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Structural changes of manganese spinel at elevated temperatures
Solid State Ionics 146 (2002) 55 – 63 www.elsevier.com/locate/ssi
Structural changes of manganese spinel at elevated temperatures Guohua Li a,*, Yukiko Iijima a, Yoshihiro Kudo b, Hideto Azuma a a
Nishi Battery Laboratories, Sony Corporation, 4-16-1 Okata, Atsugi, Kanagawa 243-0021, Japan Technical Support Center, Sony Corporation, 4-16-1 Okata, Atsugi, Kanagawa 243-0021, Japan
b
Received 15 May 2001; received in revised form 7 September 2001; accepted 8 October 2001
Abstract A chemical synthesis route to Cr-doped and undoped Mn spinel was developed for the purpose of detailed structural analysis for elucidating the relationship between storage performance and structural changes at elevated temperatures. We identified a two-phase segregation in the lithium compositional range of 0.6 < x < 1.0 for LixMn2O4, which is in the same region wherein severe degradation upon storage at elevated temperatures was observed for electrochemical cells. These two phases also coexist in Cr-doped spinel in the lithium compositional range of 0.4 < x < 0.9 for LixCr0.15Mn1.85O4. The investigation by X-ray diffraction (XRD) indicated that the crystallinity of the spinel decreased after storage at elevated temperatures. X-ray absorption fine structure (XAFS) analysis revealed that the Cr-doped samples showed less change in the local structure after storage than the undoped spinel samples. These results suggest that the Cr-doped spinel has higher structural stability at elevated temperatures than the undoped spinel. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Spinel; Storage performance; Phase segregation; X-ray diffraction; EXAFS
1. Introduction Among the known 4-V cathode materials in rechargeable lithium ion batteries, the spinel LixMn2O4 has been extensively studied as a promising cathode material because of its low cost, environmental merits and safety. Unfortunately, its poor storage performance at elevated temperatures stands in the way of the practical use of the spinel [1– 4]. Several groups have devoted much research in this issue. Much attention has been focused on the storage performance of the
*
Corresponding author. Tel.: +81-46-227-2341; fax: +81-46227-2222. E-mail address: [email protected] (G. Li).
spinel in the discharged or fully charged state [5– 7], but data on the dependence of storage performance on the discharge state at elevated temperatures are limited [8,9]. Though many models have been proposed, the reasons for the capacity loss at elevated temperatures are not well understood yet [1 – 10]. The models include: (1) electrolyte decomposition, (2) dissolution of the spinel into the electrolyte, (3) structural instability of the Mn spinel, and (4) the Jahn –Teller effect. Although improvements in the cycle performance at room temperature have been achieved by partial substitution of manganese by other metal ions, such as Li + , Co3 + , Cr3 + , Al3 + , Ni2 + , Mg2 + , Zn2 + , etc., [11– 20] efforts are still being made to improve the storage performance at elevated temperatures. Hatanaka et al. [21] reported the storage performance of
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the LixMn2O4/carbon cell. They reported that the severest capacity fade occurred at 20% of the charge state and that the storage performance was improved by Cr doping. Saito et al. [22] reported a detailed study of the dependence of capacity retention on the depth of discharge (DOD) of Mn spinel using the Li/LixMn2O4 cell. The capacity fade after elevated temperature storage varied with the DOD, and the maximum capacity fade occurred in the range centered around 75% DOD in the Li/LixMn2O4 cell. The storage performance of the Mn spinel was improved by doping the spinel with other metal ions. The Cr-doped phase had acceptable storage performance among the doped spinels. Recently, Saito et al. [23] reported their work on the dependence of capacity fade and manganese dissolution of LiMn2O4. However, because the capacity fade depends on the DOD, i.e., the lithium composition of LixMn2O4, the intrinsic structural feature of this material probably takes an important role for the origin of this capacity fade characteristic of the spinel at elevated temperatures besides manganese dissolution. In our previous investigation, we showed that two metastable phases coexist in the composition range of 0.6 < x < 1 in chemically prepared non-equilibrium LixMn2O4 samples [24]. This was consistent with the results obtained from in situ X-ray diffraction studies by Yang et al. [25]. Because these two phases reach the equivalent volume fraction at x = 0.75 for LixMn2O4, it is reasonable to suggest that this two-phase coexistence may be related to the storage characteristics of the spinel at elevated temperatures. The primary purpose of this study was to identify the structural changes of the spinel in the absence of electrolyte at elevated temperatures. In order to accomplish this goal, Cr-doped and undoped spinel samples with various lithium compositions were prepared by a chemical synthesis route. The structural changes during storage were characterized by X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) measurements. The origin of the severest capacity fade at 75% DOD at elevated temperatures was attributed to the phase segregation occurring in the range of 0.6 < x < 1.0 for LixMn2O4. The improvement in the storage performance for the Cr-doped spinel was attributed to the enhanced structural stability from Cr doping.
