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Electrochemical and in situ synchrotron X-ray diffraction studies of Li[Li0.3Cr0.1Mn0.6]O2 cathode materials
Solid State Ionics 167 (2004) 183 – 189 www.elsevier.com/locate/ssi
Electrochemical and in situ synchrotron X-ray diffraction studies of Li[Li0.3Cr0.1Mn0.6]O2 cathode materials G.X. Wang a,*, Z.P. Guo a, X.Q. Yang b, J. McBreen b, H.K. Liu a, S.X. Dou a a
ISEM, University of Wollongong, Northfield AV, Wollongong, NSW 2500, Australia b Brookhaven National Laboratory, Upton, NY 11973, USA
Received 23 September 2003; received in revised form 10 December 2003; accepted 10 December 2003
Abstract Layered Li[Li0.3Cr0.1Mn0.6]O2 cathode material with a hexagonal structure was synthesized by a solid-state reaction. The structural changes of this material were studied using a synchrotron-based in situ X-ray diffraction (XRD) technique during charge/discharge cycles. The results of in situ X-ray diffraction indicated that the layer structure and the hexagonal symmetry of this material were preserved through the phase transition between H1 and H2 during the charge/discharge cycling. When cycled in the voltage range of 2.0 – 4.5 V, the changes in lattice parameters a and c are smaller than those for the LiNiO2 layered material. When charged to a high voltage at 5.1 V, the hexagonal phase H3, which is commonly formed at voltages higher than 4.3 V in LiNiO2 with a very short c-axis, is not observed in the Li[Li0.3Cr0.1Mn0.6]O2 cathode, indicating a possible high thermal stability in the fully charged state. Cyclic voltammograms show a single pair of oxidation and reduction peaks, consistent with a reversible phase transition between H1 and H2 observed from the in situ X-ray diffraction data. D 2004 Elsevier B.V. All rights reserved. Keywords: Lithium intercalation; Solid solution; In situ X-ray diffraction; Cathode
1. Introduction The lithium manganese oxide-based cathode material systems have attracted worldwide attention due to their low cost and low toxicity. Two types of lithium manganese oxide compounds have been widely studied as cathode materials for lithium-ion batteries. One of them is LiMn2O4 with spinel structure and another is LiMnO2 with the a-NaFeO 2 -type layered crystal structure. LiMn2O4 spinel has a limited theoretical capacity and a poor cyclability at elevated temperatures. These disadvantages have hindered wide commercial applications of this material [1 –4]. Layer structured LiMnO2 can deliver a specific capacity of 285 mA h/g, which is twice of that for LiMn2O4 spinel. However, it is difficult to synthesise layered LiMnO2, because LiMnO2 tends to form an orthorhombic structure under equilibrium conditions [5– 7]. Layered LiMnO2 with the space group of C2/m was first prepared by ion exchange from NaMnO2
* Corresponding author. Tel.: +61-242215726; fax: +61-242215731. E-mail address: [email protected] (G.X. Wang). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2003.12.005
[8]. This preparation approach was tedious and the obtained LiMnO 2 was metastable. Recently, doped LiMxMn1 xO2 compounds with layered structure have been successfully synthesised, using Al, Co and Cr as doping elements [9 – 11]. Doped LiMxMn1 xO2 compounds have shown satisfactory electrochemical properties when used as cathode materials in Li-ion cells. Studies on the structural changes of different types of layered LiMnO2 have been reported. The monoclinic or orthorhombic structured LiMnO2 compounds usually transform into a spinel-type structure during charge/discharge cycling. In situ X-ray diffraction revealed that the undoped orthorhombic LiMnO2 irreversibly converts to a cubic phase with a spinel structure during the initial charge, and then transforms to a tetragonal phase in the 3 V plateau during the subsequent discharge [12]. The layer structure can be stabilized by doping and varying the doping level. As an example, powder neutron diffraction investigation demonstrated that the layered monoclinic LiCo0.1Mn0.9O2 cathodes undergo a structural transformation to a spinel structure in the initial cycling, while the highly doped LiCo0.3Mn0.7O2 can preserve the layered structure up to 30 cycles [11]. Previously, we
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˚ ). Fig. 1. X-ray diffraction pattern of Li[Li0.3Cr0.1Mn0.6]O2 (k = 1.54 A
have reported the synthesis and electrochemical properties of layered LiCrxMn1 xO2 compounds with a monoclinic structure [13,14]. In this paper, we report the synthesis and electrochemical properties of the layered Li[Li0.3Cr0.1Mn0.6]O2 material with a hexagonal structure. The structural changes of the Lix[Li0.3Cr0.1Mn0.6]O2 cathode in a lithium half-cell were studied by in situ synchrotron X-ray diffraction.
2. Experimental Sample Li[Li0.3Cr0.1Mn0.6]O2 powders were prepared by a solid-state reaction. Reagents of MnCO3 (Aldrich
Chemicals, 99%), Cr2O3 (Aldrich Chemicals, 99%) and Li2CO3 (Pacific Lithium, 99.9%) were mixed thoroughly in deionised water by using a planetary ball-milling machine. The slurry was then dried. The precursor mixtures consist of uniformly distributed Li2CO3, Cr2O3 and MnCO3. The precursors were loaded in an alumina crucible and then sintered at 1000 jC for 20 h in a tube furnace under flowing argon gas. X-ray diffraction (XRD) was performed on Li[Li0.3Cr0.1Mn0.6]O2 powders using a Philips 1730 diffractometer with Cu Ka radiation. For electrochemical testing, cathodes were made by dispersing 85% active materials, 10% acetylene carbon black and 5% poly(vinylidene fluoride) in a solvent of dimethyl phthalate. The slurry was then coated on an Al
Fig. 2. The first charge curve of a Li/Li[Li0.3Cr0.1Mn0.6]O2 in situ cell from 2.0 to 4.6 V at a C/9 rate.
