Positive electrode active material for lithium-ion secondary batteries and method for manufacturing the same

Abstract

To provide a positive electrode active material for a lithium ion secondary battery, having a high cycle capacity-keeping rate.SOLUTION: A positive electrode active material for a lithium ion secondary battery comprises: particles of lithium nickel composite oxide; and a coating layer on the surface of each particle. The particles have a crystal structure belonging to a space group R-3m, and contains at least Li, Ni, Mn and an element M; the composition of the elements is given by Li:Ni:Mn:M:Nb=a:(1-x-y):x:y:z(0.95≤a≤1.10, 0<x≤0.5, 0<y≤0.5, 0<z<0.05, and 0<x+y+z≤0.7); the Li-space share is 92% or more and 98% or less; D50 is equal to or smaller than 8 μm; a quantity of eluted lithium ions is 0.10 mass% or more and 0.50 mass% or less to a total quantity of the lithium nickel composite oxide particles. The coating layer contains Li, and at least one element selected from a group consisting of Al, Si, Ti, V, Ga, Ge, Zr, Nb, Mo, Ta and W.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material for lithium-ion secondary batteries and a method for producing the same. [Background technology]

[0002] In recent years, with growing environmental awareness, there has been a shift from gasoline-powered vehicles to hybrid and electric vehicles. In particular, there is a strong demand for the development of small, lightweight rechargeable batteries with high energy capacity, which are essential for the widespread adoption of electric vehicles. Lithium-ion rechargeable batteries are one such type of rechargeable battery.

[0003] Currently, typical lithium-ion secondary batteries use lithium transition metal composite oxides such as LiCoO2, LiNiO2, and LiMn2O4 as the positive electrode active material, and lithium metal, lithium alloys, metal oxides, carbon, etc. as the negative electrode active material.

[0004] Furthermore, when using a non-aqueous electrolyte, for example, an electrolyte is used in which a lithium salt such as LiClO4 or LiPF6 is dissolved as a supporting salt in an organic solvent such as ethylene carbonate, dimethyl carbonate, or ethyl methyl carbonate.

[0005] While research and development of such lithium-ion secondary batteries are currently underway, lithium-ion secondary batteries using layered or spinel-type lithium metal composite oxides as the cathode material are particularly gaining practical application as batteries with high energy density, as they can achieve high voltages of around 4V.

[0006] The main materials proposed so far include lithium cobalt composite oxide (LiCoO2), which is relatively easy to synthesize, lithium metal composite oxide (LiNiO2), which uses nickel, which is cheaper than cobalt, and lithium nickel cobalt manganese composite oxide (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3O2), lithium manganese composite oxide using manganese (LiMn2O4), and the like can be mentioned.

[0007] For example, in Patent Document 1, Li x (Ni y Co 2(1-y ) / 5Mn 3(1-y) / 5)1-z M z O2 (1.00≦x≦1.10, 0.65<y<0.82, 0≦z≦0.05, M is at least one element selected from Al, Zr and Mg), which describes Li-Ni composite oxide particle powder. Further, it is stated that a positive electrode containing a positive electrode active material made of this Li-Ni composite oxide particle powder has a high initial discharge capacity and excellent initial charge-discharge efficiency. PRIOR ART DOCUMENT PATENT DOCUMENT

[0008] Patent Document 1 International Publication No. 2014-061654 SUMMARY OF THE INVENTION PROBLEM TO BE SOLVED BY THE INVENTION

[0009] By the way, for achieving high energy density of lithium ion secondary batteries, it is preferable to charge and discharge the battery at a high voltage, and it is preferable to use a positive electrode active material having a high average operating voltage. As such a positive electrode active material, LiNi 0.33 Co 0.33 Mn 0.33 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, and other positive electrode active materials that contain Mn and have a Ni ratio of 30% or more can be mentioned.

[0010] Accordingly, the inventors have studied the applicability of the above-described positive electrode active material to lithium ion secondary batteries. As a result, the inventors have found that the conventional positive electrode active material described above has a problem in that, when charging and discharging are repeated at high voltage, the capacity after repeated charge-discharge cycles is significantly reduced compared to the capacity before repeated charge-discharge cycles (initial capacity).

[0011] For example, Patent Document 1 mentions the initial discharge capacity and initial charge-discharge efficiency of a positive electrode active material, but does not mention anything about cycle characteristics at high voltage.

[0012] In view of the above problems, an object of the present invention is to provide a positive electrode active material that has improved charge-discharge cycle characteristics when used as a positive electrode active material for lithium ion secondary batteries. [Means for Solving the Problem]

[0013] According to a first aspect of the present invention, there is provided a positive electrode active material for a lithium ion secondary battery, comprising lithium nickel composite oxide particles and a coating layer covering surfaces of the particles, wherein the lithium nickel composite oxide particles have a crystal structure belonging to the space group R-3m, contain at least Li, Ni, Mn and an element M, and are represented by a substance amount ratio of each element: Li:Ni:Mn:M:Nb = a:(1-x-y):x:y:z (0.95 ≦ a ≦ 1.10, 0 < x ≦ 0.5, 0 < y ≦ 0.5, 0 < z < 0.05, 0 < x+y+z ≦ 0.7, wherein the element M is Co, or Co and at least one selected from the group consisting of Al, Zr, Si, Zn, Nb and Ti), a Li site occupancy is 92% or more and 98% or less, a particle diameter (D50) corresponding to a cumulative volume fraction of 50% in a cumulative volume distribution curve of particle size distribution is 8 μm or less, a crystallite diameter calculated by the Scherrer method from a diffraction peak attributed to the (003) plane measured by XRD is 70 nm or more and 140 nm or less, an amount of eluted lithium ions determined by neutralization titration is 0.20 mass% or more and 1.00 mass% or less based on a total amount of the lithium nickel composite oxide particles, and the coating layer is a composite oxide containing Li and at least one element selected from the group consisting of Al, Si, Ti, V, Ga, Ge, Zr, Nb, Mo, Ta and W.

[0014] Further, the lithium nickel composite oxide particles include secondary particles formed by aggregation of a plurality of primary particles, have a porous structure having a plurality of void portions where no primary particles exist in the secondary particles, and have a specific surface area measured by a nitrogen adsorption BET method of 0.3 m 2 / g or more and 2.0 m 2 / g or less. Further, it is preferable that at least part of niobium contained in the lithium nickel composite oxide particles is segregated at an interface of the primary particles. Further, it is preferable that an average thickness of the coating layer is 1 nm or more and 15 nm or less.

[0015] A second aspect of the present invention provides a method for producing a positive electrode active material for a lithium-ion secondary battery, comprising: a mixing step of mixing a nickel composite compound, a niobium compound, and a lithium compound to obtain a mixture; a firing step of firing the mixture to obtain lithium nickel composite oxide particles; and a coating step of depositing a coating liquid containing at least one element selected from the group consisting of Al, Si, Ti, V, Ga, Ge, Zr, Nb, Mo, Ta, and W onto the surface of the lithium nickel composite oxide particles to form a coating layer.

[0016] Furthermore, the nickel composite compound preferably includes a nickel composite oxide, and preferably comprises an oxidation roasting step in which the nickel composite hydroxide, prepared by a crystallization reaction, is oxidized and roasted to obtain the nickel composite oxide. Furthermore, it is preferable to include a heat treatment step after the coating step in which the lithium nickel composite oxide particles, on which the coating layer has been formed on the surface, are heat-treated at 300°C or higher. [Effects of the Invention]

[0017] The positive electrode active material of the present invention improves the charge-discharge cycle characteristics when used as the positive electrode active material in a lithium-ion secondary battery. Furthermore, the manufacturing method of the present invention allows for the highly productive production of this positive electrode active material. [Brief explanation of the drawing]

[0018] [Figure 1] Figure 1 is a schematic diagram showing an example of a positive electrode active material according to this embodiment. [Figure 2] Figure 2 shows an example of a method for producing the positive electrode active material according to this embodiment. [Figure 3] Figure 3 shows an example of a method for producing a nickel composite compound according to this embodiment. [Figure 4] Figure 4 is an explanatory diagram of the cross-sectional configuration of the evaluation coin-type battery used for battery evaluation. [Figure 5] Figure 5 is an explanatory diagram of the laminated battery used for evaluation. [Modes for carrying out the invention]

[0019] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that, in the drawings, in order to make each configuration easy to understand, some parts are emphasized or simplified, and the actual structure, shape, scale, etc. may differ. In addition, the present invention is not limited to the following embodiments, and various modifications and substitutions can be made to the following embodiments without departing from the scope of the present invention.

[0020] 1. Positive electrode active material for lithium ion secondary batteries First, an example of the configuration of the positive electrode active material for lithium ion secondary batteries (hereinafter also referred to as "positive electrode active material") according to the present embodiment will be described.

[0021] FIG. 1 is a diagram schematically showing an example of the positive electrode active material according to the present embodiment. As shown in FIG. 1, the positive electrode active material 10 includes particles 1 of a lithium-nickel composite oxide, and a coating layer 2 that covers the surface of the particles 1. Hereinafter, each constituent element will be described.

[0022] (1) Particles of lithium-nickel composite oxide The particles 1 of the lithium-nickel composite oxide have a crystal structure belonging to the space group R-3m, and are a composite oxide containing at least lithium (Li), nickel (Ni), manganese (Mn), the element M, and niobium (Nb).

[0023] (Composition) When the molar ratio of each element contained in the lithium-nickel composite oxide particles 1 is expressed as Li:Ni:Mn:M:Nb = a:(1-x-y-z):x:y:z, the conditions 0.95≦a≦1.10, 0<x≦0.5, 0<y≦0.5, 0<z<0.05, and 0<x+y+z≦0.7 are satisfied. Further, it is preferable that the molar ratio satisfies 0.97≦a≦1.08, 0<x≦0.3, 0<y≦0.3, 0<z≦0.02, and 0<x+y+z≦0.5.

