JP7273260B2 - Positive electrode active material for lithium ion secondary battery, method for producing the same, and lithium ion secondary battery - Google Patents

Description

The present invention relates to a positive electrode active material for lithium ion secondary batteries, a method for producing the same, and a lithium ion secondary battery using this positive electrode active material for lithium ion secondary batteries as a positive electrode active material.

In recent years, with the spread of personal digital assistants such as smart phones and tablet PCs, there is a strong demand for the development of small, lightweight non-aqueous electrolyte secondary batteries with high energy density. Also, there is a strong demand for the development of high-output secondary batteries as power sources for electric vehicles such as hybrid electric vehicles, plug-in hybrid electric vehicles, and battery-powered electric vehicles.

As a secondary battery that satisfies such requirements, there is a lithium ion secondary battery. This lithium ion secondary battery is composed of a negative electrode, a positive electrode, an electrolytic solution, and the like, and a material capable of desorbing and intercalating lithium is used as an active material used as a material for the negative electrode and the positive electrode.

Among these lithium ion secondary batteries, a lithium ion secondary battery using a layered or spinel type lithium transition metal composite oxide as a positive electrode active material can obtain a voltage of 4V class, so it is a battery with high energy density. At present, research and development are actively carried out, and some of them are also being put to practical use.

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

By the way, in order to obtain a lithium ion secondary battery with excellent cycle characteristics and output characteristics, it is necessary that the positive electrode active material be composed of particles having a small particle size and a narrow particle size distribution. This is because particles with a small particle size have a large specific surface area, and when used as a positive electrode active material, not only can a sufficient reaction area with the electrolyte be ensured, but also the positive electrode can be formed thin, and lithium ions can be produced. This is because the movement distance between the positive electrode and the negative electrode can be shortened, so that the resistance of the positive electrode can be reduced. In addition, since particles with a narrow particle size distribution can uniformize the voltage applied to the particles in the electrode, it is possible to suppress a decrease in battery capacity due to selective deterioration of the fine particles.

For example, Patent Documents 1 to 4 disclose a crystallization reaction that is clearly separated into two stages: a nucleation step in which nucleation is mainly performed and a particle growth step in which particles are mainly grown. A method for producing transition metal composite hydroxide particles is disclosed. In these methods, a precursor having a small particle size and a narrow particle size distribution is obtained by appropriately adjusting the pH value and the reaction atmosphere in the nucleation step and the particle growth step. A positive electrode active material with a narrow particle size distribution is obtained. Furthermore, by controlling the atmospheres of the nucleation step in which nucleation is mainly performed and the atmosphere of the particle growth step in which particles are mainly grown, a low-density core composed of fine primary particles and a high-density core composed of plate-like or needle-like primary particles can be obtained. A positive electrode active material having a hollow structure is obtained using such precursors.

JP 2011-116580 A JP 2012-246199 A JP 2013-147416 A WO2012/131881

However, the production methods described in these patent documents can improve the output characteristics by improving the particle structure, but it is required to control the specific surface area of the positive electrode active material and further improve the battery capacity and cycle characteristics. ing.

In view of the above problems, the present invention provides a positive electrode for a lithium ion secondary battery that can further improve battery capacity and cycle characteristics by controlling the specific surface area of a positive electrode active material when a secondary battery is configured. An object of the present invention is to provide an active material and a method for producing the same. Another object of the present invention is to provide a lithium ion secondary battery using this positive electrode active material.

The present inventors have conducted intensive research to further improve the battery capacity and cycle characteristics of positive electrode active materials for lithium ion secondary batteries (hereinafter also referred to as "positive electrode active materials") based on prior art such as Patent Document 3. repeated. As a result, in the step of firing the mixture of the precursor and the lithium compound, by firing after calcining at a specific temperature, sintering aggregation of the secondary particles constituting the positive electrode active material can be suppressed. We have found that it is possible to improve the characteristics when used in a battery. The present invention has been completed based on these findings.

That is, one embodiment of the present invention is a method for producing a positive electrode active material for a lithium ion secondary battery, which comprises a method for producing a positive electrode active material of general formula (A): Ni _{x Mny} Co _z M _t (OH) _2+a (x+y+z+t=1, 0. 3≤x≤0.8, 0.05≤y≤0.55, 0≤z≤0.4, 0≤t≤0.1, 0≤a≤0.5, M is Nb, Mo, Ta , Zr, and one or more additional elements selected from W), and is calculated by the cumulative 90 volume% diameter (d90) and the cumulative 10 volume% diameter (d10), and the volume-based average particle diameter (MV) transition metal composite hydroxide particles whose [(d90-d10)/MV], which is an index indicating the spread of the particle size distribution, is 0.65 or less , and/or converted from the transition metal composite hydroxide particles A mixing step of mixing transition metal composite oxide particles and a lithium compound to form a lithium mixture, and a calcining step of calcining the lithium mixture formed in the mixing step at 550° C. to 745° C. in an oxidizing atmosphere. and a firing step of firing the lithium mixture calcined in the calcining step at 760 ° C. to 1050 ° C. in an oxidizing atmosphere, and the transition metal composite hydroxide particles are composed of a plurality of aggregated primary particles. It is a substantially spherical secondary particle formed by and in the calcining step, the carbon content in the lithium mixture after calcining is controlled to be 1.2% by mass or less.

According to one aspect of the present invention, by calcining after calcining at a specific temperature, it is possible to suppress sintering aggregation of secondary particles constituting the positive electrode active material, and the characteristics when used in a battery can be improved.

By controlling the carbon content within the above range, the specific surface area of the positive electrode active material can be within an appropriate range.

Moreover, one aspect of the present invention may further include a pre-firing crushing step of crushing the lithium mixture calcined in the calcining step before the calcining step.

In this way, by pulverizing the lithium mixture calcined in the calcining step, it is possible to suppress sintering aggregation of the secondary particles constituting the positive electrode active material after the sintering step.

Further, in one aspect of the present invention, in the mixing step, the lithium mixture is mixed so that the ratio of the sum of the number of atoms of metals other than lithium contained in the lithium mixture to the number of atoms of lithium is 1:0.95 to 1. 0.5 may be adjusted.

By doing so, it is possible to obtain a positive electrode active material having a desirable composition range.

Further, one aspect of the present invention may further include a heat treatment step of heat-treating the transition metal composite hydroxide particles at 105° C. to 750° C. before the mixing step.

By doing so, the amount of water remaining until after the baking process can be reduced to a certain amount, so that variations in the composition of the obtained positive electrode active material can be suppressed.

Another aspect of the present invention is a positive electrode active material for a lithium ion secondary battery comprising lithium transition metal composite oxide particles, which has the general formula (B): Li _1+u Ni _{x Mny} Co _{z MtO 2} (− 0.05≦u≦0.50, x+y+z+t=1, 0.3≦x≦0.8, 0.05≦y≦0.55, 0≦z≦0.4, 0≦t≦0.1, M is represented by one or more additional elements selected from Nb, Mo, Ta, Zr, and W), has a hexagonal crystal structure with a layered structure, and is formed by aggregation of a plurality of primary particles The secondary particles have a hollow structure consisting of an outer shell portion and a hollow portion present inside thereof, and when the positive electrode active material is observed using a scanning electron microscope , the ratio of the number of aggregated particles in which 5 or more secondary particles are aggregated to the number of secondary particles in the observation field is 0.7% or less, and the specific surface area is 0.3 m ² /g to 4.0 m ² /g, which is an index showing the spread of the particle size distribution calculated from the cumulative 90 volume% diameter (d90), the cumulative 10 volume% diameter (d10), and the volume-based average particle diameter (MV) [( d90−d10)/MV] is 0.55 or less, and the crystallite diameter is 100 nm to 160 nm.

According to another aspect of the present invention, since the specific surface area is controlled and aggregation of the secondary particles constituting the positive electrode active material is suppressed, output characteristics when used in a secondary battery are improved.

In this way, when used as a positive electrode active material for a secondary battery, the voltage applied to the particles in the electrode can be made uniform, so that the decrease in battery capacity due to selective deterioration of the fine particles can be suppressed. .

In another aspect of the present invention, the secondary particles may have an average particle size of 3 μm to 15 μm.

In this way, when used as a positive electrode active material for a secondary battery, the battery capacity per unit volume of the secondary battery can be increased, and safety and output characteristics can be improved.

Another aspect of the present invention is a lithium ion secondary battery comprising at least a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the positive electrode active material for the positive electrode is the positive electrode for a lithium ion secondary battery described above. A lithium ion secondary battery using an active material.

According to another aspect of the present invention, there is provided a lithium ion secondary battery in which output characteristics such as battery capacity and positive electrode resistance are further improved.

According to the present invention, it is possible to provide a positive electrode active material that can control the specific surface area and further improve the battery capacity and cycle characteristics when a secondary battery is formed. Batteries can be provided. Therefore, the industrial significance of the present invention is extremely large.

FIG. 1 is a process drawing showing an outline of a method for producing a positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic cross-sectional view of a coin-type battery used for battery evaluation of Examples of positive electrode active materials for lithium ion secondary batteries according to one embodiment of the present invention. (A) is a Nyquist plot showing measurement results of impedance evaluation of an example of a positive electrode active material for a lithium ion secondary battery according to an embodiment of the present invention, and (B) is used for analysis of the impedance evaluation. 1 is a schematic explanatory diagram of an equivalent circuit.

Hereinafter, the positive electrode active material for lithium ion secondary batteries, the method for producing the same, and the lithium ion secondary battery according to the present invention will be described in the following order. The present invention is not limited to the following examples, and can be arbitrarily modified without departing from the gist of the present invention.
1. Transition metal composite hydroxide particles 2 . Method for producing transition metal composite hydroxide particles3. Method for producing positive electrode active material for lithium ion secondary battery 3-1. Heat treatment process 3-2. Mixing step 3-3. Calcination process 3-4. Crushing step before firing 3-5. Firing step 3-6. Crushing process 4 . 4. Positive electrode active material for lithium ion secondary battery; lithium ion secondary battery

<1. Transition metal composite hydroxide particles>
First, the transition metal composite hydroxide particles (hereinafter referred to as "composite hydroxide particles") used as a precursor of the positive electrode active material for lithium ion secondary batteries according to one embodiment of the present invention will be described. The composite hydroxide particles are preferably composed of secondary particles formed by aggregation of a plurality of plate-like primary particles. Furthermore, the secondary particles preferably have a central portion formed by agglomeration of fine primary particles, and an outer peripheral portion formed by agglomeration of plate-like primary particles outside the central portion. It is more preferable that the central portion contains the additive element at a higher concentration than the outer peripheral portion. By using composite hydroxide particles having such a central portion, it is possible to obtain a positive electrode active material with a hollow structure having higher output characteristics. Also, the average particle size of the secondary particles is preferably 3 μm to 15 μm.

