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 16805 mA h g_carbon^-1, which is higher than that of commercial KB600J carbon (5493 mA h g_carbon^-1), and in improved cycle performance.