G.X. Wang et al. / Solid State Ionics 167 (2004) 183–189
185
Fig. 3. In situ XRD patterns in the (003) to (113) region of a Li/Li[Li0.3Cr0.1Mn0.6]O2 cathode during the first charge at a C/9 rate (1.2 h for each 2h scan, ˚ ). k = 1.195 A
foil. CR2032 coin cells were assembled in an argonfilled glove box. Each cell contains a lithium foil as anode and 1 M LiPF6 in a mixture of 50 vol.% ethylene carbonate (EC) and 50 vol.% dimethyl carbonate (DMC) as electrolyte. These cells were cycled between 2.0 and 4.5 V at room temperature. For in situ X-ray diffraction studies, cathodes were prepared by slurrying the active material powder with 10 wt.% PVDF (KynarFlex 2801, Atochem), and 10 wt.% acetylene black in an n-methyl pyrolidone (NMP) solution, then coating the mixture onto an Al foil. After vacuum drying at 100 jC for 12 h, the electrode disks (2.8 cm2) were punched and weighed (34 mg of active material for the data presented in this paper). The cathodes were incorporated into cells with a Li foil negative electrode, a Celgard separator and a 1 M LiPF6
electrolyte in a 1:1 EC/DMC solvent (LP 30 from EM Industries). The cells were assembled in an argon-filled glove box. Mylar windows were used in these in situ cells which have been described in detail elsewhere [15]. In situ XRD patterns were collected on beam line X18A ˚ wavelength) at National Synchrotron (using k = 1.195 A Light Source (NSLS) located at Brookhaven National Lab. The step size of 2h scan was 0.02j. The XRD patterns were collected in the transmission mode.
3. Results and discussion The starting materials containing Li, Mn and Cr were mixed homogeneously after intensive ball milling. This was very important in forming single-phase solid solu-
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tion during the sintering process. Fig. 1 shows the X-ray diffraction pattern of Li[Li0.3Cr0.1Mn0.6]O2 powders. Single-phase Li[Li0.3Cr0.1Mn0.6]O2 material with hexagonal crystal structure was obtained. Layered LiMnO2 usually adopts monoclinic or orthorhombic structure due to Jahn –Teller distortion of Mn3 +. In Li[Li0.3Cr0.1Mn0.6]O2 structure, the presence of low valence Li+ in Mn site forces the Mn3 +to Mn4 +, inducing the dilution of the effect of the Jahn – Teller ion. A hexagonal structure was obtained for Li[Li0.3Cr0.1Mn0.6]O2 compound. The pristine Li[Li0.3Cr0.1Mn0.6]O2 compound has a well-defined hexagonal structure (space group of R-3m), similar to LiCoO2 and LiNiO 2 compounds. In this hexagonal structure, each Li and Mn/Cr plane is sandwiched between two oxygen planes, the sandwiched slabs containing Li and Mn/Cr are packed alternatively along the c-direction. This structure is favourable for lithium insertion and extraction. The (hkl) indices for the hexagonal Li[Li0.3Cr0.1Mn0.6]O2 phase are indicated in Fig. 1. ˚ The unit cell parameters were computed as: a = 2.899 A ˚ ˚ and c = 14.472 A. (The lattice parameters are a = 2.819 A ˚ ˚ and c = 14.066 A for LiCoO2, and a = 2.882 A and ˚ for LiNiO2 [16].) c = 14.194 A The first charge curve for the in situ cell is plotted in Fig. 2. The cell was charged to 4.6 V at a constant current of 0.8 mA for 9 h. During the first charge, eight XRD scans were continuously collected and indicated in Fig. 2. The last scan (scan 8) was taken during rest time after the cell had reached 4.6 V limit. The in situ XRD patterns are plotted in Fig. 3 for the Bragg reflections from (003) to (113). The scan numbers marked in Fig. 3 are corresponding to those numbers labeled on the charge curve in Fig. 2. In addition to the eight scans, an XRD scan collected for another cell (40 mg active material loading) charged to 5.1 V was also plotted in Fig. 3. Each XRD scan took about 1.2 h. The missing data in scan 3 were due to the X-ray beam being unavailable. Since the charging process continued during this period, the incomplete XRD pattern were retained in the plot. All the in situ XRD patterns in this paper are
Table 1 Observed and calculated d-spacing for the H1 hexagonal unit cell using data from scan 1 in Fig. 3 ˚) ˚) ˚) (hkl) 2hobs (j) dobs (A dcalc (A Delta d (A (003) (101) (006) (102) (104) (105) (107) (108) (110) (113)
14.23 27.95 28.66 29.18 33.68 36.72 43.98 48.05 48.68 51.00
4.824 2.474 2.414 2.372 2.062 1.897 1.596 1.468 1.450 1.388
4.824 2.474 2.412 2.372 2.063 1.897 1.596 1.468 1.449 1.388
0.0002 0.0006 0.0020 0.0002 0.0001 0.0004 0.0003 0.0001 0.0003 0.0002
˚ , a = 2.899 A ˚ , e.s.d. = 0.0005 A ˚ , c = 14.472 A ˚ , e.s.d. = 0.0007 A ˚. k = 1.195 A
Table 2 Observed and calculated d-spacing for the H2 hexagonal unit cell using data from scan 8 in Fig. 3 ˚) ˚) ˚) dobs (A dcalc (A Delta d (A (hkl) 2hobs (j) (003) (101) (102) (104) (105) (107) (108) (110) (113)
