Relationship between Phase Transition Involving Cationic Exchange

Jan 15, 2014 - ... low cost. Li2FeSiO4 exhibits complex polymorphism and undergoes significant phase transformations during charge and discharge react...
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Relationship between Phase Transition Involving Cationic Exchange and Charge−Discharge Rate in Li2FeSiO4 Titus Masese,† Yuki Orikasa,*,† Cédric Tassel,‡,§ Jungeun Kim,∥ Taketoshi Minato,⊥ Hajime Arai,⊥ Takuya Mori,† Kentaro Yamamoto,† Yoji Kobayashi,‡ Hiroshi Kageyama,‡,# Zempachi Ogumi,⊥ and Yoshiharu Uchimoto† †

Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501, Japan ‡ Graduate School of Engineering, Kyoto University, Katsura-cho, Nishikyo-ku, Kyoto 615-8510, Japan § The Hakubi Center for Advanced Research, Kyoto University, Yoshida-Ushinomiya-cho, Sakyo-ku, Kyoto 606-8302, Japan ∥ SPring-8/JASRI, 1-1-1 Kouto, Sayo-cho, Sayo-ku, Hyogo 679-5198, Japan ⊥ Office of Society-Academia Collaboration for Innovation, Kyoto University, Gokasho, Uji 611-0011, Japan # Institute for Integrated Cell-Material Sciences, Kyoto University, Yoshida-Ushinomiya-cho, Sakyo-ku, Kyoto 606-8302, Japan S Supporting Information *

ABSTRACT: Li2FeSiO4 is considered a promising cathode material for the next-generation Li-ion battery systems owing to its high theoretical capacity and low cost. Li2FeSiO4 exhibits complex polymorphism and undergoes significant phase transformations during charge and discharge reaction. To elucidate the phase transformation mechanism, crystal structural changes during charge and discharge processes of Li2FeSiO4 at different rates were investigated by X-ray diffraction measurements. The C/50 rate of lithium extraction upon initial cycling leads to a complete transformation from a monoclinic Li2FeSiO4 to a thermodynamically stable orthorhombic LiFeSiO4, concomitant with the occurrence of significant Li/Fe antisite mixing. The C/10 rate of lithium extraction and insertion, however, leads to retention of the parent Li2FeSiO4 (with the monoclinic structure as a metastable phase) with little cationic mixing. Here, we experimentally show the presence of metastable and stable LiFeSiO4 polymorphic phases caused by lithium extraction and insertion. tions11,12 suggest different redox potentials in the various Li2FeSiO4 polymorphs.11,12 As-prepared Li2FeSiO4 undergoes irreversible phase transformation during the first charge, which accompanies a characteristic potential drop.3,13 X-ray diffraction (XRD) study and theoretical calculation suggest structural rearrangements involving the interchange of some of the Fe with Li sites.13,14 Some researchers reported that the initial Li2FeSiO4 structure is transformed to another phase of LiFeSiO415 and then to a different phase of FeSiO4 during the initial charge reaction.16 Armstrong et al. have reported that the structure of Li2FeSiO4 changes from the initial monoclinic (P21/n) phase to an orthorhombic (Pmn21) phase.17 In contradiction to this result, Kojima et al. have reported a preservation of the monoclinic (P21/n) space group for the cycled Li2FeSiO4.15 From this, a more comprehensive study of the phase transition mechanism in Li2FeSiO4 is required. Clarification of the crystal

1. INTRODUCTION In an intensive search for alternative cathode materials for Liion batteries, Li2FeSiO4 is reported to be an exciting intercalation host on the basis of its superior theoretical energy density.1−3 The presence of two Li atoms per SiO4 polyanion unit provides a high theoretical capacity of 331 mA h g−1. In addition, Li2FeSiO4 exhibits remarkable thermal stability4 and low cost since resources to prepare the material (i.e., Fe and Si) are abundant in the Earth’s crust. The crystal structure of Li2FeSiO4 was initially reported by Nyten et al.3 to be the Pmn21 space group which is isostructural with Li3PO4. Nishimura et al. prepared high quality Li2FeSiO4 samples and performed high resolution synchrotron XRD analysis, resulting in the P21 space group of Li2FeSiO4,5 which was simplified by the author to higher symmetry P21/n.6 Sirisopanaporn et al. subsequently proposed the other polymorph which is the Pmnb space group prepared at higher temperature.7 They reported that the polymorph of Li2FeSiO4 depends on the preparation temperature.8,9 Yabuuchi et al. also reported the Pmn21 space group as the low temperature phase.10 Experimental observations9 and theoretical calcula© 2014 American Chemical Society

