WO2024135536A1

WO2024135536A1 - Lithium ion secondary battery positive electrode active material, lithium ion secondary battery mixed positive electrode active material, electrode foil, and lithium ion secondary battery

Info

Publication number: WO2024135536A1
Application number: PCT/JP2023/044902
Authority: WO
Inventors: 貴文黒川; 文彦槙
Original assignee: 日本化学産業株式会社
Priority date: 2022-12-20
Filing date: 2023-12-14
Publication date: 2024-06-27

Abstract

[Problem] To provide a positive electrode active material that is for a lithium ion secondary battery that has favorable cycle characteristics but uses as little of the rare metal Co as possible, the positive electrode active material for a lithium ion secondary battery including an Li-Ni-Co-Al or Li-Ni-Co-Mn complex oxide as a principal component. [Solution] According to the present invention, a lithium ion secondary battery positive electrode active material that has excellent charge/discharge cycle characteristics and rate characteristics is characterized by having a preferred orientation in which the (0001) faces of positive electrode active material crystals that comprise hexagonal crystals are gathered on one side in the axial direction, the maximum value of a degree of preferred orientation (MUD) that indicates the crystal orientation of the (0001) faces of the positive electrode active material crystals on a pole figure created under the condition in expression (1) on the basis of crystal orientation information for the (0001) faces of the positive electrode active material crystals as measured by electron backscatter diffraction (EBSD) satisfying expression (2).　(1) Half width value=10°. (2) 30≤MUD≤200.

Description

Positive electrode active material for lithium ion secondary battery, mixed positive electrode active material for lithium ion secondary battery, electrode foil and lithium ion secondary battery

The present invention relates to a positive electrode active material for lithium ion secondary batteries, a mixed positive electrode active material for lithium ion secondary batteries, an electrode foil, and a lithium ion secondary battery. More specifically, the present invention relates to a positive electrode active material for lithium ion secondary batteries that is mainly composed of a Li-Ni-Co-Al-based or Li-Ni-Co-Mn-based composite oxide. In particular, the present invention relates to a positive electrode active material for lithium ion secondary batteries that has a specific preferred orientation and can be suitably used for lithium ion secondary batteries that have high capacity, excellent charge/discharge cycle characteristics, and rate characteristics.

Lithium-ion secondary batteries are used in a variety of applications, including as power sources for mobile devices such as laptops and mobile phones, and for power tools. Their use is expected to continue to expand in the future in light of the need to build a low-carbon society and promote energy security, and there is a strong demand for improved performance.

In recent years, the demand for lithium-ion secondary batteries as power sources for hybrid and electric vehicles (hereinafter collectively referred to as "EVs"), or as storage materials for renewable power generation, is rapidly expanding. In these applications, lithium-ion secondary batteries are particularly desired to have high capacity and high charge/discharge cycles. For this reason, there is an urgent need to improve the materials of lithium-ion secondary batteries. Among the materials constituting lithium-ion secondary batteries, _LiCoO2 , which is mainly composed of cobalt (Co), has been widely used as a positive electrode active material. However, in the current situation where the demand for EV power sources is rapidly expanding, there is a concern that Co, which is a rare metal, will be depleted as a resource and that the cost will be high.

Therefore, as a positive electrode active material to replace LiCoO ₂ , a positive electrode active material made of LiMn ₂ O ₄ containing Mn as the main component and a Ni-Co-Mn ternary composite oxide have been proposed. However, these positive electrode active materials have both advantages and disadvantages in terms of battery characteristics, and at present, they do not fully meet the requirements for use as a power source for power tools and EVs.

Under such circumstances, lithium ion secondary batteries using Li-Ni-Co-Al composite oxides mainly composed of Ni, which also have a large charge/discharge capacity, as a positive electrode active material have been studied. As a conventional technique, in order to increase capacity and improve charge/discharge retention rate, for example, a technique has been proposed in which an oxide containing Zn and Al is deposited on the surface of a LiNiO ₂ positive electrode active material to improve conductivity and extend life (Patent Document 1). Furthermore, a technique has been proposed to improve the charge/discharge capacity, packability, and storage stability of Li-Ni-Co-Al composite oxides by reducing the rate of change in specific surface area before and after compression and the content of sulfate ions (Patent Document 2). In addition, various improvement techniques have been proposed for positive electrode active material particles for lithium ion secondary batteries, including LiCoO ₂ particle materials that contain wire-shaped LiCoO ₂ particles and have fast electronic conduction to improve the output characteristics of the battery (Non-Patent Document 1).

In addition, the positive electrode active material composed of Li _x Ni _(1-yz) Co _y Al _z O ₂ proposed in the above Patent Documents 1 and 2 is considered to have a coating layer made of a surface modifier to stabilize the crystal structure, thereby improving the electronic conductivity and achieving a high capacity and long life. In addition, in the technology proposed in Patent Document 3, composite hydroxide secondary particles having different aggregation states at the center and outer periphery of the particle are synthesized, and this is used as a precursor to obtain a positive electrode active material having a hollow structure, thereby improving the cycle characteristics. However, even with these improvement technologies, sufficient effects have not yet been obtained. For this reason, the market is always demanding the development of a positive electrode active material for lithium ion secondary batteries with better battery characteristics than the positive electrode active materials for lithium ion secondary batteries that have been proposed in the past.

Furthermore, in response to the above demands, the present applicant has proposed a positive electrode active material made of nearly spherical Li _x Ni _(1-yz) Co _y Al _z O ₂ as a positive electrode active material that has excellent electrical conductivity and can be suitably used for lithium ion secondary batteries that can achieve high capacity and improved charge/discharge retention rate (Patent Document 4, International Publication No. 2016/143844).

On the other hand, a positive electrode active material for lithium ion secondary batteries has been proposed in which the above-mentioned conventional LiCoO ₂ crystal is made into a hexagonal barrel-shaped or hexagonal plate-shaped crystal, and it has been disclosed that the lithium ion secondary battery has high rate characteristics (Non-Patent Document 2, Patent Document 5). However, as described above, even in Li _x Ni _{(1-yz) Co y Al z O 2 and Li x Ni (1-yz)} Co _y _Mn _z O ₂ mainly composed of Ni, which has a high charge/discharge voltage and a large charge/discharge capacity, instead of Co, which is a rare metal and is a concern for depletion of resources, _it is questionable whether the above-mentioned hexagonal barrel-shaped or plate-shaped crystal can be produced due to the characteristics of Ni _, _Mn _, and Al.

JP 2011-129258 A JP 2008-166269 A JP 2020-71899 A WO 2016/143844 WO 2019/189801

In light of the above-mentioned conventional technologies, the present invention aims to provide a positive electrode active material for lithium ion secondary batteries that has good cycle characteristics while minimizing the amount of rare metal Co used, and that is primarily composed of Li-Ni-Co-Al or Li-Ni-Co-Mn composite oxides.

That is, the positive electrode active material for a secondary battery according to the present invention, which advantageously solves the above problems, is
[1] A positive electrode active material for a lithium ion secondary battery having excellent charge/discharge cycle characteristics and rate characteristics,
The (0001) plane of the positive electrode active material crystal made of a hexagonal crystal has a preferred orientation in which it is assembled in one axial direction, and a pole figure created under the conditions of the following relational formula (1) based on crystal orientation information of the (0001) plane of the positive electrode active material crystal measured by electron backscatter diffraction (EBSD) method is:
The positive electrode active material is characterized in that the maximum value of the degree of preferred orientation (MUD) of the (0001) plane of the crystals of the positive electrode active material satisfies the following relational expression (2).
[Equation 1]
Half width (half width) = 10° ... (1)
[Equation 2]
30≦MUD≦200... (2)
[2] The positive electrode active material for a lithium ion secondary battery according to [1], which is composed of a chemical composition represented by the following general formula (3):
[Chemical formula 1]
LiNi _1-xyz C _x A _y B _z O ₂ ... (3)
In the above general formula (1), A represents Mn or Al, B represents Mg or Zr, x represents 0.00≦x≦0.33, y represents 0.00≦y≦0.33, and z represents 0.00≦y≦0.10.
[3] A mixed positive electrode active material for a lithium ion secondary battery, comprising (a) the positive electrode active material for a lithium ion secondary battery according to [2] and (b) a mixed positive electrode active material for a lithium ion secondary battery, wherein the mixed positive electrode active material for a lithium ion secondary battery contains (a) the positive electrode active material for a lithium ion secondary battery in an amount of 10 mass% or more.
[4] An electrode foil comprising the positive electrode active material for a lithium ion secondary battery according to [2] or [3].
[5] A lithium ion secondary battery comprising the electrode foil according to [3].

