Investigation of Solid Electrolyte Interphase Layer Formation and Electrochemical Reversibility of Magnetite, Fe₃O₄, Electrodes: A Combined X‑ray Absorption Spectroscopy and X‑ray Photoelectron Spectroscopy Study

Magnetite (Fe₃O₄) is a promising electrode material for the next generation of Li-ion batteries with multiple electron transfers per metal center and a theoretical capacity of 924 mA h/g. However, multiple phase conversions during (de)­lithiation of Fe₃O₄ and formation of a solid electrolyte interphase (SEI) contribute to capacity fade. In this study, X-ray absorption spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to determine the surface chemistry, redox chemistry, and the impact on the electrochemical reversibility in the presence and absence of fluoroethylene carbonate (FEC) solvent. With FEC, improved capacity retention and enhanced reversibility are observed. In contrast, electrodes cycled with no FEC exhibit decreased reversibility where the active material remains as reduced Fe⁰. XPS results reveal LiF and lower quantities of oxygen-containing species, especially carbonates at the electrode surface tested in FEC. The improvement in electrochemical reversibility with FEC is attributed to the formation of a solid electrolyte interphase which forms prior to initiation of the conversion reaction limiting SEI growth on the reduced products, Fe⁰ and Li₂O. In contrast, ethylene carbonate-based carbonate electrolyte forms SEI at a potential where the formation of Fe⁰ and Li₂O has already initiated, resulting in SEI formation on Fe⁰ nanograins