Hierarchical Mesoporous/Macroporous Co-Doped NiO Nanosheet Arrays as Free-Standing Electrode Materials for Rechargeable Li-O2 Batteries

Lithium-oxygen (Li-O-2) batteries have been widely recognized as appealing power systems for their extremely high energy density versus common Li-ion batteries. However, there are still lots of issues that need to be addressed toward the practical application. Here, free-standing Co-doped NiO three-dimensional nanosheets were prepared by a hydrothermal synthesis method and directly employed as the air-breathing cathode of the Li-O-2 battery. The morphological phenomenon and electrochemical performance of the as-prepared cathode material were characterized by high-resolution scanning electron microscopy, X-ray diffraction, cyclic voltammetry, galvanostatic charge-discharge tests, and electrochemical impedance spectroscopy measurements. The Co-doped NiO electrode delivered a maximum discharge capacity of around 12 857 mA h g(-1) with a low overpotential (0.82 V) at 200 mA g(-1). Under upper-limit specific capacities of 500 mA h g(-1) at 400 mA g(-1), the Li-O-2 batteries exhibited a long cycle life of 165 cycles. Compared with the undoped NiO electrode, the Li-O-2 battery based on the Co-doped NiO cathode showed significantly higher oxygen reduction reaction and oxygen evolution reaction activities. This superior electrochemical performance is because of the partial substitution of Ni2+ in the NiO matrix by Co2+ to improve the p-type electronic conductivity of NiO. In addition, the morphology and specific surface area of NiO are affected by Co doping, which can expand the electrode-electrolyte contact area and lead to sufficient space for Li2O2 deposition. This approach harnesses the great potential of Co-doped NiO nanosheets for practical applications as advanced electrodes for rechargeable Li-O-2 batteries.