The present invention makes it possible to obtain a lithium-ion secondary battery that has not only a high initial discharge capacity, but also high capacity, high charge/discharge cycle characteristics, and high rate characteristics.

This is information obtained by EBSD analysis of the precursor particles for the positive electrode active material for lithium ion secondary batteries obtained in Example 1, and the analysis results of a total of three particles from the particle group in Example 1 are shown in fields 1 to 3. (A) An electron microscope photograph at 5,000 times magnification of the positive electrode active material for lithium secondary batteries obtained in Example 1. (B) An electron microscope photograph at 10,000 times magnification of the positive electrode active material for lithium secondary batteries obtained in Example 1. (C) An electron microscope photograph at 20,000 times magnification of the positive electrode active material for lithium secondary batteries obtained in Example 1. This is information obtained by EBSD analysis of the particles of the positive electrode active material for lithium ion secondary batteries obtained in Example 1, and the analysis results of a total of three particles in the particle group of Example 1, visual fields 1 to 3, are shown. 1 is a graph showing the discharge capacity (cycle characteristics) per cycle number obtained in Example 1 and Comparative Example 1. 1 is a graph showing the discharge capacity retention rate (cycle characteristics) depending on the number of cycles obtained in Example 1 and Comparative Example 1. 1 shows the rate characteristics obtained in Example 1 and Comparative Example 1, where (A) is the rate characteristic obtained in Example 1 and (B) is the rate characteristic obtained in Comparative Example 1. 4 is an electron microscope photograph showing a method for measuring the particle size of a positive electrode active material obtained using the precursor produced in Example 1. This is information obtained by EBSD analysis of particles of the positive electrode active material for a lithium ion secondary battery obtained in Comparative Example 1, and the analysis results of a total of three particles in fields 1 to 3 from the particle group of Comparative Example 1 are shown. 1 is an electron microscope photograph for measuring the particle size of a positive electrode active material obtained by using the precursor produced in Comparative Example 1, using the same measuring method as in Example 1.

Below, the embodiments of the present invention are specifically described. Note that the drawings are schematic and may differ from the actual product. Furthermore, the following embodiments are intended to exemplify devices and methods for embodying the technical ideas of the present invention, and are not intended to specify the configurations as described below. In other words, the technical ideas of the present invention can be modified in various ways within the technical scope described in the claims.

[First embodiment]
The positive electrode active material for lithium ion secondary batteries according to this embodiment will be described. The positive electrode active material for lithium ion secondary batteries according to this embodiment (hereinafter, sometimes referred to as "positive electrode active material") can provide a lithium ion secondary battery with excellent charge/discharge cycle characteristics and rate characteristics, and is characterized by having a crystal orientation that satisfies the relationships of the following relational formulas (1) and (2). That is, the positive electrode active material for lithium ion secondary batteries according to this embodiment has a preferred orientation in which the (0001) plane of the positive electrode active material crystal made of a hexagonal crystal is assembled in one axial direction, and is characterized in that in a pole figure created under the conditions of the following relational formula (1) based on the crystal orientation information of the (0001) plane of the positive electrode active material crystal measured by electron backscatter diffraction (EBSD), the maximum value of the preferred orientation degree (MUD) indicating the crystal orientation of the (0001) plane of the positive electrode active material crystal satisfies the following relational formula (2).

[Equation 1]
Half width (half width) = 10° ... (1)

[Equation 2]
30≦MUD≦200... (2)

First, the process leading up to the development of the positive electrode active material for lithium ion secondary batteries according to this embodiment will be described. First, in order to manufacture lithium ion secondary batteries with high capacity and high cycle characteristics, it has been essential to increase the packing density of the positive electrode active material that constitutes the lithium ion secondary battery.

For this reason, it was considered important that (I) the shape of the positive electrode active material be a nearly spherical particle, (II) all particles constituting the positive electrode active material have a uniform composition and shape, and (III) the shape of the positive electrode active material is not that of extremely small particles, but has a certain degree of size and high particle strength.

However, it is speculated that a lithium ion secondary battery manufactured using the cathode active material having the special wire-like shape described in Non-Patent Document 1 has extremely superior rate characteristics and cycle characteristics compared to a lithium ion secondary battery manufactured using a cathode active material having a shape closer to a sphere.

Secondly, in the Ni _(1-xy) _CoxMny _{composite hydroxide} or _oxide , which is the precursor of _the LiNi _{(1-xy)CoxAlyO2-based and LiNi(1-xy)} _CoxAlyO2 _- _based positive electrode active material, the _plate - _like crystals constituting the precursor are aggregated and grow in random directions. As a result, the positive electrode active material produced using such a precursor has cracks (fissures) inside the positive electrode active material particles when the lithium ion secondary battery is repeatedly charged and discharged.

Cracks that occur inside the particles that make up the positive electrode active material are believed to be one of the causes of the deterioration of the cycle characteristics of lithium-ion secondary batteries that contain that positive electrode active material. In other words, the deterioration of the cycle characteristics of lithium-ion secondary batteries is caused by the fact that the secondary particles made up of the positive electrode active material are composed of a collection of primary particle crystals that are oriented in random directions. In other words, it is thought that cracks occur inside the particles because there is no regularity in the direction of expansion and contraction of the crystals of the particles that make up the positive electrode material as the lithium-ion secondary battery is charged and discharged.

In other words, cracks that occur inside the positive electrode active material are considered to be one of the factors that deteriorate the cycle characteristics of lithium-ion secondary batteries that contain that positive electrode active material. The cracks are said to be caused by the expansion and contraction of the crystals of the positive electrode active material that accompanies the charge and discharge reactions of the lithium-ion secondary battery. In particular, with secondary particle-like positive electrode active material particles that are an aggregate of countless primary particles, it can be said that cracks are likely to occur when a lithium-ion secondary battery using the particles of that positive electrode active material is charged and discharged because the countless primary particles that make up the secondary particles each individually expand and contract in different directions.

However, Li-Ni-Co-Al-based positive electrode active materials and Li-Ni-Co-Mn-based positive electrode active materials, which have high capacity and low Co usage, are generally secondary particles consisting of countless crystal grains, unlike _LiCoO2 . The crystal orientation of the crystal grains constituting such positive electrode active materials is randomly oriented, and it is considered that it is extremely difficult to control the orientation. The reason why it is difficult to control the orientation of the crystal orientation of the crystal grains is due to the crystal habit of the precursor composite hydroxides such as Ni-Co, Ni-Co-Al, and Ni-Co-Mn.

From this technical perspective, a material with a high Ni content is used for the positive electrode active material required to manufacture lithium-ion secondary batteries with excellent high capacity. By controlling the crystal orientation of the positive electrode active material particles and controlling the direction of volume change when the positive electrode active material particles undergo charge and discharge reactions, the researchers investigated how to suppress the occurrence of cracks that occur inside the particles, which are believed to be the cause of reduced cycle characteristics.

As a result, they discovered that even with Li-Ni-Co-Al and Li-Ni-Co-Mn positive electrode active materials, which have a high Ni content and are expected to have high capacity, and which are secondary particles consisting of multiple crystal grains, it is possible to obtain a lithium-ion secondary battery with excellent cycle characteristics by giving the positive electrode active material crystals a preferred orientation in which the (0001) planes are oriented in the same axial direction.

The inventors have confirmed that it is extremely difficult to achieve the above-mentioned preferred orientation using Li-Ni-Co-Al or Li-Ni-Co-Mn composite oxides, which are positive electrode active materials for lithium ion secondary batteries.

Therefore, the Ni--Co based, Ni--Co--Al based and Ni--Co--Mn based composite hydroxides, which are precursors of this positive electrode active material, were investigated in detail.
As a result, by controlling the nucleation and crystal growth processes, composite hydroxide particles with a preferred orientation in which the (0001) faces of hexagonal composite hydroxide crystals are concentrated in the same axial direction could be obtained.
The inventors also discovered that by using hydroxide particles having a preferred orientation in which the (0001) planes of the composite hydroxide crystals are concentrated in the same axial direction as a precursor of a positive electrode active material for a lithium ion secondary battery, a positive electrode active material for a lithium ion secondary battery having a preferred orientation in the (0001) plane of the positive electrode active material crystals can be obtained, and that the positive electrode active material has excellent battery characteristics.

