Atomic and Electronic Structures of Li_0.44MnO₂ Nanowires and Li₂MnO₃ Byproducts in the Formation Process of LiMn₂O₄ Nanowires

High-quality single crystalline LiMn2O4 nanowires, which can be synthesized by the conversion of Na0.44MnO2 nanowires, are promising electrode materials for high-power lithium ion batteries. Understanding the conversion mechanism is crucial for further improvement of the quality of LiMn2O4 nanowires. In this paper, using advanced techniques of transmission electron microscopy and electron energy-loss spectroscopy, we investigate the atomic and electronic structures of both Li0.44MnO2 nanowires and its byproduct formed after the conversion of Na0.44MnO2 nanowires into Li0.44MnO2 nanowires, as the first half of the process in the conversion of Na0.44MnO2 nanowires into LiMn2O4 nanowires. Results show that Li0.44MnO2 nanowires have a well-defined single-crystalline nature. The byproduct is identified as nanoparticles of Li2MnO3 (space group P3112) different from conventional Li2MnO3 (space group C2/m), formed on the surfaces of Li0.44MnO2 nanowires with the specific crystallographic relationship. The formation mechanism of Li2MnO3 nanoparticles and their role in the conversion of Li0.44MnO2 nanowires into LiMn2O4 nanowires are discussed

Synergistically Activated Pd Atom in Polymer-Stabilized Au₂₃Pd₁ Cluster

Single Pd atom doped Au23Pd1 clusters stabilized by polyvinylpyrrolidone (Au23Pd1:PVP) were selectively synthesized by kinetically controlled coreduction of the Au and Pd precursor ions. The geometric structure of Au23Pd1:PVP was investigated by density functional theory calculation, aberration-corrected transmission electron microscopy, extended X-ray absorption fine structure analysis, Fourier transform infrared spectroscopy of adsorbed CO, and hydrogenation catalysis. These results showed that Au23Pd1:PVP takes polydisperse but the same atomic arrangements as undoped Au24:PVP while exposing all the atoms including the Pd atom on the surface. Au23Pd1:PVP exhibited a significantly higher catalytic activity than Au24:PVP for the aerobic oxidation of p-substituted benzyl alcohols. The kinetic studies showed that the rate-determining step was the hydride abstraction from the α-carbon of the alkoxides for both systems. The activation energy for hydride abstraction by Au23Pd1:PVP was lower than that by Au24:PVP, indicating that the doped Pd atom acts as the active center

Chemical States of Overcharged LiCoO₂ Particle Surfaces and Interiors Observed Using Electron Energy-Loss Spectroscopy

Deterioration mechanisms of LiCoO₂ electrode materials for lithium ion batteries remain unclear. Using electron energy-loss spectroscopy and transmission electron microscopy, this study investigated chemical states of LiCoO₂ particles on first overcharging. We present a scheme for quantification of the Li/Co atomic ratio. Using quantitative Li mapping and comprehensive probing of Li–K, Co–M_2,3, Co–L₃, and O–K edges, we observed that overcharging causes the progression of Co³⁺/Co²⁺ reduction with oxygen extraction from the particle surface to the interior. A gradual change in the chemical composition at and around the particle surfaces after charging of 60% revealed the presence of Co₃O₄-like and CoO-like phases at surface regions. We also observed nanocracks with deficient Li ions. These results are key factors affecting degradation on overcharging

Single Crystallization of Olivine Lithium Phosphate Nanowires using Oriented Attachments

Electrospinning enables fabrication of nanowires (NWs) of various materials from a polymer solution. Nevertheless, few reports have described single crystallization of oxide and polyanion NWs. Its mechanism remains unknown. This report presents transmission electron microscopy observations of conversion from electrospun amorphous NWs to single-crystalline olivine lithium phosphate NWs. After nucleation and grain growth, single crystallization is achieved by the attachment of adjacent crystal grains with common crystallographic orientations in an amorphous phase confined to self-forming carbon shells. The present NW axes have no specific orientation. These results imply that self-forming shells play a key role in achieving single-crystalline NWs in electrospinning

Tubular Flame Combustion for Nanoparticle Production

Requirements for nanoparticle processing based on energy and cost-effective technologies have increased in recent years. Flame synthesis is widely used on an industrial scale and is superior to gas phase one-step processes for producing nanoparticles; however, further improvements are required from the viewpoint of energy efficiency. In this work, we present a new aerodynamically stable nanoparticle processing method with extremely low energy losses, based on the use of a tubular flame. The developed tubular flame apparatus is inexpensive and simple to set up, allowing for a uniform and high temperature field with a high energy efficiency. Consequently, highly crystalline and uniform WO3 nanoparticles were successfully synthesized with a high production rate. The simple and energy-effective process proposed in this work has potential for application to various types of functional nanoparticle production

AgFeOF₂: A Fluorine-Rich Perovskite Oxyfluoride

We synthesized a silver iron oxyfluoride AgFeOF₂ by using a high-pressure reaction. Synchrotron X-ray and neutron diffraction, X-ray absorption, and ⁵⁷Fe Mössbauer spectroscopy indicate that AgFeOF₂ crystallizes in the ideal perovskite structure with iron in a trivalent state, although electron microscopy revealed weak super-reflections. A possible partial ordering in the FeO₂F₄ octahedron is inferred from Mössbauer spectroscopy. The synthesis of the fluorine-rich sample offers an opportunity to study a composition-property relation in <i>A</i>Fe^IIIO_3–<i>n</i>F_<i>n</i> (<i>n</i> = 0, 1, and 2). AgFeOF₂ exhibits a <i>G</i>-type antiferromagnetic ordering below <i>T</i>_N ≈ 480 K, which is much lower than the <i>n</i> = 0 and 1 cases, suggesting a weaker superexchange interaction between Fe moments via F 2p orbitals (vs O 2p orbitals)