Manganese-based　lithium-rich　layered　oxides　(Mn-LLOs)　are　promising　candidate　cathode　materials　for　lithium-ion　batteries,　however,　the　severe　voltage　decay　during　cycling　is　the　most　concern　for　their　practical　applications.　Herein,　an　Mn-based　composite　nanostructure　constructed　Li2MnO3　(LMO@Li2MnO3)　is　developed　via　an　ultrathin　amorphous　functional　oxide　LixMnOy　coating　at　the　grain　surface.　Due　to　the　thin　and　universal　LMO　amorphous　surface　layer　etched　from　the　lithiation　process　by　the　high-concentration　alkaline　solution,　the　structural　and　interfacial　stability　of　Li2MnO3　are　enhanced　apparently,　showing　the　significantly　improved　voltage　maintenance,　cycle　stability,　and　energy　density.　In　particular,　the　LMO@Li2MnO3　cathode　exhibits　zero　voltage　decay　over　200　cycles.　Combining　with　ex　situ　spectroscopic　and　microscopic　techniques,　the　Mn2+/4+　coexisted　behavior　of　the　amorphous　LMO　is　revealed,　which　enables　the　stable　electrochemistry　of　Li2MnO3.　This　work　provides　new　possible　routes　for　suppressing　the　voltage　decay　of　Mn-LLOs　by　modifying　with　the　composite　functional　unit　construction.