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Crystal structure analysis of γ-Fe2O3 with chemical lithium insertion
Solid State Ionics 255 (2014) 50–55
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Crystal structure analysis of γ-Fe2O3 with chemical lithium insertion Seungwon Park, Tamito Matsui, Shigeomi Takai, Takeshi Yao ⁎ Department of Fundamental Energy Science, Graduate School of Energy Science, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto, 606-8501, Japan
a r t i c l e
i n f o
Article history: Received 28 April 2013 Received in revised form 11 November 2013 Accepted 15 November 2013 Available online 12 December 2013 Keywords: γ-Fe2O3 Lithium insertion The Rietveld analysis Cation distribution Spinel structure Li-ion secondary battery
a b s t r a c t To better understand the lithium insertion mechanism on γ-Fe2O3, we investigated the crystal structure changes of γ-Fe2O3 in the process of chemical lithium insertion by using the X-ray Rietveld analysis. We newly found that the lithium inserted sample was a mixture of two phases both belonging to the space group Fd-3m, one is rich in lithium and the other is lean in lithium. It was revealed that the chemical lithium insertion process into γ-Fe2O3 occurs via a two phase reaction, i.e. inserted lithium forms a Li-rich phase in which the quantity of lithium is constant, and the overall lithium concentration is controlled by the mole fraction changes between the Li-rich phase and Li-lean phase. The Li-rich phase is thought to be the same phase previously reported by Bonnet et al. and Pernet et al., in which the iron occupancy of the 16c site was not equalized with that of the 16d site indicating the space group not to be Fm-3m but Fd-3m. Fast lithium insertion could form the two phases, Lilean and Li-rich, even for an electrochemical lithium insertion process. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Iron-based materials represent one of the most ideal electrode materials for lithium ion batteries in terms of their low cost, resource abundance and non-toxicity. LiFePO4 [1] has been extensively studied and iron oxides such as γ-Fe2O3 [2–8], α-Fe2O3 [9–12], Fe3O4 [9,12,13], and LiFeO2 [14,15] have also been investigated as active materials. To improve the performance of cells with these materials, they have been synthesized in various ways focusing on nano-sizing and/or conductive coating to enhance their intrinsic low ionic and electronic conductivities, and their electrochemical properties were evaluated. For γ-Fe2O3, Hibino et al. [4] recently prepared a composite of nano-sized γ-Fe2O3 and carbon by using aqueous solution method in order to decrease the diffusion length of lithium ions and increase the electronic conductivity at the same time. The results showed high coulombic efficiency and rapid discharge–charge characteristics as a cathode material. Nagao et al. [8] designed a new electrode structure based on a three dimensional mesoporous matrix. They introduced γ-Fe2O3 nanoparticles into mesoporous carbon using electrolysis and it exhibited good electrochemical behavior for both cathode and anode. On the other hand, unfavorable energetics due to the Fe2+/Fe3+ redox couple and relatively low potential as a cathode also limited their applications for lithium ion batteries. Hahn et al. [6] substituted the iron ion in γ-Fe2O3 to highly oxidized Mo6+ to increase the lithium ion capacity and the lithium insertion potential. Insertion of lithium into γ-Fe2O3 is also interesting as H. Sun et al. reported the performance improvement of hard carbon as an electrode by lithium insertion [16,17]. ⁎ Corresponding author. Tel.: +81 75 753 4865; fax: +81 75 753 4734. E-mail addresses: [email protected] (S. Park), [email protected] (T. Matsui), [email protected] (S. Takai), [email protected] (T. Yao). 0167-2738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2013.11.045
γ-Fe2O3 has a defective spinel structure with partial vacancies at the 16d site (Space group: Fd-3m) [18–21]. Oxygen ions are located at the 32e site forming cubic close packed array. Iron ions occupy the tetrahedral 8a site and octahedral 16d site. Another tetrahedral 8b site and the octahedral 16c site are generally empty. In the spinel structure, the tetrahedral 8a site shares common faces with four neighboring empty 16c sites. Similarly, the tetrahedral 8b site and octahedral 16d site are adjacent to each other. From these structural characteristics, it is considered that cation migration between the 8a site and 16c site, or between the 8b site and 16d site will be easy. Crystal structure study of γ-Fe2O3 in terms of lithium insertion has also been investigated. Bonnet et al. [18] and Pernet et al. [19,20] previously investigated the crystal structure of γ-Fe2O3 by chemical and electrochemical lithium insertion. They reported that iron diffuses from tetrahedral sites to octahedral sites accompanying crystal structure change from spinel to rocksalt belonging to the space group Fm-3m. This crystal transformation is an irreversible reaction leading to poor cathode characteristics. We also examined the detailed crystal structure of γ-Fe2O3 in nano-γ-Fe2O3/carbon composite, showing Table 1 Lithium content x in LixFe2O3 obtained by ICP analysis. Li-ion concentration in n-butyl lithium hexane solution/mol L−1
Lithium content x in LixFe2O3
0.10 0.25 0.50 0.75 1.00
(FeI010) (FeI025) (FeI050) (FeI075) (FeI100)
γ-Fe2O3 I
γ-Fe2O3 II
0.22(2) 0.47(3) 0.86(5) 1.24(6) 1.74(9)
(FeII010) (FeII025) (FeII050) (FeII075) (FeII100)
γ-Fe2O3 III
0.24(1) 0.49(2) 0.83(3) 1.31(7) 1.73(18)
(FeIII010) (FeIII025) (FeIII050) (FeIII075) (FeIII100)
0.20(1) 0.40(2) 0.89(5) 1.03(5) 1.74(2)
S. Park et al. / Solid State Ionics 255 (2014) 50–55
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Fig. 1. XRD patterns of the lithium inserted γ-Fe2O3 I samples. (b) is the magnified 440 reflection peak of (a).
good performance as a cathode electrode, before and during electrochemical lithium insertion as a function of the amount of lithium insertion [21]. We investigated the iron occupancies at 8a, 8b, 16c and 16d sites by means of X-ray diffraction (XRD) and the Rietveld analysis. As the lithium insertion amount increased, the iron occupancy of the 8a
site decreased and that of the 16c site increased. It was considered that lithium was inserted at the 8a site and iron at the 8a site migrated to the 16c site in the process of lithium insertion. For the most highly lithium inserted sample, iron occupancy at the 8a site decreased to 0 and that at the 16c site increased to 0.53, however iron occupancies at
Fig. 2. The Rietveld results of the lithium inserted γ-Fe2O3 I samples. The observed and calculated patterns are shown in the top part by the dots and solid line, respectively. The vertical marks in the middle part show positions calculated for Bragg reflection. The trace in the bottom part (ΔY) is a plot of the difference: observed minus calculated.