(1) Particle Structure a) Structure of Secondary Particle The composite hydroxide particles suitable for use in the present invention are substantially spherical secondary particles formed by aggregation of a plurality of plate-like primary particles. Further, it is preferable to have a structure in which the inside of the particle has a central portion made of fine primary particles, and an outer peripheral portion made of plate-like primary particles larger than the fine primary particles outside the central portion. Due to such a structure in which a plurality of plate-like primary particles are aggregated, in the sintering step of forming the lithium-nickel-manganese composite oxide, which is the positive electrode active material according to one embodiment of the present invention, diffusion of lithium into the particles is suppressed. Since the reaction is carried out sufficiently, a positive electrode active material having a uniform distribution of lithium and having a good quality can be obtained. Such composite hydroxide particles and methods for producing the same are disclosed in detail in Patent Document 1 and Patent Document 2, for example.

Here, since the central part has a structure with many gaps in which fine primary particles are connected, compared to the outer peripheral part made of plate-shaped primary particles that are larger and thicker, shrinkage due to sintering in the firing process starts at a low temperature. Occur. For this reason, sintering progresses from a low temperature during firing, shrinking from the center of the particles toward the outer periphery where sintering progresses slowly, creating a space in the center. In addition, since the central portion is considered to have a low density and a large shrinkage rate, the central portion becomes a space having a sufficient size. Thereby, the positive electrode active material obtained after firing has a hollow structure.

Further, it is more preferable if the plate-like primary particles aggregate in random directions to form secondary particles. By aggregating the plate-like primary particles in random directions, gaps are generated almost uniformly between the primary particles. is sufficiently diffused. In addition, since the composite hydroxide particles having the central part are agglomerated in random directions, the central part shrinks evenly in the firing process, so that the inside of the positive electrode active material has a sufficient size. It is possible to form a space with

In order to form spaces during firing, the fine primary particles preferably have an average particle diameter of 0.01 to 0.3 μm, more preferably 0.1 to 0.3 μm. The plate-like primary particles preferably have an average particle size of 0.3 to 3 μm, more preferably 0.4 to 1.5 μm, particularly 0.4 to 1.0 μm. preferable.

(2) Average particle size The composite hydroxide particles used for the positive electrode active material for lithium ion secondary batteries according to one embodiment of the present invention have an average secondary particle size of 3 μm to 15 μm, preferably 3 μm to 12 μm. , more preferably 3 μm to 10 μm. The average particle size of the secondary particles correlates with the average particle size of the positive electrode active material whose precursor is the composite hydroxide particles. Therefore, by controlling the average particle size of the secondary particles within such a range, it is possible to control the average particle size of the positive electrode active material having the composite hydroxide particles as a precursor within a predetermined range. Become.

In the present invention, the average particle size of secondary particles means the volume-based average particle size (MV), which can be obtained from, for example, a volume integrated value measured with a laser beam diffraction/scattering particle size analyzer.

(3) Particle size distribution The composite hydroxide particles used in the positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention are an index showing the spread of the particle size distribution [(d90-d10)/average particle size ] is adjusted to be 0.65 or less, preferably 0.55 or less, more preferably 0.50 or less.

The particle size distribution of the positive electrode active material is strongly influenced by its precursor, the composite hydroxide particles. Therefore, when composite hydroxide particles containing many fine particles and coarse particles are used as a precursor, the positive electrode active material also contains many fine particles and coarse particles. Safety, cycle characteristics and output characteristics cannot be sufficiently improved. On the other hand, if the [(d90−d10)/average particle diameter] is adjusted to be 0.65 or less at the stage of the composite hydroxide particles, the positive electrode active material using this as a precursor can be obtained. The particle size distribution can be narrowed, making it possible to avoid the problems described above. However, assuming industrial-scale production, it is not realistic to use composite hydroxide particles with an excessively small [(d90-d10)/average particle size]. Therefore, considering cost and productivity, the lower limit of [(d90-d10)/average particle diameter] is preferably about 0.25.

Note that d10 is the number of particles in each particle size accumulated from the smaller particle size side, and the particle size at which the cumulative volume is 10% of the total volume of all particles, and d90 is the number of particles. It means the particle size whose cumulative volume is 90% of the total volume of all particles. d10 and d90 can be obtained from volume integrated values measured with a laser light diffraction/scattering particle size analyzer in the same manner as the average particle size.

(4) Composition The composite hydroxide particles used in the positive electrode active material for lithium ion secondary batteries according to one embodiment of the present invention have the general formula (A): Ni _{x Mny} Co _z M _t (OH) _2+a (x + y + z + t = 1, 0.3 ≤ x ≤ 0.8, 0.05 ≤ y ≤ 0.55, 0 ≤ z ≤ 0.4, 0 ≤ t ≤ 0.1, 0 ≤ a ≤ 0.5, M is represented by one or more additive elements selected from Nb, Mo, Ta, Zr and W). By using such composite hydroxide particles as a precursor, it is possible to easily obtain the positive electrode active material represented by the general formula (B) described later, and to achieve higher battery performance.

In the composite hydroxide particles represented by the general formula (A), the composition range and critical significance of nickel, manganese, cobalt and the additive element M constituting the composite hydroxide particles are represented by the general formula (B) described later. It is the same as the positive electrode active material used. Therefore, description of these matters is omitted here.

<2. Method for producing transition metal composite hydroxide particles>
Next, a method for producing the transition metal composite hydroxide particles described above will be described. The composite hydroxide particles used for the positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention are prepared, for example, by placing an aqueous raw material solution containing at least a transition metal and an aqueous solution containing an ammonium ion donor in a reaction tank. is supplied to form a reaction aqueous solution, and a crystallization reaction is performed to produce transition metal composite hydroxide particles that serve as a precursor of a positive electrode active material for a lithium ion secondary battery.

In particular, in the crystallization reaction, the pH value of the reaction aqueous solution at a liquid temperature of 25° C. is adjusted to the range of 12.0 to 14.0 to generate nuclei, and the nuclei obtained in the nucleation step are The pH of the reaction aqueous solution containing the It is preferable to make a clear distinction between two stages. In addition, the reaction atmosphere at the beginning of the nucleation step and the grain growth step is adjusted to an oxidizing atmosphere with an oxygen concentration exceeding 5% by volume, and in the grain growth step, the reaction atmosphere is reduced from the oxidizing atmosphere to 5% by volume or less. It is preferable to switch to a non-oxidizing atmosphere.

The method for producing the composite hydroxide particles is not limited by its composition as long as the structure, average particle size and particle size distribution described above can be realized, but the composite hydroxide represented by the general formula (A) It can be suitably applied to particles.

(1) Crystallization Reaction In the method for producing composite hydroxide particles, the crystallization reaction is clearly separated into two stages, a nucleation step in which nucleation is mainly performed and a particle growth step in which particles are mainly grown. By adjusting the crystallization conditions in the process, particularly by switching the reaction atmosphere in the particle growth process, it is possible to efficiently obtain composite hydroxide particles having the above-described particle structure, average particle size, and particle size distribution. and

[Nucleation step]
In the nucleation step, first, a transition metal compound as a raw material in this step is dissolved in water to prepare a raw material aqueous solution. At the same time, an alkaline aqueous solution and an aqueous solution containing an ammonium ion donor are supplied and mixed in the reaction vessel, and the pH value measured at a liquid temperature of 25° C. is 12.0 to 14.0, and the ammonium ion concentration is 3 g/g/. Prepare a pre-reaction aqueous solution that is L-25 g/L. 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, the raw material aqueous solution is supplied. As a result, an aqueous solution for nucleation, which is a reaction aqueous solution in the nucleation step, is formed in the reaction tank. Since the pH value of the aqueous solution for nucleation is within the range described above, nucleation takes place preferentially with little growth of nuclei in the nucleation step. In the nucleation step, the pH value and the concentration of ammonium ions in the aqueous solution for nucleation change with nucleation. It is necessary to control the pH in the range of 12.0 to 14.0 and the ammonium ion concentration in the range of 3 g/L to 25 g/L on a °C basis.

In addition, in the nucleation step, the reaction atmosphere is adjusted to an oxidizing atmosphere in which the oxygen concentration exceeds 5% by volume. As a result, a central portion formed by agglomeration of fine primary particles can be formed inside the composite hydroxide particles, and the positive electrode active material having a hollow structure by using the composite hydroxide particles as a precursor. can be obtained, the reaction area with the electrolytic solution can be further increased.

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

At this time, the amount of nuclei generated can be determined from the amount of the metal compound contained in the raw material aqueous solution supplied to the nucleation aqueous solution. The amount of nuclei generated in the nucleation step is not particularly limited, but in order to obtain composite hydroxide particles with a narrow particle size distribution, the metal It is preferably 0.1 atomic % to 2 atomic %, more preferably 0.1 atomic % to 1.5 atomic %, relative to the metal element in the compound.

[Particle growth step]
After completion of the nucleation step, the pH value of the aqueous solution for nucleation in the reaction tank is adjusted to 10.5 to 12.0 based on the liquid temperature of 25° C. to form the aqueous solution for particle growth, which is the reaction aqueous solution in the particle growth step. do. The pH value can also be adjusted by stopping the supply of the alkaline aqueous solution, but in order to obtain composite hydroxide particles with a narrow particle size distribution, the pH value is adjusted by temporarily stopping the supply of all the aqueous solution. is preferred. Specifically, after stopping the supply of all the aqueous solution, it is preferable to adjust the pH value by supplying the same inorganic acid as the acid that constitutes the metal compound as a raw material to the aqueous solution for nucleation.

