Mn-based　oxides　have　attracted　extensive　attention　as　electrode　materials.　However,　the　irreversible　phase　transition　and　Mn2+　dissolution　result　in　their　structure　instability　and　performance　decay.　Here,　we　report　dual　interphase　layers　in　situ　formed　on　P2-K0.67MnO2　(P2-KMO)　in　6.0　M　of　potassium　bis(fluorosulfonyl)amide　in　diglyme　(KFSI/G2)　during　charging.　It　is　composed　of　a　solid-electrolyte　interphase　(SEI)　and　K-poor　spinel　interlayer　on　P2-KMO,　which　are　derived　from　the　simultaneous　decomposition　of　6.0　M　KFSI/G2　and　disproportionation　of　surface　Mn3+　They　cooperatively　enable　the　reversible　phase　transition　of　P2　＜-＞　P　＇＇　2　in　the　bulk　P2-KMO　and　mitigate　Mn　loss.　This　leads　to　a　high　capacity　retention　of　90.5%　and　a　Coulombic　efficiency　of　100%　after　300　cycles.　The　investigation　highlights　the　significance　of　interphase　chemistry　of　electrode　materials　for　potassium-ion　batteries　and　beyond.