The positive electrode active material for lithium ion secondary batteries of the present invention is a structure having a preferred orientation in which the (0001) planes of the positive electrode active material crystals constituting the positive electrode active material particles made of hexagonal crystals are oriented in the same axial direction, and is obtained by using a precursor having a preferred orientation in the (0001) plane as a precursor of the positive electrode active material crystals based on the above findings. Furthermore, the positive electrode active material for lithium ion secondary batteries of the present invention is obtained by mixing this precursor with a Li compound and, if necessary, an Al compound, a Zr compound, an Mg compound, etc., and firing it, and has a structure in which crystal grains made of a large number of composite oxides are assembled. The positive electrode active material for lithium ion secondary batteries of the present invention has a preferred orientation despite being a secondary particle body.

The positive electrode active material for lithium ion secondary batteries of this embodiment may be a crystal grain made of a composite oxide or a structure formed by bonding multiple crystal grains, and may have a preferred orientation in the (0001) plane of the positive electrode active material particles made of a composite oxide.

The positive electrode active material for lithium ion secondary batteries according to this embodiment has a specific crystal orientation represented by the following relational formulas (1) and (2). That is, the positive electrode active material for lithium ion secondary batteries according to this embodiment has a preferred orientation in which the (0001) planes of the positive electrode active material crystals made of hexagonal crystals are assembled in one axial direction, and the characteristics of the crystal orientation are obtained from a pole figure created with the half-width setting represented by the following relational formula (1) based on crystal orientation information of the (0001) planes of the positive electrode active material crystals measured by electron backscatter diffraction (EBSD),
The positive electrode active material is characterized in that the maximum value of a preferred orientation degree (MUD (multiple uniform density)) indicating the crystal orientation of the (0001) plane of the positive electrode active material crystal satisfies the following relational expression (2).

[Equation 1]
Half width (half width) = 10° ... (1)

[Equation 2]
30≦MUD≦200... (2)

The positive electrode active material for lithium ion secondary batteries according to this embodiment is made of a hexagonal crystal. The positive electrode active material for lithium ion secondary batteries is preferably made of a hexagonal crystal, since it facilitates lithium intercalation reaction. Here, the lithium intercalation reaction means that lithium ions can enter and leave the crystal during charge and discharge reactions. Furthermore, the positive electrode active material for lithium ion secondary batteries according to this embodiment has a preferred orientation in which the (0001) plane of the positive electrode active material crystal is assembled in a uniaxial direction. By preferentially orienting the (0001) plane of the positive electrode active material crystal, it is expected to improve the lithium intercalation reaction efficiency and suppress cracks due to repeated charge and discharge reactions, which is preferable. Here, the uniaxial direction may be any spatial coordinate axis direction regardless of the shape of the positive electrode active material particle. For example, the uniaxial direction is the x-axis, y-axis, z-axis, or any orthogonal coordinate axis direction that combines these, which is defined in an orthogonal coordinate system in a three-dimensional space with the center of the target particle as the origin.
In this way, the (0001) planes of the positive electrode active material for a lithium ion secondary battery are assembled in one axial direction, so that the positive electrode active material for a lithium ion secondary battery according to this embodiment has a specific preferred orientation.

Next, the positive electrode active material for a lithium ion secondary battery according to this embodiment has a peak point where the preferred orientation degree (MUD (multiple uniform density)) indicating the crystal orientation of the (0001) plane of the positive electrode active material crystal, obtained from a pole figure created with the half-width setting shown in the following relational expression (1) based on the crystal orientation information of the (0001) plane of the positive electrode active material crystal measured by electron backscatter diffraction (EBSD), satisfies the following relational expression (2).

[Equation 1]
Half width (half width) = 10° ... (1)

[Equation 2]
30≦MUD≦200... (2)

The half-width value is also defined as the half-width. The half-width value (°) is a value that sets the spread of the width of a normal distribution, etc., and any value can be used as necessary. In this embodiment, the half-width value was limited to 10° for evaluation. Electron backscatter diffraction (EBSD) can obtain information about the crystal system and crystal orientation. In addition, a pole figure can be created based on the obtained analytical information. From the pole figure, it is possible to visualize whether there is a texture (a collection of specific crystal planes) on the sample cross section. In this embodiment, a pole figure was created and evaluated with the half-width value set to 10° for the (0001) plane.

The reason for limiting the half width to 10° is as follows.
The half-width value (°) is one of the default values used to create pole figures. The setting of this half-width value (°) affects the results of creating pole figures and the numerical value of MUD (multiple uniform density). For this reason, when comparing multiple pieces of data, it is necessary to use the same half-width value for all pole figures. Therefore, in this embodiment, the evaluation was performed with the condition fixed at the default half-width value of 10°. If the half-width value is less than 10°, the information indicating the orientation of the (0001) plane when creating the pole figures is easily affected by noise due to roughness of the measurement surface, which is not preferable.
If the half-width exceeds 10°, the information indicating the orientation of the (0001) plane becomes unclear when creating a pole figure, and it becomes impossible to correctly determine whether or not there is a preferred orientation, which is not preferable.

The pole figure is created by converting the crystal information of each point obtained from the Kikuchi pattern acquired by the EBSD detector into an area using a Gaussian function and stereoscopically projecting it. Here, the Kikuchi pattern acquired by the EBSD detector means a figure obtained by irradiating a crystalline sample with electrons and projecting the reflected electrons.
The preferred orientation degree (MUD (multiple uniform density)) of the positive electrode active material particles for lithium ion secondary batteries according to this embodiment is evaluated using a pole figure created focusing on the (0001) plane. The preferred orientation degree (MUD (multiple uniform density)) indicating the crystal orientation of the (0001) plane of the positive electrode active material crystal is a numerical value calculated from the information of the pole figure. It indicates the distribution of the (0001) plane on the plane to be evaluated (a sample cross section in this embodiment described later), and is defined as an index for evaluating the degree of orientation.

The MUD of the positive electrode active material particles of the positive electrode active material for lithium ion secondary batteries of this embodiment is preferably 30 or more and less than 200. If the MUD of the positive electrode active material particles of the above positive electrode active material is 30 or more, the preferential orientation of the (0001) plane of the positive electrode active material crystals can be made regular in the orientation of the expansion and contraction of the crystals accompanying charge and discharge, and the occurrence of cracks can be suppressed, which is preferable. However, a positive electrode active material with a MUD of 200 or more is not preferable because the productivity during synthesis decreases.

The reason why the positive electrode active material for lithium ion secondary batteries according to this embodiment is superior to conventionally proposed positive electrode active materials that do not have a preferred orientation in the (0001) plane of the positive electrode active material crystal is presumably due to the following reasons.

When a lithium-ion secondary battery is charged or discharged, lithium ions are inserted and removed only from specific crystal planes of the positive electrode active material. The volume of the positive electrode active material expands and contracts in the c-axis direction of the crystal axis in response to the amount of lithium ions inserted into the crystal. Generally, positive electrode active materials made of Li-Ni-Co-Al or Li-Ni-Co-Mn composite oxides have a secondary particle-like structural feature composed of multiple crystal grains. In these general positive electrode active materials made of Li-Ni-Co-Al or Li-Ni-Co-Mn composite oxides, the orientation of the c-axis of each crystal grain is irregular, so there is no regularity in the direction of the volume change accompanying the charge and discharge reaction. This causes adjacent crystal grains to interfere with each other, causing cracks due to stress.

On the other hand, the positive electrode active material according to this embodiment is a positive electrode active material made of Li-Ni-Co-Al or Li-Ni-Co-Mn composite oxides that have secondary particle-like structural characteristics composed of multiple crystal grains, but because the orientation of the c-axis (0001) plane of the positive electrode active material crystals is aligned in a uniaxial direction, there is a uniform regularity in the orientation of the volume change. This reduces interference between crystal grains and suppresses the occurrence of cracks, making it superior to lithium ion secondary batteries that use conventional positive electrode active materials for lithium ion secondary batteries. This improves cycle characteristics.

Furthermore, in the positive electrode active material for lithium ion secondary batteries according to this embodiment, the crystal grains constituting the material have regularity in their crystal orientation, and therefore it is clear that lithium ions can easily diffuse at the interfaces between adjacent crystal grains.

As a result, the positive electrode active material for lithium ion secondary batteries according to this embodiment has higher rate characteristics than lithium ion secondary batteries that use conventional positive electrode active materials for lithium ion secondary batteries.

Next, a method for producing a positive electrode active material for a lithium ion secondary battery according to the present embodiment will be described. The method for producing a positive electrode active material for a lithium ion secondary battery according to the present embodiment includes a step (i) of producing a precursor of the positive electrode active material for a lithium ion secondary battery, a step (ii) of dry-mixing the precursor with a Li compound to obtain a dry mixed raw material, and a step (iii) of firing the dry mixed raw material.
Hereinafter, each step included in the method for producing a positive electrode active material for a lithium ion secondary battery according to this embodiment will be described.