Received: September 14, 2013 Revised: January 13, 2014 Published: January 15, 2014 1380

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group with lattice parameters (see Figure S2 and Table S1 in Supporting Information) in good agreement with those reported in the literature.5,6 Figure 1 shows the charge and discharge profile of the Li2FeSiO4 conducted at room temperature at C/10 and C/50

structure and reaction mechanism upon cycling is crucial for material design of Li2FeSiO4. In this study, we focus on the current flux dependency of the crystal structure change between Li2FeSiO4 and LiFeSiO4. Sirisopanaporn et al. reported that the phase transition is related with the applied current density, based on electrochemical observations.9 However, the crystal structural change caused by various current fluxes has not yet been experimentally revealed by diffraction techniques. The previous diffraction studies reported the crystal structures of Li2−xFeSiO4 as the stable crystal structure.13,15−17 This study investigates the crystal structures of electrochemically prepared Li2FeSiO4 and LiFeSiO4 at various charge and discharge rates by synchrotron ex situ XRD measurements. Our work focuses on the transient structural change between Li2FeSiO4 and LiFeSiO4 and shows both a metastable and the stable crystal structure and refined structural parameters of LiFeSiO4.

2. METHODS Stoichiometric amounts of amorphous SiO2 (99.9%, Kanto Chemical), FeC2O4·2H2O (99%, Junsei Chemical) and Li2CO3 (99%, Wako pure chemical) powders were weighed in a molar ratio of 1:1:1, and Ketjen carbon black (10 wt %) added to improve the electronic conductivity of Li2FeSiO4. The powders were mixed with 50 mL of acetone by using a planetary ball mill. Mixing was performed at 400 rpm for 6 h. The powder was pelletized and calcined at 800 °C for 6 h with a fixed Ar flux. The obtained powder was thereafter transferred to an argonfilled glovebox, owing to the inherent sensitivity of the material upon air exposure.18 Scanning electron microscopy measurements were performed with a JSM-890 (Hitachi) operating at 15 kV. Electrodes were prepared from the active material (Li2FeSiO4) to which Ketjen carbon black was added and ball-milled at 600 rpm for 30 min. Polyvinylidenedifluoride (PVdF) binder was thereafter added to get a final weight ratio of 8:1:1, and subsequently slurry was made with 1-methyl-2-pyrrolidone as the solvent. The resulting paste was cast onto an aluminum foil, followed by drying at about 80 °C in a vacuum oven overnight. Two-electrode cells were prepared by using metallic lithium as counter electrode. Two microporous polypropylene membrane separators were used. The electrolyte used was a 1 mol· dm−3 solution of LiPF6 in ethylene carbonate/dimethyl carbonate (3:7 ratio by volume, all received from Kishida chemical). Charge− discharge tests were performed within one lithium reaction, which corresponds to 166 mAh g−1 at room temperature. In addition, the low cutoff voltage was set to 1.5 V. Crystal structures of the as-prepared Li2FeSiO4 and charged− discharged phases were characterized using a high-resolution synchrotron XRD (BL02B2 beamline, SPring-8, Japan). For analysis, ex situ measurements were carried out because high quality data was needed. Attempts to measure by using in situ technique did not provide adequate results, owing to the difficulty posed in quantitatively analyzing the obtained diffraction data. The charged LiFeSiO4 and discharged Li2FeSiO4 electrodes were taken out from the cells, rinsed several times using dimethyl carbonate (DMC), and thereafter followed by drying in vacuum. Powder samples were then put in a glass capillary, and the capillaries were sealed with resin in an argonfilled glovebox to eliminate the exposure to air of the samples. Measurements were carried out immediately after sealing the powders in the glass capillary. XRD data was collected at room temperature using a large Debye−Scherrer camera with an imaging plate. The wavelength of the X-ray was λ = 0.49995 Å calculated by using CeO2 as a standard. The crystal structure analysis was performed with the program JANA2006 using the pseudo-Voigt function of Finger et al.19

