JP7464102B2 - Metal composite hydroxide and its manufacturing method, positive electrode active material for non-aqueous electrolyte secondary battery and its manufacturing method, and non-aqueous electrolyte secondary battery using the same - Google Patents

Description

The present invention relates to a metal composite hydroxide and a method for producing the same, a positive electrode active material for a non-aqueous electrolyte secondary battery and a method for producing the same, and a non-aqueous electrolyte secondary battery using the same.

In recent years, with the widespread use of portable electronic devices such as smartphones, tablet devices, and notebook computers, there is a strong demand for the development of small, lightweight nonaqueous electrolyte secondary batteries with high energy density. There is also a strong demand for the development of high-output secondary batteries as power sources for 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 other components, 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 metal composite oxides as the positive electrode active 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 the positive electrode active material for such lithium ion secondary batteries, lithium metal composite oxides 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 uses _manganese , have been proposed.

In order to further improve the output characteristics of lithium-ion secondary batteries, a method of increasing the specific surface area of the positive electrode active material is generally known. If the specific surface area of the positive electrode active material is increased, a sufficient reaction area with the electrolyte can be secured when the positive electrode active material is incorporated into a secondary battery. Therefore, several technologies have been proposed to improve the output characteristics by controlling the particle structure of the positive electrode active material.

For example, Patent Documents 1 to 3 propose a method for producing a positive electrode active material using a composite hydroxide obtained by a two-stage crystallization process as a precursor. The positive electrode active materials described in these patent documents are said to have a small particle size, a narrow particle size distribution, and a hollow structure or space inside the particles, resulting in a high specific surface area and excellent output characteristics. However, for example, as a power source for driving vehicles such as hybrid automobiles, a positive electrode active material with higher output characteristics is required.

On the other hand, as a means of further reducing the reaction resistance and realizing a positive electrode active material with higher output characteristics, the addition of a different element to the lithium metal composite oxide that constitutes the positive electrode active material is being considered. As such a different element, for example, transition metals that can assume high valence, such as Mo, Nb, W, and Ta, have been proposed.

For example, Patent Document 4 describes a lithium transition metal compound powder obtained by adding at least one additive that inhibits grain growth and sintering during firing to the main component raw material in a ratio of 0.01 mol % or more and less than 2 mol % based on the total molar amount of the transition metal elements in the main component raw material, and then firing the main component raw material. It also describes that the additive is an oxide containing at least one element selected from Mo, W, Nb, Ta, and Re, and that the atomic ratio of the total of the additive elements to the total of Li and metal elements other than the additive elements in the surface portion of the primary particles is 5 times or more the atomic ratio of the total of the additive elements to the total of the metal elements other than the additive elements in the surface portion of the primary particles.

Patent Document 5 also proposes a production method that includes a step of adding an alkaline aqueous solution in which a specific proportion of a tungsten compound is dissolved to a lithium metal composite oxide powder and mixing the powder to disperse W on the surfaces of the primary particles of the powder, and a step of 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 less than 700°C to form 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 surfaces of the lithium metal composite oxide powder or on the surfaces of the primary particles of the powder.

Patent Document 6 also proposes a positive electrode active material made of lithium metal composite oxide, which is made 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 the electrolyte can penetrate, and has a compound layer containing lithium with tungsten enriched on the surface or grain boundaries of the lithium metal composite oxide and having a layer thickness of 20 nm or less. A preferred method for obtaining the powder is to mix the composite hydroxide or composite oxide with a lithium compound, add a tungsten compound, mix, and sinter to obtain a lithium metal composite oxide, and in this method, it is preferable that the particle size of the tungsten compound is 1/5 or less of the average particle size of the manganese composite hydroxide or manganese composite hydroxide.

Patent Document 7 also proposes a method for producing a transition metal composite hydroxide, which includes a process for producing composite hydroxide particles by separating the nucleation process and the particle growth stage by pH control in a crystallization reaction, and a coating process for forming a tungsten-containing coating on the surface of the obtained composite hydroxide particles, and a positive electrode active material using the hydroxide as a precursor.

Patent Document 8 proposes a positive electrode for a lithium secondary battery in which a layer of a first positive electrode active material represented by Li _x1 Ni _a1 Mn _b1 Co _c1 O ₂ is applied to the surface of a current collector, and a layer of a second positive electrode active material represented by Li _x2 Ni _a2 Mn _b2 Co _c2 M _d O ₂ (M is Mo, W, or Nb) is applied to the surface of the first positive electrode active material.

Patent Document 9 also describes a method for producing a nickel-cobalt composite hydroxide, which includes a first crystallization step in which a solution containing nickel, cobalt, and manganese, a complex ion-forming agent, and a basic solution are separately and simultaneously supplied to one reaction vessel to obtain nickel-cobalt composite hydroxide particles, and after the first crystallization step, a solution containing nickel, cobalt, and manganese, a complex ion-forming agent, a basic solution, and a solution containing element M are separately and simultaneously supplied to the nickel-cobalt composite hydroxide particles to obtain nickel, cobalt, manganese, and element M (Al, Mg, Ca, The proposed manufacturing method includes a second crystallization step of crystallizing composite hydroxide particles containing at least one element selected from the group consisting of Ti, Zr, Nb, Ta, Cr, Mo, W, Fe, Cu, Si, Sn, Bi, Ga, Y, Sm, Er, Ce, Nd, La, Cd, and Lu, in which the total moles of nickel, cobalt, and manganese supplied in the first crystallization step are MOL(1), and the total moles of nickel, cobalt, and manganese supplied in the second crystallization step are MOL(2), and 0.30≦MOL(1)/{MOL(1)+MOL(2)}<0.95.

JP 2012-246199 A JP 2013-147416 A JP 2016-094307 A JP 2008-305777 A JP 2013-125732 A JP 2014-197556 A JP 2012-252844 A JP 2012-079608 A JP 2016-210674 A

However, the method described in Patent Document 4 has the problem that some of the added elements such as Mo, Ta, and W are substituted for Ni arranged in layers in the crystal, deteriorating battery characteristics such as battery capacity and cycle characteristics.

In addition, although the lithium metal composite oxide produced by the manufacturing method described in Patent Document 5 has improved output characteristics, if the thickness of the tungsten microparticles formed on the particle surface is not uniform, the cycle characteristics may be insufficient.

In addition, the method described in Patent Document 6 is not suitable for industrial use because it is necessary to crush the tungsten compound once to obtain a nano-order tungsten compound, and is also not easy to handle. In addition, the particle size of the obtained fine particles is non-uniform, which causes variation in the thickness of the compound layer, so that the reduction in reaction resistance is insufficient and the effect of improving output characteristics is limited. It is also described that tungsten is added as an additive element to manganese composite hydroxide to obtain the powder, but this method makes it difficult to form a compound layer in which tungsten is concentrated on the nano-order.

In addition, the method described in Patent Document 7 adds a step of coating tungsten, which increases the number of steps and is not industrially preferable. In addition, tungsten is precipitated by controlling the pH in the coating step, but since the thickness of the coating layer is not uniform due to fluctuations in the controlled pH, it is difficult to coat uniformly. In addition, when a composite oxide obtained by mixing a composite hydroxide non-uniformly coated with tungsten with lithium is used, there is a problem that the output is reduced because it becomes a resistor.

In addition, the positive electrode active material prepared by the method described in Patent Document 8 has a problem that, since the first positive electrode active material and the second positive electrode active material are different phases, when used in a secondary battery, there is a discrepancy in the structural changes that occur during the charge/discharge reaction, and cracks occur during repeated charge/discharge, deteriorating the cycle characteristics.

It is also described that the composite hydroxide and positive electrode active material obtained by the method described in Patent Document 9 have a peak for element M in the SEM-EDX spectrum in the second layer that exists at a depth ratio in the radial direction of 5% or more and less than 50%. Since element M, which contains tungsten, is contained inside the secondary particles from the surface layer in the positive electrode active material, the crystallinity is reduced, and there is a possibility that the battery characteristics will be deteriorated when the material is used in a secondary battery.

In view of these problems, the present invention aims to provide a positive electrode active material that, when used in the positive electrode of a secondary battery, reduces reaction resistance, provides higher output, and has high crystallinity, as well as a metal composite hydroxide that is its precursor.

In a first embodiment of the present invention, a metal composite hydroxide is provided which contains nickel, manganese, and tungsten, and optionally cobalt and an element M, and in which the atomic ratio of the respective metal elements is represented by Ni:Mn:Co:W:M=x:y:z:a:b (x+y+z=1, 0.3≦x≦0.95, 0.05≦y≦0.55, 0≦z≦0.4, 0<a≦0.1, 0≦b≦0.1, M is one or more elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, and Ta). The present invention relates to a method for producing a metal composite hydroxide particle, the method comprising: a first crystallization step of supplying a first raw material aqueous solution containing a metal element and an ammonium ion donor to a reaction tank, adjusting the pH of the reaction aqueous solution in the reaction tank, and carrying out a crystallization reaction to obtain first metal composite hydroxide particles; and a second raw material aqueous solution containing a metal element and containing more tungsten than the first raw material aqueous solution and an ammonium ion donor to the reaction aqueous solution containing the first metal composite hydroxide particles, and adjusting the pH of the reaction aqueous solution to carry out the crystallization reaction. and a second crystallization step of forming a tungsten-enriched layer on the surface of the first metal composite hydroxide particles to obtain second metal composite hydroxide particles, wherein in particle growth in the first crystallization step and the second crystallization step, the reaction atmosphere is switched multiple times to either a non-oxidizing atmosphere having an oxygen concentration of 1 volume % or less or an oxidizing atmosphere having an oxygen concentration of more than 1 volume %, and the metal element in the first raw material aqueous solution is supplied to the reaction tank in a range of 50 mass % to 95 mass % with respect to the total amount of metal added in the first crystallization step and the second crystallization step, and then the second raw material aqueous solution is supplied in the second crystallization step, the metal composite hydroxide includes secondary particles in which a plurality of primary particles are aggregated, and the secondary particles have a multilayer structure including, from the center of the particle toward the surface, a central portion where the primary particles are densely arranged, a void portion where the primary particles are sparsely arranged compared to the central portion, and a solid portion where the primary particles are densely arranged, and the tap density of the metal composite hydroxide is 0.75 g/cm The present invention provides a method for producing a metal composite hydroxide having a specific surface area of ³ or more and 1.35 g/cm3 or ^less .

The first crystallization step preferably includes a nucleation step for performing nucleation and a particle growth step for performing particle growth, and the second crystallization step preferably includes performing particle growth following the particle growth step, and the nucleation in the first crystallization step is performed in a non-oxidizing atmosphere with an oxygen concentration of 1% by volume or less, and the particle growth in the first crystallization step and the second crystallization step preferably include adjusting the pH of the reaction aqueous solution to be lower than the pH value of the reaction aqueous solution in the nucleation step, and switching the reaction atmosphere to either a non-oxidizing atmosphere with an oxygen concentration of 1% by volume or less or an oxidizing atmosphere with an oxygen concentration of more than 1% by volume four times.

The second crystallization step preferably includes forming a tungsten-enriched layer having a thickness of 100 nm or less in a direction from the surface to the center of the second metal composite hydroxide. The second raw material aqueous solution is preferably added in the second crystallization step when 50% to 95% of the total time during which particles grow in the first and second crystallization steps has elapsed. The second raw material aqueous solution is preferably supplied by separately supplying the first raw material aqueous solution and the tungsten-containing aqueous solution to the reaction aqueous solution. The tungsten concentration in the tungsten-containing aqueous solution is preferably 18% by mass or more relative to the total tungsten-containing aqueous solution.

In a second embodiment of the present invention, a metal composite hydroxide is provided, which contains nickel, manganese, and tungsten, and optionally cobalt and an element M, and in which the atomic ratio of each metal element is represented by Ni:Mn:Co:W:M=x:y:z:a:b (x+y+z=1, 0.3≦x≦0.95, 0.05≦y≦0.55, 0≦z≦0.4, 0<a≦0.1, 0≦b≦0.1, M is one or more elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, and Ta). The metal composite hydroxide includes secondary particles in which a plurality of primary particles are aggregated, and the secondary particles have a multilayer structure including at least a central portion in which the primary particles are densely arranged, a void portion in which the primary particles are more sparsely arranged than the central portion, and a solid portion in which the primary particles are densely arranged, from the center of the secondary particles to the surface, and has a tungsten-concentrated layer in the surface layer within the secondary particles, and has a tap density of 0.75 g/cm In accordance with the present invention, there is provided a metal composite hydroxide having a specific surface area ^{of 3} or more and 1.35 g/cm3 or ^less .

It is also preferable that the thickness of the tungsten-enriched layer is 100 nm or less. It is also preferable that the average particle size of the metal composite hydroxide is 4.0 μm or more and 9.0 μm or less, and that the index showing the spread of the particle size distribution, [(d90-d10)/average particle size], is 0.65 or less.

In a third embodiment of the present invention, there is provided a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, comprising the steps of: mixing at least one of the metal composite hydroxide obtained by the above-mentioned production method and the metal composite oxide obtained by heat-treating the metal composite hydroxide with a lithium compound to obtain a lithium mixture; and calcining the lithium mixture to obtain a lithium metal composite oxide.

In addition, in the manufacturing method of the positive electrode active material, it is preferable to adjust the value of (d50 of positive electrode active material/d50 of metal composite hydroxide), which is an index showing the degree of aggregation between particles of lithium metal composite oxide, to 0.95 or more and 1.05 or less.

