Sodium　metal　has　been　regarded　as　one　of　excellent　candidates　of　anode　materials　for　the　next-generation　high-energy　sodium-ion　batteries　owing　to　its　low　redox　potential,　low　cost,　and　high　theoretical　capacity.　However,　the　poor　reversibility　and　the　dendrite　growth　of　Na　anode　on　cycling　have　significantly　hindered　the　practical　application　of　sodium　metal　anodes　(SMAs).　Herein,　we　report　that　a　mixed-ion/electron-conducting　interface　(MIECI)　layer,　in-situ　generated　through　the　conversion　of　CuP2　into　the　mixed　conductor　of　Cu　and　Na3P,　enable　dendrite-free　Na　plating/stripping　in　ether-based　electrolyte.　The　MIECI　layer　can　significantly　improve　the　affinity　with　the　deposited　Na,　homogenize　the　Na+　flux,　and　reduce　the　local　current　density.　The　MIECI　layer　enables　Coulombic　efficiency　(CE)　of　Na　plating/stripping　as　high　as　99.71%　over　420　cycles　at　2　mA　h　cm(-2)　and　2　mA　cm(-2)　as　well　as　the　Na||Na　cell　to　stably　cycle　for　1200　h　(300　cycles)　with　a　depth　of　discharge　of　33.33　%　at　2　mA　h　cm(-2).　It　is　found　that　the　current　density　plays　a　greater　influence　on　the　SMA　stability.　The　in-situ　construction　of　MIECI　buffer　layer　opens　up　a　simple　and　facile　avenue　to　stabilize　SMAs.