Solid-state lithium metal batteries (SLMBs) require quasi solid polymer electrolytes (QSSPEs) with high ionic conductivity, interfacial stability, and oxidative resistance. In this study, a QSSPE membrane (MP46, MG₃₀:LiTFSI:succinonitrile=10:4:6 by weight) with a wide electrochemical window of 5.1 V is designed to address these challenges. Complementary infrared spectroscopy, small-angle X-ray scattering and electron microscopy analysis reveals a hierarchical ionic conductive network, comprising sphere-like nanostructures embedded in microphase-segregated architectures. This architecture enhances lithium-ion transport while maintaining mechanical integrity. The strong interfacial adhesion of MP46 with lithium metal supports stable lithium plating and stripping for over 800 h at 0.2 mA·cm^–2, mitigating dendrite formation. When paired with LiFePO₄ and LiCoO₂ cathodes, MP46 sustains prolonged cycling, with capacity retention of 80.1% after 1400 cycles at 2 C and 92.1% after 200 cycles at 4.5 V, respectively. Pouch-type cells further demonstrate mechanical flexibility and operational safety under deformation. These results indicate that MP46 enables stable high-energy-density SLMBs, providing insights into the design of next-generation polymer electrolytes.