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

Lithium-oxygen (Li-O) 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 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 with a low overpotential (0.82 V) at 200 mA g. Under upper-limit specific capacities of 500 mA h g at 400 mA g, the Li-O batteries exhibited a long cycle life of 165 cycles. Compared with the undoped NiO electrode, the Li-O 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 Ni in the NiO matrix by Co 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 LiO deposition. This approach harnesses the great potential of Co-doped NiO nanosheets for practical applications as advanced electrodes for rechargeable Li-O batteries.