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S. Park et al. / Solid State Ionics 255 (2014) 50–55
the 16c site and 16d site were not equalized, which means that the crystal structure belongs to Fd-3m rather than to Fm-3m. In recent years, for chemically lithium inserted γ-Fe 2 O 3 (Li0.843Fe2O3), we newly discovered that it featured two phase reaction both belonging to the space group Fd-3m, one is rich in lithium and the other is lean in lithium [22]. In this study, to clarify more about the lithium insertion mechanism in γ-Fe2O3 in the process of lithium insertion, lithium was chemically inserted into γ-Fe2O3 using n-butyl lithium hexane solutions with various lithium ion concentrations and its crystal structure was analyzed by means of XRD and the Rietveld method. Based on the experimental results, we also compared the chemically lithium inserted samples with electrochemically lithium inserted γ-Fe2O3. 2. Experimental 2.1. Materials and sample preparation Three kinds of commercial γ-Fe2O3 powders with different particle sizes, 8–10 nm (Alfa Aesar Co., Hereafter γ-Fe2O3 I), 50 nm (Aldrich Co., Hereafter γ-Fe2O3 II), and 1000 nm (Soekawa Chemical Co., Hereafter γ-Fe2O3 III), were used as starting powders for lithium insertion. Using these materials, chemical lithium insertion was carried out by immersing and stirring the γ-Fe2O3 in n-butyl lithium hexane solution with various lithium ion concentrations (0.10, 0.25, 0.50, 0.75, and 1.00 molL− 1). The reaction was carried out under argon atmosphere for three days at room temperature. After the reaction, the products were thoroughly washed with hexane to remove the excess n-butyl lithium and then the sample was obtained. Lithium content x in terms of LixFe2O3 was examined by inductively coupled plasma atomic emission spectroscopy (ICP: ICPS-7510, Shimadzu, Japan). 2.2. Structural characterization Samples were set in a sealed holder (2391A201, Rigaku Corp., Ltd.) under argon atmosphere. This was set to a diffractometer for XRD measurement. The diffractometer RINT-TTR (Rigaku Corp., Japan) was used for γ-Fe2O3 I and II, and Ultima IV (Rigaku Corp., Japan) was used for γ-Fe2O3 III. For γ-Fe2O3 I and II, XRD patterns were recorded from 25° to 135° in 2θ by step-scanning method with 0.04° step width and 5 s sampling time by using CuKα radiation. Tube voltage and current were set to 40 kV and 200 mA, respectively. Three continuous patterns were summed to decrease noise and improve the precision of the Rietveld analysis. For γ-Fe2O3 III, XRD patterns were measured from 25° to 135° in 2θ at a rate of 2° per min with 0.04° step width by using CuKα radiation. Tube voltage and current were set to 40 kV and 40 mA, respectively. Ten continuous patterns were summed. Obtained XRD patterns were analyzed by the Rietveld method using the RIEVEC program [21–28]. Fd-3m was employed as the space group to represent a structure model, and iron occupancies at 8a, 8b, 16c and 16d sites were calculated. It was assumed that the composition of iron oxide was constant both before and after lithium insertion. Diffraction from lithium was ignored. Lattice parameter and scale factor were also refined by the Rietveld method. 2.3. Electrochemical lithium insertion Lithium was electrochemically inserted into γ-Fe2O3 I for comparison with the chemical lithium insertion samples. We used an argonsealed three-electrode-type beaker cell for the electrochemical lithium insertion. For the working electrode, γ-Fe2O3 I powder was mixed with AB (Acetylene Black, Surface area: 133 m 2 g − 1 , Denkikagaku Kogyo Corp., Ltd) as a supplemental conductor and PTFE (polytetrafluoroethylene) powder as an adhesive agent in a weight ratio of 0.70:0.30:0.05. The mixture was ground, spread and pressed onto a nickel mesh as a current collector. Lithium metal was used for
the counter and reference electrodes. 1 M ethylene carbonate and a 1,2-dimethoxyethane solution (1:1, v/v) of lithium perchlorate (LiClO4 EC/DME, Kishida chemical Corp., Ltd) was used for the electrolyte. The electrode fabrication and cell assembly were carried out under argon atmosphere. The working electrode was discharged from the natural potential of approximately 3.0 V (vs. Li/Li+) for lithium insertion. The same amount of lithium was inserted with different current densities of 0.1 and 0.01 Ag−1 corresponding to the lithium insertion of 0.60 and 0.06 mol into γ-Fe2O3 over an hour. The amount of lithium insertion was adjusted to x = 1.3 in terms of LixFe2O3 which was calculated by integrating the current. When the required electrochemical conditions were attained, the circuit was opened and the working electrode was removed from the cell immediately under argon atmosphere to avoid the local cell reaction between the electrode material and the current collector or the supplemental conductor [26–28]. XRD patterns were measured by using RINT-TTR (Rigaku Corp., Japan) and analyzed by the Rietveld method in a similar manner as used with the chemical lithium insertion samples. Peaks derived from the AB and the Ni mesh collector included in the XRD patterns were evaluated as background and excluded from the Rietveld analysis [21]. 3. Results and discussion 3.1. Chemical Li insertion process on γ-Fe2O3 ICP measurement results are listed in Table 1. Lithium content x in terms of LixFe2O3 was linearly dependent on the concentration of lithium-ions in n-butyl lithium hexane solution. Hereafter, the samples were designated based on the kind of γ-Fe2O3 powder and the lithium concentration of the solution, e.g. FeI025 means that lithium was inserted into γ-Fe2O3 I sample using 0.25 M solution of n-butyl lithium hexane. Fig. 1 shows the observed XRD patterns of the lithium inserted γ-Fe 2 O 3 I samples. In the XRD patterns, some split peaks appeared with the increase of the amount of lithium insertion. Similar split of Table 2 Reliability R factors and GOF value obtained by the Rietveld analysis.
γ-Fe2O3 I Before lithium insertion FeI010 Li-lean FeI025 Li-lean FeI050 Li-lean Li-rich FeI075 Li-lean Li-rich FeI100 Li-lean Li-rich γ-Fe2O3 II Before lithium insertion FeII010 Li-lean FeII025 Li-lean FeII050 Li-lean Li-rich FeII075 Li-lean Li-rich FeII100 Li-lean Li-rich γ-Fe2O3 III Before lithium insertion FeIII010 Li-lean FeIII025 Li-lean FeIII050 Li-lean Li-rich FeIII075 Li-lean Li-rich FeIII100 Li-rich
RWP/%
RF/%
RB/%
GOF
6.40 5.84 6.30 5.77
4.08 3.61 2.72 1.70 1.65 2.20 2.12 3.51 2.69
5.96 5.23 3.74 2.53 1.75 2.91 3.25 5.24 4.80
1.34 1.24 1.18 1.09
6.80 6.48 5.65 3.93 2.86 2.78 2.47 7.04 3.03
6.06 5.38 4.30 3.28 4.72 2.73 3.61 7.11 2.84
1.51 1.50 1.48 1.32
5.01 5.21 4.71 2.79 2.83 3.91 3.04 2.06
3.41 3.73 4.25 2.94 2.82 4.91 3.01 2.05
1.87 1.80 1.83 1.44
5.97 5.65
7.88 7.78 7.75 6.25 6.53 6.23
10.90 10.61 11.04 8.56 8.41 5.59
1.10 1.12
1.25 1.19
1.45 1.33
S. Park et al. / Solid State Ionics 255 (2014) 50–55
Fig. 3. Detailed drawing of the Rietveld result of the sample (FeI050).
