JP7547764B2 - Lithium nickel manganese cobalt composite oxide, lithium ion secondary battery, and manufacturing method thereof - Google Patents

Description

The present invention relates to secondary particles formed by agglomeration of primary particles containing lithium, nickel, manganese, and cobalt, or a lithium nickel manganese cobalt composite oxide composed of the primary particles and the secondary particles, a lithium ion secondary battery, and a method for manufacturing the same . This application claims priority based on Japanese Patent Application No. 2019-077519, filed on April 16, 2019, and is incorporated herein by reference.

In recent years, with the widespread use of portable electronic devices such as smartphones, tablet devices, and laptop computers, there has been a growing need to develop high-power secondary batteries, including small, lightweight non-aqueous electrolyte secondary batteries with high energy density, as well as batteries for hybrid and electric vehicles.

Lithium-ion secondary batteries are an example of a secondary battery that can meet such needs. Lithium-ion secondary batteries are composed of a positive electrode, a negative electrode, and an electrolyte, and the active materials used for the positive and negative electrodes are materials that can extract and insert lithium. Lithium-ion secondary batteries are still being actively researched and developed, and among these, lithium-ion secondary batteries that use layered or spinel-type lithium metal composite oxides as the positive electrode active material can obtain a high voltage of about 4V, and are therefore being put into practical use as batteries with high energy density.

Among these, lithium nickel manganese cobalt composite oxide has attracted attention as a material that has good cycle characteristics of battery capacity and can provide high output with low resistance. In recent years, it is also suitable as a power source for electric vehicles and hybrid vehicles, which are constrained by mounting space, and is considered important as an on-board power source. Generally, lithium nickel manganese cobalt composite oxide is produced by mixing the precursor nickel manganese cobalt composite hydroxide with a lithium compound and baking it.

This nickel manganese cobalt composite hydroxide contains impurities such as sulfate radicals, chlorine radicals, and sodium derived from the raw materials and chemicals used in the manufacturing process. These impurities induce side reactions and worsen the reaction with lithium during the process of mixing the nickel manganese cobalt composite hydroxide with a lithium compound and firing it, thereby reducing the crystallinity of the lithium nickel manganese cobalt composite oxide, which has a layered structure.

When lithium nickel manganese cobalt composite oxide, which has low crystallinity due to the influence of impurities, is used as a positive electrode active material to construct a battery, it inhibits the diffusion of lithium in the solid phase, resulting in a decrease in battery capacity. In addition, since these impurities hardly contribute to the charge/discharge reaction, it is necessary to use extra negative electrode material in the battery to cover the irreversible capacity of the positive electrode material. As a result, the capacity per weight or volume of the entire battery becomes smaller, and excess lithium accumulates in the negative electrode as irreversible capacity, which poses a safety problem.

Furthermore, when sodium, potassium, calcium, magnesium, and other elements dissolve in the lithium sites, the particles of the lithium nickel manganese cobalt composite oxide tend to sinter and aggregate, and the reactivity of lithium ion secondary batteries made using this deteriorates, resulting in reduced output characteristics and battery capacity.

Impurities include sulfate radicals, chlorine radicals, and sodium, and technologies for removing these impurities have been disclosed.

For example, Patent Document 1 discloses a method for reducing sulfate and chlorine radicals by performing a crystallization process to obtain a niobium-containing transition metal composite hydroxide and washing the obtained niobium-containing transition metal composite hydroxide with an aqueous solution of a carbonate such as potassium carbonate, sodium carbonate, or ammonium carbonate.

Patent Document 2 also discloses that in the process of producing nickel manganese cobalt composite hydroxide from a crystallization reaction, the alkaline solution used for adjusting the pH is a mixed solution of an alkali metal hydroxide and a carbonate, thereby reducing the impurities of sulfate, chlorine, and carbonate.

Patent Documents 3 and 4 disclose that nickel-manganese composite hydroxide particles or nickel composite hydroxide particles having a void structure inside the particles obtained in the crystallization process are washed with an aqueous solution of a carbonate such as potassium carbonate, sodium carbonate, potassium hydrogen carbonate, or sodium hydrogen carbonate, thereby reducing the sulfate radicals, chlorine radicals, and sodium.

Patent Document 5 also discloses that an aqueous solution or dispersion containing nickel-cobalt-M element obtained by mixing a nickel ammine complex, a cobalt ammine complex, and an M element source is heated to thermally decompose the nickel ammine complex and the cobalt ammine complex, and a nickel-cobalt-M element-containing composite compound having a low content of impurities such as sulfate radicals, chlorine radicals, sodium, and iron is used.

JP 2015-122269 A JP 2016-117625 A International Publication No. 2015/146598 JP 2015-191848 A International Publication No. 2012/020768

However, Patent Documents 1 and 2 make no mention of sodium removal. In Patent Documents 3 and 4, even in the precursors with a low solid level of 2-4% porosity, sodium still remains at 0.001-0.015 mass%, and sodium reduction is insufficient. Furthermore, in Patent Document 5, a nickel-cobalt-M element-containing composite compound is obtained by thermal decomposition, so it is questionable whether it will provide sufficient battery characteristics when used as a positive electrode active material in terms of the spherical shape of the particles, particle size distribution, and specific surface area. In addition to removing impurities, it is expected that the porosity will be increased to further improve battery characteristics and suppress sintering agglomeration.

Therefore, an object of the present invention is to provide a lithium nickel manganese cobalt composite oxide, which is a positive electrode active material that reliably reduces the content of impurities, particularly sodium, which hardly contributes to charge/discharge reactions, thereby suppressing sintering agglomeration and increasing the porosity, thereby enabling further improvement of battery characteristics, and a lithium ion secondary battery and a method for manufacturing the same .

A lithium nickel manganese cobalt composite oxide according to one aspect of the present invention is a lithium nickel manganese cobalt composite oxide composed of secondary particles in which primary particles containing lithium, nickel, manganese, and cobalt are aggregated, or the primary particles and the secondary particles, characterized in that the lithium nickel manganese cobalt composite oxide has a sodium content of less than 0.0005 mass%, the lithium nickel manganese cobalt composite oxide has a porosity of 20 to 50%, a ratio of an average particle size of the lithium nickel manganese cobalt composite oxide divided by an average particle size of a precursor nickel manganese cobalt composite hydroxide is 0.95 to 1.05, and an Me site occupancy rate of 93.0% or more . A method for producing a lithium nickel manganese cobalt composite oxide according to one aspect of the present invention is a method for producing secondary particles formed by agglomeration of primary particles containing lithium, nickel, manganese, and cobalt, or a lithium nickel manganese cobalt composite oxide constituted by the primary particles and the secondary particles, the method comprising: a crystallization step of obtaining a transition metal composite hydroxide by crystallization in a reaction solution obtained by adding a raw material solution containing nickel, manganese, and cobalt, a solution containing an ammonium ion donor, and an alkaline solution; a washing step of washing the transition metal composite hydroxide obtained in the crystallization step with a washing liquid; a heat treatment step of heat-treating the nickel manganese cobalt composite hydroxide obtained in the washing step; a mixing step of mixing the metal composite oxide obtained in the heat treatment step with a lithium compound; and a calcination step of calcining the lithium mixture obtained in the mixing step, wherein the alkaline solution in the crystallization step is a mixed solution of an alkali metal hydroxide and a carbonate, and the ratio of the carbonate to the alkali metal hydroxide in the mixed solution is [CO ₃ ²⁻ ]/[OH ⁻ the crystallization step switches the atmosphere between an oxidizing atmosphere and a non-oxidizing atmosphere in two stages to perform crystallization; the washing liquid in the washing step is an ammonium bicarbonate solution having a concentration of 0.05 mol/L or more; the sodium content in the lithium nickel manganese cobalt composite oxide obtained in the baking step is less than 0.0005 mass %; the porosity of the lithium nickel manganese cobalt composite oxide is 20 to 50%; and a ratio of an average particle size of the lithium nickel manganese cobalt composite oxide divided by an average particle size of the nickel manganese cobalt composite hydroxide, which is a precursor, is 0.95 to 1.05.

In this way, it is possible to provide a lithium nickel manganese cobalt composite oxide, which is a positive electrode active material for lithium ion secondary batteries that reliably reduces the sodium content, suppresses sintering agglomeration, increases the porosity, has high packing properties, and allows for high capacity.

In this case, in one embodiment of the present invention, when 100 or more randomly selected particles of the lithium nickel manganese cobalt composite oxide are observed with a scanning electron microscope, the number of particles in which secondary particle agglomerations are observed may be 5% or less of the total number of secondary particles observed.

In this way, it is possible to provide a lithium nickel manganese cobalt composite oxide, which is a positive electrode active material for lithium ion secondary batteries that suppresses sintering agglomeration, has high packing properties, and allows for high capacity.

In this case, in one embodiment of the present invention, the lithium nickel manganese cobalt composite oxide may have a sulfate group content of 0.15 mass % or less and a chlorine group content of 0.005 mass % or less .

In this way, it is possible to reliably reduce the content of sulfate radicals, chlorine radicals, and sodium, and provide a lithium nickel manganese cobalt composite oxide, which is a positive electrode active material for lithium ion secondary batteries that can achieve high capacity.

In this case, in one embodiment of the present invention, the content of at least one of potassium, calcium, and magnesium contained in the lithium nickel manganese cobalt composite oxide may be less than 0.0005 mass%.

In this way, it is possible to provide a lithium nickel manganese cobalt composite oxide, which is a positive electrode active material for lithium ion secondary batteries that can reduce the impurity content and increase capacity.

In this case, in another aspect of the present invention, a lithium ion secondary battery may be provided, which is characterized by having at least a positive electrode containing the lithium nickel manganese cobalt composite oxide. Also, in another aspect of the present invention, a method for producing a lithium ion secondary battery may be provided, which is characterized by obtaining a positive electrode using at least the positive electrode active material for a lithium ion secondary battery produced by the method for producing a lithium nickel manganese cobalt composite oxide, and obtaining a lithium ion secondary battery using the positive electrode, a negative electrode, and a non-aqueous electrolyte.

In this way, it is possible to provide a lithium-ion secondary battery equipped with a positive electrode containing a lithium nickel manganese cobalt composite oxide that reliably reduces the sodium content, suppresses sintering agglomeration, increases the porosity, has high packing properties, and allows for a high capacity.

According to the present invention, it is possible to provide a lithium nickel manganese cobalt composite oxide, which is a positive electrode active material capable of reliably reducing the sodium content, preventing sintering aggregation, and increasing the porosity, thereby enabling further improvement in battery characteristics, and a lithium ion secondary battery and methods for producing the same .

