Growth of LiCoO2 Single Crystals by the TSFZ Method - Crystal

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Growth of LiCoO single crystals by the TSFZ method Shigenobu Nakamura, Andrey Maljuk, Yuki Maruyama, Masanori Nagao, Satoshi Watauchi, Takeshi Hayashi, Yutaka Anzai, Yasunori Furukawa, Chris D. Ling, Guochu Deng, Maxim Avdeev, Bernd Buechner, and Isao Tanaka Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01503 • Publication Date (Web): 08 Nov 2018 Downloaded from http://pubs.acs.org on November 14, 2018

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Crystal Growth & Design

Growth of LiCoO2 single crystals by the TSFZ method

Shigenobu Nakamura1, Andrey Maljuk1,2, Yuki Maruyama1, Masanori Nagao1, Satoshi Watauchi1, Takeshi Hayashi3, Yutaka Anzai3, Yasunori Furukawa3, Chris D. Ling2,4, Guochu Deng5, Maxim Avdeev5, Bernd Büchner2, Isao Tanaka1*

1

Center for Crystal Science and Technology, University of Yamanashi, Miyamae 7-32, Kofu,

Yamanashi 400-8511, Japan 2 IFW-Dresden, 3 Oxide

Helmholtzstraße 20, 01069 Dresden, Germany

Corporation, 1747-1 Mukawa, Hokuto, Yamanashi 408-0302, Japan

4 School

of Chemistry, The University of Sydney, Sydney 2006, Australia

5Australian

Centre for Neutron Scattering, ANSTO, New Illawarra Road, Lucas Heights 2234,

Australia

ABSTRACT We have grown LiCoO2 single crystals by the traveling solvent floating zone (TSFZ) growth with Li-rich solvent, having observed incongruent melting behavior of LiCoO2 between 1100˚C and 1300 ˚C. The optimum growth conditions in terms of atmosphere and solvent composition were determined to be Ar flow and an atomic ratio Li:Co=85:15, respectively. The crystals grown using a conventional mirror-type furnace contained periodic inclusions of a Co-O phase, due to the influence of Co-O phase segregation on the stability of the molten zone during growth. By using tilted-mirrors FZ furnace, inclusion-free LiCoO2 crystals of about 5 mm φ and 70 mm long were obtained at a tilting angle θ = 10˚. The grown crystals were confirmed to be single-domain by neutron Laue diffraction.

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INTRODUCTION The layered lithium cobalt oxide LiCoO2 was first proposed for use as a positive electrode material in Li-ion batteries (LIB) in the 1980’s 1, and featured in the first commercial batteries in the 1990’s. Its combination of efficiency and production cost is such that most modern batteries still use what are essentially doped variants of the Li1–xCoO2 system. Unsurprisingly, it is one of the most heavily studied in all of solid-state chemistry and physics – but, surprisingly, there remain significant gaps in our understanding of its phase behavior. LiCoO2 has rhombohedral 𝑅3𝑚 symmetry at room temperature (a = 2.8, b = 14.0 Å). If more than half of the Li is removed from the structure during battery charging (Li1–xCoO2, x > 0.57) it transforms to monoclinic C2/m symmetry (a = 4.9, b = 2.8, c = 5.0 Å; β = 109 °)

2-5.

There is

evidence from electron diffraction that the transition can be induced by cooling to ~100 K at slightly lower value of x 6. The nature of the transition is not fully understood but is thought to be related to Li ordering – possibly into chains

5,7

– with obvious important implications for Li

conductivity. Thus, the physics and crystallography of this phase transition are of real practical and commercial significance. A powerful experimental technique that has not yet been brought to bear on this problem is inelastic neutron scattering (INS), which can be used to map phonon dispersion curves as a function of temperature in the search for soft modes that precede this transition. Although this transition is only observed in Li-deficiency samples, there should still be evidence for it in the soft modes of stoichiometric samples. INS experimental data could thus be compared to the predictions of ab initio calculations for different models of Li ordering in the phase transition, to identify the best such model. The phonon dispersion curves for LiCoO2 have been calculated (albeit only very recently) 8, but no experimental data exist. This reflects the fact that the experiments are difficult,


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at least partly due to the lack of high quality and large-size single crystals and to the very weak nature of the INS signals concerned. Therefore, sizeable and high-quality samples are highly requested for INS measurements. Small LiCoO2 single crystals have been previously obtained using the flux technique 9. Relatively large samples have been grown by a crucible-free floating zone (FZ) technique

10,11.

