Positive electrode active material for lithium ion secondary batteries

Abstract

Provided is a positive electrode active material for lithium ion secondary batteries which, when used in a lithium ion secondary battery, achieves improvement in weather resistance, especially against deterioration due to moisture absorption, and has features such as high discharge capacity, low reaction resistance, and extended cycle characteristics, thus exhibiting high battery characteristics.　This positive electrode active material for lithium ion secondary batteries is composed of a coated lithium metal composite oxide containing secondary particles in which primary particles are aggregated. The positive electrode active material: is characterized by having a coated layer that contains a compound having a structure other than the spinel crystal structure, and contains at least one selected from lithium, cobalt, and nickel on the surfaces of the primary particles and the surfaces of the secondary particles; and is characterized in that the secondary particles have a solid structure, the porosity of the secondary particles is 6% or less, and the cobalt content of the secondary particles is 0.6 mass% or less.

Description

Positive electrode active material for lithium-ion secondary batteries

[0001] The present invention relates to a positive electrode active material for a lithium ion secondary battery, and more particularly to a positive electrode active material for a lithium ion secondary battery that exhibits excellent weather resistance when used in the lithium ion secondary battery.

[0002] Global environmental issues have now become a global problem, with international treaties on environmental protection being signed. Although some time has passed since global warming first came to the forefront due to increases in greenhouse gases such as carbon dioxide, methane, and chlorofluorocarbons, it is difficult to say that the situation has improved, and various efforts to reduce greenhouse gas emissions are still being made in various countries.

[0003] Among these, automobiles that run on fossil fuels, such as gasoline and diesel automobiles, emit large amounts of carbon dioxide and are therefore considered a problematic source of air pollutants.Hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and electric vehicles (EVs), which use electricity stored in lithium-ion secondary batteries, have attracted attention as environmentally friendly next-generation automobiles, and research and development toward their practical application has been progressing from an early stage.The lithium-ion secondary batteries installed in these next-generation automobiles are required to have even higher energy density, longer life, and lower cost, and research into improving battery characteristics is becoming increasingly active.

[0004] Furthermore, as it is difficult to build new thermal power plants that burn fossil fuels, one effective way to respond to the growing demand for electricity is to store potentially surplus nighttime electricity in lithium-ion secondary batteries installed in ordinary homes and use it during the daytime when electricity consumption is high, thereby leveling the load.In addition, efforts to use home storage batteries made of lithium-ion secondary batteries together with solar power generation systems are gradually spreading, and stored clean electricity can be used not only during the day but also at night, and there are high expectations for this as an emergency power source in the event of a power outage caused by a disaster such as an earthquake or typhoon.

[0005] The most common lithium-ion secondary batteries are non-aqueous electrolyte secondary batteries, and recently, all-solid-state batteries using non-flammable solid electrolytes have also attracted a great deal of attention. However, both types of batteries have problems that must be solved. One of the common factors that cause these problems is the deterioration of weather resistance, such as moisture absorption by the positive electrode active material and carbonation of the surface. Therefore, as a countermeasure to the deterioration of weather resistance, positive electrode active materials have been developed in which the surface of a lithium metal composite oxide is coated with a compound to improve its characteristics.

[0006] For example, Patent Document 1 discloses a composite positive electrode active material including at least one secondary particle including an aggregate of a plurality of primary particles, the secondary particle including a plurality of primary particles including a lithium transition metal oxide having a layered crystal structure, and a coating film disposed on the surface of the secondary particle and between the plurality of primary particles, the coating film including a lithium cobalt composite oxide having a spinel crystal structure, and the lithium cobalt composite oxide including cobalt (Co) and a Group 2, Group 12, or Group 13 element, or a combination thereof.

[0007] Specifically, Patent Document 1 focuses on lithium-cobalt composite oxides having a spinel crystal structure as compounds for improving characteristics, and describes a technology in which compounds containing these are present on the surfaces of secondary particles of a positive electrode active material and between primary particles. This technology reduces the content of residual lithium on the surface (lithium hydroxide, lithium carbonate, etc.), suppresses deterioration of the positive electrode active material due to carbonation, and when the battery is fabricated, suppresses the generation of gases such as carbon dioxide, thereby improving the thermal stability of the battery.

[0008] Japanese Patent Application Laid-Open No. 2019-046795

[0009] However, the positive electrode active material described in Patent Document 1 is susceptible to moisture absorption, and the positive electrode active material reacts with moisture in the air, causing extraction of lithium from inside the crystal, which deteriorates the positive electrode active material.When used in a battery, this results in a significant decrease in discharge capacity and an increase in reaction resistance, as well as the elution of constituent elements of the compound into the negative electrode, resulting in problems such as decreased durability (cycling characteristics).

[0010] The present invention has been made in view of the problems of the conventional art, and an object of the present invention is to provide a positive electrode active material for a lithium ion secondary battery which, when used in a lithium ion secondary battery, has improved weather resistance, in particular, improved degradation due to moisture absorption, and has excellent battery characteristics such as high discharge capacity, low reaction resistance, and long cycle characteristics.

[0011] The present inventors conducted extensive research to solve the above-mentioned problems, and as a result, discovered that by disposing a coating layer containing one or more compounds selected from lithium, cobalt, and nickel other than a spinel crystal structure on the surfaces of primary particles and secondary particles of a lithium metal composite oxide, a network structure of the coating layer is formed in the cross-sectional shape of the secondary particles, and deterioration due to moisture absorption in the positive electrode active material can be suppressed, leading to the completion of the present invention. Furthermore, in the secondary particles of the lithium metal composite oxide that serve as the base material for the positive electrode active material, the cobalt content is reduced to the impurity level (e.g., derived from raw materials or contamination from manufacturing equipment), and thus a positive electrode active material that maintains good battery characteristics and can be produced at low cost has been obtained.

[0012] That is, according to one aspect of the present invention for solving the above-mentioned problems, a first aspect of the present invention is a positive electrode active material for a lithium ion secondary battery, comprising a coated lithium metal composite oxide including secondary particles formed by aggregation of primary particles, wherein the surfaces of the primary particles and the surfaces of the secondary particles have coating layers containing a compound other than a spinel crystal structure, which contains one or more selected from lithium, cobalt, and nickel, the secondary particles have a solid structure, the porosity of the secondary particles is 6% or less, and the cobalt content of the secondary particles is 0.6% by mass or less.

[0013] A second aspect of the present invention is the positive electrode active material for a lithium ion secondary battery according to the first aspect, wherein a network structure of the coating layer is confirmed in a cross-sectional morphology of the secondary particles when observed by a STEM-EDS method.