2. Experimental LiMn2O4 and Cr-doped spinel were prepared by direct solid-state reaction according to the procedure previously described [24]. Stoichiometric spinel LiMn2O4 was prepared from a stoichiometric mixture of Li2CO3 (High Purity Chemicals, 99.99%) and MnCO3 (Soekawa Chemicals, 99.99%) in a molar ratio of 1:4 which was mixed and finely ground in an agate mortar. The mixture was first calcined at 600 C for 12 h and reground, then sintered at 800 C for 24 h in air. This process was repeated three times with intermittent grinding and was finally followed by controlled cooling to room temperature at about 1.1 C/min. The Cr-doped spinel was prepared by the same procedure with Cr2O3 used as the chromium source. In this study, a nominal composition of LiCr0.15Mn1.85O4 was selected. l-MnO2 was prepared by chemical delithiation from LiMn2O4 as reported earlier [24,26]. The LiMn2O4 spinel was put into a solution of HCl/H2O = 1:33 and stirred for 5 h at room temperature. The precipitate was filtered and washed several times with distilled water and then dried under vacuum at room temperature. A chemically delithiated Cr-doped spinel (also referred to as l-type oxide in this study) was prepared by the same procedure. LixMn2O4 samples (0 < x < 1) were prepared by reacting l-MnO2 with various amounts of LiI (High Purity Chemicals, 99.9%) in acetonitrile. The hygroscopic LiI powder was treated in a dry atmosphere and the ratio of acetonitrile/l-MnO2 was 500 ml/g. The solution was stirred for 24 h at room temperature. The product was filtered and washed several times with acetonitrile to ensure the purity of the solid phase before it was dried under vacuum at room temperature. Following the same procedure, LixCr0.15Mn1.85O4 (0 < x < 1) was prepared by chemical lithiation by reacting l-type oxide with various amounts of LiI in acetonitrile. All of the samples obtained were characterized by X-ray diffraction analysis (XRD, Rigaku, Rint-2500 V) and inductively coupled plasma spectroscopy – atomic emission spectroscopy (ICP-AES, Shimazu, ICPS-8000). The morphology and the particle size of the samples were characterized using a scanning electron microscope (SEM, HITACHI S-800). To investigate the structural changes during storage at elevated temperatures, a storage experiment was carried out for the chemically prepared samples. The
G. Li et al. / Solid State Ionics 146 (2002) 55–63
spinel powder was stored in air at 80 C and the samples were characterized by XRD at 3-day and at 1week intervals for 5 weeks. X-ray absorption fine structure (XAFS) measurements were carried out using the synchrotron radiation at beam line BL16B2 in the SPring-8 (Super Photon ring-8 GeV). The Mn K-edge EXAFS spectra were collected in transmission mode using a Si(111) doublecrystal monochromator. The energy scale was calibrated using thin Cu foil. XAFS measurements were performed for samples before storage and after storage at 80 C for 3 weeks. The analysis of the Mn K-edge XAFS spectra was carried out using the REX2000 program package (Rigaku) [27]. The XAFS oscillations were isolated after background subtraction of the raw data using the cubic spline method and converted into k space by the Fourier transforms, where k is the photoelectron wave vector. The data were weighted by k3 to compensate for the diminishing amplitude of the XAFS at high k. In all cases, the Fourier transforms ˚ 1. were taken over the k range of 3– 11.9 A
3. Results and discussion The chemical composition of the prepared Mn spinel was LiMn2O4 (Li: 3.76 wt.%, Mn: 60.5 wt.%) by ICP-AES analysis. The composition of the Crdoped spinel was determined to be LiCr0.15Mn1.85O4. SEM observation revealed that the samples consist of 0.5 – 5-mm particles. All the LiCryMn2 yO4 ( y = 0, 0.15) and l-type oxide samples were confirmed by XRD analysis to be a single phase of the cubic spinel with the space group of Fd3m. The composition and the lattice constant of the spinel and the l-type oxide samples are shown in Table 1. The particle size of the delithiated l-type oxide samples was almost the same as that of the pristine spinel, i.e., 0.50– 5 mm from the SEM observation.
Table 1 Composition and lattice constant of the samples Sample
Composition
˚) Lattice constant (A
Spinel l-MnO2 Cr-doped spinel Cr-doped l-type oxide
LiMn2O4 Li0.15Mn2O4 LiCr0.15Mn1.85O4 Li0.25Cr0.15Mn1.85O4
8.247 8.050 8.231 8.073
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Fig. 1. The results of the composition analysis by ICP-AES for LixCr0.15Mn1.85O4 prepared by chemical lithiation. The calculated composition is based on reaction (1).
In our previous investigation, we showed that all the lithium supplied by LiI was completely incorporated into l-MnO2 during the chemical intercalation reaction [24]. For the Cr-doped spinel, the reaction can be written as Eq. (1). Li0:25 Cr0:15 Mn1:85 O4 þ ðx 0:25ÞLiI ! Lix Cr0:15 Mn1:85 O4 þ ½ðx 0:25Þ=2I2
ð1Þ
Fig. 1 shows the results of the ICP-AES composition analysis for the LixCr0.15Mn1.85O4 samples from the chemical lithiation reaction. This result strongly supports reaction (1), i.e., that all of the Li + in acetonitrile supplied by LiI is incorporated into Li0.25Cr0.15Mn1.85O4, and the reaction was completed within 24 h. Thus, LixCr0.15Mn1.85O4 of desired composition x (0 < x < 1) can be synthesized very precisely by reaction (1) once the distinct composition of the l-type oxide has been determined, which is similar to the case of the preparation of LixMn2O4 [24]. As previously pointed out, the 24-h lithiated sample is in a non-equilibrium state, which is similar to the fast charge–discharge processes in an electrochemical cell.
