The　electrode　kinetics　of　Li-ion　batteries,　which　are　important　for　battery　utilization　in　electric　vehicles,　are　affected　by　the　grain　size,　crystal　orientation,　and　surface　structure　of　electrode　materials.　However,　the　kinetic　influences　of　the　grain　interior　structure　and　element　segregation　are　poorly　understood,　especially　for　Li-rich　layered　oxides　with　complex　crystalline　structures　and　unclear　electrochemical　phenomena.　In　this　work,　cross-sectional　thin　transmission　electron　microscopy　specimens　are　＂anatomized＂　from　pristine　Li1.2Mn0.567Ni0.167Co0.067O2　powders　using　a　new　argon　ion　slicer　technique.　Utilizing　advanced　microscopy　techniques,　the　interior　configuration　of　a　single　grain,　multiple　monocrystal-like　domains,　and　nickel-segregated　domain　boundaries　are　clearly　revealed;　furthermore,　a　randomly　distributed　atomic-resolution　Li2MnO3-like　with　an　intergrown　LiTMO2　(TM　=　transitional　metals)　＂twin　domain＂　is　demonstrated　to　exist　in　each　domain.　Further　theoretical　calculations　based　on　the　Li2MnO3-like　crystal　domain　boundary　model　reveal　that　Li+　migration　in　the　Li2MnO3-like　structure　with　domain　boundaries　is　sluggish,　especially　when　the　nickel　is　segregated　in　domain　boundaries.　Our　work　uncovers　the　complex　configuration　of　the　crystalline　grain　interior　and　provides　a　conceptual　advance　in　our　understanding　of　the　electrochemical　performance　of　several　compounds　for　Li-ion　batteries.