Recent progress in hierarchically structured O2-cathodes for Li-O2 batteries

In the last decade, Li-O2 batteries have been a research focus due to its ultrahigh energy density as chemical power sources and being rechargeable through oxygen reduction and evolution reactions. Besides catalyzing the oxygen reduction and evolution, the rationally designed air-cathode to accommodate the insoluble discharge products and allow efficient transfer of reactants is critical for Li-O2 batteries. To date, the development of a wide variety of electrocatalyst has driven the application-directed research in Li-O2 batteries. This review provides a comprehensive overview of the O2-electrodes for Li-O2 batteries, with an emphasis on the O2-electrodes synthesis, working mechanism, and overall performance evaluation. The aim of this review is to afford a better understanding of Li-O2 cathodes and to provide guidelines for researchers to design and construct high-performance, easy-to-use cathodes for Li-O2 batteries.