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3O2 solid solutions
Solid State Ionics 178 (2007) 849 – 857 www.elsevier.com/locate/ssi
Structural and electrochemical properties of LiNi1/3Co1/3Mn1/3O2–LiMg1/3Co1/3Mn1/3O2 solid solutions Yasuhiro Fujii a,b,⁎, Hiroshi Miura a , Naoto Suzuki a , Takayuki Shoji a , Noriaki Nakayama b,⁎ a
b
Tosoh Co., Ltd, 4560 Kaisei-cho, Syunan, Yamaguchi 746-8501, Japan Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube, 755-8611, Japan
Received 7 September 2006; received in revised form 19 February 2007; accepted 8 March 2007
Abstract The complete solid solutions in the pseudo-binary LiNi1/3Co1/3Mn1/3O2–LiMg1/3Co1/3Mn1/3O2 system with α-NaFeO2 type layered rocksalt structure have been successfully synthesized. The replacement of Ni with Mg atoms has enhanced the diffraction intensity due to cation ordering in α-NaFeO2 type structure. Powder X-ray diffraction (XRD) patterns of LiMg1/3Co1/3Mn1/3O2 show a broad and diffuse peak with an intensity maximum at around d = 4.2 Å indicating an in-plane [√3 × √3] R30° type ordering. Electron diffraction (ED) patterns also show clear and intense superlattice spots due to the in-plane [√3 × √3] R30° type ordered layers. However, the [√3 × √3] R30° type ordered layers are almost randomly stacked as evidenced by diffuse scattering in the powder XRD and ED patterns. The TEM lattice image clearly reveals the random stacking. XRD and ED patterns agree with the simulated ones using the DIFFaX program based on the above structural model. Solid solution LiNi1/3−xMg xCo1/3Mn1/3O2 (0.0 ≤ x ≤ 0.33) also show a similar two dimensional cation ordering. Electrochemical measurements of LiNi1/3−xMgxCo1/3Mn1/3O2 indicate that not only Ni but also Co can be active as redox species in this solution system. The redox potential of Co in LiMg1/3Co1/3Mn1/3O2 is about 4.1 V. © 2007 Elsevier B.V. All rights reserved. Keywords: Li-ion batteries; Cation ordering; Diffuse scattering; Layer stacking; Turbostatic
1. Introduction Recently, compounds in the Li–Ni–Co–Mn–O system have been proposed as possible alternatives to LiCoO2 that is widely used in current Li-ion batteries [1–3]. Among them, LiNi1/3Co1/3Mn1/3O2 is the most possible candidate [4,5]. The remarkable properties of these materials are related to the valence state of transition metals; 2+, 3+, and 4+ for Ni, Co, and Mn, respectively [6,7]. This material crystallizes in α-NaFeO2 type layered structure based on cubic close-packed oxygen atoms, in which lithium layers and transition metal (111) layers are alternated. Some superstructure models for the cation ordering in the α-NaFeO2 type structure have been reported [8–10]. Ohzuku et al. have proposed a [√3 × √3] R30° type cation ⁎ Corresponding authors. Fujii is to be contacted at Tosoh Co., Ltd, 4560 Kaisei-cho, Syunan, Yamaguchi 746-8501, Japan. Tel.: +81 834 63 9914; fax: +81 834 63 9896. Nakayama, Fax: +81 836 85 9601. E-mail addresses: [email protected] (Y. Fujii), [email protected] (N. Nakayama). 0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2007.03.002
ordering for LiNi1/3Co1/3Mn1/3O2 with a space group symmetry of P3112 and have observed weak superlattice reflections of a [√3 × √3] R30° type ordering in the electron diffraction patterns [9]. On the other hand, P.S. Whitfield et al. reported that there was no evidence of superstructure and that the data were consistent with a random distribution of Ni, Co, and Mn over R3¯m 3b-sites based on the synchrotron XRD and neutron diffraction measurements [10]. Therefore, no strong evidence has been reported for superstructure and the cationic arrangements in these phases are still controversial. One of the reasons why the structures and the cationic distribution have not been confirmed adequately is the similarity of X-ray atomic scattering factors among Ni, Co, and Mn. The substitution of Mg for Ni in LiNi1/3Co1/3Mn1/3O2 is interesting in the above context. The X-ray atomic scattering factor of Mg is much smaller than that of transition metals. If LiMg1/3Co1/3Mn1/3O2 can be synthesized, their cation orderings may be easily characterized by normal powder XRD and they must show some resemblances to those in LiNi1/3Co1/3Mn1/3O2. In fact, LiNi1/2Mn3/2O4 and LiMg1/2Mn3/2O4 with normal
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Table 1 The chemical compositions and oxidation states of LiNi1/3−xMgxCo1/3Mn1/3O2 measured by an ICP-AES and an iodometric titration, respectively Sample
x = 0.0 x = 0.05 x = 0.1 x = 0.2 x = 0.33
Average transition metals valence
LiNi0.34Co0.33Mn0.33O2 LiNi0.29Mg0.05Co0.33Mn0.33O2 LiNi0.24Mg0.10Co0.33Mn0.33O LiNi0.14Mg0.20Co0.33Mn0.33O2 LiMg0.34Co0.33Mn0.33O2
Target stoichiometry
Li1.00Ni0.33Co0.33Mn0.34O2 Li1.00Ni0.29Mg0.05Co0.33Mn0.33O Li1.00Ni0.24Mg0.10Co0.33Mn0.33O2 Li1.01Ni0.13Mg0.20Co0.33Mn0.33O2 Li1.01Mg0.33Co0.33Mn0.34O2
spinel structures (AB2O4) show the same cation ordering in the octahedral B-sublattice [11]. Then we have tried to prepare the solid solutions in the pseudo-binary LiNi1/3−xMgxCo1/3Mn1/3O2 system prepared in order to increase the difference in atomic scattering factors among cations and characterized them by using powder XRD and an electron diffraction. The electrochemical properties of LiNi1/3−xMgxCo1/3Mn1/3O2 are also interesting. It is still controversial whether the Co3+/4+ redox in LiNi1/3Co1/3Mn1/3O2 can be active or not, because the redox potential of Co3+/4+ partially overlaps that of Ni2+/4+ [6,12]. The redox potential of Co3+/4+ in a similar structure can
Measured stoichiometry Predicted
Measured
3.00 3.08 3.15 3.33 3.50
3.02 3.09 3.17 3.32 3.55
be measured without the redox potential of Ni overlapping in virtue of successfully synthesizing LiMg1/3Co1/3Mn1/3O2. 2. Experimental Samples with a nominal composition of LiNi1/3−xMgxCo1/3 Mn1/3O2 (x = 0.0, 0.05, 0.1, 0.2 and 0.33) were prepared by heating the mixtures of LiOH·H2O and (Ni,Mg,Co,Mn)(OH)2. Calcinations were made at 900 °C for 12 h in air stream. Excess lithium hydroxide was added to compensate for the Li loss by volatilization. A heating and cooling rate of 100 °C h− 1 was
Fig. 1. XRD patterns of LiNi1/3−xMgxCo1/3Mn1/3O2. (a) x = 0.0, (b) x = 0.05, (c) x = 0.1, (d) x = 0.2 and (e) x = 0.33. Open circles, solid lines and vertical marks show observed data, calculated profiles and peak positions, respectively. The difference plots are also added. The structural parameters and the R-factor are given in Table 1.
