JP7447401B2 - Nickel manganese cobalt-containing composite hydroxide and its manufacturing method, positive electrode active material for lithium ion secondary battery and its manufacturing method, and lithium ion secondary battery - Google Patents

Description

The present invention relates to a nickel-manganese-cobalt-containing composite hydroxide that is a precursor of a positive electrode active material for lithium ion secondary batteries, a method for producing the same, and a lithium-nickel-nickel-manganese-cobalt product obtained using the nickel-manganese-cobalt-containing composite hydroxide as a precursor. The present invention relates to a positive electrode active material for a lithium ion secondary battery comprising a composite oxide, a method for producing the same, and a lithium ion secondary battery using the positive electrode active material for a lithium ion secondary battery as a positive electrode material.

In recent years, with the spread of mobile electronic devices such as smartphones, tablet terminals, digital cameras, and notebook computers, there is a strong desire to develop small and lightweight secondary batteries with high energy density. Furthermore, there is a strong desire to develop 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.

A lithium ion secondary battery is a secondary battery that meets such requirements. This lithium ion secondary battery is composed of a negative electrode, a positive electrode, a non-aqueous electrolyte, a solid electrolyte, etc. The active material used for the negative and positive electrodes includes a material that can desorb and insert lithium. used. As a non-aqueous electrolyte, there is a non-aqueous electrolyte made by dissolving a lithium salt as a supporting salt in an organic solvent, and as a solid electrolyte, there is an inorganic or organic solid electrolyte that is nonflammable and has lithium ion conductivity. be.

Among these lithium ion secondary batteries, lithium ion secondary batteries that use a lithium transition metal-containing composite oxide with a layered rock salt type or spinel type structure as the positive electrode material can obtain a voltage of 4V class, so they have high energy. Research and development and practical application of batteries with high density are progressing.

As positive electrode materials for such lithium ion secondary batteries, 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, and lithium nickel Manganese cobalt composite oxide (LiNi _1/3 Mn _1/3 Co _1/3 O ₂ ), lithium manganese composite oxide using manganese (LiMn ₂ O ₄ ), lithium nickel manganese composite oxide (LiNi _0.5 Mn A positive electrode active material made of a lithium transition metal-containing composite oxide such as _0.5 O ₂ ) has been proposed.

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

Regarding lithium-transition metal-containing composite oxides, the focus of development has been on increasing their output by reducing their internal resistance. NCA) is also attracting attention. However, lithium-nickel-manganese-cobalt-containing composite oxides have better weather resistance and are easier to handle than lithium-nickel-cobalt-aluminum-containing composite oxides, so lithium-transition metal-containing composite oxides is given the utmost importance in the development of

For ternary positive electrode active materials made of lithium nickel manganese cobalt-containing composite oxides, there is a high level of demand for higher output by further reducing internal resistance, especially in power source applications for electric vehicles.

In order to improve the output characteristics and cycle characteristics of a lithium nickel manganese cobalt-containing composite oxide, first, the lithium nickel manganese cobalt-containing composite oxide is composed of particles with a small particle size of about 3 μm to 10 μm and a narrow particle size distribution. It is necessary that the By making the particles small in size, they have a large specific surface area, and when used as a positive electrode active material, a sufficient reaction area with the electrolyte can be secured. - Since the moving distance between the negative electrodes can be shortened, it is possible to reduce the positive electrode resistance. In addition, by forming particles with a narrow particle size distribution, it is possible to equalize the voltage applied to the particles within the electrode, thereby making it possible to suppress a decrease in battery capacity due to selective deterioration of the fine particles.

Furthermore, in order to further improve the output characteristics, research and development is also underway to improve the particle structure of lithium nickel manganese cobalt-containing composite oxides. For example, in order to improve the output characteristics, it is considered effective to form a space inside the positive electrode active material into which the electrolyte can enter. By adopting such a structure, the reaction area with the electrolyte can be increased compared to a positive electrode active material with a solid structure with a similar particle size, which can significantly reduce the positive electrode resistance. It becomes possible. It is known that the positive electrode active material inherits the particle properties of the transition metal-containing composite hydroxide that is its precursor. That is, 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 thereof. .

For example, WO2014/181891 and JP2018-104275 disclose that the pH value of an aqueous nucleation solution containing at least a metal compound containing a transition metal and an ammonium ion donor is 12.0 to 14.0. In the nucleation step, the pH value of the particle growth aqueous solution containing the generated nuclei is controlled to be lower than the pH value of the nucleation step, and from 10.5 to 12.0. The method includes a particle growth step in which growth is controlled so that A method for producing a transition metal-containing composite hydroxide is disclosed, which is characterized in that atmosphere control is performed twice to switch to a non-oxidizing atmosphere. According to this method, the particle size is small and the particle size distribution is narrow, and the core is formed by agglomeration of plate-shaped or needle-shaped primary particles, and the fine primary particles are agglomerated outside the center. It is possible to obtain a transition metal-containing composite hydroxide comprising two secondary particles having a laminated structure in which low-density layers formed by agglomeration and high-density layers formed by agglomeration of plate-shaped primary particles are alternately laminated. A positive electrode active material using a transition metal-containing composite hydroxide having such a structure as a precursor has a small particle size, a narrow particle size distribution, and a porous structure having spaces. In a secondary battery using a positive electrode active material having such a porous structure, it is possible to improve output characteristics as well as capacity characteristics and cycle characteristics.

WO2014/181891 publication Japanese Patent Application Publication No. 2018-104275

However, according to the studies of the present inventors, in order to further increase the energy density in a positive electrode active material made of a lithium nickel manganese cobalt-containing composite oxide made of porous secondary particles, it is necessary to increase the nickel ratio. However, in comparison with a lithium nickel manganese cobalt-containing composite oxide in which the ratio of nickel, manganese and cobalt is 1:1:1, the deterioration of the particle structure progresses more quickly due to contact with the electrolyte. It has been found that the durability of the positive electrode active material is impaired, resulting in an increase in internal resistance (positive electrode interfacial resistance) and a decrease in output characteristics.

In view of these problems, the present invention improves the durability of a positive electrode active material made of a lithium nickel manganese cobalt-containing composite oxide with a high nickel ratio, so that the effect of improving output characteristics can be sufficiently achieved. and to realize a particle structure of a nickel manganese cobalt-containing composite hydroxide as a precursor to realize the structure of such a positive electrode active material. The purpose of the present invention is to provide a means for industrially and efficiently producing a cathode active material having a structure and its precursor.

The present invention provides a ternary positive electrode active material precursor for a lithium ion secondary battery consisting of a lithium nickel manganese cobalt composite oxide (NMC) containing at least nickel, manganese, and cobalt as transition metals. A nickel-manganese-cobalt-containing composite hydroxide and a method for producing the same; a ternary positive electrode active material obtained using the nickel-manganese-cobalt-containing composite hydroxide as a precursor and a method for producing the same; This article relates to a lithium ion secondary battery used as a material.

The nickel-manganese-cobalt-containing composite hydroxide according to the first aspect of the present invention is a precursor of a positive electrode active material for lithium ion secondary batteries, and includes a plurality of plate-shaped primary particles and smaller particles than the plate-shaped primary particles. Consists of secondary particles formed by agglomeration of fine primary particles.

The secondary particles constituting the nickel-manganese-cobalt-containing composite hydroxide of the present invention have a central portion mainly formed by agglomeration of the plate-like primary particles, and a central portion mainly formed by the fine primary particles outside the central portion. a first low-density layer formed by agglomeration; a high-density layer formed by mainly aggregation of the plate-shaped primary particles outside the first low-density layer; and an outside of the high-density layer. A second low-density layer formed mainly by agglomeration of the fine primary particles, and an outer shell layer formed mainly by aggregation of the plate-shaped primary particles outside the second low-density layer.

In particular, in the nickel-manganese-cobalt-containing composite hydroxide of the present invention, the center, the first low-density layer, and the high-density layer have a nickel ratio that is an atomic ratio of nickel to the total of nickel, manganese, and cobalt. has a composition with a high nickel ratio exceeding 0.4, and the second low density layer and the outer shell layer have a composition with a low nickel ratio where the nickel ratio exceeds 0.1 and is 0.4 or less. has.

The average particle size MV of the secondary particles is in the range of 4 μm to 11 μm, and [(d90-d10)/average particle size MV], which is an index indicating the spread of the particle size distribution of the secondary particles, is 0. It is preferable that it is .65 or less.

The radius of the center is in the range of 20% to 40% of the particle size of the secondary particles, and the thickness of the first low density layer is in the range of 3% to 15% of the particle size of the secondary particles. The thickness of the high-density layer is in the range of 2% to 10% of the particle size of the secondary particles, and the thickness of the second low-density layer is in the range of 2% to 10% of the particle size of the secondary particles. The thickness of the outer shell layer is preferably in the range of 2% to 10% of the particle size of the secondary particles.

The particle size of the plate-shaped primary particles is preferably in the range of 0.3 μm to 3 μm, and the particle size of the fine primary particles is preferably in the range of 0.01 μm to 0.3 μm.

The nickel-manganese-cobalt-containing composite hydroxide has a general formula (A): Ni _{x Mny} Co _z M _t (OH) _2+a (x+y+z+t=1, 0.4≦x≦0.70, 0.15≦y≦ 0.4, 0.15≦z≦0.3, 0≦t≦0.1, 0≦a≦0.5, M is Mg, Al, Si, Ca, Ti, V, Cr, Zr, Nb , Mo, Hf, Ta, and W).

The method for producing a nickel manganese cobalt-containing composite hydroxide according to the second aspect of the present invention involves supplying a raw material aqueous solution containing at least a transition metal and an aqueous solution containing an ammonium ion donor into a reaction tank. The method includes a step of forming an aqueous solution and performing a crystallization reaction to obtain a nickel manganese cobalt-containing composite hydroxide.

The method for producing a nickel manganese cobalt-containing composite hydroxide of the present invention suppresses nucleation by controlling the pH value of the reaction aqueous solution at a liquid temperature of 25° C. to be in the range of 12.0 to 14.0. The nucleation step to be performed and the pH value of the reaction aqueous solution containing the nuclei obtained in the nucleation step, based on the liquid temperature of 25° C., are lower than the pH value of the nucleation step and 10.5 to 12.0. and a particle growing step of growing the nuclei by controlling the particles to fall within the range of .

In the method for producing a nickel manganese cobalt-containing composite hydroxide of the present invention, (1) the reaction atmosphere in the first stage (initial stage) of the nucleation step and the particle growth step is a non-oxidizing atmosphere with an oxygen concentration of 5% by volume or less. (2) After the first stage of the particle growth step, the reaction atmosphere is switched from the non-oxidizing atmosphere to an oxidizing atmosphere in which the oxygen concentration exceeds 5% by volume, and the particle growth (3) Next, the reaction atmosphere is switched from the oxidizing atmosphere to the non-oxidizing atmosphere to form the third step of the particle growth step; (4) Further, the reaction atmosphere is changed to the non-oxidizing atmosphere. (5) Finally, switching the reaction atmosphere from the oxidizing atmosphere to the non-oxidizing atmosphere as the fourth step of the particle growth step. , the fifth step of the particle growth process.

Particularly, in the method for producing a nickel manganese cobalt-containing composite hydroxide of the present invention, the raw material aqueous solution supplied in the first, second, and third stages of the nucleation step and the particle growth step is a nickel-manganese cobalt-containing composite hydroxide. The raw material has a high nickel ratio composition in which the nickel ratio, which is the atomic ratio of nickel to the sum of , manganese, and cobalt, exceeds 0.4, and is supplied in the fourth and fifth stages of the particle growth process. The aqueous solution has a composition with a low nickel ratio, in which the nickel ratio is more than 0.1 and less than 0.4.

In addition, the switching of the reaction atmosphere is defined by the ratio of the amount of metal added at each stage to the total amount of metal added in the grain growth step, and the crystallization at each stage for the entire grain growth step is defined as Regarding the reaction rate, the first stage is in the range of 8% to 20%, the second stage is in the range of 2% to 20%, the third stage is in the range of 12% to 40%, and the fourth stage is 3%. It is preferable to set the amount to 40%, and set the fifth stage to 10% to 50%.

When switching the reaction atmosphere, it is preferable to introduce the oxidizing gas and the non-oxidizing gas using a diffuser tube.

The nickel-manganese-cobalt-containing composite hydroxide has a general formula (A): Ni _{x Mny} Co _z M _t (OH) _2+a (x+y+z+t=1, 0.4≦x≦0.70, 0.15≦y≦ 0.4, 0.15≦z≦0.3, 0≦t≦0.1, 0≦a≦0.5, M is Mg, Al, Si, Ca, Ti, V, Cr, Zr, Nb , Mo, Hf, Ta, and W).

The positive electrode active material for a lithium ion secondary battery according to the third aspect of the present invention is a ternary positive electrode active material made of a lithium nickel manganese cobalt-containing composite oxide, and is formed by agglomeration of a plurality of primary particles. It is composed of secondary particles.

In the positive electrode active material for a lithium ion secondary battery of the present invention, the secondary particles have a porous structure, and specifically, the secondary particles have an outer shell portion formed by the aggregated primary particles. , an agglomerated part that exists inside the outer shell part, is formed by the aggregated primary particles, and is electrically connected to the outer shell part, and a space that exists dispersedly within the agglomerated part. It is equipped with a section.

In particular, in the positive electrode active material for a lithium ion secondary battery of the present invention, the primary particles constituting the agglomerated portion have a nickel ratio, which is an atomic ratio of nickel to the total of nickel, manganese, and cobalt, exceeding 0.4. The agglomerated portion has a composition with a high nickel ratio, and the agglomerated portion includes a surface portion that is graded to a composition with a low nickel ratio in which the nickel ratio is greater than 0.1 and less than or equal to 0.4, and the outer shell portion includes: The secondary particles have a composition with a low nickel ratio of more than 0.1 and less than or equal to 0.4, and the secondary particles are subjected to energy dispersive X-ray spectroscopy (STEM-) using a scanning electron microscope. In the compositional analysis of the cross section of the secondary particle by EDS or STEM-EDX), the area ratio of the low nickel ratio layer in which the nickel ratio is more than 0.1 and 0.4 or less is determined by the entire cross section of the secondary particle. It is characterized by being in the range of 25% to 45% of the area of .

The thickness of the outer shell portion of the secondary particles is preferably in the range of 0.1 μm to 1.0 μm.

The average particle diameter MV of the secondary particles is in the range of 3 μm to 10 μm, and [(d90-d10)/average particle diameter MV], which is an index indicating the spread of the particle size distribution of the secondary particles, is 0. It is preferable that it is .7 or less.

The tap density of the positive electrode active material is preferably in the range of 1.1 g/cm ³ to 1.8 g/cm ³ .

The BET specific surface area of the positive electrode active material is preferably in the range of 2.0 m ² /g to ^{5.0 m 2} /g.

The oil absorption amount of the positive electrode active material is preferably in the range of 35 ml/100 g to 60 ml/100 g.

The crystallite diameter of the primary particles, determined using the Scherrer equation from the half width of the peak of the (003) plane by X-ray diffraction of the (003) plane of the positive electrode active material, is preferably in the range of 300 Å to 1500 Å. .

The positive electrode active material for non-aqueous electrolyte secondary batteries of the present invention has the general formula (B): Li _1+u Ni _{x Mny} Co _z M _t O ₂ (-0.05≦u≦0.50, x+y+z+t=1,0 .4≦x≦0.70, 0.15≦y≦0.4, 0.15≦z≦0.3, 0≦t≦0.1, M is Mg, Al, Si, Ca, Ti, A lithium nickel manganese cobalt-containing composite oxide represented by one or more additive elements selected from V, Cr, Zr, Nb, Mo, Hf, Ta, and W) and having a hexagonal crystal structure with a layered structure. It is preferable to consist of.

A method for producing a positive electrode active material for a lithium ion secondary battery according to a fourth aspect of the present invention includes a mixing step of mixing the nickel manganese cobalt-containing composite hydroxide and a lithium compound to form a lithium mixture; The present invention is characterized by comprising a firing step of firing the lithium mixture formed in the mixing step at a temperature in the range of 650° C. to 920° C. in an oxidizing atmosphere.

In the mixing step, the lithium mixture is adjusted such that the ratio of the sum of the atoms of metals other than lithium contained in the lithium mixture and the number of lithium atoms is 1:0.95 to 1.5. It is preferable to do so.

It is preferable to further include a heat treatment step of heat treating the nickel manganese cobalt-containing composite hydroxide at a temperature in the range of 105° C. to 750° C. before the mixing step.

The positive electrode active material for lithium ion secondary batteries has a general formula (B): Li _1+u Ni _x Mn _y Co _z M _t O ₂ (-0.05≦u≦0.50, x+y+z+t=1, 0.4≦ x≦0.70, 0.15≦y≦0.4, 0.15≦z≦0.3, 0≦t≦0.1, M is Mg, Al, Si, Ca, Ti, V, Cr , Zr, Nb, Mo, Hf, Ta, and W), and is preferably made of a hexagonal lithium-nickel-manganese composite oxide having a porous structure.

A lithium ion secondary battery according to a fifth aspect of the present invention includes a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte (nonaqueous electrolyte secondary battery), or a positive electrode, a negative electrode, and a solid electrolyte (solid (electrolyte secondary battery), characterized in that the positive electrode active material for a lithium ion secondary battery according to the third aspect of the present invention is used as the positive electrode active material used in the positive electrode.

