Analytical Observation of Cathodic Zinc Deposition in High-Capacity Zinc Oxide Electrodes for Rechargeable Zinc-based Batteries: Influence of the Current Rate in the First Charging

Abstract

The effects of the current rate used during the first charging (pre-charging: so-called “formation”) on the cathodic deposition of metallic zinc (Zn) were analyzed for the high capacity (thick) zinc oxide (ZnO) electrode in rechargeable Zn-based batteries. Pre-charging at a lower current rate (1.875 mA cm⁻²) enabled greater electrode performances for the subsequent charge-discharge cycles. The Zn deposition profiles were investigated by conventional postmortem X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy using a scanning electron microscope, as well as in situ synchrotron XRD and ex situ synchrotron X-ray computed tomography. The results revealed significant differences in the deposition profiles of the metallic Zn depending on the current rates used during pre-charging. The higher rate (18.75 mA cm⁻²) resulted in an inhomogeneous deposition of Zn, whereas the lower rate yielded finer Zn particles dispersed homogeneously throughout the thick ZnO electrode. These morphological and spatial variations in the Zn deposition during pre-charging affected the subsequent cycling behavior of the thick ZnO electrode.

1. Introduction

Batteries using zinc (Zn)-based negative electrodes with aqueous alkaline electrolyte solutions are considered to be inexpensive and safe for energy storage systems.¹ In particular, Zn-air batteries that exploit oxygen in the air as a positive electrode active material have a relatively high energy density among the systems using aqueous electrolytes. Thus, small-sized primary batteries have been used in practical applications. The development of rechargeable Zn-air batteries has received attention for many years, but the poor cyclability of the Zn negative electrode hinders its commercialization.² To fabricate a useful Zn-air battery with a sufficient energy density, thicker Zn-based composite electrodes are necessary, which can support high-capacity current loading. These electrodes are generally composed of powdered active materials (e.g., Zn/ZnO), electron-conductive carbon (e.g., carbon black and carbon nano-tubes), and a polymeric binder (e.g., poly(tetrafluoro-ethylene)), in addition to auxiliary additive reagents that ensure the electron conduction and ionic conduction pathways in the electrode.^3–5 Rechargeable Zn-air batteries typically require thick composite electrodes with a high-capacity loading of 100 mAh cm⁻² or higher, which results in an electrode thickness of several mm. However, it is generally difficult for such bulky composite electrodes to homogeneously maintain the electrochemical reaction in the depth direction. For example, shape-change phenomena and inactivation of the active material frequently occur in the thick Zn/ZnO electrodes after repeated charge/discharge cycles.⁶ These phenomena decrease the utilization of the active material and Coulombic efficiency of the electrodes, which prevent their practical application.¹

The overall electrode reaction of Zn/ZnO couple in an alkaline electrolyte solution is expressed by Eq. 1,

  

\begin{equation} \text{ZnO} + \text{H$_{2}$O} + \text{2e} = \text{Zn} + \text{2OH$^{-}$} \end{equation} (1)

which proceeds through a dissolution/deposition mechanism via dissolved Zn species, shown in Eqs. 1a and 1b,⁷

  

\begin{equation} \text{ZnO} + \text{2OH$^{-}$} + \text{H$_{2}$O} = \text{Zn(OH)$_{4}{}^{2-}$} \end{equation} (1a)

  

\begin{equation} \text{Zn(OH)$_{4}{}^{2-}$} + \text{2e} = \text{Zn} + \text{4OH$^{-}$} \end{equation} (1b)

where, Zn(OH)₄²⁻ is represented by the dissolved zinc species and is known to exist as several types of water-hydroxyl complexes, [Zn(OH)_n(H₂O)_4−n]^m− (n = 0 to 4, m = n − 2), depending on the concentration of the zinc species and the alkaline concentration (pH) of the electrolyte.^8,9 To perform reversible charging and discharging, these reactions must proceed uniformly in thick high capacity electrodes. To control the Zn species in the solution, several types of electrolyte additives have been used to improve the charge/discharge performances of the Zn/ZnO electrode.^4,6,10–16 Moreover, the addition of organic and inorganic compounds in the electrode is an effective way to enhance the reversibility of the cycles.^{1,3,6,7,13,15} Geometric design and surface modification of Zn/ZnO have also been reported as effective ways to mitigate the shape change of the electrode during the charge/discharge cycling.^{3,6,7,13,17–24}