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Table 1 also shows the average valence of transition metals measured by an iodometric titration. Experimental valences agree with the calculated ones from the composition. X-ray powder diffraction patterns indicate that complete solid solutions are obtained in the pseudo-binary LiNi1/3Co1/3Mn1/3O2– LiMg1/3Co1/3Mn1/3O2 system. 3.2. XRD patterns
Fig. 2. Lattice parameters and the Li occupancy at 3a-sites of LiNi1/3−xMgxCo1/3 Mn1/3O2.
applied for all temperature settings. Samples were identified and characterized by powder XRD and ED. Structural refinements were made by using a Rietveld program, Rietan-2000 [13]. Simulations of the diffuse scattering were also performed with the DIFFaX program written by Newsam and Treacy [14,15]. For ED measurements, samples were dispersed in ethanol by applying an ultrasonic wave and were collected on a holly microgrid supported on a copper grid mesh. A field emission type TEM (JEOL JEM2010F) operated at 200 kV was used. The chemical composition and the average oxidation state of transition metals were determined by ICP-AES and iodometric titration, respectively [16]. Electrochemical properties were examined with CR2032 type coin cells. The cell was comprised of a cathode and a lithium metal anode separated by a polypropylene separator and glass fiber mat. The cathode consisted of 25 mg of LiNi1/3−xMgxCo1/3Mn1/3O2 powders and 12 mg of conducting binder pressed on a stainless screen. The electrolyte solution was 1 M LiPF6/EC and DMC. The EC and DMC were mixed in a 1:2 volume ratio. The cell was charged and discharged in the voltage range of 2.5–4.3 V at a current density of 0.4 mA cm− 2 at 23 °C. 3. Results and discussion 3.1. Chemical analysis The chemical compositions of all samples measured by ICP-AES, as listed in Table 1, correspond to the expected ones.
Fig. 1 shows X-ray diffraction patterns of the pseudo-binary solid solutions. The line plots in Fig. 1 show the confidence plots of Rietveld analysis. The differences between observed and calculated patterns were also shown below the confidence plots. The XRD patterns indicate that all the samples are single phase materials with α-NaFeO2 type structure. All the peaks can be indexed by assuming the hexagonal setting of the space group symmetry of R3¯m. The 00.6/01.2 and the 10.8/11.0 pairs of reflections, which are typical of a well developed α-NaFeO2 type layered structure, are well resolved for all samples. Rietveld refinements were performed in order to evaluate lattice parameters and site occupancy of metal atoms. The lattice parameters in the hexagonal setting and Li occupancy at 3a-site in the hexagonal setting are plotted in Fig. 2 as a function of the Mg composition x. The structural parameters obtained by Rietveld refinements are also listed in Table 2. The parameters, Rwp and S, represent a reliability factor and a scale factor, respectively. The latter parameter indicates goodness of fitting [13]. Although unit cell parameters do not show significant compositional dependence because of the similarity in the ionic radii between Ni2+ (0.84 Å) and Mg2+ (0.86 Å), the a-axis length decreases monotonically with increasing Mg composition. The c-axis length of Mg-substituted samples is almost independent of the composition and is larger than that of LiNi1/3Co1/3Mn1/3O2. The change of unit cell parameters may be related to the behaviors of the cation exchange. Ideally, Li atoms occupy 3a-sites, and Mg, Ni, Co and Mn atoms are randomly placed on 3b-sites. This model does not reproduce the observed profiles. The site exchange of Li atoms with other metal atoms was assumed. The results shown in Table 2 were obtained by assuming that Ni or Mg atoms preferentially occupy 3a-sites. Since the ionic radii of Ni2+
Table 2 Structural and fitting parameters obtained by Rietveld analysis of XRD patterns based on layered rock-salt structure (s.g. symmetry: R3¯m) Sample a (Å)
c (Å)
c/a
zox a
g(Li) at Rwp (%) S 3a-site b
x = 0.0 x = 0.05 x = 0.10 x = 0.20 x = 0.33
14.253 (1) 14.263 (1) 14.261 (1) 14.263 (1) 14.262(1)
4.974 4.978 4.978 4.984 4.989
0.2410 (2) 0.2408 (2) 0.2411 (2) 0.2413 (5) 0.2415 (1)
0.985 0.956 0.943 0.946 0.936
2.8659 (2) 2.8653 (1) 2.8650 (1) 2.8619 (1) 2.8587 (3)
13.6 10.6 10.4 11.6 12.5
1.75 1.85 1.81 1.95 1.49
Rietveld analysis has been performed starting from the structure in which Li and other metal atoms are located at 3a and 3b-sites, respectively. a zox: z value of oxygen atom at 6c-site. b g(Li) at 3a-site: Li content at 3a-site lithium layer.
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Fig. 3. Logarithmic plots of XRD patterns for LiNi1/3−xMgxCo1/3Mn1/3O2. (a) x = 0.0, (b) x = 0.05, (c) x = 0.1, (d) x = 0.2 and (e) x = 0.33. All patterns were measured using a sample folder made of quartz single crystal, to avoid the background intensity from the sample holder.
(0.84 Å) and Mg2+ (0.86 Å) are close to those of Li+ (0.88 Å), a small amount of Ni or Mg may occupy 3a-sites. The calculation was also constrained assuming that Mg2+ at 3b-sites predom-
inantly exchanges for Li at 3a-site rather than Ni2+, since both ionic radii and electronic structure of Li+ are more similar to those of Mg2+ than those of Ni2+, respectively [17]. Furthermore, C. Pouillerie et al. have confirmed the preference of Mg2+ than Ni2+ to occupy 3a-sites in the LiNi1−xMgxO2 system by their magnetic study [18]. The refinement gives a Li/Ni site exchange ratio of 1.5% for x =0.0 sample and Li/Mg site exchange ratios in the range from 4.5% to 6.3% for all the Mg-substituted samples. The cationic exchange ratios were evaluated to be higher for Mg-substituted samples than LiNi1/3Co1/3Mn1/3O2 and are almost constant. No sharp superlattice peak indicating [√3 × √3] R30° type ordering as expected from the P3112 model structure proposed by Ohzuku et al. [9] was observed. Fig. 3 shows XRD patterns measured using a sample folder made of quartz single crystal, to avoid the background intensity from the sample holder. The XRD pattern of LiMg1/3Co1/3Mn1/3O2 shows a fairly intense diffuse peak in the 2θ range from 18 to 25°. The shape of the diffuse peak is characteristic of LiMg1/3Co1/3Mn1/3O2 compared with that in the related phase like Li2MnO3 [15]. In the case of Li2MnO3 (Li[Li1/3Mn2/3]O2), fairly sharp superlattice peaks and their tails on the higher angle side are observed. The intensity of the diffuse scattering tends to decrease with decreasing Mg composition. The diffraction pattern for x = 0.05 shows no diffuse peak similar to the one for x = 0.0. One may suspect some amorphous components as the origin of diffuse peaks. TEM observations indicated that the diffuse scattering was not caused by the amorphous component but originated from the two dimensional cation ordering. We have also analyzed the content of impurities (silicon, sodium, and
Fig. 4. Electron diffraction patterns of LiNi1/3Co1/3Mn1/3O2 and LiMg1/3Co1/3Mn1/3O2. (a) observed [00.1] zone ED pattern of LiNi1/3Co1/3Mn1/3O2, (b) observed [00.1] zone ED pattern of LiMg1/3Co1/3Mn1/3O2, (c) (00.1)⁎ reciprocal lattice plane calculated for LiMg1/3Co1/3Mn1/3O2 (s.g. symmetry: P3112), (d) observed ¯.0)⁎ reciprocal lattice plane calculated for [11¯.0] zone ED pattern of LiNi1/3Co1/3Mn1/3O2, (e) observed [11¯.0] zone ED pattern of LiMg1/3Co1/3Mn1/3O2 and (f) (11 LiMg1/3Co1/3Mn1/3O2 (s.g. symmetry: P3112). The diffraction spots have been indexed assuming the R3¯m structure in hexagonal setting. Extra spots and diffuse streaks can be seen for an observed pattern in addition to fundamental spots.
