Synthesis of yolk-shell-structured iron monosulfide-carbon microspheres and understanding of their conversion reaction for potassium-ion storage

Nanostructured transition metal chalcogenides (TMCs) have attracted attention as potential anode materials owing to the anticipation of next-generation battery systems, particularly potassium-ion batteries (PIBs). In this study, the electrochemical reaction mechanism of iron monosulfide (FeS) with potassium ions was systemically investigated for the first time. To enhance the electrochemical properties by improving the structural stability and electrical conductivity during cycling, rational strategies of introducing a yolk-shell architecture via an aerosol process and pitch solution infiltration were applied, resulting in the formation of yolk-shell-structured FeS/pitch-derived carbon composite microspheres (Y-FeS@C). From in situ and ex situ investigations of electrodes in fully discharged and charged states, the following reaction mechanism was determined from the second cycle onward: 4Fe + 4K₂S ↔ K_xFe₂S₃ + FeS + Fe + (8 − x)K⁺ + (8 − x)e⁻. The inhibition of substantial FeS crystal growth thanks to the pitch-derived carbon encapsulation as well as the formation of metallic Fe after the initial conversion reaction contributed to the excellent electrochemical kinetic properties of Y-FeS@C. Y-FeS@C showed stable cycle performance (256.5 mA h g⁻¹ for the 100th cycle at 0.5 A g⁻¹) and high rate capability (113.8 mA h g⁻¹ at 5.0 A g⁻¹) as an anode material for PIBs.