Synthesis and Characterization of CuFe₂O₄ Nano/Submicron Wire-Carbon Nanotube Composites as Binder-free Anodes for Li-Ion Batteries

A series of one-dimensional CuFe₂O₄ (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 CuFe₂O₄ (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 Cu⁰, as well as limited reduction of Fe³⁺ to Fe²⁺, not Fe⁰. The results (1) shed fundamental insight into the reversibility of CuFe₂O₄ 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.