JP2019140093A - Coated positive electrode active material for lithium ion secondary battery, and manufacturing and evaluation methods thereof - Google Patents

Abstract

To provide a coated positive electrode active material which is superior in weather resistance and long-term shelf life, which can suppress the gelation when kneading a positive electrode mixture paste, and which allows a satisfactorily high initial discharge capacity to be retained when being incorporated in a lithium ion secondary battery.SOLUTION: A coated positive electrode active material 10 for a lithium ion secondary battery comprises: particles 1 of a lithium metal composite oxide containing lithium, nickel, cobalt and aluminum and primarily including secondary particles having a porosity of less than 20%; and a coating layer 3 formed on the surface of each lithium metal composite oxide particle 1, made of an ion-conducting polymer containing electrically conductive particles and having an average thickness of 0.005-0.1 μm.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a coated positive electrode active material for a lithium ion secondary battery, a method for producing the same, and a method for evaluating the coated positive electrode active material.

  Lithium ion secondary batteries, which are charged and discharged by the movement of lithium ions between the positive and negative electrodes, have higher energy density than other secondary batteries, and have excellent electromotive force and long life. Because of its characteristics, it is a compact and lightweight high-capacity battery for portable electronic devices such as smartphones and tablet terminals, notebook computers, video cameras, and high-power batteries for hybrid and electric vehicles. As the demand grows, the uses are further expanding.

This lithium ion secondary battery generally has a structure in which a separator impregnated with an organic solvent electrolyte or a polymer electrolyte is disposed between a pair of positive and negative electrodes. Electrochemical energy can be obtained by utilizing the oxidation-reduction reaction during insertion and removal of. Therefore, a material capable of reversible insertion / extraction of lithium ions is used for the positive electrode and the negative electrode. For example, as the negative electrode active material constituting the negative electrode, a carbon-based material or metallic lithium is mainly used, and as the positive electrode active material constituting the positive electrode, lithium-cobalt composite oxide (LiCoO ₂ ) that is relatively easy to manufacture. And lithium / nickel composite oxide (LiNiO ₂ ) using nickel, which is cheaper than cobalt, and lithium / nickel / manganese / cobalt composite oxide (LiNi _1/3 Mn _1/3 Co _1/3 ) using manganese. O ₂ ) and lithium metal composite oxides such as lithium-manganese composite oxide (LiMn ₂ O ₄ ) are mainly used. For example, Patent Document 1 shows an example using a lithium / nickel / cobalt / aluminum composite oxide in which a part of nickel is replaced with cobalt or aluminum.

  Among these positive electrode active materials, the lithium / nickel / manganese / cobalt composite oxide and the lithium / nickel / cobalt / aluminum composite oxide of Patent Document 1 have good cycle characteristics in battery capacity, low resistance and high output. In addition, it is highly regarded as a material that can be obtained, and is also attracting attention as an on-vehicle power source because it is suitable for the power source of electric vehicles and hybrid vehicles with limited mounting space.

  As a general method for producing the lithium metal composite oxide used as the positive electrode active material, a metal composite hydroxide that is a precursor of the metal composite oxide is prepared by neutralization crystallization, and then this precursor is prepared. A lithium metal composite oxide is prepared by mixing a lithium compound such as lithium hydroxide or lithium carbonate serving as a lithium source and firing the resulting lithium metal mixture. Examples of the lithium metal composite oxide thus prepared include unreacted lithium hydroxide and lithium carbonate, lithium carbonate produced by carbonation of lithium hydroxide, and lithium sulfate produced from impurities derived from various raw materials (hereinafter referred to as “lithium sulfate”). These lithiums are also collectively referred to as “excess lithium”).

  In contrast to this lithium metal composite oxide containing surplus lithium, Patent Document 2 proposes a technique for removing surplus lithium by adding washing water to the synthesized lithium / nickel composite oxide and washing it with stirring. Has been. In Patent Document 3, the synthesized lithium / nickel composite oxide is gas-treated at an atmospheric temperature of 150 ° C. or lower using an atmospheric gas having a carbon dioxide concentration of 0.1% by volume or higher and a dew point of −15 ° C. or lower. Technology has been proposed.

