J. Phys. Chem. B 2004, 108, 18547-18551
18547
Effects of Ni Doping on [MnO6] Octahedron in LiMn2O4 Y. J. Wei,† L. Y. Yan,† C. Z. Wang,† X. G. Xu,† F. Wu,‡ and G. Chen*,† Department of Materials Science, College of Materials Science and Engineering, Jilin UniVersity, Changchun 130023, P. R. China and Beijing Institute of Technology, Beijing 100081, P. R. China ReceiVed: May 13, 2004; In Final Form: September 13, 2004
LiNixMn2-xO4 (x e 0.5) powders were synthesized using a sol-gel technique. It was found that partial Ni atoms occupy the 8a sites in heavy doped LiNixMn2-xO4 via X-ray diffraction. XPS results showed an increase and a decrease in the average valence state of Mn and Ni ions, respectively, with the nickel content. Five Raman modes of LiNixMn2-xO4 were observed. The A1g band was observed being shifted to higher frequency for x e 0.2 and shifted to lower frequency for x > 0.2. It was indicated that the most rigid [MnO6] octahedron occurs at x ) 0.2. [MnO6] octahedron in LiNi0.2Mn1.8O4 possesses the strongest rigidity with respect to the other LiNixMn2-xO4 (x < 0.5 and * 0.2).
1. Introduction Spinel LiMn2O4 is of great interest as a cathode material for lithium ion batteries because of its high voltage, low cost, and low toxicity.1-7 However, LiMn2O4 shows disadvantages related to poor cycling behavior because of a fast capacity fading in the 3 V range because of the Jahn-Teller distortion of [MnO6] octahedron and also in the 4 V range because of the Mn3+ dissolution during lithium ions intercalation/deintercalation.8 Reducing the amount of Mn3+ ions in spinel LiMn2O4 is helpful to improve its cycling performance. Ti, Co, Mg, Ni, and so forth have been considered as the possible candidates for the substitution.9-10 Thackeray et al.11 have pointed out that the substitution enhances the structural stability of LiMn2O4 spinel. M. Wakihara12 also suggested that the improvement of the cyclability attributes to the stronger M-O bonding of [MO6] octahedron of partially substituted LiMxMn2-xO4 (M ) Cr, Co, Ni) in comparison to that of Mn-O of LiMn2O4. Several works showed that the light-doping LiMxMn2-xO4 (x < 0.2) would show better capacity retention than the undoped counterpart.13-15 In this work, we try to answer why light-doping LiNixMn2-xO4 (x < 0.2) gives rise to a better capacity retention via structural and valence state studies using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy techniques. 2. Experimental Section LiNixMn2-xO4 powders were prepared by reacting a stoichiometric mixture of LiNO3, Ni(NO3)2‚6H2O, and Mn(CH3COO)2‚ 4H2O with different x values of 0, 0.1...0.5. The three agents were mixed in a certain atomic ratio, then dissolved in distilled water, and stirred at 60 °C for about 5 h. Then, the stirred solution was dried in 120 °C and sintered at 700 °C for 20 h, followed by quenching to room temperature, yielding a dark powder. Crystal structure of the powders was identified by powder XRD, using an Rigaku D/max-rA diffractometer with Cu KR * To whom correspondence should be addressed. E-mail: GCHEN@ jlu.edu.cn. † Jilin University. ‡Beijing Institute of Technology.
