Synergistic Bulk and Grain-Boundary Regulation in NASICON Solid-State Electrolytes for Sodium Metal Batteries

Solid-state sodium metal batteries are promising for large-scale energy storage owing to their high safety, abundant sodium resources, and low material cost. However, NASICON-type solid-state electrolytes, such as Na3Zr2Si2PO12, still suffer from limited room-temperature ionic conductivity and poor ceramic densification. Herein, a multivalent co-doping strategy using Zn2+, Sc3+, Hf4+, and Nb5+ was employed to regulate the crystal structure, sintering behavior, and Na+ transport properties of NASICON electrolytes. The effects of isovalent and aliovalent dopants were systematically investigated by XRD, Rietveld refinement, SEM-EDS, bond-valence-site calculations, and electrochemical impedance spectroscopy. The optimized monoclinic Na3.167Zn0.167Hf0.167Zr1.5Nb0.167Si2PO12 electrolyte delivered a room-temperature total ionic conductivity of 1.16 mS cm−1 and an activation energy of 0.35 eV, mainly due to optimized Na+ migration-channel geometry and enhanced ceramic densification. Na||Na symmetric cells exhibited stable cycling for 500 h with a maximum polarization voltage of 20 mV. Furthermore, solid-state Na||Na3V2(PO4)3 cells retained 95% capacity after 624 cycles at 1 C. This work provides a feasible doping strategy for advanced NASICON electrolytes and solid-state sodium metal batteries.