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Fig. 2 shows the XRD patterns of the prepared Li x Cr 0.15Mn1.85O4 samples. Peak splitting can be observed in the range of 0.4 < x < 0.9. It shows that two metastable phases even coexist in the Cr-doped spinel in the lithium compositional range of 0.6 < x < 1.0 for chemically prepared non-equilibrium LixCr0.15Mn1.85O4. It is similar to the non-equilibrium LixMn2O4 although the two-phase coexistence region is different [24]. However, the completely separated two phases, which were observed in the lithium compositional range of 0.15 < x < 0.5 for Li x Mn 2 O 4 , were not observed in the Cr-doped spinel. The XRD investigation showed that the XRD patterns of the spinel samples varied after storage at elevated temperature and varied with storage time. Peak deconvolution analysis using the pseudo-Voigt function was carried out for each of the diffraction peaks (standard deviation: r < 5) based on the twophase model with cubic symmetry. The peak position, peak height and full width of the half maximum intensity (FWHM) of the two phases were treated as variable parameters, whereas the ratio Gaussian/Lorentzian = 1 and the intensity ratio Ka2/Ka1 = 0.497 were fixed. Although XRD data were collected at 2h = 10– 90 and the peak deconvolution analyses were performed for all the diffraction peaks at 2h>45, for clarity, the results for the strongest diffraction peak
Fig. 2. XRD patterns of the LixCr0.15Mn1.85O4 samples prepared by chemical lithiation.
(440) among the higher diffraction angle range are shown here. Fig. 3 shows the variations in the XRD patterns of LixMn2O4 during the storage for x = 0.64 and 0.74. After storage at elevated temperature, the diffraction intensity drastically decreased while the FWHM increased. These changes indicate that the crystallinity of the material decreased after storage at elevated temperatures. As demonstrated previously, two metastable phases coexist in the lithium compositional range of 0.6 < x < 1 for LixMn2O4 [24]. They are referred to as phase A and phase B. During the first week of storage, the diffraction intensity decreased and the FWHM increased in both phase A and phase B. However, as Fig. 3 shows, after storage beyond 1 week, the diffraction intensity of phase A decreased continuously with storage time while phase B relatively changed a little. This indicates that phase B has a higher structural stability than phase A in this lithium compositional range. This feature has a relevance to the severe capacity fade observed in the range centered around 75% DOD after storage at elevated temperatures. The XRD patterns ((440) peak) of LixMn2O4 with x = 0.15 and 0.34 during storage are shown in Fig. 4. A decrease in the diffraction intensity and an increase in FWHM were observed during the first week of storage. However, in contrast to the cases shown in Fig. 3, both the intensity and the FWHM changed a little during storage beyond 1 week. This indicates that the material has a higher structural stability in the lithium compositional range of 0 < x < 0.4 in spite of the two coexistent phases. Fig. 5 shows the XRD patterns of Li1.0Mn2O4 during storage. Even after storage at 80 C for 3 weeks, the diffraction intensity and FWHM changed a little. These suggest that the Li1.0Mn2O4 spinel has higher structural stability at elevated temperature than the delithiated phases. In general, all phases in the range of 0.6 < x < 1 showed poorer structural stability compared to Li1.0Mn2O4 (equivalent to the fully discharged state in an electrochemical cell) or LixMn2O4 with a smaller x value (equivalent to the highly charged state in an electrochemical cell). Thus, it suggests that the two-phase coexistence in 0.6 < x < 1 is related to the severe capacity loss in the range centered around 75% DOD in Li/LixMn2O4 cells after storage at elevated temperatures.
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Fig. 3. Variations in the XRD patterns of LixMn2O4 with the storage time: (a) x = 0.64 and (b) x = 0.74. Deconvolution analysis was carried out by using the pseudo-Voigt function.
Fig. 4. Variations in the XRD patterns of LixMn2O4 with storage time: (a) x = 0.15 and (b) x = 0.34.
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Fig. 7. Comparison of the variation in the integral X-ray diffraction intensity with storage time for Li0.74Mn2O4 and Li0.76Cr0.15Mn1.85 O4. I0 refers to the integral intensity before storage. Fig. 5. Variations in the XRD patterns of LiMn2O4 with storage time.
Fig. 6 shows the variations of the XRD patterns of Li0.76Cr0.15Mn1.85O4 with storage time. As can be seen, even at the composition x 0.75 in which a severe capacity loss was observed in LixMn2O4, both the diffraction and the FWHM of the Cr-doped spinel
Fig. 6. Variations in the XRD patterns of Li0.76Cr0.15Mn1.85O4 with storage time.
varied a little even after storage at 80 C for 3 weeks in spite of the two coexistent phases. This suggests that the Cr-doped spinel maintains its crystallinity after storage at elevated temperatures. A comparison of variation of the integral diffraction intensity as a function of storage time between Li0.74Mn2O4 and Li0.76Cr0.15Mn1.85O4 is shown in Fig. 7. The integral diffraction intensity of Li0.76Cr0.15Mn1.85O4 decreased slightly during the initial stage of storage, while the integral diffraction intensity of Li0.74Mn2O4 decreased drastically. Moreover, as shown in Fig. 8,
Fig. 8. Comparison of the variation in FWHM with storage time for Li0.74Mn2O4 and Li0.76Cr0.15Mn1.85O4. FWHM0 refers to the FWHM value before storage.