Japanese Patent Laid-Open No. 05-242891 JP 2007-273108 A Japanese Patent Application Laid-Open No. 10-302779

  Among the above-mentioned surplus lithium, unreacted lithium hydroxide is a main cause of causing gelation of the positive electrode mixture paste when the positive electrode active material is kneaded as a positive electrode mixture paste, and the positive electrode active material is in a high temperature environment. When it is charged at the bottom, it can cause oxidative decomposition and gas generation. On the other hand, lithium carbonate produced by carbonation of the above lithium hydroxide and lithium sulfate caused by impurities derived from various raw materials hardly contribute to the charge / discharge reaction. Therefore, when the battery is constructed, the irreversible capacity of this positive electrode active material The negative electrode material corresponding to is extra used for batteries. As a result, the capacity per unit weight and unit volume of the battery as a whole is reduced. Furthermore, if the excess lithium increases, the salt concentration tends to increase during the preparation of the positive electrode mixture paste, and gelation may be promoted by increasing the viscosity of the paste.

  Although it seems that the above problem of excess lithium can be suppressed to some extent by the method of Patent Document 2, the positive electrode active material obtained by the method of Patent Document 2 exists in the vicinity of the surface of the lithium / nickel composite oxide particles. There is a risk that even lithium ions that are washed away are washed away during washing. This lithium ion is necessary for the intercalation and deintercalation reaction of the lithium ion generated at the positive electrode during charging / discharging of the lithium ion secondary battery. May cause problems. Moreover, the weather resistance as a positive electrode active material falls remarkably, and it becomes easy to extract lithium ion from the inside of the crystal | crystallization of the particle | grain surface under the influence of the carbon dioxide gas and water | moisture content in air | atmosphere at the time of the handling in an air atmosphere. Furthermore, when incorporated in a lithium ion secondary battery, the initial discharge capacity may be reduced.

  Moreover, since the technique of the above-mentioned Patent Document 3 is a method for enhancing the storage characteristics in a high-temperature environment by carbonating lithium hydroxide remaining on the surface of the lithium / nickel composite oxide particles into lithium carbonate, It is difficult to sufficiently remove unreacted lithium hydroxide, and there is no particular description regarding removal of lithium sulfate or the like generated from impurities derived from various raw materials. The present invention has been made in view of the problems of the lithium metal composite oxide of the above-described conventional lithium ion secondary battery, and is excellent in weather resistance and long-term storage properties and gelled at the time of kneading the positive electrode mixture paste. An object of the present invention is to provide a coated positive electrode active material for a lithium ion secondary battery that can be suppressed and can maintain an initial discharge capacity sufficiently high when incorporated in a lithium ion secondary battery.

  In order to achieve the above object, the present inventor has made extensive research on a lithium metal composite oxide used for a positive electrode active material for a lithium ion secondary battery and a manufacturing method thereof, and as a result, mainly composed of secondary particles. By forming a coating layer having a predetermined thickness on the surface of the lithium metal composite oxide particles using a fluidized bed fine particle coating method, the positive electrode active material can be provided with excellent weather resistance and long-term storage properties, and positive electrode It has been found that gelation at the time of kneading the composite paste can be suppressed, and further that the initial discharge capacity when incorporated in a lithium ion secondary battery can be maintained sufficiently high, and the present invention has been completed.

  That is, the coated positive electrode active material for a lithium ion secondary battery according to the present invention contains lithium, nickel, cobalt, and aluminum, and is composed mainly of secondary particles having a porosity of less than 20%. And a coating layer having an average thickness of 0.005 to 0.1 μm made of an ion conductive polymer including electron conductive particles formed on the surface of the lithium metal composite oxide particles. It is said.

  The method for producing a coated positive electrode active material for a lithium ion secondary battery according to the present invention comprises a lithium metal composite mainly composed of secondary particles containing lithium, nickel, cobalt, and aluminum and having a porosity of less than 20%. A firing step for forming oxide particles by firing, and a preparation step for preparing a slurry for coating particles by dispersing electron conductive particles in an ion conductive polymer solution obtained by dissolving an ion conductive polymer in an organic solvent. And spraying the particle coating slurry while allowing the lithium metal composite oxide particles to flow in an air stream, and then applying the droplets to the surface of the lithium metal composite oxide particles by air drying. And a coating step of forming a coating layer having an average thickness of 0.005 to 0.1 μm made of an ion conductive polymer containing particles.