Next, supply of the raw material aqueous solution is resumed while stirring the particle growth aqueous solution. At this time, since the pH value of the aqueous solution for particle growth is within the range described above, almost no new nuclei are formed, nuclei (particles) grow, and composite hydroxide particles having a predetermined particle size are formed. be. Also in the particle growth process, the pH value and ammonium ion concentration of the aqueous solution for particle growth change as the particles grow. Therefore, the alkaline aqueous solution and the ammonia aqueous solution are supplied at appropriate times to maintain the pH value and the ammonium ion concentration within the above ranges. It is necessary to

In particular, in the method for producing composite hydroxide particles, the reaction atmosphere is switched from an oxidizing atmosphere to a non-oxidizing atmosphere having an oxygen concentration of 5% by volume or less during the particle growth step. This makes it possible to obtain composite hydroxide particles having the above-described particle structure.

In such a method for producing composite hydroxide particles, in the nucleation step and the particle growth step, the metal ions are precipitated as primary particles forming nuclei or composite hydroxide particles. Therefore, the ratio of the liquid component to the metal component in the nucleation aqueous solution and the particle growth aqueous solution is increased. As a result, the concentration of the raw material aqueous solution is apparently lowered, and particularly in the particle growth step, the growth of the composite hydroxide particles may be stagnant. Therefore, in order to suppress an increase in the liquid component, it is preferable to discharge a part of the liquid component of the aqueous solution for particle growth to the outside of the reaction tank during the particle growth process after the end of the nucleation process. Specifically, the supply and agitation of the raw material aqueous solution, the alkaline aqueous solution, and the aqueous solution containing the ammonium ion donor are temporarily stopped, and the nuclei and composite hydroxide particles in the particle growth aqueous solution are allowed to settle. It is preferable to drain the supernatant liquid. Such an operation can increase the relative concentration of the mixed aqueous solution in the aqueous solution for particle growth, thereby preventing stagnation of particle growth and controlling the particle size distribution of the resulting composite hydroxide particles within a suitable range. In addition, the density of the secondary particles as a whole can be improved.

[Addition of additive elements]
Regarding the concentration of the additive element relative to the elements other than the additive element in the raw material aqueous solution, the concentration in the oxidizing atmosphere is higher than the concentration in the non-oxidizing atmosphere. The concentration in the oxidizing atmosphere is preferably two times or more, more preferably five times or more, the concentration in the non-oxidizing atmosphere. Particularly preferably, the aqueous solution containing the additional element is added in the oxidizing atmosphere, and the addition of the aqueous solution containing the additional element is stopped in the non-oxidizing atmosphere. The central portion where fine primary particles formed in an oxidizing atmosphere are aggregated shrinks inside the outer peripheral portion formed by aggregation of plate-like primary particles formed in a non-oxidizing atmosphere when manufacturing the positive electrode active material. to form a hollow portion. Therefore, the additive element added in an oxidizing atmosphere diffuses to the outer peripheral portion and is present in a large amount on the surface of the primary particles forming the outer shell portion of the positive electrode active material, whereas the additive element added in a non-oxidizing atmosphere is A large amount of the additive element is present inside the primary particles, and a concentrated layer of the additive element is formed on the surfaces of the primary particles. By forming the concentrated layer on the surfaces of the primary particles that come into contact with the electrolytic solution, the additive element that improves the output characteristics can effectively function. By making the concentration of the additive element added in the oxidizing atmosphere higher than the concentration added in the non-oxidizing atmosphere, the amount of the additive element in the concentrated layer can be increased, and it functions more effectively. be able to.

Furthermore, in the grain growth step, the concentration of the additive element is increased to 35% in the range of 35% to 75% with respect to the time from after switching from the oxidizing atmosphere to the non-oxidizing atmosphere until the end of the grain growth step. By setting the concentration higher than the range of 75% to 75%, the concentration of the additive element on the surface side can be made higher than that on the inner side of the outer peripheral portion by 50% to 80% of the thickness of the outer peripheral portion. By forming such a layer containing the additive element at a higher concentration than the inner side on the surface side of the outer peripheral portion, the formation of the concentrated layer in the positive electrode active material can be made more uniform and the battery characteristics can be improved. .

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

[Another Embodiment of Crystallization Reaction]
In the method for producing composite hydroxide particles, a component-adjusting aqueous solution adjusted to a pH value and an ammonium ion concentration suitable for the particle growth step is prepared separately from the nucleating aqueous solution, and the component-adjusting aqueous solution is added to the nucleating solution. An aqueous solution for nucleation after the step, preferably an aqueous solution for nucleation after the step of nucleation from which part of the liquid component has been removed is added and mixed, and this is used as an aqueous solution for particle growth to perform the particle growth step. good too.

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

(2) Supply aqueous solution a) Raw material aqueous solution In one aspect of the present invention, the ratio of the metal element in the raw material aqueous solution is generally the composition ratio of the resulting composite hydroxide particles. Therefore, it is necessary to appropriately adjust the content of each metal element in the raw material aqueous solution according to the desired composition of the composite hydroxide particles. For example, when trying to obtain the composite hydroxide particles represented by the general formula (A) described above, the ratio of the metal elements in the raw material aqueous solution is changed to Ni:Mn:Co:M=x:y:z : t (however, x + y + z + t = 1, 0.3 ≤ x ≤ 0.80, 0.05 ≤ y ≤ 0.55, 0 ≤ z ≤ 0.4, 0 ≤ t ≤ 0.1) It is necessary to

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

Further, when the additive element M (M is one or more additive elements selected from Nb, Mo, Ta, Zr, and W) is contained in the composite hydroxide particles, the additive element M is supplied. As the compound of (1), similarly water-soluble compounds are preferred, and for example, niobium oxalate, ammonium molybdate, sodium tantalate, sodium tungstate, ammonium tungstate, etc. can be preferably used.

The concentration of the raw material aqueous solution is preferably 1 mol/L to 2.6 mol/L, more preferably 1.5 mol/L to 2.2 mol/L, in terms of the total concentration of metal compounds. If the concentration of the raw material aqueous solution is less than 1 mol/L, the amount of crystallized material per reaction tank is small, resulting in low productivity. On the other hand, when the concentration of the raw material aqueous solution exceeds 2.6 mol/L, the concentration exceeds the saturation concentration at room temperature, and crystals of each metal compound re-precipitate, which may clog pipes and the like.

The metal compound described above does not necessarily have to be supplied to the reaction tank as an aqueous raw material solution. For example, when performing a crystallization reaction using a metal compound that reacts to produce a compound other than the desired compound when mixed, separate Alternatively, an aqueous metal compound solution may be prepared in the above step and supplied to the reaction vessel at a predetermined ratio as an aqueous solution of individual metal compounds.

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

b) Alkaline aqueous solution The alkaline aqueous solution for adjusting the pH value in the reaction aqueous solution is not particularly limited, and a general alkali metal hydroxide aqueous solution such as sodium hydroxide or potassium hydroxide can be used. Although the alkali metal hydroxide can be added directly to the reaction aqueous solution, it is preferable to add it as an aqueous solution for ease of pH control. In this case, the concentration of the aqueous alkali metal hydroxide solution is preferably 20% to 50% by mass, more preferably 20% to 30% by mass. By regulating the concentration of the alkali metal aqueous solution within such a range, it is possible to suppress the amount of solvent (water amount) supplied to the reaction system and prevent the pH value from locally increasing at the addition position. , it becomes possible to efficiently obtain 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 aqueous solution does not become locally high and is maintained within a predetermined range. For example, while sufficiently stirring the reaction aqueous solution, it may be supplied by a pump capable of controlling the flow rate such as a metering pump.

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

When aqueous ammonia is used as the ammonium ion donor, its concentration is preferably 20% to 30% by mass, more preferably 22% to 28% by mass. By regulating the concentration of ammonia water within such a range, the loss of ammonia due to volatilization can be minimized, and thus production efficiency can be improved.

The aqueous solution containing the ammonium ion donor can also be supplied by a pump whose flow rate can be controlled in the same manner as the alkaline aqueous solution.

(3) pH value In the method for producing composite hydroxide particles, the pH value at a liquid temperature of 25 ° C. is in the range of 12.0 to 14.0 in the nucleation step, and 10.5 in the particle growth step. It is necessary to control in the range of ~12.0. In any step, it is preferable to control the fluctuation range of the pH value during the crystallization reaction within ±0.2. If the fluctuation range of the pH value is large, the ratio of the amount of nucleation and particle growth will not be constant, making it difficult to obtain composite hydroxide particles with a narrow particle size distribution.

a) Nucleation step In the nucleation step, the pH value of the reaction aqueous solution (nucleation aqueous solution) is 12.0 to 14.0, preferably 12.3 to 13.5, based on the liquid temperature of 25 ° C. Preferably, it is necessary to control in the range of 12.5-13.3. As a result, it is possible to suppress the growth of nuclei and give priority to nucleation, so that the nuclei generated in this step can be homogeneous and have a narrow particle size distribution. On the other hand, if the pH value is less than 12.0, the growth of nuclei (particles) proceeds along with nucleation, so that the composite hydroxide particles obtained have non-uniform particle diameters and a poor particle size distribution. On the other hand, when the pH value exceeds 14.0, the generated nuclei become too fine, which causes a problem of gelation of the aqueous solution for nucleation.

b) Particle Growth Step In the particle growth step, the pH value of the reaction aqueous solution (particle growth aqueous solution) is 10.5 to 12.0, preferably 11.0 to 12.0, more preferably 11.0 to 12.0, based on a liquid temperature of 25°C. must be controlled within the range of 11.5 to 12.0. As a result, the generation of new nuclei is suppressed, it becomes possible to give priority to particle growth, and the resulting composite hydroxide particles can be made homogeneous and have a narrow particle size distribution. On the other hand, if the pH value is less than 10.5, the concentration of ammonium ions increases and the solubility of metal ions increases, which not only slows down the crystallization reaction but also increases the amount of metal ions remaining in the reaction aqueous solution. and productivity suffers. On the other hand, if the pH value exceeds 12.0, the amount of nuclei generated during the particle growth process increases, the particle size of the resulting composite hydroxide particles becomes non-uniform, and the particle size distribution deteriorates.

