Improved Cyclic Performance of Lithium-Ion Batteries: An Investigation

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Improved Cyclic Performance of Lithium-Ion Batteries: An Investigation of Cathode/Electrolyte Interface via In Situ TotalReflection Fluorescence X‑ray Absorption Spectroscopy Kentaro Yamamoto,† Taketoshi Minato,*,‡ Shinichiro Mori,† Daiko Takamatsu,‡ Yuki Orikasa,† Hajime Tanida,‡ Koji Nakanishi,‡ Haruno Murayama,‡ Titus Masese,† Takuya Mori,† Hajime Arai,‡ Yukinori Koyama,‡ Zempachi Ogumi,‡ and Yoshiharu Uchimoto† †

Graduate School of Human and Environmental Studies, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Office of Society-Academia Collaboration for Innovation, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan

‡

ABSTRACT: For the further development of lithium-ion batteries, improvement of their cyclic performance is crucial. However, the mechanism underlying the deterioration of the battery cyclic performance is not fully understood. We investigated the effects of the electronic structure at the electrode/electrolyte interface on the cyclic performance of the cathode materials via in situ total-reflection fluorescence X-ray absorption spectroscopy. In a LiCoO2 thin-film electrode that exhibits gradual deterioration upon subsequent Li ion extractions and insertions (cycling), the reduction of Co ions at the electrode/electrolyte interface was observed upon immersion in an organic electrolyte, with subsequent irreversible changes after cycling. In contrast, in a LiFePO4 thin-film electrode, the electronic structure at the electrode/electrolyte interface was stable and reversible upon electrolyte immersion with subsequent cycling. The increased stability of the electronic structure at the LiFePO4/electrolyte interface affects its cycling performance.

1. INTRODUCTION Lithium-ion batteries (LiBs) are commonly used in portable electronic equipment.1 To extend the use of LiBs to vehicular applications and smart grid systems, further improvements in battery performance are necessary. Improvement in the cyclic performance is crucial to the pivotal aspects for the further development of LiBs. However, to improve the cyclic performance, the deterioration mechanism must be elucidated. In the previously proposed mechanism for the deterioration (decreasing of the capacity with cycling) of LiBs, the electrode/ electrolyte interface is thought to play a significant role. For example, the formation of a solid electrolyte interface,2 changes in the crystal structure3 on the surface of active materials during Li ion extraction/insertion, and improvement in the cyclic performance through surface coating of the active materials4 have all been reported. X-ray absorption spectroscopy (XAS),5 X-ray photoelectron spectroscopy,2 Raman spectroscopy,6 transmission electron microscopy (TEM),7 and atomic force microscopy (AFM)8 have been used to analyze the electrode/ electrolyte interface. However, determining the specific cause of the interface stability is quite difficult because the geometric and electronic structures must be tracked with a resolution of less than a few nanometers to reveal the phenomena at the electrode/electrolyte interface. To further elucidate the deterioration mechanism of LiBs, it is necessary to clarify the electronic structure at the electrode/electrolyte interface under operating conditions. © 2014 American Chemical Society

In this work, we investigated the correlation between the stability of the electronic structure at the electrode/electrolyte interface and the LiB performance using model electrodes. LiCoO2 and LiFePO4 thin-film electrodes were selected for this work. LiCoO2 is a typical cathode material in LiBs and has a capacity that is reported to fade with cycling.9,10 In contrast, LiFePO4 is a cathode material that has a more stable capacity upon cycling.11,12 In situ total-reflection fluorescence X-ray absorption spectroscopy (TRF-XAS) was used to analyze the electronic structure at the electrode/electrolyte interface under battery operation conditions.13 Using direct observations of the electronic structure at the electrode/electrolyte interfaces, we successfully demonstrate a correlation between the stability of the electronic structure at the electrode/electrolyte interface and the cyclic performance of LiBs.

