Na Intercalation in Bilayer Graphene: Formation of a Close-Packed Trilayer

Sodium (Na) intercalation into bulk graphite is predicted to be energetically unfavorable and has therefore long been considered difficult to achieve. Here, we report the successful intercalation of Na into bilayer graphene (BLG), confirmed by post-intercalation STEM characterization performed under inert conditions. Our results reveal that Na forms not a single layer but a trilayer structure with close-packed face-centered cubic (fcc) (111) stacking within the van der Waals gap of BLG. Density functional theory (DFT) calculations further confirm that the trilayer configuration is energetically more favorable than single or bilayer arrangements. This finding adds a missing piece to the understanding of alkali metal intercalation in low-dimensional systems and highlights the potential of Na and BLG as promising candidates for energy storage and nanoelectronic applications, offering advantages in safety and stability over lithium-based systems.