  According to the present invention, excellent weather resistance and long-term storage stability can be imparted to the positive electrode active material, and gelation can be suppressed during kneading of the positive electrode mixture paste, and further, the initial stage when incorporated in a lithium ion secondary battery. The discharge capacity can be maintained sufficiently high.

It is typical sectional drawing of the covering positive electrode active material of embodiment of this invention. It is a cross-sectional SEM image of the lithium metal complex oxide particle used as the raw material of the covering positive electrode active material of embodiment of this invention. It is a cross-sectional TEM image which partially shows the coating layer of the covering positive electrode active material of embodiment of this invention. It is a cross-sectional SEM image of the metal composite hydroxide particle | grains of the precursor of the metal composite oxide used as the raw material of the covering positive electrode active material of embodiment of this invention.

  Hereinafter, a coated positive electrode active material for a lithium ion secondary battery according to an embodiment of the present invention, a method for producing the same, and a lithium ion secondary battery using the coated positive electrode active material are described in “1. Coating for Lithium Ion Secondary Battery”. The positive electrode active material ”,“ 2. Production method of coated positive electrode active material for lithium ion secondary battery ”, and“ 3. Lithium ion secondary battery ”will be described in detail in this order. The coated positive electrode active material for a lithium ion secondary battery and the method for producing the same of the present invention are not limited to the following contents, and various modifications and alternative examples are possible without departing from the spirit of the present invention. Is included.

1. Coated positive electrode active material for lithium ion secondary battery 1.1 Structure and features Coated positive electrode active material for lithium ion secondary battery according to an embodiment of the present invention includes primary particles including lithium, nickel, cobalt, and aluminum. Lithium metal composite oxide particles mainly composed of secondary particles in which particles are aggregated, and an ion conductive polymer coating layer including electron conductive particles formed on the surface of the lithium metal composite oxide particles . In the coated positive electrode active material for a lithium ion secondary battery according to an embodiment of the present invention, the inside of the secondary particles has a porosity of less than 20%. That is, as shown in the schematic cross-sectional view of FIG. 1, the coated positive electrode active material 10 for a lithium ion secondary battery according to the embodiment of the present invention is such that the lithium metal composite oxide particles 1 are secondary particles. In the interior, one or a plurality of voids 2 exist within a range of a porosity of less than 20% as a whole. A coating layer 3 having an average thickness of 0.005 to 0.1 μm made of an ion conductive polymer containing electron conductive particles is formed on the surface of the lithium metal composite oxide particle 1 mainly composed of the secondary particles. Is formed.

  Since the coated cathode active material for a lithium ion secondary battery according to the embodiment of the present invention has an average thickness of 0.005 to 0.1 μm as described above, the coated cathode active material is When a positive electrode mixture paste is produced by kneading with a conductive material or a binder, an increase in viscosity that can be caused by dissolving a coating layer having a thickening action in the paste can be minimized. As described above, since the coated cathode active material of the embodiment of the present invention has the average thickness of the coating layer adjusted to an optimum range, when the coated cathode active material is handled in the atmosphere, moisture, carbon dioxide gas, etc. It is possible to suppress the induction of gelation due to an increase in paste viscosity at the time of kneading the positive electrode mixture paste, while preventing adverse effects due to. Next, the lithium metal composite oxide particles and the coating layer constituting the coated positive electrode active material of the embodiment of the present invention will be specifically described.

1.2 Lithium metal composite oxide particles [Composition]
Lithium-metal composite oxide particles constituting the coated positive electrode active material of embodiments of the present invention preferably have the general _{formula: Li a (Ni 1-x} -y Co x Al y) have a composition represented by _{O 2} A, x, and y are 0.98 ≦ a ≦ 1.20, 0.05 ≦ x ≦ 0.35, and 0.01 ≦ y ≦ 0.20, respectively, and x + y <0.40. It is.