In the positive electrode active material for lithium ion secondary batteries of the present invention, of the outer shell part and the agglomerated part that constitute the porous structure of the secondary particles, the outer shell part has a composition with a low nickel ratio to prevent contact with the electrolyte. In addition to protecting the internal agglomerated part, the primary particles that make up the internal agglomerated part are also made to have a composition with a high nickel ratio, but the surface part of the agglomerated part on the outer shell side has a low nickel ratio. By grading the composition, the nickel ratio exceeds 0.1 and is 0.4 or less in compositional analysis of the cross section of the secondary particles by energy dispersive X-ray spectroscopy using a scanning electron microscope. By controlling the ratio of the area of the low nickel ratio layer to the area of the entire cross section of the secondary particles to be 25% to 45%, the primary particles constituting the agglomerated part can also be separated from the electrolyte that has entered through the outer shell part. The structure prevents deterioration due to contact with the Therefore, although it is a composite oxide containing lithium nickel manganese cobalt with a high nickel ratio as a whole, it is possible to increase the nickel ratio and improve energy density without impairing the porous structure of the positive electrode active material, which has excellent output characteristics. Even in the case of extreme conditions, it is possible to ensure its durability. This makes it possible to improve the battery performance of a lithium ion secondary battery in comparison with conventional positive electrode active materials having a porous structure and a uniform composition.

The positive electrode active material having such a structure is composed of secondary particles having a layered structure consisting of a center, a first low density layer, a high density layer, a second low density layer, and an outer shell layer. , a nickel-manganese-cobalt-containing composite oxide precursor having a structure in which the first low-density layer and the high-density layer have a composition with a high nickel ratio, and the second low-density layer and the outer shell layer have a composition with a low nickel ratio. It is possible to produce it industrially by making it into a body.

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 for power sources of electric vehicles, it is possible to achieve better durability and Since it is possible to provide high output characteristics, it can be said to have great industrial significance.

FIG. 1 is a cross-sectional view schematically showing the structure of secondary particles constituting the nickel manganese cobalt-containing composite hydroxide of the present invention. FIG. 2 is a chart showing the steps for producing the nickel manganese cobalt-containing composite hydroxide of the present invention. FIG. 3 is a cross-sectional view schematically showing the structure of secondary particles constituting a lithium nickel manganese cobalt-containing composite oxide, which is a positive electrode active material for a lithium ion secondary battery of the present invention. FIG. 4 is a nickel distribution map obtained by compositional analysis by energy dispersive X-ray spectroscopy using a scanning electron microscope on a cross section of secondary particles constituting the lithium nickel manganese cobalt-containing composite oxide of Example 1. FIG. 5 is a nickel distribution map obtained by compositional analysis by energy dispersive X-ray spectroscopy using a scanning electron microscope on a cross section of secondary particles constituting the lithium nickel manganese cobalt-containing composite oxide of Comparative Example 1. FIG. 6 is a schematic cross-sectional view of a 2032 type coin battery used for battery evaluation. FIG. 7 is a schematic explanatory diagram of an example of measurement for impedance evaluation and an equivalent circuit used for analysis.

The present inventors attempted to increase the nickel ratio in order to further increase the energy density in a positive electrode active material made of a lithium nickel manganese cobalt-containing composite oxide consisting of secondary particles with a porous structure. In comparison with a lithium nickel manganese cobalt-containing composite oxide in which the manganese and cobalt ratio is 1:1:1, the durability of the positive electrode active material is reduced due to the faster deterioration of the particle structure due to contact with the electrolyte. It was found that the internal resistance (positive electrode interface resistance) increases and the output characteristics deteriorate.

The present inventors have intensively investigated the above-mentioned problems that positive electrode active materials with porous structures have, and found that instead of making the secondary particles of porous structures that constitute the positive electrode active materials have a uniform composition, is made to have a composition with a high nickel ratio, while the outer shell part has a composition with a low nickel ratio, and the surface part of the agglomerated part is scanned by grading the composition from a composition with a high nickel ratio to a composition with a low nickel ratio. In compositional analysis of the cross section of the secondary particles by energy dispersive X-ray spectroscopy (STEM-EDS or STEM-EDX) using an electron microscope, the nickel ratio is more than 0.1 and less than 0.4. By controlling the area ratio of the low nickel ratio layer to be 25% to 45% of the area of the entire cross section of the secondary particles, the electrolyte (of the secondary particles) is This study revealed that it is possible to protect primary particles with a high nickel ratio that exist outside the surface of the agglomerated part from the electrolyte that has penetrated into the interior, thereby preventing the structure of the secondary particles from being destroyed. Obtained.

In addition, in order to obtain a positive electrode active material with such a structure, in the process of manufacturing the nickel manganese cobalt-containing composite hydroxide, which is its precursor, it is necessary to In the step of forming the first low-density layer and the high-density layer, a raw material aqueous solution having a composition with a high nickel ratio is used to form a second layer with a sparse particle density among the layered structures constituting the internal structure. It has been found that the process of forming the low-density layer and the outer shell layer can be achieved by using a raw material aqueous solution having a composition with a low nickel ratio.

The present invention was completed based on such knowledge. Hereinafter, the present invention will be explained in the following order: a nickel manganese cobalt-containing composite hydroxide, a method for producing the same, a positive electrode active material, a method for producing the same, and a lithium ion secondary battery.

1. Nickel-manganese-cobalt-containing composite hydroxide (1) Particle structure of nickel-manganese-cobalt-containing composite hydroxide The nickel-manganese-cobalt-containing composite hydroxide of the present invention (hereinafter referred to as "composite hydroxide") is a lithium ion secondary It is a precursor of a positive electrode active material for batteries, and is composed of a plurality of plate-shaped primary particles and secondary particles formed by agglomeration of fine primary particles smaller than the plate-shaped primary particles. The secondary particles have a rounded mass shape including a substantially spherical shape, an ellipsoidal shape, and the like.

As shown in the conceptual diagram of FIG. 1, the secondary particles constituting the composite hydroxide of the present invention are mainly composed of a central part 1 formed by agglomeration of plate-like primary particles, and an outer part of the central part 1. A first low-density layer 2 formed by aggregation of fine primary particles, a high-density layer 3 formed by mainly aggregation of plate-shaped primary particles on the outside of the first low-density layer 2, and a high-density layer 3 formed by aggregation of mainly plate-shaped primary particles. On the outside of the layer 3, there is a second low-density layer 4 formed mainly by agglomeration of fine primary particles; A shell layer 5 is provided.

The center portion 1, the high-density layer 3, and the outer shell layer 5 are basically composed of plate-shaped primary particles, but fine primary particles may be included therein. On the other hand, the first low-density layer 2 and the second low-density layer 4 are basically composed of fine primary particles, but plate-shaped primary particles may be included therein. In particular, in the first low-density layer 2 and the second low-density layer 4, a high-density region formed by agglomeration of plate-shaped primary particles is contained in a low-density region formed by aggregation of fine primary particles. part can exist. More specifically, the first low-density layer 2 is formed between the surface of the aggregate of plate-like primary particles forming the center portion 1 and the surface of the aggregate of plate-like primary particles forming the high-density layer 3. The second low-density layer 4 is formed by depositing an aggregate of fine primary particles on the surface of the aggregate of plate-shaped primary particles forming the high-density layer 3 and the plate-shaped aggregate forming the outer shell layer 5. It is formed by depositing aggregates of fine primary particles between the surfaces of aggregates of primary particles. Since the surface of the aggregate of plate-shaped primary particles is uneven, the first low-density layer 2 and the second low-density layer 4 are substantially composed of aggregates of both plate-shaped primary particles and fine primary particles. It may become a formed structure.

In this case, in the low-density layer, a part of the aggregate of plate-shaped primary particles in the center and a part of the aggregate of plate-shaped primary particles in the high-density layer, or a part of the aggregate of plate-shaped primary particles in the high-density layer A part of the aggregate of the plate-shaped primary particles of the outer shell layer may be in contact with a part of the aggregate of the plate-shaped primary particles.

When the secondary particles with such a structure that make up the composite hydroxide are fired, the aggregates of the plate-like primary particles form the primary particles by sintering, and while maintaining the connection between the primary particles, the The density layer contracts and is absorbed by the primary particles. For this reason, in the obtained positive electrode active material, as described below, the outer shell portion in which primary particles aggregate in the circumferential direction, the aggregated portion of primary particles present inside the outer shell portion, and the presence between the aggregated portion. and a space consisting of pores of appropriate size. There is electrical conduction between the outer shell and the agglomerated parts of primary particles existing inside the outer shell, and between the agglomerated parts of primary particles inside, and the cross-sectional area of the path is Sufficiently secured. As a result of the positive electrode active material having such a porous structure, it is possible to maintain a high energy density while reducing the internal resistance (positive electrode interface resistance) of the positive electrode active material.

In particular, the nickel-manganese-cobalt-containing composite hydroxide of the present invention has such a particle structure, and the center, the first low-density layer, and the high-density layer contain a total of nickel, manganese, and cobalt. The second low-density layer and the outer shell layer have a composition with a high nickel ratio in which the atomic ratio of nickel to nickel exceeds 0.4, and the second low-density layer and the outer shell layer have a nickel ratio in excess of 0.1 to 0. It is characterized by having a composition with a low nickel ratio of 4 or less.

With such a configuration, when a composite hydroxide made of secondary particles having such a structure is fired, the interior of the obtained positive electrode active material basically extends from the center to the Depending on the structure up to the high-density layer, the outer shell part, which is composed of an agglomerated part having a composition with a high nickel ratio and a space part existing between them, and which covers the inside thereof, has a composition with a low nickel ratio.

However, during sintering, fine primary particles constituting the second low-density layer in the precursor and having a composition with a low nickel ratio are removed from the surface of the agglomerated part whose matrix is the high-density layer in the precursor. It forms the surface layer when absorbed by the primary particles that make up the . Therefore, the primary particles present in the surface portion of the agglomerated portion have a surface layer portion whose composition is graded from a high nickel ratio composition to a low nickel ratio composition. Therefore, in the compositional analysis of the cross section of the secondary particles by energy dispersive X-ray spectroscopy using a scanning electron microscope, the nickel ratio of the secondary particles is greater than 0.1 and less than or equal to 0.4. The area ratio of the low nickel ratio layer is in the range of 25% to 45% of the area of the entire cross section of the secondary particles. The presence of the low nickel ratio layer consisting of the outer shell portion and the surface portion of the agglomerated portion protects the primary particles with a high nickel ratio that constitute the agglomerated portion.

As a result, since the outer shell part, which has the most opportunity to come into contact with the electrolyte, has a composition with a low nickel ratio, the inner agglomerated part is protected without further deterioration due to contact with the electrolyte, and Even against the electrolyte that has penetrated between the outer shells, each of the primary particles constituting the surface of the agglomerated part is protected by the surface layer, and the composition with a high nickel ratio constituting the inside of the agglomerated part is protected. The primary particles having the agglomerated portion are also protected by the surface portion of the agglomerated portion. Therefore, it becomes possible to effectively protect and maintain the structure of the secondary particles.

In a composition with a high nickel ratio, the nickel ratio is in a range of more than 0.4 and 1 or less. The nickel ratio is preferably in a range of more than 0.4 and 0.8 or less, and more preferably in a range of 0.45 to 0.7. If the composition of the layer constituting the primary particles (agglomerated parts) inside the positive electrode active material after firing becomes 0.4 or less, the final positive electrode active material cannot have a composition with a high nickel ratio as a whole. .

On the other hand, in a composition with a low nickel ratio, the nickel ratio is in a range of more than 0.1 and 0.4 or less. The nickel ratio is preferably in a range of more than 0.1 and 0.38 or less, and more preferably in a range of 0.2 to 0.36. Normally, it is sufficient that the hydroxide has a composition in which the nickel ratio is 0.33, that is, the ratio of nickel, manganese, and cobalt is 1:1:1. If the nickel ratio in the composition of the outer shell layer constituting the outer shell particles of the positive electrode active material after firing exceeds 0.4, the effect of suppressing deterioration of the outer shell layer will be insufficient. On the other hand, if the nickel ratio is 0.1 or less, the final positive electrode active material cannot have a composition with a high nickel ratio as a whole.

In the present invention, the difference between the nickel ratio in the high nickel ratio composition and the nickel ratio in the low nickel ratio composition is preferably 0.10 or more, more preferably 0.15 or more. If the difference in the nickel ratio is less than 0.10, a composition with a high nickel ratio may not be obtained in the entire positive electrode active material, or the effect of suppressing the deterioration of secondary particles may not be sufficient. There is a possibility that it will not be possible.

(2) Average particle size MV
In the composite hydroxide of the present invention, the average particle size MV of the secondary particles is adjusted to a range of 3 μm to 12 μm, preferably a range of 4 μm to 11 μm, and more preferably a range of 5 μm to 10 μm. The average particle diameter MV of the secondary particles correlates with the average particle diameter MV of the positive electrode active material using this composite hydroxide as a precursor. Therefore, by controlling the average particle diameter MV of the secondary particles constituting the composite hydroxide within such a range, the average particle diameter MV of the positive electrode active material using this composite hydroxide as a precursor can be adjusted to a predetermined value. It becomes possible to control within a range.

In addition, in the present invention, the average particle diameter MV of secondary particles means a volume-based average particle diameter (Mean Volume Diameter), and can be determined from, for example, a volume integrated value measured with a laser beam diffraction scattering particle size analyzer. Can be done.

(3) Particle size distribution The secondary particles of the composite hydroxide of the present invention have an index indicating the spread of particle size distribution [(d90-d10)/average particle size MV] of 0.65 or less, preferably 0.65 or less, preferably 0. It is adjusted to be 55 or less, more preferably 0.50 or less.

The particle size distribution of the positive electrode active material is strongly influenced by the particle properties of the composite hydroxide that is its precursor. Therefore, if a composite hydroxide containing many fine particles and coarse particles is used as a precursor, the positive electrode active material will also contain many fine particles and coarse particles, making it safe for secondary batteries using this material. performance, cycle characteristics, and output characteristics cannot be sufficiently improved. On the other hand, if [(d90-d10)/average particle size MV] is adjusted to 0.65 or less at the stage of composite hydroxide, the positive electrode active material using this as a precursor can be The particle size distribution can be narrowed, and the above-mentioned problems can be avoided. However, assuming industrial scale production, it is not practical to use a composite hydroxide with an excessively small [(d90-d10)/average particle size MV]. Therefore, in consideration of cost and productivity, the lower limit of [(d90-d10)/average particle diameter MV] is preferably about 0.25.

In addition, d10 is the particle size at which the volume of each particle is accumulated from the small particle size side and the cumulative volume is 10% of the total volume of all particles (the cumulative curve of the particle size distribution is calculated by setting the total volume as 100%). Similarly, the volume of each particle is accumulated from the smaller particle size side, and the cumulative volume is 90% of the total volume of all particles. % (when calculating the cumulative curve of particle size distribution with the total volume as 100%, the particle size at the point where the cumulative curve becomes 90%). Like the average particle diameter MV, d10 and d90 can be determined from the volume integrated value measured with a laser beam diffraction scattering particle size analyzer.

(4) Size of the center and thickness of the first low-density layer, high-density layer, second low-density layer, and outer shell layer In the composite hydroxide of the present invention, the secondary particles of the composite hydroxide of the present invention are By appropriately controlling the size of the center, the thickness of the first low-density layer, high-density layer, second low-density layer, and outer shell layer, the aggregated primary particles The formed outer shell part, the agglomerated part that exists inside the outer shell part and is formed by the aggregated primary particles and is electrically conductive with the outer shell part, and the aggregated part that is dispersed in the aggregated part. A positive electrode active material consisting of secondary particles constituted by the existing spaces is obtained.

In order to obtain such a structure, first, in the composite hydroxide having the structure of the present invention, the average ratio of the radius of the center to the particle size of the secondary particles is preferably in the range of 20% to 40%. , more preferably in the range of 25% to 35%. By controlling the size of the center with respect to the secondary particles within the above range, the first low density layer, high density layer, and second low density layer formed on the outside thereof are made to have appropriate thicknesses. This makes it possible to obtain a desired porous structure in which the interior is composed of aggregated parts and spaces in the obtained positive electrode active material. When the size of the center exceeds the above range, the thicknesses of the first low density layer, high density layer, second low density layer, and outer shell layer cannot be controlled within a predetermined range in the composite hydroxide. As a result, the desired porous structure may not be obtained in the resulting positive electrode active material.

Further, the thickness of the first low density layer is preferably in the range of 3% to 15% of the particle size of the secondary particles, and more preferably in the range of 5% to 10%. Further, the thickness of the high-density layer is preferably in the range of 2% to 10%, more preferably in the range of 4% to 8%, of the particle size of the secondary particles. The thickness of the second low density layer is preferably in the range of 2% to 10%, more preferably in the range of 4% to 8%, of the particle size of the secondary particles. Furthermore, the thickness of the outer shell layer is preferably in the range of 2% to 10%, more preferably in the range of 4% to 8%, of the particle size of the secondary particles.

By setting the thicknesses of the first low-density layer, high-density layer, second low-density layer, and outer shell layer to the above ranges, a composite hydroxide with such a structure can be used as a precursor. The obtained cathode active material has an outer shell portion of an appropriate thickness that forms the skeleton of the secondary particles, and an agglomerated portion formed inside the outer shell portion, and is present dispersed inside the secondary particles. A structure having spaces (pores) of appropriate size is obtained. If the first low-density layer and the second low-density layer are smaller than their respective ranges, a sufficiently large space will not be created during firing of the composite hydroxide in the obtained positive electrode active material, and the There is a possibility that the structure will be similar to a solid structure. On the other hand, if the thickness of each low-density layer is too large, the low-density layer will not shrink to form a pore structure during firing of the composite hydroxide, and the resulting positive electrode active material will have, for example, a central part. This results in a structure in which large voids exist between the high-density layer and the outer shell, and the outer shell and the internal agglomerated parts may not be sufficiently connected or integrated, making it impossible to obtain the desired structure. be. In this case, an electrically conductive path having a sufficient cross-sectional area is not formed between the outer shell portion and the internal agglomerated portion, and the effect of reducing the positive electrode resistance cannot be obtained.

