Battery Relevant Electrochemistry of Ag₇Fe₃(P₂O₇)₄: Contrasting Contributions from the Redox Chemistries of Ag⁺ and Fe³⁺

Ag₇Fe₃(P₂O₇)₄ is an example of an electrochemical displacement material which contains two different electrochemically active metal cations, where one cation (Ag⁺) forms metallic silver nanoparticles external to the crystals of Ag₇Fe₃(P₂O₇)₄ via an electrochemical reduction displacement reaction, while the other cation (Fe³⁺) is electrochemically reduced with the retention of iron cations within the anion structural framework concomitant with lithium insertion. These contrasting redox chemistries within one pure cathode material enable high rate capability and reversibility when Ag₇Fe₃(P₂O₇)₄ is employed as cathode material in a lithium ion battery (LIB). Further, pyrophosphate materials are thermally and electrically stable, desirable attributes for cathode materials in LIBs. In this paper, a bimetallic pyrophosphate material Ag₇Fe₃(P₂O₇)₄ is synthesized and confirmed to be a single phase by Rietveld refinement. Electrochemistry of Ag₇Fe₃(P₂O₇)₄ is reported for the first time in the context of lithium based batteries using cyclic voltammetry and galvanostatic discharge–charge cycling. The reduction displacement reaction and the lithium (de)­insertion processes are investigated using <i>ex situ</i> X-ray absorption spectroscopy and X-ray diffraction of electrochemically reduced and oxidized Ag₇Fe₃(P₂O₇)₄. Ag₇Fe₃(P₂O₇)₄ exhibits good reversibility at the iron centers indicated by ∼80% capacity retention over 100 cycles following the initial formation cycle and excellent rate capability exhibited by ∼70% capacity retention upon a 4-fold increase in current

Probing the Li Insertion Mechanism of ZnFe₂O₄ in Li-Ion Batteries: A Combined X‑Ray Diffraction, Extended X‑Ray Absorption Fine Structure, and Density Functional Theory Study

We report an extensive study on fundamental properties that determine the functional electrochemistry of ZnFe₂O₄ spinel (theoretical capacity of 1000 mAh/g). For the first time, the reduction mechanism is followed through a combination of in situ X-ray diffraction data, synchrotron based powder diffraction, and ex-situ extended X-ray absorption fine structure allowing complete visualization of reduction products irrespective of their crystallinity. The first 0.5 electron equivalents (ee) do not significantly change the starting crystal structure. Subsequent lithiation results in migration of Zn²⁺ ions from 8a tetrahedral sites into vacant 16c sites. Density functional theory shows that Li⁺ ions insert into 16c site initially and then 8a site with further lithiation. Fe metal is formed over the next eight ee of reduction with no evidence of concurrent Zn²⁺ reduction to Zn metal. Despite the expected formation of LiZn alloy from the electron count, we find no evidence for this phase under the tested conditions. Additionally, upon oxidation to 3 V, we observe an FeO phase with no evidence of Fe₂O₃. Electrochemistry data show higher electron equivalent transfer than can be accounted for solely based on ZnFe₂O₄ reduction indicating excess capacity ascribed to carbon reduction or surface electrolyte interphase formation

Structural and Electrochemical Characteristics of Ca-Doped “Flower-like” Li₄Ti₅O₁₂ Motifs as High-Rate Anode Materials for Lithium-Ion Batteries

Doped motifs offer an intriguing structural pathway toward improving conductivity for battery applications. Specifically, Ca-doped, three-dimensional “flower-like” Li_4–<i>x</i>Ca_<i>x</i>Ti₅O₁₂ (“<i>x</i>” = 0, 0.1, 0.15, and 0.2) micrometer-scale spheres have been successfully prepared for the first time using a simple and reproducible hydrothermal reaction followed by a short calcination process. The products were experimentally characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) mapping, inductively coupled plasma optical emission spectrometry (ICP-OES), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge testing. Calcium dopant ions were shown to be uniformly distributed within the LTO structure without altering the underlying “flower-like” morphology. The largest lattice expansion and the highest Ti³⁺ ratios were noted with XRD and XPS, respectively, whereas increased charge transfer conductivity and decreased Li⁺-ion diffusion coefficients were displayed in EIS for the Li_4–<i>x</i>Ca_<i>x</i>Ti₅O₁₂ (“<i>x</i>” = 0.2) sample. The “<i>x</i>” = 0.2 sample yielded a higher rate capability, an excellent reversibility, and a superior cycling stability, delivering 151 and 143 mAh/g under discharge rates of 20C and 40C at cycles 60 and 70, respectively. In addition, a high cycling stability was demonstrated with a capacity retention of 92% after 300 cycles at a very high discharge rate of 20C. In addition, first-principles calculations based on density functional theory (DFT) were conducted with the goal of further elucidating and understanding the nature of the doping mechanism in this study. The DFT calculations not only determined the structure of the Ca-doped Li₄Ti₅O₁₂, which was found to be in accordance with the experimentally measured XPD pattern, but also yielded valuable insights into the doping-induced effect on both the atomic and electronic structures of Li₄Ti₅O₁₂

Two-Dimensional Holey Nanoarchitectures Created by Confined Self-Assembly of Nanoparticles <i>via</i> Block Copolymers: From Synthesis to Energy Storage Property

Advances in liquid-phase exfoliation and surfactant-directed anisotropic growth of two-dimensional (2D) nanosheets have enabled their rapid development. However, it remains challenging to develop assembly strategies that lead to the construction of 2D nanomaterials with well-defined geometry and functional nanoarchitectures that are tailored to specific applications. Here we report a facile self-assembly method leading to the controlled synthesis of 2D transition metal oxide (TMO) nanosheets containing a high density of holes. We utilize graphene oxide sheets as a sacrificial template and Pluronic copolymers as surfactants. By using ZnFe₂O₄ (ZFO) nanoparticles as a model material, we demonstrate that by tuning the molecular weight of the Pluronic copolymers we can incorporate the ZFO particles and tune the size of the holes in the sheets. The resulting 2D ZFO nanosheets offer synergistic characteristics including increased electrochemically active surface areas, shortened ion diffusion paths, and strong inherent mechanical properties, leading to enhanced lithium-ion storage properties. Postcycling characterization confirms that the samples maintain structural integrity after electrochemical cycling. Our findings demonstrate that this template-assisted self-assembly method is a useful bottom-up route for controlled synthesis of 2D nanoarchitectures, and these holey 2D nanoarchitectures are promising for improving the electrochemical performance of next-generation lithium-ion batteries