Layered Birnessite Cathode with a Displacement/Intercalation Mechanism for High-Performance Aqueous Zinc-Ion Batteries.

Mn-based rechargeable aqueous zinc-ion batteries (ZIBs) are highly promising because of their high operating voltages, attractive energy densities, and eco-friendliness. However, the electrochemical performances of Mn-based cathodes usually suffer from their serious structure transformation upon charge/discharge cycling. Herein, we report a layered sodium-ion/crystal water co-intercalated Birnessite cathode with the formula of NaMnO·0.57HO (NMOH) for high-performance aqueous ZIBs. A displacement/intercalation electrochemical mechanism was confirmed in the Mn-based cathode for the first time. Na and crystal water enlarge the interlayer distance to enhance the insertion of Zn, and some sodium ions are replaced with Zn in the first cycle to further stabilize the layered structure for subsequent reversible Zn/H insertion/extraction, resulting in exceptional specific capacities and satisfactory structural stabilities. Additionally, a pseudo-capacitance derived from the surface-adsorbed Na also contributes to the electrochemical performances. The NMOH cathode not only delivers high reversible capacities of 389.8 and 87.1 mA h g at current densities of 200 and 1500 mA g, respectively, but also maintains a good long-cycling performance of 201.6 mA h g at a high current density of 500 mA g after 400 cycles, which makes the NMOH cathode competitive for practical applications.