| Facing with the crisis of energy and resources and the pollution of the environment,global scientific research workers try to explore new sources of energy.The rechargeable non-aqueous Li-O2 batteries have aroused intensive research interests because of the high theoretical energy density(5200 Wh kg-1)and the property of environmental friendliness,along with the promising application prospect in electric vehicles.However,the Li-O2 batteries have also suffered from the sluggish kinetics of the oxygen reduction reaction?ORR,2Li++2O2?Li2O2?and the oxygen evolution reaction?OER,Li2O2?O2+2Li+?during the discharging and charging process,resulting in the low coulombic efficiency,poor rate capability,short cycle life and high charge-discharge voltage gap,and thus constraining the practical applications of Li-O2 batteries.The key to solve the above problems is to prepare an effective bifunctional electrocatalysts for oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?.As we all know,the morphology,structure and the chemical composition of the electrocatalysts have tremendous impacts in the performances of Li-O2 batteries.We have tried to use the improved Hummers method,hydrothermal process and calcination method,synthesizing the different structures of oxides loaded on reduced graphene oxide?rGO?.So,the purpose of us is to an efficient cathode catalyst for Li-O2 batteries.Firstly,ultrathin Co3O4 nanosheets grown on the reduced graphene oxide?Co3O4/rGO?was synthesized by a simple hydrothermal method and was investigated as a cathode in a Li-O2 battery.Benefited from the synergistic effect between Co3O4and rGO,the hybrid exhibits a high initial capacity of 10528 mAh g-1 along with a high coulombic efficiency?84.4%?at 100 mA g-1.In addition,the batteries show an enhanced cycling stability and after 113 cycles,the cut-off discharge voltage remains above 2.5 V.Although the battery with the Co3O4/rGO cathode is evaluated with a restriction of the capacity to 3000 mAh g-1,the battery maintained this high capacity for 28 cycles.The outstanding performance is intimately related to the high surface area of rGO,which not only provide carbon skeleton for the uniform distribution of Co3O4 nanosheets but also facilitate the reversible formation and decomposition of insoluble Li2O2.The results of electrochemical tests confirm that the Co3O4/rGO hybrid is a promising candidate for the Li-O2 batteries.Secondly,we synthesize wheat-like mesoporous CoO nanorods by hydrothermal process and calcination methode,and synthesize CoO/rGO composite by mounting CoO nanorods on graphene.Owing to synergistic effect between CoO and rGO,the CoO/rGO hybrid exhibits a good initial capacity of 20254 mAh g-11 along with a high coulombic efficiency?98.9%?at 200 mA g-1.In addition,the batteries show an excellent rate capability(13952 mAh g-11 at 800 mA g-1)and enhanced cycling stability(69 cycles with the capacity limited to 1000 mAh g-11 at 200 mA g-1).The electrochemical performance is intimately related to the unique architecture?i.e.,hierarchical mesoporous structure?,facilitating the reversible formation and decomposition of insoluble Li2O2.Finally,we synthesize mesoporous MnxCo3-xO4/rGO nanorods by hydrothermal process and calcination method through change the mass ratio of Co?NO3?3·6H2O and Mn?NO3?2·6H2O.With the change of mass ratio,the morphologies of synthetic compounds has also undergone tremendous changes.Among them,the synthesized MnCo2O4/rGO complex exhibits excellent electrochemical properties.When the battery with the MnCo2O4/rGO cathode is evaluated with a restriction of the capacity to 2000 mAh g-1,the battery maintained this high capacity for 32 cycles.Even after charging and discharging 32 cycles,the battery termination voltage is only 4.2 V,showing excellent OER catalytic activity. |
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