When the pH value is 12.0, it is a boundary condition between nucleation and nucleus growth, so depending on the presence or absence of nuclei in the reaction aqueous solution, the condition may be either the nucleation step or the particle growth step. can be done. That is, when the pH value of the nucleation step is set higher than 12.0 to generate a large amount of nuclei, and then the pH value of the particle growth step is set to 12.0, a large amount of nuclei are present in the reaction aqueous solution, resulting in particles Growth occurs preferentially, and 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 set to 12.0, since there are no growing nuclei in the reaction aqueous solution, nucleation occurs preferentially. , the generated nuclei grow and good composite hydroxide particles can be obtained. In any case, the pH value in the grain growth step should be controlled at a value lower than the pH value in the nucleation step. It is preferably 0.5 or more lower than the pH value of the production step, more preferably 1.0 or more lower.

(4) Reaction Atmosphere The preferred structure in which the composite hydroxide particles have a central portion as described above controls the pH value of the reaction aqueous solution in the nucleation step and the particle growth step as described above, and the reaction in these steps Formed by controlling the atmosphere. Therefore, in such a method for producing composite hydroxide particles, the reaction atmosphere is controlled as well as the pH value in each step. That is, by controlling the pH value in each step as described above and adjusting the initial reaction atmosphere of the nucleation step and the particle growth step to an oxidizing atmosphere, a central portion where the fine primary particles are aggregated is formed. be. Further, by switching from an oxidizing atmosphere to a non-oxidizing atmosphere in the middle of the particle growth process, an outer peripheral portion in which plate-like primary particles are aggregated is formed outside the central portion.

In such control of the reaction atmosphere, the fine primary particles that make up the central portion are usually plate-shaped and/or needle-shaped, but depending on the composition of the composite hydroxide particles, they may be cuboidal, ellipsoidal, or rhombohedral. A shape such as a shape can also be adopted. In this regard, the same applies to the plate-like primary particles forming the outer peripheral portion. Therefore, the reaction atmosphere in each step is appropriately controlled according to the desired composition of the composite hydroxide particles.

a) Oxidizing Atmosphere At the stage of forming the core of the composite hydroxide particles, the reaction atmosphere is controlled to be an oxidizing atmosphere. Specifically, the oxygen concentration in the reaction atmosphere is controlled so as to exceed 5% by volume, preferably 10% by volume or more, more preferably an atmospheric atmosphere (oxygen concentration: 21% by volume). By controlling the oxygen concentration in the reaction atmosphere to such a range, the average particle diameter of the primary particles is in the range of 0.01 μm to 0.3 μm, so that a central portion formed of fine primary particles can be formed. can be done.

The upper limit of the oxygen concentration in the reaction atmosphere at this stage is not particularly limited. It may not be the size. Therefore, the oxygen concentration is preferably 30% by volume or less.

b) Non-Oxidizing Atmosphere On the other hand, the reaction atmosphere in the step of forming the outer peripheral portion of the composite hydroxide particles is controlled to be a weakly oxidizing atmosphere or a non-oxidizing atmosphere. Specifically, the mixed atmosphere of oxygen and inert gas is controlled so that the oxygen concentration in the reaction atmosphere is 5% by volume or less, preferably 2% by volume or less, more preferably 1% by volume or less. As a result, the nuclei generated in the nucleation step can grow to a certain range while suppressing unnecessary oxidation. It is possible to form a structure in which plate-like primary particles are aggregated, and to form an outer peripheral portion having a sufficient difference in primary particle size from the central portion described above.

c) Timing of Atmosphere Control In the particle growth step, it is necessary to perform the above-described atmosphere control at an appropriate timing so that composite hydroxide particles having the desired particle structure are formed.

The change of the atmosphere in the particle growth step is carried out by changing the size of the central part of the composite hydroxide particles so that in the finally obtained positive electrode active material, a hollow part is obtained to the extent that fine particles are not generated and the cycle characteristics are not deteriorated. , the timing is determined. For example, with respect to the entire grain growth process time, the time range from the start of the grain growth process is 0 to 40%, preferably 0 to 30%, more preferably 0 to 25%. If the switching is performed at a point exceeding 40% of the total grain growth process time, the central portion formed becomes large, and the thickness of the outer peripheral portion becomes too thin with respect to the diameter of the secondary grains. On the other hand, if the switching is performed before the grain growth step, that is, during the nucleation step, the central portion becomes too small or secondary grains having the above structure are not formed.

d) Switching method Conventionally, the reaction atmosphere is switched during the crystallization process by circulating the atmosphere gas in the reaction vessel or by inserting a conduit with an inner diameter of about 1 mm to 50 mm into the reaction aqueous solution and bubbling the atmosphere gas. I was going with However, in such a conventional technique, it takes a long time to switch the reaction atmosphere, so it is necessary to stop the supply of the raw material aqueous solution during the switching. Therefore, when switching the reaction atmosphere while supplying the raw material aqueous solution, it is preferable to use an air diffuser. The diffuser tube is composed of a conduit having a large number of fine holes on its surface, and can release a large number of fine gases (bubbles) into the liquid, making it possible to switch the reaction atmosphere in a short time. Therefore, it is not necessary to stop the supply of the raw material aqueous solution when switching the reaction atmosphere, and the production efficiency can be improved.

As such an air diffuser, it is preferable to use a ceramic diffuser that has excellent resistance in a high pH environment. In addition, the smaller the pore diameter of the diffuser tube, the more minute air bubbles can be released, so that the reaction atmosphere can be switched with high efficiency. In the present invention, it is preferable to use an air diffuser having a pore size of 100 μm or less, more preferably 50 μm or less.

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

If the ammonium ion concentration fluctuates during the crystallization reaction, the solubility of the metal ions fluctuates and uniform composite hydroxide particles are no longer formed. For this reason, it is preferable to control the fluctuation width of the ammonium ion concentration within a certain range through the nucleation step and the grain growth step, and specifically, it is preferable to control the fluctuation width within ±5 g/L.

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

(7) Coating step A coating step may be provided to add the additive element to the positive electrode active material. The coating method is not particularly limited as long as the composite hydroxide particles can be uniformly coated with the compound containing the additive element M. For example, the composite hydroxide particles are slurried, and the pH value thereof is controlled within a predetermined range. By precipitating the compound containing the additive element M, composite hydroxide particles uniformly coated with the compound containing the additive element can be obtained. In this case, instead of the aqueous solution for coating, an alkoxide solution of the additive element may be added to the slurried composite hydroxide particles. Alternatively, the composite hydroxide particles may be coated by spraying and drying an aqueous solution or slurry in which a compound containing an additive element is dissolved, without slurrying the composite hydroxide particles. Furthermore, coating is performed by a method such as spray-drying a slurry in which the composite hydroxide particles and the compound containing the additive element are suspended, or by mixing the composite hydroxide particles and the compound containing the additive element by a solid phase method. You can also

When the surface of the composite hydroxide particles is coated with the additive element, the raw material aqueous solution and the It is necessary to appropriately adjust the composition of the coating aqueous solution. Moreover, the coating step may be performed on the heat-treated particles after heat-treating the composite hydroxide particles.

(8) Production Apparatus There are no particular restrictions on the crystallization apparatus (reaction tank) for producing composite hydroxide particles, as long as the reaction atmosphere can be switched by the above-described air diffuser. do not have. However, it is preferred to use a batch crystallizer in which the precipitated product is not collected until the crystallization reaction is complete. With such a crystallizer, unlike a continuous crystallizer that recovers the product by an overflow method, the growing particles are not recovered at the same time as the overflow liquid. particles can be easily obtained. In addition, in the method for producing composite hydroxide particles of the present invention, it is necessary to appropriately control the reaction atmosphere during the crystallization reaction, so it is preferable to use a closed crystallizer.

<3. Method for producing positive electrode active material for lithium ion secondary battery>
Next, a method for manufacturing a positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention will be described. FIG. 1 is a process drawing showing an outline of a method for producing a positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention. A method for producing a positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention is represented by general formula (A): Ni _{x Mny} Co _z M _t (OH) _2+a (x+y+z+t=1, 0.3 ≤ x ≤ 0.8, 0.05 ≤ y ≤ 0.55, 0 ≤ z ≤ 0.4, 0 ≤ t ≤ 0.1, 0 ≤ a ≤ 0.5, M is Nb, Mo, Ta, One or more additive elements selected from Zr and W), and [(d90-d10)/MV], which is an index showing the spread of the particle size distribution, is 0.65 or less. and a lithium compound to form a lithium mixture; a calcination step S3 of calcining the lithium mixture formed in the mixing step S2 at 550 ° C. to 745 ° C. in an oxidizing atmosphere; and a firing step S5 of firing the lithium mixture calcined in the firing step S3 at 760° C. to 1050° C. in an oxidizing atmosphere. In addition, before the mixing step S2, a heat treatment step S1 for heat-treating the transition metal composite hydroxide particles may be included, and before the firing step S5, the lithium mixture calcined in the calcining step S3 is pulverized. It is preferable to have a pre-firing crushing step S4. According to such a production method, a positive electrode active material, which will be described later, can be easily obtained. Each step will be described in detail below.

(3-1. Heat treatment step)
In the method for producing a positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention, optionally, a heat treatment step S1 is provided before the mixing step S2 to convert the composite hydroxide particles into heat-treated particles. It may be mixed with a lithium compound. Here, the heat-treated particles include not only composite hydroxide particles from which excess moisture has been removed in the heat treatment step, but also transition metal composite oxide particles converted to oxides in the heat treatment step (hereinafter referred to as "composite oxide particles ), or mixtures thereof.

The heat treatment step S1 is a step of removing excess moisture contained in the composite hydroxide particles by heating the composite hydroxide particles to 105° C. to 750° C. for heat treatment. As a result, the amount of water remaining until after the baking step S5 can be reduced to a certain amount, and variations in the composition of the obtained positive electrode active material can be suppressed.

The heating temperature in the heat treatment step S1 is set to 105.degree. C. to 750.degree. If the heating temperature is less than 105° C., excess moisture in the composite hydroxide particles cannot be removed, and variations may not be sufficiently suppressed. On the other hand, even if the heating temperature exceeds 750° C., no further effect can be expected, and the production cost increases.

