Scalable, thin asymmetric composite solid electrolyte for high‐performance all‐solid‐state lithium metal batteries

All-solid-state Li metal batteries (ASSLMBs) have been considered the most promising candidates for next-generation energy storage devices owing to their high-energy density and safety. However, some obstacles such as thick solid electrolyte (SSEs) and unstable interface between the solid-state electrolytes (SSEs) and the electrodes have restricted the practical application of ASSLBs. Here, the scalable polyimide (PI) film reinforced asymmetric ultra-thin (~20 μm) composite solid electrolyte (AU-CSE) with a ceramic-rich layer and polymer-rich layer is fabricated by a both-side casting method and rolling process. The ceramic-rich layer not only acts as a “securer” to inhibit the lithium dendrite growth but also redistributes Li-ions uniform deposition, while the polymer-rich layer improves the compatibility with cathode materials. As a result, the obtained AU-CSE demonstrates an ionic conductivity of 1.44 × 10−4 S cm−1 at 35°C. The PI-reinforced AU-CSE enables Li/Li symmetric cell stable cycling over 1200 h at 0.2 mA cm−2 and 0.2 mAh cm−2. Li/LiNi0.6Co0.2Mn0.2O2 and Li/LiFePO4 ASSLMBs achieve superior performances at 35°C. This study provides a new way of solving the interface problems between SSEs and electrodes and developing high-energy-density ASSLMBs for practical applications.