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Fig. 5. TEM images and the corresponding electron diffraction pattern of LiMg1/3Co1/3Mn1/3O2 viewed along the [11¯.0] direction. (a) A low magnification image of the observed particle, (b) a lattice image of the thin region marked in (a). The inset shows the corresponding ED pattern and the objective aperture size is shown in it.
boron) in order to evaluate the contamination during synthesis. Since no above impurities were detected, the diffuse peaks in XRD patterns were irrelevant to the impurities. 3.3. Electron diffraction and lattice images Fig. 4(a), (b), (d) and (e) shows observed [00.1] and [11¯.0] zone ED patterns (EDPs) of two end-members of the binary
solid solution system, LiMg1/3Co1/3Mn1/3O2 and LiNi1/3Co1/3 Mn1/3O2. Fig. 4(c) and (f) shows calculated [00.1] and [11¯.0] zone EDPs of LiMg1/3Co1/3Mn1/3O2 with a space group symmetry of P3112. All the indexes are in the hexagonal setting of the R3¯m space group. No amorphous component was observed during TEM observation. TEM–EDX analysis for several particles indicated that the atomic ratio of Mg:Ni:Mn is close to 1.0: 1.0: 1.0.
Fig. 6. Schemes generated by stacking [√3 × √3] R30° type layers of LiMg1/3Co1/3Mn1/3O2. (a) [√3 × √3] R30° type unit cell, a1, a2, and a3. The Ni, Co, and Mn ions are denoted by round, triangular and square symbols, respectively. (b) The stacking order generated by a space group symmetry of P3112 with 3-fold screw axis which is superlattice of α-NaFeO2 type structure. (c) The scheme generated by random stacking with 3-[√3 × √3] R30° type unit cells. There are three equal probabilities for the next addition of the first layer.
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Observed EDPs for both samples show extra spots and diffuse streaks in addition to the fundamental spots of αNaFeO2 type structure with an R3¯m symmetry. The [00.1] zone EDPs of LiMg1/3Co1/3Mn1/3O2 show clear extra spots due to the [√3 × √3] R30° type ordering in the transition metal layers. These EDPs are consistent with the calculated [00.1] zone EDP with a P3112 symmetry. Although the intensity is weak, the EDP of LiNi1/3Co1/3Mn1/3O2 shows similar superlattice spots. The smaller electron scattering factor of Mg compared with
Fig. 8. Charge and discharge curves of Li/LiNi1/3−xMgxCo1/3Mn1/3O2 cells operated at 0.4 mA cm− 2 in the voltage range of 2.5–4.3 V at 23 °C. (a) x = 0.0, (b) x = 0.05, (c) x = 0.1, (d) x = 0.2 and (e) x = 0.33.
transition metals has enhanced the intensity of superlattice spots. The [11¯.0] zone EDPs show the diffuse streaks along the c⁎-axis, indicating the severe stacking disorder of [√3 × √3] R30° type ordered transition metal layers. The positions of diffuse streaks in the reciprocal c⁎-plane agree with the extra spots in the observed [00.1] EDPs. A lattice image in Fig. 5 reveals the layer stacking of LiMg1/3Co1/3Mn1/3O2 viewed along the [11¯.0] direction of rhombo-hexagonal R3¯m lattice. Fig. 5(a) is a low magnification image showing the particle shape and Fig. 5(b) shows an enlarged image of the region marked in Fig. 5(a). The inset in Fig. 5(b) shows the corresponding EDP. The objective aperture size used for the imaging is marked in it. The diffuse scatterings were incorporated for the imaging but the [11.L]⁎ fundamental spots were not. The regular (00.3) lattice fringes with spacing of 0.46 nm are visible. The (00.3) lattice fringes show a contrast modulation with a period of about 0.5 nm along the [11.0] direction. In some parts, the dot-like contrasts are visible, indicating the in-plane ordering. However, the arrangement of the dot-like contrast is not regular and is almost random, as marked by circles in Fig. 5 (b). This image clearly reveals the almost random stacking of [√3 × √3] R30° type ordered layers. Fig. 7. Calculated XRD and ED patterns using DIFFaX program based on the model structure shown in Fig. 6. (a) Calculated XRD pattern 1: space group symmetry of P3112 for the regular stacking, (b) calculated XRD pattern 2 for the random stacking, (c) observed XRD pattern for comparison and (d) calculated (11¯.0)⁎ reciprocal lattice plane corresponding to the calculated XRD pattern (b) for the random stacking.
3.4. Cationic ordering and simulation of the diffuse scattering As mentioned in the above sections, XRD and TEM measurements indicate the in-plane [√3 × √3] R30° type orderings
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and the almost random stacking of the ordered layers. The problem is the ordering in the triangular lattice plane. The following two models are plausible; (1) the ordering of Mg and (2) the ordering of Li atoms that migrated in the (Ni1−xMgx)1/3Co1/3Mn1/3 lattice plane. However, the following experimental results indicate that the former model is more plausible. The occupancy of Li atoms in the (Ni1−xMgx)1/3Co1/3Mn1/3 lattice plane is almost independent of the Mg composition according to the Rietveld analysis. However, the diffuse scattering intensity in the powder XRD pattern increases with an increasing Mg composition. If Li atoms are ordered the diffuse scattering intensity should be almost independent of the Mg composition. The observed intensity of diffuse scattering for LiMg1/3Co1/3 Mn1/3O2 was compared with the calculated one. DIFFaX program [14,15] was used for the calculation. For convenience, a complete [√3 × √3] R30° type ordering of Mg, Co and Mn and three types of unit layers were assumed as shown in Fig. 6(a). The lattice dimension and the atomic positions in the 3 unit layers are derived from the results of Rietveld analysis for LiMg1/3Co1/3Mn1/3O2. The cell parameters are identical for 3 unit layers and are a = 4.95 Å and c = 4.748 Å. A wavelength of 1.5405 Å and a pseudo-Voigt profile function (u = 0.8, v = −0.3, w = 0.1 and σ = 0.6) were used. The unit cell of P3112 type ordering was divided into three unit layers (a1, a2 and a3), since there are three ways of arranging Mg, Co and Mn atoms in the [√3 × √3] R30° type cell. We chose cell parameters of a = 4.95 Å and c = 4.748 Å, which were obtained by indexing the observed XRD pattern
Fig. 9. Differential capacity vs. voltage of Li/LiNi1/3−xMgxCo1/3Mn1/3O2 cells cycled at 0.4 mA cm− 2 in the voltage range of 2.5–4.3 V at 23 °C. (a) x = 0.0, (b) x = 0.05, (c) x = 0.1, (d) x = 0.2 and (e) x = 0.33.
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Fig. 10. Intermittent charge and discharge curves (1 h on and 12 h off) of (a) Li/ LiNi1/3Co1/3Mn1/3O2 cell and (b) Li/LiMg1/3Co1/3Mn1/3O2 cell operated at 0.1 mA cm− 2 in the voltage range of 2.5–4.5 V at 23 °C.
based on the P3112 cell. The superstructure with space group symmetry of P3112 cell is illustrated in Fig. 6(b). The stacking scheme of this structure can be described as a regular sequence, a1-b1-c1. The probability from one layer to the next layer is absolutely 1.0. A stacking fault is introduced as shown in the scheme of Fig. 6(c). There are a number of different ways of generating faults in the stacking of transition metal layers. For example, one stacking sequence is represented as a1 → (b1, b2, or b3) → (c1, c2, or c3). All the probabilities were set to 1/3. Fig. 7 shows the observed XRD pattern and simulated XRD patterns of both P3112 and random stacking models for LiMg1/3Co1/3Mn1/3O2. The intensity was normalized so that the 00.3 peak height is to be 100. Fig. 7(a) for the P3112 model, shows sharp 10L superlattice reflections (L = 0, 1, 2, 3) due to the [√3 × √3] R30° type ordering. Fig. 7(b) for the random stacking model shows a peak at d = 4.2 Å with the tails on the higher angle side. The observed profile in Fig. 7(c) shows some similarity with the simulated profile for the random stacking model. However, the intensity of diffuse scattering and the peak shape are
Fig. 11. Open-circuit voltage (OCV) of Li/LiNi1/3−xMgxCo1/3Mn1/3O2 cells (0 = x = 1/3) at the charge state.
