Article pubs.acs.org/cm
Transient Phase Change in Two Phase Reaction between LiFePO4 and FePO4 under Battery Operation Yuki Orikasa,*,† Takehiro Maeda,† Yukinori Koyama,‡ Haruno Murayama,‡ Katsutoshi Fukuda,‡ Hajime Tanida,‡ Hajime Arai,‡ Eiichiro Matsubara,§ Yoshiharu Uchimoto,† and Zempachi Ogumi‡ †
Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501, Japan ‡ Office of Society-Academia Collaboration for Innovation, Kyoto University, Gokasho Uji, Kyoto 611-0011, Japan § Department of Materials Science and Engineering, Kyoto University, Yoshida-honmachi, Kyoto 606-8501, Japan S Supporting Information *
ABSTRACT: Transient states of phase transition in LiFePO4/FePO4 for lithium ion battery positive electrodes are investigated by timeresolved measurements. To directly detect changes in electronic and crystal structures under battery operation, in situ time-resolved X-ray absorption and diffraction measurements are performed, respectively. The phase fraction change estimated by the iron valence change is similar to the electrochemically expected change. The transient change of lattice constant during two phase reaction is clearly observed by the time-resolved X-ray diffraction measurement. The nonequilibrium lithium extraction behavior deviates from the thermodynamic diagram of the two phase system, resulting in continuous phase transition during electrochemical reactions. KEYWORDS: lithium ion battery, phase transition, nonequilibrium state, time-resolved measurement
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INTRODUCTION Phase transition mechanisms of electrode active materials for lithium ion batteries (LIBs) can provide design guides to improve their performance.1 Phase transitions for these materials have generally been investigated by static measurements in which thermodynamically stable states are characterized.2 From these analyses, the charge capacity and/or the stability were discussed and their properties improved. The other important property of LIBs is rate capability. As the dominant factor governing the rate capability, the diffusion path of lithium ion has been analyzed.3−5 Under real battery operating conditions, however, the crystal phase dynamically changes during lithium extraction/insertion (charge/discharge reaction). The dynamic phase transition is one of the critical parameters for rate capability because the charge−discharge process of LIBs proceeds under nonequilibrium conditions. This study reveals the transient state of electrode materials under battery operation. Polyanion compound LiFePO4 is selected as a model material to investigate phase transition dynamics. The reaction of this material proceeds in a two phase manner between Lirich Li1−αFePO4 (LFP) and Li-poor LiβFePO4 (FP).6 Because of the large volume difference between the two phases, the experimentally estimated phase boundary migration coefficient is reported to be smaller than 10−13 cm2/s.7,8 However, the actual LiFePO4 system shows a very high rate performance,9,10 which suggests a fast phase transition. Considerable effort has been expended to understand this phenomenon. One well© 2013 American Chemical Society
known phase transition model is the shrinkage-core model, in which the reaction proceeds isotropically from the external or internal side of the crystal.11,12 Although this model adequately simulates the charge−discharge profile and is useful on the secondary particle scale, it cannot explain anisotropic lithium ion diffusivity in structural points of LiFePO4 on the crystalline scale.13−15 In the anisotropic model, phase boundary movement along the a-axis with lithium ion extraction along the baxis was proposed based on TEM measurements at the phase boundary in the bc-plane.16,17 This model was extended to the domino-cascade model, in which phase growth is postulated to be faster than nucleation, resulting in a domino-like movement of the phase boundary.18 Evidence for this model is the nearly constant peak width (which corresponds to crystallite size) in the XRD patterns of various LixFePO4 obtained after charge discharge experiments, and TEM observation of only a pure LFP or FP phase in each single particle. However, these mechanisms are derived from information for the state after the reaction and do not directly reflect the dynamic phase transition. Recently, theoretical calculations have shown that the phase transition of LiFePO4 proceeds through a single phase (solid solution) reaction under nonequilibrium conditions.19,20 Although it is essential to track the transient phase change between LFP and FP phases, the experimental Received: October 22, 2012 Revised: March 11, 2013 Published: March 14, 2013 1032