radiation (λ ) 1.5406 Å). Diffraction data were collected by step scanning over an angular range of 15-70° with a step width of 0.05°. Lattice parameters were calculated by a least-squares refinement method. X-ray photoelectron spectra (XPS) were collected on an ESCALAB5 spectrometer (VG Scientific) with a nonmonochromatic Mg KR (1253.6 eV). For electronic conductivity measurements, pellets of the samples (diameter ) 10 mm; thickness ) 2 mm) were prepared under 20 MPa and sintered at 600 °C for 10 h. Silver electrodes were attached to opposite pellet faces using liquid silver paste. Then, the pellets were dried at 140 °C for half an hour. The measurements were performed using the ac complex impedance method at frequencies ranging from 1 Hz to 1 MHz on a Solarton 1260 impedance/ gain phase analyzer. Raman spectra were collected at room temperature, using a Renishaw RM1000 instrument at a resolution of 2 cm-1. Laser line with the wavelength of 514.5 nm from an argon-ion laser was used as the excitation light. To avoid photodecomposition of the materials during the collection of Raman spectra, we took a light power as low as 10 mW. 3. Results and Discussion 3.1. X-ray Diffraction. The X-ray powder diffraction patterns of the LiNixMn2-xO4 (x ) 0, 0.1...0.5) are shown in Figure 1. All patterns can be indexed as cubic spinel phases, except for x ) 0.5 which contains some extra peaks. Therefore, the maximum substitution content of Ni for Mn in LiMn2O4 is lower than 0.5 for our synthesis routines. The lattice parameter, a, calculated by a least-squares method, is listed in Table 1. The lattice parameter decreases from 8.2437 to 8.1738 Å as the Ni content increases from 0 to 0.4. Such change in the lattice parameter may reflect a change in the M (Mn, Ni)-O bonding. The M (Mn, Ni)-O bonding is stronger than Mn-O, which corresponds to a shorter M (Mn, Ni)-O bonding length and thus gives rise to a smaller lattice parameter. A new diffraction peak appears at about 2θ ) 30.9° for LiNi0.4Mn1.6O4, which can be indexed as the (220) diffraction line of the compounds.16 The (220) line corresponds to the diffraction of 8a sites of spinel.16 In nondoped LiMn2O4 spinel, only Li ions occupy the 8a sites. The diffraction intensity of (220) line of LiMn2O4 is relatively low and cannot be observed on the condition of our measurement because of the very low
10.1021/jp0479522 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/04/2004
18548 J. Phys. Chem. B, Vol. 108, No. 48, 2004
Wei et al.
Figure 2. Full-scale XPS spectra of LiNixMn2-xO4 (x ) 0, 0.2, and 0.4). The inset is the magnification of Ni 2p region. Figure 1. X-ray powder diffraction patterns of LiNixMn2-xO4 (x ) 0, 0.1...0.5). The inset is the magnification of the 28-34° region.
TABLE 1: Lattice Parameters of LiNixMn2-xO4 (x ) 0, 0.1...0.4) lattice constant, a (Å) LiMn2O4 LiNi0.1Mn1.9O4 LiNi0.2Mn1.8O4 LiNi0.3Mn1.7O4 LiNi0.4Mn1.6O4
8.2437 ( 0.0002 8.2221 ( 0.0004 8.2056 ( 0.0003 8.1844 ( 0.0003 8.1738 ( 0.0002
X-ray scattering ability of lithium. The appearance of (220) line indicates that there must be some extra ions, with much stronger scattering ability than Li ions, which occupy the 8a sites. The occurrence of (220) line in LiNi0.4Mn1.6O4 indicates that the 8a sites are partially occupied by Ni ions. This indicates that Ni ions tend to partially occupy the 8a sites in heavy-doped LiMn2O4. The occupancy of Ni on the 8a sites in heavy-doped LiMn2O4 should be related to the lowering of the average valence of Ni ions, because ions with lower valence state tend to occupy the four-coordinated 8a sites of spinel compounds.17 The XPS results will confirm the lowering of the average valence state of Ni ions in heavy-doped LiMn2O4. Because partial occupancy of Ni ions on 8a sites is not good for the cycling performance of the material, heavy-doped LiNixMn2-xO4 (x g 0.4) is not suggested to be the candidate for cathode materials. 3.2. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy has been widely used to characterize valence state of materials. Binding energies (BE) are used to identify different elements and their valence states. Moreover, using the relative area under the deconvoluted XPS bands, we can obtain a semiquantitative estimation of the valence states of the elements in the mixed-valent compounds. Figure 2 presents the full-scale XPS spectra of three LiNixMn2-xO4 (x ) 0, 0.2, and 0.4). The low-energy band located at about 54 eV is assigned to Mn 3p and Li 1s.18 The band at about 530 eV is assigned to O 1s. Mn 2p3/2 and Mn 2p1/2 are responsible for the bands at about 643 and 654 eV, respectively. The bands at approximately 854 and 872 eV correspond to Ni 2p3/2 and Ni 2p1/2, respectively. The band at about 743 eV corresponds to the Auger peak of OKLL. High-resolution XPS spectra of O 1s, Ni 2p, and Mn 2p for LiNixMn2-xO4 (x ) 0, 0.2, and 0.4) are shown in Figures 3-5. Table 2 lists their peak positions, which correspond to their BE.
Figure 3. O 1s XPS spectra of LiNixMn2-xO4 (x ) 0, 0.2, and 0.4).
Figure 4. Ni 2p XPS spectra of LiNixMn2-xO4 (x ) 0.2 and 0.4).
