CHEM. RES. CHINESE UNIVERSITIES 2010, 26(2), 283—286
Comparative Studies on Structure and Electronic Properties Between Thermal Lithiated Li0.5MnO2 and LiMn2O4 WANG Deng-pan1, CHEN Hong1,2, DU Fei3,4, BIE Xiao-fei1, LIU Li-na1, WEI Ying-jin3, CHEN Gang3,4 and WANG Chun-zhong3* 1. Department of Materials Science, College of Materials Science and Engineering, Jilin University, Changchun 130012, P. R. China; 2. College of Physics, Beihua University, Jilin 132013, P. R. China; 3. College of Physics, 4. State Key Laboratory of Superhard Materials, Jilin University , Changchun 130012, P. R. China Abstract Monoclinic Li0.5MnO2 was synthesized by solid state reaction and the spectral and magnetic properties were studied in comparison with those of spinel LiMn2O4. The XRD pattern and Raman spectrum of Li0.5MnO2 are different from those of LiMn2O4, which indicate the different long-range and short-range crystal structure. XPS result shows the binding energies of 2p3/2 and 2p1/2 in Li0.5MnO2 are located at 642.3 and 653.6 eV, respectively. Through fitting the XPS spectra, the valence state of Mn ion in Li0.5MnO2 coincides with that in LiMn2O4. The high-temperature susceptibility of Li0.5MnO2 can be fitted by Curie-Weiss law whose Curie and Weiss constants are 33 A·m2·K/(mol·T) and –277(6) K, respectively. Although Li0.5MnO2 shows spin glass ground state, the transition temperature of Li0.5MnO2 is about 9 K lower than that of LiMn2O4. Keywords Li0.5MnO2; Raman; XPS; Spin glass Article ID 1005-9040(2010)-02-283-04
1
Introduction
Lithium ion batteries have been extensively used in portable electronics. Manganese-based oxides are considered to be one of the most promising alternative cathode materials due to their economical and environmental advantages[1]. Moreover, the basic understanding of the physical properties of manganesebased oxides is also of significant interest because many of electrochemical properties such as charge capacity and capacity retention upon cycling are closely related to cation ordering and phase stability[2,3]. Spinel LiMn2O4 is a typical cathode material exhibiting not only good electrochemical performance but also interesting magnetic ground state[4,5]. For example LiMn2O4 has space group Fd3 m and the average valence of Mn ions is 3.5 based on the X-ray photoemission spectrum results. Spin glass state is suggested at a low temperature of 25 K due to the frustration competition among ferromagnetic Mn4+― O2–―Mn4+, antiferromagnetic Mn3+―O2–―Mn4+ and
direct interaction Mn3+/4+-Mn3+/4+[4]. Recently, a new manganese-based compound, Li0.33MnO2, has been synthesized as 3 V cathode material by low-temperature solid state synthesis, which delivers a reversible discharge capacity of 140―190 mA·h·g–1[6]. The arrangement of [MnO6] octahedron within the hexagonal close-packed oxygen lattice provided two different sites for Mn ions, named Mn(1) and Mn(2), respectively. The formal valency of Mn ions in Li0.33MnO2 is suggested to be +3.67 based on the study of Mn K-edge XANES, which is higher than that of LiMn2O4(+3.5). In this work, we synthesized Li0.5MnO2 material by low-temperature solid state method brought forward in Ref. [6] and studied the structural, spectral and magnetic properties by XRD, Raman scattering, XPS and SQUID in contrast with those of spinel LiMn2O4.
2
Experimental
Li0.5MnO2 was prepared by the conventional solid state reaction. Details of the sample preparation
——————————— *Corresponding author. E-mail:
[email protected] Received August 24, 2009; accepted October 16, 2009. Supported by the National Natural Science Foundation of China(No.50672031), the Special Funds for Major State Basic Research Project of China(No.2009CB220104), Program for Changjiang Scholar and Innovative Research Team in Universities of China(No.IRT0625) and Jilin Province Project of Research and Development, China(Nos.20060511 and 20075007).
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were reported in Ref. [6]. Powder X-ray diffraction (XRD) was employed to analyze the produced phase and the crystal structure on a Bruker AXS diffractometer with Cu Kα1 radiation(λ=0.15406 nm). The X-ray photoemission spectrum(XPS) was performed on an ESCALAB spectrometer(VG Scientific) equipped with XR5 Monochromated X-ray Gun(15 kV, 150 W). Magnetic characterization was performed by a superconducting quantum interference device magnetometer(Quantum Design MPMS-XL).
