Composites of Metal Nanoparticles and Reduced Graphene Oxide for Rechargeable Li-O2 Battery

A metal-air battery functions involving the principles of both batteries and fuel cells. The anode of a metal–air cell is stored inside the cell and O₂ for the air-electrode is supplied either from atmosphere or a tank. Zinc-air cells are commercially successful. In recent years, research activities on rechargeable Li-air cells have become intense due to their anticipated high energy density, which is close to specific energy of gasoline. Development of rechargeable Li-air cells involves several issues related to the negative Li electrode, the positive O₂ electrode as well as the electrolyte. Among these problems, studies on rechargeable O₂ electrode kinetics in non-aqueous electrolytes are important. Development of an appropriate catalyst, which allows both O₂ reduction and its evolution with fast kinetics and high efficiency, is a challenging problem.

 Reduced graphene oxide (RGO) and its composites have emerged as important materials for energy storage systems because of high electronic conductivity and high surface area of RGO. In the present study, graphite is converted to graphite oxide by Hummer’s method and converted to graphene oxide by ultra-sonication. Graphene oxide is chemically reduced to RGO by NaBH₄. Au-RGO is also prepared by a similar route. RGO and Au-RGO are characterized by physicochemical studies. Au nanoparticles of average size of 5.2 nm are distributed uniformly over RGO sheets (Figure: 1). Further characterization studies of RGO and Au-RGO are carried out by powder XRD, UV-Visible, Raman spectroscopy and XPS. Li-O₂(RGO) and Li-O₂(Au-RGO) cells are assembled in Ar atmosphere using Li foil as the negative electrode, the catalyst coated carbon paper as the positive electrode and 1.0 M LiPF₆ dissolved in TEGDME as the electrolyte. A glass absorbing mat is used as the separator. Typical charge-discharge curves of Li-O₂(RGO) and Li-O₂(Au-RGO) are shown in Fig. 2. The discharge capacities obtained for the first discharge are 956 and 1738 mAh g^-1, respectively, for Li-O₂(RGO) and Li-O₂(Au-RGO) cells at a discharge current of 0.2 mA cm^-2. Li-O₂(Au-RGO) cells are cycled at 0.6 mA cm^-2 over about 120 charge-discharge cycles. Although about 1500 mAh g^-1 discharge capacity is obtained for the cycle, there is a rapid decrease of capacity in the initial stages of cycling, and then there is a gradual decrease. Discharge capacity of about 500 mAh g^-1 is obtained for the 120th cycle. The moderate stability of the Li-O₂(Au-RGO) cells is attributed to nanoparticles of Au, which are uniformly distributed on RGO sheets. Powder XRD pattern of Au-RGO catalyst used for cycling of Li-O₂(Au-RGO) cells indicate the presence of Li₂O and Li₂O₂, which are the products of oxygen reduction in the non-aqueous electrolyte. Results of these investigations will be presented.

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