High-Performance Aqueous Zinc-Manganese Battery with Reversible Mn²⁺/Mn⁴⁺ Double Redox Achieved by Carbon Coated MnO_x Nanoparticles

There is an urgent need for low-cost, high-energy-density, environmentally friendly energy storage devices to fulfill the rapidly increasing need for electrical energy storage. Multi-electron redox is considerably crucial for the development of high-energy-density cathodes. Here we present high-performance aqueous zinc-manganese batteries with reversible Mn²⁺/Mn⁴⁺ double redox. The active Mn⁴⁺ is generated in situ from the Mn²⁺-containing MnO_x nanoparticles and electrolyte. Benefitting from the low crystallinity of the birnessite-type MnO₂ as well as the electrolyte with Mn²⁺ additive, the MnO_x cathode achieves an ultrahigh energy density with a peak of 845.1 Wh kg^-1 and an ultralong lifespan of 1500 cycles. The combination of electrochemical measurements and material characterization reveals the reversible Mn²⁺/Mn⁴⁺ double redox (birnessite-type MnO₂ ↔ monoclinic MnOOH and spinel ZnMn₂O₄ ↔ Mn²⁺ ions). The reversible Mn²⁺/Mn⁴⁺ double redox electrode reaction mechanism offers new opportunities for the design of low-cost, high-energy-density cathodes for advanced rechargeable aqueous batteries.