Resonant Photoemission Spectroscopy of the Cathode Material

Therefore, the intensity in the difference spectra obtained by the same way as valence band spectra at the photon energy of 600 eV is due ..... 2009, ...
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Resonant Photoemission Spectroscopy of the Cathode Material LixFePO4 for Lithium Ion Battery Shodai Kurosumi,† Naoka Nagamura,† Satoshi Toyoda,† Koji Horiba,*,†,‡,§ Hiroshi Kumigashira,†,‡,^ Masaharu Oshima,†,‡,§ Sho Furutsuki,|| Shin-ichi Nishimura,|| Atsuo Yamada,|| and Noritaka Mizuno† †

Department of Applied Chemistry, The University of Tokyo, Japan Synchrotron Radiation Research Organization, The University of Tokyo, Japan § Core Research for Evolutional Science and Technology, Japan Science and Technology Agency, Japan ^ Precursory Research for Embryonic Science and Technology, Japan Science and Technology Agency, Japan Department of Chemical System Engineering, The University of Tokyo, Japan

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‡

ABSTRACT: The change of Fe 3d states accompanied with the Li intercalation/deintercalation process has been successfully revealed by resonant photoemission spectroscopy. The main peak shift and expansion of Fe 3d bands through the Li deintercalation reflect the strong hybridization between Fe 3d states and O 2p states as the Fe O bond lengths decrease. From the antiresonance spectra, O 2p partial density of states also changes, suggesting the interaction between Fe and O atoms still remains in LiFePO4. Furthermore, density functional theory calculations results strongly support these experimental results. The framework structure of LiFePO4 is more suitable than that of LiCoO2 for the good rechargeable battery, because the change of the electronic structure in LiFePO4 valence band is rather small because of the strong covalent P O bond.

1. INTRODUCTION Since the lithium ion battery provides high voltage and energy density, this material has been used in small portable devices for the past decade.1 Recently, there has been a growing interest in applying the lithium ion battery to power sources for the larger devices, such as plug-in hybrid vehicles. However, in developing larger-sized devices, there remain several problems in the currently used cathode material, LiCoO2, such as high cost and instability at high temperatures.2,3 In particular, this electrode material suffers from deteriorations in the use beyond normal operating potentials, because irreversible structural changes occur and the valence band spectra drastically change when large amounts of the lithium are extracted and inserted.4 8 On the other hand, the olivine-type iron compound LiFePO4 is one of the most promising candidates for new cathode materials, because of its prominent properties such as low cost, high level of safety, and huge power generation.9,10 The use of the strongly covalent P O bond in polyanion (PO4)3 makes LiFePO4 not only an excellent rechargeable battery but also a safe system when the battery is fully charged.11 Furthermore, through the Fe O P inductive effect, the presence of polyanion (PO4)3 in LiFePO4 makes the transition metal Fe atom strongly ionic and stabilizes the antibonding state, generating high voltage of 3.4 V (vs Li/Li+).12,13 To understand the electronic conduction properties and charge/discharge mechanism, many researchers have devoted their efforts to the study of the electronic structure in LiFePO4.14 18 r 2011 American Chemical Society

The previous theoretical study has shown that the major contribution to the density of states (DOS) across the whole valence band comes from Fe 3d states and O 2p states.19,20 Moreover, spin-polarized density functional theory (DFT) calculations for the 3d6 Fe2+ in LiFePO4 have displayed that spin-up Fe 3d bands are fully occupied by five electrons, while one spin-down Fe 3d band is partially occupied by one electron, just below the Fermi level. In the charge/discharge process, the conventional concept was that the charge compensation concerns only this one spindown Fe 3d state. However, recently, the other theoretical report has suggested that not only Fe 3d spin-down states but also Fe 3d spin-up states change in the charge/discharge reaction.21 Fe 3d bands expand into high binding energy region through the charge process. The previous experimental study of Castro et al. indicated the change of the valence band spectra during the charge/discharge process.22 In comparison of the valence band spectra with the partial DOS of the Fe 3d state, they reported that the peak assignable to the spin-down Fe 3d state disappears and Fe 3d bands expand into high binding energy region through the charge process. However, there are no experimental studies for investigating directly the electronic structure of LiFePO4 by the Received: August 22, 2011 Revised: November 2, 2011 Published: November 21, 2011 25519

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element-selective experimental method. Therefore, in this work, we focus on revealing experimentally the change of the electronic structure of LiFePO4, especially in the Fe 3d states through the Li intercalation/deintercalation by Fe 2p 3d resonant photoemission spectroscopy (PES).