peaks was also present for other two samples, γ-Fe2O3 II and γ-Fe2O3 III. For one example, peak splitting on 440 reflection is also shown in Fig. 1(b). This splitting could not be explained by Kα1 and Kα2 splitting because the peak separation did not coincide with the energy differences between Kα1 and Kα2, and because the intensity ratios of the splitting peaks were not constant peak to peak, but could be explained by the coexistence of two phases belonging to the same space group. The two phases were named the “Li-lean phase” for the smaller lattice parameter and the “Li-rich phase” for the larger lattice parameter. The results of the Rietveld refinement for the observed XRD patterns of γ-Fe2O3 I samples are shown in Fig. 2(a) to (f). The calculated patterns agreed well with the observed ones. All the samples of lithium
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inserted γ-Fe2O3 II and γ-Fe2O3 III also fit well. Table 2 shows the resulting reliability R factors and GOF (Goodness of fit) value obtained by the Rietveld analysis. The values of RWP, RF and RB, R-weighted pattern, R-structure factor and R-Bragg factor, respectively, were sufficiently small and GOF value was close to 1 for all samples [29]. A detailed drawing of the Rietveld result of the sample (FeI050) consisting of both Li-lean phase and Li-rich phase is shown in Fig. 3. The fitting was precise and it was revealed that the samples with split peaks in the XRD patterns were a mixture of two phases both belonging to the space group Fd-3m. The Rietveld analysis assuming a single phase could not fit all of the diffraction peaks. Table 3 shows the refined structure parameters for all samples. Overall temperature factors were all well refined. The lattice parameter of the Li-lean phase slightly increased compared to the sample before lithium insertion. In contrast, for the Li-rich phase, the lattice parameter was larger than the Li-lean phase indicating that the Li-rich phase contained more lithium. Once the Li-rich phase formed, the lattice parameters of the Li-lean and Li-rich phase remained almost constant for all samples regardless of the amount of lithium insertion. These results indicated that the inserted lithium produced the Li-rich phase in which inserted lithium amount was constant and overall lithium concentration was controlled by the mole fraction changes between the Li-lean phase and Li-rich phase. It was considered that the amount of the Li-rich phase increases, and that of the Li-lean phase decreases, with the increase of the amount of lithium insertion. As the lattice parameter increased, the iron occupancy of the 8a site decreased and that of the 16c site increased. The iron occupancy of the 16d site was almost constant and that of the 8b site was nearly zero for all samples. It was indicated that, by lithium insertion, iron moved from the 8a site to the 16c site, and then it was considered that lithium was inserted at the 8a site and iron at the 8a site migrated to the 16c site. These are very similar phenomena that occurred for the electrochemical lithium insertion process [21]. In terms of the Li-rich phase, the iron occupancy of the 8a site fully decreased and that of the 16c site increased
Table 3 Refined structure parameters for all samples. γ-Fe2O3 before chemical lithium insertion was also analyzed by the X-ray Rietveld method. Lattice parameter/nm
γ-Fe2O3 I sample Before Li insertion (FeI010) Li0.21Fe2O3 (FeI025) Li0.47Fe2O3 (FeI050) Li0.86Fe2O3 (FeI075) Li1.24Fe2O3 (FeI100) Li1.74Fe2O3 γ-Fe2O3 II sample Before Li insertion (FeII010) Li0.24Fe2O3 (FeII025) Li0.49Fe2O3 (FeII050) Li0.83Fe2O3 (FeII075) Li1.31Fe2O3 (FeII100) Li1.73Fe2O3 γ-Fe2O3 III sample Before Li insertion (FeIII010) Li0.20Fe2O3 (FeIII025) Li0.40Fe2O3 (FeIII050) Li0.89Fe2O3 (FeIII075) Li1.03Fe2O3 (FeIII100) Li1.74Fe2O3
Iron occupancy
Overall temperature factor
8a site
16c site
16d site
8b site
Li-lean Li-lean Li-lean Li-rich Li-lean Li-rich Li-lean Li-rich
0.83336(5) 0.83523(4) 0.83537(3) 0.83737(2) 0.84434(2) 0.83758(1) 0.84485(2) 0.83630(2) 0.84513(2)
0.995(5) 0.961(5) 0.814(4) 0.823(11) 0.081(31) 0.731(8) 0.069(25) 0.821(33) 0.000(2)
0.000(3) 0.002(3) 0.080(3) 0.068(6) 0.463(23) 0.116(6) 0.376(15) 0.000(20) 0.462(16)
0.836(3) 0.851(3) 0.847(3) 0.854(5) 0.830(23) 0.851(6) 0.923(15) 0.923(20) 0.871(16)
0.000(5) 0.000(4) 0.000(4) 0.000(7) 0.000(31) 0.000(10) 0.000(18) 0.000(29) 0.000(12)
1.08(9) 1.28(9) 1.41(10) 1.31(12) 1.15(25) 1.99(24) 2.00(19) 0.80(42) 2.36(13)
Li-lean Li-lean Li-lean Li-rich Li-lean Li-rich Li-lean Li-rich
0.834579(4) 0.835419(7) 0.835411(9) 0.836020(8) 0.843392(10) 0.835816(7) 0.842826(9) 0.836586(3) 0.843843(3)
0.994(15) 0.985(3) 0.842(3) 0.845(5) 0.000(74) 0.881(7) 0.000(16) 0.964(32) 0.012(4)
0.000(10) 0.000(2) 0.058(2) 0.041(3) 0.593(12) 0.000(4) 0.476(13) 0.000(17) 0.425(3)
0.836(10) 0.841(2) 0.854(2) 0.870(3) 0.731(56) 0.892(4) 0.821(13) 0.851(17) 0.902(3)
0.000(14) 0.000(1) 0.000(3) 0.000(4) 0.018(74) 0.000(5) 0.073(16) 0.000(24) 0.000(56)
1.66(6) 1.72(6) 1.20(7) 1.45(9) 0.75(23) 1.16(10) 0.97(14) 4.28(64) 1.44(6)
Li-lean Li-lean Li-lean Li-rich Li-lean Li-rich Li-rich
0.834823(9) 0.835472(10) 0.835980(9) 0.836686(10) 0.844195(9) 0.837015(10) 0.844045(9) 0.844087(4)
1.000(3) 1.000(3) 0.900(3) 0.993(9) 0.000(10) 1.000(15) 0.009(7) 0.000(14)
0.008(2) 0.004(2) 0.056(2) 0.000(5) 0.461(7) 0.000(8) 0.440(5) 0.436(12)
0.825(2) 0.830(2) 0.828(2) 0.837(5) 0.858(7) 0.833(8) 0.879(5) 0.897(10)
0.000(3) 0.000(3) 0.000(3) 0.000(7) 0.028(10) 0.000(12) 0.021(6) 0.000(24)
2.06(7) 2.01(8) 2.20(8) 2.09(15) 1.68(10) 3.30(28) 1.29(7) 1.58(3)
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S. Park et al. / Solid State Ionics 255 (2014) 50–55
depending on the amount of the inserted lithium. It was therefore confirmed that chemical lithium insertion process on γ-Fe2O3 is characterized by two phase reaction between the Li-lean phase and Li-rich phase both belonging to the space group Fd-3m. 3.2. Comparison with electrochemical Li insertion process on γ-Fe2O3
Fig. 4. Relative mole fraction of Li-lean phase and Li-rich phase with the increase of lithium insertion amount for the γ-Fe2O3 I samples. Error bars indicate the estimated standard deviations.