On the other hand, if the thickness of the high-density layer and the outer shell layer is too small, the prescribed structure of the secondary particles will not be maintained during the manufacturing or filling stage of the positive electrode active material, and the internal structure of the secondary particles may be destroyed. There is a possibility that a space of sufficient size may not be formed. On the other hand, if the thickness of the high-density layer and the outer shell layer is too large, a sufficiently large space will not be formed inside the secondary particles during firing of the composite hydroxide, and Infiltration of electrolytes and conductive aids may be insufficient.

The composite hydroxide of the present invention has a structure including two laminate structures in which a low-density layer and a high-density layer are laminated on the outside of the center. When three or more of these laminated structures are provided, each low-density layer may not be formed to a sufficient thickness, and the effect of improving the specific surface area may not be sufficiently obtained. On the other hand, if only one laminated structure is provided, the proportion of voids in the obtained positive electrode active material will be insufficient, and the desired characteristics such as sufficient specific surface area and low internal resistance may not be obtained. .

In the present invention, as long as each of the core, first low-density layer, high-density layer, second low-density layer, and outer shell layer has a predetermined thickness, a composite of such a structure can be used. When the hydroxide is fired, a structure is obtained in which the center, the high-density layer, and the outer shell layer are substantially integrated by sintering shrinkage, and there are dispersed spaces inside. The space existing inside can communicate with the outside through grain boundaries or voids between the primary particles forming the aggregate. Therefore, in such a structure, while maintaining the resistance of the overall structure of the obtained positive electrode active material to collapse, sufficient electrical conduction paths between the primary particles constituting the secondary particles are secured, and the spaces and As a result of sufficiently securing communication with the outside, it becomes possible to further reduce the positive electrode resistance.

The size of the center and the thickness of the first low-density layer, high-density layer, second low-density layer, and outer shell layer are based on the cross section of the secondary particles constituting the composite hydroxide. can be determined by observing with a scanning electron microscope (SEM) such as a field emission scanning electron microscope (FE-SEM). However, since the overall shape of the secondary particles constituting the composite hydroxide is distorted and the shape of each layer is also complicated, it is not easy to specify the thickness of each layer by SEM observation. Therefore, the size of the center and the thickness of the first low-density layer, high-density layer, second low-density layer, and outer shell layer in this specification are determined in the manufacturing process of the composite hydroxide. From the ratio of the amount of each metal added in a non-oxidizing atmosphere or an oxidizing atmosphere to form the center and each layer to the total amount of metal added in the entire grain growth process. , is theoretically required.

(5) Primary particles In the composite hydroxide of the present invention, the size of the plate-shaped primary particles constituting the center, high-density layer, and outer shell layer, and the size of the first low-density layer and the second low-density layer. The size of the fine primary particles constituting the composite hydroxide can be determined by embedding the secondary particles of the composite hydroxide in a resin, etc., making it possible to observe the cross section by cross-section polishing, etc. When observed using a SEM such as SEM, the difference in structure is such that the difference in structure can be clearly understood on imaging.

In the composite hydroxide of the present invention, the plate-like primary particles are formed to have an average particle size in the range of 0.3 μm to 3 μm. Preferably, the size of the plate-like primary particles is in the range of 0.4 μm to 1.5 μm in average particle size. If the average particle size of the plate-shaped primary particles is less than 0.3 μm, volume shrinkage of the plate-shaped primary particles also occurs in the low temperature range in the firing process for producing the positive electrode active material. a density layer and a second low density layer. Since the difference in the amount of volumetric shrinkage between the central portion, the high-density layer, and the outer shell layer becomes small, there is a possibility that a sufficient number of spaces cannot be obtained inside the positive electrode active material. On the other hand, when the average particle size of the plate-shaped primary particles is larger than 3 μm, firing at a higher temperature is required to increase the crystallinity of the positive electrode active material in the firing process when producing the positive electrode active material, and the composite Sintering between the secondary particles constituting the hydroxide progresses, making it difficult to set the average particle size MV and particle size distribution of the positive electrode active material within a predetermined range.

On the other hand, fine primary particles are formed to a size that can be sufficiently distinguished from plate-like primary particles by SEM observation or the like. Although it may be difficult to fully grasp the size of fine primary particles through SEM observation, fine primary particles have an average particle size smaller than the average particle size of plate-shaped primary particles. , 0.01 μm to 0.3 μm, and the difference from the average particle size of the plate-shaped primary particles is 0.1 μm or more. Usually, the average particle size of the fine primary particles has a difference of 0.3 μm or more from the average particle size of the plate-shaped primary particles. If the average particle size of the fine primary particles is less than 0.01 μm, a satisfactory thickness of the low-density layer cannot be obtained. On the other hand, if the average particle size of the fine primary particles is larger than 0.3 μm, the volume shrinkage due to heating will not proceed sufficiently during firing in the low temperature range in the firing process for producing the positive electrode active material, resulting in a low density. Since the difference in the amount of volumetric shrinkage between the layer, the center portion, and the high-density layer becomes small, a sufficiently large space may not be formed inside the secondary particles of the positive electrode active material.

The shape of such fine primary particles may not be able to be fully grasped or specified even by SEM observation, and basically they can have any shape, but are preferably acicular. This is because acicular primary particles have a shape with one-dimensional directionality, and when the particles aggregate, they can form a structure with many gaps, that is, a structure with low density.

As mentioned above, it is sufficient that the sizes of fine primary particles and plate-like primary particles differ to the extent that the difference in the layer or part formed by their aggregation can be understood by SEM observation, and it is necessary to specifically measure them. There isn't. In particular, with regard to fine primary particles, the particle size may be so small that the shape thereof cannot be fully grasped. If both the shape and size of the fine primary particles and the plate-like primary particles can be observed by SEM, they can be determined as follows. That is, the maximum outer diameter (major axis diameter) of 10 or more fine primary particles or plate-shaped primary particles present in the cross section of the secondary particle is measured, the average value is determined, and this value is The particle size shall be that of fine primary particles or plate-shaped primary particles. Next, for 10 or more secondary particles, the particle sizes of fine primary particles and plate-shaped primary particles are determined in the same manner. Finally, by determining the average particle size of these secondary particles, the average particle size of the fine primary particles or plate-like primary particles in the entire composite hydroxide is determined.

(4) Composition The composite hydroxide of the present invention is a precursor of a ternary positive electrode active material consisting of a lithium nickel manganese cobalt-containing composite oxide (NMC) containing at least nickel, manganese, and cobalt as transition metals. It is the body. As long as it has the above-mentioned particle structure, average particle size, and particle size distribution, its composition is not limited. However, since the present invention is particularly suitably applied to a positive electrode active material having a composition with a high nickel ratio, the nickel ratio, which is the atomic ratio of nickel to the total of nickel, manganese, and cobalt, in the composite hydroxide as a whole is preferably greater than 0.4.

In particular, the composite hydroxide of the present invention has the general formula (A): Ni _{x Mny} Co _z M _t (OH) _2+a (x+y+z+t=1, 0.4≦x≦0.70, 0.15≦y≦ 0.4, 0.15≦z≦0.3, 0≦t≦0.1, 0≦a≦0.5, M is Mg, Al, Si, Ca, Ti, V, Cr, Zr, Nb , Mo, Hf, Ta, and W). By using a composite hydroxide having such a composition as a precursor, a positive electrode active material represented by the general formula (B) described below can be easily obtained, and higher battery performance can be achieved. .

In addition, in the composite hydroxide represented by the general formula (A), the composition range of nickel, manganese, cobalt, and the additive element M that constitute this and its critical significance are represented by the general formula (B). It is the same as the positive electrode active material. Therefore, description of these matters will be omitted here.

2. Method for producing a composite hydroxide containing nickel manganese cobalt (1) Crystallization reaction The method for producing a composite hydroxide of the present invention includes a raw material aqueous solution containing at least a transition metal and an ammonium ion donor in a reaction tank. In this method, a reaction aqueous solution is formed by supplying an aqueous solution, and a composite hydroxide, which becomes a precursor of a positive electrode active material, is produced by a crystallization reaction.

In the method for producing a composite hydroxide of the present invention, the crystallization reaction is clearly separated into two stages: a nucleation step in which nucleation is mainly performed, and a particle growth step in which particle growth is mainly performed, and the crystallization reaction in each step is By adjusting the analysis conditions, especially in the particle growth process, by switching the reaction atmosphere and changing the composition of the raw material aqueous solution while continuing to supply the raw material aqueous solution, the above-mentioned particle structure, average particle size MV, Moreover, it is possible to efficiently obtain a composite hydroxide having a particle size distribution.

The method for producing a composite hydroxide of the present invention includes a nucleation step in which the crystallization reaction is performed by adjusting the pH value of the reaction aqueous solution to a range of 12.0 to 14.0 based on the liquid temperature of 25°C. , the pH value of the reaction aqueous solution containing the nuclei obtained in the nucleation step, based on the liquid temperature of 25° C., is controlled to be lower than the pH value of the nucleation step and 10.5 to 12.0. There is a clear distinction between two stages: growing the nucleus and growing the grain.

In addition, by circulating an inert gas in the reaction tank before or at the start of the nucleation process, the reaction atmosphere at the initial stage of the nucleation process and particle growth process can be maintained at an oxygen concentration of 5% by volume or less. In addition to adjusting the reaction atmosphere to a non-oxidizing atmosphere of After switching to an oxidizing atmosphere, by reintroducing a non-oxidizing gas, the reaction atmosphere is switched from a non-oxidizing atmosphere to a non-oxidizing atmosphere with an oxygen concentration of 5% by volume or less, and atmosphere control is performed twice. It is characterized by

More specifically, as shown in FIG. 2, the method for producing a composite hydroxide of the present invention is as follows: (2) After the first stage of the particle growth process, the reaction atmosphere is switched from the non-oxidizing atmosphere to an oxidizing atmosphere in which the oxygen concentration exceeds 5% by volume. , as the second stage of the particle growth process, (3) next, the reaction atmosphere is switched from an oxidizing atmosphere to a non-oxidizing atmosphere, as the third stage of the particle growth process, (4) further, the reaction atmosphere is (5) Finally, the reaction atmosphere is switched from an oxidizing atmosphere to a non-oxidizing atmosphere, resulting in a fifth step of the particle growth process. It is composed of steps.

At that time, 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 grain growth process, and the crystallization reaction at each stage relative to the entire grain growth process is controlled. The ratio is preferably as follows. That is, the first stage is in the range of 8% to 20%, the second stage is in the range of 2% to 20%, the third stage is in the range of 12% to 40%, and the fourth stage is in the range of 3% to 40%. It is preferable that the fifth stage is 10% to 50%.

Basically, by switching the reaction atmosphere as described above, secondary particles of composite hydroxide having a desired multilayer structure can be obtained.

Further, in the method for producing a composite hydroxide of the present invention, the composition of the raw material aqueous solution is changed during the crystallization reaction. Specifically, the nickel ratio, which is the atomic ratio of nickel to the total of nickel, manganese, and cobalt, as the raw material aqueous solution supplied in the first, second, and third stages of the nucleation process and particle growth process. A raw material aqueous solution having a composition with a high nickel ratio exceeding 0.4 is used. On the other hand, as the raw material aqueous solution supplied in the fourth and fifth stages of the particle growth process, a raw material aqueous solution having a composition with a low nickel ratio of more than 0.1 and less than or equal to 0.4 is used. By switching the raw material aqueous solution in this way, the plate-shaped primary particles that make up the center and the high-density layer, as well as the fine primary particles that make up the first low-density layer, have a high nickel ratio of more than 0.4. The fine primary particles constituting the second low-density layer and the primary particles constituting the outer shell layer are composed of low nickel with a nickel ratio of more than 0.1 and less than 0.4. The composition can be of a ratio.

Note that the method for producing a composite hydroxide of the present invention is not limited by its composition as long as it can achieve the structure, average particle size MV, and particle size distribution described above; It can be suitably applied to complex hydroxides.

The crystallization step will be explained in more detail below.

[Nucleation process]
In the nucleation step, first, a transition metal compound serving as a raw material in this step is 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 the reaction tank, and the pH value measured based on the liquid temperature of 25°C is 12.0 to 14.0, and the ammonium ion concentration is 3 g/ Prepare a pre-reaction aqueous solution having a concentration of L to 25 g/L. Note that the pH value of the pre-reaction aqueous solution can be measured with a pH meter, and the ammonium ion concentration can be measured with an ion meter.

Next, a raw material aqueous solution is supplied while stirring this pre-reaction aqueous solution. As a result, a nucleation aqueous solution , which is a reaction aqueous solution in the nucleation step, is formed in the reaction tank. Since the pH value of this aqueous solution for nucleation is within the above-mentioned range, in the nucleation step, nucleation occurs preferentially, with almost no nuclei growing. In addition, in the nucleation process, the pH value and ammonium ion concentration of the nucleation aqueous solution change as the nucleation occurs, so the alkaline aqueous solution and the ammonia aqueous solution are supplied in a timely manner so that the pH value of the solution in the reaction tank reaches 25% of the liquid temperature. It is necessary to control the pH to a range of 12.0 to 14.0 and the ammonium ion concentration to a range of 3 g/L to 25 g/L on a temperature basis.

In the nucleation step, it is necessary to circulate an inert gas in the reaction tank and adjust the reaction atmosphere to a non-oxidizing atmosphere with an oxygen concentration of 5% by volume or less. It is usually preferable to adjust the reaction atmosphere before starting the supply of the raw material aqueous solution. Thereby, agglomerated parts are sufficiently formed inside the positive electrode active material using this composite hydroxide as a precursor, and it becomes possible to suppress a decrease in particle density due to the formation of spaces.

In the nucleation step, new nuclei are continuously generated by supplying an aqueous solution containing a raw material aqueous solution, an alkaline aqueous solution, and an ammonium ion donor to the nucleation aqueous solution. Then, when a predetermined amount of nuclei are generated in the aqueous solution for nucleation, the nucleation step is ended.

At this time, the amount of nuclei generated can be determined from the amount of metal compound contained in the raw material 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 hydroxide with a narrow particle size distribution, it is necessary to It is preferably 0.1 atomic % to 2.0 atomic %, more preferably 0.1 atomic % to 1.5 atomic %, based on the metal element contained in the metal compound. Note that the reaction time in the nucleation step is usually about 0.2 minutes to 5 minutes.

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

Next, while stirring this particle growth aqueous solution, supply of the raw material aqueous solution is restarted. At this time, since the pH value of the aqueous solution for particle growth is within the range mentioned above, new nuclei are hardly generated, and the growth of nuclei (particles) progresses, resulting in secondary particles of composite hydroxide having a predetermined particle size. is formed. In addition, in the particle growth process, the pH value and ammonium ion concentration of the particle growth aqueous solution change as the particles grow, so an alkaline aqueous solution and an ammonia aqueous solution are supplied in a timely manner to maintain the pH value and ammonium ion concentration within the above range. It is necessary to do so.

In particular, in the method for producing a composite hydroxide of the present invention, during the particle growth process, the supply of two types of raw material aqueous solutions is alternately switched, and while the supply continues, two types of atmospheric gases are introduced. An operation in which the reaction atmosphere is switched from a non-oxidizing atmosphere to an oxidizing atmosphere with an oxygen concentration of more than 5% by volume, or from this non-oxidizing atmosphere to a non-oxidizing atmosphere with an oxygen concentration of 5% by volume or less. I do.

In the particle growth process, an inert gas and/or oxidizing gas is passed through the reaction aqueous solution in the reaction tank using an aeration tube to switch from an oxidizing atmosphere to a non-oxidizing atmosphere, or to switch from an oxidizing atmosphere to a non-oxidizing atmosphere. It is preferable to quickly switch from an oxidizing atmosphere to an oxidizing atmosphere. Although it is possible to supply the inert gas and/or oxidizing gas to the reaction aqueous solution in the reaction tank, it is also possible to supply the inert gas and/or oxidizing gas to the space in the reaction tank that is in contact with the reaction aqueous solution. It is preferable to adopt a method of directly supplying the oxidizing gas to the reaction aqueous solution. This makes it possible to shorten the atmosphere switching time, and to appropriately set the size of the center, the thickness of the first low-density layer, high-density layer, second low-density layer, and outer shell layer. becomes possible.

In addition, in the particle growth process, the reaction atmosphere is switched between the first non-oxidizing atmosphere, the first oxidizing atmosphere, and the second non-oxidizing atmosphere (first stage, second stage, and third stage). The crystallization reaction is continued using a raw material aqueous solution having a composition with a high nickel ratio as the raw material aqueous solution. At the same time as switching from the second non-oxidizing atmosphere to the second oxidizing atmosphere (at the time of transition from the third stage to the fourth stage), the raw material aqueous solution is changed from a raw material aqueous solution having a composition with a high nickel ratio to a low nickel ratio. Switch to a raw material aqueous solution having the composition. When switching from the second oxidizing atmosphere to the third non-oxidizing atmosphere (transitioning from the fourth stage to the fifth stage), the raw material aqueous solution is not switched, and the fifth stage is a low-oxidizing atmosphere. A crystallization reaction using a raw material aqueous solution having a composition with a nickel ratio is continued. In this case, the composition of the primary particles constituting each layer can be appropriately controlled by providing two supply routes for the raw material aqueous solutions into the reaction tank and quickly switching between the raw material aqueous solutions supplied into the reaction tank. becomes possible.

