Sc/Mg Co‐Doping in Na ₃ Zr ₂ Si ₂ PO ₁₂ 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 Sc³⁺ and Mg²⁺ dopants are directly introduced into the Na₃Zr₂Si₂PO₁₂ solid-state electrolyte (NZSP SSE) to optimize composition and regulate interfacial chemistry. The resulting co-doped electrolyte, Na_3.7Zr_1.45Sc_0.4Mg_0.15Si₂PO₁₂ (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 ScPO₄/Mg₃(PO₄)₂-rich interphase layer at the Na metal/SSE interface. Furthermore, Na//NSZSP-0.15Mg//Na₃V₂(PO₄)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.