Enhancing the cathodic performance of FeS in lithium-ion batteries via sulfurization treatment.

Pyrite (FeS) thin films have attracted significant attention as promising cathode materials for lithium-ion batteries due to their high theoretical capacity, natural abundance, and environmental benignity. However, due to sulfur volatility during film deposition, achieving stoichiometric FeS remains challenging. In this study, we investigate the effects of post-deposition sulfurization treatment on the chemical composition, crystal structure, and electrochemical performance of FeS thin films synthesized by RF magnetron sputtering. The as-deposited films exhibit notable sulfur deficiency, with an S: Fe atomic ratio of 0.27:1, while a 4-hour sulfurization process improves the ratio to 1.97:1, approaching ideal stoichiometry. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy confirm the formation of the pyrite phase, and X-ray diffraction (XRD) and field effect emission scanning electron microscopy (FESEM) reveal enhanced crystallinity and polycrystalline growth. Electrochemical analysis using cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) demonstrates that sulfurized films deliver superior initial and long-term (100-cycle) discharge specific capacities compared to as-deposited samples. The improved performance is attributed to optimized surface stoichiometry, increased reducibility, and the formation of stable intermediate phases. These findings underscore the critical role of sulfurization in tuning the structural and electrochemical properties of FeS thin-film cathodes and present a viable strategy for enhancing the performance and stability of conversion-type cathodes in lithium-ion batteries.