2. EXPERIMENTAL METHODS The model LiCoO2 and LiFePO4 thin-film electrodes were prepared using a pulsed laser deposition technique (PLD). The LiCoO2 thin-films were prepared on mechanically polished Pt polycrystalline substrates at 873 K under O2 (0.01 Pa). The PLD target consisted of LiCoO2 (Sigma-Aldrich Co., LLC) with 15 wt % Li2O (Sigma-Aldrich Co., LLC) to supply Received: January 31, 2014 Revised: March 13, 2014 Published: April 25, 2014 9538

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in a 1:1 volumetric mixture of ethylene carbonate/diethyl carbonate or propylene carbonate were used as the electrolyte. To exclude the influence of unstable phases14 in the LiFePO4 thin-film electrode on the electrochemical behavior, the in situ TRF-XAS of LiFePO4 thin-film electrode during Li ion extraction/insertion were conducted on the thin-films after obtaining one cycle of a cyclic voltammogram. Electrochemical Li ion extraction/insertion was performed by controlling the voltage with a potentiostat (HZ-5000, Hokuto Denko Corp.) at a scan rate of 0.1 mV/s.

sufficient Li during the deposition. The LiFePO4 thin-films were prepared on mechanically polished Au polycrystalline substrates at 773 K under Ar (0.0001 Pa). The LiFePO4 target consisted of LiFePO4 with 3 wt % Li3PO4 (Wako Pure Chemical Industries, Ltd.). The as-prepared LiCoO2 and LiFePO4 thin-films were polycrystalline with thicknesses of 50 and 70 nm and surface roughness values (Ra) of 2.0 and 2.7 nm, respectively, as determined by X-ray diffraction (Figure 1a,d), TEM (Figure 1b,e), and AFM (Figure 1c,f). The distance

3. RESULTS AND DISCUSSION 3.1. Cyclic Performance of Thin-Film Electrodes. Figure 2a,b presents the cyclic voltammograms obtained for the as-

Figure 1. (a,d) XRD patterns (peaks with Miller indices and asterisks are signals emanating from thin-films and substrate, respectively); (b,e) cross-sectional TEM images; (c,f) 1 μm × 1 μm AFM images of LiCoO2/Pt and LiFePO4/Au, respectively. (a) XRD, (b) TEM, and (c) AFM data for LiCoO2 electrode were reproduced using data from ref 13.

Figure 2. (a,b) Cyclic voltammograms; (c,d) charge (filled circles) and discharge (open circles) capacities of LiCoO2 and LiFePO4 thin films, respectively.

between the target and substrate was 40 or 35 mm. A Nd:YAG 4HG laser with a wavelength of 266 nm was used as the light source. The pulse irradiation frequency was fixed at 10 Hz, and the laser power was adjusted to 200 mW. The cyclability of the as-prepared thin-film electrodes was measured via cyclic voltammetry (CV) using a three electrode cell. The in situ TRF-XAS measurements were carried out at beamlines BL01B1, BL28XU, and BL37XU at SPring-8, Japan using a custom two electrode cell purged with ultrapure He (99.99995%).13 CV measurements were also performed by the custom cell to confirm the electrochemical behavior of the thinfilm electrodes during in situ TRF-XAS measurements. The in situ TRF-XAS measurements were conducted via X-ray absorption near-edge structure (XANES) under total-reflection conditions (with an X-ray incident angle of 0.17−0.20°). The estimated penetration depth was ∼3.0 nm from the LiCoO2 or LiFePO4 surface.13 The XANES spectra obtained under totalreflection and nontotal-reflection (2.17−2.20°) conditions are denoted as the surface XANES and bulk XANES spectra, respectively. The energy at half of the normalized intensity is denoted as E0.5 for each XANES spectrum. The obtained TRFXAS spectra were calibrated using the Co K-edge and Fe Kedge of Co (7708.9 eV) and Fe (7112.0 eV) foil, respectively. The counter electrode was Li metal with a microporous membrane (CELGARD3501) as the separator. LiClO4 (1 M)

