Superconductivity at 31 K in a Stage-1 Sodium-Intercalated Graphite.

Graphite intercalation compounds (GICs) serve as a highly tunable platform for exploring phonon-mediated superconductivity in lightweight materials. While theoretical models have long predicted that densely packed, first-stage (stage-1) sodium-intercalated graphite (Na-GIC) could host elevated critical temperatures (Tc), its experimental synthesis has remained a formidable challenge. Here, we report the realization of stage-1 Na-GIC that exhibits bulk superconductivity and achieves a maximum onset Tc of ∼31 K, setting a record for all known GIC systems. By employing a room-temperature mechanical synthesis with an excess sodium reservoir, followed by lattice compression, we force a sequential staging transition in Na-GIC─from an initial stage-8, through an intermediate stage-2, and ultimately to the densely intercalated stage-1 phase at 15.6 GPa. By correlating room-temperature in situ synchrotron X-ray diffraction with evolutionary structural searches, we identify the host of this high-Tc state as an orthorhombic NaC3 structure with Imma symmetry. First-principles calculations reveal a remarkably strong electron-phonon coupling (λ ∼ 2.0), dominated by interactions between out-of-plane carbon π electrons and low-frequency Na/C vibrations. Our findings not only capture the elusive stage-1 Na-GIC but also establish pressure-driven compositional tuning as a robust strategy to unlock high-Tc states in carbon-based superlattices.