Figure 1. Charge and discharge profiles for Li2FeSiO4 during cycling at (a) C/10 and (b) C/50 rate.

rates. The charge and discharge profiles during cycling at a C/ 20 rate are shown in Supporting Information (Figure S3). The initial charge profile is different from the second charge profiles regardless of the rate, which has been ascribed to the structural rearrangements.13 The voltage observed at the initial charge is higher than the reported value which was about 3.1 V.3 This difference is derived from the measurement temperature condition. While the reported voltage of 3.1 V was observed at 60 °C, our measurement was conducted at room temperature. Lower temperature operation might cause the overpotential, resulting in a higher initial charge voltage. The higher voltage profile in the initial charge at room temperature was also reported by Zhang et al.20 In this study, even at a C/10 rate under room temperature operation, the Li2FeSiO4 exhibits an initial discharge capacity of about 166 mAh g −1 corresponding to one-lithium insertion per formula unit. The rate capability is shown in Figure S4 (Supporting Information). By setting the cells at open-circuit voltage (OCV) at the end of the initial charging process, the voltages after 10 min were 4.374, 4.353, and 4.180 V at C/10, C/20, and C/50 rate, respectively. This large difference suggests that the phase morphology obtained at the various rates could be different. To better understand the mechanism underlying the lithium extraction/insertion process in this material, we have carefully monitored the crystal structural evolution on cycling at various

3. RESULTS AND DISCUSSION The particle size of the carbon-coated Li2FeSiO4 was about 50 nm (Figure S1 in the Supporting Information). The asprepared samples were indexed to the monoclinic P21/n space 1381

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Armstrong et al.17 Rietveld refinement of our sample revealed that the lattice parameters are very close to the reported values.17 The above results indicate that the stable structures of LiFeSiO4 and Li2FeSiO4 under charge−discharge reaction at C/50 rate are orthorhombic Pnma and an orthorhombic Pmn21 structures, respectively. Crystal structural evolution at higher C/10 rate is also studied. Rietveld analysis of the discharged Li2FeSiO4 at C/10 rate is difficult owing to the overlap of an unidentifiable phase on the XRD pattern. Therefore, we are basing our discussion only on the lattice parameters change of discharged Li2FeSiO4. Table 3 shows the structural parameters of the charged LiFeSiO4 and discharged Li2FeSiO4 during initial cycling at C/ 10 rate. The structural parameters obtained from Rietveld refinement of LiFeSiO4 are shown in Table 4. In contrast to what is observed on cycling at the C/50 rate, subtle structural changes occur in the monoclinic structure upon the initial charging at C/10 rate. Both end members, charged LiFeSiO4 and discharged Li2FeSiO4, can be described by the monoclinic P21/n space group and have very similar lattice parameters. While the space group transition is observed at C/50 rate, the monoclinic P21/n space group is maintained at higher C/10 rate. At moderate rates of C/20 (Figures S7 and S8 in Supporting Information), the mixture of diffraction peaks from both orthorhombic and monoclinic phase is observed. These results strongly indicate that the phase transformation behavior between Li2FeSiO4 and LiFeSiO4 depends on the charge− discharge rate. We investigated the stability upon cycling of the monoclinic P21/n structure of LiFeSiO4 obtained at C/10 rate. Figure 3 shows the XRD pattern of LiFeSiO4 obtained with charging at C/10 rate both before and after relaxation at open-circuit potential state for 300 h. The XRD pattern drastically changes upon the relaxation for 300 h. Details regarding the complete refinement of the relaxed structure are elaborated in Supporting Information (Figure S9 and Table S3). The monoclinic P21/n LiFeSiO4 structure undergoes a reconstructive transformation to the orthorhombic Pnma LiFeSiO4 given sufficient time (for about 300 h). Therefore the monoclinic P21/n LiFeSiO4 structure is metastable and transforms to the orthorhombic Pnma LiFeSiO4 structure as a thermodynamically more stable phase. Previous theoretical calculation predicted the phase transformation in LiFeSiO4 to the more stable phase due to the difference of the stable energy in the polymorph.11 Although the charged LiFeSiO4 and discharged Li2FeSiO4 obtained at C/10 rate shows the retention of the initial monoclinic structure of Li2FeSiO4 from the XRD results, the voltage profile for the second charge is different from that for the first charge. This is because the monoclinic LiFeSiO4 phase is metastable and gradually transforms to the stable orthorhombic phase during subsequent charge−discharge cycles even at C/10 rate. The gradual phase transformation under C/10 rate is confirmed by the change in the OCVs in the subsequent charge reactions at C/10 rate (Figure S10, Supporting Information). Therefore, since the charge reaction

rates by carrying out XRD measurements and Rietveld analysis. The representative Rietveld profile is shown in Figure 2, which

