Investigation of the Charge Compensation Mechanism on the Electrochemically Li-Ion Deintercalated Li1-xCo1/3Ni1/3Mn1/3O2 Electrode System by Combination of Soft and Hard X-ray Absorption Spectroscopy Won-Sub Yoon,*,�� Mahalingam Balasubramanian,��� Kyung Yoon Chung,�� Xiao-Qing Yang,�� James McBreen,�� Clare P. Grey,�� and Daniel A. Fischer�� Contribution from the Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973, AdVanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, Department of Chemistry, State UniVersity of New York at Stony Brook, Stony Brook, New York 11794-3400, and National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received May 10, 2005 E-mail: wonsuby@bnl.gov Abstract: In situ hard X-ray absorption spectroscopy (XAS) at metal K-edges and soft XAS at O K-edge and metal L-edges have been carried out during the first charging process for the layered Li1-xCo1/3Ni1/3Mn1/3O2 cathode material. The metal K-edge XANES results show that the major charge compensation at the metal site during Li-ion deintercalation is achieved by the oxidation of Ni2+ ions, while the manganese ions and the cobalt ions remain mostly unchanged in the Mn4+ and Co3+ state. These conclusions are in good agreement with the results of the metal K-edge EXAFS data. Metal L-edge XAS results at different charge states in both the FY and PEY modes show that, unlike Mn and Co ions, Ni ions at the surface are oxidized to Ni3+ during charge, whereas Ni ions in the bulk are further oxidized to Ni4+ during charge. From the observation of O K-edge XAS results, we can conclude that a large portion of the charge compensation during Li-ion deintercalation is achieved in the oxygen site. By comparison to our earlier results on the Li1-xNi0.5Mn0.5O2 system, we attribute the active participation of oxygen in the redox process in Li1-xCo1/3Ni1/3Mn1/3O2 to be related to the presence of Co in this system. Introduction LiCoO2 is the most widely used cathode material for commercial secondary lithium batteries due to its advantages, including easy preparation and good cyclability.1-3 Numer- ous research on the cathode materials has been carried out to identify an alternative nontoxic cathode material with higher capacity, lower cost, and increased safety to replace LiCoO2. Layer-structured lithium cobalt nickel manganese oxides (Li[NixCo1-2xMnx]O2) have recently been shown to be one of the most promising alternative materials for LiCoO2 since their electrochemical and safety characteristics are comparable or better than those of LiCoO2.4,5 These materials crystallize in a layer structure isotypic with R-NaFeO2 based on a close-packed network of oxygen atoms with alternating predominantly lithium layers, and layers containing Mn4+, Ni2+, and Co3+. Extensive research on the electronic structure of Li-ion intercalated cathode materials has been carried out to elucidate the reaction mechanism of the electrochemical process in the cathode material during cycling. Hard X-ray absorption spec- troscopy (XAS) has been employed in order to examine the electronic and local structure of transition metal ions in the electrode materials for use in Li rechargeable batteries.6-10 The absorption peak features of the transition metal K-edge XAS provide useful structural information, such as the oxidation state of chemical species, their site symmetries, and covalent bond strength. Soft XAS (200-1000 eV), using synchrotron radiation, has been applied to investigate the electronic structure of specific ions in the electrode materials for Li rechargeable batteries, especially low z elements such as oxygen and fluorine that cannot be directly investigated by hard XAS (above 1000 eV).11-15 Recent theoretical calculations on electrode materials �� Brookhaven National Laboratory. ��� Argonne National Laboratory. �� State University of New York at Stony Brook. �� National Institute of Standards and Technology. (1) Mizushima, K. Jones, P. C. Wiseman, P. C. Goodenough, J. B. Mater. Res. Bull. 1980, 15, 783. (2) Ozawa, K. Solid State Ionics 1994, 69, 212. (3) Nagaura, T. Tozawa, K. Prog. Batt. Solar Cells 1991, 9, 209. (4) Ohzuku, T. Makimura, Y. Chem. Lett. 2001, 642. (5) Lu, Z. MacNeil, D. D. Dahn, J. R. Electrochem. Solid State Lett. 2001, 4, A200. (6) Delmas, C. Peres, J. P. Rougier, A. Demourgues, A. Weill, F. Chadwick, A. Broussely, M. Perton, F. Biensan, Ph. Willmann, P. J. Power Sources 1997, 68, 120. (7) Nakai, I. Nakagome, T. Electrochem. Solid State Lett. 1998, 1, 259. (8) Yoon, W.