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somewhat different. The detailed profile analysis indicated that the observed profile of diffuse scattering can be divided into three components as shown in the figure. This means that the stacking of [√3 × √3] R30° type ordered layers is not completely random and is short-range-ordered. Also the weaker observed intensity indicates that the in-plane ordering is not perfect. Anyway, the observed features of diffuse scattering can be almost reproduced by the random stacking model of [√3 × √3] R30° type ordered layers. The calculated electron diffraction pattern for the random stacking model is shown in Fig. 7(d). Observed EDPs in Fig. 4(d) and (e) show remarkable resemblance with the calculated EDPs of the random stacking model shown in Fig. 7(d). The formation of the complete solid solution suggests a similar structure for LiNi1/3Co1/3Mn1/3O2, although there may exist some difference in the order parameters.
LiNi1/3Co1/3Mn1/3O2 and LiMg1/3Co1/3Mn1/3O2. In contrast to the small polarization observed for LiNi1/3Co1/3Mn1/3O2, LiMg1/3Co1/3Mn1/3O2 shows a larger cell polarization. This polarization becomes more significant at the discharge state which may be ascribable to the structural change as seen in LiNi1−xMgxO2. Subsequently, thus the obtained open-circuit voltages measured at the charge state are plotted in Fig. 11. The open-circuit voltages for the samples with x = 0.0, 0.05 and 0.1 are almost identical in the capacity range below 180 mAh g− 1, although the theoretical capacities due to Ni2+/4+ redox are 185.3, 160.3 and 134.5 mAh g− 1, respectively. The potential for the samples with x = 0.2 and x = 0.33 is higher than that for other samples with x = 0.1. This may indicate that the charge and discharge within the capacity range below 180 mAh g− 1 can be affected not only by the Ni2+/4+ redox, but also by Co3+/4+. 4. Conclusions
3.5. Electrochemical properties The first five charge and discharge curves of the Li/LiNi1/3−x MgxCo1/3Mn1/3O2 cells (x = 0.0, 0.05, 0.1, 0.2 and 0.33) measured at 23 °C are shown in Fig. 8. The measurements were carried out in the voltage range from 2.5 to 4.3 V at a constant current density of 0.4 mA cm− 2. The specific capacity decreases monotonically with the Mg content, since Mg2+ ions do not participate in the redox process, and the theoretical capacity also decreases from 278 mAh g− 1 in LiNi1/3Co1/3Mn1/ 3O2 to 105 mAh g− 1 in LiMg1/3Co1/3Mn1/3O2. A large loss of reversibility is also observed at the end of first cycle for the Mgsubstituted samples. In order to investigate the crystal structure of LiMg1/3Co1/3Mn1/3O2 after the first cycle, the cell was decomposed and the XRD measurement of the electrode containing LiMg1/3Co1/3Mn1/3O2 was carried out. The crystal structure of LiMg1/3Co1/3Mn1/3O2 after the first cycle still kept a layered rock-salt structure. A structural study of the LiNi1−xMgxO2 system implies that the irreversible Mg migration from transition metal layers to lithium layers occurs during the first charge, leading to a significant irreversible capacity at the first cycle [18]. Although there is a possibility that such a structural change occurs for LiNi1/3−xMgxCo1/3Mn1/3O2, the reason of the irreversible capacity is not well understood at the present stage. Fig. 9 shows the differential capacity vs. the voltage curves of the Li/LiNi1/3−xMgxCo1/3Mn1/3O2 cells obtained from the charge and discharge curves in Fig. 8. There appear two broad peaks in the differential centered at around 3.75 V for x = 0.0, x = 0.05, and x = 0.1 samples. As the Mg content increases from x = 0.1 to x = 0.33, the voltage peaks shift to higher potentials. The redox potential of LiMg1/3Co1/3Mn1/3O2 stays at 4.1 V and Co ions can be active. The redox potential of Li1−xCoO2 (x = 0.5) has been also confirmed to be around 4.1 V in many literatures [3,19]. It was found that both potentials are quite similar. Furthermore, in order to examine the open-circuit voltage of Li/LiNi1/3−xMgxCo1/3Mn1/3O2 cells, intermittent charge and discharge measurements were carried out. The current was automatically switched on for 1 h and off for 12 h. Fig. 10 shows the intermittent charge and discharge curves for
Single phase of LiMg1/3Co1/3Mn1/3O2 and LiNi1/3−xMgxCo1/3 Mn1/3O2 (0.0 b x b 0.33) solid solutions were successfully synthesized for the first time. LiMg1/3Co1/3Mn1/3O2 forms a well-ordered [√3 × √3] R30° type superlattice in the transition metal layers. However, the stacking of the ordered layers along the c-axis is almost random. The ordering for LiMg1/ 3Co1/3Mn1/3O2 is two dimensional similar to the turbostatic structure of graphite [20]. The solid solution LiNi1/3−xMgxCo1/ 3Mn1/3O2 also shows a similar two dimensional ordering. The cationic ordering in LiNi1/3Co1/3Mn1/3O2 seems to be similar, although there may exist some difference in the order parameters. Hereafter, a more precise synthesis and an intimate evaluation of the cation arrangement for LiNi1/3Co1/3Mn1/3O2 are necessary. Electrochemical measurements indicate that the substitution of Mg for Ni in LiNi1/3Co1/3Mn1/3O2 leads to higher redox potentials. In this solution system, Co ions can be active and show similar voltage to those in Li1−xCoO2, where the redox reaction of Co3+/4+ takes place. References [1] E. Rossen, C.D.W. Jones, J.R. Dahn, Solid State Ionics 57 (1992) 311. [2] Z. Lu, D.D. Macneil, J.R. Dahn, Electrochem. Solid-state Lett. 4 (2001) 200. [3] Y. Koyama, I. Tanaka, H. Adachi, Y. Makimura, T. Ohzuku, J. Power Sources 119–121 (2003) 644. [4] T. Ohzuku, Y. Makimura, Chem. Lett. (2001) 744. [5] N. Yabuuchi, T. Ohzuku, J. Power Sources 119–121 (2003) 171. [6] H. Kobayashi, Y. Arachi, S. Emura, H. Kageyama, K. Tatsumi, K.T. Kamiyama, J. Power Sources 146 (2005) 640. [7] Y. Koyama, N. Yabuuchi, I. Tanaka, H. Adachi, T. Ohzuku, J. Electrochem. Soc. 151 (2004) A1545. [8] A. Van der Ven, G. Ceder, Electrochem. Commun. 6 (2004) 1045. [9] N. Yabuuchi, Y. Koyama, N. Nakayama, T. Ohzuku, J. Electrochem. Soc. 152 (2005). [10] P.S. Whitfield, I.J. Davidson, L.M.D. Cranswick, I.P. Swainson, P.W. Stephens, Solid State Ionics 176 (2005) 463. [11] G. Blasse, J. Phys. Chem. Solids 27 (1966) 383. [12] Y.W. Tsai, B.J. Hwang, G. Ceder, H.S. Sheu, D.G. Liu, J.F. Lee, Chem. Mater. 17 (2005) 3191. [13] F. Izumi, in: R.A. Young (Ed.), The Rietveld Method, Oxford Univ. Press, Oxford, 1993, ch. 13.
Y. Fujii et al. / Solid State Ionics 178 (2007) 849–857 [14] M.M.J. Treacy, J.M. Newsam, M.W. Deem, Proc. R. Soc. Lond. A433 (1991) 499. [15] J. Breger, M. Jiang, N. Dupre, Y. S.-Horn, G. Ceder, P. Grey, J. Solid State Chem. 178 (2005) 2575. [16] Von M. Jansen, R. Hoppe, Z. Anorg. Allg. Chem. 397 (1973) 279. [17] R.D. Shannon, C.T. Prewitt, Acta Crystallogr. B25 (1969) 925.
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[18] C. Pouillerie, L. Croguennec, Ph Biensan, C. Willmann, C Delmas, J. Electrochem. Soc. 147 (2000) 2061. [19] G.G. Amatucci, J.M. Tarascon, L.C Klein, J. Electrochem. Soc. 143 (1996) 1114. [20] E. Warren, Phys. Rev. 59 (1941) 693.
