10 research outputs found
Ce–O Covalence in Silicate Oxyapatites and Its Influence on Luminescence Dynamics
Cerium substituting gadolinium in
Ca<sub>2</sub>Gd<sub>8</sub>(SiO<sub>4</sub>)<sub>6</sub>O<sub>2</sub> occupies two intrinsic sites of distinct coordination. The coexistence
of an ionic bonding at a 4F site and an ionic–covalent mixed
bonding at a 6H site in the same crystalline compound provides an
ideal system for comparative studies of ion–ligand interactions.
Experimentally, the spectroscopic properties and photoluminescence
dynamics of this white-phosphor are investigated. An anomalous thermal
quenching of the photoluminescence of Ce<sup>3+</sup> at the 6H site
is analyzed. Theoretically, ab initio calculations are conducted to
reveal the distinctive properties of the Ce–O coordination
at the two Ce<sup>3+</sup> sites. The calculated eigenstates of Ce<sup>3+</sup> at the 6H site suggest a weak Ce–O covalent bond
formed between Ce<sup>3+</sup> and one of the coordinated oxygen ions
not bonded with Si<sup>4+</sup>. The electronic energy levels and
frequencies of local vibrational modes are correlated with specific
Ce–O pairs to provide a comparative understanding of the site-resolved
experimental results. On the basis of the calculated results, we propose
a model of charge transfer and vibronic coupling for interpretation
of the anomalous thermal quenching of the Ce<sup>3+</sup> luminescence.
The combination of experimental and theoretical studies in the present
work provides a comprehensive understanding of the spectroscopy and
luminescence dynamics of Ce<sup>3+</sup> in crystals of ionic–covalent
coordination
Disproportionation in Li–O<sub>2</sub> Batteries Based on a Large Surface Area Carbon Cathode
In
this paper we report on a kinetics study of the discharge process
and its relationship to the charge overpotential in a Li–O<sub>2</sub> cell for large surface area cathode material. The kinetics
study reveals evidence for a first-order disproportionation reaction
during discharge from an oxygen-rich Li<sub>2</sub>O<sub>2</sub> component
with superoxide-like character to a Li<sub>2</sub>O<sub>2</sub> component.
The oxygen-rich superoxide-like component has a much smaller potential
during charge (3.2–3.5 V) than the Li<sub>2</sub>O<sub>2</sub> component (∼4.2 V). The formation of the superoxide-like
component is likely due to the porosity of the activated carbon used
in the Li–O<sub>2</sub> cell cathode that provides a good environment
for growth during discharge. The discharge product containing these
two components is characterized by toroids, which are assemblies of
nanoparticles. The morphologic growth and decomposition process of
the toroids during the reversible discharge/charge process was observed
by scanning electron microscopy and is consistent with the presence
of the two components in the discharge product. The results of this
study provide new insight into how growth conditions control the nature
of discharge product, which can be used to achieve improved performance
in Li–O<sub>2</sub> cell
Exploring Stability of Nonaqueous Electrolytes for Potassium-Ion Batteries
Recently nonaqueous
potassium-ion batteries (KIBs) have attracted tremendous attention,
but a systematic study about the electrolytes remains lacking. Here,
the stability of a commonly used electrolyte (KPF<sub>6</sub> in ethylene
carbonate (EC) and diethyl carbonate (DEC)) at the anodes (e.g., graphite,
solid K, and liquid Na–K alloy) was studied. Interesting results
show that the linear DEC is unstable. Possibly attributed to stronger
reducibility against the anodes for KIBs, the decomposition of DEC
is initiated by the CÂ(H<sub>2</sub>)–O bond breaking of the
solvent molecule. This study shows that a systematic study to look
for a more stable electrolyte is critically important for KIBs
Deciphering the Formation and Accumulation of Solid-Electrolyte Interphases in Na and K Carbonate-Based Batteries
The
continuous solid-electrolyte interphase (SEI) accumulation
has been blamed for the rapid capacity loss of carbon anodes in Na
and K ethylene carbonate (EC)/diethyl carbonate (DEC) electrolytes,
but the understanding of the SEI composition and its formation chemistry
remains incomplete. Here, we explain this SEI accumulation as the
continuous production of organic species in solution-phase reactions.
