JP7484283B2 - Positive electrode active material for lithium ion secondary battery, manufacturing method thereof, and lithium ion secondary battery - Google Patents

Description

Article 30, paragraph 2 of the Patent Act applies 1. Website posting Date of website posting: Tuesday, August 27, 2019 Website address: https://confit.atlas.jp/guide/event/ecsj2019f/top https://confit.atlas.jp/guide/event/ecsj2019f/subject/1I16/class?cryptoId= Distributor: Huang Songkai, Hayashi Tetsutaro, Suganuma Shinsuke, Kato Toshihiro 2. Conference presentation Date of publication Thursday, September 5, 2019 Exhibition name and location 2019 Electrochemistry Fall Conference, Kofu Campus, University of Yamanashi (4-4-37 Takeda, Kofu City, Yamanashi Prefecture) Publishers Huang Songkai, Hayashi Tetsutaro, Suganuma Shinsuke, Kato Toshihiro

The present invention relates to a positive electrode active material for lithium ion secondary batteries, more specifically, to a positive electrode active material for lithium ion secondary batteries consisting of a lithium nickel-containing composite oxide coated with a compound containing W and Li, a method for producing the same, and a lithium ion secondary battery using the positive electrode active material for lithium ion secondary batteries as a positive electrode material.

In recent years, with the widespread use of portable electronic devices such as smartphones, tablet terminals, digital cameras, and notebook computers, there is a strong demand for the development of small, lightweight secondary batteries with high energy density. There is also a strong demand for the development of high-capacity, high-output secondary batteries as power sources for electric vehicles such as hybrid electric vehicles, plug-in hybrid electric vehicles, and battery-powered electric vehicles.

Lithium ion secondary batteries are a type of secondary battery that meets these requirements. These lithium ion secondary batteries are composed of a negative electrode, a positive electrode, and a non-aqueous or solid electrolyte, and the active materials used for the negative and positive electrodes are materials that can extract and insert lithium. The non-aqueous electrolyte includes a non-aqueous electrolyte solution made by dissolving a lithium salt, which serves as a supporting salt, in an organic solvent, and the solid electrolyte includes inorganic or organic solid electrolytes that are non-flammable and have lithium ion conductivity.

Among lithium-ion secondary batteries, those that use lithium transition metal-containing composite oxides with layered rock salt or spinel structures as the positive electrode material can achieve a voltage of about 4 V, and are therefore being researched, developed, and commercialized as batteries with high energy density.

As positive electrode materials for lithium-ion secondary batteries, positive electrode active materials have been proposed that are made of lithium transition metal-containing composite oxides, such as lithium cobalt composite oxide (LiCoO ₂ ), which is relatively easy to synthesize, lithium nickel composite oxide (LiNiO ₂ ), which uses nickel, which is cheaper than cobalt, lithium nickel manganese cobalt composite oxide (LiNi _1/3 Mn _1/3 Co _1/3 O ₂ ), lithium manganese composite oxide (LiMn ₂ O ₄ ), which uses manganese, and lithium nickel manganese composite oxide (LiNi _0.5 Mn _0.5 O ₂ ).

In recent years, among these lithium transition metal-containing composite oxides, a ternary positive electrode active material made of lithium nickel manganese cobalt-containing composite oxide (NMC) containing at least nickel, manganese, and cobalt as transition metals, including lithium nickel manganese cobalt composite oxide (LiNi _1/3 Mn _1/3 Co _1/3 O ₂ ), has been attracting attention as a material that has excellent thermal stability, high capacity, good cycle characteristics of battery capacity, and can obtain high output with low resistance. The lithium nickel manganese cobalt-containing composite oxide is a compound having a layered crystal structure, like the lithium cobalt composite oxide and the lithium nickel composite oxide.

The focus of development of lithium transition metal-containing composite oxides is on increasing output by reducing their internal resistance. From this perspective, lithium nickel cobalt aluminum-containing composite oxides (NCA), which have improved properties compared to lithium nickel composite oxides, are also attracting attention. Lithium nickel manganese cobalt-containing composite oxides are considered the most important in the development of lithium transition metal-containing composite oxides, because they are more weather resistant and easier to handle than lithium nickel cobalt aluminum-containing composite oxides.

As such, there is a high demand for ternary positive electrode active materials made of lithium nickel manganese cobalt-containing composite oxides to achieve higher output by further reducing internal resistance, particularly in applications as power sources for electric vehicles.

In order to improve the output characteristics and cycle characteristics of the lithium nickel manganese cobalt-containing composite oxide, it is first necessary that the lithium nickel manganese cobalt-containing composite oxide is composed of particles with a small particle size of 3 μm or more and 10 μm or less and a narrow particle size distribution. By using particles with a small particle size, the specific surface area is large, and when used as a positive electrode active material, a sufficient reaction area with a non-aqueous electrolyte can be secured. Furthermore, the positive electrode can be made thin, and the movement distance of lithium ions between the positive electrode and the negative electrode can be shortened, so that the positive electrode resistance can be reduced. In addition, by using particles with a narrow particle size distribution, the voltage applied to the particles in the electrode can be made uniform, making it possible to suppress the decrease in battery capacity due to selective deterioration of the fine particles.

In addition, research and development is also being conducted on improving the particle structure of the lithium nickel manganese cobalt-containing composite oxide in order to further improve the output characteristics. For example, it is considered effective to control the form of the positive electrode active material and form a space inside the positive electrode active material into which the non-aqueous electrolyte can enter in order to improve the output characteristics. By adopting such a structure, the reaction area with the non-aqueous electrolyte can be increased compared to a positive electrode active material with a solid structure having a similar particle size, making it possible to significantly reduce the positive electrode resistance. It is known that the positive electrode active material inherits the particle properties of its precursor, the transition metal-containing composite hydroxide. In other words, in order to obtain the above-mentioned positive electrode active material, it is necessary to appropriately control the particle size, particle size distribution, particle structure, etc. of the secondary particles of the transition metal-containing composite hydroxide, which is the precursor.

For example, JP 2018-104273 A discloses a method for producing a transition metal-containing composite hydroxide, which is characterized in that the crystallization reaction is divided into a nucleation process in which the pH value of the reaction aqueous solution is adjusted to a range of 12.0 to 14.0 to generate nuclei, and a particle growth process in which the pH value of the reaction aqueous solution containing the generated nuclei is adjusted to a range of 10.5 to 12.0 lower than the pH value of the nucleation process to grow particles, and the initial atmosphere of the nucleation process and the particle growth process is a non-oxidizing atmosphere, and at a predetermined timing in the particle growth process, the atmosphere is switched to an oxidizing atmosphere, and then switched back to a non-oxidizing atmosphere, and this atmosphere control is performed two or more times.

According to this method, a transition metal-containing composite hydroxide can be obtained, which is made up of secondary particles having a small particle size and a narrow particle size distribution, a central portion formed by the aggregation of plate-like or needle-like primary particles, and two or more laminated structures in which a low-density layer formed by the aggregation of fine primary particles and a high-density layer formed by the aggregation of plate-like primary particles are alternately laminated on the outside of the central portion. A positive electrode active material using a transition metal-containing composite hydroxide of this structure as a precursor has a small particle size, a narrow particle size distribution, and a porous structure with spaces. A secondary battery using a positive electrode active material of this porous structure can improve output characteristics as well as capacity characteristics and cycle characteristics.

On the other hand, in order to further reduce the internal resistance of lithium-ion batteries and thereby increase their output, a method of adding a tungsten compound to a lithium transition metal-containing composite oxide is being investigated.

For example, in JP 2015-216105 A, a lithium mixture obtained by mixing a nickel compound and a lithium compound is fired in an oxidizing atmosphere at a temperature of 700° C. or higher and 780° C. or lower for 1 hour to 6 hours to obtain a lithium oxide having a general formula: Li _b Ni _1-x-y Co _x M _y O ₂ (wherein M represents at least one element selected from Mg, Al, Ca, Ti, V, Cr, Mn, Nb, Zr, and Mo, and 0.95≦b≦1.03, 0<x≦0.15, 0<y≦0.07, x+y≦0.16), a lithium-nickel-containing composite oxide composed of primary particles and secondary particles formed by agglomeration of the primary particles is obtained as a base material, a tungsten compound is added during or after a water-washing treatment of the base material, W is dispersed on the surfaces of the primary particles of the base material, and the resulting mixture is heat-treated at a temperature of 100° C. or higher and 600° C. or lower in an oxygen atmosphere or a vacuum atmosphere, and a positive electrode active material for a lithium ion secondary battery has been proposed.

JP 2017-084513 A proposes a positive electrode active material for lithium ion secondary batteries, which is obtained by adding a tungsten compound during or after the water washing treatment of a base material obtained in the same manner as in JP 2015-216105 A to control the moisture content of the base material to 6.5 mass % or more and 11.5 mass % or less, dispersing W on the surfaces of the primary particles of the base material, and heat treating the surface at a temperature of 100°C or more and 600°C or less in an oxygen atmosphere or a vacuum atmosphere, and has a coating containing W and Li with a thickness of 1 nm or more and 200 nm or less on the surfaces of the primary particles of the base material.

By having fine particles and/or a coating of a compound containing W and Li, such as lithium tungstate, present on the surface of the primary particles that make up the lithium transition metal-containing composite oxide base material, these fine particles or coatings prevent contact between the lithium transition metal-containing composite oxide and the electrolyte and function as a protective film that suppresses the formation of deposits such as phosphates, and because these fine particles or coatings have a crystal structure that has Li diffusion paths, it is believed that interface resistance can be reduced.

It is important that the fine particles and/or coating of the compound containing W and Li, such as lithium tungstate, are uniformly present on the surface of the primary particles constituting the lithium transition metal-containing composite oxide base material. It is also important that the fine particles and/or coating of the compound containing W and Li, such as lithium tungstate, are uniformly formed between the lithium transition metal-containing composite oxides.

From this perspective, JP 2018-186065 A proposes to obtain a positive electrode active material for a lithium ion secondary battery by mixing a lithium transition metal-containing composite oxide base material with tungsten oxide, further spraying and mixing with water to obtain a lithium mixture, and drying the lithium mixture, which contains secondary particles formed by agglomeration of multiple primary particles and lithium tungstate, and in which the amount of tungsten present on the surface is 0.1% by mass to 1.0% by mass relative to the total, and the amount of tungsten present inside is 0.1% by mass to 1.0% by mass relative to the total. This method is said to make it possible to more uniformly proceed with the neutralization reaction between surplus lithium, such as free lithium hydroxide, and tungsten oxide on the surface of the lithium transition metal-containing composite oxide base material in the presence of water.

JP 2018-104273 A JP 2015-216105 A JP 2017-084513 A JP 2018-186065 A

In order to further increase the output of a positive electrode active material made of a porous lithium nickel manganese cobalt-containing composite oxide, the inventors have investigated how to obtain a positive electrode active material made of a W- and Li-containing compound-coated lithium nickel manganese cobalt-containing composite oxide by dry-mixing a porous lithium nickel manganese cobalt-containing composite oxide as a base material with a tungsten compound, spraying water on the mixture, and drying the mixture.

As a result, it was discovered that by forming a coating of a compound containing W and Li, such as lithium tungstate, on the surface of a lithium nickel manganese cobalt-containing composite oxide under specified conditions, it is possible to further improve the uniformity of the fine particles and coating of the compound containing W and Li, which is the coating form.

The present invention aims to achieve a higher output from a porous lithium nickel manganese cobalt-containing composite oxide coated with a compound containing W and Li by making the fine particles and coating of a compound containing W and Li, such as lithium tungstate, more uniform in size or thickness.

The present invention relates to a ternary positive electrode active material for lithium ion secondary batteries, which is made of a porous W- and Li-containing compound-coated lithium nickel manganese cobalt-containing composite oxide (NMC) containing at least nickel, manganese, and cobalt as transition metals, and which is made of secondary particles with a porous structure, in which fine particles and a coating of a compound containing W and Li, such as lithium tungstate, are present on at least a portion of the surface of the primary particles that make up the secondary particles, and a method for producing the same, as well as a lithium ion secondary battery that uses the positive electrode active material for lithium ion secondary batteries as a positive electrode material.

The positive electrode active material for lithium ion secondary batteries of the present invention is a ternary positive electrode active material made of a lithium nickel manganese cobalt-containing composite oxide, which is composed of secondary particles formed by agglomeration of a plurality of primary particles, and relates to a positive electrode active material for lithium ion secondary batteries made of a lithium nickel manganese cobalt-containing composite oxide coated with a compound containing W and Li, in which fine particles and a coating of a compound containing W and Li are present on at least a portion of the surface of at least a portion of the primary particles constituting the secondary particles.

In the positive electrode active material for lithium ion secondary batteries of the present invention, the secondary particles have a porous structure. Specifically, the secondary particles have an outer shell formed by the aggregated primary particles, an aggregate portion present inside the outer shell, formed by the aggregated primary particles, and electrically conductive with the outer shell, and a space portion present and dispersed within the aggregate portion.

In particular, the positive electrode active material for lithium ion secondary batteries of the present invention is characterized in that the porosity measured by cross-sectional observation of the secondary particles is 10% or more and 50% or less.

The particle diameter of the fine particles of the compound containing W and Li present on the surface of the secondary particles, as determined by observing the surfaces of the secondary particles using a scanning electron microscope, is preferably 1 nm or more and 430 nm or less, and more preferably 5 nm or more and 400 nm or less , and more preferably 10 nm or more and 350 nm or less .

On the other hand, it is preferable that the average thickness of the coating of the compound containing W and Li present on the surface of the secondary particles, as determined by cross-sectional observation of the secondary particles using a transmission electron microscope, is 1 nm or more and 200 nm or less.

The average particle size of the microparticles is preferably 30 nm or more and 100 nm or less.

The compound containing _W and Li is preferably lithium _tungstate . More preferably, the lithium tungstate contains _7Li2WO4.4H2O .

It is preferable that the average particle size MV of the secondary particles is 3 μm or more and 10 μm or less, and that the index showing the spread of the particle size distribution of the secondary particles, [(d90-d10)/average particle size MV], is 0.7 or less.

The thickness of the outer shell of the secondary particle is preferably 0.1 μm or more and 1.5 μm or less.

The tap density of the positive electrode active material is preferably 1.0 g/cm ³ or more, more specifically, 1.1 g/cm ³ or more and 1.8 g/cm ³ or less.

The BET specific surface area of the positive electrode active material is preferably 2.0 m ² /g or more and 5.0 m ² /g or less.

The positive electrode active material for a non-electrolyte secondary battery of the present invention is preferably composed of a lithium _{nickel manganese cobalt- containing} composite oxide represented by general formula (A): Li1 _{+ uNixMnyCozWsMtO2} (-0.05≦u≦0.50, x+y+z+s+t=1, 0.3≦x _≦ 0.7, 0.15≦y≦0.4, 0.15≦z≦0.4, 0.0005≦s≦0.03, 0≦t≦0.1, M is one or more added elements selected from Mg, Al, Si, Ca, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) and having a hexagonal layered rock salt crystal structure.

The method for producing a positive electrode active material for a lithium ion secondary battery of the present invention comprises the steps of:
a dry mixing step of mixing a tungsten compound in an amount of 0.1 mass % or more and 5 mass % or less with respect to a total mass of a lithium nickel manganese cobalt-containing composite oxide as a base material, the secondary particles being composed of secondary particles formed by agglomeration of a plurality of primary particles, the secondary particles having a porous structure including an outer shell portion formed by the agglomerated primary particles, an agglomeration portion present inside the outer shell portion, formed by the agglomerated primary particles and electrically conductive with the outer shell portion, and a space portion present in a dispersed state within the agglomeration portion;
a water spray mixing step of spraying water onto the mixture in an amount of 1% by mass or more and 30% by mass or less with respect to the total mass of the mixture while stirring the mixture, thereby mixing the mixture;
A heat treatment step of subjecting the mixture after the water spray mixing step to a heat treatment at a temperature of 500° C. or less; and
a drying step of drying the mixture after the heat treatment step at a temperature of 500° C. or less to obtain a lithium nickel manganese cobalt-containing composite oxide coated with a compound containing W and Li, in which fine particles and a coating of a compound containing W and Li are present on the surfaces of the primary particles;
Equipped with.

In particular, the method for producing a positive electrode active material for a lithium ion secondary battery of the present invention is characterized in that, at least in the drying step, the drying is performed in a vacuum atmosphere using a vacuum drying and mixing device.

In the water spray mixing step, it is preferable that the stirring during the water spraying is performed at a peripheral speed of 4 m/s to 30 m/s, the water spraying is performed at a water spray rate of 0.01 ml/min to 0.1 ml/min per 1 g of mixture, and after the water spraying, the stirring is performed at a peripheral speed in the range of 4 m/s to 30 m/s, while the mixing is continued for a time period of 5 minutes to 120 minutes.

It is more preferable that the water spray is performed using multiple nozzles, and that the water spray rate of each of the multiple nozzles is 0.005 ml/sec or more and 0.02 ml/min or less per gram of mixture.

In the heat treatment step, it is preferable to perform the heat treatment at a temperature of 40°C to 200°C for a period of 15 minutes to 120 minutes while stirring the mixture at a peripheral speed of 4 m/s to 30 m/s.

In the drying step, it is preferable to carry out the drying at a temperature of 70°C to 200°C for a period of 60 minutes to 240 minutes while stirring the mixture at a peripheral speed of 4 m/s to 30 m/s.

After the drying step, it is preferable to have a cooling step in which the mixture is cooled to 25°C for a period of 60 minutes to 240 minutes while stirring the composite oxide at a peripheral speed of 1 m/s to 20 m/s in a vacuum atmosphere.