14.33 28.21 29.45 33.95 36.99 44.32 48.45 49.15 51.48
4.790 2.452 2.351 2.047 1.884 1.584 1.456 1.437 1.376
4.792 2.452 2.351 2.046 1.881 1.584 1.457 1.437 1.376
0.0010 0.0000 0.0007 0.0008 0.0021 0.0003 0.0005 0.0001 0.0003
˚ , a = 2.873 A ˚ , e.s.d. = 0.0005 A ˚ , c = 14.375 A ˚ , e.s.d. = 0.0010 A ˚. k = 1.195 A
presented in the same way. Two hexagonal phases (H1, H2) can be identified and the Bragg peaks related to each of them are indexed in Fig. 3. The lattice parameters obtained from least square refinements and the data used for fitting are listed in Table 1 for H1 (using data from scan 1) and in Table 2 for H2 (using data from scan 8). In Table 3, the lattice parameters for all the eight scans obtained from least square refinements are listed. In this table, we noticed that from scan 1 to scan 4, the lattice parameter a contracted from 2.899 to 2.882 ˚ , while c expanded from 14.472 to 14.512 A ˚ . From A scan 4 to scan 8, the lattice parameter c changed the course from expanding to contracting, decreased from ˚ , while a continuously decreased 14.512 to 14.375 A ˚ . This general trend of the lattice from 2.882 to 2.873 A parameter changes is similar like what we have observed in LiNiO2 system [17]. However, the clear peak separations of the (003), (101), and (110) reflections between H1 and H2 phase observed in LiNiO2 system are totally disappeared in Fig. 3 for Li[Li0.3Cr0.1Mn0.6]O2 material system. This peak separation is considered as clear evidence of the two-phase coexistence during the charge curve plateau. Without the observation of this type of peak separations, it is difficult to tell whether the process is a continued lattice parameter change in one H1 phase or a phase transition from H1 to H2, since both H1 and H2 have the same hexagonal symmetry. The main reason we claim that a phase transition from H1 to H2 had
Table 3 Lattice parameters obtained form least square refinements using data from scan 1 to scan 8 in Fig. 3 ˚) ˚) ˚) ˚) Scan number A (A e.s.d. (A c (A e.s.d. (A 1 2 3 4 5 6 7 8
2.899 2.889
0.0005 0.0005
14.472 14.496
0.0007 0.0012
2.882 2.882 2.878 2.875 2.873
0.0005 0.0005 0.0005 0.0005 0.0005
14.512 14.507 14.480 14.446 14.375
0.0009 0.0011 0.0012 0.0012 0.0010
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Fig. 4. In situ XRD patterns in the (003) to (113) region of a Li/Li[Li0.3Cr0.1Mn0.6]O2 cathode during the first discharge at a C/9 rate from 4.6 to 0.5 V (1.2 h for ˚ ). each 2h scan, k = 1.195 A
occurred is based on the c parameter changing during charge. If the so-called H1 and H2 are the same phase as H0, the c parameter should have been contracting continuously through the whole process. However, it expanded first, and then contracted during charge, behave almost the same as we observed in LiNiO2 system, where the H1, H2, and H3 phases were defined. Another important reason is the flat nature of the plateau in the charging curve, which is a clear evidence for a twophase coexistence region for the charging process. The trouble of identifying the two phase coexistence was caused by the reduction of the peak separation. However, this type of reduction in peak separation has been observed in LiMg0.125Ti0.125Ni0.75O2 system [18] where higher degree of cation mixing was expected. This cation
mixing is expected in our Li[Li0.3Cr0.1Mn0.6]O2 system too. Therefore, a phase transition from hexagonal H1 to hexagonal H2 is assigned to this Li[Li0.3Cr0.1Mn0.6]O2 system during charge. Another interesting phenomenon is the hexagonal H3 phase suppression at a voltage as high as 5.1 V, as demonstrated in the XRD pattern plotted at the top in Fig. 3. This so-called H3 phase with very ˚ ) have been studied by several ex short c-axis ( < 13.5 A situ [19] and in situ [17,20] XRD techniques in LiNiO2 cathode. It is formed when LiNiO 2 is charged to voltages higher than 4.3 V and is considered as responsible for the poor thermal stability of LiNiO2 at fully charged state. Quite interestingly, the suppression of H3 formation was also observed in LiMg0.125Ti0.125Ni0.75O2 system where the better thermal stability was obtained
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Table 4 Observed and calculated d-spacing for the H1 hexagonal unit cell using data from scan 7 in Fig. 4 ˚) ˚) ˚) dobs (A dcalc (A Delta d (A (hkl) 2hobs (j) (003) (101) (102) (104) (105) (107) (108) (110) (113)
14.25 28.12 29.35 33.82 36.86 44.10 48.15 49.05 51.30
4.817 2.459 2.359 2.054 1.890 1.592 1.465 1.439 1.380
4.825 2.458 2.358 2.054 1.890 1.592 1.465 1.440 1.380
0.0079 0.0014 0.0004 0.0005 0.0003 0.0004 0.0001 0.0006 0.0004
˚ , a = 2.880 A ˚ , e.s.d. = 0.0005 A ˚ , c = 14.475 A ˚ e.s.d. = 0.0009 A ˚. k = 1.195 A Fig. 6. The charge/discharge curves of a Li[Li0.3Cr0.1Mn0.6]O2 electrode.
