Article pubs.acs.org/cm
Dependence of Structural Defects in Li2MnO3 on Synthesis Temperature Toshiyuki Matsunaga,*,† Hideyuki Komatsu,† Keiji Shimoda,† Taketoshi Minato,† Masao Yonemura,‡ Takashi Kamiyama,‡ Shunsuke Kobayashi,§ Takeharu Kato,§ Tsukasa Hirayama,§ Yuichi Ikuhara,§,∥ Hajime Arai,† Yoshio Ukyo,† Yoshiharu Uchimoto,⊥ and Zempachi Ogumi† †
Office of Society-Academia Collaboration for Innovation, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Neutron Science Laboratory, Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan § Nanostructures Research Laboratory, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta, Nagoya, Aichi 456-8587, Japan ∥ Institute of Engineering Innovation, University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan ⊥ Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-nihonmatsu-cho, Sakyo-ku, Kyoto, Kyoto 606-8501, Japan ‡
S Supporting Information *
ABSTRACT: Li2MnO3, an electrode material for Li ion batteries, belongs to the C2/m space group and is known to have a cubic-closepacked (ABC...) layered structure, in which the transition-metal layer is supposed to have an ordered atomic arrangement with Li atoms at the 2b site and Mn atoms at the 4g site. However, recently, it has been reported that this compound usually does not exhibit such an ideal structure and instead contains a large number of structural defects, not only stacking faults but also mixing of Li and Mn atoms between the 2b and 4g sites. To elucidate the effect of such structural defects on the electrochemical behavior, we examined the crystal structure of Li2MnO3 synthesized at various temperatures by simultaneously analyzing the stacking faults and cation mixing using FAULTS, a Rietveld code. Our examination showed that the crystals consist of both disordered and ordered domains; the disordered domains contain a large number of stacking faults along the c axis and have considerable Li/Mn atomic mixing within the transition-metal layer, whereas the ordered domains have almost no defects. At low synthesis temperatures, the disordered domains are dominant. However, the ordered domains increase at the expense of the disordered domains above 770 °C and become dominant at higher temperatures. It is also found that the degree of cation mixing in the disordered domains remains almost constant irrespective of synthesis temperature. The crystalline defects such as stacking faults or Li/Mn cation mixing are expected to promote the formation of smooth Li percolation paths. The decreasing of the disordered domains leads to dramatically decreased capacity. This indicates that the observed capacities of Li2MnO3 can be determined by the relative amounts of the ordered/disordered domains in the structure.
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INTRODUCTION LiCoO2, LiMnO2, LiNiO2, and their solid solutions, such as LiMn1/3Ni1/3Co1/3O2, are widely used as cathode materials for Li ion rechargeable batteries. These LiMO2 [M: transition metal (TM)] compounds have rocksalt-type layered structures that comprise Li, M, and O layers. In recent years, Li-rich layered compounds such as Li[Li1/3M2/3]O2 or Li2MO3 have been extensively investigated because their specific capacities of higher than 200 mAh/g exceed that of LMO2; among them, a simple system, Li2MnO3, has been widely examined1−8. Such high capacity implies that Li in the TM layer is utilized and thus the topological arrangements of atoms in the crystals is a key to ensure Li movement in the charge−discharge processes; however, their crystals generally contain serious structural defects, such as atomic mixing or stacking faults,2 which makes their precise structural analysis difficult. © XXXX American Chemical Society
Many structural examinations of Li2MnO3 have been performed to date. Strobel and Lambert-Andron9 clarified that Li2MnO3 belongs to the C2/m space group and has a cubic-close-packed (ABC...) layered structure (Figure 1). They observed that the TM layer consisted of Li and Mn atoms in an ordered arrangement occupying the 2b and 4g sites. However, Boulineau2 et al. recently showed that Li2MnO3 generally does not exhibit its ideal structure; instead, it contains a large number of stacking faults along the interlayer direction, and there is considerable cation (Li and Mn atomic) mixing in the TM layer. They examined the stacking faults in specimens synthesized at 650 and 850 °C using the simulation program Received: January 1, 2016 Revised: June 6, 2016
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DOI: 10.1021/acs.chemmater.5b05041 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 1. Structure model for ideally ordered Li2MnO3 (Li: dark green, O: aqua, Mn: pink). The unit cell of this crystal consists of cubic closepacked TM, O, Li, and O layers. To the right of the model, the atomic configuration of the TM layer is shown, where the open and filled circles indicate Li and Mn, respectively. The rectangle represents the a−b unit cells. In this layer, Li atoms forming a triangular net are surrounded by Mn atoms in a honeycomb lattice.