The method for producing a positive electrode active material for a lithium ion secondary battery according to this embodiment includes a step (i) of producing a precursor of the positive electrode active material for a lithium ion secondary battery. The step (i) of producing a precursor of the positive electrode active material for a lithium ion secondary battery includes a step of synthesizing a hydroxide made of Ni-Co, Ni-Co-Al, or Ni-Co-Mn, a step of washing the hydroxide with water, a step of classifying the hydroxide, and a step of drying the hydroxide.

The process for synthesizing hydroxides consisting of Ni-Co, Ni-Co-Al, or Ni-Co-Mn involves preparing a raw material solution using Ni compounds and Co compounds, and if necessary, Al compounds and Mn compounds, in order to achieve the desired composition ratio. There is no particular restriction on the compounds used to prepare the raw material solution as long as they are water-soluble, but sulfates, nitrates, chlorides, etc. are preferably used.

The concentration of the raw material solution is adjusted so that the total content of Ni, Co, Al, and Mn contained therein is 10 to 300 g/L (hereinafter referred to as "aqueous Me solution"). The target hydroxide is synthesized by supplying this aqueous Me solution to an aqueous solution (hereinafter referred to as "initial mother liquor") adjusted to 30 to 90°C, pH 9 to 13, and an _NH4 ⁺ concentration of 5 to 15.0 g/L. The supply flow rate of the aqueous Me solution is not particularly specified, but it is preferably supplied at a flow rate (L/min) equivalent to 0.05% to 0.5% of the total amount (L) of the initial mother liquor.

The reaction tank used for synthesizing Me hydroxide is a SUS reaction tank with a lid and an overflow port. During the supply of the Me aqueous solution, the initial mother liquid is stirred at 100 to 1200 rpm, the liquid temperature is 30 to 90°C, the pH is 9 to 13, the _NH4 ⁺ concentration is 5 to 15.0 g/L, and the nitrogen atmosphere is maintained.

The pH and _NH4 ⁺ concentration of this initial mother liquid and the nitrogen atmosphere are controlled by supplying an aqueous solution of NaOH and an aqueous solution of ammonium salt such as ( _NH4 ) _2SO4 (hereinafter referred to as " _NH4 ⁺ source") and nitrogen gas. The concentrations and flow rates of the NaOH aqueous solution, _NH4 ⁺ source, _and nitrogen gas are not particularly specified.

By continuously carrying out the above-mentioned Me hydroxide synthesis operation, a slurry with a solids concentration of 10 to 200 g/L is continuously produced, which is composed of Me hydroxide (hereinafter referred to as "first Me hydroxide") and an aqueous solution of water-soluble by-products (hereinafter referred to as "reaction mother liquor"). The continuously produced slurry flows out from the overflow port of the reaction tank and is sent to a service tank. The slurry in the reaction tank or service tank obtained by continuing this operation for 12 hours or more (hereinafter referred to as "first slurry") is used in the next process.

The solids concentration of the first slurry is concentrated to 350 to 1200 g/L (hereinafter referred to as "second slurry"). The first slurry is concentrated by removing the reaction mother liquor or adding first Me hydroxide.

The reaction mother liquid of the second slurry is adjusted to an NH ₄ ⁺ concentration of 5 to 15.0 g/L at 30 to 90°C and pH 9 to 13. The adjustment method is not particularly specified, but is preferably performed using ammonia water and NaOH aqueous solution. A reaction tank with a lid and no overflow port is used. An aqueous Me solution is supplied to the second slurry described above. The supply flow rate of the aqueous Me solution is not particularly specified, but is preferably supplied at a flow rate (L/min) equivalent to 0.001% to 0.3% of the total amount (L) of the second slurry. The composition ratio of metal elements in the aqueous Me solution must be the same as that of the desired precursor, but does not need to be the same as that of the first hydroxide.

During the supply of the Me aqueous solution, the reaction mother liquid is stirred at 500-1500 rpm, the liquid temperature is maintained at 30-90°C, the pH is 9-13, the _NH4 ⁺ concentration is 5-20.0 g/L, and the second slurry concentration is maintained at 350 g/L or more. The atmosphere is not particularly specified, but a nitrogen atmosphere is preferable. The pH and _NH4 ⁺ concentration of the second slurry, as well as the nitrogen atmosphere, are controlled by supplying an NaOH aqueous solution and nitrogen gas.
The concentrations and flow rates of the NaOH aqueous solution, NH ₄ ⁺ source, and nitrogen gas are not particularly specified. The concentration of the second slurry is controlled by removing the reaction mother liquor continuously or palindrically.

By continuing this operation in a continuous or palindrome, a slurry (hereinafter referred to as "third slurry") can be obtained in which a hydroxide having a plate-like or columnar shape (hereinafter referred to as "second Me hydroxide") is newly generated in the second slurry. The supply of the Me aqueous solution is continued until the particle size of the second Me hydroxide reaches the desired size.
The dimensions of the 2Me hydroxide are measured by sampling at appropriate times, separating the 2Me hydroxide into solid and liquid, washing with water, and drying, and then observing the 2Me hydroxide with a scanning electron microscope. At this time, the dimensions and shape of the 1Me hydroxide that is mixed in are not measured.

When the dimensions of the second Me hydroxide reach the desired level, the supply of the Me aqueous solution is stopped. The third slurry in the reaction tank is then washed with water to remove the reaction mother liquor. After washing with water, solid-liquid separation and drying can be performed to obtain a powder (first precursor) in which the first Me hydroxide and the second Me hydroxide are mixed.

Next, a classification operation is performed to separate and collect the second Me hydroxide from the first precursor. The classification method is not particularly specified, but it is preferable to be able to remove 70% or more of the first Me hydroxide by weight. The reason for removing the first Me hydroxide is that the first Me hydroxide has a nearly spherical shape, and the battery characteristics of a lithium-ion secondary battery using this as a raw material for the positive electrode active material are lower than those of a positive electrode active material using the second Me hydroxide as a raw material. By the above-mentioned classification operation, a hydroxide powder (hereinafter referred to as "precursor") containing 70% or more by weight of the second Me hydroxide having a columnar shape can be obtained.

The aforementioned precursors can be evaluated for their crystal orientation characteristics by electron backscatter diffraction (hereinafter referred to as "EBSD"). Samples for EBSD analysis can be prepared, for example, as follows: The precursor is dispersed in a commercially available room temperature curing epoxy resin that is a two-part mixture consisting of a base agent and a hardener, and cured in a vacuum while promoting degassing. The resin block containing the cured precursor is cut and the cut surface is smoothed with sandpaper. This cut surface is then processed with a cross-session polisher to further smooth it, thereby obtaining a precursor cross-section sample for EBSD analysis.

The precursor cross-section sample for EBSD analysis prepared by the above-mentioned operations is evaluated by EBSD analysis. The obtained orientation information of the sample is constructed on analysis software, and the crystal orientation and pole figures of the sample are evaluated by acquiring a mapping image. From the obtained pole figure information, the preferred orientation degree (MUD) of the (0001) plane of the precursor crystal is evaluated. Note that no particular device or analysis software is specified for use in the EBSD analysis.

Furthermore, the manufacturing method of the positive electrode active material for lithium ion secondary batteries according to this embodiment includes a step (ii) of dry-mixing the precursor with a Li compound to obtain a dry mixed raw material (hereinafter referred to as the "mixture"). Step (ii) includes dry-mixing the precursor with a Li compound. If the positive electrode active material is a Ni-Co-Al system, a Li compound and, if necessary, an Al compound are further dry-mixed, and if the positive electrode active material is a Li-Ni-Co-Mn system, a Li compound is further dry-mixed.

In step (ii), the precursor produced in step (i) is dry-mixed with Li compounds (hydroxides, carbonates, halides, etc., Li compounds that can be oxides at high temperatures, with an average particle size of about 50 μm or less), and, if necessary, Al compounds, Zr compounds, or Mg compounds (commercially available Al, Zr, or Mg compounds with an average particle size of 10 μm or less, which can be oxides at high temperatures, such as oxides, hydroxides, sulfates, nitrates, etc.) in a ratio that stoichiometrically satisfies the relationship of a specified composition formula to prepare a dry mixed raw material.

