Critical Descriptor for the Rational Design of Oxide-Based Catalysts in Rechargeable Li–O2 Batteries: Surface Oxygen Density

Li−O2 batteries provide high-capacity energy storage, but for aprotic Li−O2 batteries, it is reported that the charge−discharge efficiency is ultimately limited by the crystal growth of insoluble Li2O2 on the porous cathode. Catalysts have been reported to improve the nucleation and morphology of Li2O2, which helps achieve high energy densities. We provide a new insight into the catalytic mechanism of the oxygen reduction reaction (ORR) in aprotic Li−O2 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-MnO2 surfaces. Lithium ions from electrolytes are found to interact with the surface oxygen sites and form surface lithium sites, facilitating further growth of Li2O2. A larger number of initial growth points with uniform distribution makes Li2O2 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−O2 cell experiments with α-MnO2 nanowire cathodes. N lithium battery systems have become a hot issue, as they appear to be promising clean power sources for energy storage systems (ESSs) and electric vehicles (EVs). Such use of clean power sources allows for the postponement of the depletion of fossil fuel and alleviates environmental concerns. Among next-generation battery systems, rechargeable aprotic (nonaqueous) lithium−oxygen (Li−O2) batteries have attracted intensive interest due to their incomparably high specific energy densities around 3500 W·h/ kgLi2O2, 8−18 outperforming state-of-the-art Li-ion batteries and serving as a reasonable alternative to gasoline. Despite its great promise, Li−O2 battery technology remains in its infancy, and several performance issues need to be addressed to ensure the development of a practical system, such as low round-trip efficiency, poor recyclability, and power capability. During the discharge process, O2 is reduced on the porous cathode and then reacts with a Li ion, yielding insoluble Li2O2 (oxygen reduction reaction, ORR). The charge process involves an electrochemical dissociation reaction of these discharge products (oxygen evolution reaction, OER). In this regard, the aforementioned drawbacks are mainly due to the high thermodynamic stability and insulating character of Li2O2, which can result in a high overpotential in OER as well as a sluggish oxygen reduction. In general, it is believed that electrocatalysts can provide a proper solution to these issues. Therefore, extensive effort has been dedicated to designing and finding suitable catalyst materials for enabling high power and high round-trip efficiency. Various types of nanostructured materials have been experimentally adopted as catalysts due to their morphological merit regarding homogeneous reactions and fast charge transport. Gradually, researchers have realized that the discharge product Li2O2 itself is the intrinsic limit for the capacity of Li−O2 cells because of its insoluble deposition and poor electronic conductivity, which indicates that the key to achieving high-performance Li−O2 cells is control of the Received: January 7, 2015 Revised: April 21, 2015 Published: April 21, 2015 Article