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Journal of the Ceramic Society of Japan 124 [6] 706-709 2016
Single crystal synthesis, crystal structure and electrochemical property of spinel-type LiCoMnO 4 as 5 V positive electrode materials Yuki HAMADA,*,** Naoki HAMAO,* Kunimitsu KATAOKA,* Naoya ISHIDA,** Yasushi IDEMOTO** and Junji AKIMOTO*,³ *National
Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1–1–1 Higashi, Tsukuba, Ibaraki 305–8565, Japan **Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278–8510, Japan
Micrometer-sized single crystals of spinel-type LiCoMnO4 were synthesized by heating a mixture of LiOH·H2O, CoCl2, and MnCl2 with a molar ratio of Li:Co:Mn = 1.5:1:1 at 750°C. X-ray diffraction (XRD) pattern and scanning electron microscopic (SEM) image showed that LiCoMnO4 has cubic spinel structure with well-formed octahedral crystal shapes. The size of the obtained LiCoMnO4 single crystal particles was about 13 ¯m. Rietveld analysis using power XRD data confirmed the cubic spinel-type structure with space group Fd-3m, and the lattice parameter of a = 8.05812(14) ¡. The tetrahedral 8a site was occupied by both Li and Co atoms with the occupancy values of Li/Co = 0.958/0.042. Electrochemical measurement exhibited the reversible Li-ion extraction and insertion reactions at high potentials. The discharge profile with the discharge capacity of 107 mAh g¹1 showed three voltage plateaus at 5.1, 4.9, and 3.9 V; the former two corresponded to the redox reaction of Co3+/Co4+ and the latter was to the redox reaction of Mn3+/Mn4+, respectively. ©2016 The Ceramic Society of Japan. All rights reserved.
Key-words : Lithium cobalt manganese oxide, Positive electrode materials, Spinel structure, Lithium ion battery [Received January 25, 2016; Accepted April 5, 2016]
1.
Introduction
Lithium cobalt manganese oxide LiCoMnO4 with spinel-type structure is one of the promising high-voltage positive electrode materials for advanced rechargeable lithium-ion batteries.1)4) Recently, all solid-state thin-film batteries with the LiCoMnO4 positive electrode showed good cycling performance at 5 V.5) However, there are few studies on LiCoMnO4 in the literature, because of the decomposition of liquid electrolytes at high potentials in the present battery system.2) LiCoMnO4 was first reported by Kawai et al.1) The charge and discharge potentials around 5 V versus Li/Li+ of this material are attributed to the Co3+/Co4+ redox couple. The polycrystalline LiCoMnO4 samples were previously prepared by both solid state method and solgel method.1),2),4) Small single-crystal morphology of electrode oxide materials has advantages in rate capability and cycling performance, as previously demonstrated in the case of LiNi0.5Mn1.5O4 for 4.7 V positive electrode materials.6)8) We successfully synthesized the well-formed and highly crystallized LiNi0.5Mn1.5O4 by a flux method at 650 and 750°C using LiOH, NiCl2 and MnCl2 as starting materials.7) However, single crystal samples of LiCoMnO4 have not been reported yet in the literature. Previously, we tried to synthesize single crystals of LiCoMnO4 by a flux method using LiCl as a flux material.9) Unfortunately, the obtained single-crystal sample showed considerably compositional change; the chemical composition of the crystals was ³ ‡
Corresponding author: J. Akimoto; E-mail:
[email protected] Preface for this article: DOI http://dx.doi.org/10.2109/jcersj2.124.P6-1
706
Li0.65Co1.29Mn1.06O4.9) In the present study, we report the synthesis of micrometersized LiCoMnO4 single crystals by a flux method using LiOH· H2O as a starting material, and structural and electrochemical properties of the samples are demonstrated for the first time.
