JP7197825B2 - Positive electrode active material for non-aqueous electrolyte secondary battery, method for producing the same, and non-aqueous electrolyte secondary battery - Google Patents

Description

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

In recent years, with the spread of mobile information devices 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 and durability. In addition, there is a strong demand for the development of high-power secondary batteries for electric vehicles such as power tools and hybrid vehicles.
Non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries are available as secondary batteries that satisfy such requirements.

This non-aqueous electrolyte 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 during charging and discharging is used as an active material for the negative electrode and the positive electrode. Non-aqueous electrolyte secondary batteries are still being actively researched and developed. Since the battery can obtain a high charging/discharging voltage of the 4V class, secondary batteries having high energy density are being put to practical use.

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

In recent years, lithium-nickel-cobalt-manganese composite oxide (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ), which has excellent thermal stability and high capacity among the positive electrode active materials, has attracted attention. . Lithium-nickel-cobalt-manganese composite oxides are layered compounds like lithium-cobalt composite oxides and lithium-nickel composite oxides, and are basically composed of nickel, cobalt, and manganese at transition metal sites. Contained in a ratio of 1:1:1. A technology to add metals such as tungsten and niobium to these lithium-transition metal composite oxides has been proposed with the aim of achieving high performance (high cycle characteristics, high capacity, and high output) as non-aqueous electrolyte secondary batteries. It is

For example, Patent Document 1 describes a general formula: Li _a Ni _{1-xy-z Co x} My Nb _z O _b (where M is one or more elements selected from the group consisting of Mn, Fe and Al; 1 ≤ a ≤ 1.1, 0.1 ≤ x ≤ 0.3, 0 ≤ y ≤ 0.1, 0.01 ≤ z ≤ 0.05, 2 ≤ b ≤ 2.2) lithium and nickel There has been proposed a positive electrode active material for non-aqueous secondary batteries comprising a composition comprising at least one compound consisting of , cobalt, the element M, niobium, and oxygen.
In this proposal, since the Li—Nb—O-based compound present near the surface or inside the positive electrode active material particles has high thermal stability, a positive electrode active material having high thermal stability and large discharge capacity is produced. is supposed to be obtained.

Further, Patent Document 2 describes a mixing step of mixing a nickel-containing hydroxide, a lithium compound, and a niobium compound having an average particle size of 0.1 to 10 μm to obtain a mixture, and and a step of calcining to obtain a lithium-transition metal composite oxide by calcining at ℃, and a non-aqueous electrolyte comprising a lithium-transition metal composite oxide composed of particles with a polycrystalline structure obtained by the manufacturing method. A positive electrode active material for a secondary battery, which has a porous structure, a specific surface area of 0.9 to 3.0 m ² /g, and a content of alkali metals other than lithium of 20 mass ppm or less. is proposed. This positive electrode active material is said to enable realization of high thermal stability, charge/discharge capacity, and excellent cycle characteristics.

Further, in Patent Document 3, a niobium salt solution and an acid are simultaneously added to a nickel-containing hydroxide slurry, and the pH of the slurry is controlled to be constant in the range of 7 to 11 at 25 ° C. a niobium coating step of obtaining a nickel-containing hydroxide coated with a niobium compound; a mixing step of mixing the nickel-containing hydroxide coated with the niobium compound with a lithium compound to obtain a mixture; and a step of firing in an oxidizing atmosphere at 700 to 830°C to obtain a lithium-transition metal composite oxide, and a lithium-transition metal composite oxide composed of polycrystalline particles obtained by the manufacturing method. A positive electrode active material for a non-aqueous electrolyte ^secondary battery comprising It is said that a non-aqueous electrolyte secondary battery having high safety, battery capacity, and excellent cycle characteristics can be obtained by using it.

JP-A-2002-151071 JP 2015-122298 A International publication WO2014/034430

The positive electrode active materials according to the prior art all exhibit high safety and improved durability, but cannot be said to be compatible with high charge/discharge capacity, output characteristics, and energy density.
The present invention has been made in view of these circumstances, and it is an object of the present invention to provide a positive electrode active material that maintains high safety and durability, has high charge-discharge capacity, and has improved output characteristics. It is intended.
Another object of the present invention is to provide a method for easily producing such a positive electrode active material on an industrial scale.

In order to solve the above problems, the present inventors have made intensive studies and found that a lithium-nickel-manganese-cobalt-niobium composite oxide containing a specific amount of niobium at a specific material amount ratio of nickel and manganese. The present inventors have completed the present invention based on the knowledge that a positive electrode active material having high thermal stability, durability, charge/discharge capacity, and output characteristics can be obtained by producing the positive electrode active material.
That is, a first invention of the present invention is a positive electrode active material for a non-aqueous electrolyte secondary battery comprising a lithium-nickel-manganese-cobalt-niobium composite oxide composed of secondary particles in which a plurality of primary particles are aggregated. Then, the lithium-nickel-manganese-cobalt-niobium composite oxide has the general formula (1): Li _d Ni _1-ab-c1 Mna _-c2 Co _b-c3 Nb _c O ₂ (0.20≦ a≦0.50, 0.20≦b≦0.60, 0.03< c≦0.08, c=c1+c2+c3, 1.03≦d≦1.20, (1-a-b-c1)/ a ≤ 2), and niobium in the lithium-nickel-manganese-cobalt-niobium composite oxide replaces nickel, manganese, and cobalt in the composite oxide, respectively, and is dissolved in the primary particles. An active material for a non-aqueous electrolyte secondary battery characterized by:

A second aspect of the present invention is a positive electrode active material for a non-aqueous electrolyte secondary battery according to the first aspect, wherein the secondary particles have a volume average particle size MV of 5 μm or more and 20 μm or less.

The third invention of the present invention uses the half-value width of the (003) plane diffraction peak of X-ray diffraction in the first and second inventions, and the crystallite diameter obtained by the Scherrer formula is such that "the lithium nickel The crystallite diameter A of the manganese-cobalt-niobium composite oxide relative to the crystallite diameter B of the lithium-nickel-manganese-cobalt composite oxide containing no niobium and having the same material ratio of nickel, manganese, and cobalt. A positive electrode active material for a non-aqueous electrolyte secondary battery, wherein a rate of change Z (%) in child diameter satisfies the relationship of the following formula (a).