By comparing the NMR spectra of SEIs and model compounds we synthesized,
alkali metal ethyl carbonate (MEC, M = Na or K), long-chain alkali
metal ethylene carbonate (LCMEC, M = Na or K), and poly(ethylene oxide)
(PEO) oligomers with ethyl carbonate ending groups are identified
in Na and K SEIs. These components can be continuously generated in
a series of solution-phase nucleophilic reactions triggered by ethoxides.
Compared with the Li SEI formation chemistry, the enhancement of the
nucleophilicity of an intermediate should be the cause of continuous
nucleophilic reactions in the Na and K cases
Molecular Sieve Induced Solution Growth of Li<sub>2</sub>O<sub>2</sub> in the Li–O<sub>2</sub> Battery with Largely Enhanced Discharge Capacity
The formation of
the insulated film-like discharge products (Li<sub>2</sub>O<sub>2</sub>) on the surface of the carbon cathode gradually hinders the oxygen
reduction reaction (ORR) process, which usually leads to the premature
death of the Li–O<sub>2</sub> battery. In this work, by introducing
the molecular sieve powder into the ether electrolyte, the Li–O<sub>2</sub> battery exhibits a largely improved discharge capacity (63
times) compared with the one in the absence of this inorganic oxide
additive. Meanwhile, XRD and SEM results qualitatively demonstrate
the generation of the toroid Li<sub>2</sub>O<sub>2</sub> as the dominated
discharge products, and the chemical titration quantifies a higher
yield of the Li<sub>2</sub>O<sub>2</sub> with the presence of the
molecular sieve additive. The addition of the molecular sieve controls
the amount of the free water in the electrolyte, which distinguishes
the effect of the molecular sieve and the free water on the discharge
process. Hence, a possible mechanism has been proposed that the adsorption
of the molecular sieves toward the soluble lithium superoxides improves
the disproportionation of the lithium superoxides and consequently
enhances the solution-growth of the lithium peroxides in the low donor
number ether electrolyte. In general, the application of the molecular
sieve triggers further studies concerning the improvement of the discharge
performance in the Li–O<sub>2</sub> battery by adding the inorganic
additives
Dendrite-Free Potassium–Oxygen Battery Based on a Liquid Alloy Anode
The safety issue
caused by the dendrite growth is not only a key research problem in
lithium-ion batteries but also a critical concern in alkali metal
(i.e., Li, Na, and K)–oxygen batteries where a solid metal
is usually used as the anode. Herein, we demonstrate the first dendrite-free
K–O<sub>2</sub> battery at ambient temperature based on a liquid
Na–K alloy anode. The unique liquid–liquid connection
between the liquid alloy and the electrolyte in our alloy anode-based
battery provides a homogeneous and robust anode–electrolyte
interface. Meanwhile, we manage to show that the Na–K alloy
is only compatible in K–O<sub>2</sub> batteries but not in
Na–O<sub>2</sub> batteries, which is mainly attributed to the
stronger reducibility of potassium and relatively more favorable thermodynamic
formation of KO<sub>2</sub> over NaO<sub>2</sub> during the discharge
process. It is observed that our K–O<sub>2</sub> battery based
on a liquid alloy anode shows a long cycle life (over 620 h) and a
low discharge–charge overpotential (about 0.05 V at initial
cycles). Moreover, the mechanism investigation into the K–O<sub>2</sub> cell degradation shows that the O<sub>2</sub> crossover effect
and the ether–electrolyte instability are the critical problems
for K–O<sub>2</sub> batteries. In a word, this study provides
a new route to solve the problems caused by the dendrite growth in
alkali metal–oxygen batteries
Achieving Low Overpotential Lithium–Oxygen Batteries by Exploiting a New Electrolyte Based on <i>N</i>,<i>N</i>′‑Dimethylpropyleneurea
Recently,
the lithium–oxygen (Li–O<sub>2</sub>) battery has attracted
much interest due to its ultrahigh theoretical energy density. However,
its potential application is limited by an unstable electrolyte system,
low round-trip efficiency, and poor cyclic performance. In this study,
we present a new electrolyte based on <i>N</i>,<i>N</i>′-dimethylpropyleneurea (DMPU) applied for the Li–O<sub>2</sub> battery. This electrolyte possesses high ionic conductivity
and achieves a low discharge/charge voltage gap of 0.6 V, which is
mainly due to the possible one-electron charge transfer mechanism.