[0014] A third aspect of the present invention is a positive electrode active material for a lithium ion secondary battery, characterized in that the water content of the coated lithium metal composite oxide in the invention described in the first or second aspect is 0.2 mass % or less.

[0015] A fourth aspect of the present invention is a positive electrode active material for a lithium ion secondary battery, characterized in that the powder pH of the coated lithium metal composite oxide according to the first to third aspects of the invention is determined under the following measurement conditions and is 11.5 to 12.5. (Note) <Measurement conditions for powder pH> Water is added to 5 g of an analysis sample to make 100 mL, and the pH is measured with a pH meter.

[0016] In a fifth aspect of the present invention, the coating layer in the invention according to the first to fourth aspects is made of lithium cobalt oxide (LiCoO ２ ) is a positive electrode active material for a lithium ion secondary battery.

[0017] The sixth aspect of the present invention is the lithium cobalt oxide (LiCoO ２ ) is detected by an XRD method.

[0018] In a seventh aspect of the present invention, the coated lithium metal composite oxide in the invention according to the first to sixth aspects is represented by the general formula: Li ａ Ni １－ｘ－ｙ M1 ｘ M2 ｙ M3 ｚ O ２＋α (0.95≦a≦1.3, 0≦x≦0.2, 0≦y≦0.1, 0.7≦1−x−y≦1, 0<z≦0.05, −0.1≦α≦0.5, M1 is one or more selected from Al and Mn, M2 is one or more selected from Na, K, Ca, Mg, Sr, Ba, Ti, Zr, V, Nb, Ta, Cr, Mo, W, B, Si, P, and S, and M3 is one or more selected from Co and Ni).

[0019] An eighth aspect of the present invention is a positive electrode active material for a lithium ion secondary battery, characterized in that the ratio of the thickness of the coating layer to the average particle size MV of the secondary particles of the coated lithium metal composite oxide in the inventions described in the first to seventh aspects is 0.01 to 10%.

[0020] A ninth aspect of the present invention is a positive electrode active material for a lithium ion secondary battery, characterized in that the coating layer in the invention according to any one of the first to eighth aspects has a thickness of 40 to 300 nm.

[0021] A tenth aspect of the present invention is a positive electrode active material for a lithium ion secondary battery, characterized in that the lithium hydroxide content of the coated lithium metal composite oxide in the invention according to any one of the first to ninth aspects is 0.1 to 1.5 mass %.

[0022] According to the present invention, it is possible to provide a positive electrode active material for a lithium ion secondary battery which, when used in a lithium ion secondary battery, has improved weather resistance, in particular, improved resistance to deterioration due to moisture absorption, and has excellent battery characteristics such as high discharge capacity, low reaction resistance, and long cycle characteristics.

[0023] 1 is a schematic cross-sectional view of a secondary particle showing one embodiment of a positive electrode active material for a lithium ion secondary battery according to the present invention; 2 is a STEM-EDS image of a cross-section of a secondary particle showing one embodiment of a positive electrode active material for a lithium ion secondary battery according to an example of the present invention; 3 is a STEM-EDS image of a cross-section of a secondary particle showing one embodiment of a positive electrode active material for a lithium ion secondary battery according to a comparative example of the present invention; 4 is a schematic cross-sectional view of a coin-type battery used in evaluating battery characteristics according to the present invention;

[0024] The cathode active material for a lithium-ion secondary battery of the present invention (hereinafter also simply referred to as "cathode active material") will be described in detail below. The present invention is not limited to the embodiments described below, and modifications to the embodiments can be made based on the knowledge of those skilled in the art without departing from the spirit of the present invention. In the following description, the expression "A to B" means "A or more and B or less."

[0025] <Positive Electrode Active Material for Lithium-Ion Secondary Battery> The present invention provides a positive electrode active material for a lithium-ion secondary battery, which is a coated lithium metal composite oxide including secondary particles formed by aggregation of primary particles, wherein the surfaces of the primary particles and the surfaces of the secondary particles have coating layers containing a compound other than a spinel crystal structure, which contains one or more selected from lithium, cobalt, and nickel, and the secondary particles have a solid structure, a porosity of 6% or less, and a cobalt content of the secondary particles of 0.6% by mass or less.

[0026] (1) Coating Layer It has been conventionally thought that if the particle surface of the lithium metal composite oxide, which is the base material of the positive electrode active material, is completely coated with a different compound, the effect of promoting intercalation / deintercalation, which is a phenomenon in which lithium ions move, is greatly limited, and as a result, the advantage of the lithium metal composite oxide, namely, high capacity, is lost.

[0027] In contrast, in the coated lithium metal composite oxide, i.e., the cathode active material, of the present invention, a coating layer containing a compound other than a spinel crystal structure, which contains one or more selected from lithium, cobalt, and nickel, is disposed on the surfaces of the primary particles and the secondary particles. Such a coating layer has high lithium ion conductivity and is effective in promoting intercalation and deintercalation. When incorporated into a secondary battery, this coating layer forms a conduction path for lithium ions at the interface with the electrolyte, thereby reducing the reaction resistance of the cathode active material (hereinafter also simply referred to as "cathode resistance") and further improving battery characteristics such as battery capacity and output characteristics. Furthermore, in the present invention, it has been newly discovered that the coating layer can suppress deterioration of the cathode active material due to moisture absorption.

[0028] If only the surfaces of the secondary particles of the lithium metal composite oxide, which serves as the base material for the positive electrode active material, were coated with the coating layer, the specific surface area of ​​the positive electrode active material would be reduced, and when incorporated into a secondary battery, the contact area with the electrolyte would be reduced. Furthermore, the lithium ion conduction paths would be reduced, increasing the positive electrode resistance and potentially resulting in reduced battery capacity and output characteristics. However, in the positive electrode active material of the present invention, the coating layer is disposed not only on the surfaces of the secondary particles but also on the surfaces of the primary particles present therein. This gives the secondary particles a cross-sectional morphology with a mesh structure, thereby suppressing deterioration due to moisture absorption and ensuring sufficient lithium ion conduction paths when incorporated into a secondary battery. As a result, lithium ion conductivity can be effectively improved, and when used as a positive electrode material for a secondary battery, positive electrode resistance can be reduced, resulting in higher capacity and higher output.

[0029] The form of the coating layer is not particularly limited, but for example, at least a portion can be particulate, or at least a portion can be film-like (layer-like), or a mixture of particulate and film-like (layer-like) layers is acceptable. In the case of the coating layer described above, when the coating layer is particulate, the particle size is preferably 1 to 50 nm. By being particulate, if the particle size is 1 nm or more, particularly high lithium ion conductivity can be obtained, and if the particle size is 50 nm or less, the particles are distributed particularly uniformly on the particle surface of the lithium metal composite oxide base material, thereby reducing the positive electrode resistance and better achieving the effects of increasing capacity and power output.

