Li-rich　Mn-based　cathode　materials　(LRM)　have　received　great　attention　owing　to　their　high　capacity　and　low　cost.　However,　the　mismatch　between　the　widely　used　carbonate　electrolyte　and　the　LRM　cathode　and　lithium　metal　anode　causes　a　series　of　problems,　such　as　electrolyte　continuous　oxidation,　cathode　structure　degradation,　and　Li　dendritic　growth.　Herein,　inorganic　oxide　B2O3　is　introduced　as　a　dual-functional　high-voltage　electrolyte　additive　to　construct　stable　cathode　electrolyte　interphase　and　solid　electrolyte　interphase　for　Li||LRM　batteries.　The　modified　interface　derived　from　the　additive　can　induce　dendrite-free　Li　deposition,　stabilize　cathode　structure,　and　inhibit　transition　metal　dissolution.　Moreover,　the　adverse　side　reactions　are　mitigated,　thus　enhancing　Li+　transport　rate　and　reducing　interface　impedance.　With　the　addition　of　B2O3　into　the　carbonate　electrolyte,　the　Li||LRM　battery　exhibits　an　enhanced　discharge　capacity　of　221　mAh　g(-1)　after　200　cycles,　equaling　a　capacity　retention　of　92.1%.　When　the　upper　cut-off　voltage　is　increased　to　5　V,　a　superior　capacity　retention　of　＞　85%　can　still　be　achieved　after　150　cycles　at　1　C.　In　addition,　the　low　cost　of　B2O3　benefits　for　commercial　application.　This　work　offers　new　guidance　for　the　research　of　low-cost,　high-voltage　dual-functional　additives　for　advanced　lithium　metal　batteries.