Secondary zinc–MnO₂ batteries represent the climax of aqueous battery technology owing to their high specific capacity and high power density. However, zinc–MnO₂ batteries suffer from serious impediments such as capacity fading, poor electrical conductivity, and ion diffusion. Herein, we conducted an oxygen deficiency treatment on an amorphous manganese oxide with preexisting defects (A-MnO₂) to maximize atom vacancies and compared it to its crystalline analogue (α-MnO₂). As a result, A-MnO₂ delivered a specific capacity of almost 600 mA h g⁻¹ at 100 mA g⁻¹, which is to our knowledge the highest specific capacity reported for undoped and non-composite manganese oxide, while α-MnO₂ only reached 150 mA h g⁻¹ under the same conditions. In addition, an in-depth investigation of the disordered material cycling mechanism using ex situ XRD and ex situ SEM validated the high specific capacity and explained the role of defect engineering and material disordering in taking aqueous batteries toward convincing high-scale commercialization.