[0024] a, which represents the content ratio of Li in the above mass ratio, satisfies 0.95≦a≦1.10, and may satisfy 0.97≦a≦1.08. When a is less than 0.97, Li is deficient from the positive electrode active material, which tends to cause a decrease in capacity as a battery material. When a exceeds 1.10, the crystal structure of particles 1 of the lithium-nickel composite oxide grows excessively, resulting in coarse primary particles and easy occurrence of cracking in the particles 1, thereby tending to impair durability.

[0025] (1-x-y-z), which represents the content ratio of Ni in the above mass ratio, is 0.3 or more and less than 1.0, preferably 0.5 or more and less than 1.0. Further, the range including the lower limit of the Ni content ratio is preferably 0.6 or more, may be 0.7 or more, or may be 0.8 or more. The higher the content ratio of (1-x-y-z), the lower the voltage required for charging, resulting in a higher battery capacity. In addition, from the viewpoint of suitably performing charge and discharge at high voltage, the upper limit of the content ratio of (1-x-y-z) may be 0.9 or less, or may be 0.85 or less. On the other hand, when (1-x-y-z) is less than 0.3, the battery capacity becomes low.

[0026] In the above mass ratio, x, which represents the content ratio of Mn, satisfies 0<x≦0.5, may satisfy 0<x≦0.4, or may satisfy 0<x≦0.3. When the Mn content ratio is within the above range, the battery can have high battery capacity and high thermal stability. In addition, the lower limit of x representing the Mn content ratio may be, for example, 0.05 or more.

[0027] In the above mass ratio, the element M is preferably at least one selected from the group consisting of Co, Al, Zr, Si, Zn, and Ti. Further, the element M preferably contains at least one element selected from cobalt (Co) and aluminum (Al), and particularly preferably contains cobalt (Co). The element M is, for example, Co, or Co and at least one selected from the group consisting of Al, Zr, Si, Zn, and Ti. Note that the element M can be appropriately selected according to the application and required performance of a secondary battery configured using the positive electrode active material 10.

[0028] y, which represents the content ratio of the element M in the above molar ratio, satisfies 0<y≦0.5, preferably 0<y≦0.3, and may be 0<y≦0.2. For example, when the element M contains Co and Co is contained within the above range, the battery has high battery capacity and is more excellent in cycle characteristics. Further, when the element M contains Co, the range of Co (y1) may be, for example, 0<y1≦0.3, or may be 0<y1≦0.2. Further, when the element M contains Al, the range of Al (y2) may be, for example, 0<y2≦0.1, or may be 0<y2≦0.07.

[0029] z, which represents the content ratio of Nb in the above molar ratio, satisfies 0<z<0.05, and preferably 0<z≦0.02. When z is within the above range, an all-solid-state battery can have high battery capacity. When z exceeds 0.05, LiNb3O8 with low activity is generated, which may cause a decrease in battery capacity. Further, for example, when z satisfies 0.001≦z≦0.01, higher battery capacity can be obtained.

[0030] (Crystal Structure) Particles 1 of the lithium-nickel composite oxide have a crystal structure belonging to the space group R-3m. When the particles 1 of the lithium-nickel composite oxide have a crystal structure belonging to the space group R-3m, an increase in internal resistance can be suppressed in a secondary battery.

[0031] The crystal structure of the particles 1 of the lithium-nickel composite oxide can be confirmed by powder X-ray diffraction (XRD) measurement. That is, it is preferable that a peak attributed to the layered rock salt type crystal structure of the "R-3m" structure (crystal structure belonging to the space group R-3m) is detected from the diffraction pattern obtained when performing powder X-ray diffraction (XRD) measurement on the particles 1 of the lithium-nickel composite oxide. In particular, it is more preferable that only peaks attributed to the layered rock salt type crystal structure of the "R-3m" structure are detected from the diffraction pattern.

[0032] The lithium nickel composite oxide particle 1 may be a single phase of lithium nickel composite oxide having a "R-3m" crystal structure, but it may not be a single phase. If it is not a single layer and other compounds (e.g., impurities) are mixed in, it is preferable that the intensity of the heterogeneous phase peaks other than the layered rock salt type structure of the "R-3m" structure does not exceed the intensity of the peaks attributed to the layered rock salt type structure of the "R-3m" structure.

[0033] (Li seat occupancy rate) The lithium (Li) site occupancy rate of the positive electrode active material is 90% to 98%, but may also be 92% to 98% or 93% to 98%. Li site occupancy rate refers to the proportion of lithium (Li) in the lithium metal composite oxide within the layered lithium layers (Li sites). When the lithium site occupancy rate of the positive electrode active material is within the above range, it is considered that cation mixing, in which metals other than lithium migrate to lithium ion sites, is occurring in the positive electrode active material. When charged and discharged at high voltage, positive electrode active materials in which cation mixing occurs at a specific rate are thought to have high charge / discharge capacity while suppressing volume expansion and contraction, thereby improving cycle capacity retention rate (durability).

[0034] The method for evaluating the lithium seat occupancy rate is not particularly limited, but it can be determined, for example, by performing Rietveld analysis on the diffraction pattern obtained by X-ray diffraction (XRD). For example, X'PertPRO (manufactured by Spectris Corporation) can be used as an X-ray diffractometer (XRD). In addition, the lithium seat occupancy rate can be determined by performing Rietveld analysis on the diffraction pattern measured from the positive electrode active material using analysis programs such as RIETAN-FP, which is commonly used, as well as High_Score_Plus (manufactured by Spectris Corporation) and JADE_PRO (manufactured by Materials_Data).

[0035] (crystallite size) The lithium nickel composite oxide particle 1 preferably has a crystallite size of 140 nm or less, and more preferably between 70 nm and 140 nm. The range including the upper limit of the crystallite size may also be 130 nm or less. The crystallite size can be calculated using the Scherrer method with the peak attributed to (003) in the above XRD diffraction pattern. If the crystallite size of the lithium nickel composite oxide particle 1 exceeds 140 nm, the solid-state diffusion distance within the crystal increases, which can reduce battery capacity. Furthermore, if the crystallite size of the lithium nickel composite oxide particle 1 is less than 70 nm, the crystal structure becomes unstable, and battery capacity tends to decrease.

[0036] (Amount of lithium ions dissolved) The lithium nickel composite oxide particles 1 have a eluted lithium ion content of 0.20% to 1.00% by mass relative to the total amount of particles 1, preferably 0.25% to 1.00% by mass, and may also be 0.30% to 0.70% by mass. The eluted lithium ion content can be determined by a neutralization titration method using hydrochloric acid, which measures the amount of lithium ions that dissolve into water when the lithium nickel composite oxide particles 1 are dispersed in water. The Warder method or the Winkler method can be used as the neutralization titration method.

[0037] If the amount of lithium ions eluted from lithium nickel composite oxide particles 1 is less than 0.20% by mass, the battery capacity may decrease.

[0038] Furthermore, if the lithium nickel composite oxide particle 1 contains niobium, the amount of eluted lithium ions increases compared to lithium nickel composite oxide that does not contain niobium. Therefore, for example, by using the manufacturing method described later to set the niobium content within the above range and adjusting the amount of eluted lithium to 0.2% by mass or more, a positive electrode active material with high discharge capacity can be obtained. However, if the amount of eluted lithium ions from the lithium nickel composite oxide particle 1 exceeds 1.00% by mass, the discharge capacity will decrease.

[0039] (Cryslite size and amount of dissolved lithium ions) Furthermore, it is preferable that the lithium nickel composite oxide particles 1 have a crystallite diameter of 140 nm or less and an eluted lithium ion content of 0.20% by mass or more.

[0040] In other words, even if the lithium nickel composite oxide particle 1 has a crystallite size of 140 nm or less, if the amount of eluted lithium ions is less than 0.20 mass%, the battery capacity may decrease. The detailed reason for this is unknown, but it can be speculated as follows:

[0041] Particle 1 of the lithium nickel composite oxide contains secondary particles composed of aggregated primary particles. The crystallite size of particle 1 of the lithium nickel composite oxide is positively correlated with the size of the primary particles that make up the secondary particles, and it is thought that the smaller the crystallite size, the more particle interfaces there are between primary particles. Furthermore, eluted lithium ions are mainly present at the particle interfaces between primary particles. Therefore, if the crystallite size is small and there are many primary particle interfaces, if the amount of eluted lithium ions present at the primary particle interfaces (surfaces) decreases too much, voids will form at the primary particle interfaces. When there are many voids at the primary particle interfaces, the positive electrode active material becomes more prone to cracking during the electrode fabrication process of the secondary battery, and the contact interface between the lithium nickel composite oxide particles and the electrolyte increases. It is thought that the side reactions occurring at this increased contact interface will cause the generated phase to interfere with the transfer of charge between the electrolyte and the positive electrode active material, thus increasing the battery's resistance and decreasing its capacity.

[0042] On the other hand, if the crystallite size of particle 1 of the lithium nickel composite oxide exceeds 140 nm, the battery capacity will decrease even if the amount of dissolved lithium ions is 0.20 mass% or more, which is undesirable. This is thought to be because the primary particles become coarser, reducing the grain boundaries between primary particles, causing the dissolved lithium ions to be scattered in clumps on the surface of the secondary particles, and the presence of these dissolved lithium ions itself becomes a resistive phase. Note that the crystallite size and the amount of dissolved lithium can be adjusted within the above range by, for example, using the method for manufacturing the positive electrode active material described later.

[0043] (particle structure) The lithium nickel composite oxide particles 1 include secondary particles formed by the aggregation of multiple primary particles. Furthermore, the lithium nickel composite oxide particles 1 may contain a single primary particle, or a mixture of a single primary particle and a secondary particle.