FIG. 1 is a cross-sectional SEM photograph of a nickel manganese cobalt composite hydroxide according to one embodiment of the present invention, showing that the internal structure is a hollow structure. FIG. 2 is a process diagram showing an outline of a method for producing a nickel manganese cobalt composite hydroxide according to one embodiment of the present invention.

The inventors have conducted extensive research to solve the above problems, and have discovered that in the production of nickel-manganese-cobalt composite hydroxide, the reaction atmosphere in the crystallization process can be controlled, the alkaline solution used in the crystallization process is a mixed solution of an alkali metal hydroxide and a carbonate, and the transition metal composite hydroxide obtained in the crystallization process is washed in the washing process with an ammonium hydrogen carbonate solution, which is a washing solution containing hydrogen carbonate (bicarbonate), thereby more efficiently reducing the impurities, sulfate radicals, chlorine radicals, and sodium, and thus completing the present invention.

The present invention was completed based on the finding that, by using the nickel manganese cobalt composite hydroxide, in which the sodium content has been reliably reduced as described above, as a precursor, it is possible to obtain a lithium nickel manganese cobalt composite oxide, which is a positive electrode active material for lithium ion secondary batteries that suppresses sintering aggregation, has high packing properties, and allows for high capacity. The following describes preferred embodiments of the present invention.

The present embodiment described below does not unduly limit the contents of the present invention described in the claims, and can be modified within the scope of the present invention. In addition, not all of the configurations described in the present embodiment are necessarily essential as a means for solving the problems of the present invention. The nickel manganese cobalt composite hydroxide, the method for producing the nickel manganese cobalt composite hydroxide, the lithium nickel manganese cobalt composite oxide, and the lithium ion secondary battery according to one embodiment of the present invention will be described in the following order.
1. Nickel manganese cobalt composite hydroxide 2. Lithium nickel manganese cobalt composite oxide 3. Method for producing nickel manganese cobalt composite hydroxide 3-1. Crystallization process 3-1-1. Nucleation process 3-1-2. Particle growth process 3-2. Washing process 4. Lithium ion secondary battery

<1. Nickel manganese cobalt composite hydroxide>
A nickel manganese cobalt composite hydroxide according to one embodiment of the present invention is a precursor of a positive electrode active material, which is composed of secondary particles in which primary particles containing nickel, manganese, and cobalt are aggregated, or the primary particles and the secondary particles.

The nickel manganese cobalt composite hydroxide has a sodium content of less than 0.0005% by mass, and the porosity of the particles of the nickel manganese cobalt composite hydroxide is 20 to 50%. The nickel manganese cobalt composite hydroxide according to one embodiment of the present invention will be specifically described below.

[Particle composition]
The nickel manganese cobalt composite hydroxide is preferably adjusted so that its composition is expressed by the general formula: Ni _x Mn _y Co _z M _t (OH) _2+a (wherein x + y + z + t = 1, 0.20 ≤ x ≤ 0.80, 0.10 ≤ y ≤ 0.90, 0.10 ≤ z ≤ 0.50, 0 ≤ t ≤ 0.10, 0 ≤ a ≤ 0.5, and M is one or more of Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, and W).

In the above general formula, x, which indicates the nickel content, is preferably 0.20≦x≦0.80. In addition, taking into consideration electrical properties and thermal stability, x, which indicates the nickel content, is more preferably x≦0.6.

In addition, y, which indicates the manganese content, is preferably 0.10≦y≦0.90. Adding manganese within this range can further improve the durability and safety of a battery when used as a positive electrode active material. If y is less than 0.10, the effect of improving the durability and safety of the battery cannot be fully obtained, while if y exceeds 0.9, the metal elements that contribute to the Redox reaction are reduced, which may reduce the battery capacity, which is not preferable.

In addition, in the above general formula, z, which indicates the cobalt content, is preferably 0.10≦z≦0.50. By adding an appropriate amount of cobalt, it is possible to improve cycle characteristics and reduce the expansion and contraction behavior of the crystal lattice due to the desorption and insertion of Li during charging and discharging. However, if z is less than 0.10, it is difficult to obtain a sufficient effect of reducing the expansion and contraction behavior of the crystal lattice, which is not preferable. On the other hand, if too much cobalt is added and z exceeds 0.5, the initial discharge capacity may decrease significantly, and there is also a problem of being disadvantageous in terms of cost, which is also not preferable.

The additive element M is one or more of Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, and W, and is added to improve battery characteristics such as cycle characteristics and output characteristics. The content of the additive element M, t, is preferably 0≦t≦0.10. If t exceeds 0.1, the amount of metal elements that contribute to the Redox reaction decreases, which may result in a decrease in battery capacity, and this is not preferable.

Other analytical methods for determining the composition of particles are not particularly limited, but can be determined, for example, by chemical analysis methods such as acid decomposition-ICP emission spectrometry.

[Particle structure]
The nickel manganese cobalt composite hydroxide according to one embodiment of the present invention is composed of secondary particles formed by agglomeration of a plurality of primary particles, or both the primary particles and the secondary particles. The primary particles constituting the secondary particles may have various shapes such as plate-like, needle-like, rectangular, elliptical, and rhombohedral. In addition, the present invention can be applied to the case where the agglomeration state of a plurality of primary particles is not only agglomerated in random directions, but also agglomerated in the major axis direction radially from the center.

As for the agglomeration state, it is preferable that plate-like or needle-like primary particles are agglomerated in random directions to form secondary particles. In the case of such a structure, almost uniform voids are generated between the primary particles, and when mixed with a lithium compound and fired, the molten lithium compound spreads throughout the secondary particles, allowing sufficient diffusion of lithium.

The method for observing the shapes of the primary particles and secondary particles is not particularly limited, but they can be measured by observing the cross section of the nickel manganese cobalt composite hydroxide using a scanning electron microscope (SEM) or the like.

[Particle internal structure]
The nickel manganese cobalt composite hydroxide according to one embodiment of the present invention has a hollow structure inside the secondary particles. When a lithium metal composite oxide having this hollow structure and using the nickel manganese cobalt composite hydroxide as a precursor is used as a positive electrode active material, the contact area with the electrolyte increases, resulting in a lithium ion secondary battery with excellent output characteristics.

Incidentally, "having a hollow structure inside the secondary particles" refers to a structure consisting of a void portion that is the space at the center of the secondary particle and a dense portion on the outside. As shown in Figure 1, this hollow structure can be confirmed by observing the cross section of nickel manganese cobalt composite hydroxide as well as lithium metal composite oxide with a scanning electron microscope, as shown in the cross-sectional SEM image.

In addition, nickel manganese cobalt composite hydroxides having a hollow structure preferably have a porosity of 10 to 90%, more preferably 20 to 50%, measured by observing the cross section of the particles. In addition, lithium metal composite oxides having a hollow structure preferably have a porosity of 10 to 90%, more preferably 20 to 50%, measured by observing the cross section of the particles. This makes it possible to obtain a sufficient contact area between the positive electrode active material and the electrolyte while maintaining the particle strength within an acceptable range without excessively reducing the bulk density of the resulting positive electrode active material, thereby enabling improved battery characteristics.

In the present invention, the nickel manganese cobalt composite hydroxide and lithium metal composite oxide are made to have a hollow structure. For example, by adjusting the crystallization conditions during the manufacturing process of the metal composite hydroxide that serves as the source of transition metals such as nickel, manganese, and cobalt during mixing and firing, it is possible to mix solid, hollow, and porous structures in various combinations and ratios, and the obtained metal composite oxide has the advantage of being able to stabilize the overall composition and particle size compared to a simple mixture of solid, hollow, and porous products.

[Average particle size (MV)]
The nickel manganese cobalt composite hydroxide is preferably adjusted to have an average particle size of 3 to 20 μm. If the average particle size is less than 3 μm, the particle packing density decreases when the positive electrode is formed. On the other hand, if the average particle size exceeds 20 μm, the specific surface area of the positive electrode active material decreases, and the interface with the electrolyte of the battery decreases. This is not preferable because it may increase the resistance of the positive electrode and reduce the output characteristics of the battery. Therefore, the nickel manganese cobalt composite hydroxide has an average particle size of 3 to 20 μm, preferably 3 to 15 μm. If the thickness is adjusted to 4 to 12 μm, it is possible to increase the battery capacity per volume in a lithium ion secondary battery using this positive electrode active material as a positive electrode material, and the battery capacity per volume can be increased, and the safety and cycle life can be improved. This makes it possible to obtain a battery with excellent characteristics.

The method for measuring the average particle size is not particularly limited, but it can be determined, for example, from the volume-based distribution measured using a laser diffraction/scattering method.

[Impurity content]
In general, nickel manganese cobalt composite hydroxide contains impurities such as sulfate radicals, chlorine radicals, sodium, potassium, calcium, magnesium, etc. These impurities cause the reaction with lithium to deteriorate and hardly contribute to the charge/discharge reaction, so it is preferable to remove them as much as possible and reduce their content. In addition, when sodium or the like is dissolved in the lithium site, the particles of the lithium nickel manganese cobalt composite oxide tend to sinter and aggregate, so it is preferable to remove them as much as possible and reduce their content. Conventionally, techniques for removing these impurities have been disclosed, but these techniques are still insufficient.

The nickel manganese cobalt composite hydroxide according to one embodiment of the present invention is characterized in that the sodium content contained therein is less than 0.0005% by mass. In this way, it is possible to provide a nickel manganese cobalt composite hydroxide that is a precursor of a positive electrode active material for a lithium ion secondary battery that can reliably reduce the sodium content and improve battery characteristics.

As mentioned above, in the conventional technology, sodium remains at 0.001 to 0.015 mass%, which is insufficient for sodium reduction. In addition, although there are documents in the conventional technology that state that the sodium content is below a certain value, no composite hydroxide or composite oxide with an extremely low sodium content of less than 0.0005 mass%, such as the nickel manganese cobalt composite hydroxide according to one embodiment of the present invention or the lithium nickel manganese cobalt composite oxide described below, has actually been disclosed. By using the manufacturing method described below, it is possible to achieve an extremely low sodium content of less than 0.0005 mass%. By doing so, it is possible to suppress sintering aggregation when the lithium nickel manganese cobalt composite oxide is produced.

It is also preferable that the sulfate radical content in the nickel manganese cobalt composite hydroxide is 0.2 mass % or less, and the chlorine radical content is 0.01 mass % or less. In this way, it is possible to reliably reduce the sulfate radical, chlorine radical, and sodium content, and provide a nickel manganese cobalt composite hydroxide that is a precursor of a positive electrode active material for a lithium ion secondary battery that can improve battery characteristics.