In addition, in such a method for producing a composite hydroxide, metal ions precipitate as nuclei or primary particles in the nucleation step and the particle growth step. Therefore, the ratio of the liquid component to the metal component in the nucleation aqueous solution and the particle growth aqueous solution increases. As a result, the concentration of the raw material aqueous solution apparently decreases, and the growth of secondary particles of the composite hydroxide may stagnate, particularly in the particle growth step. Therefore, in order to suppress the increase in the liquid component, it is preferable to discharge a part of the liquid component of the aqueous solution for particle growth to the outside of the reaction tank after the completion of the nucleation step and during the particle growth step. Specifically, the supply and stirring of the aqueous solution containing the raw material aqueous solution, aqueous alkaline solution, and ammonium ion donor are temporarily stopped, and the nuclei and composite hydroxides in the aqueous solution for particle growth are precipitated, and the supernatant of the aqueous solution for particle growth is Preferably, the liquid is drained. Such operations can increase the relative concentration of the mixed aqueous solution in the aqueous solution for particle growth, thereby preventing stagnation of particle growth and adjusting the particle size distribution of secondary particles of the resulting composite hydroxide to a suitable range. Not only can it be controlled, but also the density of the secondary particles as a whole can be improved.

[Particle size control of secondary particles of composite hydroxide]
The particle size of the secondary particles of the composite hydroxide obtained as described above depends on the time of the particle growth process and nucleation process, the pH value of the nucleation aqueous solution and the particle growth aqueous solution, and the supply amount of the raw material aqueous solution. can be controlled. For example, by performing the nucleation process at a high pH value or by lengthening the time of the particle generation process, the amount of metal compounds contained in the supplied raw material aqueous solution can be increased, the amount of nuclei generated can be increased, and the amount of nuclei produced can be increased. The particle size of the secondary particles of the composite hydroxide can be reduced. On the other hand, by suppressing the amount of nuclei generated in the nucleation step, the particle size of the secondary particles of the resulting composite hydroxide can be increased.

[Another embodiment of crystallization reaction]
In the method for producing a composite hydroxide of the present invention, an aqueous solution for component adjustment adjusted to a pH value and ammonium ion concentration suitable for the particle growth step is prepared separately from the aqueous solution for nucleation, and this aqueous solution for component adjustment is added to the aqueous solution for component adjustment. , adding and mixing an aqueous solution for nucleation after the nucleation step, preferably from which a part of the liquid component has been removed from the aqueous solution for nucleation after the nucleation step, and using this as an aqueous solution for particle growth, perform the particle growth step. You may do so.

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

(2) Supply aqueous solution In the method for producing a composite hydroxide of the present invention, the reaction aqueous solution is supplied by supplying a raw material aqueous solution containing at least nickel, manganese, and cobalt and an aqueous solution containing an ammonium ion donor into a reaction tank. A complex hydroxide is obtained by a crystallization reaction while adjusting the pH value of the reaction aqueous solution to a predetermined range using a pH adjuster.

a) Raw Material Aqueous Solution In the present invention, the ratio of metal elements contained in the two raw material aqueous solutions generally corresponds to the composition ratio of the resulting composite hydroxide. For this reason, it is necessary to change the nickel ratio of the two raw material aqueous solutions and to adjust the metal element content of each raw material aqueous solution as appropriate depending on the composition of the target composite hydroxide.

For example, when trying to obtain a composite hydroxide represented by the above-mentioned general formula (A), the ratio of metal elements contained in the two raw material aqueous solutions is set to Ni:Mn:Co:M=x: y:z;t (however, x+y+z+t=1, 0.4≦x≦0.70, 0.15≦y≦0.4, 0.15≦z≦0.3, 0≦t≦0.1) At the same time, the nickel ratio contained in one raw material aqueous solution is adjusted to a composition with a high nickel ratio exceeding 0.4, and the nickel ratio contained in the other raw material aqueous solution is adjusted to 0.1. It is necessary to adjust the composition to a low nickel ratio exceeding 0.4 or less.

More specifically, the nickel ratio of the raw material aqueous solutions supplied in the first stage, second stage, and third stage is set to a range of more than 0.4 and 1 or less, preferably more than 0.4. It should be in the range of 0.8 or less, more preferably in the range of 0.45 to 0.7. On the other hand, the nickel ratio of the raw material aqueous solution supplied in the fourth stage and the fifth stage is in the range of more than 0.1 and less than 0.4, preferably in the range of more than 0.1 and less than 0.38. More preferably, it is in the range of 0.2 to 0.36. For example, if the ratio of nickel, manganese, and cobalt in the raw material aqueous solutions supplied in the first, second, and third stages is 5:3:2 to 7:2:1, the fourth stage, The ratio of nickel to manganese to cobalt in the raw material aqueous solution supplied in the fifth stage can be 3.5 to 4:3 to 3.5:3 to 3.5.

In addition, when introducing the additive element M in a separate process, the additive element M is not included in the raw material aqueous solution. Further, in the nucleation step and the particle growth step, it is also possible to change whether or not the additional element M is added, or the content ratio of the transition metal or the additional element M.

The transition metal compound for preparing the raw material aqueous solution is not particularly limited, but it is preferable to use water-soluble nitrates, sulfates, chlorides, etc. from the viewpoint of ease of handling, and the cost and halogen From the viewpoint of preventing contamination, it is particularly preferable to use sulfates.

Additionally, an additive element M (M is one or more additive elements selected from Mg, Al, Si, Ca, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) is added to the composite hydroxide. ), 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, peroxotitanic acid. Ammonium, potassium titanium oxalate, vanadium sulfate, ammonium vanadate, chromium sulfate, potassium chromate, zirconium sulfate, niobium oxalate, ammonium molybdate, hafnium sulfate, sodium tantalate, sodium tungstate, ammonium tungstate, etc. are preferably used. Can be used.

The concentration of the raw material aqueous solution is preferably 1 mol/L to 2.6 mol/L, more preferably 1.5 mol/L to 2.2 mol/L in total of the metal compounds. If the concentration of the raw material aqueous solution is less than 1 mol/L, the amount of crystallized material per reaction tank will decrease, resulting in a decrease in productivity. On the other hand, if the concentration of the mixed aqueous solution exceeds 2.6 mol/L, the saturated concentration at room temperature will be exceeded, so there is a risk that crystals of the respective metal compounds will re-precipitate and clog pipes and the like.

The above-mentioned metal compound does not necessarily need to be supplied to the reaction tank as a raw material aqueous solution. For example, when performing a crystallization reaction using metal compounds that react when mixed to produce a compound other than the desired compound, individually Alternatively, an aqueous solution of the metal compound may be prepared and supplied into the reaction tank at a predetermined ratio as an aqueous solution of each metal compound.

In addition, the supply amount of the raw material aqueous solution is such that the concentration of the product in the particle growth aqueous solution is preferably 30 g/L to 200 g/L, more preferably 80 g/L to 150 g/L at the end of the particle growth step. I will make it happen. If the concentration of the product is less than 30 g/L, the aggregation of primary particles may be insufficient. On the other hand, if it exceeds 200 g/L, the metal salt aqueous solution for nucleation or the metal salt aqueous solution for particle growth will not be sufficiently diffused in the reaction tank, which may cause uneven particle growth.

b) Alkaline aqueous solution The alkaline aqueous solution for adjusting the pH value in the reaction aqueous solution is not particularly limited, and common alkali metal hydroxide aqueous solutions such as sodium hydroxide and potassium hydroxide can be used. Although the alkali metal hydroxide can be added directly to the reaction aqueous solution, it is preferably added as an aqueous solution for ease of pH control. In this case, the concentration of the aqueous alkali metal hydroxide solution is preferably 20% by mass to 50% by mass, more preferably 20% by mass to 30% by mass. By regulating the concentration of the alkali metal aqueous solution within this range, it is possible to suppress the amount of solvent (water amount) supplied to the reaction system and prevent the pH value from becoming locally high at the addition location. , it becomes possible to efficiently obtain secondary particles of composite hydroxide with a narrow particle size distribution.

Note that the method of supplying the alkaline aqueous solution is not particularly limited as long as the pH value of the reaction aqueous solution does not locally increase and is maintained within a predetermined range. For example, the aqueous reaction solution may be sufficiently stirred and supplied using a pump such as a metering pump that can control the flow rate.

c) Aqueous solution containing ammonium donor The aqueous solution containing ammonium ion donor is not particularly limited, and for example, aqueous ammonia, or an aqueous solution of ammonium sulfate, ammonium chloride, ammonium carbonate, or ammonium fluoride may be used. Can be done.

When aqueous ammonia is used as the ammonium ion donor, its concentration is preferably 20% by mass to 30% by mass, more preferably 22% by mass to 28% by mass. By regulating the concentration of ammonia water within such a range, loss of ammonia due to volatilization etc. can be suppressed to a minimum, thereby making it possible to improve production efficiency.

Note that the aqueous solution containing the ammonium ion donor can also be supplied using a pump that can control the flow rate, similarly to the alkaline aqueous solution.

[pH value]
In the method for producing a composite hydroxide of the present invention, the pH value of the reaction aqueous solution based on the liquid temperature of 25°C is in the range of 12.0 to 14.0 in the nucleation step and 10.5 in the particle growth step. It is necessary to control it within the range of ~12.0. In addition, in any step, it is preferable to control the fluctuation range of the pH value during the crystallization reaction to within ±0.2. When the range of pH value fluctuation is large, the ratio of the amount of nucleation to the particle growth is not constant, making it difficult to obtain secondary particles of composite hydroxide with a narrow particle size distribution. Note that the pH value of the reaction aqueous solution can be measured with a pH meter.

a) Nucleation step In the nucleation step, the pH value of the reaction aqueous solution (nucleation aqueous solution) is set to 12.0 to 14.0, preferably 12.3 to 13.5, based on the liquid temperature of 25°C. It is necessary to control it preferably within the range of 12.5 to 13.3. This makes it possible to suppress the growth of nuclei and give priority to nucleation, making it possible to make the nuclei produced in this step homogeneous and with a narrow particle size distribution. On the other hand, if the pH value is less than 12.0, the growth of nuclei (particles) progresses as well as nucleation, so that the particle size of the secondary particles of the resulting composite hydroxide becomes non-uniform and the particle size distribution deteriorates. Moreover, when the pH value exceeds 14.0, the generated nuclei become too fine, resulting in the problem that the aqueous nucleation solution gels.

b) Particle growth step In the particle growth step, the pH value of the aqueous reaction solution ( aqueous solution for particle growth ) is set to 10.5 to 12.0, preferably 11.0 to 12.0, based on the liquid temperature of 25°C. It is necessary to control it preferably within the range of 11.5 to 12.0. This suppresses the generation of new nuclei, makes it possible to give priority to particle growth, and makes it possible to make the obtained secondary particles of the composite hydroxide homogeneous and have a narrow particle size distribution. On the other hand, if the pH value is less than 10.5, the ammonium ion concentration increases and the solubility of metal ions increases, which not only slows down the crystallization reaction rate but also increases the amount of metal ions remaining in the reaction aqueous solution. and productivity deteriorates. Moreover, when the pH value exceeds 12.0, the amount of nucleation during the particle growth process increases, the particle size of the secondary particles of the obtained composite hydroxide becomes non-uniform, and the particle size distribution deteriorates.

In addition, when the pH value is 12.0, it is a boundary condition between nucleation and nucleus growth, so depending on the presence or absence of nuclei in the reaction aqueous solution, it can be used as a condition for either the nucleation process or the particle growth process. Can be done. In other words, if the pH value in the nucleation step is made higher than 12.0 to generate a large amount of nucleation, and then the pH value in the particle growth step is set to 12.0, a large amount of nuclei will be present in the reaction aqueous solution, and the particles will be Growth occurs preferentially, and secondary particles of composite hydroxide with a narrow particle size distribution can be obtained. On the other hand, when the pH value of the nucleation step is set to 12.0, there are no nuclei to grow in the reaction aqueous solution, so nucleation occurs preferentially. , the generated nuclei grow, and good secondary particles of composite hydroxide can be obtained. In either case, the pH value of the particle growth process should be controlled to a value lower than the pH value of the nucleation process, and in order to clearly separate nucleation and particle growth, It is preferable to lower the pH value by 0.5 or more, more preferably by 1.0 or more than the pH value in the production step.

[Reaction atmosphere]
The structure of the secondary particles of the composite hydroxide of the present invention is formed by controlling the pH value of the reaction aqueous solution in the nucleation step and the particle growth step as described above, and by controlling the reaction atmosphere in these steps. Ru. Therefore, in the method for producing a composite hydroxide of the present invention, control of the reaction atmosphere as well as control of the pH value in each step has important significance.

In such reaction atmosphere control, the fine primary particles constituting the first low-density layer and the second low-density layer are usually plate-shaped and/or needle-shaped, but depending on the composition of the composite hydroxide. Depending on the case, the shape may be a rectangular parallelepiped, an ellipse, a rhombohedral, or the like. The same applies to the plate-shaped primary particles that constitute the center, high-density layer, and outer shell layer. Therefore, in the method for producing a composite hydroxide of the present invention, it is necessary to appropriately control the reaction atmosphere at each stage depending on the composition of the target composite hydroxide.

a) Non-oxidizing atmosphere In the production 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 particles of composite hydroxide is a non-oxidizing atmosphere. Specifically, by introducing a non-oxidizing gas such as an inert gas, the oxygen concentration in the reaction atmosphere is 5% by volume or less, preferably 2% by volume or less, more preferably 1% by volume or less. It is necessary to control the atmosphere to be a mixture of oxygen and inert gas so as to create an oxidizing atmosphere. This allows the nuclei generated in the nucleation process to grow to a certain extent while suppressing unnecessary oxidation, allowing the core, high-density layer, and outer shell layer of the secondary particles of composite hydroxide to grow. can have a structure in which plate-shaped primary particles aggregate.

b) Oxidizing Atmosphere On the other hand, in the step of forming the first low-density layer and the second low-density layer of secondary particles of composite hydroxide, the reaction atmosphere is controlled to be an oxidizing atmosphere. Specifically, the oxygen concentration in the reaction atmosphere is controlled so as to exceed 5% by volume, preferably 10% by volume or more, and more preferably an atmospheric atmosphere (oxygen concentration: 21% by volume). By controlling the oxygen concentration in the reaction atmosphere within this range, particle growth is suppressed and the average particle size of the primary particles is in the range of 0.01 μm to 0.3 μm. A first low-density layer and a second low-density layer having a sufficient density difference from the outer shell layer can be formed.

Note that the upper limit of the oxygen concentration in the reaction atmosphere at this stage is not particularly limited, but if the oxygen concentration is excessively high, the average particle size of the primary particles will be less than 0.01 μm, and the low density layer will not be sufficient. It may not be the same size. For this reason, the oxygen concentration is preferably 30% by volume or less.

c) Switching of reaction atmosphere In the particle growth process, it is necessary to perform the above-mentioned atmosphere control so that a composite hydroxide having the desired particle structure is formed.

In the method for producing a composite hydroxide of the present invention, when atmospheric gas is directly supplied into the reaction aqueous solution using an aeration tube, the amount of oxygen dissolved in the reaction aqueous solution, which is the reaction atmosphere, i.e., the reaction site, is Changes without delay in response to changes in oxygen concentration in the tank. Therefore, the atmosphere switching time can be confirmed by measuring the oxygen concentration in the reaction tank. On the other hand, when supplying atmospheric gas to the space in contact with the reaction aqueous solution in the reaction tank, there is a time lag between the change in the amount of dissolved oxygen in the reaction aqueous solution and the change in the oxygen concentration in the reaction tank. The amount of dissolved oxygen in the reaction aqueous solution cannot be immediately confirmed as the correct value until the change in oxygen concentration in the reaction tank stabilizes; It is possible to do so. In this way, in any case, the switching time of the atmosphere obtained based on the oxygen concentration in the reaction tank can be used as the switching time of the amount of oxygen dissolved in the reaction aqueous solution, which is the reaction site, and therefore, the reaction The reaction atmosphere can be appropriately controlled over time based on the oxygen concentration in the tank.

In the present invention, the time required for changing the atmosphere is approximately 1% to 2% of the entire particle growth process. This time is also common when switching from a non-oxidizing atmosphere to an oxidizing atmosphere or from an oxidizing atmosphere to a non-oxidizing atmosphere. Therefore, although it is possible to strictly control the atmosphere switching time alone, it is usually sufficient to include it in the non-oxidizing atmosphere or oxidizing atmosphere time after changing the atmosphere.

More specifically, the crystallization reaction in the non-oxidizing atmosphere in the first stage (initial stage) of the grain growth stage is preferably in the range of 8% to 20%, more preferably in the range of 8% to 20% of the entire grain growth process time. is in the range of 10% to 18%.

The crystallization reaction in the oxidizing atmosphere in the second stage (including the time for switching from a non-oxidizing atmosphere to an oxidizing atmosphere) is preferably in the range of 2% to 20%, and more preferably in the range of 2% to 20% of the total particle growth process time. It is preferably in the range of 3% to 15%. However, the ratio of the entire crystallization reaction in the oxidizing atmosphere in the second and fourth stages is preferably within the range of 4% to 45%, more preferably within the range of 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 in the range of 12% to 40% of the total particle growth process time, More preferably in the range of 14% to 34%, and the second switching to the oxidizing atmosphere is in the range of 3% to 40%, preferably from the start of the particle growth step to the entire particle growth step. It is carried out in the range of 4% to 25%.

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 performed at a preferable ratio of the crystallization reaction in the second stage to the entire particle growth process time. is 1.2 times to 2.5 times, more preferably 1.5 times to 2.0 times, preferably in the range of 3% to 40%, more preferably in the range of 4% to 25%.

The crystallization reaction in the non-oxidizing atmosphere in the fifth stage (including the time for switching from the oxidizing atmosphere to the non-oxidizing atmosphere) is preferably in the range of 10% to 50% of the entire particle growth process time, More preferably, the content is in the range of 25% to 45% to form a sufficient skeleton of the outer shell. After a predetermined period of time has elapsed, the crystallization reaction is finally terminated.