[0044] When observed with a scanning electron microscope (SEM) or transmission electron microscope (TEM), it is preferable that the average particle size of the secondary particles is between 3.0 μm and 8.0 μm. Furthermore, it is preferable that these secondary particles are formed by the aggregation of many primary particles with a particle size between 0.1 μm and 2.0 μm. If a single primary particle is included, it is preferable that the primary particle has a particle size between 1.0 μm and 8.0 μm. The average particle size of each particle can be obtained, for example, by calculating the average of the area-circle equivalent diameter of 20 or more particles.

[0045] (Average particle size D50) The lithium nickel composite oxide particles 1 preferably have a particle size (D50, hereinafter also referred to as "average particle size D50") corresponding to 50% of the cumulative volume fraction in the cumulative volume distribution curve of the particle size distribution, which is 8 μm or less, more preferably 2 μm to 7 μm, and even more preferably 3 μm to 7 μm. The average particle size (D50) refers to the particle size (D50) corresponding to 50% of the cumulative volume fraction in the cumulative volume distribution curve of the particle size distribution measured with a laser light diffraction scattering particle size analyzer.

[0046] When the average particle size D50 of the lithium nickel composite oxide particles 1 is 8 μm or less, a secondary battery using the positive electrode active material 10 as the positive electrode can improve the capacity retention rate after high-voltage cycle measurement, and can also obtain excellent battery characteristics such as thermal stability and high output. However, when the average particle size D50 is 2 μm or less, aggregation is more likely to occur when applying the coating layer 2.

[0047] (Broadness of particle size distribution) The index [(d90-d10) / volume-average particle size Mv], which indicates the spread of the particle size distribution of the lithium nickel composite oxide particles 1, is not particularly limited, but from the viewpoint of homogenizing the particle size, it may be 0.7 or less, 0.6 or less, or 0.55 or less. When the particle size is relatively uniform, it becomes easy to uniformly coat the surface of the lithium nickel composite oxide particles 1 with the coating layer 2, and good output characteristics can be obtained in the secondary battery. The lower limit of [(d90-d10) / volume-average particle size Mv] is not particularly limited, but for example, it is 0.3 or more. Also, from the viewpoint of packing, [(d90-d10) / volume-average particle size Mv] may be 0.7 or more, and the coating layer 2 can be coated relatively uniformly by using the method for manufacturing the positive electrode active material described later.

[0048] Note that d10 refers to the particle size at which the cumulative volume, calculated by accumulating the number of particles at each particle size from the smallest particle size, equals 10% of the total volume of all particles. Similarly, d90 refers to the particle size at which the cumulative volume, calculated by accumulating the number of particles in the same way, equals 90% of the total volume of all particles. Furthermore, d10, d90, and the volume-average particle size Mv can be determined from the integrated volume values ​​measured with a laser diffraction scattering particle size analyzer, similar to the average particle size D50.

[0049] (specific surface area) The specific surface area of ​​the lithium nickel composite oxide particle 1 is not particularly limited, for example, 0.3 m². 2 / g or more 2.0m 2 It may be less than / g, and 0.3m 2 / g or more 1.0m 2 It may be less than / g. When the specific surface area is within the above range, the output characteristics are good. The specific surface area can be measured by the nitrogen adsorption BET method.

[0050] (2) Covering layer The positive electrode active material 10 has a coating layer 2 on the surface of lithium nickel composite oxide particles 1. By having a coating layer 2 on the surface of the particles 1, direct contact between the lithium nickel composite oxide particles 1 and the electrolyte can be prevented in a secondary battery equipped with a positive electrode containing the positive electrode active material 10, thereby suppressing surface degradation of the lithium nickel composite oxide particles 1.

[0051] The coating layer 2 is a composite oxide containing lithium (Li) and one or more elements selected from the group consisting of Al, Si, Ti, V, Ga, Ge, Zr, Nb, Mo, Ta, and W. The constituent elements of the coating layer 2, excluding lithium (Li) and oxygen (O), may be one type or two or more types. For example, the coating layer 2 may be a composite oxide composed of Li and Ti, or a composite oxide composed of Li and Nb.

[0052] (Content of constituent elements in the coating layer) The amount of coating layer 2 is not particularly limited, but the specific surface area (m²) of the lithium nickel composite oxide particles 1 to be coated is limited. 2 The amount of coating can be adjusted according to the amount ( / g). For example, the coating layer 2 has a surface area of ​​1 m² of lithium nickel composite oxide particles 1. 2 Preferably, the coating layer 2 contains the constituent elements (excluding Li and O) in a proportion of 30 μmol to 600 μmol, and more preferably 50 μmol to 400 μmol.

[0053] Surface area of ​​lithium nickel composite oxide particle 1: 1 m² 2 If the content of constituent elements (excluding Li and O) in the coating layer 2 per particle is 30 μmol or more, the coating layer 2 can be uniformly distributed across the entire surface of the lithium nickel composite oxide particle 1.

[0054] Furthermore, the addition of a coating layer may increase the internal resistance of the secondary battery. (Surface area of ​​lithium nickel composite oxide particle 1 m²) 2If the content of constituent elements (excluding Li and O) in the coating layer 2 per unit is 600 μmol or less, the coating layer 2 can suppress interference with the intercalation / deintercalation reaction of lithium to the lithium nickel composite oxide particles 1, thereby reducing internal resistance.

[0055] In coating layer 2, the method for evaluating and calculating the content of constituent elements (excluding Li and O) in coating layer 2 is not particularly limited, but for example, it can be determined as follows.

[0056] First, the content of constituent elements (excluding Li and O) in the coating layer 2 per gram of positive electrode active material is measured by methods such as chemical analysis. Chemical analysis methods include ICP (Inductively Coupled Plasma) emission spectroscopy.

[0057] On the other hand, the specific surface area of ​​the lithium nickel composite oxide particles 1 before coating with coating layer 2 is measured by nitrogen adsorption BET method or the like.

[0058] Next, the content of constituent elements (excluding Li and O) of the coating layer 2 in 1 g of positive electrode active material is given by the specific surface area (m²) of the lithium nickel composite oxide particle 1. 2 By dividing by ( / g), the surface area of ​​1 particle of lithium nickel composite oxide is 1 m². 2 The content of constituent elements (excluding Li and O) in each layer 2 can be determined.

[0059] Furthermore, if the lithium nickel composite oxide particles 1 contain constituent elements (excluding Li and O) of the coating layer 2, the difference in the content of constituent elements (excluding Li and O) of the coating layer 2 before and after coating can be used as the content of constituent elements (excluding Li and O) of the coating layer 2 used for coating.

[0060] (Average thickness of the coating layer) The average thickness of the coating layer is preferably, for example, 2 nm to 20 nm, more preferably 2 nm to 15 nm, and even more preferably 5 nm to 15 nm.

[0061] The average thickness of the coating layer 2 can be calculated by observing it with a scanning electron microscope (SEM) or transmission electron microscope (TEM), or by analyzing it with a spectrometer such as an energy-dispersive X-ray spectrometer (EDS) or electron energy loss spectroscopy (EELS) attached to these microscopes, and measuring the layer uniformly formed on the surface of the lithium nickel composite oxide particles 1. If there is variation in the thickness of the coating layer 2 depending on the measurement site, the thickness of the coating layer 2 refers to the average value obtained when measuring multiple sites.

[0062] (Arrangement of coating layers) Furthermore, it is preferable that the coating layer 2 exists adjacent to the surface of the lithium nickel composite oxide particles 1. Whether or not the coating layer 2 exists adjacent to the surface of the particles 1 can be determined by whether or not the compounds containing the constituent elements of the coating layer 2 are free from the surface of the lithium nickel composite oxide particles 1. If the coating layer 2 is free from the surface of the lithium nickel composite oxide particles 1, it does not contribute to the battery capacity electrochemically, and therefore becomes a factor that reduces the battery capacity per unit weight.

[0063] Furthermore, there does not need to be a clear boundary line between the coating layer 2 and the surface of the lithium nickel composite oxide particles 1. For example, if the lithium nickel composite oxide particles 1 before coating do not contain the constituent elements of the coating layer 2 (excluding Li and O), the coating layer 2 refers to the region where the constituent elements of the coating layer 2 (excluding Li and O) are detected, and may include a region where both the constituent elements of the coating layer 2 (excluding Li and O) and the elements constituting the lithium nickel composite oxide particles 1 are detected. Also, if the lithium nickel composite oxide particles 1 before coating contain the constituent elements of the coating layer 2 (excluding Li and O), the coating layer 2 refers to the region on the surface side of the particles constituting the positive electrode active material 10 where the concentration of the constituent elements of the coating layer 2 (excluding Li and O) is higher than that of the central part of the lithium nickel composite oxide particles 1.

[0064] Furthermore, the constituent elements of the coating layer 2 (excluding Li and O) may be partially dissolved in solid solution from the surface to the interior of the lithium nickel composite oxide particles. For example, a heat treatment step (S40) can be performed after the coating step (S30), and depending on the conditions at that time, the constituent elements of the coating layer can be diffused into the lithium nickel composite oxide.

[0065] For example, if the coating layer 2 contains Ti and / or Nb, the Ti and / or Nb may be solid-dissolved from the surface into the interior of the lithium nickel composite oxide particles 1. In addition, it is preferable to adjust the degree of solid solution in the positive electrode active material 10 so that the effect of improving the cycle characteristics is fully realized.

[0066] 2. Method for producing positive electrode active material for lithium-ion secondary batteries Next, a method for manufacturing the positive electrode active material for lithium-ion secondary batteries (hereinafter also referred to as "positive electrode active material") according to this embodiment will be described. By using the manufacturing method of this embodiment, the above-mentioned positive electrode active material 10 can be manufactured with high productivity.