A fourth aspect of the present invention is the general formula (1): Li _d Ni _1-a-b-c1 Mna _-c2 Co _b-c3 Nb _c O ₂ (0.20≦a≦0.50, 0.20≦a≦0.50; 20 ≤ b ≤ 0.60, 0.03 < c ≤ 0.08, c = c1 + c2 + c3, 1.03 ≤ d ≤ 1.20, (1-ab-c1) / a ≤ 2), Lithium-nickel-manganese-cobalt, which is composed of secondary particles in which a plurality of primary particles are aggregated, and niobium replaces nickel, manganese, and cobalt in the composite oxide, respectively, and is solid-solved in the primary particles. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery made of a niobium composite oxide, wherein the general formula (2): Ni _{1-ab Mna Co b (} OH) _2+α (0.20≦a≦ 0.50, 0.20≦b≦0.60, 0≦α≦0.4), a niobium compound, and a lithium compound. a mixing step, and a firing step of obtaining a lithium-nickel-manganese-cobalt-niobium composite oxide by firing the mixture at 850° C. or higher and 1000° C. or lower in an oxidizing atmosphere for 5 hours or longer and 10 hours or shorter. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, comprising:

A fifth aspect of the present invention is the manufacturing of a positive electrode active material for a non-aqueous electrolyte secondary battery, wherein the volume average particle diameter MV of the niobium compound in the fourth aspect is 0.01 μm or more and 10 μm or less. The method.

The sixth invention of the present invention uses the half-value width of the (003) plane diffraction peak of X-ray diffraction in the fourth and fifth inventions, and the crystallite diameter obtained by Scherrer's formula is such that "the lithium nickel The crystallite diameter A of the manganese-cobalt-niobium composite oxide relative to the crystallite diameter B of the lithium-nickel-manganese-cobalt composite oxide containing no niobium and having the same material ratio of nickel, manganese, and cobalt. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, characterized in that the rate of change Z (%) in child diameter satisfies the relationship of the following formula (b).

A seventh invention of the present invention is a non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of the first to third inventions. .

By using the positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention for the positive electrode, a lithium ion secondary battery having high thermal stability and durability, high charge/discharge capacity and output characteristics can be obtained. Moreover, the method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention can be easily implemented even in industrial-scale production, and can be said to be of extremely high industrial value.

1 is a schematic cross-sectional view of a coin-type battery 10 used for battery evaluation; FIG.

Hereinafter, about the present embodiment, (1) a positive electrode active material for a non-aqueous electrolyte secondary battery, (2) a method for producing the same, and (3) a non-aqueous electrolyte secondary battery using the positive electrode active material according to the present invention explain.

(1) Positive electrode active material for non-aqueous electrolyte secondary batteries The positive electrode active material for non-aqueous electrolyte secondary batteries according to the present embodiment (hereinafter referred to as "positive electrode active material") is composed of particles with a polycrystalline structure, It is composed of a lithium-nickel-manganese-cobalt-niobium composite oxide (hereinafter referred to as "lithium transition metal composite oxide") composed of secondary particles in which a plurality of primary particles are aggregated. Further, the lithium transition metal composite oxide has the general formula (1): Li _d Ni _1-ab-c1 Mna _-c2 Co _b-c3 Nb _c O ₂ (0.20≦a≦0.50, 0.2 ≤ b ≤ 0.60, 0.03 < c ≤ 0.08, c = c1 + c2 + c3, 1.03 ≤ d ≤ 1.20, (1-a-b-c) / a ≤ 2) Niobium (Nb) replaces nickel, manganese, and cobalt in the composite oxide, respectively, and dissolves in the primary particles.
The positive electrode active material of the present embodiment contains a specific amount of niobium, and the niobium is a lithium transition metal composite oxide in which the niobium is solidly dissolved in the primary particles. A non-aqueous electrolyte secondary battery using this positive electrode active material (hereinafter, "Secondary battery") has high thermal stability and durability, high charge/discharge capacity and output characteristics.

In the general formula (1), the range of a indicating the content of Mn is 0.20≦a≦0.50, preferably 0.25≦a≦0.45. When the value of a exceeds 0.50, a spinel phase or a lithium-excess crystalline phase is generated, making it difficult to obtain a single layered compound phase. On the other hand, if it is less than 0.20, it becomes difficult to obtain the effect of promoting grain growth by adding Nb, and the effect of improving output characteristics cannot be obtained.

The range of c, which indicates the content of Nb, is 0.02 ≤ c ≤ 0.08, preferably 0.03 ≤ c ≤ 0.05, where c = c1 + c2 + c3 (c1 is Ni, c2 is Mn, c3 is the amount to be replaced with Co), and 0<c1, c2, c3.
When the range of c is within the above range, extremely good durability can be obtained, and oxygen release can be suppressed when used for the positive electrode of a secondary battery, and high thermal stability can be obtained. If the value of c is less than 0.02, the amount of solid solution in the primary particles of niobium is small, and it is difficult to obtain the effect of improving various properties. Moreover, when the value of c exceeds 0.08, not all niobium is dissolved in the lithium-transition metal composite oxide, and a lithium-niobium compound is produced, resulting in a significant decrease in battery capacity. Furthermore, from the viewpoint of achieving both higher durability and thermal stability, the range of c is more preferably 0.03≦c≦0.05.

In the positive electrode active material of this embodiment, niobium is solid-dissolved in the primary particles, and the effect of improving the thermal stability and durability is considered to be due to the solid-solution of niobium in the primary particles. Furthermore, the effect of improving the output characteristics is due to the effect of promoting grain growth due to solid solution of niobium. Here, the solid solution of niobium means, for example, that niobium is detected by ICP emission spectrometry, and that niobium is detected by surface analysis of the primary particle cross section using EDX in a scanning transmission electron microscope (S-TEM). It refers to a state in which niobium is detected in primary particles, and it is preferable that niobium is detected over the entire surface of the primary particles.