Figure 2. Rietveld refinement pattern of the synchrotron XRD data for LiFeSiO4 obtained after initial charging at C/10 rate. Black ticks indicate the position of the Bragg peaks of the phase.

is the result from LiFeSiO4 charged at the C/10 rate. Although the charge−discharge reaction lowers the crystallinity of Li2−xFeSiO4, resulting in decrease of peak intensity and broadening, the XRD profiles are well fitted. Table 1 shows the structural parameters of the charged LiFeSiO4 and discharged Li2FeSiO4 during initial cycling at C/50 rate obtained from Rietveld refinement. Refined parameters of LiFeSiO4 are shown in Table 2. The refined structural parameters for the discharged Li2FeSiO4 are shown in Figure S6 and Table S2 in Supporting Information. During the initial charge at C/50 rate, the crystal structure of Li2FeSiO4 is transformed from the initial monoclinic to an orthorhombic (Pnma) LiFeSiO4 phase. During this phase transformation, the site occupancy in the Li site is changed. As shown in Table 2, the Li/Fe ratio on the shared site of LiFeSiO4 charged at C/50 rate yields a value of 0.505:0.495. This indicates that half of the Fe ions move into vacant Li sites, which is consistent with the experimentally suggested cationic mixing process.13 For the reaction from LiFeSiO4 to Li2FeSiO4 at C/50, the crystal structure is transformed to an orthorhombic Pmn21 phase. During this discharging process, the Li/Fe antisite exchange ratio is maintained. In the case of LiFePO4, the Li/Fe mixing drastically decreases the capacity, due to the inherent limitation posed by the one-dimensional diffusion pathway of Li+ in its structure which tends to be blocked by defects.21 On the other hand, Li2FeSiO4 has a two-dimensional diffusion pathway of Li+ in its structure.12 Even when Li/Fe mixing occurs, the other diffusion pathway is available. Therefore the capacity is still maintained in Li2FeSiO4, despite the huge Li/Fe antisite mixing that has occurred. Kojima et al. recently reported XRD analysis of the charged LiFeSiO4 structure, with a P21/n space group.15 The use of this monoclinic space group gave poor fitting agreement values, and instead, we used an orthorhombic Pnma system that provided a good Rietveld fit. For the discharged Li 2 FeSiO 4 , an orthorhombic Pmn21 structure has recently been reported by

Table 1. Structural Parameters for the Charged LiFeSiO4 and Discharged Li2FeSiO4 Phase Obtained at C/50 Rate

as-prepared Li2FeSiO4 charged LiFeSiO4 discharged Li2FeSiO4 (cycled)

symmetry

space group

Fe in Li site

a (Å)

b (Å)

c (Å)

β (deg)

V (Å3)

monoclinic orthorhombic orthorhombic

P21/n Pnma Pmn21

0.025(1) 0.505(3) 0.497(5)

8.2374(2) 10.3731(10) 6.252(3)

5.0097(1) 6.5982(7) 5.416(3)

8.2140(2) 5.0378(3) 5.012(1)

99.19 90.00 90.00

334.6 344.8 169.7

1382

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Table 2. Structural Parameters for the Charged LiFeSiO4 after Initial Charging at C/50 Rate Obtained from Rietveld Refinementa,b atom

g

x

y

z

U

Li/Fe Si O1 O2 O3

0.505/0.495(3) 1.0 1.0 1.0 1.0

0.6550(3) 0.4269(5) 0.4075(13) 0.5642(10) 0.3398(8)

0.4966(6) 0.250 0.250 0.250 0.0712(8)

0.2483(17) 0.2867(17) 0.6125(18) 0.2138(36) 0.1935(19)

0.0080(1) 0.0590(2) 0.0038(5) 0.0215(5) 0.0126(6)

a g and U denote the occupancy and isotropic thermal factor, respectively. The agreement indices used are Rwp = [∑wi(yio − yic)2/∑wi(yio)2]1/2 and Rp = ∑|yio − yic|/∑yio and the goodness of fit is χ2 = [Rwp/Rexp]2 where Rexp = [(N − P)/∑wiyio2]1/2, yio and yic are the observed and calculated intensities, wi is the weighting factor, N is the total number of yio data when the background is refined, and P is the number of adjusted parameters. b Rwp = 1.26%, Rp = 0.95%, χ2= 0.71.