Note that the ratio of the 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 step. In actual operation, by constantly supplying the raw material aqueous solution and keeping the amount of metal added constant, each crystallization reaction time is kept at a predetermined ratio to the entire particle growth process. It can be controlled so that

d) Switching method Switching of the reaction atmosphere during the crystallization process is performed by circulating atmospheric gas in the reaction tank or by inserting a conduit with an inner diameter of about 1 mm to 50 mm into the reaction aqueous solution and bubbling with atmospheric gas. This is common. In this case, since it takes a long time to switch the reaction atmosphere, it is necessary to stop the supply of the raw material aqueous solution during the switching. If the supply of the raw material aqueous solution is not stopped, a gentle density gradient may be formed inside the composite hydroxide particles. In this case, in particular, the formation of the first low density layer and the second low density layer may be insufficient.

On the other hand, if the reaction atmosphere is switched using a diffuser tube, the diffuser tube is composed of a conduit with many fine holes on its surface and can release a large number of fine gases (bubbles) into the liquid, allowing it to be used in a short period of time. It is possible to switch the reaction atmosphere. Therefore, it is not necessary to stop the supply of the raw material aqueous solution when switching the reaction atmosphere, and production efficiency can be improved. In this case, even if the rate of crystallization reaction in an oxidizing atmosphere becomes short, it is possible to sufficiently form a low-density layer.

As the diffuser tube, it is preferable to use one made of ceramic, which has excellent resistance in a high pH environment. Furthermore, the smaller the pore diameter of the air diffuser tube, the more fine air bubbles can be emitted, so that the reaction atmosphere can be switched with higher efficiency. It is preferable to use an aeration tube with a pore diameter of 100 μm or less, more preferably an aeration tube with a pore diameter of 50 μm or less.

(4) Ammonium ion concentration The ammonium ion concentration in the reaction aqueous solution is preferably maintained at a constant value within the range of 3 g/L to 25 g/L, more preferably 5 g/L to 20 g/L. Since ammonium ions function as a complexing agent in the reaction aqueous solution, if the ammonium ion concentration is less than 3 g/L, the solubility of metal ions cannot be maintained constant, and the reaction aqueous solution tends to gel, causing the shape to change. It becomes difficult to obtain secondary particles of composite hydroxide with a uniform particle size. On the other hand, if the ammonium ion concentration exceeds 25 g/L, the solubility of metal ions becomes too high, and the amount of metal ions remaining in the reaction aqueous solution increases, causing compositional deviation. Note that the ammonium ion concentration of the reaction aqueous solution can be measured with an ion meter.

Note that if the ammonium ion concentration changes during the crystallization reaction, the solubility of metal ions will change, and uniform secondary particles of composite hydroxide will not be formed. Therefore, it is preferable to control the fluctuation range of ammonium ion concentration within a certain range through the nucleation step and the particle growth step, and specifically, it is preferable to control the fluctuation range to ±5 g/L.

(5) Reaction Temperature The temperature of the reaction aqueous solution (reaction temperature) needs to be controlled preferably at 20°C or higher, more preferably in the range of 20°C to 60°C, throughout the nucleation step and particle growth step. If the reaction temperature is less than 20°C, nucleation tends to occur due to the low solubility of the aqueous reaction solution, making it difficult to control the average particle size MV and particle size distribution of secondary particles of the resulting composite hydroxide. becomes. The upper limit of the reaction temperature is not particularly limited, but if it exceeds 60°C, the volatilization of ammonia will be accelerated, and the ammonium ion donor supplied to control the ammonium ions in the reaction aqueous solution within a certain range. The amount of aqueous solution containing will increase, and the production cost will increase.

(6) Coating step In the method for producing a composite hydroxide of the present invention, by adding a compound containing the additive element M to the raw material aqueous solution, a secondary composite hydroxide in which the additive element M is dispersed inside the particles is formed. particles can be obtained. However, when trying to obtain the effect of adding the additive element M with a smaller amount, the surface of the secondary particles of the composite hydroxide is coated with a compound containing the additive element M after the particle growth step. Preferably, a coating step is performed.

The coating method is not particularly limited as long as the secondary particles of the composite hydroxide can be coated with the compound containing the additive element M. For example, after slurrying the composite hydroxide and controlling its pH value within a predetermined range, an aqueous solution (aqueous coating solution) in which a compound containing the additive element M is dissolved is added to form a slurry of the secondary particles of the composite hydroxide. By precipitating the compound containing the additive element M on the surface, secondary particles of composite hydroxide coated with the compound containing the additive element M can be obtained. In this case, instead of the aqueous coating solution, an alkoxide solution of the additive element M may be added to the slurry of the composite hydroxide. Alternatively, the composite hydroxide may be coated by spraying and drying an aqueous solution or slurry in which a compound containing the additive element M is dissolved, without slurrying the composite hydroxide. Furthermore, coating is performed by spray drying a slurry in which the composite hydroxide and the compound containing the additive element M are suspended, or by mixing the composite hydroxide and the compound containing the additive element M by a solid phase method. You can also.

In addition, when coating the surface of the secondary particles of the composite hydroxide with the additive element M, the raw materials should be It is necessary to adjust the composition of the aqueous solution and the coating aqueous solution as appropriate. Further, the coating step may be performed on heat-treated particles after heat-treating the composite hydroxide.

(7) Production device The crystallization device (reaction tank) for producing the composite hydroxide of the present invention is not particularly limited as long as it can switch the reaction atmosphere and the raw material aqueous solution. Never. However, it is preferable to use a batch type crystallizer in which the precipitated product is not recovered until the crystallization reaction is completed. With this type of crystallizer, unlike continuous crystallizers that collect products using an overflow method, growing particles are not collected at the same time as the overflow liquid. Secondary particles of substances can be easily obtained. Further, in the method for producing a composite hydroxide of the present invention, it is necessary to appropriately control the reaction atmosphere during the crystallization reaction, so it is preferable to use a closed type crystallizer.

3. Positive electrode active material for lithium ion secondary batteries (1) Particle structure The positive electrode active material of the present invention is a ternary material consisting of a lithium nickel manganese cobalt-containing composite oxide containing at least nickel, manganese, and cobalt as transition metals. This positive electrode active material for lithium ion secondary batteries 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 schematically shown in FIG. An agglomerated part 7 which is made of similarly agglomerated primary particles and is electrically connected to the outer shell part 6, and a space part 8 which is dispersed between the agglomerated parts 7 inside the outer shell part 6. It has a porous structure.

In particular, in the positive electrode active material of the present invention, in the secondary particles having such a porous structure, the agglomerated portions 7 have a high nickel content in which the nickel ratio, which is the atomic ratio of nickel to the total of nickel, manganese, and cobalt, exceeds 0.4. The surface portion 9 has a composition having a high nickel ratio and is graded from a high nickel ratio composition to a low nickel ratio composition. On the other hand, the outer shell portion 6 has a composition with a low nickel ratio of more than 0.1 and less than 0.4.

Here, "electrically conductive" means that the agglomerated parts of the primary particles are structurally connected to each other and are in a state where they can be electrically conductive. Further, the "outer shell portion" corresponds to a portion where the outer shell layer formed on the outermost side of the secondary particles of the composite hydroxide that is the precursor is sintered and shrunk. In addition, the "agglomerated part" corresponds to the part where the central part of the secondary particles of the composite hydroxide that is the precursor, the first low density layer, the high density layer, and the second low density layer are sintered and shrunk. . Furthermore, the "space part" is a space where the fine primary particles constituting the first low density part and the second low density part of the secondary particles of the composite hydroxide, which is the precursor, are absorbed by the surrounding plate-shaped primary particles. This is a pore structure formed by These spaces are separated from each other due to the presence of aggregates, but they communicate with the outside and each other through grain boundaries and voids between primary particles, preventing the electrolyte and conductive agent from entering the spaces. is possible.

Due to such a particle structure, the positive electrode active material of the present invention achieves both a larger specific surface area and a higher tap density compared to a conventional positive electrode active material having a hollow structure. In the positive electrode active material having the particle structure of the present invention, the electrolyte penetrates into the interior of the secondary particles through the grain boundaries, voids, or spaces between the primary particles. Lithium can be removed and inserted even inside the particles. Moreover, in this positive electrode active material, the outer shell part and the agglomerated part are electrically conductive, and the cross-sectional area of the path is sufficiently large, so it is possible to significantly reduce the resistance inside the particles (internal resistance). can.

In the positive electrode active material of the present invention, among the outer shell part and the agglomerated part that form the skeleton of the secondary particles, the primary particles constituting the agglomerated part, excluding the primary particles present on the surface part of the agglomerated part, have a nickel ratio of As a composition with a high nickel ratio exceeding 0.4, by increasing the nickel ratio in the overall composition of the ternary positive electrode active material, the energy density is improved, and at the same time, a high nickel ratio is added to the surface of the agglomerated part. The surface part has a sloped composition with a low nickel ratio in which the nickel ratio is more than 0.1 and 0.4 or less, and the outer shell part has a nickel ratio of more than 0.1 and 0.4. The following composition with a low nickel ratio is defined as a primary particle constituting an agglomerated part having a composition with a high nickel ratio that exists inside the secondary particle due to the presence of a low nickel ratio layer consisting of an outer shell part and a surface part of the agglomerated part. By protecting the secondary particles, the structure of the secondary particles is prevented from deteriorating due to contact with the electrolyte.

In a composition with a high nickel ratio in the agglomerated portion, the nickel ratio is in a range of more than 0.4 and 1 or less. The nickel ratio is preferably in the range of more than 0.4 and 0.8 or less, and more preferably in the range of 0.45 to 0.7. If the nickel ratio is 0.4 or less, the nickel ratio in the overall composition of the positive electrode active material cannot be made sufficiently higher than the conventional 0.33 (the ratio of nickel, manganese, and cobalt is 1:1:1). . Further, it is preferable to set the nickel ratio in the agglomerated portion so that the nickel ratio in the overall composition of the positive electrode active material exceeds 0.4.

The agglomerated portion has a surface portion that is graded from a high nickel ratio to a low nickel ratio in a range of more than 0.1 and less than or equal to 0.4. This surface part can be formed on the entire surface of the agglomerated part, or a surface layer part whose composition is graded with such a low nickel ratio is formed on the surface of the primary particles present in the surface part of the agglomerated part. By doing so, a surface portion that is graded to have a low nickel ratio may be formed.

In a composition with a low nickel ratio in the outer shell, the nickel ratio is in a range of more than 0.1 and 0.4 or less. The nickel ratio is preferably in the range of more than 0.1 and 0.38 or less, and more preferably in the range of 0.2 to 0.36. Usually, the nickel ratio can be set to 0.33 (the ratio of nickel to manganese to cobalt is 1:1:1). If the nickel ratio of the outer shell exceeds 0.4, the protection of the internal agglomerated parts by the outer shell becomes insufficient, and the effect of suppressing the deterioration of the secondary particles becomes insufficient. On the other hand, if the nickel ratio in the outer shell is 0.1 or less, the final positive electrode active material cannot have a composition with a high nickel ratio as a whole.

Due to the structure of the surface part and the outer shell part of the agglomerated part, regardless of the structure of the surface part of the agglomerated part, the cross section of the secondary particles constituting the positive electrode active material can be measured using an energy dispersive electron microscope using a scanning electron microscope. When the composition is analyzed by X-ray spectroscopy (STEM-EDS or STEM-EDX), the area ratio of the low nickel ratio layer with a nickel ratio of more than 0.1 and 0.4 or less is the entire cross section of the secondary particle. 25% to 45% of the area of Regarding the area ratio of the low nickel ratio layer in the positive electrode active material, for 10 or more secondary particles, energy dispersive X-ray spectroscopy (STEM-EDS or A distribution map (mapping) of nickel (Ni) was obtained by compositional analysis using STEM-EDX), and a low nickel ratio layer with a nickel ratio of more than 0.1 and 0.4 or less, shown in blue in Figure 4, was determined. It is determined by determining the area and dividing it by the area of the entire cross section of the secondary particle. By calculating the average of the area ratios of the low nickel ratio layers, the area ratio of the low nickel ratio layers of the entire positive electrode active material is determined.

If the area ratio of the low nickel ratio layer is less than 25%, the protection of internal agglomerated parts by the low nickel ratio layer will be insufficient, and the effect of suppressing the deterioration of secondary particles will be insufficient. On the other hand, if the area ratio of the low nickel ratio layer exceeds 45%, the final positive electrode active material cannot have a composition with a high nickel ratio as a whole. From this point of view, the area ratio of the low nickel ratio layer is preferably in the range of 25% to 45%.

When a lithium ion secondary battery is constructed using a positive electrode active material having such a structure as a positive electrode material, the durability of the positive electrode active material improves, and the output characteristics deteriorate due to the resistance of the positive electrode interface resistance due to its deterioration. This makes it possible to further improve output characteristics without impairing battery capacity or cycle characteristics.

(2) Average particle size MV
The positive electrode active material of the present invention is adjusted so that the average particle size MV is in the range of 3 μm to 10 μm, preferably in the range of 4 μm to 9 μm, and more preferably in the range of 4 μm to 8 μm. If the average particle size of the positive electrode active material is within this range, it is possible not only to increase the battery capacity per unit volume of a secondary battery using this positive electrode active material, but also to improve safety and output characteristics. can do. On the other hand, if the average particle diameter MV is less than 3 μm, the filling property of the positive electrode active material decreases, making it impossible to increase the battery capacity per unit volume. On the other hand, if the average particle size MV exceeds 10 μm, the reaction area of the positive electrode active material decreases and the interface with the electrolyte decreases, making it difficult to improve the output characteristics.

Note that the average particle diameter MV of the positive electrode active material means the volume-based average particle diameter (Mean Volume Diameter) as in the above-mentioned composite hydroxide, and for example, it is measured with a laser beam diffraction scattering particle size analyzer. It can be determined from the volume integrated value.

(3) Particle size distribution The positive electrode active material of the present invention has an index indicating the spread of particle size distribution [(d90-d10)/average particle size MV] of 0.70 or less, preferably 0.60 or less, more preferably is 0.55 or less, and is composed of secondary particles with an extremely narrow particle size distribution. Such a positive electrode active material has a small proportion of fine particles and coarse particles, and a secondary battery using this material has excellent safety, cycle characteristics, and output characteristics.

On the other hand, 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 will easily generate heat due to local reactions of the fine particles, which will not only reduce safety, but also cause selective deterioration of the fine particles. The characteristics become inferior. Furthermore, if the proportion of coarse particles is high, a sufficient reaction area between the electrolyte and the positive electrode active material cannot be ensured, resulting in poor output characteristics.

Assuming production on an industrial scale, it is not realistic to use a positive electrode active material with an excessively small [(d90-d10)/average particle size MV]. Therefore, in consideration of cost and productivity, the lower limit of [(d90-d10)/average particle diameter MV] is preferably about 0.25.

Note that the meanings of d10 and d90 in the index indicating the spread of particle size distribution [(d90-d10)/average particle size MV] and how to obtain them are the same as in the case of the composite hydroxide described above.

(4) Primary particles In the positive electrode active material of the present invention, the primary particles constituting the outer shell portion and the agglomerated portion are formed to have an average particle size in the range of 0.01 μm to 0.3 μm. As with the case of composite hydroxide, the size of the primary particles is determined by embedding the secondary particles in resin, etc., making it possible to observe the cross section by cross-section polishing, etc., and then examining the cross section by FE- It is obtained by observing using a SEM such as a SEM, measuring the maximum outer diameter (major axis diameter) of 10 or more primary particles present in the cross section of the secondary particle, and calculating the average value thereof. The positive electrode active material is produced by firing the composite hydroxide precursor under appropriate firing conditions, so that the plate-shaped primary particles forming the precursor shrink by sintering while absorbing fine primary particles. Primary particles of the above size are formed. If the average particle diameter of the primary particles is less than 0.01 μm, the problem may occur that the primary particles become brittle and sufficient battery performance cannot be obtained. On the other hand, if the average particle diameter of the primary particles exceeds 0.3 μm, the diffusion distance within the solid within the particles becomes long, which may cause a problem that sufficient battery performance cannot be obtained.

In the positive electrode active material of the present invention, each of the primary particles constituting the outer shell has a generally uniform composition within the structure. In addition, each of the primary particles constituting the agglomerated portion has a generally uniform composition within the structure, except for the primary particles on the surface. However, although each of the primary particles constituting the surface part of the agglomerated part or its vicinity basically has a particle structure with a composition with a high nickel ratio, the surface layer of the primary particles has a composition with a low nickel ratio. It can be said that The presence of such a surface layer and its composition can be measured by scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS or STEM-EDX).

The surface layer whose composition is graded from a composition with a high nickel ratio to a composition with a low nickel ratio is not clearly distinguishable from other parts and has a composition with a high nickel ratio, as schematically shown in Figure 3. It is determined that the presence of a composition with a low nickel ratio is confirmed on the surface of the primary particles in the agglomerated part using scanning transmission electron microscopy/energy dispersive X-ray spectroscopy. It is.

Note that the entire primary particles present in the surface portion of the agglomerated portion may be composed of a low nickel composition, or the entire primary particles or a portion thereof may have a surface layer portion having a low nickel ratio.

(5) Outer Shell The outer shell that constitutes the secondary particles is composed of aggregates of primary particles having a composition with a low nickel ratio. The thickness of the outer shell is preferably in the range of 0.1 μm to 1.0 μm. When the thickness of the outer shell portion is less than 0.1 μm, the strength of the secondary particles of the positive electrode active material is not sufficiently ensured. On the other hand, if the thickness of the outer shell portion exceeds 1.0 μm, the number of primary particles having a composition with a low nickel ratio increases too much, making it difficult to form the entire positive electrode active material into a composition with a high nickel ratio. The thickness of the outer shell portion is preferably in the range of 0.1 μm to 0.5 μm, more preferably in the range of 0.12 μm to 0.3 μm.

