Inorganic　oxide　solid-state　electrolytes　have　attracted　significant　attention　as　critical　components　for　solid-state　batteries　due　to　their　low　cost　and　high　mechanical　strength.　However,　their　commercialization　has　been　hindered　by　intrinsically　low　ionic　conductivity　and　high　interfacial　impedance.　In　this　study,　Na3.6Zr1.6Sc0.2Mg0.2Si2PO12　(ScMg22-NZSP)　solid-state　electrolytes　were　synthesized　via　Sc3+/Mg2+　co-doping　at　Zr4+　sites　using　a　high-temperature　solid-state　method　to　enhance　performance　for　sodium　solid-state　batteries　(SSBs).　The　ScMg22-NZSP　exhibits　a　room-temperature　ionic　conductivity　of　2.13　mS　cm−1,　a　6.7-fold　improvement　over　the　undoped　counterpart　(0.316　mS　cm−1).　Sodium　symmetric　cells　demonstrate　ultralow　interfacial　resistance　(24.6　Ω　cm2　vs.　177.5　Ω　cm2　for　pristine　NZSP)　and　a　high　critical　current　density　of　0.65　mA　cm−2　(vs.　0.35　mA　cm−2),　along　with　stable　Na　plating/stripping　over　3600　h　at　0.1　mA　cm−2.　Furthermore,　Na|ScMg22-NZSP|Na3V2(PO4)3　full　cells　deliver　96.6　%　capacity　retention　after　1000　cycles　at　room　temperature　and　can　maintain　stable　operation　even　under　high　mass-loading　(200　cycles　at　4.7　mg　cm−2,　50　cycles　at　8.3　mg　cm−2).　Notably,　solid-state　batteries　employing　a　layered　oxide　cathode　achieve　a　discharge　capacity　of　75.47　mAh　g−1　at　10C　rate　with　88.1　%　capacity　retention　after　200　cycles　at　3C,　demonstrating　significant　potential　for　application　in　high-performance　solid-state　electrochemical　energy　storage　systems.　©　2025