Hi. I, Tomoya Kawaguchi, Ph.D. is a materials scientist and assistant professor of Institute for Materials Research, Tohoku University in Japan.
See Career for more detail.
My research interests are developing oxide materials for rechargeable batteries such as lithium-ion batteries and magnesium rechargeable batteries based on a concept of nanodomain microstructure.
See Research for more detail.
Sustainable society and electric vehicles are facilitated by battery technology; improving battery capacity is one of the social urgent issues today. Transition metal oxide materials are currently used for cathodes of lithium-ion batteries and magnesium rechargeable batteries, facing the intrinsic capacity limit determined by valence change of constituent transition metals. To tackle this problem, I study on strain effect yielded by a nano-domain microstructure in the oxide electrodes, which is expected to unlock large capacity delivered by anionic (e.g., oxygen and sulfur) reduction/oxidation (redox) reaction as well as the conventional cationic reaction.
Unveil subtle difference in “colors” of atoms
“Colors” of an atom in an X-ray region tell us a chemical state of the atom, which is one of the most important clues to understand the origin of materials properties. I have studied on resonant X-ray diffraction spectroscopy, which is a so-called coupling of X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS and also called as XAFS). This technique enables to reveal valence states and local structure of an element at each crystallographic site and phase. I developed a fast measurement technique of diffraction anomalous fine structure (DAFS) and a direct analysis using the logarithmic dispersion relation. This technique was applied to a battery material for the first time, which revealed a degradation mechanism of the first charge/discharge cycle of lithium-ion batteries.
Image material surface and inside
An electrochemical reaction proceeds surface of a material while it also drastically changes inside of the material. Understanding both of them are of great importance to design the material. I developed a direct analysis technique for crystal truncation rod (CTR) analysis, which is so sensitive to the surface that even a single layer change on the surface can be detected. I am also working with Bragg coherent diffraction imaging (BCDI) of an electrode material.
@article{Tanimura2021,
title = {(Review) Relaxation Behavior an Heterogeneous Structure of Metallic Glasses},
author = {Hiroshi Tanimura and Tomoki Hayashi and Martin Luckabauer and Tomoya Kawaguchi and Masato Wakeda and Hidemi Kato and Tetsu Ichitsubo},
url = {https://www.jstage.jst.go.jp/article/jsms/70/5/70_374/_pdf/-char/ja},
doi = {https://doi.org/10.2472/jsms.70.374},
year = {2021},
date = {2021-05-01},
journal = {Journal of the Society of Materials, Japan},
volume = {70},
number = {5},
pages = {374-380},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
@article{Rao2021a,
title = {pH- and Cation-Dependent Water Oxidation on Rutile RuO_{2}(110)},
author = {Reshma R. Rao and Botao Huang and Yu Katayama and Jonathan Hwang and Tomoya Kawaguchi and Jaclyn R. Lunger and Jiayu Peng and Yirui Zhang and Asuka Morinaga and Hua Zhou and Hoydoo You and Yang Shao-Horn},
url = {https://pubs.acs.org/doi/10.1021/acs.jpcc.1c00413},
doi = {https://doi.org/10.1021/acs.jpcc.1c00413},
year = {2021},
date = {2021-04-13},
journal = {The Journal of Physical Chemistry C},
volume = {125},
number = {15},
pages = {8195-8207},
abstract = {Noncovalent interactions at electrified interfaces are key to improving activity for the oxygen evolution reaction (OER). Here, we showed that on RuO_{2}(110) in alkaline solutions, OER activity is cation-dependent, being largest in 0.1 M KOH compared to LiOH and NaOH. Using crystal truncation rod analysis, −O is detected on the coordinatively unsaturated site at 1.5 V RHE in 0.1 M KOH, suggesting that the rate-determining step is −O + OH^{–} → -OOH + e^{–}, which is different from that in acid involving the final deprotonation of −OOH. The ordering of interfacial water in base was found to decrease with increasing potential and independent of cations. Using surface-enhanced infrared spectroscopy, the density of isolated water molecules (zero H-bonds) was found to increase, and the density of icelike water molecules (four H-bonds) decreases from Li^{+} to K^{+} at OER potentials. The higher activity of more isolated interfacial OH^{–} ions in the case of K^{+} and the lesser stabilization of −O intermediates by hydration water of K^{+} compared to Na^{+} and Li^{+} can result in higher OER activity for KOH. This work provides molecular details of the interface as a function of potential and electrolyte and enables the design of more active electrochemical interfaces.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Noncovalent interactions at electrified interfaces are key to improving activity for the oxygen evolution reaction (OER). Here, we showed that on RuO2(110) in alkaline solutions, OER activity is cation-dependent, being largest in 0.1 M KOH compared to LiOH and NaOH. Using crystal truncation rod analysis, −O is detected on the coordinatively unsaturated site at 1.5 V RHE in 0.1 M KOH, suggesting that the rate-determining step is −O + OH– → -OOH + e–, which is different from that in acid involving the final deprotonation of −OOH. The ordering of interfacial water in base was found to decrease with increasing potential and independent of cations. Using surface-enhanced infrared spectroscopy, the density of isolated water molecules (zero H-bonds) was found to increase, and the density of icelike water molecules (four H-bonds) decreases from Li+ to K+ at OER potentials. The higher activity of more isolated interfacial OH– ions in the case of K+ and the lesser stabilization of −O intermediates by hydration water of K+ compared to Na+ and Li+ can result in higher OER activity for KOH. This work provides molecular details of the interface as a function of potential and electrolyte and enables the design of more active electrochemical interfaces.
@article{shimokawa2021a,
title = {Structure Design of Long-Life Spinel-Oxide Cathode Materials for Magnesium Rechargeable Batteries},
author = {Kohei Shimokawa and Taruto Atsumi and Norihiko L. Okamoto and Tomoya Kawaguchi and Susumu Imashuku and Kazuaki Wagatsuma and Masanobu Nakayama and Kiyoshi Kanamura and Tetsu Ichitsubo},
url = {https://onlinelibrary.wiley.com/doi/10.1002/adma.202007539},
doi = {https://doi.org/10.1002/adma.202007539},
year = {2021},
date = {2021-01-18},
journal = {Advanced Materials},
volume = {33},
number = {7},
pages = {2007539},
abstract = {Development of metal-anode rechargeable batteries is a challenging issue. Especially, magnesium rechargeable batteries are promising in that Mg metal can be free from dendrite formation upon charging. However, in case of oxide cathode materials, inserted magnesium tends to form MgO-like rocksalt clusters in a parent phase even with another structure, which causes poor cyclability. Here, a design concept of high-performance cathode materials is shown, based on: i) selecting an element to destabilize the rocksalt-type structure and ii) utilizing the defect-spinel-type structure both to avoid the spinel-to-rocksalt reaction and to secure the migration path of Mg cations. This theoretical and experimental work substantiates that a defect-spinel-type ZnMnO_{3} meets the above criteria and shows excellent cycle performance exceeding 100 cycles upon Mg insertion/extraction with high potential (≈2.5 V vs Mg^{2+}/Mg) and capacity (≈100 mAh g^{−1}). Thus, this work would provide a design guideline of cathode materials for various multivalent rechargeable batteries.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Development of metal-anode rechargeable batteries is a challenging issue. Especially, magnesium rechargeable batteries are promising in that Mg metal can be free from dendrite formation upon charging. However, in case of oxide cathode materials, inserted magnesium tends to form MgO-like rocksalt clusters in a parent phase even with another structure, which causes poor cyclability. Here, a design concept of high-performance cathode materials is shown, based on: i) selecting an element to destabilize the rocksalt-type structure and ii) utilizing the defect-spinel-type structure both to avoid the spinel-to-rocksalt reaction and to secure the migration path of Mg cations. This theoretical and experimental work substantiates that a defect-spinel-type ZnMnO3 meets the above criteria and shows excellent cycle performance exceeding 100 cycles upon Mg insertion/extraction with high potential (≈2.5 V vs Mg2+/Mg) and capacity (≈100 mAh g−1). Thus, this work would provide a design guideline of cathode materials for various multivalent rechargeable batteries.
