Positive electrode active material precursor for lithium-ion secondary batteries and method for producing a positive electrode active material precursor for lithium-ion secondary batteries

Abstract

This invention provides a cobalt-free positive electrode active material precursor for lithium-ion secondary batteries and a method for producing the positive electrode active material precursor for lithium-ion secondary batteries, which can be used to obtain lithium-ion secondary batteries with higher discharge capacity. [Solution] A positive electrode active material precursor for a lithium-ion secondary battery containing a metal composite hydroxide, wherein the metal composite hydroxide is in the form of particles having a core-shell structure with its surface coated with an iron compound, the core-shell structure is composed of a core portion and a shell portion, the core portion contains nickel and aluminum as metal elements, the shell portion contains iron as a metal element, and the composition of the core portion is Ni (1-x) Al x A precursor of positive electrode active material for lithium-ion secondary batteries, wherein, when expressed in terms of (OH)2, 0.01 ≤ x ≤ 0.2, and the molar ratio (M2 / M1) of the molar amount M1 of the metal element in the core portion to the molar amount M2 of the metal element in the shell portion is M2 / M1 = 1 / 100 to 1 / 5.

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material precursor for lithium-ion secondary batteries and a method for producing a positive electrode active material precursor for lithium-ion secondary batteries. [Background technology]

[0002] In recent years, research and development has been conducted on secondary batteries that contribute to energy efficiency. In particular, lithium-ion secondary batteries are becoming increasingly important as a power source for electric vehicles (EVs), hybrid electric vehicles (HEVs), and other similar vehicles.

[0003] Cathode active material precursors are attracting attention as important components that determine the capacity of lithium-ion secondary batteries, and development is progressing. Conventionally, cobalt-based materials have been used as cathode active material precursors for lithium-ion secondary batteries. However, cobalt-based materials are expensive and pose resource risks. Therefore, there is a need to develop materials based on elements that can replace cobalt. For example, Non-Patent Literature 1 reports a cathode active material precursor produced by coprecipitation of nickel, iron, and aluminum hydroxides as a cobalt-free material. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Nitin Muralidharan, et al., “LiNixFeyAlzO2, A New Cobalt-Free Layered Cathode Material For Advanced Li-ion Batteries” Journal of Power Sources. Volume 471, 228389, 30 September 2020 [Overview of the project] [Problems that the invention aims to solve]

[0005] Non-patent document 1 reports that a lithium-ion secondary battery fabricated using a positive electrode active material precursor achieved a high capacity of 190 mAh / g at a rate of 0.1C. However, there is still room for improvement in terms of lithium-ion rechargeable battery capacity.

[0006] This invention was made to solve the above-mentioned problems, and aims to provide a cobalt-free positive electrode active material precursor for lithium-ion secondary batteries and a method for producing the positive electrode active material precursor for lithium-ion secondary batteries, which will result in lithium-ion secondary batteries with higher discharge capacity. Ultimately, this will contribute to energy efficiency. [Means for solving the problem]

[0007] To achieve the above objective, the present invention provides the following means. [1] A precursor of a positive electrode active material for a lithium-ion secondary battery containing a metal composite hydroxide, The aforementioned metal composite hydroxide is in the form of particles having a core-shell structure with a surface coated with an iron compound. The aforementioned core-shell structure is composed of a core part and a shell part. The core portion contains nickel and aluminum as metallic elements. The aforementioned shell portion contains iron as a metallic element. The composition of the core portion is Ni (1-x) Al x When expressed as (OH)2, 0.01 ≤ x ≤ 0.2 And, The molar ratio (M2 / M1) of the molar amount M1 of the metal element in the core and the molar amount M2 of the metal element in the shell is, M2 / M1 = 1 / 100 ~ 1 / 5 This is a precursor of positive electrode active material for lithium-ion secondary batteries.

[0008] The lithium-ion secondary battery positive electrode active material precursor (hereinafter also simply referred to as "precursor") described in [1] has its surface coated with an iron compound. Therefore, it is possible to suppress the surface of the lithium-ion secondary battery positive electrode active material (hereinafter also simply referred to as "positive electrode active material") obtained by calcining the precursor from becoming aluminum-rich. As a result, the discharge capacity of the lithium-ion secondary battery (hereinafter also simply referred to as "secondary battery") using this positive electrode active material can be increased. Consequently, the number of batteries required in the secondary battery can be reduced, contributing to cost reduction. In other words, it can contribute to energy efficiency.

[0009] [2] The positive electrode active material precursor for lithium-ion secondary batteries according to [1], wherein the core portion does not contain the same crystal structure as alpha-type nickel hydroxide.

[0010] The precursor in [2] does not have the same crystal structure in its core as alpha-type nickel hydroxide. This means that the core is composed solely of beta-type nickel hydroxide. Beta-type nickel hydroxide has a low content of anionic impurities such as sulfate ions and a high bulk density. As a result, the reaction (calcination reaction) when the precursor is calcined becomes uniform, and the distribution of iron in the positive electrode active material becomes uniform. Therefore, the discharge capacity of the secondary battery can be further increased.

[0011] [3] The positive electrode active material precursor for lithium-ion secondary batteries according to [1] or [2], wherein the shell portion comprises one or more iron compounds selected from iron(II) hydroxide, iron(III) hydroxide, iron oxyhydroxide and triiron tetroxide.

[0012] The precursor described in [3] has a shell containing a specific mixture of iron compounds. This allows for greater uniformity of the iron distribution in the shell. As a result, the discharge capacity of the secondary battery can be further increased.

[0013] [4] A positive electrode active material precursor for lithium-ion secondary batteries as described in any of [1] to [3], wherein the average particle size is 1 to 25 μm and the tap density is 1.30 g / mL or more.

[0014] [4] The precursor according to [4] has a specific average particle diameter and a specific tap density. Therefore, the electrical characteristics during high-rate discharge such as 5C in the fired positive electrode active material can be enhanced, and the electrode filling density can be enhanced. Thus, the discharge capacity of the secondary battery can be further enhanced.

[0015] [5] The specific surface area is 2 to 50 m 2 / g, and the precursor for a positive electrode active material for a lithium ion secondary battery according to any one of [1] to [4].

[0016] [5] The precursor according to [5] has a specific specific surface area. Therefore, the firing reaction when firing the precursor proceeds sufficiently, and the discharge capacity of the secondary battery can be further enhanced. [[ID=I2]]

[0017] [6] A method for producing a precursor for a positive electrode active material for a lithium ion secondary battery according to any one of [1] to [5], including a step of coating a hydroxide containing nickel and aluminum with an iron compound in an inert gas atmosphere, the method for producing a precursor for a positive electrode active material for a lithium ion secondary battery.

[0018] [6] The method for producing the precursor according to [6] includes a step of coating with an iron compound in an inert gas atmosphere. Therefore, oxidation of iron (II) ions can be suppressed, and a shell portion having a uniform layer can be obtained. Thus, the discharge capacity of the secondary battery can be further enhanced.

