Freestanding graphene/MnO₂ cathodes for Li-ion batteries

Different polymorphs of MnO₂ (α-, β-, and γ-) 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/α-, β-, and γ-MnO₂ freestanding cathodes was found to be 321 mAhg^-1, 198 mAhg^-1, and 251 mAhg^-1, respectively. Finally, the graphene/α-MnO₂ cathode displayed the best cycling performance due to the low charge transfer resistance and higher electrochemical reaction behavior. Graphene/α-MnO₂ freestanding cathodes exhibited a specific capacity of 229 mAhg^-1 after 200 cycles with 72% capacity retention.