Li-　and　Mn-rich　layered　oxides　are　among　the　most　promising　cathode　materials　for　Li-ion　batteries　with　high　theoretical　energy　density.　Its　practical　application　is,　however,　hampered　by　the　capacity　and　voltage　fade　after　long　cycling.　Herein,　a　finite　difference　method　for　near-edge　structure　(FDMNES)　code　was　combined　with　in　situ　X-ray　absorption　spectroscopy　(XAS)　and　transmission　electron　microscopy/electron　energy　loss　spectroscopy　(TEM/EELS)　to　investigate　the　evolution　of　transition　metals　(TMs)　in　fresh　and　heavily　cycled　electrodes.　Theoretical　modeling　reveals　a　recurring　partially　reversible　LiMn2O4-like　sub-nanodomain　formation/dissolution　process　during　each　charge/discharge,　which　accumulates　gradually　and　accounts　for　the　Mn　phase　transition.　From　the　modeling　of　spectra　and　maps　of　the　valence　state　over　large　regions　of　the　cathodes,　it　was　found　that　the　phase　change　is　size-dependent.　After　prolonged　cycling,　the　TMs　displayed　different　levels　of　inactivity.