Table 3. Structural Parameters for the Charged LiFeSiO4 and Discharged Li2FeSiO4 Phase Obtained at C/10 Ratea as-prepared Li2FeSiO4 charged LiFeSiO4 discharged Li2FeSiO4 (cycled) a

symmetry

space group

Fe in Li site

a (Å)

b (Å)

c (Å)

β (deg)

V (Å3)

monoclinic monoclinic monoclinic

P21/n P21/n P21/n

0.025(1) 0.025(3) N/A

8.2374(2) 8.2361(3) 8.2747(3)

5.0097(1) 4.9930(2) 5.0057(2)

8.2140(2) 8.2046(3) 8.2294(2)

99.19 99.24 99.15

334.6 333.1 336.5

The parameters of the discharged Li2FeSiO4 are determined based on Le Bail analysis.

On the basis of these results, we conceptualize the phase transformation process between Li2FeSiO4 and LiFeSiO4 as shown in Figure 4. Lithium extraction and insertion at C/50 rate allow the system to attain equilibrium state. At C/50 rate, the original monoclinic Li2FeSiO4 structure transforms to the orthorhombic LiFeSiO4 structure which is thermodynamically stable during initial lithium extraction. In addition, half of the Fe ions move into vacant Li site. During the lithium insertion process, the orthorhombic structure and the Li/Fe cationic mixing ratio are maintained. On the other hand, the C/10 rate of lithium extraction and insertion appear not to alter the asprepared monoclinic structure without Li/Fe cationic mixing. The crystal structure of monoclinic LiFeSiO4 obtained at C/10 rate is unstable and undergoes reconstructive polymorphic phase transformation to the thermodynamically stable orthorhombic LiFeSiO4 structure given sufficient relaxation time. Although the crystal structures of Li2FeSiO4 and LiFeSiO4 do not transform to their stable structures at C/10 rate, lithium extraction and insertion can proceed. This indicates the existence of a metastable monoclinic LiFeSiO4 structure, which is observed in the reaction at higher C/10 rate. At the intermediate rate (C/20), both the orthorhombic and the monoclinic structures are formed, which supports the metastable phase formation. Another point that we would like to iterate is the cationic mixing processes at different current fluxes. The mixing process strongly depends on the current flux and the time scale. This implies that the cationic mixing is a rather slow process during the initial charging process. Our results suggest that the low cationic mobility for Li/Fe antisite exchange can be a kinetic barrier that impedes the phase transformation.

Table 4. Structural Parameters for the Charged LiFeSiO4 after Initial Charging at C/10 Rate Obtained from Rietveld Refinementa,b atom

g

x

y

z

U

Li1 Li2 Fe1 Si1 O1 O2 O3 O4

0.5 0.5 0.975(3) 1.0 1.0 1.0 1.0 1.0

0.6637 0.5913 0.2951(8) 0.0487(14) 0.8228(29) 0.4289(33) 0.6894(30) 0.9662(25)

0.8144 0.214 0.7732(17) 0.7987(21) 0.7782(59) 0.2079(42) 0.7498(64) 0.8475(27)

0.6811 0.0768 0.5332(9) 0.7819(14) 0.8444(36) 0.8651(34) 0.4415(34) 0.2082(32)

0.0093 0.0095 0.0083 0.0067 0.0065 0.0084 0.0093 0.0096

a

g and U denote the occupancy and isotropic thermal factor, respectively. The atomic positions of Li and isotropic thermal factors of atoms were fixed to those of the pristine Li2FeSiO4. bRwp = 1.24%, Rp = 0.87%, χ2 = 0.68.

Figure 3. Reconstructive phase transformation of charged LiFeSiO4 upon leaving the cell under open-cell voltage (OCV) state for 300 h after initial charging at C/10 rate. Rietveld refinement data for the relaxed LiFeSiO4 phase is furnished in Table S3 in Supporting Information.