The main growth difficulty was a considerable Li-evaporation from a molten zone (MZ) that made a molten zone highly unstable. In order to compensate for Li evaporation, some authors 10,11 added from 30% to 100% of Li-excess to feed rods. This improved MZ stability during growth, but full control of the MZ composition remained unrealizable. As a result, as-grown ingots were typically 10 cm long, but only ~1-2 cm of these boules had the correct single-phase hexagonal structure and were impurity-free

10-11.

Single-crystalline grains with volumes up to 0.2 cm3 could be cleaved

from as-prepared ingots, but the sample composition was quite Li-deficient, namely Li0.87CoO2

11.

In this paper we report the first successful growth of large and stoichiometric LiCoO2 crystals by the traveling solvent floating zone (TSFZ) method using a Li-rich solvent. By using a suitable solvent composition we could dramatically decrease the growth temperature, avoiding Lievaporation from the molten zone. We could thereby achieve a highly stable MZ composition and produce high-quality samples with volumes up to 0.8 cm3 in a reproducible manner.

EXPERIMENTAL DETAILS Precursor powders of high-purity Li2CO3 (4N) and Co3O4 (3N) were used as starting materials and mixed together in a stoichiometric ratio. The mixture was fired at 750 ˚C for 8 h in air using a Pt crucible with a lid. The calcined powder was then pressed into feed rods of diameter 6 mm and length up to 10 cm under a hydrostatic pressure of 250 MPa. Finally, feed rods were sintered at 1050 ˚C for 6 h in oxygen flow. For solvent, the calcined LiCoO2 powder and LiCO3 powder were



mixed in the Li:Co atomic ratio of 70:30 ~ 85:15, formed into a cylindrical shape using a hydrostatic press, and then used as a solvent for TSFZ growth without sintering. Prior to the growth, a solvent disk of mass ~0.5-0.6 g was attached to the top of a feed rod. Single crystals of LiCoO2 compound were grown using a conventional four-mirror type image furnace (Crystal Systems Inc. model FZ-T-4000-H) and an image furnace with tilted mirrors (Crystal Systems Inc. model TLFZ-4000-H-VPO). The both furnaces were equipped with 300 W halogen lamps and operated at University of Yamanashi. The tilting angle was varied from zero to fifteen degrees. The growth conditions were: atmosphere of air, O2 or Ar flow; growth rate of 5.0 mm/h; and rotation of the feed and seed rods of 20 rpm in the opposite directions. The grown crystals were characterized as follows. The texture and composition were analyzed by scanning electron microscopy (SEM) together with energy dispersive X-ray spectroscopy (EDS) and electron probe microanalysis (EPMA; JEOL Co. model JXA-8200). A CoO single crystal was used for qualitative analysis by EPMA. The crystallographic quality of the grown crystals was characterized by Laue neutron diffraction.

RESULTS AND DISCUSSION 1. Incongruent melting behavior of LiCoO2 Although the LiCoO2 material is widely used in battery industry, the corresponding Li2O-CoOx phase diagram has not been established. Indeed, even the melting point of this compound cannot be found in any database. As the melting behavior and temperature are the key parameters for the correct choice of growth conditions of any material, we determined these characteristics using an in-house (IFW-Dresden) high-temperature optical microscope (HTOM). Using this technique one can directly observe a sample behavior at high temperatures and simultaneously monitor a sample


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temperature. A small quantity of LiCoO2 powder sample of (~50 mg) was placed in a Pt crucible with diameter 4 mm, heated in 10 ˚C steps above 1100 ˚C to 1300 ˚C and held for 10-15 min at each temperature. The temperature accuracy is ± 5 ˚C. The results of measurements are shown on Figure 1. The sample began to melt over a 1100-1205 ˚C temperature range and completely melted close to 1300 ˚C.