[0067] Figures 2 and 3 show an example of a method for producing a positive electrode active material according to this embodiment. As shown in Figure 2, the method for producing a positive electrode active material according to this embodiment comprises a mixing step (S10) of mixing a nickel composite compound, a niobium compound, and a lithium compound to obtain a mixture; a firing step (S20) of firing the mixture to obtain lithium nickel composite oxide particles; and a coating step (S30) of applying a coating liquid to the surface of the lithium nickel composite oxide particles to form a coating layer. Furthermore, after the coating step (S30), a heat treatment step (S40) may be included in which the lithium nickel composite oxide particles with the coating layer formed on their surface are heat-treated at 300°C or higher.

[0068] Furthermore, the nickel composite compound may also be a nickel composite oxide obtained by oxidative roasting of a nickel composite hydroxide prepared by a crystallization reaction. For example, the nickel composite compound can be produced by a method comprising a crystallization step (S1) and an oxidative roasting step (S2), as shown in Figure 3. The steps will be described in detail below. Note that the following description is just one example of a production method and does not limit the production method.

[0069] (Crystallization process: S1) In the crystallization step (S1), nickel composite hydroxide, which is a precursor of lithium nickel composite oxide, is prepared by a crystallization reaction.

[0070] For example, using water-soluble compounds of each element (metal compounds), a raw material aqueous solution is prepared such that the molar ratio of each element is equal to the molar ratio of each element contained in the particles of the target lithium nickel composite oxide. The prepared raw material aqueous solution, along with an alkali metal aqueous solution and an ammonium ion supply, is supplied together to a reaction vessel and subjected to a neutralization crystallization reaction to obtain nickel composite hydroxide.

[0071] The raw materials for each element may, for example, be dissolved simultaneously in water to produce a mixed aqueous solution of the raw materials. Alternatively, separate aqueous solutions may be prepared for each elemental raw material to produce individual raw material aqueous solutions. If there are disadvantages to preparing the raw material aqueous solution as a mixed aqueous solution, it is preferable to prepare individual raw material aqueous solutions for each raw material. For example, if the aqueous solutions of each raw material are acidic and basic, it is preferable to prepare individual raw material aqueous solutions for each raw material.

[0072] The metal compounds used as raw materials for each element only need to be water-soluble; sulfates, chlorides, nitrates, etc., can be used, but from a cost perspective, inexpensive sulfates are preferred. If a suitable water-soluble metal compound cannot be found for element M, etc., it may be added in the oxidation roasting process (S2) or mixing process (S10) described later, rather than being added to the mixed aqueous solution of the raw materials.

[0073] The alkali metal aqueous solution is not particularly limited, but preferably one or more selected from the group consisting of sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, and potassium carbonate can be used.

[0074] The ammonium ion supplier is not particularly limited, but preferably one or more selected from aqueous ammonia, aqueous ammonium carbonate, aqueous ammonium chloride, and aqueous ammonium sulfate can be used.

[0075] The shape of the reaction vessel is not particularly limited, but a cylindrical container with baffles inside, equipped with a stirrer and a temperature controller is preferred. The stirrer is preferably equipped with a motor, shaft, and stirring blades. The temperature controller is preferably of a type that circulates a heat transfer medium around the outside of the cylindrical container to heat or cool the cylindrical container.

[0076] In the neutralization crystallization reaction between the raw material aqueous solution, alkali metal aqueous solution, and ammonium ion supply in the reaction vessel, it is preferable to maintain the pH and ammonia concentration at constant values.

[0077] It is preferable to adjust the pH of the aqueous solution in the reaction vessel to be between 11.0 and 12.2 based on a liquid temperature of 25°C. For example, when preparing nickel composite hydroxides, impurities caused by anions constituting the metal compounds contained in the raw material aqueous solution may be mixed into the nickel composite hydroxide. However, by setting the pH of the initial aqueous solution (inside the reaction vessel) to 11.0 or higher, the mixing of impurities caused by anions can be suppressed. Furthermore, by setting the pH of the initial aqueous solution to 12.2 or lower, the micronization of the resulting nickel composite hydroxide can be suppressed, and a composite hydroxide with a particle size suitable for the charge-discharge reaction can be obtained.

[0078] The ammonia concentration in the aqueous solution in the reaction vessel is preferably adjusted to between 5 g / L and 20 g / L. When the ammonia concentration is 5 g / L or higher, the Ni in the raw material aqueous solution (mixed aqueous solution) forms an ammonium complex, and the precipitation rate from the liquid phase to the solid phase as hydroxide decreases, thus increasing the sphericity of the resulting nickel composite hydroxide particles. On the other hand, when the ammonia concentration is 20 g / L or lower, the excessive increase in the solubility of nickel forming the ammonium complex is suppressed, and the molar ratio of the resulting nickel composite hydroxide can be more reliably achieved at the target molar ratio. Furthermore, excessive consumption of ammonia can be suppressed, which is industrially preferable.

[0079] The atmosphere inside the reaction vessel is preferably a non-oxidizing atmosphere, for example, an atmosphere with an oxygen concentration of 1% by volume or less. A non-oxidizing atmosphere inside the reaction vessel suppresses the oxidation of the raw material compounds. For example, it prevents the precipitation of oxidized cobalt or manganese as fine particles.

[0080] The temperature inside the reaction vessel during the crystallization step (S1) is preferably maintained at 40°C or higher and 60°C or lower, more preferably at 45°C or higher, and even more preferably at 55°C or lower.

[0081] Since the temperature of the reaction vessel rises due to the heat of reaction and the Joule heating from stirring, maintaining the temperature inside the reaction vessel above 40°C avoids the consumption of extra energy for cooling. Furthermore, by keeping the temperature inside the reaction vessel below 60°C, the evaporation of ammonia from the initial aqueous solution and the reaction aqueous solution can be suppressed, making it easier to maintain the target ammonia concentration.

[0082] Lithium nickel composite oxide particles (powder) are preferably characterized by a narrow particle size distribution and uniform particle size. To produce such particles, it is necessary to obtain particles with uniform particle size in the nickel composite hydroxide precursor. Specific examples of methods for obtaining such particles can be found in Patent Document 3.

[0083] (Oxidation roasting process: S2) An oxidation roasting step (S2) may be performed after the precursor crystallization step (S1). In the oxidation roasting step (S2), the nickel composite hydroxide obtained in the precursor crystallization step (S1) is oxidized and roasted to obtain a nickel composite oxide. In the oxidation roasting step (S2), the nickel composite oxide can be obtained by heat treatment in an oxygen-containing atmosphere and then cooling to room temperature.

[0084] The roasting conditions in the oxidation roasting process (S2) are not particularly limited, but it is preferable to roast in an oxygen-containing atmosphere or an air atmosphere at a temperature of 500°C to 700°C for 1 to 12 hours. When the roasting temperature is 500°C or higher, the nickel composite hydroxide can be completely converted to nickel composite oxide. Furthermore, it is preferable to roast the temperature to 700°C or lower because it is possible to suppress the specific surface area of ​​the nickel composite oxide from becoming excessively small.

[0085] By extending the roasting time to one hour or more, the temperature inside the roasting container can be made uniform, allowing the reaction to proceed uniformly, which is preferable. Furthermore, since no significant change is observed in the resulting nickel composite oxide even when roasting is performed for longer than 12 hours, from the viewpoint of energy efficiency, it is preferable to limit the roasting time to 12 hours or less.

[0086] The oxygen concentration in the oxygen-containing atmosphere during roasting is preferably equal to or greater than the oxygen concentration in air, i.e., an oxygen concentration of 20% by volume or more. Since an oxygen-only atmosphere can also be used, the upper limit of the oxygen concentration in the oxygen-containing atmosphere can be 100% by volume.

[0087] Furthermore, if, for example, the compound containing element M cannot be coprecipitated in the crystallization step (S1), the compound containing element M may be added to the nickel composite hydroxide subjected to the oxidative roasting step S2 in the same molar ratio as the target compound and then calcined. The compound containing element M to be added is not particularly limited, and for example, oxides, hydroxides, carbonates, or mixtures thereof can be used.

[0088] Furthermore, if slight sintering is observed in the nickel composite oxide obtained after the oxidation roasting process (S2), a crushing treatment may be added. Note that in the oxidation roasting process (S2), it is sufficient for at least a portion of the nickel composite hydroxide to be converted into nickel composite oxide; it is not necessary to convert all of the nickel composite hydroxide into oxide.

[0089] (Mixing process: S10) The mixing step (S10) is a step in which a nickel composite compound, a niobium compound, and a lithium compound are mixed to obtain a lithium mixture.

[0090] The nickel composite compound is preferably at least one of a nickel composite hydroxide and a nickel composite oxide, and more preferably a nickel composite oxide. Furthermore, the nickel composite compound is preferably obtained by a method comprising the crystallization step (S1) and / or the oxidative roasting step (S2) described above.

[0091] Niobium compounds can include, for example, niobium acid, niobium oxide, niobium nitrate, and niobium pentachloride. Among these, niobium hydroxide or niobium oxide is preferred from the viewpoint of ease of availability and avoiding the contamination of the calcined lithium nickel composite oxide with impurities. The niobium compound is preferably added as a solid-phase additive. Compared to conventional methods of co-precipitation or coating of niobium in a known crystallization process, solid-phase addition of niobium is an environmentally friendly and highly productive method because it does not require chemicals.

[0092] When niobium is added as a solid phase, the reactivity may change depending on the particle size of the added niobium compound. In the integrated volume distribution curve of the particle size distribution of the niobium compound, the particle size (D90) corresponding to 90% of the integrated volume fraction is preferably 0.1 μm to 20 μm, more preferably 0.1 μm to 10 μm, and even more preferably 0.1 μm to 5 μm. If the D90 of the niobium compound is smaller than 0.1 μm, there is a problem that the powder becomes very difficult to handle. If the D90 of the niobium compound is larger than 20 μm, the reactivity during calcination decreases, and the diffusion of niobium into the lithium nickel composite oxide particles may be insufficient, making it impossible to ensure thermal stability. In addition, if the D90 of the niobium is too large, the formation of the coating layer 2 may become uneven. The particle size of the niobium compound can be appropriately adjusted within the above particle size range so that a positive electrode active material with the desired properties can be obtained.

