Reaction of Metal, Carbide, and Nitride of Tungsten with Hydrogen Peroxide Characterized by ¹⁸³W Nuclear Magnetic Resonance and Raman Spectroscopy

Chemical species formed by reactions of tungsten metal, carbide, and nitride with hydrogen peroxide were characterized by multinuclear nuclear magnetic resonance and Raman spectroscopy and gas chromatography. The results not only showed formation of previously known tetraperoxoditungstate, but also strongly suggested formation of diperoxomonotungstate, its protonated form, monoperoxomonotungstate, and diperoxoditungstate. Carbon in tungsten carbide was only oxidized, and oxalate ion, carbon monoxide, and carbon dioxide were produced. On the other hand, both oxidation and hydrolysis of nitrogen were observed for tungsten nitride. It was suggested that the difference of reactivity was explained by the ionicity of tungsten-heteroatom bond

Electrochemical Study of High Electrochemical Double Layer Capacitance of Ordered Porous Carbons with Both Meso/Macropores and Micropores

Porous carbons with large meso/macropore surface areas were prepared by the colloidal-crystal-templating technique. The porous carbons exibited extremely high specific electrochemical double layer (EDL) capacitance of 200−350 F g-1 in an aqueous electrolyte (1 M H2SO4). The pore structure dependence of the capacitance was studied mainly by means of cyclic voltammetry and is discussed in detail. From the sweep rate dependence of the series resistance and capacitance, it was found that the ion-penetration depth at the porous electrode surface was finite and decreased with an increasing sweep rate. Peaks around the point of zero charge, which were observed in addition to typical rectangular voltammograms, were explained well by the potential drop in pores. The surface area dependence of the capacitance revealed that the contribution of the meso/macropore surface is as great as that of the plane electrodes and that only the part of the micropore surface adjacent to the opening mouths is effective

Synthesis of Single Crystalline Spinel LiMn₂O₄ Nanowires for a Lithium Ion Battery with High Power Density

How to improve the specific power density of the rechargeable lithium ion battery has recently become one of the most attractive topics of both scientific and industrial interests. The spinel LiMn2O4 is the most promising candidate as a cathode material because of its low cost and nontoxicity compared with commercial LiCoO2. Moreover, nanostructured electrodes have been widely investigated to satisfy such industrial needs. However, the high-temperature sintering process, which is necessary for high-performance cathode materials based on high-quality crystals, leads the large grain size and aggregation of the nanoparticles which gives poor lithium ion battery performance. So there is still a challenge to synthesize a high-quality single-crystal nanostructured electrode. Among all of the nanostructures, a single crystalline nanowire is the most attractive morphology because the nonwoven fabric morphology constructed by the single crystalline nanowire suppresses the aggregation and grain growth at high temperature, and the potential barrier among the nanosize grains can be ignored. However, the reported single crystalline nanowire is almost the metal oxide with an anisotropic crystal structure because the cubic crystal structure such as LiMn2O4 cannot easily grow in the one-dimentional direction. Here we synthesized high-quality single crystalline cubic spinel LiMn2O4 nanowires based on a novel reaction method using Na0.44MnO2 nanowires as a self-template. These single crystalline spinel LiMn2O4 nanowires show high thermal stability because the nanowire structure is maintained after heating to 800 °C for 12 h and excellent performance at high rate charge−discharge, such as 20 A/g, with both a relative flat charge−discharge plateau and excellent cycle stability

Large Reversible Li Storage of Graphene Nanosheet Families for Use in Rechargeable Lithium Ion Batteries

The lithium storage properties of graphene nanosheet (GNS) materials as high capacity anode materials for rechargeable lithium secondary batteries (LIB) were investigated. Graphite is a practical anode material used for LIB, because of its capability for reversible lithium ion intercalation in the layered crystals, and the structural similarities of GNS to graphite may provide another type of intercalation anode compound. While the accommodation of lithium in these layered compounds is influenced by the layer spacing between the graphene nanosheets, control of the intergraphene sheet distance through interacting molecules such as carbon nanotubes (CNT) or fullerenes (C60) might be crucial for enhancement of the storage capacity. The specific capacity of GNS was found to be 540 mAh/g, which is much larger than that of graphite, and this was increased up to 730 mAh/g and 784 mAh/g, respectively, by the incorporation of macromolecules of CNT and C60 to the GNS

Suppressed Activation Energy for Interfacial Charge Transfer of a Prussian Blue Analog Thin Film Electrode with Hydrated Ions (Li⁺, Na⁺, and Mg²⁺)

Interfacial charge transfer is one of the most important fundamental steps in the charge and discharge processes of intercalation compounds for rechargeable batteries. In this study, temperature-dependent electrochemical impedance spectroscopy was carried out to clarify the origin of the high power output of aqueous batteries with Prussian blue analog electrodes. The activation energy for the interfacial charge transfer, <i>E</i>_a, was estimated from the temperature dependence of the interfacial charge transfer resistance. The <i>E</i>_a values with Li⁺ and Na⁺ aqueous electrolytes were considerably smaller than those with organic electrolytes. The small <i>E</i>_a values with aqueous electrolytes could result from the fact that the Coulombic repulsion at the interface is largely suppressed by the screening effect of hydration

