JP7676851B2 - Transition metal composite hydroxide particles, method for producing transition metal composite hydroxide particles, positive electrode active material for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Description

The present invention relates to transition metal composite hydroxide particles, a method for producing transition metal composite hydroxide particles, a positive electrode active material for lithium ion secondary batteries, and a lithium ion secondary battery.

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

One type of secondary battery that meets these requirements is the lithium-ion secondary battery, which is a type of non-aqueous electrolyte secondary battery. Lithium-ion secondary batteries are composed of a negative electrode, a positive electrode, an electrolyte, and the like, and the active materials used for the negative and positive electrodes are materials that can extract and insert lithium.

Among lithium-ion secondary batteries, those that use layered or spinel-type lithium transition metal composite oxides as the positive electrode material can achieve a voltage of about 4 V, and are currently the subject of vigorous research and development as batteries with high energy density, with some of these already being put to practical use.

As positive electrode materials for lithium-ion secondary batteries, lithium transition metal composite oxides have been proposed, such as lithium cobalt composite oxide ( _LiCoO2 ), which is relatively easy to synthesize, lithium nickel composite oxide ( _LiNiO2 ), which uses nickel, which is cheaper than cobalt, lithium nickel cobalt manganese composite oxide (LiNi1 _{/ 3Co1/ 3Mn1/ 3O2 )} , lithium manganese composite oxide ( _LiMn2O4 ), and lithium nickel manganese composite _oxide ( _{LiNi0.5Mn0.5O2} ) _, which use manganese.

By the way, in order to obtain a lithium-ion secondary battery with excellent output characteristics, it is common to increase the specific surface area of the positive electrode active material. This is because when it is used as a positive electrode active material, a sufficient reaction area with the electrolyte can be secured.

As a method for increasing the specific surface area of the positive electrode active material, for example, lithium nickel manganese composite oxide particles with a high specific surface area due to their hollow structure have been proposed (Patent Document 1, Patent Document 2).

However, the output characteristics required, especially for electric vehicles, are higher than ever before, and further performance improvements are required.

As a means of achieving the low resistance required for high output when applied to lithium-ion secondary batteries, the addition of different elements to the positive electrode active material is being considered, and in particular the addition of transition metals that can assume high valence, such as Mo, Nb, W, and Ta.

For example, Patent Document 3 discloses a lithium transition metal compound powder in which the atomic ratio of the total of the additive elements to the total of metal elements other than Li and one or more additive elements selected from Mo, W, Nb, Ta, and Re in the surface portion of the primary particles is 5 times or more the atomic ratio of the additive elements in the entire particle.

Patent Document 4 discloses a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, which includes a first step of dispersing W on the surface of the lithium metal composite oxide powder or on the surfaces of the primary particles of the powder by adding and mixing an alkaline solution in which a predetermined ratio of a tungsten compound is dissolved into a lithium metal composite oxide powder, and a second step of forming fine particles containing lithium tungstate represented by any one of Li ₂ WO ₄ , Li ₄ WO ₅ , and Li ₆ W ₂ O ₉ on the surface of the lithium metal composite oxide powder or on the surfaces of the primary particles of the powder by heat-treating the mixed alkaline aqueous solution in which the tungsten compound is dissolved and the lithium metal composite oxide powder at a temperature in the range of 100 to 800° C.

Patent Document 5 discloses a positive electrode active material for a non-aqueous electrolyte secondary battery, which is composed of primary particles and secondary particles formed by agglomeration of the primary particles, has voids near the surface and inside the secondary particles through which an electrolyte can penetrate, and has a compound layer of a predetermined thickness containing tungsten-enriched lithium on the surface or grain boundaries of the lithium metal composite oxide.

Patent Document 6 discloses a method for producing a transition metal composite hydroxide, which comprises a coating step in which a coating containing tungsten and a metal oxide or metal hydroxide of an additive element is formed on the surface of the composite hydroxide particles by mixing a predetermined composite hydroxide particle with an aqueous solution containing at least a tungsten compound to form a slurry and controlling the pH value of the slurry.

JP 2012-246199 A JP 2013-147416 A JP 2008-305777 A JP 2013-125732 A JP 2014-197556 A JP 2012-252844 A

However, as mentioned above, there has been a demand in recent years for further improvements in output characteristics. There has also been a demand for transition metal composite hydroxide particles that can improve output characteristics when used as a positive electrode active material in lithium ion secondary batteries.

In view of the problems inherent in the above-mentioned conventional techniques, one aspect of the present invention aims to provide transition metal composite hydroxide particles that exhibit high output characteristics when used as a positive electrode active material in a lithium ion secondary battery.

In order to solve the above problems, according to one aspect of the present invention, nickel (Ni), manganese (Mn), cobalt (Co), and an element A (A) are contained in a ratio of amounts of substances of Ni:Mn:Co:A=x:y:z:t (x+y+z=1, 0.3≦x≦0.95, 0.05≦y≦0.55, 0≦z≦0.4, 0<t≦0.1, the element A is tungsten ),
a core portion including primary particles and an outer shell portion disposed outside the core portion and in which the primary particles are more densely arranged than in the core portion;
the abundance ratio (t _h ) of the element A in the central portion is smaller than the abundance ratio (t _s ) of the element A in the outer shell portion;
The present invention provides transition metal composite hydroxide particles, in which the tungsten concentration in the center portion is 0.1% by mass or more and 9.0% by mass or less .

According to one aspect of the present invention, transition metal composite hydroxide particles that have high output characteristics when used as a positive electrode active material in a lithium ion secondary battery can be provided.

FIG. 1 is a flow diagram of a method for producing transition metal composite hydroxide particles according to one embodiment of the present disclosure. FIG. 2 is a flow diagram of a method for producing a positive electrode active material for a lithium ion secondary battery according to one embodiment of the present disclosure. FIG. 3 is a cross-sectional view of a lithium-ion secondary battery according to one embodiment of the present disclosure.

The following describes the embodiments of the present invention with reference to the drawings. However, the present invention is not limited to the following embodiments, and various modifications and substitutions can be made to the following embodiments without departing from the scope of the present invention.

The following describes in the following order: (1) transition metal composite hydroxide particles, (2) manufacturing method for transition metal composite hydroxide particles, (3) positive electrode active material for lithium ion secondary batteries, (4) manufacturing method for positive electrode active material for lithium ion secondary batteries, and (5) lithium ion secondary batteries.

(1) Transition metal composite hydroxide particles (1-1) Composition The transition metal composite hydroxide particles of the present embodiment (hereinafter, also simply referred to as "composite hydroxide particles") can contain nickel (Ni), manganese (Mn), cobalt (Co), and element A (A) in a ratio, in terms of the amount of substances, of Ni:Mn:Co:A=x:y:z:t.

The above x, y, z, and t preferably satisfy the relationships: x+y+z=1, 0.3≦x≦0.95, 0.05≦y≦0.55, 0≦z≦0.4, and 0<t≦0.1. The element A can be one or more elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, La, Hf, Ta, and W.

The composite hydroxide particles of this embodiment can be expressed, for example, by the general formula (A): Ni _x Mn _y Co _{z At} (OH) _2+a . Since x, y, z, t, and element A in the general formula (A) have already been explained, explanations are omitted here. In the general formula (A), a can be, for example, 0≦a≦0.5.

By using composite hydroxide particles having the above composition as a precursor, it is possible to easily obtain a positive electrode active material having the composition described below, thereby achieving higher battery performance.

The preferred content ranges of nickel, manganese, cobalt, and element A contained in the composite hydroxide particles are the same as those of the positive electrode active material for lithium ion secondary batteries (hereinafter, simply referred to as the "positive electrode active material") described below. Therefore, the preferred content ranges of each of these elements will be described together in the section on the positive electrode active material.

(1-2) Particle Structure The transition metal composite hydroxide particles of the present embodiment can have a core portion containing primary particles, and an outer shell portion that is arranged outside the core portion and in which the primary particles are arranged more densely than in the core portion.

As described above, the outer shell portion can have a dense structure in which the primary particles are denser than the center portion, i.e., the primary particles are arranged at a high density. The center portion can have a sparse structure in which the density of primary particles is lower than that of the outer shell portion. The center portion and the outer shell portion can be arranged so as to be in direct contact with each other.

As mentioned above, the density of the primary particles arranged in the center and the outer shell is different, so this can be easily identified by observing the cross section of the particle with a scanning electron microscope (SEM), for example.

The outer shell can include, for example, a plurality of plate-shaped primary particles. The center can be formed, for example, by agglomeration of primary particles smaller than the plate-shaped primary particles of the outer shell. The composite hydroxide particles can be secondary particles formed by agglomeration of primary particles as described above.

In addition, the composite hydroxide particles preferably have an abundance ratio (t _h ) of element A in the center smaller than an abundance ratio (t _s ) of element A in the outer shell. That is, it is preferable that t _s >t _h . The abundance ratio (t _h ) of element A in the center means the proportion of element A contained in the composite hydroxide particles that is contained in the center. In addition, the abundance ratio (t _s ) of element A in the outer shell means the proportion of element A contained in the composite hydroxide particles that is contained in the outer shell. Therefore, the sum of t _h and t _s is 100%. It is preferable that both the center and the outer shell contain element A. Therefore, it is preferable to satisfy t _s >t _h >0.

As described below, the positive electrode active material is obtained by firing a mixture of a composite hydroxide and a lithium compound.

The composite hydroxide of this embodiment can contain element A as described above, and the positive electrode active material obtained by baking a mixture of such a composite hydroxide and a lithium compound contains, for example, a compound containing element A and lithium in part. Since such a compound containing element A and lithium has high lithium ion conductivity, when the positive electrode active material is used in a lithium ion secondary battery, it suppresses the reaction resistance (positive electrode resistance) and greatly contributes to improving the output characteristics.

According to the study by the inventors of the present invention, when a positive electrode active material is prepared using the composite hydroxide particles of this embodiment containing element A, the compound containing element A and lithium is formed on the surface layer and grain boundaries of the lithium nickel manganese composite oxide particles contained in the positive electrode active material. Since the surface layer and grain boundaries of the lithium nickel manganese composite oxide particles are in contact with the electrolyte, the conductivity of lithium ions can be increased by disposing the compound containing element A and lithium in these areas, and the output characteristics of the lithium ion secondary battery using the positive electrode active material can be particularly improved.

In particular, by arranging element A so that _ts > _th as described above, the compound containing element A and lithium can be sufficiently arranged in the portion in contact with the electrolyte, such as the surface of the hollow portion of the positive electrode active material prepared using the composite hydroxide particles of this embodiment, which is preferable because it can particularly improve the output characteristics of the lithium ion secondary battery using the positive electrode active material.

In this specification, the grain boundary means an interparticle boundary that exists between two or more small crystal grains. The surface layer means the surface of the particle, specifically the inner surface that can come into contact with the electrolyte inside the cavity of the particle, and the outer surface of the particle.

According to the study by the inventors of the present invention, compounds containing lithium and tungsten have particularly excellent lithium ion conductivity. For this reason, it is preferable that the aforementioned element A contains tungsten, and it is more preferable that it is tungsten.

