Low-cost and flexible solid polymer electrolytes are promising in all-solid-state Li-metal batteries with high energy density and safety. However, both the low room-temperature ionic conductivities and the small Li⁺ transference number of these electrolytes significantly increase the internal resistance and overpotential of the battery. Here, we introduce Gd-doped CeO₂ nanowires with large surface area and rich surface oxygen vacancies to the polymer electrolyte to increase the interaction between Gd-doped CeO₂ nanowires and polymer electrolytes, which promotes the Li-salt dissociation and increases the concentration of mobile Li ions in the composite polymer electrolytes. The optimized composite polymer electrolyte has a high Li-ion conductivity of 5 × 10⁻⁴ S cm⁻¹ at 30 °C and a large Li⁺ transference number of 0.47. Moreover, the composite polymer electrolytes have excellent compatibility with the metallic lithium anode and high-voltage LiNi_0.8Mn_0.1Co_0.1O₂ (NMC) cathode, providing the stable cycling of all-solid-state batteries at high current densities.