Method for manufacturing single crystal by traveling solvent floating zone method and single crystal

Abstract

To provide a single crystal having low impedance.SOLUTION: A method for manufacturing a single crystal being a lithium cobaltate single crystal obtained by doping metal atoms A selected from a group consisting of alkaline earth metal and transition metal having a valence of 5 or more by a traveling solvent floating zone method comprises a step of forming a melting zone including a solvent having a composition different from that of a raw material rod between the raw material rod and a seed crystal to grow the single crystal on the seed crystal. When setting the content of metal atoms A in the raw material rod to total 100 at% of Co atoms and the metal atoms A in the raw material rod to FRA at%, FRA is less than 2.0 at%; and when setting the content of metal atoms A in the solvent to total 100 at% of Co atoms and the metal atoms A in the solvent to SRA at%, SRA is less than 2.0 at%.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a single crystal by a solvent transfer floating zone melting method and a single crystal.

Background Art

[0002] Lithium cobalt oxide (LiCoO2) is widely used as a cathode material for next-generation lithium-ion secondary batteries. Further, from the viewpoint of developing a cathode material substrate for next-generation lithium-ion secondary batteries such as all-solid-state lithium-ion batteries, the development of single crystals of lithium cobalt oxide has been studied.

[0003] Patent Document 1 discloses a bulk single crystal of a compound represented by the chemical formula A x CoO2 (0 < x ≦ 1, A = Li or Na), characterized in that the length, width, and height of the single crystal are each at least 1 mm or more. Further, Patent Document 2 discloses heating and melting A x CoO2 powder at a temperature equal to or higher than its melting point and then cooling it, characterized in that the length, width, and height of the single crystal are each at least 1 mm or more, and a method for manufacturing a bulk single crystal of a compound represented by the chemical formula A x CoO2 (0 < x ≦ 1, A = Li or Na).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the lithium cobaltate single crystal obtained by the prior art does not have a sufficiently low impedance. From the perspective of developing a cathode material substrate for next-generation lithium-ion secondary batteries such as all-solid-state lithium-ion batteries, in order to perform accurate electrochemical evaluation, the development of a single crystal with an even lower impedance has been desired.

[0006] The present invention has been made in view of such circumstances, and provides a single crystal with a low impedance.

Means for Solving the Problems

[0007] According to the present invention, there is provided a method for manufacturing a single crystal by a solvent transfer floating zone melting method, wherein the single crystal is a lithium cobaltate single crystal doped with a metal atom A, and the metal atom A is selected from the group consisting of alkaline earth metals and transition metals having a valence of 5 or more. The manufacturing method includes a step of forming a molten zone containing a solvent having a composition different from that of the raw material rod between the raw material rod and the seed crystal, and growing the single crystal on the seed crystal. When the content of the metal atom A in the raw material rod is F RA at%, with respect to 100 at% in total of Co atoms and metal atom A in the raw material rod, F RA is less than 2.0 at%, and when the content of the metal atom A in the solvent is S RA at%, with respect to 100 at% in total of Co atoms and metal atom A in the solvent, S RA is less than 2.0 at%, a manufacturing method is provided.

[0008] As a result of intensive studies, the present inventors have found that by making the single crystal a single crystal in which a specific metal atom A is doped into a lithium cobaltate single crystal, and by setting the concentration of the metal atom A in the solvent and in the single crystal within a specific range in the manufacturing method by the solvent transfer floating zone melting method, a single crystal with a low impedance can be obtained, leading to the completion of the present invention.

[0009] Hereinafter, various embodiments of the present invention will be exemplified. The embodiments shown below can be combined with each other. [1] A method for manufacturing a single crystal by the solvent moving floating zone melting method, wherein the single crystal is a lithium cobaltate single crystal doped with a metal atom A, and the metal atom A is selected from the group consisting of alkaline earth metals and transition metals having a valence of 5 or more. The manufacturing method includes a step of forming a molten zone containing a solvent having a composition different from that of the raw material rod between the raw material rod and the seed crystal, and growing the single crystal on the seed crystal. The metal atom A content of the raw material rod with respect to a total of 100 at% of Co atoms and metal atom A in the raw material rod is F RA in at%, where F RA is less than 2.0 at%, and the metal atom A content of the solvent with respect to a total of 100 at% of Co atoms and metal atom A in the solvent is S RA in at%, where S RA is less than 2.0 at%. A manufacturing method [2] When the metal atom A content C of the single crystal with respect to a total of 100 at% of Li atoms, Co atoms and metal atom A in the single crystal is RA in at%, 2C RA is F RA and S RA and is smaller than that of [1]. The manufacturing method according to [1]. [3] When the Li atom concentration of the raw material rod with respect to a total of 100 at% of Li atoms, Co atoms and metal atom A contained in the raw material rod is F Li in at%, the Li atom concentration of the solvent with respect to a total of 100 at% of Li atoms, Co atoms and metal atom A contained in the solvent is S Li in at%, and the Li atom concentration of the single crystal with respect to a total of 100 at% of Li atoms, Co atoms and metal atom A contained in the single crystal is C Li in at%, S Li is larger than F Li and / or S Li is larger than C Li The manufacturing method according to [1] or [2]. [4] A single crystal, wherein the single crystal is a lithium cobaltate single crystal doped with a metal atom A, and the single crystal is Li (1-X) Co (1-Y) A (X+Y)A single crystal represented by O2, wherein the metal atom A is an alkaline earth metal, and X + Y is greater than 0 and less than or equal to 0.015. [5] The number of heterogeneous phases of 10 μm or more in the cross section of the single crystal is 1 piece / mm 2 The single crystal according to [4], wherein the number is less than or equal to the following. [6] Let the ionic conductivity in the a-axis direction of the single crystal be I a S / cm, and the ionic conductivity in the c-axis direction be I c S / cm, when I a / I c is 500 or more, the single crystal according to [4] or [5]. [7] A lithium ion battery including a positive electrode material, a solid electrolyte, and a negative electrode material, wherein the positive electrode material contains the single crystal according to any one of [4] to [6].