1100 ˚C

1144 ˚C

1160 ˚C

1205 ˚C

Figure 1. Optical images of LiCoO2 powder sample (4N) at different temperatures in air: 1100 ˚C, 1144 ˚C, 1160 ˚C and 1205 ˚C.

We observed significant shrinkage of the powder over the temperature range 1100-1160 ˚C, and the appearance of a liquid phase above 1150 ˚C. However, a solid phase still co-existed with the melt even above 1200 ˚C. Such melting behavior is a typical hallmark of an incongruently melting material. Therefore, these data are clear evidence that LiCoO2 melts incongruently above 1100 ˚C in air. It is worth mentioning that Li-evaporation was clearly evident and strong above 1200 ˚C. From this point a solvent-based technique like the TSFZ method is required for controllable LiCoO2 growth.



2. Optimization of Growth Conditions

for TSFZ growth

The growth atmosphere had a strong effect on the stability of the MZ during TSFZ growth of LiCoO2. Figure 2 shows the form of the MZ under O2 and Ar flow in the conventional mirror type furnace using a solvent of atomic ratio Li:Co=70:30 (Li70). In the case of air and O2, bubbles induced, grew up and busted in MZ. Furthermore, the feed rod melted non-uniformly as shown in Figure 2a. As the result, the MZ was not maintained stably and continuously for a long time. In contrast, the MZ was stable in Ar flow as shown in Figure 2b. In general, melt growth of an oxide in a low oxygen partial pressure lower growth temperature and enhances the melt viscosity. In this case, the MZ in Ar flow was stable compared to in oxygen or air.

a)

Feed

b)

Molten zone Crystal Figure 2. Molten zone (MZ) in (a) O2 flow and (b) Ar flow.

Solvent composition is also important to maintain a stable MZ and to reduce Li vaporization from the MZ, since LiCoO2 melts incongruently as described above. TSFZ growth was carried with various solvent compositions as shown in Figure 3. The obtained crystals were black and typically 5 mm in diameter and 25 mm long. All of the grown crystals contained inclusions of a Co-O phase such as CoO or Co3O4 determined by EPMA. Segregation of the inclusions occurred easily when the MZ became unstable during crystal growth, as described later. EDS images in


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Figure 3 indicates that the composition of the inclusions in the crystals grown using the Li85 solvent was Co-poor and close to that of the LiCoO2 matrix compared to crystals grown using Li 70 and Li75 solvents. Therefore, the optimum solvent composition was determined to be Li85.

As-grown 10 mm growth direction

Co-rich phase

BEI 40µm EDS images 40µm ■ : O ■ : Co

a) Li70

b) Li75

c) Li85

Figure 3. As-grown, back-scattering images and EDS compositional images of LiCoO2 crystals grown using solvent of (a) Li:Co=70:30 (Li70), (b) Li:Co=75:25 (Li75) and (c) Li:Co=85:15 (Li85).

3. TSFZ Growth using tilted-mirrors type furnace Stability of the MZ is very important for the growth of inclusion-free single crystals of LiCoO2. As shown in Figure 4, the growth using conventional mirrors type furnace causes fluctuations of the crystal diameter and periodical segregation of Co-rich and O-poor phase throughout the grown crystals. We have reported crystal growth of silicon, rutile and La2-xSrxCuO4 using tilted-mirrors type furnace 12-16. Growth using the tilted-mirror type furnace is effective for the growth of large crystals and changing solid-liquid interfaces. We applied the tilted-mirror type furnace to TSFZ growth of LiCoO2. As the result, MZ was stable remarkably throughout TSFZ growth using the tilted-mirror type furnace at the mirror-tilting angle θ=0˚. We obtained LiCoO2 crystals with



