Investigation of Solid Electrolyte Interphase Layer
Formation and Electrochemical Reversibility of Magnetite, Fe<sub>3</sub>O<sub>4</sub>, Electrodes: A Combined X‑ray Absorption Spectroscopy
and X‑ray Photoelectron Spectroscopy Study
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Abstract
Magnetite
(Fe<sub>3</sub>O<sub>4</sub>) 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 Fe<sub>3</sub>O<sub>4</sub> 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 Fe<sup>0</sup>. 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,
Fe<sup>0</sup> and Li<sub>2</sub>O. In contrast, ethylene carbonate-based
carbonate electrolyte forms SEI at a potential where the formation
of Fe<sup>0</sup> and Li<sub>2</sub>O has already initiated, resulting
in SEI formation on Fe<sup>0</sup> nanograins