• An alloying-degree control strategy successfully fabricates Co/Co7Fe3 heterojunctions on nitrogen-doped carbon spheres (CoFe@NCS), enabling tunable phase composition.
• The optimal CoFe0.08@NCS catalyst exhibits excellent bifunctional activity: half-wave potential of 0.80 V for ORR and overpotential of 283 mV at 10 mA·cm−2 for OER.
• In liquid-state zinc-air batteries, CoFe0.08@NCS achieves a high peak power density of 157 mW·cm−2 and stable voltage gap over 150 h, surpassing Pt/C+RuO2 benchmarks.
• Solid-state flexible ZABs with CoFe0.08@NCS show reliable performance under various bending conditions, highlighting practical flexibility.