Sc/Mg Co‐Doping in Na₃Zr₂Si₂PO₁₂ Solid‐State Electrolytes Enables Outstanding Performance of Sodium Metal Batteries

All solid‐state sodium metal batteries offer a transformative opportunity for more sustainable energy storage, with the potential to significantly improve both energy density and safety compared to conventional sodium‐ion batteries. However, their practical application is hindered by challenges such as dendrite formation and limited ion conductivity. In this study, a novel strategy is proposed in which Sc 3+ and Mg 2+ dopants are directly introduced into the Na 3 Zr 2 Si 2 PO 12 solid‐state electrolyte (NZSP SSE) to optimize composition and regulate interfacial chemistry. The resulting co‐doped electrolyte, Na 3.7 Zr 1.45 Sc 0.4 Mg 0.15 Si 2 PO 12 (NSZSP‐0.15Mg), exhibits a significantly enhanced ionic conductivity of 1.3 mS cm −1 at room temperature and improved interfacial compatibility. Moreover, symmetric Na//NSZSP‐0.15Mg//Na cells demonstrate stable Na stripping/plating for over 400 h (0.5 mA cm −2 , 0.5 mAh cm −2 ), attributed to the formation of a dynamically stable ScPO 4 /Mg 3 (PO 4 ) 2 ‐rich interphase layer at the Na metal/SSE interface. Furthermore, Na//NSZSP‐0.15Mg//Na 3 V 2 (PO 4 ) 3 full cell batteries exhibit excellent rate capability and long‐term cycling stability, maintaining 75 mAh g −1 over 3000 cycles at 2 C at room temperature. This work presents a robust approach for enabling practical solid‐state sodium metal batteries with high conductivity, enhanced interfacial stability, high energy density, and long‐term cyclability, advancing the development of next‐generation SSEs for high‐performance sodium metal batteries.