2 research outputs found
Lithiation of Magnetite (Fe<sub>3</sub>O<sub>4</sub>): Analysis Using Isothermal Microcalorimetry and Operando X‑ray Absorption Spectroscopy
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
Synthesis and Characterization of CuFe<sub>2</sub>O<sub>4</sub> Nano/Submicron Wire–Carbon Nanotube Composites as Binder-free Anodes for Li-Ion Batteries
A series
of one-dimensional CuFe<sub>2</sub>O<sub>4</sub> (CFO) nano/submicron wires possessing different diameters, crystal phases,
and crystal sizes have been successfully generated using a facile
template-assisted coprecipitation reaction at room temperature, followed
by a short postannealing process. The diameter and crystal structure
of the resulting CuFe<sub>2</sub>O<sub>4</sub> (CFO) wires were judiciously
tuned by varying the pore size of the template and the postannealing
temperature, respectively. Carbon nanotubes (CNTs) were incorporated
to generate CFO-CNT binder-free anodes, and multiple characterization
techniques were employed with the goal of delineating the relationships
between electrochemical behavior and the properties of both the CFO
wires (crystal phase, wire diameter, crystal size) and the electrode
architecture (binder-free vs conventionally prepared approaches).
The study reveals several notable findings. First, the crystal phase
(cubic or tetragonal) did not influence the electrochemical behavior
in this CFO system. Second, regarding crystallite size and wire diameter,
CFO wires with larger crystallite sizes exhibit improved cycling stability,
whereas wires possessing smaller diameters exhibit higher capacities.
Finally, the electrochemical behavior is strongly influenced by the
electrode architecture, with CFO-CNT binder-free electrodes demonstrating
significantly higher capacities and cycling stability compared to
conventionally prepared coatings. The mechanism(s) associated with
the high capacities under low current density but limited electrochemical
reversibility of CFO electrodes under high current density were probed
via X-ray absorption spectroscopy mapping with submicron spatial resolution
for the first time. Results suggest that the capacity of the binder-free
electrodes under high rate is limited by the irreversible formation
of Cu<sup>0</sup>, as well as limited reduction of Fe<sup>3+</sup> to Fe<sup>2+</sup>, not Fe<sup>0</sup>. The results (1) shed fundamental
insight into the reversibility of CuFe<sub>2</sub>O<sub>4</sub> materials
cycled at high current density and (2) demonstrate that a synergistic
effort to control both active material morphology and electrode architecture
is an effective strategy for optimizing electrochemical behavior