Structural and electrochemical properties of LiNi1/3Co1/3Mn1/3O2–LiMg1/3Co1/3Mn1/3O2 solid solutions Yasuhiro Fujii a,b,⁎, Hiroshi Miura a , Naoto Suzuki a , Takayuki Shoji a , Noriaki Nakayama b,⁎ a
b
Tosoh Co., Ltd, 4560 Kaisei-cho, Syunan, Yamaguchi 746-8501, Japan Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube, 755-8611, Japan
Received 7 September 2006; received in revised form 19 February 2007; accepted 8 March 2007
Abstract The complete solid solutions in the pseudo-binary LiNi1/3Co1/3Mn1/3O2–LiMg1/3Co1/3Mn1/3O2 system with α-NaFeO2 type layered rocksalt structure have been successfully synthesized. The replacement of Ni with Mg atoms has enhanced the diffraction intensity due to cation ordering in α-NaFeO2 type structure. Powder X-ray diffraction (XRD) patterns of LiMg1/3Co1/3Mn1/3O2 show a broad and diffuse peak with an intensity maximum at around d = 4.2 Å indicating an in-plane [√3 × √3] R30° type ordering. Electron diffraction (ED) patterns also show clear and intense superlattice spots due to the in-plane [√3 × √3] R30° type ordered layers. However, the [√3 × √3] R30° type ordered layers are almost randomly stacked as evidenced by diffuse scattering in the powder XRD and ED patterns. The TEM lattice image clearly reveals the random stacking. XRD and ED patterns agree with the simulated ones using the DIFFaX program based on the above structural model. Solid solution LiNi1/3−xMg xCo1/3Mn1/3O2 (0.0 ≤ x ≤ 0.33) also show a similar two dimensional cation ordering. Electrochemical measurements of LiNi1/3−xMgxCo1/3Mn1/3O2 indicate that not only Ni but also Co can be active as redox species in this solution system. The redox potential of Co in LiMg1/3Co1/3Mn1/3O2 is about 4.1 V. © 2007 Elsevier B.V. All rights reserved. Keywords: Li-ion batteries; Cation ordering; Diffuse scattering; Layer stacking; Turbostatic
1. Introduction Recently, compounds in the Li–Ni–Co–Mn–O system have been proposed as possible alternatives to LiCoO2 that is widely used in current Li-ion batteries [1–3]. Among them, LiNi1/3Co1/3Mn1/3O2 is the most possible candidate [4,5]. The remarkable properties of these materials are related to the valence state of transition metals; 2+, 3+, and 4+ for Ni, Co, and Mn, respectively [6,7]. This material crystallizes in α-NaFeO2 type layered structure based on cubic close-packed oxygen atoms, in which lithium layers and transition metal (111) layers are alternated. Some superstructure models for the cation ordering in the α-NaFeO2 type structure have been reported [8–10]. Ohzuku et al. have proposed a [√3 × √3] R30° type cation ⁎ Corresponding authors. Fujii is to be contacted at Tosoh Co., Ltd, 4560 Kaisei-cho, Syunan, Yamaguchi 746-8501, Japan. Tel.: +81 834 63 9914; fax: +81 834 63 9896. Nakayama, Fax: +81 836 85 9601. E-mail addresses: [email protected] (Y. Fujii), [email protected] (N. Nakayama). 0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2007.03.002
ordering for LiNi1/3Co1/3Mn1/3O2 with a space group symmetry of P3112 and have observed weak superlattice reflections of a [√3 × √3] R30° type ordering in the electron diffraction patterns [9]. On the other hand, P.S. Whitfield et al. reported that there was no evidence of superstructure and that the data were consistent with a random distribution of Ni, Co, and Mn over R3¯m 3b-sites based on the synchrotron XRD and neutron diffraction measurements [10]. Therefore, no strong evidence has been reported for superstructure and the cationic arrangements in these phases are still controversial. One of the reasons why the structures and the cationic distribution have not been confirmed adequately is the similarity of X-ray atomic scattering factors among Ni, Co, and Mn. The substitution of Mg for Ni in LiNi1/3Co1/3Mn1/3O2 is interesting in the above context. The X-ray atomic scattering factor of Mg is much smaller than that of transition metals. If LiMg1/3Co1/3Mn1/3O2 can be synthesized, their cation orderings may be easily characterized by normal powder XRD and they must show some resemblances to those in LiNi1/3Co1/3Mn1/3O2. In fact, LiNi1/2Mn3/2O4 and LiMg1/2Mn3/2O4 with normal
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Table 1 The chemical compositions and oxidation states of LiNi1/3−xMgxCo1/3Mn1/3O2 measured by an ICP-AES and an iodometric titration, respectively Sample
x = 0.0 x = 0.05 x = 0.1 x = 0.2 x = 0.33
Average transition metals valence
LiNi0.34Co0.33Mn0.33O2 LiNi0.29Mg0.05Co0.33Mn0.33O2 LiNi0.24Mg0.10Co0.33Mn0.33O LiNi0.14Mg0.20Co0.33Mn0.33O2 LiMg0.34Co0.33Mn0.33O2
Target stoichiometry
Li1.00Ni0.33Co0.33Mn0.34O2 Li1.00Ni0.29Mg0.05Co0.33Mn0.33O Li1.00Ni0.24Mg0.10Co0.33Mn0.33O2 Li1.01Ni0.13Mg0.20Co0.33Mn0.33O2 Li1.01Mg0.33Co0.33Mn0.34O2
spinel structures (AB2O4) show the same cation ordering in the octahedral B-sublattice [11]. Then we have tried to prepare the solid solutions in the pseudo-binary LiNi1/3−xMgxCo1/3Mn1/3O2 system prepared in order to increase the difference in atomic scattering factors among cations and characterized them by using powder XRD and an electron diffraction. The electrochemical properties of LiNi1/3−xMgxCo1/3Mn1/3O2 are also interesting. It is still controversial whether the Co3+/4+ redox in LiNi1/3Co1/3Mn1/3O2 can be active or not, because the redox potential of Co3+/4+ partially overlaps that of Ni2+/4+ [6,12]. The redox potential of Co3+/4+ in a similar structure can
Measured stoichiometry Predicted
Measured
3.00 3.08 3.15 3.33 3.50
3.02 3.09 3.17 3.32 3.55
be measured without the redox potential of Ni overlapping in virtue of successfully synthesizing LiMg1/3Co1/3Mn1/3O2. 2. Experimental Samples with a nominal composition of LiNi1/3−xMgxCo1/3 Mn1/3O2 (x = 0.0, 0.05, 0.1, 0.2 and 0.33) were prepared by heating the mixtures of LiOH·H2O and (Ni,Mg,Co,Mn)(OH)2. Calcinations were made at 900 °C for 12 h in air stream. Excess lithium hydroxide was added to compensate for the Li loss by volatilization. A heating and cooling rate of 100 °C h− 1 was
Fig. 1. XRD patterns of LiNi1/3−xMgxCo1/3Mn1/3O2. (a) x = 0.0, (b) x = 0.05, (c) x = 0.1, (d) x = 0.2 and (e) x = 0.33. Open circles, solid lines and vertical marks show observed data, calculated profiles and peak positions, respectively. The difference plots are also added. The structural parameters and the R-factor are given in Table 1.