3
Results and Discussion
3.1 XRD and SEM Fig.1(A) illustrates the powder XRD pattern of Li0.5MnO2 material obtained by low-temperature solid state method, which reveals that the sample is a single phase with monoclinic space group C/2m[6,7]. As can be seen in Fig.1(A), the XRD peak is broad, indicating the bad crystallization of the material due to the low sintered temperature. In order to study the process of Li0.5MnO2 phase formation, the samples sintered at different temperatures were obtained with the XRD patterns listed in Fig.1(B). It has been found that the higher the sintered temperature, the sharper the peak intensity and the narrower the half-peak breadth. When the sintered temperature increased to 660 °C, the sample changed into the spinel structure LiMn2O4 with space group Fd3m. After indexing, the lattice parameters of LiMn2O4 are a = b = c = 0.82409 nm, similar to those in the Refs. [1] and [2].
error of 1% from ICP measurement. The compositional formula of our prepared material is suggested to be Li0.5MnO2, where the oxygen content is set to be 2.0. In summary, although the low-temperature synthesized sample has the similar molecular formula to that of LiMn2O4, the long-range crystal structure is different. Fig.2 shows the SEM photos of two samples sintered at different temperatures. The SEM photo of Li0.5MnO2 forms large particle cluster which is in correspondence with the broad peak in the XRD pattern and bad crystallization. While the sample of LiMn2O4 shows good crystallization and particle size at about 100 nm.
Fig.2
SEM images of Li0.5MnO2(A, 360 °C) and LiMn2O4(B, 660 °C)
3.2 Raman Scattering Fig.3 shows the Raman patterns of two samples sintered at different temperatures. It can be seen that Raman pattern of Li0.5MnO2 displays one broad peak at 634 cm–1, belonging to the Ag mode, which is in correspondence with the stretch vibration of Mn―O―Mn in the [MnO6] octahedra. In comparison with the Raman patterns of Li0.33MnO2[7], the peak position of Li0.5MnO2 shows a blue-shift. Such a phenomenon indicates the stronger interaction between Mn and O ions, which may induce better electrochemical performance than that of Li0.33MnO2. Furthermore, the Raman pattern of LiMn2O4 is also presented in Fig.3. It shows a similar pattern to the previous reported one[8] in the line shape and peak position. In comparison with that of the Raman spectrum
Fig.1 XRD patterns of Li0.5MnO2(A) and samples sintered at different temperatures(B) a. 460 °C; b. 560 °C; c. 660 °C.
Furthermore, ICP was employed to ascertain the ion ratio of the low-temperature synthesized sample. The mass percentages of Li and Mn ions were calculated to be 3.27% and 54.86%, respectively, with an
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Fig.3
Raman spectra of Li0.5MnO2(a, 360 °C) and LiMn2O4(b, 660 °C)
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of LiMn2O4, peak position of Li0.5MnO2 displays a blue shift, which may imply [MnO6] octahedra in the monoclinic phase is more stable than that in the orthorhombic system. 3.3 XPS Study X-ray photoelectron spectroscopy has been widely used to characterize the valence state of materials. Binding energies 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. Fig.4 presents the high-resolution XPS spectra of Mn2p for Li0.5MnO2. The Mn2p core-level spectra show a typical two-peak structure(2p3/2 and 2p1/2) due to the spin-orbit splitting. The XPS spectrum is referenced to the C1s line, which is located at 285 eV. After standardizing with C1s peak position, the peaks at 642.3 and 653.5 eV are attributed to Mn2p3/2 and Mn2p1/2, respectively. Chowdari et al.[8] has reported that the Mn2p3/2 XPS binding energy of Mn3+ and Mn4+ ions are 641.9 and 643.2 eV, respectively. In our experiment, the Mn2p3/2 binding energy of Li0.5MnO2 is in this scope, which indicates the mixed valence state of Mn ions. The relative amount of Mn3+ and Mn4+ ions in Li0.5MnO2 can be estimated via deconvoluting the asymmetric Mn2p3/2 XPS spectra, using the dominant Mn3+ and Mn4+ binding energy values of 641.9 and 643.2 eV, respectively, as shown in Fig.4. The percentages of Mn3+ and Mn4+ ions of the Li0.5MnO2 are 43% and 57%, respectively, with the Mn average valence state of 3.57, giving a satisfied semiquantitative estimation of stoichiometric LiMn2O4 with an error of within 5%.
