Lithiation of Magnetite (Fe<sub>3</sub>O<sub>4</sub>): Analysis Using
Isothermal Microcalorimetry and Operando X‑ray
Absorption Spectroscopy
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Abstract
Conversion
electrodes, such as magnetite (Fe<sub>3</sub>O<sub>4</sub>), 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 Fe<sub>3</sub>O<sub>4</sub> 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 Fe<sub>3</sub>O<sub>4</sub>) exceeded the heat of reaction predicted
for full lithiation of Fe<sub>3</sub>O<sub>4</sub> (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