[0093] The D90 of niobium compounds can be adjusted to the above range by grinding the raw material niobium compound using a pulverizer such as a ball mill, planetary ball mill, jet mill, bead mill, or pin mill. If necessary, classification may also be performed using a dry classifier or sieving machine. The D90 of niobium compounds can also be measured by laser scattering diffraction.

[0094] The niobium compound is mixed in an amount that provides the desired niobium content relative to the total number of Ni, Mn, and element M atoms contained in the nickel composite compound. Since the niobium content does not change before and after the calcination process, the amount of niobium compound added is equivalent to the amount of niobium added to the positive electrode active material.

[0095] The lithium compound is not particularly limited, and for example, lithium hydroxide, lithium nitrate, or lithium carbonate, or mixtures thereof can be used. From the viewpoint of having a low melting point and high reactivity, lithium hydroxide is preferred as the lithium compound.

[0096] The lithium compound may be mixed in an amount such that, for example, the lithium content is 95 atomic% to 110 atomic% or 97 atomic% to 108 atomic% relative to the sum of Ni, Mn, element M, and Nb (Me).

[0097] (Firing process: S20) The firing process (S20) is a process of firing the obtained lithium mixture to obtain lithium nickel composite oxide particles 1. The firing conditions are not particularly limited, but for example, it is preferable to fire in an oxygen-containing atmosphere at a temperature of 700°C to 1000°C for 1 hour to 24 hours. Alternatively, after firing, the mixture may be cooled to room temperature to obtain lithium nickel composite oxide particles 1. The firing conditions, such as firing temperature, firing time, and firing atmosphere, can be appropriately adjusted within the above range to obtain desired properties (Li site occupancy, crystallite size, amount of dissolved lithium ions, etc.).

[0098] When the firing temperature is set to 700°C or higher, the crystal structure of lithium nickel composite oxide particle 1 can be sufficiently grown. Furthermore, when the firing temperature is set to 1000°C or lower, the excessive incorporation of Ni atoms into the Li sites in the resulting lithium nickel composite oxide particle 1 can be suppressed.

[0099] A firing time of 1 hour or more is preferable because it allows for a uniform temperature within the firing vessel, enabling the reaction to proceed uniformly. Furthermore, since no significant change is observed even if firing is performed for longer than 24 hours, from the viewpoint of energy efficiency, the firing time is preferably 24 hours or less, but may also be 12 hours or less, 10 hours or less, or 6 hours or less.

[0100] Furthermore, the oxygen-containing atmosphere is preferably one that contains 80% by volume or more of oxygen. This is preferable because a concentration of 80% by volume or more of oxygen in the atmosphere allows for sufficient growth of the crystalline structure of the lithium nickel composite oxide particles 1. Since an oxygen-only atmosphere is also possible, the upper limit of the oxygen concentration in the oxygen-containing atmosphere can be set to 100% by volume.

[0101] Furthermore, if slight sintering is observed in the lithium nickel composite oxide particles 1 obtained after the firing process (S20), a crushing treatment may be added.

[0102] (Coating process: S30) The coating step (S30) is a step in which a coating solution is applied to the surface of the obtained lithium nickel composite oxide particles 1 to form a coating layer 2.

[0103] The coating layer 2 is formed, for example, by mixing lithium nickel composite oxide particles 1 with a coating solution and drying it to form the coating layer 2 on the surface of the lithium nickel composite oxide particles 1. Alternatively, as will be described later, a heat treatment step (S40) may be optionally performed in an oxygen-containing atmosphere after coating. An example of the coating step (S30) will be described below.

[0104] First, a predetermined amount of coating solution is prepared (coating agent preparation step). The coating agent is the specific surface area (m²) of the lithium nickel composite oxide particles 1 obtained in the calcination process (S20). 2 It can be prepared according to the content of the constituent elements (excluding Li and O) of the coating layer 2 per g.

[0105] The coating solution contains at least one element selected from the group consisting of Al, Si, Ti, V, Ga, Ge, Zr, Nb, Mo, Ta, and W. For example, the coating solution can be prepared by dissolving a raw material compound containing the constituent elements (excluding Li and O) of the target coating layer 2 in a solvent.

[0106] Examples of starting compounds include one or more selected from the group consisting of alkoxides, chelates using complexes equipped with carbonyl groups, peroxy groups, etc.

[0107] From the viewpoint of uniform coating, the coating solution only needs to be in liquid state when it is applied to the surface of the lithium nickel composite oxide particles 1. For example, it may be prepared by dissolving a compound containing the constituent elements of the coating layer 2 in a solvent and being liquid at room temperature, or it may be a compound containing the constituent elements of the coating layer 2 with a low melting point that dissolves with low-temperature heat treatment.

[0108] The coating solution may or may not contain Li. If the coating solution does not contain Li, in the coating step (S30) and / or the heat treatment step (S40), the Li present in the lithium nickel composite oxide particles 1 reacts with the compound containing the above constituent elements in the coating solution to form the coating layer 2.

[0109] Next, the coating solution is applied to the surface of the lithium nickel composite oxide particles 1. The coating solution may be applied, for example, by mixing the lithium nickel composite oxide particles 1 with the coating solution (mixture preparation step). A general mixer can be used for mixing. Alternatively, drying may be performed after mixing (drying step).

[0110] Furthermore, from the viewpoint of forming a more uniform coating layer 2 with a specific thickness, it is preferable to carry out the mixture preparation step and the drying step in parallel, and it is preferable to use a rolling flow coating apparatus.

[0111] Because the coating solution shrinks upon drying, gaps may form in the resulting coating layer 2 if the mixture preparation step and drying step are only performed once each. However, when using a rolling flow coating apparatus, the coating solution is sprayed onto lithium nickel composite oxide particles 1 that are flowing due to a heated airflow within the apparatus. As a result, the mixture preparation step and drying step are repeated in parallel, and a uniform coating layer without gaps is obtained, which is preferable.

[0112] In the drying step, it is preferable to dry at a temperature that is sufficient to remove the solvent and other components of the coating agent. For example, when using a rolling flow coating apparatus, the supply air temperature may be set to 80°C or higher and less than 300°C. Alternatively, additional drying may be performed in a separate stationary dryer after the coating process.

[0113] The atmosphere during the drying step is not particularly limited, but an inert atmosphere such as air supplied from a compressor equipped with a dryer, nitrogen, and argon gas is preferred in order to prevent the lithium nickel composite oxide particles 1 from reacting with moisture in the atmosphere.

[0114] (Heat treatment process: S40) Furthermore, if necessary, the process may include a heat treatment step (S40) after the coating step (S30), in which the lithium nickel composite oxide particles 1 with the coating layer 2 formed on their surface are heat-treated at 300°C or higher. The heat treatment step (S40) can further strengthen the bond between the coating layer 2 and the lithium nickel composite oxide particles 1.

[0115] The heat treatment conditions for the heat treatment step (S40) are not particularly limited, but it is preferable to perform the heat treatment in an oxygen-containing atmosphere at a temperature of 300°C to 600°C for 1 hour to 5 hours. The oxygen-containing atmosphere may be, for example, an air atmosphere.

[0116] The oxygen concentration in the oxygen-containing atmosphere during the heat treatment process (S40) is preferably equal to or greater than the oxygen concentration in the air atmosphere, i.e., an oxygen concentration of 20% by volume or more. By making the oxygen-containing atmosphere during heat treatment equal to or greater than the oxygen concentration in the air atmosphere, the occurrence of oxygen defects inside the resulting positive electrode active material 10 can be further suppressed. The oxygen-containing atmosphere may be an oxygen atmosphere, and the upper limit of the oxygen concentration in the oxygen-containing atmosphere is 100% by volume.

[0117] When the heat treatment temperature is 300°C or higher, the retention of impurities contained in the coating solution inside the positive electrode active material 10 can be further suppressed. Furthermore, when the heat treatment temperature is 600°C or lower, excessive diffusion of the components of the coating layer 2 can be suppressed, and the shape of the coating layer 2 can be maintained.

[0118] When the heat treatment time is 1 hour or longer, the retention of impurities contained in the coating solution inside the positive electrode active material 10 can be further suppressed. Furthermore, even when the heat treatment time is longer than 5 hours, no significant changes are observed in the resulting positive electrode active material 10. Therefore, from the viewpoint of energy efficiency, a heat treatment time of 5 hours or less is preferable.

[0119] After the heat treatment process (S40), the material is cooled to room temperature to obtain a positive electrode active material having lithium nickel composite oxide particles 1 and a coating layer 2 on its surface as the final product.

[0120] The heat treatment step (S40) does not need to be performed. In other words, the positive electrode active material 10 may be manufactured by performing only up to the coating step (S30). This is because even without performing the heat treatment step (S40), a uniform and strong coating layer can be formed on the surface of the lithium nickel composite oxide particles. Even if the heat treatment step is not performed, it is preferable to perform drying as needed to reduce or remove the solvent and moisture in the coating agent.

[0121] If slight sintering is observed in the positive electrode active material 10 obtained after the coating process (S30) and / or the heat treatment process (S40), a further crushing process may be performed.

[0122] 3. Lithium-ion rechargeable batteries The lithium-ion secondary battery according to this embodiment (hereinafter also referred to as "secondary battery") comprises, for example, a positive electrode, a negative electrode, and an electrolyte, and the positive electrode contains the positive electrode active material described above. A lithium-ion secondary battery made by fabricating a positive electrode using the positive electrode active material and incorporating it exhibits excellent charge-discharge cycle characteristics. Furthermore, it is particularly preferable to use lithium nickel composite oxide particles obtained by the above manufacturing method as the positive electrode active material in the lithium-ion secondary battery.