There is no change in BE for O 1s bands for all three materials, which indicates that there is no effect of Ni doping on the valence state of O ions. For LiNi0.2Mn1.8O4, the Ni 2p3/2 band shows the peak position at 855.2 eV and then decreases to 854.7 eV for LiNi0.4Mn1.6O4. The Ni3+ and Ni2+ ions give rise to the Ni 2p3/2 peak positions at 855.8 and 854.5 eV, respectively.19 The changing of BE between LiNi0.2Mn1.8O4 and LiNi0.4-
Effects of Ni Doping on [MnO6] Octahedron
J. Phys. Chem. B, Vol. 108, No. 48, 2004 18549
Figure 5. Mn 2p XPS spectra of LiNixMn2-xO4 (x ) 0, 0.2, and 0.4).
TABLE 2: XPS Peak Positions of LiNixMn2-xO4 (x ) 0, 0.2 and 0.4) peak position (eV) O 1s
Ni 2p1/2
Ni 2p3/2
Mn 2p1/2
Mn 2p3/2
LiMn2O4 529.9 ( 0.1 654.0 ( 0.1 642.2 ( 0.1 LiNi0.2- 529.9 ( 0.1 872.3 ( 0.1 855.2 ( 0.1 654.2 ( 0.1 642.6 ( 0.1 Mn1.8O4 LiNi0.4- 529.9 ( 0.1 872.0 ( 0.1 854.7 ( 0.1 654.4 ( 0.1 642.8 ( 0.1 Mn1.6O4
Mn1.6O4 indicates that the average valence state of Ni ions in LiNixMn2-xO4 decreases with the Ni content.20 The decreasing of Ni valence state causes Mn valence raising to keep charge balance in the system. Therefore, more Mn3+ cations will transfer to Mn4+ state in the higher Ni content compound, which can be examined by the Mn 2p3/2 XPS spectra. Chowdari has reported that the Mn 2p3/2 XPS binding energy of Mn3+ and Mn4+ ions are at 641.9 and 643.2 eV, respectively.21 In our experiment, the Mn 2p3/2 binding energy of all three samples are in this region, which indicates that the Mn valence in LiNixMn2-xO4 is in the mixed valence state. Moreover, the binding energy of Mn 2p3/2 increases from 642.2 to 642.8 eV with the Ni content increasing from x ) 0 to 0.4, which indicates that the average valence state of Mn ions in LiNixMn2-xO4 increases with nickel doping. The relative amounts of Mn3+ and Mn4+ ions in LiNixMn2-xO4 can be estimated by deconvoluting the asymmetric Mn 2p3/2 XPS spectra, using the dominant Mn3+ and Mn4+ BE values of 641.9 and 643.2 eV, respectively, as shown in Figure 6. The percentage of Mn3+ and Mn4+ ions of the undoped LiMn2O4 are 54% and 46%, with the Mn average valence state of 3.46, giving a satisfied semiquantitative estimation of stoichiometric LiMn2O4 with the error within 10%. It has been reported that the LiMn2O4 spinels with average Mn valence state below 3.5 will induce a Jahn-Teller effect.22 For LiNi0.2Mn1.8O4, the Mn3+ and Mn4+ amounts are about 40% and 60%, respectively, giving rise to a Mn average valence state of approximately 3.60. When Ni content increases to x ) 0.4, the Mn3+ and Mn4+ amounts become 38% and 62% and the Mn average valence state increases to approximately 3.62. The higher Mn average valence state in LiNixMn2-xO4 (x ) 0.2 and 0.4) is expected to restrain the Jahn-Teller effect of [MnO6] octahedron, which is helpful to the stability of the spinel framework.23 3.3. Raman Spectroscopy. Raman scattering technique is capable of probing directly the near-neighbor environment of
Figure 6. Deconvoluted profile of Mn 2p3/2 XPS spectra of LiNixMn2-xO4 (x ) 0, 0.2, and 0.4).
oxygen coordination around the Mn ions. The vibrational modes of spinel LiMn2O4 can be represented by24
Γ ) A1g + Eg + F1g + 3F2g + 2A2u + 2Eu + 4F1u + 2F2u where symmetric stretching mode (A1g), symmetric deformation mode (Eg), and three symmetric bending modes (F2g) are Raman-active and four asymmetric bending modes (F1u) mode are IR-active. Figure 7 shows the Raman spectra of LiNixMn2-xO4 (x ) 0, 0.1...0.4). The intensity of Raman bands increases with x because of the increase of the electronic conductivity. It is well known that in the high-conductivity materials, high concentration of carriers reduces the optical skin depth of the incident laser beam, resulting in a decrease in RS intensity.