It is preferable that the content of at least one of potassium, calcium, and magnesium contained in the nickel manganese cobalt composite hydroxide is less than 0.0005 mass %. In this way, it is possible to provide a nickel manganese cobalt composite hydroxide that is a precursor of a positive electrode active material for a lithium ion secondary battery, which can further improve the battery characteristics by further reducing the impurity content and increasing the porosity.

The content of each impurity can be determined, for example, by the following analytical method. In addition to sodium, potassium, calcium, magnesium, etc. can be determined by acid decomposition-atomic absorption spectrometry, acid decomposition-ICP emission spectrometry, etc. The sulfate radical can be determined by analyzing the total sulfur content of the nickel manganese cobalt composite hydroxide by combustion infrared absorption method, acid decomposition-ICP emission spectrometry, etc., and converting this total sulfur content into sulfate radical (SO ₄ ^2- ). The chlorine radical can be determined by analyzing the nickel manganese cobalt composite hydroxide directly, or by separating the chlorine radical contained in the nickel manganese cobalt composite hydroxide in the form of silver chloride or the like by distillation, and analyzing it by X-ray fluorescence (XRF) analysis.

[Particle size distribution]
The nickel manganese cobalt composite hydroxide is preferably adjusted so that the index showing the spread of the particle size distribution of the particles, [(d90-d10)/average particle size], is 0.55 or less.

If the particle size distribution is wide and the index showing the spread of the particle size distribution, [(d90-d10)/average particle size], exceeds 0.55, there will likely be many fine particles whose particle size is very small compared to the average particle size, and many particles whose particle size is very large compared to the average particle size (large diameter particles).

These characteristics of the particle size distribution at the precursor stage also have a significant effect on the positive electrode active material obtained after the firing process. If a positive electrode is formed using a positive electrode active material containing many fine particles, not only may there be a risk of heat generation due to local reactions of the fine particles, which may reduce safety, but also the fine particles with a large specific surface area may be selectively deteriorated, which may result in poor cycle characteristics, which is undesirable. On the other hand, if a positive electrode is formed using a positive electrode active material containing many large-diameter particles, there may not be enough reaction area between the electrolyte and the positive electrode active material, which may result in a decrease in battery output due to an increase in reaction resistance, which is undesirable.

Therefore, in the particle size distribution of the precursor nickel manganese cobalt composite hydroxide, it is preferable that [(d90-d10)/average particle size] is 0.55 or less, and since the proportion of fine particles and large particles is reduced when it is used as a positive electrode active material, a lithium ion secondary battery using this positive electrode active material in the positive electrode is safer and can obtain good cycle characteristics and battery output.

In the index showing the spread of particle size distribution [(d90-d10)/average particle size], d10 means the particle size at which the cumulative volume of the number of particles at each particle size, when accumulated from the smallest particle size, is 10% of the total volume of all particles. In contrast, d90 means the particle size at which the cumulative volume of the number of particles at each particle size, when accumulated from the smallest particle size, is 90% of the total volume of all particles. There are no particular limitations on the method for determining the average particle size, d90, and d10, but they can be determined, for example, from a volume-based distribution measured using a laser diffraction/scattering method.

[Specific surface area]
The nickel manganese cobalt composite hydroxide is preferably adjusted to have a specific surface area of 10 to 80 m ² /g, and if the particles are solid and have no voids, it is more preferably adjusted to 10 to 20 m ² /g in order to further stabilize the battery characteristics. If the specific surface area is within the above range, a sufficient particle surface area can be secured to be in contact with the molten lithium compound when mixed with the lithium compound and fired, and the particle strength when formed into a positive electrode active material can also be satisfied.

On the other hand, if the specific surface area is less than 10 ^m2 /g, when mixed with a lithium compound and fired, the contact with the molten lithium compound becomes insufficient, the crystallinity of the resulting lithium nickel manganese cobalt composite oxide decreases, and when used as a positive electrode material to form a lithium ion secondary battery, there is a concern that the diffusion of lithium in the solid phase is hindered and the battery capacity decreases. Also, if the specific surface area exceeds 80 ^m2 /g, when mixed with a lithium compound and fired, the crystal growth proceeds too much, causing cation mixing in which nickel is mixed into the lithium layer of the lithium transition metal composite oxide, which is a layered compound, and the charge/discharge capacity may decrease, which is not preferable.

Furthermore, in the case of a hollow structure having a space at the center of the secondary particle, such as the nickel manganese cobalt composite hydroxide according to one embodiment of the present invention, it is more preferable that the specific surface area is adjusted to 30 to 40 m ² /g. If the specific surface area is less than 30 m ² /g, it is difficult to obtain a predetermined porosity, and the reaction area with the lithium compound during firing cannot be sufficiently secured. If the specific surface area exceeds 40 m ² /g, the filling property may be deteriorated when a positive electrode active material having a hollow structure is obtained.

The method for measuring the specific surface area is not particularly limited, but it can be determined, for example, by the BET multipoint method, the BET single point method, or the nitrogen gas adsorption/desorption method.

Figure 1 shows a cross-sectional SEM photograph of a nickel manganese cobalt composite hydroxide according to one embodiment of the present invention. As shown in Figure 1, the nickel manganese cobalt composite hydroxide according to one embodiment of the present invention has a hollow internal structure.

The nickel manganese cobalt composite hydroxide according to one embodiment of the present invention can provide a precursor for a positive electrode active material for a lithium ion secondary battery that can reliably reduce the sodium content and improve the battery characteristics. In addition, by using a nickel manganese cobalt composite hydroxide with a reliably reduced sodium content as described above as a precursor, a lithium nickel manganese cobalt composite oxide can be obtained that is a positive electrode active material for a lithium ion secondary battery that is suppressed from sintering and has high packing properties and can be made to have a high capacity.

<2. Lithium Nickel Manganese Cobalt Composite Oxide>
A lithium nickel manganese cobalt composite oxide according to one embodiment of the present invention is composed of secondary particles formed by agglomeration of primary particles containing lithium, nickel, manganese, and cobalt, or the primary particles and the secondary particles, and is characterized in that the lithium nickel manganese cobalt composite oxide has a sodium content of less than 0.0005 mass % and a porosity of 20 to 50%.

It is also preferable that the content of sulfate radicals contained in the lithium nickel manganese cobalt composite oxide is 0.15 mass% or less, the content of chlorine radicals is 0.005 mass% or less, and the Me site occupancy is 93.0% or more.

The ratio of the average particle size of the lithium nickel manganese cobalt composite oxide to the average particle size of the precursor nickel manganese cobalt composite hydroxide, i.e., "MV of lithium nickel manganese cobalt composite oxide/MV of nickel manganese cobalt composite hydroxide" (hereinafter also referred to as "MV ratio"), can be evaluated as an index showing sintering agglomeration. The range of the MV ratio is 0.95 to 1.05, and preferably 0.97 to 1.03.

When the MV ratio is within the above range, the positive electrode active material is composed of a lithium nickel manganese cobalt composite oxide in which the secondary particles are hardly aggregated together due to sintering. A secondary battery using such a positive electrode active material has high packing properties, high capacity, and excellent uniformity in characteristics with little variation.

On the other hand, if the MV ratio exceeds 1.05, the specific surface area and packing property may decrease due to sintering aggregation. A secondary battery using such a positive electrode active material may have a decrease in output characteristics and battery capacity due to a deterioration in reactivity. Furthermore, when the positive electrode is repeatedly charged and discharged, selective collapse may occur from the weak parts where the secondary particles are aggregated together in the positive electrode, which may significantly impair the cycle characteristics. Therefore, to be safe, the MV ratio is 1.05 or less, and preferably 1.03 or less.

Furthermore, if the MV ratio is less than 0.95, it is believed that part of the primary particles is missing from the secondary particles during the manufacturing process of the lithium nickel manganese cobalt composite oxide, resulting in a decrease in particle size, which may result in a broader particle size distribution. Therefore, the MV ratio is 0.95 or more, and preferably 0.97 or more.

The MV of the nickel manganese cobalt composite hydroxide refers to the MV of the nickel manganese cobalt composite hydroxide used as a precursor when producing the lithium nickel manganese cobalt composite oxide. In addition, the MV of the lithium nickel manganese cobalt composite oxide refers to the MV of the lithium nickel manganese cobalt composite oxide after the crushing process, if a crushing process is performed. The MV of each particle can be measured using a laser diffraction/scattering particle size analyzer, and refers to the particle size at which the number of particles at each particle size is accumulated from the smallest particle size side, and the accumulated volume is the average value of the total volume of all particles.

When 100 or more randomly selected particles of lithium nickel manganese cobalt composite oxide are observed with a scanning electron microscope (SEM), the number of particles in which secondary particle aggregation is observed may be 5% or less, 3% or less, or 2% or less of the total number of secondary particles observed. When the number of particles in which secondary particle aggregation is observed is within the above range, it indicates that sintering aggregation of secondary particles is sufficiently suppressed. When the MV of the positive electrode active material is within the above range, the number of particles in which secondary particle aggregation is observed can be easily set to the above range. The magnification when observing with a scanning electron microscope (SEM) is, for example, about 1000 times.

When the number of secondary particle aggregates observed is 5% or less of the total number of secondary particles observed, the positive electrode active material is composed of a lithium nickel manganese cobalt composite oxide in which there is almost no aggregation of secondary particles due to sintering aggregation. A secondary battery using such a positive electrode active material has high filling properties, high capacity, and excellent uniformity with little variation in characteristics.

On the other hand, if the number of secondary particle aggregates observed exceeds 5% of the total number of secondary particles observed, the specific surface area and packing property may decrease due to sintering aggregation. A secondary battery using such a positive electrode active material may have a decrease in output characteristics and battery capacity due to a deterioration in reactivity. Furthermore, when repeatedly charged and discharged, selective collapse may occur in the weak parts of the positive electrode where the secondary particles are aggregated, which may significantly impair the cycle characteristics, so a safe estimate is preferably 5% or less.

It is preferable that the content of at least one of potassium, calcium, and magnesium contained in the lithium nickel manganese cobalt composite oxide is less than 0.0005 mass %. In this way, it is possible to provide a lithium nickel manganese cobalt composite oxide that is a positive electrode active material for lithium ion secondary batteries that can further reduce the impurity content and increase the capacity.

The nickel manganese cobalt composite hydroxide can be mixed with a lithium compound and fired to produce a lithium nickel manganese cobalt composite oxide. The lithium nickel manganese cobalt composite oxide can be used as a raw material for the positive electrode active material of lithium ion secondary batteries.

The lithium nickel manganese cobalt composite oxide used as the positive electrode active material is obtained by mixing the precursor nickel manganese cobalt composite hydroxide with lithium compounds such as _lithium carbonate ( _Li2CO3 : melting point 723°C), lithium hydroxide (LiOH: melting point 462°C), lithium nitrate ( _LiNO3 : melting point 261°C), lithium chloride (LiCl: melting point 613°C), and _lithium sulfate ( _Li2SO4 : melting point 859°C), and then subjecting the mixture to a firing process.