[18]. In order to use the same cell for the subsequent discharge study, the 4.6 V charging limit was used to avoid the irreversible structural changes in cathode and decomposition of electrolyte at high voltage. To confirm the H3 phase was not formed at voltages higher than 4.6 V, a second cell was used for collecting data at charging voltage as high as 5.1 V. The reversibility of the structural changes was studied in the subsequent discharge process. The cell (34 mg loading) was discharged to 0.5 V at a constant current of 0.8 mA for 9 h. The eight XRD scans continuously collected during the first discharge are plotted in Fig. 4. The duration of each scan is indicated in the inset in Fig. 4. Scan 8 was recorded at voltages near 0.5 V. Since some data were missing in scan 8, the lattice parameters were calculated from scan 7 and listed in Table 4. From scan 1 to scan 7 during discharge in Fig. ˚ , but 4, a-axis reversibly changed from 2.873 to 2.880 A ˚ not quite back to the original value of 2.899 A obtained in scan 1 of Fig. 3. At the same time, c-axis reversibly ˚ which is very close to changed from 14.375 to 14.475 A ˚ the original value of 14.472 A. For most of the layered LiMnO2 systems, the irreversible structural conversion
from orthorhombic to cubic spinel during the initial charge/discharge cycle was considered as the main problem for preserving the structural integrity and retaining the capacity [12]. The doped Li[Li0.3Cr0.1Mn0.6]O2, which has hexagonal structure at the pristine state, undergoes a reversible structural change within the hexagonal frame during cycling. This is very desirable. The cyclic voltammograms of Li[Li0.3Cr0.1Mn0.6]O2 electrode are shown in Fig. 5. A pair of redox peaks was observed in the voltage range of 3.0 –4.5 V. The oxidation peak is much broader than the reduction peak. The reason is not quite clear to us yet. These CV curves are quite similar as those for LiNiO2 and LiCoO2 having reversible phase transition between H1 and H2 in the same voltage range. These results are in very good agreement with the structural changes assigned as phase transition between H1 and H2 through in situ XRD. The cycling test was performed in the voltage range of 2.0– 4.5 V versus a lithium reference electrode. The cell was first charged and discharged at a constant current density of 8.3 mA/g in the first cycle. It delivered a discharge capacity of 185 mA h/g. Starting from the second cycle, the current density was increased to 24 mA/g. The Li[Li0.3Cr0.1Mn0.6]O2 electrode delivered a discharge capacity of 145 mA h/g at this new current density. Intermittently, the current density was switched back to the low level of 8.3 mA/g. The capacity of Li[Li0.3Cr0.1Mn0.6]O2 electrode is strongly affected by the current density of charge and discharge. The charge/discharge curves are shown in Fig. 6. The shape of the first discharge curve differs from the rest of cycles, having a higher average potential. After the 10th cycle, the changes in discharge curves become negligible. 4. Conclusions
Fig. 5. Cyclic voltammograms of a Li[Li0.3Cr0.1Mn0.6]O2 electrode in Liion cell.
A Li[Li0.3Cr0.1Mn0.6]O2 compound with a hexagonal structure in the pristine state has been synthesized by a solid-state reaction. Based on the in situ XRD patterns
G.X. Wang et al. / Solid State Ionics 167 (2004) 183–189
collected during the charge/discharge process and the cyclic voltammograms, a reversible phase transformation between hexagonal phases H1 and H2 has been identified for Li[Li0.3Cr0.1Mn0.6]O2 cathode material. Since this phase transition takes place within the hexagonal symmetry and the changes in lattice parameters are much smaller than in other layered systems, the integrity of the crystal structure is preserved during cycling. Therefore, Li[Li0.3Cr0.1Mn0.6]O2 cathode has a good cyclability when charged/discharged in a wide voltage range of 2.0 –4.5 V. References [1] M.M. Thackeray, J. Electrochem. Soc. 142 (1995) 2559. [2] P. Arora, B.N. Popov, R.E. White, J. Electrochem. Soc. 145 (1998) 807. [3] R.J. Gummow, A. de Kock, M.M. Thackeray, Solid State Ionics 69 (1994) 59. [4] G.X. Wang, D.H. Bradhurst, H.K. Liu, S.X. Dou, Solid State Ionics 120 (1999) 95. [5] G. Ceder, S.K. Mishra, Electrochem, Solid-State Lett. 2 (1999) 550. [6] M.M. Thackeray, J. Electrochem. Soc. 142 (1995) 2558.