2. 2.1
Experimental
Synthesis
Single crystal samples of LiCoMnO4 were synthesized by a flux method using MnCl2, CoCl2, and LiOH·H2O as starting materials. First, MnCl2 and CoCl2 were ground and mixed in an agate mortar for a certain time under a dry air in a nominal atomic ratio of Co/Mn = 1:1, and then they were mixed with LiOH·H2O powder in an atomic ratio of Mn/Li = 1:1.5. An excess amount of lithium hydroxide monohydrate was used as a self-flux material. The mixture was heated at 750°C for 48 h in alumina crucibles and gradually cooled to room temperature at a cooling rate of 6°C/h. The products were easily separated from the flux by rinsing the crucible in water for several hours. The obtained sample was observed by using an optical microscope and a scanning electron microscope (SEM, JEOL JCM6000). Chemical analysis of Li, Co and Mn contents was performed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Shimadzu ICPS-8000). The phase purity and crystal structure of the samples were characterized by powder X-ray diffraction (XRD) profiles measured with Cu K¡ radiation using a Rigaku RINT2550V diffractometer equipped with a curved graphite monochromator. The powder XRD intensity data was collected for 1 s at each 0.03° step in the 2ª range of 5120°. The Rietveld analysis was carried out using Rietan-FP program.10) ©2016 The Ceramic Society of Japan DOI http://dx.doi.org/10.2109/jcersj2.16020
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Journal of the Ceramic Society of Japan 124 [6] 706-709 2016
2.2
Electrochemical Properties
Electrochemical Li-ion extraction and insertion properties were evaluated using the CR2032-type coin cells at 25°C. The working electrode was made from a mixture of the sample (5 mg), acetylene black (5 mg), and polytetrafluoroethylene (PTFE) (1 mg) powder. Aluminum mesh having a diameter of 15 mm was used as a current collector. The counter electrode was a Li foil having a thickness of 0.2 mm. The separator was a microporous polypropylene sheet. A solution of 1 M LiPF6 in a 1:1 mixture by volume of ethylene carbonate (EC) and diethyl carbonate (DEC) (Kishida Chemical Co., Ltd.) was used as the electrolyte. The cells were constructed in an argon-filled globe box. Electrochemical charge and discharge cycle tests were carried out at a constant current density of 10 mA g¹1. The cell was first charged at a constant current density of 10 mA g¹1 for 24 h, and then discharged to 3.0 V at the same current density.
3. 3.1
Results and discussion
Synthesis
Figure 1 shows the powder XRD pattern of the LiCoMnO4 sample prepared by a flux method at 750°C in air. The pattern showed high crystallinity and was well indexed to be a single phase of the spinel-type structure having a cubic crystal system and space group Fd-3m. The well-formed octahedral shaped single-crystal morphology of LiCoMnO4 with well-developed {111} faces can be observed in the SEM photographs, as shown in Fig. 2. The size of the obtained LiCoMnO4 single crystal particles was about 13 ¯m. Chemical formula, determined by ICP-AES analysis, was Li0.97Co1.00Mn1.03O4, which suggested the nearly stoichiometric composition. We can speculate on the present synthetic route of the spinel crystals as follows. The starting chlorides and lithium hydroxide partly melt by heating above 650°C. Because these chlorides are unstable in air at these temperatures, the oxidation reaction should then proceed. Then, LiCoMnO4 would precipitate together with the crystal growth. We tried to synthesize larger single crystals of LiCoMnO4 by changing the flux materials and the flux-content, and synthetic temperatures, unfortunately these experiments were failures. In the case of the use of LiCl as a flux material, the crystal size was achieved to be 100 ¯m, but the chemical composition was considerably shifted to the lithium-poor one, as reported previously.9) On the other hand, the lithium-rich starting composition resulted in the production of Li2MnO3 phase.
Fig. 1. Powder XRD pattern of the LiCoMnO4 single crystal sample synthesized by a flux method.