In a fourth embodiment of the present invention, a composition containing lithium, nickel, manganese, and tungsten, and optionally cobalt and an element M, and an atomic ratio of each metal element is represented by Li:Ni:Co:Mn:W:M=1+u:x:y:z:a:b (x+y+z=1, -0.05≦u≦0.50, 0.3≦x≦0.95, 0.05≦y≦0.55, 0≦z≦0.4, 0<a≦0.1, 0≦b≦0.1, M is one or more elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, and Ta). The lithium metal composite oxide includes secondary particles formed by agglomeration of a plurality of primary particles, the secondary particles having a multilayer structure including, from the center of the secondary particles toward the surface, at least a central portion where the primary particles are densely arranged, a void portion where the primary particles are more sparsely arranged than the central portion, and a solid portion where the primary particles are densely arranged, and a compound containing tungsten and lithium is present in a concentrated state in surface layers of the primary particles present on the surface or inside of the secondary particles and in grain boundaries between the primary particles, the positive electrode active material having a tap density of 1.00 g/cm3 ^or more and 2.00 g/ ^cm3 or less and a BET specific surface area of 1.45 ^m2 /g or more and 5.40 ^m2 /g or less.

The positive electrode active material preferably has a crystallite diameter of 120 nm or more in the (003) plane obtained by powder X-ray diffraction measurement.

In a fourth embodiment of the present invention, a non-aqueous electrolyte secondary battery is provided that includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, and the positive electrode contains the positive electrode active material for non-aqueous electrolyte secondary batteries.

According to the present invention, it is possible to provide a positive electrode active material that has reduced reaction resistance, high output, and high crystallinity when used in the positive electrode of a secondary battery, and a metal composite hydroxide that is its precursor. In addition, according to the present invention, it is possible to provide a secondary battery that includes such a positive electrode active material. Furthermore, according to the present invention, it is possible to provide a method that can easily produce such a positive electrode active material and metal composite hydroxide on an industrial scale. For this reason, the industrial significance of the present invention is extremely great.

1(A) and 1(B) are schematic diagrams showing an example of a metal composite hydroxide, and FIG. 1(C) is an example of a cross-sectional SEM image of the metal composite hydroxide. FIG. 2 is a diagram showing an example of a method for producing a metal composite hydroxide. FIG. 3 is a diagram showing an example of a method for producing a metal composite hydroxide. FIG. 4 is a diagram illustrating the start point of addition of the tungsten-containing aqueous solution. 5(A) to 5(D) are schematic diagrams showing an example of a lithium metal composite oxide. FIG. 6 is a diagram showing an example of a method for producing a lithium metal composite oxide. FIG. 7 is a schematic diagram showing a coin-type battery for evaluation used in the examples. FIG. 8 is a drawing-substitute photograph (upper figure) showing an example of the distribution of W by area analysis using EDX of a metal composite hydroxide having a tungsten-enriched layer, and an explanatory diagram (lower figure) explaining the tungsten-enriched layer in the drawing-substitute photograph.

Below, a description will be given of a metal composite hydroxide and its manufacturing method, a positive electrode active material for a non-aqueous electrolyte secondary battery and its manufacturing method, and a non-aqueous electrolyte secondary battery according to an embodiment, with reference to the drawings. Note that the present invention is not limited to the embodiments described below. In addition, in the drawings, in order to make each component easier to understand, some parts are emphasized or simplified, and the actual structure, shape, scale, etc. may differ.

(Tungsten concentrated layer)
FIG. 1(A) is a schematic diagram showing an example of a metal composite hydroxide according to this embodiment. As shown in FIG. 1(A), the metal composite hydroxide 10 (secondary particle 2) has a tungsten-enriched layer 3 in which tungsten is concentrated on its surface. The tungsten-enriched layer 3 is a layered region formed on the surface of the metal composite hydroxide 10, in which tungsten is more concentrated than inside the particle. When the metal composite hydroxide 10 with the tungsten-enriched layer 3 formed thereon is used as a precursor, a positive electrode active material having high crystallinity, reduced reaction resistance, and high output can be obtained. The tungsten-enriched layer 3 can be confirmed by detecting the W distribution by area analysis using an energy dispersive X-ray analyzer (EDX), for example, as shown in FIG. 8. Note that the primary particles 1 are not shown in FIG. 1(A).

As described below, the tungsten-enriched layer 3 forms a compound 23 (e.g., lithium tungstate) containing tungsten and lithium in the lithium metal composite oxide 20 (see FIG. 5). The compound 23 containing tungsten and lithium is formed in a process of mixing the metal composite hydroxide 10 (precursor) with a lithium compound and baking it to obtain the lithium metal composite oxide 20 (see FIG. 5).

In the lithium metal composite oxide 20 (positive electrode active material), the compound 23 containing tungsten and lithium is formed on the surface layer of the primary particles 21 or on the grain boundaries between the primary particles 21. The compound 23 containing tungsten and lithium has high ionic conductivity, so when the lithium metal composite oxide 20 (positive electrode active material) is used in the positive electrode of a secondary battery, the presence of the compound 23 containing tungsten and lithium on the surface layer of the primary particles 21 that comes into contact with the electrolyte or on the grain boundaries between the primary particles 21 reduces the reaction resistance of the positive electrode active material, which can greatly contribute to improving the output.

When a conventional method for producing a metal composite hydroxide is used, the tungsten contained in the metal composite hydroxide (precursor) may inhibit sintering during firing. Therefore, the conventional precursor containing tungsten has a contradictory problem that the tungsten in the obtained positive electrode active material contributes to reducing the reaction resistance, while the crystallinity of the lithium metal composite oxide constituting the positive electrode active material decreases, making it difficult to realize a lithium metal composite oxide with excellent output characteristics and high crystallinity. However, the metal composite hydroxide 10 according to this embodiment forms a tungsten-concentrated layer 3 on the surface layer, so that there is almost no effect of inhibiting the sintering of tungsten during firing, and the reaction resistance can be reduced and the crystallinity of the resulting lithium metal composite oxide 20 can be increased.

The thickness of the tungsten enriched layer 3 in the direction from the surface of the metal composite hydroxide 10 toward the center C can be, for example, 200 nm, preferably 100 nm or less, more preferably 10 nm or more and 100 nm or less, and even more preferably 20 μm or more and 50 μm or less. When the thickness of the tungsten enriched layer 3 is 100 nm or less, the crystallinity of the lithium metal composite oxide 20 can be made higher, and when the lithium metal composite oxide 20 is used in the positive electrode of a secondary battery, the reaction resistance can be further reduced and the output characteristics can be improved.

The thickness of the tungsten-enriched layer 3 is measured by cutting the composite hydroxide 10 embedded in resin or the like to prepare a sample of the cross section of the secondary particle 2, and performing surface analysis of the distribution of W using EDX. Specifically, from the sample of the cross section of the secondary particle 2, 20 cross sections of the secondary particle 2 that are 80% or more of the volume average particle diameter (MV) measured using a laser light diffraction scattering type particle size analyzer are randomly selected, and in each selected secondary particle 2, the thickness (width) of the part (tungsten-enriched layer 3) where W is detected thickly in the direction from the surface of the metal composite hydroxide 10 toward the center C is measured at five or more points, and the average thickness of the tungsten-enriched layer 3 in each secondary particle 2 is obtained. Then, the average thickness of each of the selected 20 secondary particles 2 can be calculated to obtain the thickness of the tungsten-enriched layer 3.

If the thickness of the tungsten-enriched layer 3 exceeds 100 nm, the effect of suppressing sintering by tungsten becomes large during firing, and the growth of the primary particles may be inhibited. Therefore, the resulting lithium metal composite oxide may have a large number of primary particles with small crystallite diameters and many grain boundaries, which may increase the reaction resistance at the positive electrode. In addition, the crystallinity of the lithium metal composite oxide 20 may decrease due to the suppression of the growth of the primary particles.
On the other hand, if the thickness of the tungsten-enriched layer 3 is less than 10 nm, the specific surface area becomes large and the metal composite hydroxides 10 easily aggregate during firing, so that the packing density of the obtained positive electrode active material decreases, and the battery capacity per volume may decrease. Also, the compound 23 containing tungsten and lithium may not be sufficiently formed in the lithium metal composite oxide 20, and the lithium ion conductivity may be insufficient.

The thickness of the tungsten-enriched layer 3 can be, for example, 3% or less of the average particle size of the metal composite hydroxide 10 in the direction from the surface of the secondary particle 2 toward the center C of the secondary particle 2. From the viewpoint of further improving the crystallinity of the lithium metal composite oxide 20 and further reducing the reaction resistance, the thickness of the tungsten-enriched layer 3 is preferably 2% or less, more preferably 0.1% to 2%, more preferably 0.1% to 1%, and even more preferably 0.1% to 0.5%. The average particle size of the metal composite hydroxide 10 refers to the volume average particle size (MV).

(Particle Structure)
FIG. 1(B) is a schematic diagram showing an example of the metal composite hydroxide according to this embodiment, and FIG. 1(C) is an example of a cross-sectional SEM image of the metal composite hydroxide according to this embodiment. As shown in FIG. 1(C), the metal composite hydroxide 10 includes secondary particles 2 formed by agglomeration of a plurality of primary particles 1. The shape of the primary particles 1 constituting the secondary particles 2 is not particularly limited, but may be, for example, a plate-like or needle-like shape, or may be a fine primary particle smaller than these. In addition to the secondary particles 2, the metal composite hydroxide 10 may include a single primary particle 1. The particle structure of the secondary particles 2 is not particularly limited, and may have a conventionally known particle structure such as a solid structure in which almost no voids are observed in the particles, a hollow structure having a hollow part in the center of the particle, and a void structure having a plurality of voids, but as shown in FIG. 1(B) and FIG. 1(C), it is preferable to have a multilayer structure having a layered void part 5 and a layered solid part 6. Hereinafter, a preferred example of the particle structure will be described with reference to FIG. 1(B) and FIG. 1(C).

As shown in FIG. 1(B), the secondary particle 2 according to this embodiment preferably has a multilayer structure including, from the center C of the secondary particle 2 toward the surface, at least a central portion 4 in which the primary particles 1 are densely arranged, a void portion 5 in which the primary particles 1 are more sparsely arranged than the central portion 4, and a solid portion 6 in which the primary particles are densely arranged. The structure in which the primary particles are densely arranged is also called a solid structure.

The secondary particle 2 may have at least one layer of voids 5 and solid portions 6 from the center C of the secondary particle 2 toward the surface, or may have two alternating layers as shown in Figures 1(B) and 1(C). When the secondary particle 2 has a multi-layer structure, the specific surface area of the resulting lithium metal composite oxide 20 (positive electrode active material) can be increased, improving the output characteristics.

The tungsten-enriched layer 3 may be formed not only on the surface of the secondary particles 2, but also in the voids 5 and solid portions 6 present therein, but is preferably formed on the surface portion of the solid portion 6 located on the outermost surface of the secondary particles 2.

The particle structure of the secondary particles 2 can be made multilayered, for example, by switching the reaction atmosphere multiple times in the particle growth process during the crystallization reaction, as described below.

(Tap Density)
The tap density of the metal composite hydroxide 10 is not particularly limited, but is preferably 0.75 g/cm 3 or more and 1.35 g/cm ³ or less, and more preferably 1 g/cm ³ or more and 1.35 g/cm ^{3 or} less, when the secondary particles 2 have a multilayer structure. The tap density of the metal composite hydroxide 10 correlates with the tap density of the lithium metal composite oxide 20 (positive electrode active material) using this composite hydroxide 10 as a precursor, and when the metal composite hydroxide 10 has a multilayer structure, the tap density of the obtained lithium metal composite oxide 20 tends to be higher than the tap density of the metal composite hydroxide 10. Therefore, by controlling the tap density of the metal composite hydroxide 10 to the above range, it is possible to control the tap density of the lithium metal composite oxide 20 (see FIG. 5) using this composite hydroxide 10 as a precursor to a range described later. When this lithium metal composite oxide 20 is used for the positive electrode, a secondary battery having a high battery capacity can be obtained. On the other hand, when the tap density of the metal composite hydroxide 10 is less than 0.75 g/ ^cm3 , for example, in the firing step (step S40, see FIG. 6 ) in producing the positive electrode active material, the pile height becomes too high when piled into a sagger, and the material may not be sufficiently fired, resulting in reduced crystallinity.

(Average particle size)
The average particle size of the metal composite hydroxide 10 is not particularly limited, but is preferably 4.0 μm or more, more preferably 4 μm or more and 9.0 μm or less, and preferably 4.0 μm or more and 7 μm or less. The average particle size of the metal composite hydroxide 10 correlates with the average particle size of the lithium metal composite oxide 20 (positive electrode active material) having the composite hydroxide 10 as a precursor. Therefore, by controlling the average particle size of the metal composite hydroxide 10 to the above range, it becomes possible to control the average particle size of the lithium metal composite oxide 20 (see FIG. 5) having the composite hydroxide 10 as a precursor to the above range.

If the average particle size of the metal composite hydroxide 10 is less than 4 μm, the specific surface area is increased, and in the firing process (step S40, see FIG. 6) when manufacturing the positive electrode active material, the particles of the metal composite hydroxide 10 may easily aggregate with each other, reducing the packing density of the resulting positive electrode active material and reducing the battery capacity per volume. In this specification, the average particle size of the secondary particles 2 means the volume average particle size (MV), which can be determined, for example, from the volume integrated value measured by a laser light diffraction/scattering particle size analyzer.