G. Li et al. / Solid State Ionics 146 (2002) 55–63
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Fig. 9. Fourier transformed Mn K-edge EXAFS spectra of LixMn2O4 before and after storage.
Li0.76Cr0.15Mn1.85O4 showed little change in the FWHM compared to Li0.74Mn2O4. These results show that the Cr-doped spinel exhibits higher structural stability than the undoped Mn spinel phase. Fig. 9 shows the Fourier transformed Mn K-edge extended XAFS (EXAFS) spectra for Li x Mn 2O 4 (x = 0.15, 0.50, 0.74 and 1.0) before and after storage at 80 C for 3 weeks. It shows a radial atomic distri-
Fig. 10. Fourier transformed Mn K-edge EXAFS spectra of Li0.76Cr0.15Mn1.85O4 before and after storage.
Fig. 11. Changes in the interatomic distances of (a) Mn – O and (b) Mn – Mn for LixMn2O4 before and after storage.
bution about the manganese ions, the first peak assigned to the contribution of the nearest six oxygen ions, and the second peak to the contribution of the six manganese ions at the second nearest sites. A decrease in magnitude, especially in the first peak, was observed for the LixMn2O4 samples after storage. It reveals some change in the local structure of the Mn spinel after storage at elevated temperature. The Fourier transformed Mn K-edge EXAFS spectra of the Cr-doped spinel before and after storage are shown in Fig. 10. For clarity, only the results for Li0.76 Cr0.15Mn1.85O4 are shown here. As can be seen, the magnitude and profiles changed a little with storage. This shows that the Cr-doped samples experience less change in the local structure with storage than the undoped samples.
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For LixMn2O4 and LixCr0.15Mn1.85O4 samples, the structural parameters were determined by fitting the inverse Fourier transformed radial distribution function. The Mn –O interatomic distance was calculated by the one-shell model with six oxygen ions at the nearest neighbor, and the Mn – Mn interatomic distance was calculated by the one-shell model with six manganese ions at the second nearest sites, which is consistent with the cubic spinel structure. For the curve-fitting refinement of the Mn – O interatomic distance, the coordination number for the first shell of oxygen ions was fixed at the crystallographic value of 6 for all the samples. In the case of the second shell consisting of manganese ions in neighboring octahedral sites, the coordination number was also fixed at 6. The variation in the interatomic distances of Mn –O
and Mn – Mn from such a fit for LixMn2O4 is shown in Fig. 11. After storage, changes in both the Mn – O and Mn – Mn interatomic distances were observed, partic˚ at ularly in Mn – O with a maximum value of 0.023 A x = 0.74 corresponding to the composition at which the severest capacity fade at elevated temperatures occurred. This reveals some changes in the local structure in LixMn2O4 after storage at elevated temperatures. On the other hand, as shown in Fig. 12 for LixCr0.15Mn1.85O4 samples, the changes in the Mn – O and Mn –Mn interatomic distances before and after storage were negligible with a maximum value of ˚ in the Mn –O distance. These results suggest 0.005 A that the local structure in the Cr-doped spinel changed a little compared to the undoped spinel samples after the same storage treatment. In the Lix(MyMn2 y)O4 spinel structure, the transition metal ions occupy the octahedral 16d site. The enhancement of the structural stability imparted by chromium doping could be attributed either to that Cr3 + (t2g3) having the highest stabilization energy for octahedral coordination among the transition metal ions [28], or to the greater strength of the Cr)O bond compared to the Mn)O bond [29].
4. Conclusions Two-phase coexistence was observed for LixMn2O4 in the range of 0.6 < x < 1. This phase segregation relates to the severe capacity fade observed in the range centered around 75% DOD in Li/LixMn2O4 cells after elevated temperature storage. Although two phases coexist in the range of 0.4 < x < 0.9 for LixCr0.15 Mn1.85O4, X-ray diffraction and X-ray absorption fine structure studies indicated that the Cr-doped spinel has a higher structural stability than LixMn2O4. The improvement in the storage performance imparted by Cr doping can be attributed to structural stability.
Acknowledgements
Fig. 12. Changes in the interatomic distances of (a) Mn – O and (b) Mn – Mn for LixCr0.15Mn1.85O4 before and after storage.
The authors wish to thank Takamitsu Saito, Yoshikatsu Yamamoto and Koji Sekai for the technical discussions. We also thank Kuang-Yu Liu for his help in the XAFS measurements and Kimiko Ishikawa for the ICP-AES analysis.