[0030] Furthermore, when the coating layer is in the form of a film (layer), the ratio of the thickness to the average particle size MV of the secondary particles of the coated lithium metal composite oxide is preferably 0.01 to 10%. If the thickness ratio is 0.01% or more, the effects of promoting intercalation / deintercalation and suppressing deterioration due to moisture absorption are fully exhibited, and if it is 10% or less, the activation barrier for lithium ion migration can be lowered, and charge transfer resistance can be reduced, thereby improving battery characteristics.

[0031] Furthermore, when the coating layer is in the form of a film (layer), its thickness is more preferably 40 to 300 nm. This is because, when the coating layer is in the form of a film (layer), a thickness of 40 nm or more provides particularly high lithium ion conductivity, and a thickness of 300 nm or less ensures sufficient lithium ion conduction paths when incorporated into a secondary battery, reducing positive electrode resistance and enabling better performance of the effects of increased capacity and increased output. The particle surface properties of such coated lithium metal composite oxides are not particularly limited, but can be confirmed, for example, by observing a cross-sectional sample of the particles using scanning transmission electron microscope-energy dispersive X-ray spectroscopy (STEM-EDS) or the like.

[0032] The coating layer contains a compound other than a spinel crystal structure, which contains one or more compounds selected from lithium, cobalt, and nickel, and is preferably lithium cobalt oxide (LiCoO ２ ), and / or lithium nickel oxide (LiNiO ２ ), and lithium cobalt oxide (LiCoO ２ It is more preferable that the compound contains a cation exchange group (C1) containing a cation exchange group (C2) containing a cation exchange group (C3) containing a cation exchange group (C4) containing a cation exchange group (C5) containing a cation exchange group (C6) containing a cation exchange group (C7) containing a cation exchange group (C8) containing a cation exchange group (C9) containing a cation exchange group (C10) containing a cation exchange group (C11) containing a cation exchange group (C12) containing a cation exchange group (C13) containing a cation exchange group (C20) containing a cation exchange group (C21) containing a cation exchange group (C3) containing a cation exchange group (C4) containing a cation exchange group (C5) containing a cation exchange group (C6) containing a cation exchange group (C7) containing a cation exchange group (C8) containing a cation exchange group (C9) containing a cation exchange group (C14) containing a cation exchange group (C15) containing a cation exchange group (C16) containing a cation exchange group (C17) containing a cation exchange group (C20) containing a cation exchange group (C21) containing a cation exchange group (C22) containing a cation exchange group (C31) containing a cation exchange group (C32) containing a cation exchange group (C41) containing a cation exchange group (C42) containing a cation exchange group (C43) containing a cation exchange group (C44) containing a cation exchange group (C45) containing a cation exchange group (C46) containing a cation exchange group (C47) containing a cation exchange group (C48) containing a cation exchange group (C49) containing a cation exchange group (C51) containing a cation exchange group (C52) containing a cation exchange group (C61) containing a cation exchange group (C62) containing a cation exchange group (C73) containing a cation exchange group (C74) containing a cation exchange group (C75) containing a

[0033] The positive electrode active material for a lithium ion secondary battery according to the present invention will be described below with reference to Fig. 1. Fig. 1 is a schematic cross-sectional view of a secondary particle showing one embodiment of the positive electrode active material for a lithium ion secondary battery according to the present invention. Fig. 2, which supplements Fig. 1, is a STEM-EDS image of a cross-section of a secondary particle showing one embodiment of the positive electrode active material for a lithium ion secondary battery according to an example of the present invention, and Fig. 3 is a STEM-EDS image of a cross-section of a secondary particle showing one embodiment of the positive electrode active material for a lithium ion secondary battery according to a comparative example of the present invention. In a coated lithium metal composite oxide, i.e., a cathode active material, according to one embodiment of the present invention, as shown in FIG. 1 , primary particles 2 are aggregated to form secondary particles 1, and further, coating layers 3 containing a compound other than a spinel crystal structure, which contains one or more elements selected from lithium, cobalt, and nickel, are disposed on the surfaces of the primary particles 2 and the surfaces of the secondary particles 1. Furthermore, as shown in FIGS. 1 and 2 , a mesh structure of the coating layer is formed in the cross section of the secondary particles (in FIG. 3 , the elements that form the coating layer are diffused substantially uniformly throughout, and the mesh structure cannot be confirmed). This suppresses deterioration due to moisture absorption, and, due to the high lithium ion conductivity of the coating layer 3, active intercalation and deintercalation of lithium ions into the crystal lattice occurs at the interface with the electrolyte solution that fills the voids 4. Therefore, when the cathode active material is used in a lithium ion secondary battery, it is considered that the cathode resistance is reduced and the battery capacity and output characteristics are improved. The network structure of the coating layer does not need to be present on the surface of all primary particles as shown in FIG. 1, and some of the primary particles may have defects, but it is desirable that the coating layer be present in the areas exposed to voids.

[0034] (2) Particle Morphology and Internal Particle Structure Most of the particles of the coated lithium metal composite oxide serving as the positive electrode active material according to the present invention have the form of secondary particles formed by agglomeration of multiple primary particles, but some primary particles may be present that are not agglomerated as secondary particles. The shapes of the primary particles constituting the secondary particles and the primary particles present alone are not particularly limited and may take various shapes, such as spherical, plate-like, needle-like, rectangular, ellipsoidal, and rhombohedral. The agglomeration form of multiple primary particles is also not particularly limited and may take various shapes, such as agglomeration in random directions, or agglomeration that is approximately uniform and radial from the center to form approximately spherical or ellipsoidal secondary particles.

[0035] The coated lithium metal composite oxide particles having the above-mentioned secondary particle form have a solid structure with few voids present inside. Here, a solid structure refers to a cross-sectional state in which the contents of the secondary particles are packed, as shown in Figure 1, and a small void is defined as a porosity measured at the cross-section of the secondary particles of 6% or less, preferably 3% or less. Furthermore, for efficient contact between the coating layer and the electrolyte, i.e., efficient interface formation, a porosity of 0.5% or more is preferable. This porosity can be determined, for example, by the following method.

