Scalable Synthesis of Porous Silicon by Acid Etching of Atomized Al-Si Alloy Powder for Lithium-Ion Batteries.

Si anodes have attracted considerable attention for their potential application in next-generation lithium-ion batteries because of their high specific capacity (Li15Si4, 3579 mAh g-1) and elemental abundance. However, Si anodes have not yet been practically applied in lithium-ion batteries because the volume change associated with lithiation and delithiation degrades their capacity during cycling. Instead of considering the active material, we focused on the structural design and developed a scalable process for producing Si anodes with excellent cycle characteristics while precisely controlling the morphology. Al-Si alloy powders were prepared by gas atomization, and porous Si with a skeletal structure was prepared by leaching Al using HCl. Porous Si (p-Si12, p-Si19) prepared from Al88Si12 and Al81Si19 comprised resinous eutectic Si, and porous Si (p-Si25) prepared from Al75Si25 comprised lumpy primary Si and resinous eutectic Si. The porosity of the Si anodes varied from 63% to 76%, depending on the Si composition. The p-Si19 anode displayed the finest pore distribution (20-200 nm), excellent rate characteristics, a reversible discharge capacity of 1607 mAh g-1 after 200 cycles at a rate of 0.1 C with a Coulombic efficiency of over 97%, and high stability. The performances of the p-Si25 and p-Si19 electrodes began to decrease after 250 and 850 cycles, respectively, with a constant-charge capacity of 1000 mAh g-1 and at a rate of 0.2 C. In contrast, the p-Si12 anode maintained its discharge capacity at 1000 mAh g-1 for up to 1000 cycles without degradation. Therefore, the developed manufacturing process is expected to produce porous Si as an active material in lithium-ion batteries for high capacity and long life at an industrial scale.

How Fluorine Introduction Solves the Spinel Transition, a Fundamental Problem of Mn-Based Positive Electrodes.

In pursuit of high-capacity Mn-based oxides as positive electrode materials for lithium-ion batteries, the changes in the charge-discharge curve due to the spinel transition still stand in the way of the cycling stability. We found in this study that Li1.12Mn0.74O1.60F0.40 (LMOF05) positive electrodes with a loose-crystalline rock salt structure (LCRS), in which F is placed near Mn, show a stable and high capacity (300 mA h g-1, 952 W h kg-1) with little change in the charge-discharge curve. We demonstrated by F K-edge soft X-ray absorption spectroscopy and X-ray diffraction (XRD) that a part of F in the LCRS positive electrode forms F-Mn bonds. Operando XRD/X-ray absorption fine structure measurements revealed the lattice size and Mn surrounding environment during charge/discharge of F-containing LCRS positive electrodes (LMOF05), LCRS-LiMnO2 (LMO), and a spinel-like Li1.1Al0.1Mn1.8O4 positive electrode (SPINEL). Micro- and macroscopic structural changes indicate how the introduction of F suppresses the local spinel transition in Mn-based positive electrodes. These findings should be an effective tool for applying Co-free positive electrode materials for lithium-ion batteries.

Synthesis of Supported Bimetal Catalysts using Galvanic Deposition Method.

Supported bimetallic catalysts have been studied because of their enhanced catalytic properties due to metal-metal interactions compared with monometallic catalysts. We focused on galvanic deposition (GD) as a bimetallization method, which achieves well-defined metal-metal interfaces by exchanging heterogeneous metals with different ionisation tendencies. We have developed Ni@Ag/SiO2 catalysts for CO oxidation, Co@Ru/Al2 O3 catalysts for automotive three-way reactions and Pd-Co/Al2 O3 catalysts for methane combustion by using the GD method. In all cases, the catalysts prepared by the GD method showed higher catalytic activity than the corresponding monometallic and bimetallic catalysts prepared by the conventional co-impregnation method. The GD method provides contact between noble and base metals to improve the electronic state, surface structure and reducibility of noble metals.

The Metal-Support Interaction Concerning the Particle Size Effect of Pd/Al2 O3 on Methane Combustion.

The particle size effect of Pd nanoparticles supported on alumina with various crystalline phases on methane combustion was investigated. Pd/θ, α-Al2 O3 with weak metal-support interaction showed a volcano-shaped dependence of the catalytic activity on the size of Pd particles, and the catalytic activity of the strongly interacted Pd/γ-Al2 O3 increased with the particle size. Based on a structural analysis of Pd nanoparticles using CO adsorption IR spectroscopy and spherical aberration-corrected scanning/transmission electron microscopy, the dependence of catalytic activity on Pd particle size and the alumina crystalline phase was due to the fraction of step sites on Pd particle surface. The difference in fraction of the step site is derived from the particle shape, which varies not only with Pd particle size but also with the strength of metal-support interaction. Therefore, this interaction perturbs the particle size effect of Pd/Al2 O3 for methane combustion.