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For in situ XAS and XRD measurements, three particle sizes (60, 150, and 1000 nm) of LiFePO4 were examined. Pouch cells constructed from LiFePO4-positive and lithium-negative electrodes were set on the synchrotron X-ray beam path, and spectra were measured continuously during the galvanostatic charge process at 1 C. XAS measurements were performed on BL01B1 and BL28XU at SPring-8 (Hyogo, Japan). The synchrotron radiation X-ray from the storage ring was monochromated by a Si (111) crystal. Iron K-edge XAS spectra were measured at room temperature in transmission mode using two ion chambers filled with 50% helium−50% nitrogen and 85% nitrogen−15% argon for incidence and transmittance, respectively. The XAS spectra were acquired every 62 s during the charge reaction. The energy scale was calibrated using Cu foils. X-ray absorption near edge structure (XANES) spectra were analyzed with the REX2000 program (Rigaku Co.). Time-resolved XRD measurements were performed on BL46XU and BL28XU at SPring-8, using parallel X-ray having λ = 0.9995 Å monochromated by a Si (111) crystal in transmittance mode at room temperature. Snapshots of the diffraction patterns were measured by using a two-dimensional hybrid pixel array detector, PILATUS 100K (DECTRIS Ltd.), in the 2θ range from 18° to 21° with a 0.5 s exposure time for each shot. During in situ measurement, both the sample and the detector were fixed and the distance between them was 1000 mm. The pixel size of the detector was 0.172 mm × 0.172 mm. This size gave the angle resolution of approximately 0.01°. The sample thickness was about 30 μm, which is negligible for the angle resolution because of being much smaller than the pixel size. The beam size used in XRD measurements was 0.5 mm (vertical) × 0.5 mm (horizontal). When the detector angle (2θ) is 20.0°, the beam height corresponds to 2.76 pixels, eventually resulting in diffraction peak resolution of 0.0272°.
investigation is a matter of luck due to the difficulty of direct observation. In the thermodynamic aspect, the crystal structure of two phases should be unchanged under two phase reaction. Electrode materials, however, react in nonequilibrium condition, which can cause different phenomena from the thermodynamic expectation. This study experimentally tracks the transient phase change of LFP and FP phases during battery operation. To detect the transient state change of LiFePO4, in situ time-resolved X-ray absorption spectroscopy (XAS) and Xray diffraction (XRD) measurements are performed. For in situ XAS, the amount of electrical charge on iron in LiFePO4, which is changed by lithium deintercalation−intercalation, is measurable.21 For in situ XRD, crystal structure changes under charge−discharge reactions were observable.22 These timeresolved observations enabled us to reveal the real phase transition dynamics of electrode active materials under battery operation.
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EXPERIMENTAL SECTION
Carbon-coated LiFePO4 was prepared using a solid-state reaction or hydrothermal method. For the solid state route, Li2CO3 (Kojundo, 99.99%), FeC2O4·2H2O (Wako, 99.6%), and (NH4)2HPO4 (Wako, 98.0%) were mixed at stoichiometric ratios in ethanol using a planetary ball mill (Fritsch P-6) with zirconia pot and balls for 12 h. After milling and drying, the precursor was mixed with 10 wt % of carbon black (Denkikagaku kogyo) in the ball mill using a rotation speed of 400 or 200 rpm to change the particle size. The mixtures were pelletized and annealed at 600 °C under 97% argon−3% hydrogen atmosphere for 6 h. For the hydrothermal reaction, LiOH·2H2O (Kojundo, 99.99%), FeSO4·7H2O (Kojundo, 99%), H3PO4 (Wako, 85.0%), and ascorbic acid (Wako, 99.6%) were mixed at a ratio of 3:1:1:0.2. The LiOH·2H2O and H3PO4 were first dissolved in N2-bubbled H2O, and then FeSO4·7H2O and ascorbic acid were added. The mixed solution was put into a quartz vessel sealed in a stainless steel autoclave (MMJ-200, OM Lab-Tech) and calcined at 190 °C for 12 h. After washing with deionized water and acetone, and drying in a vacuum oven, the product was annealed at 700 °C in 97% argon−3% hydrogen atmosphere for 1 h. The products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The laboratory XRD measurements were performed on a Rigaku Rint-2200 diffractometer using Cu Kα radiation at room temperature. The diffraction data were collected in a 2θ range of 10° to 60° and step size of 0.04°. SEM micrographs were recorded with a JSM-890 (JEOL) operated at 15 kV. For electrochemical measurements, LiFePO4/C, carbon black, and polyvinylidene fluoride (Kureha) were mixed at a ratio of 75:15:10 with 1-methyl-2-pyrrolidone (Wako, 99%). The slurry was coated onto an aluminum foil current collector and dried in a vacuum oven at 80 °C. The dried electrodes were pressed at 40 MPa. For the laboratory charge−discharge measurements, the prepared LiFePO4 electrode, lithium metal, and electrolyte-soaked separator (Celgard 2500) were constructed into a stainless steel flat cell (HS, HOSEN). The electrolyte was a 1 mol dm−3 solution of LiPF6 in ethylene carbonate/ ethyl methyl carbonate (3:7 volume ratio, Kishida). The cell construction process was performed in an Ar-atmosphere glovebox. Galvanostatic charge−discharge measurements were performed using battery test equipment (HJ1001SD8, Hokuto) at room temperature. The charged electrodes at 0.1 C after 7 days relaxation were washed with ethyl methyl carbonate and dried in a vacuum. The products were characterized with ex situ X-ray diffraction and transmission electron microscopy (TEM) without air exposure. X-ray diffraction measurements were performed at BL02B2 in SPring-8 (Hyogo, Japan) with a Debye−Scherrer camera and imaging plate detectors. The wavelength of the incident X-ray was 0.4994 Å. TEM observations were performed with an HR-9000 (Hitachi) operated at 200 kV.
