3D Printing of Free-Standing "O2 Breathable" Air Electrodes for High-Capacity and Long-Life Na-O2 Batteries

Superoxide-based Na-O-2 batteries have been considered as some of the most promising candidates for next-generation energy storage systems due to their high theoretical energy density and energy efficiency. However, to fully realize the advantages of Na-O-2 batteries, the underutilization of air electrodes and poor cycling performance caused by limited O-2 transport in the air electrodes must be addressed. In this work, 3D printing of a reduced graphene oxide (rGO)-based air electrode with a hierarchical porous structure was first demonstrated as being "O-2 breathable" for Na-O-2 batteries. The unique cathode structure features noncompetitive and continuous pathways for O-2, Na+ ions, and electrons. The macropores provide smooth passages to facilitate O-2 access across the whole electrode, while the micropores between rGO sheets serve as electrolyte reservoirs and accommodate NaO2. The efficiently packed rGO sheets ensure sufficient electronic conductivity within the 3D architecture. Na-O-2 batteries using these "O-2 breathable" electrodes can achieve a high capacity of 13484.6 mAh g(-1) at 0.2 A g(-1) and a stable cycling performance over 120 cycles with a cutoff capacity of 500 mAh g(-1) at 0.5 A g(-1). The 3D-printed "O-2 breathable" electrodes evidently demonstrate the importance of a refined balance between electronic conductivity and O-2 transport for high-performance Na-O-2 batteries.