This is because the element A contains tungsten, and when a positive electrode active material made using the composite hydroxide is applied to a lithium ion secondary battery, the output characteristics of the lithium ion secondary battery can be particularly improved.

In the particle growth steps of the method for producing composite hydroxide particles described below, the central portion can be formed by the first crystallization step, the inner peripheral side of the outer shell portion by the second crystallization step, and the outer peripheral side of the outer shell portion by the third crystallization step. Therefore, for example, the ratio of the amount of substance of element A supplied during the first crystallization step to the amount of substance of element A supplied during the third crystallization step can be set as the ratio of the abundance ratio of element A in the central portion (t _h ) to the abundance ratio of element A in the outer shell portion (t _s ).

In addition, for example, in the cross section of a composite hydroxide particle, the concentrations of element A in the center and outer shell are measured and calculated by EDX (Energy Dispersive X-ray spectroscopy), and multiplied by the area (area ratio) of each region to calculate and compare the abundance ratio of element A in the center (t _h ) and the abundance ratio of element A in the outer shell (t _s ).

When the concentration of element A varies between the center and the outer shell, the concentration of element A can be measured by EDX at multiple points along the radial direction of the composite hydroxide particle in its cross section, and the average concentration of element A in the center and outer shell can be calculated. There are no particular limitations on the points to be measured, but for example, the concentration of element A can be measured at two points within the center that can divide the center into multiple regions at equal intervals in the radial direction of the particle, and at two points within the outer shell that can divide the outer shell into multiple regions at equal intervals in the radial direction of the particle, and the average concentration of element A in each region can be calculated. The average concentration of each region can then be multiplied by the area (area ratio) of each region to calculate and compare the abundance ratio of element A in the center and the abundance ratio of element A in the outer shell.

Among the outer shells, it is preferable that the abundance ratio of element A in the outer periphery of the outer shell is high. That is, it is preferable that the abundance ratio of element A in the outer periphery of the outer shell ( _{ts '} ) is greater than the abundance ratio of element A in the outer shell (ts), and it is preferable that the relationship is ts _' > _ts . By making the abundance ratio of element A in the outer periphery of the outer shell (ts _' ) greater than the abundance ratio of element A in the outer shell ( _ts ), in the positive electrode active material prepared using the composite hydroxide particles, a compound containing element A and lithium can be arranged at a particularly high concentration on the outer surface side of the particles of the positive electrode active material. Since the outer surface of the positive electrode active material is the part that is particularly in contact with the electrolyte, by arranging a compound containing element A and lithium having high lithium ion conductivity in this part at a high concentration, the reaction resistance can be particularly reduced and the output characteristics can be particularly improved.

As mentioned above, it is preferable that element A contains tungsten.

In this case, the tungsten concentration in the center of the composite hydroxide particles of this embodiment is preferably 0.1% by mass or more and 9.0% by mass or less. This is because by setting the tungsten concentration in the center within the above range, the output characteristics of a lithium ion secondary battery using a positive electrode active material prepared using the composite hydroxide particles can be particularly improved.

The concentration of tungsten in the center of the composite hydroxide can be evaluated, for example, by collecting particles after the first crystallization step (described later) during the manufacturing process and using ICP (Inductively Coupled Plasma) or the like. It can also be evaluated, for example, using EDX.

In addition, when element A contains tungsten, the composite hydroxide particle of this embodiment preferably has a tungsten-enriched layer of 10 nm to 100 nm on the outer surface of the outer shell. The tungsten-enriched layer here means a layer in which the tungsten concentration is locally higher than in other parts. By disposing a tungsten-enriched layer on the outer surface of the outer shell of the composite hydroxide particle, i.e., on the outer surface of the particle, in a positive electrode active material produced using the composite hydroxide as a precursor, a lithium-tungsten compound can be disposed at a particularly high concentration on the outer surface of the particle of the positive electrode active material. And, as described above, the lithium-tungsten compound has particularly excellent lithium ion conductivity, so that the output characteristics of the positive electrode active material can be particularly improved.
It is more preferable that the tungsten concentrated layer is also provided on the inner surface of the cavity of the particle that can come into contact with the electrolyte, that is, the tungsten concentrated layer is more preferably disposed on the surface layer of the outer shell of the composite hydroxide particle.

The thickness of the tungsten-enriched layer can be evaluated, for example, by SEM-EDX.

(1-3) Average Particle Size The average particle size of the composite hydroxide particles of this embodiment is not particularly limited, but the average particle size is preferably, for example, from 4.0 μm to 9.0 μm.

By setting the average particle size of the composite hydroxide particles of this embodiment within the above range, the battery capacity per unit volume can be increased in a lithium ion secondary battery using a positive electrode active material produced using the composite hydroxide particles as a precursor. In addition, the thermal stability and output characteristics can be particularly improved.

Specifically, by making the average particle size 4.0 μm or more, the filling property is improved when preparing a positive electrode using a positive electrode active material produced using the composite hydroxide particles as a precursor, and the battery capacity per unit volume can be increased.

On the other hand, by setting the average particle size to 9.0 μm or less, the reaction area with the electrolyte can be increased for the positive electrode active material produced using the composite hydroxide particles as a precursor, and the output characteristics can be particularly improved when used in a lithium ion secondary battery.

The average particle size means the cumulative median diameter, and can be calculated, for example, from the volume integrated value measured using a laser light diffraction scattering particle size analyzer.

(1-4) Particle Size Distribution The distribution index, which indicates the spread of the particle size distribution of the composite hydroxide particles of this embodiment, is not particularly limited, but is preferably 0.60 or less.

The distribution index can be calculated using the following formula (1):

(Distribution index) = [(d ₉₀ - _{d 10} )/average particle size]...(1)
The powder characteristics of the positive electrode active material are strongly influenced by the powder characteristics of the composite hydroxide particles, which are the precursors. Therefore, the positive electrode active material produced using the composite hydroxide particles, which have a distribution index in the above range, as the precursor can have a narrow particle size distribution. In other words, the positive electrode active material can have a small proportion of fine particles and coarse particles.

For example, if a positive electrode is made using a positive electrode active material containing many fine particles, heat may be generated due to local reactions of the fine particles. This may result in reduced thermal stability or selective degradation of the fine particles, resulting in reduced capacity of the lithium-ion secondary battery, i.e., reduced cycle characteristics. Also, if a positive electrode is made using a positive electrode active material containing many coarse particles, the reaction area between the electrolyte and the positive electrode active material cannot be sufficiently secured, resulting in poor output characteristics.

In contrast, as described above, by using a positive electrode active material with a narrow particle size distribution and a low proportion of fine particles and coarse particles, it is possible to particularly improve cycle characteristics, output characteristics, and thermal stability.

However, using a particle with an excessively small distribution index is not realistic from the standpoint of productivity. Therefore, it is preferable that the distribution index of the composite hydroxide particles is, for example, 0.25 or more.

In addition, _d10 in formula (1) means a particle size where the cumulative volume is 10% of the total volume of all particles when the number of particles at each particle size is accumulated from the smallest particle size. Also, _d90 means a particle size where the cumulative volume is 90% of the total volume of all particles when the number of particles at each particle size is accumulated from the smallest particle size. There is no particular limitation on the method of determining the average particle size, _d90 , and _d10 , but they can be determined, for example, from the integrated volume measured by a laser light diffraction scattering type particle size analyzer. As the average particle size, the median diameter, i.e., _d50 , can be used as described above. As _d50 , a particle size where the cumulative volume is 50% of the total particle volume can be used as in the case of _d90 .
(2) Method for Producing Transition Metal Composite Hydroxide Particles The method for producing the transition metal composite hydroxide particles of the present embodiment can include the following steps.

The first crystallization process is a process in which a raw material aqueous solution containing at least nickel and manganese, an additive aqueous solution containing element A, a complexing agent, and a basic aqueous solution are supplied to a reaction aqueous solution in a reaction tank that has an oxidizing atmosphere and contains a reaction aqueous solution that contains nuclei for particle growth.

The second crystallization process switches the atmosphere in the reaction tank to a non-oxidizing atmosphere.

The third crystallization process supplies a raw material aqueous solution containing at least nickel and manganese, an additive aqueous solution containing element A, a complexing agent, and a basic aqueous solution into a reaction tank.

According to the manufacturing method of the transition metal composite hydroxide particles of this embodiment, it is possible to manufacture transition metal composite hydroxide particles containing nickel (Ni), manganese (Mn), cobalt (Co), and element A (A) in a ratio of Ni:Mn:Co:A=x:y:z:t in terms of the ratio of the amounts of substances. It is preferable that the above x, y, z, and t satisfy x+y+z=1, 0.3≦x≦0.95, 0.05≦y≦0.55, 0≦z≦0.4, and 0<t≦0.1. In addition, element A can be one or more elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, La, Hf, Ta, and W.

Each step of the method for producing composite hydroxide particles of this embodiment will be described below. The method for producing composite hydroxide particles of this embodiment is not limited as long as it can produce the composite hydroxide particles described above, but it can be suitably applied to composite hydroxide particles having the composition described above. The composite hydroxide can be produced by the method for producing composite hydroxide of this embodiment. For this reason, some of the matters already described will not be described.

The method for producing composite hydroxide particles of this embodiment is a method for producing composite hydroxide particles that serve as a precursor of a positive electrode active material by a crystallization reaction. Figure 1 shows a flow 10 of the method for producing composite hydroxide particles of this embodiment. In Figure 1, (A) is the nucleation process, and (B) is the particle growth process.

In the method for producing composite hydroxide particles of this embodiment, the crystallization reaction is clearly separated into two stages, a nucleation process in which nucleation is mainly performed, and a particle growth process in which particle growth is mainly performed, and the crystallization conditions in each process are adjusted, and the method can be carried out by this. This makes it possible to obtain composite hydroxide particles having a predetermined structure. Furthermore, in the particle growth process, the radial distribution of element A within the particles can be controlled by starting and stopping the supply of an additive aqueous solution containing element A at a predetermined timing.

If the reaction aqueous solution containing the nuclei to be used in the particle growth process is prepared in advance, the particle growth process can be started without carrying out the nucleation process.

(2-1) Each step The operation of each step is described below. The aqueous solutions such as the raw aqueous solution supplied in each step, and the suitable conditions such as pH value, ammonium ion concentration, temperature, etc. will be described later.

(2-1-1) Nucleation step (S11)
In the nucleation step, a reaction aqueous solution containing nuclei for particle growth to be supplied to the first crystallization step can be prepared.

It is preferable to prepare a pre-reaction aqueous solution in the reaction vessel before the nucleation step is started. Therefore, first, the pre-reaction aqueous solution preparation step for preparing the pre-reaction aqueous solution will be described.
(Pre-reaction aqueous solution preparation step)
The pre-reaction aqueous solution can be prepared, for example, by supplying and mixing a basic aqueous solution and a complexing agent. Water can also be added to the pre-reaction aqueous solution as necessary.

It is preferable to prepare the pre-reaction aqueous solution so that the pH value measured at a liquid temperature of 25°C is 12.0 to 14.0, and the ammonium ion concentration is 3 g/L to 25 g/L.