almost uniform diameter and about 5 mm and 40 mm long by TSFZ growth using the mirror-tilted type furnace as shown in Figure 4. The composition of the grown crystals was almost uniform without Co-rich inclusion although cracks and subgrains were included in the grown crystals because of no-seed growth. The difference of the MZ stability between the conventional and tiltedmirrors type furnaces may be due to the temperature gradient and shape of the solid-liquid interface, which is an affected by the mirror shape. The mirror shape of the tilted-mirror type furnace is vertically asymmetric, with part of the upper side scraped off to prevent the mirrors hitting each other when tilted up to 30 degrees, whereas the conventional mirrors were symmetric halfellipsoidal. The mirror-tilting effects on the solid- liquid interface and MZ length during TSFZ growth of LiCoO2 will be discussed elsewhere.

Conventional mirror type

Tilted-mirror type

As-grown

Concentration Co distribution High Low

O Growth direction

10 mm

Figure 4. As-grown and concentration distribution images of the LiCoO2 crystals grown using conventional mirror type furnace and tilted-mirror type furnace. The concentration distribution of cobalt and oxygen was measured by EPMA.

Figure 5 shows the concentration distribution along the growth direction in the LiCoO2 crystals grown using the conventional and titled-mirrors type furnaces according to quantitative analysis


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by EPMA. The concentrations of cobalt and oxygen in the crystals grown in argon flow using the conventional mirrors type furnace were determined to be 24.0±0.5 at% and 47±3 at%, respectively while those in the crystal grown under argon flow using the tilted-mirrors type furnace were determined to be 24.0±0.6 at% and 50±2 at%, respectively. The measured value for cobalt was close to that for lithium, and almost half the measured value of oxygen (although the measured error for oxygen was one digit larger than for cobalt). Both crystals were almost uniform and stoichiometric in composition.

Figure 5. Concentration distribution along the growth direction in the LiCoO2 crystal grown (a) in Ar flow using conventional mirror type furnace and (b) in Ar flow using tilted-mirror type furnace. The Li concentration CLi was calculated by the equation CLi=100-CCo-CO.

Next, the dependence of the mirrors tilting angle (θ) on the TSFZ growth of LiCoO2 was investigated. In our growth experiments, the following growth parameters were fixed: feed rod diameter, solvent composition and weight, growth rate and atmosphere. Only the mirror-tilting



angle (θ) was varied from 0˚ to 15˚. The image furnace with tilted mirrors has partly cut-off reflectors that facilitates direct visualization of the growth process (Figure 6) due to the large gap between mirrors. Consequently, some evidence of Li-evaporation could be seen on the quartz tube (Figure 6), and Li-evaporation was observed during melting of the solvent disk only. Once a molten zone was formed, no visible traces of evaporation were detected for a particular solvent

Figure 6. Hanging feed rod (grey, upper) and growing crystal (black, bottom) inside a quartz tube. composition. Based on our HTOM measurements this composition was completely molten above 900 ˚C in air.


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The best LiCoO2 crystal was prepared at θ=10˚ as shown on Fig.7. The end-portion of as-grown boule of about 30 mm in length was single-domain, and contained two large, shiny and visible facets.

Figure 7. As-grown ingot at 𝜃= 10˚, ceramic seed. A quartz tube after the growth is given on Figure 8. Only a small amount of Li-evaporation is visible on the upper part of the tube. This was formed during solvent disk melting, and therefore did not affect the growth conditions.

Feed side

Crystal side

Figure 8. Quartz tube after the growth experiment. A tiny trace of Lievaporation.



Thus, using a carefully chosen Li-rich solvent composition, LiCoO2 crystal growth was almost Li-evaporation free. The MZ was stable in all growth runs. Usually, mirror tilting is used to avoid contact between the feed rod and growing crystal due to the highly convex molten solid-liquid interfaces

12-14.

