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Herein, hierarchically structured microgrid frameworks of Co3O4 and carbon composite deposited on reduced graphene oxide (Co3O4@C/rGO) are demonstrated through the three‐dimensioinal (3D) printing method, where the porous structure is controllable and the height and width are scalable, for dendrite‐free Na metal deposition. The sodiophilicity, facile Na metal deposition kinetics, and NaF‐rich solid electrolyte interphase (SEI) formation of cubic Co3O4 phase are confirmed by combined spectroscopic and computational analyses. Moreover, the uniform and reversible Na plating/stripping process on 3D‐printed Co3O4@C/rGO host is monitored in real time using in situ transmission electron and optical microscopies. In symmetric cells, the 3D printed Co3O4@C/rGO electrode achieves a long‐term stability over 3950 at 1 mA cm−2 and 1 mAh cm−2 with a superior Coulombic efficiency (CE) of 99.87% as well as 120 h even at 20 mA cm−2 and 20 mAh cm−2, far exceeding the previously reported carbon‐based hosts for Na metal anodes. Consequently, the full cells of 3D‐printed Na@Co3O4@C/rGO anode with 3D‐printed Na3V2(PO4)3@C‐rGO cathode (≈15.7 mg cm−2) deliver the high specific capacity of 97.97 mAh g−1 after 500 cycles with a high CE of 99.89% at 0.5 C, demonstrating the real operation of flexible Na metal batteries. [ABSTRACT FROM AUTHOR]

Sodium metal batteries are promising next‐generation energy storage technology by using energy‐dense and affordable Na metal anodes, yet suffering uncontrollable Na dendritic growth issues. Herein, Au nanoparticle@hollow amorphous carbon tube yolk/shell arrays (Au/HCT‐CC) is rationally designed on carbon cloth as a dynamic host. In situ transmission electron microscopy observations reveal a regulated dendrite‐free Na metal plating/stripping within the Au/HCT‐CC host. The self‐confinement of Na metal deposition in the hollow carbon can further stabilize the electrolyte/electrode interface and homogenize Na ion flux, as evidenced by rigorous experimental and theoretical characterizations, thus successfully accommodating the hurdles to Na metal anodes. When cycling in half cells, the Au/HCT‐CC electrodes deliver remarkably high coulombic efficiencies (CEs) of 99.96% over 2200 h at 5 mA cm−2. The high CE of 99.54% is preserved even under harsh cycling conditions of 10 mA cm−2 and 20 mAh cm−2 for 250 cycles. These values rival the state‐of‐the‐art electrochemical performance for Na metal anodes in literature. Finally, the practical feasibility of the new anode is demonstrated by cycling in Na3V2(PO4)3@C||Na‐Au/HCT‐CC full cells over 900 cycles with an extremely low capacity degradation rate of 0.017% per cycle. [ABSTRACT FROM AUTHOR]