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[19] F. Le Cars, D. Bloch, M. Anne, P. Strobel, Solid State Ionics 89 (1996) 203. [20] J.H. Lee, J.K. Hong, D.H. Jang, Y.K. Sun, S.M. Oh, J. Power Sources 89 (2000) 7. [21] H. Hatanaka, K. Okami, J. Yamaura, Y. Kogetsu, N. Tanaka, T. Saito, 41st Battery Symposium of Japan, Nagoya, Japan, The Committee of Battery Technology, The Electrochemical Society of Japan, Kyoto University, Japan, October, 2000, pp. 438, Extended Abstract. [22] T. Saito, M. Machida, Y. Yamamoto, M. Nagamine, Abstract of the 68th Meeting of The Electrochemical Society of Japan, The Electrochemical Society of Japan, 4-8-30 Kudannan, Chiyoda-ku, Tokyo, 102-0074, Japan, April, 2001, pp. 236. [23] T. Saito, M. Machida, Y. Yamamoto, M. Nagamine, The 200th Meeting of The Electrochemical Society, and the 52nd Annual Meeting of the International Society of Electrochemistry, San Francisco, USA, The Electrochemical Society, Inc., 65 South Main Street, Pennington, NJ 08534-2839, USA, September 2 – 7, 2001, Abstract No. 180. [24] G. Li, A. Yamada, Y. Fukushima, K. Yamaura, T. Saito, T. Endo, H. Azuma, K. Sekai, Y. Nishi, Solid State Ionics 130 (2000) 221. [25] X.Q. Yang, X. Sun, S.J. Lee, J. McBreen, S. Mukerjee, M.L. Daroux, X.K. Xing, Electrochem. Solid-State Lett. 2 (1999) 157. [26] J.C. Hunter, J. Solid State Chem. 39 (1981) 142. [27] Manual for XAFS Analysis Software Rex2000, Rigaku. [28] A.F. Wells, Structural Inorganic Chemistry, 3rd edn., Clarendon Press, Oxford, 1962, p. 91. [29] M. Wakihara, Li Guohua, H. Ikuta, in: M. Wakihara, O. Yamamoto (Eds.), Lithium Ion Batteries, Kodansha-Wiley VCH, Tokyo, 1998, p. 26.
Structural changes of manganese spinel at elevated temperatures Guohua Li a,*, Yukiko Iijima a, Yoshihiro Kudo b, Hideto Azuma a a
Nishi Battery Laboratories, Sony Corporation, 4-16-1 Okata, Atsugi, Kanagawa 243-0021, Japan Technical Support Center, Sony Corporation, 4-16-1 Okata, Atsugi, Kanagawa 243-0021, Japan
b
Received 15 May 2001; received in revised form 7 September 2001; accepted 8 October 2001
Abstract A chemical synthesis route to Cr-doped and undoped Mn spinel was developed for the purpose of detailed structural analysis for elucidating the relationship between storage performance and structural changes at elevated temperatures. We identified a two-phase segregation in the lithium compositional range of 0.6 < x < 1.0 for LixMn2O4, which is in the same region wherein severe degradation upon storage at elevated temperatures was observed for electrochemical cells. These two phases also coexist in Cr-doped spinel in the lithium compositional range of 0.4 < x < 0.9 for LixCr0.15Mn1.85O4. The investigation by X-ray diffraction (XRD) indicated that the crystallinity of the spinel decreased after storage at elevated temperatures. X-ray absorption fine structure (XAFS) analysis revealed that the Cr-doped samples showed less change in the local structure after storage than the undoped spinel samples. These results suggest that the Cr-doped spinel has higher structural stability at elevated temperatures than the undoped spinel. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Spinel; Storage performance; Phase segregation; X-ray diffraction; EXAFS
1. Introduction Among the known 4-V cathode materials in rechargeable lithium ion batteries, the spinel LixMn2O4 has been extensively studied as a promising cathode material because of its low cost, environmental merits and safety. Unfortunately, its poor storage performance at elevated temperatures stands in the way of the practical use of the spinel [1– 4]. Several groups have devoted much research in this issue. Much attention has been focused on the storage performance of the
*
Corresponding author. Tel.: +81-46-227-2341; fax: +81-46227-2222. E-mail address: [email protected] (G. Li).
spinel in the discharged or fully charged state [5– 7], but data on the dependence of storage performance on the discharge state at elevated temperatures are limited [8,9]. Though many models have been proposed, the reasons for the capacity loss at elevated temperatures are not well understood yet [1 – 10]. The models include: (1) electrolyte decomposition, (2) dissolution of the spinel into the electrolyte, (3) structural instability of the Mn spinel, and (4) the Jahn –Teller effect. Although improvements in the cycle performance at room temperature have been achieved by partial substitution of manganese by other metal ions, such as Li + , Co3 + , Cr3 + , Al3 + , Ni2 + , Mg2 + , Zn2 + , etc., [11– 20] efforts are still being made to improve the storage performance at elevated temperatures. Hatanaka et al. [21] reported the storage performance of
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the LixMn2O4/carbon cell. They reported that the severest capacity fade occurred at 20% of the charge state and that the storage performance was improved by Cr doping. Saito et al. [22] reported a detailed study of the dependence of capacity retention on the depth of discharge (DOD) of Mn spinel using the Li/LixMn2O4 cell. The capacity fade after elevated temperature storage varied with the DOD, and the maximum capacity fade occurred in the range centered around 75% DOD in the Li/LixMn2O4 cell. The storage performance of the Mn spinel was improved by doping the spinel with other metal ions. The Cr-doped phase had acceptable storage performance among the doped spinels. Recently, Saito et al. [23] reported their work on the dependence of capacity fade and manganese dissolution of LiMn2O4. However, because the capacity fade depends on the DOD, i.e., the lithium composition of LixMn2O4, the intrinsic structural feature of this material probably takes an important role for the origin of this capacity fade characteristic of the spinel at elevated temperatures besides manganese dissolution. In our previous investigation, we showed that two metastable phases coexist in the composition range of 0.6 < x < 1 in chemically prepared non-equilibrium LixMn2O4 samples [24]. This was consistent with the results obtained from in situ X-ray diffraction studies by Yang et al. [25]. Because these two phases reach the equivalent volume fraction at x = 0.75 for LixMn2O4, it is reasonable to suggest that this two-phase coexistence may be related to the storage characteristics of the spinel at elevated temperatures. The primary purpose of this study was to identify the structural changes of the spinel in the absence of electrolyte at elevated temperatures. In order to accomplish this goal, Cr-doped and undoped spinel samples with various lithium compositions were prepared by a chemical synthesis route. The structural changes during storage were characterized by X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) measurements. The origin of the severest capacity fade at 75% DOD at elevated temperatures was attributed to the phase segregation occurring in the range of 0.6 < x < 1.0 for LixMn2O4. The improvement in the storage performance for the Cr-doped spinel was attributed to the enhanced structural stability from Cr doping.