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Table 1 also shows the average valence of transition metals measured by an iodometric titration. Experimental valences agree with the calculated ones from the composition. X-ray powder diffraction patterns indicate that complete solid solutions are obtained in the pseudo-binary LiNi1/3Co1/3Mn1/3O2– LiMg1/3Co1/3Mn1/3O2 system. 3.2. XRD patterns
Fig. 2. Lattice parameters and the Li occupancy at 3a-sites of LiNi1/3−xMgxCo1/3 Mn1/3O2.
applied for all temperature settings. Samples were identified and characterized by powder XRD and ED. Structural refinements were made by using a Rietveld program, Rietan-2000 [13]. Simulations of the diffuse scattering were also performed with the DIFFaX program written by Newsam and Treacy [14,15]. For ED measurements, samples were dispersed in ethanol by applying an ultrasonic wave and were collected on a holly microgrid supported on a copper grid mesh. A field emission type TEM (JEOL JEM2010F) operated at 200 kV was used. The chemical composition and the average oxidation state of transition metals were determined by ICP-AES and iodometric titration, respectively [16]. Electrochemical properties were examined with CR2032 type coin cells. The cell was comprised of a cathode and a lithium metal anode separated by a polypropylene separator and glass fiber mat. The cathode consisted of 25 mg of LiNi1/3−xMgxCo1/3Mn1/3O2 powders and 12 mg of conducting binder pressed on a stainless screen. The electrolyte solution was 1 M LiPF6/EC and DMC. The EC and DMC were mixed in a 1:2 volume ratio. The cell was charged and discharged in the voltage range of 2.5–4.3 V at a current density of 0.4 mA cm− 2 at 23 °C. 3. Results and discussion 3.1. Chemical analysis The chemical compositions of all samples measured by ICP-AES, as listed in Table 1, correspond to the expected ones.
Fig. 1 shows X-ray diffraction patterns of the pseudo-binary solid solutions. The line plots in Fig. 1 show the confidence plots of Rietveld analysis. The differences between observed and calculated patterns were also shown below the confidence plots. The XRD patterns indicate that all the samples are single phase materials with α-NaFeO2 type structure. All the peaks can be indexed by assuming the hexagonal setting of the space group symmetry of R3¯m. The 00.6/01.2 and the 10.8/11.0 pairs of reflections, which are typical of a well developed α-NaFeO2 type layered structure, are well resolved for all samples. Rietveld refinements were performed in order to evaluate lattice parameters and site occupancy of metal atoms. The lattice parameters in the hexagonal setting and Li occupancy at 3a-site in the hexagonal setting are plotted in Fig. 2 as a function of the Mg composition x. The structural parameters obtained by Rietveld refinements are also listed in Table 2. The parameters, Rwp and S, represent a reliability factor and a scale factor, respectively. The latter parameter indicates goodness of fitting [13]. Although unit cell parameters do not show significant compositional dependence because of the similarity in the ionic radii between Ni2+ (0.84 Å) and Mg2+ (0.86 Å), the a-axis length decreases monotonically with increasing Mg composition. The c-axis length of Mg-substituted samples is almost independent of the composition and is larger than that of LiNi1/3Co1/3Mn1/3O2. The change of unit cell parameters may be related to the behaviors of the cation exchange. Ideally, Li atoms occupy 3a-sites, and Mg, Ni, Co and Mn atoms are randomly placed on 3b-sites. This model does not reproduce the observed profiles. The site exchange of Li atoms with other metal atoms was assumed. The results shown in Table 2 were obtained by assuming that Ni or Mg atoms preferentially occupy 3a-sites. Since the ionic radii of Ni2+
Table 2 Structural and fitting parameters obtained by Rietveld analysis of XRD patterns based on layered rock-salt structure (s.g. symmetry: R3¯m) Sample a (Å)
c (Å)
c/a
zox a
g(Li) at Rwp (%) S 3a-site b
x = 0.0 x = 0.05 x = 0.10 x = 0.20 x = 0.33
14.253 (1) 14.263 (1) 14.261 (1) 14.263 (1) 14.262(1)
4.974 4.978 4.978 4.984 4.989
0.2410 (2) 0.2408 (2) 0.2411 (2) 0.2413 (5) 0.2415 (1)
0.985 0.956 0.943 0.946 0.936
2.8659 (2) 2.8653 (1) 2.8650 (1) 2.8619 (1) 2.8587 (3)
13.6 10.6 10.4 11.6 12.5
1.75 1.85 1.81 1.95 1.49
Rietveld analysis has been performed starting from the structure in which Li and other metal atoms are located at 3a and 3b-sites, respectively. a zox: z value of oxygen atom at 6c-site. b g(Li) at 3a-site: Li content at 3a-site lithium layer.
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Fig. 3. Logarithmic plots of XRD patterns for LiNi1/3−xMgxCo1/3Mn1/3O2. (a) x = 0.0, (b) x = 0.05, (c) x = 0.1, (d) x = 0.2 and (e) x = 0.33. All patterns were measured using a sample folder made of quartz single crystal, to avoid the background intensity from the sample holder.
(0.84 Å) and Mg2+ (0.86 Å) are close to those of Li+ (0.88 Å), a small amount of Ni or Mg may occupy 3a-sites. The calculation was also constrained assuming that Mg2+ at 3b-sites predom-
inantly exchanges for Li at 3a-site rather than Ni2+, since both ionic radii and electronic structure of Li+ are more similar to those of Mg2+ than those of Ni2+, respectively [17]. Furthermore, C. Pouillerie et al. have confirmed the preference of Mg2+ than Ni2+ to occupy 3a-sites in the LiNi1−xMgxO2 system by their magnetic study [18]. The refinement gives a Li/Ni site exchange ratio of 1.5% for x =0.0 sample and Li/Mg site exchange ratios in the range from 4.5% to 6.3% for all the Mg-substituted samples. The cationic exchange ratios were evaluated to be higher for Mg-substituted samples than LiNi1/3Co1/3Mn1/3O2 and are almost constant. No sharp superlattice peak indicating [√3 × √3] R30° type ordering as expected from the P3112 model structure proposed by Ohzuku et al. [9] was observed. Fig. 3 shows XRD patterns measured using a sample folder made of quartz single crystal, to avoid the background intensity from the sample holder. The XRD pattern of LiMg1/3Co1/3Mn1/3O2 shows a fairly intense diffuse peak in the 2θ range from 18 to 25°. The shape of the diffuse peak is characteristic of LiMg1/3Co1/3Mn1/3O2 compared with that in the related phase like Li2MnO3 [15]. In the case of Li2MnO3 (Li[Li1/3Mn2/3]O2), fairly sharp superlattice peaks and their tails on the higher angle side are observed. The intensity of the diffuse scattering tends to decrease with decreasing Mg composition. The diffraction pattern for x = 0.05 shows no diffuse peak similar to the one for x = 0.0. One may suspect some amorphous components as the origin of diffuse peaks. TEM observations indicated that the diffuse scattering was not caused by the amorphous component but originated from the two dimensional cation ordering. We have also analyzed the content of impurities (silicon, sodium, and
Fig. 4. Electron diffraction patterns of LiNi1/3Co1/3Mn1/3O2 and LiMg1/3Co1/3Mn1/3O2. (a) observed [00.1] zone ED pattern of LiNi1/3Co1/3Mn1/3O2, (b) observed [00.1] zone ED pattern of LiMg1/3Co1/3Mn1/3O2, (c) (00.1)⁎ reciprocal lattice plane calculated for LiMg1/3Co1/3Mn1/3O2 (s.g. symmetry: P3112), (d) observed ¯.0)⁎ reciprocal lattice plane calculated for [11¯.0] zone ED pattern of LiNi1/3Co1/3Mn1/3O2, (e) observed [11¯.0] zone ED pattern of LiMg1/3Co1/3Mn1/3O2 and (f) (11 LiMg1/3Co1/3Mn1/3O2 (s.g. symmetry: P3112). The diffraction spots have been indexed assuming the R3¯m structure in hexagonal setting. Extra spots and diffuse streaks can be seen for an observed pattern in addition to fundamental spots.