Sungwook Choi, Myungwoo Chung, Dongjin Kim, Sungwon Kim, Kyuseok Yun, Wonsuk Cha, Ross Harder, Tomoya Kawaguchi, Yihua Liu, Andrew Ulvestad, Hoydoo You, Mee Kyung Song, and Hyunjung Kim
@article{Choi2020,
title = {In Situ Strain Evolution on Pt Nanoparticles during Hydrogen Peroxide Decomposition},
author = {Sungwook Choi and Myungwoo Chung and Dongjin Kim and Sungwon Kim and Kyuseok Yun and Wonsuk Cha and Ross Harder and Tomoya Kawaguchi and Yihua Liu and Andrew Ulvestad and Hoydoo You and Mee Kyung Song and and Hyunjung Kim},
url = {https://pubs.acs.org/doi/10.1021/acs.nanolett.0c03005},
doi = {https://doi.org/10.1021/acs.nanolett.0c03005},
year = {2020},
date = {2020-11-28},
journal = {Nano Letters},
abstract = {Fundamental understanding of structural changes during catalytic reactions is crucial to understanding the underlying mechanisms and optimizing efficiencies. Surface energy and related catalytic mechanisms are widely studied. However, the catalyst lattice deformation induced by catalytic processes is not well understood. Here, we study the strain in an individual platinum (Pt) nanoparticle (NP) using Bragg coherent diffraction imaging under in situ oxidation and reduction reactions. When Pt NPs are exposed to H_{2}O_{2}, a typical oxidizer and an intermediate during the oxygen reduction reaction process, alternating overall strain distribution near the surface and inside the NP is observed at the (111) Bragg reflection. In contrast, relatively insignificant changes appear in the (200) reflection. Density functional theory calculations are employed to rationalize the anisotropic lattice strain in terms of induced stress by H_{2}O_{2} adsorption and decomposition on the Pt NP surface. Our study provides deeper insight into the activity–structure relationship in this system.},
keywords = {},
pubstate = {forthcoming},
tppubtype = {article}
}
Fundamental understanding of structural changes during catalytic reactions is crucial to understanding the underlying mechanisms and optimizing efficiencies. Surface energy and related catalytic mechanisms are widely studied. However, the catalyst lattice deformation induced by catalytic processes is not well understood. Here, we study the strain in an individual platinum (Pt) nanoparticle (NP) using Bragg coherent diffraction imaging under in situ oxidation and reduction reactions. When Pt NPs are exposed to H2O2, a typical oxidizer and an intermediate during the oxygen reduction reaction process, alternating overall strain distribution near the surface and inside the NP is observed at the (111) Bragg reflection. In contrast, relatively insignificant changes appear in the (200) reflection. Density functional theory calculations are employed to rationalize the anisotropic lattice strain in terms of induced stress by H2O2 adsorption and decomposition on the Pt NP surface. Our study provides deeper insight into the activity–structure relationship in this system.
@article{Oishi2020,
title = {Disordered Cubic Spinel Structure in the Delithiated Li2MnO3 Revealed by Difference Pair Distribution Function Analysis},
author = {Masatsugu Oishi and Keiji Shimoda and Koji Ohara and Daiki Kabutan and Tomoya Kawaguchi and Yoshiharu Uchimoto},
url = {https://pubs.acs.org/doi/10.1021/acs.jpcc.0c07124},
doi = {https://doi.org/10.1021/acs.jpcc.0c07124},
year = {2020},
date = {2020-10-27},
journal = {Journal of Physical Chemistry C},
volume = {124},
pages = {24081−24089},
abstract = {An archetypical Li-rich layered oxide, Li_{2}MnO_{3}, shows a large initial charge capacity of ∼350 mAh g^{–1} with little oxidation of the constituent Mn ions; yet, the crystal structure of delithiated Li_{2}MnO_{3} is still unclarified because the structural disorder induced by the considerable Li extraction makes the analysis challenging. X-ray pair distribution function (PDF) analysis is a powerful tool to experimentally elucidate the structure of the disordered phase. Here, we conducted a comprehensive analysis with a focus on PDF analysis in combination with X-ray powder diffraction (XRPD), transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAS) to reveal the disordered crystalline structure of the electrochemically delithiated Li_{2}MnO_{3}. The XRPD and TEM analyses clarified the formation of a low-crystallinity phase in the light of the average structure. The XAS and PDF analyses further revealed that the MnO_{6}-based framework was rearranged with maintenance of the MnO_{6} octahedral coordination after the initial charge. The difference pair distribution function (d-PDF) technique was therefore employed to extract the structural information of the low-crystallinity disordered phase. The delithiated phase was found to have a structure similar to that of the cubic spinel, LiMn_{2}O_{4}, rather than that of delithiated LiMn_{2}O_{4} (λ-MnO_{2}). In addition, the middle-range order of the delithiated phase deteriorated after the charge, indicating a decrease of coherent domain size to a single nm order. The composite structure formed after the first charge, therefore, consists of the disordered cubic spinel structure and unreacted Li_{2}MnO_{3}. The formation of the composite structure “activates” the electrode material structurally and eventually induces characteristic large capacity of this material.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
An archetypical Li-rich layered oxide, Li2MnO3, shows a large initial charge capacity of ∼350 mAh g–1 with little oxidation of the constituent Mn ions; yet, the crystal structure of delithiated Li2MnO3 is still unclarified because the structural disorder induced by the considerable Li extraction makes the analysis challenging. X-ray pair distribution function (PDF) analysis is a powerful tool to experimentally elucidate the structure of the disordered phase. Here, we conducted a comprehensive analysis with a focus on PDF analysis in combination with X-ray powder diffraction (XRPD), transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAS) to reveal the disordered crystalline structure of the electrochemically delithiated Li2MnO3. The XRPD and TEM analyses clarified the formation of a low-crystallinity phase in the light of the average structure. The XAS and PDF analyses further revealed that the MnO6-based framework was rearranged with maintenance of the MnO6 octahedral coordination after the initial charge. The difference pair distribution function (d-PDF) technique was therefore employed to extract the structural information of the low-crystallinity disordered phase. The delithiated phase was found to have a structure similar to that of the cubic spinel, LiMn2O4, rather than that of delithiated LiMn2O4 (λ-MnO2). In addition, the middle-range order of the delithiated phase deteriorated after the charge, indicating a decrease of coherent domain size to a single nm order. The composite structure formed after the first charge, therefore, consists of the disordered cubic spinel structure and unreacted Li2MnO3. The formation of the composite structure “activates” the electrode material structurally and eventually induces characteristic large capacity of this material.
