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

Simultaneous Reduction of Co³⁺ and Mn⁴⁺ in P2-Na_2/3Co_2/3Mn_1/3O₂ As Evidenced by X‑ray Absorption Spectroscopy during Electrochemical Sodium Intercalation

Sodium intercalation in P2-Na_2/3Co_2/3Mn_1/3O₂ (obtained by a coprecipitation method) was investigated by ex situ and in situ X-ray absorption spectroscopy. The electronic transitions at the O K-edge and the charge compensation mechanism, during the sodium intercalation process, were elucidated by combining Density Function Theory (DFT) calculations and X-ray absorption spectroscopy (XAS) data. The pre-edge of the oxygen K-edge moves to higher energy while the integrated intensity dramatically decreases, indicating that the population of holes in O 2p states is reduced with increasing numbers of sodium ions. From the K-edge and L-edge observations, the oxidation states of pristine Co and Mn were determined to be +III and +IV, respectively. The absorption energy shifts to lower positions during the discharging process for both the Co and the Mn edges, suggesting that the redox pairs, that is, Co³⁺/Co²⁺ and Mn⁴⁺/Mn³⁺, are both involved in the reaction

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

Highly Active and Stable Hybrid Catalyst of Cobalt-Doped FeS₂ Nanosheets–Carbon Nanotubes for Hydrogen Evolution Reaction

Hydrogen evolution reaction (HER) from water through electrocatalysis using cost-effective materials to replace precious Pt catalysts holds great promise for clean energy technologies. In this work we developed a highly active and stable catalyst containing Co doped earth abundant iron pyrite FeS₂ nanosheets hybridized with carbon nanotubes (Fe_1–<i>x</i>Co_<i>x</i>S₂/­CNT hybrid catalysts) for HER in acidic solutions. The pyrite phase of Fe_1–<i>x</i>Co_<i>x</i>S₂/­CNT was characterized by powder X-ray diffraction and absorption spectroscopy. Electrochemical measurements showed a low overpotential of ∼0.12 V at 20 mA/cm², small Tafel slope of ∼46 mV/decade, and long-term durability over 40 h of HER operation using bulk quantities of Fe_0.9Co_0.1S₂/CNT hybrid catalysts at high loadings (∼7 mg/cm²). Density functional theory calculation revealed that the origin of high catalytic activity stemmed from a large reduction of the kinetic energy barrier of H atom adsorption on FeS₂ surface upon Co doping in the iron pyrite structure. It is also found that the high HER catalytic activity of Fe_0.9Co_0.1S₂ hinges on the hybridization with CNTs to impart strong heteroatomic interactions between CNT and Fe_0.9Co_0.1S_2. This work produces the most active HER catalyst based on iron pyrite, suggesting a scalable, low cost, and highly efficient catalyst for hydrogen generation

Kinetically Controlled Autocatalytic Chemical Process for Bulk Production of Bimetallic Core–Shell Structured Nanoparticles

Although bimetallic core@shell structured nanoparticles (NPs) are achieving prominence due to their multifunctionalities and exceptional catalytic, magnetic, thermal, and optical properties, the rationale underlying their design remains unclear. Here we report a kinetically controlled autocatalytic chemical process, adaptable for use as a general protocol for the fabrication of bimetallic core@shell structured NPs, in which a sacrificial Cu ultrathin layer is autocatalytically deposited on a dimensionally stable noble-metal core under kinetically controlled conditions, which is then displaced to form an active ultrathin metal-layered shell by redox–transmetalation. Unlike thermodynamically controlled under-potential deposition processes, this general strategy allows for the scaling-up of production of high-quality core–shell structured NPs, without the need for any additional reducing agents and/or electrochemical treatments, some examples being Pd@Pt, Pt@Pd, Ir@Pt, and Ir@Pd. Having immediate and obvious commercial potential, Pd@Pt NPs have been systematically characterized by <i>in situ</i> X-ray absorption, electrochemical-FTIR, transmission electron microscopy, and electrochemical techniques, both during synthesis and subsequently during testing in one particularly important catalytic reaction, namely, the oxygen reduction reaction, which is pivotal in fuel cell operation. It was found that the bimetallic Pd@Pt NPs exhibited a significantly enhanced electrocatalytic activity, with respect to this reaction, in comparison with their monometallic counterparts