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Fig. 5. TEM images and the corresponding electron diffraction pattern of LiMg1/3Co1/3Mn1/3O2 viewed along the [11¯.0] direction. (a) A low magnification image of the observed particle, (b) a lattice image of the thin region marked in (a). The inset shows the corresponding ED pattern and the objective aperture size is shown in it.
boron) in order to evaluate the contamination during synthesis. Since no above impurities were detected, the diffuse peaks in XRD patterns were irrelevant to the impurities. 3.3. Electron diffraction and lattice images Fig. 4(a), (b), (d) and (e) shows observed [00.1] and [11¯.0] zone ED patterns (EDPs) of two end-members of the binary
solid solution system, LiMg1/3Co1/3Mn1/3O2 and LiNi1/3Co1/3 Mn1/3O2. Fig. 4(c) and (f) shows calculated [00.1] and [11¯.0] zone EDPs of LiMg1/3Co1/3Mn1/3O2 with a space group symmetry of P3112. All the indexes are in the hexagonal setting of the R3¯m space group. No amorphous component was observed during TEM observation. TEM–EDX analysis for several particles indicated that the atomic ratio of Mg:Ni:Mn is close to 1.0: 1.0: 1.0.
Fig. 6. Schemes generated by stacking [√3 × √3] R30° type layers of LiMg1/3Co1/3Mn1/3O2. (a) [√3 × √3] R30° type unit cell, a1, a2, and a3. The Ni, Co, and Mn ions are denoted by round, triangular and square symbols, respectively. (b) The stacking order generated by a space group symmetry of P3112 with 3-fold screw axis which is superlattice of α-NaFeO2 type structure. (c) The scheme generated by random stacking with 3-[√3 × √3] R30° type unit cells. There are three equal probabilities for the next addition of the first layer.
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Observed EDPs for both samples show extra spots and diffuse streaks in addition to the fundamental spots of αNaFeO2 type structure with an R3¯m symmetry. The [00.1] zone EDPs of LiMg1/3Co1/3Mn1/3O2 show clear extra spots due to the [√3 × √3] R30° type ordering in the transition metal layers. These EDPs are consistent with the calculated [00.1] zone EDP with a P3112 symmetry. Although the intensity is weak, the EDP of LiNi1/3Co1/3Mn1/3O2 shows similar superlattice spots. The smaller electron scattering factor of Mg compared with
Fig. 8. Charge and discharge curves of Li/LiNi1/3−xMgxCo1/3Mn1/3O2 cells operated at 0.4 mA cm− 2 in the voltage range of 2.5–4.3 V at 23 °C. (a) x = 0.0, (b) x = 0.05, (c) x = 0.1, (d) x = 0.2 and (e) x = 0.33.
transition metals has enhanced the intensity of superlattice spots. The [11¯.0] zone EDPs show the diffuse streaks along the c⁎-axis, indicating the severe stacking disorder of [√3 × √3] R30° type ordered transition metal layers. The positions of diffuse streaks in the reciprocal c⁎-plane agree with the extra spots in the observed [00.1] EDPs. A lattice image in Fig. 5 reveals the layer stacking of LiMg1/3Co1/3Mn1/3O2 viewed along the [11¯.0] direction of rhombo-hexagonal R3¯m lattice. Fig. 5(a) is a low magnification image showing the particle shape and Fig. 5(b) shows an enlarged image of the region marked in Fig. 5(a). The inset in Fig. 5(b) shows the corresponding EDP. The objective aperture size used for the imaging is marked in it. The diffuse scatterings were incorporated for the imaging but the [11.L]⁎ fundamental spots were not. The regular (00.3) lattice fringes with spacing of 0.46 nm are visible. The (00.3) lattice fringes show a contrast modulation with a period of about 0.5 nm along the [11.0] direction. In some parts, the dot-like contrasts are visible, indicating the in-plane ordering. However, the arrangement of the dot-like contrast is not regular and is almost random, as marked by circles in Fig. 5 (b). This image clearly reveals the almost random stacking of [√3 × √3] R30° type ordered layers. Fig. 7. Calculated XRD and ED patterns using DIFFaX program based on the model structure shown in Fig. 6. (a) Calculated XRD pattern 1: space group symmetry of P3112 for the regular stacking, (b) calculated XRD pattern 2 for the random stacking, (c) observed XRD pattern for comparison and (d) calculated (11¯.0)⁎ reciprocal lattice plane corresponding to the calculated XRD pattern (b) for the random stacking.
3.4. Cationic ordering and simulation of the diffuse scattering As mentioned in the above sections, XRD and TEM measurements indicate the in-plane [√3 × √3] R30° type orderings
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and the almost random stacking of the ordered layers. The problem is the ordering in the triangular lattice plane. The following two models are plausible; (1) the ordering of Mg and (2) the ordering of Li atoms that migrated in the (Ni1−xMgx)1/3Co1/3Mn1/3 lattice plane. However, the following experimental results indicate that the former model is more plausible. The occupancy of Li atoms in the (Ni1−xMgx)1/3Co1/3Mn1/3 lattice plane is almost independent of the Mg composition according to the Rietveld analysis. However, the diffuse scattering intensity in the powder XRD pattern increases with an increasing Mg composition. If Li atoms are ordered the diffuse scattering intensity should be almost independent of the Mg composition. The observed intensity of diffuse scattering for LiMg1/3Co1/3 Mn1/3O2 was compared with the calculated one. DIFFaX program [14,15] was used for the calculation. For convenience, a complete [√3 × √3] R30° type ordering of Mg, Co and Mn and three types of unit layers were assumed as shown in Fig. 6(a). The lattice dimension and the atomic positions in the 3 unit layers are derived from the results of Rietveld analysis for LiMg1/3Co1/3Mn1/3O2. The cell parameters are identical for 3 unit layers and are a = 4.95 Å and c = 4.748 Å. A wavelength of 1.5405 Å and a pseudo-Voigt profile function (u = 0.8, v = −0.3, w = 0.1 and σ = 0.6) were used. The unit cell of P3112 type ordering was divided into three unit layers (a1, a2 and a3), since there are three ways of arranging Mg, Co and Mn atoms in the [√3 × √3] R30° type cell. We chose cell parameters of a = 4.95 Å and c = 4.748 Å, which were obtained by indexing the observed XRD pattern
Fig. 9. Differential capacity vs. voltage of Li/LiNi1/3−xMgxCo1/3Mn1/3O2 cells cycled at 0.4 mA cm− 2 in the voltage range of 2.5–4.3 V at 23 °C. (a) x = 0.0, (b) x = 0.05, (c) x = 0.1, (d) x = 0.2 and (e) x = 0.33.
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Fig. 10. Intermittent charge and discharge curves (1 h on and 12 h off) of (a) Li/ LiNi1/3Co1/3Mn1/3O2 cell and (b) Li/LiMg1/3Co1/3Mn1/3O2 cell operated at 0.1 mA cm− 2 in the voltage range of 2.5–4.5 V at 23 °C.
based on the P3112 cell. The superstructure with space group symmetry of P3112 cell is illustrated in Fig. 6(b). The stacking scheme of this structure can be described as a regular sequence, a1-b1-c1. The probability from one layer to the next layer is absolutely 1.0. A stacking fault is introduced as shown in the scheme of Fig. 6(c). There are a number of different ways of generating faults in the stacking of transition metal layers. For example, one stacking sequence is represented as a1 → (b1, b2, or b3) → (c1, c2, or c3). All the probabilities were set to 1/3. Fig. 7 shows the observed XRD pattern and simulated XRD patterns of both P3112 and random stacking models for LiMg1/3Co1/3Mn1/3O2. The intensity was normalized so that the 00.3 peak height is to be 100. Fig. 7(a) for the P3112 model, shows sharp 10L superlattice reflections (L = 0, 1, 2, 3) due to the [√3 × √3] R30° type ordering. Fig. 7(b) for the random stacking model shows a peak at d = 4.2 Å with the tails on the higher angle side. The observed profile in Fig. 7(c) shows some similarity with the simulated profile for the random stacking model. However, the intensity of diffuse scattering and the peak shape are
Fig. 11. Open-circuit voltage (OCV) of Li/LiNi1/3−xMgxCo1/3Mn1/3O2 cells (0 = x = 1/3) at the charge state.