Fig.4
3.4
Mn2p XPS spectra of Li0.5MnO2
Magnetic Properties The temperature dependences of the Zero-field
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cooled(ZFC) magnetization and field cooled(FC) magnetization in an applied field of 0.05 T for Li0.5MnO2 are shown in Fig.5, together with the reciprocal ZFC/FC curve. The high-temperature region of reciprocal ZFC curve can be well fitted by Curie-Weiss law x=C/(T–θ) (1) where x is the susceptibility, C is Curie constant and θ is Weiss constant[9]. The Weiss constant was calculated to be –277(6) K which indicates strong antiferromagnetic interaction. While the Curie constant was fitted to be 33 A·m2·K/(mol·T) with which the effective moment μeff=5.1(4) μB/f.u. is obtained by means of Eq.(2) C=Nμ2eff /3kB (2) where N is the number density of magnetic ions per unit gram, kB is the Boltzmann’s constant[10,11]. The ZFC/FC curves increase with the decrease of temperature, and show a cusp at about 9 K in the ZFC branch just below the bifurcation temperature of 15 K. The presence of bifurcation between MZFC and MFC indicates history dependence of the magnetization processes. These features suggest spin glass state in Li0.5MnO2 at low temperatures.
Fig.5
ZFC and FC susceptibility as a function of temperature between 5 K and 300 K in an applied magnetic field of 0.05 T
Inset shows the reciprocal susceptibility, including a Curie-Weiss law fitting to the data above 200 K
Furthermore, it is important to compare the magnetic behavior of Li0.5MnO2 with that of spinel LiMn2O4. It is well known that spinel LiMn2O4 shows spin glass ground state due to the frustrated exchange interaction among Mn ions[12―14]. The spin glass transition temperature is about 25 K higher than that of Li0.5MnO2, which indicates the different magnetic behavior between the two compounds. Although Li0.5MnO2 has similar valence state of magnetic Mn ions as LiMn2O4, the difference in long-range and short-range structures would induce the different spin-glass behavior. In addition, a simple method to
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estimate the effect of geometrical frustration is to calculate |θ|/Tf, where θ is Weiss constant and Tf is the peak temperature in the ZFC curve. It has been proposed that the somewhat arbitrary condition, |θ|/Tf >10, be taken as a criterion for the presence of frustration[15,16]. In Li0.5MnO2, the frustration parameter f was calculated to be about 30, indicating that the magnetic state of Li0.5MnO2 is dominated by geometrical frustration effect. In contrast, although geometrical frustration has been suggested in cubic LiMn2O4, the frustration parameter is comparatively lower than that of Li0.5MnO2[13,14]. References [1] Gong J., Wang C. Z., Liu W., et al., Chem. J. Chinese Universities, 2002, 23(8), 1462 [2] Wei Y. J., Kying W. N., Kwang B. K., et al., Solid State Ionics, 2006, 177, 29 [3] Chen B., Fu Q., Huang X. W., et al., Chem. J. Chinese Universities, 2003, 24(12), 2260 [4] Wei Y. J., Yan L. Y., Wang C. Z., et al., J. Phys. Chem., 2004,
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108(48), 18547 [5] Hon Y. M., Chung H. Y., Fung K. Z., et al., J. Solid State Chem., 2001, 106, 368 [6] Wei Y. J., Ehrenberg H., Kim K. B., et al., J. Alloys and Compounds, 2009, 470, 273 [7] Julien C. M., Banov B., Monchilov A., et al., J. Power Sources, 2006, 159, 1365 [8] Wei Y. J., Nam K. W., Kim K. B., et al., Solid State Ionics, 2006, 177, 29 [9] Shaju K. M., Rao G. V. S., Chowdari B. V. R., Solid State Ionics, 2002, 69, 152 [10] Huang Z. F., Du F., Wang C. Z., et al., Phys. Rev. B, 2007, 75, 054411 [11] Sugiyama J., Nozaki H., Brewer J. H., et al., Phys. Rev. B, 2005, 72, 144424 [12] Julien C. M., Salah A. A., Mauger A., et al., Ionics, 2006, 12, 21 [13] Jang Y. I., Huang B. Y., Chou F. C., et al., Journal of Applied Physics, 2000, 87(10), 7382 [14] Jang Y. I., Chou F. C., Chiang Y. M., Applied Physics Letters, 1999, 74(17), 2504 [15] Greedan J. E., J. Mater. Chem., 2001, 11, 37 [16] Du F., Bie X. F., Chen Y., et al., J. Appl. Phys., 2009, 106, 053904