In addition, in the heat treatment step S1, it is sufficient that the water content can be removed to the extent that the number of atoms of each metal component in the positive electrode active material and the ratio of the number of Li atoms do not vary. There is no need to convert to oxide particles. However, in order to reduce the variation in the number of atoms of each metal component and the ratio of the number of Li atoms, all the composite hydroxide particles are converted to composite oxide particles by heating to 400 ° C. or higher. preferably. Incidentally, by determining the metal components contained in the composite hydroxide particles according to the heat treatment conditions in advance by analysis and determining the mixing ratio with the lithium compound, the above-described variations can be further suppressed.

The atmosphere in which the heat treatment is performed is not particularly limited as long as it is a non-reducing atmosphere.

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

(3-2. Mixing step)
The mixing step S2 is a step of mixing a lithium compound with the composite hydroxide particles or heat-treated particles described above to obtain a lithium mixture.

In the mixing step S2, the ratio of the sum of the number of atoms (Me) of the metal atoms other than lithium in the lithium mixture, specifically nickel, cobalt, manganese and the additive element M, to the number of lithium atoms (Li) (Li/Me) is 0.95 to 1.5, preferably 1.0 to 1.5, more preferably 1.0 to 1.35, still more preferably 1.0 to 1.2 , it is necessary to mix the lithium compound with the composite hydroxide particles or heat-treated particles. That is, since Li/Me does not change before and after the baking step S5, the composite hydroxide particles or heat-treated particles and lithium It is necessary to mix the compounds.

Although the lithium compound used in the mixing step S2 is not particularly limited, it is preferable to use lithium hydroxide, lithium nitrate, lithium carbonate, or a mixture thereof in terms of availability. In particular, considering ease of handling and stability of quality, it is preferable to use lithium hydroxide or lithium carbonate, and it is more preferable to use lithium carbonate.

It is preferable that the composite hydroxide particles or the heat-treated particles and the lithium compound are sufficiently mixed to the extent that fine powder is not generated. Insufficient mixing may cause variations in Li/Me among individual particles, making it impossible to obtain sufficient battery characteristics. In addition, a general mixer can be used for mixing. For example, shaker mixers, Loedige mixers, Julia mixers, V-blenders, etc. can be used.

(3-3. Temporary firing process)
In the calcining step S3, the lithium mixture formed in the mixing step S2 is calcined at 550°C to 745°C, preferably 580°C to 720°C, in an oxidizing atmosphere. This makes it possible to sufficiently diffuse and react lithium in the composite hydroxide particles or the heat-treated particles, thereby suppressing sintering and agglomeration of the secondary particles in the subsequent firing step S5. That is, by calcining, the lithium compound is consumed by the reaction with the composite hydroxide particles or the heat-treated particles, and the lithium compound remaining after calcining is greatly reduced. In the subsequent firing step S5, it is necessary to maintain the lithium transition metal composite oxide particles at a high temperature in order to increase the crystallinity of the particles. Sintering agglomeration progresses rapidly. By significantly reducing the amount of lithium compounds by calcination, sintering aggregation of secondary particles can be suppressed.

The holding time at the above temperature is preferably 1 to 10 hours, more preferably 3 to 6 hours. The atmosphere in the calcining step S3 is preferably an oxidizing atmosphere, more preferably an atmosphere with an oxygen concentration of 18% by volume to 100% by volume, as in the firing step S5 described later.

Furthermore, the carbon content of the calcined particles is controlled to preferably 1.2% by mass or less, more preferably 0.3% to 1.0% by mass. The carbon content is derived, for example, from lithium carbonate or the like as a raw material. As a result, the residual lithium compound is further reduced, the specific surface area of the positive electrode active material is controlled within a preferable range, and sintering and agglomeration of the secondary particles of the lithium-transition metal composite oxide in the firing step S5 can be further suppressed. In addition, the crystallinity can be sufficiently improved. Since the carbon content can be measured by the high frequency-infrared combustion method, the carbon content can be easily controlled by adjusting the calcining temperature and time so that the carbon content falls within the above range.

(3-4. Crushing process before firing)
Aggregation or mild sintering may occur in the lithium composite oxide particles obtained in the calcination step S3. In such a case, it is preferable to crush the aggregates or sintered bodies of the lithium composite oxide particles in the pre-firing crushing step S4 before the firing step S5. Thereby, the average particle size and particle size distribution of the obtained positive electrode active material can be adjusted to a suitable range.

As a crushing method, known means can be used as in the crushing step S6, which will be described later. For example, a pin mill or a hammer mill can be used. At this time, it is preferable to adjust the crushing force to an appropriate range so as not to destroy the secondary particles.

(3-5. Firing process)
In the firing step S5, the lithium mixture is fired under predetermined conditions to diffuse lithium into the composite hydroxide particles or the heat-treated particles to increase the crystallinity of the reacted particles, thereby obtaining lithium-transition metal composite oxide particles. It is a process.

The composite hydroxide particles are secondary particles having a central portion formed by agglomeration of fine primary particles and an outer peripheral portion formed by agglomeration of plate-like primary particles outside the central portion. In this case, in this firing step S5 and the previous calcination step S3, the central portion and high-density layer of the composite hydroxide particles and the heat-treated particles undergo sintering shrinkage, forming an aggregated portion of the primary particles in the positive electrode active material. . On the other hand, since the low-density layer is composed of fine primary particles, sintering starts at a lower temperature than the central portion and the high-density layer, which are composed of plate-like primary particles larger than the fine primary particles. Moreover, the low-density layer shrinks more than the central portion and the high-density layer. For this reason, the fine primary particles forming the low-density layer shrink toward the central portion where sintering progresses slowly and toward the high-density layer side, forming a space of an appropriate size. At this time, the high-density portion in the low-density layer shrinks during sintering while maintaining connection with the central portion and the high-density layer. , and a sufficient cross-sectional area for the path can be ensured. As a result, the internal resistance of the positive electrode active material is greatly reduced, and when a secondary battery is constructed, it becomes possible to improve the output characteristics without impairing the battery capacity and cycle characteristics.

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

The furnace used in the firing step S5 is not particularly limited as long as it can heat the lithium mixture in the air or in an oxygen stream. However, from the viewpoint of maintaining a uniform atmosphere in the furnace, an electric furnace that does not generate gas is preferable, and either a batch type or a continuous type electric furnace can be suitably used. Regarding this point, the same applies to the furnaces used in the heat treatment step S1 and the calcining step S3.

a) Firing temperature The firing temperature of the lithium mixture must be higher than the calcining temperature and be 760°C to 1050°C, preferably 790°C to 1000°C. If the firing temperature is less than 760° C., lithium does not sufficiently diffuse into the composite hydroxide particles or the heat-treated particles, and excess lithium or unreacted composite hydroxide particles or heat-treated particles remain, or the obtained lithium composite The crystallinity of the oxide particles becomes insufficient. On the other hand, if the firing temperature exceeds 1050° C., the lithium composite oxide particles are severely sintered, causing abnormal grain growth and increasing the proportion of amorphous coarse particles.

In addition, the heating rate in the firing step S5 is preferably 2° C./min to 10° C./min, more preferably 5° C./min to 10° C./min. Furthermore, during the firing step S5, it is preferable to hold the temperature near the melting point of the lithium compound for preferably 1 to 5 hours, more preferably 2 to 5 hours. Thereby, the composite hydroxide particles or the heat-treated particles and the lithium compound can be reacted more uniformly.

b) Firing Time Of the firing time, the holding time at the firing temperature described above is preferably at least 2 hours or longer, more preferably 4 to 24 hours. When the holding time at the firing temperature is less than 2 hours, lithium does not sufficiently diffuse into the composite hydroxide particles or the heat-treated particles, and excess lithium or unreacted composite hydroxide particles or heat-treated particles remain or are not obtained. The resulting lithium composite oxide particles may have insufficient crystallinity.

The cooling rate from the firing temperature to at least 200° C. after the holding time is preferably 2° C./min to 10° C./min, more preferably 3° C./min to 7° C./min. By controlling the cooling rate within such a range, it is possible to secure productivity and prevent equipment such as a sagger from being damaged by rapid cooling.

c) Firing atmosphere The atmosphere during firing is preferably an oxidizing atmosphere, more preferably an atmosphere with an oxygen concentration of 18% by volume to 100% by volume, and a mixed atmosphere of oxygen and an inert gas having the above oxygen concentration. is particularly preferred. That is, it is preferable to carry out the calcination in the atmosphere or in an oxygen stream. If the oxygen concentration is less than 18% by volume, the crystallinity of the lithium composite oxide particles may be insufficient.

(3-6. Crushing process)
Aggregation or mild sintering may occur in the lithium composite oxide particles obtained in the firing step S5. In such a case, it is preferable to crush the aggregates or sintered bodies of the lithium composite oxide particles in the crushing step S6. Thereby, the average particle size and particle size distribution of the obtained positive electrode active material can be adjusted to a suitable range. Crushing means applying mechanical energy to agglomerates consisting of a plurality of secondary particles generated by sintering necking between secondary particles during firing, without almost destroying the secondary particles themselves. It means the operation of separating and loosening aggregates.

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

<4. Positive electrode active material for lithium ion secondary battery>
Next, a positive electrode active material for a lithium ion secondary battery manufactured by the method for manufacturing a positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention described above will be described. One aspect of the present invention is
A positive electrode active material for a lithium ion secondary battery comprising lithium transition metal composite oxide particles, represented by the general formula (B): Li _1+u Ni _{x Mny} Co _z M _t O ₂ (−0.05≦u≦ 0.50, x + y + z + t = 1, 0.3 ≤ x ≤ 0.8, 0.05 ≤ y ≤ 0.55, 0 ≤ z ≤ 0.4, 0 ≤ t ≤ 0.1, M is Nb, Mo , Ta, Zr, and W), has a hexagonal crystal structure with a layered structure, and is formed by agglomeration of a plurality of primary particles. When the positive electrode active material is observed using a scanning electron microscope, the ratio of the number of aggregated particles in which 5 or more secondary particles are aggregated to the number of secondary particles in the observation field is 0.7%. and a specific surface area of 0.3 m ² /g to 4.0 ^{m 2} /g. Hereinafter, the characteristics of the positive electrode active material for lithium ion secondary batteries according to one embodiment of the present invention will be described in detail.

