6 research outputs found
DFT+U Calculations and XAS Study: Further Confirmation of the Presence of CoO<sub>5</sub> Square-Based Pyramids with IS-Co<sup>3+</sup> in Li-Overstoichiometric LiCoO<sub>2</sub>
LiCoO<sub>2</sub>, one of the major
positive electrode materials
for Li-ion batteries, can be synthesized with excess Li. Previous
experimental work suggested the existence of intermediate spin (IS)
Co<sup>3+</sup> ions in square-based pyramids to account for the defect
in this material. We present here a theoretical study based on density
functional theory (DFT) calculations together with an X-ray absorption
spectroscopy (XAS) experimental study. In the theoretical study, a
hypothetical Li<sub>4</sub>Co<sub>2</sub>O<sub>5</sub> material, where
all the Co ions are in pyramids, was initially considered as a model
material. Using DFT+U, the intermediate spin state of the Co<sup>3+</sup> ions is found stable for U values around 1.5 eV. The crystal and
electronic structures are studied in detail, showing that the defect
must actually be considered as a pair of such square-based pyramids,
and that CoāCo bonding can explain the position of Co in the
basal plane. Using a supercell corresponding to more diluted defects
(as in the actual material), the calculations show that the IS state
is also stabilized. In order to investigate experimentally the change
in the electronic structure in the Li-overstoichiometric LiCoO<sub>2</sub>, we used X-ray absorption near edge structure (XANES) spectroscopy
and propose an interpretation of the O Kedge spectra based on the
DFT+U calculations, that fully supports the presence of pairs of intermediate
spin state Co<sup>3+</sup> defects in Li-overstoichiometric LiCoO<sub>2</sub>
Stabilizing Nanosized Si Anodes with the Synergetic Usage of Atomic Layer Deposition and Electrolyte Additives for Li-Ion Batteries
A substantial increase in charging
capacity over long cycle periods
was made possible by the formation of a flexible weblike network via
the combination of Al<sub>2</sub>O<sub>3</sub> atomic layer deposition
(ALD) and the electrolyte additive vinylene carbonate (VC). Transmission
electron microscopy shows that a weblike network forms after cycling
when ALD and VC were used in combination that dramatically increases
the cycle stability for the Si composite anode. The ALDāVC
combination also showed reduced reactions with the lithium salt, forming
a more stable solid electrolyte interface (SEI) absent of fluorinated
silicon species, as evidenced by X-ray photoelectron spectroscopy. Although the bare Si composite
anode showed only an improvement from a 56% to a 45% loss after 50
cycles, when VC was introduced, the ALD-coated Si anode showed an
improvement from a 73% to a 11% capacity loss. Furthermore, the anode
with the ALD coating and VC had a capacity of 630 mAh g<sup>ā1</sup> after 200 cycles running at 200 mA g<sup>ā1</sup>, and the
bare anode without VC showed a capacity of 400 mAh g<sup>ā1</sup> after only 50 cycles. This approach can be extended to other Si
systems, and the formation of this SEI is dependent on the thickness
of the ALD that affects both capacity and stability
Understanding the Role of Ni in Stabilizing the Lithium-Rich High-Capacity Cathode Material Li[Ni<sub><i>x</i></sub>Li<sub>(1ā2<i>x</i>)/3</sub>Mn<sub>(2ā<i>x</i>)/3</sub>]O<sub>2</sub> (0 ā¤ <i>x</i> ā¤ 0.5)
The lithium-rich high-capacity cathode
material LiĀ[Ni<sub><i>x</i></sub>Li<sub>(1ā2<i>x</i>)/3</sub>Mn<sub>(2ā<i>x</i>)/3</sub>]ĀO<sub>2</sub> was investigated
by X-ray absorption spectroscopy (XAS) to understand the role of Ni
in the lithium-rich layered line in the pseudoternary system. The
electronic structural changes that occur at different charged states
were examined with soft XAS to understand the role of the secondary
transition metal in controlling the stability. The Mn L<sub>II,III</sub>-edges reveal a relationship between Ni and the degree of structural
transformation after the first cycle. Close examination of Li<sub>2</sub>MnO<sub>3</sub> shows that Mn changes its oxidation state
slightly during charging; however, there is a more dramatic change
upon reduction from 4+ to a value close to 3+ at discharge, where
it participates in the subsequent charge-compensation mechanism. The
Ni L<sub>II,III</sub>-edges reveal that nickel acts as a stabilizer
preventing the complete transformation of Mn by substitution reduction
that is thought to couple with the anionic redox couple during the
extended oxygen-activation plateau. The O K-edge reveals that loss
of stability might be related to diminished dāsp hybridization
occurring after oxygen activation, which is improved by the incorporation
of nickel as a stabilizing agent. These findings clarify the complex
relationships among nickel, manganese, and oxygen within lithium-rich
high-capacity cathode materials in controlling stability while simultaneously
obtaining high capacities exceeding 250 mAĀ·h/g