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RESULT AND DISCUSSION The average valence change of iron caused by lithium extraction can be tracked by the X-ray absorption near edge structure (XANES) at the iron K-edge.23 The iron K-edge XANES spectra of 1000 nm LiFePO4 under 1 C charge is shown in Figure 1a. The absorption edge corresponds to the electronic structure of iron. During the charge reaction, the absorption edge shifts toward higher energy. At 7126 eV, a clear isosbestic point is observed. This indicates a two phase reaction between LFP and FP.24−26 In this case, there are two possible scenarios for the existence of the two phases, namely, a mixture of LFP and FP crystals or one crystal containing both LFP and FP phases. The change in the spectra indicates that a fraction of each phase changes during the charge reaction. In principle, the fraction of each phase during the charge−discharge reactions can be estimated by decomposing the spectra into the contributions of the two end members (i.e., LiFePO4 and FePO4).27 The phase fraction change as a function of lithium content during 1 C charge is shown in Figure 1b. The dotted line corresponds to the phase fractions estimated using the total electrochemical charge capacity. The phase fraction determined from XANES is consistent with the electrochemically expected value. The synchronized change between the charge current and XANES spectra for LiFePO4 system is consistent with the previously reported results.28,29 Those results indicate that the in situ cell works well and can be used to investigate phase transition under battery operation conditions. Previous literature reports that reaction distribution occurs under charging for the LiFePO4 system.30 To minimize this reaction distribution effect, the slurry used for the composite electrode was well homogenized. This process can improve charge state homogeneity. Furthermore, the charge state was checked by spectral measurements and the applicable points were selected before in situ time-resolved measurement. The phase fraction change, as shown in Figure 1b, reflects the ideal 1033
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Figure 1. (a) XANES spectra at the iron K-edge of LixFePO4 during a charge reaction at 1 C rate. Enlarged view around the isosbestic point is shown in the inset. (b) Phase fraction change of LiFePO4 and FePO4 during a charge reaction at 1 C rate estimated by twocomponent analysis. Dashed lines correspond to the phase fraction calculated from the electric current. Solid lines denote the galvanostatic charge profile of LixFePO4 for the in situ XANES measurements.
valence change of iron. Therefore, the distribution effect is minimized enough to discuss the phase transition process. The transition behavior of crystalline phase was further assessed by XRD measurement. Figures 2a and 2b show the XRD data collected during the 1 C charge reaction of 60 and 1000 nm LiFePO4, respectively. Voltage profiles of 60 and 1000 nm LiFePO4 are shown in Figures 2c and 2d. Before the in situ XRD measurement, the cell was charged slowly to 3.35 V, which is just below the plateau potential of LiFePO4/FePO4. This treatment can exclude the influence of the single phase reaction for the LFP phase. At this composition, only a merged peak at 19.15° from LFP (211) and (020) is observed. During the charge reaction, new peaks appear at 19.5 and 19.9° that correspond to FP (211) and (020), respectively. This result indicates that the two phase reaction proceeds with the 1 C charge rate. Previous time-resolved XRD studies of LiFePO4 investigated the phase fraction change by calculating the diffraction peak area.29,31,32 However, the diffraction peak from very small domain sizes usually has poor intensity, resulting in a difficulty of interpreting the phase fraction. In this study, the peak width and peak position estimated from Gauss function fitting are used to discuss phase transition phenomena at the crystal size level.