The nucleation process is preferably carried out in an oxidizing atmosphere. For this reason, it is also preferable to control the reaction atmosphere in the reaction tank to an oxidizing atmosphere in the pre-reaction aqueous solution preparation process.

Here, an oxidizing atmosphere means an atmosphere in which the oxygen concentration in the reaction atmosphere is sufficiently high; specifically, for example, the oxygen concentration is preferably higher than 5% by volume. In the case of an oxidizing atmosphere, it is also possible to use an atmosphere containing only oxygen, so the upper limit of the oxygen concentration can be set to 100% by volume or less.

In the oxidizing atmosphere, the components other than oxygen are preferably rare gases or inert gases such as nitrogen gas. In this case, the oxygen concentration in the oxidizing atmosphere can be adjusted by controlling the mixture atmosphere with the inert gas.

The pH value of the aqueous solution before the reaction and the aqueous reaction solution described below can be measured with a pH meter, and the ammonium ion concentration can be measured with an ion meter.
(Nucleation process)
In the nucleation step, the raw material aqueous solution, and if necessary, a basic aqueous solution and a complexing agent can be supplied to and mixed with the reaction vessel.

In the nucleation step, the raw material aqueous solution can be supplied to the pre-reaction aqueous solution prepared in the pre-reaction aqueous solution preparation step while stirring the pre-reaction aqueous solution. This forms an aqueous solution for nucleation, which is the aqueous reaction solution in the nucleation step, in the reaction tank.

The pH value of the nucleation aqueous solution is preferably 12.0 or more and 14.0 or less. By setting the pH value in this range, nuclei hardly grow during the nucleation process, and nucleation occurs preferentially.

In the nucleation process, the pH value of the nucleation aqueous solution and the concentration of ammonium ions change as nucleation occurs. This allows the basic aqueous solution and complexing agent to be supplied at the appropriate time. It is therefore preferable to control the pH value of the nucleation aqueous solution in the reaction tank to be in the range of pH 12.0 to 14.0, based on a liquid temperature of 25°C, and to maintain the ammonium ion concentration in the range of 3 g/L to 25 g/L.

In the nucleation process, new nuclei are continuously generated by supplying the raw material aqueous solution, the basic aqueous solution, and the complexing agent to the nucleation aqueous solution. The nucleation process ends when a predetermined amount of nuclei is generated in the nucleation aqueous solution. The amount of nuclei generated can be determined from the amount of metal compounds contained in the raw material aqueous solution supplied to the nucleation aqueous solution.

(2-1-2) Particle Growth Step The particle growth step can have the following first to third crystallization steps.
(First crystallization step (S12))
In the first crystallization step, a raw material aqueous solution containing at least nickel and manganese, an additive aqueous solution containing element A, a complexing agent, and a basic aqueous solution can be supplied to a reaction aqueous solution in a reaction tank which has an oxidizing atmosphere and which contains an aqueous reaction solution containing nuclei for particle growth.

During the particle growth process, the pH value of the aqueous solution for particle growth is preferably adjusted to 10.5 or more and 12.0 or less at a liquid temperature of 25°C. Therefore, when starting the first crystallization process, it is preferable that the pH value of the reaction aqueous solution containing nuclei for particle growth is adjusted to the above range.

Therefore, for example, the pH value of the nucleation aqueous solution obtained in the nucleation step described above can be adjusted to prepare a particle growth aqueous solution, which is a reaction aqueous solution, before starting the first crystallization step. The pH value of the nucleation aqueous solution can be adjusted by adding an acid to the nucleation aqueous solution or by stopping the supply of the basic aqueous solution added in the nucleation step. When adjusting the pH value of the reaction aqueous solution by adding an acid to the nucleation aqueous solution, it is preferable to use an inorganic acid of the same type as the acid used to form the metal compound when preparing the raw aqueous solution, from the viewpoint of suppressing the inclusion of impurities. For example, when a sulfate salt is used when preparing the raw aqueous solution, it is preferable to use sulfuric acid as the acid for adjusting the pH value of the reaction aqueous solution.

When adjusting the pH value of the aqueous solution for nucleation, it is preferable to stop the supply of all aqueous solutions added to the aqueous solution for nucleation in the nucleation step, adjust the pH value to prepare an aqueous solution for particle growth, and then start the particle growth step. By temporarily stopping the supply of all aqueous solutions when adjusting the pH, it is possible to increase the uniformity of the particle size of the resulting composite hydroxide particles, which is preferable.

As mentioned above, if the aqueous reaction solution containing the nuclei to be subjected to the particle growth process is prepared in advance, it is not necessary to carry out the nucleation process, and the pH value of the aqueous reaction solution containing the nuclei can be adjusted to a predetermined range and subjected to the first crystallization process.

In the composite hydroxide obtained by the method for producing a composite hydroxide of this embodiment, it is preferable that the abundance ratio of element A in the center portion is lower than the abundance ratio of element A in the outer shell portion.

However, it is preferable that the center portion also contains a certain percentage of element A, and since the size of the center portion is usually smaller than the size of the outer shell portion, it is preferable that element A is contained in a high concentration in the center portion. In order to contain element A in a high concentration in the center portion, it is necessary to add a solution containing element A in the first crystallization step of the particle growth step. By increasing the amount of solution added, the amount of added element in the sparse center portion can be increased.
(Second crystallization step (S13))
In the second crystallization step, the atmosphere in the reaction tank can be switched to a non-oxidizing atmosphere.

The sparse central portion and dense outer shell portion of the composite hydroxide particles are formed by controlling the pH, ammonia concentration, and atmosphere in the reaction tank during the particle growth process. In simple terms, the sparse central portion and dense outer shell portion are formed by the following process. For example, in the first crystallization process of the particle growth process in an oxidizing atmosphere, a region of low-density particles with many voids formed by fine primary particles is formed. On the other hand, in the particle growth process after the second crystallization process in which the weakly oxidizing atmosphere is changed to a non-oxidizing atmosphere, a region of large, dense, high-density particles is formed by the primary particles. That is, by making the nucleation process and the initial part of the particle growth process an oxidizing atmosphere, a central portion containing fine primary particles is formed. By subsequently switching the particle growth process from an oxidizing atmosphere to an atmosphere in the range of weakly oxidizing to non-oxidizing, a particle structure can be formed having an outer shell portion containing plate-like primary particles larger than the fine primary particles outside the central portion. The central portion may be composed of the fine primary particles, and the outer shell portion may be composed of the plate-like primary particles.

Here, a non-oxidizing atmosphere means an atmosphere in which the oxygen concentration in the reaction atmosphere is sufficiently low; specifically, for example, the oxygen concentration is preferably 5% by volume or less, more preferably 2% by volume or less, and even more preferably 1% by volume or less.

In the non-oxidizing atmosphere, the components other than oxygen are preferably inert gases such as rare gases and nitrogen gas. In this case, the oxygen concentration in the non-oxidizing atmosphere can be adjusted by controlling the mixed atmosphere of the inert gas. By making the atmosphere in the reaction tank in the second crystallization step and the third crystallization step described below a non-oxidizing atmosphere, unnecessary oxidation of the crystallized product can be suppressed while the nuclei generated in the nucleation step can be grown to a certain extent, and the outer shell can have a structure in which plate-like primary particles with a narrow particle size distribution are aggregated.

In the second crystallization step, after the atmosphere in the reaction tank is switched to a non-oxidizing atmosphere, a raw material aqueous solution containing at least nickel and manganese can be supplied to the reaction aqueous solution in the reaction tank. When the raw material aqueous solution is added to the aqueous solution for particle growth, the pH value and ammonia concentration of the aqueous solution for particle growth also change, so a complexing agent and a basic aqueous solution can also be added to the aqueous solution for particle growth.

However, in the second crystallization step, it is preferable not to add an aqueous solution containing element A. By supplying the raw material aqueous solution in the second crystallization step in this way and not adding an aqueous solution containing element A, the abundance ratio of element A on the inner periphery of the outer shell can be suppressed, and the abundance ratio of element A in the outer periphery can be made greater than the abundance ratio of element A in the outer shell.

The aqueous solution containing element A is preferably added mainly in the first crystallization step described above, which forms the central portion of the composite hydroxide particles, or in the third crystallization step, which forms the outside of the outer shell of the composite hydroxide particles. For this reason, if most of the raw aqueous solution added throughout the particle growth step is added in the second crystallization step, there is a risk that element A will not be sufficiently incorporated into the composite hydroxide particles. For this reason, when the total amount of metal substance in the raw aqueous solution supplied in the first crystallization step is M1, the total amount of metal substance in the raw aqueous solution supplied in the second crystallization step is M2, and the total amount of metal substance in the raw aqueous solution supplied in the third crystallization step is M3, it is preferable to satisfy the relationship of the following formula (2).

0≦M2/(M1+M2+M3)≦0.90...(2)
That is, the ratio of the total amount of metals in the raw aqueous solution supplied in the second crystallization step to the total amount of metals in the raw aqueous solution supplied during the particle growth step, such as nickel, manganese, and in some cases cobalt, is preferably 90% or less. By making the ratio of the total amount of metals in the raw aqueous solution supplied in the second crystallization step to the total amount of metals in the raw aqueous solution supplied during the particle growth step 90% or less, it is possible to incorporate almost all of the added element A into the composite hydroxide particles.

Since it is not necessary to add a raw material aqueous solution in the second crystallization step, the above M2/(M1+M2+M3) can be set to 0 or more.
(Third crystallization step (S14))
In the third crystallization step, a raw material aqueous solution containing at least nickel and manganese, an additive aqueous solution containing element A, a complexing agent, and a basic aqueous solution can be supplied into a reaction tank.

The third crystallization step can be carried out subsequent to the second crystallization step, with the atmosphere in the reaction tank being a non-oxidizing atmosphere. By carrying out the third crystallization step, the outer shell of the metal hydroxide particles can be formed.

When the predetermined amount of raw material aqueous solution and additive aqueous solution containing element A have been supplied in the third crystallization process, the supply of all aqueous solutions is stopped and the particle growth process can be terminated.

(2-2) Aqueous solutions to be supplied Each aqueous solution that can be supplied in the above-mentioned nucleation step and particle growth step will be described. Although each aqueous solution may have a different composition within the range described below for each step, it is preferable to use the same aqueous solution from the viewpoints of productivity and suppressing the inclusion of impurities.

(Raw material aqueous solution)
The ratio of the metal elements in the raw aqueous solution is approximately the same as the composition ratio of the resulting composite hydroxide particles. Therefore, it is preferable to appropriately adjust the content of each metal element in the raw aqueous solution according to the composition of the desired composite hydroxide particles.

For example, when trying to obtain composite hydroxide particles having the composition already described, it is preferable to adjust the ratio of the amounts of metal elements in the raw material aqueous solution to Ni:Mn:Co = x:y:z. In the above case, x, y, and z satisfy x + y + z = 1, 0.3 ≤ x ≤ 0.95, 0.05 ≤ y ≤ 0.55, and 0 ≤ z ≤ 0.4, as already described.

