JP4911617B2 - Process for producing bulk single crystal of alkali cobalt oxide - Google Patents

Description

  The present invention relates to a novel alkali cobalt oxide bulk single crystal useful as a thermoelectric conversion material and a lithium ion secondary battery material, and a method for producing the same.

Conventionally, lithium cobalt oxide that has been put to practical use as a lithium battery material uses a polycrystal. However, as a material constituting a thin film battery, a micro battery, an all-solid-state lithium battery, etc., a single crystal material is preferable from the viewpoint of energy density and diffusion in a solid, and a single crystal growth technique together with a single crystal film preparation technique. Need to be established.
On the other hand, sodium cobaltate, which is attracting attention as a thermoelectric material, is known to improve thermoelectric conversion characteristics when a single crystal is used, and it is necessary to establish a single crystal growth technique.
With regard to single-crystal growth technology for these alkali-cobalt oxides having a layered crystal structure, there are only examples of synthesizing small, flaky single crystals by applying the flux method so far, and lithium battery materials and thermoelectric conversion. Establishment of a single crystal growth technique for obtaining a bulk single crystal of an alkali cobaltate having a layered crystal structure that can be used as a material or the like has been demanded.

  Accordingly, an object of the present invention is to provide a useful method for producing an alkali cobalt oxide bulk single crystal having a layered crystal structure, which has excellent battery characteristics and thermoelectric conversion performance.

As a result of intensive studies, the present inventors have found that an alkali cobalt oxide single crystal can be grown as a bulk single crystal by a melting method, and completed the present invention.
That is, the present invention has the following configuration.
1. The A _x CoO ₂ powder is melted by heating to a temperature equal to or higher than its melting point, and then cooled, each of the vertical, horizontal and height of the single crystal is at least 1 mm or more, and the chemical formula A _x CoO ₂ A method for producing a bulk single crystal of a compound represented by (0 <x ≦ 1, A = Li or Na).
2. 2. The method for producing a bulk single crystal according to 1, wherein the heating atmosphere is in oxygen gas or air.
3. The bulk single crystal according to 1 or 2, wherein the A _x CoO ₂ powder is stored in a cylindrical container, and the single crystal is grown by gradually moving the melting region of the A _x CoO ₂ . Production method.
4). 3. Production of bulk single crystal according to 3, wherein the cylindrical container is composed of alumina (Al ₂ O ₃ ), magnesia (MgO), zirconia (ZrO ₂ ) or platinum (Pt). Method.
5. The method for producing a bulk single crystal according to 3 or 4, wherein the cylindrical container is locally heated.
6). The method for producing a bulk single crystal according to any one of 1 to 5, wherein heating is performed using a single crystal growth apparatus using a halogen lamp or a laser light source.

In the present invention, a bulk single crystal of A _x CoO ₂ is obtained by a melt crystal growth method in which A _x CoO ₂ powder is used as a raw material, put into a high-temperature holding container, sufficiently melted by heating, and then cooled. Examples of the melt crystal growth method include zone melting and bridgeman method.
That is, the starting material was put in a ceramic container such as alumina, magnesia, zirconia, or a container such as platinum, and it was confirmed that the starting material was sufficiently melted by holding at a heating temperature of 1300 ° C. or higher in an oxygen gas atmosphere or in the air. Later, by cooling, a bulk single crystal is obtained. For the growth of this single crystal, a general-purpose large single crystal growth apparatus can be used.

The composition and crystal structure of the obtained bulk single crystal can be confirmed by morphological observation by SEM-EDX, chemical analysis, X-ray diffraction, and the like.
As for the chemical composition of the single crystal, when an alumina container is used, a single crystal in which a part of cobalt of the chemical formula A _x CoO ₂ is substituted with aluminum is obtained. Such a thing is also included.

Although the shape and size of the single crystal vary depending on the production conditions, it can usually be obtained as a rectangular solid of about 1 mm square, and can be obtained as a bulk of about 5 mmφ × 3 mm at the maximum.
In the conventional flux method, a single crystal is obtained by mixing LiCoO ₂ or Na _x CoO ₂ powder with LiCl or NaCl as a flux, heating at a relatively low temperature and then gradually cooling. Although crystals were obtained, bulk single crystals could not be produced.
The bulk single crystal of A _x CoO _{2 according to} the present invention can be manufactured by an industrially advantageous melt crystal growth method using a general-purpose large single crystal growth apparatus. Therefore, it is a material having high practical value as a lithium battery material and a thermoelectric conversion material.

Next, an apparatus for producing an A _x CoO ₂ bulk single crystal of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic view showing an example of a bulk single crystal production apparatus of the present invention. The apparatus 1 includes a quartz tube 3 that houses a cylindrical container 2 made of alumina, magnesia, zirconia, or platinum, and an elliptical reflecting mirror 4 that is provided so as to cover the periphery of the quartz tube 3. The quartz tube 3 is provided with an oxygen gas inlet 5 and an oxygen gas outlet 6 and holding means 7 for holding the container 2 so as to be movable up and down. In addition, the elliptical reflecting mirror 4 is provided with a halogen lamp 8 for heating the container 2 by irradiating infrared rays. A laser light source may be used in place of the halogen lamp.