DIFFaX,10 which can estimate the contribution of stacking faults to the diffraction intensities and profiles, and revealed that stacking faults and atomic mixing increase when the synthesis temperature decreases. The specific capacity of Li2MnO3 is known to vary with synthesis temperature, with high capacities observed at lower temperatures of around 600 °C.1 This implies that the structural features, which depend on the heat treatment temperature, greatly affect the electrode behavior. Therefore, it is desirable to analyze both crystallographic data and quantitative information on the stacking faults simultaneously. Unlike other codes for Rietveld analysis,11 the FAULTS program created by Casas-Cabanas12 et al., which is a Rietveld code based on DIFFaX, allows not only simulation but also refinement of the experimental pattern: i.e., this program allows us to perform Rietveld refinement to analyze different amounts and types of stacking faults as well as to determine crystallographic structural parameters for multiple phases. In this study, to elucidate the relationship between synthetic conditions, structure, and electrode performance, we examine a number of Li2MnO3 specimens synthesized at a wide range of temperatures from 450 to 1000 °C, scrutinize their structures and structural defects using the FAULTS program, and discuss the effect of the structural features on electrode performance.
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>99.9%, Honjo Metal) was used as the counter and reference electrodes. These components were assembled along with the polyolefin separator and soaked in the electrolyte solution in an Arfilled glovebox, which was sealed in an aluminum-coated laminate-type cell. The 1 M LiPF6 dissolved in a 3:7 (v:v) mixture of anhydrous ethylene carbonate (EC) and ethylmethyl carbonate (EMC) (Kishida Chemical) was used as the electrolyte. The electrochemical measurements were performed at room temperature on an automatic cycling and data recording system (HJ1001SD8, Hokuto Denko). The cells were galvanostatically cycled between 4.8 and 2.0 V vs Li/Li+ at a current rate of 10 mA/g. Powder XRD patterns were collected between 15° and 125° at a scan rate of 1°/min (in 2θ) on a Rigaku SmartLab diffractometer (Cu Kα radiation) equipped with a Johansson-type monochromator and D/teX Ultra, a high-speed one-dimensional X-ray detector.13 The applied isotropic thermal displacement parameters (Biso) for Li, Mn, and O were fixed at 1.0, 0.5, and 0.8 Å2, respectively, to prevent divergence of the least-squares refinement.2 The atomic configuration of the material synthesized at 700 °C was directly observed on an aberration-corrected STEM (scanning transmission electron microscopy, JEM-2100F, JEOL Ltd.) operated at 200 kV, using crushed polycrystalline particles supported on holey carbon films. A radial Wiener filter (HREM Filters, HREM Research Inc.) was applied to the images for noise reduction. HAADF (highangle annular dark-field) STEM image simulations were conducted using MacTempasX (Total Resolution LLC).