[0036] That is, after the particle group of the coated lithium metal composite oxide to be measured is embedded in a resin, the particle group is cut using a cross-section polisher (CP) and argon sputtering to expose the cross section of the particle group, and the cross section of this exposed particle group is imaged using a scanning electron microscope (SEM) or the like. The obtained cross-sectional image of the particle group is then analyzed using image analysis software to identify void areas as black areas and dense areas as white areas, and the porosity can be determined for the cross sections of any 20 or more particles by calculating "area of ​​black areas / (area of ​​black areas + area of ​​white areas) × 100".

[0037] As described above, particles of the coated lithium metal composite oxide having a solid structure with few internal voids have high strength, and therefore, when used as a positive electrode active material, it is possible to ensure a sufficient contact area between the positive electrode active material and the electrolyte without excessively reducing the bulk density. Furthermore, the shape of the primary particles and the morphology of the secondary particles of the coated lithium metal composite oxide, as well as the internal structure of the secondary particles, can be understood, for example, by observing the cross section of the particles using a scanning electron microscope (SEM) or the like.

[0038] In addition, the average particle size of the secondary particles of the coated lithium metal composite oxide serving as the positive electrode active material according to the present invention is preferably controlled to be within the range of 2 to 20 μm, more preferably within the range of 4 to 18 μm. In this way, if the average particle size of the coated lithium metal composite oxide is within the range of 2 to 20 μm, a secondary battery incorporating this in a positive electrode can have a large battery capacity per volume, improved safety, and good cycle characteristics.

[0039] If this average particle size is less than 2 μm, the particle packing density may decrease when the positive electrode is fabricated, and the battery capacity per volume of the positive electrode may decrease. Conversely, if the average particle size exceeds 20 μm, the specific surface area of ​​the positive electrode active material may decrease, and the interface with the electrolyte of the secondary battery may decrease. As a result, the resistance of the positive electrode may increase, and the output characteristics of the battery may decrease.

[0040] Furthermore, the average particle size of the coated lithium metal composite oxide can be controlled by the nucleation time in the crystallization step in the production process of the metal composite hydroxide, as well as the supply amount of the raw material solution and pH. That is, when the average particle size is less than 2 μm, the supply amount of the raw material solution (metal compound) may be reduced to shorten the nucleation time in the crystallization step, or the pH may be controlled to be lower. This reduces the amount of nuclei generated that serve as particle seeds, allowing the particle size of the resulting metal composite hydroxide to be larger. Conversely, when the average particle size exceeds 20 μm, the supply amount of the raw material solution (metal compound) may be increased to lengthen the nucleation time in the crystallization step, or the pH may be controlled to be higher. This increases the amount of nuclei generated that serve as particle seeds, allowing the particle size of the resulting metal composite hydroxide to be smaller.

[0041] (3) Composition The composition of the coated lithium metal composite oxide serving as the positive electrode active material according to the present invention is represented by the general formula: Li ａ Ni １－ｘ－ｙ M1 ｘ M2 ｙ M3 ｚ O ２＋α It is preferable that the elements are controlled so as to be expressed as follows: (0.95≦a≦1.3, 0≦x≦0.2, 0≦y≦0.1, 0.7≦1−x−y≦1, 0<z≦0.05, −0.1≦α≦0.5, M1 is one or more selected from Al and Mn, M2 is one or more selected from Na, K, Ca, Mg, Sr, Ba, Ti, Zr, V, Nb, Ta, Cr, Mo, W, B, Si, P, and S, and M3 is one or more selected from Co and Ni).

[0042] In the above general formula, 1-x-y, which indicates the nickel (Ni) content, is in the range of 0.7 to 1, preferably 0.75 to 1, and particularly preferably 0.8 to 1. By doing so, when used as a positive electrode active material, it is possible to achieve a high potential and a high capacity of a lithium ion secondary battery. If 1-x-y is less than 0.7, the above-mentioned high potential and high capacity cannot be sufficiently achieved.

[0043] M1 is an element that contributes to the durability, thermal stability, and safety of the lithium-ion secondary battery and is optionally added. The value of "x," which indicates the molar ratio of M1, may be 0. However, in order to fully obtain the above-mentioned effects, the value of "x" is preferably within the range of more than 0 and not more than 0.2, more preferably more than 0 and not more than 0.19, and particularly preferably more than 0 and not more than 0.18. If the value of "x" is 0, the durability, thermal stability, and safety of the lithium-ion secondary battery cannot be improved. On the other hand, if the value of "x" exceeds 0.2, the molar ratio of nickel decreases, and the effect cannot be fully obtained. As such M1, one or more elements selected from aluminum (Al) and manganese (Mn) can be used.

[0044] M2 is an element that contributes to the output characteristics, cycle characteristics, durability, etc. of the lithium ion secondary battery, and is added optionally. The value of "y", which indicates the molar ratio of M2, may be 0, but in order to fully obtain the above-mentioned effects, the value of "y" is preferably within the range of more than 0 and not more than 0.1, more preferably more than 0 and not more than 0.05, and particularly preferably more than 0 and not more than 0.01. If the value of "y" is 0, the output characteristics, cycle characteristics, durability, etc. of the lithium ion secondary battery cannot be improved. On the other hand, if the value of "y" exceeds 0.1, the molar ratio of nickel decreases, and the effect cannot be fully obtained. As such M2, one or more elements selected from sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), strontium (Sr), barium (Ba), titanium (Ti), zirconium (Zr), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W), boron (B), silicon (Si), phosphorus (P), and sulfur (S) can be used.

[0045] M1 and M2 may be added at any time during the production of the metal composite hydroxide (precursor) or during the production of the lithium metal composite oxide (base material).

[0046] M3 is an element that contributes to weather resistance by being contained in the coating layer of the positive electrode active material and contributes to various battery characteristics, including the output characteristics of a lithium-ion secondary battery. The value of "z," which indicates the molar ratio of M3, is greater than 0 and less than 0.05, preferably 0.001 to 0.04, more preferably 0.003 to 0.03, and particularly preferably 0.005 to 0.02. If the value of "z" is 0, the battery characteristics, including the output characteristics of a lithium-ion secondary battery, cannot be improved. On the other hand, if the value of "z" exceeds 0.05, the effect cannot be fully obtained. As such M3, one or more elements selected from cobalt (Co) and nickel (Ni) can be used.

[0047] (4) Cobalt Content of Secondary Particles The cobalt content of the secondary particles of the coated lithium metal composite oxide serving as the positive electrode active material according to the present invention is 0.6% by mass or less, preferably 0.5% by mass or less, and particularly preferably 0.4% by mass or less. Cobalt (Co) is an element that contributes to the cycle characteristics and durability of lithium ion secondary batteries, but by reducing the cobalt content to the impurity level, it is possible to obtain a low-cost lithium ion secondary battery.

