Optimizing the transport properties of sulfide-based composite cathode for sodium solid state batteries: Effects of composition and ball-milling parameters

Sodium solid-state batteries are a promising next-generation energy storage technology; however, further progress relies on improving ionic transport in solid electrolytes and addressing the complex design requirements of composite cathodes. This work investigates how composition, microstructure, and ball-milling parameters govern transport properties. Conducting matrix mixtures containing Na 3 PS 4 as the solid electrolyte and vapor-grown carbon fiber (VGCF) as the conductive additive were prepared with varying conductive additive contents (2–12 wt%). Composite cathodes were formed with different fractions of NaCrO 2 as the active material (40–70 wt%). The sequence of mixing the components and the milling time were systematically altered. Results show that the conducting matrix mixtures exhibit percolation-like behavior with a percolation region of 4–6 wt.% VGCF. Adding NaCrO 2 to the CM reduces both ionic and electronic conductivities. Optimal transport in composite cathodes is achieved at 50 wt% NaCrO 2 , resulting in high and comparable ionic and electronic conductivities while maintaining a relatively high fraction of active material. The mixing sequence shows no significant effect on conductivity, whereas the milling time significantly influences the electronic conductivity of composite cathodes. These findings offer valuable insights for optimizing cathode microstructures and ball-milling to improve sodium solid-state battery performance.