The metal composite hydroxide 10 has an index of the spread of the particle size distribution, [(d90-d10)/average particle size] of 0.65 or less. The particle size distribution of the lithium metal composite oxide 20 (positive electrode active material) is strongly influenced by the composite hydroxide 10, which is its precursor. For this reason, when the composite hydroxide 10 containing many fine particles and coarse particles is used as a precursor, the lithium metal composite oxide 20 also contains many fine particles and coarse particles. In a secondary battery using such a lithium metal composite oxide 20 as a positive electrode active material, the battery characteristics such as thermal stability, cycle characteristics, and output characteristics may be deteriorated. Therefore, when the [(d90-d10)/average particle size] of the metal composite hydroxide 10 is adjusted to the above range, the particle size distribution of the lithium metal composite oxide 20 obtained by using it as a precursor can be narrowed, and the inclusion of fine particles and coarse particles can be suppressed.
In addition, the lower limit of the [(d90-d10)/average particle size] of the composite hydroxide 10 is not particularly limited, but is preferably about 0.25 or more from the viewpoint of cost and productivity. In the case of industrial-scale production, it is not realistic to use a composite hydroxide 10 having an excessively small [(d90-d10)/average particle size].

Note that d10 refers to the particle size at which the number of particles at each particle size is accumulated from the smallest particle size to the particle size at which the cumulative volume is 10% of the total volume of all particles, and d90 refers to the particle size at which the number of particles is accumulated in the same way to a particle size at which the cumulative volume is 90% of the total volume of all particles. Also, d10 and d90 can be calculated from the volume integrated value measured with a laser light diffraction scattering particle size analyzer, just like the average particle size.

(composition)
The composition of the metal composite hydroxide 10 is not particularly limited, but for example, it is preferable that the metal composite hydroxide 10 contains Ni, Co, and W, and optionally Mn and M, and the ratio (A) of the atomic numbers of the respective metal elements is Ni:Co:Mn:W:M=x:y:z:a:b (x+y+z=1, 0.3≦x≦0.95, 0.05≦y≦0.55, 0≦z≦0.4, 0<a≦0.1, 0≦b≦0.1, M is one or more metal elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, and Ta).

The atomic ratio (A) of the respective metal elements in the metal composite hydroxide 10 is maintained in the lithium metal composite oxide 20, and therefore, in the composite hydroxide 10 represented by the ratio (A) of the metal elements, the composition ranges of nickel, manganese, cobalt, tungsten, and element M constituting the composite hydroxide 10 and their critical meanings are the same as those of the positive electrode active material represented by the ratio (B) described below. Therefore, a description of these matters will be omitted here.
The metal composite hydroxide 10 may be represented by the general formula (A1): Ni _x Mn _y Co _z W _a M _b (OH) _{2 + α} (x + y + z = 1, 0.3 ≦ x ≦ 0.95, 0.05 ≦ y ≦ 0.55, 0 ≦ z ≦ 0.4, 0 < a ≦ 0.1, 0 ≦ b ≦ 0.1, 0 ≦ α ≦ 0.5, M is one or more metal elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, and Ta).

In the above general formula (A1), from the viewpoint of further improving the capacity characteristics of a secondary battery using the obtained positive electrode active material, it is preferable that the composition is the general formula (A2): NixMnyCozWaMb(OH)2+α (x+y+z=1, 0.7<x≦0.95, 0.05≦y≦0.1, 0≦z≦0.2, 0<a≦0.1, 0≦b≦0.1, 0≦α≦0.5, M is one or more elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, and Mo). Furthermore, from the viewpoint of achieving both thermal stability and battery capacity, it is more preferable that the value of x in the above general formula (A2) is 0.7<x≦0.9, and even more preferable that it is 0.7<x≦0.85.

In the above general formula (A1), from the viewpoint of further improving the thermal stability of a secondary battery using the obtained positive electrode active material, it is preferable that the composition be represented by general formula (A3): Ni _x Mn _y Co _z W _a M _b (OH) _{2 + α} (x + y + z = 1, 0.3 ≤ x ≤ 0.7, 0.1 ≤ y ≤ 0.55, 0 ≤ z ≤ 0.4, 0 < a ≤ 0.1, 0 ≤ b ≤ 0.1, 0 ≤ α ≤ 0.5, M is one or more elements selected from Al, Ti, V, Cr, Zr, Nb, Mo, Hf, and Ta).

2. Manufacturing method of metal composite hydroxide Figures 2 to 3 are diagrams showing an example of a manufacturing method of a metal composite hydroxide according to this embodiment. The manufacturing method according to this embodiment is a method for manufacturing a metal composite hydroxide containing nickel, manganese, and tungsten, and optionally cobalt and element M, and in which the atomic ratio of each metal element is represented by Ni:Mn:Co:W:M = x:y:z:a:b (x + y + z = 1, 0.3 ≦ x ≦ 0.95, 0.05 ≦ y ≦ 0.55, 0 ≦ z ≦ 0.4, 0 < a ≦ 0.1, 0 ≦ b ≦ 0.1, M is one or more elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, and Ta) by a crystallization reaction. By the manufacturing method according to this embodiment, the metal composite hydroxide 10 having the above characteristics can be easily manufactured on an industrial scale.

[Crystallization reaction]
As shown in FIG. 2 , the method for producing the metal composite hydroxide 10 includes a first crystallization step (step S10) in which a first raw material aqueous solution containing nickel (Ni) and manganese (Mn), and optionally cobalt (Co) and/or a metal element (M), and an ammonium ion donor are supplied into a reaction tank to carry out a crystallization reaction to obtain first metal composite hydroxide particles, and a second crystallization step (step S20) in which a second raw material aqueous solution containing more tungsten than the first raw material aqueous solution and an ammonium ion donor are supplied to the reaction aqueous solution containing the first metal composite hydroxide particles to carry out a crystallization reaction to form a tungsten-concentrated layer on the surface of the first metal composite hydroxide particles, thereby obtaining second metal composite hydroxide particles.

In the method for producing a metal composite hydroxide according to this embodiment, in the second crystallization step (step S20), a tungsten-enriched layer 3 is formed on the surface of the first composite hydroxide particle obtained by the first crystallization step (step S10), thereby obtaining a second metal composite hydroxide particle. The inside of the second metal composite hydroxide particle is composed of the first metal composite hydroxide particle that does not contain tungsten or has a low tungsten content, so that the lithium metal composite oxide 20 (positive electrode active material) obtained using this as a precursor has high crystallinity inside the particle, and the compound 23 containing tungsten and lithium derived from the tungsten-enriched layer 3 is present on the surface of the primary particles or on the grain boundaries between the primary particles, resulting in excellent output characteristics.

As shown in FIG. 3, the first crystallization step (step S10) preferably further includes a nucleation step (step S11) in which nucleation is mainly performed, and a particle growth step (step S12) in which particle growth is mainly performed. The nucleation step (step S11) and the particle growth step (step S12) can be clearly separated, for example, by controlling the pH of the reaction aqueous solution, and a metal composite hydroxide 10 having a narrow particle size distribution and a uniform particle size can be obtained. The second crystallization step (step S20) is a step following the particle growth step (step S12) in which particle growth is mainly performed.

The method for producing nickel composite hydroxide including such a two-stage crystallization process is disclosed in, for example, Patent Document 2 and Patent Document 3, and the detailed conditions can be adjusted appropriately by referring to these documents. In addition, the method for producing metal composite hydroxide according to this embodiment can form a tungsten-enriched layer 3 having a desired film thickness using the conditions of a known crystallization method, as described later, and therefore can be easily applied to industrial-scale production.

Hereinafter, with reference to FIG. 3, each step of the manufacturing method, including the nucleation step (step S11) and the particle growth step (step S12), will be described. Note that the following description is an example of a method for manufacturing metal composite hydroxide 10, and is not limited to this method.

(1) First crystallization step (step S10)
(Nucleation process)
First, the first raw aqueous solution and an ammonium ion donor are supplied, and the pH of the reaction aqueous solution (aqueous solution for nucleation) in the reaction tank is controlled to a predetermined range to perform nucleation (step S11). The first raw aqueous solution is adjusted, for example, by dissolving a compound containing a transition metal as a raw material in water. In the method for producing a metal composite hydroxide described below, the composition ratio of the metal composite hydroxide formed by crystallization in each step is the same as the composition ratio of each metal in the raw aqueous solution, so that the composition ratio of each metal in the raw aqueous solution can be the composition ratio of the transition metal of the target metal composite hydroxide. In addition, the first raw aqueous solution may contain a small amount of tungsten or may not contain tungsten.

First, an aqueous solution containing an alkaline aqueous solution and an ammonium ion donor is supplied to and mixed with a reaction tank to prepare a pre-reaction aqueous solution having a pH value of 12.0 to 14.0 measured at a liquid temperature of 25°C and an ammonium ion concentration of 3 g/L to 25 g/L. When the particle center has a solid structure (when the primary particles 1 are densely arranged), the reaction atmosphere in the reaction tank is preferably a non-oxidizing atmosphere. The atmosphere is controlled by, for example, introducing nitrogen gas. The pH value of the pre-reaction aqueous solution can be measured with a pH meter, and the ammonium ion concentration can be measured with an ion meter.

Next, while stirring the pre-reaction aqueous solution in the reaction tank, the first raw material aqueous solution is supplied into the reaction tank to form a reaction aqueous solution (nucleation aqueous solution). In the nucleation step (step S11), the pH value and ammonium ion concentration of the nucleation aqueous solution change with the nucleation in the reaction aqueous solution, so it is preferable to timely supply an alkaline aqueous solution and an ammonia aqueous solution to control the pH value of the liquid in the reaction tank to be maintained in the range of pH 12.0 to 14.0 at a liquid temperature of 25°C, and the ammonium ion concentration to be maintained in the range of 3 g/L to 25 g/L. When the pH value of the reaction aqueous solution (nucleation aqueous solution) is within the above range, nuclei hardly grow and nucleation occurs preferentially.

The pH value of the reaction aqueous solution (aqueous solution for nucleation) measured at a liquid temperature of 25°C is preferably in the range of 12.0 to 14.0, more preferably 12.3 to 13.5, and even more preferably 12.5 to 13.3. When the pH is in the above range, the growth of the nuclei can be suppressed and nucleation can be prioritized, and the nuclei generated in the nucleation process can be made homogeneous and have a narrow particle size distribution. On the other hand, when the pH value is less than 12.0, the growth of the nuclei (particles) proceeds along with the nucleation, so that the particle size of the obtained metal composite hydroxide particles becomes non-uniform and the particle size distribution deteriorates. In addition, when the pH value exceeds 14.0, the nuclei generated become too fine, and the reaction aqueous solution (aqueous solution for nucleation) may gel.

It is preferable that the pH value of the reaction aqueous solution (aqueous solution for nucleation) fluctuate within ±0.2. If the pH value fluctuates widely, the ratio of the amount of nucleation and the particle growth will not be constant, making it difficult to obtain metal composite hydroxide particles with a narrow particle size distribution.

The ammonium ion concentration of the reaction aqueous solution (aqueous solution for nucleation) is preferably adjusted to a range of 3 g/L to 25 g/L, more preferably 5 g/L to 20 g/L. Since ammonium ions function as a complexing agent in the reaction aqueous solution, if the ammonium ion concentration is less than 3 g/L, the solubility of the metal ions cannot be maintained constant or the reaction aqueous solution is prone to gelling, making it difficult to obtain metal composite hydroxide particles with a uniform shape and particle size. On the other hand, if the ammonium ion concentration exceeds 25 g/L, the solubility of the metal ions becomes too high, so the amount of metal ions remaining in the reaction aqueous solution increases, which may cause composition deviations, etc.

If the ammonium ion concentration fluctuates during the crystallization reaction, the solubility of the metal ions will fluctuate, and uniform metal composite hydroxide particles will not 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 (step S11) and the particle growth process (step S12), and more specifically, it is preferable to control the fluctuation range to ±5 g/L.

In the nucleation step (step S11), the first raw material aqueous solution, the alkali aqueous solution, and the aqueous solution containing the ammonium ion donor are supplied to the reaction aqueous solution (nucleation aqueous solution), so that new nuclei are continuously generated. Then, the nucleation step is terminated when a predetermined amount of nuclei is generated in the nucleation aqueous solution. At this time, the amount of nuclei generated can be determined from the amount of metal compound contained in the raw material aqueous solution supplied to the nucleation aqueous solution.
The amount of nuclei generated in the nucleation step (step S11) is not particularly limited, but from the viewpoint of obtaining metal composite hydroxide particles having a narrow particle size distribution, it is preferably 0.1 atomic % or more and 2 atomic % or less, and more preferably 0.1 atomic % or more and 1.5 atomic % or less, relative to the metal elements in the metal compounds contained in the raw material aqueous solution supplied through the crystallization steps (including the first crystallization step and the second crystallization step).

In the nucleation step, the upper limit of the temperature of the reaction aqueous solution (nuclear generation aqueous solution) is not particularly limited, but is preferably 60°C or less, and more preferably 50°C or less. If the temperature of the reaction aqueous solution (nuclear generation aqueous solution) exceeds 60°C, distortion may occur in the primary crystals, causing the tap density to start to decrease.

(Particle growth process)
Next, particle growth is performed in a reaction aqueous solution (particle growth aqueous solution) whose pH has been adjusted to a specific range (step S12). The reaction aqueous solution (particle growth aqueous solution) is formed by supplying a first raw material aqueous solution, an alkaline aqueous solution, and an aqueous solution containing an ammonium ion donor to the reaction aqueous solution containing the generated nuclei. The reaction aqueous solution (particle growth aqueous solution) is preferably adjusted to have a pH value of 10.5 to 12.0 and an ammonium ion concentration of 3 g/L to 25 g/L, measured at a liquid temperature of 25° C. as a reference. This allows particle growth to be performed preferentially over nucleation in the reaction aqueous solution (particle growth aqueous solution).