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RESULTS AND DISCUSSION 1. Electrochemical Performance. Figure 2 shows the first cycle charge/discharge profiles of the powdered Li2MnO3
EXPERIMENTAL SECTION
Li2MnO3 specimens were synthesized by a solid-state reaction using Li2CO3 and MnCO3 in a molar ratio of 1.05:1 as precursors. The precursors were thoroughly mixed by milling in acetone for 2 h and then dried at 100 °C. The dried precursors were then pelletized at 5 MPa and calcined at 450 °C for 48 h. X-ray diffraction (XRD) measurements confirmed that the pellets contained only Li2MnO3 (space group: C2/m). The pellets were powdered and pressed again to form new pellets, which were then annealed at various temperatures between 500 and 1000 °C for 12 h in air. The heating and cooling rates were both 300 °C/h for all the annealing temperatures. Additionally, some specimens were prepared using LiOH·H2O and MnCO3 (2.1:1), but changing the precursor did not have any effect on the structural features. Inductively coupled plasma (for metals) and iodometric redox titration (for oxygen) analyses revealed that the compositions (in at%) for two of the synthesized products were Li, 32.7; Mn, 14.3; O, 47.7 (annealing at 700 °C); and Li, 33.6; Mn, 15.4; O, 48.8 (annealing at 900 °C). For fabricating the positive electrode, a mixture of the active material, acetylene black (Denki Kagaku Kogyo), and polyvinylidene difluoride (PVDF, Kureha) in a weight ratio of 80:10:10 was spread onto an aluminum foil with 1-methyl-2-pyrrolidone (NMP) and then dried overnight at 80 °C under vacuum. The electrode was pressed to a typical thickness of 30−35 μm. Metallic Li foil (0.2 mm in thickness,
Figure 2. First charge/discharge curves of Li2MnO3 electrodes synthesized at 450, 700, and 900 °C. The measurements were carried out between 2.0 and 4.8 V (vs Li/Li+) at a rate of 10 mA/g at room temperature. B
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Figure 3. (Left) Pseudotrigonal cell for FAULTS analysis with the original C2/m (monoclinic) cell and (right) a rigid slab viewed along the pseudotrigonal c axis. A rigid slab consists of four atomic layers (−O layer −TM layer−O layer−Li layer−). It is assumed that stacking faults can occur only in the area between the slabs (shown by open red arrows). Shift vectors of the stacking faults are shown by thin red arrows.
for charging/discharging is significantly decreased between 700 and 900 °C, which suggests that structural modification of the material has occurred. 2. Simultaneous Analysis of Stacking Faults and Li/ Mn Mixing. The powder diffraction patterns obtained for the synthesized Li2MnO3 materials are shown in Figure S1 (see Supporting Information). Structural pre-examination of various possible atomic arrangements in this compound confirmed that the obtained specimens contained only a few vacancies at any atomic site; this is in agreement with the chemical composition analysis results, which suggest that the number of cations nearly equals that of anions. However, a large amount of Li and Mn atomic mixing at the 2b and 4g sites, previously reported by Boulineau2 et al., was also observed in our Li2MnO3 crystals. Consequently, we then included the site occupancy g for the Mn atoms at the 4g site (g4g Mn) in the structure refinement. When g4g Mn = 1.0 (100% Mn at the 4g site), the 2b and 4g sites are exclusively occupied by Li and Mn atoms, respectively, which corresponds to the model described by Strobel and Lambert-Andron.9 In addition, a large number of stacking faults were also observed in our specimens; fairly intense diffuse peaks arising from the stacking faults were observed, especially at the bottom of the 020 and 110 reflections, as shown in Figure S1 (Supporting Information; also see Figures S2 and 4). These diffuse peaks are more noticeable in the diffraction patterns of the specimens synthesized at lower temperatures. In the stacking fault analysis using FAULTS, we assumed a structural model of the pseudotrigonal cell shown in Figure 3 (space group: P1̅) that comprised three rigid slabs of −O−TM−O− Li− layers, wherein stacking faults were allowed only in between the slabs and could never be generated inside the slabs. According to Boulineau2 et al., in order to express all possible atomic configurations generated by the stacking faults, the three vectors for describing the stacking faults are defined as [0, 0, 1], [1/2, −1/6, 1], and [1/6, −1/6, 1] in the unit cell ([−1/3, −1/3, 1/3], [1/3, 0, 1/3], and [0, −1/3, 1/3] in the pseudotrigonal cell); the stacking probabilities associated with these stacking vectors are given by three α parameters, viz., α11, α12, and α13, respectively. These parameters indicate the probabilities of normal (cubic) stacking, the other cubic stacking, and hexagonal close-packed (ABAB...) sequences, as seen in the right illustration in Figure 3. If the TM layer has the
Figure 4. Observed (+) and calculated (red line) X-ray diffraction profiles of Li2MnO3 synthesized at (a) 700 °C and (b) 900 °C. A difference curve (observed minus calculated, blue line) is shown at the top of each figure; under the diffraction profiles, reflection markers are indicated by spikes. The background profiles are shown by green lines. Some peaks on the low-angle side are labeled with their Miller indices, and strong peaks from unreacted Li2CO3 are shown with arrows.