-S. Lee, K.-K. Kim, K.-B. J. Electrochem. Soc. 2000, 147, 2023. (9) Balasubramanian, M. Sun, X. Yang, X. Q. McBreen, J. J. Electrochem. Soc. 2000, 147, 2903. (10) Yoon, W.-S. Grey, C. P. Yang, X.-Q. Balasubramanian, M. McBreen, J. Chem. Mater. 2003, 15, 3161. (11) Montoro, L. A. Abbate, M. Rosolen, J. M. Electrochem. Solid State Lett. 2000, 3, 410. (12) Uchimoto, Y. Sawada, H. Yao, T. J. Power Sources 2001, 97, 326. (13) Yoon, W.-S. Kim, K.-B. Kim, M.-G. Lee, M.-K. Shin, H.-J. Lee, J.- M. Lee, J.-S. Yo, C.-H. J. Phys. Chem. B 2002, 106, 2526. Published on Web 11/18/2005 10.1021/ja0530568 CCC: $30.25 �� 2005 American Chemical Society J. AM. CHEM. SOC. 2005, 127, 17479-17487 9 17479

for Li batteries indicate that electron exchange in cathode materials (i.e., oxidation and reduction) may involve the participation of the oxygen 2p band, in addition to charge compensation by the metal ions. On the basis of the earlier research of the electronic structure of late transition metal oxides, the ground state of the metal ion exists as a mixed electronic state between 3dn and 3dn+1L due to the M-O covalent bond character, where L means the oxygen ligand hole state by the charge transfer of oxygen 2p electrons to the metal ion.16-19 The charge of the oxygen ion is not the fixed -2 value but a less negative value, corresponding to the more oxidative state by the charge transfer. The oxygen site can contribute to the electron exchange for charge compensation in the Li-ion intercalation-deintercalation of the cathode, which gives rise to the variation of the working voltage of the lithium battery. Soft XAS at O K-edge could provide crucial experimental evidence for the oxygen contribution to charge compensation in the Li-ion intercalation-deintercalation process. Soft XAS spectra can be obtained in both the electron yield (EY) and fluorescence yield (FY) modes. The electron yield mode is surface sensitive, with the total electron yield (TEY) mode probing a depth of ���100 �� and the partial electron yield (PEY) probing a depth of ���50 ��. The fluorescence yield (FY) mode probes the bulk to a depth of more than ���2000 ��.20,21 Unfortunately, most of the soft XAS results reported in the literature on lithium battery materials have been obtained by the electron yield method only.11-13 Although soft XAS, using electron yield method, gives useful information about the electronic structure of the transition metal and oxygen ions, the results are limited only to probing the surface or near surface, which may not reflect what is happening in the bulk. In contrast, our previous soft XAS study using both the FY and PEY modes clearly showed that the surface of nickel-based compounds has a different electronic structure from the bulk.15 Therefore, both FY and PEY detection modes were used simultaneously in this paper in order to obtain more complete information about the electronic structure. Although studies on the charge compensation mechanism for the cathode materials of lithium batteries have been reported in the literature, the techniques used in most of those studies are limited to hard XAS only. The lack of systematic study of the charge compensation mechanism for the cathode materials of lithium batteries during Li+ intercalation/deintercalation left many important questions unanswered, such as what is the role of oxygen, what are the correlations between transition metal ions and the surrounding oxygen ions. These are the issues we are trying to address in this paper, by using the combination of hard and soft XAS techniques, together with the simultaneous data collection with both PEY and FY modes in soft XAS. The electronic structure and local environment of transition metals and oxygen in the electrochemically Li-ion deintercalated Li1-xCo1/3Ni1/3Mn1/3O2 electrode system were studied. The major charge compensation mechanisms in the Li1-xCo1/3Ni1/3Mn1/3O2 electrode system on Li-ion deinterca- lation are established. This combination of hard and soft XAS analysis gives a better understanding of the charge compensation mechanism of lithium transition metal oxides during Li inter- calation/deintercalation. Experimental Section LiCo1/3Ni1/3Mn1/3O2 powders were synthesized by reacting stoichio- metric quantities of a coprecipitated double hydroxide