Advantages of the Invention

[0010] According to the method for producing a single crystal of the present invention, a single crystal with low impedance can be obtained.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

[0100] and

[0001] of the LiCo0.9913Mg0.0087O2 single crystal at room temperature according to Example 1 are shown.

Figure 7

Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present invention will be exemplified and the present invention will be described in detail. The present invention is not limited in any way by these descriptions. Each feature of the embodiments of the present invention shown below can be combined with each other. Also, an invention can be established independently for each feature.

[0013] 1. Method for Producing Single Crystal by Solvent-Moving Floating Zone Melting Method The present invention relates to a method for producing a single crystal by a solvent-moving floating zone melting method. In the present invention, the single crystal is a lithium cobalt oxide single crystal doped with a metal atom A, and the metal atom A is selected from the group consisting of alkaline earth metals and transition metals having a valence of 5 or more. Further, the production method according to the present invention includes a step of forming a molten zone containing a solvent having a composition different from that of the raw material rod between the raw material rod and the seed crystal, and growing the single crystal on the seed crystal. Also, when the metal atom A content of the raw material rod is F RA at% with respect to a total of 100 at% of Co atoms and metal atom A in the raw material rod, F RA is less than 2.0 at%, and when the metal atom A content of the solvent is S RA at% with respect to a total of 100 at% of Co atoms and metal atom A in the solvent, S RA is less than 2.0 at%.

[0014] First, the solvent-moving floating zone melting method (TSFZ method) will be described. The floating zone melting (FZ) method, which is a higher - level concept than the traveling solvent floating zone melting (TSFZ) method, is a method of heating a part of a polycrystalline raw material rod as a raw material, creating a melting zone between a single crystal serving as a seed crystal and the raw material rod, moving the melting zone while supporting it by surface tension, and cooling the molten part zone to obtain a single crystal. Since the FZ method is a manufacturing method that does not use a crucible, it has the advantage of avoiding impurity contamination from the crucible and obtaining a single crystal with high purity.

[0015] The TSFZ method is a type of FZ method. A solvent with a composition different from that of the raw material rod is placed between the raw material rod and the seed crystal. The solvent is heated and melted to form a melting zone, and while supporting the melting zone by surface tension, it is relatively moved with respect to the raw material rod and the seed crystal, and a target single crystal is grown on the seed crystal. That is, the traveling solvent floating zone (TSFZ) method is an FZ method that uses a solvent with a composition different from that of the raw material rod in the melting zone part.

[0016] FIG. 1 shows a schematic diagram of a crystal growth process by the TSFZ method according to an embodiment of the present invention. In the single - crystal manufacturing method by the TSFZ method according to an embodiment of the present invention, a raw material rod 1 is supported and installed at its upper part. Also, on the extension line in the longitudinal direction (vertical direction) of the raw material rod, a seed crystal 2 serving as a base for growing a single crystal is installed. Before the start of the crystal growth process, a solvent is adhered to the seed - crystal side of the end of the raw material rod. After installing the raw material rod 1 and the seed crystal 2, by heating the solvent with a heating means 3, the solvent dissolves, and further, a part of the raw material rod in contact with the solvent dissolves, and a melting zone 4 is formed. A single crystal precipitates on the seed crystal 2 from the formed melting zone 4. During heating, in order to stabilize the growth of the crystal, it is preferable to rotate the raw material rod and the seed crystal in opposite directions relatively to heat the melting zone uniformly. As an example, the rotation speed of the raw material rod can be 3 - 17 rpm, and the rotation speed of the seed crystal can be 20 - 40 rpm.

[0017] The heating means may include a light source and a condenser. As the light source, a halogen lamp or the like can be used. As the condenser, an elliptical mirror (hereinafter also simply referred to as a mirror) or the like can be used. Such heating means can condense the light from the light source by the condenser to dissolve the solvent. In the manufacturing method according to an embodiment of the present invention, it is preferable to use an inclined mirror type single crystal growth apparatus capable of inclining the above mirror. Fig. 2 shows a schematic diagram (side view) of an inclined mirror type FZ apparatus. As shown in Fig. 2, in the inclined mirror type FZ apparatus, the straight line connecting the light source of the halogen lamp and the focus of the mirror can be inclined at an angle α° with respect to the horizontal direction. As an example, the inclination can be in the direction in which the light beam faces downward (the direction in which the inclination angle α is formed upward, the direction represented by the arrow in Fig. 2).

