The data that support the findings of this study are available from the corresponding author upon reasonable request.

Understanding the thermal runaway mechanism of solid-state electrolytes is critical for the development of all-solid-state Li-metal batteries (ASLMBs). Herein we employ multiscale methods including in situ optical microscopy and cryo-transmission electron microscopy combining with density functional theory to reveal the failure mechanism of Li 1.3 Al 0.3 Ti 1.7 P 3 O 12 (LATP). Li reacts with LATP to form an amorphous phase at elevated temperatures, which then crystallizes into Li 3 PO 4 and LiP with additional Li 3 P and Li 0.5 TiO 2 at even higher temperatures. The instability of the corner-sharing PO 4 tetrahedra and TiO 6 octahedra against Li at high temperature is the root cause of thermal runaway for LATP. Li diffusion into LATP causes the collapse of the PO 4 tetrahedra and TiO 6 octahedra, forming Li−O, Li−P−O, and Li−Ti−O species which release a large amount of heat, triggering thermal runaway of LATP. This work provides atomic-scale understanding of the thermal runaway of LATP, which offers an important clue to mitigate failure of ASLMBs.

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High interfacial impedance is a major obstacle in the application of solidstate Li metal batteries (SSLMBs). Understanding the atomic-scale structure of the interfaces in SSLMBs is thus critical to their practical implementations. However, due to the beam sensitivity of battery materials, such information is not accessible by conventional electron microscopy (EM). Herein, by using cryogenic-EM (cryo-EM), the atomic-scale structures of interfaces in garnet electrolyte based SSLMBs are revealed. A LiF-rich interlayer exhibiting intimate contacts with both Li and LLZTO is shown, thus rendering uniform Li + transport across the interface in turn inhibiting Li dendrite growth. Consequently, the Li symmetric cell based on the LiF-rich interlayer exhibits a high critical current density of 3.2 mA cm −2 and a long lifespan over 1800 cycles at 1 mA cm −2 . Moreover, a full cell with a LiNi 0.88 Co 0.1 Al 0.02 O 2 cathode at a high mass loading ≈12 mg cm −2 reached over 400 cycles at 1.2 mA cm −2 , which represents a major progress in the performance of the garnet-type SSLMBs. This study provides atomic-scale understanding of interfaces in SSLMBs and an effective strategy to design dendrite-free SSLMBs for practical applications.