Examples of Li compounds include _LiOH.H2O , LiOH, and _Li2CO3 . _{Examples of Al compounds include Al2O3 and Al(OH)3} _. _Examples of Zr compounds include _ZrO2 . Examples of Mg compounds include _MgO and _MgCO3 .
The composition ratio of these metal compounds is Li _x Ni _(1-y-α-γ) Co _y Al _αB _γ O ₂ (wherein, in the composition formula, 0.9≦x≦1.1, 0.03≦y≦0.3, 0.00<α≦0.05, B is one or more elements selected from Zr and Mg, and 0.00≦γ≦0.10), or Li _x Ni _(1-y-β-γ) Co _y Mn _βB _γ O ₂ (wherein, in the composition formula, 0.9<x<1.1, 0≦y≦0.33, 0.00<β≦0.33), B is one or more elements selected from Zr and Mg, and 0.00≦γ≦0.10).

The dry mixing in step (ii) is desirably carried out for approximately 0.5 to 1.5 hours under normal temperature, normal pressure, and closed conditions (e.g., closing the raw material inlet of the powder mixing device).

The method for producing a positive electrode active material for a lithium ion secondary battery according to this embodiment includes step (iii) of calcining the mixture obtained in step (ii). In step (iii), the raw material mixture prepared as described above is calcined in an oxidizing atmosphere at 700 to 800° C. for 5 to 20 hours. After the calcination, the mixture is rapidly cooled outside the calcination furnace or slowly cooled inside the furnace.
The heating conditions during firing are not particularly limited, but for example, the temperature is raised for 5 to 15 hours, preferably 8 to 12 hours, from the start of heating the furnace.
In this manner, the positive electrode active material for a lithium ion secondary battery according to the present embodiment shown in FIGS. 1 and 2 can be produced by steps (i) to (iii).

The positive electrode active material particles obtained by the above-mentioned step (iii) are evaluated by EBSD analysis. Samples for EBSD analysis can be prepared as follows. The positive electrode active material is dispersed in a commercially available room temperature curing epoxy resin of a two-liquid mixture type consisting of a base agent and a hardener, and cured in a vacuum while promoting degassing. The resin block containing the cured positive electrode active material is cut and the cut surface is smoothed with sandpaper. The cut surface can be further smoothed by cross-session polishing to obtain a cross-sectional sample of the positive electrode active material for EBSD analysis. Note that the positive electrode active material reacts with moisture and carbon dioxide to generate impurities on the crystal surface, so this series of pretreatments should be performed in a low humidity environment, and the cross-sectional sample of the positive electrode active material for EBSD analysis should be stored under an inert gas such as Ar.

The cross-sectional sample of the positive electrode active material for EBSD analysis prepared by the above-mentioned procedure is subjected to EBSD analysis. The obtained orientation information is constructed on an analysis software, and the crystal orientation and pole figures of the sample are evaluated by acquiring a mapping image. The preferred orientation degree (MUD) is evaluated from the pole figure information of the (0001) plane of the obtained positive electrode active material crystal. Note that no particular device or analysis software is specified for use in the EBSD analysis.

As described above, by adopting the positive electrode active material for lithium ion secondary batteries according to this embodiment and controlling its crystal orientation, it is possible to provide a lithium ion secondary battery that can improve cycle characteristics and rate characteristics.

[Second embodiment]
The positive electrode active material for a lithium ion secondary battery according to the second embodiment is characterized in that, in the positive electrode active material for a lithium ion secondary battery according to the above embodiment, it is composed of a chemical composition represented by the following general formula (3).

[Chemical formula 1]
LiNi _1-xyz C _x A _y B _z O ₂ ... (3)
In the above general formula (1), A represents Mn or Al, B represents Mg or Zr, x represents 0.00≦x≦0.33, y represents 0.00≦y≦0.33, and z represents 0.00≦y≦0.10.

The positive electrode active material of the present invention is composed of a Li-Ni-Co-Al based or Li-Ni-Co-Mn based composite oxide, and specific examples thereof include composite oxides represented by the composition formula Li _x Ni _1-yz Co _y Al _z O ₂ (where 0.9≦x≦1.1, 0.00≦y≦0.3, 0.00<z≦0.05), Li _x Ni _1-yz Co _y Mn _z O ₂ (where 0.9<x<1.1, 0.00≦y≦0.33, 0.00<z≦0.33), etc. Specific _examples of Li-Ni-Co- _Al _based _composite _oxides include _{LiNi0.86Co0.11Al0.03O2} _and _{LiNi0.90Co0.05Al0.05O2.Specific} examples of _Li -Ni _- Co _- _Mn based composite oxides include _{LiNi0.8Co0.1Mn0.1O2} _and _{LiNi0.5Co0.2Mn0.3O2} _.

The Li-Ni-Co-Al based composite oxide constituting the positive electrode active material particles for lithium ion secondary batteries according to this embodiment is not particularly limited, but may be, for example, LiNi _0.86 Co _0.11 Al _0.03 O ₂ or LiNi _0.90 Co _0.05 Al _0.05 O _2. The Li-Ni-Co-Mn based composite oxide constituting the positive electrode active material particles for lithium ion secondary batteries is not particularly limited, but may be, for example, LiNi _0.8 Co _0.1 Mn _0.1 O ₂ or LiNi _0.5 Co _0.2 Mn _0.3 O ₂ .

The precursor is not particularly limited as long as it is _a metal hydroxide _that produces an oxide by firing, and examples thereof include _{Ni0.86Co0.11Al0.03} (OH) ₂ _, _Ni0.89Co0.11 (OH) ₂ , and _{Ni0.5Co0.2Mn0.3} (OH) ₂ . The _positive _electrode _active _material precursor for lithium ion secondary batteries _is not _particularly limited as long as it is a composite carbonate _, and examples _thereof include _{Ni0.86Co0.11Al0.03CO3} _, _{Ni0.89Co0.11CO3} _, and _{Ni0.5Co0.2Mn0.3CO3} .

[Third embodiment]
The third embodiment is a mixed positive electrode active material for lithium ion secondary batteries, comprising (a) the positive electrode active material for lithium ion secondary batteries obtained in the above embodiment, and (b) a mixed positive electrode active material for lithium ion secondary batteries. That is, the mixed positive electrode active material for lithium ion secondary batteries of this embodiment is a mixed positive electrode active material for lithium ion secondary batteries, comprising (a) the positive electrode active material for lithium ion secondary batteries obtained in the above embodiment, and (b) a mixed positive electrode active material for lithium ion secondary batteries, characterized in that the content of the positive electrode active material for lithium ion secondary batteries (a) contained in the mixed positive electrode active material for lithium ion secondary batteries is 10 mass% or more.

The (a) positive electrode active material for lithium ion secondary batteries obtained in the above embodiment can be used as a positive electrode active material in the same manner even when mixed with other positive electrode active material particles for lithium ion secondary batteries having different crystal orientation characteristics or composition. In other words, the mixed positive electrode active material for lithium ion secondary batteries according to this embodiment is composed of the (a) positive electrode active material for lithium ion secondary batteries obtained in the above embodiment and (b) mixed positive electrode active material for lithium ion secondary batteries.

In other words, by mixing the (a) lithium ion secondary battery positive electrode active material obtained in the above embodiment with another positive electrode active material (additive positive electrode active material) and using it as a mixed positive electrode active material for a lithium ion secondary battery, a positive electrode active material with improved cycle characteristics and rate characteristics can be obtained.

Furthermore, it is preferable that the content of the (a) mixed positive electrode active material for lithium ion secondary batteries contained in the mixed positive electrode active material for lithium ion secondary batteries according to this embodiment is 10% by mass or more and 98% by mass or less. The reason is that if the mixed ratio of the positive electrode active material for lithium ion secondary batteries of the present invention is 10% by mass or more and 98% by mass or less, the cycle characteristics and rate characteristics can be improved by 10% or more. Note that the (b) mixed positive electrode active material for lithium ion secondary batteries that can be mixed and used with the (a) positive electrode active material for lithium ion secondary batteries is a conventional positive electrode active material for lithium ion secondary batteries, for example, the positive electrode active material for lithium ion secondary batteries described in WO2016-143844A1 or other commercially available Co-based, Ni-Co-Al-based, or Ni-Co-Mn-based positive electrode active materials.

Among these, (b) the positive electrode active material for a lithium ion secondary battery to be mixed is preferably a positive electrode active material having the same composition ratio of Ni, Co, Al, and Mn as the positive electrode active material for a lithium ion secondary battery to be used (a). The composition of the positive electrode active material to be mixed is one type, or a mixture of two or more types.

