Display omitted •Bi-RGO was synthesized by one-pot supercritical acetone method.•Unique deoxygenation properties and ultrafast Bi formation of scACT formed Bi-RGO.•Nanosize Bi tightly anchored on basal plane of RGO with a heterogeneous nucleation.•Reversible capacity of 170 mAh g−1 and the high volumetric capacity of 60 Ah L−1 were achieved.•Bi transformed to h-Na3Bi or c-Na3Bi phases depending on the Bi particle size. A high-energy–density Bi-reduced graphene oxide (Bi-RGO) composite was synthesized using the quick one-pot supercritical acetone (scACT) method. The unique deoxygenation properties and ultrafast particle formation in scACT led to the simultaneous reduction of graphene oxide to reduced graphene oxide (RGO) and the formation of heterogeneously nucleated, nanosized Bi particles 10–20 nm in size that were tightly anchored to the basal plane of RGO and homogeneously nucleated, large Bi particles 250–950 nm in size. When tested as the anode of Na-ion batteries, the Bi-RGO composite delivered the high reversible capacity of 200 mAh g−1 at 50 mA g−1 and the high volumetric capacity of 60 Ah L−1. In situ X-ray diffraction analysis revealed that the two-step alloying–dealloying Bi → NaBi → hexagonal Na3Bi or cubic Na3Bi reactions depended on the Bi particle size.