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

Abstract 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.