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

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 Na_0.55Mn₂O₄·0.57H₂O (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 Zn²⁺, and some sodium ions are replaced with Zn²⁺ in the first cycle to further stabilize the layered structure for subsequent reversible Zn²⁺/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^-1 at current densities of 200 and 1500 mA g^-1, respectively, but also maintains a good long-cycling performance of 201.6 mA h g^-1 at a high current density of 500 mA g^-1 after 400 cycles, which makes the NMOH cathode competitive for practical applications.