The dry mixing step is preferably carried out for 5 minutes to 50 minutes while stirring at a peripheral speed of 5 m/s to 30 m/s. The dry mixing step is also preferably carried out using the vacuum dry mixing device.

The positive _electrode active material for lithium ion secondary batteries is preferably composed of _a lithium _{nickel manganese} composite oxide represented by general _formula (A): Li1 _{+ uNixMnyCozWsMtO2} (-0.05≦u≦0.50, x+y+z+s+t=1, 0.3≦x≦0.7, 0.15≦y≦0.4, 0.15≦z≦0.4, 0.0005≦s≦0.03, 0≦t≦0.1, M is one or more additive elements selected from Mg, Al, Si, Ca, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W), and has a hexagonal layered rock salt crystal structure.

The lithium ion secondary battery of the present invention comprises a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte (non-aqueous electrolyte secondary battery), or comprises a positive electrode, a negative electrode, and a solid electrolyte (solid electrolyte secondary battery), and is characterized in that the positive electrode active material for the lithium ion secondary battery of the present invention is used as the positive electrode active material used in the positive electrode.

The present invention provides a positive electrode active material for lithium ion secondary batteries, which is made of a lithium nickel manganese cobalt-containing composite oxide coated with a compound containing W and Li, in which fine particles of a compound containing W and Li are present more finely and more uniformly on the surface of the secondary particles constituting the porous lithium nickel manganese cobalt-containing composite oxide base material, and a coating of a compound containing W and Li is present more uniformly.

The positive electrode active material of the present invention has better battery characteristics than conventional porous lithium nickel manganese cobalt-containing composite oxides due to its ternary composition. In addition, since fine particles of a compound containing W and Li are present on at least a portion of the surface of at least a portion of the primary particles that make up the secondary particles, and a coating of a compound containing W and Li is present uniformly, it is possible to further reduce resistance, making it possible to provide a lithium ion secondary battery with higher output.

Therefore, by applying the positive electrode active material for lithium ion secondary batteries of the present invention to the positive electrode material of lithium ion secondary batteries used as power sources for electric vehicles, it becomes possible to provide a battery with greater durability and higher output characteristics than conventional batteries, and this has great industrial significance.

FIG. 1 is a flow chart showing an example of a process for producing a positive electrode active material for a lithium ion secondary battery according to the present invention. FIG. 2 is a cross-sectional view that shows a schematic structure of an example of the lithium nickel manganese cobalt-containing composite oxide coated with a compound containing W and Li, which is the positive electrode active material for a lithium ion secondary battery of the present invention. FIG. 3 is a SEM photograph of the surface of a secondary particle constituting the positive electrode active material for a lithium ion secondary battery of Example 1 of the present invention. FIG. 4 is an SEM photograph of the surface of a secondary particle of the positive electrode active material for a lithium ion secondary battery of Comparative Example 1. FIG. 5 is a schematic cross-sectional view of a 2032 type coin battery used for battery evaluation. FIG. 6 is a schematic explanatory diagram of an example of impedance evaluation measurement and an equivalent circuit used in the analysis.

The present invention will be described in the following order: a positive electrode active material for a lithium ion secondary battery, a method for producing the same, and a lithium ion secondary battery.

The positive electrode active material for a lithium ion secondary battery of the present invention (hereinafter referred to as "positive electrode active material") relates to a lithium nickel manganese cobalt-containing composite oxide coated with a compound containing W and Li (hereinafter referred to as "composite oxide"), which is composed of secondary particles having a porous structure, and in which fine particles and a coating of a compound containing W and Li, such as lithium tungstate, are present on at least a portion of the surface of at least a portion of the primary particles constituting the secondary particles.

(1) Particle structure The positive electrode active material of the present invention is a ternary system containing at least nickel, manganese, and cobalt as transition metals. This positive electrode active material is composed of secondary particles formed by agglomeration of a plurality of primary particles. In the positive electrode active material of the present invention, as shown in FIG. 2, the secondary particles have a porous structure including an outer shell 1 made of agglomerated primary particles, an agglomeration portion 2 present inside the outer shell 1, made of agglomerated primary particles similar to the outer shell 1, and electrically conductive with the outer shell 1, and a space portion 3 present inside the outer shell 1 and dispersed between the agglomeration portions 2.

Here, "electrically conductive" means that the aggregates of the primary particles are structurally connected to each other and can be electrically conductive. The spaces that make up the pore structure are separated from each other due to the presence of the aggregates, but they communicate with the outside and with each other through the grain boundaries and voids between the primary particles, allowing the non-aqueous electrolyte and conductive additive to penetrate into the spaces.

The positive electrode active material of the present invention, due to such a particle structure, achieves both a larger specific surface area and a higher tap density compared to conventional positive electrode active materials having a hollow structure. In the positive electrode active material having the particle structure of the present invention, the non-aqueous electrolyte penetrates into the interior of the secondary particles through the grain boundaries, voids, or spaces between the primary particles, making it possible to remove and insert lithium not only on the surface of the secondary particles but also inside the secondary particles. Moreover, in this positive electrode active material, the outer shell and the aggregates are electrically connected, and the cross-sectional area of the path is sufficiently large, so that the resistance inside the particles (internal resistance) is significantly reduced.

When a lithium ion secondary battery (hereinafter referred to as "secondary battery") is constructed using a positive electrode active material having such a structure as the positive electrode material, its durability is improved, and deterioration of output characteristics due to the resistance of the positive electrode interface caused by its degradation is prevented, making it possible to further improve output characteristics without impairing battery capacity or cycle characteristics. In addition, because the tap density is high, the filling property of the positive electrode active material is high, and the capacity per volume of the secondary battery can be maintained high.

(2) Fine particles and coating of compound containing W and Li In the positive electrode active material of the present invention, as shown in FIG. 2, a coating consisting of fine particles 4 and coating 5 of a compound containing W and Li is present on at least a portion of the surface of at least a portion of the primary particles constituting the secondary particles of the composite oxide having a porous structure serving as the base material.

In particular, in the positive electrode active material for lithium ion secondary batteries of the present invention, the particle diameter of the fine particles of the compound containing W and Li present on the surface of the secondary particles, as determined by observing the surfaces of the secondary particles with a scanning electron microscope (SEM), is preferably 1 nm or more and 430 nm or less. Also, the average thickness of the coating of the compound containing W and Li present on the surface of the secondary particles, as determined by observing the cross-sections of the secondary particles with a transmission electron microscope (TEM), is preferably 1 nm or more and 200 nm or less.

Generally, if the surface of the positive electrode active material is completely covered with a different compound, the movement (intercalation) of lithium ions is greatly restricted, and as a result, the advantage of the high capacity of ternary composite oxides is eliminated.

In contrast, in the present invention, fine particles and coatings of compounds containing W and Li are present on at least a portion of the surface of at least a portion of the primary particles that make up the secondary particles of the composite oxide.

Fine particles and coatings of compounds containing W and Li have high lithium ion conductivity and promote the movement of lithium ions. Therefore, by having fine particles and coatings of compounds containing W and Li present on the surface of the primary particles, a Li conduction path is formed at the interface with the electrolyte, making it possible to reduce the reaction resistance of the positive electrode active material (also called "positive electrode resistance") and improve its output characteristics.

In other words, by reducing the positive electrode resistance, the voltage lost in the secondary battery is reduced, and the voltage actually applied to the load side is relatively higher, resulting in high output. In addition, by increasing the voltage applied to the load side, lithium is sufficiently inserted and removed from the positive electrode, improving the battery capacity. Furthermore, by reducing the positive electrode resistance, the load on the positive electrode active material during charging and discharging is also reduced, improving the cycle characteristics.

The effect of having such fine particles and coatings of compounds containing W and Li present on the surface of primary particles can be effectively achieved by having finer particles and coatings of an appropriate thickness more uniformly present on the surface of secondary particles.

In the present invention, finer particles, i.e., particles having a predetermined particle size and a coating having a predetermined thickness, made of a compound containing W and Li, are formed on at least a portion of the surface of at least the primary particles so that these particles and coatings are more uniformly present on the surface of the secondary particles. This makes it possible to effectively improve lithium ion conduction, suppress a decrease in charge/discharge capacity, and reduce reaction resistance without excessively reducing the electrode surface or significantly increasing the bulk resistance of the electrode.

If the fine particles and coating of the compound containing W and Li are present in excess on the surface of the primary particles that make up the secondary particles of the positive electrode active material, the BET specific surface area of the positive electrode active material will decrease, and even if the compound containing W and Li has high lithium ion conductivity, the contact area between the positive electrode active material and the electrolyte will become smaller, which will likely lead to a decrease in charge/discharge capacity and an increase in reaction resistance. In addition, although the presence of the compound containing W and Li reduces the reaction resistance, the compound containing W and Li itself has low electronic conductivity, and therefore reduces the electronic conductivity of the electrode, leading to a decrease in the output characteristics of the secondary battery.

From this viewpoint, in the positive electrode active material of the present invention, it is sufficient that the fine particles and coating of the compound containing W and Li are present on at least a part of the surface of at least a part of the primary particles constituting the secondary particles. And, if the amount of the compound containing W and Li is the same, the particle diameter of the fine particles of the compound containing W and Li is set to 1 nm or more and 430 nm or less, so that the fine particles of the compound containing W and Li are finer than those of the conventional positive electrode active material coated with the compound containing W and Li, and are more uniformly and more evenly present on the surface of the secondary particles. Also, the average thickness of the coating of the compound containing W and Li is set to 1 nm or more and 200 nm or less, so that the coating of the compound containing W and Li is thinner than those of the conventional positive electrode active material coated with the compound containing W and Li, and are more uniformly and more evenly present on the surface of the secondary particles.

When the particle diameter of the fine particles of the compound containing W and Li is less than 1 nm, the fine particles may not have sufficient lithium ion conductivity, and when the particle diameter exceeds 430 nm, the coating of the fine particles may be formed non-uniformly, and the effect of reducing the reaction resistance may not be sufficiently obtained.

The particle diameter of the microparticles is preferably 10 nm or more and 350 nm or less, and more preferably 20 nm or more and 300 nm or less. It is preferable that 50% or more of the total number of microparticles formed on the surface of the secondary particles have a particle diameter of 10 nm or more and 350 nm or less. In this case, a higher effect of improving the battery characteristics can be obtained.

For the same reason, the average particle size of the W- and Li-containing compound particles is preferably 30 nm or more and 100 nm or less, and more preferably 40 nm or more and 80 nm or less. By regulating the average particle size in this way, it is possible to make the particles finer and to have them present more uniformly and more evenly on the surface of the secondary particles than in conventional W- and Li-containing compound-coated positive electrode active materials.

The particle diameter of the microparticles is obtained by observing the surfaces of any number (e.g., 10) of secondary particles using a scanning electron microscope (SEM) and measuring the maximum diameter of each of the microparticles. The average particle diameter of the microparticles is calculated, for example, by observing the surfaces of any number (e.g., 10) of secondary particles using a scanning electron microscope (SEM), measuring the particle diameters of any number (e.g., 10) of microparticles present on the surface of each secondary particle, and calculating the average value.

If the average thickness of the coating of the compound containing W and Li is less than 1 nm, the coating may not be formed sufficiently or may not have sufficient lithium ion conductivity. If the average thickness of the coating exceeds 200 nm, the coating may become too thick, causing a decrease in the BET specific surface area of the positive electrode active material and a decrease in the contact area between the positive electrode active material and the electrolyte, which may result in an insufficient reduction in reaction resistance.

The average thickness of the coating is preferably 1 nm or more and 100 nm or less, and more preferably 1 nm or more and 80 nm or less. In this case, a greater effect of improving the battery characteristics can be obtained.

The average thickness of the coating can be determined by observing the cross-section of the secondary particles using a transmission electron microscope (TEM). The average thickness of the coating can also be determined by measuring the average thickness of any number (e.g., 10 locations) of coatings present on the surfaces of any number (e.g., 10) of primary particles on which coatings exist in the secondary particles, and then averaging the values obtained for any number (e.g., 10) of secondary particles.

Since contact with the electrolyte occurs on the surface of the primary particles, it is important that fine particles and a coating of a compound containing W and Li are present on at least a portion of the surface of at least some of the primary particles, preferably at least a portion of all of the primary particles. Here, the surface of the primary particles in the present invention includes both the primary particle surface exposed on the outer surface of the secondary particles and the primary particle surface exposed in the vicinity of the surface and in the internal voids of the secondary particles through which the electrolyte can penetrate and communicate with the outside of the secondary particles. Furthermore, even if the grain boundary between primary particles is incompletely bonded and in a state in which the electrolyte can penetrate, such a location is also included in the surface of the primary particles.

By forming finer particles and coatings of compounds containing W and Li on at least a portion of the surface of the primary particles so that the particles and coatings of compounds containing W and Li are more uniformly and evenly present on the surfaces of the secondary particles, it becomes possible to further promote the movement of lithium ions and further reduce the reaction resistance of the particles that make up the positive electrode active material.

That is, the fine particles of the compound containing W and Li do not have to be present on the surface of all the primary particles constituting the secondary particles that can come into contact with the electrolyte. As shown in FIG. 3, the fine particles of the compound containing W and Li may be scattered on a part of the secondary particles, that is, on the surface of any primary particle. The coating of the compound containing W and Li does not have to be present on the surface of all the primary particles constituting the secondary particles that can come into contact with the electrolyte. The fine particles of the compound containing W and Li may cover a part of the secondary particles, that is, a part of the surface of any primary particle. However, the coating of the compound containing W and Li preferably covers most or the entire surface of any primary particle, and may cover the surface of all the primary particles.

The fine particles of the compound containing W and Li may be scattered on the surface of the secondary particles. In other words, if the fine particles of the compound containing W and Li are present on at least a portion of the surface of the primary particles exposed to the outer surface and internal voids of the secondary particles constituting the positive electrode active material, the effect of reducing reaction resistance can be sufficiently obtained together with the presence of the coating of the compound containing W and Li.

However, the higher the proportion of fine particles of the compound containing W and Li on the surfaces of the secondary particles and primary particles, and the more uniformly they are present, the more likely it is that the effect of reducing reaction resistance will be achieved. In this regard, in the positive electrode active material of the present invention shown in Figure 3, when the compound containing W and Li is the same, finer fine particles are more uniformly present on the surfaces of the secondary particles or primary particles compared to the conventional positive electrode active material shown in Figure 4, and therefore such an effect can be fully obtained.

If fine particles and coatings are present unevenly between the particles that make up the lithium nickel-containing composite oxide, the movement of lithium ions between the particles will be uneven, placing a strain on certain particles and making it easy for cycle characteristics to deteriorate and reaction resistance to increase. Therefore, it is preferable that fine particles and coatings are present uniformly between the particles that make up the lithium nickel-containing composite oxide.

In this way, in the positive electrode active material of the present invention, fine particles of a compound containing W and Li are mixed with a coating of a compound containing W and Li and formed on at least a portion of the surface of the primary particles, so that a high effect on the battery characteristics can be obtained. This form provides a sufficient contact area with the electrolyte and effectively improves lithium ion conductivity, thereby improving the charge/discharge capacity and more effectively reducing reaction resistance.

The properties of the primary particles, including the fine particles and coating of the compound containing W and Li, can be determined, for example, by surface observation and cross-sectional observation using a scanning electron microscope (SEM) such as a field emission scanning electron microscope (FE-SEM), cross-sectional element mapping by EDX analysis using a scanning transmission electron microscope (STEM), cross-sectional observation using a transmission electron microscope (TEM), etc. By these means, it is possible to confirm that the fine particles and coating of the compound containing W and Li are formed on the surfaces of the primary particles that make up the composite oxide for the positive electrode active material of the present invention.

In the present invention, the compound containing W and Li that constitutes _the fine particles and coating of the compound containing W and Li is _preferably lithium tungstate.The lithium tungstate _{is preferably in} the form _of at least one selected from _Li2WO4 , _{Li4WO5 , Li6WO6 , Li2W4O13 , Li2W2O7 , Li6W2O9 , Li2W2O7 , Li2W5O16 , Li9W19O55 , Li3W10O30 , Li18W5O15 ,} and their _{hydrates.More preferably ,} the _lithium tungstate contains _{7Li2WO4.4H2O .} By forming such lithium tungstate, the lithium ion conductivity is further increased, and the effect of reducing the reaction resistance is further increased.

The number of W atoms contained in the compound containing W and Li is preferably 0.05 atomic % or more and 3.0 atomic % or less, more preferably 0.05 atomic % or more and 1.0 atomic % or less, and even more preferably 0.05 atomic % or more and 0.5 atomic % or less, relative to the total number of atoms of metals other than Li, i.e., Ni, Mn, Co, and the additive element M, contained in the particles constituting the positive electrode active material. This makes it possible to achieve both high charge/discharge capacity and output characteristics.

If the W content is less than 0.05 atomic %, the effect of improving the output characteristics may not be sufficient. If the W content exceeds 3.0 atomic %, the amount of coating with the fine particles and coating of the compound containing W and Li becomes too large, the BET specific surface area of the positive electrode active material decreases, and the bulk resistance of the electrode increases, so that a sufficient reaction resistance reduction effect may not be obtained.

The amount of Li contained in the fine particles and coating of the compound containing W and Li is not particularly limited, and as long as Li is contained, the effect of improving lithium ion conductivity can be obtained. However, it is preferable that this amount of Li is sufficient to form lithium tungstate.