The raw aqueous solution can be prepared, for example, by adding a metal compound to water. There are no particular limitations on the type of metal compound used to prepare the raw aqueous solution, but from the perspective of ease of handling, it is preferable to use one or more types selected from water-soluble nitrates, sulfates, hydrochlorides, etc. From the perspective of cost and preventing the inclusion of halogens, it is particularly preferable to use sulfates as the metal compound.

The concentration of the raw material aqueous solution, in terms of the total metal compounds, is preferably 0.5 mol/L or more and 2.0 mol/L or less, and more preferably 1.0 mol/L or more and 1.5 mol/L or less.

By setting the concentration of the raw material aqueous solution to 0.5 mol/L or more, the amount of crystallized material per reaction tank can be sufficiently increased, improving productivity. On the other hand, by setting the concentration of the raw material aqueous solution to 2.0 mol/L or less, it is possible to prevent the crystals of each metal compound from re-precipitating, and more reliably prevent the clogging of pipes, etc.

The raw aqueous solution can contain at least nickel and manganese as described above, but it is not necessary to prepare a single raw aqueous solution. For example, when mixing multiple metal compounds, if certain metal compounds react with each other to produce unnecessary compounds, raw aqueous solutions can be prepared for each type of metal compound. Then, the prepared individual raw aqueous solutions may be added simultaneously to the pre-reaction aqueous solution or the reaction aqueous solution in a predetermined ratio. In this case, it is preferable to adjust and add the raw aqueous solutions so that the total concentration of the metal compounds in the individually added raw aqueous solutions is within the concentration range of the raw aqueous solutions described above.

The amount of the raw aqueous solution supplied during the particle growth step is not particularly limited, but it can be supplied so that the concentration of the product in the aqueous solution for particle growth at the end of the particle growth step is preferably 30 g/L or more and 200 g/L or less, more preferably 80 g/L or more and 150 g/L or less. By setting the product concentration to 30 g/L or more, the aggregation of primary particles is sufficiently promoted, and composite hydroxide particles having the desired structure can be obtained with a good yield. Furthermore, by setting the product concentration to 200 g/L or less, the raw aqueous solution can be sufficiently diffused in the reaction tank, and particularly uniform particle growth can be promoted.
(Additive aqueous solution containing element A)
As described above, the composite hydroxide particles can contain the element A. Therefore, in the particle growth step, an additive aqueous solution containing the element A can be added to the reaction aqueous solution.

As described above, element A may be one or more selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, La, Hf, Ta, and W. For this reason, a water-soluble compound containing the element A to be added can be preferably used as a metal compound for preparing an aqueous solution containing element A.

Suitable examples of water-soluble compounds containing element A include magnesium sulfate, calcium sulfate, aluminum sulfate, titanium sulfate, ammonium peroxotitanate, potassium titanium oxalate, vanadium sulfate, ammonium vanadate, chromium sulfate, potassium chromate, zirconium sulfate, niobium oxalate, ammonium molybdate, hafnium sulfate, sodium tantalate, sodium tungstate, and ammonium tungstate.
It is particularly preferable that the added aqueous solution containing element A has a high concentration. This is because by increasing the concentration of element A, when an aqueous solution containing element A is added in the first crystallization step or the third crystallization step described above, element A precipitates in a short time in the reaction aqueous solution, making it easier to arrange element A in a desired region of the particle. For this reason, it is preferable that the aqueous solution containing element A has a concentration of element A close to the solubility of element A, for example. However, it is preferable that the concentration of element A is equal to or lower than the solubility of element A so that element A does not precipitate in the aqueous solution containing element A.
When element A is tungsten, for example, the concentration of tungsten is preferably 1.37 mol/L or less, which is the solubility of tungsten at room temperature (25° C.), and is preferably, for example, 0.5 mol/L or more and 1.0 mol/L or less.

The additive aqueous solution containing element A can be mixed with the raw material aqueous solution described above, but as described above, it is preferable not to add an aqueous solution containing element A in, for example, the second crystallization step. For this reason, it is preferable to prepare an aqueous solution separate from the raw material aqueous solution and supply it to the reaction tank.

The element A is preferably tungsten. When the element A is tungsten, the water-soluble compound containing the element A may be, for example, sodium tungstate or ammonium tungstate as described above, but sodium tungstate is preferred.

When element A is tungsten, the total amount of tungsten contained in the additive aqueous solution supplied in the first crystallization step is W1, and the total amount of tungsten contained in the additive aqueous solution supplied in the third crystallization step is W3.

When the total amount of tungsten contained in the additive aqueous solution supplied to the reaction aqueous solution during the first crystallization step to the third crystallization step is W, it is preferable that the relationship between the following formula (3) and formula (4) is satisfied.

0.05≦W1/W<0.5...(3), 0.5≦W3/W<0.95...(4)
This is because, by satisfying the relationship between formula (3) and formula (4), the proportion of tungsten arranged on the outer periphery of the outer shell of the obtained composite hydroxide particles can be increased, and when a positive electrode active material obtained using the composite hydroxide particles is used in a lithium ion secondary battery, the output characteristics can be particularly improved.

(Basic aqueous solution)
The basic aqueous solution for adjusting the pH value in the reaction aqueous solution is not particularly limited and various basic aqueous solutions can be used, for example, an aqueous solution containing one or more selected from common alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, etc. The alkali metal hydroxide can be added directly to the reaction aqueous solution, but it is preferable to add it as an aqueous solution from the viewpoint of ease of pH control.

When an alkali metal hydroxide aqueous solution is used as the basic aqueous solution, the concentration of the alkali metal hydroxide aqueous solution is preferably 20% by mass or more and 50% by mass or less, and more preferably 20% by mass or more and 30% by mass or less.

By setting the concentration of the alkali metal aqueous solution within the above range, it is possible to prevent the pH value from becoming locally high at the addition position while suppressing the amount of solvent (amount of water) supplied to the reaction tank. This makes it possible to efficiently obtain composite hydroxide particles with a narrow particle size distribution.

The method of supplying the basic aqueous solution is not particularly limited, so long as the pH value of the reaction aqueous solution does not become locally high and is maintained within a specified range. For example, the reaction aqueous solution can be supplied using a pump capable of flow control, such as a metering pump, while thoroughly stirring the reaction aqueous solution.

(Complexing Agent)
The complexing agent is not particularly limited, but for example, an aqueous solution containing an ammonium ion donor can be preferably used.

As an aqueous solution containing an ammonium ion donor, for example, an aqueous solution containing one or more selected from ammonia water, ammonium sulfate, ammonium chloride, ammonium carbonate, ammonium fluoride, etc. can be used.

When ammonia water is used as the aqueous solution containing the ammonium ion donor, the ammonia concentration is not particularly limited, but is preferably 20% by mass or more and 30% by mass or less, and more preferably 22% by mass or more and 28% by mass or less. By setting the concentration of the ammonia water in this range, it is possible to minimize the loss of ammonia due to volatilization, etc.

The method of supplying the complexing agent is not particularly limited, but it can be supplied, for example, by a pump capable of controlling the flow rate, in the same manner as the basic aqueous solution.

(2-3) pH Value of Aqueous Reaction Solution The pH value of the aqueous reaction solution in the nucleation step and particle growth step of the method for producing composite hydroxide particles of this embodiment is not particularly limited, and can be arbitrarily selected depending on the reaction in each step.

The pH value at a standard liquid temperature of 25°C is preferably set in the range of 12.0 to 14.0 in the nucleation process. In the particle growth process, it is preferably controlled in the range of 10.5 to 12.0.

In each process, it is preferable that the pH value fluctuation range during the crystallization reaction is small, for example, within ±0.2. By keeping the pH value fluctuation range within the above range, it is possible to suppress fluctuations in the amount of nucleation and the rate of particle growth, and to obtain composite hydroxide particles with a particularly narrow particle size distribution. The suitable pH value for each process is described in detail below.

(Nucleation process)
In the nucleation step, the pH value of the reaction aqueous solution (aqueous solution for nucleation) is preferably 12.0 or more and 14.0 or less, more preferably 12.3 or more and 13.5 or less, and even more preferably 12.5 or more and 13.3 or less, based on a liquid temperature of 25°C.

By setting the pH value of the reaction aqueous solution within the above range, it is possible to suppress the growth of nuclei and prioritize nucleation, and the nuclei generated in the nucleation process can be made particularly homogeneous and have a narrow particle size distribution.

Specifically, by setting the pH value to 12.0 or more, the progression of nucleus (particle) growth can be particularly suppressed. This allows the particle size of the resulting composite hydroxide particles to be particularly uniform, and the particle size distribution to be particularly narrow. Furthermore, by setting the pH value to 14.0 or less, it is particularly possible to prevent the nuclei generated from becoming excessively fine and the aqueous solution for nucleation from gelling.

(Particle growth process)
In the particle growth step, the pH value of the reaction aqueous solution (aqueous solution for particle growth) is preferably 10.5 or more and 12.0 or less, more preferably 11.0 or more and 12.0 or less, and even more preferably 11.5 or more and 12.0 or less, based on a liquid temperature of 25° C.

By setting the pH value of the reaction aqueous solution within the above range, the formation of new nuclei is suppressed and particle growth is prioritized, making it possible to obtain composite hydroxide particles that are particularly homogeneous and have a narrow particle size distribution.

Specifically, by setting the pH value to 10.5 or more, the ammonium ion concentration can be suppressed, and the solubility of metal ions can be suppressed, so that the rate of the crystallization reaction can be sufficiently promoted. Furthermore, the amount of metal ions remaining in the reaction aqueous solution can be suppressed, and productivity can be increased. In addition, by setting the pH value to 12.0 or less, the amount of nucleation during the particle growth process can be suppressed, and the particle size of the resulting composite hydroxide particles can be made uniform, so that the particle size distribution can be particularly narrow.

In addition, since the pH value of 12.0 is the boundary condition between nucleation and nucleus growth, it can be set as the condition of either the nucleation process or the particle growth process depending on the presence or absence of nuclei in the reaction aqueous solution. That is, if the pH value of the nucleation process is made higher than 12.0 to generate a large amount of nuclei, and then the pH value of the particle growth process is set to 12.0, a large amount of nuclei are present in the reaction aqueous solution, so particle growth occurs preferentially, and composite hydroxide particles with a narrow particle size distribution can be obtained. On the other hand, if the pH value of the nucleation process is set to 12.0, nuclei that grow do not exist in the reaction aqueous solution, so nucleation occurs preferentially, and by making the pH value of the particle growth process smaller than 12.0, the generated nuclei grow, and good composite hydroxide particles can be obtained. In either case, it is sufficient to control the pH value of the particle growth process 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 make the pH value of the particle growth process 0.5 or more lower than the pH value of the nucleation process, and more preferably to make it 1.0 or more lower.

(2-4) Ammonium ion concentration The ammonium ion concentration in the reaction aqueous solution is not particularly limited, but is preferably 3 g/L or more and 25 g/L or less, and more preferably 5 g/L or more and 20 g/L or less, in both the nucleation step and the particle growth step. It is preferable to suppress the fluctuation of the ammonium ion concentration within the above range, and more preferably to maintain it at a constant value.