in proportion to the decrease in the amount of iron at the 8a site. These results also indicated that the Li-rich phase is rich in lithium. Both the lattice parameters and iron occupancies of the Li-rich phase were very close to those previously given by Bonnet et al. [18] and Pernet et al. [19,20]. The Li-rich phase is thought to be the same phase described by them. The iron occupancy of the 16c site was not equalized with that of the 16d site for the Li-rich phase. It therefore belongs to Fd-3m not to Fm-3m. Using the refined scale factors and lattice parameters, the relative mole fraction of both phases was calculated as a function of the amount of lithium insertion. Generally, the relative mole fraction of phase p in a mixture of n phases is calculated by the following equation [30].
n X Mp ¼ Sp ðZV Þp = Si ðZV Þi i¼1
where MP, S, Z, and V are the relative mole fraction of phase p in a mixture of n phases, the Rietveld scale factor, the number of formula units per unit cell, and the unit cell volume (in Å3), respectively. γ-Fe2O3 has eight formula units per unit cell and the unit cell volume can be calculated by the refined lattice parameter. Fig. 4 shows the relative mole fraction of Li-lean phase and Lirich phase with the increase of the amount of lithium insertion for the γ-Fe2O3 I samples. For all samples of the γ-Fe2O3 I, II, and III, the mole fraction of the Li-lean phase decreased and that of the Li-rich phase increased, and the amount of each phase monotonously changed
The electrochemical lithium insertion process as was previously reported [21] featured single phase reaction. It was considered that the chemical lithium insertion reaction proceeds rapidly because it was carried out by immersing and stirring the powder in the lithium solution. On the other hand, for the electrochemical lithium insertion process, lithium was gradually inserted into the electrode in accordance with the current density. We considered that, when lithium was rapidly inserted under a large current density, two phases were expected to form even for the electrochemical lithium insertion process. In this regard, lithium was electrochemically inserted into γ-Fe2O3 I with different current densities of 0.1 (hereafter, sample (A)) and 0.01 Ag−1 (hereafter, sample (B)). Fig. 5 shows the observed XRD patterns. For comparison, the chemically lithium inserted sample (FeI075) which had a similar lithium insertion amount to the electrochemically lithium inserted samples of (A) and (B) is also shown in Fig. 5. Focusing on the reflection peak of 440 as shown in Fig. 5(b), the right side of the peak for sample (A) was broader than that of sample (B). And the peak shape of sample (A) was similar to that of sample (FeI075). It was considered that the two phases, Li-lean phase and Li-rich phase, were present in sample (A), on the other hand, only the Li-rich phase was present in sample (B). The XRD patterns were analyzed by the Rietveld method using both single phase and two phase models belonging to the space group Fd-3m. The diffraction peaks for the sample (A) could fit condition with conditions of either single phase or two phase refinement, however, that of sample (B) could only fit with the conditions of single phase refinement. For sample (A), two phase Rietveld analysis was more appropriate based on the reliability factors. These results indicated that the electrochemical lithium insertion process could undertake two phase reaction under fast lithium insertion conditions. The refined structure parameters and calculated mole fraction are listed in Table 4. The values for sample (A) were close to those of sample (FeI075). 4. Conclusion Lithium was chemically inserted into γ-Fe2O3 using n-butyl lithium hexane solution with various lithium ion concentrations and the detailed crystal structure in the process of lithium insertion was examined.
Fig. 5. XRD patterns of the electrochemically lithium inserted γ-Fe2O3 I samples with different current densities. For comparison, the chemically lithium inserted sample (FeI075) which had a similar lithium insertion amount to the samples of (A) and (B) is also shown. (b) is the magnified 440 reflection peak from (a). Asterisk mark (*) indicates the nickel mesh collector.
S. Park et al. / Solid State Ionics 255 (2014) 50–55
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Table 4 Refined structure parameters and calculated mole fraction of the electrochemically lithium inserted γ-Fe2O3 I samples. γ-Fe2O3 electrode before electrochemical lithium insertion was also analyzed by the X-ray Rietveld method. γ-Fe2O3 I sample
Before Li insertion (A) Li1.3Fe2O3_0.1 (B) Li1.3Fe2O3_0.01
Lattice parameter/nm
Li-lean Li-rich Li-rich
0.83336(5) 0.83586(5) 0.84389(5) 0.84200(7)
Iron occupancy 8a site
16c site
16d site
8b site
0.995(5) 0.602(32) 0.193(34) 0.424(8)
0.000(3) 0.151(24) 0.364(30) 0.248(7)
0.836(3) 0.881(24) 0.873(30) 0.873(7)
0.000(5) 0.000(33) 0.000(36) 0.000(13)
We newly found that the chemically lithium inserted sample was a mixture of two phases both belonging to the space group Fd-3m, one is rich in lithium and the other is lean in lithium. It was revealed that the chemical lithium insertion process in γ-Fe2O3 undertakes a two phase reaction, i.e. inserted lithium forms a Li-rich phase in which the lithium amount is constant, and the overall lithium concentration is controlled by the mole fraction changes between the Li-rich phase and Li-lean phase. The Li-rich phase is thought to be the same phase previously reported by Bonnet et al. and Pernet et al., in which the iron occupancy of the 16c site was not equalized with that of the 16d site. This phase belongs to Fd-3m not to Fm-3m. Fast lithium insertion could form the two phases of Li-lean and Li-rich even for the electrochemical lithium insertion process. Acknowledgment This work was partly supported by the “Energy Science in the Age of Global Warming” Global Center of Excellence (G-COE) program (J-051) of the Ministry of Education, Culture, Sports, Science and Technology of Japan and partly supported by a Grant-in-Aid for Challenging Exploratory Research (24656581) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT). References [1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188–1194. [2] C.W. Kwon, M. Quintin, S. Mornet, C. Barbieri, O. Devos, G. Campet, M.H. Delville, J. Electrochem. Soc. 151 (2004) A1445–A1449. [3] S. Kanzaki, T. Inada, T. Matsumura, N. Sonoyama, A. Yamada, M. Takano, R. Kanno, J. Power Sources 146 (2005) 323–326.