(3) Composition The positive electrode active material of the present invention is made of a lithium nickel manganese cobalt-containing composite oxide having a ternary composition containing at least nickel, manganese, and cobalt, and the composition is not limited as long as it has a structure in which the fine particles and coating of the compound containing W and Li described above are present on at least a part of the surface of at least a part of the primary particles constituting the secondary particles.

However, when the amount of W in the fine particles and coating of the compound containing W and Li is 0.05 atomic % or more and _3.0 atomic % or less, the positive electrode active material of the present invention is preferably composed of a composite _{oxide represented} by general formula (A): Li1 _{+ uNixMnyCozWsMtO2} (-0.05≦u≦0.50, x+y+z+s+t=1, 0.3≦x _≦ 0.7, 0.15 _≦ y≦0.4, 0.15≦z≦0.4, 0.0005≦s≦0.03, 0≦t≦0.1, M is one or more added elements selected from Mg, Al, Si, Ca, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) and having a crystal structure of a hexagonal layered rock salt structure. It should be noted that W can be contained within the composite oxide as an additive element M. In this case, the amount of W (s in the composition formula) does not include the amount of W contained as the additive element M.

The value of u, which indicates the excess amount of lithium (Li), is preferably -0.05 or more and 0.50 or less, more preferably 0 or more and 0.50 or less, and even more preferably 0 or more and 0.35 or less. By restricting the value of u to the above range, it is possible to improve the output characteristics and battery capacity of a secondary battery using this positive electrode active material as a positive electrode material. In contrast, if the value of u is less than -0.05, the positive electrode resistance of the secondary battery becomes large, making it impossible to improve the output characteristics. On the other hand, if the value of u exceeds 0.50, not only does the initial discharge capacity decrease, but the positive electrode resistance also becomes large.

Nickel (Ni) is an element that contributes to increasing the potential and capacity of secondary batteries. From the viewpoint of forming a ternary positive electrode active material with a high nickel ratio, the value of x, which indicates its content, is preferably 0.3 to 0.7, more preferably 0.4 to 0.65, and even more preferably 0.5 to 0.6. If the value of x is less than 0.3, the energy density of the secondary battery cannot be sufficiently improved. On the other hand, if the value of x exceeds 0.7, the content of other elements decreases, and the effect of a ternary positive electrode active material cannot be obtained.

Manganese (Mn) is an element that contributes to improving thermal stability, and the value of y, which indicates its content, is preferably 0.15 to 0.4, more preferably 0.2 to 0.35. If the value of y is less than 0.15, the thermal stability of a secondary battery using this positive electrode active material cannot be improved. On the other hand, if the value of y exceeds 0.4, Mn will dissolve from the positive electrode active material during high-temperature operation, deteriorating the charge-discharge cycle characteristics.

Cobalt (Co) is an element that contributes to improving the charge-discharge cycle characteristics, and the value z, which indicates the content of Co, is preferably 0.15 to 0.4, more preferably 0.2 to 0.35. If the value of z exceeds 0.4, the initial discharge capacity of the secondary battery will decrease significantly.

In order to further improve the durability and output characteristics of the secondary battery, the positive electrode active material of the present invention may contain an additive element M in addition to the above-mentioned metal elements. As such an additive element M, one or more elements selected from magnesium (Mg), aluminum (Al), silicon (Si), calcium (Ca), titanium (Ti), vanadium (V), chromium (Cr), zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta), and tungsten (W) can be used.

The value of t, which indicates the content of the added element M, is preferably 0 to 0.1, more preferably 0.001 to 0.05. If the value of t exceeds 0.1, the amount of metal elements that contribute to the Redox reaction decreases, and the battery capacity of the secondary battery decreases.

Such an additive element M may be dispersed inside the composite oxide particles, or may coat the surface of the composite oxide particles. Furthermore, it may be dispersed inside the particles and then coat the surface. In either case, it is necessary to control the content of the additive element M so that it falls within the above range.

(4) Average particle size MV
The positive electrode active material of the present invention is adjusted so that the average particle size MV is 3 μm or more and 10 μm or less, preferably 4 μm or more and 9 μm or less, more preferably 4 μm or more and 8 μm or less. If the average particle size of the positive electrode active material is in such a range, not only can the battery capacity per unit volume of the secondary battery using this positive electrode active material be increased, but also the safety and output characteristics can be improved. On the other hand, if the average particle size MV is less than 3 μm, the filling property of this positive electrode active material is reduced, and the battery capacity per unit volume cannot be increased. On the other hand, if the average particle size MV exceeds 10 μm, the reaction area of this positive electrode active material is reduced, and the interface with the non-aqueous electrolyte is reduced, making it difficult to improve the output characteristics.

The mean particle size MV of the positive electrode active material means the volume-based mean particle size (Mean Volume Diameter), and can be calculated, for example, from the integrated volume value measured with a laser light diffraction/scattering particle size analyzer.

(5) Particle size distribution The positive electrode active material of the present invention is composed of secondary particles having a very narrow particle size distribution, with [(d90-d10)/average particle size MV] being 0.70 or less, preferably 0.60 or less, more preferably 0.55 or less. Such a positive electrode active material has a low ratio of fine particles and coarse particles, and a secondary battery using this has excellent safety, cycle characteristics, and output characteristics.

In contrast, when [(d90-d10)/average particle size MV] exceeds 0.70, the proportion of fine particles and coarse particles in the positive electrode active material increases. For example, if the proportion of fine particles is high, the secondary battery is more likely to generate heat due to local reactions of the fine particles, not only reducing safety but also resulting in poor cycle characteristics due to selective deterioration of the fine particles. In addition, if the proportion of coarse particles is high, the reaction area between the non-aqueous electrolyte and the positive electrode active material cannot be sufficiently secured, resulting in poor output characteristics.

When considering industrial-scale production, it is not realistic to use a positive electrode active material with an excessively small [(d90-d10)/average particle size MV]. Therefore, taking into account costs and productivity, it is preferable to set the lower limit of [(d90-d10)/average particle size MV] to approximately 0.25.

d10 means the particle size at which the cumulative volume of each particle, calculated from the smaller particle size side, is 10% of the total volume of all particles (the particle size at which the cumulative curve is 10% when the total volume is 100% and the cumulative curve of the particle size distribution is calculated). Similarly, d90 means the particle size at which the cumulative volume of each particle, calculated from the smaller particle size side, is 90% of the total volume of all particles (the particle size at which the cumulative curve is 90% when the total volume is 100% and the cumulative curve of the particle size distribution is calculated). d10 and d90 can be calculated from the volume integrated value measured by a laser light diffraction scattering particle size analyzer, just like the average particle size MV.

(6) Primary Particles In the positive electrode active material of the present invention, the primary particles constituting the outer shell and the aggregate are formed with an average particle size of 0.02 μm or more and 0.5 μm or less. The size of the primary particles is obtained by embedding the secondary particles in a resin or the like, making the cross-section observable by cross-section polishing, observing the cross-section using an SEM such as an FE-SEM, measuring the maximum outer diameter (major axis diameter) of 10 or more primary particles present in the cross-section of the secondary particles, and calculating the average value. If the average particle size of the primary particles is less than 0.02 μm, the particles may become brittle and may not provide sufficient battery performance. On the other hand, if the average particle size of the primary particles is more than 0.5 μm, the solid diffusion distance in the particles may become long, and may not provide sufficient battery performance. In the positive electrode active material of the present invention, each primary particle has a generally uniform composition.

(7) Shell The shell constituting the secondary particles is composed of an aggregate of primary particles. The thickness of the shell is preferably 0.1 μm or more and 1.5 μm or less. If the thickness of the shell is less than 0.1 μm, the strength of the secondary particles is not sufficiently guaranteed. On the other hand, if the thickness of the shell exceeds 1.5 μm, problems may occur such as an appropriate size of space and aggregates not being formed inside, and non-aqueous electrolyte not being sufficiently permeated inside the secondary particles. The thickness of the shell is preferably 0.1 μm or more and 1.2 μm or less, more preferably 0.12 μm or more and 1.0 μm or less.

(8) Tap density In order to extend the usage time of portable electronic devices and the driving distance of electric vehicles, it is important to increase the capacity of secondary batteries. On the other hand, the thickness of the electrodes of secondary batteries is required to be about several μm due to problems of packing and electronic conductivity of the entire battery. For this reason, it is necessary not only to use a high-capacity positive electrode active material, but also to improve the filling property of the positive electrode active material and increase the capacity of the entire secondary battery.

From this viewpoint, in the positive electrode active material of the present invention, even if it has a ternary composition containing at least nickel, manganese, and cobalt and has a porous structure, the tap density, which is an index of packing property (sphericity of secondary particles constituting the positive electrode active material), is preferably 1.0 g/cm ³ or more, more preferably 1.1 g/cm ³ or more and 1.8 g/cm ³ or less. When the tap density is less than 1.0 g/cm ³ , even if the BET specific surface area is increased, the packing property is low, and the battery capacity of the entire secondary battery may not be sufficiently improved. On the other hand, the upper limit of the tap density is not particularly limited, but in the case of the composition and particle structure of the present invention, the upper limit under normal manufacturing conditions is about 1.8 g/cm ^3. The tap density is preferably 1.2 g/cm ³ or more, more preferably 1.4 g/cm ³ or more.

Tap density is the bulk density of a powder sample collected in a container after tapping it 100 times, based on JIS Z-2504, and can be measured using a shaking specific gravity measuring device.

(9) BET specific surface area The positive electrode active material of the present invention is characterized in that the specific surface area is improved by the presence of spaces formed inside the secondary particles. As the specific surface area of the positive electrode active material in the present invention, for example, the BET specific surface area measured by the BET method using nitrogen gas adsorption is used. In the positive electrode active material of the present invention, as long as the above-mentioned secondary particle structure is maintained, the BET specific surface area is preferably as large as possible. This is because the larger the BET specific surface area, the larger the contact area with the non-aqueous electrolyte, and the output characteristics of the secondary battery using this can be significantly improved. Specifically, the BET specific surface area of the positive electrode active material of the present invention is preferably 2.0 m ² /g or more and 5.0 m ² /g or less. If the specific surface area of the positive electrode active material is less than 2.0 m ² /g, when a secondary battery is constructed using this positive electrode active material as a positive electrode material, the reaction area with the non-aqueous electrolyte cannot be sufficiently secured, and it is difficult to sufficiently improve the output characteristics. The BET specific surface area is more preferably 2.5 m ² /g or more and 4.5 m ² /g or less, and further preferably 3.0 m ² /g or more and 4.0 m ² /g or less.

(10) Porosity The positive electrode active material of the present invention has a porosity of 10% or more and 50% or less, which is measured by observing the cross section of the secondary particles and is an index of the ratio of the voids formed inside the secondary particles. By controlling the porosity of the secondary particles within this range, it is possible to obtain a high-density positive electrode while ensuring a contact area with the electrolyte by achieving both a larger specific surface area and a higher tap density. This allows a secondary battery having a high output density per volume to be obtained. The porosity of the secondary particles is preferably 15% or more and 45%, and more preferably 20% or more and 40% or less.

The porosity of secondary particles can be measured by observing an arbitrary cross section of the secondary particles using a scanning electron microscope (SEM) and performing image analysis. Specifically, multiple secondary particles are embedded in resin or the like, a cross-section sample is prepared by cross-section polishing, etc., and the cross section of the secondary particles can be observed using a scanning electron microscope. Then, for any 20 or more secondary particles, image analysis software (WinRoof 6.1.1, etc.) is used to detect the voids (spaces) in the secondary particles in black and the dense parts within the outline of the secondary particles in white, and the total area of the black and white parts of the 20 or more secondary particles is measured, and the porosity can be calculated by calculating the area ratio of [black parts/(black parts + white parts)]. The porosity of the secondary particles after coating is not affected by the presence of the fine particles and coating of W and Li compounds, and is equivalent to the porosity of the secondary particles (base material) before coating.

2. Method for Producing Positive Electrode Active Material for Lithium Ion Secondary Battery A second aspect of the present invention relates to a method for producing a positive electrode active material for a lithium ion secondary battery, i.e., a lithium nickel manganese cobalt-containing composite oxide coated with a compound containing W and Li, in which fine particles and a coating of a compound containing W and Li are present on at least a portion of the surface of at least a portion of the primary particles constituting the secondary particles.

In the method for producing a positive electrode active material for a lithium ion secondary battery of the present invention, a ternary composite oxide having a porous structure having the above-mentioned structure, except for the presence of fine particles and a coating of a compound containing W and Li, is used as a base material. That is, a ternary composite oxide is used as a base material, which is composed of secondary particles formed by agglomeration of a plurality of primary particles, and the secondary particles have a porous structure including an outer shell portion formed by the agglomerated primary particles, an agglomeration portion present inside the outer shell portion, formed by the agglomerated primary particles, and electrically conductive with the outer shell portion, and a space portion present dispersed within the agglomeration portion.

In particular, the method for producing a positive electrode active material for a lithium ion secondary battery of the present invention comprises the steps of:
a dry mixing step of mixing a tungsten compound in an amount of 0.1 mass % to 5 mass % based on the total mass of the composite oxide to obtain a mixture;
a water spray mixing step of spraying water onto the mixture in an amount of 1% by mass or more and 30% by mass or less with respect to the total mass of the mixture while stirring the mixture, thereby mixing the mixture;
A heat treatment step of subjecting the mixture after the water spray mixing step to a heat treatment at a temperature of 500° C. or less; and
a drying step of drying the mixture after the heat treatment step at a temperature of 500° C. or less to obtain the lithium nickel manganese cobalt-containing composite oxide coated with the compound containing W and Li, in which fine particles of the compound containing W and Li are present on the surfaces of the primary particles;
Equipped with
At least in the drying step, a vacuum drying and mixing device is used to perform the drying in a vacuum atmosphere.

The manufacturing method of the positive electrode active material for lithium ion secondary batteries of the present invention will be described below with reference to Figure 1.

(1) Dry mixing step The dry mixing step is a step of dry mixing a composite oxide and a tungsten compound to disperse the tungsten compound on the surfaces of the primary particles of the composite oxide. By dry mixing these powders together before mixing by water spraying, the tungsten compound can be more uniformly dispersed in the composite oxide.

The amount of tungsten compound added is preferably 0.1 mass% or more and 5 mass% or less based on the total mass of the composite oxide. This allows the amount of W dispersed on the surface of the composite oxide to be 0.05 atomic % or more and 3.0 atomic % or less based on the total number of atoms of metals other than lithium contained in the composite oxide, i.e., Ni, Mn, Co, and the additive element M.

That is, if the amount of the tungsten compound added is less than 0.1% by mass relative to the total mass of the composite oxide, the fine particles of the compound containing W and Li may be too fine or the coating may be too thin in the final positive electrode active material, resulting in insufficient lithium ion conductivity, or the formation of the coating by the fine particles and coating may be uneven, resulting in insufficient reduction in reaction resistance. If the amount of the tungsten compound added is more than 5% by mass, the amount of coating by the fine particles and coating of the compound containing W and Li may be too thick in the final positive electrode active material, resulting in a decrease in the BET specific surface area of the positive electrode active material and an increase in the bulk resistance of the electrode, resulting in insufficient reduction in reaction resistance.

The amount of tungsten compound added is preferably 0.2% by mass or more and 4% by mass or less, and more preferably 0.2% by mass or more and 2% by mass or less, relative to the total mass of the composite oxide. This allows the amount of W dispersed on the surface of the composite oxide to be 0.05 atomic % or more and 1.0 atomic % or less, and even more preferably 0.05 atomic % or more and 0.5 atomic % or less.

By adding a tungsten compound in this range, it is possible to more uniformly form fine particles and coatings of compounds containing W and Li on the surfaces of the primary particles of the positive electrode active material, forming a Li conduction path at the interface with the electrolyte and further reducing the reaction resistance of the positive electrode active material.

The tungsten compound may be in the form of either an alkaline solution in which a tungsten compound is dissolved (hereinafter referred to as "alkaline solution (W)") or a tungsten compound.

When added as an alkaline solution (W), the tungsten compound may be one that is soluble in an alkaline solution, and it is preferable to use a tungsten compound that is easily soluble in alkali, such as tungsten oxide, lithium tungstate, or ammonium tungstate.

As the alkali used in the alkaline solution (W), in order to obtain a high charge/discharge capacity, a general alkaline solution that does not contain impurities that are harmful to the positive electrode active material, such as ammonia or lithium hydroxide, can be used. It is preferable to use lithium hydroxide in order to make it possible to supply a sufficient amount of Li from the surplus Li and the alkaline solution (W) to form a compound containing W and Li, and from the viewpoint of not inhibiting the intercalation of Li.

On the other hand, when added in the form of a tungsten compound, it is preferable to use a tungsten compound _that is soluble in alkali as the tungsten compound, it is more preferable to use a tungsten compound containing lithium, and it is even more preferable to use _{lithium tungstate. As the lithium tungstate, at least one selected from Li2WO4, Li4WO5 , and Li6W2O9 can} be used.

In either case, the processing conditions and means for adding the tungsten compound must be selected so that the amount of W in the fine particles and coating of the compound containing W and Li, such as lithium tungstate, formed on the surface of the particles that make up the positive electrode active material, falls within the range of the present invention. Note that these processing conditions and means for adding are publicly known, so a detailed description will be omitted here.

It is preferable to carry out the dry mixing step for 5 to 50 minutes while stirring at a peripheral speed of 5 to 30 m/s. If either the peripheral speed of stirring or the process time in the dry mixing step is below the lower limit, the tungsten compound may not be sufficiently and uniformly dispersed on the surface of the primary particles of the composite oxide. On the other hand, if either the peripheral speed of stirring or the process time in the dry mixing step is above the upper limit, the positive electrode active material may be pulverized, resulting in an excessively small particle size.