Fast Li-Ion Insertion into Nanosized LiMn₂O₄ without Domain Boundaries

The effect of crystallite size on Li-ion insertion in electrode materials is of great interest recently because of the need for nanoelectrodes in higher-power Li-ion rechargeable batteries. We present a systematic study of the effect of size on the electrochemical properties of LiMn2O4. Accurate size control of nanocrystalline LiMn2O4, which is realized by a hydrothermal method, significantly alters the phase diagram as well as Li-ion insertion voltage. Nanocrystalline LiMn2O4 with extremely small crystallite size of 15 nm cannot accommodate domain boundaries between Li-rich and Li-poor phases due to interface energy, and therefore lithiation proceeds via solid solution state without domain boundaries, enabling fast Li-ion insertion during the entire discharge process

Crystalline Grain Interior Configuration Affects Lithium Migration Kinetics in Li-Rich Layered Oxide

The electrode kinetics of Li-ion batteries, which are important for battery utilization in electric vehicles, are affected by the grain size, crystal orientation, and surface structure of electrode materials. However, the kinetic influences of the grain interior structure and element segregation are poorly understood, especially for Li-rich layered oxides with complex crystalline structures and unclear electrochemical phenomena. In this work, cross-sectional thin transmission electron microscopy specimens are “anatomized” from pristine Li1.2Mn0.567Ni0.167Co0.067O2 powders using a new argon ion slicer technique. Utilizing advanced microscopy techniques, the interior configuration of a single grain, multiple monocrystal-like domains, and nickel-segregated domain boundaries are clearly revealed; furthermore, a randomly distributed atomic-resolution Li2MnO3-like with an intergrown LiTMO2 (TM = transitional metals) “twin domain” is demonstrated to exist in each domain. Further theoretical calculations based on the Li2MnO3-like crystal domain boundary model reveal that Li+ migration in the Li2MnO3-like structure with domain boundaries is sluggish, especially when the nickel is segregated in domain boundaries. Our work uncovers the complex configuration of the crystalline grain interior and provides a conceptual advance in our understanding of the electrochemical performance of several compounds for Li-ion batteries

Fabrication of a Cyanide-Bridged Coordination Polymer Electrode for Enhanced Electrochemical Ion Storage Ability

Host frameworks with the ability to store guest ions are very important in a wide range of applications including electrode materials for Li-ion batteries. In this report, we demonstrate that the ion storage ability of the cyanide-bridged coordination polymer (Prussian blue analogue, PBA) can be enhanced by suppressing vacancy formation. K-ions in the vacancy-suppressed PBA framework K_1.72Mn­[Mn­(CN)₆]_0.93·□_0.07·0.65H₂O (□: a [Mn­(CN)₆]^4– defect) were electrochemically extracted. The open circuit voltages (OCVs) during K-ion extraction exhibited two specific plateaus at 3.0 and 3.7 V vs Li/Li⁺. Ex situ X-ray diffraction and IR spectroscopy revealed drastic structural and electronic changes during K-ion extraction. Furthermore, after K-ion extraction, the vacancy-suppressed PBA framework was applied to the cathode material for a Li-ion battery. The charge/discharge experiments revealed that the framework can accommodate a large amount of Li-ions

Configuration-Interaction Full-Multiplet Calculation to Analyze the Electronic Structure of a Cyano-Bridged Coordination Polymer Electrode

To understand the electronic-structure changes of electrode materials during the charge/discharge processes is one of the most important fundamental aspects to improve the battery performance. Soft X-ray absorption spectroscopy (XAS) was used to study a bimetallic NiFe Prussian blue analogue electrode. XA spectra were obtained during the charge/discharge and were analyzed by the configuration-interaction full-multiplet (CIFM) calculation, in which the strong charge transfer due to the σ/π-donation and back-donation of cyanide was taken into account. The CIFM calculation revealed that the metal-to-ligand charge transfer (MLCT) played an important role in the electronic state of Ni–N bond. The Fe³⁺–C bond in the charged state is dominated by both the MLCT and ligand-to-metal charge transfer (LMCT), whereas only the MLCT strongly affects the Fe²⁺–C bond in the discharged state

Electrospinning Synthesis of Wire-Structured LiCoO₂ for Electrode Materials of High-Power Li-Ion Batteries

An application of the Li-ion batteries to advanced transportation systems essentially requires the enhancement of the rate capability; thus, the fabrication of nanostructured cathode materials with the large surface area and short Li-ion diffusion length is particularly important. In this study, an electrospinning method was adopted for the synthesis of wire-structured LiCoO₂. The diameter of the as-spun fiber obtained from the precursor solution with multiwalled carbon nanotubes (vapor-grown carbon fiber, VGCF) was thinner than that of as-spun fiber obtained from the solution without VGCF. After the heat treatment, wire-structured LiCoO₂ was successfully obtained regardless of the existence of dispersed VGCF in the precursor solution, although the particle size of LiCoO₂ fabricated with VGCF was smaller than that of LiCoO₂ fabricated without VGCF. The charge/discharge and rate-capability experiments revealed that both resulting materials show the reversible Li-ion insertion/extraction reaction. However, due to the existence of a small irreversible capacity at the initial cycles, the interfacial resistance increases, resulting in the poor cyclability and lower charge/discharge rate capability, especially for nanowire LiCoO₂ fabricated with VGCF