3.2
Crystal structure of LiCoMnO4
The crystal structure of LiCoMnO4 was refined by Rietveld method using the powder XRD data with an initial structure model of (Li1¹xCox)8a[Co1¹xMnLix]16dO4 and space group Fd-3m (No. 227), as previously reported.2) In the present refinement, the chemical formula was fixed to be Li0.97Co1.00Mn1.03O4 analyzed by ICP-AES. Figure 3 shows the observed, calculated, and difference patterns for the Rietveld refinement of LiCoMnO4. The resultant Rvalues reached Rwp = 16.9% and Rp = 11.1%, with a fit indicator of S = Rwp/Re = 1.38. This result accorded with the observed XRD and calculated XRD patterns well. The refined atomic coordinates are listed in Table 1. The lattice parameter of the sample was refined to be a = 8.05812(14) ¡. This lattice parameter was similar to the reported values for the LiCoMnO4 powder samples.1),2) The occupation value of the Co atom in 8a site was determined to be about 4%. This value was comparable to the reported value (7%) of the sample prepared by a solid state synthetic route.2)
3.3
Electrochemical properties
Figure 4 represents the charge and discharge curves of the LiCoMnO4 sample for the initial three cycles with a constant current density of 10 mA g¹1. It was confirmed that the Li-ion extraction and insertion reactions occurred in the LiCoMnO4 single crystal samples having the crystal size of several micrometers. The discharge capacity was achieved to be 107 mAh g¹1 in the second cycle, which was about 74% of the theoretical capacity of 145 mAh g¹1. The obtained capacity was well consist-
Fig. 2.
SEM images of the LiCoMnO4 single crystal sample.
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Hamada et al.: Single crystal synthesis, crystal structure and electrochemical property of spinel-type LiCoMnO4 as 5 V positive electrode
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materials
Fig. 3.
Observed, calculated, and difference patterns for the Rietveld refinement using the powder XRD data of LiCoMnO4.
Table 1. Structural parameters Li0.97Co1.00Mn1.03O4. Rwp = 16.9%, Rp = 11.1%. Space group Fd-3m, a = 8.05812(14) ¡, V = 523.240(15) ¡3, Z=8
atom
site
Li1 Co1 Co2 Mn1 Li2 O1
8a 8a 16d 16d 16d 32e
x/a
y/b
z/c
1/8 1/8 1/8 1/8 1/8 1/8 1/2 1/2 1/2 1/2 1/2 1/2 1/2 1/2 1/2 0.2630(2) 0.2630(2) 0.2630(2)
Uiso (¡2) occupancy 0.013 0.013 0.0036(3) 0.0036(3) 0.0036(3) 0.0051(8)
0.958(3) 0.042(3) 0.479(5) 0.515 0.006(5) 1
Fig. 5.
dQ/dE plots of the LiCoMnO4 single crystal sample.
consistent with the chemical composition determined by both the ICP-AES and structure analyses.
4.
Fig. 4.
Charge and discharge profiles of the LiCoMnO4 single crystal
sample.
ent with the previous data.1),2),4) The irreversible capacity was probably due to the decomposition of the electrolyte at high potentials. The Li-ion extraction and insertion potentials were carefully examined by the dQ/dE plots of the LiCoMnO4 sample, as shown in Fig. 5. Three oxidation and reduction peaks were observed in this figure. Two peaks at 5.1 and 4.9 V were attributed to be the redox reactions of Co3+/Co4+, suggesting the lithiumsite ordering in the intermediate composition.11) On the other hand, the peak at 3.9 V corresponded to the redox reaction of Mn3+/Mn4+, such as LiMn2O4 and LiNi0.4Mn1.6O4.12) This fact indicated the existence of Mn3+ ions in the sample, and well 708
Conclusion
In summary, we successfully synthesized micrometer-sized single crystals of spinel-type LiCoMnO4 by a flux method at 750°C in air. The well-formed and high crystallinity LiCoMnO4 sample was first synthesized in the present study. The crystal structure was refined by Rietveld analysis using powder XRD data. The electrochemical measurement revealed the Li-ion extraction and insertion properties of the sample. The rate capability and charge and discharge cycling properties are now investigating in the all solid-state lithium ion battery configuration, and will be published in the near future. References 1) 2)
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Journal of the Ceramic Society of Japan 124 [6] 706-709 2016
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