Since ammonium ions function as a complexing agent in the reaction aqueous solution, the solubility of metal ions can be kept constant by setting the ammonium ion concentration to 3 g/L or more. In addition, gelation of the reaction aqueous solution can be suppressed, and composite hydroxide particles with uniform shape and particle size can be obtained.

On the other hand, by setting the ammonium ion concentration to 25 g/L or less, the solubility of metal ions is prevented from becoming excessively high, and the amount of metal ions remaining in the reaction aqueous solution is reduced, making it possible to more reliably obtain composite hydroxide particles of the desired composition.

As described above, it is preferable that the ammonium ion concentration during the crystallization reaction is suppressed from fluctuating and maintained at a constant value. By suppressing fluctuations in the ammonium ion concentration, fluctuations in the solubility of metal ions can be suppressed, and composite hydroxide particles with particularly uniform composition can be formed. For this reason, it is preferable to control the fluctuation range of the ammonium ion concentration to a certain range throughout the nucleation process and the particle growth process, and specifically, it is preferable to control the fluctuation range to within ±5 g/L.

(2-5) Temperature of Aqueous Reaction Solution The temperature of the aqueous reaction solution (reaction temperature) is preferably controlled to 20° C. or higher throughout the nucleation step and the particle growth step, and more preferably controlled in the range of 20° C. or higher and 60° C. or lower.

By setting the reaction temperature at 20°C or higher, the solubility of the reaction aqueous solution can be increased and excessive nucleation can be suppressed. This makes it easy to control the average particle size and particle size distribution of the resulting composite hydroxide particles.

The upper limit of the reaction temperature is not particularly limited, but it is preferable to set the reaction temperature at 60°C or less, since this suppresses the volatilization of ammonia and reduces the amount of aqueous solution containing an ammonium ion donor that is supplied to control the ammonium ion concentration in the reaction aqueous solution.

(2-6) Manufacturing Apparatus In the manufacturing method of composite hydroxide particles, it is preferable to use an apparatus that does not recover the product until the reaction is completed, for example, a batch reaction tank. With such an apparatus, unlike a continuous crystallizer that recovers the product by an overflow method, the growing particles are not recovered simultaneously with the overflow liquid, and composite hydroxide particles with a narrow particle size distribution can be easily obtained.

In addition, since it is necessary to control the reaction atmosphere during the crystallization reaction, it is preferable to use an apparatus capable of controlling the atmosphere, such as a sealed apparatus. With such an apparatus, the reaction atmosphere in the nucleation process and particle growth process can be appropriately controlled, so that composite hydroxide particles having the above-mentioned particle structure and a narrow particle size distribution can be easily obtained.

(3) Positive Electrode Active Material for Lithium Ion Secondary Battery The positive electrode active material of the present embodiment can contain lithium (Li), nickel (Ni), manganese (Mn), cobalt (Co), and element A (A) in a ratio of Li:Ni:Mn:Co:A=1+u:x:y:z:t in terms of the amount of substances.

The positive electrode active material of this embodiment can contain hexagonal lithium nickel manganese composite oxide particles.

The positive electrode active material of this embodiment can also be composed of the above-mentioned lithium nickel manganese composite oxide particles.

It is preferable that u, x, y, z, and t satisfy the relationships -0.05≦u≦0.50, x+y+z=1, 0.3≦x≦0.95, 0.05≦y≦0.55, 0≦z≦0.4, and 0<t≦0.1. The element A can be one or more elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, La, Hf, Ta, and W.

In addition, the positive electrode active material of this embodiment can be a mixture of transition metal composite hydroxide particles and a lithium compound that has been fired.

(3-1) Particle Structure The lithium nickel manganese composite oxide particles (hereinafter, simply referred to as "composite oxide particles") contained in the positive electrode active material of this embodiment may contain secondary particles formed by agglomeration of a plurality of primary particles. The secondary particles may have agglomerated portions of electrically conductive primary particles and hollow portions where no primary particles are present, which are dispersed. In such composite oxide particles, the hollow portions do not need to be formed partially, and may be formed over the entire area between the center and the outer shell. In addition, the aggregated portions may be in a state in which, for example, plate-like primary particles are aggregated to form a plurality of secondary particles connected together. In this embodiment, "electrically conductive" means that the high-density portions of the composite oxide particles are directly structurally connected to each other and can be electrically conductive.

In composite oxide particles having such a particle structure, the electrolyte penetrates into the interior of the secondary particles through the grain boundaries or hollow spaces between the primary particles. This allows lithium to be desorbed and inserted not only on the outer surface of the secondary particles, but also inside the secondary particles. In addition, since the composite oxide particles have many electrically conductive paths inside the secondary particles, the resistance inside the particles can be made sufficiently small. Therefore, when a secondary battery is constructed using the positive electrode active material of this embodiment containing such composite oxide particles, it is possible to significantly improve the output characteristics without impairing the capacity characteristics or cycle characteristics.

In addition, it is preferable that the compound containing element A and lithium is disposed in a portion selected from the surface layer and grain boundary of the composite oxide particle.

Generally, when the surface layer of the positive electrode active material is completely covered with a different compound, the movement (intercalation) of lithium ions is greatly restricted. As a result, the advantage of the high capacity of the composite oxide is generally eliminated. In contrast, in this embodiment, a compound containing element A and lithium having lithium conductivity is arranged at one or more locations selected from the surface layer and grain boundary of the composite metal oxide particle. Therefore, a lithium conduction path can be formed at the interface with the electrolyte. Therefore, the reaction resistance of the active material can be reduced and the output characteristics can be improved. In particular, when a compound containing element A and lithium is formed inside the hollow structure, it is possible to further improve the output characteristics.

As described above, element A preferably contains tungsten, and more preferably is tungsten. In other words, it is preferable that the composite oxide particles have a compound containing tungsten and lithium disposed in a portion selected from the surface layer and grain boundary of the particle.

Compounds containing tungsten and lithium have high lithium ion conductivity and therefore have the effect of promoting the movement of lithium ions. Therefore, the reaction resistance of the active material can be particularly reduced, and the output characteristics can be improved. Compounds containing tungsten and lithium are not particularly limited, and include, for example, Li ₂ WO ₄ , Li ₄ WO ₅ , Li ₂ W ₂ O _7, and any compound may be used.

(3-2) Composition As described above, the positive electrode active material of the present embodiment can contain lithium (Li), nickel (Ni), manganese (Mn), cobalt (Co), and element A (A) in a ratio of substance amounts of Li:Ni:Mn:Co:A=1+u:x:y:z:t.

The positive _{electrode active material of this embodiment can be expressed, for example, by the general formula (B): Li1+uNixMnyCozAtO2 + B . The} elements _u , x, y, z, t, and element A have already been described, and therefore will not be described here. B can be, for example, -0.05≦B≦0.25.

The value of u, which indicates the excess amount of lithium (Li), is preferably -0.05 or more and 0.50 or less, more preferably 0 or more and 0.50 or less, and even more preferably 0 or more and 0.35 or less. By adjusting the value of u within the above range, it is possible to improve the output characteristics and capacity characteristics of a secondary battery using this positive electrode active material as a positive electrode material. By setting the value of u to -0.05 or more, it is possible to suppress the positive electrode resistance of the secondary battery and improve the output characteristics. On the other hand, by setting the value of u to 0.50 or less, it is possible to increase the initial discharge capacity and suppress the positive electrode resistance.

Nickel (Ni) is an element that contributes to increasing the potential and capacity of a secondary battery, and the value of x, which indicates its content, is preferably 0.30 to 0.95, more preferably 0.30 to 0.90. By making the value of x 0.30 or more, it is possible to improve the capacity characteristics of a secondary battery using the positive electrode active material. On the other hand, by making the value of x 0.95 or less, it is possible to sufficiently secure the content of other elements and obtain the effects of adding other elements.

Manganese (Mn) is an element that contributes to improving thermal stability, and the value of y, which indicates its content, is preferably 0.05 to 0.55, more preferably 0.10 to 0.40. By setting the value of y to 0.05 or more, the thermal stability of a secondary battery using the positive electrode active material can be improved. On the other hand, by setting the value of y to 0.55 or less, it is possible to suppress the elution of Mn from the positive electrode active material even during high-temperature operation, and to improve the charge-discharge cycle characteristics.

Cobalt (Co) is an element that contributes to improving the charge-discharge cycle characteristics, and the value of z, which indicates the content of Co, is preferably 0 to 0.4, more preferably 0.10 to 0.35. By setting the value of z to 0.4 or less, the initial discharge capacity of a secondary battery using the positive electrode active material can be improved.

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

The value of t indicating the content of element A is preferably greater than 0 and equal to or less than 0.1, more preferably equal to or greater than 0.001 and equal to or less than 0.05. By setting the value of t to 0.1 or less, it is possible to sufficiently secure the metal element that contributes to the oxidation-reduction reaction (Redox reaction), thereby increasing the battery capacity.
The composition of the positive electrode active material shown above is merely an example, and does not limit the composition of the positive electrode active material when it has the above-mentioned structure.

(3-3) Lattice Constant In the positive electrode active material of this embodiment, the lattice constant a of the lithium nickel manganese composite oxide obtained by X-ray diffraction is preferably 2.585981 Å or more and 2.860002 Å or less.

The lattice constant a can be calculated by the Rietveld method or the like from the powder X-ray diffraction pattern measured for the positive electrode active material.

(4) Method for Producing Positive Electrode Active Material for Lithium Ion Secondary Battery The method for producing the positive electrode active material in this embodiment can include the following two steps.

A mixing process in which a transition metal complex compound and a lithium compound are mixed to form a lithium mixture.

A firing process in which the lithium mixture formed in the mixing process is fired.

The transition metal composite hydroxide particles described above can be used as the transition metal composite compound to be subjected to the mixing step, but the transition metal composite hydroxide particles can also be heat-treated in a heat treatment step before use. When such a heat treatment step is performed, the manufacturing method for the positive electrode active material of this embodiment can include three steps including the heat treatment step. Figure 2 shows a flow 20 of the manufacturing method for the positive electrode active material when it includes three steps.

The method for producing the positive electrode active material of this embodiment makes it easy to obtain the above-mentioned positive electrode active material, in particular the positive electrode active material represented by general formula (B).

Each process is explained below.

(Heat treatment step (S21))
In the method for producing a positive electrode active material of this embodiment, a heat treatment step may be optionally performed before the mixing step to convert the composite hydroxide particles into "heat-treated particles" and then mixed with a lithium compound. Here, the heat-treated particles include not only composite hydroxide particles from which excess moisture has been removed in the heat treatment step, but also transition metal composite oxide particles converted into oxides by the heat treatment step, or a mixture thereof.

The heat treatment process is a process in which the composite hydroxide particles are heated to 105°C or higher and 750°C or lower to remove excess moisture contained in the composite hydroxide particles. By carrying out the heat treatment before the mixing process in this way, it is possible to suppress variation in the composition of the resulting positive electrode active material.