The introduction of the antioxidant butylatedhydroxytoluene (BHT)
as an additive stabilizes the superoxide radical by chemical adsorption
and improves the cyclic performance remarkably. Thus, this new electrolyte
system may be one of the candidates for Li–O<sub>2</sub> batteries
Graphene-Directed Formation of a Nitrogen-Doped Porous Carbon Sheet with High Catalytic Performance for the Oxygen Reduction Reaction
A nitrogen
(N)-doped porous carbon sheet is prepared by in situ
polymerization of pyrrole on both sides of graphene oxide, following
which the polypyrrole layers are then transformed to the N-doped porous
carbon layers during the following carbonization, and a sandwich structure
is formed. Such a sheet-like structure possesses a high specific surface
area and, more importantly, guarantees the sufficient utilization
of the N-doping active porous sites. The internal graphene layer acts
as an excellent electron pathway, and meanwhile, the external thin
and porous carbon layer helps to decrease the ion diffusion resistance
during electrochemical reactions. As a result, this sandwich structure
exhibits prominent catalytic activity toward the oxygen reduction
reaction in alkaline media, as evidenced by a more positive onset
potential, a larger diffusion-limited current, better durability and
poison-tolerance than commercial Pt/C. This study shows a novel method
of using graphene to template the traditional porous carbon into a
two-dimensional, thin, and porous carbon sheet, which greatly increases
the specific surface area and boosts the utilization of inner active
sites with suppressed mass diffusion resistance
Interfacial Effects on Lithium Superoxide Disproportionation in Li-O<sub>2</sub> Batteries
During the cycling of Li-O<sub>2</sub> batteries the
discharge process gives rise to dynamically evolving agglomerates
composed of lithium–oxygen nanostructures; however, little
is known about their composition. In this paper, we present results
for a Li-O<sub>2</sub> battery based on an activated carbon cathode
that indicate interfacial effects can suppress disproportionation
of a LiO<sub>2</sub> component in the discharge product. High-intensity
X-ray diffraction and transmission electron microscopy measurements
are first used to show that there is a LiO<sub>2</sub> component along
with Li<sub>2</sub>O<sub>2</sub> in the discharge product. The stability
of the discharge product was then probed by investigating the dependence
of the charge potential and Raman intensity of the superoxide peak
with time. The results indicate that the LiO<sub>2</sub> component
can be stable for possibly up to days when an electrolyte is left
on the surface of the discharged cathode. Density functional calculations
on amorphous LiO<sub>2</sub> reveal that the disproportionation process
will be slower at an electrolyte/LiO<sub>2</sub> interface compared
to a vacuum/LiO<sub>2</sub> interface. The combined experimental and
theoretical results provide new insight into how interfacial effects
can stabilize LiO<sub>2</sub> and suggest that these interfacial effects
may play an important role in the charge and discharge chemistries
of a Li–O<sub>2</sub> battery
Raman Evidence for Late Stage Disproportionation in a Li–O<sub>2</sub> Battery
Raman spectroscopy is used to characterize
the composition of toroids
formed in an aprotic Li–O<sub>2</sub> cell based on an activated
carbon cathode. The trends in the Raman data as a function of discharge
current density and charging cutoff voltage provide evidence that
the toroids are made up of outer LiO<sub>2</sub>-like and inner Li<sub>2</sub>O<sub>2</sub> regions, consistent with a disproportionation
reaction occurring in the solid phase. The LiO<sub>2</sub>-like component
is found to be associated with a new Raman peak identified in the
carbon stretching region at ∼1505 cm<sup>–1</sup>, which
appears only when the LiO<sub>2</sub> peak at 1123 cm<sup>–1</sup> is present. The new peak is assigned to distortion of the graphitic
ring stretching due to coupling with the LiO<sub>2</sub>-like component
based on density functional calculations. These new results on the
LiO<sub>2</sub>-like component from Raman spectroscopy provide evidence
that a late stage disproportionation mechanism can occur during discharge
and add new understanding to the complexities of possible processes
occurring in Li–O<sub>2</sub> batteries