18550 J. Phys. Chem. B, Vol. 108, No. 48, 2004
Figure 7. Raman spectra of LiNixMn2-xO4 (x ) 0, 0.1...0.4).
Wei et al.
Figure 9. Variation of A1g mode with the Ni content x of LiNixMn2-xO4 (x ) 0, 0.1...0.4).
TABLE 3: Raman Peak Positions of LiNixMn2-xO4 (x ) 0, 0.1...0.4) band position (cm-1) LiMn2O4 LiNi0.1Mn1.9O4 LiNi0.2Mn1.8O4 LiNi0.3Mn1.7O4 LiNi0.4Mn1.6O4
A1g
F2g(1)
F2g(2)
Eg
F2g(3)
617 ( 2 625 ( 2 638 ( 2 631 ( 2 628 ( 2
588 ( 2 593 ( 2 599 ( 2 593 ( 2 588 ( 2
485 ( 2 498 ( 2 494 ( 2 488 ( 2 488 ( 2
weak weak weak 403 ( 2 403 ( 2
weak weak weak 390 ( 2 390 ( 2
The RS peak positions of each mode of LiNixMn2-xO4 are listed in Table 3 from which we can see for Ni-doped LiNixMn2-xO4, the A1g mode exhibits a blue shift with the Ni content, x, up to x ) 0.2 and then a red shift for x > 0.2 as shown in Figure 9. Frequency of the A1g symmetric stretching mode is determined by function:25 Figure 8. Electronic conductivity of LiNixMn2-xO4 (x ) 0, 0.1...0.4).
To confirm this explanation of the variation of Raman intensity with x, we measured the change of the electronic conductivity of the material with x, as shown in Figure 8. The measured conductivity indeed supports this relationship between Raman intensity and conductivity. The Raman spectra of LiNixMn2-xO4 (x ) 0, 0.1...0.4) are dominated by a strong and wide band at approximately 625 cm-1, with a shoulder band at about 590 cm-1. They are assigned to the A1g and F2g(1) species, respectively. The RS band with medium intensity located at about 490 cm-1 has F2g(2) symmetry.24 The A1g, F1g(1), and F1g(2) vibration modes correspond to the different vibration modes of [MnO6] octahedron. Two overlapped weak bands are observed at about 403 cm-1 and 390 cm-1; they are assigned to Eg and F2g(3) symmetry, respectively. Julien et al. have suggested24 that the symmetric Mn-O stretching vibration of [MnO6] octahedron will split into two bands because of the structural difference between the isotropic [Mn4+O6] octahedron and the locally distorted [Mn3+O6] octahedron. They considered the intensity of the RS shoulder band (F2g(1)) is closely related to the Mn average valence state. From Figure 7, we can see that the intensity of F2g(1) band increases with the Ni content, which is related to the raising of the Mn average valence state in LiNixMn2-xO4. Therefore, the RS confirms the higher average valence state of Mn ions in LiNixMn2-xO4.
νi )
1 2π
x
kii µii
where kii and µii are the local force constant and effective mass of M (Mn, Ni)-O bond, respectively. If Ni doped into LiMn2O4, the µii slightly increases because of the larger atom mass of Ni than that of Mn. The blue shift of ν1 must be attributed to the stronger interaction of M (Mn, Ni)-O than that of Mn-O, which can give a larger kii of M-O. The stronger M (Mn, Ni)-O bonding can strengthen the [MnO6] octahedron and restrain the Jahn-Teller effect in LiMn2O4 spinel. Such a strengthened M (Mn, Ni)-O bond arrives at the maximum in LiNi0.2Mn1.8O4 (see Figure 9). Better capacity retention is decided by the rigidity of the [MnO6] octahedron. From the RS study, we know the most rigid [MnO6] octahedron is obtained at LiNi0.2Mn1.8O4. On the other hand, XRD and XPS results reveal that partial Ni ions occupy the 8a sites in heavy-doped LiNixMn2-xO4, which is not good for the cycling performance of the material. So, it is reasonable to claim that light-doping LiNixMn2-xO4 (x < 0.2) gives rise to a better capacity retention, which fits well to those reported papers.13,26 4. Conclusions XRD study showed pure spinel LiNixMn2-xO4 compounds could be obtained within x < 0.5, and the lattice parameter decreased with nickel doping. Ni ions partially occupy 8a sites in heavy-doped LiNixMn2-xO4 (x g 0.4). XPS study showed
