Vertically self-standing C@NiCo2O4 nanoneedle arrays as effective binder-free cathodes for rechargeable Na−O2 batteries

Sodium-oxygen (Na−O2) batteries are being extensively studied because of their higher energy efficiency and lower cost compared to those of lithium-oxygen (Li-O2) batteries. However, highly efficient and durable cathode catalysts are very limited, thus considerably hindering the application of Na-O2 batteries. Herein, a vertically self-standing C@NiCo2O4 nanoneedle arrays (C@NiCo2O4-NAs) have been delicately designed and fabricated by a facile hydrothermal method. The NiCo2O4 nanoneedles are in-situ grown on the carbon paper, forming an unique urchin-like structure, which is directly used to Na-O2 batteries as a self-standing and binder-free cathode for the first time. Under a current density of 50 mA g−1, the Na-O2 battery containing C@NiCo2O4-NAs cathode shows a high initial discharge capacity of 6500 mAh g−1 (based on NiCo2O4 material mass) and low over-potential of 0.6 V due to the numerous catalytic sites and high electron mobility of C@NiCo2O4-NAs. SEM, XPS and FTIR tests have confirmed the formation of thin film-like Na2O2 discharge product on the surface and gaps of NiCo2O4 nanoneedles, and the special film-like structure can facilitate the decomposition of Na2O2 in the subsequent charging process and reduce the over-potential notably. Meanwhile, the robust urchin-like structure of C@NiCo2O4-NAs renders the Na-O2 battery a high cyclic stability up to 120 cycles when the specific capacity is limited to be 800 mAh g−1.