Carbon Nanotube@MnO2 Hybrid Catalysts for Li-O2 Batteries

Abstract

Rechargeable lithium-oxygen (Li-O2) battery has attracted enormous research attention due to its remarkably high theoretical energy capacity. However, the sluggish oxygen reduction and oxygen evolution reaction (ORR and OER) kinetics cause the high over potential, poor electrolyte stability and short cycling life.[1] Thus, the development of efficient catalysts are required to effectively improve the Li-O2 battery performance. The materials based on MnO2 are recognized as low-cost and efficient catalysts for Li-O2 battery cathodes.[2] Here, in-situ growth of branched MnO2 nanosheets on multi-walled carbon nanotubes (MWCNTs) were generated through a one-pot reaction process and applied as cathode catalyst in a rechargeable Li-O2 battery. The MnO2 with the morphology of curved petal-like walls (about 50 nm in thickness) were homogeneously coated outside the CNT (about 40 nm in diameter) to form a core–shell nanowires.[3] The CNT@MnO2 hybrid showed excellent catalytic activity for both ORR and OER processes in Li-O2 batteries. Owing to its ultra-high specific surface area, it delivered a high reversible capacity of 1500 mAh·g-1 based on the mass of the hybrid. This core-shell structure can effectively prevent direct contact between the CNT and the discharge product Li2O2, thus avoiding or reducing the decomposition of CNT and the formation of Li2CO3 during charge process, which can also improve the conductivity of the cathode and increase the cycling performance of Li-O2 batteries. As a promising cathode catalyst in Li-O2 batteries, the branched CNT@MnO2 nanocomposite cathode exhibits a stable full discharge and charge cycling performance. As shown in figure 1b, about 250 cycles were achieved with a fixed capacity of 500 mAh·g-1 at a current of 100 mA·g-1. The results shown that such a design of a core-shell architecture with a high conductive carbon nanotube in the center and MnO2 nanosheets uniformly coated on its surface both achieved the high conductivity and excellent catalytic activity in Li-O2 batteries. Figure 1. Schematic illustrations of the transfer of electrons in CNT@MnO2 cathode (a). discharging and charging profiles (b) and cycle performance of Li-O2 battery with CNT@MnO2 cathode(c).