[0018] In the manufacturing method according to an embodiment of the present invention, it is preferable that the mirror inclination angle α is 2 to 20°, more preferably 5 to 15°, and even more preferably 7 to 13°. By adjusting the inclination angle of the mirror, the length of the molten zone can be controlled. In particular, by setting it within the above numerical range, a stable molten zone suitable for growing a large-diameter single crystal can be formed.

[0019] In the manufacturing method according to an embodiment of the present invention, it is preferable to use heating means capable of forming a molten zone of 5 mm or more. The length of the molten zone formed by the heating means is also related to the size and shape of the filament of the lamp. As an example, it is preferable to use a lamp in which the size of the filament in a direction perpendicular to the straight line connecting the light source of the lamp and the focus of the mirror is 4 mm or more in the vertical plane. Further, regarding the shape of the filament of the lamp, as an example, a halogen lamp with a flat type or cylindrical type filament can be used, and it is preferable to use a halogen lamp with a cylindrical type filament. Furthermore, it is more preferable to use a halogen lamp in which the size in a direction perpendicular to the straight line connecting the light source of the lamp and the focus of the mirror is 4 mm or more and which has a cylindrical type filament. By using the above heating means, a long and stable molten zone suitable for growing a large-diameter single crystal can be formed.

[0020] The atmosphere during crystal growth is preferably an inert gas atmosphere. For example, it can be an argon atmosphere. The pressure can be normal pressure. For example, an inert gas can be flowed at a flow rate of 0.5 to 5.0 L / min. The crystal growth rate can be, for example, 1 to 10 mm / h as an example. The temperature of the molten zone during crystal growth is preferably kept lower than the melting point of the oxide to be manufactured. In the manufacturing method according to an embodiment of the present invention, it is preferable to make the temperature of the molten zone during crystal growth lower than the melting point of the oxide constituting the target oxide single crystal. As an example, the temperature of the molten zone during crystal growth can be 1200°C or lower.

[0021] 1.1 Single crystal In the present invention, the single crystal is a lithium cobalt oxide single crystal doped with metal atom A. The single crystal according to an embodiment of the present invention preferably has a crystal structure similar to that of a lithium cobalt oxide single crystal, that is, an α-NaFeO2 type structure, and preferably has a layered structure in which Li layers and CoO6 octahedral layers are alternately laminated in the C-axis direction.

[0022] The single crystal to be obtained by the manufacturing method according to the present invention is a lithium cobalt oxide single crystal doped with a metal atom A. The metal atom A can be at least one selected from the group consisting of alkaline earth metals and transition metals having a valence (ionic valence) of 5 or more, and can also be at least one selected from alkaline earth metals. When the metal atom A contains an alkaline earth metal, the obtained single crystal can have more excellent ionic conductivity. Further, when the metal atom A contains a transition metal having a high valence such as Nb or Ta, the obtained single crystal can have more excellent electrical characteristics (charge and discharge characteristics, etc.). Examples of the alkaline earth metal include Be, Mg, Ca, Sr, and Ba. Examples of the transition metal having a valence of 5 or more include a transition metal having a valence of 5 and a transition metal having a valence of 6, and specifically, V, Nb, Ta, Bi, and Mo can be mentioned. The metal atom A can be one or more selected from the group consisting of Be, Mg, Ca, Sr, Ba, V, Nb, Ta, Bi, and Mo, and preferably one or more selected from the group consisting of Mg, Ca, Sr, Ba, V, Nb, Ta, and Mo.

[0023] The target single crystal can be a lithium cobalt oxide single crystal in which Li and / or Co is replaced by the metal atom A, Li (1-X) Co (1-Y) A (X+Y) It can be represented by O2. Here, X + Y is preferably greater than 0 and less than or equal to 0.015. X + Y is, for example, 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01, 0.011, 0.012, 0.013, 0.014, 0.015, and may be within the range between any two of the numerical values exemplified here.

[0024] That is, in one embodiment of the present invention, for the target single crystal, when the content C RA at% of the metal atom A in the single crystal is based on the total 100 at% of the Li atoms, Co atoms, and metal atom A in the single crystal, 2C RA is preferably greater than 0 and less than or equal to 1.5 at%. 2C RAis, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5 at%, and may be within the range between any two of the values exemplified herein. Regarding other properties of the single crystal, it can be as described later in 2. Single crystal.

[0025] 1.2 Raw material rod and solvent In the manufacturing method according to the present invention, when the content rate of the metal atom A of the solvent with respect to the total 100 at% of the Co atom and the metal atom A in the solvent is S RA in at%, S RA is less than 2.0 at%. S RA is, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9 at%, and may be within the range between any two of the values exemplified herein. Further, in the manufacturing method according to the present invention, when the content rate of the metal atom A of the raw material rod with respect to the total 100 at% of the Co atom and the metal atom A in the raw material rod is F RA in at%, F RA is less than 2.0 at%. F RA is, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9 at%, and may be within the range between any two of the values exemplified herein.

