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　(AlF3))　with　dual　functionality　is　introduced　into　poly(ethylene　oxide)　(PEO)-based　polymer　electrolyte.　The　AlF3　facilitates　the　dissociation　of　lithium　salt,　increasing　the　ion-transfer　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　LiFePO4　full　cell　with　the　composite　electrolyte　displays　a　much　higher　Coulombic　efficiency　(98.4%　after　150　cycles)　than　that　of　the　electrolyte　without　the　AlF3　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.