(6) Crystallite diameter determined from the X-ray diffraction pattern of the (003) plane. When determining the crystallite diameter, the crystallite diameter is in the range of 300 Å to 1500 Å, preferably in the range of 400 Å to 1300 Å, and more preferably in the range of 700 Å to 1250 Å. A positive electrode active material having a crystallite diameter in such a range has extremely high crystallinity, and can reduce the positive electrode resistance of a secondary battery and improve its output characteristics.

On the other hand, if the crystallite diameter of the (003) plane is less than 300 Å, the primary particles are so fine that the pores existing between the primary particles in the positive electrode active material become too fine, causing Since it becomes difficult for the electrolyte to penetrate, the reaction area with the electrolyte decreases, and the output characteristics of the secondary battery decrease. On the other hand, if the crystallite diameter of the (003) plane exceeds 1500 Å, the primary particles will become too coarse, the proportion of pores in the secondary particles will be extremely reduced, and the infiltration route for the electrolyte will be reduced. The reaction area with the electrolyte is reduced, and the output characteristics of the secondary battery are reduced.

(7) Tap density Increasing the capacity of secondary batteries has become an important issue in order to extend the usage time of portable electronic devices and the driving distance of electric vehicles. On the other hand, the thickness of the electrode of a secondary battery is required to be approximately several micrometers due to the packing of the entire battery and problems with electronic conductivity. Therefore, it is necessary not only to use a high-capacity cathode active material, but also to improve the filling properties of the cathode active material to increase the capacity of the secondary battery as a whole.

From this point of view, in the positive electrode active material of the present invention, the tap density, which is an index of packing property (sphericity of secondary particles constituting the positive electrode active material), is set to 1.1 g/cm ³ to 1.8 g/cm. It is preferable to maintain it within the range of ³ . When the tap density is less than 1.1 g/cm ³ , even if the BET specific surface area is increased, the filling 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 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.

Note that the tapped density refers to the bulk density after a sample powder collected in a container is tapped 100 times based on JIS Z-2504, and can be measured using a shaking specific gravity meter.

(8) BET Specific Surface Area The positive electrode active material of the present invention is characterized in that the specific surface area is improved due to 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, it is preferable that the BET specific surface area is as large as possible as long as the structure of the secondary particles described above is maintained. This is because the larger the BET specific surface area, the larger the contact area with the electrolyte, and the output characteristics of a 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 in the range of 2.0 ^{m 2} /g to 5.0 ^{m 2} /g. In the present invention, 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, a sufficient reaction area with the electrolyte cannot be secured, and the output characteristics cannot be sufficiently improved. It becomes difficult to do so. The BET specific surface area is more preferably 2.5 m ² /g to 5.0 m ² /g, and even more preferably 3.0 m ^{2 /} g to 6.0 m ² /g.

(9) Oil Absorption The positive electrode active material of the present invention is characterized in that it has improved output characteristics by having a porous structure with spaces formed inside secondary particles. In the present invention, it is possible to use the oil absorption amount as an index indicating the abundance ratio of spaces present in secondary particles (hereinafter referred to as "space ratio").

The oil absorption amount is determined by operating according to the procedure described in "JIS K 6217-4 (Carbon black for rubber - Basic properties - Part 4: How to determine the oil absorption amount (including compressed samples))". However, since the operation process is complicated, the oil absorption amount is generally measured using an oil absorption amount (absorption amount) measuring device marketed in accordance with the above-mentioned JIS. Note that di-n-butyl phthalate (di-n-butyl phthalate, DBP) is usually used as the oil for measurement, and the measurement results are calculated based on the amount of oil absorbed per 100 g of sample. Therefore, the unit is "ml/100g".

The oil absorption amount of the positive electrode active material of the present invention is preferably in the range of 35 ml/100 g to 60 ml/100 g, more preferably in the range of 40 ml/100 g to 55 ml/100 g. As the oil absorption increases, the occupation rate of the space occupied by each secondary particle constituting the positive electrode active material, that is, the space ratio increases, and the specific surface area tends to increase. Therefore, it can be evaluated that if the content is less than 35 ml/100 g, sufficient space will not be formed and the effect of increasing the specific surface area will not be obtained. On the other hand, when the content exceeds 60 ml/100 g, many voids are structurally present inside the secondary particles, and the proportion of agglomerated parts becomes too low, making it impossible to obtain desired characteristics.

(10) Composition _The composition of the ternary positive electrode active material of the present invention is not limited as long as it has the above- _{mentioned structure} . Co _z M _t O ₂ (-0.05≦u≦0.50, x+y+z+t=1, 0.4≦x≦0.70, 0.15≦y≦0.4, 0.15≦z≦0. 3, 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, W) It can be suitably applied to a positive electrode active material made of a lithium nickel manganese cobalt-containing composite oxide having a hexagonal crystal structure with a layered structure.

In this positive electrode active material, the value of u indicating the excess amount of lithium (Li) is preferably in the range of -0.05 to 0.50, more preferably in the range of 0 to 0.50, even more preferably 0 to 0. .35 range. By regulating the value of u within 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. On the other hand, if the value of u is less than -0.05, the positive electrode resistance of the secondary battery increases, making it impossible to improve the output characteristics. On the other hand, when it exceeds 0.50, not only the initial discharge capacity decreases but also the positive electrode resistance increases.

Nickel (Ni) is an element that contributes to high potential and high capacity of secondary batteries, and from the viewpoint of using a ternary positive electrode active material with a high nickel ratio, the value of x indicating its content is: It is preferably in the range of 0.4 to 0.7, more preferably in the range of 0.43 to 0.65, even more preferably in the range of 0.5 to 0.6. If the value of x is less than 0.4, the energy density of a secondary battery using this positive electrode active material cannot be sufficiently improved. On the other hand, when the value of x exceeds 0.7, the content of other elements decreases, making it impossible to obtain the effect as a ternary positive electrode active material.

Manganese (Mn) is an element that contributes to improving thermal stability, and the value of y indicating its content is preferably in the range of 0.15 to 0.4, more preferably 0.2 to 0.35. be within the range of 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 be eluted from the positive electrode active material during high-temperature operation, and the charge-discharge cycle characteristics will deteriorate.

Cobalt (Co) is an element that contributes to improving charge-discharge cycle characteristics, and the value of z indicating its content is preferably in the range of 0.15 to 0.3, more preferably 0.2 to 0. The range is 25. If the value of z exceeds 0.3, the initial discharge capacity of a secondary battery using this positive electrode active material will be significantly reduced.

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

The value of t indicating the content of the additive element M is preferably in the range of 0 to 0.1, more preferably in the range of 0.001 to 0.05. When the value of t exceeds 0.1, the metal elements contributing to the Redox reaction decrease, resulting in a decrease in battery capacity.

Such an additive element M may be dispersed inside the particles of the positive electrode active material, or may be coated on the surface of the particles of the positive electrode active material. Furthermore, after being dispersed inside the particles, the surfaces thereof may be coated. In any case, it is necessary to control the content of the additive element M to be within the above range.

4. Method for manufacturing a positive electrode active material for lithium ion secondary batteries A method for manufacturing a positive electrode active material of the present invention uses the above-mentioned composite hydroxide as a precursor, and uses a positive electrode having a predetermined particle structure, average particle size MV, and particle size distribution. There are no particular limitations as long as the active material can be synthesized. However, when production is assumed to be on an industrial scale, the above-mentioned composite hydroxide is mixed with a lithium compound to obtain a lithium mixture, and the resulting lithium mixture is mixed in an oxidizing atmosphere at 650°C to It is preferable to synthesize the positive electrode active material by a manufacturing method including a firing step of firing at 920°C. Note that, if necessary, steps such as a heat treatment step and a calcination step may be added to the above-mentioned steps. According to such a manufacturing method, the above-mentioned positive electrode active material, particularly the positive electrode active material represented by general formula (B), can be easily obtained.

(1) Heat treatment step In the method for producing a positive electrode active material of the present invention, a heat treatment step may optionally be provided before the mixing step to form the composite hydroxide into heat-treated particles and then mix them with the lithium compound. Here, the heat-treated particles include not only composite hydroxide from which excess moisture has been removed in the heat treatment process, but also nickel-manganese-cobalt-containing composite oxide (hereinafter referred to as "composite oxide") that has been converted into an oxide through the heat treatment process. ), or mixtures thereof.

The heat treatment step is a step of removing excess moisture contained in the composite hydroxide by heating the composite hydroxide to 105° C. to 750° C. and heat-treating the composite hydroxide. Thereby, it is possible to reduce the remaining moisture to a certain amount until after the firing process, and it is possible to suppress variations in the composition of the obtained positive electrode active material.

The heating temperature in the heat treatment step is 105°C to 750°C. If the heating temperature is less than 105° C., excess water in the composite hydroxide cannot be removed, and variations may not be sufficiently suppressed. On the other hand, even if the heating temperature exceeds 700° C., not only no further effect can be expected, but also production costs increase.

In addition, in the heat treatment process, it is sufficient to remove water to the extent that there is no variation in the number of atoms of each metal component in the positive electrode active material and the ratio of the number of Li atoms, so not all composite hydroxides are necessarily mixed with composite oxidation. There is no need to convert it into something. However, in order to reduce the variation in the number of atoms of each metal component and the ratio of the number of Li atoms, all the composite hydroxides are converted to composite oxides by heating to 400°C or higher. It is preferable. Note that the above-mentioned variations can be further suppressed by determining the metal components contained in the composite hydroxide under heat treatment conditions in advance by analysis and determining the mixing ratio with the lithium compound.

The atmosphere in which the heat treatment is carried out is not particularly limited, and may be any non-reducing atmosphere, but it is preferable to carry out the heat treatment in an air stream because it can be easily carried out.

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

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

In the mixing step, the ratio (Li/ Me) is 0.95 to 1.5, preferably 1.0 to 1.5, more preferably 1.0 to 1.35, even more preferably 1.0 to 1.2. It is necessary to mix the oxide or heat-treated particles with the lithium compound. That is, since Li/Me does not change before and after the firing process, the composite hydroxide or heat-treated particles and the lithium compound are mixed so that the Li/Me in the mixing process becomes the desired Li/Me of the positive electrode active material. It is necessary to do so.

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 thereof from the viewpoint of easy availability. In particular, considering ease of handling and stability of quality, it is preferable to use lithium hydroxide or lithium carbonate.

It is preferable that the composite hydroxide or the heat-treated particles and the lithium compound are sufficiently mixed to such an extent that no fine powder is generated. If mixing is insufficient, Li/Me may vary between individual particles, and sufficient battery characteristics may not be obtained. Note that a general mixer can be used for mixing. For example, a shaker mixer, Loedige mixer, Julia mixer, V-blender, etc. can be used.

(3) Calcination process When using lithium hydroxide or lithium carbonate as the lithium compound, after the mixing process and before the firing process, the lithium mixture is heated at a temperature lower than the firing temperature described below and from 350°C to A calcination step may be performed at 800°C, preferably 450°C to 780°C. Thereby, lithium can be sufficiently diffused into the composite hydroxide or heat-treated particles, and a more uniform lithium composite oxide can be obtained.

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

(4) Firing process The firing process is a process in which the lithium mixture obtained in the mixing process is fired under predetermined conditions to diffuse lithium into the composite hydroxide or heat-treated particles to obtain a lithium composite oxide. .

In this firing process, the plate-like primary particles constituting the center, high-density layer, and high-density part of the composite hydroxide and heat-treated particles undergo sintering shrinkage while absorbing fine primary particles, and after sintering, The primary particles form an outer shell portion and an agglomerated portion in the positive electrode active material. Note that the fine primary particles start sintering at a lower temperature than the plate-like primary particles, and have a larger amount of shrinkage than the plate-like primary particles, so the first low-density layer and the second low-density layer are It is absorbed into the central part, high-density layer, and outer shell layer where formation is slow, and moderately sized spaces are formed as pores. At this time, the plate-shaped primary particles undergo sintering shrinkage while maintaining connection with adjacent plate-shaped primary particles, so in the resulting positive electrode active material, there is an electrical connection between the outer shell part and the agglomerated part. It is possible to ensure conduction and a sufficient cross-sectional area of the path. As a result, the internal resistance of the positive electrode active material is significantly reduced, and when a secondary battery is constructed, it becomes possible to improve the output characteristics without impairing the battery capacity or cycle characteristics.

In the lithium composite oxide constituting the positive electrode active material of the present invention, the plate-like primary particles of the outer shell layer, which have a composition with a low nickel ratio among the composite hydroxide and heat-treated particles, basically have a composition with a low nickel ratio. Among the fine primary particles constituting the outer shell having a composition of , constitutes the surface part of the agglomerated part.

The particle structure of such a positive electrode active material is basically determined according to the particle structure of the composite hydroxide that is the precursor, but it may be affected by its composition, firing conditions, etc. After conducting a preliminary test, it is preferable to adjust each condition as appropriate so as to obtain a desired structure.

Note that the furnace used in the firing process is not particularly limited, and may be any furnace that can heat the lithium mixture in the atmosphere or in an oxygen stream. However, from the viewpoint of maintaining a uniform atmosphere in the furnace, an electric furnace that does not generate gas is preferable, and either a batch type or a continuous type electric furnace can be suitably used. Regarding this point, the same applies to the furnace 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 to 920°C. If the firing temperature is less than 650°C, lithium will not diffuse sufficiently into the composite hydroxide or heat-treated particles, and excess lithium or unreacted composite hydroxide or heat-treated particles may remain, or the resulting positive electrode active material may The crystallinity may be 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 collapsed, and the secondary particles of the positive electrode active material may be violently sintered, resulting in abnormal grain growth. This results in an increase in the proportion of irregularly shaped coarse particles. From the viewpoint of controlling the agglomerated portions and spaces constituting the secondary particles within appropriate size ranges, the firing temperature of the lithium mixture is preferably 700°C to 920°C, and 800°C to 900°C. It is more preferable to do so.

Further, the temperature increase rate in the firing step is preferably 2° C./min to 10° C./min, more preferably 5° C./min to 10° C./min. Furthermore, during the firing step, it is preferable to maintain the temperature near the melting point of the lithium compound for preferably 1 to 5 hours, more preferably 2 to 5 hours. Thereby, the composite hydroxide or the heat-treated particles and the lithium compound can be reacted more uniformly.

b) Firing time Among the firing times, the holding time at the above-mentioned firing temperature is preferably at least 2 hours, more preferably from 4 hours to 24 hours. If the holding time at the firing temperature is less than 2 hours, lithium will not diffuse sufficiently into the composite hydroxide or heat-treated particles, and excess lithium or unreacted composite hydroxide or heat-treated particles may remain or the resulting positive electrode There is a possibility that the crystallinity of the active material may become insufficient.

The cooling rate from the firing temperature to at least 200°C after the holding time is preferably 2°C/min to 10°C/min, more preferably 33°C/min to 77°C/min. By controlling the cooling rate within such a range, it is possible to ensure productivity and prevent equipment such as saggers from being damaged due to rapid cooling.

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

(5) Crushing step The secondary particles constituting the positive electrode active material obtained by the firing step may be agglomerated or slightly sintered. In such a case, it is preferable to crush the aggregate or sintered body. Thereby, the average particle size and particle size distribution of the obtained positive electrode active material can be adjusted to a suitable range. Note that crushing is a process in which mechanical energy is applied to an aggregate consisting of multiple secondary particles that is generated due to sinter necking between secondary particles during firing, without destroying the secondary particles themselves. It means the operation of separating and loosening aggregates.

As a crushing method, a known means can be used, for example, a pin mill, a hammer mill, etc. can be used. In addition, at this time, it is preferable to adjust the crushing force to an appropriate range so as not to destroy the secondary particles.

5. 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 constituent members 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 constituent members such as a positive electrode, a negative electrode, and a solid electrolyte. That is, the present invention is widely applicable to a secondary battery that is charged and discharged by desorption and insertion of lithium ions, from non-aqueous electrolyte secondary batteries to all-solid lithium secondary batteries. Note that the embodiments described below are merely illustrative, and the present invention is applied to lithium ion secondary batteries with various changes and improvements based on the embodiments described in this specification. Is possible.

(1) Components a) Positive electrode Using the above-described positive electrode active material, a positive electrode of a lithium ion secondary battery is produced, for example, in the following manner.

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

The obtained positive electrode mixture paste is applied to the surface of a current collector made of aluminum foil, for example, and dried to scatter the solvent. If necessary, pressure may be applied using a roll press or the like to increase the electrode density. In this way, a sheet-like positive electrode can be produced. The sheet-like positive electrode can be cut into an appropriate size depending on the intended battery and used for battery production. Note that the method for producing the positive electrode is not limited to the above-mentioned example, and other methods may be used.

As the conductive material, for example, graphite (natural graphite, artificial graphite, expanded graphite, etc.) and carbon black-based materials such as acetylene black and Ketjen black can be used.

The binder plays a role in binding the active material particles, and includes, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose resin, or polyacrylic resin. Acids can be used.

In addition, a solvent that disperses the positive electrode active material, the conductive material, and the activated carbon and dissolves the binder can be added to the positive electrode mixture, if necessary. Specifically, organic solvents such as N-methyl-2-pyrrolidone can be used as the solvent. Moreover, activated carbon can also be added to the positive electrode composite material in order to increase the electric double layer capacity.

b) Negative electrode Metallic lithium, lithium alloy, etc. can be used for the negative electrode. In addition, a negative electrode active material that can absorb and deintercalate lithium ions is mixed with a binder and an appropriate solvent is added to form a paste, which is then applied to the surface of a metal foil current collector such as copper. , dried and compressed to increase the electrode density if necessary.