[0123] The secondary battery comprises, for example, a positive electrode, a negative electrode, and a non-aqueous electrolyte. Furthermore, the secondary battery may be any secondary battery that performs charging and discharging by the desorption and insertion of lithium ions, such as a non-aqueous electrolyte secondary battery or an all-solid-state lithium secondary battery. Note that the embodiments described below are merely illustrative, and the secondary battery according to this embodiment can be applied to various modified and improved forms based on the embodiments described herein.

[0124] (1) Components a) Positive electrode Using the positive electrode active material described above, the positive electrode of a lithium-ion secondary battery can be fabricated, for example, as follows.

[0125] First, the positive electrode active material of the present invention is mixed with a conductive material and a binder, and activated carbon and solvents such as viscosity adjusters are added as needed, and these are kneaded to produce a positive electrode composite paste. At this time, the mixing ratio of each component in the positive electrode composite paste is an important factor in determining the performance of the lithium-ion secondary battery. For example, if the solid content of the positive electrode composite material excluding the solvent is 100 parts by mass, the content of the positive electrode active material can be 60 to 95 parts by mass, 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, similar to the positive electrode of a general lithium-ion secondary battery.

[0126] The resulting positive electrode composite paste is applied to the surface of a current collector, for example, made of aluminum foil, and dried to remove the solvent. If necessary, pressure may be applied using a roll press or the like to increase the electrode density. In this way, a sheet-like positive electrode can be produced. The sheet-like positive electrode can be cut to an appropriate size depending on the intended battery and used to manufacture the battery. Note that the method for producing the positive electrode is not limited to the example given above, and other methods may be used.

[0127] As conductive materials, for example, graphite (natural graphite, artificial graphite, expanded graphite, etc.) or carbon black-based materials such as acetylene black and Ketjenblack can be used. The binder serves to hold the active material particles together, and examples of binders that can be used include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, cellulose resin, or polyacrylic acid.

[0128] In addition, if necessary, a solvent that disperses the positive electrode active material, conductive material, and activated carbon, and dissolves the binder, can be added to the positive electrode mixture. Specifically, organic solvents such as N-methyl-2-pyrrolidone can be used as the solvent. Furthermore, activated carbon can also be added to the positive electrode mixture to increase the electrical double layer capacitance.

[0129] b) Negative electrode For the negative electrode, metallic lithium or lithium alloys can be used. Alternatively, a negative electrode composite material can be used, which is made by mixing a binder with a negative electrode active material capable of intercalating and deintercalating lithium ions, adding a suitable solvent to form a paste, applying this paste to the surface of a metal foil current collector such as copper, drying it, and compressing it as needed to increase the electrode density. As the negative electrode active material, for example, lithium-containing substances such as metallic lithium and lithium alloys, natural graphite capable of intercalating and deintercalating lithium ions, calcined organic compounds such as artificial graphite and phenolic resins, and powdered carbon materials such as coke can be used. In this case, as with the positive electrode, a fluororesin such as PVDF can be used as the negative electrode binder, and an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent for dispersing these active materials and binders.

[0130] c) Separator A separator is placed between the positive and negative electrodes and has the function of separating the positive and negative electrodes and holding the electrolyte. Such a separator can be a thin membrane made of polyethylene or polypropylene, for example, with numerous fine pores, but it is not particularly limited as long as it has the above-mentioned function.

[0131] d) Non-aqueous electrolyte Non-aqueous electrolytes are prepared by dissolving lithium salts, which act as supporting salts, in an organic solvent. As organic solvents, one or more selected from cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and trifluoropropylene carbonate; linear carbonates such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate; ether compounds such as tetrahydrofuran, 2-methyltetrahydrofuran, and dimethoxyethane; sulfur compounds such as ethyl methyl sulfone and butanesultone; and phosphorus compounds such as triethyl phosphate and trioctyl phosphate can be used alone or in combination of two or more. Supporting salts that can be used include LiPF6, LiBF4, LiClO4, LiAsF6, LiN(CF3SO2)2, and their composite salts. The non-aqueous electrolyte may also contain radical scavengers, surfactants, and flame retardants.

[0132] (2) Lithium-ion rechargeable batteries The lithium-ion secondary battery, composed of the positive electrode, negative electrode, separator, and non-aqueous electrolyte described above, can be made into various shapes, such as cylindrical or stacked. Regardless of the shape, the positive electrode and negative electrode are stacked with a separator in between to form an electrode body, the resulting electrode body is impregnated with a non-aqueous electrolyte, the positive electrode current collector and the positive electrode terminal that is open to the outside, and the negative electrode current collector and the negative electrode terminal that is open to the outside are connected using current collector leads, and the battery is sealed in a battery case to complete the lithium-ion secondary battery.

[0133] (3) Characteristics of lithium-ion secondary batteries The lithium-ion secondary battery according to this embodiment has a high cycle capacity retention rate (durability) and can be suitably used when charging and discharging at high voltages (for example, with an upper limit voltage of 4.5V or higher based on metallic lithium). For example, using the laminated battery shown in Figure 5 used in the example, after conditioning, the current density was set to 2.7mA / cm² in a constant temperature bath maintained at 60°C. 2 When the cycle of charging to a cutoff voltage of 4.5V, resting for 1 hour, and then discharging to a cutoff voltage of 3.0V is repeated 200 times, the cycle capacity retention rate (discharge capacity after 200 cycles / initial discharge capacity (%)) is preferably 80% or more, and more preferably 90% or more.

[0134] The conditioning is performed in a constant temperature bath maintained at 25°C with a current density of 0.33 mA / cm². 2 The process involves charging to a cutoff voltage of 4.5V, resting for one hour, and then discharging to a cutoff voltage of 3.0V. This cycle is repeated five times. Additionally, the discharge capacity obtained in the first cycle after conditioning can be defined as the initial discharge capacity, and the capacity retention rate can be calculated by dividing the initial discharge capacity by the discharge capacity obtained in the 200th cycle after conditioning.

[0135] Furthermore, the laminate-type battery used to evaluate the cycle capacity retention rate uses, for example, a sheet formed by coating copper foil with a negative electrode composite paste made of a mixture of graphite powder and polyvinylidene fluoride as the negative electrode.

[0136] Furthermore, because the above-mentioned positive electrode active material is used as the positive electrode material, the lithium-ion secondary battery according to this embodiment has excellent capacity characteristics and also excellent output characteristics.

[0137] For example, when the positive electrode active material according to this embodiment is used as the positive electrode to construct a 2032 type coin cell (for evaluation) as shown in Figure 4, the initial discharge capacity can be preferably 180 mAh / g or more, more preferably 185 mAh / g or more, and more preferably 195 mAh / g or more. Furthermore, the initial discharge capacity is determined by a current density of 0.1 mA / cm². 2 This represents the discharge capacity when the battery is charged to a cutoff voltage of 4.5V, left idle for 1 hour, and then discharged to a cutoff voltage of 3.0V.

[0138] (4)Applications As described above, the lithium-ion secondary battery of the present invention has excellent capacity characteristics, output characteristics, and cycle characteristics, and can be suitably used as a power source for small portable electronic devices (such as notebook personal computers and mobile phone terminals) where these characteristics are required at a high level. Furthermore, the lithium-ion secondary battery of the present invention also has excellent safety, and not only can it be miniaturized and have high output, but it can also simplify expensive protection circuits, making it suitable for use as a power source for transportation equipment where mounting space is limited. [Examples]

[0139] The present invention will be described in detail below using examples and comparative examples.

[0140] [Example 1] 1. Manufacturing of lithium nickel composite oxide Lithium nickel composite oxide was manufactured using the following process.

[0141] (a) Crystallization process In a reaction vessel with an internal volume of 60 L, 10 L of pure water was added and stirred while maintaining the vessel temperature at 50°C. During this time, the reaction vessel was under a nitrogen atmosphere with an oxygen concentration of 1% by volume or less. In this reaction vessel, appropriate amounts of 25% by mass sodium hydroxide aqueous solution and 25% by mass aqueous ammonia were added to prepare the initial aqueous solution so that the pH value at a liquid temperature of 25°C was 12.8 and the ammonia concentration of the solution in the reaction vessel was 15 g / L.

[0142] Simultaneously, nickel sulfate, cobalt sulfate, and manganese sulfate were dissolved in pure water in a molar ratio of nickel, cobalt, and manganese of Ni:Co:Mn = 0.78:0.11:0.11 to prepare 25 L of a 2.0 mol / L nickel-cobalt mixed aqueous solution. In addition, 5 L of a 0.37 mol / L aluminum sulfate aqueous solution was prepared.

[0143] A nickel-cobalt-manganese mixed aqueous solution was added dropwise at a rate of 109 mL / min to the initial aqueous solution in the reaction vessel at a rate of 66 mL to prepare the reaction solution. At the same time, 25% by mass aqueous ammonia and 25% by mass aqueous sodium hydroxide solutions were also added dropwise to the initial aqueous solution at a constant rate, and the pH value of the reaction solution was controlled to be maintained at 12.8 based on a liquid temperature of 25°C.

[0144] Next, sulfuric acid was added dropwise to the reaction vessel to adjust the pH of the reaction solution to 11.5. This procedure was intended to lower the pH, thereby reducing the rate at which the Ni, Co, Mn, and Al composite hydroxide precipitated from the liquid phase to the solid phase in the subsequent precursor crystallization step, and thereby improving the uniformity of the resulting particle size distribution and the sphericity of the particles.

