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In this work, an in-situ synthetic, pyrolytic and carbonized method is elaborately demonstrated to prepare the nickel cobalt sulphide (NiCo 2 S 4) nanoparticles (NPs)/carbon nitrogen nanosheets (CNNs) composites as a highly efficient bifunctional oxygen electrocatalyst for rechargeable Zn-air batteries (ZABs). Specifically, the in-situ synthesized NiCo 2 S 4 NPs confined in cages of CNNs create a well-defined electron transfer hetero-interface with more exposed active sites. The combined NiCo 2 S 4 NPs with the CNNs generate a synergistic effect, which provides effective electron transfer pathways to enhance the electrocatalytic behaviors, yielding a more positive half-wave potential (0.83 V) for oxygen reduction reaction (ORR) and a lower overpotential (360 mV at 10 mA cm−2) for oxygen evolution reaction (OER). As a proof of concept, the equipped ZABs exhibit a high peak power density of 92 mW cm−2 and a superior energy density of 1025 Wh kg−1 with robust cycling stability over 1000 cycles for 180 h, which are better than that of a commercially available Pt/C-RuO 2 catalyst. The findings highlight the practical viability of the NiCo 2 S 4 /CNNs composites in rechargeable ZABs and provide a new approach for the efficient synthesis of bifunctional oxygen electrocatalyst in the future. Image, graphical abstract An in-situ synthetic, pyrolytic and carbonized method was elaborately developed to prepare the nickel cobalt sulfide/carbon nitrogen nanosheets (NiCo 2 S 4 /CNNs) composites as highly efficient bifunctional oxygen electrocatalyst for rechargeable Zn-air batteries (ZABs). The equipped ZABs display a high peak power density (92 mW cm−2), yielding superior energy density of 1025 Wh kg−1 with robust cycling stability over 1000 cycles for 180 h, which are better than that of a commercial available Pt/C-RuO 2 catalyst. [ABSTRACT FROM AUTHOR]