Under　the　background　of　surging　global　demand　for　batteries　and　scarcity　of　Li　resources,　sodium-ion　batteries　(SIBs)　are　attracting　attention　as　a　potential　alternative　with　their　unique　advantages,　and　the　layered　transition　metal　(TM)　oxides　therein　are　considered　to　be　one　of　the　most　promising　cathode　materials.　In　this　paper,　firstly,　the　diversity　of　cathode　materials　for　SIBs　is　systematically　introduced,　as　well　as　the　layered　oxide　structures　among　them　are　categorized,　and　then　it　focuses　on　the　O3-type　sodium-rich　Na2MO3,　which　is　promising　for　large-scale　commercial　applications,　illustrating　the　development　and　mechanism　of　anion　redox.　Excess　Na　transforms　the　TM　layer　into　the　mixed　Na1/3M2/3O2　layer,　leading　to　the　formation　of　localized　configuration　Na-O-Na.　Thereby,　isolated　nonhybridized　O　2p　states　are　introduced,　which　participate　in　the　charge　compensation　process　(O2-/On-)　under　high-voltage　conditions　and　provide　the　battery　with　additional　capacity　beyond　the　cation　redox　reaction.　Therefore,　the　Na2MO3　formed　by　its　TM　element　located　in　different　periods　are　classified,　discussed　and　summarized　in　terms　of　structural　change　characteristics,　electrochemical　properties　and　anion-redox　mechanism.　However,　this　particular　redox　mechanism　is　also　accompanied　by　the　challenges　such　as　voltage　hysteresis,　irreversible　oxygen　loss,　TM　migration,　capacity　decay　and　poor　air　stability.　Therefore,　to　address　these　challenges,　various　improvement　strategies　have　been　proposed,　including　doping　of　large　radius　metal　ions,　light　metal　ions,　TM　ions　with　high　covalency　with　O,　nonmetal　ions,　formation　of　mixed　phases,　and　surface　modification.　This　work　is　expected　to　provide　new　ways　to　find　and　design　novel　high-capacity　Na-rich　layered　oxide　cathode　materials.