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[7] F. Leroux, D. Guyomard, Y. Piffard, Solid State Ionics 80 (1995) 299. [8] A. Robert Armstrong, P.G. Bruce, Nature 381 (1996) 499. [9] Y.-M. Chiang, D.R. Sadoway, Y.-I. Jang, B. Huang, Electrochem, Solid-State Lett. 2 (1999) 107. [10] B. Ammundsen, J. Desilvestro, T. Groutso, D. Hassell, J.B. Metson, E. Regan, R. Steiner, P.J. Pickering, J. Electrochem. Soc. 147 (2000) 4078. [11] A.R. Armstrong, A.D. Robertson, P.G. Bruce, Electrochim. Acta 45 (1999) 285. [12] I.M. Kotschau, J.R. Dahn, J. Electrochem. Soc. 145 (1998) 2672. [13] Z.P. Guo, G.X. Wang, H.K. Liu, S.X. Dou, Solid State Ionics 148 (2002) 359. [14] Z.P. Guo, S. Zhong, G.X. Wang, G. Walter, H.K. Liu, S.X. Dou, J. Electrochem. Soc. 149 (2002) A792. [15] M. Balasubramanian, X. Sun, X.Q. Yang, J. McBreen, J. Power Sources 92 (1) (2001) 1. [16] T. Ohzuku, A. Ueda, Solid State Ionics 69 (1994) 201. [17] X.Q. Yang, X. Sun, J. McBreen, Electrochem. Commun. 1 (1999) 227. [18] X.Q. Yang, X. Sun, J. McBreen, Electrochem. Commun. 2 (2000) 733. [19] T. Ohzuku, A. Ueda, M. Nagayama, J. Electrochem. Soc. 140 (1993) 1862. [20] W. Li, J.N. Reimers, J.R. Dahn, Solid State Ionics 67 (1993) 123.
Electrochemical and in situ synchrotron X-ray diffraction studies of Li[Li0.3Cr0.1Mn0.6]O2 cathode materials G.X. Wang a,*, Z.P. Guo a, X.Q. Yang b, J. McBreen b, H.K. Liu a, S.X. Dou a a
ISEM, University of Wollongong, Northfield AV, Wollongong, NSW 2500, Australia b Brookhaven National Laboratory, Upton, NY 11973, USA
Received 23 September 2003; received in revised form 10 December 2003; accepted 10 December 2003
Abstract Layered Li[Li0.3Cr0.1Mn0.6]O2 cathode material with a hexagonal structure was synthesized by a solid-state reaction. The structural changes of this material were studied using a synchrotron-based in situ X-ray diffraction (XRD) technique during charge/discharge cycles. The results of in situ X-ray diffraction indicated that the layer structure and the hexagonal symmetry of this material were preserved through the phase transition between H1 and H2 during the charge/discharge cycling. When cycled in the voltage range of 2.0 – 4.5 V, the changes in lattice parameters a and c are smaller than those for the LiNiO2 layered material. When charged to a high voltage at 5.1 V, the hexagonal phase H3, which is commonly formed at voltages higher than 4.3 V in LiNiO2 with a very short c-axis, is not observed in the Li[Li0.3Cr0.1Mn0.6]O2 cathode, indicating a possible high thermal stability in the fully charged state. Cyclic voltammograms show a single pair of oxidation and reduction peaks, consistent with a reversible phase transition between H1 and H2 observed from the in situ X-ray diffraction data. D 2004 Elsevier B.V. All rights reserved. Keywords: Lithium intercalation; Solid solution; In situ X-ray diffraction; Cathode
1. Introduction The lithium manganese oxide-based cathode material systems have attracted worldwide attention due to their low cost and low toxicity. Two types of lithium manganese oxide compounds have been widely studied as cathode materials for lithium-ion batteries. One of them is LiMn2O4 with spinel structure and another is LiMnO2 with the a-NaFeO 2 -type layered crystal structure. LiMn2O4 spinel has a limited theoretical capacity and a poor cyclability at elevated temperatures. These disadvantages have hindered wide commercial applications of this material [1 –4]. Layer structured LiMnO2 can deliver a specific capacity of 285 mA h/g, which is twice of that for LiMn2O4 spinel. However, it is difficult to synthesise layered LiMnO2, because LiMnO2 tends to form an orthorhombic structure under equilibrium conditions [5– 7]. Layered LiMnO2 with the space group of C2/m was first prepared by ion exchange from NaMnO2
* Corresponding author. Tel.: +61-242215726; fax: +61-242215731. E-mail address: [email protected] (G.X. Wang). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2003.12.005
[8]. This preparation approach was tedious and the obtained LiMnO 2 was metastable. Recently, doped LiMxMn1 xO2 compounds with layered structure have been successfully synthesised, using Al, Co and Cr as doping elements [9 – 11]. Doped LiMxMn1 xO2 compounds have shown satisfactory electrochemical properties when used as cathode materials in Li-ion cells. Studies on the structural changes of different types of layered LiMnO2 have been reported. The monoclinic or orthorhombic structured LiMnO2 compounds usually transform into a spinel-type structure during charge/discharge cycling. In situ X-ray diffraction revealed that the undoped orthorhombic LiMnO2 irreversibly converts to a cubic phase with a spinel structure during the initial charge, and then transforms to a tetragonal phase in the 3 V plateau during the subsequent discharge [12]. The layer structure can be stabilized by doping and varying the doping level. As an example, powder neutron diffraction investigation demonstrated that the layered monoclinic LiCo0.1Mn0.9O2 cathodes undergo a structural transformation to a spinel structure in the initial cycling, while the highly doped LiCo0.3Mn0.7O2 can preserve the layered structure up to 30 cycles [11]. Previously, we
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˚ ). Fig. 1. X-ray diffraction pattern of Li[Li0.3Cr0.1Mn0.6]O2 (k = 1.54 A
have reported the synthesis and electrochemical properties of layered LiCrxMn1 xO2 compounds with a monoclinic structure [13,14]. In this paper, we report the synthesis and electrochemical properties of the layered Li[Li0.3Cr0.1Mn0.6]O2 material with a hexagonal structure. The structural changes of the Lix[Li0.3Cr0.1Mn0.6]O2 cathode in a lithium half-cell were studied by in situ synchrotron X-ray diffraction.
