Harnessing Thin‐Film Solid‐State Electrolytes: Enabling Breakthroughs in All‐Solid‐State Batteries

The pursuit of safer and higher energy‐density batteries has positioned all‐solid‐state lithium batteries (ASSLBs) at the forefront of next‐generation energy storage technologies. The solid‐state electrolyte (SSE) serves as the pivotal component, with its thin‐film fabrication being critical for minimizing inactive material mass and maximizing energy density—a decisive step toward commercial viability. However, the transition from fundamental materials discovery to high‐performance, ultrathin SSE membranes faces significant challenges, including insufficient ionic conductivity, poor interfacial stability, and inadequate mechanical integrity at reduced dimensions. This review provides a comprehensive overview and critical analysis of the latest advancements in the design and manufacturing of thin‐film composite SSEs. We first delineate the intrinsic limitations of conventional inorganic and polymer SSEs, establishing the imperative for composite strategies. The core of the review systematically navigates advanced fabrication methodologies—spanning wet, dry, and emerging processes—and architectural innovations, with a focus on nanofiller engineering, layered structures, and 3D scaffolds. We emphasize the synergistic interplay between processing techniques and multiscale structures in simultaneously enhancing ionic transport, mechanical robustness, and electrode compatibility. By framing a comprehensive “processing–structure–performance” paradigm, this review aims to guide future research endeavors and accelerate the development of industrially relevant thin‐film SSEs for practical high‐energy‐density ASSLBs.