Calcium-intercalated birnessite MnO₂ anchored on carbon nanotubes as high-performance cathodes for aqueous zinc-ion...

Aqueous Zn-ion batteries (ZIBs) show great potential in energy storage systems because of their high theoretical capacities, high safety, low cost, and environmental friendliness. The lack of suitable cathode materials for sustaining the Zn²⁺ intercalation/deintercalation severely restricts their further application. Herein, calcium-intercalated birnessite MnO₂ anchored on carbon nanotubes (CNTs) was designed as a cathode for ZIBs. The cathode material can be facilely produced by a simple one-pot reaction process. The external calcium-intercalated MnO₂ with large layer spacing affords a fast ionic migration rate and the internal CNTs serving as a structural framework endow the electrode with better electrical conductivity. Benefiting from the larger interlayer spacing and the enhanced electrical conductivity, the CNT-CaMO cathode shows a high specific capacity of 351.8 mA h g^-1 at 200 mA g^-1 and a long cycle life over 6000 cycles. Besides, the H⁺ and Zn²⁺ co-intercalation storage mechanism was confirmed by ex situ XRD, SEM, and XPS analyses. This work opens up a new way to develop aqueous ZIB cathode materials with a high reversible capacity and long cycle life.