Advantages of the Invention

[0019] According to the precursor for a positive electrode active material for a lithium ion secondary battery and the method for producing a precursor for a positive electrode active material for a lithium ion secondary battery of the present invention, a lithium ion secondary battery with a further enhanced discharge capacity can be obtained.

Brief Description of the Drawings

[0020] [Figure 1] It is a cross-sectional view schematically showing the lithium ion secondary battery produced in the example. [Figure 2]This is an example of a scanning electron microscope (SEM) image of the precursor from Example 1. [Figure 3] This is an example of a scanning electron microscope energy-dispersive X-ray fluorescence (SEM-EDX) image of the precursor from Example 1. [Figure 4] This figure shows the X-ray diffraction (XRD) pattern of the precursor from Example 1. [Figure 5] This figure shows the XRD pattern of the precursor from Example 2. [Figure 6] This figure shows the XRD pattern of the precursor of Comparative Example 1. [Figure 7] This figure shows the XRD pattern of the precursor of Comparative Example 2. [Figure 8] This figure shows the XRD pattern of the positive electrode active material in Example 3. [Figure 9] This figure shows the XRD pattern of the positive electrode active material in Example 4. [Figure 10] This figure shows the XRD pattern of the positive electrode active material of Comparative Example 3. [Figure 11] This figure shows the XRD pattern of the positive electrode active material of Comparative Example 4. [Figure 12] This figure shows the discharge curve of the lithium-ion secondary battery of Example 3. [Figure 13] This figure shows the discharge curve of the lithium-ion secondary battery of Example 4. [Figure 14] This figure shows the discharge curve of the lithium-ion secondary battery of Comparative Example 3. [Figure 15] This figure shows the discharge curve of the lithium-ion secondary battery of Comparative Example 4. [Modes for carrying out the invention]

[0021] Preferred embodiments of the present invention will be described in detail below.

[0022] [Precursor material for positive electrode active material in lithium-ion secondary batteries] The positive electrode active material precursor for lithium-ion secondary batteries of this embodiment (hereinafter also simply referred to as "precursor") contains a metal composite hydroxide. The metal composite hydroxide exists as particles having a core-shell structure with its surface coated with an iron compound. The precursor of this embodiment is an aggregate of metal composite hydroxide particles and is a material that serves as a raw material for lithium-ion secondary battery positive electrode active material (hereinafter also simply referred to as "positive electrode active material") used in the positive electrode of lithium-ion secondary batteries (hereinafter also simply referred to as "secondary batteries"). In the precursor, the metal composite hydroxide particles may be primary particles or secondary particles formed by aggregation of primary particles.

[0023] The content of metal composite hydroxide particles in the precursor is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and may be 100% by mass. The precursor may contain components other than metal composite hydroxide particles as long as the function of the present invention is not impaired.

[0024] Metal composite hydroxide particles have a core-shell structure consisting of a core portion and a shell portion. The core contains nickel and aluminum as metallic elements. The shell contains iron as a metallic element. The core and shell can be easily distinguished from scanning electron microscope energy-dispersive X-ray fluorescence (SEM-EDX) images, etc.

[0025] The core composition is Ni (1-x) Al x It can be represented as (OH)2. In this case, x satisfies 0.01 ≤ x ≤ 0.2. x represents the number of moles of aluminum when hydroxyl groups are assumed to be 2 moles. If x is greater than or equal to the lower limit above, the thermal stability of the positive electrode active material obtained from the precursor can be increased. If x is less than or equal to the upper limit above, the discharge capacity of the secondary battery using the positive electrode active material obtained from the precursor can be increased. x is preferably 0.02 to 0.15, and more preferably 0.03 to 0.1. x can be adjusted by the molar ratio of the nickel compound to the aluminum compound. x can be determined, for example, by inductively coupled plasma (ICP) emission spectroscopy.

[0026] If M1 is the molar amount of the metal element in the core and M2 is the molar amount of the metal element in the shell, then the molar ratio (M2 / M1) satisfies M2 / M1 = 1 / 100 to 1 / 5. If M2 / M1 is above the lower limit, it is possible to suppress the surface of the positive electrode active material obtained from the precursor from becoming aluminum-rich, thereby increasing the discharge capacity of the secondary battery using the positive electrode active material. If M2 / M1 is below the upper limit, it is possible to maintain a sufficient nickel ratio, thereby increasing the discharge capacity of the secondary battery using the positive electrode active material obtained from the precursor. The ratio of M2 / M1 is preferably 1 / 100 to 1 / 10, and more preferably 1 / 100 to 3 / 20. M2 / M1 can be determined, for example, by inductively coupled plasma (ICP) emission spectroscopy. The size of the core and the thickness of the shell can be adjusted as appropriate within the range that satisfies the aforementioned molar ratio (M2 / M1 = 1 / 100 to 1 / 5).

[0027] Preferably, the core portion does not contain the same crystal structure as alpha-type nickel hydroxide. Alpha-type nickel hydroxide contains many anionic impurities such as sulfate ions, which leads to an uneven reaction (calcination reaction) when the precursor is calcined, resulting in a decrease in the discharge capacity of secondary batteries using the positive electrode active material obtained from the precursor. "The core portion does not contain the same crystal structure as alpha-type nickel hydroxide" means that the core portion is composed solely of beta-type nickel hydroxide. Beta-type nickel hydroxide has a low content of anionic impurities such as sulfate ions and a high bulk density. Therefore, the reaction (calcination reaction) when the precursor is calcined becomes uniform, and the distribution of iron in the positive electrode active material becomes uniform. Thus, the discharge capacity of secondary batteries can be further increased. In this specification, the phrase "the core portion does not contain the same crystal structure as alpha-type nickel hydroxide" does not mean that it does not contain any crystal structure identical to alpha-type nickel hydroxide at all, but rather that it allows for the inevitable inclusion of a crystal structure identical to alpha-type nickel hydroxide. The crystal structure of the core can be determined by X-ray diffraction (XRD).

[0028] The shell portion preferably contains one or more iron compounds selected from iron(II) hydroxide, iron(III) hydroxide, iron oxyhydroxide, and triiron tetroxide. By including specific iron compounds in the shell portion, the uniformity of the iron distribution within the shell portion can be further enhanced. Therefore, the discharge capacity of lithium-ion secondary batteries using positive electrode active materials obtained from precursors can be further increased. The iron compounds included in the shell are preferably iron(II) hydroxide, iron oxyhydroxide, and triiron tetroxide, with iron(II) hydroxide being more preferred. The iron compound contained in the shell may be one type or two or more types. The shell portion may contain small amounts of impurities other than iron compounds. The type of iron compound contained in the shell can be identified by Raman spectroscopy.

[0029] The average particle size of the precursor is preferably 1 to 25 μm, more preferably 2 to 20 μm, and even more preferably 3 to 15 μm. If the average particle size of the precursor is above the lower limit, the productivity of the precursor can be increased. If the average particle size of the precursor is below the upper limit, the electrical characteristics during high-rate discharge such as 5C can be improved. The average particle size of the precursor refers to D50, which is measured, for example, by a laser diffraction particle size distribution analyzer.