As described above, in the secondary battery using the positive electrode active material of the present embodiment, niobium dissolves in the primary particles, making it difficult for oxygen to escape from the crystal during charging, improving thermal stability. be able to.
In addition, niobium in a solid solution has the effect of suppressing the phase transition of the crystalline phase due to overcharging, which also contributes to the improvement of thermal stability. In addition, the suppression of the phase transition of the crystalline phase is effective in suppressing deterioration of the positive electrode active material during repeated charging and discharging, and contributes to improving the durability of the secondary battery.
Furthermore, the effect of promoting grain growth by adding niobium improves the crystallinity inside the positive electrode active material, making it easier for lithium ions to move during charging and discharging, and the output characteristics are also improved compared to when niobium is not added.

When the volume average particle diameter MV of the positive electrode active material particles is 5 μm or more and 20 μm or less, when used for the positive electrode of a secondary battery, high output characteristics and battery capacity, and high fillability to the positive electrode are compatible. Further, if the thickness is 5 μm or more and 15 μm or less, further improvement can be expected, which is more preferable. However, when the volume average particle diameter MV of the secondary particles is less than 5 μm, high filling properties into the positive electrode may not be obtained, and when the volume average particle diameter MV exceeds 20 μm, high output characteristics and battery capacity are obtained. may not be The volume-average particle diameter MV can be obtained, for example, from a volume integrated value measured by a laser beam diffraction/scattering particle size distribution analyzer.

The crystallinity of the lithium transition metal composite oxide is determined by the Scherrer formula (τ = Kλ / β cos θ, τ: crystallite diameter (Å), K: shape factor (usually 0.9), λ: X-ray wavelength (Å), β: peak half width (rad), θ: diffraction angle (rad)). When determining the crystallite size of a lithium-transition metal composite oxide, it is common to use the (003) plane peak, which is the first peak. (Hereinafter, the crystallite diameter determined from the (003) plane diffraction peak is referred to as the (003) crystallite diameter.)

The (003) crystallite diameter A of the lithium-transition metal composite oxide produced by adding niobium is the same as the (003) crystallite diameter A of the lithium-transition metal composite oxide containing no niobium and having the same nickel:manganese:cobalt substance amount ratio. The rate of change Z with respect to the crystallite diameter B is determined by the following formula (c) and has a relationship of 5% or more and 12% or less, and is preferably large within that range. Furthermore, more preferably, it is large in the range of 5% or more and 10.5% or less.

If the increase in the crystallite size change rate Z is less than 5%, the crystal growth effect due to the addition of niobium cannot be said to be sufficient, and the desired effect of improving battery characteristics cannot be obtained. On the other hand, when the increase in the crystallite size change rate Z exceeds 10.5%, cation mixing, in which transition metal ions are mixed into the lithium sites, tends to occur, and the charge/discharge capacity decreases. The increase in the ratio Z is preferably 12% or less, more preferably 10.5% or less. In addition, the significant coarsening of the primary particles reduces the reaction field between the positive electrode active material and the electrolytic solution, which may lead to deterioration of the output characteristics.
Such a change in crystallite diameter can be made within the above range by adjusting the substance amount ratio of nickel and manganese, the amount of niobium added, the firing temperature, the firing time, and the like.

(2) Manufacturing method of positive electrode active material for non-aqueous electrolyte secondary battery By the manufacturing method of the present embodiment, general formula (1): Li _d Ni _1-ab-c1 Mn _a-c2 Co _b-c3 Nb _c O ₂ (0.20≦a≦0.50, 0.2≦b≦0.60, 0.03< c≦0.08, c=c1+c2+c3, 1.03≦d≦1.20, (1− represented by a-b-c1) / a ≤ 2), composed of secondary particles in which a plurality of primary particles are aggregated, and niobium replaces nickel, manganese, and cobalt in the composite oxide, respectively, to form primary particles can be easily obtained on an industrial scale.

The production method of the present embodiment includes a mixing step of preparing a mixture containing nickel-manganese composite hydroxide particles having a specific composition, a niobium compound, and a lithium compound, and As described above, it includes a firing step of obtaining a lithium-transition metal composite oxide by firing at 1000° C. or less. A method for manufacturing the positive electrode active material of this embodiment will be described below. In addition, the following description is an example of the manufacturing method, and does not limit the manufacturing method.

(Crystallization step)
First, nickel-manganese-cobalt composite hydroxide particles (hereinafter also referred to as "composite hydroxide particles") are prepared. This composite hydroxide particle has a general formula (2): Ni _{1-ab Mna Co b (} OH) _2+α (0.20≦a≦0.50, 0.20≦b≦0.60, 0 ≦α≦0.4).
Since the material amount ratio of the metals (Ni, Mn, Co) in the composite hydroxide particles is substantially maintained even in the lithium-transition metal composite oxide, the material amount ratio of each metal (Ni, Mn, Co) does not meet the objective. It is preferable that the ratio is the same as the material amount ratio of each metal in the lithium-transition metal composite oxide.

A mixed aqueous solution containing nickel, manganese and cobalt having the same substance quantity ratio as the substance quantity ratio of each metal element in the target lithium-transition metal composite oxide is, for example, a sulfate solution, a nitrate solution, or the like of nickel, manganese and cobalt. Chloride solutions can be prepared by mixing.
A neutralizing agent is added to this mixed solution, and a composite hydroxide can be obtained by a neutralization reaction.
An alkaline aqueous solution can be used as the neutralizing agent, and for example, sodium hydroxide, potassium hydroxide, or the like can be used.

Moreover, it is preferable to add a complexing agent to the mixed aqueous solution together with the neutralizing agent.
The complexing agent to be added is not particularly limited as long as it can form a soluble complex by bonding with nickel ions and other transition metal ions in the reaction system, and known ones can be used. A supplier can be used. The ammonium ion donor is not particularly limited, but for example, ammonia, ammonium sulfate, ammonium chloride, ammonium carbonate, ammonium fluoride and the like can be used.
By adding this complexing agent, the solubility of metal ions in the reaction system can be adjusted, and composite hydroxide particles having desired physical properties can be obtained.