  In the above general formula, x indicating the cobalt content is in the range of 0.05 to 0.35, more preferably in the range of 0.07 to 0.25, and most preferably in the range of 0.10 to 0.20. Within the range, the cycle characteristics can be improved, and the expansion and contraction behavior of the crystal lattice due to the insertion / extraction of lithium accompanying charging / discharging can be reduced. If the effect of the characteristics and expansion / contraction behavior cannot be obtained, and x exceeds 0.35, the amount of cobalt added is too large, resulting in a large decrease in the initial discharge capacity and a disadvantage in cost. Therefore, it is not preferable.

  In addition, when y indicating the aluminum content is in the range of 0.01 or more and 0.20 or less, when the lithium metal composite oxide particles having this composition are used as the positive electrode active material of the secondary battery, the durability is improved. Performance and safety can be improved, and capacity reduction can be suppressed. In particular, if aluminum is distributed almost uniformly in the inside of the particles, the above effect can be obtained over the entire particle, so that the above effect can be obtained even with a small addition amount. If this y is less than 0.01, the above effect cannot be obtained. Conversely, if this y exceeds 0.20, the amount of aluminum added becomes too large and contributes to the redox reaction (Redox reaction). This is not preferable because the metal element decreases and the battery capacity decreases. In addition, there is no limitation in particular in the analysis method regarding said composition, For example, it can obtain | require from the chemical analysis method by acid decomposition-ICP (inductively coupled plasma) emission spectroscopy analysis etc. In addition, the uniform distribution in the particle | grains of this aluminum is mentioned later, The molar ratio of sodium with respect to aluminum is 1.0-1.0 in the aluminum solution produced by adding sodium hydroxide to the solution which melt | dissolved sodium aluminate. It can adjust by producing so that it may become 3.0. When the amount of sodium, that is, the amount of sodium hydroxide is out of the range of 1.0 to 3.0 in terms of the above molar ratio, the stability of the aluminum solution decreases and immediately after being added to the reaction vessel, or Aluminum hydroxide tends to precipitate as fine particles before it is added. As a result, a coprecipitation reaction with nickel hydroxide and cobalt hydroxide hardly occurs, and a nickel-cobalt-aluminum composite hydroxide having a wide particle size distribution and a dispersed particle size is formed, and the dispersion of aluminum in the particles is not allowed. This is not preferable because problems such as uniformity may occur.

[Particle morphology and internal structure]
Most of the lithium metal composite oxide particles constituting the coated positive electrode active material of the embodiment of the present invention are in the form of secondary particles in which a plurality of primary particles are aggregated, but partially aggregated as secondary particles. Primary particles in a state of not being included may be included. The shape 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 a spherical shape, a plate shape, a needle shape, a rectangular parallelepiped shape, an elliptical shape, and a rhombohedral shape. . In addition, there are no particular limitations on the form of aggregation of the plurality of primary particles, and forms that aggregate in a random direction, or aggregate substantially radially from the center to form substantially spherical or elliptical secondary particles. It can take form.

  The lithium metal composite oxide particles of the embodiment of the present invention are excellent in particle strength because there are few voids present inside the secondary particles. In addition, the shape of the primary particles, the shape of the secondary particles, and the internal structure of the secondary particles can be grasped by observing the cross section of the particles using a scanning electron microscope (SEM), for example. can do. For example, FIG. 2 is a cross-sectional SEM image of a specific example of the secondary particles of the lithium metal composite oxide used in the coated positive electrode active material of the present invention, and it can be seen that the internal structure has few voids. In particular, the porosity measured in the cross section of the secondary particles is preferably less than 20%, and more preferably 5% or less. Thereby, the intensity | strength of the positive electrode active material formed from the lithium metal complex oxide particle can also be raised. In addition, a sufficient contact area between the positive electrode active material and the electrolytic solution can be secured without excessively reducing the bulk density of the positive electrode active material. This porosity can be determined by the following method. That is, after embedding a particle group of a lithium metal composite oxide to be measured in a resin, a cross section of the particle group is exposed by cutting with a cross section polisher (CP) and argon sputtering, and the cross section of the exposed particle group Are imaged using a scanning electron microscope. Then, by analyzing the cross-sectional image of the obtained particle group by image analysis software, the void portion is identified as a black region, and the dense portion is identified as a white region. The void ratio can be obtained by calculating “area of area / (area of black area + area of white area) × 100”.