[0030] The tap density of the precursor is preferably 1.30 g / mL or higher, more preferably 1.32 g / mL or higher, and even more preferably 1.35 g / mL or higher. If the tap density of the precursor is above the lower limit, the electrode packing density can be increased. Therefore, the discharge capacity of the secondary battery can be further increased. The upper limit of the tap density of the precursor is not particularly limited, but for example, it is 2.50 g / mL. The tap density of the precursor can be determined, for example, by the following method.

[0031] <Method for measuring tap density> Tap density is the increased bulk density obtained after mechanically tapping a container containing a powder sample. Tap density is obtained by mechanically tapping a measuring container containing a powder sample. After measuring the initial volume or mass of the powder, the measuring container is mechanically tapped, and the volume or mass is read until almost no change in volume or mass is observed. Mechanical tapping is performed by lifting the container and dropping it under its own weight for a predetermined distance using the method described below.

[0032] (Operation method) Using an appropriate tap density tester, tap the measurement container 50-60 times per minute. After 200 taps, calculate the tap density (g / mL) from the sample weight (g) / filled volume after tapping (mL), and record the average of three measurements using three different samples. Include the test conditions, including tap height, in the results section.

[0033] The specific surface area of ​​the precursor is 2 to 50 m². 2 / g is preferred, 5-40m 2 / g is more preferable, 10-30m 2 / g is even more preferable. If the specific surface area of ​​the precursor is greater than or equal to the lower limit above, the calcination reaction during calcination of the precursor proceeds sufficiently, and the discharge capacity of the secondary battery can be further increased. If the specific surface area of ​​the precursor is less than or equal to the upper limit above, peeling of the shell portion during the mixing process with the lithium source can be suppressed. The specific surface area of ​​the precursor can be determined by the BET method.

[0034] [Method for producing precursors] The method for producing the precursor of this embodiment includes a step of coating a hydroxide containing nickel and aluminum with an iron compound under an inert gas atmosphere (hereinafter also referred to as the "coating step"). By including the coating step in the method for producing the precursor of this embodiment, metal composite hydroxide particles having a core-shell structure with the surface coated with an iron compound can be obtained. The coating process will be explained below.

[0035] <Coating process> Nickel and aluminum hydroxide (hereinafter also referred to as "nickel-aluminum coprecipitation hydroxide") is obtained by the coprecipitation process described later. When coating nickel-aluminum coprecipitate hydroxide with an iron compound, it is preferable to do so under an inert gas atmosphere. By performing the coating process under an inert gas atmosphere, the oxidation of iron(II) ions can be suppressed, resulting in a shell with a uniform layer. Therefore, the discharge capacity of the secondary battery can be further increased. On the other hand, if the coating process is carried out in the presence of oxygen gas, the dissolved oxygen may oxidize the iron(II) ions, resulting in an uneven layer and potentially exposing the core of the particle. In other words, the particle may lack a core-shell structure.

[0036] Examples of inert gases used in the coating process include oxygen-free gases such as nitrogen gas, helium gas, and argon gas. From an economic standpoint, nitrogen gas is preferred as the inert gas used in the coating process. The flow rate of the inert gas is not particularly limited, as long as it can prevent the introduction of oxygen gas.

[0037] In the coating process, it is preferable to mix the nickel-aluminum coprecipitate hydroxide with water to prepare a slurry. Using a slurry makes it easier to uniformly coat the nickel-aluminum coprecipitate hydroxide with an iron compound. This makes it easier to obtain metal composite hydroxide particles having a uniform shell layer. The pH of the slurry is preferably 8 to 10 at 75°C. pH can be adjusted using sulfuric acid solution, sodium hydroxide solution, etc. pH can be measured using a pH meter or similar device, with the sample temperature set to 75°C.

[0038] The temperature of the slurry liquid during the coating process is preferably 70-80°C. Examples of iron compounds used in the coating process include iron(II) sulfate and iron(II) chloride. The valency of the iron ions in the iron compound is preferably 2 rather than 3. Coating with an iron compound containing divalent iron ions makes it easier to obtain metal composite hydroxide particles having a uniform shell layer.

[0039] The method for producing the precursor in this embodiment may include steps other than the coating step. Other processes besides the coating process include, for example, the coprecipitation process, dehydration process, drying process, and sieving process.

[0040] <Co-precipitation process> Nickel-aluminum coprecipitated hydroxide is obtained by coprecipitating a nickel compound and an aluminum compound in an alkaline solution (coprecipitation step). Examples of nickel compounds include nickel sulfate, nickel nitrate, and nickel chloride. Examples of aluminum compounds include aluminum sulfate, aluminum nitrate, and aluminum chloride. Examples of alkaline solutions include aqueous ammonia, aqueous sodium hydroxide solution, and mixed solutions thereof.

[0041] In the coprecipitation process, the molar ratio of the nickel compound to the aluminum compound is preferably 99:1 to 80:20, and more preferably 98:2 to 70:30, based on the molar ratio of nickel element to aluminum element. By keeping the molar ratio of the nickel compound to the aluminum compound within the above range in the coprecipitation process, a positive electrode active material with high thermal stability can be obtained, and the discharge capacity of the secondary battery using this positive electrode active material can be further increased.

[0042] The pH of the alkaline solution in the coprecipitation process is preferably 10 to 12 at 50°C. pH can be adjusted using sulfuric acid solution, sodium hydroxide solution, etc. pH can be measured using a pH meter or similar device, with the sample temperature set at 50°C.

[0043] The reaction temperature in the coprecipitation step is preferably 40 to 60°C. The reaction time in the coprecipitation step is preferably 30 to 50 hours. In the coprecipitation step, it is preferable to dehydrate and wash the slurry obtained by the reaction with water.

[0044] <Dehydration process> The dehydration step is a process of dehydrating the slurry liquid obtained in the coprecipitation step or coating step. The dehydration method in the dehydration process is not particularly limited and can be carried out by conventionally known methods. The dehydration process is preferably carried out under an inert gas atmosphere from the viewpoint of suppressing the oxidation of iron compounds.

[0045] <Drying process> The drying process involves drying the metal composite hydroxide after dehydrating the slurry to obtain powdered metal composite hydroxide particles. Obtaining powdered metal composite hydroxide particles makes it easier to prepare the cathode active material from the precursor. The drying method in the drying process is not particularly limited and can be carried out by conventionally known methods. The drying process is preferably carried out under an inert gas atmosphere from the viewpoint of suppressing the oxidation of iron compounds.

[0046] The drying temperature in the drying process is preferably 100 to 120°C. The drying time in the drying process is preferably 5 to 15 hours.