prepared LiCoO2 and LiFePO4 electrodes. For the LiCoO2 electrodes, the anodic and cathodic current peaks corresponding to the first-order phase transition between the two hexagonal phases of LiCoO2 are observed at 3.92 and 3.90 V, respectively (Figure 2a). The anodic peaks at approximately 4.08 and 4.18 V are ascribed to the phase transition of LiCoO2 between its hexagonal and monoclinic phases.3,15 For the LiFePO4 electrode, the anodic and cathodic peaks originating from two-phase transitions16 of Li1−αFePO4/LiβFePO4 were observed at 3.47 and 3.37 V, respectively (Figure 2b). The voltammograms match the previous results,15,16 confirming that the thin-film electrodes are appropriate models.17 Figure 2c,d shows the charge and discharge capacities for the LiCoO2 and LiFePO4 electrodes based on CV measurements. The charge/discharge capacity of the LiCoO2 electrode decreased gradually with repeated cycling from 0.940 to 0.500 μAh after 20 cycles (Figure 2c). In contrast, the capacity of the LiFePO4 electrode decreased only slightly with repeated cycling, maintaining an almost constant discharge capacity of ∼0.8 μAh by the 20th cycle (Figure 2d). These results indicate that the cyclability of the LiFePO4 electrode is superior to that of the LiCoO2 electrode. 3.2. Electronic Structure at the Electrolyte/Electrode Interface during Electrolyte Immersion. To investigate the electronic structure of the electrode/electrolyte interface during battery operation, in situ TRF-XAS measurements of the 9539

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LiCoO2 and LiFePO4 electrodes were made. Figure 3a presents the Co K-edge surface XANES spectra of the LiCoO2 electrode

Figure 4. Co K-edge (a,b) surface and (c,d) bulk XANES spectra of the LiCoO2 electrode during Li ion (a,c) extraction and (b,d) insertion. Some spectra were reproduced using data from ref 13. (R3.8 V is the voltage applied during Li-ion insertion.)

Figure 3. Co K-edge (a) surface and (c) bulk XANES spectra of LiCoO2 and Fe K-edge (b) surface and (d) bulk XANES spectra of LiFePO4 before (black line) and after (red line) electrolyte immersion, obtained via in situ TRF-XAS. (Some spectra for the LiCoO2 electrode were reproduced using the data from ref 13.)

insertion. The E0.5 values of the Co K-edge surface XANES spectra during Li ion extraction/insertion are plotted in Figure 6a (red circles). Because of Li ion extraction at 4.2 V, the E0.5 value shifted from 7716.1 to 7718.1 eV (from the black curve to the red curve in Figure 4a); the shift is attributed to the oxidation of Co ions.13,20,25 However, after Li ion insertion at 3.8 V, E0.5 shifted slightly to 7717.9 eV (from the red curve to the black curve in Figure 4b) and did not revert to its original value (7716.1 eV), indicating that the Co ions at the surface were not reduced to their original valence states during the Li ion extraction/insertion cycle. In contrast to the surface spectra, the bulk XANES spectra exhibited reversible behavior during Li ion extraction/insertion processes (Figure 4c,d), which indicates that the Co ions at the surface fail to reduce to their original valence states during cycling. This irreversible behavior of the electronic structure at the LiCoO2 surface is related to the decrease in capacity with repeated cycling (Figure 2c). The electronic structure of the LiFePO4/electrolyte interface exhibited distinct features during Li ion extraction/insertion. Figure 5 presents the Fe K-edge surface and bulk XANES spectra of the LiFePO4/electrolyte interface during Li ion extraction/insertion. The E0.5 values for the Fe K-edge surface and bulk XANES spectra during Li ion extraction/insertion are plotted in Figure 6b. Because of Li ion extraction at 4.0 V (from the black curve to the red curve in Figure 5a), E0.5 shifted by 2.6 eV (from 7116.6 to 7119.2 eV), which can be explained by the screening effects on the Fe 1s electrons, which are excited to unoccupied states, leading to stronger bonding with the Fe ions.24 The increased energy indicates that the Fe ions at the LiFePO4/electrolyte interface were oxidized.24 Insertion of Li ions at 3.0 V caused the Fe K-edge surface XANES spectrum of the LiFePO4/electrolyte interface to shift toward a lower energy (from the red curve to the black curve in Figure 5b). E0.5 shifted by 2.7 eV (from 7119.2 to 7116.5 eV); and the shift can be attributed to reduction of the Fe ions at the LiFePO4/ electrolyte interface. The small difference between E0.5 before Li ion extraction and after Li ion insertion (0.1 eV) clearly demonstrates that the change in the electronic structure of the