@article{Kawaguchi2020c,
title = {In-situ to ex-situ in-plane structure evolution of stern layers on Pt(111) surface: Surface X-ray scattering studies},
author = {Tomoya Kawaguchi and Yihua Liua and Evguenia A.Karapetrova and Vladimir Komanicky and Hoydoo You},
url = {https://authors.elsevier.com/c/1cBNy5bbJ5c-uK
https://www.sciencedirect.com/science/article/pii/S1572665720307220},
doi = {https://doi.org/10.1016/j.jelechem.2020.114495},
year = {2020},
date = {2020-07-24},
journal = {Journal of Electroanalytical Chemistry},
volume = {875},
number = {15},
pages = {114495},
abstract = {In-situ to ex-situ evolution of the Cs+ in-plane structure in electrochemical Stern layers are investigated on Pt(111) surface with surface X-ray scattering studies. The Cs+ single-sublattice (2 × 2) structure of the Stern layer formed in situ in 0.1 M CsF electrolyte [Liu et al., J. Phys. Chem. Lett., 9 (2018) 1265] evolves eventually to the two-sublattice (2 × 2) structure upon emersion from the electrolyte. While the Cs+ layers maintain the (2 × 2) symmetry, the ex situ layer increases the density progressively over several minutes by incorporating Cs+ ions of the electrolyte during the emersion process.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
In-situ to ex-situ evolution of the Cs+ in-plane structure in electrochemical Stern layers are investigated on Pt(111) surface with surface X-ray scattering studies. The Cs+ single-sublattice (2 × 2) structure of the Stern layer formed in situ in 0.1 M CsF electrolyte [Liu et al., J. Phys. Chem. Lett., 9 (2018) 1265] evolves eventually to the two-sublattice (2 × 2) structure upon emersion from the electrolyte. While the Cs+ layers maintain the (2 × 2) symmetry, the ex situ layer increases the density progressively over several minutes by incorporating Cs+ ions of the electrolyte during the emersion process.
@article{kawaguchi2020b,
title = {Stern layers on RuO_{2} (100) and (110) in electrolyte: Surface X-ray scattering studies},
author = {Tomoya Kawaguchi and Reshma R. Rao and Jaclyn R. Lunger and Yihua Liu and Donald Walko and Evguenia A. Karapetrova and Vladimir Komanicky and Yang Shao-Horn and Hoydoo You },
url = {https://authors.elsevier.com/c/1cBNy5bbJ5c-Eo
https://linkinghub.elsevier.com/retrieve/pii/S1572665720304562},
doi = {10.1016/j.jelechem.2020.114228},
year = {2020},
date = {2020-05-20},
journal = {Journal of Electroanalytical Chemistry},
volume = {875},
number = {15},
pages = {114228},
abstract = {Electrochemical Stern layers are observed on the surfaces of RuO_{2} single crystals in 0.1 M CsF electrolyte. The Stern layers formed at the interfaces of RuO_{2} (110) and (100) are compared to the previously reported Stern layer on Pt (111) [Liu et al., J. Phys. Chem. Lett., 9 (2018) 1265].While the Cs^{+} density profiles at the potentials close to hydrogen evolution reactions are similar, the hydration layers intervening the surface and the Cs^{+} layer are significantly denser on RuO_{2} surfaces than that on Pt(111) surface, reflecting the oxygen termination of RuO_{2} surfaces. The overall similarities between Stern layers on ruthenium surfaces and platinum surface suggest the universal presence of Stern layers in all well-defined solid-electrolyte interfaces.
},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Electrochemical Stern layers are observed on the surfaces of RuO2 single crystals in 0.1 M CsF electrolyte. The Stern layers formed at the interfaces of RuO2 (110) and (100) are compared to the previously reported Stern layer on Pt (111) [Liu et al., J. Phys. Chem. Lett., 9 (2018) 1265].While the Cs+ density profiles at the potentials close to hydrogen evolution reactions are similar, the hydration layers intervening the surface and the Cs+ layer are significantly denser on RuO2 surfaces than that on Pt(111) surface, reflecting the oxygen termination of RuO2 surfaces. The overall similarities between Stern layers on ruthenium surfaces and platinum surface suggest the universal presence of Stern layers in all well-defined solid-electrolyte interfaces.
@article{Kimura2020,
title = {Development of a half-cell for X-ray structural analysis of liquid electrolytes in rechargeable batteries},
author = {Koji Kimura and Hisao Kiuchi and Masahito Morita and Tomoya Kawaguchi and Kazuki Yoshii and Hikari Sakaebe and Kouichi Hayashi },
url = {https://aip.scitation.org/doi/10.1063/1.5124797},
doi = {https://doi.org/10.1063/1.5124797},
year = {2020},
date = {2020-03-24},
journal = {Review of Scientific Instruments},
volume = {91},
pages = {033907},
abstract = {A half-cell of the rechargeable Li-ion battery was developed to characterize an electrolyte structure using high energy X-ray total scattering measurements in combination with a two-dimensional X-ray detector. The scattering pattern consisted of strong Bragg peaks from electrodes and diffuse scatterings from sapphire windows, in addition to a weak halo pattern from the electrolyte. By selectively removing the signals of the electrodes and windows using specific numerical procedures, we could successfully extract the structural information of the electrolyte, which was in reasonable agreement with a reference data obtained from the electrolyte in a glass capillary. The present demonstration with a half-cell is expected to shed new light on operand characterization of the electrolyte structure during charging and discharging. },
keywords = {},
pubstate = {published},
tppubtype = {article}
}
A half-cell of the rechargeable Li-ion battery was developed to characterize an electrolyte structure using high energy X-ray total scattering measurements in combination with a two-dimensional X-ray detector. The scattering pattern consisted of strong Bragg peaks from electrodes and diffuse scatterings from sapphire windows, in addition to a weak halo pattern from the electrolyte. By selectively removing the signals of the electrodes and windows using specific numerical procedures, we could successfully extract the structural information of the electrolyte, which was in reasonable agreement with a reference data obtained from the electrolyte in a glass capillary. The present demonstration with a half-cell is expected to shed new light on operand characterization of the electrolyte structure during charging and discharging.
