Thin solid‐state electrolytes with nonflammability, high ionic conductivity, low interfacial resistance, and good processability are urgently required for next‐generation safe, high energy density lithium metal batteries. Here, a 3D Li6.75La3Zr1.75Ta0.25O12 (LLZTO) self‐supporting framework interconnected by polytetrafluoroethylene (PTFE) binder is prepared through a simple grinding method without any solvent. Subsequently, a garnet‐based composite electrolyte is achieved through filling the flexible 3D LLZTO framework with a succinonitrile solid electrolyte. Due to the high content of garnet ceramic (80.4 wt%) and high heat‐resistance of the PTFE binder, such a composite electrolyte film with nonflammability and high processability exhibits a wide electrochemical window of 4.8 V versus Li/Li+ and high ionic transference number of 0.53. The continuous Li+ transfer channels between interconnected LLZTO particles and succinonitrile, and the soft electrolyte/electrode interface jointly contribute to a high ambient‐temperature ionic conductivity of 1.2 × 10−4 S cm−1 and excellent long‐term stability of the Li symmetric battery (stable at a current density of 0.1 mA cm−2 for over 500 h). Furthermore, as‐prepared LiFePO4|Li and LiNi0.5Mn0.3Co0.2O2|Li batteries based on the thin composite electrolyte exhibit high discharge specific capacities of 153 and 158 mAh g−1 respectively, and desirable cyclic stabilities at room temperature.

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Solid-state polymer electrolytes (SPEs) with flexibility, easy processability, and low cost have been regarded as promising alternatives for conventional liquid electrolytes in next-generation high-safety lithium metal batteries. However, SPEs generally suffer poor strength to block Li dendrite growth during the charge/discharge process, which severely limits their wide practical applications. Here, a rational design of 3D cross-linked network asymmetric SPE modified with a metal-organic framework (MOF) layer on one side is proposed and prepared through an in-situ polymerization process. In such unique asymmetric SPEs, the nanoscale MOF layer acts as a shield that effectively suppresses the growth of Li dendrites and regulates the uniform Li + transport, and the polymer electrolyte can be scattered in the whole cell to endow the smooth transmission of Li +. As a result, the asymmetric SPE exhibits high ionic conductivity, wide electrochemical window, high thermal stability and safety, which endows the Li/Li symmetrical cell with outstanding cyclic stability (operate well over 800 h at a current density of 0.1 mA cm −2 for the capacity of 0.1 mAh cm −2).

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