Microstructural and Interfacial Engineering of NASICON‐Type Na 1 + x Zr 2 Si x P 3 − x O 12 Electrolytes for Solid‐State Sodium Metal Batteries: A Review

Solid‐state sodium metal batteries (SSSMBs) are promising candidates for next‐generation energy storage systems due to their high theoretical energy density, inherent safety, and abundant sodium (Na) reserves. The realization of practical SSSMBs relies on the development of advanced inorganic solid electrolytes (ISEs). Among the ISEs investigated, NASICON‐type Na 1 + x Zr 2 Si x P 3 − x O 12 (0 ≤ x ≤ 3, NZSP) solid electrolytes have attracted significant attention for their excellent chemical stability, environmental robustness, and wide electrochemical stability window. However, their practical implementation remains constrained by insufficient room‐temperature Na + conductivity, stemming from sluggish bulk transport and high grain‐boundary impedance, and by an unstable Na/NZSP interface that elevates interfacial resistance and promotes dendrite‐induced failure during cycling. This review examines the structural and electrochemical features of NZSP solid electrolytes and critically analyzes the key factors governing Na + transport across the bulk crystal lattice, grain boundaries, and interfacial regions. Building on this foundation, it surveys recent advances in doping, grain‐boundary engineering, and interfacial modification aimed at enhancing the ionic conductivity of NZSP solid electrolytes and stabilizing the Na/NZSP interface. Overall, NZSP modification strategies are inherently multifunctional, and integrated designs concurrently addressing bulk transport, grain‐boundary conduction, and interfacial stability offer a particularly promising route toward practical, dendrite‐free SSSMBs.