Electrochemically　charging　induced　phase　transition　is　a　common　and　thermodynamically-driven　phenomenon　for　variety　of　cathode　materials,　which　couples　with　chemical　and　mechanical　effects　leading　to　performance　degradation.　Phase　transition　is　particularly　complex　for　layered　sodium　transition　metal　oxides　and　its　related　detrimental　effects　remain　elusive.　Herein,　we　take　P2-type　Na2/3Ni1/3Mn2/3O2　(P2-NNM)　as　an　example　to　scrutiny　the　detrimental　consequences　upon　high　voltage　cycling.　We　find　that　repeated　P2-O2　phase　transition　breaks　down　cathode　primary　grains　by　generating　high　density　of　intragranular　cracks,　which　is　qualitatively　proved　to　be　the　main　cause　of　performance　decay.　Intriguingly,　the　nucleation　and　growth　of　intragranular　crack　is　through　loss　of　atoms　rather　than　cleavage,　resembling　the　stress　corrosion　cracking　mechanism　which　preferentially　nucleates　at　P2/O2　phase　boundary.　Moreover,　we　find　the　P2-structured　cathode　is　not　sensitive　to　surface　degradation,　which　explains　the　superior　performance　of　P2-NNM　cathode　when　cycling　at　low　voltage.