before and after electrolyte immersion. The observed edge feature is attributed to a transition from the Co 1s core orbital to the Co 4p state.20,21 After electrolyte immersion, the XANES spectrum shifted toward a lower energy (from the black line to the red line in Figure 3a). The E0.5 value of the surface XANES spectrum shifted 1.0 eV, i.e., from 7717.1 to 7716.1 eV, upon electrolyte immersion. This change in the Co K-edge surface XANES spectrum is attributed to reduction of the Co ions (from Co3+ to Co2+ with increasing concentration of antisite Co ions at Li sites) coupled with the formation of reaction layers.13,22,23 The LiCoO2 surface is therefore unstable under electrolyte immersion. In contrast, the Co K-edge of the bulk XANES spectrum did not exhibit any significant changes after electrolyte immersion (Figure 3c). These results indicate that only the LiCoO2 surface is unstable upon electrolyte immersion. In contrast, the Fe ions at the LiFePO4 surface were not reduced upon electrolyte immersion. Figure 3b presents the Fe K-edge surface XANES spectra of LiFePO4 before and after electrolyte immersion. The edge feature is attributed to transitions from Fe 1s to Fe 4p and the continuum states.24 A comparison with the Fe K-edge XANES spectra of standard materials revealed a Fe2+ oxidation state at the LiFePO4 surface before electrolyte immersion. After electrolyte immersion, the Fe K-edges of the surface XANES spectrum did not exhibit shifts in the edge or E0.5 (from the black line to the red line in Figure 3b) as well as bulk (from the black line to the red line in Figure 3d), which indicates that the LiFePO4 electrode surface was unaffected by the electrolyte. The differences observed in the edge shift between LiFePO4 and LiCoO2 demonstrated experimentally that the LiFePO4 surface is more stable than the LiCoO2 surface upon contact with an electrolyte. 3.3. Electronic Structure at the Electrolyte/Electrode Interface during Li Ion Extraction/Insertion Processes. Figure 4 shows the Co K-edge surface and bulk XANES spectra of the LiCoO2/electrolyte interface during Li ion extraction/ 9540

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electrolyte to the electrode. Therefore, Co ions are reduced on the LiCoO2 surface (Figure 7a). The formation of the space

Figure 5. Fe K-edge (a,b) surface and (c,d) bulk XANES spectra of the LiFePO4 electrode during Li ion (a,c) extraction and (b,d) insertion. (R3.0 V is the voltage applied during Li ion insertion.)

Figure 7. Schematic illustrations of the changes in the electronic structure at the (a,c) LiCoO2/electrolyte and (b,d) LiFePO4/ electrolyte interfaces upon electrolyte immersion and during Li ion extraction/insertion. (ΦS and ΦL are the inner potentials of the electrode and electrolyte, respectively. To simplify, we assume that the plane of polycrystalline LiCoO2 and LiFePO4 facing the electrolyte is perpendicular to (001) and parallel to (010), respectively.)

Figure 6. E0.5 values of (a) Co K-edge surface (red) and bulk (white) XANES spectra of the LiCoO2 electrode and (b) Fe K-edge surface (red) and bulk (white) XANES spectra of the LiFePO4 electrode during Li ion extraction and insertion. (R3.8 V and R3.0 V are the applied voltages during the Li ion insertion process. Error bars were calculated using the accuracy of the incident X-ray energy (0.01%).)

charge layer results in irreversible Li ion extraction/insertion, which is the origin of the LiCoO2 electrode deterioration (Figure 7c). During further cycling, the irreversible layers continue to grow from the surface to the bulk, and the discharge capacity fades. In contrast to the situation in LiCoO2, the Fe ions at the LiFePO4 surface are not reduced upon contact with the electrolyte, indicating that the change in the electrochemical potential inside the space charge layer at the LiFePO4 surface is not dramatic. Because LiFePO4 has a wider band gap (∼3.7 eV) than LiCoO2,28,29 the bottom of the conduction band energy level of LiFePO4 would be much higher than that of LiCoO2, so fewer electrons are transferred to the LiFePO4 surface. Therefore, the potential change in the space charge layer in LiFePO4 is much smaller than that in LiCoO2 (Figure 7b). The difference between the electrochemical potentials of the electrode and electrolyte is compensated mainly by the electrical double layer in the electrolyte. Because the potential change in the space charge layer is not dramatic, the LiFePO4 surface is stable under electrolyte immersion. For Li ion extraction from LiFePO4 to FePO4, the band gap is narrowed to ∼1.9 eV.28,29 However, the energy of the bottom of the FePO4 conduction band still exceeds that of LixCoO2 because the redox potential of Co2+/Co3+ is more positive than that of Fe2+/Fe3+ (e.g., the difference between LiCoPO4 and LiFePO4 is 1.3 V).30 This suppresses a large potential change in the space charge layer at the FePO4 surface during Li ion extraction/insertion. The lack of a large potential change in the space charge layer at the LiFePO4 surface is the origin of