Dong Young Chung, Pietro P. Lopes, Pedro Farinazzo Bergamo Dias Martins, Haiying He, Tomoya Kawaguchi, Peter Zapol, Hoydoo You, Dusan Tripkovic, Dusan Strmcnik, Yisi Zhu, Soenke Seifert, Sungsik Lee, Vojislav R. Stamenkovic, Nenad M. Markovic
@article{Dongyoung2020,
title = {Dynamic stability of active sites in hydr(oxy)oxides for the oxygen evolution reaction},
author = {Dong Young Chung and Pietro P. Lopes and Pedro Farinazzo Bergamo Dias Martins and Haiying He and Tomoya Kawaguchi and Peter Zapol and Hoydoo You and Dusan Tripkovic and Dusan Strmcnik and Yisi Zhu and Soenke Seifert and Sungsik Lee and Vojislav R. Stamenkovic and Nenad M. Markovic},
url = {https://www.nature.com/articles/s41560-020-0576-y},
doi = {https://doi.org/10.1038/s41560-020-0576-y},
year = {2020},
date = {2020-03-16},
journal = {Nature Energy},
volume = {5},
pages = {222-230},
abstract = {The poor activity and stability of electrode materials for the oxygen evolution reaction are the main bottlenecks in the water-splitting reaction for H_{2} production. Here, by studying the activity–stability trends for the oxygen evolution reaction on conductive M^{1}O_{x}H_{y}, Fe–M^{1}O_{x}H_{y} and Fe–M^{1}M^{2}O_{x}H_{y} hydr(oxy)oxide clusters (M^{1} = Ni, Co, Fe; M^{2} = Mn, Co, Cu), we show that balancing the rates of Fe dissolution and redeposition over a MO_{x}H_{y} host establishes dynamically stable Fe active sites. Together with tuning the Fe content of the electrolyte, the strong interaction of Fe with the MO_{x}H_{y} host is the key to controlling the average number of Fe active sites present at the solid/liquid interface. We suggest that the Fe–M adsorption energy can therefore serve as a reaction descriptor that unifies oxygen evolution reaction catalysis on 3d transition-metal hydr(oxy)oxides in alkaline media. Thus, the introduction of dynamically stable active sites extends the design rules for creating active and stable interfaces.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The poor activity and stability of electrode materials for the oxygen evolution reaction are the main bottlenecks in the water-splitting reaction for H2 production. Here, by studying the activity–stability trends for the oxygen evolution reaction on conductive M1OxHy, Fe–M1OxHy and Fe–M1M2OxHy hydr(oxy)oxide clusters (M1 = Ni, Co, Fe; M2 = Mn, Co, Cu), we show that balancing the rates of Fe dissolution and redeposition over a MOxHy host establishes dynamically stable Fe active sites. Together with tuning the Fe content of the electrolyte, the strong interaction of Fe with the MOxHy host is the key to controlling the average number of Fe active sites present at the solid/liquid interface. We suggest that the Fe–M adsorption energy can therefore serve as a reaction descriptor that unifies oxygen evolution reaction catalysis on 3d transition-metal hydr(oxy)oxides in alkaline media. Thus, the introduction of dynamically stable active sites extends the design rules for creating active and stable interfaces.
@article{Kawaguchi2020a,
title = {Direct observation of elastic softening immediately after femtosecond-laser excitation in a phase-change material},
author = {Tomoya Kawaguchi and Kazuya Tokuda and Seiya Okada and Makina Yabashi and Tetsu Ichitsubo and Noboru Yamada and Eiichiro Matsubara},
url = {https://journals.aps.org/prb/abstract/10.1103/PhysRevB.101.060302},
doi = {https://doi.org/10.1103/PhysRevB.101.060302},
year = {2020},
date = {2020-02-24},
journal = {Physical Review B Rapid Communication},
volume = {101},
pages = {060302(R)},
abstract = {The generation and propagation of photoexcited elastic waves in crystalline Ge_{2}Sb_{2}Te_{5} were analyzed by picosecond time-resolved X-ray diffraction using a femtosecond (fs)-laser pump and an X-ray free-electron laser probe technique. The crystalline lattice anisotropically expanded initially in approximately 20 ps after the excitation. This was followed by a periodic oscillation of the lattice strain. The elastic stiffness along the cubic <111> direction had significantly softened during the initial expansion, and the strain magnitude was the largest in the <100> and <110> directions. This indicates that fs-laser excitation creates a shallower interlayer potential between the Te and Ge-Sb layers and eventually leads to softening of the elastic stiffness along the cubic <111> direction. Furthermore, this softened state increases the system’s sensitivity to an external stress field. This residual internal stress in a thin film enhances the selective formation of a particular type of variant during the symmetry change from cubic to rhombohedra. This causes the subsequent anisotropic expansion. These phenomena are quite interesting and align with the ultrafast amorphization of this material.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The generation and propagation of photoexcited elastic waves in crystalline Ge2Sb2Te5 were analyzed by picosecond time-resolved X-ray diffraction using a femtosecond (fs)-laser pump and an X-ray free-electron laser probe technique. The crystalline lattice anisotropically expanded initially in approximately 20 ps after the excitation. This was followed by a periodic oscillation of the lattice strain. The elastic stiffness along the cubic <111> direction had significantly softened during the initial expansion, and the strain magnitude was the largest in the <100> and <110> directions. This indicates that fs-laser excitation creates a shallower interlayer potential between the Te and Ge-Sb layers and eventually leads to softening of the elastic stiffness along the cubic <111> direction. Furthermore, this softened state increases the system’s sensitivity to an external stress field. This residual internal stress in a thin film enhances the selective formation of a particular type of variant during the symmetry change from cubic to rhombohedra. This causes the subsequent anisotropic expansion. These phenomena are quite interesting and align with the ultrafast amorphization of this material.
Tomoya Kawaguchi, Thomas F. Keller, Henning Runge, Luca Gelisio, Christoph Seitz, Young Y. Kim, Evan R. Maxey, Wonsuk Cha, Andrew Ulvestad, Stephan O. Hruszkewycz, Ross Harder, Ivan A. Vartanyants, Andreas Stierle, Hoydoo You
@article{Kawaguchi2019a,
title = {Gas-induced segregation in Pt-Rh alloy nanoparticles observed by in situ Bragg coherent diffraction imaging},
author = {Tomoya Kawaguchi and Thomas F. Keller and Henning Runge and Luca Gelisio and Christoph Seitz and Young Y. Kim and Evan R. Maxey and Wonsuk Cha and Andrew Ulvestad and Stephan O. Hruszkewycz and Ross Harder and Ivan A. Vartanyants and Andreas Stierle and Hoydoo You },
url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.123.246001},
doi = {10.1103/PhysRevLett.123.246001},
year = {2019},
date = {2019-12-13},
journal = {Physical Review Letters},
volume = {123},
pages = {246001},
abstract = {Bimetallic catalysts can undergo segregation or redistribution of the metals driven by oxidizing and reducing environments. Bragg coherent diffraction imaging (BCDI) was used to relate displacement fields to compositional distributions in crystalline Pt-Rh alloy nanoparticles. 3D images of internal composition showed that the radial distribution of compositions reverses partially between the surface shell and the core when gas flow changes between O_{2} and H_{2}. Our observation suggests that the elemental segregation of nanoparticle catalysts should be highly active during heterogeneous catalysis and can be a controlling factor in synthesis of electrocatalysts. In addition, our study exemplifies applications of BCDI for in situ 3D imaging of internal equilibrium compositions in other bimetallic alloy nanoparticles. },
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Bimetallic catalysts can undergo segregation or redistribution of the metals driven by oxidizing and reducing environments. Bragg coherent diffraction imaging (BCDI) was used to relate displacement fields to compositional distributions in crystalline Pt-Rh alloy nanoparticles. 3D images of internal composition showed that the radial distribution of compositions reverses partially between the surface shell and the core when gas flow changes between O2 and H2. Our observation suggests that the elemental segregation of nanoparticle catalysts should be highly active during heterogeneous catalysis and can be a controlling factor in synthesis of electrocatalysts. In addition, our study exemplifies applications of BCDI for in situ 3D imaging of internal equilibrium compositions in other bimetallic alloy nanoparticles.
