3D-Printed Hierarchically Microgrid Frameworks of Sodiophilic Co₃O₄@C/rGO Nanosheets for Ultralong Cyclic Sodium Metal Batteries

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.