Oxygen Reaction of Nonlayered Tetrahedral KFeO₂ Positive Electrode for Potassium-Ion Battery Using an FSA-based Ionic Liquid Electrolyte

Abstract

K-ion batteries (KIBs) that use ionic liquid (IL) electrolytes are promising candidates for post-Li-ion batteries because of the abundance of potassium resources and safety of ILs. We successfully synthesized stoichiometric KFeO₂ using a solid-state method and evaluated its charge–discharge performance as a KIB positive electrode material, with an amide-based IL electrolyte at 298 K. Transmission electron microscopy, X-ray photoelectron spectroscopy, synchrotron soft X-ray absorption spectroscopy, and energy-dispersive X-ray spectroscopy data showed that the bulk redox and surface oxidation of oxygen, rather than those of iron, contribute to the reversible and irreversible capacities, respectively. Capacity decay occurred upon repeated cycling, owing to the surface irreversible oxidation of oxygen ions to form O₂ and K₁−ₓFeO₂−ₓ/₂, which blocks the pathways of K⁺ transfer to KFeO₂ particles. This study provides a vital platform for constructing novel KIBs and elucidates the important role of oxygen in KFeO₂ positive electrode.