2. Experimental Sample Li[Li0.3Cr0.1Mn0.6]O2 powders were prepared by a solid-state reaction. Reagents of MnCO3 (Aldrich
Chemicals, 99%), Cr2O3 (Aldrich Chemicals, 99%) and Li2CO3 (Pacific Lithium, 99.9%) were mixed thoroughly in deionised water by using a planetary ball-milling machine. The slurry was then dried. The precursor mixtures consist of uniformly distributed Li2CO3, Cr2O3 and MnCO3. The precursors were loaded in an alumina crucible and then sintered at 1000 jC for 20 h in a tube furnace under flowing argon gas. X-ray diffraction (XRD) was performed on Li[Li0.3Cr0.1Mn0.6]O2 powders using a Philips 1730 diffractometer with Cu Ka radiation. For electrochemical testing, cathodes were made by dispersing 85% active materials, 10% acetylene carbon black and 5% poly(vinylidene fluoride) in a solvent of dimethyl phthalate. The slurry was then coated on an Al
Fig. 2. The first charge curve of a Li/Li[Li0.3Cr0.1Mn0.6]O2 in situ cell from 2.0 to 4.6 V at a C/9 rate.
G.X. Wang et al. / Solid State Ionics 167 (2004) 183–189
185
Fig. 3. In situ XRD patterns in the (003) to (113) region of a Li/Li[Li0.3Cr0.1Mn0.6]O2 cathode during the first charge at a C/9 rate (1.2 h for each 2h scan, ˚ ). k = 1.195 A
foil. CR2032 coin cells were assembled in an argonfilled glove box. Each cell contains a lithium foil as anode and 1 M LiPF6 in a mixture of 50 vol.% ethylene carbonate (EC) and 50 vol.% dimethyl carbonate (DMC) as electrolyte. These cells were cycled between 2.0 and 4.5 V at room temperature. For in situ X-ray diffraction studies, cathodes were prepared by slurrying the active material powder with 10 wt.% PVDF (KynarFlex 2801, Atochem), and 10 wt.% acetylene black in an n-methyl pyrolidone (NMP) solution, then coating the mixture onto an Al foil. After vacuum drying at 100 jC for 12 h, the electrode disks (2.8 cm2) were punched and weighed (34 mg of active material for the data presented in this paper). The cathodes were incorporated into cells with a Li foil negative electrode, a Celgard separator and a 1 M LiPF6
electrolyte in a 1:1 EC/DMC solvent (LP 30 from EM Industries). The cells were assembled in an argon-filled glove box. Mylar windows were used in these in situ cells which have been described in detail elsewhere [15]. In situ XRD patterns were collected on beam line X18A ˚ wavelength) at National Synchrotron (using k = 1.195 A Light Source (NSLS) located at Brookhaven National Lab. The step size of 2h scan was 0.02j. The XRD patterns were collected in the transmission mode.
3. Results and discussion The starting materials containing Li, Mn and Cr were mixed homogeneously after intensive ball milling. This was very important in forming single-phase solid solu-
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tion during the sintering process. Fig. 1 shows the X-ray diffraction pattern of Li[Li0.3Cr0.1Mn0.6]O2 powders. Single-phase Li[Li0.3Cr0.1Mn0.6]O2 material with hexagonal crystal structure was obtained. Layered LiMnO2 usually adopts monoclinic or orthorhombic structure due to Jahn –Teller distortion of Mn3 +. In Li[Li0.3Cr0.1Mn0.6]O2 structure, the presence of low valence Li+ in Mn site forces the Mn3 +to Mn4 +, inducing the dilution of the effect of the Jahn – Teller ion. A hexagonal structure was obtained for Li[Li0.3Cr0.1Mn0.6]O2 compound. The pristine Li[Li0.3Cr0.1Mn0.6]O2 compound has a well-defined hexagonal structure (space group of R-3m), similar to LiCoO2 and LiNiO 2 compounds. In this hexagonal structure, each Li and Mn/Cr plane is sandwiched between two oxygen planes, the sandwiched slabs containing Li and Mn/Cr are packed alternatively along the c-direction. This structure is favourable for lithium insertion and extraction. The (hkl) indices for the hexagonal Li[Li0.3Cr0.1Mn0.6]O2 phase are indicated in Fig. 1. ˚ The unit cell parameters were computed as: a = 2.899 A ˚ ˚ and c = 14.472 A. (The lattice parameters are a = 2.819 A ˚ ˚ and c = 14.066 A for LiCoO2, and a = 2.882 A and ˚ for LiNiO2 [16].) c = 14.194 A The first charge curve for the in situ cell is plotted in Fig. 2. The cell was charged to 4.6 V at a constant current of 0.8 mA for 9 h. During the first charge, eight XRD scans were continuously collected and indicated in Fig. 2. The last scan (scan 8) was taken during rest time after the cell had reached 4.6 V limit. The in situ XRD patterns are plotted in Fig. 3 for the Bragg reflections from (003) to (113). The scan numbers marked in Fig. 3 are corresponding to those numbers labeled on the charge curve in Fig. 2. In addition to the eight scans, an XRD scan collected for another cell (40 mg active material loading) charged to 5.1 V was also plotted in Fig. 3. Each XRD scan took about 1.2 h. The missing data in scan 3 were due to the X-ray beam being unavailable. Since the charging process continued during this period, the incomplete XRD pattern were retained in the plot. All the in situ XRD patterns in this paper are