Figure 2. Time-resolved X-ray diffraction pattern of (a) 60 nm and (b) 1000 nm LixFePO4 particles during galvanostatic charge reaction at 1 C rate. These figures are inclined view, and black color corresponds to background. Diffraction peaks of LiFePO4 (211) and (020) at 19.15° overlap, whereas those of FePO4 at 19.5 and 19.9° are separate. Galvanostatic charge profile of (c) 60 nm and (d) 1000 nm LixFePO4 for the in situ XRD measurements.
In the present in situ time-resolved XRD, the peaks of LFP (211) and (020) appear to shift and broaden during the two phase reaction as shown in Figure 2a, while ex situ XRD measurements have shown constant peak position and full 1034
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Figure 3. (a) Lattice constant and (b) full width at half-maximum (fwhm) of LFP and FP as a function of lithium concentration x estimated from the electrochemical charge current during a galvanostatic charge reaction at 1 C rate. These values are calculated from the diffraction peak at LFP (211) and (020), and FP (020) by using Gauss function.
Figure 5. (a) Ex situ XRD patterns of electrochemically delithiated and relaxed LixFePO4 of various particle size. (b) Calculated lattice constant (Li stoichiometry) of LFP and FP from ex situ XRD patterns. These values are calculated from the diffraction peaks for LFP (211) and (020), and FP (020).
compared to conventional proposed reaction mechanism of LIBs system as follows. The peak position shift is mainly related to the transient change of the lattice constant. To analyze the lattice constant, several diffraction peaks had better be considered. In this study time-resolved XRD measurements in the wide angle were also performed (Figure S5 in the Supporting Information). The position shifts of other peaks in two phase reaction are also apparent for x < 0.6 in LixFePO4. However, the diffraction peak of FP phase for x > 0.6 in LixFePO4 is not fully clear because of feeble diffraction intensity. Therefore, we focus on the diffraction peak observed at about 19.5°. The calculated lattice constant of both phases from the (020) reflection as a function of x in LixFePO4 is plotted in Figure 3a. The x value represents the overall average concentration of lithium in LixFePO4, which is estimated by the electrochemical charge current. At
Figure 4. Gibbs energy diagram for the two phase reaction between LFP and FP.
width at half-maximum (fwhm).18 The peak shift is clearly observed especially for the diffraction of FP (020) in 60 nm LiFePO4. The width of these peaks and their positions are helpful for discussing this anomalous phenomenon as 1035
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change of lattice constant is much smaller than that observed for FP. The degree of lattice constant b decrease in FP at the end of the charge reaction is also greater for the smaller particles. The fwhm of both phases as a function of x in LixFePO4 is plotted in Figure 3b. The fwhm of LFP slightly increases during the charge process, whereas that of FP considerably decreases during the reaction. Although it is not clear which factors cause the observed change, the phase transition continuously proceeds inside the composite electrode under 1 C charge reaction. Even during the 1 C charge reaction, which corresponds to a relatively slow lithium deintercalation process, the change in fwhm for the LFP and FP phases is clearly observable. This indicates that the phase transition between LFP and FP is not so fast. At the early state of transformation from the initial phase to another phase in the LiFePO4 system, nucleation should occur. The nucleus grows via further electrochemical reaction, and consequently another phase is formed. During this so-called two phase reaction, the crystallite size of both phases is in an inversely proportional relationship. When the fwhm of LFP and FP diffraction is converted to the average crystallite size by using Scherrer’s equation, the crystallite size of LFP slightly decreases during the charge process, whereas that of FP increases during the reaction (Figure S6 in the Supporting Information). In addition the lattice strain from the phase boundary can be induced, which gives rise to the broadening of XRD peaks in the transient state. In addition to the thermodynamically stable LFP and FP phase, a transient change of lattice constant is observed. According to Vegard’s formula, the lattice constants correspond to the apparent “lithium concentration” in the LFP and FP phases. The present results can be explained by using the Gibbs energy diagram shown in Figure 4. In a two phase coexistence system, the phase change is expressed by the two Gibbs free energy curves and the common tangent. This scheme can be applied to the individual particle for both phase separation and two phase coexistence cases. When the lithium ion is extracted from the solution limit of Li1−αFePO4, phase separation between LFP and FP can occur. The chemical potential line of this stage is expressed by the tangent line of Li1−α′FePO4. The initial lithium concentration of the FP phase formed in a Li1−α′FePO4 particle is