Potassium-ion　batteries　(KIBs)　are　emerging　as　one　of　the　most　promising　candidates　for　large-scale　energy　storage　owing　to　the　natural　abundance　of　the　materials　required　for　their　fabrication　and　the　fact　that　their　intercalation　mechanism　is　identical　to　that　of　lithium-ion　batteries.　However,　the　larger　ionic　radius　of　K+　is　likely　to　induce　larger　volume　expansion　and　sluggish　kinetics,　resulting　in　low　specific　capacity　and　unsatisfactory　cycle　stability.　A　new　Ni/Mn-based　layered　oxide,　P2-type　K0.44Ni0.22Mn0.78O2,　is　designed　and　synthesized.　A　cathode　designed　using　this　material　delivers　a　high　specific　capacity　of　125.5　mAh　g(-1)　at　10　mA　g(-1),　good　cycle　stability　with　capacity　retention　of　67%　over　500　cycles　and　fast　kinetic　properties.　In　situ　X-ray　diffraction　recorded　for　the　initial　two　cycles　reveals　single　solid-solution　processes　under　P2-type　framework　with　small　volume　change　of　1.5%.　Moreover,　a　cathode　electrolyte　interphase　layer　is　observed　on　the　surface　of　the　electrode　after　cycling　with　possible　components　of　K2CO3,　RCO2K,　KOR,　KF,　etc.　A　full　cell　using　K0.44Ni0.22Mn0.78O2　as　the　cathode　and　soft　carbon　as　the　anode　also　exhibits　exceptional　performance,　with　capacity　retention　of　90%　over　500　cycles　as　well　as　superior　rate　performance.　These　findings　suggest　P2-K0.44Ni0.22Mn0.78O2　is　a　promising　candidate　as　a　high-performance　cathode　for　KIBs.