@article{Kawaguchi2019b,
title = {Study of the Internal Compositions of Binary Alloy Pd-Rh Nanoparticles by Using Bragg Coherent Diffraction Imaging},
author = {Tomoya Kawaguchi and Wonsuk Cha and Vitalii Latyshev and Serhii Vorobiov and Vladimir Komanicky and Hoydoo You},
url = {https://link.springer.com/article/10.3938/jkps.75.528},
doi = {https://doi.org/10.3938/jkps.75.528},
year = {2019},
date = {2019-10-10},
journal = {Journal of the Korean Physical Society},
volume = {75},
number = {7},
pages = {528-533},
abstract = {Bragg coherent diffraction imaging (BCDI), a well-established technique for imaging the internal strain of nanoparticles, was used to image the internal compositional distribution of binary alloys in thermal equilibrium. The images experimentally obtained for Pd-Rh alloy nanoparticles are presented and discussed. A direct correspondence between the lattice strain and the compositional deviation is discussed with the derivation of the BCDI displacement field aided by illustrations. The correspondence suggests that the longitudinal derivative of the displacement field, the strain induced by compositional heterogeneity, can be quantitatively converted to 3D images of the compositional deviation from the particle average by using Vegard’s law. It also suggests that the transverse derivative can be qualitatively associated with the disorder of Bragg planes. The studied Pd-Rh alloy nanoparticle exhibited internal composition heterogeneity; the Rh composition tends to be high at edges and corners between facets and gradually decreases from the surface to the core of the particle.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Bragg coherent diffraction imaging (BCDI), a well-established technique for imaging the internal strain of nanoparticles, was used to image the internal compositional distribution of binary alloys in thermal equilibrium. The images experimentally obtained for Pd-Rh alloy nanoparticles are presented and discussed. A direct correspondence between the lattice strain and the compositional deviation is discussed with the derivation of the BCDI displacement field aided by illustrations. The correspondence suggests that the longitudinal derivative of the displacement field, the strain induced by compositional heterogeneity, can be quantitatively converted to 3D images of the compositional deviation from the particle average by using Vegard’s law. It also suggests that the transverse derivative can be qualitatively associated with the disorder of Bragg planes. The studied Pd-Rh alloy nanoparticle exhibited internal composition heterogeneity; the Rh composition tends to be high at edges and corners between facets and gradually decreases from the surface to the core of the particle.
@article{Murakami2019,
title = {High Anionic Conductive Form of Pb_{x}Sn_{2–x}F_{4}},
author = {Miwa Murakami and Yoshiyuki Morita and Masao Yonemura and Keiji Shimoda and Masahiro Mori and Yukinori Koyama and Tomoya Kawaguchi and Katsutoshi Fukuda and Yoshihisa Ishikawa and Takashi Kamiyama and Yoshiharu Uchimoto and Zempachi Ogumi},
url = {https://pubs.acs.org/doi/10.1021/acs.chemmater.9b02623},
doi = {https://doi.org/10.1021/acs.chemmater.9b02623},
year = {2019},
date = {2019-08-28},
journal = {Chemistry of Materials},
volume = {31},
number = {18},
pages = {7704-7710},
abstract = {A high anionic conductivity of ∼3.5 × 10^{–3} S cm^{–1} at room temperature is achieved for Pb_{x}Sn_{2–x}F_{4} (x = 1.21) obtained by annealing a mechanically milled PbF_{2}/SnF_{2} mixture at 400 °C. The observed synchrotron X-ray diffraction patterns indicate formation of a new tetragonal phase at x = 1.1–1.3. The Rietveld analysis of the neutron diffraction patterns leads to a unique structure consisting of two alternating layers, namely, a double Pb layer and a triple layer, each flanked by a single Sn layer. As the Rietveld analysis does not fully converge, the authors further apply high-resolution solid-state NMR (^{19}F, ^{119}Sn, and ^{207}Pb) to confirm the structure. Further, the ^{19}F-^{207}Pb cross-polarization experiment shows that most F^{–} ions, except for those that lie between the double Pb layers, contribute to its high ionic conductivity. The high conductivity is also attributed to structural flexibility of the triple Pb layers, indicated by temperature-dependent ^{207}Pb NMR spectra.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
A high anionic conductivity of ∼3.5 × 10–3 S cm–1 at room temperature is achieved for PbxSn2–xF4 (x = 1.21) obtained by annealing a mechanically milled PbF2/SnF2 mixture at 400 °C. The observed synchrotron X-ray diffraction patterns indicate formation of a new tetragonal phase at x = 1.1–1.3. The Rietveld analysis of the neutron diffraction patterns leads to a unique structure consisting of two alternating layers, namely, a double Pb layer and a triple layer, each flanked by a single Sn layer. As the Rietveld analysis does not fully converge, the authors further apply high-resolution solid-state NMR (19F, 119Sn, and 207Pb) to confirm the structure. Further, the 19F-207Pb cross-polarization experiment shows that most F– ions, except for those that lie between the double Pb layers, contribute to its high ionic conductivity. The high conductivity is also attributed to structural flexibility of the triple Pb layers, indicated by temperature-dependent 207Pb NMR spectra.
Zhihai H. Zhu, Joerg Strempfer, Reshma R. Rao, Connor A. Occhialini, Jonathan Pelliciari, Yongseong Choi, Tomoya Kawaguchi, Hoydoo You, John F. Mitchell, Yang Shao-Horn, and Riccardo Comin
@article{Zhu2018,
title = {Anomalous Antiferromagnetism in Metallic RuO_{2} Determined by Resonant X-ray Scattering},
author = {Zhihai H. Zhu and Joerg Strempfer and Reshma R. Rao and Connor A. Occhialini and Jonathan Pelliciari and Yongseong Choi and Tomoya Kawaguchi and Hoydoo You and John F. Mitchell and Yang Shao-Horn and and Riccardo Comin},
url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.122.017202},
doi = {https://doi.org/10.1103/PhysRevLett.122.017202},
year = {2019},
date = {2019-01-03},
journal = {Phycal Review Letters},
volume = {122},
pages = {017202},
abstract = {We studied the magnetic ordering of both thin films and bulk crystals of rutile RuO_{2} using resonant X-ray scattering near the Ru L_{2} absorption edge. Combining a polarization analysis and azimuthal-angle dependence of the resonant reflection characteristic of the antiferromagentic ordering, we have established the G-type antiferromagnetism in RuO_{2} with T_{N} > 300 K. In addition to igniting an inquiry into the itinerant antiferromagnetism, the revelation of the endurance even in a nanometer-thick film lays the foundations for potential applications of RuO_{2} in antiferromagnetic spintronics},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We studied the magnetic ordering of both thin films and bulk crystals of rutile RuO2 using resonant X-ray scattering near the Ru L2 absorption edge. Combining a polarization analysis and azimuthal-angle dependence of the resonant reflection characteristic of the antiferromagentic ordering, we have established the G-type antiferromagnetism in RuO2 with TN > 300 K. In addition to igniting an inquiry into the itinerant antiferromagnetism, the revelation of the endurance even in a nanometer-thick film lays the foundations for potential applications of RuO2 in antiferromagnetic spintronics
@article{C9CP04461B,
title = {Electrochemical phase transformation accompanied with Mg extraction and insertion in a spinel MgMn_{2}O_{4} cathode material},
author = {Takuya Hatakeyama and Norihiko L Okamoto and Kohei Shimokawa and Hongyi Li and Aiko Nakao and Yoshiharu Uchimoto and Hiroshi Tanimura and Tomoya Kawaguchi and Tetsu Ichitsubo},
url = {http://dx.doi.org/10.1039/C9CP04461B},
doi = {10.1039/C9CP04461B},
year = {2019},
date = {2019-01-01},
journal = {Phys. Chem. Chem. Phys.},
volume = {21},
pages = {23749-23757},
publisher = {The Royal Society of Chemistry},
abstract = {One of the key challenges when developing magnesium rechargeable batteries (MRB) is to develop Mg-intercalation cathodes exhibiting higher redox potentials with larger specific capacities. Although Mg-transition-metal spinel oxides have been shown to be excellent candidates as MRB cathode materials by utilizing the valence change from trivalent to divalent of transition metals starting from Mg insertion, there is no clear evidence to date that Mg can be indeed extracted from the initial spinel hosts by utilizing the change from trivalent to quadrivalent. In this work, we clearly present various experimental evidences of the electrochemical extraction of Mg from spinel MgMn_{2}O_{4}. The present electrochemical charge, i.e., extraction treatment of Mg, was performed in an ionic liquid at 150 °C to ensure Mg hopping in the spinel host. Our analyses show that Mg can be extracted from Mg_{1−x}Mn_{2}O_{4} up to x = 0.4 and, afterwards, successively be inserted into the Mg-extracted (demagnesiated) host via a two-phase reaction between tetragonal and cubic spinels. Finally, we also discuss the difference in electrochemical features between LiMn_{2}O_{4} and MgMn_{2}O_{4}.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
One of the key challenges when developing magnesium rechargeable batteries (MRB) is to develop Mg-intercalation cathodes exhibiting higher redox potentials with larger specific capacities. Although Mg-transition-metal spinel oxides have been shown to be excellent candidates as MRB cathode materials by utilizing the valence change from trivalent to divalent of transition metals starting from Mg insertion, there is no clear evidence to date that Mg can be indeed extracted from the initial spinel hosts by utilizing the change from trivalent to quadrivalent. In this work, we clearly present various experimental evidences of the electrochemical extraction of Mg from spinel MgMn2O4. The present electrochemical charge, i.e., extraction treatment of Mg, was performed in an ionic liquid at 150 °C to ensure Mg hopping in the spinel host. Our analyses show that Mg can be extracted from Mg1−xMn2O4 up to x = 0.4 and, afterwards, successively be inserted into the Mg-extracted (demagnesiated) host via a two-phase reaction between tetragonal and cubic spinels. Finally, we also discuss the difference in electrochemical features between LiMn2O4 and MgMn2O4.