2. Experimental LiMn2O4 and Cr-doped spinel were prepared by direct solid-state reaction according to the procedure previously described [24]. Stoichiometric spinel LiMn2O4 was prepared from a stoichiometric mixture of Li2CO3 (High Purity Chemicals, 99.99%) and MnCO3 (Soekawa Chemicals, 99.99%) in a molar ratio of 1:4 which was mixed and finely ground in an agate mortar. The mixture was first calcined at 600 C for 12 h and reground, then sintered at 800 C for 24 h in air. This process was repeated three times with intermittent grinding and was finally followed by controlled cooling to room temperature at about 1.1 C/min. The Cr-doped spinel was prepared by the same procedure with Cr2O3 used as the chromium source. In this study, a nominal composition of LiCr0.15Mn1.85O4 was selected. l-MnO2 was prepared by chemical delithiation from LiMn2O4 as reported earlier [24,26]. The LiMn2O4 spinel was put into a solution of HCl/H2O = 1:33 and stirred for 5 h at room temperature. The precipitate was filtered and washed several times with distilled water and then dried under vacuum at room temperature. A chemically delithiated Cr-doped spinel (also referred to as l-type oxide in this study) was prepared by the same procedure. LixMn2O4 samples (0 < x < 1) were prepared by reacting l-MnO2 with various amounts of LiI (High Purity Chemicals, 99.9%) in acetonitrile. The hygroscopic LiI powder was treated in a dry atmosphere and the ratio of acetonitrile/l-MnO2 was 500 ml/g. The solution was stirred for 24 h at room temperature. The product was filtered and washed several times with acetonitrile to ensure the purity of the solid phase before it was dried under vacuum at room temperature. Following the same procedure, LixCr0.15Mn1.85O4 (0 < x < 1) was prepared by chemical lithiation by reacting l-type oxide with various amounts of LiI in acetonitrile. All of the samples obtained were characterized by X-ray diffraction analysis (XRD, Rigaku, Rint-2500 V) and inductively coupled plasma spectroscopy – atomic emission spectroscopy (ICP-AES, Shimazu, ICPS-8000). The morphology and the particle size of the samples were characterized using a scanning electron microscope (SEM, HITACHI S-800). To investigate the structural changes during storage at elevated temperatures, a storage experiment was carried out for the chemically prepared samples. The
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spinel powder was stored in air at 80 C and the samples were characterized by XRD at 3-day and at 1week intervals for 5 weeks. X-ray absorption fine structure (XAFS) measurements were carried out using the synchrotron radiation at beam line BL16B2 in the SPring-8 (Super Photon ring-8 GeV). The Mn K-edge EXAFS spectra were collected in transmission mode using a Si(111) doublecrystal monochromator. The energy scale was calibrated using thin Cu foil. XAFS measurements were performed for samples before storage and after storage at 80 C for 3 weeks. The analysis of the Mn K-edge XAFS spectra was carried out using the REX2000 program package (Rigaku) [27]. The XAFS oscillations were isolated after background subtraction of the raw data using the cubic spline method and converted into k space by the Fourier transforms, where k is the photoelectron wave vector. The data were weighted by k3 to compensate for the diminishing amplitude of the XAFS at high k. In all cases, the Fourier transforms ˚ 1. were taken over the k range of 3– 11.9 A
3. Results and discussion The chemical composition of the prepared Mn spinel was LiMn2O4 (Li: 3.76 wt.%, Mn: 60.5 wt.%) by ICP-AES analysis. The composition of the Crdoped spinel was determined to be LiCr0.15Mn1.85O4. SEM observation revealed that the samples consist of 0.5 – 5-mm particles. All the LiCryMn2 yO4 ( y = 0, 0.15) and l-type oxide samples were confirmed by XRD analysis to be a single phase of the cubic spinel with the space group of Fd3m. The composition and the lattice constant of the spinel and the l-type oxide samples are shown in Table 1. The particle size of the delithiated l-type oxide samples was almost the same as that of the pristine spinel, i.e., 0.50– 5 mm from the SEM observation.
Table 1 Composition and lattice constant of the samples Sample
Composition
˚) Lattice constant (A
Spinel l-MnO2 Cr-doped spinel Cr-doped l-type oxide
LiMn2O4 Li0.15Mn2O4 LiCr0.15Mn1.85O4 Li0.25Cr0.15Mn1.85O4
8.247 8.050 8.231 8.073
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Fig. 1. The results of the composition analysis by ICP-AES for LixCr0.15Mn1.85O4 prepared by chemical lithiation. The calculated composition is based on reaction (1).