[Volume average particle size (MV)]
The lithium metal composite oxide particles constituting the coated positive electrode active material of the embodiment of the present invention preferably have a volume average particle size (MV) of 3 to 20 μm, more preferably 3 to 15 μm. Most preferably, it is 12 μm. Thus, if the volume average particle size of the lithium metal composite oxide particles is in the range of 3 to 20 μm, a secondary battery incorporating a positive electrode active material produced by using the lithium metal composite oxide particles as a positive electrode is a battery per volume. The capacity can be increased, safety is increased, and cycle characteristics are improved.

  When the volume average particle size is less than 3 μm, there is a risk that when the positive electrode is produced, the packing density of the particles is reduced, and the battery capacity per volume of the positive electrode may be reduced. Conversely, the volume average particle size exceeds 20 μm. In addition, the specific surface area of the positive electrode active material is decreased, and the interface with the electrolyte of the secondary battery is decreased. As a result, the resistance of the positive electrode is increased and the output characteristics of the battery are decreased. In addition, the volume average particle diameter of the lithium metal composite oxide particles is adjusted by the supply amount and pH of the raw material solution in addition to the nucleation time in the crystallization process in the metal composite hydroxide production process described later. be able to. That is, when the volume average particle size is less than 3 μm, the supply amount of the raw material solution (metal compound) may be reduced to shorten the time for nucleation in the crystallization step, or the pH may be controlled to be lower. Thereby, since the production amount of the nucleus used as a seed of particles decreases, the particle size of the obtained metal composite hydroxide can be increased. Conversely, when the volume average particle diameter exceeds 20 μm, the supply amount of the raw material solution (metal compound) may be increased to extend the time for nucleation in the crystallization step, or the pH may be controlled to be higher. . Thereby, since the production amount of the nucleus used as a seed of particles increases, the particle size of the obtained metal composite hydroxide can be reduced. Moreover, said volume average particle diameter is calculated | required from the volume reference distribution measured using the laser diffraction and the scattering method.

[Specific surface area]
The specific surface area of the lithium metal composite oxide particles constituting the coated positive electrode active material of the embodiment of the present invention is not particularly limited, but is preferably in the range of 1.0 to 7.0 m ² / g. More preferably, it is in the range of 0.0 to 2.0 m ² / g. If this specific surface area is 1.0-7.0 m < ² > / g, the particle | grain contact surface which can contact electrolyte solution can fully be ensured. When the specific surface area is less than 1.0 m ² / g, the particle contact surface becomes too small, and a sufficient charge / discharge capacity may not be obtained. On the other hand, when the specific surface area exceeds 7.0 m ² / g, the particle contact surface becomes excessive, and the surface activity may be too high. In addition, there is no limitation in particular in the measuring method of said specific surface area, For example, it can obtain | require by the nitrogen gas adsorption / desorption method by a BET multipoint method or a BET 1 point method.

[Lithium hydroxide content and lithium carbonate content]
The lithium metal composite oxide particles constituting the coated positive electrode active material of the embodiment of the present invention preferably have an excess lithium lithium hydroxide content of 1.5% by mass or less, and 1.0% by mass or less. More preferably. When the lithium hydroxide content exceeds 1.5% by mass, as described later, the lithium metal composite oxide particles are coated with a coating layer of an ion conductive polymer including electronic conductive particles to produce a positive electrode active material. Later, when this is kneaded as a positive electrode mixture paste, gelation easily occurs. Further, when the positive electrode active material is charged in a high temperature environment, lithium hydroxide is oxidatively decomposed and gas is easily generated.

  The lithium metal composite oxide particles constituting the positive electrode active material of the embodiment of the present invention preferably have a lithium carbonate content of 1.5% by mass or less, which is other surplus lithium, and is 1.0% by mass. The following is more preferable. When the lithium carbonate content exceeds 1.5% by mass, when the positive electrode active material is charged in a high temperature environment, gas is likely to be generated as in the case of lithium hydroxide. When the lithium carbonate content in the lithium metal composite oxide particles is high, it can be said that the lithium hydroxide is carbonated. Therefore, the lithium metal composite oxide particles themselves also contain carbon dioxide gas in the atmosphere. There is a high possibility of being affected by moisture, and there is a concern that the positive electrode active material produced from this lithium metal composite oxide will deteriorate in weather resistance.