2. EXPERIMENTAL SECTION Pristine LiFePO4 powder was synthesized by solid state reactions of lithium carbonate (Li2CO3, Wako, 99.9%), iron oxalate dehydrate (Fe2C2O4 3 2H2O, junsei, 99%) and diammonium hydrogen phosphate ((NH4)2HPO4, Wako, 99%). These starting materials and 10 wt % (as a final product) conductive carbon (ECP ketjenblack, Lion) were poured into a 250-mL Cr-hardened stainless steel (Cr-ss) container together with a mixture of Cr-ss balls (10  10 mmØ, 16  5 mmØ). The precursors were thoroughly mixed and ground by a conventional planetary milling apparatus (Itoh, LA-PO4) for 12 h. The selfassembled LiFePO4/C composite was synthesized by sintering at 600 C for 6 h under Ar gas flow. To obtain FePO4, chemical oxidation was carried out by reacting LiFePO4 with nitronium tetrafluoroborate (NO2BF4, Alfa Aesar, 96%) in dehydrated acetonitrile. After NO2BF4 (twice the amount required for the reaction) was dissolved in acetonitrile, LiFePO4 was added and stirred for 18 h. The product was filtered and washed with acetonitrile to remove impurities before drying under vacuum. Partially lithiated sample Li0.6FePO4 was prepared by reacting FePO4 with LiI (Alfa Aesar, 99.9%) in acetonitrile for 20 h in Ar atmosphere. The product was filtered and washed with acetonitrile to remove impurities before drying under vacuum. The characterizations were performed by the X-ray diffraction measurement and atomic absorption spectrometry. Experiments were performed at BL-2C of the photon factory (PF) in High Energy Accelerator Research Organization (KEK). To probe the electronic structure of Fe 3d states in LixFePO4 (x = 0, 0.6, 1.0), we have performed Fe 2p 3d resonant PES. By resonant PES, we can extract the information on Fe 3d partial DOS by gigantic enhancement of photoemission spectra excited with the photons of the energy close to the absorption threshold of Fe 2p core level.23 25 The total energy resolution of PES and X-ray absorption spectroscopy (XAS) measurements was set to 200 meV. DFT calculations within generalized gradient approximation + U were carried out using the full-potential linearized augmented planewave method as implemented in the WIEN 2k code. For the sampling of the irreducible wedge of the Brillouin zone, we used 126 k points. Within this approach, to include the effect of the Fe 3d electron correlation, we considered the onsite coulomb term U and the exchange term J. The value U J = 4.3 eV was used in the calculations. 3. RESULTS AND DISCUSSION Figure 1 shows valence band spectra of LixFePO4 at the photon energy of 600 eV. These valence band spectra include the O 2s core level at the binding energy of about 25 eV, and this peak is used for normalization. As described by the previous studies, the peak located at the binding energy of about 2 eV can be assigned to the spin-down Fe 3d state.21 The spectrum of FePO4 also displays a weak component at this binding energy region. This is probably due to the small proportion of Fe2+ remaining at the surface of the FePO4 particle, because PES measurement is a surface-sensitive technique. The difference

Figure 1. Valence band spectra of LixFePO4 at the photon energy of 600 eV.