In our previous investigation, we showed that all the lithium supplied by LiI was completely incorporated into l-MnO2 during the chemical intercalation reaction [24]. For the Cr-doped spinel, the reaction can be written as Eq. (1). Li0:25 Cr0:15 Mn1:85 O4 þ ðx 0:25ÞLiI ! Lix Cr0:15 Mn1:85 O4 þ ½ðx 0:25Þ=2I2
ð1Þ
Fig. 1 shows the results of the ICP-AES composition analysis for the LixCr0.15Mn1.85O4 samples from the chemical lithiation reaction. This result strongly supports reaction (1), i.e., that all of the Li + in acetonitrile supplied by LiI is incorporated into Li0.25Cr0.15Mn1.85O4, and the reaction was completed within 24 h. Thus, LixCr0.15Mn1.85O4 of desired composition x (0 < x < 1) can be synthesized very precisely by reaction (1) once the distinct composition of the l-type oxide has been determined, which is similar to the case of the preparation of LixMn2O4 [24]. As previously pointed out, the 24-h lithiated sample is in a non-equilibrium state, which is similar to the fast charge–discharge processes in an electrochemical cell.
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Fig. 2 shows the XRD patterns of the prepared Li x Cr 0.15Mn1.85O4 samples. Peak splitting can be observed in the range of 0.4 < x < 0.9. It shows that two metastable phases even coexist in the Cr-doped spinel in the lithium compositional range of 0.6 < x < 1.0 for chemically prepared non-equilibrium LixCr0.15Mn1.85O4. It is similar to the non-equilibrium LixMn2O4 although the two-phase coexistence region is different [24]. However, the completely separated two phases, which were observed in the lithium compositional range of 0.15 < x < 0.5 for Li x Mn 2 O 4 , were not observed in the Cr-doped spinel. The XRD investigation showed that the XRD patterns of the spinel samples varied after storage at elevated temperature and varied with storage time. Peak deconvolution analysis using the pseudo-Voigt function was carried out for each of the diffraction peaks (standard deviation: r < 5) based on the twophase model with cubic symmetry. The peak position, peak height and full width of the half maximum intensity (FWHM) of the two phases were treated as variable parameters, whereas the ratio Gaussian/Lorentzian = 1 and the intensity ratio Ka2/Ka1 = 0.497 were fixed. Although XRD data were collected at 2h = 10– 90 and the peak deconvolution analyses were performed for all the diffraction peaks at 2h>45, for clarity, the results for the strongest diffraction peak
Fig. 2. XRD patterns of the LixCr0.15Mn1.85O4 samples prepared by chemical lithiation.
(440) among the higher diffraction angle range are shown here. Fig. 3 shows the variations in the XRD patterns of LixMn2O4 during the storage for x = 0.64 and 0.74. After storage at elevated temperature, the diffraction intensity drastically decreased while the FWHM increased. These changes indicate that the crystallinity of the material decreased after storage at elevated temperatures. As demonstrated previously, two metastable phases coexist in the lithium compositional range of 0.6 < x < 1 for LixMn2O4 [24]. They are referred to as phase A and phase B. During the first week of storage, the diffraction intensity decreased and the FWHM increased in both phase A and phase B. However, as Fig. 3 shows, after storage beyond 1 week, the diffraction intensity of phase A decreased continuously with storage time while phase B relatively changed a little. This indicates that phase B has a higher structural stability than phase A in this lithium compositional range. This feature has a relevance to the severe capacity fade observed in the range centered around 75% DOD after storage at elevated temperatures. The XRD patterns ((440) peak) of LixMn2O4 with x = 0.15 and 0.34 during storage are shown in Fig. 4. A decrease in the diffraction intensity and an increase in FWHM were observed during the first week of storage. However, in contrast to the cases shown in Fig. 3, both the intensity and the FWHM changed a little during storage beyond 1 week. This indicates that the material has a higher structural stability in the lithium compositional range of 0 < x < 0.4 in spite of the two coexistent phases. Fig. 5 shows the XRD patterns of Li1.0Mn2O4 during storage. Even after storage at 80 C for 3 weeks, the diffraction intensity and FWHM changed a little. These suggest that the Li1.0Mn2O4 spinel has higher structural stability at elevated temperature than the delithiated phases. In general, all phases in the range of 0.6 < x < 1 showed poorer structural stability compared to Li1.0Mn2O4 (equivalent to the fully discharged state in an electrochemical cell) or LixMn2O4 with a smaller x value (equivalent to the highly charged state in an electrochemical cell). Thus, it suggests that the two-phase coexistence in 0.6 < x < 1 is related to the severe capacity loss in the range centered around 75% DOD in Li/LixMn2O4 cells after storage at elevated temperatures.
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Fig. 3. Variations in the XRD patterns of LixMn2O4 with the storage time: (a) x = 0.64 and (b) x = 0.74. Deconvolution analysis was carried out by using the pseudo-Voigt function.
Fig. 4. Variations in the XRD patterns of LixMn2O4 with storage time: (a) x = 0.15 and (b) x = 0.34.
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Fig. 7. Comparison of the variation in the integral X-ray diffraction intensity with storage time for Li0.74Mn2O4 and Li0.76Cr0.15Mn1.85 O4. I0 refers to the integral intensity before storage. Fig. 5. Variations in the XRD patterns of LiMn2O4 with storage time.
