The manganese dissolution leading to sharp capacity decline as well as the sluggish reaction kinetic are still major issues for manganese‐based materials as aqueous zinc‐ion batteries (ZIBs) cathodes. Here, a potassium‐ion‐stabilized and oxygen‐defect K0.8Mn8O16 is reported as a high‐energy‐density and durable cathode for neutral aqueous ZIBs. A new insight into suppressing manganese dissolution via incorporation of K+ ions to intrinsically stabilize the Mn‐based cathodes is provided. A comprehensive study suggests that oxygen defects improve electrical conductivity and open the MnO6 polyhedron walls for ion diffusion, which plays a critical role in the fast reaction kinetics and capacity improvement of K0.8Mn8O16. In addition, direct evidence for the mechanistic details of simultaneous insertion and conversion reaction based on H+‐storage mechanism is demonstrated. As expected, a significant energy output of 398 W h kg−1 (based on the mass of cathode) and an impressive durability over 1000 cycles with no obvious capacity fading are obtained. Such a high‐energy Zn‐K0.8Mn8O16 battery, as well as the basic understanding of manganese dissolution and oxygen defects may open new opportunities toward high‐performance aqueous ZIBs.

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