A two-electron transfer mechanism of the Zn-doped delta-MnO2 cathode toward aqueous Zn-ion batteries with ultrahigh capacity

Neutral aqueous zinc-ion batteries (ZIBs) have attracted considerable attention due to their safe and green features. As one typical cathode, birnessite MnO2 (delta-MnO2) suffers from low conductivity and structural instability, and its energy storage mechanism is still not well established yet. Herein, we developed a Zn-doped delta-MnO2 material via a facile and effective microwave-assisted method for the cathode in aqueous ZIBs. By incorporating Zn to modify the microstructure and promote reaction kinetics, the Zn-doped delta-MnO2 electrode demonstrates significantly enhanced electrochemical performance with an ultrahigh reversible capacity of 455 mA h g(-1) and excellent specific energy of 628 W h kg(-1). In addition, the successive insertion of H+ and Zn2+ and deep two-electron transfer routes are revealed systematically by ex situ experiments. The two-electron transfer route (Mn4+/Mn3+ and Mn3+/Mn2+) mechanism of Zn-doped delta-MnO2 electrodes explains the exceedingly high capacity and opens new opportunities to develop high-energy aqueous ZIBs.