[0048] (5) Water Content The water content of the coated lithium metal composite oxide serving as the positive electrode active material according to the present invention is preferably 0.2% by mass or less. The water content is an important indicator of weather resistance, and a high water content adversely affects the charge / discharge reaction of the secondary battery. Therefore, it is preferable to remove as much water as possible to reduce the water content.

[0049] (6) Powder pH The powder pH of the coated lithium metal composite oxide serving as the positive electrode active material according to the present invention is preferably 11.5 to 12.5. However, lithium hydroxide (LiOH) and lithium carbonate (Li ２ CO ３ The excess lithium (also referred to as residual lithium) constituted by the above-mentioned components serves as a lithium supply source for forming the coating layer. If the powder pH is less than 11.5, there is a possibility that the excess lithium was insufficient during the production of the positive electrode active material, whereas if the powder pH is greater than 12.5, there is an excess of lithium remaining, which may cause gas generation during the charge-discharge reaction of the secondary battery.

[0050] (7) Lithium Hydroxide Content The lithium hydroxide content of the coated lithium metal composite oxide serving as the positive electrode active material according to the present invention is preferably 1.5% by mass or less. If the lithium hydroxide content exceeds 1.5% by mass, it may cause gelation when kneading a positive electrode composite paste using the positive electrode active material, and may also cause gas generation during the charge / discharge reaction of a secondary battery. On the other hand, the lower limit of the lithium hydroxide content is not particularly limited, but is, for example, 0.1% by mass or more.

[0051] <Evaluation of Positive Electrode Active Material for Lithium-Ion Secondary Batteries> (1) Sample Analysis Method 1) Composition The composition can be evaluated by acid decomposition-ICP (inductively coupled plasma) atomic emission spectrometry. Furthermore, a multi-type ICP atomic emission spectrometry analyzer, ICPE-9000 (manufactured by Shimadzu Corporation), can be used for the analysis. From the obtained results, Li / Me (the ratio of the number of lithium (Li) atoms to the sum of the number of atoms of metal elements contained in the coated lithium metal composite oxide (positive electrode active material), the "ratio value") can be determined.

[0052] 2) Cobalt content of secondary particles The cobalt content of secondary particles can be evaluated by acid decomposition-ICP (inductively coupled plasma) atomic emission spectrometry. The analysis can be performed using a multi-type ICP atomic emission spectrometry analyzer, ICPE-9000 (manufactured by Shimadzu Corporation).

[0053] 3) Coating Layer Compounds The coating layer compounds can be identified by X-ray diffraction (XRD). Specifically, a sample placed in a sample holder is measured using CuKα radiation as the radiation source at a measurement speed of 2° / min, a tube voltage of 45 kV, a tube current of 40 mA, and a measurement range of 2θ = 10 to 100°. The compounds can then be identified by comparing the standard diffraction pattern of the compound with the diffraction pattern of the sample using the PDF (Powder Diffraction File) database in the ICDD (International Centre for Diffraction Data). Furthermore, an X'PertPRO (Spectris Corporation) X-ray diffractometer (XRD) can be used for the analysis.

[0054] 4) Mesh structure of the coating layer The mesh structure of the coating layer can be confirmed by scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS). In other words, the STEM-EDS method can obtain HAADF-STEM (High_angle_Annular_Dark_Field_Scanning_TEM) images with Z (atomic number) contrast, making it suitable for analyzing the composition of compounds and semiconductors. In addition, in the analysis, a cross-section sample of particles processed from positive electrode active material was obtained using a cross-section polisher (CP) IB-19530CP (manufactured by JEOL Ltd.), and the cross-sectional morphology was observed using a scanning transmission electron microscope (STEM) ARM200F (manufactured by JEOL Ltd.) equipped with an energy dispersive X-ray spectroscopy device (manufactured by EDAX).

[0055] 5) Coating Layer Thickness The coating layer thickness can be evaluated using scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS). In other words, STEM-EDS produces images that reflect compositional information, allowing for accurate confirmation of coating layer thicknesses down to nanometers. Furthermore, in the analysis, a cross-section sample of particles processed from positive electrode active material was obtained using a cross-section polisher (CP) IB-19530CP (manufactured by JEOL Ltd.), and the cross section was observed using a scanning transmission electron microscope (STEM) ARM200F (manufactured by JEOL Ltd.) equipped with an energy dispersive X-ray spectroscopy (manufactured by EDAX). Specifically, the coating layer thickness was measured at five randomly selected locations on each of 10 randomly selected particles, and the 50 values ​​obtained were averaged to determine the thickness.

[0056] 6) Particle Structure The particle structure of secondary particles can be confirmed using a scanning electron microscope (SEM). Furthermore, for analysis, a cross-section sample of particles processed from positive electrode active material using a cross-section polisher (CP) IB-19530CP (manufactured by JEOL Ltd.) can be used, and the cross section can be observed using a Schottky field emission scanning electron microscope (SEM) JSM-7001F (manufactured by JEOL Ltd.). Furthermore, the average particle size (MV) of the secondary particles of the lithium metal composite oxide (base material) and the coated lithium metal composite oxide (positive electrode active material) can be evaluated using the volume-based distribution measured using a laser diffraction / scattering method. Furthermore, for analysis, a laser diffraction / scattering particle size distribution analyzer, the Microtrac MT3300EXII (manufactured by Microtrac Bell Corporation), can be used.

[0057] 7) Porosity The porosity of secondary particles can be evaluated using a scanning electron microscope (SEM). Furthermore, in the analysis, a cross-section sample of particles processed from a positive electrode active material using a cross-section polisher (CP) IB-19530CP (manufactured by JEOL Ltd.) was used, and the cross section was observed using a Schottky field emission scanning electron microscope (SEM) JSM-7001F (manufactured by JEOL Ltd.). Specifically, the obtained image was analyzed using image analysis and measurement software WinRoof 6.1.1 (manufactured by Mitani Shoji Co., Ltd.), with the voids in the particle cross section measured as black regions and the dense portions of the particles measured as white regions. For each of 20 arbitrarily selected particles, the "area of ​​the black region / (area of ​​the black region + area of ​​the white region) × 100" was calculated, and the porosity was determined by averaging these values.

[0058] 8) Water Content The water content can be evaluated by Karl Fischer titration at 300° C. The analysis can be performed using a coulometric Karl Fischer moisture meter, MKC-710D (manufactured by Kyoto Electronics Manufacturing Co., Ltd.).

[0059] 9) Powder pH 5 g of the positive electrode active material was dispersed in 100 mL of pure water, allowed to stand for 10 minutes, and the pH of the supernatant was measured using a pH meter to evaluate the powder pH. An Orion Star A214 pH meter (manufactured by Thermo Fisher Scientific Co., Ltd.) was also used for the analysis.