As for the lithium compound, it is particularly preferable to use lithium carbonate or lithium hydroxide, taking into consideration ease of handling and stability of quality.

In this firing process, the carbonate, hydroxyl, nitrate, chlorine, and sulfate groups, which are also components of the lithium compound, volatilize, but a small portion remains in the positive electrode active material. In addition to the non-volatile components such as sodium, the particle size distribution, specific surface area, and hollow structure of the secondary particles, the characteristics of the precursor nickel manganese cobalt composite hydroxide are almost identical.

The lithium nickel manganese cobalt composite oxide according to one embodiment of the present invention can provide a positive electrode active material for a lithium ion secondary battery that can reliably reduce the sodium content and increase the porosity, thereby further improving the battery characteristics.

<3. Method for producing nickel manganese cobalt composite hydroxide>
Next, a method for producing a nickel manganese cobalt composite hydroxide according to one embodiment of the present invention will be described with reference to Fig. 2. The method for producing a nickel manganese cobalt composite hydroxide according to one embodiment of the present invention is a method for producing a precursor of a positive electrode active material, which is composed of secondary particles formed by agglomeration of primary particles containing nickel, manganese, and cobalt, or the primary particles and the secondary particles. As shown in Fig. 2, the method includes a crystallization step S10 and a washing step S20.

In the crystallization step S10, a raw material solution containing nickel, manganese, and cobalt, a solution containing an ammonium ion donor, and an alkaline solution are added to obtain a reaction solution, and crystallization occurs to obtain a transition metal composite hydroxide. In the washing step S20, the transition metal composite hydroxide obtained in the crystallization step S10 is washed with a washing solution.

The alkaline solution in the crystallization step S10 is a mixed solution of an alkali metal hydroxide and a carbonate, and the molar ratio of the carbonate to the alkali metal hydroxide in the mixed solution, [CO ₃ ²⁻ ]/[OH ⁻ ], is 0.002 to 0.050, the crystallization step S10 switches the atmosphere between an oxidizing atmosphere and a non-oxidizing atmosphere in two stages to perform crystallization, and the washing solution in the washing step S20 is an ammonium bicarbonate solution having a concentration of 0.05 mol/L or more. Each step will be described in detail below.

<3-1. Crystallization process>
In the crystallization step S10, a raw material solution containing nickel, manganese, and cobalt, a solution containing an ammonium ion donor, and an alkaline solution are added to obtain a reaction solution, and crystallization is performed in the reaction solution to obtain a transition metal composite hydroxide. get.

The crystallization step S10 preferably further includes a nucleation step S11 and a particle growth step S12. In the nucleation step S11, an alkaline solution is added to generate nuclei in the reaction solution so that the pH measured at a liquid temperature of 25°C is 12.0 to 14.0, and in the particle growth step S12, an alkaline solution is preferably added to the reaction solution containing the nuclei formed in the nucleation step S11 so that the pH measured at a liquid temperature of 25°C is 10.5 to 12.0. Details will be described later.

In conventional continuous crystallization methods, the nucleation reaction and the nucleus growth reaction proceed simultaneously in the same reaction tank, resulting in a wide range of particle size distribution of the precursor. In contrast, the method for producing nickel manganese cobalt composite hydroxide in the present invention clearly separates the time during which the nucleation reaction mainly occurs (nucleation process) from the time during which the particle growth reaction mainly occurs (particle growth process), thereby obtaining a transition metal composite hydroxide with a narrow particle size distribution even if both processes are carried out in the same reaction tank. In addition, by using a mixed solution of an alkali metal hydroxide and a carbonate as the alkaline solution, impurities such as sulfate radicals can be reduced.

The materials and conditions used in the method for producing nickel manganese cobalt composite hydroxide of the present invention are described in detail below.

[Raw material solution containing nickel, manganese, and cobalt]
Metal salts such as nickel salts, manganese salts, and cobalt salts used in the raw material solution containing nickel, manganese, and cobalt are not particularly limited as long as they are water-soluble compounds, and sulfates, nitrates, chlorides, etc. can be used. For example, nickel sulfate, manganese sulfate, and cobalt sulfate are preferably used.

If necessary, compounds containing one or more additive elements M can be mixed in a predetermined ratio to prepare the raw material solution. In this case, in the crystallization step S10, it is preferable to use a compound containing one or more of Al, Ti, V, Cr, Zr, Nb, Mo, and W, and examples of compounds that can be used include titanium sulfate, ammonium peroxotitanate, potassium titanium oxalate, vanadium sulfate, ammonium vanadate, chromium sulfate, potassium chromate, zirconium sulfate, zirconium nitrate, niobium oxalate, ammonium molybdate, sodium tungstate, and ammonium tungstate.

The nickel manganese cobalt composite hydroxide obtained by crystallization may be mixed with an aqueous solution containing one or more additive elements M to form a slurry, and the pH may be adjusted to coat the nickel manganese cobalt composite hydroxide with a compound containing one or more additive elements M.

The concentration of the raw material solution is preferably 1.0 to 2.6 mol/L, more preferably 1.0 to 2.2 mol/L, in terms of the total metal salts. If the concentration is less than 1.0 mol/L, the resulting hydroxide slurry will have a low concentration and will be less productive. On the other hand, if the concentration exceeds 2.6 mol/L, crystal precipitation or freezing will occur at temperatures below -5°C, which may clog the piping of the equipment, and the piping will need to be kept warm or heated, which will be costly.

Furthermore, the amount of raw material solution supplied to the reaction tank is preferably such that the crystallized product concentration at the end of the crystallization reaction is approximately 30 to 250 g/L, and more preferably 80 to 150 g/L. If the crystallized product concentration is less than 30 g/L, the primary particles may not aggregate sufficiently, and if it exceeds 250 g/L, the raw material solution added may not diffuse sufficiently in the reaction tank, causing uneven particle growth.

[Ammonium ion donor]
The ammonium ion donor in the reaction solution is not particularly limited as long as it is a water-soluble compound, and examples of the donor include ammonia water, ammonium sulfate, ammonium chloride, ammonium carbonate, and ammonium fluoride. For example, it is preferable to use ammonia water or ammonium sulfate.

The ammonium ion concentration in the reaction solution is adjusted to preferably 3 to 25 g/L, more preferably 5 to 20 g/L, and even more preferably 5 to 15 g/L. The presence of ammonium ions in the reaction solution causes metal ions, particularly nickel ions, to form ammine complexes, increasing the solubility of the metal ions and promoting the growth of primary particles, making it easier to obtain dense nickel manganese cobalt composite hydroxide particles. Furthermore, because the solubility of the metal ions is stable, nickel manganese cobalt composite hydroxide particles with a uniform shape and particle size are easily obtained. And, by setting the ammonium ion concentration in the reaction solution to 3 to 25 g/L, it is easier to obtain composite hydroxide particles that are denser and have a uniform shape and particle size.

If the ammonium ion concentration in the reaction solution is less than 3 g/L, the solubility of the metal ions may become unstable, and primary particles with uniform shape and particle size may not be formed, and gel-like nuclei may be generated, resulting in a wide particle size distribution. In contrast, if the ammonium ion concentration exceeds 25 g/L, the solubility of the metal ions may become too high, and the amount of metal ions remaining in the reaction solution may increase, causing a deviation in the composition. The concentration of ammonium ions can be measured using an ion electrode method (ion meter).

[Alkaline solution]
The alkaline solution is prepared by mixing an alkali metal hydroxide and a carbonate. The alkaline solution has a molar ratio of the alkali metal hydroxide to the carbonate, [CO ₃ ²⁻ ]/[OH ⁻ ] of 0.002 to 0.050, more preferably 0.005 to 0.030, and further preferably 0.010 to 0.025.

By using a mixed solution of an alkali metal hydroxide and a carbonate as the alkaline solution, it is possible to replace and remove anions such as sulfate and chlorine that remain as impurities in the nickel manganese cobalt composite hydroxide obtained in the crystallization step S10 with carbonate. Carbonate is more easily volatilized by ignition than sulfate and chlorine, and is preferentially volatilized in the process of mixing and firing the nickel manganese cobalt composite hydroxide with a lithium compound, so that almost no carbonate remains in the lithium nickel manganese cobalt composite oxide, which is the positive electrode material.

If [CO ₃ ²⁻ ]/[OH ⁻ ] is less than 0.002, the replacement of sulfate radicals and chlorine radicals, which are impurities derived from the raw materials, with carbonate ions becomes insufficient in the crystallization step S10, and these impurities become easily incorporated into the nickel manganese cobalt composite hydroxide. On the other hand, even if [CO ₃ ²⁻ ]/[OH ⁻ ] exceeds 0.050, the reduction in sulfate radicals and chlorine radicals, which are impurities derived from the raw materials, does not change, and the addition of an excessive amount of carbonate increases costs.

The alkali metal hydroxide is preferably one or more of lithium hydroxide, sodium hydroxide, and potassium hydroxide, and compounds that are easily soluble in water are preferred because the amount added is easy to control.

The carbonate is preferably one or more of sodium carbonate, potassium carbonate, and ammonium carbonate, and compounds that are easily soluble in water are preferred because the amount added is easy to control.

The method for adding the alkaline solution to the reaction tank is not particularly limited, and it may be added using a pump capable of controlling the flow rate, such as a metering pump, so that the pH of the reaction solution is maintained within the range described below.

[pH Control]
It is more preferable that the crystallization step S10 comprises a nucleation step S11 in which an alkaline solution is added to generate nuclei so that the pH of the reaction solution measured at a liquid temperature of 25° C. is 12.0 to 14.0, and a particle growth step S12 in which an alkaline solution is added to the particle growth solution containing the nuclei formed in the nucleation step S11 so that the pH of the solution measured at a liquid temperature of 25° C. is 10.5 to 12.0, thereby growing the nuclei.

In other words, the nucleation reaction and the particle growth reaction do not proceed at the same time in the same tank, but rather the time during which the nucleation reaction (nucleation step S11) mainly occurs and the time during which the particle growth reaction (particle growth step S12) mainly occurs are clearly separated. The nucleation step S11 and the particle growth step S12 are described in detail below.

<3-1-1. Nucleation process>
In the nucleation step S11, it is preferable to control the pH of the reaction solution to be in the range of 12.0 to 14.0 at a liquid temperature of 25° C. If the pH exceeds 14.0, the nuclei generated If the pH is less than 12.0, the nuclei will grow together with the nuclei formation, resulting in a wide range of particle size distribution of the nuclei. It may become inhomogeneous.