In the crystallization step, when the pH in the reaction system exceeds 12, the obtained composite hydroxide particles become fine particles, filterability is deteriorated, and spherical particles may not be obtained. On the other hand, when the pH of the reaction aqueous solution is lower than 10, the production rate of composite hydroxide particles is significantly slowed, nickel remains in the filtrate at a high concentration, and the composition of the obtained composite hydroxide deviates from the target composition. As a result, the desired ratio of lithium-transition metal composite oxide may not be obtained.

When an ammonium ion donor (complexing agent) is used, the temperature in the reaction system is preferably 30° C. or higher and 60° C. or lower, and the pH of the reaction aqueous solution is 10 or higher and 13 or lower (25 °C standard).

Further, the ammonia concentration in the reaction aqueous solution is preferably maintained at a constant value within the range of 3 g/L or more and 25 g/L or less.
If the ammonia concentration is less than 3 g/L, the solubility of the metal ions is low, so primary particles of the composite hydroxide with a regular shape and particle size may not be formed, and gel precipitates are likely to form. , the particle size distribution of the obtained composite hydroxide particles tends to be widened.
On the other hand, if the ammonia concentration exceeds 25 g/L, the solubility of metal ions becomes too large, the amount of metal ions remaining in the reaction aqueous solution increases, and the composition of the resulting composite hydroxide particles tends to vary. . If the ammonia concentration fluctuates, the solubility of metal ions fluctuates and uniform hydroxide particles are not formed. Therefore, it is preferable to maintain the ammonia concentration at a constant value during the crystallization reaction. For example, the ammonia concentration is preferably maintained at a desired concentration with a width of about 5 g/L between the upper limit and the lower limit.

Furthermore, when no complexing agent is used, the temperature of the reaction aqueous solution (liquid temperature) is preferably in the range of 60° C. to 80° C., and the pH in the reaction system in that temperature range is It is preferably 10 or more and 12 or less (at 25°C).
When the temperature in the reaction system exceeds 60° C., the solubility of nickel increases, the amount of precipitated nickel deviates from the target composition, and the phenomenon that coprecipitation does not occur can be avoided. On the other hand, when the temperature of the reaction aqueous solution exceeds 80°C, the slurry concentration (concentration of the reaction aqueous solution) increases due to a large amount of evaporation of water, crystals such as sodium sulfate are generated in the filtrate, and the concentration of impurities increases. , the charge/discharge capacity of the positive electrode active material may decrease.

In addition, the crystallization process may use a crystallization method by a batch system, or may use a continuous crystallization method.
For example, in the case of a batch-type crystallization method, after the reaction aqueous solution in the reaction vessel reaches a steady state, the precipitate can be collected, filtered and washed with water to obtain composite hydroxide particles. In the case of the continuous crystallization method, a mixed aqueous solution, an alkaline aqueous solution, and optionally an aqueous solution containing an ammonium ion donor are continuously supplied to overflow the reaction vessel to collect the precipitate, which is then filtered, washed with water, and dried. composite hydroxide particles can be obtained.

(Mixing process)
The mixing step is a step of mixing the composite hydroxide particles obtained above, the niobium compound, and the lithium compound to obtain a mixture.
As the niobium compound, known compounds containing niobium can be used, such as niobic acid, niobium oxide, niobium nitrate, niobium pentachloride, and niobium nitrate. Among these, niobic acid, niobium oxide, or a mixture thereof is preferable from the viewpoint of availability and avoiding contamination of the lithium-transition metal composite oxide with impurities. When impurities are mixed in the lithium-transition metal composite oxide, the heat stability, battery capacity, and cycle characteristics of the obtained secondary battery may be deteriorated.

Also, the niobium compound is preferably mixed in particles (solid phase).
When niobium is added in a solid phase, the particle size of the niobium compound changes the reactivity of the niobium compound in the subsequent firing step, so the particle size of the niobium compound used is one of the important manufacturing conditions. The average particle size of the niobium compound is preferably 0.01 μm or more and 10 μm or less, more preferably 0.05 μm or more and 3.0 μm or less, and still more preferably 0.08 μm or more and 1.0 μm or less.

If the average particle size is less than 0.01 μm, there is a problem that the handling of the powder becomes very difficult, and the niobium compound scatters during the mixing process and the firing process, and the amount of niobium in the lithium-transition metal composite oxide after firing decreases. A problem that does not reach the target amount may occur. On the other hand, if the average particle size is more than 10 μm, solid solution of niobium in the lithium-transition metal composite oxide during firing may be insufficient, and a sufficient effect of improving secondary battery characteristics may not be obtained. As the average particle diameter, a volume average particle diameter MV can be used, and for example, it can be obtained from a volume integrated value measured by a laser light diffraction/scattering particle size distribution analyzer.

Such a niobium compound may be pulverized in advance using various pulverizers such as a ball mill, a planetary ball mill, a jet mill/nano jet mill, a bead mill, and a pin mill so as to have a particle size within the above range. Moreover, the niobium compound may be classified by a dry classifier or sieving, if necessary. For example, only particles smaller than 0.01 μm can be obtained with a wind classifier.

The lithium compound is not particularly limited, and known compounds containing lithium can be used. For example, lithium carbonate, lithium hydroxide, lithium nitrate, or mixtures thereof are used. Among these, lithium carbonate, lithium hydroxide, or a mixture thereof is preferable because it is less affected by residual impurities and melts at a temperature equal to or lower than the firing temperature.

The method of mixing the composite hydroxide particles, the lithium compound, and the niobium compound is not particularly limited, and the composite hydroxide particles, the lithium compound, and the niobium compound are mixed to the extent that the particle shape of the composite hydroxide particles, etc. is not destroyed. It should be well mixed.
As a mixing method, for example, a common mixer can be used for mixing, for example, a shaker mixer, Loedige mixer, Julia mixer, V blender, or the like can be used. It is preferable that this mixture is sufficiently mixed before the firing step described later. If the mixture is not sufficiently mixed, the ratio of Li to the transition metal element Me other than Li ( Li/Me) may vary, causing problems such as insufficient battery characteristics.