The general formulas showing the mixed positive electrode active materials for lithium ion secondary batteries, Co-based, Ni-Co-Al-based, and Ni-Co-Mn-based, are Li _x CoO ₂ for the Co-based, Li _x Ni _(1-y-α) Co _y Al _α O ₂ for the Ni-Co-Al-based, and Li _x Ni _(1-y-β) Co _y Mn _β O ₂ for the Ni-Co-Mn-based (wherein, in the composition formulas, 0.9≦x≦1.1, 0.03≦y≦0.33, 0.00<α≦0.05, 0.00<β≦0.33).

As described above, the mixed positive electrode active material containing the positive electrode active material for lithium ion secondary batteries according to this embodiment can improve the cycle characteristics and rate characteristics of the lithium ion secondary battery even when a commercially available positive electrode active material for lithium ion secondary batteries is used.

Then, the positive electrode active material obtained as described above and other materials can be combined to produce an electrode foil (hereinafter, sometimes referred to as "positive electrode foil"). The positive electrode current collector of the positive electrode foil of the lithium ion secondary battery is preferably made of aluminum, which is processed into a thin film, although there is no particular limitation thereto. The method of supporting the positive electrode mixture on the positive electrode current collector is, without particular limitation thereto, for example, a method of pressurizing the positive electrode mixture on the positive electrode current collector. Alternatively, the positive electrode mixture may be made into a paste using an organic solvent, and the resulting paste of the positive electrode mixture may be applied to at least one side of the positive electrode current collector, dried, and pressed to adhere.

The composition of the paste of the positive electrode mixture is not particularly limited, but is preferably composed of a positive electrode active material, a conductive assistant, a binder, and a dispersion medium. The conductive assistant is not particularly limited, but carbon black (e.g., acetylene black) may be used. The binder is not particularly limited, but it is more preferable to use polyvinylidene fluoride, which may be used alone or in combination of two or more types. The dispersion medium is not particularly limited, but N-methyl-2-pyrrolidone is more preferable.

The method for applying the paste of the positive electrode mixture to the positive electrode current collector is not particularly limited, but examples include slit die coating, screen coating, curtain coating, knife coating, gravure coating, and electrostatic spraying. Positive electrodes can be manufactured by the methods listed above.

A lithium ion secondary battery can be obtained by combining the positive electrode foil obtained as described above with other materials. One example of a lithium ion secondary battery using the positive electrode active material for lithium ion secondary batteries of the present invention has a positive electrode foil and a negative electrode foil, a separator sandwiched between the positive electrode foil and the negative electrode foil, and an electrolyte placed between the positive electrode foil and the negative electrode foil.

The separator is not particularly limited, but may be, for example, a material having a form such as a porous film, nonwoven fabric, or woven fabric, made of a material such as a polyolefin resin, such as polyethylene or polypropylene, a fluororesin, or a nitrogen-containing aromatic polymer. One or more of these materials may be used.

The negative electrode is not particularly limited as long as it is capable of doping and dedoping lithium ions at a lower potential than the positive electrode. Examples of the negative electrode foil include an electrode in which a negative electrode mixture containing a negative electrode active material is supported on a negative electrode current collector, and an electrode made of a negative electrode active material alone. If necessary, a binder may be included.

The electrolyte of the lithium ion secondary battery contains an electrolyte and an organic solvent. The electrolyte contained in the electrolyte is not particularly limited, but it is preferable to use an electrolyte containing at least one selected from the group consisting of fluorine-containing LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN(SO ₂ CF ₃ ) ₂ and LiC(SO ₂ CF ₃ ) _3. The lithium salt used in the electrolyte may be one or more.

The organic solvent contained in the electrolyte is not particularly limited, but examples include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and 4-trifluoromethyl-1,. These may be used alone or in combination of two or more.

[Other embodiments]
Although the present invention has been described above with reference to the embodiment, the present invention is not limited to the above embodiment. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the technical scope of the present invention.

Example 1
<Production of Ni-Co coprecipitated hydroxide>
A Ni-Co aqueous solution (hereinafter referred to as "Me _aqueous solution" ₎ was prepared at room temperature by adjusting the molar ratio of Ni:Co between _NiSO4 and _CoSO4 to 89:11 and the total content of _NiSO4.6H2O and _CoSO4.7H2O to 265 g/L. Meanwhile, pure water, 25% NaOH aqueous solution, and ( _NH4 ) _2SO4 crystals were added to a SUS reaction tank (capacity 50 L) equipped with a lid and an overflow port, _and the mixture was heated and stirred to prepare an aqueous solution (hereinafter referred to as "initial mother liquor") adjusted to 60°C, pH 12.0, and _NH4 ⁺ concentration 12.0 g/L.

Nitrogen gas was introduced into the reaction vessel to maintain a positive pressure environment, and an aqueous Me solution was supplied to the initial mother liquid at a flow rate of 40 ml/min. The initial mother liquid was controlled to have a liquid temperature of 60°C, a stirring speed of 400 rpm with a propeller blade of 20 cm in diameter, a pH of 12.0, and an _NH4 ⁺ concentration of 12.0 g/L. These controls were performed by supplying a 25% aqueous NaOH solution and a 20% aqueous ammonia solution.

By continuously continuing the above-mentioned Me aqueous solution supply operation, a slurry (hereinafter referred to as "first slurry") with a solid content concentration of 80 g/L was continuously produced, which was composed of a solid phase of spherical hydroxide with a Ni:Co molar ratio of 89:11 (hereinafter referred to as "first Me hydroxide") and a liquid phase of an aqueous solution in which sodium sulfate, sodium hydroxide, and ammonia were dissolved (hereinafter referred to as reaction mother liquor). The continuously produced first slurry flows out from the overflow port of the reaction tank and is sent to a service tank. The first slurry in the service tank obtained by continuing this operation for 48 hours is used in the next process.

The solid content of the first slurry was concentrated to 500 g/L (hereinafter referred to as "second slurry"). The obtained second slurry was charged into another lidded SUS reactor (capacity 5 L), and nitrogen gas was introduced into the reactor to maintain a positive pressure environment, while Me aqueous solution was supplied to the second slurry at a flow rate of 2 ml/min. At this time, the second slurry was controlled to have a liquid temperature of 55°C, a stirring speed of 1000 rpm with a turbine blade having a diameter of 5 cm, pH of 12.0, and an _NH4 ⁺ concentration of 12.0 g/L. These controls were performed by supplying a 25% NaOH aqueous solution and a 20% ammonia aqueous solution.

Furthermore, the reaction mother liquor was periodically removed so that the solids concentration of the second slurry was maintained in the range of 500 to 1000 g/L. A slurry (hereinafter referred to as "third slurry") was obtained in which a plate-like or caterpillar-like hydroxide (hereinafter referred to as "second Me hydroxide") was newly generated in the second slurry by supplying the Me aqueous solution. The dimensions of the second Me hydroxide contained in the third slurry were measured by sampling 10 ml every 7 hours, separating the solid and liquid, washing with water, and drying, and then observing the particle shape at 1500x magnification using a microscope (JEOL Ltd., product name "JSM-6700F" was used).

The dimensions of the 2Me hydroxide were measured every 7 hours from the start of stirring. Dimensions were measured by selecting particles whose dimensions could be clearly recognized from among the particles present within a randomly selected 1500x field of view, and measuring them. When the average dimensions of the major axis G and minor axis T of more than 70% of the sample particles within the field of view, calculated by number, reached minor axis T = 1.8 μm and major axis G = 4.1 μm, respectively, the supply of the Me aqueous solution was stopped.

Then, the third slurry containing the first Me hydroxide and the second Me hydroxide was taken out and washed with water until the conductivity of the washing water was 300 mS/cm or less, and the reaction mother liquor components, which are impurities in the third slurry containing the first Me hydroxide and the second Me hydroxide, were removed. The solid matter was then dehydrated and dried to obtain a mixture of the first Me hydroxide, which is the conventional precursor, and the second Me hydroxide, which is a precursor having a preferred orientation. The reason for washing with water until the conductivity of the washing water was 300 mS/cm or less is to confirm the removal target when removing the reaction mother liquor components in the third slurry containing the mixture of the precursors first Me hydroxide and second Me hydroxide by washing with water in the manufacturing process, by using the conductivity of the washed water, and in the present invention, the target value is 300 mS/cm or less.

Next, in order to classify the first Me hydroxide, which is the conventional precursor, and the second Me hydroxide, which is the precursor having a preferred orientation, by utilizing the difference in sedimentation speed, 100 g of solids consisting of a mixture of the first Me hydroxide, which is the conventional precursor, and the second Me hydroxide, which is the caterpillar-shaped precursor, were dispersed in 1000 mL of pure water. After that, it was left to stand for 5 minutes, and the solids that were still floating were collected, dehydrated, and dried to obtain a precursor consisting of the second Me hydroxide exhibiting a caterpillar-shaped form with a Ni:Co molar ratio of 89:11.

