Birnessite Nanosheet Arrays with High K Content as a High-Capacity and Ultrastable Cathode for K-Ion Batteries.

Potassium-ion batteries (PIBs) are one of the emerging energy-storage technologies due to the low cost of potassium and theoretically high energy density. However, the development of PIBs is hindered by the poor K transport kinetics and the structural instability of the cathode materials during K intercalation/deintercalation. In this work, birnessite nanosheet arrays with high K content (K MnO ⋅0.23H O) are prepared by "hydrothermal potassiation" as a potential cathode for PIBs, demonstrating ultrahigh reversible specific capacity of about 134 mAh g at a current density of 100 mA g , as well as great rate capability (77 mAh g at 1000 mA g ) and superior cycling stability (80.5% capacity retention after 1000 cycles at 1000 mA g ). With the introduction of adequate K ions in the interlayer, the K-birnessite exhibits highly stabilized layered structure with highly reversible structure variation upon K intercalation/deintercalation. The practical feasibility of the K-birnessite cathode in PIBs is further demonstrated by constructing full cells with a hard-soft composite carbon anode. This study highlights effective K -intercalation for birnessite to achieve superior K-storage performance for PIBs, making it a general strategy for developing high-performance cathodes in rechargeable batteries beyond lithium-ion batteries.