  The content of lithium hydroxide or lithium carbonate can be determined by a neutralization titration method. That is, lithium hydroxide and lithium carbonate as surplus lithium existing on the surface of the lithium metal composite oxide particles are dissolved in water, whereby hydroxide ions and carbonate ions are ionized from the lithium ions, respectively. Therefore, it is possible to separate and quantify lithium hydroxide and lithium carbonate by titrating these ionized anions with an inorganic acid or the like. This quantitative analysis method is sequential titration by the so-called RB Warder method, and the first end point (pH: about 8.3) is a change in pH when the total amount of lithium hydroxide reacts with half of the lithium carbonate. The second end point (pH: about 3.8) is the change in pH when half of the remaining lithium carbonate has reacted. In order to detect the pH inflection point (end point) at which pH change occurs, a method of visually determining the discoloration of an indicator such as phenolphthalein or methyl orange, or a method of reading the change in potential difference due to the pH composite electrode is used. It is done.

[moisture]
The lithium metal composite oxide particles constituting the coated positive electrode active material according to the embodiment of the present invention preferably have a water content of 0.10% by mass or less, and more preferably 0.05% by mass or less. The lithium metal composite oxide having a water content of more than 0.10% by mass is coated with an ion conductive polymer coating layer containing electron conductive particles to produce a positive electrode active material, and using this, a lithium ion secondary battery Incorporating into the metal layer, water dissolves in the electrolyte together with the coating layer and reacts with the electrolyte lithium hexafluorophosphate (lithium hexafluorophosphate: LiPF ₆ ) to generate hydrofluoric acid. May cause damage. In addition, dry weight method (also referred to as loss on drying method), Karl Fischer titration method, distillation method, etc. can be used for the above moisture measurement method, but when there is a large amount of sample that can be collected for analysis, It is preferable to use a dry weight method which is not affected by the coexisting elements.

1.3 Coating Layer The coating layer formed on the surface of the lithium metal composite oxide particles constituting the coated positive electrode active material of the above-described embodiment of the present invention has an average thickness of 0 in which electronic conductive particles are dispersed almost uniformly. It consists of 0.005-0.1 micrometer ion conductive polymer. The average thickness of the coating layer is preferably in the range of 0.01 to 0.08 μm. When the average thickness of the coating layer is greater than 0.1 μm, when the coated positive electrode active material is kneaded as a positive electrode mixture paste, the amount of the coating layer having a thickening action in the paste is excessively dissolved, The viscosity of the paste increases, and gelation is easily induced by the influence. On the other hand, when the average thickness is less than 0.005 μm, the effect of suppressing adverse effects due to moisture, carbon dioxide gas, etc. in the air atmosphere becomes insufficient when the coated positive electrode active material is handled in the air atmosphere. The covering layer preferably has an ionic conductivity (20 ° C.) of 10 ⁻⁹ to 10 ⁻³ S / cm, and an electronic conductivity (20 ° C.) of 10 ⁻⁸ to 10 ⁻¹ S / cm. Preferably there is. This ionic conductivity is determined by fixing both sides of the coating layer with a separator having ionic conductivity and non-electron conductivity so that only ions pass through, and then placing electrodes on the separators on both sides and setting a resistance value (separator It can be obtained by measuring the resistance value of only the blank test value). On the other hand, the electron conductivity can be obtained by performing the same operation as described above except that a separator having nonionic conductivity and electron conductivity is used and only the electrons pass.

[Ion conductive polymer]
The type of ion conductive polymer used for the coating layer is not particularly limited, but it should be one or more selected from polyethylene glycol (PEG), polyethylene oxide (PEO), derivatives or salts thereof. Is preferred. In addition to good ion conductivity, these polymer polymers are readily soluble in water, an organic solvent used as a raw material for the positive electrode mixture paste, and an electrolyte contained in the lithium ion secondary battery. Has a feature that it is difficult to absorb moisture and carbon dioxide. Therefore, the positive electrode active material coated with the coating layer made of this ion conductive polymer is excellent in weather resistance and hardly gels when the positive electrode mixture paste is kneaded.