In the dry mixing process, the peripheral speed of stirring is preferably 6 m/s or more and 25 m/s or less. The process time of the dry mixing process is preferably 10 minutes or more and 30 minutes or less.

In the dry mixing step, a known mixing device can be used as long as it is capable of mixing the composite oxide and the tungsten compound while stirring them. From the viewpoint of process continuity, it is preferable to use a vacuum dry mixing device, which will be described later, in this step as well.

(2) Water spray mixing step The water spray mixing step is a step of spraying water onto the mixture of the obtained composite oxide and the tungsten compound while stirring the mixture, thereby mixing the mixture. This makes it possible to more uniformly proceed with the neutralization reaction between excess lithium such as lithium hydroxide liberated on the surfaces of the primary particles of the composite oxide and the tungsten compound such as tungsten oxide in the presence of water.

It is preferable to use pure water. In the water of the mixture, the unreacted lithium compounds present in the composite oxide and the excess lithium present in the crystals dissolve, and the mixed tungsten compounds such as tungsten oxide also dissolve.

The amount of water to be sprayed (total amount) is preferably 1% by mass or more and 30% by mass or less, more preferably 4% by mass or more and 20% by mass or less, and even more preferably 5% by mass or more and 15% by mass or less, based on the entire composite oxide. If the amount of water to be sprayed is less than 1% by mass, the effect of adding water is not fully exerted, and tungsten compounds may remain. On the other hand, if the amount of water to be sprayed exceeds 30% by mass, the amount of lithium (excess lithium amount) eluted from the composite oxide, which is the base material, into the water during the period from water spraying to drying becomes excessive, and the lithium content in the obtained positive electrode active material decreases, and the effect of reducing resistance may not be fully obtained. In addition, if the amount of water to be sprayed exceeds 30% by mass, a long drying time is required to reduce the moisture content in the obtained positive electrode active material, and productivity may decrease. In addition, the mixture may aggregate, the viscosity of the mixture may increase, and the resistance during stirring may increase, resulting in the positive electrode active material being crushed.

The water to be sprayed is preferably in a mist state with an average particle size of 300 μm or less. When mist water is added, the neutralization reaction between the excess lithium and the tungsten compound on the surface of the primary particles of the composite oxide can proceed more uniformly, and the particle size and average film thickness of the coating of the fine particles containing W and Li formed on the surface of the primary particles constituting the positive electrode active material can be set to a suitable range. If the average particle size of the water to be sprayed is larger than 300 μm, the amount of lithium elution differs between the secondary particles to which moisture is directly supplied by spraying and the secondary particles to which moisture is supplied by stirring after spraying, which may cause non-uniformity in the reaction.

In order to form the fine particles and coating of the compound containing W and Li on the surface of the primary particles, the moisture content of the powder after water spray mixing and before drying is preferably 3.0% by mass or more and 10% by mass or less, and more preferably 3.0% by mass or more and less than 6.5% by mass. By restricting this moisture content to the above range, the fine particles and coating of the compound containing W and Li can be formed on the surface of the primary particles in a mixed state. When the moisture content of the powder is 6.5% by mass or more, there is a tendency that the coating is formed more than the fine particles of the compound containing W and Li on the surface of the primary particles, and when it exceeds 10% by mass, the fine particles of the compound containing W and Li may become coarse and the coating may become thick.

The moisture content of the powder before drying after water spray mixing is calculated from the mass of the powder before drying after water spray mixing and the mass of the powder after drying at 180°C for 3 hours using the formula: "moisture content = (mass of powder before drying after water spray mixing - mass of powder after drying) / (mass of powder before drying after water spray mixing)."

In particular, in the method for producing a positive electrode active material for a lithium ion secondary battery of the present invention, in the water spray mixing step, a vacuum dry mixer is used to spray water at a water spray rate of 0.01 ml/min to 0.1 ml/min per 1 g of the mixture while stirring at a peripheral speed of 4 m/sec to 30 m/sec, and after the water spraying, it is preferable to continue mixing for 5 minutes to 120 minutes while stirring at a peripheral speed of 4 m/sec to 30 m/sec.

In the water spray mixing process, water can be sprayed using multiple nozzles to shorten the spray time. In this case, the water spray rate of each nozzle is set so that the total water spray rate of all nozzles is 0.01 ml/min to 0.1 ml/min. It is preferable to set the water spray rate per nozzle to 0.005 ml/min to 0.05 ml/min.

By carrying out such a treatment, it becomes possible to ensure that the tungsten compound is present sufficiently and uniformly on the surface of the primary particles of the composite oxide, and it is also possible to appropriately regulate the moisture content, resulting in the formation of finer and more uniformly dispersed microparticles of the compound containing W and Li, and a coating of the compound containing W and Li that is more appropriately thick and more uniformly formed.

In the water spray mixing process, particularly in the process of continuing to mix the mixture with water after water spraying, if either the peripheral speed of stirring or the water spraying speed falls below the lower limit, the tungsten compound cannot be sufficiently and uniformly dispersed on the surface of the primary particles of the composite oxide until the particle diameter of the W and Li-containing compound fine particles is 5 nm or more and 400 nm or less, or until the average film thickness of the W and Li-containing compound coating is 1 nm or more and 200 nm or less, or the reaction with lithium becomes insufficient, and a large amount of tungsten compounds such as tungsten oxide may remain. On the other hand, if either the peripheral speed of stirring or the water spraying speed in the water spray mixing process exceeds the upper limit, the positive electrode active material is pulverized and the water does not reach the mixture uniformly, which may cause the reaction to become biased and cause segregation of the W and Li-containing compound fine particles and coating.

In the water spray mixing process, the peripheral speed of the stirring water spray is preferably 6 m/s or more and 24 m/s or less, and more preferably 6 m/s or more and 20 m/s or less.

The water spray rate is preferably 0.015 ml/min to 0.07 ml/min, more preferably 0.02 ml/min to 0.06 ml/min. When spraying water using multiple nozzles, the total water spray rate of all nozzles is preferably 0.01 ml/min to 0.08 ml/min, more preferably 0.015 ml/min to 0.07 ml/min. The water spray rate of each nozzle is preferably 0.007 ml/min to 0.03 ml/min.

The process time for water spraying is determined by the amount of water sprayed and the water spraying speed, but is preferably from 5 to 30 minutes, and more preferably from 10 to 25 minutes.

The peripheral speed of the stirring in the subsequent continued mixing is preferably 6 m/s to 25 m/s, more preferably 6 m/s to 20 m/s. The process time for continuing this mixing is more preferably 5 minutes to 90 minutes, even more preferably 10 minutes to 45 minutes.

In the water spray mixing process, a known mixing device can be used as long as it is possible to spray water and mix while stirring the composite hydroxide and the tungsten compound, but from the viewpoint of process continuity, it is preferable to use a vacuum dry mixing device, which will be described later, in this process as well.

(3) Heat Treatment Step The heat treatment step is a step of performing a heat treatment at a temperature of 500° C. or less on a mixture obtained by mixing a composite oxide, a tungsten compound, and water before drying under reduced pressure, which will be described later. In the heat treatment step, it is preferable to perform heat treatment at a temperature of 40° C. or more and 200° C. or less for a time of 15 minutes to 120 minutes in an air atmosphere while stirring at a peripheral speed of 4 m/s to 30 m/s. By performing such a treatment, the water in the tank can be evaporated before drying under reduced pressure, so that the water in the positive electrode active material can be removed more efficiently.

The peripheral speed of the stirring during the heat treatment is more preferably 6 m/s or more and 25 m/s or less. The heat treatment temperature is more preferably 80°C or more and 120°C or less. The process time for carrying out the heat treatment is more preferably 30 minutes or more and 90 minutes or less.

In the heat treatment step, a known heat treatment device can be used, but from the viewpoint of processing efficiency, it is preferable to use the vacuum drying and mixing device used in the drying step.

(4) Drying Step The drying step is a step for obtaining a positive electrode active material made of a compound-coated complex oxide containing W and Li by drying a mixture obtained by mixing a complex oxide, a tungsten compound, and water. The compound-coated complex oxide containing W and Li is basically made of a compound containing W and Li, such as lithium tungstate, formed by reacting a tungsten compound, such as tungsten oxide, with excess lithium in the base compound oxide in the presence of water.

Specifically, in the drying step, the mixture is heat-treated at a temperature of 500°C or less to dry the mixture, and a lithium nickel manganese cobalt-containing composite oxide coated with the compound containing W and Li is obtained, in which fine particles and a coating of the compound containing W and Li are present on the surfaces of the primary particles.

The temperature of the drying process is not particularly limited as long as the moisture content is sufficiently reduced, but it is preferable to dry at a temperature of 500°C or less. If the drying temperature exceeds 500°C, lithium will be further liberated from the inside of the primary particles, and sufficient slurry stability will not be obtained. The lower limit of the drying temperature is not particularly limited, but from the viewpoint of efficiency, it is preferable that it is 70°C or higher. A more preferable temperature for the drying process is a temperature of 70°C or higher and 200°C or lower.

In particular, the method for producing a positive electrode active material for a lithium ion secondary battery of the present invention is characterized in that drying is performed under a vacuum atmosphere using a vacuum drying and mixing device. It is preferable to perform reduced pressure drying under a vacuum atmosphere at a temperature of 70° C. to 200° C. for a time of 60 minutes to 300 minutes while stirring at a peripheral speed of 4 m/s to 30 m/s.

By carrying out such a treatment, it becomes possible to sufficiently and uniformly react the tungsten compound with the surface of the primary particles that make up the secondary particles of the complex oxide, and the friction between the secondary particles that make up the positive electrode active material, which occurs due to the stirring during reduced pressure drying, causes the fine particles of the compound containing W and Li to be dispersed more finely and more uniformly, and a compound coating containing W and Li is formed in a thin and uniform coating, forming a complex oxide coated with the compound containing W and Li.

If any of the stirring speed, processing temperature, and process time during reduced pressure drying falls below the lower limit, the tungsten compound may not be able to react sufficiently and uniformly with the surface of the primary particles of the composite oxide. Furthermore, if stirring and mixing is not performed during reduced pressure drying, the degree of evaporation of the water attached to the positive electrode active material varies, and segregation of the fine particles and coating of the compound containing W and Li is likely to occur in areas where water is difficult to evaporate, and the particle size of the fine particles of the compound containing W and Li may locally increase, or the average film thickness of the coating of the compound containing W and Li may become too thick.

On the other hand, if any of the stirring speed, processing temperature, and processing time in the drying process exceeds the upper limit, the secondary particles that make up the positive electrode active material are pulverized, and particles with small particle diameters are generated, which may deteriorate the particle size distribution of the positive electrode active material.

During reduced pressure drying, the peripheral speed of the stirring is preferably 6 m/s or more and 25 m/s or less, and even more preferably 6 m/s or more and 20 m/s or less.

During reduced pressure drying, the processing temperature is preferably in the range of 75°C to 180°C, and more preferably 80°C to 160°C.

The process time for drying under reduced pressure is preferably 80 minutes or more and 200 minutes or less, and more preferably 90 minutes or more and 150 minutes or less.

The pressure of the vacuum atmosphere during reduced pressure drying is 0.5 MPa or less, preferably 0.1 MPa or less, and more preferably 0.05 MPa or less. If the pressure during reduced pressure drying is higher than 0.5 MPa, the moisture content of the mixture may not be sufficiently reduced within the process time range described above.

(5) Cooling Step After the reduced pressure drying in the drying step, it is preferable to carry out reduced pressure cooling in which the mixture is cooled to 25° C. for 60 to 240 minutes while stirring at a peripheral speed of 1 to 20 m/s in a vacuum atmosphere using a vacuum drying and mixing device. By carrying out such a process, it is possible to minimize the moisture content of the mixture.

It is more preferable that the peripheral speed during reduced pressure cooling in a vacuum atmosphere is 2 m/s or more and 15 m/s or less. It is more preferable that the process time for this reduced pressure cooling is 90 minutes or more and 150 minutes or less.

(6) Vacuum Drying and Mixing Apparatus In the method for producing a positive electrode active material for a lithium ion secondary battery of the present invention, it is preferable to use a vacuum drying and mixing apparatus as an apparatus for mixing the mixture.

Any vacuum drying and mixing device can be used as long as it can mix the composite oxide and tungsten compound within the device, can spray water, and can reduce the pressure to a vacuum atmosphere of 0.1 MPa or less.

Examples of such vacuum drying equipment include a Henschel mixer, a vacuum mixing dryer, and a vibration dryer.

In the present invention, a ternary composite oxide having the porous structure described above, except for the presence of fine particles and coating of a compound containing W and Li, is used as the base material. That is, the ternary composite oxide is composed of secondary particles formed by agglomeration of a plurality of primary particles, and the secondary particles have a porous structure with an outer shell formed by the agglomerated primary particles, an agglomeration part inside the outer shell formed by the agglomerated primary particles and electrically conductive with the outer shell, and a space part dispersed within the agglomeration part. The ternary composite oxide has a moderate strength and its hardness is not excessive for the use of a vacuum dry mixing device, so that the vacuum dry mixing device can be applied continuously in the water spray mixing process, heat treatment process, and drying process. It is also possible to use the vacuum dry mixing device continuously in all of the dry mixing process, water spray mixing process, heat treatment process, drying process, and cooling process.

By using the vacuum dry mixing device in this way, the mixture is mixed and dried continuously while being mixed, so that the mixture is sufficiently stirred, and the tungsten compound can be sufficiently and uniformly reacted on the surface of the primary particles of the composite oxide, and the moisture content can be appropriately regulated, making it possible to form finer and more uniformly dispersed W and Li-containing compound particles and a more uniformly formed coating of the W and Li-containing compound. In addition, by using the vacuum dry mixing device to mix the mixture under the same conditions, it is possible to suppress the variation in product quality from lot to lot. Furthermore, by using the vacuum dry mixing device, mechanical mixing is possible while spraying water, improving the mixing efficiency and making it possible to uniformly react the tungsten compound. In addition, the vacuum dry mixing device is a device that integrates a mixer and a dryer, and is a device that can mix while drying.

In the present invention, by taking advantage of these two features, it is possible to obtain a lithium nickel manganese cobalt-containing composite oxide coated with a compound containing W and Li, which is coated with fine particles and a coating of the compound containing W and Li in a highly dispersed state. This has resulted in a dramatic improvement in the dispersibility of the fine particles of the compound containing W and Li after coating, and the uniformity of the coating of the compound containing W and Li, which were low in the conventional manufacturing method due to the lack of stirring during drying.

In the method for producing a positive electrode active material for a lithium ion secondary battery of the present invention, the powder finally obtained after the cooling step is sieved to finally obtain a positive electrode active material made of a lithium nickel manganese cobalt-containing composite oxide coated with a compound containing W and Li, the surface of which is coated with fine particles and a coating of a compound containing W and Li.

(7) Difference in average particle size MV of secondary particles before and after coating with compound containing W and Li By applying the manufacturing method of the present invention, the ratio of the average particle size MV of secondary particles constituting the positive electrode active material after coating with the compound containing W and Li to the average particle size MV of secondary particles constituting the complex oxide before coating with the compound containing W and Li is 90% or more. This ratio is preferably 95% or more.

In the present invention, when forming a compound coating containing W and Li on a base material, the strength of the secondary particles of the base material is high, so that the secondary particles of the base material are hardly pulverized in the dry mixing process, water spray mixing process, and drying process using a vacuum dry mixing device. Therefore, the average particle size MV of the secondary particles of the base material, the index showing the spread of the particle size distribution of the secondary particles [(d90-d10)/average particle size MV], and the tap density generally inherit the characteristics of the base material, and in particular, the decrease in the average particle size MV is suppressed.

(8) Manufacturing Method of Lithium Nickel Manganese Cobalt-Containing Composite Oxide as Base Material The manufacturing method of the positive electrode active material for lithium ion secondary batteries of the present invention is characterized in that a ternary composite oxide with a porous structure is used as a base material, that is, a ternary composite oxide composed of secondary particles formed by agglomeration of a plurality of primary particles, the secondary particles having a porous structure including an outer shell portion formed by the agglomerated primary particles, an agglomeration portion present inside the outer shell portion, formed by the agglomerated primary particles and electrically conductive with the outer shell portion, and a space portion present and dispersed within the agglomeration portion; however, the manufacturing method of this composite oxide is not limited.

However, the manufacturing method for such porous composite oxides will be briefly explained below.

(8-1) Method for Producing Nickel Manganese Cobalt-Containing Composite Hydroxide The particle structure of such a composite oxide having a porous structure inherits the particle structure of the precursor nickel manganese cobalt-containing composite hydroxide (hereinafter referred to as "composite hydroxide"). The composite hydroxide having this particle structure is preferably produced by the following crystallization reaction.

(a) Crystallization Reaction The composite hydroxide is obtained by supplying a raw material aqueous solution containing at least a transition metal and an aqueous solution containing an ammonium ion donor into a reaction vessel to form a reaction aqueous solution, and then carrying out a crystallization reaction.

The process of carrying out the crystallization reaction includes a nucleation process in which nuclei are generated by controlling the pH value of the reaction aqueous solution at a standard liquid temperature of 25°C to 12.0 or more and 14.0 or less, and a particle growth process in which the nuclei are grown by controlling the pH value of the reaction aqueous solution containing the nuclei obtained in the nucleation process at a standard liquid temperature of 25°C to a value lower than the pH value of the nucleation process and to 10.5 or more and 12.0 or less.