(1) Particle Structure a) Structure of Secondary Particles A positive electrode active material according to an embodiment of the present invention is composed of secondary particles formed by aggregation of a plurality of primary particles. In order to have higher output characteristics, the secondary particles preferably have an outer shell portion and a hollow portion in which no primary particles are present inside the outer shell portion.

In the positive electrode active material having a hollow structure, the electrolyte penetrates through the grain boundaries or voids between the primary particles, and lithium is intercalated and deintercalated at the reaction interface on the surface of the primary particles on the hollow side inside the particles. , the movement of electrons is not hindered, and the output characteristics can be improved.

b) Outer Shell In the positive electrode active material having a hollow structure, the thickness of the outer shell is preferably 5 to 45%, more preferably 8 to 38%, in terms of the ratio of the particle size of the secondary particles. preferable. Further, the absolute value is more preferably in the range of 0.4 to 2.5 μm, particularly preferably in the range of 0.5 to 2.0 μm. If the ratio of the thickness of the outer shell is less than 5%, the strength of the secondary particles is reduced, so that the particles are broken and fine particles are generated when the powder is handled and used as the positive electrode of the battery, and the characteristics are deteriorated. make worse. On the other hand, if the ratio of the thickness of the outer shell exceeds 45%, the amount of the electrolyte decreases from the grain boundaries or voids where the electrolyte can enter the hollow part inside the particle, and the surface area that contributes to the battery reaction decreases. , the positive electrode resistance increases and the output characteristics deteriorate. The ratio of the thickness of the outer shell to the diameter of the secondary particles can be determined in the same manner as for the composite hydroxide particles.

(2) Average Particle Size The average particle size of the positive electrode active material according to an embodiment of the present invention is adjusted to preferably 3 μm to 15 μm, more preferably 3 μm to 10 μm. If the average particle size of the positive electrode active material is in such a range, not only can the battery capacity per unit volume of the secondary battery using this positive electrode active material be increased, but also safety and output characteristics can be improved. can do. On the other hand, if the average particle diameter is less than 3 μm, the filling property of the positive electrode active material is lowered, and the battery capacity per unit volume may not be sufficiently increased. On the other hand, if the average particle size exceeds 15 μm, the reaction area of the positive electrode active material is reduced, and the interface with the electrolyte is reduced, so that the output characteristics may not be sufficiently improved.

The average particle diameter of the positive electrode active material means the volume-based average particle diameter (MV), as in the case of the composite hydroxide particles described above. can be obtained from the value.

(3) Particle size distribution In the positive electrode active material according to one embodiment of the present invention, when observed using a scanning electron microscope, 5 or more secondary particles aggregate with respect to the number of secondary particles in the observation field. The ratio of the number of agglomerated particles is 0.7% or less. When measuring the ratio of the number of aggregated particles in which 5 or more secondary particles are aggregated, the total number of secondary particles to be measured should be sufficient to be considered as the average of all secondary particles of the positive electrode active material. , too many particles make counting difficult. For example, when observed at a magnification of 1000 times, about 800 to 1100 secondary particles are included in the entire field of view. In that case, the total number of aggregated particles in which the secondary particles are aggregated is preferably 6 pieces/view or less, more preferably 3 pieces/view or less, and the ratio of the aggregated secondary particles is preferably 0.7. % or less, more preferably 0.4% or less. By reducing the aggregated particles, the distribution of the positive electrode active material particles in the positive electrode becomes uniform, the Li insertion/extraction and current flow can be made uniform, and the battery capacity and cycle characteristics can be improved. When the aggregated particles increase, the distribution of the positive electrode active material particles in the positive electrode becomes uneven, making it difficult to improve the above battery characteristics.

Furthermore, [(d90-d10)/average particle diameter (MV)], which is an index showing the spread of the particle size distribution, is preferably 0.55 or less, more preferably 0.50 or less, and the particles having an extremely narrow particle size distribution Configured. Such a positive electrode active material has a low ratio of fine particles and coarse particles, and a secondary battery using this has superior safety, cycle characteristics, and output characteristics.

On the other hand, when [(d90-d10)/average particle size] exceeds 0.55, the ratio of fine particles and coarse particles in the positive electrode active material increases. For example, in a secondary battery using a positive electrode active material containing a large proportion of fine particles, the secondary battery tends to generate heat due to the local reaction of the fine particles. Selective degradation of the particles can result in poor cycling performance. Moreover, in a secondary battery using a positive electrode active material with a large proportion of coarse particles, a sufficient reaction area between the electrolyte and the positive electrode active material cannot be ensured, and the output characteristics may be inferior.

On the other hand, 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, the lower limit of [(d90-d10)/average particle diameter] is preferably about 0.25.

Note that the meaning of d10 and d90 in the index [(d90-d10)/average particle size] indicating the spread of the particle size distribution, and the method for obtaining these are the same as for the composite hydroxide particles described above. is omitted.

(4) Crystallite Size The positive electrode active material of the present invention preferably has a crystallite size of 100 nm to 160 nm, more preferably 110 nm to 150 nm. By controlling the crystallite size within the above range, the cycle characteristics and output can be further improved.

(5) Composition The positive electrode active material of the present invention has the general formula (B): Li _1+u Ni _{x Mny} Co _z M _t O ₂ (−0.05≦u≦0.50, x+y+z+t=1, 0. 3≤x≤0.80, 0.05≤y≤0.55, 0≤z≤0.4, 0≤t≤0.1, M is selected from Nb, Mo, Ta, Zr, W one or more additive elements).

In this positive electrode active material, the value of u, which indicates the excess amount of lithium (Li), is preferably −0.05 or more and 0.50 or less, more preferably 0 or more and 0.50 or less, and still more preferably 0 or more and 0.35. Below. By limiting the value of u to the above range, the output characteristics and battery capacity 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 it exceeds 0.50, not only the initial discharge capacity decreases, but also the positive electrode resistance increases.

Nickel (Ni) is an element that contributes to increasing the potential and capacity of secondary batteries, and the value of x, which indicates its content, is 0.3 or more and 0.80 or less, more preferably 0.3 or more. 0.60 or less. If the value of x is less than 0.3, the secondary battery capacity using this positive electrode active material cannot be improved. On the other hand, if the value of x exceeds 0.80, the content of other elements will decrease and the effect cannot be obtained.

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

Cobalt (Co) is an element that contributes to the improvement of charge-discharge cycle characteristics, and the value of z indicating its content is preferably 0 or more and 0.4 or less, more preferably 0.10 or more and 0.35 or less. do. When the value of z exceeds 0.4, the initial discharge capacity of the secondary battery using this positive electrode active material is significantly reduced.

In order to further improve the durability and output characteristics of the secondary battery, the positive electrode active material according to one embodiment of the present invention may contain an additive element M in addition to the metal elements described above. Such additive elements M include magnesium (Mg), calcium (Ca), aluminum (Al), titanium (Ti), vanadium (V), chromium (Cr), zirconium (Zr), niobium (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta), lanthanum (La), and tungsten (W) can be used. Among these, it is particularly preferable to use one or more selected from niobium (Nb), molybdenum (Mo), tantalum (Ta), zirconium (Zr), and tungsten (W).

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

(6) Specific Surface Area The positive electrode active material according to an embodiment of the present invention preferably has a specific surface area of 0.3 m ² /g to 4.0 m ² /g, more preferably 0.5 m ² /g to 3.0 m 2 /g. More preferably, it is 0 m ² /g. A positive electrode active material having a specific surface area within this range has a large contact area with an electrolytic solution, and can greatly improve the output characteristics of a secondary battery using the same. On the other hand, when the specific surface area of the positive electrode active material is less than 0.3 m ² /g, the reaction area with the electrolytic solution cannot be secured when the secondary battery is constructed, and the output characteristics are sufficiently improved. It becomes difficult to let On the other hand, when the specific surface area of the positive electrode active material exceeds 4.0 m ² /g, the reactivity with the electrolytic solution becomes too high, which may reduce the thermal stability.

The specific surface area of the positive electrode active material can be measured, for example, by the BET method using nitrogen gas adsorption.

(7) Tap density Increasing the capacity of secondary batteries is an important issue in order to extend the usage time of portable electronic devices and the driving distance of electric vehicles. On the other hand, the thickness of the electrode of the secondary battery is required to be about several microns from the viewpoint of the packing and electronic conductivity of the battery as a whole. For this reason, it is necessary not only to use a high-capacity positive electrode active material, but also to improve the filling property of the positive electrode active material to increase the capacity of the secondary battery as a whole. From such a point of view, in the positive electrode active material of the present invention, the tap density, which is an index of fillability, is preferably 1.0 g/cm ³ or more, more preferably 1.2 g/cm ³ or more. . If the tap density is less than 1.0 g/cm ³ , the fillability is low, and the overall battery capacity of the secondary battery may not be sufficiently improved. On the other hand, the upper limit of the tap density is not particularly limited, but the upper limit under normal manufacturing conditions is about 3.0 g/cm ³ .

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

5. Lithium Ion Secondary Battery Finally, a lithium ion secondary battery according to an embodiment of the present invention will be described. A lithium ion secondary battery according to an embodiment of the present invention includes at least a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, and the positive electrode for a lithium ion secondary battery described above is used as a positive electrode active material for the positive electrode. Active material is used. In addition, although the said positive electrode active material can be used suitably for a lithium ion secondary battery, it is not necessarily limited to the secondary battery to be used. A lithium-ion secondary battery according to an embodiment of the present invention includes components such as a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, which are similar to those of a general lithium-ion secondary battery. Hereinafter, each component and the like will be described, but the embodiments described below are merely examples, and the lithium ion secondary battery according to one embodiment of the present invention is based on the embodiments described in this specification. Therefore, it is also possible to apply various modifications and improvements.

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

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

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

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

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

In addition, if necessary, a solvent for dispersing the positive electrode active material, conductive material and activated carbon and dissolving the binder can be added to the positive electrode mixture. As the solvent, specifically, an organic solvent such as N-methyl-2-pyrrolidone can be used. Activated carbon can also be added to the positive electrode mixture in order to increase the electric double layer capacity.

b) Negative Electrode Metallic lithium, lithium alloys, or the like can be used for the negative electrode. In addition, a binder is mixed with a negative electrode active material capable of intercalating and deintercalating lithium ions, and an appropriate solvent is added to form a paste. , dried and optionally formed by compression to increase electrode density.