In our experiments the ratio of MZ length to feed rod diameter, ɑ, was always

monitored: smaller ɑ means shorter MZ length. Slight contact was observed at the end of the growth run at θ=0˚ when this ratio was ɑ=0.73. No contact was detected for tilting angles θ=5˚ and 10˚, although ɑ was 0.69 and 0.67 at the end of the growth runs, respectively. This shows that mirror-tilting improved MZ stability at small tilting angles and solved the contact problem. Contact was again observed at the end of growth at θ=15˚ for ɑ=0.70. It was also much more difficult to melt a solvent disk homogeneously at θ=15˚ compared with smaller tilting angles, making the seeding step more complicated and less controllable. Therefore, tilting angles of 5 and 10 degrees are preferable ones for the stable growth of LiCoO2 by the TSFZ method.

4. Neutron diffraction

We used neutron diffraction to assess the quality of the grown boule. In addition to being a nondestructive technique, the negligible absorption of neutrons by the sample (compared to X-rays) mean that this is a true measurement of the bulk average crystal structure. Furthermore, the favorable neutron scattering length of lithium (cf. its very low atomic number and therefore X-ray scattering power) means that crystallographic refinement against neutron diffraction data can be used to reliably determine the stoichiometry of the bulk. An initially assessment was carried out using the Laue neutron diffractometer JOEY at the OPAL research reactor, Lucas Heights, Australia. JOEY uses a “white” thermal neutron covering a


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wavelength range 0.8–3.2 Å, and a flat neutron scintillator in conjunction with wide-area CCD cameras to detect backscattered neutrons. Figure 9 shows Laue images taken on JOEY at three positions, using a 2 mm aperture for the incident beam, along the 3 cm boule in the same orientation. They clearly show that a single crystalline domain dominates the entire boule. A smaller, only slightly mis-oriented, domain present near the beginning of the growth (5 mm) has grown out by the end (25 mm). All spots can be indexed to the 𝑅3𝑚 unit cell of LiCoO2, with the c axis oriented approximately perpendicular to the direction of growth along the boule.

Figure 9. Laue images taken on the Laue neutron diffractometer JOEY at OPAL, at three positions along the 30 mm growth direction of the boule in the same orientation. (The growth began at 0 mm and the tip is at 30 mm.)

A crystal fragment ~0.5 mm3 in volume was broken from the main crystal rod and mounted on an aluminium pin using fluorinated grease. Single-crystal neutron Laue diffraction data were collected at 5 K in a helium closed-cycle cryostat on the KOALA instrument at the OPAL research reactor, Lucas Heights, Australia. KOALA uses a polychromatic neutron beam from the thermal neutron guide and is equipped with an image plate detector. The data set consisted of 11 Laue images taken with a 3 mm diameter beam, 1.5 h exposure, and rotation steps of 15° perpendicular to the incident beam. A representative image is shown in Figure 10, showing small and sharp spots that are not broadened beyond the characteristic divergence of the beam from sample to detector.



Figure 10. A typical single-crystal neutron Laue diffraction pattern of the fragment from the boule, collected at 5 K on the KOALA diffractometer at OPAL. The transmitted incident neutron beam exits the detector through the central hole, and the weak diffuse scattering in the central region is white-beam powder diffraction from the polycrystalline aluminium cryostat heat shields. All discrete spots could be indexed to the R3m unit cell of LiCoO2. The inset shows the intensity profile across the spot indicated with arrow.