of manganese and nickel with lithium hydroxide at 900 ��C for 24 h in O2. The crystal structure of the LiCo1/3Ni1/3Mn1/3O2 powder was characterized by X-ray diffraction (XRD), by using a Scintag powder X-ray diffractometer (with Cu KR radiation and a flat-plate data collection geometry). Cathode specimens were prepared by mixing the LiCo1/3Ni1/3Mn1/3O2 powders with 10 wt % acetylene black and 10 wt % PVDF (poly- vinylidene fluoride) in NMP (n-methyl pyrrolidone) solution 1 M LiPF6 in a 1:1 ethyl carbonate:dimethyl carbonate (EC:DMC) solution was used as the electrolyte. The cell was assembled in an argon-filled glovebox. The detailed design of the spectroelectrochemical cell used in situ XAS measurement has been described elsewhere.22 Hard XAS measurements were performed in transmission mode at beamline X18B of the National Synchrotron Light Source (NSLS) using a Si(111) double-crystal monochromator detuned to 35-45% of its original intensity to eliminate the high order harmonics. The in situ Mn, Co, and Ni K-edge XAS data were obtained in two separate cells. The Mn XAS spectra were collected using one cell, while the Co and Ni XAS spectra were collected in tandem using the second cell. Energy calibration was carried out by using the first inflection point of the spectrum of Mn and Ni metal foil as a reference (i.e., Mn K-edge ) 6539 eV and Ni K-edge ) 8333 eV). Reference spectra were simultaneously collected for each in situ spectrum by using Mn or Ni metal foils. The EXAFS data analysis was carried out using standard procedures. The measured absorption spectrum below the pre-edge region was fitted to a straight line. The background contribution above the post-edge region, ��o(E), was fitted to a fourth order polynomial (cubic spline). The fitted polynomials were extrapolated through the total energy region and subtracted from the total absorption spectra. The background- subtracted absorption spectra were normalized for the above energy region, l(E) ) {��(E) - ��o(E)}/��o(E). The normalized l(E) spectra were converted to l(k) in k space, where k ) [8��2m(E - Eo)/h2]1/2. The l(k) spectra were k3-weighted to magnify the small signal in the higher k space. The normalized k3-weighted EXAFS spectra, k3l(k), were Fourier transformed (FT) in k space with integration limits of 2.5-11.0 ��-1 for the Mn data, 3.0-12.5 ��-1 for the Co data, and 3.0-13.5 ��-1 for the Ni data. EXAFS structural parameters were obtained by nonlinear least-squares analysis of the data using phase and amplitude functions generated from the FEFF6 code.23,24 The least-squares fits were carried out in r space using FEFFIT. The amplitude reduction factor S02 was scaled to a fixed value of 0.69 for the Mn edge and 0.72 for the Co edge, respectively, after preliminary refinements. Soft XAS measurements were performed in both fluorescence yield (FY) and partial electron yield (PEY) modes at beamline U7A of the NSLS. The beam size was 1 mm in diameter. The estimated incident X-ray energy resolution was ���0.15 eV at the oxygen K-edge (E/��E ��� 3500). Monochromator absorption features and beam instabilities were normalized out by dividing the detected FY and PEY signals by the (14) Balasubramanian, M. Lee, H. S. Sun, X. Yang, X.-Q. Moodenbaugh, A. R. McBreen, J. Fischer, D. A. Fu, Z. Electrochem. Solid State Lett. 2002, 5, A22. (15) Yoon, W.-S. Balasubramanian, M. Yang, X.-Q. Fu, Z. Fischer, D. A. McBreen, J. J. Electrochem. Soc. 2004, 151, A246. (16) Zaanen, J. Sawatzky, G. A. Allen, J. W. Phys. ReV. Lett. 1985, 55, 418. (17) de Groot, F. M. F. Grioni, M. Fuggle, J. C. Ghijsen, J. Sawatzky, G. A. Petersen, H. Phys. ReV. B 1989, 40, 5715. (18) Hu, Z. Mazumdar, C. Kaindl, G. de Groot, F. M. F. Warda, S. A. Reinen, D. Chem. Phys. Lett. 1998, 297, 321. (19) van der Laan, G. Henderson, C. M. B. Pattrick, R. A. D. Dhesi, S. S. Schofield, P. F. Dudzik, E. Vaughan, D. J. Phys. ReV. B 1999, 59, 4314. (20) de Groot, F. M. F. J. Electron Spectrosc. Relat. Phenom. 1994, 67, 529. (21) Stohr, J. NEXAFS Spectroscopy Springer-Verlag: Berlin, 1992. (22) Balasubramanian, M. Sun, X. Yang, X. Q. McBreen, J. J. Power Sources 2001, 92, 1. (23) Rehr, J. J. Zabinsky, S. I. Albers, R. C. Phys. ReV. Lett. 1992, 69, 3397. (24) O���Day, P. A. Rehr, J. J. Zabinsky, S. I. Brown, G. E., Jr. J. Am. Chem. Soc. 1994, 116, 2938. A R T I C L E S Yoon et al. 17480 J. AM. CHEM. SOC. 9 VOL. 127, NO. 49, 2005