[0047] <Sieving process> For the aggregate of powdered metal composite hydroxide particles (precursor) obtained in the drying process, it is preferable to adjust the particle size using a sieve (sieving process). Including a sieving process makes it easier to adjust the average particle size of the precursor, and in secondary batteries using the positive electrode active material obtained from the precursor, the electrical characteristics during high-rate discharge such as 5C can be further improved. The sieves used in the screening process are not particularly limited, and conventionally known sieves can be used. The mesh size of the sieve used in the sieving process is not particularly limited, but a mesh size such that the average particle size of the precursor is 1 to 25 μm is preferred. Examples of such sieves include those with a mesh size of 100 to 200.

[0048] [Cathode active material] The positive electrode active material of this embodiment mainly consists of a lithium transition metal composite oxide and is used as the positive electrode of a lithium-ion secondary battery. "Mainly consisting of a lithium transition metal composite oxide" means that the content of the lithium transition metal composite oxide relative to the total mass of the positive electrode active material is 75% by mass or more, preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more, and may be 100% by mass. The positive electrode active material may contain components other than the main component, as long as they do not impair the function of the present invention.

[0049] The positive electrode active material of this embodiment may contain only one type of lithium transition metal composite oxide, or it may contain two or more types, as long as it is mainly composed of lithium transition metal composite oxides.

[0050] When a positive electrode active material is manufactured using lithium transition metal composite oxide as the main component, the overall composition ratio (Li:Ni:Al:Fe) of the lithium transition metal composite oxide is maintained in the resulting positive electrode active material. When a positive electrode active material obtained using lithium transition metal composite oxide with such a composition is used in a secondary battery, high capacity can be achieved. Furthermore, the composition ratio of the lithium transition metal composite oxide is adjusted to be similar to the composition ratio required for the desired positive electrode active material.

[0051] <Lithium transition metal composite oxide> The lithium transition metal composite oxide of this embodiment is a layered rock salt type oxide and is in the form of particles having an outer layer on the surface.

[0052] In this specification, the average particle diameter of the particles of the lithium transition metal composite oxide is not particularly limited. For example, 2 to 30 μm is preferable, 3 to 20 μm is more preferable, and 5 to 15 μm is even more preferable. When the average particle diameter of the particles of the lithium transition metal composite oxide is at least the above lower limit value, the productivity of the positive electrode active material can be further enhanced. When the average particle diameter of the particles of the lithium transition metal composite oxide is at most the above upper limit value, the electrochemical characteristics of the secondary battery can be made better. The average particle diameter of the particles of the lithium transition metal composite oxide means, for example, D50 measured by a laser diffraction type particle size distribution measuring device or the like.

[0053] The lithium transition metal composite oxide of this embodiment is represented by the following formula (1). Li x Ni y Al z Fe w O2(1) In formula (1), x is in the range of 0.95 ≤ x ≤ 1.05, y is in the range of 0.78 ≤ y ≤ 0.95, z is in the range of 0.01 ≤ z ≤ 0.15, and w is in the range of 0.01 ≤ w ≤ 0.15, and x + y + z + w = 2.

[0054] The lithium transition metal composite oxide of this embodiment more preferably has x in the range of 0.97 ≤ x ≤ 1.03, y in the range of 0.80 ≤ y ≤ 0.92, z in the range of 0.04 ≤ z ≤ 0.09, and w in the range of 0.04 ≤ w ≤ 0.12 in formula (1).

[0055] The chemical composition of the lithium transition metal composite oxide of this embodiment can be determined by inductively coupled plasma (ICP) emission spectroscopy.

[0056] <Surface composition> The particles of the lithium transition metal composite oxide have an outer layer on the surface. In this specification, the "outer layer" refers to the region extending from the surface of the particle to the interior of the particle up to 25 nm. If the particle diameter is less than 50 nm, the particle is assumed to have a single-layer structure consisting only of the outer layer.

[0057] In this embodiment, it is preferable that the lithium transition metal composite oxide particles have a uniform ratio of the number of Fe atoms to the number of Ni atoms in the outer layer (Fe / Ni ratio) and a uniform ratio of the Fe / Ni ratio in the entire particle (hereinafter also referred to as the "Fe / Ni ratio in the outer layer / entire particle"). More specifically, the Fe / Ni ratio in the 'outer layer / whole particle' is preferably 0.7 to 1.7, more preferably 0.9 to 1.5, and even more preferably 1.1 to 1.3. When the Fe / Ni ratio in the 'outer layer / whole particle' is within the above numerical range, it does not hinder the movement of lithium ions, and when used as a positive electrode active material, it can further increase the discharge capacity of the secondary battery.

[0058] The Fe / Ni ratio can be determined by quantitative analysis using X-ray electron spectroscopy (XPS). XPS allows for the analysis of the transition metal element composition across the entire particle. In other words, the analysis results obtained by XPS represent the composition across the entire surface of the particle, not just a localized composition on the entire surface of the particle.

[0059] The lithium transition metal composite oxide particles in this embodiment may be primary or secondary particles. It is preferable that the lithium transition metal composite oxide particles be secondary particles formed by the aggregation of multiple primary particles, as this yields relatively dense particles.

[0060] <Lattice constant> The lithium transition metal composite oxide of this embodiment is a rhombohedral layered compound and has a crystal structure of the space group R-3m. The lattice constants of the positive electrode active material of this embodiment mainly composed of the lithium transition metal composite oxide preferably have an a-axis length of 2.860 Å to 2.890 Å and a c-axis length of 14.18 Å to 14.28 Å. When the lattice constants are within the above range, in the primary particles of the positive electrode active material, lithium ions are likely to diffuse and the resistance is low. The lattice constants of the crystal can be obtained by measuring the X-ray diffraction pattern of the positive electrode active material and using the least squares method with each index and its interplanar spacing.

[0061] <X-ray Diffraction (XRD) Pattern> In the positive electrode active material of this embodiment, the ratio (I1 / I2) of the integrated intensity (I1) of the diffraction peak of the 003 plane to the integrated intensity (I2) of the diffraction peak of the 104 plane in the space group R-3m measured by X-ray diffraction (XRD) is preferably 1.15 or more and 1.35 or less, more preferably 1.20 or more and 1.30 or less, and even more preferably 1.23 or more and 1.27 or less. When the integrated intensity ratio (I1 / I2) is within the above numerical range, cation mixing is less and lithium ions are likely to diffuse. Therefore, the discharge capacity and the average discharge voltage of the secondary battery can be further increased. The integrated intensity ratio (I1 / I2) is obtained by analyzing the XRD pattern.

[0062] [Manufacturing Method of Positive Electrode Active Material] The positive electrode active material of this embodiment can be synthesized using the precursor of this embodiment. For example, a composite hydroxide of a nickel compound, an aluminum compound, and an iron compound is prepared as a precursor, and this precursor and a lithium compound are mixed to form a raw material mixture (mixing step), and this raw material mixture is heat-treated (for example, fired) at a predetermined temperature for a predetermined time in a predetermined atmosphere to be synthesized (heat treatment step).

[0063] <Mixing Step> The mixing step is a step of mixing a lithium compound and the precursor of this embodiment to obtain a raw material mixture.