The lithium compound is mixed in an amount such that Li/Me in the lithium-niobium mixture is 1.03 or more and 1.20 or less, and Li/Me in the mixture is the same as Li/Me of the desired positive electrode active material. prepared to be This is because Li/Me in the lithium-transition metal composite oxide obtained after the firing step is approximately equal to Li/Me in the mixture. Further, the niobium compound is such that the niobium content in the mixture is 0.03 at% or more and 3 at% or less with respect to the total of metal elements (Ni, Mn, Co, Nb) other than Li in the mixture. mixed.

(Baking process)
The firing step is a step of firing the mixture at 850° C. or higher and 1000° C. or lower in an oxidizing atmosphere.
When the mixture is calcined, lithium in the lithium compound diffuses into the composite hydroxide particles, forming a lithium-transition metal composite oxide composed of particles with a polycrystalline structure. The lithium compound melts at the firing temperature and penetrates into the composite hydroxide particles to form a lithium transition metal composite oxide. At this time, the niobium compound penetrates into the secondary particles together with the molten lithium compound. In addition, if there is a crystal grain boundary or the like in the primary particles, it penetrates. The permeation promotes the diffusion of niobium inside the primary particles, and the niobium uniformly dissolves in the primary particles.

The firing temperature is 850° C. or higher and 1000° C. or lower in an oxidizing atmosphere. If the firing temperature is less than 850° C., the diffusion of lithium and niobium into the nickel-manganese composite hydroxide particles is insufficient, and the crystal structure is not sufficiently arranged, resulting in insufficient battery characteristics. The problem arises that there is no Further, if the firing temperature exceeds 1000° C., intense sintering may occur between the formed lithium-transition metal composite oxide particles, and abnormal grain growth may occur. If abnormal grain growth occurs, the particles after firing may become coarse and may not be able to maintain the particle shape, and when the positive electrode active material is formed, the specific surface area decreases and the resistance of the positive electrode increases. A problem arises that the battery capacity decreases.

The firing time is preferably at least 3 hours or longer, more preferably 6 hours or longer and 24 hours or shorter. If the firing time is less than 3 hours, the lithium-transition metal composite oxide may not be produced sufficiently.

Furthermore, the atmosphere during firing is preferably an oxidizing atmosphere, particularly an atmosphere with an oxygen concentration of 20 to 100% by volume. That is, the calcination is preferably carried out in the air, in an oxygen stream, or in a mixed gas thereof.
This is because if the oxygen concentration is less than 20% by volume, the lithium-transition metal composite oxide may not be sufficiently oxidized and the crystallinity of the lithium-transition metal composite oxide may be insufficient. In particular, considering battery characteristics, it is preferable to carry out in an oxygen stream.

In addition, the firing furnace is not particularly limited as long as it can fire the lithium-niobium mixture in the air or in an oxygen stream, but it is preferable to use an electric furnace that does not generate gas, and a batch or continuous furnace. can be used.

The lithium-transition metal composite oxide obtained by firing is not strongly sintered secondary particles, but may form coarse particles due to weak sintering or agglomeration. In such a case, the sintering and agglomeration can be eliminated by pulverization to adjust the particle size distribution.

(3) Non-Aqueous Electrolyte Secondary Battery The non-aqueous electrolyte secondary battery of the present invention (hereinafter also referred to as "secondary battery") uses the positive electrode active material described above for the positive electrode. An example of the secondary battery of the present embodiment will be described below for each component.
The secondary battery of the present invention includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, and is composed of components similar to those of a general lithium ion secondary battery. It should be noted that the embodiments described below are merely examples, and the non-aqueous electrolyte secondary battery can be implemented in various modified and improved forms based on the knowledge of those skilled in the art, including the following embodiments. can. Moreover, the secondary battery is not particularly limited in its application.

(positive electrode)
A positive electrode of a secondary battery is produced using the above positive electrode active material. An example of the method for producing the positive electrode will be described below. First, the positive electrode active material (powder), conductive material, and binder are mixed, and if necessary, activated carbon and a solvent for viscosity adjustment are added, and the mixture is kneaded to form a positive electrode. Prepare a mixture paste.
Since the mixing ratio of each material in the positive electrode mixture is a factor that determines the performance of the lithium secondary battery, it can be adjusted according to the application. The mixture ratio of the materials can be the same as that of the positive electrode of a known lithium secondary battery. It can contain up to 95% by mass, 1 to 20% by mass of the conductive material, and 1 to 20% by mass of the binder.

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

Examples of conductive materials that can be used include graphite (natural graphite, artificial graphite, expanded graphite, etc.) and carbon black-based materials such as acetylene black and ketjen black.
The binding agent (binder) plays a role of binding the active material particles, and examples thereof include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, and cellulose. Resin, polyacrylic acid, and the like can be used.
If necessary, the positive electrode active material, the conductive material and the activated carbon are dispersed, and a solvent that dissolves the binder is added to the positive electrode mixture. As the solvent, specifically, an organic solvent such as N-methyl-2-pyrrolidone can be used. In addition, activated carbon can be added to the positive electrode mixture in order to increase the electric double layer capacity.

(negative electrode)
Metallic lithium, a lithium alloy, or the like can be used for the negative electrode. In addition, the negative electrode is prepared by mixing a negative electrode active material capable of intercalating and deintercalating lithium ions with a binder, adding an appropriate solvent to make a paste, and applying the negative electrode mixture to the surface of a metal foil current collector such as copper. It may be applied, dried, and compressed to increase the electrode density as necessary.

As the negative electrode active material, for example, natural graphite, artificial graphite, sintered organic compounds such as phenol resin, and powdery carbon substances such as coke 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.

(separator)
A separator is interposed between the positive electrode and the negative electrode. The separator separates the positive electrode from the negative electrode and holds the electrolyte, and a known one can be used. For example, a thin film such as polyethylene or polypropylene having a large number of fine pores can be used. can.

