4 research outputs found
Battery Relevant Electrochemistry of Ag<sub>7</sub>Fe<sub>3</sub>(P<sub>2</sub>O<sub>7</sub>)<sub>4</sub>: Contrasting Contributions from the Redox Chemistries of Ag<sup>+</sup> and Fe<sup>3+</sup>
Ag<sub>7</sub>Fe<sub>3</sub>(P<sub>2</sub>O<sub>7</sub>)<sub>4</sub> is
an example of an electrochemical displacement material which
contains two different electrochemically active metal cations, where
one cation (Ag<sup>+</sup>) forms metallic silver nanoparticles external
to the crystals of Ag<sub>7</sub>Fe<sub>3</sub>(P<sub>2</sub>O<sub>7</sub>)<sub>4</sub> via an electrochemical reduction displacement
reaction, while the other cation (Fe<sup>3+</sup>) 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 Ag<sub>7</sub>Fe<sub>3</sub>(P<sub>2</sub>O<sub>7</sub>)<sub>4</sub> 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 Ag<sub>7</sub>Fe<sub>3</sub>(P<sub>2</sub>O<sub>7</sub>)<sub>4</sub> is
synthesized and confirmed to be a single phase by Rietveld refinement.
Electrochemistry of Ag<sub>7</sub>Fe<sub>3</sub>(P<sub>2</sub>O<sub>7</sub>)<sub>4</sub> 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 Ag<sub>7</sub>Fe<sub>3</sub>(P<sub>2</sub>O<sub>7</sub>)<sub>4</sub>. Ag<sub>7</sub>Fe<sub>3</sub>(P<sub>2</sub>O<sub>7</sub>)<sub>4</sub> 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 ZnFe<sub>2</sub>O<sub>4</sub> 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 ZnFe<sub>2</sub>O<sub>4</sub> 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 Zn<sup>2+</sup> ions from 8a tetrahedral sites into vacant 16c sites. Density functional
theory shows that Li<sup>+</sup> 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 Zn<sup>2+</sup> 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 Fe<sub>2</sub>O<sub>3</sub>. Electrochemistry data show higher electron equivalent transfer
than can be accounted for solely based on ZnFe<sub>2</sub>O<sub>4</sub> reduction indicating excess capacity ascribed to carbon reduction
or surface electrolyte interphase formation
Structural and Electrochemical Characteristics of Ca-Doped “Flower-like” Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub> 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<sub>4–<i>x</i></sub>Ca<sub><i>x</i></sub>Ti<sub>5</sub>O<sub>12</sub> (“<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 Ti<sup>3+</sup> ratios were noted with XRD and XPS, respectively, whereas increased
charge transfer conductivity and decreased Li<sup>+</sup>-ion diffusion
coefficients were displayed in EIS for the Li<sub>4–<i>x</i></sub>Ca<sub><i>x</i></sub>Ti<sub>5</sub>O<sub>12</sub> (“<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
Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub>, 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 Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub>
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 ZnFe<sub>2</sub>O<sub>4</sub> (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