An EBSD (electron backscatter diffraction) analysis was performed on the hydroxide precursor obtained in this Example 1. The sample to be subjected to the EBSD analysis was prepared as follows.
The precursor hydroxide was dispersed in a commercially available room temperature curing epoxy resin of a two-liquid mixture type consisting of a base agent and a curing agent, and cured while promoting degassing in a vacuum. The resin block containing the cured precursor hydroxide was cut and the cut surface was smoothed with sandpaper. The cut surface was then processed with a cross-session polisher to further smoothen it, obtaining a cross-sectional sample of the precursor hydroxide for EBSD analysis.

The cross-sectional sample of the hydroxide, which was the precursor for EBSD analysis prepared by the above-mentioned procedure, was analyzed using the JSM-7001F manufactured by JEOL Ltd., and orientation analysis was performed using the analysis software AZtec Crystal manufactured by Oxford Instruments Ltd.

The MUD was calculated using the pole figure creation function of the analysis software, with the (0001) plane being evaluated and the half-width set to 10°. Analysis was performed on three randomly selected fields of view for each particle being evaluated, and the average maximum MUD was 115.3. The EBSD results for the precursor hydroxide are shown in Figure 1.

<Production and analysis of positive electrode active material for lithium ion secondary battery>
150 g of the precursor thus obtained was dry-mixed with 71.8 g of commercially available lithium hydroxide monohydrate pulverized product (D50: 30 μm) and 2.3 g of commercially available alumina to prepare a mixture. The above raw material mixture was fired at 730° C. for 15.3 hours in an oxygen atmosphere to obtain a positive electrode active material for lithium ion secondary batteries having the composition formula LiNi _0.86 Co _0.11 Al _0.03 O ₂ . The molar ratio of Ni:Co:Al (ratio of gram atoms of each element) was measured by an inductively coupled plasma (ICP) emission spectrometer (manufactured by Thermo Fisher Scientific Co., Ltd. under the trade name "ICAP6500").

Fig. 2 is an electron microscope photograph of the positive electrode active material for lithium ion secondary batteries obtained in Example 1. Specifically, Fig. 2(A) is an enlarged photograph at 5000 times, Fig. 2(B) is an enlarged photograph at 10000 times, and Fig. 2(C) is an enlarged photograph at 20000 times. As shown in the microscope photograph in Fig. 2, the shape of the obtained positive electrode active material for lithium ion secondary batteries exhibited a columnar structure.
Furthermore, the particle dimensions of the positive electrode active material for lithium ion secondary batteries were measured using an electron microscope, and the measured values were that the minor axis T was 1.6 μm on average and the major axis G was 4.2 μm on average.

An EBSD analysis (electron backscatter diffraction method) was performed on the positive electrode active material particles obtained in this Example 1. The sample to be subjected to the EBSD analysis was prepared as follows.
The positive electrode active material is dispersed in a two-liquid mixture of room temperature curing epoxy resin consisting of a base agent and a hardener, and cured under vacuum while promoting degassing. The resin block containing the cured positive electrode active material is cut, and the cut surface is smoothed with sandpaper. The cut surface is then processed with a cross-session polisher to further smooth it, thereby obtaining a cross-sectional sample of the positive electrode active material for EBSD analysis. Since the positive electrode active material reacts with humidity and carbon dioxide to generate impurities on the crystal surface, this series of pretreatments is performed in a low humidity environment, and the cross-sectional sample of the positive electrode active material for EBSD analysis is stored under an inert gas such as Ar.

The cross-sectional sample of the positive electrode active material for EBSD analysis prepared by the above-mentioned procedure was analyzed using the JSM-7001F manufactured by JEOL Ltd., and orientation analysis was performed using the analysis software AZtec Crystal manufactured by Oxford Instruments Ltd.

The MUD was calculated using the pole figure creation function of the analysis software, with the (0001) plane as the evaluation target and the half-width set to 10°. The particle to be evaluated was analyzed in three randomly selected fields of view, with the average maximum MUD being 96. The EBSD results for the positive electrode active material obtained from the hydroxide precursor described above are shown in Figure 3.

The electrode foil was produced using the positive electrode active material obtained as described above. The electrode foil material was mixed in a ratio of positive electrode active material: conductive assistant: binder: dispersion medium = 45:2.5:2.5:50. Denka Black manufactured by Denka was used as the conductive assistant, and Solef5130 manufactured by Solvay Japan was used as the binder and dispersion medium. The electrode foil material was mixed using a homodisper manufactured by Primix at 6000 rpm for 5 minutes to form a paste. The paste-like electrode foil material was then applied to an aluminum foil with a thickness of 7 mil by the doctor blade method. The aluminum foil coated with the paste-like electrode foil material was heated at 110°C for 4 hours, after which the NMP was removed, and the electrode foil was obtained by roll pressing at 0.04 mm.

A lithium ion secondary battery was produced using the obtained electrode foil. The produced lithium ion secondary battery was composed of a positive electrode, a separator (glass fiber filter paper), a metallic lithium negative electrode, and an electrolyte (1 mol/L LiPF ₆ /PC), and was produced in the argon atmosphere. This lithium ion secondary battery was repeatedly charged and discharged 80 times at a measurement temperature of 20°C, a voltage range of 4.25 to 2.5 V, and a voltage rate of 1 C, and the cycle characteristics (discharge capacity for each cycle and discharge capacity retention rate) were evaluated. The rate characteristic test was carried out at a voltage range of 4.25 to 2.5 V and a voltage rate of 0.1 C to 5 C.

Table 1 and Figures 4 to 6 show the measurement results of the cycle characteristics (discharge capacity for each cycle or the rate of capacity decrease for each cycle relative to the discharge capacity at the initial discharge) and rate characteristics during charge and discharge of the lithium ion secondary battery obtained in Example 1. In Figures 4 and 5, ●5 is the data for Example 1. Also, in Figure 6, (A) is the data for Example 1.

Figure 7 shows the method for measuring the dimensions of positive electrode active material particles for lithium ion secondary batteries using an electron microscope. The dimensions of a specified amount of product taken from appropriately selected locations of the positive electrode active material for lithium ion secondary batteries were measured and averaged to calculate the average short axis T and long axis G of the positive electrode active material particles for lithium ion secondary batteries. Note that when measuring the dimensions from the microscope image, a field of view with low particle density was selected so that the particle shape could be clearly confirmed.

(Comparative Example 1) (same as Example 1 of WO2016-143844A1)
A Ni-Co aqueous solution was prepared at room temperature with a molar ratio of Ni:Co between _NiSO4 and _CoSO4 of 89:11. Meanwhile, pure water was placed in a SUS reaction tank (capacity 50 L) with a lid and an overflow port, _and the agitator was operated at 60°C. While maintaining this state, _N2 gas was introduced, and the Ni-Co aqueous solution, ( _NH4 ) _2SO4 , and NaOH aqueous solution were dropped, and stirring was continued for 10 hours at a tip speed of 4.1 m/s.

During this stirring, it was confirmed that _N2 gas passing through the reaction tank was continuously entrained in the Ni-Co aqueous solution. The amount of ( _NH4 ) _2SO4 _dropped was adjusted so that the Ni-Co aqueous solution in the tank had an ammonia concentration of 12.0 g/L, and the amount of NaOH dropped was adjusted so that the Ni-Co aqueous solution in the tank had a pH of 12.0. The resulting precipitate was taken out in a slurry state, dehydrated, and dried at 110°C for 16 hours to obtain Ni-Co coprecipitated hydroxide.

950g (molar ratio 0.97) of the Ni-Co coprecipitated hydroxide, 160g (molar ratio 0.03) of alumina (average particle size: 10μm), and 445g (molar ratio 1.03) of pulverized lithium hydroxide monohydrate (D50: 30μm) were dry mixed in a blender for 1 hour. After mixing, the raw material powders of the Ni-Co coprecipitated hydroxide, alumina, and lithium hydroxide were sintered in an oxidizing atmosphere at 750°C for 20 hours, including the heating time, in an electric furnace. After sintering, the raw material was taken out of the furnace when the temperature inside the furnace reached 200°C, and was allowed to cool to room temperature, and a positive electrode active material for lithium ion secondary batteries was obtained. It was found that the positive electrode active material for lithium ion secondary batteries was a positive electrode active material with an aspect ratio of more than 0.9, nearly spherical particles, and a smooth surface.

