▎ 摘　　要

To reduce the charge overpotential and improve the cyclability of Lithium-Oxygen (Li-O-2) batteries, a novel carbon composite material consisting of graphene and activated carbon (G/AC) is developed for the oxygen electrode (cathode). In the G/AC, the graphene forms a three-dimensional (3D) network with good electrical conductivity and excellent mechanical strength and flexibility, while the AC layer on the graphene surface provides numerous meso/micropores with diameters of less than several nanometers that act as active sites (or nucleation sites) for the discharge reaction. The particle size of the discharge product Li2O2 becomes much smaller (similar to 10 nm), and the particles are distributed more homogeneously in the G/AC cathode, whereas toroidal particles as large as 100-200 nm occur in the pristine graphene cathode. The charge voltage for the G/AC cathode gradually increases from similar to 3 V to 4 V, while the voltage for the graphene cathode shows a steep increase to a plateau at 4.2-4.3 V and eventually exceeds 4.5 V. The G/AC cathode also exhibits much better cyclability than the graphene. These observations indicate that the refinement of the Li2O2 particles achieved by increasing the number of nucleation sites effectively lowers the charge overpotential and improves the cyclability of the Li-O-2 batteries. (C) 2015 Elsevier Ltd. All rights reserved.