expressed as the intersection between the tangent lines of the Gibbs energy curve for LFP at Li1−α′FePO4 and the Gibbs energy curve for FP, as shown in Figure 4. This is because the driving force for FP formation is generated only for β′ > x > β, and the lithium concentration should change continuously from x = 1 − α to x = β during the charge reaction. That is, the formation of two thermodynamically stable phases (Li1−αFePO4 and LiβFePO4) is delayed under nonequilibrium conditions during battery operation. The instantaneous phase corresponds to Liβ′FePO4, which has a larger lithium concentration than the thermodynamically stable LiβFePO4. Although the lithium concentration of Li1−α′FePO4 is almost the same as that of Li1−αFePO4, the concentration difference between Liβ′FePO4 and LiβFePO4 is conspicuous. The second phase gradually changes to LiβFePO4 because the energy of the second phase is not thermodynamically stable. Noticeable decrease in lattice constant b of FP after formation of the second phase can be caused as a result. The fwhm of the FP phase also changes rapidly within this range. This dynamic behavior is observed regardless of the particle size, which indicates that the phase transition process during battery operation can be generically explained by the Gibbs energy
Figure 6. Transmission electron image and electron diffraction pattern of electrochemically delithiated Li0.5FePO4 of (a, b) 60 nm and (c) 1000 nm particle relaxed for 1 week after electrochemical treatment. The similar lattice constant between the different spot sizes (A-1 and A-2, A-3 and A-4) of the small particle indicates individual phases of LFP or FP. Different lattice constants are observed for the large particle, indicating coexistence of LFP and FP in one particle (not in one crystal).
approximately x = 0.9, where the FP phase starts to form, the lattice constant b in FP is much larger than that of the equilibrium phase, regardless of the particle size. During further lithium extraction, the lattice constant b in FP decreases markedly to x = 0.6 and then gradually approaches the equilibrium value. This inflection point is consistent with the point of slope change for fwhm discussed later with Figure 3b. The change of lattice constant b in the LFP phase is dependent on the particle size. For large-particle LFP, the lattice constant b is almost the same as the initial value and decreases slightly with lithium extraction. The lattice constant b of the 60 nm LFP gradually decreases with lithium extraction. However, the 1036
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Figure 7. Phase transition model of LixFePO4. Ex situ/in situ static measurements have observed only the initial state and the relaxed state (white arrow route). Synchrotron in situ time-resolved measurement directly provides a snapshot of the phase transformation (black arrow route).
diagram. The continuous yet gradual changes of the FP phase can only be observed by making snapshot measurements during battery operation. The particle size dependency can be explained by the recently reported interfacial effects that are detectable in a two phase system.33 Neutron diffraction of chemically delithiated LixFePO4 has revealed the particle size dependence of the miscibility gap in delithiated LixFePO4.33 The interfacial effect is pronounced, especially in nanoparticles. In fact, the changes of lattice constant b for 0.6 < x < 1.0 in LFP and for 0.2 < x < 0.4 in FP are clearly observed for 60 nm LFP. This phenomenon is less pronounced for larger particles. The above results are only observed under the transient state. To analyze the static crystal phase, ex situ XRD patterns of electrochemically delithiated and relaxed LixFePO4 are shown in Figure 5a. Similar to the data analysis of the in situ measurements, the peak position and lattice constant of the b axis were calculated, and the results are shown in Figure 5b. The lattice constant of both LFP and FP phases is almost constant under two phase composition. Although the lattice constant slightly decreases with x in LixFePO4 for the 60 nm particle, the amount of change is less than 0.01 of the value of lattice constant b. Therefore, the relaxed crystal structure can be expressed with the two Gibbs free energy curves and the common tangent shown in Figure 4. A TEM image and electron diffraction pattern for Li0.5FePO4 obtained after a long relaxation time (1 week) are shown in Figure 6. For each particle size measurement, 16 particles were selected at random. The three representative results are shown in Figure 6, and