Fig. 6 shows the variations of the XRD patterns of Li0.76Cr0.15Mn1.85O4 with storage time. As can be seen, even at the composition x 0.75 in which a severe capacity loss was observed in LixMn2O4, both the diffraction and the FWHM of the Cr-doped spinel
Fig. 6. Variations in the XRD patterns of Li0.76Cr0.15Mn1.85O4 with storage time.
varied a little even after storage at 80 C for 3 weeks in spite of the two coexistent phases. This suggests that the Cr-doped spinel maintains its crystallinity after storage at elevated temperatures. A comparison of variation of the integral diffraction intensity as a function of storage time between Li0.74Mn2O4 and Li0.76Cr0.15Mn1.85O4 is shown in Fig. 7. The integral diffraction intensity of Li0.76Cr0.15Mn1.85O4 decreased slightly during the initial stage of storage, while the integral diffraction intensity of Li0.74Mn2O4 decreased drastically. Moreover, as shown in Fig. 8,
Fig. 8. Comparison of the variation in FWHM with storage time for Li0.74Mn2O4 and Li0.76Cr0.15Mn1.85O4. FWHM0 refers to the FWHM value before storage.
G. Li et al. / Solid State Ionics 146 (2002) 55–63
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Fig. 9. Fourier transformed Mn K-edge EXAFS spectra of LixMn2O4 before and after storage.
Li0.76Cr0.15Mn1.85O4 showed little change in the FWHM compared to Li0.74Mn2O4. These results show that the Cr-doped spinel exhibits higher structural stability than the undoped Mn spinel phase. Fig. 9 shows the Fourier transformed Mn K-edge extended XAFS (EXAFS) spectra for Li x Mn 2O 4 (x = 0.15, 0.50, 0.74 and 1.0) before and after storage at 80 C for 3 weeks. It shows a radial atomic distri-
Fig. 10. Fourier transformed Mn K-edge EXAFS spectra of Li0.76Cr0.15Mn1.85O4 before and after storage.
Fig. 11. Changes in the interatomic distances of (a) Mn – O and (b) Mn – Mn for LixMn2O4 before and after storage.
bution about the manganese ions, the first peak assigned to the contribution of the nearest six oxygen ions, and the second peak to the contribution of the six manganese ions at the second nearest sites. A decrease in magnitude, especially in the first peak, was observed for the LixMn2O4 samples after storage. It reveals some change in the local structure of the Mn spinel after storage at elevated temperature. The Fourier transformed Mn K-edge EXAFS spectra of the Cr-doped spinel before and after storage are shown in Fig. 10. For clarity, only the results for Li0.76 Cr0.15Mn1.85O4 are shown here. As can be seen, the magnitude and profiles changed a little with storage. This shows that the Cr-doped samples experience less change in the local structure with storage than the undoped samples.
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For LixMn2O4 and LixCr0.15Mn1.85O4 samples, the structural parameters were determined by fitting the inverse Fourier transformed radial distribution function. The Mn –O interatomic distance was calculated by the one-shell model with six oxygen ions at the nearest neighbor, and the Mn – Mn interatomic distance was calculated by the one-shell model with six manganese ions at the second nearest sites, which is consistent with the cubic spinel structure. For the curve-fitting refinement of the Mn – O interatomic distance, the coordination number for the first shell of oxygen ions was fixed at the crystallographic value of 6 for all the samples. In the case of the second shell consisting of manganese ions in neighboring octahedral sites, the coordination number was also fixed at 6. The variation in the interatomic distances of Mn –O
and Mn – Mn from such a fit for LixMn2O4 is shown in Fig. 11. After storage, changes in both the Mn – O and Mn – Mn interatomic distances were observed, partic˚ at ularly in Mn – O with a maximum value of 0.023 A x = 0.74 corresponding to the composition at which the severest capacity fade at elevated temperatures occurred. This reveals some changes in the local structure in LixMn2O4 after storage at elevated temperatures. On the other hand, as shown in Fig. 12 for LixCr0.15Mn1.85O4 samples, the changes in the Mn – O and Mn –Mn interatomic distances before and after storage were negligible with a maximum value of ˚ in the Mn –O distance. These results suggest 0.005 A that the local structure in the Cr-doped spinel changed a little compared to the undoped spinel samples after the same storage treatment. In the Lix(MyMn2 y)O4 spinel structure, the transition metal ions occupy the octahedral 16d site. The enhancement of the structural stability imparted by chromium doping could be attributed either to that Cr3 + (t2g3) having the highest stabilization energy for octahedral coordination among the transition metal ions [28], or to the greater strength of the Cr)O bond compared to the Mn)O bond [29].
4. Conclusions Two-phase coexistence was observed for LixMn2O4 in the range of 0.6 < x < 1. This phase segregation relates to the severe capacity fade observed in the range centered around 75% DOD in Li/LixMn2O4 cells after elevated temperature storage. Although two phases coexist in the range of 0.4 < x < 0.9 for LixCr0.15 Mn1.85O4, X-ray diffraction and X-ray absorption fine structure studies indicated that the Cr-doped spinel has a higher structural stability than LixMn2O4. The improvement in the storage performance imparted by Cr doping can be attributed to structural stability.
Acknowledgements
Fig. 12. Changes in the interatomic distances of (a) Mn – O and (b) Mn – Mn for LixCr0.15Mn1.85O4 before and after storage.
The authors wish to thank Takamitsu Saito, Yoshikatsu Yamamoto and Koji Sekai for the technical discussions. We also thank Kuang-Yu Liu for his help in the XAFS measurements and Kimiko Ishikawa for the ICP-AES analysis.
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