[0026] In the conventional TSFZ method, raw materials and the like are usually supplied according to the composition of the single crystal to be manufactured. That is, conventionally, during crystal growth, raw materials having the composition of the desired single crystal are continuously supplied from the raw material rod to the melting zone. However, according to the present invention, by setting the composition of the solvent, particularly the compositions of the solvent and the raw material rod, to be different from the composition of the single crystal to be manufactured, and by defining the content ratio of metal atom A in the solvent and in the raw material rod, it is possible to adjust the composition of the obtained single crystal, improve the quality of the grown crystal, and it is presumed that the generation of defects such as inclusions (heterophases) and cracks can be suppressed. Such a crystal has low impedance, and particularly has very high ionic conductivity in the a-axis direction and very high ionic conductivity anisotropy.

[0027] In one embodiment of the present invention, the Li atom concentration of the raw material rod with respect to 100 at% of the total of Li atoms, Co atoms, and metal atom A contained in the raw material rod is F Li at%, the Li atom concentration of the solvent with respect to 100 at% of the total of Li atoms, Co atoms, and metal atom A contained in the solvent is S Li at%, and the Li atom concentration of the single crystal with respect to 100 at% of the total of Li atoms, Co atoms, and metal atom A contained in the single crystal is C Li at%. When Li S Li is preferably larger than F Li . Also, S Li is preferably larger than C As shown in the examples described later, by making S Li larger than F Li and / or C Li , a highly stable melting zone can be formed, a melting zone with a more sufficient length can be formed, more stable crystal growth becomes possible, and the quality of the crystal is improved.

[0028] C Li can be 50 at% or less. That is, the target single crystal can have a cation ratio of Li:(Co + A) = 49.5:51.5 to 50:50. C Liis, for example, 49.5, 49.6, 49.7, 49.8, 49.9, 50.0, 50.1, 50.2, 50.3, 50.4, 50.5, 50.6, 50.7, 50.8, 50.9, 51, 51.1, 51.2, 51.3, 51.4, 51.5 at%, and may be within the range between any two of the values exemplified herein. Also, F Li can be 50 at% or more. That is, the raw material rod can have a cation ratio of Li:(Co + A) = 50~98:2~50. F Li is, for example, 50, 55, 60, 65, 70, 75, 80, 85, 90, 91, 92, 93, 94, 95, 96, 97, 98 at%, and may be within the range between any two of the values exemplified herein. S Li can be more than 50 at%, preferably 85 at% or more, and more preferably 87 at% or more. S Li is, for example, 51, 55, 60, 65, 70, 75, 80, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98 at%, and may be within the range between any two of the values exemplified herein. That is, the solvent can have a cation ratio of Li:(Co + A) = 51:49~98:2, and for example, can have a cation ratio of Li:(Co + Mg) = 51:49~98:2.

[0029] 1.3 Melting zone In the manufacturing method according to an embodiment of the present invention, it is important to form a stable molten zone having an appropriate length between the raw material rod and the seed crystal. The length of the molten zone is preferably adjusted according to the diameter of the target single crystal, and / or the diameter of the raw material rod supporting the molten zone, and / or the diameter of the seed crystal, etc. For example, when the diameter of the single crystal to be manufactured is D mm, the length of the molten zone can be 0.5D mm or more, more preferably 0.6D mm or more, and even more preferably 0.7D mm or more. Also, the molten zone is preferably a length that can be maintained by surface tension, and the upper limit of the length of the molten zone can be, for example, 1.2D mm or less, 1.1D mm or less, and 1.0D mm or less. The length of the molten zone can be, for example, 0.5D, 0.6D, 0.7D, 0.8D, 0.9D, 1.0D, 1.1D, 1.2D mm, and can also be within the range between any two of the numerical values exemplified here.

[0030] As an example, the length of the molten zone is preferably 5 mm or more, more preferably 6 mm or more, and even more preferably 7 mm or more with respect to the diameter of the single crystal to be manufactured of 10 mm. The length of the molten zone can be 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 mm, and can also be within the range between any two of the numerical values exemplified here.

[0031] In the present invention, the length of the molten zone means the length of the molten zone in the vertical direction, that is, the length in the direction in which the crystal grows, and the length in the longitudinal direction of the raw material rod. The length of the molten zone can be measured using a scale by taking a photograph of the state near the molten zone during crystal growth. By setting the length of the molten zone within a certain numerical range according to the diameter of the target single crystal, and / or the diameter of the raw material rod supporting the molten zone, and / or the diameter of the seed crystal, etc., it is possible to more reliably prevent the raw material rod and the single crystal growing on the seed crystal from coming into contact during crystal growth, and a single crystal with a larger diameter and excellent quality can be obtained with a more highly stable molten zone. The length of the molten zone can be controlled by adjusting the composition and amount of the raw material rod and solvent used, the heating means during molten zone formation, etc.

