The development of high-performance solid polymer electrolytes is crucial for producing all-solid-state lithium metal batteries with high safety and high energy density. However, the low ionic conductivity of solid polymer electrolytes and their unstable electrolyte/electrode interfaces have hindered their widespread utilization. To address these critical challenges, a strong Lewis acid (aluminum fluoride (AlF)) with dual functionality is introduced into polyethylene oxide) (PEO)-based polymer electrolyte. The AlF facilitates the dissociation of lithium salt, increasing the iontransfer efficiency due to the Lewis acid-base interaction; further the in-situ formation of lithium fluoride-rich interfacial layer is promoted, which suppresses the uneven lithium deposition and continuous undesired reactions between the Li metal and PEO matrix. Benefiting from our rational design, the symmetric Li/Li battery with the modified electrolyte exhibits much longer cycling stability (over 3600 h) than that of the pure PEO/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) electrolyte (550 h). Furthermore, the all-solid-state LiFePO full cell with the composite electrolyte displays a much higher Coulombic efficiency (98.4% after 150 cycles) than that of the electrolyte without the AlF additive (63.3% after 150 cycles) at a large voltage window of 2.4–4.2 V, demonstrating the improved interface and cycling stability of solid polymer lithium metal batteries.