Critical Descriptor for the Rational Design of Oxide-Based Catalysts in Rechargeable Li–O₂Batteries: Surface Oxygen Density

Li–O 2 batteries provide high-capacity energy storage, but for aprotic Li–O 2 batteries, it is reported that the charge–discharge efficiency is ultimately limited by the crystal growth of insoluble Li 2 O 2 on the porous cathode. Catalysts have been reported to improve the nucleation and morphology of Li 2 O 2, which helps achieve high energy densities. We provide a new insight into the catalytic mechanism of the oxygen reduction reaction (ORR) in aprotic Li–O 2 batteries—the oxygen sites on the surface play a more important role than the exposed metal sites—via a study based on the density functional theory (DFT) examining α-MnO 2 surfaces. Lithium ions from electrolytes are found to interact with the surface oxygen sites and form surface lithium sites, facilitating further growth of Li 2 O 2 . A larger number of initial growth points with uniform distribution makes Li 2 O 2 well dispersed, forming small particles, which benefit both the ORR and oxygen evolution reactions (OER). This design concept for oxygen sites has been successfully validated by the real Li–O 2 cell experiments with α-MnO 2 nanowire cathodes.