Fe ions at the LiFePO4/electrolyte interface is highly reversible. The stability of the capacity with repeated Li ion extraction/ insertion (Figure 2d) was related to the reversible behavior of the electronic structure at the LiFePO4/electrolyte interface. Clearly, the reversible behavior of the LiFePO4 surface (Figures 5a,b and 6b) differs from the behavior of LiCoO2 surface, which is irreversible upon subsequent Li ion extraction/insertion processes (Figures 4a,b and 6a). 3.4. Relationship between the Cyclability and the Electronic Structure at the Electrolyte/Electrode Interface. The observed relationship between the cyclability and the reversible behavior of the electronic structure at the electrolyte/ electrode interface is explained by the electrochemical potential. The electrochemical potential in the electrodes differs from that in the electrolytes. At the electrode/electrolyte interface, the difference between the two electrochemical potentials is compensated. The change in the potential around the interface forms a space charge layer on the electrode side and an electrical double layer on the electrolyte side.26 The reduction of Co ions at the LiCoO2 surface, observed by in situ TRF-XAS (Figures 4a,b and 6a), represents a space charge layer. LiCoO2 is a semiconductor with a relatively small band gap of ∼1.5 eV.27 The energy level of the bottom of the conduction band in LiCoO2 is low enough to allow an electron transfer from the 9541

