Interplay of Composition, Microstructure, and Ionic Conductivity in NaSICON-Type Solid Electrolytes from Scalable Spray-Flame Synthesis

Abstract

Solid-state sodium batteries are attractive next-generation energy-storage systems, combining the safety of inorganic solid electrolytes with the resource and cost benefits of sodium-based chemistries. Among Na+ conductors, NaSICON-type Na3Zr2Si2PO12 (NZSP) is particularly promising due to its high conductivity and stability, yet its practical implementation is challenged by energy-intensive ceramic processing, typically requiring prolonged high-temperature sintering for densification. Here, spray-flame synthesis (SFS) is introduced as a scalable route to nanoparticulate NZSP precursors, offering nanometer-scale compositional homogeneity beyond conventional solid-state synthesis and greater compositional flexibility than sol–gel approaches. Rather than forming crystalline NZSP directly, SFS yields a nanostructured intermediate consisting of ∼5 nm ZrO2 domains embedded in an amorphous Na–Si–P–O matrix, enabling NaSICON formation at ∼980 °C and densification to ∼97% within 2 h at 1140 °C. Systematic variation of the Si/P ratio and substitution of Zr4+ with Mg2+ or Ca2+ increase the Na content per formula unit, resulting in reduced lattice distortion and lower activation energies. Si-rich and Mg-substituted compositions reach conductivities up to 1.16 mS cm–1 with activation energies of 0.299(2)–0.327(6) eV. We have discovered that macroscopic conductivity is governed not by composition alone but by the interplay of the crystal structure, phase stability, densification, and grain-boundary chemistry.