We employ a MnCo 2 O 4 -graphene hybrid material as the cathode catalyst for Li-O 2 batteries with a non-aqueous electrolyte. The hybrid is synthesized by direct nucleation and growth of MnCo 2 O 4 nanoparticles on reduced graphene oxide, which controls the morphology, size and distribution of the oxide nanoparticles and renders strong covalent coupling between the oxide nanoparticles and the electrically conducting graphene substrate. The inherited excellent catalytic activity of the hybrid leads to lower overpotentials and longer cycle lives of Li-O 2 cells than other catalysts including noble metals such as platinum. We also study the relationships between the charging-discharging performance of Li-O 2 cells and the oxygen reduction and oxygen evolution activity of catalysts in both aqueous and non-aqueous solutions.
Lithium ion batteries (LIBs) have become the main power source for today's portable electronics and are being actively pursued for propelling electric vehicles in the near future. 1-4 However, challenges remain for LIBs to become a major energy supply device for transportation. In particular, the energy density of LIBs should be increased by at least 3 times in order to support a driving range of more than 500 km with a single charge. 5 Also, the cost of LIBs should be lowered in order to be competitive with respect to other sources of energy. Limited by the insufficient capacity of electrode materials, the current LIB systems are not likely to reach the specific energy level needed for electric transportation in the long run. [1][2][3][4][5] It is necessary to develop alternative types of batteries that are capable of delivering higher energy density.
Li-O 2 batteries have recently attracted renewed interest due to significantly higher gravimetric energy density than LIBs. [5][6][7] With a theoretical specific energy of $3500 W h kg À1 based on the mass of Li and O 2 , Li-O 2 batteries are estimated to provide $500 to 900 W h kg À1 in practical devices, which is more than 3 times higher than that of a typical LIB. [5][6][7] In an ideal Li-O 2 cell, a Li metal anode is oxidized while O 2 is reduced at the cathode during discharging, producing Li 2 O 2 in an aprotic electrolyte or LiOH in an alkaline solution. During charging, Li 2 O 2 or LiOH is supposed to be oxidized to generate O 2 at the cathode and Li is plated back onto the anode. Despite that numerous cathode catalysts including carbon, metal oxides and noble metals have been applied to enhance the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the cathode, Li-O 2 cells obtained thus far have exhibited high overpotential and short cycle lives. [5][6][7][8][9][10][11][12] It is important to understand the operating principles of Li-O 2 cells and to explore new oxygen electrode materials with higher catalytic