8 research outputs found
An Organic Coprecipitation Route to Synthesize High Voltage LiNi<sub>0.5</sub>Mn<sub>1.5</sub>O<sub>4</sub>
High-voltage cathode material LiNi<sub>0.5</sub>Mn<sub>1.5</sub>O<sub>4</sub> has been prepared with a novel
organic coprecipitation
route. The as-prepared sample was compared with samples produced through
traditional solid state method and hydroxide coprecipitation method.
The morphology was observed by scanning electron microscopy, and the
spinel structures were characterized by X-ray diffraction and Fourier
transform infrared spectroscopy. Besides the ordered/disordered distribution
of Ni/Mn on octahedral sites, the confusion between Li and transition
metal is pointed out to be another important factor responsible for
the corresponding performance, which is worthy further investigation.
Galvanostatic cycles, cyclic voltammetry, and electrochemical impedance
spectroscopy are employed to characterize the electrochemical properties.
The organic coprecipitation route produced sample shows superior rate
capability and stable structure during cycling
<sup>2</sup>H and <sup>27</sup>Al Solid-State NMR Study of the Local Environments in Al-Doped 2‑Line Ferrihydrite, Goethite, and Lepidocrocite
Although substitution of aluminum
into iron oxides and oxyhydroxides
has been extensively studied, it is difficult to obtain accurate incorporation
levels. Assessing the distribution of dopants within these materials
has proven especially challenging because bulk analytical techniques
cannot typically determine whether dopants are substituted directly
into the bulk iron oxide or oxyhydroxide phase or if they form separate,
minor phase impurities. These differences have important implications
for the chemistry of these iron-containing materials, which are ubiquitous
in the environment. In this work, <sup>27</sup>Al and <sup>2</sup>H NMR experiments are performed on series of Al-substituted goethite,
lepidocrocite, and 2-line ferrihydrite in order to develop an NMR
method to track Al substitution. The extent of Al substitution into
the structural frameworks of each compound is quantified by comparing
quantitative <sup>27</sup>Al MAS NMR results with those from elemental
analysis. Magnetic measurements are performed for the goethite series
to compare with NMR measurements. Static <sup>27</sup>Al spin–echo
mapping experiments are used to probe the local environments around
the Al substituents, providing clear evidence that they are incorporated
into the bulk iron phases. Predictions of the <sup>2</sup>H and <sup>27</sup>Al NMR hyperfine contact shifts in Al-doped goethite and
lepidocrocite, obtained from a combined first-principles and empirical
magnetic scaling approach, give further insight into the distribution
of the dopants within these phases
Single-Phase Lithiation and Delithiation of Simferite Compounds Li(Mg,Mn,Fe)PO<sub>4</sub>
Understanding
the phase transformation behavior of electrode materials
for lithium ion batteries is critical in determining the electrode
kinetics and battery performance. Here, we demonstrate the lithiation/delithiation
mechanism and electrochemical behavior of the simferite compound,
LiMg<sub>0.5</sub>Fe<sub>0.3</sub>Mn<sub>0.2</sub>PO<sub>4</sub>.
In contrast to the equilibrium two-phase nature of LiFePO<sub>4</sub>, LiMg<sub>0.5</sub>Fe<sub>0.3</sub>Mn<sub>0.2</sub>PO<sub>4</sub> undergoes a one-phase reaction mechanism as confirmed by ex situ
X-ray diffraction at different states of delithiation and electrochemical
measurements. The equilibrium voltage measurement by galvanostatic
intermittent titration technique shows a continuous change in voltage
at Mn<sup>3+</sup>/Mn<sup>2+</sup> redox couple with addition of Mg<sup>2+</sup> in LiMn<sub>0.4</sub>Fe<sub>0.6</sub>PO<sub>4</sub> olivine
structure. There is, however, no significant change in the Fe<sup>3+</sup>/Fe<sup>2+</sup> redox potential
Li<sub>3</sub>Mo<sub>4</sub>P<sub>5</sub>O<sub>24</sub>: A Two-Electron Cathode for Lithium-Ion Batteries with Three-Dimensional Diffusion Pathways
The structure of the novel compound
Li<sub>3</sub>Mo<sub>4</sub>P<sub>5</sub>O<sub>24</sub> has been solved
from single crystal X-ray
diffraction data. The Mo cations in Li<sub>3</sub>Mo<sub>4</sub>P<sub>5</sub>O<sub>24</sub> are present in four distinct types of MoO<sub>6</sub> octahedra, each of which has one open vertex at the corner
participating in a Moî—»O double bond and whose other five corners
are shared with PO<sub>4</sub> tetrahedra. On the basis of a bond
valence sum difference map (BVS-DM) analysis, this framework is predicted
to support the facile diffusion of Li<sup>+</sup> ions, a hypothesis
that is confirmed by electrochemical testing data, which show that
Li<sub>3</sub>Mo<sub>4</sub>P<sub>5</sub>O<sub>24</sub> can be utilized
as a rechargeable battery cathode material. It is found that Li can
both be removed from and inserted into Li<sub>3</sub>Mo<sub>4</sub>P<sub>5</sub>O<sub>24</sub>. The involvement of multiple redox processes
occurring at the same Mo site is reflected in electrochemical plateaus
around 3.8 V associated with the Mo<sup>6+</sup>/Mo<sup>5+</sup> redox
couple and 2.2 V associated with the Mo<sup>5+</sup>/Mo<sup>4+</sup> redox couple. The two-electron redox properties of Mo cations in
this structure lead to a theoretical capacity of 198 mAh/g. When cycled
between 2.0 and 4.3 V versus Li<sup>+</sup>/Li, an initial capacity
of 113 mAh/g is observed with 80% of this capacity retained over the
first 20 cycles. This compound therefore represents a rare example
of a solid state cathode able to support two-electron redox capacity
and provides important general insights about pathways for designing
next-generation cathodes with enhanced specific capacities
Composition-Structure Relationships in the Li-Ion Battery Electrode Material LiNi<sub>0.5</sub>Mn<sub>1.5</sub>O<sub>4</sub>
A study of the correlations between the stoichiometry,
secondary
phases, and transition metal ordering of LiNi<sub>0.5</sub>Mn<sub>1.5</sub>O<sub>4</sub> was undertaken by characterizing samples synthesized
at different temperatures. Insight into the composition of the samples
was obtained by electron microscopy, neutron diffraction, and X-ray
absorption spectroscopy. In turn, analysis of cationic ordering was
performed by combining neutron diffraction with Li MAS NMR spectroscopy.