Vincent Esposito, Laurenz Rettig, Elisabeth M. Bothschafter, Yunpei Deng, Christian Dornes, Lucas Huber, Tim Huber, Gerhard Ingold, Yuichi Inubushi, Tetsuo Katayama, Tomoya Kawaguchi, Henrik Lemke, Kanade Ogawa, Shigeki Owada, Milan Radovic, Mahesh Ramakrishnan, Zoran Ristic, ValerioScagnoli, Yoshikazu Tanaka, Tadashi Togashi, Kensuke Tono, Ivan Usov, Yoav W. Windsor, Makina Yabashi, Steven L. Johnson, Paul Beaud, Urs Staub
@article{Esposito2018,
title = {Dynamics of the Photoinduced Insulator-to-Metal Transition in a Nickelate Film},
author = {Vincent Esposito and Laurenz Rettig and Elisabeth M. Bothschafter and Yunpei Deng and Christian Dornes and Lucas Huber and Tim Huber and Gerhard Ingold and Yuichi Inubushi and Tetsuo Katayama and Tomoya Kawaguchi and Henrik Lemke and Kanade Ogawa and Shigeki Owada and Milan Radovic and Mahesh Ramakrishnan and Zoran Ristic and ValerioScagnoli and Yoshikazu Tanaka and Tadashi Togashi and Kensuke Tono and Ivan Usov and Yoav W. Windsor and Makina Yabashi and Steven L. Johnson and Paul Beaud and Urs Staub},
url = {https://aca.scitation.org/doi/10.1063/1.5063530},
doi = {https://doi.org/10.1063/1.5063530},
year = {2018},
date = {2018-12-28},
journal = {Structural Dynamics},
volume = {5},
pages = {064501},
abstract = {The control of materials properties with light is a promising approach towards the realization of faster and smaller electronic devices. With phases that can be controlled via strain, pressure, chemical composition or dimensionality, nickelates are good candidates for the development of a new generation of high performance and low consumption devices. Here we analyze the photoinduced dynamics in a single crystalline NdNiO_{3} film upon excitation across the electronic gap. Using time-resolved reflectivity and resonant x-ray diffraction, we show that the pump pulse induces an insulator-to-metal transition, accompanied by the melting of the charge order. Finally we compare our results to similar studies in manganites and show that the same model can be used to describe the dynamics in nickelates, hinting towards a unified description of these photoinduced phase transitions.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The control of materials properties with light is a promising approach towards the realization of faster and smaller electronic devices. With phases that can be controlled via strain, pressure, chemical composition or dimensionality, nickelates are good candidates for the development of a new generation of high performance and low consumption devices. Here we analyze the photoinduced dynamics in a single crystalline NdNiO3 film upon excitation across the electronic gap. Using time-resolved reflectivity and resonant x-ray diffraction, we show that the pump pulse induces an insulator-to-metal transition, accompanied by the melting of the charge order. Finally we compare our results to similar studies in manganites and show that the same model can be used to describe the dynamics in nickelates, hinting towards a unified description of these photoinduced phase transitions.
@article{sakuda2018,
title = {A Reversible Rocksalt to Amorphous Phase Transition Involving Anion Redox},
author = {Atsushi Sakuda and Koji Ohara and Tomoya Kawaguchi and Katsutoshi Fukuda and Koji Nakanishi and Hajime Arai and Yoshiharu Uchimoto and Toshiaki Ohta and Eiichiro Matsubara and Zempachi Ogumi and Kentaro Kuratani and Hironori Kobayashi and Masahiro Shikano and Tomonari Takeuchi and Hikari Sakaebe},
url = {http://www.nature.com/articles/s41598-018-33518-4},
doi = {10.1038/s41598-018-33518-4},
isbn = {4159801833518},
year = {2018},
date = {2018-10-10},
journal = {Scientific Reports},
volume = {8},
number = {1},
pages = {15086},
abstract = {The charge-discharge capacity of lithium secondary batteries is dependent on how many lithium ions can be reversibly extracted from (charge) and inserted into (discharge) the electrode active materials. In contrast, large structural changes during charging/discharging are unavoidable for electrode materials with large capacities, and thus there is great demand for developing materials with reversible structures. Herein, we demonstrate a reversible rocksalt to amorphous phase transition involving anion redox in a Li_{2}TiS_{3} electrode active material with NaCl-type structure. We revealed that the lithium extraction during charging involves a change in site of the sulfur atom and the formation of S−S disulfide bonds, leading to a decrease in the crystallinity. Our results show great promise for the development of long-life lithium insertion/extraction materials, because the structural change clarified here is somewhat similar to that of optical phase-change materials used in DVD-RW discs, which exhibit excellent reversibility of the transition between crystalline and amorphous phase.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The charge-discharge capacity of lithium secondary batteries is dependent on how many lithium ions can be reversibly extracted from (charge) and inserted into (discharge) the electrode active materials. In contrast, large structural changes during charging/discharging are unavoidable for electrode materials with large capacities, and thus there is great demand for developing materials with reversible structures. Herein, we demonstrate a reversible rocksalt to amorphous phase transition involving anion redox in a Li2TiS3 electrode active material with NaCl-type structure. We revealed that the lithium extraction during charging involves a change in site of the sulfur atom and the formation of S−S disulfide bonds, leading to a decrease in the crystallinity. Our results show great promise for the development of long-life lithium insertion/extraction materials, because the structural change clarified here is somewhat similar to that of optical phase-change materials used in DVD-RW discs, which exhibit excellent reversibility of the transition between crystalline and amorphous phase.