Regarding the reaction atmosphere in the reaction tank, when forming secondary particles having a multilayer structure in which the tap density of the metal composite hydroxide is 0.75 g/cm3 ^or more and 1.35 g/ ^cm3 or less, it is preferable to appropriately switch between a non-oxidizing atmosphere with an oxygen concentration of 1 volume % or less and an oxidizing atmosphere with an oxygen concentration of more than 1 volume %. The atmosphere is controlled by, for example, introducing nitrogen gas.

Specifically, after the nucleation step is completed, the pH value of the aqueous solution for nucleation in the reaction tank is adjusted to 10.5 or more and 12.0 or less at a liquid temperature of 25°C to form an aqueous solution for particle growth, which is a reaction aqueous solution in the particle growth step. The pH value can be adjusted by stopping the supply of only the alkaline aqueous solution, but from the viewpoint of increasing the uniformity of the particle size, it is preferable to stop the supply of all aqueous solutions once and adjust the pH value. The pH value may also be adjusted by supplying the same type of inorganic acid as the acid constituting the compound containing the raw material transition metal, for example, sulfuric acid when a sulfate of a transition metal is used as the raw material, to the aqueous solution for nucleation.

Next, while stirring the reaction aqueous solution (particle growth aqueous solution), the supply of the first raw material aqueous solution is resumed. At this time, since the pH value of the particle growth aqueous solution is within the above-mentioned range, almost no new nuclei are generated, and nuclei (particles) grow, forming first metal composite hydroxide particles having a predetermined particle size. Note that, in the particle growth process (step S12), the pH value and ammonium ion concentration of the particle growth aqueous solution change with particle growth, so it is necessary to supply an alkaline aqueous solution and an ammonia aqueous solution at appropriate times to maintain the pH value and ammonium ion concentration within the above-mentioned range.

The pH value of the reaction aqueous solution (particle growth aqueous solution) is controlled to a range of 10.5 to 12.0, preferably 11.0 to 12.0, more preferably 11.5 to 11.9, based on a liquid temperature of 25°C. When the pH is in the above range, the generation of new nuclei is suppressed, particle growth is prioritized, and the resulting metal composite hydroxide can be made homogeneous and have a narrow particle size distribution. On the other hand, when the pH value is less than 10.5, the ammonium ion concentration increases and the solubility of the metal ions increases, so not only does the crystallization reaction slow down, but the amount of metal ions remaining in the reaction aqueous solution increases, and productivity may deteriorate. In addition, when the pH value exceeds 12.0, the amount of nuclei generated during the particle growth process increases, and the particle size of the resulting metal composite hydroxide particles becomes non-uniform, and the particle size distribution may deteriorate.

It is preferable that the pH value in the reaction aqueous solution (particle growth aqueous solution) fluctuate within ±0.2. If the pH value fluctuates widely, the ratio of the amount of nucleation and particle growth will not be constant, making it difficult to obtain a metal composite hydroxide with a narrow particle size distribution.

The pH value of the particle growth process (step S12) is preferably adjusted to a value lower than the pH value of the nucleation process (step S11). In order to clearly separate nucleation and particle growth, the pH value of the particle growth process (step S12) is preferably 0.5 or more lower than the pH value of the nucleation process (step S11), and more preferably 0.8 or more lower.

For example, when the pH value of the reaction aqueous solution (nucleation step and/or particle growth step) is 12.0, this is the boundary condition between nucleation and nucleation growth, so the condition can be either the nucleation step or the particle growth step depending on the presence or absence of nuclei in the reaction aqueous solution. That is, if the pH value of the nucleation step is made higher than 12.0 to generate a large amount of nuclei, and then the pH value of the particle growth step is made 12.0, a large amount of nuclei are present in the reaction aqueous solution, so particle growth occurs preferentially, and metal composite hydroxide particles with a narrow particle size distribution can be obtained. On the other hand, when the pH value of the nucleation step is made 12.0, there are no nuclei to grow in the reaction aqueous solution, so nucleation occurs preferentially, and by making the pH value of the particle growth step smaller than 12.0, the generated nuclei grow, and good metal composite hydroxide particles can be obtained.

The ammonium ion concentration in the reaction aqueous solution (particle growth aqueous solution) can be set to the same range as the preferred range of the ammonium ion concentration in the reaction aqueous solution (nucleation aqueous solution). The fluctuation range of the ammonium ion concentration can also be set to the same range as the preferred range in the reaction aqueous solution (nucleation aqueous solution).

The reaction atmosphere in the particle growth process (step S12) may be switched by adjusting the oxygen concentration. For example, if the center of the secondary particles is to have a solid structure, following the nucleation process (step S11), particle growth may be performed in a non-oxidizing atmosphere with an oxygen concentration of 1% by volume or less, then switched to an oxidizing atmosphere with an oxygen concentration of more than 1% by volume (first switch), particle growth may be performed, and then further switched to a non-oxidizing atmosphere with an oxygen concentration of 1% by volume or less (second switch), particle growth may be performed. By switching the reaction atmosphere, secondary particles having the above-mentioned multilayer structure can be obtained.

When producing secondary particles having a multilayer structure, the reaction atmosphere may be switched multiple times in the particle growth process (overall) in the first crystallization process and the second crystallization process. The reaction atmosphere may be switched multiple times in the second crystallization process without switching the reaction atmosphere in the particle growth process (step S12), or the reaction atmosphere may be switched multiple times in the particle growth process (step S12). The reaction atmosphere may be switched two or more times, or four or more times in the particle growth process (step S12).

(2) Second Crystallization Step Next, a second raw material aqueous solution containing metal elements and containing more tungsten than the first raw material aqueous solution, and an ammonium ion donor are supplied to the reaction aqueous solution containing the first composite metal hydroxide particles to carry out a crystallization reaction (step S20). This produces second composite metal hydroxide particles having a tungsten-enriched layer formed on the surface of the first composite metal hydroxide particles.
Since the particles grow from secondary particles formed by agglomeration of a plurality of primary particles, the second crystallization step (step S20) can be carried out by supplying a second raw material aqueous solution containing more tungsten than the first raw material aqueous solution and an ammonium ion donor to the reaction aqueous solution containing the first metal composite hydroxide particles, consecutively to the particle growth step (step S12). As a result, a tungsten-enriched layer is formed on the outer periphery of the secondary particles constituting the first composite oxide particles.

The method for producing a metal composite hydroxide of this embodiment can easily control the thickness of the tungsten-enriched layer formed on the outer periphery of the first metal composite hydroxide particle by, for example, adjusting the timing at which the second raw material aqueous solution starts to be added (i.e., the timing at which the second crystallization process starts).

The second crystallization reaction (step S20) can be started by supplying the second raw material aqueous solution when the metal elements in the first raw material aqueous solution are supplied to the reaction tank in an amount of, for example, 10 mass% or more of the total amount of metal added in the first crystallization step and the second crystallization step. This makes it possible to easily form a tungsten-enriched layer on the surface of the first metal composite hydroxide particles.

In addition, from the viewpoint of obtaining a positive electrode active material with higher crystallinity and further reduced reaction resistance when used in the positive electrode of a secondary battery, the second raw material aqueous solution can be supplied and the second crystallization step (step S20) can be started at the point when the metal elements in the first raw material aqueous solution are supplied to the reaction tank in a range of preferably 50% by mass to 95% by mass, more preferably 75% by mass to 90% by mass, based on the total amount of metal added in the first crystallization step and the second crystallization step.

The second crystallization step (step S20), like the particle growth step (step S12), is a step in which particle growth occurs, so the pH, temperature, and ammonium ion concentration of the reaction aqueous solution, as well as the atmosphere in the reaction tank, can be the same as those in the particle growth step (step S12). By continuously performing the second crystallization step (step S20) under the same conditions as those in the particle growth step (step S12), a tungsten-enriched layer can be formed on the surface of the first metal composite hydroxide particles in a simple and highly productive manner.

The second raw material aqueous solution may be supplied by separately preparing a raw material aqueous solution containing a metal element other than tungsten and an aqueous solution containing tungsten, and supplying each of them to the reaction tank. By separately supplying the aqueous solution containing tungsten, the tungsten-enriched layer can be formed more simply and uniformly. As shown in FIG. 2, the second raw material aqueous solution may contain the first raw material aqueous solution and an aqueous solution containing tungsten (W), and each aqueous solution may be supplied separately to the reaction aqueous solution. In this way, by changing the concentration of the aqueous solution containing tungsten and the flow rate when the aqueous solution containing tungsten is supplied to the reaction tank, the tungsten-enriched layer can be formed uniformly, and the thickness of the tungsten-enriched layer can be easily adjusted.

The tungsten-containing aqueous solution can be prepared, for example, by dissolving a tungsten compound in water. The tungsten compound used is not particularly limited, and any compound containing tungsten without containing lithium can be used, but sodium tungstate is preferably used.

The tungsten (W) concentration of the tungsten-containing aqueous solution is not particularly limited, but is, for example, 0.1 mol/l to 0.5 mol/l, and preferably 0.2 mol/l to 0.4 mol/l. The addition flow rate of the tungsten-containing aqueous solution is not particularly limited, but is, for example, 5 L/min to 20 L/min, and preferably 10 L/min to 15 L/min.

The thickness of the tungsten-enriched layer can also be controlled by adjusting the concentration of the tungsten-containing aqueous solution or the flow rate of the aqueous solution added. For example, if the concentration and flow rate of the tungsten-containing aqueous solution are constant, the thickness and concentration of the tungsten-enriched layer formed can be more accurately and easily adjusted by adjusting the time when the addition of the tungsten-containing aqueous solution begins.

Figure 4 is a diagram showing a preferred example of the manufacturing method of this embodiment. Figure 4 is a diagram explaining the start time of addition of the aqueous solution containing tungsten (the start time of the second crystallization step) in a case where the first crystallization step (step S10) comprises a nucleation step (step S11) and a particle growth step (step S12) separately, and the second crystallization step (step S20) is performed by supplying an aqueous solution containing tungsten together with the first aqueous raw material solution, consecutive to the particle growth step (step S12).

Figure 4 (A) is a diagram showing an example of the case where an aqueous solution of a tungsten compound is added 75% of the time required for particle growth from the start of particle growth to the end of particle growth (the end of the crystallization process). In this case, as shown in the lower part of Figure 4 (A), a tungsten-enriched layer 3 can be formed on the surface (outer periphery) of the secondary particles (first metal composite hydroxide particles). Note that if an aqueous solution of a tungsten compound is added throughout the entire process from the start to the end of particle growth, a tungsten-enriched layer 3 will not be formed.

Figure 4 (B) shows an example of the case where an aqueous solution of a tungsten compound is added 87.5% of the total time for particle growth from the start of particle growth to the end of particle growth (end of the crystallization process). In this case, as shown in the lower part of Figure 4 (B), a more concentrated tungsten layer can be formed on the surface (outer periphery) of the secondary particles (first metal composite hydroxide particles) compared to the case where the addition is started 75% of the time.

When the second crystallization step (step S20) is performed by supplying a tungsten-containing aqueous solution together with the first raw aqueous solution as described above, the supply of the tungsten-containing aqueous solution (second raw aqueous solution) can be performed, for example, at a time when 10% or more has elapsed, preferably at a time when 50% or more and 95% or less has elapsed, more preferably at a time when 75% or more and 90% or less has elapsed, relative to the entire time from the start to the end of the particle growth in the first and second crystallization steps. When the time to start the supply is within the above range, a tungsten-concentrated layer can be easily and productively obtained. In addition, when the time to start the addition of the tungsten-containing aqueous solution is later within the above range, the crystallite size of the obtained positive electrode active material tends to be larger. The addition of the tungsten-containing aqueous solution continues until the crystallization reaction is completed.
The raw materials and conditions preferably used in the crystallization step will be described below.

(First and second raw material aqueous solutions)
The first and second aqueous raw material solutions contain nickel and manganese, and optionally cobalt, element M, and tungsten. The first aqueous raw material solution may not contain tungsten. In the second crystallization step, when the first aqueous raw material solution and an aqueous solution containing tungsten are used as the second aqueous raw material solution, the ratio of metal elements in the first aqueous raw material solution is the composition ratio (excluding tungsten) of the metal composite hydroxide finally obtained. Therefore, the content of each metal element in the first aqueous raw material solution can be appropriately adjusted according to the composition of the target metal composite hydroxide. For example, when trying to obtain metal composite hydroxide particles represented by the above-mentioned ratio (A), the ratio of metal elements in the aqueous raw material solution can be adjusted to Ni:Mn:Co:M=x:y:z:b (where x+y+z=1, 0.3≦x≦0.95, 0.05≦y≦0.55, 0≦z≦0.4, 0≦b≦0.1). The first and second aqueous raw material solutions used in the first and second crystallization steps may have different compositions. In this case, the total content of each metal element in each aqueous raw material solution used in each crystallization step can be the composition ratio of the resulting metal composite hydroxide.

The compounds of metal elements (transition metals) used to prepare the first and second raw aqueous solutions are not particularly limited, but from the viewpoint of ease of handling, it is preferable to use water-soluble nitrates, sulfates, and hydrochlorides, and from the viewpoint of cost and prevention of halogen contamination, it is particularly preferable to use sulfates.

When the metal composite hydroxide contains element M (wherein M is one or more elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W), the compound for supplying element M is preferably a water-soluble compound, such as 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.