Then, (1) adjust the reaction atmosphere in the nucleation process and the first stage (initial stage) of the particle growth process to a non-oxidizing atmosphere with an oxygen concentration of 2% by volume or less, (2) switch the reaction atmosphere from the non-oxidizing atmosphere to an oxidizing atmosphere with an oxygen concentration of more than 2% by volume and not more than 5% by volume after the first stage of the particle growth process, to form a second stage of the particle growth process, (3) switch the reaction atmosphere from the oxidizing atmosphere to the non-oxidizing atmosphere to form a third stage of the particle growth process, (4) switch the reaction atmosphere from the non-oxidizing atmosphere to the oxidizing atmosphere to form a fourth stage of the particle growth process, and (5) finally switch the reaction atmosphere from the oxidizing atmosphere to the non-oxidizing atmosphere to form a fifth stage of the particle growth process.

The switching of the reaction atmosphere is defined as the ratio of the amount of metal added at each stage to the total amount of metal added in the particle growth process. The ratio of the crystallization reaction at each stage to the entire particle growth process is preferably 8% to 20% in the first stage, 2% to 20% in the second stage, 12% to 40% in the third stage, 3% to 40% in the fourth stage, and 10% to 50% in the fifth stage.

[Nucleation process]
In the nucleation step, a compound of a transition metal, which is the raw material in this step, is first dissolved in water to prepare a raw material aqueous solution. At the same time, an alkaline aqueous solution and an aqueous solution containing an ammonium ion donor are supplied and mixed into a reaction tank to prepare a pre-reaction aqueous solution having a pH value of 12.0 to 14.0 measured at a liquid temperature of 25° C. and an ammonium ion concentration of 3 g/L to 25 g/L. The pH value of the pre-reaction aqueous solution can be measured by a pH meter, and the ammonium ion concentration can be measured by an ion meter.

Next, while stirring this pre-reaction aqueous solution, the raw material aqueous solution is supplied. As a result, an aqueous solution for nucleation, which is the aqueous solution for reaction in the nucleation process, is formed in the reaction tank. Since the pH value of this aqueous solution for nucleation is in the above-mentioned range, in the nucleation process, nuclei do not grow much and nucleation occurs preferentially. In the nucleation process, the pH value and ammonium ion concentration of the aqueous solution for nucleation change with nucleation, so it is necessary to supply an alkaline aqueous solution and an ammonia aqueous solution at an appropriate time to maintain the pH value of the liquid in the reaction tank at pH 12.0 to 14.0 at a liquid temperature of 25°C and the ammonium ion concentration at 3 g/L to 25 g/L.

In the nucleation step, it is necessary to circulate an inert gas through the reaction vessel and adjust the reaction atmosphere to a non-oxidizing atmosphere with an oxygen concentration of 2% by volume or less. The reaction atmosphere is usually adjusted preferably before starting to supply the raw material aqueous solution. This allows sufficient formation of agglomerates within the positive electrode active material using this composite hydroxide as a precursor, making it possible to suppress a decrease in particle density due to the formation of voids.

In the nucleation process, new nuclei are continuously generated by supplying the aqueous solution for nucleation with an aqueous solution containing the raw material aqueous solution, an aqueous alkali solution, and an aqueous solution containing an ammonium ion donor. Then, when a predetermined amount of nuclei are generated in the aqueous solution for nucleation, the nucleation process is terminated.

In this case, the amount of nuclei generated can be determined from the amount of metal compounds contained in the raw aqueous solution supplied to the aqueous solution for nucleation. The amount of nuclei generated in the nucleation process is not particularly limited, but in order to obtain secondary particles of composite hydroxides with a narrow particle size distribution, it is preferable to set the amount of nuclei to 0.1 atomic % or more and 2 atomic % or less, and more preferably 0.1 atomic % or more and 1.5 atomic % or less, of the metal elements in the metal compounds contained in the raw aqueous solution supplied through the nucleation process and the particle growth process. The reaction time in the nucleation process is usually 0.2 minutes or more and 5 minutes or less.

[Particle growth process]
After the nucleation step is completed, the pH value of the aqueous solution for nucleation in the reaction tank is adjusted to 10.5 or more and 12.0 or less based on a liquid temperature of 25°C, to form an aqueous solution for particle growth, which is a reaction aqueous solution in the particle growth step. The pH value can also be adjusted by stopping the supply of the aqueous alkaline solution, but in order to obtain secondary particles of composite hydroxide with a narrow particle size distribution, it is preferable to stop the supply of all aqueous solutions once and adjust the pH value. Specifically, it is preferable to adjust the pH value by supplying the aqueous solution for nucleation with the same type of inorganic acid as the acid constituting the raw metal compound after stopping the supply of all aqueous solutions.

Next, while stirring the aqueous solution for particle growth, the supply of the raw material aqueous solution is resumed. At this time, since the pH value of the aqueous solution for particle growth is within the above-mentioned range, almost no new nuclei are generated, and nucleus (particle) growth progresses, forming secondary particles of composite hydroxide having a predetermined particle size. Note that, even during the particle growth process, the pH value and ammonium ion concentration of the aqueous solution for particle growth change with particle growth, so it is necessary to supply an alkaline aqueous solution and an ammonia aqueous solution at appropriate times to maintain the pH value and ammonium ion concentration within the above-mentioned range.

In particular, in the method for producing a composite hydroxide of the present invention, during the particle growth step, the supply of the raw material aqueous solution is continued while atmospheric gas is introduced to switch the reaction atmosphere from a non-oxidizing atmosphere to an oxidizing atmosphere in which the oxygen concentration is more than 2% by volume and not more than 5% by volume, or from this oxidizing atmosphere to a non-oxidizing atmosphere in which the oxygen concentration is 2% by volume or less.

In the particle growth process, it is preferable to use an air diffuser to circulate an inert gas and/or an oxidizing gas through the aqueous reaction solution in the reaction tank, and to quickly switch from an oxidizing atmosphere to a non-oxidizing atmosphere, or from a non-oxidizing atmosphere to an oxidizing atmosphere. The inert gas and/or the oxidizing gas can be supplied to the aqueous reaction solution in the reaction tank by supplying it to the space in the reaction tank that is in contact with the aqueous reaction solution, but it is preferable to use an air diffuser to directly supply the inert gas and/or the oxidizing gas into the aqueous reaction solution. This shortens the time required to switch the atmosphere, and makes it possible to appropriately set the size of the center, the thickness of the first low-density layer, the high-density layer, the second low-density layer, and the outer shell layer.

In such a method for producing a composite hydroxide, in the nucleation step and the particle growth step, the metal ions precipitate as nuclei or primary particles. Therefore, the ratio of liquid components to metal components in the aqueous solution for nucleation and the aqueous solution for particle growth increases. As a result, the apparent concentration of the raw material aqueous solution decreases, and in particular, in the particle growth step, the growth of secondary particles of the composite hydroxide may stagnate. Therefore, in order to suppress the increase in the liquid components, it is preferable to discharge a part of the liquid components of the aqueous solution for particle growth outside the reaction tank after the end of the nucleation step and during the particle growth step. Specifically, it is preferable to temporarily stop the supply and stirring of the aqueous solution containing the raw material aqueous solution, the alkaline aqueous solution, and the ammonium ion donor, settle the nuclei and composite hydroxide in the aqueous solution for particle growth, and discharge the supernatant of the aqueous solution for particle growth. By such an operation, the relative concentration of the mixed aqueous solution in the aqueous solution for particle growth can be increased, preventing stagnation of particle growth and controlling the particle size distribution of the secondary particles of the obtained composite hydroxide within a suitable range, as well as improving the density of the secondary particles as a whole.

[Particle size control of secondary particles of composite hydroxide]
The particle size of the secondary particles of the composite hydroxide can be controlled by the time of the particle growth step or nucleation step, the pH value of the aqueous solution for nucleation or the aqueous solution for particle growth, or the supply amount of the raw aqueous solution. For example, by performing the nucleation step at a high pH value or by lengthening the time of the particle generation step, the amount of metal compound contained in the raw aqueous solution supplied can be increased, the amount of nuclei generated can be increased, and the particle size of the secondary particles of the composite hydroxide obtained can be reduced. Conversely, by suppressing the amount of nuclei generated in the nucleation step, the particle size of the secondary particles of the composite hydroxide obtained can be increased.

[Another embodiment of the crystallization reaction]
In the method for producing a composite hydroxide of the present invention, a component adjusting aqueous solution adjusted to a pH value and ammonium ion concentration suitable for the particle growth step may be prepared separately from the nucleation aqueous solution, and the nucleation aqueous solution after the nucleation step, preferably the nucleation aqueous solution after the nucleation step from which some of the liquid components have been removed, may be added to and mixed with this component adjusting aqueous solution to form the particle growth aqueous solution, and the particle growth step may be carried out using this aqueous solution.

In this case, the nucleation process and the particle growth process can be separated more reliably, so that the reaction aqueous solution in each process can be controlled to an optimal state. In particular, the pH value of the aqueous solution for particle growth can be controlled to an optimal range from the start of the particle growth process, so that the particle size distribution of the secondary particles of the resulting composite hydroxide can be made narrower.

(8-2) Aqueous Solution to be Supplied a) Raw Aqueous Solution In the present invention, the ratio of metal elements contained in the raw aqueous solution is approximately the composition ratio of the resulting composite hydroxide.

The transition metal compound used to prepare the raw aqueous solution is not particularly limited, but from the standpoint of ease of handling, it is preferable to use water-soluble nitrates, sulfates, chlorides, etc., and from the standpoint of cost and preventing the inclusion of halogens, it is particularly preferable to use sulfates.

In addition, when the composite hydroxide contains an additive element M (wherein M is one or more additive elements selected from Mg, Al, Si, Ca, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W), the compound for supplying the additive element M is preferably a water-soluble compound, such as magnesium sulfate, aluminum sulfate, sodium silicate, calcium sulfate, titanium sulfate, ammonium peroxotitanate, potassium titanium oxalate, vanadium sulfate, ammonium vanadate, chromium sulfate, potassium chromate, zirconium sulfate, niobium oxalate, ammonium molybdate, hafnium sulfate, sodium tantalate, sodium tungstate, and ammonium tungstate.

The concentration of the raw material aqueous solution, in terms of the total metal compounds, is preferably 1 mol/L or more and 2.6 mol/L or less, and more preferably 1.5 mol/L or more and 2.2 mol/L or less.

The amount of raw material aqueous solution supplied is preferably such that the concentration of the product in the aqueous solution for particle growth at the end of the particle growth process is 30 g/L or more and 200 g/L or less, more preferably 80 g/L or more and 150 g/L or less.

b) Alkaline aqueous solution The alkaline aqueous solution for adjusting the pH value in the reaction aqueous solution is not particularly limited, and a general aqueous solution of an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide can be used. The alkali metal hydroxide can be added directly to the reaction aqueous solution, but it is preferable to add it as an aqueous solution from the viewpoint of ease of pH control. In this case, the concentration of the aqueous solution of the alkali metal hydroxide is preferably 20% by mass or more and 50% by mass or less, more preferably 20% by mass or more and 30% by mass or less.

There are no particular limitations on the method of supplying the alkaline aqueous solution, so long as the pH value of the reaction solution does not become locally high and is maintained within a specified range. For example, the reaction solution may be supplied using a pump capable of flow control, such as a metering pump, while thoroughly stirring the reaction solution.

c) Aqueous Solution Containing Ammonium Donor The aqueous solution containing the ammonium ion donor is not particularly limited, and for example, an aqueous solution of ammonia water, ammonium sulfate, ammonium chloride, ammonium carbonate, ammonium fluoride, or the like can be used.

When using ammonia water as the ammonium ion donor, its concentration is preferably 20% by mass or more and 30% by mass or less, more preferably 22% by mass or more and 28% by mass or less. By restricting the concentration of ammonia water to this range, it is possible to minimize the loss of ammonia due to evaporation, etc., and therefore it is possible to improve production efficiency.

The aqueous solution containing the ammonium ion donor can be supplied using a pump capable of controlling the flow rate, similar to the alkaline aqueous solution.

[pH value]
In the method for producing a composite hydroxide of the present invention, it is necessary to control the pH value of the reaction aqueous solution at a liquid temperature of 25°C to 12.0 or more and 14.0 or less in the nucleation step, and to 10.5 or more and 12.0 or less in the particle growth step. In either step, it is preferable to control the fluctuation range of the pH value during the crystallization reaction within ±0.2. If the fluctuation range of the pH value is large, the ratio of the amount of nucleation and the particle growth will not be constant, making it difficult to obtain secondary particles of a composite hydroxide with a narrow particle size distribution. The pH value of the reaction aqueous solution can be measured by a pH meter.

[Reaction atmosphere]
a) Non-oxidizing atmosphere In the manufacturing method of the present invention, the reaction atmosphere in the stage of forming the center, high density layer, and outer shell layer of the secondary particle of the composite hydroxide is a non-oxidizing atmosphere. Specifically, by introducing a non-oxidizing gas such as an inert gas, the reaction atmosphere is controlled to a mixed atmosphere of oxygen and inert gas so that the oxygen concentration in the reaction atmosphere is 2% by volume or less, preferably 1% by volume or less.

b) Oxidizing atmosphere On the other hand, in the stage of forming the first low-density layer and the second low-density layer of the secondary particles of the composite hydroxide, the reaction atmosphere is controlled to be an oxidizing atmosphere. Specifically, the oxygen concentration in the reaction atmosphere is controlled to be more than 2% by volume and less than 5% by volume, preferably more than 3% by volume. By controlling the oxygen concentration in the reaction atmosphere to be in such a range, particle growth is suppressed, and the average particle size of the fine primary particles is 0.02 μm or more and 0.3 μm or less, so that the first low-density layer and the second low-density layer having a sufficient density difference with the center, the high-density layer, and the outer shell layer can be formed. When the oxygen concentration in the reaction atmosphere exceeds 5% by volume, in a ternary composite hydroxide having a composition containing at least nickel, manganese, and cobalt, the average particle size of the fine primary particles is less than 0.02 μm, and the total amount of the fine primary particles absorbed in the high-density layer and the outer shell layer becomes insufficient, and in the lithium nickel manganese cobalt-containing composite oxide obtained using such a composite hydroxide as a precursor, the particle strength is not sufficiently improved. On the other hand, if the oxygen concentration in the reaction atmosphere is 2% by volume or less, the average particle size of the fine primary particles will exceed 0.3 μm, and the difference in particle size between the plate-like primary particles constituting the center, high-density layer, and outer shell layer and the fine primary particles constituting the first low-density layer and the second low-density layer will become too small, which may result in insufficient formation of spaces due to shrinkage of the low-density layer.

For the same reason, the difference in oxygen concentration between the non-oxidizing atmosphere and the oxidizing atmosphere is preferably 1.0% by volume or more, and more preferably 2.0% by volume or more.

c) Switching of reaction atmosphere The crystallization reaction in a non-oxidizing atmosphere in the first stage (initial stage) of the particle growth stage is preferably 8% or more and 20% or less, more preferably 10% or more and 18% or less, of the entire particle growth process time.

The crystallization reaction in the oxidizing atmosphere in the second stage (including the time to switch from the non-oxidizing atmosphere to the oxidizing atmosphere) is preferably 2% to 20% and more preferably 3% to 15% of the total particle growth process time. However, the proportion of the total crystallization reaction in the oxidizing atmosphere in the second and fourth stages should preferably not deviate from the range of 4% to 45%, and more preferably 5% to 35%.

The crystallization reaction in the non-oxidizing atmosphere in the third stage (including the time for switching from the oxidizing atmosphere to the non-oxidizing atmosphere) is preferably 12% to 40% and more preferably 14% to 34% of the total particle growth process time, and the second switch to the oxidizing atmosphere is performed from the start of the particle growth process for 3% to 40% and more preferably 4% to 25% of the total particle growth process.

The crystallization reaction in the oxidizing atmosphere in the fourth stage (including the time for switching from a non-oxidizing atmosphere to an oxidizing atmosphere) is preferably 1.2 to 2.5 times, more preferably 1.5 to 2.0 times, the rate of the crystallization reaction in the second stage, and is preferably 3% to 40%, more preferably 4% to 25% of the total particle growth process time.

The crystallization reaction in the non-oxidizing atmosphere in the fifth stage (including the time to switch from the oxidizing atmosphere to the non-oxidizing atmosphere) is preferably 10% to 50%, more preferably 25% to 45%, of the total particle growth process time, so that a sufficient outer shell skeleton is formed. After a predetermined time has elapsed, the crystallization reaction is finally terminated.

The rate of crystallization reaction at each stage is defined as the ratio of the amount of metal added at each stage to the total amount of metal added in the particle growth process. In actual operation, these can be controlled so that the time of each crystallization reaction is a specified ratio to the total time of the particle growth process by supplying a constant amount of raw material aqueous solution and keeping the amount of metal added constant.

By such a crystallization reaction,
The secondary particles are formed by agglomeration of a plurality of plate-like primary particles and fine primary particles smaller than the plate-like primary particles,
the secondary particles have a center portion mainly formed by the aggregation of the plate-like primary particles, a first low-density layer formed mainly by the aggregation of the fine primary particles outside the center portion, a high-density layer formed mainly by the aggregation of the plate-like primary particles outside the first low-density layer, a second low-density layer formed mainly by the aggregation of the fine primary particles outside the high-density layer, and an outer shell layer formed mainly by the aggregation of the plate-like primary particles outside the second low-density layer;
The tap density is 0.95 g/ ^cm3 or more,
The average particle size MV of the secondary particles is 3 μm or more and 12 μm or less, and the index showing the spread of the particle size distribution of the secondary particles, [(d90-d10)/average particle size MV], is 0.65 or less.
A composite hydroxide is obtained.