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somewhat different. The detailed profile analysis indicated that the observed profile of diffuse scattering can be divided into three components as shown in the figure. This means that the stacking of [√3 × √3] R30° type ordered layers is not completely random and is short-range-ordered. Also the weaker observed intensity indicates that the in-plane ordering is not perfect. Anyway, the observed features of diffuse scattering can be almost reproduced by the random stacking model of [√3 × √3] R30° type ordered layers. The calculated electron diffraction pattern for the random stacking model is shown in Fig. 7(d). Observed EDPs in Fig. 4(d) and (e) show remarkable resemblance with the calculated EDPs of the random stacking model shown in Fig. 7(d). The formation of the complete solid solution suggests a similar structure for LiNi1/3Co1/3Mn1/3O2, although there may exist some difference in the order parameters.
LiNi1/3Co1/3Mn1/3O2 and LiMg1/3Co1/3Mn1/3O2. In contrast to the small polarization observed for LiNi1/3Co1/3Mn1/3O2, LiMg1/3Co1/3Mn1/3O2 shows a larger cell polarization. This polarization becomes more significant at the discharge state which may be ascribable to the structural change as seen in LiNi1−xMgxO2. Subsequently, thus the obtained open-circuit voltages measured at the charge state are plotted in Fig. 11. The open-circuit voltages for the samples with x = 0.0, 0.05 and 0.1 are almost identical in the capacity range below 180 mAh g− 1, although the theoretical capacities due to Ni2+/4+ redox are 185.3, 160.3 and 134.5 mAh g− 1, respectively. The potential for the samples with x = 0.2 and x = 0.33 is higher than that for other samples with x = 0.1. This may indicate that the charge and discharge within the capacity range below 180 mAh g− 1 can be affected not only by the Ni2+/4+ redox, but also by Co3+/4+. 4. Conclusions
3.5. Electrochemical properties The first five charge and discharge curves of the Li/LiNi1/3−x MgxCo1/3Mn1/3O2 cells (x = 0.0, 0.05, 0.1, 0.2 and 0.33) measured at 23 °C are shown in Fig. 8. The measurements were carried out in the voltage range from 2.5 to 4.3 V at a constant current density of 0.4 mA cm− 2. The specific capacity decreases monotonically with the Mg content, since Mg2+ ions do not participate in the redox process, and the theoretical capacity also decreases from 278 mAh g− 1 in LiNi1/3Co1/3Mn1/ 3O2 to 105 mAh g− 1 in LiMg1/3Co1/3Mn1/3O2. A large loss of reversibility is also observed at the end of first cycle for the Mgsubstituted samples. In order to investigate the crystal structure of LiMg1/3Co1/3Mn1/3O2 after the first cycle, the cell was decomposed and the XRD measurement of the electrode containing LiMg1/3Co1/3Mn1/3O2 was carried out. The crystal structure of LiMg1/3Co1/3Mn1/3O2 after the first cycle still kept a layered rock-salt structure. A structural study of the LiNi1−xMgxO2 system implies that the irreversible Mg migration from transition metal layers to lithium layers occurs during the first charge, leading to a significant irreversible capacity at the first cycle [18]. Although there is a possibility that such a structural change occurs for LiNi1/3−xMgxCo1/3Mn1/3O2, the reason of the irreversible capacity is not well understood at the present stage. Fig. 9 shows the differential capacity vs. the voltage curves of the Li/LiNi1/3−xMgxCo1/3Mn1/3O2 cells obtained from the charge and discharge curves in Fig. 8. There appear two broad peaks in the differential centered at around 3.75 V for x = 0.0, x = 0.05, and x = 0.1 samples. As the Mg content increases from x = 0.1 to x = 0.33, the voltage peaks shift to higher potentials. The redox potential of LiMg1/3Co1/3Mn1/3O2 stays at 4.1 V and Co ions can be active. The redox potential of Li1−xCoO2 (x = 0.5) has been also confirmed to be around 4.1 V in many literatures [3,19]. It was found that both potentials are quite similar. Furthermore, in order to examine the open-circuit voltage of Li/LiNi1/3−xMgxCo1/3Mn1/3O2 cells, intermittent charge and discharge measurements were carried out. The current was automatically switched on for 1 h and off for 12 h. Fig. 10 shows the intermittent charge and discharge curves for
Single phase of LiMg1/3Co1/3Mn1/3O2 and LiNi1/3−xMgxCo1/3 Mn1/3O2 (0.0 b x b 0.33) solid solutions were successfully synthesized for the first time. LiMg1/3Co1/3Mn1/3O2 forms a well-ordered [√3 × √3] R30° type superlattice in the transition metal layers. However, the stacking of the ordered layers along the c-axis is almost random. The ordering for LiMg1/ 3Co1/3Mn1/3O2 is two dimensional similar to the turbostatic structure of graphite [20]. The solid solution LiNi1/3−xMgxCo1/ 3Mn1/3O2 also shows a similar two dimensional ordering. The cationic ordering in LiNi1/3Co1/3Mn1/3O2 seems to be similar, although there may exist some difference in the order parameters. Hereafter, a more precise synthesis and an intimate evaluation of the cation arrangement for LiNi1/3Co1/3Mn1/3O2 are necessary. Electrochemical measurements indicate that the substitution of Mg for Ni in LiNi1/3Co1/3Mn1/3O2 leads to higher redox potentials. In this solution system, Co ions can be active and show similar voltage to those in Li1−xCoO2, where the redox reaction of Co3+/4+ takes place. References [1] E. Rossen, C.D.W. Jones, J.R. Dahn, Solid State Ionics 57 (1992) 311. [2] Z. Lu, D.D. Macneil, J.R. Dahn, Electrochem. Solid-state Lett. 4 (2001) 200. [3] Y. Koyama, I. Tanaka, H. Adachi, Y. Makimura, T. Ohzuku, J. Power Sources 119–121 (2003) 644. [4] T. Ohzuku, Y. Makimura, Chem. Lett. (2001) 744. [5] N. Yabuuchi, T. Ohzuku, J. Power Sources 119–121 (2003) 171. [6] H. Kobayashi, Y. Arachi, S. Emura, H. Kageyama, K. Tatsumi, K.T. Kamiyama, J. Power Sources 146 (2005) 640. [7] Y. Koyama, N. Yabuuchi, I. Tanaka, H. Adachi, T. Ohzuku, J. Electrochem. Soc. 151 (2004) A1545. [8] A. Van der Ven, G. Ceder, Electrochem. Commun. 6 (2004) 1045. [9] N. Yabuuchi, Y. Koyama, N. Nakayama, T. Ohzuku, J. Electrochem. Soc. 152 (2005). [10] P.S. Whitfield, I.J. Davidson, L.M.D. Cranswick, I.P. Swainson, P.W. Stephens, Solid State Ionics 176 (2005) 463. [11] G. Blasse, J. Phys. Chem. Solids 27 (1966) 383. [12] Y.W. Tsai, B.J. Hwang, G. Ceder, H.S. Sheu, D.G. Liu, J.F. Lee, Chem. Mater. 17 (2005) 3191. [13] F. Izumi, in: R.A. Young (Ed.), The Rietveld Method, Oxford Univ. Press, Oxford, 1993, ch. 13.
Y. Fujii et al. / Solid State Ionics 178 (2007) 849–857 [14] M.M.J. Treacy, J.M. Newsam, M.W. Deem, Proc. R. Soc. Lond. A433 (1991) 499. [15] J. Breger, M. Jiang, N. Dupre, Y. S.-Horn, G. Ceder, P. Grey, J. Solid State Chem. 178 (2005) 2575. [16] Von M. Jansen, R. Hoppe, Z. Anorg. Allg. Chem. 397 (1973) 279. [17] R.D. Shannon, C.T. Prewitt, Acta Crystallogr. B25 (1969) 925.
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[18] C. Pouillerie, L. Croguennec, Ph Biensan, C. Willmann, C Delmas, J. Electrochem. Soc. 147 (2000) 2061. [19] G.G. Amatucci, J.M. Tarascon, L.C Klein, J. Electrochem. Soc. 143 (1996) 1114. [20] E. Warren, Phys. Rev. 59 (1941) 693.