Layered　transition-metal　oxides　are　promising　cathode　candidates　for　sodium-ion　batteries.　However,　the　inferior　interphase　formation　and　particulate　fracture　during　sodiation/desodiation　result　in　structure　degradation　and　poor　stability.　Herein,　the　interface　chemistry　of　P2-Na0.640Ni0.343Mn0.657O2　in　an　electrolyte　of　1.0　mol/L　NaPF6　in　diglyme　is　unveiled　to　enable　highly　reversible　Na　extraction　and　intercalation.　The　uniform　and　robust　cathode-electrolyte　interphase　layer　is　in　situ　formed　with　decomposition　of　diglyme　molecules　and　anions　in　initial　cycles.　The　NaF-　and　CO-rich　CEI　film　exhibits　high　mechanical　strength　and　ionic　conductivity,　which　suppresses　the　reconstruction　of　its　electrode　interphase　from　P2　phase　to　spinel-like　structure　and　reinforces　its　structure　integrity　without　cracks.　This　favours　facile　Na+　transport　and　stable　bulk　redox　reactions.　It　is　demonstrated　to　show　long　cycling　stability　with　capacity　retention　of　94.4%　for　180　cycles　and　superior　rate　capability.　This　investigation　highlights　the　cathode　interphase　chemistry　in　sodium-ion　batteries.