(9) Method for Producing Lithium Nickel Manganese Cobalt-Containing Composite Oxide as Base Material The composite oxide as base material is obtained by a mixing step of mixing the composite hydroxide with a lithium compound to form a lithium mixture, and a firing step of firing the lithium mixture formed in the mixing step at a temperature of 650° C. or higher and 920° C. or lower in an oxidizing atmosphere.

(9-1) Heat Treatment Step This is an optional step in which the composite hydroxide is heat-treated into particles (composite hydroxide from which excess water has been removed and/or composite oxide converted into oxide) before being mixed with the lithium compound.

The heat treatment temperature is between 105°C and 750°C, which reduces the amount of moisture remaining in the heat-treated particles after the firing process to a certain level and suppresses variation in the composition of the composite oxide, which is the base material obtained.

The heat treatment time is not particularly limited, but from the viewpoint of sufficiently removing excess water in the composite hydroxide, it is preferable to set it to at least 1 hour, and more preferably to 5 hours or more and 15 hours or less.

(9-2) Mixing Step The mixing step is a step of mixing the composite hydroxide or the heat-treated particles with a lithium compound to obtain a lithium mixture.

In the mixing step, the composite hydroxide or heat-treated particles and the lithium compound are mixed so that the ratio (Li/Me) of the sum of the number of atoms of metal atoms other than lithium in the lithium mixture, specifically, nickel manganese cobalt and the added element M (Me) to the number of lithium atoms (Li) is 0.95 or more and 1.5 or less, preferably 1.0 or more and 1.5 or less, more preferably 1.0 or more and 1.35 or less, and even more preferably 1.0 or more and 1.2 or less.

The lithium compound used in the mixing step is not particularly limited, but it is preferable to use lithium hydroxide, lithium nitrate, lithium carbonate, or a mixture of these because of their ease of availability. In particular, it is preferable to use lithium hydroxide or lithium carbonate because of their ease of handling and quality stability.

It is preferable to mix the composite hydroxide or heat-treated particles and the lithium compound thoroughly enough to avoid the formation of fine powder. If the mixing is insufficient, the Li/Me ratio will vary between individual particles, and sufficient battery characteristics may not be obtained. A general mixer can be used for mixing. For example, a shaker mixer, a Loedige mixer, a Julia mixer, a V blender, etc. can be used.

(9-3) Calcination Step When lithium hydroxide or lithium carbonate is used as the lithium compound, after the mixing step and before the calcination step, a calcination step may be performed in which the lithium mixture is calcined at a temperature lower than the calcination temperature described below and at 350° C. or higher and 800° C. or lower, preferably 450° C. or higher and 780° C. or lower. This allows lithium to be sufficiently diffused in the composite hydroxide or heat-treated particles, and a more uniform lithium composite oxide can be obtained.

The holding time at the above temperature is preferably 1 hour to 10 hours, more preferably 3 hours to 6 hours. The atmosphere in the calcination step is preferably an oxidizing atmosphere, similar to the firing step described below, and more preferably an atmosphere with an oxygen concentration of 18% by volume to 100% by volume.

(9-4) Calcination Step The calcination step is a step in which the lithium mixture obtained in the mixing step is calcined under predetermined conditions to diffuse lithium into the composite hydroxide or the heat-treated particles, thereby obtaining a lithium composite oxide.

In this firing process, the central portion, the high density layer, and the plate-like primary particles constituting the high density layer of the composite hydroxide and heat-treated particles shrink during sintering while absorbing the fine primary particles, and the primary particles after sintering form the outer shell and aggregate portions of the positive electrode active material. Note that the fine primary particles begin to sinter at a lower temperature range than the plate-like primary particles, and shrink more than the plate-like primary particles, so the first low density layer and the second low density layer are absorbed by the central portion, the high density layer, and the outer shell layer, where sintering progresses more slowly, and moderately sized spaces are formed as pores. At this time, the plate-like primary particles shrink during sintering while maintaining the connection with the adjacent plate-like primary particles, so that in the obtained positive electrode active material, electrical conduction is established between the outer shell and the aggregate portions, and the cross-sectional area of the path can be sufficiently secured. As a result, the internal resistance of the positive electrode active material is significantly reduced, and when a secondary battery is constructed, it is possible to improve the output characteristics without impairing the battery capacity or cycle characteristics.

The particle structure of such composite oxides is basically determined by the particle structure of the precursor composite hydroxide, but it can be affected by the composition and sintering conditions, so it is preferable to conduct preliminary tests and then adjust the respective conditions appropriately to obtain the desired structure.

The furnace used in the firing step is not particularly limited, but it is preferable to use a batch or continuous electric furnace. The same applies to the furnaces used in the heat treatment step and the calcination step.

a) Firing temperature The firing temperature of the lithium mixture needs to be 650°C or more and 920°C or less. If the firing temperature is less than 650°C, lithium does not diffuse sufficiently in the composite hydroxide or heat-treated particles, and excess lithium or unreacted composite hydroxide or heat-treated particles remain, or the crystallinity of the resulting positive electrode active material becomes insufficient. On the other hand, if the firing temperature exceeds 920°C, the pores in the secondary particles of the positive electrode active material may be crushed, and the secondary particles of the positive electrode active material may be sintered violently, causing abnormal grain growth, and increasing the proportion of amorphous coarse particles. From the viewpoint of controlling the agglomeration and space constituting the secondary particles within the appropriate size range, the firing temperature of the lithium mixture is preferably 700°C or more and 920°C or less, and more preferably 750°C or more and 900°C or less.

The heating rate in the baking step is preferably 1.5 ° C./min to 10° C./min, more preferably 5° C./min to 10° C./min. Furthermore, during the baking step, it is more preferable to maintain the temperature near the melting point of the lithium compound for preferably 1 hour to 5 hours, more preferably 2 hours to 5 hours.

b) Firing Time During the firing time, the holding time at the above-mentioned firing temperature is preferably at least 2 hours, and more preferably 4 hours to 24 hours. If the holding time at the firing temperature is less than 2 hours, lithium does not sufficiently diffuse into the composite hydroxide or heat-treated particles, and excess lithium or unreacted composite hydroxide or heat-treated particles may remain, or the crystallinity of the obtained positive electrode active material may be insufficient.

After the holding time has ended, the cooling rate from the firing temperature to at least 200° C. is preferably 2° C./min to 10° C./min, more preferably 3° C./min to 7° C./min.

c) Firing atmosphere The firing atmosphere is preferably an oxidizing atmosphere, more preferably an atmosphere with an oxygen concentration of 18% by volume or more and 100% by volume or less, and particularly preferably a mixed atmosphere of oxygen and an inert gas (air or oxygen stream) with the above oxygen concentration. If the oxygen concentration is less than 18% by volume, the crystallinity of the positive electrode active material may be insufficient.

(9-5) Crushing step This is an optional step for crushing the aggregates or sintered bodies when the secondary particles constituting the composite oxide obtained by the firing step are aggregated or lightly sintered. This allows the average particle size and particle size distribution of the obtained positive electrode active material to be adjusted to a suitable range.

Disintegration is carried out using known means, such as a pin mill or hammer mill, by adjusting the disintegration force to an appropriate range so as not to destroy the secondary particles.

3. Lithium-ion secondary battery The lithium-ion secondary battery of the present invention can have the same configuration as a general non-aqueous electrolyte secondary battery, including components such as a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. Alternatively, the lithium-ion secondary battery of the present invention can have the same configuration as a general solid electrolyte secondary battery, including components such as a positive electrode, a negative electrode, and a solid electrolyte. That is, the present invention can be widely applied to non-aqueous electrolyte secondary batteries and all-solid-state lithium secondary batteries, as long as the secondary battery is one that charges and discharges by desorption and insertion of lithium ions. Note that the embodiments described below are merely examples, and the present invention can be applied to lithium-ion secondary batteries in various forms that have been modified and improved based on the embodiments described in this specification.

(1) Constituent Members a) Positive Electrode Using the above-mentioned positive electrode active material, a positive electrode of a lithium ion secondary battery is produced, for example, as follows.

First, the conductive material and binder are mixed with the positive electrode active material of the present invention, and activated carbon and a solvent for viscosity adjustment, etc. are added as necessary, and these are kneaded to prepare a positive electrode composite paste. At this time, the mixing ratio of each in the positive electrode composite paste is also an important factor that determines the performance of the lithium ion secondary battery. For example, if the solid content of the positive electrode composite excluding the solvent is 100 parts by mass, the content of the positive electrode active material can be 60 parts by mass or more and 95 parts by mass or less, the content of the conductive material can be 1 part by mass or more and 20 parts by mass or less, and the content of the binder can be 1 part by mass or more and 20 parts by mass or less, as in the case of the positive electrode of a general lithium ion secondary battery.

The obtained positive electrode composite 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. The method for producing the positive electrode is not limited to the above-mentioned example, and other methods may also be used.

Examples of conductive materials that can be used include graphite (natural graphite, artificial graphite, expanded graphite, etc.) and carbon black-based materials such as acetylene black and ketjen black.

The binder serves to hold the active material particles together, and may be, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose-based resin, or polyacrylic acid.

In addition, if necessary, a solvent that disperses the positive electrode active material, conductive material, and activated carbon and dissolves the binder can be 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, metallic lithium, lithium alloys, etc. may be used. In addition, a negative electrode active material capable of absorbing and desorbing lithium ions may be mixed with a binder, and an appropriate solvent may be added to form a paste of the negative electrode mixture, which may be applied to the surface of a metal foil collector such as copper, dried, and compressed to increase the electrode density as necessary.

As the negative electrode active material, for example, a material containing lithium such as metallic lithium or a lithium alloy, natural graphite capable of absorbing and desorbing lithium ions, 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, a fluorine-containing resin 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 In a non-aqueous electrolyte secondary battery, a separator is disposed between a positive electrode and a negative electrode, and has a function of separating the positive electrode and the negative electrode and retaining the non-aqueous electrolyte. As such a separator, for example, a thin membrane such as polyethylene or polypropylene having a large number of fine pores can be used, but there is no particular limitation as long as it has the above function.

d) Electrolyte The nonaqueous electrolyte used in the nonaqueous electrolyte secondary battery is, for example, a nonaqueous electrolytic solution obtained by dissolving a lithium salt, which serves as a supporting electrolyte, in an organic solvent.

The organic solvent used in the non-aqueous electrolyte may be one selected from the following: 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. These may be used alone or in combination of two or more.

As _the supporting salt, _LiPF6 , _LiBF4 , _LiClO4 , _LiAsF6 , LiN( _CF3SO2 ) ₂ , and composite salts thereof can be used.

The non-aqueous electrolyte may contain radical scavengers, surfactants, flame retardants, etc.

On the other hand, as a solid electrolyte used in _a solid electrolyte secondary battery such as _an all-solid-state lithium secondary battery, _{Li1.3Al0.3Ti1.7} ( _PO4 ) ₃ or _Li2S - _SiS2 can be used. The solid electrolyte has a property of being able to withstand high voltage. The solid electrolyte includes an inorganic solid electrolyte and an organic solid electrolyte.

Inorganic solid electrolytes include oxide solid electrolytes and sulfide solid electrolytes.

As the oxide solid electrolyte, an oxide that contains oxygen (O) and has lithium ion conductivity and electronic insulation properties can be used. For example, 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), LiTi ₂ ( _PO4 ) _{3 , Li3XLa2 /3-XTiO3 ( 0 ≦} X≦ ₂ / ₃ ) _{, Li5La3Ta2O12 , Li7La3Zr2O12 , Li6BaLa2Ta2O12, Li3.6Si0.6P0.4O4 , and the like can} be used _.

As the sulfide solid electrolyte, a sulfide containing sulfur (S) and having lithium ion conductivity and electronic insulation can be used. For example, 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 , LiI-Li ₃ PO ₄ -P ₂ S ₅ and the like can be used.

As the inorganic solid electrolyte other than the oxide solid electrolyte and the sulfide solid electrolyte, for example, Li ₃ N, LiI, Li ₃ N-LiI-LiOH, etc. can be used.

As the organic solid electrolyte, a polymeric compound exhibiting ion conductivity can be used. For example, polyethylene oxide, polypropylene oxide, and copolymers thereof can be used. The organic solid electrolyte can also contain a supporting salt (lithium salt).

When using a solid electrolyte, the solid electrolyte can be mixed into the positive electrode material to ensure contact between the electrolyte and the positive electrode active material.

(2) Structure of Lithium-Ion Secondary Battery The structure of the lithium-ion secondary battery is not particularly limited, and may be a non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, a separator, a non-aqueous electrolyte, etc., or a solid electrolyte secondary battery having a positive electrode, a negative electrode, a solid electrolyte, etc. The shape of the secondary battery is not particularly limited, and may be a cylindrical shape, a laminated shape, or various other shapes.

In the case of a non-aqueous electrolyte secondary battery, for example, a positive electrode and a negative electrode are laminated with a separator between them to form an electrode body, the resulting electrode body is impregnated with a non-aqueous electrolyte, and the positive electrode current collector and the positive electrode terminal connected to the outside, and the negative electrode current collector and the negative electrode terminal connected to the outside are connected using current collecting leads or the like, and the battery is sealed in a battery case to complete the lithium ion secondary battery.

(3) Characteristics of Lithium-Ion Secondary Battery As described above, the lithium-ion secondary battery of the present invention uses a ternary positive electrode active material with a porous structure in which the W- and Li-containing compound particles are present on at least a part of the surface of at least a part of the primary particles constituting the secondary particles, the W- and Li-containing compound particles are finer and more uniform, and the W- and Li-containing compound coating is more appropriately thick and more uniform, and therefore has excellent capacity characteristics, output characteristics, and cycle characteristics. Therefore, compared with a conventional lithium-ion secondary battery using a ternary positive electrode active material with a porous structure that does not have a compound coating containing W and Li, or a ternary positive electrode active material that has a compound coating containing W and Li but does not have a porous structure, it is possible to exhibit battery characteristics of lower resistance and higher output.

(4) Uses of Lithium-Ion Secondary Battery As described above, the lithium-ion secondary battery of the present invention has excellent capacity characteristics, output characteristics, and cycle characteristics, and can be suitably used as a power source for small portable electronic devices (such as notebook personal computers, smartphones, tablet terminals, and digital cameras) that require a high level of these characteristics. In addition, the lithium-ion secondary battery of the present invention can be not only miniaturized and has high output, but also has excellent safety and durability, and can simplify expensive protection circuits, so that it can also be suitably used as a power source for transportation equipment such as electric vehicles that are limited in mounting space.

The present invention will be described in detail below using examples and comparative examples. In the following examples and comparative examples, unless otherwise specified, special-grade reagent samples manufactured by Wako Pure Chemical Industries, Ltd. were used to prepare the composite hydroxide and the positive electrode active material. In addition, throughout the nucleation process and particle growth process, the pH value of the reaction aqueous solution was measured using a pH controller (NPH-690D manufactured by Nisshin Rika Co., Ltd.), and the supply amount of the sodium hydroxide aqueous solution was adjusted based on this measurement value to control the fluctuation range of the pH value of the reaction aqueous solution in each process to within the range of ±0.2.

Example 1
a) Production of composite hydroxide [Nucleation step]
First, 14 L of water was put into the reaction tank and the temperature in the tank was set to 40° C. while stirring. At this time, nitrogen gas was circulated in the reaction tank for 30 minutes, and the reaction atmosphere was made a non-oxidizing atmosphere with an oxygen concentration of 2% by volume or less. Next, an appropriate amount of 25% by mass aqueous sodium hydroxide solution and 25% by mass aqueous ammonia were supplied into the reaction tank, and the pH value was adjusted to 12.6 at a liquid temperature of 25° C. and the ammonium ion concentration was adjusted to 10 g/L to form a pre-reaction aqueous solution.

At the same time, nickel sulfate, manganese sulfate, and cobalt sulfate were dissolved in water so that the molar ratio of each metal element was Ni:Mn:Co = 1:1:1 to prepare a 2 mol/L raw material aqueous solution.

Next, the raw material aqueous solution was fed to the pre-reaction aqueous solution at 115 ml/min to form an aqueous solution for the nucleation process, and nucleation was carried out for 1 minute. At this time, 25% by mass of sodium hydroxide aqueous solution and 25% by mass of ammonia water were fed at appropriate times to maintain the pH value and ammonium ion concentration of the aqueous solution for nucleation within the above-mentioned ranges.

[Particle growth process]
After the nucleation was completed, the supply of all the aqueous solutions was temporarily stopped, and sulfuric acid was added to adjust the pH value to 11.2 at a liquid temperature of 25° C., thereby forming an aqueous solution for particle growth. After it was confirmed that the pH value had reached a predetermined value, the raw material aqueous solution was supplied at a constant rate of 115 ml/min, the same as in the nucleation step, to grow the nuclei (particles) generated in the nucleation step.

In the first stage, crystallization in a non-oxidizing atmosphere was continued for 35 minutes (14.6% of the entire particle growth process) from the start of the particle growth process, and then, while continuing to supply the raw material aqueous solution, air was circulated into the reaction tank using a ceramic air diffuser with a pore size of 20 μm to 30 μm (manufactured by Kinoshita Rika Kogyo Co., Ltd.) to adjust the reaction atmosphere to an oxidizing atmosphere with an oxygen concentration of 5% by volume (switching operation 1).

In the second stage, crystallization in an oxidizing atmosphere was continued for 20 minutes (8.3% of the entire particle growth process) from switching operation 1, and then, while continuing to supply the raw material aqueous solution, nitrogen was circulated into the reaction tank using an aeration tube to adjust the reaction atmosphere to a non-oxidizing atmosphere with an oxygen concentration of 2% by volume or less (switching operation 2).