(Non-aqueous electrolyte)
The non-aqueous electrolytic solution is obtained by dissolving a lithium salt as a supporting salt in an organic solvent. Examples of 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; One selected from ether compounds such as methyltetrahydrofuran and dimethoxyethane, sulfur compounds such as ethylmethylsulfone and butane sultone, and phosphorus compounds such as triethyl phosphate and trioctyl phosphate, and the like, or a mixture of two or more. can be done.

As the supporting salt, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiN(CF ₃ SO ₂ ) ₂ , composite salts thereof, and the like can be used. Furthermore, the non-aqueous electrolytic solution may contain radical scavengers, surfactants, flame retardants, and the like.

(Shape and configuration of secondary battery)
The non-aqueous electrolyte secondary battery of the present invention, which is composed of the positive electrode, negative electrode, separator and non-aqueous electrolytic solution described above, can have various shapes such as a cylindrical shape and a laminated shape. Regardless of which shape is adopted, the positive electrode and the negative electrode are laminated with a separator interposed to form an electrode body, the obtained electrode body is impregnated with a non-aqueous electrolytic solution, and communicated 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 the battery case to complete the non-aqueous electrolyte secondary battery. .

(Characteristics of secondary battery)
The positive electrode active material of the present invention can be obtained by the industrial production method as described above, and the secondary battery according to the present invention using the positive electrode active material has high thermal stability and output characteristics. Since both can be achieved, it is suitable as a power source for small portable electronic devices (smartphones, tablet PCs, etc.) that always require high capacity.
Furthermore, in comparison with batteries using conventional lithium-cobalt-based oxides or lithium-nickel-based oxide positive electrode active materials, they are superior in terms of safety, capacity, and durability. Therefore, it is possible to reduce the size and increase the capacity, and it is suitable as a power source for an electric vehicle, which is limited in mounting space. The secondary battery of the present invention can be used not only as a power source for electric vehicles that are driven purely by electrical energy, but also as a power source for so-called hybrid vehicles that use a combustion engine such as a gasoline engine or a diesel engine. .

EXAMPLES The present invention will be described in more detail below with reference to Examples and Comparative Examples of the present invention, but the present invention is not limited by these Examples. The analysis method of the metal contained in the positive electrode active material and the various evaluation methods of the positive electrode active material in Examples and Comparative Examples are as follows.
(1) Analysis of elemental composition: Measured by ICP emission spectrometry.
(2) Volume average particle size MV, and [(D90-D10)/volume average particle size MV]:
It was measured with a laser diffraction scattering type particle size distribution analyzer (Microtrac HRA manufactured by Nikkiso Co., Ltd.).
(3) Qualitative evaluation of crystal structure and lithium-niobium compound, and crystallite size: XRD diffraction device (manufactured by PANalytical, X'Pert PRO). From the XRD measurement results, the peak of the (003) plane present near 2θ=18° was analyzed, and the crystallite size was calculated using Scherrer's formula.

(4) Initial charge capacity and initial discharge capacity:
The initial charge capacity and initial discharge capacity were measured by leaving the coin-type battery for about 24 hours, and after the open circuit voltage OCV (open circuit voltage) was stabilized, the current density for the positive electrode was cut to 0.1 mA/cm ² . The battery was charged to an off-voltage of 4.3 V to obtain an initial charge capacity, and after resting for 1 hour, the battery was discharged to a cut-off voltage of 3.0 V. The initial discharge capacity was obtained. A multi-channel voltage/current generator (R6741A, manufactured by Advantest Co., Ltd.) was used to measure the discharge capacity.

(5) Evaluation of thermal stability The thermal stability of the positive electrode was evaluated by quantifying the amount of oxygen released by heating the positive electrode active material in an overcharged state.
A coin-type battery was prepared in the same manner as in (4), and CCCV charged (constant current-constant voltage charge) at a 0.2 C rate to a cutoff voltage of 4.5V. Thereafter, the coin battery was disassembled, and only the positive electrode was taken out so as not to short-circuit, washed with DMC (dimethyl carbonate), and dried. Approximately 2 mg of the dried positive electrode was weighed and heated from room temperature to 450° C. at a temperature elevation rate of 10° C./min using a gas chromatograph mass spectrometer (GCMS, Shimadzu Corporation, QP-2010plus). Helium was used as carrier gas.

The generation behavior of oxygen (m/z = 32) generated during heating was measured, and the amount of oxygen generated was semi-quantified from the obtained maximum oxygen generation peak height and peak area, and these were used as evaluation indices for thermal stability. . The semi-quantitative value of the amount of oxygen generated was calculated by injecting pure oxygen gas into the GCMS as a standard sample and extrapolating the calibration curve obtained from the measurement results.

(6) Output evaluation Output evaluation was performed by charging a coin-type battery whose temperature was adjusted to the measurement temperature at a charging potential of 4.1 V and measuring the reaction resistance value by the AC impedance method.

(Crystallization step)
A nickel-manganese-cobalt composite hydroxide was produced by a known method as described below. A predetermined amount of pure water was put into a reaction tank (60 L), and the temperature inside the tank was set to 45° C. while stirring. At this time, N ₂ gas was flowed into the reaction tank so that the dissolved oxygen concentration in the reaction tank liquid was 0.8 mg/L.
A 2.0 M mixed aqueous solution of nickel sulfate, cobalt sulfate, and manganese sulfate prepared so that the molar ratio of nickel:manganese:cobalt was 0.50:0.30:0.20, and an alkaline solution were placed in the reaction tank. A 25% by mass sodium hydroxide solution as a solution and a 25% by mass aqueous ammonia solution as a complexing agent were simultaneously and continuously added to the reactor. At this time, the flow rate of each solution was adjusted so that the residence time for the crystallization reaction was 4 hours, the pH in the reaction tank was 11.1 to 11.3, and the ammonia concentration in the reaction tank was 9 to 12 g/L. , a nickel-manganese-cobalt composite hydroxide was produced.

