Designing carbon wetting layers with inorganic salts as additives for enhanced adhesion and molten sodium wettability in electrochemical application

β″ alumina solid-state electrolyte (BASE) is considered one of the most promising materials for molten sodium-based batteries due to its high ionic conductivity and exceptional stability with molten sodium. However, at lower operating temperatures, the poor physical contact at the liquid-solid interface between molten sodium and BASE results in high interfacial resistance. In this study, we explore the optimization of carbon wetting layers with various salt additives to enhance adhesion and improve molten sodium wettability. Using a spray-coating method, carbon layers incorporating sodium hexametaphosphate (Na 6 (PO 3 ) 6 , SHMP) as an additive demonstrates improved adhesion, mechanical stability, and enhanced wettability with molten sodium compared to other conventional sodium salts such as NaCl, NaNO 3 , and Na 2 CO 3 . Scanning Electron Microscopy (SEM) analysis reveals that SHMP preserves the porous carbon network required for efficient sodium transport while minimizing structural defects such as cracks and voids. Electrochemical testing using Na symmetric cells confirms that SHMP-modified carbon layers reduce interfacial resistance and overpotential, outperforming No-salt and other salts counterparts. These findings highlight that the strategic incorporation of suitable additives, such as SHMP, in the design of microscale carbon layers could enhance the performance of molten sodium batteries at lower operating temperatures.