Despite the numerous attempts, as mentioned above, the improvements in battery performances remain limited, and a definitive method to produce practical Zn/ZnO electrodes has not been established.^1,2,13 In the most cases, the mechanisms of performance improvements have also not been clarified. Thus, advanced analyses of the electrochemical processes that occur during the charge/discharge cycles become necessary to elucidate what happens in the bulk of thick electrodes. In addition to conventional postmortem analyses, computer simulations and modeling have applied to the analyses of the electrochemical processes during operation.²⁵

Recently, in situ techniques have been developed to analyze chemical and structural changes in the bulk Zn/ZnO electrode during the charge/discharge cycles, as well as morphology changes during cathodic Zn deposition. For example, in situ X-ray diffraction (XRD) has been adopted to investigate the effect of Bi₂O₃ addition on the cycling behavior of a thick ZnO electrode in an alkaline solution.²⁶ As an in situ technique, soft X-ray microscopy can be used to monitor the electrochemical deposition/dissolution of Zn in an eutectic solvent electrolyte.²⁷ More recently, Sasaki and co-workers directly observed morphological changes during electrochemical Zn deposition/dissolution by employing in situ transmission electron microscopy.^28,29 We have successfully visualized the cathodic behavior of dendritic Zn deposition accompanied by a redistribution of Zn and hydrogen evolution by in situ X-ray fluorescence imaging.³⁰ Additionally, the combination of in situ XRD and X-ray computational tomography (X-CT) was found to be a particularly effective way to simultaneously monitor the changes in the chemistry and morphology of Zn/ZnO electrodes during the charge/discharge cycles.³¹ We have also demonstrated the local compositional changes in a thick Zn/ZnO electrode by in situ synchrotron XRD mapping.³²

In the present work, we adopted synchrotron in situ XRD and ex situ X-ray-CT (X-CT) technique, combined with postmortem energy-dispersive X-ray spectroscopy using a scanning electron microscope (SEM/EDX), to clarify the effect of the initial charging rate (current density) on the cathodic deposition of Zn in the composite electrode. It has previously been reported that anodic passivation and morphological changes of Zn electrodes in alkaline electrolytes depend much on the discharging rate.^1,33–37 We have also observed that the initial charging rate determines the subsequent cycle performances of Zn/ZnO electrode.³⁸ However, it was necessary to directly observe the chemical and structural changes of the bulk materials in the electrode during the first charging process. Thus, this report provides useful insight for the further development of thick Zn/ZnO electrodes for rechargeable Zn-air batteries.

2. Experimental

2.1 The test cell and electrochemical measurements

The test cell was a three-electrode type consisting of a ZnO-based working electrode and a nickel hydroxide (Ni(OH)₂)-based counter electrode with excess capacity, by which the electrochemical behavior of the Zn electrode can be monitored.³² The working electrode was prepared from ZnO powder mixed with 5.0 wt% of carbon black as a conductive support, 2.0 wt% of PbO as an additive, and 1.5 wt% of poly(tetrafluoroethylene) as a polymer binder. The mixture was loaded on a Cu mesh current collector to form a sheet. The size of ZnO working electrode was 20 mm × 20 mm, and the loaded capacity was 300 mAh. The counter electrode consisted of a paste-type Ni(OH)₂ sheet (20 mm × 45 mm) with a loaded capacity of 400 mAh, where the reason for the use of an Ni(OH)₂ sheet instead of an air electrode is to avoid any influences induced from less rechargeability of the home-made air-electrode. These two sheet electrodes were arranged to oppose each other using separator sheets (polypropylene non-woven sheet and graft membrane) in a rectangular cell-case made of acrylic resin. The electrolyte was ZnO-saturated 8 mol dm⁻³ (M) KOH aqueous solution. An Hg/HgO electrode was used as a reference electrode to monitor the potential of the working electrode.

