Enhancing Electronic Conductivity in Iron Oxyfluoride Cathodes via Sulfur Substitution for Lithium-Ion Batteries

Conventional lithium ion batteries (LIB) rely on intercalation of lithium ions into a limited number of interstitial sites within open-framework transition metal cathodes, restricting energy density [1]. Transition metal fluoride (TMF) conversion cathodes offer a 200-300% increase in theoretical energy density by storing multiple Li-ions per metal center due to a multielectron conversion reaction [1]. However, their large band gaps and poor electronic conductivity hinder their practical application. Iron oxyfluoride (FeOF) is a promising cathode material that reduces voltage hysteresis compared to FeF₃ due to increased lattice defects, but its wide band gap still limits charge transport [2,3]. This work investigates sulfur substitution in FeOF as a strategy to improve electronic conductivity and reduce voltage hysteresis, thereby improving electrochemical performance. In this presentation, I will discuss FeO (1-x) FS x synthesis via a fluorosilicic acid aqueous solution approach, prepared by incorporating sulfur precursors during synthesis [4]. We use a combination of X-ray diffraction (XRD) and scanning electron microscopy (SEM) to illuminate cathode phase purity, structural changes, and morphology. Using X-ray photoelectron spectroscopy (XPS) we see shifts in Fe 3d binding energies, indicating successful sulfur substitution from an increased electron density due to sulfur coordination [5]. Further, we elucidate electronic conductivity and electrochemical performance by employing 4-point probe technique and electronic impedance spectroscopy (EIS). References: [1] Lorenz, V., et al. Curr. Op. Electrochem. 30 , 100779 ( 2021 ). [2] Xiulin, F., et al. Nat. Commun. 9 , 2324 ( 2018 ). [3] Xiulin, F., et al. Nano Lett. 15 (11), 7650-7656 ( 2015 ). [4] Pereira, N. et al. J. Electrochem. Soc. 156 , A407 ( 2009 ). [5] Lin, Y., Xia, Y., et al. Nano Energy. 130 (Part B), 109944 ( 2024 ).