[0145] After pH control, 26.2 L of nickel-cobalt-manganese mixed aqueous solution was added dropwise to the reaction aqueous solution in the reaction vessel at a rate of 109.2 mL / min, and simultaneously, 5.9 L of aluminum sulfate aqueous solution was added dropwise at a rate of 24.8 mL / min. At this time, 25% by mass aqueous ammonia and 25% by mass aqueous sodium hydroxide solutions were also added dropwise to the initial aqueous solution at a constant rate, so that the pH value of the reaction aqueous solution was maintained at 11.5 and the ammonia concentration at 15 g / L based on a liquid temperature of 25°C.

[0146] After adding the entire nickel-cobalt-manganese mixed aqueous solution and the aluminum sulfate aqueous solution dropwise, the pH of the reaction solution in the reaction vessel was raised to 13.0 based on a liquid temperature of 25°C. This operation is intended to precipitate nickel ions, which are complexed with ammonia and dissolved in the liquid phase, onto the hydroxide, thereby obtaining the desired chemical composition.

[0147] Subsequently, the reaction aqueous solution was subjected to solid-liquid separation using a Buchner funnel, a filter, and a vacuum pump / vacuum filter. Furthermore, the obtained solid phase was dispersed in 20 L of pure water at 40°C, and the solid-liquid separation process was repeated twice to remove water-soluble impurities such as sodium sulfate from the nickel complex hydroxide. After washing and solid-liquid separation, the cake-like solid phase was dried in a stationary dryer at 120°C for 24 hours under an air atmosphere, and then passed through a sieve with a mesh size of 100 μm to obtain a powdered nickel composite hydroxide.

[0148] (b) Oxidation roasting process Using an atmospheric firing furnace (Siliconit Co., Ltd., BM-50100M), the prepared composite hydroxide was fired at 600°C for 2 hours in an air atmosphere with an oxygen concentration of 20% by volume, and then cooled to room temperature to obtain a nickel composite oxide.

[0149] (c) Mixing process Niobium acid (Nb2O3·xH2O) powder manufactured by Mitsuwa Chemical Co., Ltd. was added to the nickel composite oxide so that the amount of Nb was 0.1% relative to the total amount of Ni, Co, Mn, and Al contained in the nickel composite oxide. Lithium hydroxide monohydrate, weighed so that the amount of Li was 103% relative to the total amount of Ni, Co, Mn, Al, and Nb, was then added and mixed using a tarbler shaker mixer (Dalton Co., Ltd., T2F) to obtain a lithium mixture.

[0150] (d) Firing process Using an atmospheric firing furnace (Siliconit Co., Ltd., BM-50100M), the obtained lithium mixture was fired at 875°C for 5 hours in an oxygen-containing atmosphere with an oxygen concentration of 90% by volume or higher, and then cooled to room temperature. This yielded lithium nickel composite oxide particles.

[0151] 2. Evaluation of lithium nickel composite oxide particles The following evaluations were performed on the obtained lithium nickel composite oxide.

[0152] (a) composition Analysis using an ICP emission spectrometer (VARIAN 725ES) confirmed that the molar ratio of Li, Ni, Co, Mn, Al, and Nb in the lithium nickel composite oxide is Li:Ni:Co:Mn:Al:Nb = 1.03:0.750:0.108:0.108:0.03:0.004.

[0153] (b) Crystal structure The crystal structure of lithium nickel composite oxide particles was measured using XRD (PANALYTICAL, X'Pert, PROMRD), and it was confirmed that it has a layered rock salt type crystal structure, with peaks attributed to the R-3m structure detected in the diffraction pattern. Furthermore, by measuring the full width at half maximum of the peak attributed to the (003) plane in the diffraction pattern and calculating the crystallite size using the Scherrer method, it was confirmed to be 113 nm.

[0154] (c) Measurement of eluted lithium ion amount The amount of lithium ions eluted from lithium nickel composite oxide was determined by titration. 2.0 g of lithium nickel composite oxide was dispersed in 125 ml of pure water, and 2 ml of 10% barium chloride solution was added. Neutralization titration was performed with 1 mol / L hydrochloric acid while stirring, and the amount of 1 mol / L hydrochloric acid required to reach the inflection point around pH 4 of the resulting titration curve was calculated as the amount of Li due to eluted lithium ions. As a result, the amount of lithium ions eluted from lithium nickel composite oxide was 0.47 wt%.

[0155] (d) Specific surface area The BET specific surface area of ​​lithium nickel composite oxide was measured using a fully automated BET specific surface area measuring device (MacSorb, manufactured by Mountec Co., Ltd.), and the result was 0.56 m². 2 I confirmed that it was / g.

[0156] (e) Particle size distribution The particle size distribution of lithium nickel composite oxide was measured using a laser diffraction scattering particle size distribution analyzer (Microtrac HRA, manufactured by Nikkiso Co., Ltd.). From the results, it was confirmed that the average particle size D50 based on volume was 4.7 μm, and the variation index ((D90-D10) / MV) calculated from D10, D90, and MV was 0.43.

[0157] 3. Coating of lithium nickel composite oxide The following coating process was performed on the obtained lithium nickel composite oxide. To a solution prepared by adding 30 ml of isopropyl alcohol (IPA) and 1.8 g of titanium tetrabutoxide (Ti-BuOH) and stirring, a solution containing 20 ml of IPA and 0.9 g of acetylacetone was added dropwise while heating and stirring at 60°C. This was done to avoid directly adding a high concentration of acetylacetone to the Ti solution. Subsequently, 10 ml of IPA mixed with 0.54 g of pure water was added to the cooled solution. Finally, 65 ml of IPA was added to the resulting solution to prepare the coating solution.

[0158] Using the above coating solution, 500g of lithium nickel composite oxide was coated using a rolling flow coating apparatus (MP-01, Powrec).

[0159] 500g of lithium nickel composite oxide was subjected to a flow rate of 0.3m in a heated environment at 120°C. 3 The lithium nickel composite oxide was circulated in a chamber with air at 1 / h, and the coating solution was sprayed onto it at a rate of 1.7 ml / min.

[0160] After spraying the entire coating solution, the lithium nickel composite oxide was recovered from the chamber and heat-treated at 400°C for 5 hours under oxygen flow using an atmospheric firing furnace (Siliconit Co., Ltd., BM-50100M). It was then cooled to room temperature to obtain lithium nickel composite oxide particles (cathode active material) having a coating layer (containing Li and Ti).

[0161] 4. Evaluation of lithium nickel composite oxide particles with a coating layer (a) composition Analysis using an ICP emission spectrometer (VARIAN 725ES) revealed that the coated lithium nickel composite oxide contains 0.88 wt% Ti, and the Ti content per unit area of ​​the base material is 370 μmol / m². 2 This was confirmed.

[0162] (b) Thickness of the coating layer Observation of thinned coated lithium nickel composite oxide samples prepared using a cryo-ion slicer (JEOL, IB-09060CIS) with a TEM (JEOL, JEM-ARM200F) confirmed that the thickness of the coating layer was 11 nm.

[0163] [Making a coin-type battery] The initial discharge capacity was evaluated using a 2032 type coin cell CBA shown in Figure 4. The electrodes of the coin cell CBA consist of a positive electrode PE, a separator SE1, and a negative electrode NE, which are stacked in this order. The positive electrode PE is in contact with the inner surface of the positive electrode can PC, and the negative electrode NE is in contact with the inner surface of the negative electrode can NC, as they are housed in a case.

[0164] The positive electrode PE was prepared as follows: 52.5 mg of the positive electrode active material obtained as described above, 15 mg of acetylene black, and 7.5 mg of PTEE were mixed, and the mixture was press-molded at a pressure of 100 MPa to a diameter of 11 mm and a thickness of 100 μm. The mixture was then dried in a vacuum dryer at 120°C for 12 hours to obtain the positive electrode PE.

[0165] Next, using this positive electrode PE, a 2032 type coin cell CBA shown in Figure 4 was fabricated in a glove box with an Ar atmosphere where the dew point was controlled to -80°C.

[0166] For the negative electrode NE, a lithium metal with a diameter of 17 mm and a thickness of 1 mm was used, and the electrolyte was an equal mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with 1 M LiClO4 as the supporting electrolyte (manufactured by Toyama Pharmaceutical Co., Ltd.). Furthermore, a polyethylene porous membrane with a thickness of 25 μm was used for the separator SE1. The 2032 type coin cell CBA has a gasket GA and is assembled into a coin-shaped battery using the positive electrode can PC and the negative electrode can NC.

[0167] [Initial discharge capacity] After fabricating a 2032 type coin cell battery (CBA), leave it for about 24 hours until the open circuit voltage (OCV) stabilizes, then set the current density to 0.1 mA / cm² for the positive electrode. 2 To determine the initial discharge capacity, a charge-discharge test was conducted in which the battery was charged until the cutoff voltage reached 4.5V, then left idle for 1 hour, and finally discharged until the cutoff voltage reached 3.0V. The results confirmed that the initial discharge capacity was 196mAh / g. A battery charge-discharge system (HJ1001SD8, manufactured by Hokuto Denko Co., Ltd.) was used to measure the initial discharge capacity.

[0168] [Making laminated batteries] The cycle characteristics were evaluated using the laminated battery LBA shown in Figure 5.

[0169] As shown in Figure 5, the laminated battery LBA has a structure in which an electrolyte is impregnated into a laminate of a positive electrode film PS, a separator SE2, and a negative electrode film NS, and then sealed with a laminate LE. The positive electrode film PS is connected to a positive-side tab lead PL, and the negative electrode film NS is connected to a negative-side tab lead NL, with the positive-side tab lead PL and the negative-side tab lead NL exposed outside the laminate LA.