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(6) Baddour-Hadjean, R.; Pereira-Ramos, J.-P. Raman Microspectrometry Applied to the Study of Electrode Materials for Lithium Batteries. Chem. Rev. 2010, 110, 1278−1319. (7) Yamamoto, K.; Iriyama, Y.; Asaka, T.; Hirayama, T.; Fujita, H.; Fisher, C. A. J.; Nonaka, K.; Sugita, Y.; Ogumi, Z. Dynamic Visualization of the Electric Potential in an All-Solid-State Rechargeable Lithium Battery. Angew. Chem., Int. Ed. 2010, 49, 4414−4417. (8) Doi, T.; Inaba, M.; Tsuchiya, H.; Jeong, S. K.; Iriyama, Y.; Abe, T.; Ogumi, Z. Electrochemical AFM Study of LiMn2O4 Thin Film Electrodes Exposed to Elevated Temperatures. J. Power Sources 2008, 180, 539−545. (9) Oh, S.; Lee, J. K.; Byun, D.; Cho, W. I.; Cho, B. W. Effect of Al2O3 Coating on Electrochemical Performance of LiCoO2 as Cathode Materials for Secondary Lithium Batteries. J. Power Sources 2004, 132, 249−255. (10) Kweon, H.-J.; Park, J. J.; Seo, J. W.; Kim, G. B.; Jung, B. H.; Lim, H. S. Effects of Metal Oxide Coatings on the Thermal Stability and Electrical Performance of LiCoO2 in a Li-ion Cell. J. Power Sources 2004, 126, 156−162. (11) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. Phospho-Olivines as Positive-Electrode Materials for Rechargeable Lithium Batteries. J. Electrochem. Soc. 1997, 144, 1188−1194. (12) Liu, J.; Wang, J.; Yan, X.; Zhang, X.; Yang, G.; Jalbout, A. F.; Wang, R. Long-Term Cyclability of LiFePO4/Carbon Composite Cathode Material for Lithium-Ion Battery Applications. Electrochim. Acta 2009, 54, 5656−5659. (13) Takamatsu, D.; Koyama, Y.; Orikasa, Y.; Mori, S.; Nakatsutsumi, T.; Hirano, T.; Tanida, H.; Arai, H.; Uchimoto, Y.; Ogumi, Z. First In Situ Observation of the LiCoO2 Electrode/Electrolyte Interface by Total-Reflection X-ray Absorption Spectroscopy. Angew. Chem., Int. Ed. 2012, 51, 11597−11601. (14) Lu, Z. G.; Lo, M. F.; Chung, C. Y. Pulse Laser Deposition and Electrochemical Characterization of LiFePO4-C Composite Thin Films. J. Phys. Chem. C 2008, 112, 7069. (15) Iriyama, Y.; Inaba, M.; Abe, T.; Ogumi, Z. Preparation of c-axis Oriented Thin Films of LiCoO2 by Pulsed Laser Deposition and Their Electrochemical Properties. J. Power Sources 2001, 94, 175−182. (16) Takahashi, M.; Tobishima, S.; Takei, K.; Sakurai, Y. Reaction Behavior of LiFePO4 as a Cathode Material for Rechargeable Lithium Batteries. Solid State Ionics 2002, 148, 283−289. (17) The capacity fading of LiFePO4 electrode depends on the experimental condition. In higher temperature, using electrolyte contanining LiPF6 and higher rate Li extraction/insertion, the capacity fading of LiFePO4 electrode with cycling is reported in previous work.18,19 Under our experimental conditions (room temperature, electrolyte prepared from LiClO4 and 0.1 mV/s), the LiFePO4 thinfilm exhibits stable capacity with cycling. (18) Zhang, Y.; Wang, C.-Y.; Tang, X. Cycling Degradation of an Automotive LiFePO4 Lithium-Ion Battery. J. Power Sources 2011, 196, 1513−1520. (19) Amine, K.; Liu, J.; Belharouak, I. High-temperature storage and cycling of C-LiFePO4/graphite Li-ion cells. Electrochem. Commun. 2005, 7, 669−673. (20) Nakai, I.; Takahashi, K.; Shiraishi, Y.; Nakagome, T.; Nishikawa, F. Study of the Jahn−Teller Distortion in LiNiO2, a Cathode Material in a Rechargeable Lithium Battery, by in Situ X-ray Absorption Fine Structure Analysis. J. Solid State Chem. 1998, 140, 145−148. (21) Koyama, Y.; Arai, H.; Ogumi, Z.; Tanaka, I.; Uchimoto, Y. Co K-edge XANES of LiCoO2 and CoO2 with a Variety of Structures by Supercell Density Functional Calculations with a Core Hole. Phys. Rev. B 2012, 85, 075129. (22) Koyama, Y.; Arai, H.; Tanaka, I.; Uchimoto, Y.; Ogumi, Z. Defect Chemistry in Layered LiMO2 (M = Co, Ni, Mn, and Li1/3Mn2/3) by First-Principles Calculations. Chem. Mater. 2012, 24, 3886−3894. (23) Takamatsu, D.; Mori, S.; Orikasa, Y.; Nakatsutsumi, T.; Koyama, Y.; Tanida, H.; Arai, H.; Uchimoto, Y.; Ogumi, Z. Effects of ZrO2 Coating on LiCoO2 Thin-Film Electrode Studied by In Situ X-ray

the stable behavior during electrolyte contact and Li ion extraction/insertion (Figure 7d).

4. CONCLUSIONS The electronic structure at the electrode/electrolyte interface during electrolyte immersion and Li ion extraction/insertion was directly monitored by in situ TRF-XAS. The LiFePO4 surface is stable upon electrolyte immersion, but the LiCoO2 surface undergoes a reduction. Additionally, the changes in the electronic structure at the LiFePO4 surface are highly reversible during the Li ion extraction/insertion, in contrast to those at the LiCoO2 surface. The stability of LiFePO4 is attributed to the lack of a large potential change in the space charge layer at the surface. The marked highly reversible behavior of the electronic structure at the LiFePO4 surface is related to its stable cycling capacity. Our results clearly show that the electronic structure at the electrode/electrolyte interface has an important role in the cyclic performance of LiB.

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AUTHOR INFORMATION

Corresponding Author

*(T.Mi.) E-mail: [email protected]. Phone: +81-774-38-4968. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the “Research and Development Initiative for Scientific Innovation of New Generation Battery (RISING project)” of the New Energy and Industrial Technology Development Organization (NEDO), Japan. The synchrotron radiation experiments were performed with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2009B1032, 2010A1017, 2010B1027, 2011A1011, 2011B1023, 2011B1037, 2012A7600, and 2012B7600).

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REFERENCES

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