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
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Abstract
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