Figure 2. Fe 2p 3d XAS spectra of LixFePO4.

spectra are obtained by subtracting the spectrum of FePO4 or Li0.6FePO4 from the spectrum of Li1.0FePO4. The difference spectra suggest that not only one spin-down Fe 3d state, but also the other states than the spin-down Fe 3d state are involved in the charge/discharge reaction. These changes in the valence band spectra are in good agreement with the previous report.22 To investigate the detailed electronic structure, we have performed Fe 2p 3d resonant PES. Figure 2 shows Fe 2p 3d XAS spectra of LixFePO4. The energy position of the main peak shifts from 708.1 eV in Li1.0FePO4 to 710.2 eV in FePO4 due to the valence change from Fe2+ to Fe3+, which is consistent with the previous report.22 However, we could not confirm a component of Fe2+ remaining in the spectrum of FePO4, because the XAS measurement is a bulk sensitive analysis technique, compared with the PES measurement. This result indicates that a component of Fe2+ in FePO4 exists only at the surface of the particle and the proportion is very low. Subsequently, we measured Fe 2p 3d resonant PES spectra for each sample with excitation energies denoted by the photon energies R1 and R2 corresponding to the absorption edges of Fe2+ and Fe3+, respectively. Figure 3 shows the Fe 2p 3d resonant PES results on LixFePO4. The maximum intensity in the resonant PES spectra was observed at the excitation energy R1 for Li0.6FePO4 and Li1.0FePO4, whereas it was observed at the 25520

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Figure 4. Valence band spectra of LixFePO4 at the photon energy of 700 eV corresponding to the excitation energy of Fe 2p 3d antiresonance. Figure 3. Fe 2p 3d resonant PES spectra of LixFePO4 measured at the photon energy of Fe 2p 3d absorption edge indicated by R1 and R2 in Figure 2.

excitation energy R2 for FePO4. In comparison of both resonant PES spectra for Li1.0FePO4 and FePO4, the spectral line shape drastically changes in the wide region of valence bands. The first feature is that the main peak shifts from 5.9 eV in Li1.0FePO4 to 7.5 eV in FePO4. The second feature is that Fe 3d bands expand into high binding energy region through the Li deintercalation. The third feature is that the intensity of the peak located at the binding energy of about 2 eV is enhanced only at the excitation energy R1 corresponding to the photon energy of Fe2+ 2p 3d threshold. This feature indicates that the peak is assignable to the one spin-down Fe 3d state, which only exists in Fe2+. These modifications of Fe 3d states are explained by the change of the framework structure through the Li deintercalation, where the framework structure of LiFePO4 shrinks and the Fe O bond length decreases from 2.15 to 2.03 Å.26 Because of the strong overlap between Fe 3d and O 2p states, this decrease in the Fe O bond length leads to the change in the extent of hybridization and intensity distribution across the whole valence band. As a result, the spectral line-shape in resonant PES spectra drastically changed. Figure 4 shows Fe 2p 3d antiresonance valence band spectra of LixFePO4 at the photon energy of 700 eV, where the contribution of Fe 3d states to the intensity across the valence bands consisting of Fe 3d and O 2p states significantly decreases. Therefore, the intensity in the difference spectra obtained by the same way as valence band spectra at the photon energy of 600 eV is due mainly to the change of the O 2p states. The difference spectra suggest that the O 2p states near the Fermi level also change through the Li intercalation/deintercalation. From the experimental results, it was found that not only Fe 3d states but also O 2p states are involved in the charge/discharge reaction. Furthermore, we carried out DFT calculations to analyze the change in the electronic structure of LiFePO4 through the charge/discharge process. Figure 5 shows the calculated DOS of Li1.0FePO4 and FePO4 along with the partial DOS of the Fe 3d state, O 2p state, P 3p state, and Li 2s and 2p states. Through the Li deintercalation process, the partial DOS of Fe 3d bands expands into high binding energy region. On the other hand,

Figure 5. Total and partial DOS for Fe 3d states, O 2p states, P 3p states, and Li 2s and 2p states of LiFePO4 and FePO4 obtained from DFT calculations. All DOS are shifted to the high binding energy by 1.7 eV for the energy position of spin-down Fe 3d state obtained by DFT calculations are reconciled with the energy position obtained by the experimental results.

