FAC-treated NZSP. (a) Digital photo of the PVDF-derived layer on the surface of NZSP. Inset is the XRD profile of PVDF-derived carbon. (b) Optical photograph of melted Na metal on top of the NZSP pellet. (c) SEM image of the FAC-treated NZSP pellet. EDX mapping of (d)–(f) revealing the distribution of F, Na, and P elements near the interface between Na metal and NZSP pellet. C1s, F1s, and Na1s XPS spectra from (g) untreated and (h) FAC-treated NZSP pellets.