Overall temperature factor
Mole fraction
1.08(9) 1.78(66) 1.38(56) 2.66(27)
– 0.436(53) 0.564(40) –
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Crystal structure analysis of γ-Fe2O3 with chemical lithium insertion Seungwon Park, Tamito Matsui, Shigeomi Takai, Takeshi Yao ⁎ Department of Fundamental Energy Science, Graduate School of Energy Science, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto, 606-8501, Japan
a r t i c l e
i n f o
Article history: Received 28 April 2013 Received in revised form 11 November 2013 Accepted 15 November 2013 Available online 12 December 2013 Keywords: γ-Fe2O3 Lithium insertion The Rietveld analysis Cation distribution Spinel structure Li-ion secondary battery
a b s t r a c t To better understand the lithium insertion mechanism on γ-Fe2O3, we investigated the crystal structure changes of γ-Fe2O3 in the process of chemical lithium insertion by using the X-ray Rietveld analysis. We newly found that the lithium inserted sample was a mixture of two phases both belonging to the space group Fd-3m, one is rich in lithium and the other is lean in lithium. It was revealed that the chemical lithium insertion process into γ-Fe2O3 occurs via a two phase reaction, i.e. inserted lithium forms a Li-rich phase in which the quantity of lithium is constant, and the overall lithium concentration is controlled by the mole fraction changes between the Li-rich phase and Li-lean phase. The Li-rich phase is thought to be the same phase previously reported by Bonnet et al. and Pernet et al., in which the iron occupancy of the 16c site was not equalized with that of the 16d site indicating the space group not to be Fm-3m but Fd-3m. Fast lithium insertion could form the two phases, Lilean and Li-rich, even for an electrochemical lithium insertion process. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Iron-based materials represent one of the most ideal electrode materials for lithium ion batteries in terms of their low cost, resource abundance and non-toxicity. LiFePO4 [1] has been extensively studied and iron oxides such as γ-Fe2O3 [2–8], α-Fe2O3 [9–12], Fe3O4 [9,12,13], and LiFeO2 [14,15] have also been investigated as active materials. To improve the performance of cells with these materials, they have been synthesized in various ways focusing on nano-sizing and/or conductive coating to enhance their intrinsic low ionic and electronic conductivities, and their electrochemical properties were evaluated. For γ-Fe2O3, Hibino et al. [4] recently prepared a composite of nano-sized γ-Fe2O3 and carbon by using aqueous solution method in order to decrease the diffusion length of lithium ions and increase the electronic conductivity at the same time. The results showed high coulombic efficiency and rapid discharge–charge characteristics as a cathode material. Nagao et al. [8] designed a new electrode structure based on a three dimensional mesoporous matrix. They introduced γ-Fe2O3 nanoparticles into mesoporous carbon using electrolysis and it exhibited good electrochemical behavior for both cathode and anode. On the other hand, unfavorable energetics due to the Fe2+/Fe3+ redox couple and relatively low potential as a cathode also limited their applications for lithium ion batteries. Hahn et al. [6] substituted the iron ion in γ-Fe2O3 to highly oxidized Mo6+ to increase the lithium ion capacity and the lithium insertion potential. Insertion of lithium into γ-Fe2O3 is also interesting as H. Sun et al. reported the performance improvement of hard carbon as an electrode by lithium insertion [16,17]. ⁎ Corresponding author. Tel.: +81 75 753 4865; fax: +81 75 753 4734. E-mail addresses: [email protected] (S. Park), [email protected] (T. Matsui), [email protected] (S. Takai), [email protected] (T. Yao). 0167-2738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2013.11.045
γ-Fe2O3 has a defective spinel structure with partial vacancies at the 16d site (Space group: Fd-3m) [18–21]. Oxygen ions are located at the 32e site forming cubic close packed array. Iron ions occupy the tetrahedral 8a site and octahedral 16d site. Another tetrahedral 8b site and the octahedral 16c site are generally empty. In the spinel structure, the tetrahedral 8a site shares common faces with four neighboring empty 16c sites. Similarly, the tetrahedral 8b site and octahedral 16d site are adjacent to each other. From these structural characteristics, it is considered that cation migration between the 8a site and 16c site, or between the 8b site and 16d site will be easy. Crystal structure study of γ-Fe2O3 in terms of lithium insertion has also been investigated. Bonnet et al. [18] and Pernet et al. [19,20] previously investigated the crystal structure of γ-Fe2O3 by chemical and electrochemical lithium insertion. They reported that iron diffuses from tetrahedral sites to octahedral sites accompanying crystal structure change from spinel to rocksalt belonging to the space group Fm-3m. This crystal transformation is an irreversible reaction leading to poor cathode characteristics. We also examined the detailed crystal structure of γ-Fe2O3 in nano-γ-Fe2O3/carbon composite, showing Table 1 Lithium content x in LixFe2O3 obtained by ICP analysis. Li-ion concentration in n-butyl lithium hexane solution/mol L−1
Lithium content x in LixFe2O3
0.10 0.25 0.50 0.75 1.00
(FeI010) (FeI025) (FeI050) (FeI075) (FeI100)
γ-Fe2O3 I
γ-Fe2O3 II
0.22(2) 0.47(3) 0.86(5) 1.24(6) 1.74(9)
(FeII010) (FeII025) (FeII050) (FeII075) (FeII100)
γ-Fe2O3 III
0.24(1) 0.49(2) 0.83(3) 1.31(7) 1.73(18)
(FeIII010) (FeIII025) (FeIII050) (FeIII075) (FeIII100)
0.20(1) 0.40(2) 0.89(5) 1.03(5) 1.74(2)
S. Park et al. / Solid State Ionics 255 (2014) 50–55
51
Fig. 1. XRD patterns of the lithium inserted γ-Fe2O3 I samples. (b) is the magnified 440 reflection peak of (a).
good performance as a cathode electrode, before and during electrochemical lithium insertion as a function of the amount of lithium insertion [21]. We investigated the iron occupancies at 8a, 8b, 16c and 16d sites by means of X-ray diffraction (XRD) and the Rietveld analysis. As the lithium insertion amount increased, the iron occupancy of the 8a
site decreased and that of the 16c site increased. It was considered that lithium was inserted at the 8a site and iron at the 8a site migrated to the 16c site in the process of lithium insertion. For the most highly lithium inserted sample, iron occupancy at the 8a site decreased to 0 and that at the 16c site increased to 0.53, however iron occupancies at
Fig. 2. The Rietveld results of the lithium inserted γ-Fe2O3 I samples. The observed and calculated patterns are shown in the top part by the dots and solid line, respectively. The vertical marks in the middle part show positions calculated for Bragg reflection. The trace in the bottom part (ΔY) is a plot of the difference: observed minus calculated.
