Li-O₂ batteries have received much attention due to the high theoretical energy density. However, they still suffer from many challenges such as unsatisfactory practical specific capacity, cycle stability, and relatively high overpotential. The electrochemical performance of Li-O₂ batteries is closely related to the reversibility of the discharge (oxygen reduction reaction, ORR) and charge (oxygen evolution reaction, OER) processes. During the discharge process, non-conductive Li₂O₂ product is formed and gradually covers on the surface of the positive electrode material, leading to the deactivation of battery. The charging process is accompanied by the electrochemical decomposition of Li₂O₂ products. Therefore, how to achieve the highly reversible formation and decomposition of the Li₂O₂ product is the key to improve the electrochemical performance of Li-O₂ batteries. To date, two strategies have been developed:(i) sp² carbon materials with large specific surface area, suitable pore structure and high conductivity are used as cathode materials to disperse/accommodate the Li₂O₂ product and promote the electron transfer; (ii) the soluble redox mediators with ORR/OER bifunctionally catalytic activities are adopted as the electrolyte additive to promote the formation and decomposition of the Li₂O₂ product and lower the overpotentials. Recently, we reported a novel 3D hierarchical carbon nanocages (hCNC) featuring on the ultrahigh specific surface area, multiscale pore structure (micro-meso-macropore coexistence), high conductivity, and abundant defects, which demonstrated the excellent electrochemical performances in energy conversion and storage. Herein, taking advantages of hCNC, the high performances of Li-O₂ batteries were fabricated, showing high full discharge specific capacity (14080 mAh·g^-1) and good cyclability. After adding acetylacetone ferrous (Fe(acac)₂) as the redox mediator to electrolyte, the electrochemical performances are further promoted. Namely, the discharge capacity reaches to 23560 mAh·g^-1 at the current density of 0.1 A·g^-1 (7.82 times of XC-72), and the cycle numbers are up to 138 cycles at the current density of 0.5 A·g^-1 and the discharge/charge depth of 800 mAh·g^-1 (far higher than 68 cycles of hCNC without Fe(acac)₂ and 13 cycles of XC-72). Especially, at the high current density of 5.0 A·g^-1, the cycle numbers still reach to 63 cycles, far higher than 21 cycles of hCNC without Fe(acac)₂. Such excellent electrochemical performances can be ascribed to:the unique structure of hCNC facilitating electron transfer, reversible conversion of 2Li⁺+O₂+2e^-⇆Li₂O₂(s), and dispersion/accommodation of the insulating Li₂O₂ product; the soluble redox mediator of Fe(acac)₂ effectively catalyzing the discharge products of Li₂O₂ to form uniformly dispersed small-sized particles and decompose completely during the charge process. This provides a promising strategy for improving the performance of Li-O₂ batteries via designing novel carbon-based positive electrode materials and adding efficient soluble redox mediators.

Key words: Li-O₂ batteries, 3D hierarchical carbon nanocages (hCNC), acetylacetone ferrous (Fe(acac)₂), Li₂O₂, discharge capacity, cycle stability