries require thin solid-state electrolytes that are dozens of micrometers thick due to the high density of ceramic electrolytes. Perovskite-type Li_0.34 La_0.56 TiO₃ (LLTO) freestanding ceramic electrolyte film with a thickness of 25 µm is prepared by tape-casting. Compared to a thick electrolyte (>200 µm) obtained by cold-pressing, the total Li ionic conductivity of this LLTO film improves from 9.6 × 10^-6 to 2.0 × 10^-5 S cm^-1 . In addition, the LLTO film with a thickness of 25 µm exhibits a flexural strength of 264 MPa. An all-solid-state Li-metal battery assembled with a 41 µm thick LLTO exhibits an initial discharge capacity of 145 mAh g^-1 and a high capacity retention ratio of 86.2% after 50 cycles. Reducing the thickness of oxide ceramic electrolytes is crucial to reduce the resistance of electrolytes and improve the energy density of Li-metal batteries.20:T5eb,

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