Examples of negative electrode active materials include materials containing lithium such as metallic lithium and lithium alloys, natural graphite capable of intercalating and deintercalating lithium ions, artificial graphite, sintered organic compounds such as phenolic resins, and carbon materials such as coke. A powder can be used. In this case, a fluorine-containing resin such as PVDF can be used as the binder for the negative electrode in the same manner as the binder for the positive electrode. Solvents can be used.

c) 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 film of polyethylene, polypropylene, or the like and having many fine pores can be used, but there is no particular limitation as long as it has the above function.

d) Non-aqueous Electrolyte The non-aqueous electrolyte is obtained by dissolving a lithium salt as a supporting salt in an organic solvent.

Organic solvents include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate and trifluoropropylene carbonate; chain carbonates such as diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate and dipropyl carbonate; and ether compounds such as dimethoxyethane, sulfur compounds such as ethyl methyl sulfone and butane sultone, and phosphorus compounds such as triethyl phosphate and trioctyl phosphate. can.

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

In addition, the non-aqueous electrolyte may contain a radical scavenger, a surfactant, a flame retardant, and the like.

In addition, it is also possible to construct a lithium ion secondary battery using a solid electrolyte instead of a non-aqueous electrolyte. The charge/discharge characteristics at the rate are improved. Therefore, the positive electrode active material of the present invention can exhibit the desired effects not only for non-aqueous electrolytes but also for lithium ion secondary batteries using solid electrolytes.

(2) Lithium-ion secondary battery A lithium-ion secondary battery according to an embodiment of the present invention, which is composed of the positive electrode, negative electrode, separator, and non-aqueous electrolyte described above, can be formed in various shapes such as cylindrical and laminated. can do. Regardless of which shape is adopted, the positive electrode and the negative electrode are laminated with a separator interposed to form an electrode body, and the obtained electrode body is impregnated with a non-aqueous electrolyte, and communicates with the positive electrode current collector and the outside. The positive electrode terminal and the negative electrode current collector and the negative electrode terminal connected to the outside are connected using a current collecting lead or the like, and sealed in a battery case to complete the lithium ion secondary battery.

(3) Characteristics of lithium ion secondary battery Since the lithium ion secondary battery according to one embodiment of the present invention uses the positive electrode active material of the present invention as a positive electrode material as described above, battery capacity and output characteristics and excellent cycle characteristics. Moreover, it can be said that the secondary battery is superior in thermal stability and safety even in comparison with a secondary battery using a positive electrode active material composed of conventional lithium-nickel-based composite oxide particles.

(4) Applications The lithium-ion secondary battery of the present invention, as described above, is excellent in battery capacity, output characteristics and cycle characteristics. It can be suitably used as a power supply for computers, mobile phones, etc.). In addition, the lithium-ion secondary battery of the present invention is excellent in safety, and not only can it be made smaller and higher in output, but it can also simplify the expensive protection circuit, so the mounting space is limited. It can also be suitably used as a power supply for transportation equipment that receives power.

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

(Example 1)
[Production of composite hydroxide particles]
(Nucleation step)
In Example 1, first, the temperature in the tank was set to 40° C. while half the amount of water was put into the reaction tank (34 L) and stirred. At this time, the inside of the reaction tank was set to an air atmosphere (oxygen concentration: 21% by volume). Appropriate amounts of 25% by mass aqueous sodium hydroxide solution and 25% by mass aqueous ammonia are added to the water in the reaction tank to adjust the pH value of the reaction solution in the tank to 13.0 at a liquid temperature of 25°C. bottom. Further, the concentration of ammonia in the reaction solution was adjusted to 10 g/L to prepare an aqueous pre-reaction solution.

At the same time, nickel sulfate, cobalt sulfate, manganese sulfate, and zirconium sulfate were mixed with water so that the molar ratio of each metal element was Ni:Mn:Co:Zr=33.1:33.1:33.1:0.2. to prepare a 1.8 mol/L raw material aqueous solution.

This mixed aqueous solution was added to the pre-reaction aqueous solution in the reaction tank at a rate of 88 ml/min to obtain a reaction aqueous solution. At the same time, 25% by mass ammonia water and 25% by mass aqueous sodium hydroxide solution were also added to this aqueous reaction solution at a constant rate, and the pH Nucleation was performed by crystallization for 2 minutes and 30 seconds while controlling the value at 13.0 (nucleation pH value).

(Particle growth step)
After the nucleation was completed, the supply of all the aqueous solution was temporarily stopped, and sulfuric acid was added to adjust the pH value to 11.6 based on the liquid temperature of 25° C., thereby forming an aqueous solution for particle growth. . To the reaction aqueous solution (aqueous solution for particle growth), the supply of the raw material aqueous solution and the 25% by mass sodium hydroxide aqueous solution was resumed, and the sodium tungstate aqueous solution was additionally supplied as the raw material aqueous solution. The pH value was kept at the above value, and crystallization was continued to grow grains. The concentration and flow rate of the sodium tungstate aqueous solution were adjusted so as to match the composition. After 30 minutes of particle growth, the liquid supply was once stopped, and nitrogen gas was circulated at 5 L/min until the oxygen concentration in the space inside the reaction vessel became a non-oxidizing atmosphere of 0.2% by volume or less. Thereafter, the supply of the raw material aqueous solution and the sodium tungstate aqueous solution was restarted, and crystallization was carried out for 2 hours in total from the start of growth while maintaining the ammonia concentration and pH value.

When the inside of the reactor became full, the crystallization was stopped, and the stirring was stopped and the mixture was allowed to stand, thereby facilitating the precipitation of the product. After that, half of the supernatant liquid was extracted from the reaction vessel, crystallization was restarted, and after 2 hours of crystallization (total of 4 hours), the crystallization was terminated.

Then, the product was washed with water, filtered and dried to obtain composite hydroxide particles. It should be noted that the switching from the air atmosphere to the nitrogen atmosphere was performed at a time point of 12.5% of the entire grain growth process time from the start of the grain growth process.

In the above crystallization, the pH was controlled by adjusting the supply flow rate of the aqueous sodium hydroxide solution using a pH controller, and the variation range was within the range of 0.2 above and below the set value.

[Analysis of composite hydroxide]
A sample of _the obtained composite _hydroxide was dissolved in an _inorganic acid and then chemically analyzed by ICP emission spectroscopy _{. 002} W _0.005 (OH) _2+a (0≦a≦0.5).

In addition, for this composite hydroxide, the value [(d90-d10)/average particle size] indicating the average particle size and particle size distribution was measured using a laser diffraction scattering particle size distribution analyzer (Microtrac HRA manufactured by Nikkiso Co., Ltd.). It was obtained by calculating from the volume integrated value measured using. As a result, the average particle diameter was 5.2 μm, and the [(d90−d10)/average particle diameter] value was 0.48.

Next, when the obtained composite hydroxide particles were observed by SEM (scanning electron microscope S-4700 manufactured by Hitachi High-Technologies Co., Ltd.) (magnification: 1000 times), the composite hydroxide particles were approximately It was confirmed that the particles were spherical and had almost uniform particle diameters.

In addition, a sample of the obtained composite hydroxide particles was embedded in a resin and subjected to cross-section polisher processing. is composed of secondary particles, and the secondary particles consist of needle-like and flake-like fine primary particles (particle size of about 0.3 μm) in the central part, and outside the central part larger than the fine primary particles It was confirmed that the outer peripheral portion was composed of plate-like primary particles (particle diameter of approximately 0.6 μm). The thickness of the outer shell with respect to the diameter of the secondary particles obtained from SEM observation of this cross section was 12%.

[Production of positive electrode active material]
Lithium hydroxide was weighed and mixed with the composite hydroxide particles such that Li/Me=1.12 to prepare a lithium mixture. Mixing was performed using a shaker-mixer device (TURBULA Type T2C manufactured by Willie & Bachoffen (WAB)).

The resulting lithium mixture was calcined at 600° C. for 4 hours in the atmosphere (oxygen: 21% by volume). After pulverizing the calcined lithium mixture, the mixture was heated at a rate of 5° C./min, baked at 970° C. for 3 hours, cooled, and pulverized to obtain a positive electrode active material.

[Measurement of carbon content after calcination]
When the carbon content (TC amount) contained in the calcined lithium mixture was measured by a high-frequency combustion-infrared absorption method using a carbon analyzer (manufactured by LECO, model number: CS-600), it was 1.1. % by mass.

[Analysis of positive electrode active material]
When the particle size distribution of the obtained positive electrode active material was measured in the same manner as for the composite hydroxide particles, the average particle size was 4.7 μm, and the [(d90−d10)/average particle size] value was 0. .48.

In addition, SEM observation and cross-sectional SEM observation of the positive electrode active material were performed in the same manner as for the composite hydroxide particles. was confirmed. In addition, when observed with a scanning electron microscope at a magnification of 1000 times, the ratio of the number of aggregated particles in which 5 or more secondary particles are aggregated to the number of secondary particles in the observation field is 0.40%. Met.

For the obtained positive electrode active material, the specific surface area was measured by a flow-type gas adsorption method specific surface area measuring device (manufactured by Yuasa Ionics Co., Ltd., Multisorb), and the tap density was measured by a tapping machine (Kuramochi Scientific Instruments Manufacturing Co., Ltd., KRS-406). , respectively, were measured. As a result, it was confirmed that the specific surface area was 1.4 m ² /g and the tap density was 1.23 g/cm ³ .

Further, when the obtained positive electrode active material was analyzed by powder X-ray diffraction using Cu—Kα rays using an X-ray diffractometer (manufactured by PANalytical, X'Pert PRO), the crystal structure of the positive electrode active material was was confirmed to consist of a single phase of hexagonal layered crystal lithium-nickel-manganese composite oxide. From the obtained X-ray diffraction pattern, the (003) plane crystallite diameter using Scherrer's formula was 121 nm.