The concentration of the first raw aqueous solution and the second raw aqueous solution is preferably 1 mol/L to 2.6 mol/L, more preferably 1.5 mol/L to 2.2 mol/L, in terms of the total metal compounds. If the concentration of the raw aqueous solutions is less than 1 mol/L, the amount of crystallized material per reaction tank will be small, resulting in reduced productivity. On the other hand, if the concentration of the mixed aqueous solution exceeds 2.6 mol/L, the concentration will exceed the saturation concentration at room temperature, and crystals of each metal compound may reprecipitate, potentially clogging piping, etc.

The above-mentioned metal compounds do not necessarily have to be supplied to the reaction tank as one type of raw material aqueous solution. For example, when a crystallization reaction is carried out using metal compounds that will react with each other to produce compounds other than the target compound when mixed, aqueous solutions of the metal compounds may be prepared individually and supplied to the reaction tank in a predetermined ratio as aqueous solutions of the individual metal compounds so that the total concentration of the aqueous solutions of all the metal compounds falls within the above range.
The supply amounts of the first and second raw aqueous solutions are set so that the concentration of the product (second metal composite hydroxide particles) in the reaction solution (particle growth aqueous solution) at the end of the crystallization step is preferably 30 g/L to 200 g/L, more preferably 80 g/L to 150 g/L. If the product concentration is less than 30 g/L, the primary particles may not be sufficiently aggregated. On the other hand, if the product concentration exceeds 200 g/L, the aqueous solution for nucleation or the aqueous solution for particle growth may not be sufficiently diffused in the reaction tank, resulting in uneven particle growth.

(Alkaline aqueous solution)
The alkaline aqueous solution for adjusting the pH value in the reaction aqueous solution is not particularly limited, and a general aqueous solution of an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide can be used. The alkali metal hydroxide can be added directly to the reaction aqueous solution, but it is preferable to add it as an aqueous solution because of the ease of pH control. In this case, the concentration of the aqueous solution of the alkali metal hydroxide is preferably 20% by mass or more and 50% by mass or less, and more preferably 20% by mass or more and 30% by mass or less. By restricting the concentration of the aqueous solution of the alkali metal to such a 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 system, and therefore it is possible to efficiently obtain metal composite hydroxide particles with a narrow particle size distribution.

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

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

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

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

(Reaction atmosphere)
In the manufacturing method of this embodiment, in the nucleation step, when the center of the particle is to have a solid structure in which primary particles are densely arranged, it is preferable to control the reaction atmosphere to a non-oxidizing atmosphere with an oxygen concentration of 1% by volume or less. In addition, in order to mainly form secondary particles having a multilayer structure and a tap density of 0.75 g/cm ³ or more and 1.35 g/cm ³ or less of the metal composite hydroxide, it is preferable to appropriately switch between a non-oxidizing atmosphere with an oxygen concentration of 1% by volume or less and an oxidizing atmosphere with an oxygen concentration of more than 1% by volume in the particle growth (step S12, step S20) in the first crystallization step and the second crystallization step. Specifically, the non-oxidizing atmosphere needs to be controlled to a mixed atmosphere of oxygen and inert gas so that the oxygen concentration in the reaction atmosphere is preferably 1% by volume or less. In addition, the oxidizing atmosphere may be an atmosphere in which the oxygen concentration in the reaction atmosphere exceeds 1% by volume, for example, the oxygen concentration may exceed 5% by volume, or may be an air atmosphere.

As described above, the particle structure, specific surface area, and tap density of the metal composite hydroxide are controlled by controlling the atmosphere in the reaction tank to be an oxidizing atmosphere or a non-oxidizing atmosphere and by controlling the number of times the reaction atmosphere is switched. The number of times the reaction atmosphere is switched during the entire particle growth (step S12 and step S20) in the first crystallization step and the second crystallization step may be, for example, two or more times, or may be four or more times. From the viewpoint of obtaining a positive electrode active material having a large BET specific surface area, it is preferable that the reaction atmosphere is switched four or more times, and from the viewpoint of productivity, it is preferable that the reaction atmosphere is switched four times. In addition, the timing of switching the reaction atmosphere can be appropriately adjusted according to the desired thickness of each layer in the multilayer structure.

(Reaction temperature)
The temperature of the reaction aqueous solution (reaction temperature) is preferably controlled to 20° C. or higher, more preferably 20° C. or higher and 60° C. or lower throughout the crystallization process (nucleation process, particle growth process, second crystallization process). If the reaction temperature is less than 20° C., the solubility of the reaction aqueous solution is reduced, which makes it easier for nucleation to occur, and it may be difficult to control the average particle size and particle size distribution of the resulting metal composite hydroxide. The upper limit of the reaction temperature is not particularly limited, but if it exceeds 60° C., the volatilization of ammonia is promoted, and the amount of the aqueous solution containing the ammonium ion donor supplied to control the ammonium ion in the reaction aqueous solution to a certain range increases, resulting in an increase in production costs. Furthermore, if the temperature exceeds 60° C., as described above, there is a risk that the primary crystals will be distorted in the nucleation process and the tap density will begin to decrease.

(Manufacturing equipment)
In the method for producing a metal composite hydroxide according to this embodiment, 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 together with the overflow liquid, and therefore metal composite hydroxide particles with a narrow particle size distribution can be easily obtained.

In addition, in the method for producing a metal composite hydroxide of this embodiment, it is preferable to control the reaction atmosphere during the crystallization reaction, so 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 metal composite hydroxide particles having the above-mentioned particle structure and a narrow particle size distribution can be easily obtained.

The composition of the metal composite hydroxide 10 obtained by the manufacturing method of this embodiment is not particularly limited as long as it can realize the above-mentioned particle structure, average particle size, and particle size distribution, but it can be suitably applied to, for example, a metal composite hydroxide represented by the composition ratio (A).

3. Positive electrode active material for non-aqueous electrolyte secondary battery The positive electrode active material according to this embodiment contains lithium, nickel, manganese, and tungsten, and optionally cobalt and element M, and contains a lithium metal composite oxide (hereinafter referred to as "lithium metal composite oxide") represented by the atomic ratio of each metal element being Li:Ni:Mn:Co:W:M=(1+u):x:y:z:a:b (x+y+z=1, -0.05≦u≦0.50, 0.3≦x≦0.95, 0.05≦y≦0.55, 0≦z≦0.4, 0<a≦0.1, 0≦b≦0.1, M is one or more elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, and Ta). In addition, the lithium metal composite oxide has a hexagonal layered crystal structure.

Figure 5 (A) is a schematic diagram showing an example of a lithium metal composite oxide according to this embodiment. As shown in Figure 5 (A), the lithium metal composite oxide 20 contains secondary particles 22 formed by agglomeration of multiple primary particles 21. The lithium metal composite oxide 20 may contain a small amount of individual primary particles 21. Details of the lithium metal composite oxide 20 will be described below with reference to Figures 5 (A) and 5 (B).

(Compounds containing tungsten and lithium)
As shown in Fig. 5(B) , in the lithium metal composite oxide 20, compounds 23 containing tungsten and lithium are present in a concentrated form in the surface layers of the primary particles 21 present on the surfaces or inside the secondary particles 22, and in the grain boundaries between the primary particles 21. The locations of the compounds 23 containing tungsten and lithium can be confirmed by detecting the W distribution in area analysis using an energy dispersive X-ray analyzer (EDX), as shown in Fig. 9 . It is also preferable that the compounds 23 containing tungsten and lithium are present in greater amounts on the surfaces (surface layers) of the secondary particles 22 than inside them.

When the surface layer of the positive electrode active material is coated with a different compound, the movement (intercalation) of lithium ions is greatly restricted, and as a result, the advantage of the lithium metal composite oxide, that is, high capacity, may not be fully exhibited. In contrast, the compound 23 containing tungsten and lithium (e.g., lithium tungstate, etc.) has a high lithium ion conductivity and therefore has the effect of promoting the movement of lithium ions. Therefore, the lithium metal composite oxide 20 according to this embodiment forms the compound 23 containing tungsten and lithium on the surface layer of the primary particles 21 near the surface and on the grain boundaries between the primary particles 21, thereby forming a conductive path of Li at the interface with the electrolyte, thereby reducing the reaction resistance of the positive electrode active material and improving the output characteristics. The compound 23 containing tungsten and lithium can be present, for example, in the form of fine particles near the surface of the lithium metal composite oxide 20.

The compound 23 containing tungsten and lithium is not particularly limited, and examples thereof include lithium tungstates such as Li ₂ WO ₄ , Li ₄ WO ₅ , and Li ₂ W ₂ O _7. These lithium tungstates are concentrated and formed on the surface layers of the primary particles 21 present on the surfaces or inside the secondary particles 22 and on the grain boundaries between the primary particles 21, thereby further increasing the lithium ion conductivity of the lithium metal composite oxide 20, and when used in the positive electrode of a secondary battery, the reaction resistance is further reduced.

(Particle Structure)
The lithium metal composite oxide in this embodiment has a multilayer structure that is characteristic of metal composite hydroxides, and while maintaining sufficient particle strength, it has an increased contact area with the electrolyte and has excellent output characteristics.

The structure of the secondary particles 22 of the lithium metal composite oxide 20 is not particularly limited, and may be, for example, a solid structure formed by agglomeration of primary particles with relatively little gaps, a void structure having multiple voids inside the secondary particles 22, or a hollow structure having a hollow portion. However, as shown in FIG. 5 (A), it is preferable that the secondary particles 22 have a central portion 24 having a solid structure, and outside the central portion 24, a void portion 26 in which the primary particles 21 are arranged more sparsely than the central portion 24, and a multilayer structure having a substantial portion 21 in which the primary particles 21 are arranged densely, which is electrically conductive with the central portion. When the lithium metal composite oxide 20 made of secondary particles having a void portion 26 in the secondary particles 22 is used as the positive electrode of a secondary battery, the electrolyte penetrates into the secondary particles, and the contact area between the primary particles 21 inside the secondary particles 22 and the electrolyte increases, so that the resistance inside the secondary particles 22 can be reduced and the output characteristics can be improved. There may be multiple solid portions 21, and for example, as described in Patent Document 3, the solid portion 21 may be composed of an outer shell portion, or an outer shell portion and an inner shell portion. The central portion 24 may also have a hollow structure.

In addition, in the lithium composite oxide, layers may be formed entirely or partially between the central portion 24 and the substantial portion 21, or between multiple substantial portions 21. The central portion may be in a state in which multiple aggregate portions formed by aggregation of plate-like primary particles are connected together. In this specification, "electrically conductive" means that the high-density portions of the lithium metal composite oxide are directly structurally connected to each other and can be electrically conductive.

When the lithium metal composite oxide 20 containing the secondary particles 22 having the above-mentioned multilayer structure is used as a positive electrode, the electrolyte penetrates into the secondary particles 22 through the grain boundaries or voids 26 between the primary particles 21, making it possible to remove and insert lithium not only on the surface of the secondary particles 22 but also inside the secondary particles 22. Moreover, since the lithium metal composite oxide 20 has many electrically conductive paths inside the secondary particles 22, the resistance inside the particles can be made sufficiently small. Therefore, when this lithium metal composite oxide 20 is used to form a positive electrode for a secondary battery, it is possible to significantly improve the output characteristics without impairing the capacity characteristics or cycle characteristics.

The secondary particles having a multilayer structure including the hollow portion 25 and the space portion 26 can be formed by appropriately adjusting the conditions in the nucleation process (step S11) and the particle growth process (step S12) described above. For example, the conditions described in Patent Documents 2 and 3 may be used.

In the positive electrode active material according to this embodiment, a second crystallization process of adding an aqueous solution containing tungsten, as described above, is added consecutively to a conventionally known particle growth process during the production of secondary particles 22 (metal composite hydroxide) having a multilayer structure, thereby obtaining a metal composite hydroxide 10 having a tungsten-enriched layer 3 formed on the surface (surface), and a lithium metal composite oxide 20 using this as a precursor. Furthermore, the lithium metal composite oxide 20 can achieve further improvement in output characteristics while maintaining the crystallinity of the positive electrode active material containing secondary particles 22 having a multilayer structure, due to the presence of a compound 23 containing lithium and tungsten on the surface of the primary particles 21 and in the grain boundaries between the primary particles 21.

(BET specific surface area)
The BET specific surface area of the positive electrode active material is not particularly limited, but is preferably 1.45 ^m2 /g or more and 5.40 ^m2 /g or less, more preferably ² m2/g or more and 5.40 ^m2 /g or less, and even more preferably ^2.5 m2/g or more and 5.40 ^m2 /g or less. When the BET specific surface area is in the above range, the contact area with the electrolyte can be increased when used in the positive electrode of a secondary battery, the positive electrode resistance can be reduced, and the output characteristics can be improved.

(Tap Density)
The tap density of the positive electrode active material is not particularly limited, but is preferably 1.00 g/cm ³ or more and 2.00 g/cm ³ or less, and more preferably 1.2 g/cm ³ or more and 2.00 g/cm ³ or less, when the secondary particles 2 have a multilayer structure. When the tap density of the positive electrode active material is in the above range, the output characteristics are improved by increasing the contact area with the electrolyte while improving the battery capacity per unit volume. On the other hand, when the tap density exceeds 2.00 g/cm ³ , there are few voids in the particle structure, and the average particle size tends to be large, and the output characteristics decrease with the decrease in the reaction area. In addition, when the particle size distribution is large, the tap density tends to increase, but in this case, fine particles may be selectively deteriorated and the cycle characteristics may decrease. On the other hand, when the tap density is less than 1.00 g/cm ³ , there are many voids in the particle structure, and the particle strength decreases, so the cycle characteristics may decrease. In addition, the filling property of the positive electrode active material decreases, making it difficult to increase the battery capacity per unit volume.

