All‐Solid‐State Thin‐Film Lithium‐Selenium Batteries

All‐solid‐state batteries (ASSBs) with high‐energy‐density and enhanced safety are ideal for next‐generation energy storage in electric transportation and Internet of Things. Fundamentally, the augmentation of their energy density relays on advanced cathode materials. This imperative has driven growing interest in Se‐based cathodes, which demonstrate a high volumetric energy density, as well as higher electrical conductivity and better environmental adaptability compared to the well‐known S cathodes. However, to ensure sufficient mechanical strength and mitigate the continuous deterioration of the solid‐solid interface caused by the substantial volume expansion of the Se, the all‐solid‐state Li‐Se batteries reported thus far typically employ thick solid electrolytes (50–200 µm), which severely limits their energy density. Here, the first successful fabrication of all‐solid‐state thin‐film Li‐Se batteries is reported, featuring an ultra‐thin (≈1.4 µm) lithium phosphorus oxynitride solid electrolyte and a hybrid Se cathode supported by vertical graphene nanoarrays (VGs). The conductive VGs, serving as the Se host, effectively mitigate the volume change during cycling and ensure stable solid‐solid contact. Consequently, the cells exhibit over 1000 stable cycles with a capacity retention rate of 89% are attained in the “all‐thin film” configuration. This study provides a novel design strategy for the development of next‐generation high‐performance ASSBs.