Prior to cycling under a constant-current, the test cell was pre-charged galvanostatically (so-called “formation” treatment in the battery technology) at 1.875 or 18.75 mA cm⁻² from 0 to 50 % of the state of charge (SOC, with respect to ZnO), followed by discharging at the same current density to 0 % of SOC. After this pre-cycling, the cell was cycled at 37.5 mA cm⁻² under 0–50 % SOC.

2.2 In situ X-ray diffraction (XRD)

Cathodic deposition of Zn in the thick ZnO electrode was examined by in situ (operando) XRD using synchrotron radiation at beamline BL28XU, SPring-8.³² In this measurement, the same composition and size described above were adopted to test the ZnO electrode in the Ni-Zn cell. The test cell was set in a multi-axis diffractometer (Huber) to keep the synchrotron radiation beam perpendicular to the ZnO electrode surface. The radiation beam was 38 keV irradiation energy, and focused to a spot size of 0.5 mm (H) × 0.5 mm (W), and an area of 24 mm × 23 mm was scanned, which only included Zn electrode surface. The X-ray beam was scanned with 1.0 mm pitch, and the irradiation (exposure) time was 1 s. A CdTe two-dimensional detector (PIATUS100K or 300K) was used.

2.3 X-ray computed tomography (X-CT)

We also applied an X-CT technique using synchrotron radiation to monitor the changes in the shape and size of Zn and ZnO in the electrode at the micrometer level.^31,39 In this measurement, the Zn/ZnO sample was prepared by electrochemical charging of the Ni-Zn cell consisting of the same components described above. After the cathodic charge under different conditions, cylindrical samples of 0.6 mmϕ were taken from the Zn/ZnO electrode. The sampling positions were classified into four groups, P-s, P-c, C-s, and C-c, according to the position in the electrode, where P and C denote periphery and center locations, respectively, and s and c mean surface (electrolyte side) and core (current collector side), respectively. These test pieces were sampled in wet states from the Zn/ZnO electrode taken out of the pre-charged cells, followed by sealed in airtight packages and hand-carried to X-CT apparatus. The samples were fixed in a test cell that was installed on a turntable for the X-CT measurement. The irradiation energy was 37 keV, and the size of beam spot was 0.7 mm (H) and 0.3 mm (W). A scintillator lens-coupled CMOS camera was used for detection, and then CT images of the Zn metal and ZnO were constructed from the differences in the linear absorption coefficients of the signals. This experiment was performed at beamline BL46XU, SPring-8.

The details of other experimental procedures are described in Supporting Information.

3. Results and Discussion

3.1 Charge/discharge characteristics of the ZnO electrode

Typical results showing the influence of the first charging rate on the subsequent cycle performances of the Zn/ZnO electrode are displayed in Fig. 1. Here the Ni-Zn test cells were cycled under the same conditions except for the current density used in the first charging (formation) process. The cell was first charged at 1.875 or 18.75 mA cm⁻² up to 50 % SOC and then cycled at 37.5 mA cm⁻². The cell with the higher initial charging current exhibits a higher overvoltage during the subsequent cycles (Figs. 1a and 1b) and a shorter cycling life (Fig. 1c). Here, the “cycle life” means the cycle number where the discharge capacity (retention) steeply decreases to lower values. It will be determined by the fact that the activity of the loaded ZnO in the electrode becomes lower than 50 %, the SOC value. This study aimed to visualize the Zn deposition behavior in the thick Zn/ZnO electrode during the charging process and determine how the initial charging rate influences the subsequent cycle performance. Thus, we examined the details of the Zn/ZnO electrode composition after charging under different current rates, as discussed in the following sections.

Figure 1.

Influences of the charging rate of pre-cycling on the following cycle performance of Zn/ZnO electrode: Charge/discharge curves at the 1st (a) and 15th (b) cycles, and the capacity retention with the cycle (c). Blue lines and marks: pre-charged at the Lower rate, Red lines and marks: pre-charged at the Standard rate.