[0032] The amount of the solvent used in the TSFZ method is preferably adjusted according to the diameter of the target single crystal, and / or the diameter of the raw material rod supporting the melting zone, and / or the diameter of the seed crystal, the length of the melting zone to be formed, etc. For example, if the diameter of the oxide single crystal to be produced is D mm, the amount of the solvent is 0.020D 2 ~0.040D 2 g, preferably 0.023D 2 ~0.031D 2 g, more preferably. The amount of the solvent is 0.020D 2 , 0.022D 2 , 0.024D 2 , 0.026D 2 , 0.028D 2 , 0.030D 2 , 0.032D 2 , 0.034D 2 , 0.036D 2 , 0.038D 2 , 0.040D 2 g, and it may also be within the range between any two of the values exemplified here.

[0033] As an example, the amount of the solvent is preferably 2.0 g or more, more preferably 2.5 g or more, and even more preferably 3.0 g or more with respect to the single crystal diameter of 10 mm to be produced. The amount of the solvent can be 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 6.0, 7.0, 8.0, 9.0, 10 g, and it may also be within the range between any two of the values exemplified here. By appropriately adjusting the amount of the solvent, a melting zone with an appropriate length according to the diameter of the raw material rod supporting the melting zone, and / or the diameter of the seed crystal, and / or the diameter of the target single crystal can be formed, and a more highly stable melting zone can be obtained.

[0034] The single crystal obtained by the above production method can have the characteristics described below and can also be used for the uses described below.

[0035] 2. Single crystal The single crystal according to one embodiment of the present invention is a lithium cobalt oxide single crystal doped with a metal atom A. The single crystal according to one embodiment of the present invention can be obtained by the above-described manufacturing method. The single crystal according to one embodiment of the present invention preferably has the same crystal structure as that of the lithium cobalt oxide single crystal, that is, an α-NaFeO2-type structure, and preferably has a layered structure in which Li layers and CoO6 octahedral layers are alternately stacked in the C-axis direction.

[0036] The single crystal according to the present invention is a lithium cobalt oxide single crystal doped with a metal atom A, and the metal atom A is one or more selected from alkaline earth metals. Examples of the alkaline earth metal include Be, Mg, Ca, Sr, and Ba.

[0037] The single crystal according to the present invention is Li (1-X) Co (1-Y) A (X+Y) It can be represented by O2. Here, X + Y is preferably more than 0 and not more than 0.015. X + Y is, for example, 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01, 0.011, 0.012, 0.013, 0.014, 0.015, and may be within the range between any two of the values exemplified here.

[0038] That is, for the single crystal according to one embodiment of the present invention, when the content C RA at% of the metal atom A in the single crystal is based on the total 100 at% of the Li atoms, Co atoms, and metal atom A in the single crystal, 2C RA is preferably more than 0 and not more than 1.5 at%. 2C RA is, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5 at%, and may be within the range between any two of the values exemplified here.

[0039] The single crystal according to an embodiment of the present invention can suppress the generation of defects such as heterogeneous phases and cracks by doping a specific type of metal atom A in a specific trace amount, has a low impedance, and in particular, has a very high ionic conductivity in the a-axis direction and a very high ionic conductivity anisotropy. The type and amount of the doped metal atoms in the single crystal can be controlled by adjusting the manufacturing method of the single crystal. As an example, it can be controlled by appropriately adjusting the type and amount of the raw material rod and the solvent mixture and forming a stable and sufficiently long molten zone.

[0040] When the Li atom concentration of the single crystal according to an embodiment of the present invention is C Li at% with respect to the total 100 at% of the Li atoms, Co atoms, and metal atoms A contained in the single crystal, C Li can be 50 at% or less. That is, the single crystal according to an embodiment of the present invention can have a cation ratio of Li:(Co + A) = 49.5:51.5 to 50:50, for example, Li:(Co + Mg) = 49.5:51.5 to 50:50. C Li can be, for example, 49.5, 49.6, 49.7, 49.8, 49.9, 50.0, 50.1, 50.2, 50.3, 50.4, 50.5, 50.6, 50.7, 50.8, 50.9, 51, 51.1, 51.2, 51.3, 51.4, 51.5 at%, and may be within the range between any two of the values exemplified herein.

[0041] When observing the cross-section of the single crystal according to an embodiment of the present invention, the number of heterogeneous phases of 10 μm or more is preferably 2 1 or less per mm, and the number of heterogeneous phases of 5 μm or more is more preferably 2 1 or less per mm. The cross-section observation can be performed with an electron microscope (for example, SEM), and specifically, in the method described in the examples, several fields of view are observed, and it is preferable that there are no such heterogeneous phases over a total field of view of 2 1 mm or more.

[0042] The single crystal according to an embodiment of the present invention has few heterophases, so it has low impedance. In particular, it has very high ionic conductivity in the a-axis direction and very high ionic conduction anisotropy. The amount of heterophase in the single crystal can be controlled by adjusting the manufacturing method of the single crystal. For example, it can be controlled by appropriately adjusting the type and amount of the raw material rod and the solvent composition, and the generation of heterophases can be suppressed by forming a stable and sufficiently long molten zone.