specimens. The charge/discharge current rate was 10 mA/g, corresponding to ca. 1/50 C, which is low enough to ensure that a quasistatic process is realized. The specimens synthesized at 450 and 700 °C show high charge/discharge capacities; on the other hand, the specimen annealed at 900 °C exhibits a very low capacity. This demonstrates that the amount of Li available C
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Table 1. Refined Structural Parameters for Li2MnO3 (space group: C2/m) Synthesized by Annealing at 700 and 900 °C (disordered domain) for 12 h at Room Temperaturea 700 °C: a = 4.9259(6) Å, b = 8.5283(5) Å, c = 5.0116(1) Å, β = 109.125(2)°, R = 1.2%, χ2 = 1.1 site
g
x
y
z
Li (1) Mn (1) Li (2) Li (3) Li (4) Mn (2) O (1) O (2)
2b 2b 2c 4h 4g 4g 4i 8j
0.813 0.187 1 1 0.093 0.907(3) 1 1
0 0 0 0 0 0 0.214(6) 0.262(5) α12
1/2 1/2 0 0.665(4) 0.159(1) 0.159 0 0.317(5)
0 0 1/2 1/2 0 0 0.215(4) 0.230(2)
α11 0.691(5)
1.0 1.0 1.0 0.5 0.8 0.8 α13
0.304(5) 0.005(1) 900 °C: a = 4.9273(2) Å, b = 8.5192(2) Å, c = 5.0143(1) Å, β = 109.120(1)°, R = 2.1%, χ2 = 5.3 B (Å2)
atom
site
g
x
y
z
Li (1) Mn (1) Li (2) Li (3) Li (4) Mn (2) O (1) O (2)
2b 2b 2c 4h 4g 4g 4i 8j
0.809 0.191 1 1 0.096 0.904(3) 1 1
0 0 0 0 0 0 0.217(2) 0.253(1) α12
1/2 1/2 0 0.673(3) 0.159(1) 0.159 0 0.305(1)
0 0 1/2 1/2 0 0 0.225(3) 0.211(2)
α11 0.827(3) a
B (Å2)
atom
1.0 1.0 1.0 0.5 0.8 0.8 α13
0.156(3)
0.017(1)
The standard deviations are shown in parentheses.
Figure 6. Occupancy factor of Mn atoms at the 4g site, g4g Mn, and translation probabilities for the three kinds of stacking faults, α11, α12, and α13, determined by Rietveld analysis as a function of annealing temperatures. The standard deviations obtained by the least-squares calculations are smaller than the symbol sizes.
Figure 5. HAADF STEM image of Li2MnO3 synthesized at 700 °C; inset shows simulated images. Solid, dashed, and dashed−dotted lines indicate the crystal orientation viewed along the [100], [110], and [11̅0] zone axes, respectively. White arrows indicate the columns with cation mixing.
The refinement results for the specimen synthesized at 700 °C are shown in Figure 4a and Table 1 (shown in the original C2/m cell configuration). As seen in this figure, the superstructure reflections, (110), (11̅ 1), (021), and (111), are almost obscured by the diffuse intensities arising from the stacking faults. However, the FAULTS program successfully determined the structure with defects, i.e., with atomic mixing
completely disordered atomic arrangement, α11 and α12 can be identical. D
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approximately proportional to the square of Z).15 Figure 5 shows the HAADF STEM image of the Li2MnO3 specimen synthesized at 700 °C. The bright spots show the positions of Mn atomic columns in Li2MnO3 along the [100], [110], and [110̅ ] zone axes, which are indicated by solid, dashed, and dashed−dotted lines, respectively. The coexistence of these three crystal orientations in a single image indicates that the compound synthesized at 700 °C definitely contains stacking faults, as has been shown in other reports.2,16 In addition to the stacking faults, defective contrasts, indicated by white arrows, were observed in the TM layers in this image. Since the specimen contains few vacancies, as deduced from the XRD and chemical composition analysis, such disordered contrasts can be attributed to intense mixing of Li and Mn. In fact, the simulated HAADF STEM image in Figure 5, made using a hypothetical structure model with heavily disordered atomic arrangement (g4g Mn was assumed to be 0.6), is in good agreement with the observations in the A−A′ region. Various other contrast disorders can also be explained by different degrees of cation mixing. It is suggested that these atomic arrangements are averaged to lead to the XRD results containing the disorder and atomic mixing. 