[0064] Lithium compounds are not particularly limited, but examples include hydroxides such as lithium hydroxide monohydrate (LiOH·H2O), carbonates such as lithium carbonate (Li2CO3), and acetates such as lithium acetate (CH3COOLi) and lithium acetate dihydrate (CH3COOLi·2H2O). These lithium compounds may be used individually or in combination of two or more.

[0065] A known mixer can be used to mix the lithium compound and the precursor. Examples of such mixing machines include shaker mixers, Redigge mixers, Julia mixers, and V-blenders. The mixing conditions in the mixing process are not particularly limited, but it is preferable to select conditions such that the raw material components are sufficiently mixed without destroying the physical structure of the raw material particles, such as precursors.

[0066] It is preferable to pre-adjust the particle size and other properties of the lithium compound and precursor so that the desired lithium transition metal composite oxide can be obtained after the heat treatment process.

[0067] It is preferable to weigh and mix the lithium compound and precursor in the raw material mixture after the heat treatment process, so that the ratio of the amounts of lithium transition metal composite oxide is Li:Ni:Al:Fe=x:y:z:w. More specifically, the lithium compound is weighed in an amount of 1% to 5% by mass, preferably 1% to 3% by mass, more than the stoichiometric ratio. Here, x, y, z, and w can be within the same range as those described for lithium transition metal composite oxides in the positive electrode active material.

[0068] <Heat treatment process> The heat treatment process involves heat-treating the raw material mixture obtained in the mixing process in a predetermined atmosphere, at a predetermined temperature, and for a predetermined time. In the heat treatment process, the raw material mixture is filled into a crucible or similar container and then heat-treated. Examples of crucibles include alumina saggers, alumina crucibles, platinum crucibles, and gold crucibles. For heat treatment of the raw material mixture, for example, a firing furnace or a roller hearth kiln is used.

[0069] The raw material mixture placed in a saggar or crucible is heated to reach the heat treatment temperature at a heating rate of 5°C / min to 25°C / min, preferably 10°C / min to 25°C / min. The heat treatment atmosphere is not particularly limited and can be in the atmosphere (air atmosphere), oxygen flow, etc. Oxygen flow is preferred as the heat treatment atmosphere. The heat treatment time can be set appropriately according to the heat treatment temperature. Note that the heat treatment time refers to the time for which the heat treatment temperature is maintained.

[0070] The heat treatment temperature is preferably 700°C to 800°C, and more preferably 720°C to 780°C. The heat treatment time is preferably 1 hour to 20 hours, and more preferably 3 hours to 15 hours.

[0071] The method for producing the positive electrode active material in this embodiment may include steps other than the mixing step and the heat treatment step. One example of such a process is a cooling process.

[0072] <Cooling process> The cooling step is a process of cooling the lithium transition metal composite oxide obtained in the heat treatment step to a predetermined temperature at a predetermined rate of cooling. In the cooling process, for example, the powder obtained in the heat treatment process is cooled to room temperature (e.g., 25°C) at a cooling rate of 5°C / min to 25°C / min, preferably 10°C / min to 25°C / min. The atmosphere used to cool the powder is not particularly limited and may include air (air atmosphere), airflow, oxygen flow, etc.

[0073] In this embodiment, the Fe / Ni ratio in the 'outer layer / entire particle' of the lithium transition metal composite oxide can be more easily adjusted to a specific range by appropriately selecting the heat treatment temperature and heat treatment time in the heat treatment process.

[0074] [Lithium-ion rechargeable battery] The lithium-ion secondary battery of this embodiment (hereinafter also simply referred to as "secondary battery") comprises a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains a positive electrode active material mainly composed of the lithium transition metal composite oxide described above. The secondary battery of this embodiment may include other battery elements as needed.

[0075] The secondary battery of this embodiment can use the same battery elements as known lithium-ion secondary batteries, except that the positive electrode contains a positive electrode active material mainly composed of the lithium transition metal composite oxide described above. The secondary battery of this embodiment may have any of the following configurations: coin type, button type, cylindrical type, prismatic type, or laminate type. Furthermore, the secondary battery of this embodiment can be applied to a wide range of applications, such as mobile devices like mobile phones and laptop computers, and in-vehicle applications.

[0076] The following description of the secondary battery of this embodiment will focus on a secondary battery using an electrolyte (coin-type lithium-ion secondary battery). Each battery element described below can also be applied to an all-solid-state lithium-ion secondary battery that does not use an electrolyte.

[0077] Figure 1 is a schematic cross-sectional view showing a lithium-ion secondary battery according to this embodiment. Figure 1 shows an example in which the lithium-ion secondary battery of this embodiment is a coin-type lithium-ion secondary battery. As shown in Figure 1, the lithium-ion secondary battery 1 of this embodiment comprises a negative electrode can (negative electrode terminal) 20, a negative electrode 3, a separator 4 impregnated with electrolyte, an insulating packing (gasket) 5, a positive electrode 2, and a positive electrode can 10.

[0078] The positive electrode can 10 is positioned below the separator 4, and the negative electrode can 20 is positioned above the separator 4. The positive electrode can 10 and the negative electrode can 20 form the outer shape of the lithium-ion secondary battery 1. Between the positive electrode can 10 and the negative electrode can 20, a positive electrode 2 and a negative electrode 3 are provided via a separator 4 impregnated with electrolyte, with the separator 4 separating the positive electrode 2 and the negative electrode 3. The positive electrode can 10 and the negative electrode can 20 are electrically insulated by an insulating packing 5.

[0079] The lithium-ion secondary battery 1 can be manufactured by preparing a positive electrode composite material by adding a conductive agent, a binder, etc., as needed to the positive electrode active material of this embodiment, and then pressing this composite material onto a current collector (not shown). Preferably, stainless steel mesh, aluminum foil, etc., can be used as the current collector. Preferably, acetylene black, ketchen black, etc., can be used as the conductive agent. Preferably, tetrafluoroethylene, polyvinylidene fluoride, etc., can be used as the binder.

[0080] The composition of the positive electrode active material, conductive agent, and binder in the positive electrode composite material is not particularly limited. The content of the conductive agent in the positive electrode composite material is preferably 1% to 15% by mass, and more preferably 0.1% to 5% by mass. The content of the binder in the positive electrode composite material is preferably 0.1% to 10% by mass, and more preferably 0.1% to 5% by mass. It is preferable to blend the positive electrode active material, conductive agent, and binder so that the remainder of the positive electrode composite material (the portion other than the positive electrode active material and conductive agent) becomes the positive electrode active material.

[0081] In a lithium-ion secondary battery 1, the negative electrode 3 relative to the positive electrode 2 can be any known material that functions as a negative electrode active material and is capable of intercalating and releasing lithium, such as metallic materials like metallic lithium or lithium alloys, carbon-based materials like graphite or MCMB (mesocarbon microbeads), or silicon-based materials like silicon (Si), Si alloys, or silicon oxide.

[0082] The separator 4 and battery container (positive electrode container 10, negative electrode container 20) can use known battery components.