That is, in the nucleation step S11, by controlling the pH of the reaction solution in the range of 12.0 to 14.0, it is possible to suppress the growth of nuclei and cause only nucleation to occur, and the nuclei formed can be made homogeneous and have a narrower particle size distribution range.

<3-1-2. Particle growth process>
In the particle growth step S12, the pH of the reaction solution is preferably 10.5 to 12.0, more preferably 11.0 to 12.0, based on a liquid temperature of 25° C. If the pH exceeds 0.0, the number of newly generated nuclei increases, resulting in the generation of fine secondary particles, and it may not be possible to obtain hydroxides with a good particle size distribution. In the case of ammonium ions, the solubility of the metal ions is high, and the amount of metal ions that remain in the solution without being precipitated increases, which may result in a decrease in production efficiency.

In other words, by controlling the pH of the reaction solution in the particle growth step S12 to a range of 10.5 to 12.0, it is possible to preferentially cause the growth of only the nuclei generated in the nucleation step S11 and suppress the formation of new nuclei, and to obtain a nickel manganese cobalt composite hydroxide that is homogeneous and has a narrower particle size distribution range.

In addition, since a pH of 12.0 is the boundary condition between nucleation and nucleus growth, the conditions can be set to either the nucleation step or the particle growth step depending on the presence or absence of nuclei in the reaction solution. That is, if the pH of the nucleation step S11 is made higher than 12.0 to generate a large amount of nuclei, and then the pH is made 12.0 in the particle growth step S12, the large amount of nuclei is present in the reaction aqueous solution, so nucleus growth takes precedence, and the above hydroxide with a narrower particle size distribution and a relatively large particle size is obtained.

On the other hand, when no nuclei are present in the reaction solution, that is, when the pH is set to 12.0 in the nucleation step S11, there are no nuclei to grow, and nucleation takes precedence. By setting the pH in the particle growth step S12 to a value less than 12.0, the generated nuclei grow, resulting in a better hydroxide.

In either case, the pH of the particle growth process S12 may be controlled to a value lower than the pH of the nucleation process S11. In order to clearly separate nucleation and particle growth, it is preferable to set the pH of the particle growth process S12 at least 0.5 lower than the pH of the nucleation process S11, and more preferably at least 1.0 lower.

As described above, by clearly separating the nucleation step S11 and the particle growth step S12 by pH, nucleation takes precedence in the nucleation step S11, with almost no nucleus growth, and conversely, in the particle growth step S12, only nucleus growth occurs, with almost no new nuclei being generated. As a result, in the nucleation step S11, nuclei with a narrow particle size distribution range can be formed, and in the particle growth step S12, nuclei can be grown uniformly. Therefore, the method for producing nickel manganese cobalt composite hydroxide can produce nickel manganese cobalt composite hydroxide particles with a narrower particle size distribution range and more uniform.

[Reaction solution temperature]
In the reaction tank, the temperature of the reaction solution is preferably set to 20 to 80° C., more preferably 30 to 70° C., and even more preferably 35 to 60° C. If the temperature of the reaction solution is less than 20° C., the solubility of metal ions is low, so nucleation is likely to occur and control is difficult. On the other hand, if the temperature exceeds 80° C., the volatilization of ammonia is promoted, so that an excess of ammonium ion donor must be added to maintain a predetermined ammonia concentration, resulting in high costs.

[Reaction atmosphere]
The particle size and particle structure of the nickel manganese cobalt composite hydroxide are also controlled by the reaction atmosphere in the crystallization step S10. Therefore, in the crystallization step S10 in the method for producing a nickel manganese cobalt composite hydroxide according to one embodiment of the present invention, crystallization is performed by switching the atmosphere between an oxidizing atmosphere and a non-oxidizing atmosphere in two stages. Specifically, crystallization is performed in an oxidizing atmosphere, and then the atmosphere in the reaction tank is switched to a non-oxidizing atmosphere and crystallization is performed.

When the atmosphere in the reaction tank during the crystallization step S10 is controlled to a non-oxidizing atmosphere, the growth of the primary particles that form the nickel manganese cobalt composite hydroxide is promoted, and the primary particles are large and dense, and secondary particles with a moderately large particle size are formed. On the other hand, when the atmosphere in the reaction tank during the crystallization step is controlled to an oxidizing atmosphere, the growth of the primary particles that form the nickel manganese cobalt composite hydroxide is suppressed, and secondary particles are formed that are made of fine primary particles and have many spaces or fine voids dispersed in the center of the particles.

In the method for producing nickel manganese cobalt composite hydroxide according to one embodiment of the present invention, in the crystallization step S10, the crystallization is performed by switching the atmosphere between an oxidizing atmosphere and a non-oxidizing atmosphere in two stages. By adjusting the timing of the atmosphere switching, it is possible to control the size of the hollow part of the particle when a hollow structure is created, and the proportion of voids when a porous structure is created.

A non-oxidizing atmosphere refers to a mixed atmosphere of oxygen and an inert gas with an oxygen concentration of 5.0% by volume or less, preferably 2.5% by volume or less, and more preferably 1.0% by volume or less. Means for maintaining such a non-oxidizing atmosphere within the reaction vessel include circulating an inert gas such as nitrogen into the reaction vessel space, and further bubbling an inert gas into the reaction solution. The preferred flow rate of bubbling in the crystallization step S10 is 3 to 7 L/min, and more preferably about 5 L/min.

On the other hand, an oxidizing atmosphere refers to an atmosphere in which the oxygen concentration exceeds 5.0% by volume, preferably 10.0% by volume or more, and more preferably 15.0% by volume or more. Means for maintaining the space inside the reaction vessel in such an oxidizing atmosphere include circulating air or the like into the space inside the reaction vessel, and further bubbling air or the like into the reaction solution.

When creating a hollow structure, it is preferable to make the atmosphere in the reaction tank an oxidizing atmosphere (usually an oxygen concentration of more than 21% by volume, for example, air atmosphere), perform the crystallization process within, for example, 0.5 hours, and stop the liquid supply after a predetermined time to stop the crystallization process. Thereafter, it is preferable to switch to an inert atmosphere or a non-oxidizing atmosphere in which the oxygen concentration is controlled to 0.2% by volume or less, perform the crystallization process within a range of 0.5 to 3.5 hours, and by changing the crystallization state, control the porosity of the resulting metal composite hydroxide to 20 to 50%, thereby producing a nickel manganese cobalt composite hydroxide having a porous structure. The total crystallization time is in the range of 0.5 to 4 hours, and is appropriately adjusted depending on the size of the hydroxide particles, the size of the voids, the thickness of the dense parts, etc.

In this way, by controlling the atmosphere in the reaction tank to be a non-oxidizing atmosphere or an oxidizing atmosphere, the porosity of the secondary particles in the resulting metal composite hydroxide can be controlled.

The nucleation step S11 and the particle growth step S12 have been described above, but the above reaction atmosphere is controlled simultaneously while nucleation and particle growth are occurring.

[Porosity]
After embedding the nickel manganese cobalt composite hydroxide in resin, the nickel manganese cobalt composite hydroxide particles are cut by argon sputtering using a cross-section polisher (CP) to expose the particle cross-section. The exposed particle cross-section is observed using a scanning electron microscope, and the image of the observed particle cross-section is analyzed using image analysis software with voids in the image as black and dense parts as white, and the porosity can be obtained by calculating the area of the black part/(black part + white part) for any 20 or more particle cross-sections.

<3-2. Cleaning process>
In the washing step S20, the transition metal composite hydroxide obtained in the crystallization step S10 is washed with a washing liquid.

[Cleaning fluid type]
In the washing step S20, washing is performed with a washing solution based on an alkali metal salt or ammonium salt of a carbonate, hydrogen carbonate (bicarbonate), or hydroxide. Preferably, the transition metal composite hydroxide is washed with a washing solution in which a carbonate, hydrogen carbonate (bicarbonate), or a mixture thereof is dissolved in water.

By doing so, it is possible to efficiently remove impurity anions such as sulfate and chlorine radicals by utilizing a substitution reaction with the carbonate ions and hydrogen carbonate ions (bicarbonate ions) in the cleaning solution. In addition, by using carbonates and hydrogen carbonates (bicarbonates), it is possible to suppress the inclusion of alkali metals such as sodium compared to when hydroxides are used. In addition, when hydroxides are used in transition metal composite hydroxides that have a porous structure, it is difficult to remove impurities inside the particles, and in this respect, too, it is more effective to use carbonates and hydrogen carbonates (bicarbonates).

As the carbonate, it is preferable to select potassium carbonate, and as the hydrogen carbonate (bicarbonate), it is preferable to select potassium hydrogen carbonate or ammonium hydrogen carbonate. Furthermore, by selecting an ammonium salt from among the carbonates and hydrogen carbonates (bicarbonates), it is possible to efficiently remove cations such as sodium, potassium, calcium, and magnesium, which are impurities, by utilizing a substitution reaction with the ammonium ions in the cleaning solution. Furthermore, by selecting ammonium hydrogen carbonate from among the ammonium salts, it is possible to most efficiently remove cations such as sodium.

This is because, in addition to the substitution reaction between cations such as sodium and ammonium ions, the superior properties of ammonium bicarbonate compared to other salts, namely the high foaming efficiency of carbon dioxide gas when used as a cleaning solution, are thought to contribute greatly to the removal of cations such as sodium.

[Concentration and pH]
The concentration of the ammonium bicarbonate solution, which is the cleaning solution, is 0.05 mol/L or more. If the concentration is less than 0.05 mol/L, the effect of removing impurities such as sulfate radicals, chlorine radicals, and sodium may decrease. If the concentration is 0.05 mol/L or more, the effect of removing these impurities remains unchanged. Therefore, adding an excess of ammonium bicarbonate (ammonium bicarbonate) increases costs and affects the environmental load such as wastewater standards, so it is preferable to set the upper limit concentration at about 1.0 mol/L.

The pH of the ammonium bicarbonate solution does not need to be adjusted if the concentration is 0.05 mol/L or more, and can be left at its natural pH. If the concentration is 0.05 to 1.0 mol/L, the pH will be in the range of approximately 8.0 to 9.0.

[Liquid temperature]
The temperature of the ammonium bicarbonate solution, which is the cleaning solution, is not particularly limited, but is preferably 15 to 50° C. If the temperature of the solution is within the above range, the substitution reaction with impurities and the bubbling effect of carbon dioxide gas generated from ammonium bicarbonate are more favorable, and the removal of impurities proceeds efficiently.

