Layered　transition-metal　oxides　have　attracted　intensive　interest　for　cathode　materials　of　sodium-ion　batteries.　However,　they　are　hindered　by　the　limited　capacity　and　inferior　phase　transition　due　to　the　gliding　of　transition-metal　layers　upon　Na+　extraction　and　insertion　in　the　cathode　materials.　Here,　we　report　that　the　large-sized　K+　is　riveted　in　the　prismatic　Na+　sites　of　P2-Na0.612K0.056MnO2　to　enable　more　thermodynamically　favorable　Na+　vacancies.　The　Mn-O　bonds　are　reinforced　to　reduce　phase　transition　during　charge　and　discharge.　0.901　Na+　per　formula　are　reversibly　extracted　and　inserted,　in　which　only　the　two-phase　transition　of　P2　＜-＞　P＇2　occurs　at　low　voltages.　It　exhibits　the　highest　specific　capacity　of　240.5　mAh　g(-1)　and　energy　density　of　654　Wh　kg(-1)　based　on　the　redox　of　Mn3+/Mn4+,　and　a　capacity　retention　of　98.2%　after　100　cycles.　This　investigation　will　shed　lights　on　the　tuneable　chemical　environments　of　transition-metal　oxides　for　advanced　cathode　materials　and　promote　the　development　of　sodium-ion　batteries.　High-capacity　and　structural　stable　cathode　materials　are　challenges　for　sodium-ion　batteries.　Here,　the　authors　report　a　layered　P2-Na0.612K0.056MnO2　with　large-sized　K+　riveted　in　the　Na-layers　to　enable　0.9　Na+　(de)insertion　with　a　reversible　phase　transition　of　P2-P＇2.