In order to produce the bulk single crystal of A _x CoO ₂ of the present invention using this apparatus 1, after the A _x CoO ₂ powder 11 as a raw material is stored in the container 2, the oxygen gas inlet 5 is used. Oxygen gas is introduced into the quartz tube 3, and infrared rays are irradiated from the halogen lamp 8 to heat and melt the A _x CoO ₂ powder 11.
From the heat-melted A _x CoO ₂ powder 11, polycrystals of A _x CoO ₂ are first formed. The container 2 is moved up and down in the quartz tube 3 by the holding means 7, and the polycrystal melting region 12 is obtained. Is gradually moved up and down to grow a single crystal. When the heating temperature is maintained at 1300 ° C. or higher and it is cooled after confirming that A _x CoO ₂ is sufficiently melted, a bulk single crystal is obtained.

EXAMPLES Next, the present invention will be further described with reference to examples, but the following specific examples are not intended to limit the present invention.
Example 1
Using an infrared condensing and heating single crystal growing apparatus (manufactured by Crystal System Co., Ltd., four elliptical mirror) having the halogen lamp 8 having the configuration of FIG. 1, LiCoO ₂ powder having a purity of 99.9% or more (particle size is several About 5 microns) was filled into a single-sealed platinum tube container 2, and the tip of the container 2 was set to the maximum temperature range of the furnace. While flowing oxygen gas as an atmospheric gas at a flow rate of 3 liters per minute, controlling the maximum temperature with a halogen lamp output, heating the sample, melting the sample, and then passing the container 2 through the high temperature part at a rate of 3 to 10 mm per hour To grow a single crystal. The obtained black single crystal had a maximum size of about 3 × 2 × 2 mm.

FIG. 2 shows a stereoscopic microscope photograph of the bulk single crystal obtained in Example 1 and having a length, width and height of about 2 mm each. In FIG. 2, one scale in the figure corresponds to 1 mm.
Moreover, it was confirmed by chemical analysis by SEM-EDX (using JSM-5400 manufactured by JEOL) that the elements constituting the container 2 were not mixed in the single crystal. The obtained EDX spectrum (acceleration voltage 20 kv, measurement time 100 seconds) is shown in FIG.
Furthermore, using a four-axis X-ray diffractometer (Rigaku Denki AFC-7S, using Mo tube X-rays), it was confirmed that the crystal structure was a layered rock-salt type with a trigonal system and a space group of R-3m. The lattice constants determined by the least square method by accurately measuring the four-axis angle for 25 reflections having a significant intensity of 2θ (Mo) = 20 to 30 ° were as follows when expressed by a hexagonal lattice.
a = 2.8159 ± 0.0007 (Å)
c = 14.0543 ± 0.0010 (Å)

(Example 2)
In Example 1, a LiCoO ₂ single crystal was grown in the same manner as in Example 1 except that an alumina (JIS standard SSA-S) tube container 2 was used instead of the platinum tube container 2. The size of the obtained single crystal was a bulk single crystal of about 2 mm square as in Example 1.
A scanning electron micrograph of the single crystal group grown on the alumina tube wall surface obtained in Example 2 is shown in FIG. In addition, by chemical analysis using SEM-EDX (using JSM-5400 manufactured by JEOL Ltd.), it was confirmed that aluminum as an element constituting the container 2 was mixed in the single crystal. FIG. 5 shows the obtained EDX spectrum (acceleration voltage 20 kv, measurement time 100 seconds).
Furthermore, as a result of single crystal X-ray structural analysis using a four-axis X-ray diffractometer (AFC-7S manufactured by Rigaku Corporation, using Mo tube X-ray), the final reliability factor (R value) was 3%. It was confirmed that the layered rock salt type crystal structure of the trigonal system, space group R-3m, and the exact chemical composition were LiAl _0.32 Co _0.68 O ₂ . The lattice constants determined by the least square method by accurately measuring the four-axis angle for 25 reflections having a significant intensity of 2θ (Mo) = 20 to 30 ° were as follows when expressed by a hexagonal lattice.
a = 2.80656 ± 0.0011 (Å)
c = 14.1079 ± 0.0015 (Å)

It is a schematic diagram which shows an example of the manufacturing apparatus of the bulk-like single crystal of this invention. 2 is a stereomicrograph of a bulk single crystal obtained in Example 1. FIG. 2 is an EDX spectrum of a bulk single crystal obtained in Example 1. 2 is a scanning electron micrograph of a bulk single crystal obtained in Example 2. FIG. 4 is an EDX spectrum of a bulk single crystal obtained in Example 2.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Bulk single crystal manufacturing apparatus 2 Cylindrical container 3 Quartz tube 4 Elliptical reflector 5 Oxygen gas inlet 6 Oxygen gas outlet 7 Holding means 8 Halogen lamp 11 LiCoO ₂ powder 12 Melting zone

Claims (6)

The A _x CoO ₂ powder is melted by heating to a temperature equal to or higher than its melting point, and then cooled, each of the vertical, horizontal and height of the single crystal is at least 1 mm or more, and the chemical formula A _x CoO ₂ A method for producing a bulk single crystal of a compound represented by (0 <x ≦ 1, A = Li or Na).   2. The method for producing a bulk single crystal according to claim 1, wherein the heating atmosphere is in oxygen gas or air. 3. The bulk single crystal according to claim 1, wherein the single crystal is grown by storing the A _x CoO ₂ powder in a cylindrical container and gradually moving the melting region of the A _x CoO _{2. 4.} Crystal production method. Cylindrical container alumina _{(Al 2 O} 3), magnesia (MgO), zirconia (ZrO ₂₎ or platinum bulk single crystal according to claim 3, characterized in that constituted by (Pt) Manufacturing method.   The method for producing a bulk single crystal according to claim 3 or 4, wherein the cylindrical container is locally heated.   The method for producing a bulk single crystal according to any one of claims 1 to 5, wherein heating is performed using a single crystal growth apparatus using a halogen lamp or a laser light source.

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