Takamasa Nonaka

X-ray absorption fine structure analysis of local structure of CeO2–ZrO2 mixed oxides with the same composition ratio (Ce/Zr=1)

Abstract Three types of CeO2–ZrO2 (Ce:Zr=1:1 molar ratio) compounds with different oxygen storage/release capacities (OSCs) were characterized by means of the Ce K-edge and Zr K-edge X-ray absorption fine structure (XAFS). In order to investigate the relationship between the OSC and local structure, the quantitative EXAFS curve-fitting analysis was applied. By enhancing the homogeneity of the Ce and Zr atoms in the CeO2–ZrO2 solid solution, the OSC performance increased. Especially, the atomically homogeneous Ce0.5Zr0.5O2 solid solution exhibited the highest OSC among these CeO2–ZrO2 samples. Additionally, the local oxygen environment around Ce and Zr was remarkably modified by enhancing the homogeneity of the CeO2–ZrO2 solid solution. It was postulated that the enhancement of the homogeneity of the CeO2–ZrO2 solid solution and the modification of the oxygen environment would be the source for the OSC improvement.

Capacity-Fading Mechanisms of LiNiO2-Based Lithium-Ion Batteries I. Analysis by Electrochemical and Spectroscopic Examination

I. Analysis by Electrochemical and Spectroscopic Examination Tsuyoshi Sasaki,* Takamasa Nonaka, Hideaki Oka, Chikaaki Okuda, Yuichi Itou, Yasuhito Kondo, Yoji Takeuchi, Yoshio Ukyo,* Kazuyoshi Tatsumi, and Shunsuke Muto* Toyota Central Research and Development Laboratories, Incorporated, Nagoakute 480-1192, Japan Department of Materials, Physics and Energy Engineering, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan

Factors affecting cycling life of LiNi0.8Co0.15Al0.05O2 for lithium-ion batteries

Factors affecting the cycling life of cylindrical lithium-ion batteries of LiNi0.8Co0.15Al0.05O2 (NCA) with graphite were examined in terms of the rechargeable capacity and polarization of NCA derivatives of LizNi0.8Co0.15Al0.05O2−δ (0.8 ≤ z ≤ 1.05). NCA derivatives with rock-salt domains in the structure were prepared by a co-precipitation method and the structures of [Li1−yNiy]3(b)[Ni,Co,Al]3(a)O26(c) based on a space group of Rm were refined by a Rietveld method of the XRD patterns. The electrochemical reactivity of the NCA derivatives with rock-salt domains was examined in non-aqueous lithium cells, and it was found that the rechargeable capacities (Q) of the samples decrease linearly as the amount of rock-salt domain (y) increases. An empirical relation is obtained to be Q = 181.4 − 725.5y in which Q reaches zero at y = 0.25, which is derived from not only the capacity loss owing to inactive rock-salt domains but also the polarization increase. The galvanostatic intermittent titration technique (GITT) measurement told us that polarization of NCA derivatives increases when the amount of rock-salt domains is above 2%, i.e., y > 0.02, and such a relation is remarkable in the lithium insertion direction into the structure, which is ascribed to slow lithium ion mobility due to nickel ions in the lithium layers. The NCA derivatives with increased rock-salt domains of above 2% deteriorate rapidly in non-aqueous lithium cells upon charge and discharge cycles, which is ascribed to the cumulative increase in polarization during charge and discharge. An extended cycling test for cylindrical lithium-ion batteries of the NCA derivatives with a graphite negative electrode at elevated temperature was performed and the quantitative relation is discussed thereof.

Structural Improvement of CaFe2O4 by Metal Doping toward Enhanced Cathodic Photocurrent

Various metal-doped p-type CaFe2O4 photocathodes were prepared in an attempt to improve the low quantum efficiency for photoreaction. CuO and Au doping enhanced the photocurrent by expansion of the absorption wavelength region and plasmon resonance, respectively. X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) analysis showed that doping with these metals further disturbed the originally distorted crystal structure of CaFe2O4. In contrast, doping with Ag relaxed the distorted crystal structure around the Fe center toward symmetry. Ag doping resulted in improvement of the carrier mobility together with a red-shift of photoabsorption with Ag-doped CaFe2O4 having a 23-fold higher photocurrent than undoped CaFe2O4.

Surface-Sensitive X-Ray Absorption Study on LiNi0.8Co0.15Al0.05O2 Cathode Material for Lithium-Ion Batteries

We have used Ni and Co K-edge conversion electron yield X-ray absorption fine structure (CEY-XAFS) and conventional XAFS in transmission mode to investigate LiNi 0.8 Co 0.15 Al 0.05 O 2 , one of the promising cathode materials for Li-ion batteries. The former technique is surface-sensitive, having a probing depth of -90 nm, while the latter is bulk-sensitive. X-ray absorption near edge structure analysis revealed that the bulk-averaged Ni valences for cycle-tested cells and aging-tested cells are lower than that for a fresh cell throughout charging. Further reduction of Ni atoms is observed at the surface of the cathode material particles, and the ranges of the Ni valence change upon charging are narrower than that for a fresh cell, indicating the presence of Ni atoms which are unaffected by charging. Extended X-ray absorption fine structure analysis revealed that the change in bond lengths [Ni-O, Ni-M (M = Ni,Co), Co-0, and Co-M] is consistent with the change in the Ni valence. These electronic and structural changes occur predominantly at the surface and are probably the main causes of capacity fading.

