Free-standing Co3O4 on nickel foam is synthesized by a simple hydrothermal route, using cobalt nitrate hexahydrate as the cobalt source, and urea as the precipitator. The effects of urea on the structural and electrochemical characteristics are investigated in detail. As the urea increased, the morphology of the free-standing Co3O4 turns from hexagonal nanosheets to nanoflakes, and finally turns to nanoflakes/nanowires hybrid structure. This hybrid structure is made up of ultrathin porous nanoflakes, and short nanowires anchored at the edge of the nanoflakes, which may enhance the diffusion rate of the electrolyte and O-2 and provide more active sites for both O-2 reduction and evolution reactions. When applied as a free-standing cathode for the Li-O-2 cells, it shows better performance for the nanoflakes/nanowires hybrid structure than the others. It exhibits a high specific capacity of 5958 mA h g(-1) at 0.1 mA cm(-2), a long cycle life of 108 cycles with a limited capacity of 500 mA h g(-1), and unique morphology of the discharge products. (C) 2019 Elsevier Ltd. All rights reserved.