Significant progress has been made to replace graphite anode materials with Li metal in next-generation Li ion batteries, called Li metal batteries (LMBs). However, the development of practical LMBs requires the suppression of Li dendrites. Owing to their ability to relax polarization, single-ion solid polymer electrolytes (SSPEs) are widely considered as an effective strategy for preventing dendrite generation. The novel SSPE membrane prepared in this work, which consists of a polymeric lithium salt modified with an electrolyte solvent, shows single-ion conducting behavior that results in the effective restriction of Li dendritic growth. The SSPE membrane delivers an ionic conductivity as high as 1.42 × 10-4 S cm-1 at room temperature. A LiFePO4 (LFP) coin cell assembled with the SSPE membrane shows excellent rate performance and outstanding cycling stability. In addition, the LFP flexible battery using the SSPE membrane exhibits good practicability and environmental adaptability.