[Liquid volume]
The amount of the ammonium bicarbonate solution, which is the cleaning solution, is preferably 1 to 20 L per 1 kg of nickel manganese cobalt composite hydroxide (slurry concentration is 50 to 1000 g/L). If the amount is less than 1 L, sufficient impurity removal effect may not be obtained. Furthermore, even if an amount of the solution exceeding 20 L is used, the effect of removing impurities remains the same, and an excessive amount of the solution increases costs, affects environmental loads such as wastewater standards, and is also a factor in increasing the load of wastewater volume in wastewater treatment.

[Washing time]
The washing time with the ammonium hydrogen carbonate solution is not particularly limited as long as it can sufficiently remove impurities, but it is usually 0.5 to 2 hours.

[Cleaning method]
The washing method may be 1) a general washing method in which nickel manganese cobalt composite hydroxide is added to an ammonium hydrogen carbonate solution, the slurry is stirred and washed, and then filtered, or 2) a liquid-passing washing method may be performed in which a slurry containing nickel manganese cobalt composite hydroxide produced by neutralization crystallization is supplied to a filter such as a filter press and an ammonium hydrogen carbonate solution is passed through the slurry. Liquid-passing washing is more preferable because it has a high impurity removal effect, can perform filtration and washing continuously in the same equipment, and is highly productive.

After washing with the ammonium bicarbonate solution, the washing liquid containing impurities washed out by the substitution reaction may adhere to the nickel manganese cobalt composite hydroxide, so it is preferable to finally wash with water. Furthermore, after washing with water, it is preferable to perform a drying process (not shown) to dry the adhering water from the filtered nickel manganese cobalt composite hydroxide.

The nickel manganese cobalt composite hydroxide obtained through the above-mentioned washing step S20 is a nickel manganese cobalt composite hydroxide, which is a precursor of a positive electrode active material, and is composed of secondary particles in which primary particles containing nickel, manganese, and cobalt are aggregated, or the primary particles and the secondary particles, and is characterized in that the sodium content contained in the nickel manganese cobalt composite hydroxide is less than 0.0005 mass%, and the porosity of the particles of the nickel manganese cobalt composite hydroxide is 20 to 50%.

The method for producing nickel manganese cobalt composite hydroxide according to one embodiment of the present invention can provide a method for producing nickel manganese cobalt composite hydroxide, which is a precursor of the positive electrode active material of a lithium ion secondary battery, capable of reliably reducing the sodium content and improving the battery characteristics.

<4. Lithium-ion secondary battery>
The lithium ion secondary battery according to one embodiment of the present invention is characterized by having a positive electrode containing the above-mentioned lithium nickel manganese cobalt composite oxide. The lithium ion secondary battery may be composed of the same components as a general lithium ion secondary battery, for example, including a positive electrode, a negative electrode, and a non-aqueous electrolyte. The embodiments described below are merely illustrative, and the lithium ion secondary battery of this embodiment may be implemented in a form in which various modifications and improvements have been made based on the embodiments described in this specification and on the knowledge of those skilled in the art. The lithium ion secondary battery of this embodiment is not particularly limited in its use.

(a) Positive electrode The positive electrode of a lithium ion secondary battery is prepared, for example, as follows, using the lithium nickel manganese cobalt composite oxide, which is the positive electrode active material described above. First, a powdered positive electrode active material, a conductive agent, and a binder are mixed, and activated carbon and a solvent for viscosity adjustment, etc. are added as necessary, and the mixture is kneaded to prepare a positive electrode mixture paste. The mixing ratio of each component in the positive electrode mixture paste is, for example, as in the case of a general lithium ion secondary battery positive electrode, when the total mass of the solid content of the positive electrode mixture excluding the solvent is 100 parts by mass, the content of the positive electrode active material is 60 to 95 parts by mass, the content of the conductive agent is 1 to 20 parts by mass, and the content of the binder is 1 to 20 parts by mass.

The obtained positive electrode mixture paste is applied to the surface of a current collector made of, for example, aluminum foil, and dried to evaporate the solvent. If necessary, pressure may be applied using a roll press or the like to increase the electrode density. In this manner, a sheet-shaped positive electrode can be produced. The sheet-shaped positive electrode can be cut to an appropriate size depending on the desired battery and used to produce the battery. However, the method for producing the positive electrode is not limited to the example, and other methods may also be used.

Examples of conductive materials that can be used for the positive electrode include graphite (natural graphite, artificial graphite, expanded graphite, etc.) and carbon black-based materials such as acetylene black and Ketjen Black (registered trademark).

The binder serves to hold the active material particles together, and examples of the binder that can be used include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose-based resin, polyacrylic acid, etc.

If necessary, a solvent that disperses the positive electrode active material, conductive agent, and activated carbon and dissolves the binder is added to the positive electrode mixture. Specifically, an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent. Activated carbon can also be added to the positive electrode mixture to increase the electric double layer capacity.

(b) Negative Electrode For the negative electrode, a negative electrode mixture is used which is prepared by mixing metallic lithium, a lithium alloy, or a negative electrode active material capable of absorbing and desorbing lithium ions with a binder, adding an appropriate solvent to form a paste, and applying the resulting mixture to the surface of a metal foil current collector such as copper, drying the mixture, and compressing it as necessary to increase the electrode density.

As the negative electrode active material, for example, natural graphite, artificial graphite, baked organic compounds such as phenolic resin, and powdered carbon materials such as coke can be used. In this case, as with the positive electrode, fluorine-containing resins such as PVDF can be used as the negative electrode binder, and an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent for dispersing these active materials and binders.

(c) Separator A separator is sandwiched between the positive electrode and the negative electrode. The separator separates the positive electrode and the negative electrode and retains the electrolyte. For example, a thin membrane of polyethylene, polypropylene, or the like having many minute pores can be used.

(d) Non-aqueous electrolyte A non-aqueous electrolyte can be used as the non-aqueous electrolyte. For example, the non-aqueous electrolyte may be a solution of a lithium salt as a supporting salt dissolved in an organic solvent. In addition, the non-aqueous electrolyte may be a solution of a lithium salt dissolved in an ionic liquid. The ionic liquid refers to a salt that is composed of cations and anions other than lithium ions and is liquid even at room temperature.

As the organic solvent, one selected from cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate, chain carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate, ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane, sulfur compounds such as ethyl methyl sulfone and butane sultone, and phosphorus compounds such as triethyl phosphate and trioctyl phosphate may be used alone or in combination of two or more.

The supporting salt may be LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiN(CF ₃ SO ₂ ) ₂ , or a composite salt thereof. Furthermore, the non-aqueous electrolyte may contain a radical scavenger, a surfactant, a flame retardant, and the like.

A solid electrolyte may also be used as the non-aqueous electrolyte. A solid electrolyte has the property of being able to withstand high voltages. Examples of solid electrolytes include inorganic solid electrolytes and organic solid electrolytes.

As inorganic solid electrolytes, oxide-based solid electrolytes, sulfide-based solid electrolytes, etc. are used.

The oxide-based solid electrolyte is not particularly limited, and any oxide-based solid electrolyte that contains oxygen (O) and has lithium ion conductivity and electronic insulation properties can be used. Examples of oxide-based solid electrolytes include lithium phosphate (Li ₃ PO ₄ ), Li ₃ PO ₄ N _x , LiBO ₂ N _x , LiNbO ₃ , LiTaO ₃ , Li ₂ SiO ₃ , Li ₄ SiO ₄ -Li ₃ PO ₄ , Li ₄ SiO ₄ -Li ₃ VO ₄ , Li ₂ O-B ₂ O ₃ -P ₂ O ₅ , Li ₂ O-SiO ₂ , Li ₂ O-B ₂ O ₃ -ZnO, Li _1+X Al _X Ti _2-X (PO ₄ ) ₃ (0≦X≦1), Li _1+X Al _X Ge _2-X (PO ₄ ) ₃ (0≦X≦ ₁ ), _LiTi2 ( _PO4 ) ₃ , _{Li3XLa2 /3- XTiO3 (} 0≦X≦2/3) _{, Li5La3Ta2O12, Li7La3Zr2O12, Li6BaLa2Ta2 Examples include O 12 , Li 3.6 Si 0.6} P _0.4 O ₄ and the like _.

The sulfide-based solid electrolyte is not particularly limited, and any electrolyte that contains sulfur (S) and has lithium ion conductivity and electronic insulation can be used. Examples of sulfide-based solid electrolytes include Li ₂ S-P ₂ S ₅ , Li ₂ S-SiS 2 , LiI _- Li ₂ S-SiS ₂ , LiI-Li ₂ S-P ₂ S ₅ , LiI-Li ₂ S-B ₂ S ₃ , Li ₃ PO ₄ -Li ₂ S-Si ₂ S, Li ₃ PO ₄ -Li ₂ S-SiS ₂ , LiPO ₄ -Li ₂ S-SiS, LiI-Li ₂ S-P ₂ O ₅ , and LiI-Li ₃ PO ₄ -P ₂ S ₅ .

As the inorganic solid electrolyte, materials other than those mentioned above may be used, for example, Li ₃ N, LiI, Li ₃ N-LiI-LiOH, etc. may be used.

The organic solid electrolyte is not particularly limited as long as it is a polymeric compound that exhibits ion conductivity, and examples of the organic solid electrolyte that can be used include polyethylene oxide, polypropylene oxide, and copolymers thereof. The organic solid electrolyte may also contain a supporting salt (lithium salt).

(e) Shape and configuration of the battery The lithium ion secondary battery according to one embodiment of the present invention is composed of, for example, the positive electrode, negative electrode, separator, and non-aqueous electrolyte as described above. The shape of the lithium ion secondary battery is not particularly limited, and can be various, such as a cylindrical type and a laminated type. Regardless of the shape, the positive electrode and the negative electrode are laminated via a separator to form an electrode body, the obtained electrode body is impregnated with a non-aqueous electrolyte, and the positive electrode current collector and the positive electrode terminal leading to the outside, and the negative electrode current collector and the negative electrode terminal leading to the outside are connected using a current collecting lead or the like, and the battery is sealed in a battery case to complete the lithium ion secondary battery.

The lithium-ion secondary battery according to one embodiment of the present invention is provided with a positive electrode made of the above-mentioned positive electrode active material, which reliably reduces the sodium content in particular, suppresses sintering agglomeration, and increases the porosity, making it possible to further improve the battery characteristics.

Next, the nickel manganese cobalt composite hydroxide, the method for producing the nickel manganese cobalt composite hydroxide, the lithium nickel manganese cobalt composite oxide, and the lithium ion secondary battery according to one embodiment of the present invention will be described in detail with reference to examples. Note that the present invention is not limited to these examples.

For Examples 1 to 13 and Comparative Examples 1 to 9, the transition metal composite hydroxide obtained in the crystallization process was washed, filtered, and dried to recover the precursor nickel manganese cobalt composite hydroxide, and then various analyses were performed using the following methods.

