Microstructure Optimization of Na ₃ SbS ₄ /Na ₃ Zr ₂ Si ₂ PO ₁₂ Composite Solid Electrolytes for Improving Cycling Stability in All‐Solid‐State Sodium Batteries

ABSTRACT Sulfide‐based solid electrolytes have attracted significant attention for all‐solid‐state batteries due to their high ionic conductivity. However, their practical application is limited by interfacial instability at the sodium metal anode, leading to side reactions that form Na ₂ S and Na ₃ Sb, and by structural defects such as voids and cracks that create electronic leakage pathways. To address these issues, a composite electrolyte was developed by incorporating Na ₃ Zr ₂ Si ₂ PO ₁₂ (NZSP), a stable NASICON‐type oxide, into Na ₃ SbS ₄ (NSS). The optimized 90–10 wt.% NSS‐NZSP composite improves microstructural integrity by filling voids and mitigating crack formation, enabling efficient Na ⁺ transport. As a result, the ionic conductivity increases from 3.7 × 10 ⁻⁴ to 3.97 × 10 ⁻⁴ S cm ⁻¹ , while the activation energy decreases from 0.25 to 0.22 eV. A half‐cell configuration (Na _2/3 Fe _1/2 Mn _1/2 O ₂ |90–10 wt.% electrolyte|Na) demonstrates stable cycling over 100 cycles at 0.05 A g ⁻¹ , delivering a discharge capacity of 118.9 mAh g ⁻¹ at room temperature.