@article{Komatsu2018,
title = {Site-Selective Analysis of Nickel-Substituted Li-Rich Layered Material: Migration and Role of Transition Metal at Charging and Discharging},
author = {Hideyuki Komatsu and Taketoshi Minato and Toshiyuki Matsunaga and Keiji Shimoda and Tomoya Kawaguchi and Katsutoshi Fukuda and Koji Nakanishi and Hajime Tanida and Shunsuke Kobayashi and Tsukasa Hirayama and Yuichi Ikuhara and Hajime Arai and Yoshio Ukyo and Yoshiharu Uchimoto and Eiichiro Matsubara and Zempachi Ogumi},
url = {http://pubs.acs.org/doi/10.1021/acs.jpcc.8b05539},
doi = {10.1021/acs.jpcc.8b05539},
issn = {1932-7447},
year = {2018},
date = {2018-08-17},
journal = {J. Phys. Chem. C},
volume = {122},
number = {35},
pages = {20099-20107},
abstract = {Li-rich type manganese oxides are one of the most promising cathodes for lithium-ion batteries in recent years; thanks to their high energy density. In these cathodes, partial substitution of manganese by other transition metals such as nickel and cobalt has been proposed and shown to be effective in improving the performance; however, the role of such metals in the battery performance has not been clarified. We examined Ni-substituted Li_{2}MnO_{3} as a model of Li_{2}MeO_{3} solid-solution cathodes to understand the effect of the substituted Ni on the electrode performances by using a combination of resonant X-ray diffraction spectroscopy (RXDS) and operando X-ray absorption spectroscopy. The capacity and cyclability were improved by substituting Ni into the Li_{2}MnO_{3} phase, which suggests its important roles in the cathodes. The change in the oxidation state and transbilayer migration of the transition metals as a function of the operating potential during the first charge–discharge processes were revealed by the site-selective analysis of RXDS. We discuss the influence of the irreversible and reversible migration of Ni and Mn ions on the electrode performance.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Li-rich type manganese oxides are one of the most promising cathodes for lithium-ion batteries in recent years; thanks to their high energy density. In these cathodes, partial substitution of manganese by other transition metals such as nickel and cobalt has been proposed and shown to be effective in improving the performance; however, the role of such metals in the battery performance has not been clarified. We examined Ni-substituted Li2MnO3 as a model of Li2MeO3 solid-solution cathodes to understand the effect of the substituted Ni on the electrode performances by using a combination of resonant X-ray diffraction spectroscopy (RXDS) and operando X-ray absorption spectroscopy. The capacity and cyclability were improved by substituting Ni into the Li2MnO3 phase, which suggests its important roles in the cathodes. The change in the oxidation state and transbilayer migration of the transition metals as a function of the operating potential during the first charge–discharge processes were revealed by the site-selective analysis of RXDS. We discuss the influence of the irreversible and reversible migration of Ni and Mn ions on the electrode performance.
@article{Kawaguchi2018a,
title = {Strain-Induced Stabilization of Charged State in Li-Rich Layered Transition Metal Oxide for Lithium Ion Batteries},
author = {Tomoya Kawaguchi and Masashi Sakaida and Masatsugu Oishi and Tetsu Ichitsubo and Katsutoshi Fukuda and Satoshi Toyoda and Eiichiro Matsubara},
url = {https://pubs.acs.org/articlesonrequest/AOR-C8SjZ5pB5xbnrbGbxeVM
http://pubs.acs.org/doi/10.1021/acs.jpcc.8b03205},
doi = {10.1021/acs.jpcc.8b03205},
issn = {1932-7447},
year = {2018},
date = {2018-08-08},
journal = {J. Phys. Chem. C},
volume = {122},
number = {34},
pages = {19298–19308},
abstract = {Li-rich layered oxide (LLO) is a promising cathode material for lithium-ion batteries because of its large capacity in comparison with conventional layered rock-salt structure materials. In contrast to the conventional materials, it is known that LLO of 3d transition metal has nanodomain microstructure; however, roles of each domain and effects of strain, induced by the microstructure, on electrode properties are still unclear. In this study, the influence of the strain on an electronic structure is studied to elucidate the stabilization mechanism of LLO material Li[Li_{0.2}Ni_{0.2}Mn_{0.6}]O_{2} in the charged state by using resonant X-ray diffraction spectroscopy (RXDS), X-ray diffraction and X-ray absorption spectroscopy (XAS) in combination with \textit{ab initio} calculation. RXDS of a superlattice peak and XAS at Mn and Ni K-edges unveil that this material has a microstructure consisting of Mn-rich and Ni-rich domains, whose structures are similar to Li_{2}MnO_{3} and LiNiO_{2}, respectively. In the Ni-rich domain, trigonal distortion in the NiO_{6} octahedral cluster is induced by an elastic constraint due to the microstructure. Hybridization between oxygen p- and nickel d-orbitals are enhanced by the distortion as revealed both by XAS and \textit{ab initio} calculation, accounting for stabilization of the charged state by alleviating the direct hole formation on oxygen p-orbital that usually destabilizes the charged material.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Li-rich layered oxide (LLO) is a promising cathode material for lithium-ion batteries because of its large capacity in comparison with conventional layered rock-salt structure materials. In contrast to the conventional materials, it is known that LLO of 3d transition metal has nanodomain microstructure; however, roles of each domain and effects of strain, induced by the microstructure, on electrode properties are still unclear. In this study, the influence of the strain on an electronic structure is studied to elucidate the stabilization mechanism of LLO material Li[Li0.2Ni0.2Mn0.6]O2 in the charged state by using resonant X-ray diffraction spectroscopy (RXDS), X-ray diffraction and X-ray absorption spectroscopy (XAS) in combination with ab initio calculation. RXDS of a superlattice peak and XAS at Mn and Ni K-edges unveil that this material has a microstructure consisting of Mn-rich and Ni-rich domains, whose structures are similar to Li2MnO3 and LiNiO2, respectively. In the Ni-rich domain, trigonal distortion in the NiO6 octahedral cluster is induced by an elastic constraint due to the microstructure. Hybridization between oxygen p- and nickel d-orbitals are enhanced by the distortion as revealed both by XAS and ab initio calculation, accounting for stabilization of the charged state by alleviating the direct hole formation on oxygen p-orbital that usually destabilizes the charged material.
@article{Koganei2018b,
title = {Analysis of the Discharge/Charge Mechanism in VS_{4} Positive Electrode Material},
author = {Kazuto Koganei and Atsushi Sakuda and Tomonari Takeuchi and Hikari Sakaebe and Hironori Kobayashi and Hiroyuki Kageyama and Tomoya Kawaguchi and Hisao Kiuchi and Koji Nakanishi and Masashi Yoshimura and Toshiaki Ohta and Toshiharu Fukunaga and Eiichiro Matsubara},
url = {https://linkinghub.elsevier.com/retrieve/pii/S0167273817307567},
doi = {10.1016/j.ssi.2018.05.010},
issn = {01672738},
year = {2018},
date = {2018-05-17},
journal = {Solid State Ionics},
volume = {323},
pages = {32--36},
abstract = {Among transition metal sulfides, VS_{4} is a promising candidate material for the positive electrode in rechargeable Li/metal-sulfide batteries, due to its long, flat plateau at about 2.0 V and its high theoretical capacity (1195 mAh g^{−1}). In this study, we prepared VS_{4} positive electrode material by heating in a sealed tube, and studied local structural changes during charge/discharge cycles via X-ray absorption and scattering spectroscopies, focusing on the reversibility of VS_{4}. The findings reveal that a VS_{4}-like local structure was formed in the first cycle, and that subsequent cycles showed reversible changes.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Among transition metal sulfides, VS4 is a promising candidate material for the positive electrode in rechargeable Li/metal-sulfide batteries, due to its long, flat plateau at about 2.0 V and its high theoretical capacity (1195 mAh g−1). In this study, we prepared VS4 positive electrode material by heating in a sealed tube, and studied local structural changes during charge/discharge cycles via X-ray absorption and scattering spectroscopies, focusing on the reversibility of VS4. The findings reveal that a VS4-like local structure was formed in the first cycle, and that subsequent cycles showed reversible changes.