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S. Park et al. / Solid State Ionics 255 (2014) 50–55
the 16c site and 16d site were not equalized, which means that the crystal structure belongs to Fd-3m rather than to Fm-3m. In recent years, for chemically lithium inserted γ-Fe 2 O 3 (Li0.843Fe2O3), we newly discovered that it featured two phase reaction both belonging to the space group Fd-3m, one is rich in lithium and the other is lean in lithium [22]. In this study, to clarify more about the lithium insertion mechanism in γ-Fe2O3 in the process of lithium insertion, lithium was chemically inserted into γ-Fe2O3 using n-butyl lithium hexane solutions with various lithium ion concentrations and its crystal structure was analyzed by means of XRD and the Rietveld method. Based on the experimental results, we also compared the chemically lithium inserted samples with electrochemically lithium inserted γ-Fe2O3. 2. Experimental 2.1. Materials and sample preparation Three kinds of commercial γ-Fe2O3 powders with different particle sizes, 8–10 nm (Alfa Aesar Co., Hereafter γ-Fe2O3 I), 50 nm (Aldrich Co., Hereafter γ-Fe2O3 II), and 1000 nm (Soekawa Chemical Co., Hereafter γ-Fe2O3 III), were used as starting powders for lithium insertion. Using these materials, chemical lithium insertion was carried out by immersing and stirring the γ-Fe2O3 in n-butyl lithium hexane solution with various lithium ion concentrations (0.10, 0.25, 0.50, 0.75, and 1.00 molL− 1). The reaction was carried out under argon atmosphere for three days at room temperature. After the reaction, the products were thoroughly washed with hexane to remove the excess n-butyl lithium and then the sample was obtained. Lithium content x in terms of LixFe2O3 was examined by inductively coupled plasma atomic emission spectroscopy (ICP: ICPS-7510, Shimadzu, Japan). 2.2. Structural characterization Samples were set in a sealed holder (2391A201, Rigaku Corp., Ltd.) under argon atmosphere. This was set to a diffractometer for XRD measurement. The diffractometer RINT-TTR (Rigaku Corp., Japan) was used for γ-Fe2O3 I and II, and Ultima IV (Rigaku Corp., Japan) was used for γ-Fe2O3 III. For γ-Fe2O3 I and II, XRD patterns were recorded from 25° to 135° in 2θ by step-scanning method with 0.04° step width and 5 s sampling time by using CuKα radiation. Tube voltage and current were set to 40 kV and 200 mA, respectively. Three continuous patterns were summed to decrease noise and improve the precision of the Rietveld analysis. For γ-Fe2O3 III, XRD patterns were measured from 25° to 135° in 2θ at a rate of 2° per min with 0.04° step width by using CuKα radiation. Tube voltage and current were set to 40 kV and 40 mA, respectively. Ten continuous patterns were summed. Obtained XRD patterns were analyzed by the Rietveld method using the RIEVEC program [21–28]. Fd-3m was employed as the space group to represent a structure model, and iron occupancies at 8a, 8b, 16c and 16d sites were calculated. It was assumed that the composition of iron oxide was constant both before and after lithium insertion. Diffraction from lithium was ignored. Lattice parameter and scale factor were also refined by the Rietveld method. 2.3. Electrochemical lithium insertion Lithium was electrochemically inserted into γ-Fe2O3 I for comparison with the chemical lithium insertion samples. We used an argonsealed three-electrode-type beaker cell for the electrochemical lithium insertion. For the working electrode, γ-Fe2O3 I powder was mixed with AB (Acetylene Black, Surface area: 133 m 2 g − 1 , Denkikagaku Kogyo Corp., Ltd) as a supplemental conductor and PTFE (polytetrafluoroethylene) powder as an adhesive agent in a weight ratio of 0.70:0.30:0.05. The mixture was ground, spread and pressed onto a nickel mesh as a current collector. Lithium metal was used for
the counter and reference electrodes. 1 M ethylene carbonate and a 1,2-dimethoxyethane solution (1:1, v/v) of lithium perchlorate (LiClO4 EC/DME, Kishida chemical Corp., Ltd) was used for the electrolyte. The electrode fabrication and cell assembly were carried out under argon atmosphere. The working electrode was discharged from the natural potential of approximately 3.0 V (vs. Li/Li+) for lithium insertion. The same amount of lithium was inserted with different current densities of 0.1 and 0.01 Ag−1 corresponding to the lithium insertion of 0.60 and 0.06 mol into γ-Fe2O3 over an hour. The amount of lithium insertion was adjusted to x = 1.3 in terms of LixFe2O3 which was calculated by integrating the current. When the required electrochemical conditions were attained, the circuit was opened and the working electrode was removed from the cell immediately under argon atmosphere to avoid the local cell reaction between the electrode material and the current collector or the supplemental conductor [26–28]. XRD patterns were measured by using RINT-TTR (Rigaku Corp., Japan) and analyzed by the Rietveld method in a similar manner as used with the chemical lithium insertion samples. Peaks derived from the AB and the Ni mesh collector included in the XRD patterns were evaluated as background and excluded from the Rietveld analysis [21]. 3. Results and discussion 3.1. Chemical Li insertion process on γ-Fe2O3 ICP measurement results are listed in Table 1. Lithium content x in terms of LixFe2O3 was linearly dependent on the concentration of lithium-ions in n-butyl lithium hexane solution. Hereafter, the samples were designated based on the kind of γ-Fe2O3 powder and the lithium concentration of the solution, e.g. FeI025 means that lithium was inserted into γ-Fe2O3 I sample using 0.25 M solution of n-butyl lithium hexane. Fig. 1 shows the observed XRD patterns of the lithium inserted γ-Fe 2 O 3 I samples. In the XRD patterns, some split peaks appeared with the increase of the amount of lithium insertion. Similar split of Table 2 Reliability R factors and GOF value obtained by the Rietveld analysis.
γ-Fe2O3 I Before lithium insertion FeI010 Li-lean FeI025 Li-lean FeI050 Li-lean Li-rich FeI075 Li-lean Li-rich FeI100 Li-lean Li-rich γ-Fe2O3 II Before lithium insertion FeII010 Li-lean FeII025 Li-lean FeII050 Li-lean Li-rich FeII075 Li-lean Li-rich FeII100 Li-lean Li-rich γ-Fe2O3 III Before lithium insertion FeIII010 Li-lean FeIII025 Li-lean FeIII050 Li-lean Li-rich FeIII075 Li-lean Li-rich FeIII100 Li-rich
RWP/%
RF/%
RB/%
GOF
6.40 5.84 6.30 5.77
4.08 3.61 2.72 1.70 1.65 2.20 2.12 3.51 2.69
5.96 5.23 3.74 2.53 1.75 2.91 3.25 5.24 4.80
1.34 1.24 1.18 1.09
6.80 6.48 5.65 3.93 2.86 2.78 2.47 7.04 3.03
6.06 5.38 4.30 3.28 4.72 2.73 3.61 7.11 2.84
1.51 1.50 1.48 1.32
5.01 5.21 4.71 2.79 2.83 3.91 3.04 2.06
3.41 3.73 4.25 2.94 2.82 4.91 3.01 2.05
1.87 1.80 1.83 1.44
5.97 5.65
7.88 7.78 7.75 6.25 6.53 6.23
10.90 10.61 11.04 8.56 8.41 5.59
1.10 1.12
1.25 1.19
1.45 1.33
S. Park et al. / Solid State Ionics 255 (2014) 50–55
Fig. 3. Detailed drawing of the Rietveld result of the sample (FeI050).