[0043] When the ionic conductivity in the a-axis direction of the single crystal according to an embodiment of the present invention is I a S / cm and the ionic conductivity in the c-axis direction is I c S / cm, it is preferable that I a / I c is 500 or more. I a / I c is, for example, 500, 1000, 1500, 2000, 2500, 3000, 3500, 4000, 4500, 5000, 5500, 6000, 6500, 7000, 7500, 8000, and may be within the range between any two of the numerical values exemplified here.

[0044] I a is preferably, for example, 10.0×10 -6 or more, and for example, 10.0×10 -6 , 20.0×10 -6 , 30.0×10 -6 , 40.0×10 -6 , 50.0×10 -6 , 60.0×10 -6 , 70.0×10 -6 , 80.0×10 -6 , 90.0×10 -6 , 100.0×10 -6 , 150.0×10 -6 , 200.0×10 -6 and may be within the range between any two of the numerical values exemplified here. I c is, for example, for example, 0.1×10 -8 , 0.5×10-8 , 1.0×10 -8 , 1.5×10 -8 , 2.0×10 -8 , 3.0×10 -8 , 4.0×10 -8 , 5.0×10 -8 , 6.0×10 -8 , 7.0×10 -8 , 8.0×10 -8 , 9.0×10 -8 , 10.0×10 -8 and may be within the range between any two of the numerical values exemplified herein. The ionic conductivity and anisotropy can be adjusted by adjusting the amount of defects such as heterogeneous phases and cracks in the single crystal, and the type and amount of doped metal atoms. Specifically, in the method for producing a single crystal, for example, it can be controlled by appropriately adjusting the type and amount of the raw material rod and the solvent composition.

[0045] The single crystal according to an embodiment of the present invention preferably has a thickness of 1 mm or more. The thickness can be, for example, 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 mm, and may be within the range between any two of the numerical values exemplified herein.

[0046] The single crystal according to an embodiment of the present invention preferably has a diameter of 1 mm or more. The diameter can be, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 mm, and may be within the range between any two of the numerical values exemplified herein. The single crystal according to an embodiment of the present invention preferably has the above thickness and diameter, and is not a so-called polycrystal. It is preferable that one crystal has the above thickness and diameter. The size of the single crystal can be controlled by appropriately adjusting, for example, the length of the melting zone and the size of the seed crystal in the method for producing the single crystal.

[0047] The single crystal according to an embodiment of the present invention having the above-described characteristics has a sufficiently low impedance, and in the development of a positive electrode material substrate for a next-generation lithium ion secondary battery such as an all-solid-state lithium ion battery, for example, accurate electrochemical evaluation can be performed. It is increasingly being mounted on IoT devices and wearable devices, and further contributes to the development of all-solid-state batteries described for deployment in multiple fields such as medical, infrastructure, aviation, and space. The single crystal according to an embodiment of the present invention having the above-described characteristics can be suitably used as a positive electrode material for a lithium ion battery. Moreover, a lithium ion battery according to an embodiment of the present invention is a lithium ion battery including a positive electrode material, a solid electrolyte, and a negative electrode material, and the positive electrode material can include the above single crystal. Experimental Example

[0048] [Influence of Solvent Composition on Crystal Growth] [Example 1] [Preparation of Raw Material Rod F-1] Li2CO3 (manufactured by Rare Metallic Co., Ltd., purity 99.9%), Co3O4 (manufactured by Rare Metallic Co., Ltd., purity 99.9%), and Mg5(CO3)4(OH)2 (manufactured by Rare Metallic Co., Ltd., purity 99.9%) were weighed and mixed so as to be Li (1-X) Co (1-Y) A (X+Y) O2 (X = 0, Y = 0.01, X + Y = 0.01). The mixed powder was heated in an intake air at 700 °C for 5 hours to synthesize Li (1-X) Co (1-Y) A (X+Y) O2. The synthesized Li (1-X) Co (1-Y) A (X+Y) O2 powder was formed into a columnar shape with a diameter of about 6 mm and a length of 50 mm and pressurized in a hydrostatic pressure of 300 MPa by the rubber plug method. The prepared raw material was sintered in oxygen at 1000 °C for 10 hours to obtain a raw material rod F-1.

[0049] [Preparation of Solvent] Li (1-X) Co (1-Y) A (X+Y)Li2CO3 was added to the O2 powder and mixed so that the cation ratio Li:(Co+Mg) was 85:15 or 90:10. The obtained mixed powder was formed into a cylindrical shape with a diameter of about 6 mm, pressurized in a hydrostatic pressure of 300 MPa by the rubberless method, cut out into a pellet shape, and used in the unsintered state.

[0050] (Preparation of seed crystal) As the seed crystal, a LiCoO2 single crystal with a diameter of 10 mm was prepared.

[0051] (Manufacture of single crystal) Using the raw material rod and the solvent obtained above, a single crystal was manufactured. Before starting the crystal growth, the solvent cut out into a pellet shape was dissolved and attached to the tip of the raw material rod. For single crystal growth, an inclined mirror type elliptical infrared heating FZ apparatus (TLFZ-4000-H-VPO manufactured by Crystal System) was used. The growth atmosphere was an argon atmosphere, and argon gas was flowed at a flow rate of 1.5 L / min. The growth orientation was

[0100] , the growth rate was 5 mm / h, the rotational speeds of the upper and lower shafts were 5 rpm and 30 rpm in opposite directions, respectively. The mirror inclination angle was 10°. As the halogen lamp for heating, a halogen lamp with a cylindrical type filament having a filament size (the size of the filament in a direction perpendicular to the straight line connecting the light source of the lamp and the focus of the mirror) of 4 mm was used.

