Understanding the Lithium Storage Mechanism in Core–Shell Fe₂O₃@C Hollow Nanospheres Derived from Metal–Organic Frameworks: An In operando Synchrotron Radiation Diffraction and in operando X-ray Absorption Spectroscopy Study

In this work, a core−shell structure of an Fe2O3@C hollow nanosphere derived from metal−organic frameworks is used as an anode material for Li-ion batteries. This material delivers a reversible capacity of 928 mAh g−1 at 0.2 A g−1 in 1 M LiPF6 in ethylene carbonate/dimethyl carbonate = 1:1. Although 1 M lithium bis(trifluoromethane sulfonyl)imide is used as a conductive salt, it delivers only 644 mAh g−1 at 0.2 A g−1. In operando synchrotron radiation diffraction revealed that the intermediate phases LixFe2O3 (R3̅m, hexagonal) and LixFe2O3 (Fd3̅m, Li-lean) form and subsequently convert to LixFe2O3 (Fd3̅m, Li-rich), which finally transforms into Fe, Li2O, and LixFe2O3 (Fd3̅m, X phase). During the delithiation process, the material does not return to the initial Fe2O3 structure; instead, the partially delithiated Lix−1Fe2O3 (Fd3̅m, X phase) and an amorphous metallic Fe phase remain. The Fe K-edge transition and the formation of Fe are confirmed by the in operando X-ray absorption spectroscopy measurement. Furthermore, the resistive contributions of this material in the two types of Li-salts are evaluated by electrochemical impedance spectroscopy, which highlights a different type of solid electrolyte interphase induced by the salt. This work provides fundamental insights into understanding the lithium-ion storage mechanism in conversion-type electrodes.