4. CONCLUSION Phase transition behavior of Li2FeSiO4 at various cycling rates is investigated using synchrotron X-ray diffraction measurements. At C/50 rate of lithium extraction and insertion, the monoclinic Li2FeSiO4 undergoes a reconstructive phase transformation to an orthorhombic structure concomitant with half of the Fe sites moving into vacant Li sites. At C/10 rate, however, the monoclinic Li2FeSiO4 structure is retained with little cationic mixing occurring. This monoclinic LiFeSiO4 structure trans-

at C/10 rate entails the gradual phase transformation to the orthorhombic phase, the voltage profile for the second charge is different from that for the first charge. 1383

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Figure 4. Schematic summary of the phase transition behavior exhibited in the Li2FeSiO4LiFeSiO4 system upon cycling at C/10 and C/50 rate. Bold and broken arrows indicate the charge and discharge processes, respectively. (7) Sirisopanaporn, C.; Boulineau, A.; Hanzel, D.; Dominko, R.; Budic, B.; Armstrong, A. R.; Bruce, P. G.; Masquelier, C. Inorg. Chem. 2010, 49, 7446. (8) Boulineau, A.; Sirisopanaporn, C.; Dominko, R.; Armstrong, A. R.; Bruce, P. G.; Masquelier, C. Dalton Trans. 2010, 39, 6310. (9) Sirisopanaporn, C.; Masquelier, C.; Bruce, P. G.; Armstrong, A. R.; Dominko, R. J. Am. Chem. Soc. 2011, 133, 1263. (10) Yabuuchi, N.; Yamakawa, Y.; Yoshii, K.; Komaba, S. Dalton Trans. 2011, 40, 1846. (11) Seo, D. H.; Kim, H.; Park, I.; Hong, J.; Kang, K. Phys. Rev. B 2011, 84. (12) Eames, C.; Armstrong, A. R.; Bruce, P. G.; Islam, M. S. Chem. Mater. 2012, 24, 2155. (13) Nyten, A.; Kamali, S.; Haggstrom, L.; Gustafsson, T.; Thomas, J. O. J. Mater. Chem. 2006, 16, 2266. (14) Larsson, P.; Ahuja, R.; Nytén, A.; Thomas, J. O. Electrochem. Commun. 2006, 8, 797. (15) Kojima, A.; Kojima, T.; Sakai, T. J. Electrochem. Soc. 2012, 159, A525. (16) Lv, D. P.; Bai, J. Y.; Zhang, P.; Wu, S. Q.; Li, Y. X.; Wen, W.; Jiang, Z.; Mi, J. X.; Zhu, Z. Z.; Yang, Y. Chem. Mater. 2013, 25, 2014. (17) Armstrong, A. R.; Kuganathan, N.; Islam, M. S.; Bruce, P. G. J. Am. Chem. Soc. 2011, 133, 13031. (18) Deng, C.; Zhang, S.; Gao, Y.; Wu, B.; Ma, L.; Sun, Y. H.; Fu, B. L.; Wu, Q.; Liu, F. L. Electrochim. Acta 2011, 56, 7327. (19) Finger, L. W.; Cox, D. E.; Jephcoat, A. P. J. Appl. Crystallogr. 1994, 27, 892. (20) Zhang, S.; Deng, C.; Yang, S. Y. Electrochem. Solid State Lett. 2009, 12, A136. (21) Axmann, P.; Stinner, C.; Wohlfahrt-Mehrens, M.; Mauger, A.; Gendron, F.; Julien, C. M. Chem. Mater. 2009, 21, 1636.

forms to the orthorhombic LiFeSiO4 given sufficient time. These findings suggest that the origin of the slow phase transition in Li2FeSiO4 is the cationic exchange process, which is influenced by the cycling rate and the relaxation time.

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ASSOCIATED CONTENT

* Supporting Information S

SEM image, electrochemical data, and XRD data for Li2‑xFeSiO4 electrodes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING) project under the auspices of New Energy and Industrial Technology Department Organization (NEDO (Japan)). The synchrotron radiation experiments were performed at the BL02B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2012B1018, 2012A1022, and 2011B1029). T.M. acknowledges the Ito Foundation Scholarship and the Honjo International Scholarship Foundation.

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REFERENCES

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