[Electron conductive particles]
There are no particular limitations on the type of electron conductive particles contained in the ion conductive polymer, but carbon particles such as Denka Black, Ketjen Black, Acetylene Black, and Carbon Black are preferable. These carbon particles preferably have a volume average particle size (MV) measured by using a laser diffraction / scattering particle size distribution measuring apparatus of 0.01 to 0.06 μm. If the volume average particle diameter is within this range, the dispersibility when preparing the particle coating slurry by adding the carbon particles to the solution of the ion conductive polymer is improved.

  As a method for forming a coating layer from this particle coating slurry, a coating method using a general drying apparatus such as a granulation dryer or a spray dryer may be used, but a fluidized bed fine particle coating method is preferred. In this method, a coating material dissolved in a predetermined solvent and / or dispersed as fine particles is prepared, and this is sprayed onto the particles to be coated and brought into contact with hot air to dry the particles simultaneously. This is a method of forming a coating layer on the surface.

[Measurement method of average thickness]
The “average thickness” of the coating layer can be obtained by a cut image analysis using a scanning electron microscope (SEM) or a transmission electron microscope (TEM), and the number of particles used for measurement is 20 or more. It is preferable. For example, FIG. 3 shows an example of a cross-sectional TEM image obtained by partially imaging the coating layer of the coated positive electrode active material. In the coated positive electrode active material of the embodiment of the present invention, it is preferable that the entire surface of the lithium metal composite oxide particles is coated with the above coating layer (that is, the coverage is 100%). When it is as thin as about 0.005 μm, the coverage may not be 100%, and a coating layer may not exist locally. Even when the surface of the particle is partially exposed without the local presence of the coating layer, the area is narrower than the area covered with the coating layer, and is determined from the object of the present invention. Thus, the case where the particle surface is considered to be substantially the same as the case where the particle surface is entirely covered is also included in the scope of the present invention. The above-mentioned coverage can be adjusted within a range of 0 to 100% by changing the conditions when performing the fluidized bed fine particle coating method described later. The coverage in this case can be determined by a transmission electron microscope, X-ray photoelectron spectroscopy (XPS), or the like, and the number of particles used for measurement is preferably 20 or more. Next, an embodiment of a method for producing a coated positive electrode active material according to the present invention will be described.

2. Manufacturing method of coated positive electrode active material for lithium ion secondary battery 2.1 Manufacturing process of metal composite hydroxide [crystallization process]
The metal composite hydroxide is one of the two main raw materials of the positive electrode active material. In the preparation thereof, first, a nickel / cobalt mixed aqueous solution obtained by mixing a nickel compound and a cobalt compound is prepared as a raw material solution. These compounds are preferably sulfates and hydrates. Furthermore, an aluminum solution containing sodium aluminate and sodium hydroxide, an alkaline solution such as a sodium hydroxide aqueous solution for adjusting pH, and ammonia water for adjusting ammonium ion concentration (NH ₄ ⁺ ) are prepared. It is not preferable to use aluminum sulfate for this aluminum solution because aluminum hydroxide precipitates at a lower pH than nickel hydroxide or cobalt hydroxide when aluminum sulfate is used. This is because it is difficult to obtain a nickel / cobalt / aluminum composite hydroxide having a narrow particle size distribution and uniform particle size.

  Next, water is received in the reaction vessel, and the water is heated to control within a range of 40 to 60 ° C. The water is preferably pure water with controlled impurity concentration such as ion exchange water. While stirring this water with a stirrer, nitrogen gas is introduced from the upper part of the reaction tank, and the inside of the reaction tank is controlled to a non-oxidizing atmosphere having an oxygen concentration of 2.0% by volume or less. Further, the alkaline solution is supplied while controlling the amount of the solution supplied so that the pH of the water in the reaction vessel is within the range of 11.0 to 12.5, and the nickel / cobalt mixed solution, the aluminum solution, and Ammonia water is supplied at a constant flow rate. By preparing the reaction solution in this manner, the crystallization reaction can proceed efficiently.