Effects of Ni Doping on [MnO6] Octahedron that the average valence state of Ni ions decreases, and the average valence state of Mn ions increases, with the Ni content in LiNixMn2-xO4. The RS intensity of F2g(1) mode increases with Ni content, which confirms the increase of the average valence state of Mn ions in LiNixMn2-xO4. The blue shift of A1g mode in LiNixMn2-xO4 with Ni content, x, up to x ) 0.2 indicates that Ni doping strengthens the M-O bonding in the [MnO6] octahedron and gives rise to the better capacity retention of the materials. Acknowledgment. This work was supported by Chinese Natural Science Foundation, under Grant # 50272023 and the Special Funds for Major State Basic research Project of China under Grant 2002CB211802. This work was also partially supported by Ministry of Education, China, a component part of Key Project 10411. References and Notes (1) Liu, Y.; Fujiwara, T.; Yukawa, H.; Morinaga, M. Electrochim. Acta 2001, 46, 1151. (2) Takada, T.; Enoki, H.; Hayakawa, H.; Akiba, E. J. Solid State Chem. 1998, 139, 290. (3) Tao, H.; Haoqing, W. J. Electrochem. Soc. 1999, 463, 24. (4) Wu, X.; Kim, S. B. J. Power Sources 2002, 109, 53. (5) Kalyani, P.; Kalaiselvi, N.; Muniyandi, N. J. Power Sources 2002, 111, 232. (6) Huang, K.; Peng, B.; Chen, Z.; Huang, P. Sol. Energy Mater. Sol. Cells 2000, 62, 177. (7) Bang, H. J.; Donepudi, V. S.; Prakash, J. Electrochim. Acta 2002, 48, 443.
J. Phys. Chem. B, Vol. 108, No. 48, 2004 18551 (8) Tarascon, J. M.; Mckinnon, W. R.; Coowar, F.; Boowner, T. N.; Amatucci, G.; Guyomard, D. J. Electrochem. Soc. 1997, 141, 22. (9) Bach, S.; Henry, M.; Buffer, N.; Livage, J. J. Solid State Chem. 1990, 88, 325. (10) Bittihin, R.; Herr, R.; Hoge, D. J. Power Sources 1993, 43-44, 223. (11) Gummow, R. J.; de Kock, A.; Thackeray, M. M. Solid State Ionics 1994, 69, 59. (12) Hayashi, N.; Ikuta, H.; Wakihara, M. J. Electrochem. Soc. 1999, 146 (4), 1351. (13) Taniguchi, I.; Song, D.; Wakihara, M. J. Power Sources 2002, 109, 333. (14) Sun, Y.-K.; Kim, D.-W.; Choi, Y.-M. J. Power Sources 1999, 79, 231. (15) Wu, S.-h.; Su, H.-J. Mater. Chem. Phys. 2002, 78, 189. (16) Lee, Y.-s.; Sun, Y.-K.; Nahm, K.-S. Solid State Ionics 1998, 109, 285. (17) Buchanan, R. C.; Park, T. Materials Crystal Chemistry; Marcel Dekker: New York, 1997. (18) Rios, E.; Chen, Y.-Y.; Gracia, M.; Marco, J. F.; Gancedo, J. R.; Gautier, J. L. Electrochim. Acta 2001, 47, 559. (19) Kim, K. S.; Winograd, N. Surf. Sci. 1974, 43, 625. (20) Liu, H.; Li, J.; Zhang, Z.; Gong, Z.; Yang, Y. Electrochim. Acta 2004, 49, 1151. (21) Shaju, K. M.; Subba Rao, G. V.; Chowdari, B. V. R. Solid State Ionics 2002, 152-153, 69. (22) Tackeray, M. M.; David, W. I. F.; Bruce, P. G.; Goodenough, J. B. Mater. Res. Bull. 1983, 18, 461. (23) Lee, J. H.; Hong, J. K.; Jang, D. H.; Sun, Y.-K.; Oh, S. M. J. Power Sources 2000, 89, 7, (24) Julien, C. M.; Massort, M. Mater. Sci. Eng., B 2003, 97, 217. (25) Urban, M. W.; Koenig, J. L. In Vibrational Spectra and Sturcture; Durig J. R., Ed.; Elsevier: New York, 1990. (26) Bang, H. J.; Donepudi, V. S.; Prakash, J. Electrochim. Acta 2002, 48, 443.
