A Self‐Limited Free‐Standing Sulfide Electrolyte Thin Film for All‐Solid‐State Lithium Metal Batteries

All‐solid‐state (ASS) lithium metal batteries (LMBs) are considered the most promising next‐generation batteries due to their superior safety and high projected energy density. To access the practically desired high energy density of ASS LMBs, an ultrathin solid‐state electrolyte (SSE) film with fast ion‐transport capability presents as an irreplaceable component to reduce the proportion of inactive materials in ASS batteries. In this contribution, an ultrathin (60 µm), flexible, and free‐standing argyrodite (Li6PS5Cl) SSE film is designed through a self‐limited strategy. A chemically compatible cellulose membrane is employed as the self‐limiting skeleton that not only defined the thinness of the sulfide SSE film but also strengthened its mechanical properties. The ionic conductivity of the SSE film reaches up to 6.3 × 10−3 S cm−1 at room temperature, enabling rapid lithium‐ion transportation. The self‐limited SSE thin films are evaluated in various ASS LMBs with different types of cathode (sulfur and lithium titanate) and anode materials (lithium and lithium‐indium alloy) at both mold‐cell and pouch‐cell levels, demonstrating a stable performance and high‐rate capability. This study provides a general strategy for the rational design of an SSE thin film towards high‐energy‐density ASS batteries. An ultrathin, flexible, and free‐standing argyrodite solid‐state electrolyte film is designed through a self‐limited strategy. The ionic conductivity of the SSE film reaches up to 6.3 × 10−3 S cm−1 at room temperature, enabling rapid lithium‐ion transportation in all‐solid‐state batteries.