The resulting slurry containing the nickel-manganese-cobalt composite hydroxide was collected as an overflow from the reaction vessel and then filtered to obtain a nickel-manganese-cobalt composite hydroxide cake. Impurities were washed by passing 1 L of pure water through 150 g of the filtered nickel-cobalt-manganese composite hydroxide in Denver. The powder after filtration was dried in a vacuum dryer at 120° C. for 12 hours, and nickel-cobalt-manganese represented by Ni _0.50 Mn _0.30 Co _0.20 (OH) _2+α (0≦α≦0.4) Composite hydroxide particles were obtained. The volume average particle diameter MV of the obtained composite hydroxide was 6.0 μm.

(Mixing process)
_Nickel : _manganese : _cobalt : so that the molar ratio of niobium is 0.475:0.285:0.190:0.05 (c1=0.025, c2=0.015, c3=0.010, c=c1+c2+c3=0.05). And after weighing so that the atomic ratio of the amount of lithium (Li) and the total metal amount (Me) of nickel, manganese, cobalt and niobium (hereinafter referred to as Li/Me) is 1.03, Thorough mixing was performed using a shaker mixer device (TURBULA Type T2C manufactured by Willie & Bachoffen (WAB)) to obtain a lithium mixture.

(Baking process)
The obtained lithium mixture was held at 900° C. for 10 hours in an air stream (oxygen: 21% by volume) and sintered, and then pulverized with a hammer mill to form a lithium-nickel-cobalt mixture in which niobium was solid-dissolved. A cathode active material comprising a manganese-niobium composite oxide was obtained.
Table 1 shows the volume average particle size MV of the obtained positive electrode active material.
The lattice constants a and c of the lithium-nickel-cobalt-manganese-niobium composite oxide according to Example 1 were found to be increased compared to the lattice constants a and c of the lithium-nickel-cobalt composite oxide containing no niobium. rice field.
From the results of STEM-EDX analysis, it was confirmed that niobium was dissolved in the crystal structure. Furthermore, the 003 crystallite size was calculated from the XRD measurement results using Scherrer's formula and was 1348 Å.

(Evaluation of electrochemical properties)
52.5 mg of the obtained positive electrode active material, 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. Evaluation electrode) 1 was produced.
After drying the prepared positive electrode 1 at 120° C. for 12 hours in a vacuum dryer, a 2032-type coin battery 10 was prepared using the dried positive electrode 1 in an Ar atmosphere glove box with a controlled dew point of −80° C. made.
Lithium (Li) metal with a diameter of 17 mm and a thickness of 1 mm is used for the negative electrode 2, and the electrolyte is an equal mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with 1 M LiClO ₄ as the supporting electrolyte ( Toyama Pharmaceutical Co., Ltd.) was used. A polyethylene porous film having a film thickness of 25 μm was used as the separator 3 .
The coin 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. Table 2 shows the measurement results of the initial charge/discharge capacity of the obtained positive electrode active material and the positive electrode resistance value at 25°C.

(Thermal stability evaluation)
The thermal stability was evaluated according to the procedure described in (5) for each evaluation of the positive electrode active material described above. A semi-quantitative value of the oxygen evolution amount was obtained from the obtained maximum oxygen evolution peak intensity and peak area. These results are shown in Table 2.

Same as Example 1, except that a 2.0 M mixed aqueous solution of nickel sulfate, cobalt sulfate, and manganese sulfate was used so that the molar ratio of nickel:manganese:cobalt was 0.40:0.40:0.20. Similarly, a positive electrode active material was obtained and evaluated.
Evaluation results are shown in Tables 1 and 2.

A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the firing temperature was 950°C.
Evaluation results are shown in Tables 1 and 2.

A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the firing temperature was 870°C.
Evaluation results are shown in Tables 1 and 2.

The obtained nickel-cobalt-manganese composite hydroxide particles, lithium carbonate, and niobic acid (Nb ₂ O ₅ ·nH ₂ O) having an average particle diameter of 1.0 µm were combined with a nickel:manganese:cobalt:niobium molar ratio of A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the weight ratio was adjusted to 0.485:0.291:0.194:0.03.
Evaluation results are shown in Tables 1 and 2.

The obtained nickel-cobalt-manganese composite hydroxide particles, lithium carbonate, and niobic acid (Nb ₂ O ₅ ·nH ₂ O) having an average particle diameter of 1.0 µm were combined with a nickel:manganese:cobalt:niobium molar ratio of A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the weight ratio was adjusted to 0.465:0.279:0.186:0.07.
Evaluation results are shown in Tables 1 and 2.

(Comparative example 1)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that niobic acid (Nb ₂ O _{5.nH 2} O) was not added.
Evaluation results are shown in Tables 1 and 2.

(Comparative example 2)
A positive electrode active material was obtained and evaluated in the same manner as in Example 2, except that niobic acid (Nb ₂ O _{5.nH 2} O) was not added.
Evaluation results are shown in Tables 1 and 2.

(Comparative Example 3)
Same as Example 1, except that a 2.0M mixed aqueous solution of nickel sulfate, cobalt sulfate, and manganese sulfate was used so that the molar ratio of nickel:manganese:cobalt was 0.50:0.20:0.30. Composite hydroxide particles were prepared in the same manner, and a positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that niobic acid (Nb ₂ O _{5.nH 2} O) was not added.
Evaluation results are shown in Tables 1 and 2.

(Comparative Example 4)
Same as Example 1, except that a 2.0M mixed aqueous solution of nickel sulfate, cobalt sulfate, and manganese sulfate was used so that the molar ratio of nickel:manganese:cobalt was 0.50:0.20:0.30. Composite hydroxide particles were similarly prepared.
The resulting composite hydroxide particles, lithium carbonate, and niobic acid (Nb ₂ O ₅ ·nH ₂ O) having an average particle size of 1.0 μm were mixed with nickel:manganese:cobalt:niobium at a molar ratio of 0.475: A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the weights were adjusted to 0.190:0.285:0.05.
Evaluation results are shown in Tables 1 and 2.

(Comparative Example 5)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the firing temperature was 800°C.
Evaluation results are shown in Tables 1 and 2.

(Comparative Example 6)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the firing temperature was set to 1050°C. Evaluation results are shown in Tables 1 and 2.

