Lithiation of Magnetite (Fe₃O₄): Analysis Using Isothermal Microcalorimetry and Operando X-ray Absorption Spectroscopy

Conversion electrodes, such as magnetite (Fe₃O₄), offer high theoretical capacities (>900 mAh/g) because of multiple electron transfer per metal center. Capacity retention for conversion electrodes has been a challenge in part because of the formation of an insulating surface electrolyte interphase (SEI). This study provides the first detailed analysis of the lithiation of Fe₃O₄ using isothermal microcalorimetry (IMC). The measured heat flow was compared with heat contributions predicted from heats of formation for the Faradaic reaction, cell polarization, and entropic contributions. The total measured energy output of the cell (7260 J/g Fe₃O₄) exceeded the heat of reaction predicted for full lithiation of Fe₃O₄ (5508 J/g). During initial lithiation (3.0-0.86 V), the heat flow was successfully modeled using polarization and entropic contributions. Heat flow at lower voltage (0.86-0.03 V) exceeded the predicted values for iron oxide reduction, consistent with heat generation attributable to electrolyte decomposition and surface electrolyte interphase (SEI). Operando X-ray absorption spectroscopy (XAS) indicated that the oxidation state of the Fe centers deviated from predicted values beginning at ∼0.86 V, supportive of SEI onset in this voltage range. Thus, these combined results from electrochemistry, IMC, and XAS indicate parasitic reactions consistent with SEI formation at a moderate voltage and illustrate an approach for deconvoluting Faradaic and non-Faradaic contributions to heat, which should be broadly applicable to the study of energy-storage materials and systems.