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.