A highly reversible aqueous rechargeable battery system is assembled by using a Zn anode, a room-temperature synthesized MnO2-birnessite cathode and a mild ZnSO4-based aqueous electrolyte with MnSO4 as an additive. Structural analysis reveal that MnO2-birnessite undergoes a structural transformation to poorly crystalline MnO2. It offers an operating voltage of 1.39 V, an energy density of 364 Wh kg−1 (cathode only) at 200 mA g−1 and shows a capacity retention of 100% over 400 cycles at 500 mA g−1. This low-cost battery system is a promising candidate for large-scale energy storage.