Examples of negative electrode active materials include materials containing lithium such as metallic lithium and lithium alloys, natural graphite that can absorb and desorb lithium ions, fired organic compounds such as artificial graphite and phenol resin, and carbon materials such as coke. A powder can be used. In this case, as the negative electrode binder, a fluororesin such as PVDF can be used as in the positive electrode, and as the solvent for dispersing these active materials and the binder, an organic resin such as N-methyl-2-pyrrolidone can be used. Solvents can be used.

c) Separator A separator is placed between a positive electrode and a negative electrode in a non-aqueous electrolyte secondary battery, and has the function of separating the positive electrode and negative electrode and retaining the non-aqueous electrolyte. As such a separator, for example, a thin film made of polyethylene or polypropylene having many fine pores can be used, but there is no particular limitation as long as it has the above-mentioned functions.

d) Electrolyte The nonaqueous electrolyte used in the nonaqueous electrolyte secondary battery includes a nonaqueous electrolyte prepared by dissolving a lithium salt serving as a supporting salt in an organic solvent.

Organic solvents used in the nonaqueous electrolyte include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; chain carbonates such as diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, and dipropyl carbonate. ; Furthermore, one selected from ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane; sulfur compounds such as ethylmethylsulfone and butane sultone; phosphorus compounds such as triethyl phosphate and trioctyl phosphate, etc.; Alternatively, two or more types can be used in combination.

As the supporting salt, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiN(CF ₃ SO ₂ ) ₂ and complex salts thereof can be used.

Note that the nonaqueous electrolyte may contain a radical scavenger, a surfactant, a flame retardant, and the like.

On the other hand, solid electrolytes used in solid electrolyte secondary batteries such as all-solid lithium secondary batteries include Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ and Li ₂ S-SiS ₂ . be able to. A solid electrolyte has the property of being able to withstand high voltage. Solid electrolytes include inorganic solid electrolytes and organic solid electrolytes.

Inorganic solid electrolytes include oxide solid electrolytes, sulfide solid electrolytes, and the like.

As the oxide solid electrolyte, an oxide containing oxygen (O) and having lithium ion conductivity and electronic insulation can be used. _For example _, lithium phosphate ( _{Li3PO4 )} , _Li3PO4NX , _{LiBO2NX , LiNbO3 , LiTaO3 , Li2SiO3 , Li4SiO4 - Li3PO4} , _Li4SiO4- 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 (PO ₄ ) _{3 ,} Li _{3 X ≦ 2 / 3 ) , Li5La3Ta2O12 , Li7La3Zr2O12 , Li6BaLa2Ta2O12 , Li3.6Si0.6P0.4O4 , etc.} 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 , etc. 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 polymer compound exhibiting ionic conductivity can be used. For example, polyethylene oxide, polypropylene oxide, copolymers thereof, etc. can be used. Further, the organic solid electrolyte can include a supporting salt (lithium salt).

Note that when a solid electrolyte is used, the solid electrolyte can also be mixed in the positive electrode material in order to ensure contact between the electrolyte and the positive electrode active material.

(2) Configuration of lithium ion secondary battery The configuration of a lithium ion secondary battery is not particularly limited, and can be a configuration consisting of a positive electrode, a negative electrode, a separator, a nonaqueous electrolyte, etc. in a nonaqueous electrolyte secondary battery, or a configuration consisting of a positive electrode, a negative electrode, a separator, a nonaqueous electrolyte , etc. in a nonaqueous electrolyte secondary battery, A configuration consisting of a positive electrode, a negative electrode, a solid electrolyte, etc. in a secondary battery can be adopted. Further, the shape of the secondary battery is not particularly limited, and may be in various shapes such as a cylindrical shape or a stacked shape.

In the case of a non-aqueous electrolyte secondary battery, for example, a positive electrode and a negative electrode are laminated with a separator in between to form an electrode body, the obtained electrode body is impregnated with a non-aqueous electrolyte, and a positive electrode is connected to a positive electrode current collector and connected to the outside. The lithium ion secondary battery is completed by connecting the battery to the terminal and between the negative electrode current collector and the negative terminal communicating with the outside using a current collecting lead or the like, and sealing it in a battery case.

(3) Characteristics of lithium ion secondary battery As described above, the lithium ion secondary battery of the present invention uses the porous structure and ternary positive electrode active material of the present invention having a composition with a high nickel ratio as the positive electrode material. Because of this, it has excellent capacity characteristics, output characteristics, and cycle characteristics. Moreover, in the positive electrode active material of the present invention, the outer shell portion having a composition with a low nickel ratio protects the internal agglomerated portion having a composition with a high nickel ratio, and the primary particles constituting the agglomerated portion have a low nickel ratio. The surface layer has a structure that protects the primary particles themselves, which have a composition with a high nickel ratio. In comparison with the next battery, the positive electrode interfacial resistance can be reduced while maintaining the same high energy density, and the discharge capacity retention rate can be further improved.

(4) Applications of lithium ion secondary batteries As mentioned above, the lithium ion secondary batteries of the present invention have excellent capacity characteristics, output characteristics, and cycle characteristics, and are suitable for small-sized batteries that require these characteristics at a high level. It can be suitably used as a power source for portable electronic devices (laptop personal computers, smartphones, tablet terminals, digital cameras, etc.). In addition, the lithium ion secondary battery of the present invention not only can be made smaller and have higher output, but also has excellent safety and durability, and can simplify expensive protection circuits, so it can be installed easily. It can also be suitably used as a power source for transportation equipment such as electric vehicles that are subject to space constraints.

Hereinafter, the present invention will be explained in detail using Examples and Comparative Examples. In the Examples and Comparative Examples below, unless otherwise specified, samples of reagent special grade 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 (manufactured by Nisshin Rika Co., Ltd., NPH-690D), and based on this measurement value, the supply amount of the sodium hydroxide aqueous solution was adjusted. By adjusting, the range of fluctuation in the pH value of the reaction aqueous solution in each step was controlled within the range of ±0.2.

(Example 1)
a) Production of composite hydroxide [Nucleation process]
First, 17 L of water was placed in a reaction tank, and the temperature inside the tank was set at 40° C. while stirring. At this time, nitrogen gas was passed through the reaction tank for 30 minutes to make the reaction atmosphere a non-oxidizing atmosphere with an oxygen concentration of 2% by volume or less. Next, appropriate amounts of 25% by mass aqueous sodium hydroxide solution and 25% by mass aqueous ammonia were supplied into the reaction tank so that the pH value was 12.6 based on the liquid temperature of 25°C and the ammonium ion concentration was 10 g/L. A pre-reaction aqueous solution was formed by adjusting the following.

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=7:2:1, and a 2 mol/L first raw material aqueous solution was prepared. was prepared. Further, nickel sulfate, manganese sulfate, and cobalt sulfate are dissolved in water so that the molar ratio of each metal element is Ni:Mn:Co=1:1:1, and a 2 mol/L second raw material aqueous solution is prepared. was prepared.

Next, the first raw material aqueous solution was supplied to the pre-reaction aqueous solution at a rate of 100 ml/min to form a nucleation aqueous solution , and nucleation was performed for 1 minute. At this time, a 25% by mass aqueous sodium hydroxide solution and a 25% by mass aqueous ammonia solution were supplied at appropriate times to maintain the pH value and ammonium ion concentration of the nucleation aqueous solution within the above range.

[Particle growth process]
After the nucleation was completed, the supply of all aqueous solutions was temporarily stopped, and sulfuric acid was added to adjust the pH value to 11.2 based on the liquid temperature of 25°C, thereby forming an aqueous solution for particle growth. . After confirming that the pH value has reached a predetermined value, the first raw material aqueous solution is supplied at a constant rate of 100 ml/min, which is the same as in the nucleation process, and the nuclei (particles) generated in the nucleation process are Made it grow.

As the first step, crystallization in a non-oxidizing atmosphere is continued for 35 minutes (14.6% of the entire particle growth process) from the start of the particle growth process, and then the supply of the first raw material aqueous solution is continued. Air was circulated in the reaction tank using a ceramic diffuser tube (manufactured by Kinoshita Rika Kogyo Co., Ltd.) with a pore size of 20 μm to 30 μm, and the reaction atmosphere was changed to an oxidizing atmosphere with an oxygen concentration of 21% by volume. Adjusted (switching operation 1).

As the second step, after continuing crystallization in an oxidizing atmosphere using the first raw material aqueous solution from switching operation 1 for 20 minutes (8.3% of the entire particle growth process), Nitrogen was passed through the reaction tank to adjust the reaction atmosphere to a non-oxidizing atmosphere with an oxygen concentration of 2% by volume or less (switching operation 2).

As the third step, after continuing crystallization in a non-oxidizing atmosphere using the first raw material aqueous solution from switching operation 2 for 65 minutes (27.1% of the entire particle growth process), the first raw material At the same time as the supply of the aqueous solution was stopped and the supply of the second raw material aqueous solution was started, air was circulated in the reaction tank using an aeration pipe to adjust the oxygen concentration to an oxidizing atmosphere of 21% by volume (switching operation 3).

As the fourth step, after continuing the crystallization reaction in an oxidizing atmosphere using the second raw material aqueous solution from switching operation 3 for 40 minutes (16.7% of the entire particle growth process), the second raw material aqueous solution is While continuing to supply the aqueous solution, nitrogen was passed through 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).

As the fifth step, after continuing the crystallization reaction in a non-oxidizing atmosphere using the second raw material aqueous solution from switching operation 4 for 110 minutes (33.3% of the entire particle growth process), the second The particle growth process was completed by stopping the supply of all aqueous solutions including the raw material aqueous solution. Thereafter, the obtained product was washed with water, filtered, and dried to obtain a powdered composite hydroxide. The rate of crystallization reaction in the entire oxidizing atmosphere was 25.0%.

In addition, in the particle growth step, 25% by mass sodium hydroxide aqueous solution and 25% by mass aqueous ammonia were supplied at appropriate times throughout this step, and the pH value and ammonium ion concentration of the particle growth aqueous solution were maintained within the above range. .

b) Evaluation of composite hydroxide [Composition]
Analysis using an ICP emission spectrometer (ICPE-9000, manufactured by Shimadzu Corporation) reveals that the composition of this composite hydroxide has the general formula: Ni _0.50 Mn _0.27 Co _0.23 (OH) ₂ It was confirmed that it is expressed as

[Particle structure]
After embedding a portion of the composite hydroxide in a resin and making the cross section observable by processing with a cross-section polisher (manufactured by JEOL Ltd., IB-19530CP), 10 or more secondary particles of the composite hydroxide are prepared. was observed using a field emission scanning electron microscope (FE-SEM: JSM-6360LA, manufactured by JEOL Ltd.). As a result, this composite hydroxide has a center formed by agglomeration of plate-shaped primary particles, and a first low-density layer formed by aggregation of fine primary particles outside the center. , a high-density layer formed by agglomeration of plate-shaped primary particles, a second low-density layer formed by aggregation of fine primary particles, and an outer shell layer formed by aggregation of plate-shaped primary particles. It was also confirmed that some of the plate-like primary particles constituting the center, high-density layer, and outer shell layer were interconnected.

The ratio of the radius of the center and the thickness of the first low density layer, high density layer, second low density layer, and outer shell layer to the particle size of the secondary particles was also measured and calculated. However, they were 25%, 10%, 5%, 5%, and 5%, respectively.

[Average particle diameter MV and particle size distribution]
Using a laser beam diffraction scattering particle size analyzer (Microtrac MT3300EXII, manufactured by Microtrac Bell Co., Ltd.), measure the average particle diameter MV of the secondary particles of the composite hydroxide, and also measure d10 and d90, [(d90-d10)/average particle diameter MV], which is an index indicating the spread of particle size distribution, was calculated. As a result, it was confirmed that the average particle size MV was 5.5 μm and [(d90-d10)/average particle size MV] was 0.50.

c) Preparation of positive electrode active material After heat-treating the composite hydroxide obtained as described above at 120°C for 12 hours in a stream of air (oxygen concentration: 21% by volume) (heat treatment step), Li/Me 1.06, was sufficiently mixed with lithium hydroxide using a shaker mixer device (TURBULA Type T2C manufactured by Willy & Bacchofen (WAB)) to obtain a lithium mixture (mixing step).

This lithium mixture was heated to 885°C at a temperature increase rate of 1.5°C/min in an oxygen (oxygen concentration: 100% by volume) stream, and was fired by holding at this temperature for 3 hours. It was cooled to room temperature at a rate of about 4° C./min (calcination step). The thus obtained positive electrode active material had agglomeration or slight sintering. Therefore, this positive electrode active material was crushed to adjust the average particle size and particle size distribution (pulverization step).

d) Evaluation of positive electrode active material [Composition]
According to an analysis using an ICP emission spectrometer (ICPE-9000, manufactured by Shimadzu Corporation), the composition of this positive electrode active material was determined by the general formula: Li _1.06 Ni _0.50 Mn _0.27 Co _0.23 O ₂ was confirmed.

[Particle structure]
A part of the positive electrode active material was embedded in a resin, and the cross section was made observable by cross-section polisher (IB-19530CP, manufactured by JEOL Ltd.), and then SEM (FE-SEM: JSM, manufactured by JEOL Ltd.) was applied. -6360LA). As a result, this positive electrode active material is composed of secondary particles formed by agglomeration of a plurality of primary particles, and the secondary particles are present in the outer shell and dispersed inside the outer shell, It was confirmed that it has an agglomerated part electrically connected to the outer shell part, and a space part consisting of a pore structure in which no primary particles exist, which exists between the agglomerated parts inside the outer shell part. . Further, from observation of a cross-sectional image containing arbitrary 10 or more secondary particles obtained from the above SEM observation, the average thickness of the outer shell portion of the secondary particles was 0.3 μm.

[Crystallite diameter]
Using an X-ray diffraction (XRD) device (X'Pert PRO, manufactured by Spectris Co., Ltd.), analysis was performed by powder X-ray diffraction using CuKα rays, and each diffraction peak was When the crystallite diameter of the (003) plane was calculated using Scherrer's equation, it was found to be 950 Å.

[Composition of outer shell part and agglomerated part]
From the results using a scanning transmission electron microscope - energy dispersive X-ray spectroscopy (STEM-EDS) device (manufactured by Hitachi High-Technologies Corporation, HD-2300A), nickel and manganese in the outer shell part of the secondary particles were detected. The ratio of nickel to cobalt is 4:3:3, the ratio of nickel to manganese to cobalt in the agglomerated part is 6:2:2, and the overall ratio of nickel to manganese to cobalt is 5.0:2. It was confirmed that the ratio was .7:2.3.

Using a scanning transmission electron microscope-energy dispersive X-ray spectroscopy (STEM-EDS) device (manufactured by Hitachi High-Technologies Corporation, HD-2300A), the nickel distribution map (mapping) in secondary particles shown in Figure 4 was created. Obtained. In the nickel distribution diagram, the part shown in blue indicates a low nickel ratio layer in which the nickel ratio is more than 0.1 and 0.4 or less. Using image analysis software (open source software, developer: Wayne Rasband, ImageJ), the ratio of the area of the low nickel ratio layer to the area of the entire cross section of the secondary particles was determined to be 43%.

Furthermore, using a scanning transmission electron microscope-energy dispersive X-ray spectroscopy (STEM-EDS) device (manufactured by Hitachi High-Technologies Corporation, HD-2300A), the nickel concentration in the outer shell and the surface of the agglomerate was measured. As a result, they were 40 atomic % and 42 atomic %.

[Average particle diameter MV and particle size distribution]
Using a laser beam diffraction scattering particle size analyzer (Microtrac MT3300EXII, manufactured by Microtrac Bell Co., Ltd.), the average particle diameter MV of the positive electrode active material was measured, as well as d10 and d90, and the spread of the particle size distribution was determined. The index [(d90-d10)/average particle diameter MV] was calculated. As a result, it was confirmed that the average particle size MV was 5.0 μm 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 method specific surface area measuring device (Macsorb 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 3.0 m ² /g and the tap density was 1.3 g/cm ³ .

[Oil absorption amount]
Oil absorption measuring device (manufactured by Asahi Souken Co., Ltd., S-500) compliant with "JIS K 6217-4 (Carbon black for rubber - Basic properties - Part 4: How to determine oil absorption (including compressed samples))" ), the oil absorption was measured using di-n-butyl phthalate (di-n-butyl phthalate, DBP) as the oil for measurement. As a result, it was confirmed that the oil absorption amount was 48.0 ml/100 g.

e) Preparation of secondary battery A 2032 type coin battery 11 as shown in FIG. 6 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 PTEE were mixed and pressed at a pressure of 100 MPa to a diameter of 11 mm and a thickness of 100 μm. After molding, the positive electrode 12 was produced by drying at 120° C. for 12 hours in a vacuum dryer.

Next, a 2032 type coin battery was fabricated using this positive electrode 12 in a glove box with an Ar atmosphere whose dew point was controlled at -80°C. The negative electrode 13 of this 2032 type coin battery uses lithium metal with a diameter of 17 mm and a thickness of 1 mm, and the non-aqueous electrolyte contains ethylene carbonate (EC) and diethyl carbonate (DEC) with 1M LiClO ₄ as a supporting electrolyte. An equal volume mixture of (manufactured by Toyama Pharmaceutical Co., Ltd.) was used. Further, as the separator 14, a porous polyethylene film having a film thickness of 25 μm was used. In this way, a 2032 type coin battery 11 having a gasket 15, a positive electrode can 16, and a negative electrode can 17 was assembled.

f) Battery evaluation [Positive electrode interface resistance]
The positive electrode interfacial resistance was measured using the impedance measurement method, by charging a 2032 type coin battery at a charging potential of 4.4 V, and using a frequency response analyzer and potentiogalvanostat (manufactured by Solartron, 1255B), as shown in Figure 7. I got a Nyquist plot. The Nyquist plot shown in Figure 7 is expressed as the sum of characteristic curves showing solution resistance, negative electrode resistance and its capacity, and positive electrode resistance (interfacial resistance) and its capacity. A fitting calculation was performed using the equivalent circuit shown to calculate the value of the positive electrode interface resistance. Regarding the positive electrode interfacial resistance, the resistance reduction rate with respect to the positive electrode active material of Comparative Example 1, which will be described later, is shown as a reference. As a result, the positive electrode interfacial resistance was reduced to 0.71 times that of Comparative Example 2.