The heat treatment temperature in the heat treatment step is not particularly limited, but can be, for example, 105°C or higher and 750°C or lower. By setting the heat treatment temperature at 105°C or higher, the excess moisture in the composite hydroxide particles can be sufficiently removed, and the variation in the composition of the positive electrode active material can be sufficiently suppressed. On the other hand, even if the heating temperature exceeds 750°C, the removal of the excess moisture is not promoted. For this reason, from the viewpoint of productivity, it is preferable that the temperature be 750°C or lower.

In the heat treatment process, it is sufficient to remove moisture to the extent that there is no variation in the atomic number of each metal component in the positive electrode active material or in the ratio of the number of Li atoms. Therefore, it is not necessary to convert all of the hydroxides contained in the composite hydroxide particles to oxides. However, in order to reduce the variation in the atomic number of each metal component and the ratio of the number of Li atoms, it is preferable to heat to 400°C or higher and convert all of the hydroxides contained in the composite hydroxide particles to oxides.

There are no particular limitations on the atmosphere in the heat treatment process. It is sufficient if it is a non-reducing atmosphere, but it may also be performed in an air stream for simplicity.

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

(Mixing step (S22))
The mixing step is a step of mixing a transition metal composite compound, which is a composite hydroxide particle or a heat-treated particle, with a lithium compound to obtain a lithium mixture.

In the mixing step, the ratio (Li/Me) of the sum of the amounts of substances of metal atoms other than lithium in the lithium mixture, specifically nickel, cobalt, manganese, and the additive element A (Me) to the amount of substance of lithium (Li) is preferably 0.95 or more and 1.5 or less, more preferably 1.0 or more and 1.5 or less, even more preferably 1.0 or more and 1.35 or less, and particularly preferably 1.0 or more and 1.2 or less. It is preferable to mix the transition metal complex compound and the lithium compound so as to obtain such a Li/Me ratio.

This is because the Li/Me ratio remains almost unchanged before and after the calcination process, so it is preferable to mix the composite hydroxide particles or heat-treated particles with the lithium compound so that the Li/Me ratio in the mixing process becomes the Li/Me of the desired positive electrode active material.

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

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

(Firing step (S23))
The calcination step is a step in which the lithium mixture obtained in the mixing step is calcined under predetermined conditions to diffuse lithium into the transition metal composite compound, thereby obtaining lithium nickel manganese composite oxide particles.

The fine primary particles, which are mainly contained in the low-density part with many voids in the transition metal complex compound, start sintering at a lower temperature than the plate-shaped primary particles, which are mainly contained in the high-density part. Compared to the high-density part containing the plate-shaped primary particles, the low-density part containing the fine primary particles shrinks to a greater extent. For this reason, the fine primary particles that make up the low-density part shrink toward the high-density part of the outer shell, where sintering proceeds more slowly, and a hollow part of appropriate size is formed.

However, the structure of the core may change depending on the composition of the composite hydroxide particles and the firing conditions. Therefore, it is preferable to conduct preliminary tests and then control the various conditions appropriately so that the core has the desired structure.

The furnace used in the firing step is not particularly limited, and may be any furnace capable of heating the lithium mixture in air or oxygen flow. 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 or continuous electric furnace can be suitably used. This also applies to the furnaces used in the heat treatment step and the calcination step described below.

The calcination temperature of the lithium mixture in the calcination step is not particularly limited, but is preferably, for example, 650°C or higher and 980°C or lower. By setting the calcination temperature at 650°C or higher, lithium can be sufficiently diffused in the transition metal complex compound, and excess lithium and unreacted transition metal complex compound can be prevented from remaining. In addition, the crystallinity of the resulting lithium nickel manganese complex oxide particles can be improved.

On the other hand, by setting the firing temperature to 980°C or less, sintering between the resulting lithium nickel manganese composite oxide particles can be suppressed, and abnormal particle growth and the associated generation of irregular coarse particles can be suppressed.

When attempting to obtain a positive electrode active material with the composition already described, it is preferable to set the firing temperature to 650°C or higher and 950°C or lower.

The rate of temperature rise in the firing process is not particularly limited, but is preferably, for example, 2°C/min to 10°C/min, and more preferably, 2.5°C/min to 10°C/min. During the firing process, it is preferable to maintain the temperature near the melting point of the lithium compound, for example, for 1 hour to 5 hours, and more preferably, for 2 hours to 5 hours. By maintaining the temperature near the melting point of the lithium compound, the transition metal complex compound and the lithium compound can be reacted more uniformly.

In the firing process, the holding time at the above firing temperature is not particularly limited, but is preferably 2 hours or more, and more preferably 4 hours or more and 24 hours or less. By holding the above firing temperature for 2 hours or more, lithium can be sufficiently diffused in the transition metal complex compound, and excess lithium and unreacted transition metal complex compound can be prevented from remaining. In addition, the crystallinity of the resulting lithium nickel manganese complex oxide particles can be particularly improved.

After the holding time is over, the cooling rate from the firing temperature to at least 200°C is preferably 2°C/min to 10°C/min, and more preferably 3°C/min to 7°C/min. By controlling the cooling rate within this range, it is possible to ensure productivity while preventing equipment such as saggers from being damaged by rapid cooling.

The firing atmosphere is preferably an oxidizing atmosphere, and more preferably an atmosphere with an oxygen concentration of 18% by volume or more and 100% by volume or less. The firing atmosphere is preferably a mixed atmosphere of oxygen with the above oxygen concentration and an inert gas.

The calcination is preferably carried out in air or an oxygen stream. By setting the oxygen concentration to 18% by volume or more, the crystallinity of the lithium nickel manganese composite oxide particles can be particularly improved.

The method for producing the positive electrode active material of this embodiment may further include an optional step.

(Pre-firing process)
When lithium hydroxide or lithium carbonate is used as the lithium compound, a calcination step may be further carried out after the mixing step and before the calcination step, in which the lithium mixture is calcined at a temperature lower than the calcination temperature described below and at 350° C. or higher and 800° C. or lower. The calcination temperature in the calcination step is more preferably 450° C. or higher and 780° C. or lower.

By carrying out the calcination process, lithium can be sufficiently diffused in the transition metal composite compound, and more uniform lithium nickel manganese composite oxide particles can be obtained.

The time for which the material is held at the calcination temperature is not particularly limited, but is preferably 1 hour or more and 10 hours or less, and more preferably 3 hours or more and 6 hours or less.

As in the firing process described above, the atmosphere in the calcination process is preferably an oxidizing atmosphere, and more preferably an atmosphere with an oxygen concentration of 18% by volume or more and 100% by volume or less.

(Crushing process)
The lithium nickel manganese composite oxide particles obtained by the firing process may be aggregated or lightly sintered. In such a case, it is preferable to crush the aggregates or sintered bodies of the lithium nickel manganese composite oxide particles. This allows the average particle size and particle size distribution of the obtained positive electrode active material to be adjusted to a suitable range. Note that crushing refers to an operation of applying mechanical energy to an aggregate consisting of a plurality of secondary particles generated by sintering necking between secondary particles during firing, separating the secondary particles themselves almost without destroying them, and loosening the aggregates.

As a crushing method, known means can be used, for example, a pin mill, a hammer mill, etc. In this case, it is preferable to adjust the crushing force within an appropriate range so as not to destroy the secondary particles.
(5) Lithium-Ion Secondary Battery The lithium-ion secondary battery (hereinafter also referred to as "secondary battery") of this embodiment can have a positive electrode containing the above-described positive electrode active material.

Hereinafter, an example of the configuration of the secondary battery of this embodiment will be described for each component. The secondary battery of this embodiment includes, for example, a positive electrode, a negative electrode, and a non-aqueous electrolyte, and is composed of the same components as a general lithium-ion secondary battery. Note that the embodiment described below is merely an example, and the lithium-ion secondary battery of this embodiment can be embodied in various forms that have been modified and improved based on the knowledge of those skilled in the art, including the following embodiment. In addition, the secondary battery is not particularly limited in its use.
(positive electrode)
The positive electrode of the secondary battery of this embodiment can contain the positive electrode active material described above.

An example of a method for manufacturing a positive electrode is described below. First, the positive electrode active material (powder form), conductive material, and binder are mixed to form a positive electrode mixture, and activated carbon or a solvent for viscosity adjustment or other purposes is added as necessary. The mixture is then kneaded to form a positive electrode mixture paste.

The mixing ratio of each material in the positive electrode mixture is a factor that determines the performance of the lithium ion secondary battery, and can be adjusted depending on the application. The mixing ratio of the materials can be the same as that of the positive electrode of a known lithium ion secondary battery. For example, if the total mass of the solid content of the positive electrode mixture excluding the solvent is taken as 100 mass%, the positive electrode active material can be contained in a ratio of 60 mass% to 95 mass%, the conductive material can be contained in a ratio of 1 mass% to 20 mass%, and the binder can be contained in a ratio of 1 mass% to 20 mass%.

The obtained positive electrode composite paste is applied to the surface of a current collector made of, for example, aluminum foil, and dried to evaporate the solvent, producing a sheet-like positive electrode. If necessary, pressure can be applied using a roll press or the like to increase the electrode density. The sheet-like positive electrode thus obtained can be cut to an appropriate size depending on the desired battery, and used to produce the battery.

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

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

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

The method for producing the positive electrode is not limited to the above-mentioned example, and other methods may be used. For example, the positive electrode may be produced by pressing a positive electrode mixture and then drying it in a vacuum atmosphere.
(Negative electrode)
The negative electrode may be made of metallic lithium, a lithium alloy, etc. The negative electrode may be made by mixing a binder with a negative electrode active material capable of absorbing and desorbing lithium ions, adding an appropriate solvent to make a paste-like negative electrode mixture, applying the mixture to the surface of a metal foil current collector such as copper, drying it, and compressing it to increase the electrode density as necessary.

As the negative electrode active material, for example, natural graphite, artificial graphite, sintered organic compounds such as phenol resin, and powder of carbon material such as coke can be used. In this case, as with the positive electrode, a fluorine-containing resin such as PVDF can be used as the negative electrode binder, and an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent for dispersing these active materials and binders.
(Separator)
A separator may be sandwiched between the positive electrode and the negative electrode as necessary. The separator separates the positive electrode and the negative electrode and holds the electrolyte, and any known separator may be used, such as a thin membrane made of polyethylene or polypropylene having many minute pores.
(Non-aqueous electrolyte)
As the non-aqueous electrolyte, for example, a non-aqueous electrolytic solution can be used.

As a non-aqueous electrolyte, for example, a solution in which a lithium salt as a supporting salt is dissolved in an organic solvent can be used. Alternatively, as a non-aqueous electrolyte, a solution in which a lithium salt is dissolved in an ionic liquid can be used. Note that an ionic liquid refers to a salt that is composed of cations and anions other than lithium ions and is liquid even at room temperature.

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

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

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

Examples of inorganic solid electrolytes include oxide-based solid electrolytes and sulfide-based solid electrolytes.