peaks was also present for other two samples, γ-Fe2O3 II and γ-Fe2O3 III. For one example, peak splitting on 440 reflection is also shown in Fig. 1(b). This splitting could not be explained by Kα1 and Kα2 splitting because the peak separation did not coincide with the energy differences between Kα1 and Kα2, and because the intensity ratios of the splitting peaks were not constant peak to peak, but could be explained by the coexistence of two phases belonging to the same space group. The two phases were named the “Li-lean phase” for the smaller lattice parameter and the “Li-rich phase” for the larger lattice parameter. The results of the Rietveld refinement for the observed XRD patterns of γ-Fe2O3 I samples are shown in Fig. 2(a) to (f). The calculated patterns agreed well with the observed ones. All the samples of lithium
53
inserted γ-Fe2O3 II and γ-Fe2O3 III also fit well. Table 2 shows the resulting reliability R factors and GOF (Goodness of fit) value obtained by the Rietveld analysis. The values of RWP, RF and RB, R-weighted pattern, R-structure factor and R-Bragg factor, respectively, were sufficiently small and GOF value was close to 1 for all samples [29]. A detailed drawing of the Rietveld result of the sample (FeI050) consisting of both Li-lean phase and Li-rich phase is shown in Fig. 3. The fitting was precise and it was revealed that the samples with split peaks in the XRD patterns were a mixture of two phases both belonging to the space group Fd-3m. The Rietveld analysis assuming a single phase could not fit all of the diffraction peaks. Table 3 shows the refined structure parameters for all samples. Overall temperature factors were all well refined. The lattice parameter of the Li-lean phase slightly increased compared to the sample before lithium insertion. In contrast, for the Li-rich phase, the lattice parameter was larger than the Li-lean phase indicating that the Li-rich phase contained more lithium. Once the Li-rich phase formed, the lattice parameters of the Li-lean and Li-rich phase remained almost constant for all samples regardless of the amount of lithium insertion. These results indicated that the inserted lithium produced the Li-rich phase in which inserted lithium amount was constant and overall lithium concentration was controlled by the mole fraction changes between the Li-lean phase and Li-rich phase. It was considered that the amount of the Li-rich phase increases, and that of the Li-lean phase decreases, with the increase of the amount of lithium insertion. As the lattice parameter increased, the iron occupancy of the 8a site decreased and that of the 16c site increased. The iron occupancy of the 16d site was almost constant and that of the 8b site was nearly zero for all samples. It was indicated that, by lithium insertion, iron moved from the 8a site to the 16c site, and then it was considered that lithium was inserted at the 8a site and iron at the 8a site migrated to the 16c site. These are very similar phenomena that occurred for the electrochemical lithium insertion process [21]. In terms of the Li-rich phase, the iron occupancy of the 8a site fully decreased and that of the 16c site increased
Table 3 Refined structure parameters for all samples. γ-Fe2O3 before chemical lithium insertion was also analyzed by the X-ray Rietveld method. Lattice parameter/nm
γ-Fe2O3 I sample Before Li insertion (FeI010) Li0.21Fe2O3 (FeI025) Li0.47Fe2O3 (FeI050) Li0.86Fe2O3 (FeI075) Li1.24Fe2O3 (FeI100) Li1.74Fe2O3 γ-Fe2O3 II sample Before Li insertion (FeII010) Li0.24Fe2O3 (FeII025) Li0.49Fe2O3 (FeII050) Li0.83Fe2O3 (FeII075) Li1.31Fe2O3 (FeII100) Li1.73Fe2O3 γ-Fe2O3 III sample Before Li insertion (FeIII010) Li0.20Fe2O3 (FeIII025) Li0.40Fe2O3 (FeIII050) Li0.89Fe2O3 (FeIII075) Li1.03Fe2O3 (FeIII100) Li1.74Fe2O3
Iron occupancy
Overall temperature factor
8a site
16c site
16d site
8b site
Li-lean Li-lean Li-lean Li-rich Li-lean Li-rich Li-lean Li-rich
0.83336(5) 0.83523(4) 0.83537(3) 0.83737(2) 0.84434(2) 0.83758(1) 0.84485(2) 0.83630(2) 0.84513(2)
0.995(5) 0.961(5) 0.814(4) 0.823(11) 0.081(31) 0.731(8) 0.069(25) 0.821(33) 0.000(2)
0.000(3) 0.002(3) 0.080(3) 0.068(6) 0.463(23) 0.116(6) 0.376(15) 0.000(20) 0.462(16)
0.836(3) 0.851(3) 0.847(3) 0.854(5) 0.830(23) 0.851(6) 0.923(15) 0.923(20) 0.871(16)
0.000(5) 0.000(4) 0.000(4) 0.000(7) 0.000(31) 0.000(10) 0.000(18) 0.000(29) 0.000(12)
1.08(9) 1.28(9) 1.41(10) 1.31(12) 1.15(25) 1.99(24) 2.00(19) 0.80(42) 2.36(13)
Li-lean Li-lean Li-lean Li-rich Li-lean Li-rich Li-lean Li-rich
0.834579(4) 0.835419(7) 0.835411(9) 0.836020(8) 0.843392(10) 0.835816(7) 0.842826(9) 0.836586(3) 0.843843(3)
0.994(15) 0.985(3) 0.842(3) 0.845(5) 0.000(74) 0.881(7) 0.000(16) 0.964(32) 0.012(4)
0.000(10) 0.000(2) 0.058(2) 0.041(3) 0.593(12) 0.000(4) 0.476(13) 0.000(17) 0.425(3)
0.836(10) 0.841(2) 0.854(2) 0.870(3) 0.731(56) 0.892(4) 0.821(13) 0.851(17) 0.902(3)
0.000(14) 0.000(1) 0.000(3) 0.000(4) 0.018(74) 0.000(5) 0.073(16) 0.000(24) 0.000(56)
1.66(6) 1.72(6) 1.20(7) 1.45(9) 0.75(23) 1.16(10) 0.97(14) 4.28(64) 1.44(6)
Li-lean Li-lean Li-lean Li-rich Li-lean Li-rich Li-rich
0.834823(9) 0.835472(10) 0.835980(9) 0.836686(10) 0.844195(9) 0.837015(10) 0.844045(9) 0.844087(4)
1.000(3) 1.000(3) 0.900(3) 0.993(9) 0.000(10) 1.000(15) 0.009(7) 0.000(14)
0.008(2) 0.004(2) 0.056(2) 0.000(5) 0.461(7) 0.000(8) 0.440(5) 0.436(12)
0.825(2) 0.830(2) 0.828(2) 0.837(5) 0.858(7) 0.833(8) 0.879(5) 0.897(10)
0.000(3) 0.000(3) 0.000(3) 0.000(7) 0.028(10) 0.000(12) 0.021(6) 0.000(24)
2.06(7) 2.01(8) 2.20(8) 2.09(15) 1.68(10) 3.30(28) 1.29(7) 1.58(3)
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S. Park et al. / Solid State Ionics 255 (2014) 50–55
depending on the amount of the inserted lithium. It was therefore confirmed that chemical lithium insertion process on γ-Fe2O3 is characterized by two phase reaction between the Li-lean phase and Li-rich phase both belonging to the space group Fd-3m. 3.2. Comparison with electrochemical Li insertion process on γ-Fe2O3
Fig. 4. Relative mole fraction of Li-lean phase and Li-rich phase with the increase of lithium insertion amount for the γ-Fe2O3 I samples. Error bars indicate the estimated standard deviations.
