▎ 摘　　要

Different polymorphs of MnO2 (alpha-, beta-, and gamma-) were produced by microwave hydrothermal synthesis, and graphene oxide (GO) nanosheets were prepared by oxidation of graphite using a modified Hummers' method. Freestanding graphene/MnO2 cathodes were manufactured through a vacuum filtration process. The structure of the graphene/MnO2 nanocomposites was characterized using X-ray diffraction (XRD) and Raman spectroscopy. The surface and cross-sectional morphologies of freestanding cathodes were investigated by scanning electron microcopy (SEM). The charge-discharge profile of the cathodes was tested between 1.5 V and 4.5 V at a constant current of 0.1 mA cm(-2) using CR2016 coin cells. The initial specific capacity of graphene/alpha-, beta-, and gamma- MnO2 freestanding cathodes was found to be 321 mAhg(-1), 198 mAhg(-1), and 251 mAhg(-1), respectively. Finally, the graphene/alpha-MnO2 cathode displayed the best cycling performance due to the low charge transfer resistance and higher electrochemical reaction behavior. Graphene/alpha-MnO2 freestanding cathodes exhibited a specific capacity of 229 mAhg(-1) after 200 cycles with 72% capacity retention.