3.2 Observation of cross-section of ZnO electrode after the first charging

To examine the evolution of metallic Zn in the ZnO electrode during the first charge, following microscopic analyses were conducted using the cross-section of the ZnO electrode after the first charge. Results of SEM-EDX and electron backscatter diffraction (EBSD) mapping for the sample charged up to 50 % SOC are shown in Figs. 2 and 3, respectively. The charging rate was either 1.875 mA cm⁻² (referred to here as the Lower rate) or 18.75 mA cm⁻² (referred to here as the Standard rate). In the EDX mapping images, several bright spots are observed for Zn species that are several tens of µm in size (Figs. 2b and 2b′), which correspond to dark spots in the O mapping (Figs. 2c and 2c′). This result indicates that the metallic Zn was deposited with homogeneous dispersion in the ZnO electrode. With respect to the first charging rate, metallic Zn particles sized between several and several tens µm are dispersed uniformly on the whole electrode pre-charged at the Lower rate (Figs. 2a–2c), whereas the Zn particles are somewhat segregated in the surface region of the electrode pre-charged at the Standard rate (Figs. 2a′–2c′).

Figure 2.

SEM-EDX images for the cross-section of ZnO electrode charged at the Lower rate (1.875 mA cm⁻², a–c) and at the Standard rate (18.75 mA cm⁻², a′–c′) to 50 % SOC. SEM images: (a) and (a′), EDX mapping for Zn: (b) and (b′), EDX mapping for O: (c) and (c′).

Figure 3.

SEM images and EBSD mapping for cross-section of ZnO electrode pre-charged at the Lower rate (1.875 mA cm⁻², a–e) and at the Standard rate (18.75 mA cm⁻², a′–e′) to 50 % SOC. SEM images: (a) and (a′), Image quality map: (b) and (b′), IPF Maps in Normal: (c) and (c′), Reference direction: (d) and (d′), Transverse direction: (e) and (e′).

The EBSD results for the metallic Zn in the SEM image are shown in Fig. 3. The inverse pole figures (IPF) maps suggest that the deposited Zn particles consist of polycrystals without specific orientation. We did not observe significant differences between the samples pre-charged at the Lower and Standard rates. Thus, the pre-charging rate does not affect the orientation of the deposited Zn crystal. In general, lattice defects and residual stresses, which influences the degree of dendric deposition of Zn metal, are considered to determine the crystal orientation.⁴⁰ The present EBSD observation suggests no significant differences in these parameters for Zn metal deposition regardless of the pre-charging current rate.

The local composition of the electrode cross-section was investigated by XRD experiments. Typical XRD patterns for the ZnO electrodes pre-charged at different current rates are shown in Fig. 4. Besides the sharp diffraction peaks ascribed to the ZnO component, with hexagonal wurtzite structure, several peaks attributed to metallic Zn are observed for both the Lower and Standard rate samples. The XRD patterns also reveal that the Zn crystals have no preferred orientation, regardless of the charging rates, which is consistent with the EBSD observation results. The numeric data for Zn(201) diffraction obtained from samples at different positions (surface, core, and current-collector sides) with different charging conditions and are summarized in Table 1 and Table S1 (Supporting Information). There were no significant differences in the lattice parameters (lattice spacing, d, and lattice constants, a and c) among the samples. The lattice constants, a = 2.665 and c = 4.950 ± 1, indicate the deposited Zn forms crystalline metallic Zn phases, regardless of the charging rate. However, the FWHM (Full Width at Half Maximum) values seemed to be little bit higher at the core and current-collector sides than at the surface side, especially for the samples charged at the Standard rate. These results suggest that the size of the metallic Zn crystallite was smaller at the core and current-collector sides than at the surface side, especially for the samples charged at the Standard rate. That is, the higher pre-charging current ends to inhibit the growth of the crystallinity of Zn metal, which seems to be consistent with general observation in the electrochemical deposition of transition metals from aqueous systems.

Figure 4.

Typical XRD patterns for ZnO electrode (surface region) charged at the Lower rate (a) and at the Standard rate (b) (SOC 50 %). “Cu (CC)” indicate diffractions based on the copper current collector.

Table 1. Summary of XRD results for Zn/ZnO electrode charged under different conditions.