(Average particle size)
The average particle size (MV) of the positive electrode active material is not particularly limited, but can be adjusted to, for example, 3 μm or more and 9 μm or less. When the average particle size is in the above range, not only can the battery capacity per unit volume of the secondary battery using this positive electrode active material be increased, but the thermal stability and output characteristics can also be improved. On the other hand, when the average particle size is less than 4 μm, the filling property of the positive electrode active material decreases, making it difficult to increase the battery capacity per unit volume. On the other hand, when the average particle size exceeds 9 μm, the reaction area of the positive electrode active material begins to decrease, so that the output characteristics may not be sufficient.

The average particle size of the positive electrode active material means the volume-based average particle size (MV), as with the metal composite hydroxide described above, and can be calculated, for example, from the integrated volume value measured with a laser light diffraction/scattering particle size analyzer.

(Particle size distribution)
The positive electrode active material of this embodiment preferably has an index of particle size distribution ((d90-d10)/average particle size) of 0.65 or less. When ((d90-d10)/average particle size) is in the above range, the positive electrode active material can be made of a lithium metal composite oxide 20 having a very narrow particle size distribution. Such a positive electrode active material has a low ratio of fine particles and coarse particles, and a secondary battery using this positive electrode has excellent thermal stability, cycle characteristics, and output characteristics.

On the other hand, when [(d90-d10)/average particle size] exceeds 0.65, the proportion of fine particles and coarse particles in the positive electrode active material increases. A secondary battery using a positive electrode active material with a wide particle size distribution may, for example, generate heat due to local reactions of fine particles, reduce thermal stability, and cause selective deterioration of fine particles, resulting in reduced cycle characteristics. In addition, a secondary battery using a positive electrode active material with a wide particle size distribution may have a high proportion of coarse particles, making it difficult to ensure a sufficient reaction area between the electrolyte and the positive electrode active material, resulting in reduced output characteristics.

In addition, assuming industrial-scale production, it is not realistic to use a positive electrode active material with an excessively small [(d90-d10)/average particle size]. Therefore, considering cost and productivity, it is preferable to set the lower limit of [(d90-d10)/average particle size] to about 0.25. The meanings of d10 and d90 in [(d90-d10)/average particle size] and the method of determining them are the same as those for the metal composite hydroxides described above.

(Sintering and agglomeration)
The positive electrode active material of this embodiment preferably has an index of sintering aggregation [lithium metal composite oxide d50/metal composite hydroxide d50 (hereinafter referred to as "d50 ratio") in the range of 0.95 to 1.05. When the d50 ratio is in the above range, the positive electrode active material can be composed of lithium metal composite oxide 20 with almost no aggregation of secondary particles associated with sintering aggregation. A secondary battery using such a positive electrode active material has high filling properties, high capacity, and excellent uniformity with little variation in characteristics.

On the other hand, when the d50 ratio is 1.05 or more, the specific surface area decreases due to sintering aggregation, and the packing property may also decrease. A secondary battery using such a positive electrode active material may have a decrease in output characteristics and a decrease in capacity due to a decrease in reactivity. Furthermore, when a secondary battery using such a positive electrode active material is repeatedly charged and discharged, the weak parts of the positive electrode where the secondary particles are aggregated may selectively collapse, which may cause a significant impairment of the cycle characteristics.

Furthermore, when the d50 ratio is less than 1.0, it means that by crushing the lithium metal composite oxide 20, some of the primary particles 21 are missing from the secondary particles 22, resulting in a decrease in particle size. In particular, when the d50 ratio is less than 0.95, a large amount of the missing primary particles 21 is present, resulting in a large amount of fine powder being generated and the particle size distribution may become broad.
The d50 of the metal composite hydroxide means the d50 of the metal composite hydroxide 10 used as a precursor in producing the lithium metal composite oxide 20. The d50 of each particle can be measured by a laser light diffraction scattering type particle size analyzer, and means the particle size at which the number of particles at each particle size is accumulated from the smallest particle size side and the accumulated volume is 50% of the total volume of all particles.

(Crystallite size)
The positive electrode active material according to this embodiment can have a larger crystallite diameter of the (003) plane obtained by powder X-ray diffraction measurement than the positive electrode active material using a metal composite hydroxide obtained by uniformly adding tungsten throughout the crystallization process as a precursor, as in the conventional manufacturing method. The crystallite diameter of the (003) plane of the positive electrode active material can be, for example, 110 nm or more, and is preferably adjusted to 120 nm or more. When the crystallite diameter of the (003) plane of the positive electrode active material is 120 nm or more, the crystallinity is high, and the secondary battery using this positive electrode active material as a positive electrode has improved output characteristics due to reduced reaction resistance, and also improves thermal stability. On the other hand, when the crystallite diameter of the (003) plane is less than 120 nm, the thermal stability of the secondary battery may decrease. The upper limit of the crystallite diameter of the (003) plane is not particularly limited, but can be, for example, 200 nm or less, and is preferably 120 nm or more and 150 nm or less. In the positive electrode active material of the present embodiment, by using the metal composite hydroxide 10 having the tungsten concentrated layer 3 on its surface as a precursor as described above, high crystallinity can be maintained, and the crystallite size of the (003) plane can be set within the above range.

(composition)
The composition of the positive electrode active material of the present embodiment is not particularly limited as long as it has the above-mentioned characteristics, and is, for example, represented by general formula (B): Li1+uNixMnyCozWaMbO2 (-0.05≦u≦0.50, x+y+z=1, 0.3≦x≦0.95, 0.05≦y≦0.55, 0≦z≦0.4, 0<a≦0.1, 0≦b≦0.1, M is one or more elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, and Ta).
In the above general formula (B), the value of u indicating the 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. When the value of u is within the above range, the output characteristics and capacity characteristics of a secondary battery using this positive electrode active material as a positive electrode material can be improved. On the other hand, when the value of u is less than -0.05, the positive electrode resistance of the secondary battery becomes large and the output characteristics cannot be improved. On the other hand, when the value of u exceeds 0.50, the initial discharge capacity may decrease or the positive electrode resistance may become large.

In the above general formula (B), the value of x, which indicates the content of nickel (Ni), is preferably 0.3 to 0.95, more preferably 0.3 to 0.9. Nickel is an element that contributes to increasing the potential and capacity of a secondary battery. If the value of x is less than 0.3, the capacity characteristics of a secondary battery using this positive electrode active material cannot be improved. On the other hand, if the value of x exceeds 0.95, the content of other elements decreases, and the effects of the other elements cannot be obtained.

In the above general formula (B), the value of y, which indicates the manganese (Mn) content, is preferably 0.05 or more and 0.55 or less, more preferably 0.10 or more and 0.40 or less. Manganese is an element that contributes to improving thermal stability. If the value of y is less than 0.05, the thermal stability of a secondary battery using this positive electrode active material cannot be improved. On the other hand, if the value of y exceeds 0.55, Mn may dissolve from the positive electrode active material during high-temperature operation, resulting in deterioration of the charge-discharge cycle characteristics.

In the above general formula (B), the value of z, which indicates the content of cobalt (Co), is preferably 0 to 0.4, more preferably 0.10 to 0.35. Cobalt is an element that contributes to improving the charge-discharge cycle characteristics. If the value of z exceeds 0.4, the initial discharge capacity of a secondary battery using this positive electrode active material may be significantly reduced.

In the above general formula (B), the value of a, which indicates the content of tungsten (W), is more than 0 and 0.1 or less, preferably 0.001 or more and 0.01 or less, and more preferably 0.0045 or more and 0.006 or less, when the total number of moles of Ni, Co, and Mn is 1. When the value of a is within the above range, the positive electrode active material has excellent output characteristics and cycle characteristics while maintaining high crystallinity. In addition, as described above, W is contained in the positive electrode active material mainly in the surface layer of the primary particles 21 near the surface of the secondary particles 22, or in the grain boundaries between the primary particles 21.

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 M may be contained. As such element M, one or more elements selected from magnesium (Mg), calcium (Ca), aluminum (Al), titanium (Ti), vanadium (V), chromium (Cr), zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium (Hf), and tantalum (Ta) can be used.

In the above general formula (B), the value of b, which indicates the content of element M, is preferably 0 to 0.1, more preferably 0.001 to 0.05, when the sum of the moles of Ni, Co, and Mn is 1. If the value of b exceeds 0.1, the amount of metal elements that contribute to the Redox reaction decreases, which may result in a decrease in battery capacity.

In the positive electrode active material represented by the general formula (B), from the viewpoint of further improving the capacity characteristics of the secondary battery, the composition is preferably the general formula (B1): Li _{1 + u} Ni _x Mn _y Co _z W _a MbO ₂ (-0.05≦u≦0.20, x + y + z = 1, 0.7 < x ≦ 0.95, 0.05 ≦ y ≦ 0.1, 0 ≦ z ≦ 0.2, 0 < a ≦ 0.1, 0 ≦ b ≦ 0.1, M is one or more elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, and Ta). Among them, from the viewpoint of achieving both thermal stability and battery capacity, the value of x in the general formula (B1) is more preferably 0.7 < x ≦ 0.9, and even more preferably 0.7 < x ≦ 0.85.

From the viewpoint of further improving the thermal stability, the composition is preferably Li1 _{+ uNixMnyCozWaMbO2} (-0.05 _≦ u≦0.50, x+y ₊ z= ₁ , 0.3≦x≦0.7, 0.1≦y≦0.55, 0≦z≦0.4, 0<a≦0.1 _, 0≦b≦0.1, _M is one or more elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, and Ta).

4. Manufacturing method of positive electrode active material for non-aqueous electrolyte secondary battery Fig. 6 is a diagram showing an example of a manufacturing method of a positive electrode active material according to the present embodiment. The manufacturing method according to the present embodiment can easily manufacture the positive electrode active material including the lithium metal composite oxide 20 described above on an industrial scale. The positive electrode active material including the lithium metal composite oxide 20 is not particularly limited as long as it has the above-mentioned specific structure, average particle size and particle size distribution, and can be obtained using a known manufacturing method.

As shown in FIG. 6, the method for producing a positive electrode active material according to this embodiment includes a step (step S30) of mixing the metal composite hydroxide obtained by the above-mentioned production method with a lithium compound to obtain a lithium mixture, and a step (step S40) of calcining the lithium mixture to obtain a lithium metal composite oxide. In addition to the above-mentioned steps, steps such as a heat treatment step and a calcination step may be added as necessary.

The tungsten in the tungsten-enriched layer formed on the surface layer of the metal composite hydroxide reacts with the lithium compound in the mixing step with the lithium compound (step S30) and the firing step (step S40) to form compounds 23 containing tungsten and lithium on the surface layer of the primary particles in the lithium metal composite oxide and in the grain boundaries between the primary particles. Below, the method for producing the positive electrode active material according to this embodiment will be described with reference to FIG. 6.

(Mixing process)
First, at least one of a metal composite hydroxide and a metal composite oxide obtained by heat-treating the metal composite hydroxide (hereinafter, these are also collectively referred to as "precursors") is mixed with a lithium compound to obtain a lithium mixture (step S30).

In the mixing step (step S30), the precursor and the lithium compound are mixed so that the ratio (Li/Me) of the sum of the atomic numbers of metal atoms other than lithium in the lithium mixture, specifically nickel, cobalt, manganese, and element M (Me) to the atomic number of lithium (Li) is 0.95 to 1.5, preferably 1.0 to 1.5, more preferably 1.0 to 1.35, and even more preferably 1.0 to 1.2. In other words, since the Li/Me ratio does not change before and after the firing step, the precursor and the lithium compound are mixed so that the Li/Me ratio in the mixing step is the Li/Me ratio of the desired positive electrode active material.

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

It is preferable to mix the precursor 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.

(Heat treatment process)
In the method for producing a positive electrode active material of this embodiment, a heat treatment step may be optionally provided before the mixing step (step S30) to heat-treat the metal composite hydroxide before mixing with the lithium compound (not shown). Here, the particles obtained after the heat treatment include not only particles of the metal composite hydroxide from which excess moisture has been removed in the heat treatment step, but also particles of a transition metal composite oxide (hereinafter referred to as "metal composite oxide") converted into an oxide by the heat treatment step, or a mixture thereof.

The heat treatment process is a process for removing excess moisture contained in the metal composite hydroxide by heating the metal composite hydroxide to 105°C or higher and 750°C or lower. This makes it possible to reduce the amount of moisture remaining after the firing process to a certain level, thereby suppressing variation in the composition of the resulting positive electrode active material.

The heating temperature in the heat treatment process is 105°C or higher and 750°C or lower. If the heating temperature is less than 105°C, the excess moisture in the metal composite hydroxide cannot be removed, and the variation may not be sufficiently suppressed. On the other hand, if the heating temperature exceeds 700°C, not only will no further effect be expected, but the production cost will also increase.

In the heat treatment process, it is not necessary to convert all of the metal composite hydroxides to composite oxides, since 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. 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 metal composite hydroxides to composite oxides. The above-mentioned variation can be further suppressed by determining in advance by analysis the metal components contained in the metal composite hydroxides under the heat treatment conditions and determining the mixing ratio with the lithium compound.

There are no particular limitations on the atmosphere in which the heat treatment is carried out, as long as it is a non-reducing atmosphere, but it is preferable to carry out the heat treatment in an air stream, which is easy to carry out.

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

(Firing process)
Next, the lithium mixture obtained in the mixing step (step S30) is calcined to obtain a lithium metal composite oxide (step S40). In this step, calcination is performed under predetermined conditions to diffuse lithium into the precursor to obtain a lithium metal composite oxide. The obtained lithium metal composite oxide may be used as it is as a positive electrode active material, or may be used as a positive electrode active material after adjusting the particle size distribution by a crushing step as described below.