[0060] 10) Lithium hydroxide content: 10 g of the positive electrode active material is stirred in 100 mL of pure water for 5 minutes, filtered, and the filtrate is titrated with 1 mol / L hydrochloric acid. The amount of alkalinity neutralized by the hydrochloric acid can be evaluated as the amount of lithium (Li) derived from lithium hydroxide (LiOH). The analysis uses an automatic titrator, COM-1750 (manufactured by Hiranuma Sangyo Co., Ltd.), and the endpoint (potential difference) can be determined using a pH composite electrode.

[0061] (2) Battery Measurement Method 1) Initial Discharge Capacity After the evaluation battery (coin-type battery CBA) shown in FIG. 4 was prepared, it was left for about 24 hours. After the open circuit voltage OCV (open_circuit_voltage) was stabilized, the current density to the positive electrode was 0.1 mA / cm ２ The initial charge capacity is determined by charging the battery to a cutoff voltage of 4.3 V, and after a one-hour rest, the battery is discharged to a cutoff voltage of 3 V. The initial discharge capacity can be evaluated as the capacity when the battery is discharged to a cutoff voltage of 3 V. The measurement can be performed using a multi-channel voltage / current generator R6741A (manufactured by Advantest Corporation).

[0062] [Method of Fabricating Evaluation Battery (Coin Cell Battery CBA)] An evaluation battery (coin cell battery CBA) can be fabricated by the following method. 52.5 mg of the above-mentioned positive electrode active material, 15 mg of acetylene black, and 7.5 mg of polytetrafluoroethylene are weighed and mixed, and press-molded at 100 MPa to a diameter of 11 mm and a thickness of 100 μm to fabricate a positive electrode PE (electrode for evaluation). The fabricated positive electrode PE is dried in a vacuum dryer at 120°C for 12 hours, and then a 2032-type coin cell battery CBA is fabricated using this positive electrode PE in a glove box with an argon gas atmosphere and a dew point controlled at -80°C. Metallic lithium with a diameter of 17 mm and a thickness of 1 mm is used for the negative electrode NE, and 1 mol of LiClO is used as the electrolyte. ４ The separator SE is a 25 μm thick polyethylene porous membrane. The coin-type battery CBA is assembled with a positive electrode can PC and a negative electrode can NC, and a gasket GA and a wave washer WW are placed in the separator SE.

[0063] 2) Reaction Resistance A test battery (coin-type battery CBA) shown in Figure 4 was prepared, and this test battery was charged at a charging potential of 4.1 V. A Nyquist plot was obtained by measuring the AC impedance method using a frequency response analyzer and a potentiogalvanostat. This Nyquist plot is expressed as the sum of characteristic curves showing the solution resistance, negative electrode resistance and its capacity, and positive electrode resistance and its capacity. The positive electrode resistance can be evaluated by performing a fitting calculation using an equivalent circuit based on the Nyquist plot.

[0064] 3) Cycle Characteristics The cycle characteristics can be evaluated based on the capacity retention rate after a cycle test using an evaluation battery (coin-type battery CBA) shown in Fig. 4. That is, in the cycle test, the battery is held at 60°C and the initial discharge capacity is measured, followed by a 10-minute pause. Similar to the measurement of the initial discharge capacity, the charge-discharge cycle (charge and discharge) is repeated 200 times, including the measurement of the initial discharge capacity, and the discharge capacity at the 200th cycle is measured. The capacity retention rate (%) can be calculated as the percentage of the discharge capacity at the 200th cycle relative to the discharge capacity at the first cycle (initial discharge capacity).

[0065] (3) Others In each of the above-described manufacturing processes, commercially available thermometers, pH meters, ammonium ion meters, oxygen concentration meters, etc. can be used to measure temperature, pH, ammonium ion concentration, oxygen concentration, etc., depending on the embodiment, and are not particularly limited.

[0066] The present invention will be specifically described below using examples and comparative examples. In the following examples and comparative examples, reagents manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. were used unless otherwise specified. Furthermore, the present invention is not limited in any way by the following examples and comparative examples.

[0067] (1) Production of Metal Composite Hydroxide (Precursor) First, 14 L of water was placed in a 60 L reaction tank, and the temperature inside the tank was set to 40°C while stirring. Nitrogen gas was introduced into the tank to control the gas phase inside the tank to a non-oxidizing atmosphere with an oxygen concentration of 0.1% by volume. Next, an alkaline solution and an appropriate amount of ammonium ion donor were added to the water in the tank to prepare a reaction solution with a pH (based on a liquid temperature of 25°C, the same applies below) of 12.8 and an ammonium ion concentration of 10 g / L.

[0068] On the other hand, a nickel-manganese raw material solution prepared by dissolving nickel sulfate hexahydrate and manganese sulfate monohydrate in water, and an aluminum raw material solution prepared by dissolving sodium aluminate in water and adding sodium hydroxide were weighed out so that the atomic percentages of nickel, aluminum, and manganese were Ni:Al:Mn=84:10:6, and were dissolved in water so that the total concentration of nickel, aluminum, and manganese was 2 mol / L to prepare raw material solutions.

[0069] The raw material solution was added to the reaction solution in the tank at a rate of 100 mL / min, and simultaneously, the alkaline solution and ammonium ion donor were also added to the reaction solution at a constant rate. Nucleation was carried out by carrying out crystallization for 1 minute while maintaining the pH of the reaction solution at 12.8 (nucleation step pH) and the ammonium ion concentration at 10 g / L. After temporarily stopping the supply of the raw material solution, alkaline solution, and ammonium ion donor, sulfuric acid was added until the pH of the reaction solution reached 11.6 (particle growth step pH). After the pH reached 11.6, the supply of the raw material solution, alkaline solution, and ammonium ion donor was resumed. While maintaining the pH at 11.6 and the ammonium ion concentration at 10 g / L, crystallization was continued for 4 hours to carry out particle growth, resulting in the production of a solid-structured nickel aluminum manganese composite hydroxide (metal composite hydroxide), Ni ０．８４ Al ０．１ Mn ０．０６ (OH) ２ A slurry containing

[0070] The obtained slurry containing the metal composite hydroxide (before washing) was charged into a filter press and pressure-filtered to recover a metal composite hydroxide cake. This metal composite hydroxide cake was returned to the reaction tank, and the tank was filled with an alkaline washing solution and stirred for 30 minutes to perform alkali washing. After that, pressure filtering was again performed using the filter press to recover the alkali-washed cake. This alkali-washed cake was returned to the reaction tank, and the tank was filled with water and stirred for 30 minutes to perform water washing. After that, pressure filtering was again performed using the filter press to recover the water-washed cake (washed cake). The recovered water-washed cake was dried at 150°C for 5 hours using an electric heating dryer to obtain a metal composite hydroxide (precursor).

