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

    Full text link
    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

    Full text link
    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
    corecore