Under the conditions chosen for the synthesis, all samples systematically
showed an excess of Mn, which was compensated by the formation of
a secondary rock-salt phase and not via the creation of oxygen vacancies.
Local deviations from the ideal 3:1 Mn:Ni ordering were found, even
for samples that show the superlattice ordering by diffraction, with
different disordered schemes also being possible. The magnetic behavior
of the samples was correlated with the deviations from this ideal
ordering arrangement. The in-depth crystal-chemical knowledge generated
was employed to evaluate the influence of these parameters on the
electrochemical behavior of the materials
Structure Stabilization by Mixed Anions in Oxyfluoride Cathodes for High-Energy Lithium Batteries
Mixed-anion oxyfluorides (<i>i.e.</i>, FeO<sub><i>x</i></sub>F<sub>2–<i>x</i></sub>) are an appealing alternative to pure fluorides as high-capacity cathodes in lithium batteries, with enhanced cyclability <i>via</i> oxygen substitution. However, it is still unclear how the mixed anions impact the local phase transformation and structural stability of oxyfluorides during cycling due to the complexity of electrochemical reactions, involving both lithium intercalation and conversion. Herein, we investigated the local chemical and structural ordering in FeO<sub>0.7</sub>F<sub>1.3</sub> at length scales spanning from single particles to the bulk electrode, <i>via</i> a combination of electron spectrum-imaging, magnetization, electrochemistry, and synchrotron X-ray measurements. The FeO<sub>0.7</sub>F<sub>1.3</sub> nanoparticles retain a FeF<sub>2</sub>-like rutile structure but chemically heterogeneous, with an F-rich core covered by thin O-rich shell. Upon lithiation the O-rich rutile phase is transformed into Li–Fe–O(−F) rocksalt that has high lattice coherency with converted metallic Fe, a feature that may facilitate the local electronic and ionic transport. The O-rich rocksalt is highly stable over lithiation/delithiation and thus advantageous to maintain the integrity of the particle, and due to its predominant distribution on the surface, it is expected to prevent the catalytic interaction of Fe with electrolyte. Our findings of the structural origin of cycling stability in oxyfluorides may provide insights into developing viable high-energy electrodes for lithium batteries
What Happens to LiMnPO<sub>4</sub> upon Chemical Delithiation?
Olivine MnPO<sub>4</sub> is the delithiated
phase of the lithium-ion-battery cathode (positive electrode) material
LiMnPO<sub>4</sub>, which is formed at the end of charge. This phase
is metastable under ambient conditions and can only be produced by
delithiation of LiMnPO<sub>4</sub>. We have revealed the manganese
dissolution phenomenon during chemical delithiation of LiMnPO<sub>4</sub>, which causes amorphization of olivine MnPO<sub>4</sub>.
The properties of crystalline MnPO<sub>4</sub> obtained from carbon-coated
LiMnPO<sub>4</sub> and of the amorphous product resulting from delithiation
of pure LiMnPO<sub>4</sub> were studied and compared. The phosphorus-rich
amorphous phases in the latter are considered to be MnHP<sub>2</sub>O<sub>7</sub> and MnH<sub>2</sub>P<sub>2</sub>O<sub>7</sub> from
NMR, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy
analysis. The thermal stability of MnPO<sub>4</sub> is significantly
higher under high vacuum than at ambient condition, which is shown
to be related to surface water removal
Electrochemical Performance of Nanosized Disordered LiVOPO<sub>4</sub>
ε-LiVOPO<sub>4</sub> is a promising multielectron cathode
material for Li-ion batteries that can accommodate two electrons per
vanadium, leading to higher energy densities. However, poor electronic
conductivity and low lithium ion diffusivity currently result in low
rate capability and poor cycle life. To enhance the electrochemical
performance of ε-LiVOPO<sub>4</sub>, in this work, we optimized
its solid-state synthesis route using in situ synchrotron X-ray diffraction
and applied a combination of high-energy ball-milling with electronically
and ionically conductive coatings aiming to improve bulk and surface
Li diffusion. We show that high-energy ball-milling, while reducing
the particle size also introduces structural disorder, as evidenced
by <sup>7</sup>Li and <sup>31</sup>P NMR and X-ray absorption spectroscopy.
We also show that a combination of electronically and ionically conductive
coatings helps to utilize close to theoretical capacity for ε-LiVOPO<sub>4</sub> at C/50 (1 C = 153 mA h g<sup>–1</sup>) and to enhance
rate performance and capacity retention. The optimized ε-LiVOPO<sub>4</sub>/Li<sub>3</sub>VO<sub>4</sub>/acetylene black composite yields
the high cycling capacity of 250 mA h g<sup>–1</sup> at C/5
for over 70 cycles