(Comparative Example 7)
The obtained nickel-cobalt-manganese composite hydroxide particles, lithium carbonate, and niobic acid (Nb ₂ O ₅ ·nH ₂ O) having an average particle diameter of 1.0 µm were combined with a nickel:cobalt:manganese:niobium molar ratio of A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the weight ratio was adjusted to 0.495:0.297:0.198:0.01.
Evaluation results are shown in Tables 1 and 2.

(Comparative Example 8)
The obtained nickel-cobalt-manganese composite hydroxide particles, lithium carbonate, and niobic acid (Nb ₂ O ₅ ·nH ₂ O) having an average particle diameter of 1.0 µm were combined with a nickel:cobalt:manganese:niobium molar ratio of A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the weight ratio was adjusted to 0.450:0.270:0.180:0.100.
Evaluation results are shown in Tables 1 and 2.

(Evaluation results)
As shown in Tables 1 and 2, the positive electrode active materials obtained in Examples have both high thermal stability and output. In addition, the 003 crystallite size is within the target range, which is presumed to affect both electrochemical properties and thermal stability.
In all of the positive electrode active materials obtained in the examples, Nb dissolves in the primary particles to form a strong bond with oxygen, suppressing oxygen release and structural phase transition during overcharge, and improving thermal stability. presumed to have improved. Furthermore, it is presumed that the crystallinity was improved and the output characteristics were improved because the grain growth was promoted by the addition of Nb.

On the other hand, the positive electrode active materials of Comparative Examples 1 and 2 have poor thermal stability because Nb is not added, and the output characteristics are also poor due to the small crystallite diameter. In the positive electrode active material of Comparative Example 3, the molar ratio of nickel and manganese was Ni/Mn=2.5, and since no Nb was added, grain growth was not observed and thermal stability was lowered. there is
The positive electrode active material of Comparative Example 4 also had Ni/Mn=2.5, but no grain growth was observed due to the addition of Nb. No improvement in properties was observed. It can be seen that the Ni/Mn ratio is important for exhibiting the grain growth effect due to the addition of Nb.
In the positive electrode active material of Comparative Example 5, since the firing temperature was low, crystal growth did not proceed sufficiently, Nb remained as a single substance and hardly dissolved in the crystal structure, and the concentration difference between the particle surface and the center was large. As a result, the reaction resistance and bulk resistance became extremely high, and the deterioration of capacity and output was observed. Thermal stability is presumed to have apparently improved due to the lower capacity.
The positive electrode active material of Comparative Example 6 had a low initial capacity due to the progress of sintering/aggregation and the occurrence of cation mixing due to the high firing temperature.
Since the positive electrode active material of Comparative Example 7 has a small amount of Nb added, no effect of improving thermal stability is observed, no change in grain growth is observed, and no improvement in output characteristics is observed.
The positive electrode active material of Comparative Example 8 shows improvement in thermal stability, but the output characteristics and initial capacity are low because the amount of Nb added is large and the Nb that cannot be fully dissolved forms a Li compound.

1 positive electrode 2 negative electrode 3 separator 4 gasket 5 wave washer 6 positive electrode can 7 negative electrode can 10 2032 type coin battery

Claims (7)

A positive electrode active material for a non-aqueous electrolyte secondary battery comprising a lithium-nickel-manganese-cobalt-niobium composite oxide composed of secondary particles in which a plurality of primary particles are aggregated,
The lithium-nickel-manganese-cobalt-niobium composite oxide is
General formula (1): Li _d Ni _1-ab-c1 Mn _a-c2 Co _b-c3 Nb _c O ₂ (0.20≦a≦0.50, 0.20≦b≦0.60, 0 .03 < c ≤ 0.08, c = c1 + c2 + c3, 1.03 ≤ d ≤ 1.20, (1-ab-c1) / a ≤ 2),
Niobium in the lithium-nickel-manganese-cobalt-niobium composite oxide is substituted with nickel, manganese, and cobalt in the composite oxide, respectively, and is dissolved in the primary particles. Active material for electrolyte secondary batteries. 2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the secondary particles have a volume average particle diameter MV of 5 μm or more and 20 μm or less. "Nickel, manganese , the crystallite diameter change rate Z (%) with respect to the crystallite diameter B of the lithium-nickel-manganese-cobalt composite oxide containing no niobium and having the same cobalt substance amount ratio is expressed by the following formula (a): The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1 or 2, characterized by comprising:

General formula (1): Li _d Ni _1-ab-c1 Mn _a-c2 Co _b-c3 Nb _c O ₂ (0.20≦a≦0.50, 0.20≦b≦0.60, 0 .03 < c ≤ 0.08, c = c1 + c2 + c3, 1.03 ≤ d ≤ 1.20, (1-abc) / a ≤ 2), and a plurality of primary particles aggregated secondary A non-aqueous electrolyte comprising a lithium-nickel-manganese-cobalt-niobium composite oxide composed of particles, wherein niobium replaces nickel, manganese, and cobalt in the composite oxide, respectively, and is dissolved in the primary particles. A method for producing a positive electrode active material for a secondary battery, comprising:
General formula (2): Ni _{1-ab Mna Co b (} OH) _2+α (0.20≦a≦0.50, 0.20≦b≦0.60, 0≦α≦0.4) a mixing step of preparing a mixture consisting of the represented nickel-manganese-cobalt composite hydroxide particles, a niobium compound, and a lithium compound;
a firing step of firing the mixture in an oxidizing atmosphere at 850° C. or higher and 1000° C. or lower for 5 hours or longer and 10 hours or shorter to obtain a lithium-nickel-manganese-cobalt-niobium composite oxide;
A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, comprising: 5. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 4, wherein the niobium compound has a volume average particle size MV of 0.01 μm or more and 10 μm or less. "Nickel, manganese , the crystallite diameter change rate Z (%) with respect to the crystallite diameter B of the lithium-nickel-manganese-cobalt composite oxide containing no niobium and having the same cobalt substance amount ratio is represented by the following formula (b): The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 4 or 5, comprising:

A non-aqueous electrolyte secondary battery comprising a positive electrode containing the positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3.

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