[Composition, calcium and magnesium content]
The composition and calcium and magnesium contents were analyzed by acid decomposition-ICP emission spectrometry, and a multi-type ICP emission spectrometry analyzer, ICPE-9000 (manufactured by Shimadzu Corporation), was used for the measurements.

[Sodium and potassium content]
The sodium and potassium contents were analyzed by acid decomposition-atomic absorption spectrometry, and an atomic absorption spectrophotometer 240AA (manufactured by Agilent Technologies, Inc.) was used as an atomic absorption analyzer for the measurements.

[Sulfate content]
The sulfate radical content was determined by analyzing the total sulfur content by acid decomposition-ICP atomic emission spectrometry and converting the total sulfur content into sulfate radical (SO ₄ ²⁻ ). The measurement was performed using a multi-type ICP atomic emission spectrometry analyzer, ICPE-9000 (manufactured by Shimadzu Corporation).

[Chlorine content]
The chlorine radical content was analyzed by X-ray fluorescence spectrometry (XRF) using the sample directly or by separating the chlorine radical contained in the sample in the form of silver chloride by distillation. Note that the measurement was performed using an X-ray fluorescence analyzer, Axios (manufactured by Spectris Corporation).

[Average particle size and particle size distribution]
The average particle size (MV) and particle size distribution [(d90-d10)/average particle size] were determined from the volume-based distribution measured using a laser diffraction/scattering method. A particle size distribution measuring device, Microtrac MT3300EXII (manufactured by Microtrac Bell Co., Ltd.), was used.

[Specific surface area]
The specific surface area was analyzed by a nitrogen gas adsorption/desorption method according to the BET single point method, and a gas flow type specific surface area measuring device, Macsorb 1200 series (manufactured by Mountec Co., Ltd.), was used for the measurement.

[Porosity]
The porosity was measured by cutting the sample particles using a cross-section preparation device, a cross-section polisher IB-19530CP (manufactured by JEOL Ltd.), and by observing the cross-section using a Schottky field emission scanning electron microscope SEM-EDS, a JSM-7001F (manufactured by JEOL Ltd.). Furthermore, the porosity was obtained by measuring the voids in the particle cross-section as black and the dense parts of the particle as white using image analysis and measurement software, WinRoof6.1.1 (manufactured by Mitani Shoji Co., Ltd.), and calculating the area of the black part/(black part + white part) for any 20 or more particles.

[Production and evaluation of positive electrode active material]
Furthermore, the lithium metal composite oxide, more specifically the lithium nickel manganese cobalt composite oxide, which is the positive electrode active material made from the nickel manganese cobalt composite hydroxide of the present invention as a raw material, was produced and evaluated by the following method.

[A. Production of positive electrode active material]
The precursor nickel manganese cobalt composite hydroxide was heat-treated at 700°C for 6 hours in an air stream (oxygen: 21% by volume), and the metal composite oxide was recovered. Next, lithium hydroxide, a lithium compound, was weighed out so that Li/Me was 1.025, and mixed with the recovered metal composite oxide to prepare a lithium mixture. A shaker mixer (TURBULA-Type T2C manufactured by WAB) was used for the mixing operation.

Next, the lithium mixture was calcined at 500°C for 4 hours in an oxygen (oxygen: 100% by volume) stream, then sintered at 730°C for 24 hours, cooled, and crushed to obtain lithium nickel manganese cobalt composite oxide.

[B. Evaluation of Positive Electrode Active Material]
The above-mentioned analytical method and analytical equipment were used to analyze the sodium content, potassium content, calcium content, magnesium content, sulfate content, chlorine content, and porosity of the obtained lithium nickel manganese cobalt composite oxide. The Me site occupancy rate, which indicates the crystallinity of the lithium nickel manganese cobalt composite oxide, was calculated by performing Rietveld analysis on the diffraction pattern measured using an X-ray diffraction analyzer (XRD). The measurement was performed using an X-ray diffraction analyzer X'Pert-PRO (manufactured by Spectris Co., Ltd.). The Me site occupancy rate indicates the presence ratio of metal elements in the metal layer (Me site) of the layered structure of nickel, manganese, cobalt, and the additive element M of the lithium nickel manganese cobalt composite oxide. The Me site occupancy rate is correlated with battery characteristics, and the higher the Me site occupancy rate, the better the battery characteristics.

The conditions for the examples and comparative examples are explained below.

Example 1
In Example 1, 0.9 L of water was placed in a crystallization reaction tank (5 L) in the crystallization step, and the temperature inside the tank was set to 40° C. while stirring.

An appropriate amount of 25% sodium hydroxide aqueous solution and 25% ammonia water, which is an ammonium ion donor, were added to the water in the reaction tank, and the pH of the reaction solution in the tank was adjusted to 12.8, measured based on a liquid temperature of 25°C. The ammonium ion concentration of the reaction solution was also adjusted to 10 g/L.

Next, nickel sulfate, manganese sulfate, and cobalt chloride were dissolved in water to prepare a 2.0 mol/L raw material solution. In this raw material solution, the elemental molar ratio of each metal was adjusted to Ni:Mn:Co = 1:1:1. Furthermore, sodium hydroxide, which is an alkali metal hydroxide, and sodium carbonate, which is a carbonate, were dissolved in water so that [CO ₃ ^2- ]/[OH ^- ] was 0.025 to prepare an alkaline solution.

The raw material solution was added to the reaction solution in the reaction tank at a rate of 12.9 mL/min. At the same time, the ammonium ion donor and alkaline solution were also added to the reaction solution at a constant rate. Nucleation was performed by controlling the pH to 12.8 (nucleation process pH) while maintaining the ammonium ion concentration in the reaction solution at 10 g/L and carrying out crystallization for 2 minutes and 30 seconds.

After that, 64% sulfuric acid was added until the pH of the reaction solution reached 11.6 (particle growth process pH) measured at a liquid temperature of 25°C. After the pH of the reaction solution reached 11.6, measured at a liquid temperature of 25°C, the supply of the raw material solution, ammonium ion donor, and alkaline solution was resumed, and the pH was controlled at 11.6 while crystallization was continued for 4 hours to grow particles, thereby obtaining a transition metal composite hydroxide.

As for the reaction atmosphere, the inside of the reaction tank was first filled with air (oxygen concentration: 21% by volume), and after 32 minutes had passed since the start of crystallization, the supply of the raw material solution, ammonia water, and alkaline solution was temporarily stopped, and nitrogen gas was circulated at a flow rate of 5 L/min to replace the oxygen concentration in the reaction tank space until it reached 0.2% by volume or less. After the oxygen concentration reached 0.2% by volume or less, the supply of the liquids was resumed and crystallization was carried out for 3.5 hours.

The obtained transition metal composite hydroxide was subjected to solid-liquid separation using a filter press filter, and then impurities were removed by passing 5 L of washing solution through the filter press filter for every 1 kg of transition metal composite hydroxide, using a 0.05 mol/L ammonium bicarbonate solution as a washing solution, and then washing was performed by passing water through the filter press filter. The water adhering to the washed transition metal composite hydroxide was then dried to obtain the precursor nickel manganese cobalt composite hydroxide.

Example 2
In Example 2, a nickel-manganese-cobalt composite hydroxide was obtained in the same manner as in Example 1, except that when nickel sulfate, manganese sulfate, and cobalt chloride were dissolved in water to prepare a 2.0 mol/L raw material solution, the molar ratio of nickel, manganese, and cobalt in the raw material solution was adjusted to Ni:Mn:Co=6:2:2.

Example 3
In Example 3, when nickel sulfate, manganese sulfate, and cobalt chloride were dissolved in water to prepare a 2.0 mol/L raw material solution, the molar ratio of nickel, manganese, and cobalt in the raw material solution was adjusted to Ni:Mn:Co=2:7:1, and a nickel-manganese-cobalt composite hydroxide was obtained in the same manner as in Example 1.

Example 4
In Example 4, a nickel manganese cobalt composite hydroxide was obtained in the same manner as in Example 1, except that when preparing the alkaline solution, the ratio [CO ₃ ²⁻ ]/[OH ⁻ ] was adjusted to 0.003.

Example 5
In Example 5, a nickel manganese cobalt composite hydroxide was obtained in the same manner as in Example 1, except that the alkaline solution was adjusted so that [CO ₃ ²⁻ ]/[OH ⁻ ] was 0.048.

Example 6
In Example 6, a nickel manganese cobalt composite hydroxide was obtained in the same manner as in Example 1, except that the pH in the nucleation step was 13.6.

(Example 7)
In Example 7, a nickel manganese cobalt composite hydroxide was obtained in the same manner as in Example 1, except that the pH in the nucleation step was 12.3.

(Example 8)
In Example 8, a nickel manganese cobalt composite hydroxide was obtained in the same manner as in Example 1, except that the pH in the particle growth step was 11.8.

Example 9
In Example 9, a nickel manganese cobalt composite hydroxide was obtained in the same manner as in Example 1, except that the pH in the particle growth step was 10.6.

Example 10
In Example 10, a nickel manganese cobalt composite hydroxide was obtained in the same manner as in Example 1, except that when preparing the alkaline solution, potassium hydroxide was used as the alkali metal hydroxide and potassium carbonate was used as the carbonate.

(Example 11)
In Example 11, a nickel manganese cobalt composite hydroxide was obtained in the same manner as in Example 1, except that, when preparing the alkaline solution, ammonium carbonate was used as the carbonate and the ammonium ion concentration was adjusted to 20 g/L.

Example 12
In Example 12, a nickel manganese cobalt composite hydroxide was obtained in the same manner as in Example 1, except that the temperature inside the tank was set to 35°C.

(Example 13)
In Example 13, a nickel manganese cobalt composite hydroxide was obtained in the same manner as in Example 1, except that an ammonium bicarbonate solution with a concentration of 1.00 mol/L was used as the cleaning liquid.

(Comparative Example 1)
In Comparative Example 1, when nickel sulfate, manganese sulfate, and cobalt chloride were dissolved in water to prepare a 2.0 mol/L raw material solution, a nickel manganese cobalt composite hydroxide was obtained in the same manner as in Example 1, except that the molar ratio of nickel, manganese, and cobalt in the raw material solution was adjusted to Ni:Mn:Co = 2:6:2 and that the ratio [CO ₃ ²⁻ ]/[OH ⁻ ] was adjusted to 0.001.

(Comparative Example 2)
In Comparative Example 2, a nickel manganese cobalt composite hydroxide was obtained in the same manner as in Example 1, except that only sodium hydroxide was used to adjust the alkaline solution and [CO ₃ ²⁻ ]/[OH ⁻ ] was not taken into consideration.