The oxide-based solid electrolyte is not particularly limited, and for example, one that contains oxygen (O) and has lithium ion conductivity and electronic insulation properties can be suitably used. Examples of oxide-based solid electrolytes include lithium phosphate (Li ₃ PO ₄ ), Li ₃ PO ₄ N _x , LiBO ₂ N _x , LiNbO ₃ , LiTaO ₃ , Li ₂ SiO ₃ , Li ₄ SiO ₄ -Li 3 PO ₄ , _{Li 4} SiO ₄ -Li ₃ VO ₄ , Li ₂ O-B ₂ O ₃ -P ₂ O ₅ , Li ₂ O-SiO ₂ , Li ₂ O-B ₂ O ₃ -ZnO, Li _1+X Al _X Ti _2-X (PO ₄ ) ₃ (0≦X≦1), Li _1+X Al _X Ge _2-X (PO ₄ ) ₃ (0≦ _X ≦1), _LiTi2 ( _PO4 ) ₃ , _{Li3XLa2 / 3- XTiO3 ( 0 ≦} X≦ ₂ / ₃ ) _{, Li5La3Ta2O12} , _Li7La3Zr2O12 , _{Li6BaLa2Ta2O12 , Li3.6Si0.6P0.4O4 , and the} like can be used.

The sulfide-based solid electrolyte is not particularly limited, and for example, one that contains sulfur (S) and has lithium ion conductivity and electronic insulation properties can be suitably used. As the sulfide-based solid electrolyte, for example, one or more types selected from Li ₂ S-P ₂ S ₅ , Li ₂ S-SiS ₂ , LiI-Li ₂ S- _{SiS 2} , LiI-Li ₂ S- _{P 2 S 5} , LiI _{-Li 2} S- _{B 2 S 3} , Li ₃ PO ₄ -Li ₂ S-Si 2 S, Li ₃ PO ₄ -Li ₂ S-SiS 2 , LiPO 4 -Li ₂ S-SiS, LiI-Li 2 S-P ₂ O ₅ , LiI-Li ₃ PO ₄ -P ₂ S ₅ and the like can be used.

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

The organic solid electrolyte is not particularly limited as long as it is a polymer compound exhibiting ion conductivity, and examples of the organic solid electrolyte that can be used include polyethylene oxide, polypropylene oxide, copolymers thereof, etc. The organic solid electrolyte may also contain a supporting salt (lithium salt).
(Shape and structure of secondary battery)
The lithium ion secondary battery of the present embodiment described above can be made into various shapes such as a cylindrical shape, a laminated shape, etc. Regardless of the shape, if the secondary battery of the present embodiment uses a non-aqueous electrolyte solution as the non-aqueous electrolyte, the positive electrode and the negative electrode are laminated via a separator to form an electrode body, the obtained electrode body is impregnated with a non-aqueous electrolyte solution, and the positive electrode current collector and the positive electrode terminal connected to the outside, and the negative electrode current collector and the negative electrode terminal connected to the outside are connected using a current collecting lead or the like, and the battery case is sealed to form a structure.

As described above, the secondary battery of this embodiment is not limited to a form using a nonaqueous electrolyte solution as the nonaqueous electrolyte, but can also be, for example, a secondary battery using a solid nonaqueous electrolyte, i.e., an all-solid-state battery. When making an all-solid-state battery, the configuration other than the positive electrode active material can be changed as necessary.

The following examples of the present invention will be described in more detail in comparison with comparative examples, but the present invention is not limited to these examples.

Unless otherwise specified, each sample of special grade reagents manufactured by Wako Pure Chemical Industries, Ltd. was used to prepare the composite hydroxide particles and the positive electrode active material. The pH value of the reaction solution was measured using a pH meter (HM-30P, manufactured by Toa DKK) throughout the nucleation process and the particle growth process. Based on this measurement, the supply amount of sodium hydroxide solution, which is a basic aqueous solution, was adjusted using a pH controller (NPM-690D, manufactured by Nisshin Rika) to control the fluctuation range of the pH value of the reaction solution in each process to within ±0.2. The ammonium ion concentration of the reaction solution was measured using an ion meter (Orion Star A324, manufactured by Thermo Fisher Scientific). Based on this measurement, the supply amount of ammonia water, which is a complexing agent, was adjusted to control the fluctuation range of the ammonium ion concentration of the reaction solution in each process to within ±5 g/L.

Example 1
(a) Production of composite hydroxide particles [Pre-reaction aqueous solution preparation step]
First, the temperature in the reaction tank was set to 40° C. while stirring the reaction tank containing 14 L of water at 300 rpm. At this time, oxygen gas was introduced into the reaction tank and circulated for 30 minutes, and the reaction atmosphere was made an oxidizing atmosphere with an oxygen concentration of more than 5% by volume. The reaction tank was kept in an oxidizing atmosphere until the first crystallization step was completed. Next, an appropriate amount of a 25% by mass aqueous solution of sodium hydroxide, which is a basic aqueous solution, and a 25% by mass aqueous ammonia, which is a complexing agent, were supplied into the reaction tank, and the pH value was adjusted to 12.5 based on a liquid temperature of 25° C., and the ammonium ion concentration was adjusted to 10 g/L to form a pre-reaction aqueous solution.

Nickel sulfate, manganese sulfate, and cobalt sulfate were dissolved in water so that the mass ratio of each metal element was Ni:Mn:Co = 40:30:30, to prepare a 2 mol/L raw material aqueous solution.

[Nucleation process]
Next, the raw material aqueous solution was fed to the pre-reaction aqueous solution at a rate of 10 mL/min to form an aqueous solution for nucleation, and nucleation was carried out for 1 minute. At this time, a 25% by mass aqueous solution of sodium hydroxide and a 25% by mass aqueous ammonia were fed in a timely manner to maintain the pH value and ammonium ion concentration of the aqueous solution for nucleation at the above-mentioned values.

[Particle growth process]
(First crystallization step)
After the nucleation was completed, the supply of all the aqueous solutions was stopped temporarily, and sulfuric acid was added to the reaction aqueous solution to adjust the pH value to 11.2 at a liquid temperature of 25° C., thereby forming an aqueous solution for particle growth. After it was confirmed that the pH value had reached a predetermined value, an aqueous tungsten compound solution, which was an additive aqueous solution containing element A, was supplied in addition to the raw material aqueous solution, and the nuclei (particles) generated in the nucleation step were allowed to grow. Note that sodium tungstate was used as the tungsten compound.

(Second crystallization step)
After the first crystallization step was completed, the supply of the raw material aqueous solution and the additive aqueous solution was stopped once in order to make the reaction tank into a non-oxidizing atmosphere. Then, oxygen gas was replaced by nitrogen gas, and the reaction atmosphere was made into a non-oxidizing atmosphere with an oxygen concentration of 2% by volume or less. The reaction tank was kept in a non-oxidizing atmosphere until the third crystallization step was completed.

(Third crystallization step)
After the second crystallization step was completed, the supply of the raw material aqueous solution and the additive aqueous solution was resumed. When the predetermined amounts of the raw material aqueous solution and the additive aqueous solution had been supplied in the third crystallization step, the supply of all the aqueous solutions was stopped, and the particle growth step was completed.

Here, the total amount of substance of metals in the raw aqueous solution supplied in the first crystallization step is M1, the total amount of substance of metals in the raw aqueous solution supplied in the second crystallization step is M2, and the total amount of substance of metals in the raw aqueous solution supplied in the third crystallization step is M3. In this case, in Example 1, the raw aqueous solution was supplied in each step so that the supply ratio of the raw aqueous solution in each crystallization step satisfied the following formula:
M1/(M1+M2+M3)=0.04
M2/(M1+M2+M3)=0
M3/(M1+M2+M3)=0.96
Also, the total amount of substance of tungsten, which is element A, in the additive aqueous solution supplied in the first crystallization step is W1, the total amount of substance of tungsten in the additive aqueous solution supplied in the second crystallization step is W2, and the total amount of substance of tungsten in the additive aqueous solution supplied in the third crystallization step is W3. In this case, in Example 1, the additive aqueous solution was supplied in each step so that the supply ratio of the additive aqueous solution in each crystallization step satisfied the following formula.

When the total amount of tungsten contained in the additive aqueous solution supplied to the reaction aqueous solution from the first crystallization step to the third crystallization step is W, W=W1+W2+W3.
W1/(W1+W2+W3)=0.09
W2/(W1+W2+W3)=0
W3/(W1+W2+W3)=0.91
After the third crystallization step, the obtained product was washed with water, filtered and dried to obtain powdered composite hydroxide particles. The first crystallization step formed the center of the composite hydroxide particles, the second crystallization step formed the inside of the outer shell of the composite hydroxide particles, and the third crystallization step formed the outside of the outer shell. Therefore, by supplying the additive aqueous solution at the above ratio, the abundance ratio of tungsten in the center (t _h ) was 9%, which corresponds to the proportion added in the first crystallization step, and the abundance ratio of tungsten in the outer shell (t _s ) was 91%, which corresponds to the proportion added in the second and third crystallization steps.

The same can be said for the other examples and comparative examples below.

For example, in the composite hydroxide obtained in Example 2, the abundance ratio of tungsten in the center (t _h ) was 27%, which corresponds to the proportion added in the first crystallization step, and the abundance ratio of tungsten in the outer shell (t _s ) was 73%, which corresponds to the proportion added in the second crystallization step and the third crystallization step.

In the composite hydroxide obtained in Example 3, the proportion of tungsten in the center (t _h ) was 45%, which corresponds to the proportion added in the first crystallization step, and the proportion of tungsten in the outer shell (t _s ) was 55%, which corresponds to the proportion added in the second crystallization step and the third crystallization step.

Examples 4 to 6 have the same results as Example 1.

The feed ratio of the raw material aqueous solution in each crystallization process and the feed ratio of the added aqueous solution in each crystallization process calculated using the above formula are shown in Table 1.

During the particle growth process, 25% by mass of sodium hydroxide aqueous solution and 25% by mass of ammonia water were supplied at appropriate times to maintain the pH value and ammonium ion concentration of the aqueous solution for particle growth at the values mentioned above.

(b) Evaluation of Composite Hydroxide Particles (b-1) Cross-Sectional Structure The composite hydroxide particles obtained were embedded in a resin and processed with a cross-section polisher to enable cross-section observation of the particles, which was then observed with an SEM.

As a result, it was confirmed that the core of the composite hydroxide particle was composed of fine primary particles, the outer shell arranged outside the core was composed of plate-like primary particles larger than the core, and the primary particles were densely arranged. It was also confirmed that the composite hydroxide particles obtained in the following Examples 2 to 6 had a similar structure.

(b-2) Composition Analysis By analysis using an ICP emission spectrometer (Shimadzu Corporation, ICPE- _9000ICPE -9000), it was confirmed that the composite hydroxide particles were represented _by the general formula: _{Ni0.40Mn0.30Co0.30W0.006} (OH) _{2 .} The results are shown in Table 2.

(b-3) Distribution Index The average particle size, _d10 , and _d90 of the composite hydroxide particles were calculated from the particle size distribution obtained using a laser light diffraction scattering particle size analyzer, and the distribution index, which is an index showing the spread of the particle size distribution, was calculated using the above-mentioned formula (1). The results are shown in the "( _D90 - _D10 )/average particle size" column in Table 2.