In the third stage, crystallization in a non-oxidizing atmosphere was continued for 65 minutes (27.1% of the entire particle growth process) from switching operation 2, and then, while continuing to supply the raw material aqueous solution, air was circulated into the reaction tank using an air diffuser to adjust the atmosphere to an oxidizing atmosphere with an oxygen concentration of 5% by volume (switching operation 3).

In the fourth step, the crystallization reaction in an oxidizing atmosphere was continued for 25 minutes (10.4% of the entire particle growth process) from switching operation 3, and then, while continuing to supply the raw material aqueous solution, nitrogen was circulated into the reaction tank using an aeration tube to adjust the reaction atmosphere to a non-oxidizing atmosphere with an oxygen concentration of 2% by volume or less (switching operation 4).

In the fifth stage, the crystallization reaction in the non-oxidizing atmosphere was continued from switching operation 4 for 95 minutes (39.6% of the entire particle growth process), and then the supply of all aqueous solutions, including the raw aqueous solution, was stopped to terminate the particle growth process. The resulting product was then washed with water, filtered, and dried to obtain 4,500 g of powdered composite hydroxide. The proportion of the crystallization reaction in the entire oxidizing atmosphere was 18.7%.

In addition, during the particle growth process, 25% by mass of sodium hydroxide aqueous solution and 25% by mass of ammonia water were supplied at appropriate times throughout the process to maintain the pH value and ammonium ion concentration of the aqueous solution for particle growth within the above-mentioned ranges.

b) Evaluation of composite hydroxide [Composition]
Analysis using an ICP optical emission spectrometer (Shimadzu Corporation, ICPE-9000) confirmed that the composition of this composite hydroxide was represented by the general formula: Ni _0.33 Mn _0.33 Co _0.33 (OH) ₂ .

[Particle structure]
A part of the composite hydroxide was embedded in a resin, and processed with a cross-section polisher (IB-19530CP, manufactured by JEOL Ltd.) to make it possible to observe the cross section, and then 10 or more secondary particles of the composite hydroxide were observed with a field emission scanning electron microscope (FE-SEM: JSM-6360LA, manufactured by JEOL Ltd.). As a result, it was confirmed that this composite hydroxide has a center portion formed by the aggregation of plate-like primary particles, and that outside the center portion, the composite hydroxide is composed of a first low-density layer formed by the aggregation of fine primary particles, a high-density layer formed by the aggregation of plate-like primary particles, a second low-density layer formed by the aggregation of fine primary particles, and an outer shell layer formed by the aggregation of plate-like primary particles, and that some of the plate-like primary particles constituting the center portion, the high-density layer, and the outer shell layer are mutually connected. The maximum outer diameters (major axis diameters) of 10 or more fine primary particles and plate-like primary particles present in the cross section of the secondary particle were measured, their average values were calculated, and this value was taken as the particle diameter of the fine primary particles or plate-like primary particles in this secondary particle. Next, the particle diameters of the fine primary particles and plate-like primary particles of 10 or more secondary particles were calculated in the same manner, and the average particle diameters obtained for these secondary particles were calculated. The average particle diameter of the plate-like primary particles was 0.53 μm, and the average particle diameter of the fine primary particles was 0.09 μm.

The ratios of the radius of the center to the diameter of the secondary particles, as well as the thicknesses of the first low-density layer, the high-density layer, the second low-density layer, and the outer shell layer, were also measured and calculated to be 25%, 10%, 5%, 4%, and 6%, respectively.

[Average particle size MV and particle size distribution]
Using a laser light diffraction scattering type particle size analyzer (Microtrac MT3300EXII, manufactured by Microtrac Bell Co., Ltd.), the average particle size MV of the secondary particles of the composite hydroxide was measured, and d10 and d90 were measured to calculate [(d90-d10)/average particle size MV], which is an index showing the spread of the particle size distribution. As a result, it was confirmed that the average particle size MV was 5.4 μm and [(d90-d10)/average particle size MV] was 0.50.

[Tap density]
The tap density was measured using a tapping machine (KRS-406, manufactured by Kuramochi Scientific Instruments Manufacturing Co., Ltd.) and was found to be 1.15 g/ ^cm3 .

c) Preparation of composite oxide as base material The composite hydroxide obtained as described above was heat-treated in an air stream (oxygen concentration: 21% by volume) at 120° C. for 12 hours (heat treatment step), and then thoroughly mixed with lithium hydroxide using a shaker mixer (TURBULA Type T2C, manufactured by WAB) so that the Li/Me ratio was 1.10, thereby obtaining a lithium mixture (mixing step).

This lithium mixture was heated to 800°C at a heating rate of 1.5°C/min in an oxygen (oxygen concentration: 100% by volume) stream, sintered by holding at this temperature for 3 hours, and then cooled to room temperature at a cooling rate of about 4°C/min (sintering process). The lithium nickel manganese cobalt-containing composite oxide thus obtained had agglomerated or lightly sintered. For this reason, the composite oxide was crushed to adjust the average particle size and particle size distribution (crushing process).

d) Evaluation of composite oxide [Composition]
Analysis using an ICP emission spectrometer (Shimadzu Corporation, ICPE-9000) confirmed that the composition of this composite oxide was represented by the general formula: Li _1.10 Ni _0.33 Mn _0.33 Co _0.33 O ₂ .

[Particle structure]
A part of the complex oxide was embedded in resin, processed with a cross-section polisher (IB-19530CP, manufactured by JEOL Ltd.) to make it possible to observe the cross section, and then observed with a SEM (FE-SEM: JSM-6360LA, manufactured by JEOL Ltd.). As a result, it was confirmed that this complex oxide is composed of secondary particles formed by agglomeration of multiple primary particles, and that this secondary particle has an outer shell part, an agglomeration part that exists in a dispersed state inside the outer shell part and is electrically conductive with the outer shell part, and a space part consisting of a pore structure in which no primary particles exist and exists between the agglomerations inside the outer shell part. In addition, from the observation of a cross-sectional image including any 10 or more secondary particles obtained from the above SEM observation, the average thickness of the outer shell part of the secondary particle was 0.8 μm.

[Average particle size MV and particle size distribution]
Using a laser light diffraction/scattering particle size analyzer (Microtrack MT3300EXII, manufactured by Microtrack Bell Co., Ltd.), the average particle size MV of the secondary particles of the composite oxide was measured, and d10 and d90 were measured to calculate [(d90-d10)/average particle size MV], which is an index showing the spread of the particle size distribution. As a result, it was confirmed that the average particle size MV was 5.1 μm and [(d90-d10)/average particle size MV] was 0.42.

[BET specific surface area, tap density, and particle strength]
The BET specific surface area was measured using a flow type gas adsorption specific surface area measuring device (Maxsorb 1200 series, manufactured by Mountec Co., Ltd.), and the tap density was measured using a tapping machine (KRS-406, manufactured by Kuramochi Scientific Instruments Manufacturing Co., Ltd.) As a result, it was confirmed that the BET specific surface area was 2.6 m ² /g and the tap density was 1.5 g/cm ³ .

e) Preparation of Positive Electrode Active Material The composite oxide (3000 g) obtained as described above was used as a base material, and tungsten oxide (56 g) was added in an amount of W of 0.35 atomic % relative to the total number of Ni, Mn, and Co atoms contained in the composite oxide. Using a Henschel mixer (FM10C/I, manufactured by Nippon Coke and Engineering Co., Ltd.), the composite oxide and tungsten oxide were thoroughly mixed under atmospheric pressure at a peripheral speed of the Henschel mixer of 8 m/sec for 10 minutes to obtain a mixture (dry mixing step).

After the dry mixing process was completed, the peripheral speed of the Henschel mixer was set to 6 m/sec and the mixture was mixed under atmospheric pressure. At the same time, a water spray nozzle (Ikeuchi Corporation, hollow cone nozzle K010) was used to spray pure water into the mixture in a mist with an average particle size of 100 μm or less at a water spray rate of 0.025 ml/min per 1 g of mixture, in an amount that was 12.3% by mass of the total mass of the mixture. The peripheral speed of the Henschel mixer was then set to 8 m/sec and mixing of the mixture was continued for another 10 minutes (water spray mixing process). After water spray mixing, a portion of the mixture was used as a sample and its moisture content was measured, which was 5%.

After the water spray mixing process was completed, the peripheral speed of the Henschel mixer was maintained at 8 m/s and the mixture was heat-treated at atmospheric pressure and at a temperature of 120°C for 30 minutes. After that, the settings of the Henschel mixer were changed so that the inside of the Henschel mixer container was a vacuum atmosphere of 0.1 MPa or less, and the mixture was dried under reduced pressure at a temperature of 150°C for 90 minutes in a vacuum atmosphere. After that, the peripheral speed of the Henschel mixer was reduced to 2 m/s and the mixture was cooled under reduced pressure to 25°C in a vacuum atmosphere for 90 minutes (drying process).

Then, the obtained positive electrode active material was sieved using a sieve with a mesh size of 38 μm.

f) Evaluation of Positive Electrode Active Material [Average Particle Size MV and Particle Size Distribution]
Using a laser light diffraction scattering type particle size analyzer (Microtrack MT3300EXII, manufactured by Microtrack Bell Co., Ltd.), the average particle size MV of the positive electrode active material was measured, and d10 and d90 were measured to calculate [(d90-d10)/average particle size MV], which is an index showing the spread of the particle size distribution. As a result, it was confirmed that the average particle size MV was 4.8 μm (a ratio to the average particle size MV of the composite oxide before coating with the compound containing W and Li: 94%), and [(d90-d10)/average particle size MV] was 0.42.

[BET specific surface area and tap density]
The BET specific surface area was measured using a flow type gas adsorption specific surface area measuring device (Maxsorb 1200 series, manufactured by Mountec Co., Ltd.), and the tap density was measured using a tapping machine (KRS-406, manufactured by Kuramochi Scientific Instruments Manufacturing Co., Ltd.) As a result, it was confirmed that the BET specific surface area was 3.1 ^m2 /g and the tap density was 1.5 g/ ^cm3 .

[composition]
The composition of the obtained positive electrode active material was analyzed by ICP emission spectrometry, and it was confirmed that the W content was 0.35 atomic % based on the total number of Ni, Mn, and Co atoms.

[Microparticles and Coatings]
From the results of powder XRD analysis using an X-ray diffraction (XRD) device (Spectris, _X'Pert PRO), it was confirmed that the fine particles of a compound containing W and Li on the surface of the primary particles consisted of lithium _tungstate ( _7Li2WO4.4H2O ).

After this positive electrode active material was prepared so that the surface could be observed using a scanning transmission electron microscope (STEM: Hitachi High-Technologies Corporation, scanning electron microscope S-4700), the vicinity of the surface of the secondary particles was observed using SEM, confirming that fine particles with an average particle size of 66 nm and particle sizes in the range of 26 nm to 301 nm were present uniformly on the surface of the secondary particles. The SEM image obtained is shown in Figure 3. Furthermore, after the cross-section was prepared so that it could be observed using a scanning transmission electron microscope, the cross-section of the secondary particles was observed using SEM, confirming that a coating with an average thickness of 3 nm was present on the entire surface and part of the interior of the secondary particles.

[Porosity]
Cross-sectional images of the secondary particles taken with a scanning transmission electron microscope were analyzed using image analysis software (WinRoof 6.1.1, etc.) to measure the porosity of the secondary particles, which was found to be 32%.

g) Preparation of secondary battery A 2032-type coin-shaped battery as shown in Fig. 5 was prepared. Specifically, 52.5 mg of the positive electrode active material obtained as described above, 15 mg of acetylene black, and 7.5 mg of PTFE were mixed and pressed at a pressure of 100 MPa to prepare a positive electrode 11 (electrode for evaluation) as shown in Fig . 5 with a diameter of 11 mm and a thickness of 100 µm. The obtained positive electrode 11 was dried in a vacuum dryer at 120°C for 12 hours.

Next, a 2032-type coin battery was produced using this positive electrode 11 in a glove box in an Ar atmosphere with a dew point controlled at -80°C. For the negative electrode 12, a negative electrode sheet was used in which graphite powder with an average particle size of about 20 μm and polyvinylidene fluoride were applied to copper foil, punched into a disk shape with a diameter of 14 mm, and an equal mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with 1M LiPF ₆ as the supporting electrolyte (manufactured by Toyama Pharmaceutical Co., Ltd.) was used for the non-aqueous electrolyte. For the separator 13, a polyethylene porous film with a thickness of 25 μm was used. In this way, a 2032-type coin battery having a gasket 14 and a wave washer 15, a positive electrode can 16, and a negative electrode can 17 was assembled.

f) Battery Evaluation [Positive Electrode Interface Resistance]
The positive electrode interface resistance was measured by an impedance measurement method, and a 2032-type coin battery was charged at a charging potential of 4.4 V, and a Nyquist plot shown in FIG. 6 was obtained by using a frequency response analyzer and a potentiogalvanostat (manufactured by Solartron, 1255B). The Nyquist plot shown in FIG. 6 is expressed as the sum of the characteristic curves showing the solution resistance, the negative electrode resistance and its capacity, and the positive electrode resistance (interface resistance) and its capacity. Therefore, based on this Nyquist plot, a fitting calculation was performed using the equivalent circuit shown in FIG. 6 to calculate the value of the positive electrode interface resistance. Note that the positive electrode interface resistance is based on the positive electrode active material of Comparative Example 1 described later, and the resistance reduction rate relative to this is shown. As a result, the positive electrode interface resistance was reduced by 12% compared to Comparative Example 1.

[Discharge capacity retention rate]
After the 2032-type coin battery was prepared, it was left for about 24 hours, and after the open circuit voltage OCV (Open Circuit Voltage) was stabilized, the current density to the positive electrode was set to 0.1 mA/ ^cm2 , and the battery was charged until the cutoff voltage reached 4.3 V. After one hour of rest, a charge-discharge test was performed to measure the discharge capacity when the battery was discharged until the cutoff voltage reached 3.0 V, and the initial discharge capacity was obtained. Furthermore, the battery was repeatedly charged and discharged 100 times, and the ratio of the second discharge capacity to the initial discharge capacity was taken as the discharge capacity retention rate. As a result, the discharge capacity retention rate was 88%.

The conditions of the dry mixing step, the water spray mixing step, and the drying step, the average particle size, particle size range, average film thickness, and porosity of the fine particles present on the surface of the secondary particles of the positive electrode active material, as well as the reduction rate of the positive electrode interface resistance and the discharge capacity retention rate of the obtained lithium ion secondary battery are shown in Tables 1 and 2. These are also shown in Tables 1 and 2 for Examples 2 to 6, Reference Examples 7 and 8 , and Comparative Example 1.

Example 2
In the water spray mixing step, the peripheral speed of the Henschel mixer during water spraying was 18 m/sec, the water spray rate was 0.016 ml/min per 1 g of mixture, and water was sprayed in an amount of 12.3 mass % with respect to the mass of the entire mixture, and thereafter, the peripheral speed of the Henschel mixer was maintained at 18 m/sec and mixing was continued for 5 minutes, and in the cooling step, the peripheral speed of the Henschel mixer during reduced pressure cooling was 8 m/sec. Except for this, a positive electrode active material was obtained in the same manner as in Example 1, and its evaluation was performed.

Example 3
A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that in the dry mixing step, the mixing time was 20 minutes, and in the water spray mixing step, the peripheral speed of the Henschel mixer during water spraying was 10 m/sec, the water spray rate was 0.033 ml/min per 1 g of the mixture, and water was sprayed in an amount of 13.1 mass % with respect to the mass of the entire mixture, and then the peripheral speed of the Henschel mixer was set to 8 m/sec, and mixing was continued for 20 minutes.

Example 4
A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that in the dry mixing step, the peripheral speed of the Henschel mixer was 15 m/sec, and in the water spray mixing step, the peripheral speed of the Henschel mixer during water spraying was 8 m/sec, the water spray rate was 0.025 ml/min per 1 g of mixture, and water was sprayed in an amount of 12.3 mass % with respect to the mass of the entire mixture, and then the peripheral speed of the Henschel mixer was maintained at 8 m/sec and mixing was continued for 15 minutes.

Example 5
A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that in the heat treatment step, the mixture was heat-treated at a temperature of 50° C. under atmospheric pressure for 20 minutes with the peripheral speed of the Henschel mixer set to 6 m/sec, and in the drying step, the mixture was dried under reduced pressure at a temperature of 100° C. for 90 minutes in a vacuum atmosphere with the peripheral speed of the Henschel mixer maintained at 6 m/sec.

Example 6
A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that in the water spray mixing step, the mixing time after water spraying was 15 minutes, in the heat treatment step, the peripheral speed of the Henschel mixer was set to 10 m/s, and the mixture was heat-treated at atmospheric pressure and a temperature of 150° C. for 90 minutes, in the drying step, the peripheral speed of the Henschel mixer was maintained at 10 m/s, and the mixture was dried under reduced pressure at a temperature of 160° C. for 180 minutes in a vacuum atmosphere, and in the cooling step, reduced pressure cooling was performed for 120 minutes.