Conditions	Position	2θ/deg	d/10−1 nm	Peak height/ count	Peak area/ counts	FWHM /deg	Integrated FWHM/deg
Lower rate SOC 50 %	Surface side	54.319	1.6875	629	6121	0.076	0.097
	Core	54.334	1.6870	725	7240	0.078	0.100
	CC side	54.329	1.6872	665	7406	0.086	0.111
Standard rate SOC 50 %	Surface side	54.333	1.6871	935	9799	0.081	0.105
	Core	54.317	1.6875	1054	14436	0.106	0.137
	CC side	54.326	1.6873	872	12258	0.109	0.141
Standard rate SOC 25 %	Surface side	54.322	1.6874	367	3331	0.071	0.091
	Core	54.326	1.6873	495	6392	0.101	0.129
	CC side	54.327	1.6872	362	4923	0.106	0.136

3.3 Observation of Zn dispersion by in situ synchrotron radiation techniques

In situ XRD measurements using synchrotron radiation were conducted to investigate the Zn deposition in a thick ZnO electrode during the first charge. The appearance of the set-up and the measuring area of the sample are shown in Fig. S1. A typical diffraction pattern for the ZnO electrode during pre-charging is shown in Fig. 5, along with the detected Debye rings. The observed diffraction patterns and Debye rings reveal that the Zn metal consists of polycrystals without a preferred crystal orientation, which is consistent with the EBSD analyses. The integrated peak intensity of Zn(102) is visualized as a two-dimensional map in Fig. 6, as a function of SOC. Here, to minimize the difference in SOC between the start and end points during the beam scanning, the measurement was limited to half of the surface of the sample charged at the Standard rate (Fig. 6b). The Zn metal deposited at the edge parts of the electrode in the early stage of charging and then spread to the central region, resulting in the inhomogeneous deposition. For the sample charged at the Lower rate, the Zn(102) diffraction mapping shows a relatively low integrated intensity as low the Zn metal density but no local deposition with the increasing of SOC (Fig. 6a).

Figure 5.

Typical XRD pattern and Debye ring obtained for ZnO electrode during the cathodic charging (pre-charging).

Figure 6.

Two-dimensional mapping of Zn(102) diffraction for the ZnO electrodes pre-charged at the Lower rate (C/80, a) and at the Standard rate (C/8, b).

As a higher spatial resolution technique, X-CT using synchrotron radiation was employed to analyze the effects of the first charge on a thick ZnO electrode. The samples were removed from ZnO electrodes with different charging conditions and set on the X-CT cell (Fig. 7). The samples were labeled as P-s, P-c, C-s, and C-c according to their position in the electrode, where P and C denote periphery and center parts, and s and c represent surface (electrolyte/separator side) and current-collector side, respectively. Typical X-CT images are shown in Fig. 8. The metallic Zn with a high absorption coefficient tends to aggregate at the surface of the electrode. The CT images also indicate that the Zn deposition proceeds preferentially at the edges of the electrode, which is consistent with the synchrotron XRD results. For the samples taken from the edge (P-s and P-c), differences in the Zn crystal dispersity are observed according to the charging conditions. Multiple fine voids are detected around the metallic Zn crystal, resulting in the formation of low density-Zn phase, for the samples charged at the Lower rate. In contrast, for the samples charged at the Standard rate, the Zn phase is denser, without voids. These observations are also consistent with the synchrotron XRD results, where the diffraction intensity for the metallic Zn is lower in the electrode charged at the Lower rate than in the sample charge at the Standard rate, despite the same SOC. The influences of the depth are that the segregation of metallic Zn was more significant at the surface layers than in the current-collector side (P-s > P-c) for the edge of the samples. Even for the current-collector side, significant differences in the crystal Zn dispersion were observed for the different pre-charging conditions.

Figure 7.

Scheme of sampling the test pieces for X-ray micro-CT measurements.

Figure 8.

Typical X-ray micro-CT images for the pre-charged ZnO: Charged at the Lower rate (C/80: a) and at the Standard rate (C/8: b).

The overall discussion is schematically summarized in Fig. 9. The Lower rate charging leads that a lower density of the active material is widely distributed, so a pore structure will be fine. The Standard rate charging leads that current is concentrated at the edge side and the surface of the electrode, which results in the denser structure of the Zn metal deposition.⁴¹ Consequently, the changes in the pre-charging rate affect the reaction active area with electrolyte by altering the deposition behavior of Zn crystals, especially in the edge of the composite electrode. The Zn metal deposition structure that formed at the Lower rate may positively contribute to the rechargeability of the Zn/ZnO composite electrode. These results will be effectively expanded not only to Zn-air battery technology but also to the developments of other Zn-based systems using alkaline electrolytes, such as an Ni-Zn battery.