(b-4) Tungsten concentration Analysis was performed using a scanning transmission electron microscope (HD-2300A manufactured by Hitachi High-Technologies Corporation) equipped with EDX. It was confirmed that the tungsten concentration in the sparse central portion was 1.2 mass%, and the tungsten concentration in the dense outer shell portion was 1.23 mass%. The evaluation results are shown in Table 3.

(c) Preparation of Positive Electrode Active Material (Mixing Step)
The composite hydroxide particles obtained as described above were thoroughly mixed with lithium carbonate using a shaker mixer (TURBULA Type T2C manufactured by WAB) so that the Li/Me ratio was 1.10, thereby obtaining a lithium mixture.

(Firing process)
This lithium mixture was heated to 950° C. at a heating rate of 2.5° C./min in an air flow (oxygen concentration: 21% by volume), sintered by holding at this temperature for 4 hours, and then cooled to room temperature at a cooling rate of about 4° C./min.

(Crushing process)
The positive electrode active material obtained after the firing step had agglomerated or slightly sintered, so the positive electrode active material was crushed to adjust the average particle size and particle size distribution.

(d) Evaluation of Positive Electrode Active Material (Composition Analysis)
_Analysis using an ICP optical emission spectrometer _confirmed that this positive _electrode active material was represented by _the general formula _{Li1.1Ni0.40Mn0.30Co0.30W0.006O2 .}

(Lattice constant)
Powder X-ray diffraction measurement was performed on the obtained positive electrode active material. The measurement was performed using an X-ray diffractometer (Spectris X' Pert PRO) using CuKα radiation (wavelength 1.54059 Å) as radiation. A graphite single crystal monochromator was used to monochromatize the X-rays, and the measurement was performed with the applied voltage set to 40 kV and the current set to 40 mA. The measurement was performed at a scanning speed of 5°/min and recorded in the angle range of 10° to 100° (2θ).

From the obtained X-ray diffraction pattern, it was confirmed that the lithium nickel manganese composite oxide contained in the positive electrode active material is a cubic crystal. The lattice constant of the a-axis length of the lithium nickel manganese composite oxide was calculated to be 2.86002 Å. The results are shown in Table 4.

(e) Fabrication of Lithium Ion Secondary Battery Using the obtained positive electrode active material, a lithium ion secondary battery 30 as shown in FIG. 3, which is CR2032 (IEC/JIS standard), that is, a 2032 type coin battery, was fabricated.

The lithium ion secondary battery 30 is composed of a case 31 and an electrode 32 housed within this case 31.

The case 31 has a hollow positive electrode can 311 with one end open, and a negative electrode can 312 that is placed in the opening of the positive electrode can 311. When the negative electrode can 312 is placed in the opening of the positive electrode can 311, a space is formed between the negative electrode can 312 and the positive electrode can 311 to accommodate the electrode 32.

The electrode 32 is composed of a positive electrode 321, a separator 322, and a negative electrode 323, which are stacked in this order and housed in the case 31 so that the positive electrode 321 contacts the inner surface of the positive electrode can 311 and the negative electrode 323 contacts the inner surface of the negative electrode can 312.

The case 31 is provided with a gasket 313, which restricts relative movement between the positive electrode can 311 and the negative electrode can 312 and fixes them in place so that they are not in contact with each other, i.e., are kept electrically insulated. The gasket 313 also seals the gap between the positive electrode can 311 and the negative electrode can 312, providing an airtight and liquid-tight barrier between the inside of the case 31 and the outside.

This lithium ion secondary battery 30 was produced as follows. First, 52.5 mg of the obtained positive electrode active material, 15 mg of acetylene black, and 7.5 mg of PTFE were mixed, and the mixture was press-molded at a pressure of 100 MPa to a diameter of 11 mm and a thickness of 100 μm. The mixture was then dried in a vacuum dryer at 120° C. for 12 hours to produce the positive electrode 321.

Using this positive electrode 321, negative electrode 323, separator 322, and electrolyte, a lithium ion secondary battery 30 was fabricated in a glove box with an Ar atmosphere whose dew point was controlled at -80°C.

A lithium metal having a diameter of 17 mm and a thickness of 1 mm was used for the negative electrode 323. An equal mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with _1M LiClO4 as a supporting electrolyte (manufactured by Toyama Pharmaceutical Co., Ltd.) was used for the electrolyte.

A polyethylene porous film with a thickness of 25 μm was used for the separator 322.

(f) Battery Evaluation [Reaction Resistance]
The resistance value of the assembled coin battery was measured by an AC impedance method at SOC 80%. SOC refers to the state of charge of the battery, and SOC 80% indicates a state where the battery is charged to 80% of the full charge capacity, which is 100%.

Then, the resistance value of the lithium ion secondary battery produced in Comparative Example 1 described below at SOC 20% was set as the reference standard (Ref.), i.e., 1, and the index value of SOC 80% evaluated in Example 1 was calculated.

The index value of reaction resistance at SOC 80% was 0.85. The results are shown in Table 5.
(Examples 2 to 6)
Composite hydroxide particles, positive electrode active materials, and lithium ion secondary batteries were produced and evaluated in the same manner as in Example 1, except that the supply ratio of the raw material aqueous solution in each crystallization step in the particle growth process and the supply ratio of the additive aqueous solution in each crystallization step were set to the values shown in Table 1. The results are shown in Tables 2 to 5.

Specifically, in Examples 2 to 3, the supply ratio of the additive aqueous solution in the first crystallization step and the third crystallization step was changed compared to Example 1.

In addition, in Examples 4 to 6, the supply ratio of the raw material aqueous solution in the second crystallization step and the third crystallization step was changed compared to Example 1.

In Examples 4 to 6, the raw material aqueous solution is supplied in the second crystallization step in which the inside of the outer shell is formed. Therefore, the inside of the outer shell includes a region that contains almost no tungsten. Therefore, the proportion of tungsten in the outer periphery of the outer shell formed in the third crystallization step is greater than the proportion of tungsten in the entire outer shell.
(Comparative Example 1 and Comparative Example 2)
Composite hydroxide particles, positive electrode active materials, and lithium ion secondary batteries were produced and evaluated in the same manner as in Example 1, except that the supply ratio of the raw material aqueous solution in each crystallization step in the particle growth process and the supply ratio of the additive aqueous solution in each crystallization step were set to the values shown in Table 1. The results are shown in Tables 2 to 5.

表５の結果から、所定の粒子構造を有し、中心部における元素Ａの存在割合（ｔ_ｈ）が、外殻部における元素Ａの存在割合（ｔ_ｓ）よりも小さい複合水酸化物粒子を前駆体として製造した正極活物質を適用したリチウムイオン二次電池において、反応抵抗を抑制できることを確認できた。すなわち出力特性を向上できることを確認できた。

From the results in Table 5, it was confirmed that in a lithium ion secondary battery using a positive electrode active material produced using a composite hydroxide particle precursor having a predetermined particle structure and in which the abundance ratio (t _h ) of element A in the center is smaller than the abundance ratio (t _s ) of element A in the outer shell, reaction resistance can be suppressed, i.e., output characteristics can be improved.

Claims (9)

The alloy contains nickel (Ni), manganese (Mn), cobalt (Co), and an element A (A) in a ratio of amounts of substances of Ni:Mn:Co:A=x:y:z:t (x+y+z=1, 0.3≦x≦0.95, 0.05≦y≦0.55, 0≦z≦0.4, 0<t≦0.1, and the element A is tungsten ),
a core portion including primary particles and an outer shell portion disposed outside the core portion and in which the primary particles are more densely arranged than in the core portion;
the abundance ratio (t _h ) of the element A in the central portion is smaller than the abundance ratio (t _s ) of the element A in the outer shell portion;
The transition metal composite hydroxide particle has a tungsten concentration in the center portion of 0.1% by mass or more and 9.0% by mass or less. 2. The transition metal composite hydroxide particle according to claim 1, wherein the abundance ratio (t _s' ) of the element A in the outer periphery of the outer shell is greater than the abundance ratio (t _s ) of the element A in the outer shell. 3. The transition metal composite hydroxide particles according to claim 1, having a distribution index, which indicates the spread of particle size distribution calculated by the following formula (1), of 0.60 or less.
(Distribution index) = [(d ₉₀ - _{d 10} )/average particle size]...(1) The transition metal composite hydroxide particle according to claim 1 , wherein a tungsten-concentrated layer having a thickness of 10 nm or more and 100 nm or less is formed on an outer surface of the outer shell portion. A method for producing transition metal composite hydroxide particles, comprising the steps of:
The transition metal composite hydroxide particles contain nickel (Ni), manganese (Mn), cobalt (Co), and an element A (A) in a substance amount ratio of Ni:Mn:Co:A=x:y:z:t (x+y+z=1, 0.3≦x≦0.95, 0.05≦y≦0.55, 0≦z≦0.4, 0<t≦0.1, and the element A is tungsten ),
a first crystallization step of supplying a raw material aqueous solution containing at least nickel and manganese, an additive aqueous solution containing the element A, a complexing agent, and a basic aqueous solution to a reaction aqueous solution in a reaction tank that is in an oxidizing atmosphere and contains nuclei for particle growth;
A second crystallization step of switching the atmosphere in the reaction tank to a non-oxidizing atmosphere;
a third crystallization step of supplying into the reaction tank a raw material aqueous solution containing at least nickel and manganese, an additive aqueous solution containing the element A, a complexing agent, and a basic aqueous solution ;
The total amount of tungsten contained in the additive aqueous solution supplied in the first crystallization step is designated as W1,
The total amount of tungsten contained in the additive aqueous solution supplied in the third crystallization step is designated as W3,
When the total amount of tungsten contained in the additive aqueous solution supplied to the reaction aqueous solution during the first crystallization step to the third crystallization step is W,
A method for producing transition metal composite hydroxide particles , wherein 0.05≦W1/W<0.5 and 0.5≦W3/W<0.95 are satisfied . A calcined mixture of the transition metal composite hydroxide particles according to any one of claims 1 to 4 and a lithium compound,
The lithium (Li), nickel (Ni), manganese (Mn), cobalt (Co), and an element A (A) are contained in a ratio of amounts of substance of Li:Ni:Mn:Co:A=1+u:x:y:z:t (-0.05≦u≦0.50, x+y+z=1, 0.3≦x≦0.95, 0.05≦y≦0.55, 0≦z≦0.4, 0<t≦0.1, the element A being one or more elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, La, Hf, Ta, and W),
A positive electrode active material for lithium ion secondary batteries, comprising hexagonal lithium nickel manganese composite oxide particles. 7. The positive electrode active material for a lithium ion secondary battery according to claim 6 , wherein the lattice constant a of the lithium nickel manganese composite oxide obtained by X-ray diffraction is 2.585981 Å or more and 2.860002 Å or less. The positive electrode active material for lithium ion secondary batteries according to claim 6 or 7, in which a compound containing tungsten and lithium is disposed in a portion selected from the surface layer and grain boundary of the particles. A lithium ion secondary battery comprising a positive electrode containing the positive electrode active material for lithium ion secondary batteries according to any one of claims 6 to 8 .

Images