[0052] (Observation results of the molten zone during crystal growth and the crystal after crystal growth) Fig. 3 shows photographs of the molten zone during crystal growth and photographs of the crystal after crystal growth in Example 1 when the composition of the solvent was Li:(Co+Mg)=85:15 and when the composition of the solvent was Li:(Co+Mg)=90:10. When the solvent with Li:(Co+Mg)=90:10 was used, the feed melted and a molten zone with a more sufficient length could be formed, and more stable crystal growth could be achieved. In the subsequent study, the composition of the solvent was set to Li:(Co+Mg)=90:10.

[0053] [Influence of the Composition of the Raw Material Rod and the Solvent on the Crystal] <Examples 2, Comparative Examples 1 to 3> (Preparation of Raw Material Rods F-2 to 5) Li (1-X) Co (1-Y) A (X+Y) Except that the raw materials were blended so that O2 (X = 0, Y = 0.50, 0.02, 0.03, 0.04, X + Y = 0.50, 0.02, 0.03, 0.04), the raw material rods F-2 (X + Y = 0.50) of Example 2 and the raw material rods F-2 (X + Y = 0.02, 0.03, 0.04) of Comparative Examples 3, 4, and 5 were obtained in the same manner as the raw material rod F-1.

[0054] Similar to Example 1, the preparation of the solvent, the preparation of the seed crystal, and the production of the single crystal were carried out. In the adjustment of the solvent, Li2CO3 was added to the synthesized Li (1-X) Co (1-Y) A (X+Y) O2 powder and mixed so that the cation ratio Li:(Co + Mg)=90:10.

[0055] <Observation of Crystal Appearance and Cross-Section>[ Figure 4 shows photographs of the crystals after crystal growth and cross-sectional SEM photographs observed with an electron microscope for the crystals according to Example 1 (Figure 4B), Example 2 (Figure 4A), Comparative Example 1 (Figure 4C), Comparative Example 2 (Figure 4D), and Comparative Example 3 (Figure 4E). As described above, all the crystals were grown using the more optimized solvent conditions of Li:(Co + Mg)=90:10. The grown crystals were black. The diameter of the crystals was about 6 mm, and the length was 35 to 40 mm. In Examples 1 and 2, that is, in the preparation of the raw material rod and the like, Li (1-X) Co (1-Y) A (X+Y) O2 (X + Y = 0.5, 0.01), and when the Mg atom content F RA = 0.5, 1.0 at%, crystals having a long and shiny surface were obtained along the crystal growth direction. The grown crystals had a metallic luster and two crystal habits appeared on the crystal side surface. On the other hand, when F RA = 2.0, 3.0, 4.0 at%, the diameters of the grown crystals were non-uniform. In addition, a black powdery part was observed on the surface of the crystals. In the electron microscope observation (SEM) of the cross-section, FRA When it is 0.5 and 1.0 at%, inclusions of 10 μm or more were not observed at all in the total observation range of 1 mm or more. Here, an inclusion means a part having a composition different from that of a single crystal in a scanning electron micrograph (SEM), and is composed of an oxide of cobalt and / or magnesium. On the other hand, the Mg atom content F of a raw material rod, etc. 2 When it is 2.0, 3.0, and 4.0 at%, a plurality of inclusions having a major axis exceeding several tens of μm were observed in the total observation range of 1 mm or more. Also, when F RA = 2.0, 3.0, 4.0 at%, a plurality of inclusions having a major axis exceeding several tens of μm were observed in the total observation range of 1 mm or more. Also, when F 2 = 4.0 at%, cracks were also observed in addition to inclusions. RA = 4.0 at%, cracks were also observed in addition to inclusions.

[0056] <Evaluation of Crystal Composition> The composition in the crystal was analyzed by EPMA (electron probe microanalyzer) (JXA-iHP200F, JEOL Ltd.). The results are shown in Fig. 5. The Mg atom content F of a raw material rod, etc. RA When it is 0.5 at%, the Mg atom content of the single crystal with respect to the total 100 at% of Co atoms and Mg atoms in the crystal = 0.44 at%. That is, when the Mg atom content of the single crystal with respect to the total 100 at% of Li atoms, Co atoms, and Mg atoms in the crystal is C RA Then, 2C RA = 0.44 at% (LiCo 0.9956 Mg 0.0044 O2). When the Mg atom content F of a raw material rod, etc. RA = 1.0 at%, the Mg atom content of the single crystal with respect to the total 100 at% of Co atoms and Mg atoms in the crystal = 0.87 at%. That is, when the Mg atom content of the single crystal with respect to the total 100 at% of Li atoms, Co atoms, and Mg atoms in the crystal is C RA Then, 2C RA = 0.87 at% (LiCo 0.9913 Mg 0.0087 O2). Also, when F RA = 2.0, 3.0, 4.0 at%, 2C RA is more than 1.5 at% (LiCo (1-Y) Mg (Y)In O2, Y > 0.015).