Table 1 Observed and calculated d-spacing for the H1 hexagonal unit cell using data from scan 1 in Fig. 3 ˚) ˚) ˚) (hkl) 2hobs (j) dobs (A dcalc (A Delta d (A (003) (101) (006) (102) (104) (105) (107) (108) (110) (113)
14.23 27.95 28.66 29.18 33.68 36.72 43.98 48.05 48.68 51.00
4.824 2.474 2.414 2.372 2.062 1.897 1.596 1.468 1.450 1.388
4.824 2.474 2.412 2.372 2.063 1.897 1.596 1.468 1.449 1.388
0.0002 0.0006 0.0020 0.0002 0.0001 0.0004 0.0003 0.0001 0.0003 0.0002
˚ , a = 2.899 A ˚ , e.s.d. = 0.0005 A ˚ , c = 14.472 A ˚ , e.s.d. = 0.0007 A ˚. k = 1.195 A
Table 2 Observed and calculated d-spacing for the H2 hexagonal unit cell using data from scan 8 in Fig. 3 ˚) ˚) ˚) dobs (A dcalc (A Delta d (A (hkl) 2hobs (j) (003) (101) (102) (104) (105) (107) (108) (110) (113)
14.33 28.21 29.45 33.95 36.99 44.32 48.45 49.15 51.48
4.790 2.452 2.351 2.047 1.884 1.584 1.456 1.437 1.376
4.792 2.452 2.351 2.046 1.881 1.584 1.457 1.437 1.376
0.0010 0.0000 0.0007 0.0008 0.0021 0.0003 0.0005 0.0001 0.0003
˚ , a = 2.873 A ˚ , e.s.d. = 0.0005 A ˚ , c = 14.375 A ˚ , e.s.d. = 0.0010 A ˚. k = 1.195 A
presented in the same way. Two hexagonal phases (H1, H2) can be identified and the Bragg peaks related to each of them are indexed in Fig. 3. The lattice parameters obtained from least square refinements and the data used for fitting are listed in Table 1 for H1 (using data from scan 1) and in Table 2 for H2 (using data from scan 8). In Table 3, the lattice parameters for all the eight scans obtained from least square refinements are listed. In this table, we noticed that from scan 1 to scan 4, the lattice parameter a contracted from 2.899 to 2.882 ˚ , while c expanded from 14.472 to 14.512 A ˚ . From A scan 4 to scan 8, the lattice parameter c changed the course from expanding to contracting, decreased from ˚ , while a continuously decreased 14.512 to 14.375 A ˚ . This general trend of the lattice from 2.882 to 2.873 A parameter changes is similar like what we have observed in LiNiO2 system [17]. However, the clear peak separations of the (003), (101), and (110) reflections between H1 and H2 phase observed in LiNiO2 system are totally disappeared in Fig. 3 for Li[Li0.3Cr0.1Mn0.6]O2 material system. This peak separation is considered as clear evidence of the two-phase coexistence during the charge curve plateau. Without the observation of this type of peak separations, it is difficult to tell whether the process is a continued lattice parameter change in one H1 phase or a phase transition from H1 to H2, since both H1 and H2 have the same hexagonal symmetry. The main reason we claim that a phase transition from H1 to H2 had
Table 3 Lattice parameters obtained form least square refinements using data from scan 1 to scan 8 in Fig. 3 ˚) ˚) ˚) ˚) Scan number A (A e.s.d. (A c (A e.s.d. (A 1 2 3 4 5 6 7 8
2.899 2.889
0.0005 0.0005
14.472 14.496
0.0007 0.0012
2.882 2.882 2.878 2.875 2.873
0.0005 0.0005 0.0005 0.0005 0.0005
14.512 14.507 14.480 14.446 14.375
0.0009 0.0011 0.0012 0.0012 0.0010
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Fig. 4. In situ XRD patterns in the (003) to (113) region of a Li/Li[Li0.3Cr0.1Mn0.6]O2 cathode during the first discharge at a C/9 rate from 4.6 to 0.5 V (1.2 h for ˚ ). each 2h scan, k = 1.195 A
occurred is based on the c parameter changing during charge. If the so-called H1 and H2 are the same phase as H0, the c parameter should have been contracting continuously through the whole process. However, it expanded first, and then contracted during charge, behave almost the same as we observed in LiNiO2 system, where the H1, H2, and H3 phases were defined. Another important reason is the flat nature of the plateau in the charging curve, which is a clear evidence for a twophase coexistence region for the charging process. The trouble of identifying the two phase coexistence was caused by the reduction of the peak separation. However, this type of reduction in peak separation has been observed in LiMg0.125Ti0.125Ni0.75O2 system [18] where higher degree of cation mixing was expected. This cation