4. Formation of Ordered Crystalline Domains at High Synthesis Temperatures. Next, we examined the structures of the specimens obtained at high temperatures. For the specimen obtained at 900 °C, the convergence of the FAULTS analysis becomes poor when a single-phase is assumed, as shown in Figure S2a. Generally, Li2MnO3 specimens obtained at high synthesis temperatures of over 800 °C show sharp (020), (110), (1̅11), (021), and (111) diffraction lines out of diffuse peaks, as shown in Figure S1; at the same time, the diffuse peaks become weak as the synthesis temperature increases. According to Boulineau2 et al., this suggests the generation of highly crystalline domains in addition to the disordered crystalline domains with stacking faults. We then performed the FAULTS analyses by assuming that these specimens consist of crystalline domains with and without stacking faults. In the latter case, we further assumed that there was no Li/Mn atomic mixing in the TM layers and the diffraction intensities were incorporated into the background to simplify the calculation. The background profile of the specimen obtained at 900 °C is shown in Figure 4b. As shown in this figure, the multiphase analysis results indicate very good agreement between the experimentally obtained and calculated patterns, showing that this specimen consists of both ordered domains without any cation mixing and stacking faults as well as disordered domains with both defects. The refined structural parameters of the disordered domain are listed in Table 1, indicating that the structural features related to the disordered domain are similar for specimens obtained at 700 and 900 °C. 5. Temperature Dependence of Structural Features. The analysis of all specimens was carried out using FAULTS, with the assumption that the specimens consist of both disordered and ordered domains. Figure 6 summarizes the structural features of the disordered domains for specimens obtained at various synthesis temperatures. The features are almost unchanged irrespective of the synthesis temperature, indicating a constant degree of disorder in the disordered domain. The volume fraction of the disordered domains was estimated by comparing the scale factors (integrated intensities) of the disordered and ordered domains with the background for the ordered domain. We also assumed that the
Figure 7. Fraction of the disordered crystalline domains estimated by FAULTS analyses. The dashed line in the figure was obtained by fitting 1 − a(T − Tc)β; T ≥ Tc to the data from the 800 to 1000 °C range. The critical temperature (Tc) and critical index (β) are 773 ± 22 °C and 1.24 ± 0.25, respectively (a = 0.00077 ± 0.00113).
Figure 8. Occupancy factor (g4g Mn) and translation probabilities (α11, α12, and α13) in Figure 6 averaged (corrected) by the disordered domain fraction in Figure 7.
and stacking faults, using these diffuse intensities and the Bragg intensities. The calculated diffraction pattern shows very good agreement with the observed one, including the region of these superstructure reflections. As shown in Table 1 (and Figure 6, to be discussed later), g was not unity; our analysis showed that about 10% of the Li/Mn mixing at the 4g site (i.e., 20% Li/Mn mixing at the 2b site) took place in the TM layer. Table 1 shows that α11 and α12 are about 70% and 30%, respectively, whereas α13 is almost zero. This shows that the hexagonal closepacked arrangement that requires short O−O pairs of 2.4 Å along the interlayer direction is quite unlikely to appear (note that the O2− ion radius is about 1.4 Å14). 3. Direct Observation of the Defects in the Li2MnO3 Crystals. We also used HAADF STEM to confirm the presence of cation-mixed structures and stacking faults in the Li2MnO3 compounds. In HAADF STEM images, the brightness can be directly correlated to the atomic number Z (it is E
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Figure 9. Stereographic views of the three individual Li atoms ((a) Li(1), (b) Li(2), and (c) Li(3); see Table 1) within an asymmetric unit in perfectly ordered Li2MnO3 with the first-nearest O atoms and second-nearest metal atoms depicted by the intersection technique. Li, O, and Mn are indicated by dark green, aqua, and pink, respectively. The metal atoms are linked by bars. When the Li ions move into neighboring metal atomic positions along any of these bars, they must break through the potential barriers formed by the two O ions and four Li/Mn ions, as shown in illustrations (d), (e), and (f). In this crystal, only these three arrangements of the six obstacle atoms are observed. The three illustrations, (d), (e), and (f), all provide two different views of the six obstacle atoms: one is normal (left) and the other is parallel (right) to the moving axes. The bottom illustration (g), which is depicted in the perfectly ordered structure model, shows a Li−Mn chain (connected by bars) with their obstacle atoms (cross-sectional view); this chain can function as a smooth Li percolation path when the Mn atoms are replaced by Li.