Figure 9.

Schematic illustration for the Zn metal deposition in the ZnO electrode under different charging rates: Charged at the Lower rate (C/80: a) and at the Standard rate (C/8: b).

4. Conclusions

The effect of the charging rate on the microstructure of the thick ZnO composite electrode during the first charge (pre-charging: “formation” treatment) was studied using synchrotron radiation. The experimental results demonstrate that the pre-charging rate affects the Zn electrodeposition, and hence, the electrode density. When pre-charging is performed at the Standard rate, fine Zn particles are deposited preferentially at the edge of the electrode and form a dense structure. Conversely, when the pre-charging rate is lowered, finer Zn particles are dispersed and deposited throughout the whole electrode, but a fine structure is observed at the edges. These differences affect the cycle characteristics of the electrode after the pre-charging.

Acknowledgments

This work was based on results obtained from a project, JPNP16001 (RISING2), commissioned by the New Energy and Industrial Technology Development Organization (NEDO). The synchrotron X-ray radiation experiments at SPring-8 were performed with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2016B7609, 2017A7609, 2017B1785, 2017B7609, 2018A1581). We thank Liwen Bianji (Edanz) (www.liwenbianji.cn) for editing the English text of a draft of this manuscript.

Data Availability Statement

The data that support the findings of this study are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.25530733.

• 1) The effects of the current rate used during the first charging (pre-charging: so-called “formation”) on the cathodic deposition of metallic zinc (Zn) were analyzed for the high capacity (thick) zinc oxide (ZnO) electrode in rechargeable Zn-based batteries. Pre-charging at a lower current rate (1.875 mA cm⁻²) enabled greater electrode performances for the subsequent charge-discharge cycles. The Zn deposition profiles were investigated by conventional postmortem X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy using a scanning electron microscope, as well as in situ synchrotron XRD and ex situ synchrotron X-ray computed tomography. The results revealed significant differences in the deposition profiles of the metallic Zn depending on the current rates used during pre-charging. The higher rate (18.75 mA cm⁻²) resulted in an inhomogeneous deposition of Zn, whereas the lower rate yielded finer Zn particles dispersed homogeneously throughout the thick ZnO electrode. These morphological and spatial variations in the Zn deposition during pre-charging affected the subsequent cycling behavior of the thick ZnO electrode.

CRediT Authorship Contribution Statement

Mitsuhiro Kishimi: Conceptualization (Lead), Data curation (Lead)

Masahito Morita: Data curation (Equal)

Tatsumi Hirano: Data curation (Equal), Formal analysis (Equal)

Hisao Kiuchi: Data curation (Equal), Formal analysis (Equal), Methodology (Lead)

Kentaro Kajiwara: Data curation (Supporting), Methodology (Supporting)

Tomoya Kawaguchi: Data curation (Supporting), Methodology (Supporting)

Akiyoshi Nakata: Investigation (Equal), Methodology (Equal)

Hajime Arai: Conceptualization (Equal), Supervision (Equal)

Eiichiro Matsubara: Funding acquisition (Lead), Supervision (Supporting)

Zempachi Ogumi: Supervision (Lead), Writing – review & editing (Supporting)

Masayuki Morita: Supervision (Equal), Writing – original draft (Lead), Writing – review & editing (Supporting)

Takeshi Abe: Funding acquisition (Equal), Supervision (Equal)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Funding

NEDO: JPNP16001

Footnotes

M. Kishimi: Present address: Research Center, Osaka Soda Co. Ltd., 9 Otakasu-cho, Amagasaki, Hyogo 660-0842, Japan

M. Morita: Present address: Office of Society-Academia Collaboration for Innovation (SACI), Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan

H. Kiuchi: Present address: Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8511, Japan

T. Kawaguchi: Present address: Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

H. Arai: Present address: School of Materials and Chemical Technology, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8502, Japan

M. Kishimi, H. Kiuchi, T. Kawaguchi, A. Nakata, H. Arai, and M. Morita: ECSJ Active Members

Z. Ogumi: ECSJ Honorary Member

T. Abe: ECSJ Fellow

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