The KOALA detector covers ~3π steradians in real space, but the coverage of reciprocal space is reduced as spots along the same direction in reciprocal space are overlapped for the Laue method. The software suite LaueG 17 was used to index, integrate 18 and apply wavelength normalization plus other intensity corrections 19 to obtain a list of integrated intensities. A total of 949 spots were integrated and accepted as being free from spot overlap and within the wavelength limits of 0.85– 1.70 Å, which reduced to 35 unique reflections above I>3*(I) after merging. Fractional atomic coordinates, anisotropic atomic displacement parameters (ADPs) and the fractional occupancy of the Li site in LiCoO2 (space group 𝑅3𝑚, #166) were refined against KOALA data using the Jana2006 software package 20. Refinement of oxygen site occupancies did


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not lead to a statistically significant deviation from 100%. The 9 free parameters were refined against 35 observed intensities, yielding Robs = 0.0567 and a goodness-of-fit GoF = 1.70. The results are shown in Table 1. The lattice parameters for the grown crystal were determined to be a = 2.8010 Å, c = 13.9850(5) Å. Uthayakumar et al. (11) reported the lattice parameters of a=b=2.812(2) Å and c= 14.071(4) Å for Li-deficiency Li0.87CoO2 crystals grown by the FZ method. Pinsard-Gaudart et al. (10) demonstrated the c-axis enlargement with Li deficiency of LiCoO2 crystals grown by the FZ method. It is suggested that the LiCoO2 single crystals grown by the TSFZ method seems to be more Li-stoichiometric as compared with those grown by the FZ method. The key results are that ADPs for all sites refine to reasonable values and the fractional occupancy for the Li site refined to 100±6%. Full details of the refinement can be found in the CIF file deposited in association with this paper. Table 1. Final refined structural parameters for LiCoO2 at 5 K. Space group R-3m (#166), a = 2.8010 Å, c = 13.9850(5) Å. Note that a was fixed at the data integration stage because the absolute values of unit cell parameters cannot be refined against Laue data, and that U13 = U23 = 0 for all sites. The neutron scattering lengths and cross-sections used were for the naturally occurring isotopic mixtures of the elements. [Neutron News, Vol. 3, No. 3, 1992, pp. 29-37] Atom Occ. Li Co O

x (a) 1.00(6) 0 1 0 1 0

y (b) 0 0 0

z (c)

U11 (Å2)

U22 (Å2)

U33 (Å2)

U12 (Å2)

0 0.011(3) 0.011(3) 0.007(3) 0.006(3) 0.5 0.0006(9) 0.0006(9) 0.0069(15) 0.0003(9) 0.23953(8) 0.0037(6) 0.0037(6) 0.0055(5) 0.0018(6)

CONCLUSION We have successfully grown LiCoO2 single crystals by the traveling solvent floating zone (TSFZ) method using a tilted-mirror type image-focus furnace. The grown crystals were inclusion-free and sizeable, being about 5 mm φ and 70 mm long. The grown crystals were confirmed to be of highquality and single-domain by neutron Laue diffraction and of uniform composition by EPMA.



Such LiCoO2 single crystals could be useful for basic research and development in the field of solid-state lithium ion batteries. AUTHOR INFORMATION Corresponding Author

Fax; +81-55-220-8625, E-mail: [email protected] Notes

The authors declare no competing financial interest.

ABBREVIATIONS LiCoO2, lithium cobaltate; LIB, lithium ion battery; INS, inelastic neutron scattering; FZ, floating zone; MZ, molten zone; TSFZ, Traveling solvent floating zone; EPMA, electron probe microanalysis; HTOM, home-made high-temperature optical microscope; Li70, Li:Co=70:30; Li75, Li:Co=75:25; Li85, Li:Co=85:15;

ACKNOWLEDGMENTS This work was supported partly by JSPS KAKENHI Grant Number JP16K05930. CDL and MA received support for this work from the Australian Research Council (DP170100269). CDL’s participation was also supported by the Alexander von Humboldt Foundation (Friedrich Wilhelm Bessel Research Award).


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REFERENCES 1. Mizushima, K.; Jones, P. C.; Wiseman, P. J.; Goodenough, J. B. “LixCoO2 (0

Growth of LiCoO2 Single Crystals by the TSFZ Method - Crystal

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