in proportion to the decrease in the amount of iron at the 8a site. These results also indicated that the Li-rich phase is rich in lithium. Both the lattice parameters and iron occupancies of the Li-rich phase were very close to those previously given by Bonnet et al. [18] and Pernet et al. [19,20]. The Li-rich phase is thought to be the same phase described by them. The iron occupancy of the 16c site was not equalized with that of the 16d site for the Li-rich phase. It therefore belongs to Fd-3m not to Fm-3m. Using the refined scale factors and lattice parameters, the relative mole fraction of both phases was calculated as a function of the amount of lithium insertion. Generally, the relative mole fraction of phase p in a mixture of n phases is calculated by the following equation [30].
n X Mp ¼ Sp ðZV Þp = Si ðZV Þi i¼1
where MP, S, Z, and V are the relative mole fraction of phase p in a mixture of n phases, the Rietveld scale factor, the number of formula units per unit cell, and the unit cell volume (in Å3), respectively. γ-Fe2O3 has eight formula units per unit cell and the unit cell volume can be calculated by the refined lattice parameter. Fig. 4 shows the relative mole fraction of Li-lean phase and Lirich phase with the increase of the amount of lithium insertion for the γ-Fe2O3 I samples. For all samples of the γ-Fe2O3 I, II, and III, the mole fraction of the Li-lean phase decreased and that of the Li-rich phase increased, and the amount of each phase monotonously changed
The electrochemical lithium insertion process as was previously reported [21] featured single phase reaction. It was considered that the chemical lithium insertion reaction proceeds rapidly because it was carried out by immersing and stirring the powder in the lithium solution. On the other hand, for the electrochemical lithium insertion process, lithium was gradually inserted into the electrode in accordance with the current density. We considered that, when lithium was rapidly inserted under a large current density, two phases were expected to form even for the electrochemical lithium insertion process. In this regard, lithium was electrochemically inserted into γ-Fe2O3 I with different current densities of 0.1 (hereafter, sample (A)) and 0.01 Ag−1 (hereafter, sample (B)). Fig. 5 shows the observed XRD patterns. For comparison, the chemically lithium inserted sample (FeI075) which had a similar lithium insertion amount to the electrochemically lithium inserted samples of (A) and (B) is also shown in Fig. 5. Focusing on the reflection peak of 440 as shown in Fig. 5(b), the right side of the peak for sample (A) was broader than that of sample (B). And the peak shape of sample (A) was similar to that of sample (FeI075). It was considered that the two phases, Li-lean phase and Li-rich phase, were present in sample (A), on the other hand, only the Li-rich phase was present in sample (B). The XRD patterns were analyzed by the Rietveld method using both single phase and two phase models belonging to the space group Fd-3m. The diffraction peaks for the sample (A) could fit condition with conditions of either single phase or two phase refinement, however, that of sample (B) could only fit with the conditions of single phase refinement. For sample (A), two phase Rietveld analysis was more appropriate based on the reliability factors. These results indicated that the electrochemical lithium insertion process could undertake two phase reaction under fast lithium insertion conditions. The refined structure parameters and calculated mole fraction are listed in Table 4. The values for sample (A) were close to those of sample (FeI075). 4. Conclusion Lithium was chemically inserted into γ-Fe2O3 using n-butyl lithium hexane solution with various lithium ion concentrations and the detailed crystal structure in the process of lithium insertion was examined.
Fig. 5. XRD patterns of the electrochemically lithium inserted γ-Fe2O3 I samples with different current densities. For comparison, the chemically lithium inserted sample (FeI075) which had a similar lithium insertion amount to the samples of (A) and (B) is also shown. (b) is the magnified 440 reflection peak from (a). Asterisk mark (*) indicates the nickel mesh collector.
S. Park et al. / Solid State Ionics 255 (2014) 50–55
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Table 4 Refined structure parameters and calculated mole fraction of the electrochemically lithium inserted γ-Fe2O3 I samples. γ-Fe2O3 electrode before electrochemical lithium insertion was also analyzed by the X-ray Rietveld method. γ-Fe2O3 I sample
Before Li insertion (A) Li1.3Fe2O3_0.1 (B) Li1.3Fe2O3_0.01
Lattice parameter/nm
Li-lean Li-rich Li-rich
0.83336(5) 0.83586(5) 0.84389(5) 0.84200(7)
Iron occupancy 8a site
16c site
16d site
8b site
0.995(5) 0.602(32) 0.193(34) 0.424(8)
0.000(3) 0.151(24) 0.364(30) 0.248(7)
0.836(3) 0.881(24) 0.873(30) 0.873(7)
0.000(5) 0.000(33) 0.000(36) 0.000(13)
We newly found that the chemically lithium inserted sample was a mixture of two phases both belonging to the space group Fd-3m, one is rich in lithium and the other is lean in lithium. It was revealed that the chemical lithium insertion process in γ-Fe2O3 undertakes a two phase reaction, i.e. inserted lithium forms a Li-rich phase in which the lithium amount is constant, and the overall lithium concentration is controlled by the mole fraction changes between the Li-rich phase and Li-lean phase. The Li-rich phase is thought to be the same phase previously reported by Bonnet et al. and Pernet et al., in which the iron occupancy of the 16c site was not equalized with that of the 16d site. This phase belongs to Fd-3m not to Fm-3m. Fast lithium insertion could form the two phases of Li-lean and Li-rich even for the electrochemical lithium insertion process. Acknowledgment This work was partly supported by the “Energy Science in the Age of Global Warming” Global Center of Excellence (G-COE) program (J-051) of the Ministry of Education, Culture, Sports, Science and Technology of Japan and partly supported by a Grant-in-Aid for Challenging Exploratory Research (24656581) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT). References [1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188–1194. [2] C.W. Kwon, M. Quintin, S. Mornet, C. Barbieri, O. Devos, G. Campet, M.H. Delville, J. Electrochem. Soc. 151 (2004) A1445–A1449. [3] S. Kanzaki, T. Inada, T. Matsumura, N. Sonoyama, A. Yamada, M. Takano, R. Kanno, J. Power Sources 146 (2005) 323–326.
Overall temperature factor
Mole fraction
1.08(9) 1.78(66) 1.38(56) 2.66(27)
– 0.436(53) 0.564(40) –
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