Carbon-coated beta-MnO2 for cathode of lithium-ion battery

beta-MnO2 with its stable tunnel structures can adapt to the insertion and extraction of Li-ions, and it has exhibited attractive potential as the cathode for a Li-ion battery. In our work, a type of hollow beta-MnO2 bipyramid was synthesized by the hydrothermal method with an initial discharge capacity of 181.3 mA h g(-1) at 20 mA g(-1). Considering the poor conductivity of manganese oxide, the samples were successfully modified with N-doped carbon by in situ synthesis of polydopamine on the surface of the samples and annealing. The carbon-coated samples (beta-MnO2@C) deliver a higher initial capacity of 235.5 mA h g(-1), better rate performance and higher cycle stability, compared to that of the bare samples. This is mainly because the carbon layer forms conductive networks on the surface of the electrodes to enhance the electrochemical kinetics, and also suppresses the volume expansion caused by the insertion and extraction of Li-ions. In addition, the mechanisms of lithium storage were studied by ex situ XRD measurement, showing that rutile beta-MnO2 irreversibly transforms to orthorhombic LixMnO2 in the initial cycle and no other phases are generated in the subsequent charge-discharge process.