( Reference Example 7)
In the water spray mixing step, the peripheral speed of the Henschel mixer during water spraying was 4 m/sec, the water spray rate was 0.039 ml/min per 1 g of the mixture, and water was sprayed in an amount of 11.8 mass% with respect to the mass of the entire mixture. Thereafter, the peripheral speed of the Henschel mixer was maintained at 4 m/sec and mixing was continued for 10 minutes. In the heat treatment step, the peripheral speed of the Henschel mixer was maintained at 4 m/sec and the mixture was heat-treated at atmospheric pressure at a temperature of 120° C. for 30 minutes. In the drying step, the peripheral speed of the Henschel mixer was maintained at 4 m/sec and the mixture was dried under reduced pressure at a temperature of 150° C. for 90 minutes in a vacuum atmosphere. Except for this, a positive electrode active material was obtained in the same manner as in Example 1, and its evaluation was performed.

( Reference Example 8)
In the water spray mixing step, the peripheral speed of the Henschel mixer during water spraying was 30 m/sec, the water spray rate was 0.066 ml/min per 1 g of the mixture, and water was sprayed in an amount of 11.8 mass% with respect to the mass of the entire mixture. Thereafter, the peripheral speed of the Henschel mixer was reduced to 20 m/sec, and mixing was continued for 10 minutes. In the heat treatment step, the peripheral speed of the Henschel mixer was maintained at 20 m/sec, and the mixture was heat-treated at atmospheric pressure and at a temperature of 120° C. for 30 minutes. In the drying step, the peripheral speed of the Henschel mixer was reduced to 16 m/sec, and the mixture was dried under reduced pressure at a temperature of 150° C. for 300 minutes in a vacuum atmosphere. Except for this, a positive electrode active material was obtained in the same manner as in Example 1, and its evaluation was performed.

(Comparative Example 1)
In the water spray mixing step, water was sprayed onto the mixture in a stationary state, in an amount equivalent to 10% by mass relative to the mass of the entire mixture, and then mixed by hand using a rubber spatula. The mixture was then left to stand for 60 minutes, dried at 120° C. under atmospheric pressure for 12 hours, and cooled to 25° C. under atmospheric pressure for 90 minutes. The positive electrode active material was obtained and evaluated in the same manner as in Example 1.

When the surface of the obtained secondary particles was observed by STEM, it was confirmed that fine particles with an average particle size of 650 nm and particle sizes in the range of 62 nm to 696 nm were uniformly present on the surface of the primary particles. The obtained STEM image is shown in Figure 4.

Example 9
As a fourth step, the crystallization reaction in the oxidizing atmosphere was continued for 40 minutes (16.7% of the entire particle growth process) from switching operation 3, and as a fifth step, the crystallization reaction in the non-oxidizing atmosphere was continued for 80 minutes (33.3% of the entire particle growth process) from switching operation 4. Therefore, the proportion of the crystallization reaction in the entire oxidizing atmosphere was 25.0%. Except for this, a positive electrode active material was obtained in the same manner as in Example 1, and its evaluation was performed.

In the composite hydroxide, the ratios of the radius of the center and the thicknesses of the first low-density layer, the high-density layer, the second low-density layer, and the outer shell layer to the particle diameter of the secondary particles were 25%, 10%, 5%, 5%, and 5%, respectively. The average particle diameter MV was 5.5 μm, [(d90-d10)/average particle diameter MV] was 0.50, and the tap density was 1.05 g/ ^cm3 .

In the composite oxide used as the base material, the average thickness of the outer shell of the secondary particles was 0.6 μm, the average particle size MV was 5.0 μm, [(d90-d10)/average particle size MV] was 0.42, the BET specific surface area was 3.0 ^m2 /g, and the tap density was 1.4 g/ ^cm3 .

The positive electrode active material had an average particle size MV of 4.6 μm (a ratio of 92% to the average particle size MV of the composite oxide before coating with the compound containing W and Li), and [(d90-d10)/average particle size MV] was 0.44. The BET specific surface area was 3.7 m ² /g, and the tap density was 1.4 g/cm ^3. In addition, SEM observation confirmed that fine particles with an average particle size of 66 nm and a particle size in the range of 21 nm to 279 nm were uniformly present on the surface of the secondary particles, and that a coating with an average film thickness of 2 nm was present on the entire surface and part of the inside of the secondary particles. Furthermore, the porosity of the secondary particles was 36%.

The conditions of the dry mixing step, the water spray mixing step, and the drying step, the average particle size, particle size range, average film thickness, and porosity of the fine particles present on the surface of the secondary particles of the positive electrode active material, as well as the reduction rate of the positive electrode interface resistance and the discharge capacity retention rate of the obtained lithium ion secondary battery are shown in Tables 3 and 4. These are also shown in Tables 1 and 2 for Examples 10 to 14 and 17, Reference Examples 15 and 16 , and Comparative Example 2.

Example 10
In the water spray mixing step, the peripheral speed of the Henschel mixer during water spraying was 5 m/sec, the water spray rate was 0.016 ml/min per 1 g of the mixture, and water was sprayed in an amount of 12.3 mass % with respect to the mass of the entire mixture. Thereafter, the peripheral speed of the Henschel mixer was maintained at 5 m/sec, and mixing was continued for 5 minutes. Except for this, a positive electrode active material was obtained in the same manner as in Example 9, and its evaluation was performed.

(Example 11)
In the water spray mixing step, the peripheral speed of the Henschel mixer during water spraying was 10 m/sec, the water spray rate was 0.033 ml/min per 1 g of the mixture, and water was sprayed in an amount of 13.1 mass % with respect to the mass of the entire mixture. Thereafter, the peripheral speed of the Henschel mixer was set to 8 m/sec, and mixing was continued for 10 minutes. Except for this, a positive electrode active material was obtained in the same manner as in Example 9, and its evaluation was performed.

Example 12
In the water spray mixing step, the peripheral speed of the Henschel mixer during water spraying was 8 m/sec, the water spray rate was 0.025 ml/min per 1 g of the mixture, and water was sprayed in an amount of 12.3 mass % with respect to the mass of the entire mixture. Thereafter, the peripheral speed of the Henschel mixer was maintained at 8 m/sec, and mixing was continued for 10 minutes. Except for this, a positive electrode active material was obtained in the same manner as in Example 9, and its evaluation was performed.

(Example 13)
A positive electrode active material was obtained and evaluated in the same manner as in Example 9, except that in the heat treatment step, the mixture was heat-treated at a temperature of 50° C. under atmospheric pressure for 20 minutes with the peripheral speed of the Henschel mixer set to 5 m/sec, and in the drying step, the mixture was dried under reduced pressure at a temperature of 100° C. for 90 minutes in a vacuum atmosphere with the peripheral speed of the Henschel mixer maintained at 5 m/sec.

(Example 14)
A positive electrode active material was obtained and evaluated in the same manner as in Example 9, except that in the heat treatment step, the peripheral speed of the Henschel mixer was set to 10 m/sec and the heat treatment of the mixture was performed at atmospheric pressure and a temperature of 150° C. for 90 minutes, and in the drying step, the peripheral speed of the Henschel mixer was maintained at 10 m/sec and the mixture was dried under reduced pressure at a temperature of 200° C. for 180 minutes in a vacuum atmosphere.

( Reference Example 15)
In the water spray mixing step, the peripheral speed of the Henschel mixer during water spraying was 4 m/sec, the water spray rate was 0.039 ml/min per 1 g of the mixture, and water was sprayed in an amount of 11.8 mass% with respect to the mass of the entire mixture. Thereafter, the peripheral speed of the Henschel mixer was maintained at 4 m/sec and mixing was continued for 10 minutes. In the heat treatment step, the peripheral speed of the Henschel mixer was maintained at 4 m/sec and the mixture was heat-treated at atmospheric pressure at a temperature of 120° C. for 30 minutes. In the drying step, the peripheral speed of the Henschel mixer was maintained at 4 m/sec and the mixture was dried under reduced pressure at a temperature of 150° C. for 90 minutes in a vacuum atmosphere. Except for this, a positive electrode active material was obtained in the same manner as in Example 9 and its evaluation was performed.

( Reference Example 16)
In the water spray mixing step, the peripheral speed of the Henschel mixer during water spraying was 16 m/sec, the water spray rate was 0.013 ml/min per 1 g of the mixture, and water was sprayed in an amount of 11.8% by mass with respect to the mass of the entire mixture. Thereafter, the peripheral speed of the Henschel mixer was maintained at 16 m/sec and mixing was continued for 10 minutes. In the heat treatment step, the peripheral speed of the Henschel mixer was maintained at 16 m/sec and the mixture was heat-treated at atmospheric pressure at a temperature of 120° C. for 30 minutes. In the drying step, the peripheral speed of the Henschel mixer was maintained at 16 m/sec and the mixture was dried under reduced pressure at a temperature of 150° C. for 300 minutes in a vacuum atmosphere. Except for this, a positive electrode active material was obtained in the same manner as in Example 9, and its evaluation was performed.

(Example 17)
In the water spray mixing step, a water spray rate of 0.020 ml/min per 1 g of the mixture from each of the two spray nozzles (total of 0.039 ml/min from the two nozzles) was set to 11.8% by mass of water relative to the total mass of the mixture, and thereafter, the peripheral speed of the Henschel mixer was maintained at 8 m/sec. Except for this, a positive electrode active material was obtained in the same manner as in Example 9, and its evaluation was performed.

(Comparative Example 2)
In the water spray mixing step, water was sprayed onto the mixture in a stationary state, in an amount equivalent to 10% by mass relative to the mass of the entire mixture, and then mixed by hand using a rubber spatula. The mixture was then left to stand for 60 minutes, dried at 120° C. under atmospheric pressure for 12 hours, and cooled to 25° C. under atmospheric pressure for 90 minutes. The positive electrode active material was obtained and evaluated in the same manner as in Example 9.

When the surface of the obtained secondary particles was observed using STEM, it was confirmed that fine particles with an average particle size of 650 nm and particle sizes in the range of 62 nm to 696 nm were uniformly distributed on the surface of the primary particles.

In the positive electrode active materials of Examples 1 to 6, 9 to 14, and 17 within the scope of the present invention, finer and more uniform fine particles of a compound containing W and Li are present on the surfaces of the primary particles constituting the secondary particles, together with a thin coating of a compound containing W and Li, such as lithium tungstate. Therefore, compared with the positive electrode active material of Comparative Example 1, it is understood that the positive electrode interface resistance is significantly reduced, high output is achieved, and the discharge capacity retention rate is also improved.

REFERENCE SIGNS LIST 1 Outer shell portion 2 Aggregation portion 3 Space portion 4 Fine particles of compound containing W and Li 5 Coating of compound containing W and Li 11 Positive electrode (electrode for evaluation)
12 Negative electrode 13 Separator 14 Gasket 15 Wave washer 16 Positive electrode can 17 Negative electrode can

Claims (20)

A positive electrode active material for a lithium ion secondary battery is made of a lithium nickel manganese cobalt-containing composite oxide coated with a compound containing W and Li, the composite oxide being composed of secondary particles formed by agglomeration of a plurality of primary particles, and fine particles and a coating of a compound containing W and Li being present on at least a portion of the surfaces of at least a portion of the primary particles constituting the secondary particles,
the secondary particles have a porous structure including an outer shell portion formed by the aggregated primary particles, an aggregate portion present inside the outer shell portion, formed by the aggregated primary particles and electrically conductive with the outer shell portion, and spaces present and dispersed within the aggregate portion;
The porosity of the secondary particles measured by cross-sectional observation is 10% or more and 50% or less ,
a particle diameter of fine particles of a compound containing W and Li present on the surface of the secondary particles, which is determined by observing the surfaces of the secondary particles using a scanning electron microscope, is 10 nm or more and 360 nm or less;
The average particle size of the fine particles is 30 nm or more and 100 nm or less, and
the average thickness of the coating of the compound containing W and Li present on the surface of the secondary particles, as determined by cross-sectional observation of the secondary particles using a transmission electron microscope, is 1 nm or more and 80 nm or less;
Positive electrode active material for lithium-ion secondary batteries. 2. The positive electrode active material for a lithium ion secondary battery according to claim 1 , wherein the coating of the compound containing W and Li has an average thickness of 2 nm or more and 5 nm or less. The positive electrode active material for a lithium ion secondary battery according to claim 1 or 2 , wherein the fine particles have an average particle size of 40 nm or more and 80 nm or less. 4. The positive electrode active material for a lithium ion secondary battery according to claim 1 , wherein the compound containing W and Li is lithium tungstate. The positive electrode active material for a lithium ion secondary battery according to claim 4 , wherein the lithium tungstate comprises 7Li ₂ WO ₄ .4H ₂ O. The positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 5, wherein the average particle size MV of the secondary particles is 3 μm or more and 10 μm or less, and an index showing the spread of the particle size distribution of the secondary particles, [(d90- d10 )/average particle size MV], is 0.7 or less. 7. The positive electrode active material for a lithium ion secondary battery according to claim 1 , wherein the thickness of the outer shell of the secondary particle is 0.1 μm or more and 1.5 μm or less. The positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 7 , having a tap density of 1.0 g/ ^cm3 or more. 9. The positive electrode active material for a lithium ion secondary battery according to claim 1 , wherein the BET specific surface area is 2.0 m ² /g or more and 5.0 m ² /g or less. 10. The positive electrode active material for lithium ion _secondary batteries according to claim ₁ , which is composed of a lithium _{nickel manganese cobalt} -containing composite oxide represented by general formula (A): Li1 _{+ uNixMnyCozWsMtO2} (-0.05≦u≦0.50, x+y+z+s+t=1, 0.3≦x≦0.7, 0.15≦y≦0.4, 0.15≦z≦0.4, 0.0005≦s≦0.03, 0≦t≦0.1, M is one or more added elements selected from Mg, Al, Si, Ca, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W), and has a crystal structure of a hexagonal layered rock salt structure. A method for producing a positive electrode active material for a lithium ion secondary battery, comprising a lithium nickel manganese cobalt-containing composite oxide coated with a compound containing W and Li, comprising:
a dry mixing step of mixing a tungsten compound in an amount of 0.1 mass % or more and 5 mass % or less with respect to a total mass of a lithium nickel manganese cobalt-containing composite oxide as a base material, the secondary particles being composed of secondary particles formed by agglomeration of a plurality of primary particles, the secondary particles having a porous structure including an outer shell portion formed by the agglomerated primary particles, an agglomeration portion present inside the outer shell portion, formed by the agglomerated primary particles and electrically conductive with the outer shell portion, and a space portion present in a dispersed state within the agglomeration portion;
a water spray mixing step of spraying water onto the mixture in an amount of 1% by mass or more and 30% by mass or less with respect to the total mass of the mixture while stirring the mixture, thereby mixing the mixture;
A heat treatment step of subjecting the mixture after the water spray mixing step to a heat treatment at a temperature of 500° C. or less; and
a drying step of drying the mixture after the heat treatment step at a temperature of 500° C. or less to obtain the lithium nickel manganese cobalt-containing composite oxide coated with the compound containing W and Li, in which fine particles of the compound containing W and Li are present on the surfaces of the primary particles;
Equipped with
At least in the drying step, a vacuum drying and mixing device is used to perform the drying under a vacuum atmosphere.
A method for producing a positive electrode active material for a lithium ion secondary battery. 12. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 11, wherein, in the water spray mixing step, the stirring during the water spraying is performed at a peripheral speed of 4 m/sec to 30 m/sec, the water spraying is performed at a water spray rate of 0.01 ml/min to 0.1 ml/min per 1 g of the mixture, and after the water spraying, the stirring is performed at a peripheral speed in the range of 4 m/sec to 30 m/sec, while the mixing is continued for a time of 5 minutes to 120 minutes. 13. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 12 , wherein the water spraying is performed using a plurality of nozzles, and the water spraying rate of each of the plurality of nozzles is 0.005 ml/min or more and 0.02 ml/min or less per 1 g of the mixture. 14. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 11, wherein in the heat treatment step, the heat treatment is carried out at a temperature of 40° C. or more and 200° C. or less for a time period of 15 minutes or more and 120 minutes or less while stirring the mixture at a peripheral speed of 4 m /sec or more and 30 m/sec or less. 15. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 11, wherein in the drying step, the drying is performed at a temperature of 70° C. or more and 200° C. or less for a time of 60 minutes or more and 240 minutes or less while stirring the mixture at a peripheral speed of 4 m / s or more and 30 m/s or less. The method for producing a positive electrode active material for a lithium ion secondary battery according to any one of claims 11 to 15, further comprising a cooling step of cooling the mixture to 25°C for a period of 60 minutes to 240 minutes in a vacuum atmosphere while stirring the composite oxide at a peripheral speed of 1 m /sec to 20 m /sec after the drying step. The method for producing a positive electrode active material for a lithium ion secondary battery according to any one of claims 11 to 16 , wherein the dry mixing step is carried out for a time period of 5 minutes to 50 minutes while stirring at a peripheral speed of 5 m/sec to 30 m/sec. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 11 , wherein the dry mixing step is carried out using the vacuum dry mixer. The method for _{producing a} positive electrode active material for a lithium ion secondary battery according to _any one of claims 11 to ₁₈ , wherein _the positive electrode active material for a lithium ion secondary battery is represented by general formula (A): Li1 _{+ uNixMnyCozWsMtO2} (-0.05≦u≦0.50, x+y+z+s+t=1, 0.3≦x≦0.7, 0.15≦y≦0.4, 0.15≦z≦0.4, 0.0005≦s≦0.03, 0≦t≦ 0.1 , M is one or more added elements selected from Mg, Al, Si, Ca, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W), and is made of a lithium nickel manganese composite oxide having a crystal structure of a hexagonal layered rock salt structure. A lithium ion secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, or comprising a positive electrode, a negative electrode, and a solid electrolyte, the positive electrode active material for lithium ion secondary batteries according to any one of claims 1 to 10 being used as a positive electrode active material for the positive electrode.

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