Synthesis of a calcium-bridged siloxene by a solid state reaction for optical and electrochemical properties

A Ca-bridged siloxene (Ca-siloxene) resulting from the deintercalation of Ca from CaSi2 was synthesized via a solid state reaction with ethanol wash using TaCl5. The presence of fragmented, two-dimensional siloxene planes with Ca bridging was confirmed. The Ca-siloxene exhibited tunable optical properties, as well as stable Li storage performance by Ca bridging.

Syntheses of Au–Cu-rich AuAg(AgCl)Cu alloy and Ag–Cu-rich AuAgCu@Cu core–shell and AuAgCu alloy nanoparticles using a polyol method

Core–shell and alloy types of nanoparticles including Au, Ag, and Cu components were prepared by reducing mixtures of HAuCl4·4H2O, AgNO3, and Cu(OAc)2·H2O in ethylene glycol (EG) in the presence of poly(vinylpyrrolidone) (PVP) at 175 °C. At a HAuCl4·4H2O : AgNO3 : Cu(OAc)2·H2O molar ratio of 1 : 2 : 1, mixtures of Au–Cu-rich AuAg(AgCl)Cu alloy nanoparticles and AgCl precipitates were formed after 2.5–35 min heating. On the other hand, at a HAuCl4·4H2O : AgNO3 : Cu(OAc)2·H2O molar ratio of 0.0065 : 2 : 1, at which the formation of AgCl precipitate was suppressed, Ag–Cu-rich AuAgCu alloy particles were prepared via AuAgCu@Cu core–shell particles after 2.5–34 min heating. The growth mechanisms of AuAg(AgCl)Cu, AuAgCu@Cu, and AuAgCu particles were examined using TEM-energy dispersed X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES), and ultraviolet (UV)-visible (Vis)-near infrared (NIR) extinction spectral data. The time dependence of UV-Vis-NIR spectral data indicated that the Cu components of AuAg(AgCl)Cu and AuAgCu alloy particles retained good anti-oxidation properties about 1 month after preparation.

In situ XAFS study on cathodic materials for lithium-ion batteries

Ni and Co K-edge X-ray absorption spectra of LiNi0.8Co0.2O2 have been collected using in situ coin cells. To investigate the electronic and structural changes accompanied by the capacity fading during electrochemical cycling and keeping batteries at high temperatures, the cells with different cycling states and keeping conditions (temperature, time) were prepared. Upon charging the cell, the Ni and Co K absorption edge shifted towards higher energy, and the good correlation between the range of chemical shifts upon charging and the capacity of the cell was observed. From quantitative analysis of EXAFS data, it was revealed that the capacity fading is closely related to the Jahn-Teller distortion of the NiO6 octahedron.

Highly crystalline β-FeOOH(Cl) nanorod catalysts doped with transition metals for efficient water oxidation

The application of the water oxidation reaction to extract electrons from water molecules is important for the future sustainable synthesis of useful chemicals such as hydrogen and organic compounds. Therefore, a cost-effective oxygen evolution reaction (OER) in alkaline, neutral or acidic solution is required, based on the use of catalysts incorporating earth abundant elements. This work demonstrates that β-FeOOH(Cl) nanorod catalysts with high crystallinity and small size (an average diameter of 3 nm and a length of 15 nm) provided the best performance in the OER activity among Fe-based oxide and (oxy)hydroxide catalysts. The pristine β-FeOOH(Cl) nanorods with the high crystallinity are realized by a new process enabling one-pot, fast, and room temperature synthesis, which is the key to forming colloidal suspensions of the highly crystallized pure-phase β-FeOOH(Cl) nanorods. The versatility of this process also enabled doping of a wide variety of transition metals. Doping of Ni2+ (at 1.2 at%) improved the OER activity and shifted the threshold potential in the negative direction by 100 mV in an alkaline electrolyte, which was comparable to that of conventional IrOx colloidal nanoparticles.

Effect of Ti compositions for efficiency enhancement of CaTiO3:Er3+,Ni2+ broadband-sensitive upconverters

Improving the efficiency of upconversion (UC) materials is a hot topic in recent days due to the important applications of UC materials in photovoltaics, photonics devices, photocatalysts, sensors, biological imaging, and therapeutics. Recently, we have reported a broadband-sensitive UC emission in Ni2+, Er3+-codoped perovskites. However, the applications of these perovskites are limited due to their low conversion efficiency. Herein, we realized highly improved UC efficiency in the CaTiO3:Er3+,Ni2+ upconverter as compared to those of the previously reported CaZrO3 and La(Ga,Sc)O3 upconverters. Ti composition plays important roles in stabilizing divalent nickel (Ni2+) in an octahedral coordination, which is the key point for sensitization to Er3+ emitters. Furthermore, oxygen vacancies and consequently tetrahedral Ni2+ ions, which kill the luminescence, are suppressed, and as a result, the UC emission intensity is dramatically increased. The 0.1 mole Ti-deficient sample with the (Ca0.8Er0.10Li0.10)(Ti0.894Ni0.002Nb0.004)O2.8 composition exhibited the most intense broadband-sensitive UC emission, which was 264-fold stronger than that of the stoichiometric sample and more than 12 folds as compared to that of the previously reported CaZrO3:Er,Ni and La(Ga,Sc)O3 upconverters. The highest UC quantum yield of ∼2.53% was realized in the optimized CaTiO3:Er3+,Ni2+ upconverter under 1490 nm laser excitation of ∼1000 W m−2.