Preparation of graphene hollow spheres from vacuum residue of ultra-heavy oil as an effective oxygen electrode for Li–O2 batteries†
A three-dimensional graphene hollow sphere is prepared via a green synthetic method by utilizing vacuum residue (VR) and is shown to function as an oxygen electrode material for Li–O2 batteries. It is noteworthy that VR, which is considered an environmental pollutant and is used primarily in asphalt-paved roads, can be utilized effectively as a novel precursor for carbon electrodes in Li–O2 batteries. The low-valued VR is converted into graphene hollow sphere carbon by using Fe3O4, which serves as both a catalyst and a template in m-xylene under supercritical conditions. Since the hollow sphere structure is associated with a multi-layer graphene feature, it provides adequate void space for storing the discharge product of Li2O2 and suppresses the formation of Li2CO3. This results in capacities reaching 16 805 mA h gcarbon−1, which is higher than that of commercial KB600J carbon (5493 mA h gcarbon−1), and in improved cycle performance.