[0057] [Influence of the type of metal element constituting the crystal on the crystal] <Comparative Example 4> As a raw material, except that Zr was used instead of Mg, Li (1-X) Co (1-Y) A (X+Y) O2 crystals were prepared.

[0058] <Crystal impedance evaluation> The impedance of the crystal was measured in the frequency range of 100 Hz to 10 MHz at room temperature using an AC impedance analyzer (PSM1735, Iwatsu Electric Co., Ltd.). The measurement results of the crystal according to Example 1 (Mg atom content F in the raw material rod, etc.) RA = 1.0 at%, Mg atom content 2C in the crystal RA = 0.87 at%, LiCo 0.9913 Mg 0.0087 O2) are shown in Fig. 6. Also, the ionic conductivity and its anisotropy of LiCo 0.9913 Mg 0.0087 O2 single crystal, LiCo 0.9974 Zr 0.0026 O2 single crystal, and unsubstituted LiCoO2 single crystal are shown in Fig. 7. In the table of Fig. 7, the ionic conductivity and ionic conduction anisotropy of each crystal are shown. The ionic conductivities in the a-axis direction and c-axis direction of the LiCo 0.9913 Mg 0.0087 O2 single crystal at room temperature are σ

[0100] = 7.2×10 -5 S / cm, σ

[0001] = 1.1×10 -8 S / cm, respectively, showing a very high ionic conduction anisotropy of about 6545. Compared with the ionic conductivity of the unsubstituted LiCoO2 single crystal, the ionic conductivity in the c-axis direction hardly changed, but the ionic conductivity in the a-axis direction was about 18 times larger. Note that the single crystals according to Comparative Examples 1 to 3 have poor crystallinity due to the influence of heterogeneous phases and the orientation cannot be identified, so the anisotropy cannot be clarified. Also, due to the influence of heterogeneous phases, the absolute value of the ionic conductivity is very low, 1.0×10-6 is less than.

Explanation of Signs

[0059] 1 raw material rod 2 seed crystals 3 heating means 4 melting zone

Claims

1. A method for manufacturing a single crystal by a solvent moving floating zone melting method, wherein the single crystal is a lithium cobaltate single crystal doped with a metal atom A, the metal atom A is selected from the group consisting of an alkaline earth metal and a transition metal having a valence of 5 or more, the manufacturing method includes a step of forming a molten zone containing a solvent having a composition different from that of the raw material rod between the raw material rod and the seed crystal, and growing the single crystal on the seed crystal, The content of the metal atom A in the raw material rod is F in terms of at% with respect to a total of 100 at% of Co atoms and the metal atom A in the raw material rod ＲＡ When it is at%, F ＲＡ is less than 2.0 at%, and When the content rate of the metal atom A in the solvent is S in terms of at% with respect to a total of 100 at% of Co atoms and the metal atom A in the solvent ＲＡ is at%, S ＲＡ is less than 2.0 at%, a production method.

2. The content ratio C of the metal atom A in the single crystal with respect to 100 at% of the total of Li atoms, Co atoms, and metal atom A in the single crystal ＲＡ in at%, 2C ＲＡ is smaller than F ＲＡ and S ＲＡ The production method according to claim 1, which is smaller.

3. Let the Li atom concentration of the raw material rod be F at% with respect to 100 at% of the total of Li atoms, Co atoms, and metal atom A contained in the raw material rod Ｌｉ at%, Let the Li atom concentration of the solvent be S in terms of at% with respect to the total 100 at% of Li atoms, Co atoms, and metal atom A contained in the solvent. Ｌｉ at%, and When the Li atom concentration of the single crystal is C in terms of at% with respect to 100 at% of the total of Li atoms, Co atoms and metal atom A contained in the single crystal Ｌｉ in at%, S Ｌｉ is greater than F Ｌｉ and / or S Ｌｉ is greater than C Ｌｉ The manufacturing method according to claim 1 or claim 2.

4. A single crystal, wherein the single crystal is a lithium cobaltate single crystal doped with a metal atom A, The single crystal is Li （１－Ｘ） Co （１－Ｙ） A （Ｘ＋Ｙ） O ２ represented by, the metal atom A is an alkaline earth metal, X + Y is greater than 0 and less than or equal to 0.015, the single crystal.

5. The number of heterogeneous phases of 10 μm or more in the cross section of the single crystal is 1 piece / mm ２ The single crystal according to claim 4, wherein the number is 1 or less.

6. Let the ionic conductivity in the a-axis direction of the single crystal be I ａ S / cm, and the ionic conductivity in the c-axis direction be I ｃ S / cm. When I ａ / I ｃ is 500 or more, the single crystal according to claim 4 or claim 5.

7. A lithium ion battery including a positive electrode material, a solid electrolyte, and a negative electrode material, wherein the positive electrode material includes the single crystal according to Claim 4 or Claim 5, the lithium ion battery.

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