specimen are diluted by the ordered domains. When volume fraction correction is applied to Figure 6, the structural features shown in Figure 8 are obtained, indicating that the defects decrease on average for specimens synthesized at high temperatures. 6. Structural Features and Their Advantages for Battery Application. The high charge−discharge capacity for the low-temperature-synthesized Li2MnO3 materials, shown in Figure 2, can be ascribed to the structural characteristics
densities and compositions were identical for both domains. The volume fraction of the disordered domains is shown in Figure 7. As seen in this figure, below ca. 770 °C, the specimens consist entirely of disordered domains, while above this temperature, the fraction of disordered domains decreases and ordered crystalline domains grow on increasing the temperature. This also shows that the proportion of stacking faults and atomic mixing in the whole specimen decreases as the synthesis temperature increases, because the defects in the F
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simple system can contribute to the development of better Lirich compounds and better batteries.
revealed herein, namely, the large number of stacking faults and significant atomic mixing occurring in these crystals. It is deemed very important for the Li-rich cathode material to make effective use of Li ions in both the TM and Li layers, and smooth Li percolation paths between the TM and Li layers are desirable for the utilization of these ions. It is expected that such Li percolation paths are formed when the sites of the Li and Mn atoms in the TM layer are exchanged, as detailed below. The stereographic views in Figure 9a−c show three individual Li atoms in the asymmetric unit of the C2/m space group with their first-nearest O atoms and second-nearest metal atoms. Before the Li ion at each center can jump into the positions of the 12 neighboring metal atoms, it has to pass through the potential barrier produced by six obstacle atoms, viz., two O ions and four Li/Mn ions, located around each jumping axis in all 12 directions [all kinds of jumping passes are included in the three polyhedra (a−c)]. In an ordered crystal with no stacking faults and no atomic mixing, the hopping Li ion necessarily encounters the potential barrier formed by the contribution of one or two Mn ions (see Figure 9d and Figure 9e) within two steps. Of the three types of obstacle atoms, Mn atoms provide the greatest hindrance to the passage of the migrating atoms through the barriers because they are tetravalent cations (Mn4+), thus producing a significantly large Coulomb potential barrier to cut off the migration paths. Consequently, when these obstructing Mn ions are replaced by Li ions, the potential barrier is expected to decrease considerably (see Figure 9f), which has been predicted by the calculations.17 In the crystal structure of Li2MnO3 with a large number of stacking faults and significant cation mixing, replacement of the obstacle Mn atoms with favorable Li atoms allows the formation of three-dimensional (3-D) chains with low Coulomb potential barriers, leading to smooth Li ion diffusion (percolation) (see Figure 9g) and enhanced Li utilization. This is considered one of the reasons for the superior performance of Li2MnO3 materials synthesized at lower temperatures. It is noted that, in contrast to the rate capability, on which the effect of the material surface area cannot be disregarded, the capacity at low rates shown in Figure 2 can clearly be correlated with such a structural effect. A similar effect is expected for other Li-rich compounds.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b05041. X-ray-diffraction patterns of Li2MnO3 synthesized at different temperatures (450−1000 °C). The patterns (taken with Cu Kα1 radiation) are shown in the range of 15° ≤ 2θ ≤ 125°; observed and calculated diffraction profiles obtained from single-phase and double-phase Rietveld refinements performed using the FAULTS code. These analyses were conducted on the Li 2 MnO 3 specimen synthesized by annealing at 900 °C for 12 h (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Fax: 81-774-38-4996. E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Research and Development Initiative for Scientific Innovation of New Generation Battery (RISING) project of the New Energy and Industrial Technology Development Organization (NEDO), Japan. The authors thank Prof. Yukinori Koyama at Kyoto University for his fruitful suggestions and Mr. T. Kakei, Mr. Y. Kamishima, and Dr. Y. Takabayashi for their experimental support. The structure models were drawn using Java Structure Viewer (JSV 1.08 lite), which was created by Dr. Steffen Weber.