[0071] (2) Production of Metal Composite Oxide (Intermediate) The obtained metal composite hydroxide (precursor) was calcined by heating in an air stream (oxygen concentration: 21% by volume) at 400°C for 2 hours using an electric furnace, and then oxidized and roasted by heating in an air stream (oxygen concentration: 21% by volume) at 550°C for 5 hours to produce a solid-structure nickel aluminum manganese composite oxide (metal composite oxide), Ni ０．８４ Al ０．１ Mn ０．０６ I got O.

[0072] (3) Production of Lithium Metal Composite Oxide (Base Material) The obtained metal composite oxide (intermediate) and lithium hydroxide, a lithium compound, were weighed out so that the ratio (Li / Me), which is the ratio of the number of lithium atoms (Li) to the number of metal atoms other than lithium (Me), was 1.03, and then thoroughly mixed to obtain a lithium mixture. This lithium mixture was calcined by heating at 500°C for 6 hours in an oxygen (oxygen concentration: 100% by volume) stream using an electric furnace, and then calcined by heating at 700°C for 8 hours in an oxygen (oxygen concentration: 100% by volume) stream to obtain a solid-structure lithium nickel aluminum manganese composite oxide (base material), Li １．０１ Ni ０．８４ Al ０．１ Mn ０．０６ O ２ obtained.

[0073] (4) Production of Coated Lithium Metal Composite Oxide (Positive Electrode Active Material) Cobalt hydroxide, a cobalt compound, was added to the obtained lithium metal composite oxide (base material) in an amount equivalent to 0.5 atomic % of cobalt, and then mixed using a mixer for 1 hour. The resulting transition metal mixture was then heat-treated in an electric furnace at 700°C for 6 hours in an air stream (oxygen concentration: 21% by volume).

[0074] Thereafter, the aggregates contained in the obtained heat-treated product are crushed to finally obtain a solid-structure coated lithium nickel aluminum manganese cobalt composite oxide (positive electrode active material), Li １．０１ Ni ０．８４ Al ０．１ Mn ０．０６ Co ０．００５ O ２ The evaluation results of the obtained coated lithium metal composite oxide are shown in Table 1.

[0075] The same procedure as in Example 1 was carried out, except that cobalt hydroxide, a cobalt compound, was added in an amount equivalent to 1 atomic % of cobalt. Finally, a solid-structure coated lithium nickel aluminum manganese cobalt composite oxide (positive electrode active material), Li １．０１ Ni ０．８４ Al ０．１ Mn ０．０６ Co ０．０１ O ２ obtained.

[0076] The same procedure as in Example 1 was carried out, except that cobalt hydroxide, a cobalt compound, was added in an amount equivalent to 1.5 atomic % of cobalt. Finally, a solid-structure coated lithium nickel aluminum manganese cobalt composite oxide (positive electrode active material), Li １．０１ Ni ０．８４ Al ０．１ Mn ０．０６ Co ０．０１５ O ２ obtained.

[0077] The same procedure as in Example 1 was carried out, except that cobalt hydroxide, a cobalt compound, was added in an amount equivalent to 2 atomic % of cobalt. Finally, a solid-structure coated lithium nickel aluminum manganese cobalt composite oxide (positive electrode active material), Li １．０１ Ni ０．８４ Al ０．１ Mn ０．０６ Co ０．０２ O ２ obtained.

[0078] The same procedure as in Example 1 was carried out, except that cobalt hydroxide, a cobalt compound, was added in an amount equivalent to 0.42 atomic % of cobalt. Finally, a solid-structure coated lithium nickel aluminum manganese cobalt composite oxide (positive electrode active material), Li １．０１ Ni ０．８４ Al ０．１ Mn ０．０６ Co ０．００４２ O ２ obtained.

[0079] The same procedure as in Example 1 was carried out, except that cobalt hydroxide, a cobalt compound, was added in an amount equivalent to 2.2 atomic % of cobalt. Finally, a solid-structure coated lithium nickel aluminum manganese cobalt composite oxide (positive electrode active material), Li １．０１ Ni ０．８４ Al ０．１ Mn ０．０６ Co ０．０２２ O ２ obtained.

[0080] The same procedure as in Example 1 was carried out, except that nickel hydroxide, a nickel compound, was added in an amount equivalent to 0.42 atomic % of nickel. Finally, a solid-structure coated lithium nickel aluminum manganese composite oxide (positive electrode active material), Li １．０１ Ni ０．８４ Al ０．１ Mn ０．０６ Ni ０．００４２ O ２ obtained.

[0081] The same procedure as in Example 1 was carried out, except that nickel hydroxide, a nickel compound, was added in an amount equivalent to 2.2 atomic % of nickel. Finally, a solid-structure coated lithium nickel aluminum manganese composite oxide (positive electrode active material), Li １．０１ Ni ０．８４ Al ０．１ Mn ０．０６ Ni ０．０２２ O ２ obtained.

[0082] The same procedure as in Example 1 was carried out, except that cobalt hydroxide, a cobalt compound, was added in an amount equivalent to 0.25 atomic % of cobalt, and nickel hydroxide, a nickel compound, was added in an amount equivalent to 0.25 atomic % of nickel. Finally, a solid-structure coated lithium nickel aluminum manganese cobalt composite oxide (positive electrode active material), Li １．０１ Ni ０．８４ Al ０．１ Mn ０．０６ Ni ０．００２５ Co ０．００２５ O ２ obtained.

[0083] The same procedure as in Example 1 was carried out, except that cobalt hydroxide, a cobalt compound, was added in an amount equivalent to 2 atomic % of cobalt, and nickel hydroxide, a nickel compound, was added in an amount equivalent to 2 atomic % of nickel. Finally, a solid-structure coated lithium nickel aluminum manganese cobalt composite oxide (positive electrode active material), Li １．０１ Ni ０．８４ Al ０．１ Mn ０．０６ Ni ０．０２ Co ０．０２ O ２ obtained.

[0084] Comparative Example 1 The same procedure as in Example 1 was carried out, except that no cobalt compound and / or nickel compound was added and no heat treatment was performed. Finally, a solid-structure lithium nickel aluminum manganese composite oxide (positive electrode active material) was obtained. １．０１ Ni ０．８４ Al ０．１ Mn ０．０６ O２ obtained.

