Cage-Type Highly Graphitic Porous Carbon-Co₃O₄ Polyhedron as the Cathode of Lithium-Oxygen Batteries

A novel cage-type highly graphitic porous carbon-Co₃O₄ (GPC-Co₃O₄) polyhedron was designed and successfully prepared for the first time by executing a two-step annealing of core-shell structured metal-organic frameworks (MOFs). The low graphitic carbon cores were selectively removed during the secondary annealing in air atmospheres, leaving the interior voids due to their lower thermal stability compared with the graphitic carbon shells. Inspired by the unique properties of the cage-type GPC-Co₃O₄ polyhedron, GPC-Co₃O₄ was assembled as an oxygen electrode for a rechargeable Li-O₂ battery without the additional conductive agent. The efficient generation of Li₂O₂ during discharging and the reversible decomposition of Li₂O₂ during charging were clearly observed by XRD patterns and SEM images. The GPC-Co₃O₄ polyhedron integrates the beneficial properties, including high electronic conductivity, the rigid cage-type structure consisting of the mesoporous walls and interior void space, as well as the uniformly embedded catalytically active Co₃O₄ nanoparticles. As a result, the GPC-Co₃O₄ cathode displays a low charge overpotential of 0.58 V, a good rate capability, and a long cycle life in a Li-O₂ battery.