(Comparative Example 3)
In Comparative Example 3, a nickel manganese cobalt composite hydroxide was obtained in the same manner as in Example 1, except that the alkaline solution was adjusted so that [CO ₃ ²⁻ ]/[OH ⁻ ] was 0.001.

(Comparative Example 4)
In Comparative Example 4, a nickel manganese cobalt composite hydroxide was obtained in the same manner as in Example 1, except that the alkaline solution was adjusted so that [CO ₃ ²⁻ ]/[OH ⁻ ] was 0.055.

(Comparative Example 5)
In Comparative Example 5, a nickel manganese cobalt composite hydroxide was obtained in the same manner as in Example 1, except that the washing step was omitted and washing with an ammonium hydrogen carbonate solution was not performed.

(Comparative Example 6)
In Comparative Example 6, a nickel-manganese-cobalt composite hydroxide was obtained in the same manner as in Example 1, except that an ammonium bicarbonate solution with a concentration of 0.02 mol/L was used as the cleaning liquid.

(Comparative Example 7)
In Comparative Example 7, a nickel manganese cobalt composite hydroxide was obtained in the same manner as in Example 1, except that an ammonium carbonate solution was used as the cleaning liquid.

(Comparative Example 8)
In Comparative Example 8, a nickel manganese cobalt composite hydroxide was obtained in the same manner as in Example 1, except that a sodium hydrogen carbonate solution was used as the cleaning liquid.

(Comparative Example 9)
In Comparative Example 9, a nickel manganese cobalt composite hydroxide was obtained in the same manner as in Example 1, except that a sodium carbonate solution was used as the cleaning liquid.

The above conditions and results are shown in Tables 1, 2 and 3.

(comprehensive evaluation)
As shown in Tables 1, 2, and 3, in Examples 1 to 13, the conditions of the crystallization step and the washing step in the nickel manganese cobalt composite hydroxide, which is the precursor, were all within the preferred range. Therefore, not only in the nickel manganese cobalt composite hydroxide, but also in the lithium nickel manganese cobalt composite oxide, which is the positive electrode active material, the sodium content, sulfate radical content, chlorine radical content, potassium, calcium, and magnesium content were sufficiently reduced in the impurity removal. Furthermore, in the lithium nickel manganese cobalt composite oxide, the Me site occupancy rate exceeded 93.0%, resulting in excellent crystallinity and improved battery characteristics.

In particular, the sodium content of both the precursor and the positive electrode active material showed very good results, with all example data being below the lower limit of quantification (analysis) (0.0005 mass%). In addition, similar results were obtained for potassium, calcium, and magnesium. Therefore, in the positive electrode active material, sodium and other elements do not form solid solutions at the lithium sites, and the MV ratio, which is an indicator of sintering aggregation, is in the range of 0.95 to 1.05. Furthermore, when 100 or more randomly selected particles were observed with a scanning electron microscope, the number of particles in which secondary particle aggregation was observed was 5% or less of the total number of secondary particles observed.

The lower limit of quantification here means the minimum amount or minimum concentration at which a target component can be analyzed (quantified) using a certain analytical method. The minimum amount (value) at which a signal of the target component can be detected in a measurement is called the detection limit, and the minimum amount (value) at which the signal of the target component obtained in a measurement is reliable is called the lower limit of measurement. Furthermore, the lower limit of quantification can be calculated by multiplying the lower limit of measurement by the dilution factor, which indicates how much the original analytical sample has been concentrated or diluted in the process of preparing the analytical sample into the measurement specimen liquid.

That is, for example, the sodium content and potassium content in the present invention are 0.05 μg/mL, which is the lower limit of measurement for an atomic absorption spectrometer, and 1 g of the analytical sample is acid-decomposed to prepare 100 mL of the measurement specimen liquid (dilution ratio: 100 times), so the lower limit of quantification is 5 ppm (μg/g), i.e., 0.0005 mass%. Also, the calcium content and magnesium content in the present invention are 0.05 μg/mL, which is the lower limit of measurement for an ICP atomic emission spectrometer, and 1 g of the analytical sample is acid-decomposed to prepare 100 mL of the measurement specimen liquid (dilution ratio: 100 times), so the lower limit of quantification is 5 ppm (μg/g), i.e., 0.0005 mass%.

In contrast, in Comparative Examples 1 to 9, the [CO ₃ ²⁻ ]/[OH ⁻ ] ratio when preparing the alkaline solution and the concentration of the ammonium bicarbonate solution used as the cleaning liquid were not within the preferred range, or a cleaning liquid other than ammonium bicarbonate solution was used, so that the excellent effects of the Examples were not obtained.

In addition, in Examples 1 to 13, the hollow structure of the lithium nickel manganese cobalt composite oxide, as well as the nickel manganese cobalt composite hydroxide, had a porosity in the preferred range of 20 to 50%, and when used as a positive electrode active material, the contact area between the positive electrode active material and the electrolyte could be made sufficient without excessively reducing the bulk density while keeping the particle strength within an acceptable range, thereby improving the battery characteristics.

As described above, it has been possible to provide a lithium nickel manganese cobalt composite oxide and a lithium ion secondary battery, which are positive electrode active materials capable of suppressing sintering aggregation, increasing the porosity, and further improving battery characteristics, in particular by reliably reducing the sodium content.
Meanwhile, for example, in the field of analytical chemistry, reagent manufacturers and the like who provide reference materials that serve as standards for analysis and testing are working on further increasing the purity of the reference materials and are conducting research to reduce impurities as much as possible. From this, it goes without saying that the lithium nickel manganese cobalt composite oxide disclosed in the present invention, in which the content of impurities including sodium is reduced as much as possible, is not simply a change in design.

Although each embodiment and each example of the present invention has been described in detail above, it will be readily apparent to those skilled in the art that many modifications are possible that do not substantially deviate from the novel features and effects of the present invention. Therefore, all such modifications are intended to be included within the scope of the present invention.

For example, a term described at least once in the specification or drawings together with a different term having a broader or similar meaning may be replaced with that different term anywhere in the specification or drawings. Furthermore, the configurations and operations of the lithium nickel manganese cobalt composite oxide and the lithium ion secondary battery are not limited to those described in the embodiments and examples of the present invention, and various modifications are possible.

S10 Crystallization process, S11 Nucleation process, S12 Particle growth process, S20 Washing process

Claims (10)

A lithium nickel manganese cobalt composite oxide comprising secondary particles formed by agglomeration of primary particles containing lithium, nickel, manganese, and cobalt, or the primary particles and the secondary particles,
The lithium nickel manganese cobalt composite oxide has a sodium content of less than 0.0005% by mass, and the lithium nickel manganese cobalt composite oxide has a porosity of 20 to 50%;
The lithium nickel manganese cobalt composite oxide is characterized in that the ratio of the average particle size of the lithium nickel manganese cobalt composite oxide divided by the average particle size of the precursor nickel manganese cobalt composite hydroxide is 0.95 to 1.05, and the Me site occupancy rate is 93.0% or more . The lithium nickel manganese cobalt composite oxide according to claim 1, characterized in that when 100 or more randomly selected particles of the lithium nickel manganese cobalt composite oxide are observed with a scanning electron microscope, the number of particles in which secondary particle agglomeration is observed is 5% or less of the total number of secondary particles observed. 3. The lithium nickel manganese cobalt composite oxide according to claim 1, wherein the lithium nickel manganese cobalt composite oxide has a sulfate group content of 0.15% by mass or less and a chlorine group content of 0.005% by mass or less . The lithium nickel manganese cobalt composite oxide according to any one of claims 1 to 3, characterized in that the content of at least one of potassium, calcium, and magnesium contained in the lithium nickel manganese cobalt composite oxide is less than 0.0005 mass%. A lithium ion secondary battery comprising a positive electrode containing at least the lithium nickel manganese cobalt composite oxide according to any one of claims 1 to 4. A method for producing secondary particles in which primary particles containing lithium, nickel, manganese, and cobalt are aggregated, or a lithium nickel manganese cobalt composite oxide composed of the primary particles and the secondary particles, comprising the steps of:
a crystallization step of crystallizing in a reaction solution obtained by adding a raw material solution containing nickel, manganese, and cobalt, a solution containing an ammonium ion donor, and an alkaline solution to obtain a transition metal composite hydroxide;
a washing step of washing the transition metal composite hydroxide obtained in the crystallization step with a washing liquid;
a heat treatment step of heat-treating the nickel manganese cobalt composite hydroxide obtained in the washing step;
a mixing step of mixing the metal composite oxide obtained in the heat treatment step with a lithium compound;
A calcination step of calcining the lithium mixture obtained in the mixing step,
The alkaline solution in the crystallization step is a mixed solution of an alkali metal hydroxide and a carbonate,
The ratio of the carbonate to the alkali metal hydroxide in the mixed solution, [CO ₃ ^2- ］/[OH ^- ] is 0.002 to 0.050,
In the crystallization step, crystallization is performed by switching the atmosphere between an oxidizing atmosphere and a non-oxidizing atmosphere in two stages,
The cleaning solution in the cleaning step is an ammonium bicarbonate solution having a concentration of 0.05 mol/L or more,
the lithium nickel manganese cobalt composite oxide obtained in the calcination step has a sodium content of less than 0.0005 mass % and a porosity of the lithium nickel manganese cobalt composite oxide of 20 to 50%;
A method for producing a lithium nickel manganese cobalt composite oxide, characterized in that a ratio obtained by dividing an average particle size of the lithium nickel manganese cobalt composite oxide by an average particle size of the nickel manganese cobalt composite hydroxide which is a precursor of the lithium nickel manganese cobalt composite oxide is 0.95 to 1.05. 7. The method for producing a lithium nickel manganese cobalt composite oxide according to claim 6, characterized in that, when 100 or more randomly selected particles of the lithium nickel manganese cobalt composite oxide are observed by scanning electron microscope, the number of particles in which secondary particle agglomeration is observed is 5% or less of the total number of secondary particles observed. 8. The method for producing a lithium nickel manganese cobalt composite oxide according to claim 6, wherein the lithium nickel manganese cobalt composite oxide has a sulfate group content of 0.15% by mass or less, a chlorine group content of 0.005% by mass or less, and a Me site occupancy rate of 93.0% or more. The method for producing a lithium nickel manganese cobalt composite oxide according to any one of claims 6 to 8, characterized in that the content of at least one of potassium, calcium and magnesium contained in the lithium nickel manganese cobalt composite oxide is less than 0.0005 mass% . Obtaining a positive electrode using a positive electrode active material for a lithium ion secondary battery produced by the method for producing a lithium nickel manganese cobalt composite oxide according to any one of claims 6 to 9;
A method for producing a lithium ion secondary battery, comprising the steps of: obtaining a lithium ion secondary battery by using the positive electrode, the negative electrode, and the non-aqueous electrolyte;