Electrochemical Performance Of Graphene And Manganese Dioxide For Li-air Battery

Lithium-air battery is one of the most attractive emerging energy storage systems for electric vehicles （EV） and hybrid electric vehicles （HEV） because of its extremely high theoretical specific capacity. But there still exists critical challenges, the energy efficiency which is limited by the sluggish kinetics of oxygen reduction reaction （ORR） and oxygen evolution reaction （OER）. The design of porous structure electrode and bifunctional catalyst are major challenge to further enhance the electrochemical performance of lithium-air batteries. Herein, unique porous structure graphene and manganese dioxide bifunctional catalyst were synthesized as cathode materials for lithium air batteries,The main research content is as follows:The thermal expansion graphene （TEGO） was produced by thermal expansion and simultaneous reduction of graphite oxide. The structures and morphologies of TEGO were respectively characterized by XRD and SEM. The results reveal that TEGO has graphene slice structure. The determined Brunauer-Emmett-Teller （BET） specific surface area is 268 m2g^-1. The galvanostatic chargre and discharge test results indicated that at a current density of 100 mAg^-1, in the voltage range 2.0-4.5 V, TEGO can deliver an initial discharge specific capacity of 9223 mAhg^-1 and an initial charge specific capacity of 6259 mAhg^-1. When restricting the cell to a cycling specific capacity of 1000 mAhg^-1, the cell sustains 29 cycles with the discharge terminal above 2.0 V. The microwave expanded graphene （MEGO） was produced by microwave expansion and simultaneous reduction of graphite oxide. The structures and morphologies of MEGO were respectively characterized by XRD and SEM. The results reveal that compared with TEGO, MEGO has better graphene slice pore structure. The determined Brunauer-Emmett-Teller （BET） specific surface area is 337 m2g^-1. The galvanostatic chargre and discharge test results indicated that at a current density of 100 mAg^-1, in the voltage range 2.0-4.5 V, MEGO can deliver an initial discharge specific capacity of 13862 mAhg^-1 and an initial charge specific capacity of 11811 mAhg^-1. When restricting the cell to a cycling specific capacity of 1000 mAhg^-1, the cell sustains 53 cycles with the discharge terminal above 2.0 V. Compared with TEGO, MEGO exhibits excellent performance, including good rate capacity, long cycle stability. This excellent electrochemical performance is attributed to the unique bimodal slice pore structure of MEGO which consists of channels facilitating rapid O₂ diffusion.Pure MnO₂ was produced by hydrothermal method. The structures and morphologies of MnO₂ were respectively characterized by XRD and SEM. The results reveal that the pure MnO₂ has good crystal morphology and nano-flower-like structure. The galvanostatic chargre and discharge test results indicated that at a current density of 100 mAg^-1, in the voltage range 2.0-4.5 V, the mixture of MEGO and MnO₂ can deliver an initial discharge specific capacity of 15473 mAhg^-1 and an initial charge specific capacity of 15440 mAhg^-1. When restricting the cell to a cycling specific capacity of 1000 mAhg^-1, the cell sustains 73 cycles with the discharge terminal above 2.0 V. The composite of MEGO@MnO₂ was produced by the reactive of C with KMnO4 in an acidic environment. The structures and morphologies of MEGO@MnO₂ were respectively characterized by XRD, Raman and SEM. The results reveal that MEGO@MnO₂ has better crystal morphology, MnO₂ evenly distributed over the slice of MEGO. The galvanostatic chargre and discharge test results indicated that at a current density of 100 mAg^-1, in the voltage range 2.0-4.5 V, MEGO@MnO₂ can deliver a initial discharge specific capacity of 17101 mAh g^-1 and an initial charge specific capacity of 16454 mAh g^-1. When restricting the cell to a cycling specific capacity of 1000 mAhg^-1, the cell sustains 96 cycles with the discharge terminal above 2.0 V. Compared with the mixture of MEGO and MnO₂, The composite of MEGO@MnO₂ exhibits excellent performance, including low charge overpotential, good rate capacity, long cycle stability up to 96 cycles with controlling capacity of 1000 mAhg^-1. A combination of the multi-scale porous network of the shell-connected MEGO support and the evenly dispersed MnO₂ nanostructure benefits the transportation of oxygen and electrolyte while the highly connected nanoscale slice pore provide a high density of reactive sites, exhibited enhanced electrochemical performance.