these characteristics are similarly observed for all the tested particles. For small particles, only a pure LFP or FP phase exists in each particle, resulting in no phase boundary inside the particle. When lithium extraction is stopped during the charge reaction process, a phase boundary between the two phases seems to exist. To stabilize the system, the disproportionation into LFP or FP particles occurs. In contrast, the coexistence of LFP and FP is observed for the large-particle Li0.5FePO4, as shown in Figure 6c. The coexistence of LFP and FP within the same particle can be attributed to the small contribution of the interfacial energy. These results are consistent with previous reports of static measurements.16−18 Recently, theoretical calculation by R. Malik et al. has predicted the single phase transformation path in “one crystal”.20 If this scenario occurs in LiFePO4, the diffraction peak gradually shifts even in the apparent two phase reaction range. Although a small peak shift is observed in this study, the entire peak shift is not observed. The single phase transformation may cause peak broadening, meaning that the expected change cannot be observed by the diffraction method. Moreover, the time-resolved XRD, which has a beam size of 0.5 × 0.5 mm, provides average crystal information from more than 108 crystals. A much smaller size of X-ray beam, less than 1 × 1 μm, will enable the demonstration of their model. M. Tang et al. showed that crystal to amorphous phase transition can happen depending on the crystal size and overpotential.34 Although they did not discuss the transient state between LFP and FP, the present peak broadening under phase transition is consistent with their results. The transient state observed in this 1037
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Chemistry of Materials study may be related to the amorphous state, which does not appear in diffraction patterns. Although they also performed in situ XRD,31 the phase fraction of the diffraction peaks is still unclear because of many factors such as crystallinity, crystal size, strain, and lithium concentration fraction. N. Sharma et al. performed time-resolved neutron diffraction measurement of LiFePO4 and showed solid solution and two phase reaction occurring concurrently.35 The solid solution they discussed corresponds to the thermodynamically expected solid solution reaction outside the miscibility gap.6 Our result suggests the possibility of concentration change inside the miscibility gap between LFP and FP phases as a transient state. Although the neutron study discussed time dependency of crystal structure, the main difference is time scale. While the previously reported neutron diffraction study investigated the charge reaction for more than 10 h, our study discusses the charge reaction in 1 h. The results from the time-resolved X-ray measurement predict the following phase transition mechanism. The white arrow path in Figure 7 shows the previously proposed model deduced from information obtained from relaxed states (using ex situ measurement). The phase transition mechanism of Li1−xFePO4 proposed in this study is represented by the black arrow path in Figure 7. First, the FP phase is formed in one of the LFP particles. In the initial stage, the FP formed instantly displays a higher lithium concentration than the thermodynamically stable phase. During further charge reaction, the lithium concentration decreases, resulting in an approach to the thermodynamically stable phase. As the charge reaction proceeds, the same phenomenon occurs with other particles. The fraction of LFP gradually decreases with progress of the charge reaction. When the reaction is stopped midstage, the coexistence of LFP and FP is diminished because of the large interfacial energy.
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REFERENCES
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CONCLUSION This study has shown the dynamic behavior of electrochemical lithium extraction for LiFePO4 by applying the time-resolved Xray measurement. The valence change observed in XANES is consistent with the electrochemically expected change. Under the nonequilibrium state during the voltage plateau (so-called two phase region), the lattice constants continuously change. This suggests the gradual decrease of lithium concentration in FP phase that can only be observed by time-resolved XRD measurements. The nonequilibrium phase gradually changes and finally reaches the thermodynamical stable state. ASSOCIATED CONTENT
S Supporting Information *
X-ray diffraction pattern and scanning electron microscopy image of LiFePO4 powders, charge discharge profile for various particle sizes of LiFePO4, typical XRD pattern of time-resolved measurement, wide angle time-resolved XRD patterns during 1 C charge reaction, and average crystallite change during 1 C charge reaction (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
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ACKNOWLEDGMENTS
This work was partially supported by the Research and Development Initiative for Scientific Innovation of New Generation Battery (RISING) project under the auspices of New Energy and Industrial Technology Development Organization (NEDO), Japan. Synchrotron radiation experiments were performed at BL01B1 and BL46XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2011A1014, 2011A1009, 2010B1896, and 2010B1028).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +81-75-7536850. Notes
The authors declare no competing financial interest. 1038
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dx.doi.org/10.1021/cm303411t | Chem. Mater. 2013, 25, 1032−1039
