A solid polymer electrolyte is expected to be useful for safe and high energy density lithium-metal batteries owing to its good flexibility and high degree of safety. The development of a polyethylene oxide (PEO) based solid electrolyte is still restrained by low ionic conductivity and unsatisfactory mechanical strength. Since MnO₂ could combine with PEO chains and Li ions could undergo long-range migration on MnO₂ nanosheets, MnO₂ nanoflakes are chosen as fillers to improve the electrochemical and mechanical properties of a solid polymer electrolyte. A PEO/MnO₂ composite solid polymer electrolyte (CSPE) displays a higher lithium ion transference number (0.378), higher ionic conductivity (1.5 times higher at 60 °C) and better tensile strength (2.3 times) than a PEO solid electrolyte. Density functional theory calculations reflect the fact that the binding energy between the PEO/Li complex and MnO₂ is small and there is easy desorption of Li from PEO and migration on MnO₂ nanosheets, indicating enhanced lithium ion transport in the electrolyte system. A solid-state lithium metal battery using a PEO/MnO₂ CSPE delivers higher capacity (143.5 mA h g⁻¹ after 300 cycles) than an electrolyte without fillers (61.2 mA h g⁻¹ after 90 cycles). Soft-package lithium metal batteries with an MnO₂ CSPE reveal high safety after cutting, nail and bending tests.