[Firing temperature]
The calcination temperature of the lithium mixture is preferably 650° C. or more and 980° C. or less. If the calcination temperature is less than 650° C., lithium does not sufficiently diffuse in the precursor, and excess lithium or unreacted metal composite hydroxide or metal composite oxide remains, or the crystallinity of the resulting lithium metal composite oxide becomes insufficient. On the other hand, if the calcination temperature exceeds 980° C., the lithium composite oxide particles are severely sintered, causing abnormal grain growth, and the proportion of amorphous coarse particles may increase.

When attempting to obtain a positive electrode active material represented by the above-mentioned general formula (B1), it is preferable to set the sintering temperature to 650°C or higher and 900°C or lower. On the other hand, when attempting to obtain a positive electrode active material represented by the general formula (B2), it is preferable to set the sintering temperature to 800°C or higher and 980°C or lower.

The rate of temperature rise in the calcination step is preferably 2°C/min to 10°C/min, more preferably 5°C/min to 10°C/min. Furthermore, during the calcination step (step S40), it is preferable to maintain the temperature near the melting point of the lithium compound used for preferably 1 hour to 5 hours, more preferably 2 hours to 5 hours. This allows the precursor and the lithium compound to react more uniformly.

[Baking time]
The retention time (calcination time) at the calcination temperature is preferably at least 2 hours, and more preferably 4 hours to 24 hours. If the retention time at the calcination temperature is less than 2 hours, lithium will not be sufficiently diffused in the precursor, and excess lithium or unreacted metal composite hydroxide particles or heat-treated particles may remain, or the crystallinity of the resulting lithium composite oxide particles may be insufficient.

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 33°C/min to 77°C/min. By controlling the cooling rate within this range, it is possible to ensure productivity while preventing damage to equipment such as saggers due to rapid cooling.

[Firing atmosphere]
The atmosphere during the calcination is preferably an oxidizing atmosphere, more preferably an atmosphere with an oxygen concentration of 18% by volume or more and 100% by volume or less, and particularly preferably a mixed atmosphere of oxygen and an inert gas with the above oxygen concentration. That is, the calcination is preferably carried out in air or an oxygen stream. If the oxygen concentration is less than 18% by volume, the crystallinity of the lithium composite oxide particles may be insufficient.

The furnace used in the firing step (step S40) is not particularly limited as long as it can heat 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 type or a continuous type electric furnace can be suitably used. This also applies to the furnaces used in the heat treatment step and the calcination step.

(Pre-firing process)
When lithium hydroxide or lithium carbonate is used as the lithium compound, a calcination step may be performed after the mixing step (step S30) and before the calcination step (step S40). The calcination step is a step of calcining the lithium mixture at a temperature lower than the calcination temperature described below and at 350° C. to 800° C., preferably 450° C. to 780° C. This allows lithium to be sufficiently diffused in the precursor, and more uniform lithium composite oxide particles can be obtained.

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

(Crushing process)
The lithium metal composite oxide obtained by the firing step (step S40) may be aggregated or lightly sintered. In such a case, it is preferable to crush the aggregate or sinter of the lithium composite oxide particles. This makes it possible to adjust the average particle size and particle size distribution of the obtained positive electrode active material 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 aggregate.

As a method of crushing, known means can be used, such as a pin mill or a hammer mill. In this case, it is preferable to adjust the crushing force to an appropriate range so as not to destroy the secondary particles.

5. Non-aqueous electrolyte secondary battery The non-aqueous electrolyte secondary battery of the present embodiment includes the same components as a general non-aqueous electrolyte secondary battery, such as a positive electrode, a negative electrode, a separator, a non-aqueous electrolyte solution, etc. Note that the embodiments described below are merely examples, and the non-aqueous electrolyte secondary battery of the present invention can be applied to various forms that are modified and improved based on the embodiments described in this specification.

(1) Component (positive electrode)
The positive electrode active material for a nonaqueous electrolyte secondary battery obtained according to this embodiment is used to fabricate a positive electrode for a nonaqueous electrolyte secondary battery, for example, as follows.

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

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

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

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

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

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

As the negative electrode active material, for example, a material containing lithium such as metallic lithium or a lithium alloy, a fired organic compound such as natural graphite, artificial graphite, or phenolic resin capable of absorbing and desorbing lithium ions, or a powder of a 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)
The separator is sandwiched between the positive electrode and the negative electrode, and has the function of separating the positive electrode and the negative electrode and retaining the electrolyte. As such a separator, for example, a thin membrane such as polyethylene or polypropylene having many fine pores can be used, but there is no particular limitation as long as it has the above function.

(Non-aqueous electrolyte)
The non-aqueous electrolyte is prepared by dissolving a lithium salt as a supporting electrolyte in an organic solvent.
The organic solvent may be one selected from the group consisting of 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, and may be used alone or in combination of two or more.
As _the supporting salt, _LiPF6 , _LiBF4 , _LiClO4 , _LiAsF6 , LiN( _CF3SO2 ) ₂ , and composite salts thereof can be used.

The non-aqueous electrolyte may further contain a radical scavenger, a surfactant, a flame retardant, etc.

(2) Nonaqueous Electrolyte Secondary Battery The nonaqueous electrolyte secondary battery of the present invention, which is composed of the above-mentioned positive electrode, negative electrode, separator and nonaqueous electrolyte solution, can be formed into various shapes such as a cylindrical shape or a laminated shape.

Regardless of the shape, the positive and negative electrodes are stacked with a separator between them to form an electrode body, the resulting electrode body is impregnated with a nonaqueous electrolyte, the positive electrode current collector and the positive electrode terminal connected to the outside, and the negative electrode current collector and the negative electrode terminal connected to the outside are connected using current collecting leads or the like, and the nonaqueous electrolyte secondary battery is completed by sealing it in a battery case.

(3) Characteristics of the Non-Aqueous Electrolyte Secondary Battery As described above, the non-aqueous electrolyte secondary battery of the present invention uses the positive electrode active material of the present invention as a positive electrode material, and therefore has excellent capacity characteristics, output characteristics, and cycle characteristics. Moreover, it can be said that the non-aqueous electrolyte secondary battery of the present invention has excellent thermal stability in comparison with a secondary battery using a positive electrode active material made of conventional lithium nickel-based oxide particles.

(4) Applications As described above, the nonaqueous electrolyte secondary battery of the present invention has excellent capacity characteristics, output characteristics, and cycle characteristics, and can be suitably used as a power source for small portable electronic devices (such as notebook personal computers and local telephone terminals) that require a high level of these characteristics. In addition, the nonaqueous electrolyte secondary battery of the present invention has excellent thermal stability, and can be made compact and have high output. In addition, expensive protection circuits can be simplified, so that the battery can be suitably used as a power source for transportation equipment that is limited by its mounting space.

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

Example 1
(a) Production of metal composite hydroxide [Nucleation step]
First, 1.2 L of water was put into the reaction tank and stirred at 790 rpm while setting the temperature inside the tank to 40 ° C. At this time, nitrogen gas was introduced into the reaction tank and circulated for 30 minutes, and the reaction atmosphere was made a non-oxidizing atmosphere with an oxygen concentration of 1 volume % or less. Next, an appropriate amount of 25 mass% sodium hydroxide aqueous solution and 25 mass% ammonia water were supplied into the reaction tank, and the pH value was adjusted to 12.5 at a liquid temperature of 25 ° C. and the ammonium ion concentration was adjusted to 10 g / L to form a pre-reaction aqueous solution.

At the same time, nickel sulfate, cobalt sulfate, manganese sulfate, and zirconium sulfate were dissolved in water so that the molar ratio of each metal element was Ni:Mn:Co:Zr = 38:30:32:0.2, to prepare a first raw material aqueous solution of 2 mol/L.

Next, the first raw water reaction solution was fed to the pre-reaction aqueous solution at 13 ml/min to form an aqueous solution for the nucleation process, and nucleation was carried out for 2.5 minutes. At this time, 25% by mass of sodium hydroxide aqueous solution and 25% by mass of ammonia water were fed at appropriate times to maintain the pH value and ammonium ion concentration of the aqueous solution for nucleation within the above-mentioned ranges.

[Particle growth process]
After the nucleation was completed, the supply of all the aqueous solutions was temporarily stopped, and sulfuric acid was added to the reaction tank to adjust the pH value to 11.6 at a liquid temperature of 25° C., thereby forming an aqueous solution for particle growth. After it was confirmed that the pH value had reached a predetermined value, the first aqueous raw material solution was supplied to the reaction tank, and the nuclei (particles) generated in the nucleation step were allowed to grow.

[Second crystallization step]
The first raw material aqueous solution and an aqueous solution containing tungsten were used as the second raw material aqueous solution. As the aqueous solution containing tungsten, sodium tungstate dihydrate was dissolved in water so that the molar ratio of each metal element of the resulting hydroxide was Ni:Co:Mn:Zr:W=38:30:32:0.2:0.6, to prepare an aqueous sodium tungstate solution.

After 19/20 hours (95%) of the total time for particle growth had elapsed, the first aqueous solution was supplied and the supply of the aqueous sodium tungstate solution to the reaction tank was started (addition range: 5%). The supply of all aqueous solutions was stopped to terminate the particle growth process. The product obtained was then washed with water, filtered and dried to obtain a powdered metal composite hydroxide.

In the first crystallization step and the particle growth step of the second crystallization step, the non-oxidizing atmosphere and the oxidizing atmosphere were switched four times. Specifically, after switching from the non-oxidizing atmosphere to the oxidizing atmosphere (first time), the atmosphere was switched back to the non-oxidizing atmosphere (second time). Furthermore, after switching to the oxidizing atmosphere (third time), the atmosphere was switched back to the non-oxidizing atmosphere (fourth time). As the non-oxidizing atmosphere, nitrogen gas was introduced and an atmosphere with an oxygen concentration of 1% by volume or less was used, and as the oxidizing atmosphere, air was used. The timing of switching the reaction atmosphere was when 20%, 40%, 60%, and 80% of the total time for particle growth had elapsed.

In the particle growth process in the first crystallization process and the second crystallization process, 25% by mass of sodium hydroxide aqueous solution and 25% by mass of ammonia water were supplied at appropriate times throughout these processes to maintain the pH value and ammonium ion concentration of the aqueous solution for particle growth within the above-mentioned ranges. The supply rate of the first raw material aqueous solution was kept constant (13 ml/min) throughout the entire crystallization process.

(b) Evaluation of Metal Composite Hydroxide By analysis using _an ICP optical emission spectrometer (Shimadzu Corporation, ICPE- _9000ICPE -9000) _, it was confirmed that this metal composite hydroxide was represented by the general formula: _{Ni0.38Mn0.30Co0.32Zr0.002W0.005} (OH) _{2 .}

In addition, a laser light diffraction/scattering particle size analyzer (Microtrac MT3300EX2, manufactured by Microtrac Bell Co., Ltd.) was used to measure the average particle size of the metal composite hydroxide, as well as d10 and d90, and the index showing the spread of the particle size distribution, [(d90-d10)/average particle size], was calculated. As a result, it was confirmed that the average particle size of the metal composite hydroxide was 5.4 μm, and [(d90-d10)/average particle size] was 0.45.

In addition, to confirm the presence or absence of a tungsten-enriched layer in the metal complex hydroxide and its thickness, a surface analysis of the cross section of the metal complex hydroxide was performed using an energy dispersive X-ray analyzer (EDX) mounted on a scanning transmission electron microscope (HD-2300A, manufactured by Hitachi High-Technologies Corporation). As a result, the formation of a region where tungsten is concentrated (tungsten-enriched layer) was confirmed on the surface layer of the metal complex hydroxide, and the thickness was confirmed to be 3 m to 8 m.

The tap density was measured using a shaking specific gravity meter (KRS-409, manufactured by Kuramochi Scientific Instruments Manufacturing Co., Ltd.) by filling a 20 ml measuring cylinder with the obtained metal composite hydroxide, and then densely filling the measuring cylinder by repeatedly subjecting the cylinder to free fall from a height of 2 cm 500 times. As a result, it was confirmed that the tap density was 0.75 g/ ^cm3 or more and 1.35 g/ ^cm3 or less.

(c) Preparation of Positive Electrode Active Material The metal composite hydroxide obtained as described above was thoroughly mixed with lithium carbonate using a shaker mixer (TURBULA Type T2C manufactured by WAB) so that the Li/Me ratio was 1.14, thereby obtaining a lithium mixture.
The lithium mixture was heated to 900°C at a heating rate of 2.52.5°C/min in an air stream (oxygen concentration: 21% by volume), and then calcined by holding at this temperature for 4 hours, and cooled to room temperature at a cooling rate of about 4°C/min. The positive electrode active material thus obtained had agglomerated or lightly sintered. Therefore, the positive electrode active material was crushed to adjust the average particle size and particle size distribution.

(d) Evaluation of the positive electrode active material Analysis using an ICP emission spectrometer confirmed that this positive electrode active material was represented by the general formula: Li _1.14 Ni _0.38 Mn _0.30 Co _0.32 Zr _0.002 W _0.005 O _2. In addition, using a laser light diffraction scattering type particle size analyzer, the average particle size of the lithium metal composite oxide was measured, and d10 and d90 were measured, and [(d90-d10)/average particle size], which is an index showing the spread of the particle size distribution, was calculated. As a result, it was confirmed that the average particle size of the lithium metal composite oxide was 5.3 μm, [(d90-d10)/average particle size] was 0.45, and the d50 ratio was 1.04.