the partial DOS of O 2p bands expands into low binding energy region. The O 2p states mainly contribute to the DOS of FePO4 near the Fermi level. Moreover, it is clearly seen that the lineshape of Fe 3d partial DOS of FePO4 at the binding energy of about 7.5 eV is almost the same as that of O 2p partial DOS, suggesting strong hybridization between Fe 3d states and O 2p states through the Li deintercalation. These DFT calculations strongly support the experimental results. From the Fe 2p 3d resonant photoemission results, we observed the change of Fe 3d and O 2p states due to the shrinkage of the framework structure and Fe O bond length through the Li deintercalation. As a good rechargeable battery, the electronic structure should remain unchanged together with the crystal structure during the Li intercalation/deintercalation process. Although the spectral line-shape changes in the wide region of valence bands in 25521

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The Journal of Physical Chemistry C LixFePO4, the change of the LixFePO4 spectral line shape in the valence band is much smaller than that of LixCoO2.7,8 Therefore, we can say that the framework structure of LiFePO4 is more suitable for the good rechargeable battery. In terms of the inductive effect, it is worth noting that the electron of O 2p states in LiFePO4 is also involved in the charge/ discharge process, which means that the interaction between Fe and O atoms still remains. It suggests that the use of the polyanion including more electron-withdrawing atom could make the neighboring transition metal atom such as Fe more ionic, generating the higher voltage than the polyanion (PO4)3 .

4. CONCLUSION We can successfully extract the information on Fe 3d partial DOS by resonant PES and experimentally reveal the change of Fe 3d states accompanied with the Li intercalation/deintercalation process. The energy position of the main peak shifts and Fe 3d bands expand into high binding energy region through the Li deintercalation. This change of Fe 3d states reflects the strong hybridization between Fe 3d states and O 2p states as the Fe O bond lengths decrease through the Li deintercalation. From the antiresonance spectra, O 2p partial DOS also changes, suggesting the interaction between Fe and O atoms still remains in LiFePO4. It indicates that substituting phosphate PO4 for the other polyanion which has larger electron negativity raises the electrode potential. Furthermore, DFT calculations results strongly support these experimental results. Comparing the electronic structure of LiFePO4 with LiCoO2, the framework structure of LiFePO4 is more suitable for the good rechargeable battery, because the change of the electronic structure in LiFePO4 valence band is rather small due to the strong covalent P O bond. New cathode materials should be designed to decrease the change of the electronic and crystal structure during the charge/ discharge process.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work is supported by the Japan Society for the Promotion of Science (JSPS) through its “Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program).” ’ REFERENCES (1) Tarascon, J. M.; Armand, M. Nature 2001, 414, 359. (2) Baba, Y.; Okada, S.; Yamaki, J. Solid State Ionics 2002, 148, 311. (3) Jiang, J.; Dahn, J. R. Electrochim. Acta 2004, 49, 2661. (4) Zhang, S. S.; Xu, K.; Jow, T. R. J. Power Sources 2006, 160, 1349. (5) Gabrisch, H.; Yazami, R.; Fultz, B. J. Power Sources 2003, 119 121, 674. (6) Yazami, R.; Ozawa, Y.; Gabrisch, H.; Fultz, B. Electrochim. Acta 2004, 50, 385. (7) Laubach, S.; Laubach, S.; Schmidt, P. C.; Ensling, D.; Schmid, S.; Jaegermann, W.; Thiβen, A.; Nikolowski, K.; Ehrenberg, H. Phys. Chem. Chem. Phys. 2009, 11, 3278. (8) van Elp, J.; Wieland, J. L.; Eskes, H.; Kuiper, P.; Sawatzky, G. A.; de Groot, F. M. F.; Turner, T. S. Phys. Rev. B 1991, 44, 6090. (9) Yamada, A.; Chung, S. C.; Hinokuma, K. J. Electrochem. Soc. 2001, 148, A224. 25522

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