Rechargeable aqueous Zn/MnO2 battery chemistry in a neutral or mildly acidic electrolyte has attracted extensive attention recently because all the components (anode, cathode, and electrolyte) in a Zn/MnO2 battery are safe, abundant, and sustainable. However, the reaction mechanism of the MnO2 cathode remains a topic of discussion. Herein, we design a highly reversible aqueous Zn/MnO2 battery where the binder-free MnO2 cathode was fabricated by in-situ electrodeposition of MnO2 on carbon fiber paper in mild acidic ZnSO4+MnSO4 electrolyte. Electrochemical and structural analysis identify that the MnO2 cathode experience a consequent H⁺ and Zn²⁺ insertion/extraction process with high reversibility and cycling stability. To our best knowledge, it is the first report on rechargeable aqueous batteries with a consequent ion-insertion reaction mechanism.

In this study, we demonstrated that the electrodeposited MnO₂ experienced the successive H⁺ and Zn²⁺ insertion processes with a distinct difference in reaction kinetics during the discharging. The electrodeposited MnO₂ has a nanocrystalline structure, in which the particle size is typically less than 10 nm, providing abundant electrode/electrolyte contact interfaces and reduced ion diffusion path and ensuring that the reaction with slow electrochemical kinetics could also happen. The as-designed Zn/MnO₂ battery delivered an excellent cycling performance with a low capacity decay rate of 0.007% per cycle for 10,000 cycles at a high rate of 6.5 C.