Low-cost birnessite as a promising cathode for high-performance aqueous rechargeable batteries

A highly reversible aqueous rechargeable battery system is assembled by using a Zn anode, a room-temperature synthesized MnO 2 -birnessite cathode and a mild ZnSO 4 -based aqueous electrolyte with MnSO 4 as an additive. Structural analysis reveal that MnO 2 -birnessite undergoes a structural transformation to poorly crystalline MnO 2 . 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.