[0170] A slurry was prepared by dispersing 20.0 g of the obtained positive electrode active material, 2.35 g of acetylene black, and 1.18 g of polyvinylidene fluoride in N-methyl-2-pyrrolidone (NMP) and spreading it on an aluminum foil for 1 cm. 2 The positive electrode active material was coated so that 7.0 mg was present per Al foil. Next, the slurry containing the positive electrode active material was coated onto the Al foil and dried in air at 120°C for 30 minutes to remove NMP. The Al foil coated with the positive electrode active material was cut into strips 66 mm wide and roll-pressed with a load of 1.2 t to produce a positive electrode film. The positive electrode film was then cut into a rectangle of 50 mm x 30 mm and dried in a vacuum dryer at 120°C for 12 hours, and used as the positive electrode film PS for a laminate-type battery LBA.

[0171] Furthermore, a negative electrode film NS was prepared by coating copper foil with a negative electrode composite paste, which is a mixture of graphite powder with an average particle size of approximately 20 μm and polyvinylidene fluoride. For the separator SE2, a polyethylene porous membrane with a thickness of 20 μm was used, and for the electrolyte, a 3:7 mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with 1 M LiPF6 as the supporting electrolyte (manufactured by Ube Industries, Ltd.) was used.

[0172] In a dry room controlled to a dew point of -60°C, the laminate of the positive electrode film PS, separator SE2, and negative electrode film NS was impregnated with an electrolyte, and then sealed with laminate LA to fabricate a laminate-type battery LBA.

[0173] [Cycle Characteristics] The cycle characteristics were evaluated by measuring the capacity retention rate after 200 charge-discharge cycles using a laminated battery LBA. Specifically, the laminated battery LBA was first subjected to a current density of 0.33 mA / cm² in a constant temperature bath maintained at 25°C. 2 First, the battery was charged to a cutoff voltage of 4.5V, then left for 1 hour, and discharged to a cutoff voltage of 3.0V. This cycle was repeated 5 times for conditioning. Next, the battery was placed in a constant temperature bath maintained at 60°C and subjected to a current density of 2.7mA / cm². 2The discharge capacity was evaluated by measuring the discharge capacity of each cycle, which was repeated 200 times in a cycle of charging to a cutoff voltage of 4.5V, resting for 1 hour, and then discharging to a cutoff voltage of 3.0V.

[0174] The capacity retention rate of the laminated battery LBA, calculated by dividing the discharge capacity obtained in the first cycle after conditioning by the discharge capacity obtained in the 200th cycle after conditioning, was 82%.

[0175] [Example 2] A coated lithium nickel composite oxide was synthesized under the same conditions as in Example 1, except that the ratio of Ni:Co:Mn:Al:Nb in the lithium nickel composite oxide synthesis step was set to 0.75:0.054:0.162:0.03:0.004. The results are shown in Table 1.

[0176] [Example 3] A coated lithium nickel composite oxide was synthesized under the same conditions as in Example 1, except that the ratio of Ni:Co:Mn:Al:Nb in the lithium nickel composite oxide synthesis step was set to 0.65:0.158:0.158:0.03:0.004. The results are shown in Table 1.

[0177] [Example 4] A coated lithium nickel composite oxide was synthesized under the same conditions as in Example 1, except that the ratio of Ni:Co:Mn:Al:Nb in the lithium nickel composite oxide synthesis step was set to 0.65:0.079:0.238:0.03:0.004. The results are shown in Table 1.

[0178] [Example 5] A coated lithium nickel composite oxide was synthesized under the same conditions as in Example 1, except that the ratio of Ni:Co:Mn:Al:Nb in the lithium nickel composite oxide synthesis process was set to 0.55:0.208:0.208:0.03:0.004. The results are shown in Table 1.

[0179] [Example 6] A coated lithium nickel composite oxide was synthesized under the same conditions as in Example 1, except that the ratio of Ni:Co:Mn:Al:Nb in the lithium nickel composite oxide synthesis process was set to 0.55:0.104:0.312:0.03:0.004. The results are shown in Table 1.

[0180] [Comparative Example 1] A coated lithium nickel composite oxide was synthesized under the same conditions as in Example 1, except that the Li / M ratio in the lithium nickel composite oxide synthesis process was set to 1.08. The results are shown in Table 1.

[0181] [Comparative Example 2] A coated lithium nickel composite oxide was synthesized under the same conditions as in Example 1, except that the average particle size D50 in the lithium nickel composite oxide synthesis process was set to 11.3 μm. The results are shown in Table 1.

[0182] [Comparative Example 3] Coated lithium nickel composite oxide was synthesized under the same conditions as in Example 2, except that the lithium nickel composite oxide particles in Example 1 were not coated. The results are shown in Table 1.

[0183] [Table 1]

[0184] [Evaluation Results] The positive electrode active material of the Examples showed a significant improvement in cycle capacity retention in lithium-ion secondary batteries compared to the positive electrode active material of Comparative Example 1 (Li seat occupancy rate of 98% or higher). Furthermore, the positive electrode active material of Example 1 showed an even greater improvement in cycle capacity retention compared to the positive electrode active material of Reference Example 1, which was manufactured under similar conditions except for the absence of Nb. In particular, the cycle capacity retention rate was significantly improved in Examples 4 and 6, which had low Li seat occupancy rates.

[0185] On the other hand, in Comparative Example 2, the positive electrode active material, which had a larger average particle size D50 (11.3 μm), showed a decrease in cycle capacity retention.

[0186] Furthermore, the positive electrode active material of Comparative Example 3, which was manufactured under the same conditions as Example 1 except for the absence of a coating layer, exhibited a very low cycle capacity retention rate in lithium-ion secondary batteries. This is thought to be because the lack of a coating layer on the surface of the lithium nickel composite oxide particles increased resistance at the interface between the positive electrode active material and the electrolyte.

[0187] According to the present invention, it is possible to provide a positive electrode active material that can be suitably used in the positive electrode of a lithium-ion secondary battery where a high battery capacity is required, and a method for producing the same. [Explanation of Symbols]

[0188] 1…Particles of lithium nickel composite oxide 2…Covering layer CBA… Coin cell battery PE... Positive electrode (evaluation electrode) NE...negative electrode SE1... Separator GA... Gasket PC... Positive electrode can NC... Negative electrode can LBA... Laminated battery PS... Positive electrode sheet NS... Negative electrode sheet SE2... Separator LA... Laminate PL... Positive side tab lead NL... Negative side tab lead

Claims

1. A positive electrode active material for a lithium-ion secondary battery, comprising lithium nickel composite oxide particles and a coating layer covering the surface of the particles, The lithium nickel composite oxide particles are It has a crystal structure belonging to the space group R-3m, It contains at least Li, Ni, Mn, element M and Nb, The molar ratio of each of the aforementioned elements is Li:Ni:Mn:M:Nb = a:(1-x-y-z):x:y:z (0.95 ≤ a ≤ 1.10, 0 < x ≤ 0.5, 0 < y ≤ 0.5, 0<x+y+z≦0.7, 0 < z < 0.05, The element M is represented as Co, or at least one selected from the group consisting of Co and Al, Zr, Si, Zn, and Ti. The seat occupancy rate for Li is between 92% and 98%. In the cumulative volume distribution curve of the particle size distribution, the particle size (D50) corresponding to a cumulative volume fraction of 50% is 8 μm or less. The crystallite size calculated by Scherrer method from the diffraction peaks attributed to the (003) plane measured by XRD is between 70 nm and 140 nm. The amount of lithium ions eluted by neutralization titration is 0.20% by mass or more and 1.00% by mass or less, relative to the total amount of lithium nickel composite oxide particles. The aforementioned coating layer is A composite oxide composed of Li and at least one element selected from the group consisting of Al, Si, Ti, V, Ga, Ge, Zr, Nb, Mo, Ta, and W. The content of elements other than Li and oxygen among the elements constituting the coating layer is the surface area of ​​the lithium nickel composite oxide particles 1 m² ２ The amount is between 30 μmol and 600 μmol per unit. Positive electrode active material for lithium-ion secondary batteries.

2. The lithium nickel composite oxide particles are It contains secondary particles composed of aggregated primary particles, The secondary particles have a porous structure having multiple voids in which the primary particles are not present, The specific surface area measured by the nitrogen adsorption BET method was 0.3 m². ２ / g or more 2.0m ２ It is less than or equal to / g. The positive electrode active material for a lithium-ion secondary battery according to claim 1.

3. The positive electrode active material for a lithium-ion secondary battery according to claim 2, wherein at least a portion of the niobium contained in the lithium nickel composite oxide particles segregates at the interface of the primary particles.

4. The positive electrode active material for a lithium-ion secondary battery according to claim 1 or claim 2, wherein the average thickness of the coating layer is 1 nm or more and 15 nm or less, and the average thickness of the coating layer is the arithmetic mean thickness.

5. A mixing step of mixing a nickel composite compound, a niobium compound, and a lithium compound to obtain a mixture, A firing step of firing the mixture to obtain the lithium nickel composite oxide particles, The process includes a coating step of applying a coating solution containing at least one element selected from the group consisting of Al, Si, Ti, V, Ga, Ge, Zr, Nb, Mo, Ta, and W to the surface of the lithium nickel composite oxide particles to form the coating layer. A method for producing a positive electrode active material for a lithium-ion secondary battery according to claim 1 or claim 2.

6. The aforementioned nickel composite compound includes a nickel composite oxide, A method for producing a positive electrode active material for a lithium-ion secondary battery according to claim 5, comprising an oxidation roasting step of obtaining the nickel composite oxide by oxidizing and roasting a nickel composite hydroxide prepared by a crystallization reaction.

7. A method for producing a positive electrode active material for a lithium-ion secondary battery according to claim 5, further comprising a heat treatment step of heat-treating the lithium nickel composite oxide particles on which the coating layer has been formed on their surface at 300°C or higher, after the coating step.

8. The positive electrode active material for a lithium-ion secondary battery according to claim 1 or 2, wherein the coating layer is a composite oxide composed of Li and Ti, or a composite oxide composed of Li and Nb.

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