@article{Kawaguchi2018,
title = {Direct Determination of One-Dimensional Interphase Structures Using Normalized Crystal Truncation Rod Analysis },
author = {Tomoya Kawaguchi and Yihua Liu and Anthony Reiter and Christian Cammarota and Michael S. Pierce and Hoydoo You},
doi = {10.1107/S1600576718004326},
year = {2018},
date = {2018-01-01},
journal = {J. Appl. Crystallogr.},
volume = {51},
pages = {1--6},
publisher = {International Union of Crystallography},
abstract = {A one-dimensional non-iterative direct method was employed for normalized crystal truncation rod analysis. The non-iterative approach, utilizing the Kramers-Kronig relation, avoids the ambiguities due to an improper initial model or incomplete convergence in the conventional iterative methods. The validity and limitations of the present method are demonstrated through both numerical simulations and experiments with Pt(111) in a 0.1 M CsF aqueous solution. The present method is compared with conventional iterative phase-retrieval methods.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
A one-dimensional non-iterative direct method was employed for normalized crystal truncation rod analysis. The non-iterative approach, utilizing the Kramers-Kronig relation, avoids the ambiguities due to an improper initial model or incomplete convergence in the conventional iterative methods. The validity and limitations of the present method are demonstrated through both numerical simulations and experiments with Pt(111) in a 0.1 M CsF aqueous solution. The present method is compared with conventional iterative phase-retrieval methods.
@article{Takeuchi2018,
title = {Structure Analyses of Fe-Substituted Li_{2}S-Based Positive Electrode Materials for Li-S Batteries},
author = {Tomonari Takeuchi and Hiroyuki Kageyama and Noboru Taguchi and Koji Nakanishi and Tomoya Kawaguchi and Koji Ohara and Katsutoshi Fukuda and Atsushi Sakuda and Toshiaki Ohta and Toshiharu Fukunaga and Hikari Sakaebe and Hironori Kobayashi and Eiichiro Matsubara},
url = {http://linkinghub.elsevier.com/retrieve/pii/S0167273817309037},
doi = {10.1016/j.ssi.2018.03.028},
issn = {01672738},
year = {2018},
date = {2018-01-01},
journal = {Solid State Ionics},
volume = {320},
number = {September 2017},
pages = {387--391},
publisher = {Elsevier},
abstract = {The structure of Fe-substituted Li_{2}S-based positive electrode material Li_{8}FeS_{5} was analyzed using high-energy X- ray total scattering measurements. Pair distribution function (PDF) analyses indicated that the mechanically milled Li_{8}FeS_{5} sample could best be described as having an anti-fluorite structure in which Fe ions partially occupy Li sites in the Fm3m unit cell. The electrochemical properties of a cell utilizing Li_{8}FeS_{5} as the positive electrode were also consistent with this structural model.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The structure of Fe-substituted Li2S-based positive electrode material Li8FeS5 was analyzed using high-energy X- ray total scattering measurements. Pair distribution function (PDF) analyses indicated that the mechanically milled Li8FeS5 sample could best be described as having an anti-fluorite structure in which Fe ions partially occupy Li sites in the Fm3m unit cell. The electrochemical properties of a cell utilizing Li8FeS5 as the positive electrode were also consistent with this structural model.
@article{Liu2018,
title = {Layering and Ordering in Electrochemical Double Layers},
author = {Yihua Liu and Tomoya Kawaguchi and Michael S Pierce and Vladimir Komanicky and Hoydoo You},
url = {http://pubs.acs.org/doi/10.1021/acs.jpclett.8b00123},
doi = {10.1021/acs.jpclett.8b00123},
issn = {1948-7185},
year = {2018},
date = {2018-01-01},
journal = {J. Phys. Chem. Lett.},
volume = {9},
pages = {1265--1271},
abstract = {Electrochemical double layers (EDL) form at electrified interfaces. Whereas the Gouy–Chapman model describes moderately charged EDL, the formation of Stern layers was predicted for highly charged EDL. Our results provide structural evidence for a Stern layer of cations at potentials close to hydrogen evolution in alkali fluoride and chloride electrolytes. Layering was observed by X-ray crystal truncation rods and atomic-scale recoil responses of Pt(111) surface layers. Ordering in the layer was confirmed by glancing-incidence in-plane diffraction measurements.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Electrochemical double layers (EDL) form at electrified interfaces. Whereas the Gouy–Chapman model describes moderately charged EDL, the formation of Stern layers was predicted for highly charged EDL. Our results provide structural evidence for a Stern layer of cations at potentials close to hydrogen evolution in alkali fluoride and chloride electrolytes. Layering was observed by X-ray crystal truncation rods and atomic-scale recoil responses of Pt(111) surface layers. Ordering in the layer was confirmed by glancing-incidence in-plane diffraction measurements.
Generally, optics based on the electromagnetics is described with the refractive index, while such index does not barely appear in X-ray diffraction (XRD) from the kinematical approach, presumably because of the different historical background; however, it is of great importance to see their relationship to understand the diffraction anomalous fine structure (DAFS) method. Read More…
The atomic form factor is a Fourier transform of the electron distribution in an atom; therefore, it is a real number and independent of photon energy. In contrast, there exists an absorption edge and a fine structure in an absorption spectrum in the x-ray region. Thus, the absorption term should be included into the scattering length as an imaginary part, which is proportional to the absorption cross-section, by assuming a more elaborated model rather than that of a cloud of free electrons. Read More…
A structure factor is calculated by summing up scattering factors of each atom with multiplying the phases at each atomic position in a unit cell. It is a facile approach to calculate the structure factor; however, the calculation becomes complicated when the unit cell includes the large number of atoms. Thus, the calculation of the structure factor should be carried out based on the crystallographic site in space group, which is more versatile and convenient. Read More…
Resident associate
Materials Science Division, Argonne National Laboratory, U.S.A., 2017-2019 Synchrotron Studies of Materials Group (supervisor: Hoydoo You)
JSPS overseas research fellow
Japan Society for Promotion of Science (JSPS), Japan, 2017-2019
Research assistant professor
Technical Director (Synchrotron Radiation Study Group) of RISING and RISING2 projects
Office of Society Academia Collaboration for Innovation, Kyoto University, Japan, 2015-2017
JSPS research fellow (DC2), JSPS, Japan, 2013 – 2015
Structural analysis of the electrochemical double layer using crystal truncation rod (CTR) analysis. Direct determination of the surface structure from CTR. Bragg coherent diffraction imaging of the internal composition of the alloy catalysis particles, Materials Science Division, Argonne National Laboratory, 2017 – 2019