Furthermore, according to analysis using an ICP emission spectrometer, this positive electrode active material is represented by the general formula: Li _1.12 Ni _0.331 Mn _0.331 Co _0.331 Zr _0.002 W _0.005 O ₂ It was confirmed that

[Manufacture of coin-type battery and evaluation of battery characteristics]
52.5 mg of positive electrode active material for lithium ion secondary batteries, 15 mg of acetylene black, and 7.5 mg of polytetrafluoroethylene resin (PTFE) were mixed and press-molded at a pressure of 100 MPa to a diameter of 11 mm and a thickness of 100 μm. A positive electrode 1 (evaluation electrode) shown was prepared. The produced positive electrode 1 was dried in a vacuum dryer at 120° C. for 12 hours. Using this positive electrode 1, a 2032-type coin-type battery 10 was produced in an Ar atmosphere glove box with a controlled dew point of -80°C.

For the negative electrode 2, a negative electrode sheet in which graphite powder with an average particle size of about 20 μm and polyvinylidene fluoride, which are punched into a disk shape with a _diameter of 14 mm, and polyvinylidene fluoride are coated on a copper foil is used. A mixture of equal amounts of ethylene carbonate (EC) and diethyl carbonate (DEC) (manufactured by Tomiyama Pharmaceutical Co., Ltd.) was used. A polyethylene porous film having a film thickness of 25 μm was used as the separator 3 . Also, the coin-type battery 10 has a gasket 4 and a wave washer 5, and is assembled into a coin-shaped battery with a positive electrode can 6 and a negative electrode can 7. FIG. The initial discharge capacity and positive electrode resistance, which indicate the performance of the manufactured coin-type battery 10, were evaluated as follows.

a) Initial Discharge Capacity The initial discharge capacity is obtained by leaving the coin-type battery 10 for about 24 hours after manufacture, and after the open circuit voltage OCV (Open Circuit Voltage) has stabilized, the current density to the positive electrode is set to 0.1 mA/cm ² . was charged to a cut-off voltage of 4.3 V, and after resting for 1 hour, the capacity when discharged to a cut-off voltage of 3.0 V was taken as the initial discharge capacity.

b) Positive Electrode Resistance The positive electrode resistance is measured by the AC impedance method by charging the coin battery 10 at 25° C. with a charging potential of 4.1 V and using a frequency response analyzer and a potentiogalvanostat (manufactured by Solartron, 1255B). Then, the Nyquist plot shown in FIG. 3(A) is obtained. Since this Nyquist plot is represented as the sum of the characteristic curves showing the solution resistance, the negative electrode resistance and its capacity, and the positive electrode resistance and its capacity, the equivalent circuit shown in FIG. A fitting calculation was performed to calculate the value of the positive electrode resistance.

c) Capacity retention rate after 500 cycles The capacity retention rate after 500 cycles is obtained by performing the charge/discharge capacity measurement described in a) above, then resting for 1 hour, and repeating the charge/discharge capacity measurement again for 500 cycles. , and the ratio (%) of the discharge capacity after 500 cycles to the initial discharge capacity. As a result, the capacity retention rate was 81.2%.

Table 1 shows the properties of the composite hydroxide obtained in this example, and Table 2 shows the properties of the positive electrode active material and each evaluation of the coin-type battery produced using this positive electrode active material. In addition, Tables 1 and 2 show the same contents for Examples 2 to 5 and Comparative Example 1 below.

(Example 2)
In Example 2, in the particle growth step, the flow rate of the sodium tungstate aqueous solution was adjusted so that the concentration of tungsten in the non-oxidizing atmosphere was 20% with respect to the concentration of the additive element (tungsten) in the raw material aqueous solution in the oxidizing atmosphere. A positive electrode active material for a lithium ion secondary battery was obtained and evaluated in the same manner as in Example 1 except for the adjustment. It _was confirmed that the composition _{of the obtained} positive _electrode active material _was represented by _{Li1.12Ni0.331Mn0.331Co0.331Zr0.002W0.005O2} .

(Example 3)
In Example 3, a positive electrode active material for a lithium ion secondary battery was obtained and evaluated in the same manner as in Example 1, except that the calcination temperature was 650°C and the firing temperature was 970°C.

(Example 4)
In Example 4, a positive electrode active material for a lithium ion secondary battery was obtained and evaluated in the same manner as in Example 1, except that the calcining temperature was 700°C and the firing temperature was 990°C.

(Example 5)
In Example 5, a positive electrode active material for a lithium ion secondary battery was obtained and evaluated in the same manner as in Example 1, except that the calcining temperature was 720°C and the firing temperature was 990°C.

(Example 6)
In Example 6, the positive electrode active material for a lithium ion secondary battery was produced in the same manner as in Example 1 except that the calcining temperature was 720° C., the firing temperature was 990° C., and the crushing before firing was not performed. We obtained the results and evaluated them.

(Comparative example 1)
In Comparative Example 1, a positive electrode active material for a lithium ion secondary battery was obtained and evaluated in the same manner as in Example 1, except that calcination was not performed and the firing temperature was set to 980°C.

(evaluation)
Since the positive electrode active materials for lithium ion secondary batteries of Examples 1 to 6 were produced according to the method for producing a positive electrode active material for lithium ion secondary batteries according to one embodiment of the present invention, the carbon content after calcination was I was able to keep it low. In addition, all of the positive electrode active materials of Examples 1 to 6 had a smaller number of aggregated particles, a higher initial discharge capacity, and a lower positive electrode resistance than Comparative Example 1, which was not calcined. Cycle characteristics (capacity retention rate after 500 cycles) are also superior to Comparative Example 1. As described above, it was found that the cathode active material for a lithium ion secondary battery according to an embodiment of the present invention has excellent characteristics such as improved battery capacity and cycle characteristics.

Note that, in Example 6, since the method for producing a positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention is followed, the carbon content after calcining can be kept low. Crushing (pre-firing crushing step S4) was not performed. For this reason, the number of aggregated particles is not as large as in Comparative Example 1, but the battery capacity and cycle characteristics are slightly inferior to those of Examples 1 to 5. Therefore, in the method for producing a positive electrode active material for a lithium ion secondary battery according to one embodiment of the present invention, performing the pre-firing crushing step S4 is more effective in improving the battery capacity and cycle characteristics. It turns out that

Although the embodiments and examples of the present invention have been described in detail as described above, it should be understood by those skilled in the art that many modifications are possible without substantially departing from the novel matters and effects of the present invention. , will be easily understood. Accordingly, all such modifications are intended to be included within the scope of the present invention.

For example, a term described at least once in the specification or drawings together with a different, broader or synonymous term can be replaced with the different term anywhere in the specification or drawings. Moreover, the configuration and operation of the positive electrode active material for the lithium ion secondary battery and the lithium ion secondary battery are not limited to those described in the embodiments and examples of the present invention, and various modifications are possible.

1 positive electrode (electrode for evaluation), 2 negative electrode, 3 separator, 4 gasket, 5 wave washer, 6 positive electrode can, 7 negative electrode can, 10 coin type battery

Claims (7)

A method for producing a positive electrode active material for a lithium ion secondary battery, comprising:
General formula (A): _{NixMnyCozMt ( OH} ) _2+a (x+y+z ₊ t=1, 0.3≤x≤0.8, 0.05≤y≤0.55, 0≤z≤0.4 , 0 ≤ t ≤ 0.1, 0 ≤ a ≤ 0.5, M is one or more additive elements selected from Nb, Mo, Ta, Zr, W), and the cumulative 90 volume% diameter ( d90), the cumulative 10 volume% diameter (d10), and the volume-based average particle diameter (MV), which is an index showing the spread of the particle size distribution [(d90-d10) / MV] is 0.65 or less. A mixing step of mixing the transition metal composite hydroxide particles and/or the transition metal composite oxide particles converted from the transition metal composite hydroxide particles and the lithium compound to form a lithium mixture;
a calcining step of calcining the lithium mixture formed in the mixing step at 550° C. to 745° C. in an oxidizing atmosphere;
A firing step of firing the lithium mixture calcined in the calcining step at 760 ° C. to 1050 ° C. in an oxidizing atmosphere;
has
The transition metal composite hydroxide particles are substantially spherical secondary particles formed by agglomeration of a plurality of primary particles. having an outer peripheral portion made of plate-shaped primary particles larger than the fine primary particles on the outside,
A method for producing a positive electrode active material for a lithium ion secondary battery, wherein in the calcining step, the carbon content in the lithium mixture after calcining is controlled to be 1.2% by mass or less. 2. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 1, further comprising a pre-firing crushing step of crushing the lithium mixture calcined in the calcining step before the calcining step. In the mixing step, the lithium mixture is adjusted so that the ratio of the sum of the number of metal atoms other than lithium contained in the lithium mixture to the number of lithium atoms is 1:0.95 to 1.5. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 1 or 2. The lithium ion secondary battery according to any one of claims 1 to 3, further comprising a heat treatment step of heat-treating the transition metal composite hydroxide particles at 105 ° C. to 750 ° C. before the mixing step. A method for producing a positive electrode active material. A positive electrode active material for a lithium ion secondary battery comprising lithium transition metal composite oxide particles,
General formula (B): _{Li1 + uNixMnyCozMtO2} (-0.05≤u≤0.50, _x +y+z+t= _{1 ,} 0.3≤x≤0.8, 0.05≤y≤ 0.55, 0 ≤ z ≤ 0.4, 0 ≤ t ≤ 0.1, M is one or more additional elements selected from Nb, Mo, Ta, Zr, W), and has a layered structure has a hexagonal crystal structure,
Consisting of secondary particles formed by aggregation of a plurality of primary particles,
The secondary particles have a hollow structure consisting of an outer shell portion and a hollow portion present inside thereof,
When observing the positive electrode active material using a scanning electron microscope, the ratio of the number of aggregated particles in which 5 or more secondary particles are aggregated to the number of secondary particles in the observation field is 0.7% or less. and has a specific surface area of 0.3 m ² /g to 4.0 m ² /g,
An index showing the spread of the particle size distribution calculated from the cumulative 90 volume% diameter (d90), the cumulative 10 volume% diameter (d10), and the volume-based average particle diameter (MV) [(d90 -d10) / MV ] is 0.55 or less and the crystallite diameter is 100 nm to 160 nm. 6. The positive electrode active material for a lithium ion secondary battery according to claim 5, wherein the secondary particles have an average particle size of 3 μm to 15 μm. A lithium ion secondary battery comprising at least a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte,
A lithium ion secondary battery, wherein the positive electrode active material for a lithium ion secondary battery according to claim 5 or 6 is used as the positive electrode active material of the positive electrode.

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