The crystallite diameter of the (003) plane was measured using an X-ray diffraction device (Spectris Corporation, X'Pert PRO) and found to be 1385 Å (138.5 nm).

In addition, to confirm the distribution of tungsten in the lithium metal composite oxide, a surface analysis of the cross section of the lithium metal composite oxide was performed using an energy dispersive X-ray analyzer (EDX) mounted on a scanning transmission electron microscope (HD-2300A, manufactured by Hitachi High-Technologies Corporation). As a result, it was confirmed that tungsten is contained in large amounts in the surface layers of the primary particles near the surfaces of the secondary particles and in the grain boundaries between the primary particles in the lithium metal composite oxide.

The tap density was evaluated under the same conditions as for the metal composite hydroxides, and the BET specific surface area was evaluated using a specific surface area measuring device (Maxsorb 1200 series, manufactured by Mountec Co., Ltd.) that employs the flow method-nitrogen gas adsorption method.

(e) Fabrication of Secondary Battery Fig. 7 is a diagram showing a 2032-type coin battery CBA used in the evaluation of battery characteristics. Hereinafter, a method for fabricating the secondary battery will be described with reference to Fig. 7.

52.5 mg of the positive electrode active material obtained as described above, 15 mg of acetylene black, and 7.5 mg of PTFE were mixed, and 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 a positive electrode PE.

Next, a 2032-type coin battery CBA was produced using this positive electrode PE in a glove box in an Ar atmosphere with a dew point controlled at -80°C. The negative electrode NE of this 2032-type coin battery was made of lithium metal with a diameter of 17 mm and a thickness of 1 mm, and the electrolyte was made of an equal mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with _1M LiClO4 as the supporting electrolyte (manufactured by Toyama Pharmaceutical Co., Ltd.). In addition, a polyethylene porous film with a thickness of 25 μm was used for the separator SE. The 2032-type coin battery CBA had a gasket GA and was assembled into a coin-shaped battery with a positive electrode can PC and a negative electrode can NC.

(f) Battery evaluation [resistance]
The resistance value of the coin battery CBA assembled above was measured by an AC impedance method at an SOC of 20%, and the relative value based on Comparative Example 1 was calculated as the resistance value for Ref., which was 93%. These results are shown in Table 1.

(Examples 2 to 8)
A metal composite hydroxide was obtained under the same conditions as in Example 1, except that the timing (addition time) of adding the aqueous solution containing tungsten during particle growth was changed as shown in Table 1. The evaluation results of the obtained metal composite hydroxide are shown in Table 1.

In addition, an energy dispersive X-ray analyzer (EDX) mounted on a scanning transmission electron microscope (Hitachi High-Technologies Corporation, HD-2300A) was used to perform surface analysis of the cross sections of the metal composite hydroxides obtained in Examples 2 to 8, and the distribution of W obtained was analyzed. As a result, it was confirmed that the metal composite hydroxides obtained in Examples 2 to 8 had a region where tungsten was concentrated on the surface layer of the secondary particles (tungsten-concentrated layer), and that the thickness was 20 nm or more and 160 nm or less.

Next, a positive electrode active material and a secondary battery were obtained under the same conditions as in Example 1, except that the obtained metal composite hydroxide was used as a precursor. The evaluation results of the obtained metal composite hydroxide and the positive electrode active material are shown in Table 2. In addition, an energy dispersive X-ray analyzer (EDX) mounted on a scanning transmission electron microscope (Hitachi High-Technologies Corporation, HD-2300A) was used to perform surface analysis of the cross section of the lithium metal composite oxide obtained in Examples 2 to 8, and the distribution of W obtained was analyzed. As a result, it was confirmed that the lithium metal composite oxide obtained in Examples 2 to 8 had a large amount of tungsten present in the surface layer of the primary particles near the surface of the secondary particles, and in the grain boundaries between the primary particles.

(Comparative Example 1)
A metal composite hydroxide was obtained under the same conditions as in Example 1, except that the tungsten compound was added from the start of the particle growth process (the addition range was 100%). The evaluation results of the obtained metal composite hydroxide are shown in Table 1. Next, a positive electrode active material and a secondary battery were produced under the same conditions as in Example 1, except that the obtained metal composite hydroxide was used as a precursor. The evaluation results of the obtained positive electrode active material and secondary battery are shown in Tables 1 and 2.

(Comparative Example 2)
A _tungsten -free metal composite hydroxide ( _{Ni0.38Mn0.30Co0.32Zr0.002} (OH) ₂ ) was used, and when the metal composite hydroxide was _mixed with lithium carbonate _, tungsten oxide was also added and mixed to obtain a lithium mixture (external addition). _{A positive} electrode active material ( _{Li1.14Ni0.38Mn0.30Co0.32Zr0.002W0.005O2} ) and a secondary battery _were produced _under the same conditions as in Example 1. The evaluation results of the obtained positive electrode active material and _secondary battery are shown in Tables ₁ and 2.

(Comparative Example 3)
In the crystallization step, except that the sodium tungstate aqueous solution was not supplied in the second crystallization step, a metal composite hydroxide, a positive electrode active material, and a secondary battery were produced under the same conditions as in Example 1. The evaluation results of the obtained metal composite hydroxide, positive electrode active material, and secondary battery are shown in Tables 1 and 2.

(Evaluation results)
It was confirmed that the metal composite hydroxide obtained in the examples formed a tungsten-enriched layer on its surface. Furthermore, the positive electrode active material obtained in the examples had a larger crystallite size and showed a lower resistance value when used as a positive electrode of a secondary battery, compared to Comparative Example 1 in which tungsten was added throughout the crystallization process.

In addition, the metal complex hydroxides obtained in Examples 1 to 6 formed a tungsten-enriched layer with a thickness of 100 nm or less, and the positive electrode active materials obtained using these metal complex hydroxides as precursors had larger crystallite diameters and showed lower resistance values when used as the positive electrode of a secondary battery, even when compared to Comparative Example 2 in which a tungsten compound was externally added.

On the other hand, the positive electrode active material obtained in Comparative Example 3, in which tungsten was not added, had a relatively large crystallite size compared to other Examples and Comparative Examples in which tungsten was added, but when used as a positive electrode for a secondary battery, it was shown to have a large resistance value and poor output characteristics.

10... Metal composite hydroxide 1... Primary particle 2... Secondary particle 3... Tungsten concentrated layer 4... Center 5... Void 6... Substance C... Center 20... Lithium metal composite oxide 21... Primary particle 22... Secondary particle 23... Compound containing tungsten and lithium 24... Center 25... Substance 26... Void CBA... Coin battery CA... Case PC... Positive electrode can NC... Negative electrode can GA... Gasket PE... Positive electrode NE... Negative electrode SE... Separator

Claims (14)

A method for producing a metal composite hydroxide comprising nickel, manganese, and tungsten, and optionally cobalt and an element M, and having an atomic ratio of the respective metal elements expressed as Ni:Mn:Co:W:M=x:y:z:a:b (x+y+z=1, 0.3≦x≦0.95, 0.05≦y≦0.55, 0≦z≦0.4, 0<a≦0.1, 0≦b≦0.1, M is one or more elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, and Ta),
a first crystallization step of supplying a first raw material aqueous solution containing the metal element and an ammonium ion donor to a reaction tank, adjusting the pH of the reaction aqueous solution in the reaction tank, and performing a crystallization reaction to obtain first metal composite hydroxide particles;
a second crystallization step of supplying a second raw material aqueous solution containing the metal element and containing more tungsten than the first raw material aqueous solution, and an ammonium ion donor to a reaction aqueous solution containing the first metal composite hydroxide particles, adjusting the pH of the reaction aqueous solution, and carrying out a crystallization reaction to form a tungsten-concentrated layer on the surface of the first metal composite hydroxide particles, thereby obtaining second metal composite hydroxide particles;
In the particle growth in the first crystallization step and the second crystallization step, the reaction atmosphere is switched multiple times to either a non-oxidizing atmosphere having an oxygen concentration of 1% by volume or less, or an oxidizing atmosphere having an oxygen concentration of more than 1% by volume,
supplying the metal element in the first raw material aqueous solution to the reaction tank in a range of 50 mass % or more and 95 mass % or less with respect to the total amount of metal added in the first crystallization step and the second crystallization step, and then supplying the second raw material aqueous solution in the second crystallization step;
The metal composite hydroxide contains secondary particles formed by agglomeration of a plurality of primary particles,
the secondary particles have a multilayer structure including, from the center to the surface of the secondary particles, a central portion in which the primary particles are densely arranged, a void portion in which the primary particles are more sparsely arranged than the central portion, and a solid portion in which the primary particles are densely arranged,
The tap density of the metal composite hydroxide is 0.75 g/ ^cm3 or more and 1.35 g/ ^cm3 or less.
A method for producing a metal composite hydroxide. The first crystallization step includes a nucleation step of performing nucleation and a particle growth step of performing particle growth, and the second crystallization step includes performing particle growth subsequent to the particle growth step,
The nucleation in the first crystallization step is carried out in a non-oxidizing atmosphere having an oxygen concentration of 1% by volume or less;
2. The method for producing a metal composite hydroxide according to claim 1, wherein the particle growth in the first crystallization step and the second crystallization step includes adjusting the pH of the reaction aqueous solution to be lower than the pH value of the reaction aqueous solution in the nucleation step, and switching the reaction atmosphere to either a non-oxidizing atmosphere having an oxygen concentration of 1 vol % or less or an oxidizing atmosphere having an oxygen concentration of more than 1 vol %, four times. 3. The method for producing a metal composite hydroxide according to claim 1 or 2, wherein the second crystallization step includes forming the tungsten concentrated layer to a thickness of 100 nm or less in a direction from a surface toward a center of the second metal composite hydroxide particle. The method for producing a metal composite hydroxide according to any one of claims 1 to 3, wherein the addition of the second raw material aqueous solution in the second crystallization step is performed when 50% to 95% of the total time during which particles grow in the first and second crystallization steps has elapsed. The method for producing a metal composite hydroxide according to any one of claims 1 to 4, wherein the second raw material aqueous solution is supplied by separately supplying the first raw material aqueous solution and the aqueous solution containing tungsten to the reaction aqueous solution. The method for producing a metal composite hydroxide according to claim 5, wherein the tungsten concentration in the tungsten-containing aqueous solution is 18 mass% or more relative to the entire tungsten-containing aqueous solution. A metal composite hydroxide comprising nickel, manganese, and tungsten, and optionally cobalt and an element M, and having an atomic ratio of the respective metal elements expressed as Ni:Mn:Co:W:M=x:y:z:a:b (x+y+z=1, 0.3≦x≦0.95, 0.05≦y≦0.55, 0≦z≦0.4, 0<a≦0.1, 0≦b≦0.1, M is one or more elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, and Ta),
The secondary particles are formed by agglomerating a plurality of primary particles.
the secondary particles have a multilayer structure including, from the center toward the surface of the secondary particles, at least a central portion in which the primary particles are densely arranged, a void portion in which the primary particles are more sparsely arranged than the central portion, and a solid portion in which the primary particles are densely arranged,
A tungsten-enriched layer is formed in a direction from the surface of the secondary particle toward the center thereof,
The tap density is 0.75 g/cm3 or ^more and 1.35 g/ ^cm3 or less.
Metal complex hydroxides. The metal complex hydroxide according to claim 7, wherein the thickness of the tungsten-enriched layer is 100 nm or less. The average particle size of the metal composite hydroxide is 4.0 μm or more and 9.0 μm or less, and the index indicating the spread of the particle size distribution, [(d90-d10)/average particle size], is 0.65 or less. The metal composite hydroxide according to claim 7 or 8. A step of mixing at least one of the metal composite hydroxide obtained by the production method according to any one of claims 1 to 6 and a metal composite oxide obtained by heat-treating the metal composite hydroxide with a lithium compound to obtain a lithium mixture;
and calcining the lithium mixture to obtain a lithium metal composite oxide. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 10, wherein the value of (d50 of the positive electrode active material/d50 of the metal composite hydroxide), which is an index showing the degree of aggregation between particles of the lithium metal composite oxide, is adjusted to be 0.95 or more and 1.05 or less. a lithium metal composite oxide containing lithium, nickel, manganese, and tungsten, and optionally cobalt and an element M, in which the atomic ratio of the respective metal elements is represented by Li:Ni:Co:Mn:W:M=1+u:x:y:z:a:b (x+y+z=1, -0.05≦u≦0.50, 0.3≦x≦0.95, 0.05≦y≦0.55, 0≦z≦0.4, 0<a≦0.1, 0≦b≦0.1, M is one or more elements selected from Mg, Ca, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, and Ta);
The lithium metal composite oxide includes secondary particles formed by agglomeration of a plurality of primary particles,
the secondary particles have a multilayer structure including, from the center toward the surface of the secondary particles, at least a central portion in which the primary particles are densely arranged, a void portion in which the primary particles are more sparsely arranged than the central portion, and a solid portion in which the primary particles are densely arranged,
a compound containing tungsten and lithium is present in a concentrated state on the surface of the secondary particles or on the surface layers of the primary particles present inside the secondary particles and in grain boundaries between the primary particles;
The tap density is 1.00 g/cm ³ or more and 2.00 g/cm ³ or less, and
The BET specific surface area is 1.45 m ² /g or more and 5.40 m ² /g or less.
Positive electrode active material for non-aqueous electrolyte secondary batteries. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 12, wherein the crystallite size of the (003) plane obtained by powder X-ray diffraction measurement is 120 nm or more. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, the positive electrode active material for non-aqueous electrolyte secondary batteries according to claim 12 or 13 being used as a positive electrode material for the positive electrode.

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