mixing is expected in our Li[Li0.3Cr0.1Mn0.6]O2 system too. Therefore, a phase transition from hexagonal H1 to hexagonal H2 is assigned to this Li[Li0.3Cr0.1Mn0.6]O2 system during charge. Another interesting phenomenon is the hexagonal H3 phase suppression at a voltage as high as 5.1 V, as demonstrated in the XRD pattern plotted at the top in Fig. 3. This so-called H3 phase with very ˚ ) have been studied by several ex short c-axis ( < 13.5 A situ [19] and in situ [17,20] XRD techniques in LiNiO2 cathode. It is formed when LiNiO 2 is charged to voltages higher than 4.3 V and is considered as responsible for the poor thermal stability of LiNiO2 at fully charged state. Quite interestingly, the suppression of H3 formation was also observed in LiMg0.125Ti0.125Ni0.75O2 system where the better thermal stability was obtained
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Table 4 Observed and calculated d-spacing for the H1 hexagonal unit cell using data from scan 7 in Fig. 4 ˚) ˚) ˚) dobs (A dcalc (A Delta d (A (hkl) 2hobs (j) (003) (101) (102) (104) (105) (107) (108) (110) (113)
14.25 28.12 29.35 33.82 36.86 44.10 48.15 49.05 51.30
4.817 2.459 2.359 2.054 1.890 1.592 1.465 1.439 1.380
4.825 2.458 2.358 2.054 1.890 1.592 1.465 1.440 1.380
0.0079 0.0014 0.0004 0.0005 0.0003 0.0004 0.0001 0.0006 0.0004
˚ , a = 2.880 A ˚ , e.s.d. = 0.0005 A ˚ , c = 14.475 A ˚ e.s.d. = 0.0009 A ˚. k = 1.195 A Fig. 6. The charge/discharge curves of a Li[Li0.3Cr0.1Mn0.6]O2 electrode.
[18]. In order to use the same cell for the subsequent discharge study, the 4.6 V charging limit was used to avoid the irreversible structural changes in cathode and decomposition of electrolyte at high voltage. To confirm the H3 phase was not formed at voltages higher than 4.6 V, a second cell was used for collecting data at charging voltage as high as 5.1 V. The reversibility of the structural changes was studied in the subsequent discharge process. The cell (34 mg loading) was discharged to 0.5 V at a constant current of 0.8 mA for 9 h. The eight XRD scans continuously collected during the first discharge are plotted in Fig. 4. The duration of each scan is indicated in the inset in Fig. 4. Scan 8 was recorded at voltages near 0.5 V. Since some data were missing in scan 8, the lattice parameters were calculated from scan 7 and listed in Table 4. From scan 1 to scan 7 during discharge in Fig. ˚ , but 4, a-axis reversibly changed from 2.873 to 2.880 A ˚ not quite back to the original value of 2.899 A obtained in scan 1 of Fig. 3. At the same time, c-axis reversibly ˚ which is very close to changed from 14.375 to 14.475 A ˚ the original value of 14.472 A. For most of the layered LiMnO2 systems, the irreversible structural conversion
from orthorhombic to cubic spinel during the initial charge/discharge cycle was considered as the main problem for preserving the structural integrity and retaining the capacity [12]. The doped Li[Li0.3Cr0.1Mn0.6]O2, which has hexagonal structure at the pristine state, undergoes a reversible structural change within the hexagonal frame during cycling. This is very desirable. The cyclic voltammograms of Li[Li0.3Cr0.1Mn0.6]O2 electrode are shown in Fig. 5. A pair of redox peaks was observed in the voltage range of 3.0 –4.5 V. The oxidation peak is much broader than the reduction peak. The reason is not quite clear to us yet. These CV curves are quite similar as those for LiNiO2 and LiCoO2 having reversible phase transition between H1 and H2 in the same voltage range. These results are in very good agreement with the structural changes assigned as phase transition between H1 and H2 through in situ XRD. The cycling test was performed in the voltage range of 2.0– 4.5 V versus a lithium reference electrode. The cell was first charged and discharged at a constant current density of 8.3 mA/g in the first cycle. It delivered a discharge capacity of 185 mA h/g. Starting from the second cycle, the current density was increased to 24 mA/g. The Li[Li0.3Cr0.1Mn0.6]O2 electrode delivered a discharge capacity of 145 mA h/g at this new current density. Intermittently, the current density was switched back to the low level of 8.3 mA/g. The capacity of Li[Li0.3Cr0.1Mn0.6]O2 electrode is strongly affected by the current density of charge and discharge. The charge/discharge curves are shown in Fig. 6. The shape of the first discharge curve differs from the rest of cycles, having a higher average potential. After the 10th cycle, the changes in discharge curves become negligible. 4. Conclusions
Fig. 5. Cyclic voltammograms of a Li[Li0.3Cr0.1Mn0.6]O2 electrode in Liion cell.
A Li[Li0.3Cr0.1Mn0.6]O2 compound with a hexagonal structure in the pristine state has been synthesized by a solid-state reaction. Based on the in situ XRD patterns
G.X. Wang et al. / Solid State Ionics 167 (2004) 183–189
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