Facile Ball-Milling Synthesis of Carbon-Coated Layered K-Birnessite for High-Rate and Long-Term Potassium-Ion Batteries

Layered manganese oxide cathode materials have been widely employed in energy storage systems because of their high energy density and low cost. However, manganese-based cathodes for potassium-ion batteries face significant challenges, including large volume changes during K+ intercalation/deintercalation processes, severe capacity loss, and sluggish K+ transport kinetics. Herein, the K0.28MnO2·0.29H2O (KMO) precursor, synthesized via a solid-state reaction, was ball-milled with Ketjen black to form the [email protected] composite. The carbon coating layer prevents direct contact between KMO and the electrolyte, effectively mitigating the Jahn–Teller effect of Mn and ensuring the structural integrity of the cathode during rapid and stable K+ intercalation/deintercalation processes. Density functional theory (DFT) calculations confirm that the carbon layer enhances the material’s electronic conductivity, reduces the electrochemical reaction resistance, and significantly improves the sample’s rate capability. The [email protected] cathode delivers an initial discharge capacity of 137.9 mAh g–1 at 20 mA g–1, a rate capability of 77.2 mAh g–1 at 500 mA g–1, and retains 81% of its capacity after 300 cycles. These findings regarding the surface protection layer structure provide valuable insights for advancing manganese-based layered cathodes with enhanced interfacial stability for potassium-ion batteries.