Except for using the above positive electrode active material, EBSD analysis (electron backscatter diffraction) of the positive electrode active material particles was performed in the same manner as in Example 1. The average maximum MUD was 5. The EBSD results of the positive electrode active material obtained in Comparative Example 1 are shown in Figure 8.

Except for using the above positive electrode active material, an electrode foil, which is a positive electrode of a lithium ion secondary battery, was prepared in the same manner as in Example 1. A lithium ion secondary battery was prepared using this electrode foil. The initial capacity (discharge capacity), discharge capacity retention rate, i.e., cycle characteristics (the ratio of discharge capacity after 80 discharges to the discharge capacity at the initial discharge), and rate characteristics of this lithium ion secondary battery were measured under the same conditions as in Example 1, and the results are shown in Table 1 and Figures 4 to 6. In Figures 4 and 5, square 6 represents the data for Comparative Example 1, and the dashed line in Figure 6 (B) represents the data for Comparative Example 1.

Example 2
A positive electrode active material precursor for a lithium secondary battery was produced in the same manner as in Example 1, except that stirring and supplying were stopped when the measurement results by an electron microscope showed average values of minor axis T = 0.6 and major axis G = 1.7 μm. A positive electrode active material for a lithium ion secondary battery was produced in the same manner as in Example 1, except that this precursor was used. An electrode foil, which is a positive electrode of a lithium ion secondary battery, and a lithium ion secondary battery including the same were produced in the same manner as in Example 1, except that this active material for a lithium ion secondary battery (average values of minor axis T = 0.6 μm and major axis G = 1.3 μm) was used. The initial capacity (discharge capacity), cycle characteristics (discharge capacity for each cycle or capacity reduction rate for each cycle relative to the discharge capacity at the time of initial discharge) and rate characteristics of this lithium ion secondary battery were measured under the same conditions as in Example 1. The measurement results are shown in Table 1. In addition, EBSD analysis (electron backscatter diffraction) of the positive electrode active material particles was performed in the same manner as in Example 1, except that the above positive electrode active material was used. The average value of the maximum MUD was 31.

Example 3
A positive electrode active material precursor for a lithium ion secondary battery was produced in the same manner as in Example 1 except that stirring and supplying were stopped when the measurement results by the electron microscope reached the average dimensions of the minor axis T = 4.2 μm and the major axis G = 19 μm. A positive electrode active material for a lithium ion secondary battery was produced in the same manner as in Example 1 except that this precursor was used. An electrode foil, which is a positive electrode of a lithium ion secondary battery, and a lithium ion secondary battery including the same were produced in the same manner as in Example 1 except that this positive electrode active material (average values of minor axis T = 5.1 μm and major axis G = 18 μm) was used. The initial capacity (discharge capacity), cycle characteristics (discharge capacity for each cycle or capacity reduction ratio for each cycle relative to the discharge capacity at the time of initial discharge), and rate characteristics of this battery were measured under the same conditions as in Example 1. The measurement results are shown in Table 1. In addition, EBSD analysis (electron backscatter diffraction) of the positive electrode active material particles was performed in the same manner as in Example 1 except that the above positive electrode active material was used. The average value of the maximum MUD was 178.

According to Table 1, it was found that the lithium ion secondary battery equipped with the electrode foil obtained using the positive electrode active material of Example 1 has excellent initial capacity, rate characteristics, and cycle characteristics. Furthermore, as shown in Figures 4 to 6, the lithium ion secondary battery obtained using the above-mentioned positive electrode active material of the present invention has excellent initial discharge capacity, cycle characteristics, and rate characteristics compared to lithium ion secondary batteries using conventional positive electrode active materials.

Figure 4 shows the discharge capacity per cycle, and Figure 5 shows the discharge capacity maintenance rate from the initial discharge capacity per cycle. In Figures 4 and 5, ●5 shows the discharge capacity per cycle and the discharge capacity maintenance rate from the initial discharge capacity per cycle depending on the number of cycles when the positive electrode active material of the present invention having the preferred orientation obtained in Example 1 was used and measured under the measurement conditions described in Example 1, and □6 shows the discharge capacity per cycle and the discharge capacity maintenance rate from the initial discharge capacity per cycle when measured under the same conditions as above using the conventional positive electrode active material obtained in Comparative Example 1.

Figure 6 shows the rate characteristics of a lithium ion secondary battery equipped with an electrode foil obtained using the positive electrode active material of Example 1. Figure 6(A) shows the results when the positive electrode active material of the columnar structure of the present invention obtained in Example 1 was used, measured under the same conditions as above, and Figure 6(B) shows the results when the conventional positive electrode active material obtained in Comparative Example 1 was used, measured under the same conditions as above.

As is clear from Figures 4 to 6, it has been found that the positive electrode active material for lithium ion secondary batteries according to the present invention is superior in various properties required for positive electrode active materials for lithium ion secondary batteries compared to conventional positive electrode active materials for lithium ion secondary batteries that do not have a preferred orientation.

(Examples 4 to 7)
An electrode foil, which is a positive electrode of a lithium ion secondary battery, and a lithium ion secondary battery including the electrode foil were prepared in the same manner as in Example 1, except that the positive electrode active material for lithium ion secondary batteries obtained in Example 1 was mixed with a conventional positive electrode active material for lithium ion secondary batteries as shown in Table 2. The initial capacity (discharge capacity), cycle characteristics (discharge capacity for each cycle or the rate of capacity decrease for each cycle relative to the discharge capacity at the time of initial discharge) and rate characteristics of these batteries were measured under the same conditions as in Example 1. The measurement results are shown in Table 2.

Fig. 7 is an electron microscope photograph showing a method for measuring the particle size of the positive electrode active material produced in Example 1. As shown in Fig. 7, it was revealed that the positive electrode active material of Example 1 is a columnar structure having a concept of a long axis and a short axis, which is significantly different from conventional positive electrode active materials. This is because the particle shape of the precursor is inherited by the particle shape of the positive electrode active material.
9 is an electron microscope photograph for measuring the particle size of particles constituting a positive electrode obtained using the positive electrode active material produced in Comparative Example 1 by the same measuring method as in Example 1. As shown in Fig. 9, it is clear that the positive electrode active material of Comparative Example 1 is spherical and has no concept of major axis and minor axis.

According to Table 2, it was found that by mixing 10% or more of the positive electrode active material for lithium ion secondary batteries of Example 1 with a conventional positive electrode active material, a positive electrode active material for lithium ion secondary batteries with excellent initial capacity, rate characteristics, and cycle characteristics was obtained.

The positive electrode active material for lithium secondary batteries, which is made of a columnar structure formed by bonding granules made of the complex oxide of the present invention, can be used in a variety of well-known applications that require high capacity at all times during use, including power sources for EVs, personal computers, mobile phones, and backup power sources, and is therefore industrially useful.

Claims

A positive electrode active material for a lithium ion secondary battery having excellent charge/discharge cycle characteristics and rate characteristics,
The (0001) plane of the positive electrode active material crystal made of a hexagonal crystal has a preferred orientation in which it is assembled in one axial direction, and a pole figure created under the conditions of the following relational formula (1) based on crystal orientation information of the (0001) plane of the positive electrode active material crystal measured by electron backscatter diffraction (EBSD) method is:
A positive electrode active material for a lithium ion secondary battery, characterized in that a maximum value of a degree of preferred orientation (MUD), which indicates a crystal orientation of the (0001) plane of the positive electrode active material crystal, satisfies the following relational expression (2):
[Equation 1]
Half width (half width) = 10° ... (1)
[Equation 2]
30≦MUD≦200... (2)
The positive electrode active material for a lithium ion secondary battery according to claim 1, characterized in that it is composed of a chemical composition represented by the following general formula (3):
[Chemical formula 1]
LiNi 1-xyz C x A y B z O 2 ... (3)
In the above general formula (1), A represents Mn or Al, B represents Mg or Zr, x represents 0.00≦x≦0.33, y represents 0.00≦y≦0.33, and z represents 0.00≦y≦0.10.
A mixed positive electrode active material for a lithium ion secondary battery comprising (a) the positive electrode active material for a lithium ion secondary battery according to claim 2 and (b) a mixed positive electrode active material for a lithium ion secondary battery,
The mixed positive electrode active material for lithium ion secondary batteries is characterized in that the content of the (a) positive electrode active material for lithium ion secondary batteries contained in the mixed positive electrode active material for lithium ion secondary batteries is 10 mass % or more.
An electrode foil comprising the positive electrode active material for a lithium ion secondary battery according to claim 2 or 3.
A lithium ion secondary battery comprising the electrode foil according to claim 4.