Tuning defect and hollow size of metallic K_xCoF₃ for ultrastable potassium storage

Potassium-ion batteries (KIBs) hold great promise as an electrochemical energy storage system owing to the abundance of potassium resource in the earth's crust, close redox potential to lithium, and similar electrochemical behavior to lithium system. In this work, the defect and hollow size of the hollow porous K_xCoF₃ (KCF) open framework are tuned using a micro-emulsion method. The optimized metallic K_0.66CoF₃ (K_0.66CF) is used for the first time as an advanced anode material for KIBs, which exhibits a high capacity of 167 mAh g⁻¹ at a current density of 50 mA g⁻¹ over 200 cycles. A high-rate performance with a capacity of 139 mAh g⁻¹ at 1000 mA g⁻¹ and an impressively ultra-long cycle life over 3000 cycles are achieved. Density functional theory (DFT) calculations show that the K defects together with the enlarged interlayer space in this open framework can boost the electronic and ionic diffusion, stabilize the crystal structure and accommodate more K ions during cycling. In addition, the unique hollow porous structure increases the contact area of K_0.66CF with electrolyte and provides shortened ion diffusion paths for K⁺. This work provides the basic understanding of tuning defects and hollow size for boosting electrochemical behavior in energy storage fields.