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REFERENCES
(1) Yu, D. Y. W.; Yanagida, K.; Kato, Y.; Nakamura, H. Electrochemical Activities in Li2MnO3. J. Electrochem. Soc. 2009, 156, A417−A424 and references therein.. (2) Boulineau, A.; Croguennec, L.; Delmas, C.; Weill, F. Structure of Li2MnO3 with Different Degrees of Defects. Solid State Ionics 2010, 180, 1652−1659 and references therein.. (3) Yu, D. Y. W.; Yanagida, K. Structural Analysis of Li2MnO3 and Related Li-Mn-O Materials. J. Electrochem. Soc. 2011, 158, A1015− A1022. (4) Xiao, R.; Li, H.; Chen, L. Density Functional Investigation on Li2MnO3. Chem. Mater. 2012, 24, 4242−4251. (5) Okamoto, Y. Ambivalent Effect of Oxygen Vacancies on Li2MnO3: A First-Principles Study. J. Electrochem. Soc. 2012, 159, A152−A157. (6) Wang, R.; He, X.; He, L.; Wang, F.; Xiao, R.; Gu, L.; Li, H.; Chen, L. Atomic Structure of Li2MnO3 after Partial Delithiation and ReLithiation. Adv. Energy Mater. 2013, 3, 1358−1367. (7) Rana, J.; Stan, M.; Kloepsch, R.; Li, J.; Schumacher, G.; Welter, E.; Zizak, I.; Banhart, J.; Winter, M. Structural Changes in Li2MnO3 Cathode Material for Li-Ion Batteries. Adv. Energy Mater. 2014, 4, 1300998. (8) Yan, P.; Xiao, L.; Zheng, J.; Zhou, Y.; He, Y.; Zu, X.; Mao, S. X.; Xiao, J.; Gao, F.; Zhang, J. G.; Wang, C. M. Probing the Degradation
CONCLUSIONS
We examined the crystal structure and defects of Li2MnO3 synthesized at various temperatures using FAULTS, a Rietveld code based on DIFFaX. Three possible stacking vectors, viz., [0, 0, 1], [1/2, −1/6, 1], and [1/6, −1/6, 1], were adopted to describe the stacking faults in the unit cell, and our analysis clarified that the first two vectors are dominant. In addition to these stacking faults, considerable Li/Mn atomic mixing was observed in the TM layers. This cation mixing as well as the large number of stacking faults favors the formation of long and smooth 3-D Li percolation paths, which leads to high capacities. These structural defects are stable up to around 770 °C; however, ordered crystalline domains begin emerging around 770 °C, and they grow with increasing temperature. This implies that Li2MnO3 changes into an ordered crystal at high synthesis temperatures, which hinders the formation of 3-D Li percolation paths in the crystal. Although Li2MnO3 itself may not be an attractive cathode material due to its poor cycle performance, we believe that a basic understanding of this G
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Chemistry of Materials Mechanism of Li2MnO3 Cathode for Li-Ion Batteries. Chem. Mater. 2015, 27, 975−982. (9) Strobel, P.; Lambert-Andron, B. Crystallographic and Magnetic Structure of Li2Mn03. J. Solid State Chem. 1988, 75, 90−98. (10) Treacy, M. M. J.; Newsam, J. M.; Deem, M. W. A General Recursion Method for Calculating Diffracted Intensities from Crystals Containing Planar Faults. Proc. R. Soc. London, Ser. A 1991, 433, 499− 520. (11) Rietveld, H. M. A Profile Refinement Method for Nuclear and Magnetic Structures. J. Appl. Crystallogr. 1969, 2, 65−71. (12) Casas-Cabanas, M.; Rodríguez-Carvajal, J.; Palacín, M. R. FAULTS, A New Program for Refinement of Powder Diffraction Patterns from Layered Structures. Z. Kristallogr. Suppl. 2006, 23, 243− 248. (13) http://www.rigaku.com/products/xrd, http://www.rigaku.com/ node/4442. (14) Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751−767. (15) Pennycook, S. J.; Jesson, D. E. High-resolution Z-contrast Imaging of Crystals. Ultramicroscopy 1991, 37, 14−38. (16) Wang, R.; He, X.; He, L.; Wang, F.; Xiao, R.; Gu, L.; Li, H.; Chen, L. Atomic Structure of Li2MnO3 after Partial Delithiation and Re-lithiation. Adv. Energy Mater. 2013, 3, 1358−1367. (17) Lee, J.; Urban, A.; Li, X.; Su, D.; Hautier, G.; Ceder, G. Unlocking the Potential of Cation-disordered Oxides for Rechargeable Lithium Batteries. Science 2014, 343, 519−522.
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