LAUSR

The construction of robust (quasi)‐solid‐state electrolyte (SSE) for flexible lithium‐metal batteries is desirable but extremely challenging. Herein, a novel, flexible, and robust quasi‐solid‐state electrolyte (QSSE) with a “tree‐trunk” design is reported for ultralong‐life lithium‐metal batteries (LMBs). An in‐situ‐grown metal–organic framework (MOF) layer covers the cellulose‐based framework to form hierarchical ion‐channels, enabling rapid ionic transfer kinetics and excellent durability. A conductivity of 1.36 × 10−3 S cm−1, a transference number of 0.72, an electrochemical window of 5.26 V, and a good rate performance are achieved. The flexible LMBs fabricated with as‐designed QSSEs deliver areal capacity of up to 3.1 mAh cm−2 at the initial cycle with high mass loading of 14.8 mg cm−2 in Li‐NCM811 cells and can retain ≈80% capacity retention after 300 cycles. An ultralong‐life of 3000 cycles (6000 h) is also achieved in Li‐LiFePO4 cells. This work presents a promising route in constructing a flexible QSSE toward ultralong‐life LMBs, and also provides a design rationale for material and structure development in the area of energy storage and conversion.