MnO₂ and Reduced Graphene Oxide as Bifunctional Electrocatalysts for Li-O₂ Batteries

An effective and cheap way to optimize the oxygen electrocatalyst of the cathode for Li-O₂ battery is to control the growth of a low amount of nanosized transition metal oxides on the surface of the carbon electrode. This approach combines the advantages of surface, interface, and nanosize engineering. In this work, a MnO₂ content of about 9 wt % is deposited on reduced graphene oxide (MnO₂@rGO). The MnO₂ particles grow uniformly on the rGO surface with a particle size smaller than 20 nm. The MnO₂@rGO composites were applied as cathode catalysts in Li-O₂ batteries, demonstrating an initial discharge capacity of 5139 mAh g^-1 and a high capacity of 4262 mAh g^-1 (80% capacity retention) after 15 full discharge-charge cycles at a current density of 100 mA g^-1. The outstanding performance can be attributed to the strong synergistic effect between the rGO framework and the nanosized MnO₂ particles on its surface. The rGO framework possesses a porous multilayer structure, which provides an excellent electrical conductivity, promotes oxygen and ion diffusion, and provides storage space for the discharge products. The nanosized MnO₂ possesses a high exposed surface, which enhances surface transport of LiO₂ species and avoids the accumulation of discharge products on electrode surface. Furthermore, a transition between lithiated and nonlithiated manganese oxide during the discharge and charge processes was observed. This transition apparently helps to promote electron transfer between discharge products and the catalyst and thereby to reduce the overpotential of the oxygen evolution reaction.