The Zn/Co₃O₄ battery is one of the few aqueous electrolyte batteries with a potential >2 V voltage. Unfortunately, so far, all reported Zn/Co₃O₄ batteries are using an alkaline electrolyte, resulting in poor cycling stability and environmental problems. Here, we report a Co(III) rich-Co₃O₄ nanorod material with vastly improved electrochemical kinetics. Zn/Co(III) rich-Co₃O₄ batteries can work well in ZnSO₄ with a CoSO₄ additive aqueous solution as a mild electrolyte, delivering a high voltage of 2.2 V, a capacity of 205 mA h g⁻¹ (Co₃O₄) and an extreme cycling stability of 92% capacity retention even after 5000 cycles. Further mechanistic study reveals a conversion reaction between in situ formed CoO and Co₃O₄, which has never been observed in an alkaline Zn/Co₃O₄ battery. Subsequently, a flexible solid-state battery is constructed and reveals high flexibility and a high energy density of 360.8 W h kg⁻¹ at a current density of 0.5 A g⁻¹. Our research initiates the first Zn/Co₃O₄ battery working in a mild electrolyte, resulting in excellent electrochemical performance. It also indicates that the electrochemical kinetics can be effectively enhanced by fine tuning the atomic structure of electrode materials, opening a new door to improve the performance of aqueous electrolyte batteries.