DFT+U Calculations and XAS Study: Further Confirmation of the Presence of CoO₅ Square-Based Pyramids with IS-Co³⁺ in Li-Overstoichiometric LiCoO₂

LiCoO₂, 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) Co³⁺ 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 Li₄Co₂O₅ 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 Co³⁺ 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 Co–Co 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 LiCoO₂, 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 Co³⁺ defects in Li-overstoichiometric LiCoO₂

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 Al₂O₃ 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 ALD–VC 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^–1 after 200 cycles running at 200 mA g^–1, and the bare anode without VC showed a capacity of 400 mAh g^–1 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_<i>x</i>Li_{(1–2<i>x</i>)/3}Mn_{(2–<i>x</i>)/3}]O₂ (0 ≤ <i>x</i> ≤ 0.5)

The lithium-rich high-capacity cathode material Li­[Ni_<i>x</i>Li_{(1–2<i>x</i>)/3}Mn_{(2–<i>x</i>)/3}]­O₂ 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_II,III-edges reveal a relationship between Ni and the degree of structural transformation after the first cycle. Close examination of Li₂MnO₃ 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_II,III-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 d–sp 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 mA·h/g

Interplay between Molybdenum Dopant and Oxygen Vacancies in a TiO₂ Support Enhances the Oxygen Reduction Reaction

In this study, molybdenum doping of anatase TiO₂, used as a Pt catalyst support, both augments resistance against the carbon corrosion that commonly occurs in oxygen reduction reaction (ORR) Pt/C catalysts and promotes the generation of oxygen vacancies that allow better electron transfer from the nanosupport to Pt, thereby facilitating the oxygen dissociation reaction. The effects of the oxygen vacancies within the Mo-doped TiO₂ nanosupport on ORR activity and stability are investigated both experimentally and by density functional theory analysis. The mass activity of Pt-supported molybdenum-doped anatase TiO₂ is shown to be 9.1 times higher than that of a commercial standard Pt/C catalyst after hydrogen reduction. The oxide-supported nanocatalysts also show improved stability against Pt sintering under during cycling, because of strong metal–support interactions