All-solid-state batteries are a potential game changer in the energy storage market; however, their practical employment has been hampered by premature short circuits caused by the lithium dendritic growth through the solid electrolyte. Here, we demonstrate that a rational layer-by-layer strategy using a lithiophilic and electron-blocking multilayer can substantially enhance the performance/stability of the system by effectively blocking the electron leakage and maintaining low electronic conductivity even at high temperature (60°C) or under high electric field (3 V) while sustaining low interfacial resistance (13.4 ohm cm 2 ). It subsequently results in a homogeneous lithium plating/stripping, thereby aiding in achieving one of the highest critical current densities (~3.1 mA cm −2 ) at 60°C in a symmetric cell. A full cell paired with a commercial-level cathode exhibits exceptionally long durability (>3000 cycles) and coulombic efficiency (99.96%) at a high current density (2 C; ~1.0 mA cm −2 ), which records the highest performance among all-solid-state lithium metal batteries reported to date.

Halide solid electrolytes have recently emerged as a promising option for cathode‐compatible catholytes in solid‐state batteries (SSBs), owing to their superior oxidation stability at high voltage and their interfacial stability. However, their day‐ to month‐scale aging at the cathode interface has remained unexplored until now, while its elucidation is indispensable for practical deployment. Herein, the stability of halide solid electrolytes (e.g., Li3InCl6) when used with conventional layered oxide cathodes during extended calendar aging is investigated. It is found that, contrary to their well‐known oxidation stability, halide solid electrolytes can be vulnerable to reductive side reactions with oxide cathodes (e.g., LiNi0.8Co0.1Mn0.1O2) in the long term. More importantly, the calendar aging at a low state of charge or as‐fabricated state causes more significant degradation than at a high state of charge, in contrast to typical lithium‐ion batteries, which are more susceptible to high‐state‐of‐charge calendar aging. This unique characteristic of halide‐based SSBs is related to the reduction propensity of metal ions in halide solid electrolytes and correlated to the formation of an interphase due to the reductive decomposition triggered by the oxide cathode in a lithiated state. This understanding of the long‐term aging properties provides new guidelines for the development of cathode‐compatible halide solid electrolytes.

The all-solid-state battery (ASSB) has become one of the most promising next-generation battery systems, since the aspect of safety has emerged as a crucial criterion for new large-scale applications such as in electric vehicles. Despite the recent remarkable progress in the performance enhancement, the real-world implementation of the ASSB still requires full comprehension/evaluation of its properties and performance under various practical operational conditions. Unlike batteries employed in conventional electronic devices, those in electric vehicles—the major application that the ASSB is expected to be employed—would be exposed to wide temperature variations (−20 to ∼70 °C) at various states of charges due to their outdoor storage and irregular discharge/rest/charge conditions depending on vehicle drivers' usage patterns. Herein, we investigate the reliability of a Li6PS5Cl-based ASSB system in practically harsh but plausible storage conditions and reveal that it is vulnerable to elevated-temperature storage as low as 70 °C, which, in contrast to the common belief, causes significant degradation of the electrolyte and consequently irreversible buildup of the cell resistance. It is unraveled that this storage condition induces the decomposition of Li6PS5Cl in contact with the cathode material, involving the SOx gas evolution particularly at charged states, which creates a detrimental porous cathode/electrolyte interface, thereby leading to the large interfacial resistance. Our findings indicate that the stability of the solid electrolyte, which has been believed to be failsafe, needs to be carefully revisited at various practical operational conditions for actual applications in ASSBs.