A noble-metal-free bifunctional electrocatalyst with outstanding activity and stability is of great importance for the development of rechargeable zinc-air batteries (ZABs). Herein, we employed a facile strategy to fabricate nitrogen-doped NiCo2O4 nanostructures on carbon paper as a binder-free air cathode for rechargeable ZABs. An optimized process of nitrogen plasma treatment has enhanced the electrical conductivity of the metal oxide and induced abundant active sites into the crystal structures, resulting in enhanced electrocatalytic activities toward oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Notably, mild plasma treatment leads to efficient N doping while the morphology and specific surface area of the catalyst both remain unchanged. The air cathode with NiCo2O4 doped with nitrogen in 2 min plasma treatment and integrated into a zinc-air battery with liquid electrolyte provided a small charge-discharge voltage gap and distinguished cycling stability superior to the air cathode with noble-metal catalyst counterparts. Furthermore, the solid-state zinc-air battery with this material displays excellent stability with just a small increase of the charge-discharge voltage gap over 20 h of operation. This work illustrates the promising potential of plasma treatment in the fabrication of high-performance catalysts.