[0085] Comparative Example 2 First, 0.75 parts by mass of a mixture of cobalt nitrate hexahydrate and aluminum nitrate nonahydrate in a molar ratio of 3:1 was prepared, and then this mixture was added to 10 parts by mass of water and stirred at room temperature (25° C.) for 1 minute to prepare an aqueous solution. Next, 100 parts by mass of a lithium metal composite oxide (base material) was added to 90 parts by mass of water, and the prepared aqueous solution was added while stirring at room temperature for 10 minutes to form a precipitate.

[0086] Next, the obtained precipitate was filtered using a filter press, and the filtered product was dried using an electric heating dryer at 150°C for 15 hours to obtain a dried product. The obtained dried product was then heat-treated using an electric furnace in an oxygen atmosphere at 720°C for 5 hours to obtain a coated lithium metal composite oxide (cathode active material) with a solid structure. Except for this, the same procedure as in Example 1 was carried out. Finally, a coated lithium nickel aluminum manganese cobalt composite oxide (cathode active material) with a solid structure, Li １．０１ Ni ０．８４ Al ０．１ Mn ０．０６ Co ０．００９５ Al ０．００６９ O ２ obtained.

[0087] Comparative Example 3 The same procedure as in Comparative Example 2 was carried out, except that 2.3 parts by mass of a mixture of cobalt nitrate hexahydrate and aluminum nitrate nonahydrate in a molar ratio of 3:1 was prepared. Finally, a solid-structure coated lithium nickel aluminum manganese cobalt composite oxide (positive electrode active material) was obtained. １．０１ Ni ０．８４ Al ０．１ Mn ０．０６ Co ０．０２９２ Al ０．０２１３ O ２ obtained.

[0088] Comparative Example 4 The same procedure as in Comparative Example 2 was carried out, except that 7.5 parts by mass of a mixture of cobalt nitrate hexahydrate and aluminum nitrate nonahydrate in a molar ratio of 3:1 was prepared. Finally, a solid-structure coated lithium nickel aluminum manganese cobalt composite oxide (positive electrode active material) was obtained. １．０１ Ni ０．８４ Al ０．１ Mn ０．０６ Co ０．０９５３ Al ０．０６９４ O ２ obtained.

[0089]

[0090] [Overall Review] When the coated lithium metal composite oxides (positive electrode active materials) of Examples 1 to 10, which are within the scope of the present invention, were used in lithium ion secondary batteries, it was confirmed that they had the effect of improving initial discharge capacity, the effect of reducing reaction resistance, and the effect of improving cycle characteristics, in all cases, compared to those of Comparative Examples 1 to 4, which deviate from the scope of the present invention.

[0091] Thus, in the present invention, by disposing a coating layer containing a compound other than a spinel crystal structure, which contains at least one selected from lithium, cobalt, and nickel, on the surfaces of the primary particles and secondary particles of a lithium metal composite oxide, a network structure of the coating layer is formed in the cross section of the secondary particle, thereby reducing the water content in the coated lithium metal composite oxide (cathode active material) and significantly improving weather resistance. In other words, according to the present invention, it is possible to provide a cathode active material for a lithium ion secondary battery that has excellent weather resistance when used in a lithium ion secondary battery, and the above evaluation results can be said to be sufficient to support this.

[0092] In contrast, in the comparative examples of the prior art, no mesh structure of the coating layer was formed in the cross section of the secondary particles, and only a localized coating layer composed of a compound with a spinel crystal structure was observed in Comparative Examples 2 to 4. Therefore, the thickness of the coating layer in Comparative Examples 2 to 4 was only localized, and the reduction in moisture content was insufficient in all Comparative Examples. Furthermore, the technical scope of the present invention is not limited to the aspects described in the above embodiment. One or more of the requirements described in the above embodiment may be omitted. The requirements described in the above embodiment may be combined as appropriate. Furthermore, to the extent permitted by law, the contents of all documents cited in this specification are incorporated by reference and incorporated herein by reference.

[0093] 1 Secondary particle 2 Primary particle 3 Coating layer 4 Void CBA Coin cell (for evaluation) PE Positive electrode (electrode for evaluation) NE Negative electrode SE Separator GA Gasket WW Wave washer PC Positive electrode can NC Negative electrode can

Claims

1. A positive electrode active material for a lithium ion secondary battery, comprising a coated lithium metal composite oxide containing secondary particles formed by aggregation of primary particles, characterized in that the surfaces of the primary particles and the surfaces of the secondary particles have coating layers containing a compound other than a spinel crystal structure, which contains one or more elements selected from lithium, cobalt, and nickel; the secondary particles have a solid structure, the porosity of the secondary particles is 6% or less, and the cobalt content of the secondary particles is 0.6% by mass or less.

2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein a mesh structure of the coating layer is confirmed in the cross-sectional morphology of the secondary particles when observed by STEM-EDS.

3. The positive electrode active material for a lithium ion secondary battery according to claim 1 or 2, characterized in that the moisture content of the coated lithium metal composite oxide is 0.2 mass % or less.

4. The positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 3, characterized in that the powder pH of the coated lithium metal composite oxide is determined under the following measurement conditions and is 11.5 to 12.

5. (Note) <Measurement conditions for powder pH> Water is added to 5 g of an analytical sample to make 100 mL, and the pH is measured with a pH meter.

5. The coating layer is made of lithium cobalt oxide (LiCoO ２ 5. The positive electrode active material for a lithium ion secondary battery according to claim 1, further comprising:

6. The lithium cobalt oxide (LiCoO ２ 6. The positive electrode active material for a lithium ion secondary battery according to claim 5, wherein the above-mentioned cations are detected by an XRD method.

7. The coated lithium metal composite oxide has the general formula: Li ａ Ni １－ｘ－ｙ M1 ｘ M2 ｙ M3 ｚ O ２＋α 7. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein: (0.95≦a≦1.3, 0≦x≦0.2, 0≦y≦0.1, 0.7≦1−x−y≦1, 0<z≦0.05, −0.1≦α≦0.5, M1 is one or more selected from Al and Mn; M2 is one or more selected from Na, K, Ca, Mg, Sr, Ba, Ti, Zr, V, Nb, Ta, Cr, Mo, W, B, Si, P, and S; and M3 is one or more selected from Co and Ni).

8. A positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 7, characterized in that the ratio of the thickness of the coating layer to the average particle size MV of the secondary particles of the coated lithium metal composite oxide is 0.01 to 10%.

9. The positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 8, wherein the thickness of the coating layer is 40 to 300 nm.

10. A positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 9, characterized in that the lithium hydroxide content of the coated lithium metal composite oxide is 0.1 to 1.5 mass %.

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