[0083] As the electrolyte, known electrolytes such as liquid electrolytes and solid electrolytes can be used. As an electrolyte, for example, an electrolyte such as lithium perchlorate or lithium hexafluoride phosphate can be dissolved in a solvent such as ethylene carbonate (EC), dimethyl carbonate (DMC), propylene carbonate (PC), or diethyl carbonate (DEC) can be used.

[0084] Furthermore, all-solid-state lithium-ion secondary batteries can have the same structure as known all-solid-state lithium-ion secondary batteries, except that they use a positive electrode active material mainly composed of the lithium transition metal composite oxide described above.

[0085] In the case of all-solid-state lithium-ion secondary batteries, the electrolyte can be a polymer-based solid electrolyte such as a polyethylene oxide-based polymer compound, a polymer compound containing at least one of a polyorganosiloxane chain or a polyoxyalkylene chain, a sulfide-based solid electrolyte, or an oxide-based solid electrolyte.

[0086] For the positive electrode of an all-solid-state lithium-ion secondary battery, for example, in addition to the positive electrode active material, conductive agent, and binder described above, a positive electrode composite material containing a solid electrolyte can be supported on a positive electrode current collector made of aluminum, nickel, stainless steel, or the like.

[0087] In this embodiment, the lithium-ion secondary battery 1 has a positive electrode 2 that contains a positive electrode active material mainly composed of the above-mentioned lithium transition metal composite oxide, thereby increasing the discharge capacity. [Examples]

[0088] The following describes embodiments of the present invention, but the present invention is not limited to the following embodiments.

[0089] [Example 1] <Preparation of nickel-aluminum coprecipitate hydroxide> A stirrer equipped with a 7cm diameter blade and a 5L sealed, flat-bottomed beaker fitted with an overflow tube with an inner diameter of 5mm were used to hold 3L of water. After heating to 50°C, 150mL of 25% by mass ammonia solution was added, and sulfuric acid solution or sodium hydroxide solution was added to adjust the pH to 11.10 (measured at 50°C). The liquid temperature in the beaker was maintained at 50°C, and while stirring at a speed of 600 rpm, a solution of nickel sulfate and aluminum sulfate, adjusted to a molar ratio (Ni:Al = 95.3:4.7) of 10% by weight in terms of metal, was continuously added at a flow rate of 300 mL / h. Subsequently, 25% by mass aqueous ammonia was added at a flow rate of 30 mL / h. In addition, 25% by mass aqueous sodium hydroxide solution was continuously added to maintain the reaction solution in the beaker at pH 11.10 ± 0.05 (measured at 50°C). Approximately 30 hours after the start of the reaction, when the slurry concentration reaches a steady state, a high-density hydroxide with a median diameter of 10.5 μm and a tap density of 1.8 g / mL is obtained. The reaction continues while controlling the pH of the reaction solution, and the slurry discharged from the overflow pipe between 30 and 50 hours after the start of the reaction is dehydrated and washed with water to obtain nickel-aluminum coprecipitated hydroxide.

[0090] <Iron hydroxide coating process> 955 g (solid weight) of the obtained nickel-aluminum coprecipitate hydroxide powder was placed in a 5 L beaker and diluted with water to a 20 wt% slurry. This slurry was then heated to 75°C. Furthermore, the slurry was adjusted with sulfuric acid aqueous solution or sodium hydroxide aqueous solution to a pH of 9.00 ± 0.05 (measured at 75°C). In addition, a certain amount of nitrogen gas purging was performed to prevent oxidation of iron. Next, while maintaining the temperature at 75°C and stirring the slurry, an aqueous solution of iron(II) sulfate, adjusted to 10% by weight (metal equivalent), was continuously added dropwise so that iron(II) hydroxide equivalent to 4.5 mol% iron would precipitate on the nickel hydroxide surface. At the same time, a 25% by mass aqueous solution of sodium hydroxide was added dropwise to maintain the pH at 9.00 ± 0.05 (measured at 75°C). This yielded a slurry of nickel-aluminum hydroxide coated with iron(II) hydroxide. The obtained slurry was then dehydrated, washed with alkali and water, dried at 110°C for 10 hours, and then passed through a 100-mesh sieve to obtain 1 kg of precursor in which the surface of nickel-aluminum hydroxide was coated with iron(II) hydroxide.

[0091] [Example 2] <Preparation of nickel-aluminum coprecipitate hydroxide> A stirrer equipped with a 7cm diameter blade and a 5L sealed flat-bottom beaker fitted with an overflow tube with an inner diameter of 5mm were used to hold 3L of water. After heating to 50°C, 150mL of 25% ammonia solution was added, and sulfuric acid solution or sodium hydroxide solution was added to adjust the pH to 11.00 (measured at 50°C). The liquid temperature in the beaker was maintained at 50°C, and while stirring at a speed of 600 rpm, a solution of nickel sulfate and aluminum sulfate, adjusted to a molar ratio (Ni:Al = 95.3:4.7) of 10% by weight in terms of metal, was continuously added at a flow rate of 300 mL / h. Subsequently, 25% by mass aqueous ammonia was added at a flow rate of 30 mL / h. In addition, 25% by mass aqueous sodium hydroxide solution was continuously added to maintain the reaction solution in the beaker at pH 11.00 ± 0.05 (measured at 50°C). While controlling the pH of the reaction solution, a slurry concentrator was connected to continuously perform solid-liquid separation, and only the filtrate was discharged from the system to increase the slurry concentration. Approximately 25 hours after the start of the reaction, a high-density hydroxide with a median diameter of 5.4 μm and a tap density of 1.8 g / mL or higher was obtained. After the reaction was completed, the slurry was dehydrated and washed with water to obtain nickel-aluminum coprecipitated hydroxide.

[0092] <Iron hydroxide coating process> 955 g (solid weight) of the obtained nickel-aluminum coprecipitate hydroxide powder was placed in a 5 L beaker and diluted with water to a 20 wt% slurry. This slurry was then heated to 75°C. Furthermore, the slurry was adjusted with sulfuric acid aqueous solution or sodium hydroxide aqueous solution to a pH of 9.00 ± 0.05 (measured at 75°C). In addition, a certain amount of nitrogen gas purging was performed to prevent oxidation of iron. Next, while maintaining the temperature at 75°C and stirring the slurry, an aqueous solution of iron(II) sulfate, adjusted to 10% by weight (metal equivalent), was continuously added dropwise so that iron(II) hydroxide equivalent to 4.5 mol% iron would precipitate on the nickel hydroxide surface. At the same time, a 25% by mass aqueous solution of sodium hydroxide was added dropwise to maintain the pH at 9.00 ± 0.05 (measured at 75°C). This yielded a slurry of nickel-aluminum hydroxide coated with iron(II) hydroxide. The obtained slurry was then dehydrated, washed with alkali and water, dried at 110°C for 10 hours, and then passed through a 100-mesh sieve to obtain 1 kg of precursor in which the surface of nickel-aluminum hydroxide was coated with iron(II) hydroxide.