[Discharge capacity maintenance rate]
After the 2032-type coin battery was produced and left for about 24 hours and the open circuit voltage OCV (Open Circuit Voltage) stabilized, the current density to the positive electrode was set to 0.1 mA/cm ² and the cutoff voltage was set to 4.3 V. After a 1-hour rest, a charge-discharge test was conducted to measure the discharge capacity when the battery was charged until the cut-off voltage reached 3.0 V, and the initial discharge capacity was determined. Furthermore, charging and discharging were repeated 100 times, and the ratio of the second discharge capacity to the initial discharge capacity was defined as the discharge capacity retention rate. As a result, the discharge capacity retention rate was 92%.

Table 1 shows the outer shell, agglomerated part, and overall composition of the obtained positive electrode active material, the area ratio and characteristics of the low nickel ratio layer, and the characteristics of the obtained lithium ion secondary battery. . Note that Comparative Example 1 and Comparative Example 2 are also shown in Table 1.

(Comparative example 1)
Nickel sulfate, manganese sulfate, and cobalt sulfate are dissolved in water so that the molar ratio of each metal element is Ni:Mn:Co=6:3:1 to prepare a 2 mol/L first raw material aqueous solution. and that in switching operation 3, the supply of the first raw material aqueous solution was continued, and in switching operation 4, the supply of the first raw material aqueous solution was stopped and the supply of the second raw material aqueous solution was started. A composite hydroxide, a positive electrode active material, and a lithium ion secondary battery were obtained in the same manner as in Example 1.

The composition of the composite hydroxide was the general formula: Ni _0.49 Mn _0.32 Co _0.19 (OH) ₂ . Furthermore, the ratio of the radius of the center, the thickness of the first low density layer, high density layer, second low density layer, and outer shell layer to the particle size of the secondary particles of the composite hydroxide is , 30%, 6%, 6%, 4%, and 4%, respectively. Furthermore, the average particle size MV was 5.5 μm, and [(d90-d10)/average particle size MV] was 0.50.

The composition of the positive electrode active material was the general formula: Li _1.06 Ni _0.49 Mn _0.32 Co _0.19 O ₂ . The average thickness of the outer shell portion of the secondary particles was 0.3 μm, and the crystallite diameter of the (003) plane was 930 Å. The ratio of nickel, manganese, and cobalt in the outer shell part of the secondary particles is 4.5:3.5:2.5, and the ratio of nickel, manganese, and cobalt in the agglomerated part is 5:3:2. The overall nickel to manganese to cobalt ratio was 4.9:3.2:1.9.

The ratio of the area of the low nickel ratio layer to the area of the entire cross section of the secondary particles was determined to be 15%. Further, the nickel concentration in the outer shell portion was 45 atomic %, but the nickel concentration in the surface portion of the agglomerated portion was 49 atomic %.

The average particle size MV was 4.9 μm, and [(d90-d10)/average particle size MV] was 0.42. Further, the BET specific surface area was 3.0 m ² /g, the tap density was 1.4 g/cm ³ , and the oil absorption was 45 ml/100 g).

In a lithium ion secondary battery using this positive electrode active material, the positive electrode interfacial resistance was reduced to 0.9 times that of Comparative Example 1. The discharge capacity retention rate was 85%.

(Comparative example 2)
Regarding the raw material aqueous solution, nickel sulfate, manganese sulfate, and cobalt sulfate were prepared without dividing into the first raw material aqueous solution and the second raw material aqueous solution, and the molar ratio of each metal element was Ni:Mn:Co=5:3:2. Except that only a 2 mol/L raw material aqueous solution was prepared by dissolving it in water so that In the same manner as in Example 1, a composite hydroxide, a positive electrode active material, and a lithium ion secondary battery were obtained.

The composition of the composite hydroxide was the general formula: Ni _0.5 Mn _0.3 Co _0.2 (OH) ₂ . Furthermore, the ratio of the radius of the center, the thickness of the first low density layer, high density layer, second low density layer, and outer shell layer to the particle size of the secondary particles of the composite hydroxide is , 25%, 10%, 5%, 5%, and 5%, respectively. Furthermore, the average particle size MV was 5.5 μm, and [(d90-d10)/average particle size MV] was 0.50.

The composition of the positive electrode active material was the general formula: Li _1.10 Ni _0.5 Mn _0.3 Co _0.2 O ₂ . The average thickness of the outer shell portion of the secondary particles was 0.3 μm, and the crystallite diameter of the (003) plane was 970 Å. The ratio of nickel to manganese to cobalt in the outer shell portion and the aggregated portion of the secondary particles was 5:2:3. Further, the average particle size MV was 5.2 μm, and [(d90-d10)/average particle size MV] was 0.42. Further, the BET specific surface area was 3.0 m ² /g, the tap density was 1.4 g/cm ³ , and the oil absorption was 45 ml/100 g.

The discharge capacity retention rate of the lithium ion secondary battery using this positive electrode active material was 80%.

In the lithium ion secondary battery using the positive electrode active material of Example 1, which is within the scope of the present invention, in comparison with Comparative Example 1 and Comparative Example 2, the positive electrode interface resistance decreased and the discharge capacity retention rate decreased. was confirmed to have improved.

1 Central part 2 First low-density layer 3 High-density layer 4 Second low-density layer 5 Outer shell layer 6 Outer shell part 7 Agglomerated part 8 Space part 9 Surface part of aggregated part 11 Coin battery 12 Positive electrode (evaluation electrode )
13 negative electrode 14 separator 15 gasket 16 positive electrode can 17 negative electrode can

Claims (20)

Nickel manganese cobalt is a precursor of a positive electrode active material for lithium ion secondary batteries, and is composed of secondary particles formed by agglomeration of a plurality of plate-shaped primary particles and fine primary particles smaller than the plate-shaped primary particles. A containing complex hydroxide,
The secondary particles include a center portion mainly formed by agglomeration of the plate-shaped primary particles, and a first low-density layer formed mainly by aggregation of the fine primary particles outside the center portion; A high-density layer formed by mainly agglomerating the plate-shaped primary particles outside the first low-density layer, and a second high-density layer mainly formed by agglomerating the fine primary particles outside the high-density layer. a low-density layer, and an outer shell layer formed mainly by agglomeration of the plate-shaped primary particles outside the second low-density layer, and
The central portion, the first low-density layer, and the high-density layer have a composition with a high nickel ratio in which the nickel ratio, which is the atomic ratio of nickel to the sum of nickel, manganese, and cobalt, exceeds 0.4, The second low density layer and the outer shell layer have a composition with a low nickel ratio in which the nickel ratio is more than 0.1 and less than or equal to 0.4.
Composite hydroxide containing nickel manganese cobalt. The average particle size MV of the secondary particles is in the range of 4 μm to 11 μm, and [(d90-d10)/average particle size MV], which is an index indicating the spread of the particle size distribution of the secondary particles, is 0. The nickel-manganese-cobalt-containing composite hydroxide according to claim 1, which has a nickel-manganese-cobalt content of .65 or less. The radius of the center is in the range of 20% to 40% of the particle size of the secondary particles, and the thickness of the first low density layer is in the range of 3% to 15% of the particle size of the secondary particles. The thickness of the high-density layer is in the range of 2% to 10% of the particle size of the secondary particles, and the thickness of the second low-density layer is in the range of 2% to 10% of the particle size of the secondary particles. The nickel manganese cobalt-containing composite according to claim 1 or 2, wherein the thickness of the outer shell layer is in the range of 2% to 10% of the particle size of the secondary particles. hydroxide. General formula (A): Ni _{x Mny} Co _z M _t (OH) _2+a (x+y+z+t=1, 0.4≦x≦0.70, 0.15≦y≦0.4, 0.15≦z≦0 .3, 0≦t≦0.1, 0≦a≦0.5, M is selected from Mg, Al, Si, Ca, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W. The nickel-manganese-cobalt-containing composite hydroxide according to any one of claims 1 to 3, having a composition represented by one or more additive elements). A step of forming a reaction aqueous solution 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 tank, and obtaining a nickel manganese cobalt-containing composite hydroxide through a crystallization reaction. A method for producing a nickel manganese cobalt-containing composite hydroxide, comprising:
In the step of obtaining the nickel-manganese-cobalt-containing composite hydroxide, the ammonium ion concentration of the reaction aqueous solution is in the range of 3 g/L to 25 g/L, and the pH value based on the liquid temperature of 25° C. is 12.0 to 14. By controlling the ammonium ion concentration to be in the range of 0, the ammonium ion concentration of the nucleation step and the reaction aqueous solution containing the nuclei obtained in the nucleation step is in the range of 3 g / L to 25 g / L, and growing the nuclei by controlling the pH value based on the liquid temperature of 25° C. to be lower than the pH value of the nucleation step and in the range of 10.5 to 12.0. and a growth process,
(1) Adjusting the reaction atmosphere in the first stage of the nucleation step and the particle growth step to a non-oxidizing atmosphere with an oxygen concentration of 5% by volume or less, (2) After the first step of the particle growth step, (3) the reaction atmosphere is switched from the non-oxidizing atmosphere to an oxidizing atmosphere in which the concentration of oxygen exceeds 5% by volume for the second stage of the particle growth step; (4) Furthermore, the reaction atmosphere is switched from the non-oxidizing atmosphere to the oxidizing atmosphere to perform the particle growing step. (5) Finally, the reaction atmosphere is switched from the oxidizing atmosphere to the non-oxidizing atmosphere, and the crystallization reaction is continued as the fifth step of the particle growth step, and ,
The raw material aqueous solution supplied in the first, second, and third stages of the nucleation step and the particle growth step has a nickel ratio of 0, which is the atomic ratio of nickel to the total of nickel, manganese, and cobalt. The raw material aqueous solution supplied in the fourth and fifth stages of the particle growth step has a composition with a high nickel ratio of more than 0.4 and has a nickel ratio of more than 0.1 and 0.4 or less. having a composition with a certain low nickel ratio,
A precursor of a positive electrode active material for lithium ion secondary batteries, which is composed of secondary particles formed by aggregation of a plurality of plate-shaped primary particles and fine primary particles smaller than the plate-shaped primary particles, and The particles include a center portion mainly formed by agglomeration of the plate-shaped primary particles, a first low-density layer formed mainly by aggregation of the fine primary particles outside the center portion, and a first low-density layer formed mainly by aggregation of the fine primary particles. A high-density layer formed by mainly agglomerating the plate-shaped primary particles outside the low-density layer, and a second low-density layer mainly formed by aggregating the fine primary particles outside the high-density layer. layer, and an outer shell layer mainly formed by agglomeration of the plate-shaped primary particles outside the second low-density layer, and the center portion, the first low-density layer, and the high-density layer. The layer has a high nickel ratio composition with a nickel ratio, which is the atomic ratio of nickel to the sum of nickel, manganese, and cobalt, greater than 0.4, and the second low density layer and the outer shell layer are Obtaining a nickel manganese cobalt-containing composite hydroxide having a composition with a low nickel ratio of more than 0.1 and less than or equal to 0.4,
A method for producing a composite hydroxide containing nickel manganese cobalt. The switching of the reaction atmosphere is defined by the ratio of the amount of metal added at each step to the total amount of metal added in the particle growth step, and the change of the reaction atmosphere is defined as the ratio of the amount of metal added at each step to the total amount of metal added in the step of particle growth. Regarding the percentage, the first stage is in the range of 8% to 20%, the second stage is in the range of 2% to 20%, the third stage is in the range of 12% to 40%, and the fourth stage is in the range of 3% to 40%. %, and the fifth step is 10% to 50%. The method for producing a nickel manganese cobalt-containing composite hydroxide according to claim 5. The nickel-manganese-cobalt-containing composite hydroxide has a general formula (A): Ni _{x Mny} Co _z M _t (OH) _2+a (x+y+z+t=1, 0.4≦x≦0.70, 0.15≦y≦ 0.4, 0.15≦z≦0.3, 0≦t≦0.1, 0≦a≦0.5, M is Mg, Al, Si, Ca, Ti, V, Cr, Zr, Nb The method for producing a nickel-manganese-cobalt-containing composite hydroxide according to claim 5 or 6, which has a composition represented by one or more additive elements selected from , Mo, Hf, Ta, and W. A positive electrode active material for a lithium ion secondary battery comprising a lithium nickel manganese cobalt-containing composite oxide,
The positive electrode active material is composed of secondary particles formed by agglomerating a plurality of primary particles, and the secondary particles have an outer shell formed by the aggregated primary particles and an inner side of the outer shell. an agglomerated part that is present in the agglomerated part, is formed by the agglomerated primary particles, and is electrically connected to the outer shell part, and a space part that exists dispersedly within the agglomerated part, and
The agglomerated portion has a composition with a high nickel ratio in which the nickel ratio, which is the atomic ratio of nickel to the sum of nickel, manganese, and cobalt, exceeds 0.4; the outer shell has a composition with a low nickel ratio of more than 0.1 and less than or equal to 0.4; and the secondary particles have a nickel ratio of more than 0.1 and 0.4 in a composition analysis of a cross section of the secondary particles by energy dispersive X-ray spectroscopy using a scanning electron microscope. The area ratio of the low nickel ratio layer is in the range of 25% to 45% of the area of the entire cross section of the secondary particles,
Positive electrode active material for lithium ion secondary batteries. The positive electrode active material for a lithium ion secondary battery according to claim 8, wherein the thickness of the outer shell portion of the secondary particles is in the range of 0.1 μm to 1.0 μm. The average particle diameter MV of the secondary particles is in the range of 3 μm to 10 μm, and [(d90-d10)/average particle diameter MV], which is an index indicating the spread of the particle size distribution of the secondary particles, is 0. The positive electrode active material for a lithium ion secondary battery according to claim 8 or 9, wherein the positive electrode active material is .7 or less. The positive electrode active material for a lithium ion secondary battery according to any one of claims 8 to 10, wherein the tap density of the positive electrode active material is in the range of 1.1 g/cm ³ to 1.8 g/cm ³ . The BET specific surface area of the positive electrode active material is in the range of 2.0 m ² /g to 5.0 m ² /g,
The positive electrode active material for a lithium ion secondary battery according to any one of claims 8 to 11. The positive electrode active material for a lithium ion secondary battery according to any one of claims 8 to 12, wherein the oil absorption amount of the positive electrode active material is in the range of 35 ml/100 g to 60 ml/100 g. The crystallite diameter of the primary particles determined using the Scherrer equation from the half width of the peak of the (003) plane by X-ray diffraction of the (003) plane of the positive electrode active material is in the range of 300 Å to 1500 Å. 14. The positive electrode active material for a lithium ion secondary battery according to any one of 8 to 13. The lithium nickel manganese cobalt-containing composite oxide has a general formula (B): Li _1+u Ni _x Mn _y Co _z M _t O ₂ (-0.05≦u≦0.50, x+y+z+t=1, 0.4≦x ≦0.70, 0.15≦y≦0.4, 0.15≦z≦0.3, 0≦t≦0.1, M is Mg, Al, Si, Ca, Ti, V, Cr, one or more additive elements selected from Zr, Nb, Mo, Hf, Ta, and W) and has a hexagonal crystal structure with a layered structure, according to any one of claims 8 to 14. Positive electrode active material for lithium ion secondary batteries. A mixing step of mixing a nickel manganese cobalt-containing composite hydroxide and a lithium compound to form a lithium mixture, and oxidizing the lithium mixture formed in the mixing step with an oxygen concentration of 18% by volume to 100% by volume. a firing step for obtaining a positive electrode active material for a lithium ion secondary battery made of a lithium nickel manganese cobalt-containing composite oxide by firing at a temperature in the range of 650° C. to 920° C. in a neutral atmosphere,
As the nickel-manganese-cobalt-containing composite hydroxide, the nickel-manganese-cobalt-containing composite hydroxide according to any one of claims 1 to 4 is used,
A method for producing a positive electrode active material for a lithium ion secondary battery. In the mixing step, the lithium mixture is adjusted so that the ratio of the sum of the number of atoms of metals other than lithium contained in the lithium mixture and the number of lithium atoms is 1:0.95 to 1.5. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 16. The lithium ion secondary battery according to claim 16 or 17, further comprising a heat treatment step of heat treating the nickel manganese cobalt-containing composite hydroxide at a temperature in the range of 105° C. to 750° C. before the mixing step. A method for producing a cathode active material for use. The positive electrode active material for lithium ion secondary batteries has a general formula (B): Li _1+u Ni _x Mn _y Co _z M _t O ₂ (-0.05≦u≦0.50, x+y+z+t=1, 0.4≦ x≦0.70, 0.15≦y≦ 0.4 , 0.15≦z≦ 0.3 , 0≦t≦0.1, M is Mg, Al, Si, Ca, Ti, V, Cr , Zr, Nb, Mo, Hf, Ta, and W) and comprises a hexagonal lithium nickel manganese composite oxide having a porous structure. A method for producing a positive electrode active material for a lithium ion secondary battery according to any one of the above. The lithium ion secondary battery according to any one of claims 8 to 15, comprising a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte, or a positive electrode, a negative electrode, and a solid electrolyte, and as a positive electrode active material used in the positive electrode. A lithium-ion secondary battery that uses a positive electrode active material.

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