Mn-based　Li-rich　layered　oxide　(Mn-LLO)　cathodes　own　promising　potential　for　electrochemical　energy-storage　(EES)　systems　due　to　their　cost-effectiveness,　structural　diversity,　and　high　energy　density.　However,　the　longevity　of　these　Mn-based　materials　remains　a　crucial　challenge　that　hampers　their　adoption　in　grid-scale　applications.　To　address　this　issue,　the　composite-structure　outside　of　grain　(CSOG)　strategy　integrated　with　layered/olivine　structures　is　proposed　for　enhancing　the　stability　of　crystal　structure,　interfacial　chemistry,　and　electrochemical　cycling　behaviors　of　Mn-LLOs.　A　series　of　cutting-edge　microscopic　and　synchrotron　techniques　present　the　CSOG　strategy　that　initiates　the　intensified　particle　surface,　stable　P-O　bonding　for　layered　structure,　and　robust　electrode-electrolyte　interphase.　This　multi-faceted　reinforcement　effectively　bolsters　interfacial　stability　for　both　pristine　and　electrochemical　cycling.　As　a　result,　the　Mn-based　CSOG　cathodes　realize　superior　capacity　retention　over　1000　cycles　with　capacity　decay　of　similar　to　0.009　%　per　cycle　in　pouch　cells.　Crucially,　enhanced　interfacial　behaviors　in　the　CSOG　cathodes　inhibit　elemental　dissolution　and　deposition　of　Mn　onto　the　anode　and　result　in　a　similar　to　39　%　improvement　in　thermal　stability　compared　to　pristine　Mn-LLOs.　This　innovative　CSOG　concept　opens　new　avenues　for　designing　highly　stable　Mn-based　cathode　materials,　accentuating　their　potential　for　grid-scale　EES　applications.