[0093] [Comparative Example 1] <Preparation of nickel-aluminum-iron coprecipitate oxide> A stirrer equipped with a 7cm diameter blade and a 5L sealed flat-bottom beaker fitted with an overflow tube with an inner diameter of 5mm were used to hold 3L of water. After heating to 50°C, 150mL of 25% ammonia solution was added, and sulfuric acid solution or sodium hydroxide solution was added to adjust the pH to 11.00 (measured at 50°C). The liquid temperature in the beaker was maintained at 50°C, and while stirring at a speed of 600 rpm, a solution of nickel sulfate, aluminum sulfate, and iron(II) sulfate, adjusted to a molar ratio (Ni:Al:Fe = 91.0:4.5:4.5) of 10% by weight in terms of metal, was continuously added at a flow rate of 300 mL / h. Subsequently, 25% by mass aqueous ammonia was added at a flow rate of 30 mL / h. In addition, 25% by mass aqueous sodium hydroxide solution was continuously added to maintain the reaction solution in the beaker at pH 11.00 ± 0.05 (measured at 50°C). While controlling the pH of the reaction solution, a slurry concentrator was connected to continuously perform solid-liquid separation, and only the filtrate was discharged from the system to increase the slurry concentration. Approximately 25 hours after the start of the reaction, a high-density hydroxide with a median diameter of 4.0 μm and a tap density of 1.57 g / mL or higher was obtained. After the reaction, the slurry was dehydrated and washed with water to obtain nickel-aluminum coprecipitated hydroxide. After the reaction, the slurry was dehydrated, washed with alkali and water, dried at 110°C for 10 hours, and then passed through a 100-mesh sieve to obtain nickel-aluminum-iron coprecipitated hydroxide.

[0094] [Comparative Example 2] <Preparation of nickel-aluminum coprecipitate hydroxide> A stirrer equipped with a 7cm diameter blade and a 5L sealed, flat-bottomed beaker fitted with an overflow tube with an inner diameter of 5mm were used to hold 3L of water. After heating to 50°C, 150mL of 25% by mass ammonia solution was added, and sulfuric acid solution or sodium hydroxide solution was added to adjust the pH to 11.10 (measured at 50°C). The liquid temperature in the beaker was maintained at 50°C, and while stirring at a speed of 600 rpm, a solution of nickel sulfate and aluminum sulfate, adjusted to a molar ratio (Ni:Al = 95.3:4.7) of 10% by weight in terms of metal, was continuously added at a flow rate of 300 mL / h. Subsequently, 25% by mass aqueous ammonia was added at a flow rate of 30 mL / h. In addition, 25% by mass aqueous sodium hydroxide solution was continuously added to maintain the reaction solution in the beaker at pH 11.10 ± 0.05 (measured at 50°C). Approximately 30 hours after the start of the reaction, when the slurry concentration reaches a steady state, a high-density hydroxide with a tap density of 1.8 g / mL is obtained. The reaction continues while controlling the pH of the reaction solution, and the slurry discharged from the overflow pipe between 30 and 50 hours after the start of the reaction is dehydrated and washed with water to obtain nickel-aluminum coprecipitated hydroxide.

[0095] <Iron hydroxide coating process> 955 g (solid weight) of the obtained nickel-aluminum coprecipitate hydroxide powder was placed in a 5 L beaker and diluted with water to a 20 wt% slurry solution. This solution was then heated to 75°C. Furthermore, the slurry solution was adjusted with sulfuric acid aqueous solution or sodium hydroxide aqueous solution to a pH of 11.00 ± 0.05 (measured at 75°C). Next, while maintaining the temperature at 75°C and stirring the slurry, an aqueous solution of iron(II) sulfate, adjusted to 10% by weight (metal equivalent), was continuously added dropwise so that iron(II) hydroxide equivalent to 4.5 mol% iron would precipitate on the nickel hydroxide surface. At the same time, a 25% by mass aqueous solution of sodium hydroxide was added dropwise to maintain the pH at 11.00 ± 0.05 (measured at 75°C). This yielded a slurry of nickel-aluminum hydroxide coated with iron hydroxide. The obtained slurry was then dehydrated, washed with alkali and water, dried at 110°C for 10 hours, and then passed through a 100-mesh sieve to obtain 1 kg of precursor in which the surface of nickel-aluminum hydroxide was coated with iron hydroxide.

[0096] (analysis) Table 1 shows the results of chemical composition analysis of the precursors obtained in Examples 1-2 and Comparative Examples 1-2 using an ICP emission spectrometer (product name: Agilent 5110 VDV, manufactured by Agilent Technologies).

[0097] The precursors obtained in Examples 1-2 and Comparative Examples 1-2 were observed using a scanning electron microscope (SEM). Figure 2 shows the SEM image of the precursor obtained in Example 1. As shown in Figure 2, it was confirmed that the precursor of Example 1 was coated with iron hydroxide on its surface. This means that the precursor of Example 1 has a core-shell structure.

[0098] Cross-sections of the precursors obtained in Examples 1-2 and Comparative Examples 1-2 were observed using scanning electron microscopy energy-dispersive X-ray fluorescence spectroscopy (SEM-EDX). Figure 3 shows the SEM-EDX image of the cross-section of the precursor obtained in Example 1. In the SEM-EDX image, the white areas indicate the presence of a large amount of iron. As shown in Figure 3, it was confirmed that the precursor of Example 1 was coated with iron hydroxide on its surface. This means that the precursor of Example 1 has a core-shell structure. From the SEM and SEM-EDX images of the precursors obtained in Examples 1-2 and Comparative Examples 1-2, it was confirmed that the presence or absence of a core-shell structure is as shown in Table 1.

[0099] The average particle size, tap density, and specific surface area of ​​the precursors obtained in Examples 1-2 and Comparative Examples 1-2 were measured under the measurement conditions shown below. The results are shown in Table 1.

[0100] <Average particle size> Model name: Particle size distribution analyzer SALD-2200 Manufacturer: Shimadzu Corporation Analysis method: Laser diffraction (wet method) Refractive index: 1.70 + 0.20i Dispersion medium: water Dispersion method: Addition of surfactant, ultrasonic irradiation

[0101] <Tap density> Model name: Tapdenser KYT-3000 Manufacturer: Seishin Corporation Cell capacity of the measuring container: 100 mL Number of taps: 200 Calculation method: Tap density (g / mL) = Sample weight (g) / Filling volume after tapping (mL)

[0102] <Specific surface area> Model name: Specific surface area analyzer NOVA-2200e Manufacturer: Quantachrome Instruments Analysis method: BET multi-point method Adsorbed gas type: Nitrogen gas Relative pressure: 0.1, 0.2, 0.3 Measurement temperature: 77K (using liquid nitrogen)

[0103] [Table 1]

[0104] The X-ray diffraction (XRD) patterns of the precursors obtained in Examples 1-2 and Comparative Examples 1-2 were measured using a powder X-ray diffractometer (product name: SmartLab, manufactured by Rigaku). Cu (copper) was used as the target for electron beam irradiation, and Kα rays were used as the characteristic X-rays. The results are shown in Figures 4 to 7. As shown in Figures 4 to 7, it was confirmed that the precursors obtained in Examples 1 and 2 and Comparative Examples 1 and 2 have the same crystal structure as beta-type nickel hydroxide.

[0105] <Fabrication of positive electrode active material> [Example 3] 1.38 g of the precursor obtained in Example 1 and 0.65 g of the lithium compound (LiOH·H2O) were dispersed in ethanol in a mortar and mixed. The mixture was then packed into a platinum crucible conforming to JIS standards. Using a firing furnace, the mixture packed in the platinum crucible was heated in air at a heating rate of 10°C / min and fired at 750°C for 10 hours. After that, the resulting powder was left to cool to room temperature (25°C) to obtain the positive electrode active material of Example 3, as shown in the chemical composition in Table 2.

[0106] [Example 4] Except for using the precursor obtained in Example 2, the cathode active material of Example 4, with the chemical composition shown in Table 2, was obtained in the same manner as in Example 3.

[0107] [Comparative Example 3] The positive electrode active material of Comparative Example 3, shown in the chemical composition of Table 2, was obtained in the same manner as in Example 3, except that the precursor obtained in Comparative Example 1 was used.

[0108] [Comparative Example 4] The positive electrode active material of Comparative Example 4, with the chemical composition shown in Table 2, was obtained in the same manner as in Example 3, except that the precursor obtained in Comparative Example 2 was used.

[0109] The X-ray diffraction (XRD) patterns of the positive electrode active materials obtained in Examples 3-4 and Comparative Examples 3-4 were measured using a powder X-ray diffractometer (product name: SmartLab, manufactured by Rigaku). Cu (copper) was used as the target for electron beam irradiation, and Kα rays were used as the characteristic X-rays. The lattice constants were determined by the least squares method using the indices and interplanar spacings of the obtained XRD patterns. The powder X-ray diffraction patterns for each example are shown in Figures 8-11. The lattice constant values ​​are shown in Table 2.

[0110] As shown in Figures 8 to 11, it was confirmed that the positive electrode active materials obtained in Examples 3 to 4 and Comparative Examples 3 to 4 have the same crystal structure as rhombohedral layered compounds.

[0111] X-ray photoelectron spectroscopy (XPS) analyzer (product name: K-Alpha + Table 2 shows the results of quantitative analysis of the surface composition of the positive electrode active materials obtained in Examples 3-4 and Comparative Examples 3-4, performed using Thermo Fisher Scientific. The measurement conditions for XPS are shown below. XPS measurement conditions Model used: Thermo Fisher Scientific, K-Alpha + (Product name) Irradiation X-rays: Single-crystal spectroscopy AlKα (12 keV, 72 W) X-ray spot diameter: 400 μm Neutralizing electron gun: Use Reference spectrum: CC, CH 284.6eV Detection depth: 6-7nm

[0112] The integrated intensity ratio (I1 / I2) was determined from the XRD patterns of the positive electrode active materials obtained in Examples 3-4 and Comparative Examples 3-4. The results are shown in Table 2.

[0113] [Table 2]

[0114] <Manufacturing of lithium-ion secondary batteries> The positive electrode active materials obtained in Examples 3-4 and Comparative Examples 3-4 were mixed with acetylene black (AB) as a conductive agent and polyvinylidene fluoride (PVDF) as a binder in a weight ratio of 8:1:1 using NMP (N-methyl-2-pyrrolidone) as the solvent to prepare a slurry. This slurry was then coated onto 15 μm thick aluminum foil and dried to produce a 14φ positive electrode. The coating area density was 4.5 mg / cm². 2 The volume density is 2.3 g / cm³. 3 The positive electrode was constructed using a lithium metal counter electrode with a thickness of 200 μm and a diameter of 16φ, and a polyethylene microporous membrane with a thickness of 20 μm and a diameter of 18φ as a separator. The electrolyte was a 1.2 mol / L solution of lithium hexafluoride phosphate (LiPF6) dissolved in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (volume ratio 3:4:3). A lithium-ion secondary battery (2032 coin-type cell) with the structure shown in Figure 1 was fabricated. The battery was fabricated according to known cell configuration and assembly methods.

[0115] <Charge / Discharge Test> For each lithium-ion secondary battery fabricated, charge-discharge tests were performed at a constant current of 0.05C or 5C (1C: 250 mA / g) at a temperature of 25°C, with a cutoff potential of 4.3V to 2.5V, to evaluate the initial discharge capacity. The charge-discharge tests started with charging. The results are shown in Table 2.

[0116] The discharge curves of lithium-ion secondary batteries for each example at 5C are shown in Figures 12 to 15. From the discharge curves in Figures 12 to 15, it was confirmed that the lithium-ion secondary batteries of Examples 3 and 4 showed improved discharge capacity and average discharge voltage compared to the lithium-ion secondary batteries of Comparative Examples 3 and 4.

[0117] From the above results, it was found that the present invention can provide a lithium-ion secondary battery with a higher discharge capacity. [Explanation of symbols]

[0118] 1…Lithium-ion rechargeable battery 2...Positive electrode 3...Negative electrode 4... Separator 5…Insulating packing (gasket) 10…Positive electrode can 20... Negative electrode can (negative electrode terminal)

Claims

1. A precursor of a positive electrode active material for lithium-ion secondary batteries containing a metal composite hydroxide, The aforementioned metal composite hydroxide is in the form of particles having a core-shell structure with a surface coated with an iron compound. The aforementioned core-shell structure is composed of a core part and a shell part. The core portion contains nickel and aluminum as metallic elements. The aforementioned shell portion contains iron as a metallic element. The composition of the core portion is Ni （１－ｘ） Al ｘ (OH) ２ When expressed as, 0.01 ≤ x ≤ 0.2 And, Molar amount M of metal elements in the core portion １ and the molar amount M of the metal element in the shell portion ２ The molar ratio (M ２ / M １ )but, M ２ / M １ =1 / 100~1 / 5 This is a precursor of positive electrode active material for lithium-ion secondary batteries.

2. The positive electrode active material precursor for lithium-ion secondary batteries according to claim 1, wherein the core portion does not contain the same crystal structure as alpha-type nickel hydroxide.

3. The positive electrode active material precursor for lithium-ion secondary batteries according to claim 1, wherein the shell portion contains one or more iron compounds selected from iron(II) hydroxide, iron(III) hydroxide, iron oxyhydroxide, and triiron tetroxide.

4. A cathode active material precursor for a lithium-ion secondary battery according to claim 1, wherein the average particle size is 1 to 25 μm and the tap density is 1.30 g / mL or more.

5. Specific surface area of ​​2 to 50 m ２ A precursor of a positive electrode active material for a lithium-ion secondary battery according to claim 1, wherein the value is / g.

6. A method for producing a positive electrode active material precursor for a lithium-ion secondary battery according to any one of claims 1 to 5, A method for producing a positive electrode active material precursor for lithium-ion secondary batteries, comprising the step of coating a hydroxide containing nickel and aluminum with an iron compound under an inert gas atmosphere.

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