  By maintaining the above crystallization reaction conditions for 4 hours, for example, a nickel / cobalt / aluminum composite hydroxide comprising secondary particles whose voids are adjusted to less than 20% as shown in the cross-sectional SEM image of FIG. Particles can be made. The reason why secondary particles of such a characteristic form are formed is that it is difficult to correctly measure the dissolved oxygen concentration with a dissolved oxygen meter or the like because the liquid phase portion has a high salt concentration due to the coexistence of heavy metals and alkalis. However, the inventor believes that the dissolved oxygen in the liquid phase changed under the influence of the oxygen concentration in the gas phase affects the morphology of the secondary particles. That is, as described above, the oxygen in the liquid phase part is dissolved in accordance with the fact that the atmosphere in the reaction tank during the crystallization reaction is an inert atmosphere or a non-oxidizing atmosphere in which the oxygen concentration is controlled to 2.0% by volume or less. Since the concentration can also be suppressed to a low concentration, it is considered that as a result, the aggregation state of the primary particles of the metal composite hydroxide is affected and the porosity of the secondary particles is controlled. As a result, the porosity of the nickel / cobalt / aluminum composite hydroxide particles can be controlled to be less than 20%, which is indirectly less than 20% of the lithium metal composite oxide produced in the subsequent process. Will be controlled. When the porosity of the metal composite hydroxide is 20% or more, it becomes difficult to maintain the strength of the secondary particles, and it becomes easy to be crushed in a subsequent process, and a positive electrode active material having desired characteristics cannot be obtained. There is a fear.

When the temperature of the reaction solution at the time of the crystallization reaction is lower than 40 ° C., the particle size of the primary particles of the metal composite hydroxide becomes too large. Conversely, when the temperature is higher than 60 ° C., the primary particles of the metal composite hydroxide This is not preferable because the particle size of the metal becomes too small, and its influence appears in the lithium metal composite oxide produced in the subsequent process. Furthermore, when the pH of the initial reaction solution is smaller than 11.0, the sulfate concentration remaining in the positive electrode active material is increased, and output characteristics when incorporated in a battery are deteriorated. Conversely, if the pH is higher than 12.5, the particle size of the primary particles becomes too small, which is not preferable. Moreover, the ammonium ion concentration (NH ₄ ⁺ ) of the reaction solution during the crystallization reaction is preferably in the range of 5 to 30 g / L, and more preferably in the range of 10 to 20 g / L. By controlling the ammonium ion concentration of the reaction solution within the range of 5 to 30 g / L, the crystallization reaction can be stabilized.

[Filtering / washing process and drying process]
Next, in the filtration / washing step, the slurry containing secondary particles of the metal composite hydroxide generated in the crystallization step is subjected to solid-liquid separation by filtration or the like, and the resulting solid metal composite hydroxide secondary The wet cake of particles is washed with an alkali solution such as a sodium hydroxide solution, whereby sulfate ions (SO ₄ ²⁻ ) and chloride ions (Cl ² ) that can be contained in the secondary particles of the metal composite hydroxide. ^- )) And the like, and rinse with pure water in which impurities such as ion exchange water are controlled to remove the remaining cleaning liquid used for alkali cleaning such as sodium ions (Na ⁺ ). Next, the washed metal composite hydroxide is put in a dryer and dried within a temperature range of 100 to 150 ° C. to obtain a metal composite hydroxide dry powder.

2.2 Lithium metal composite oxide production process [Lithium compound grinding process]
Next, a lithium compound serving as a lithium source which is the other of the two main raw materials of the positive electrode active material is prepared. This lithium compound is pulverized in the pulverization step to form a fine powder having many new surfaces with little surface degradation due to moisture absorption or the like. Thus, by making the lithium compound into a fine powder form, the contact surface and reactivity to the metal composite hydroxide are increased, and a positive electrode active comprising a lithium metal composite oxide produced through a mixing and firing process described later. The substance has good low-temperature output characteristics when incorporated into a secondary battery.

  The finely powdered lithium compound after the pulverization treatment preferably has a maximum particle size of 10.0 μm or less and a volume average particle size (MV) of 5.0 μm or less. If it is this particle size, it can produce by using common grinders, such as a jet mill and a ball mill. The lower limit of the particle size is not particularly limited, but the volume average particle size is about 0.1 μm in consideration of the performance of the above pulverizer. The maximum particle size and the volume average particle size are measured using the above-described laser diffraction / scattering particle size distribution measuring apparatus.

There is no particular limitation on the kind of the lithium compound, lithium carbonate _(Li 2 CO _3: mp 723 ° C.), lithium hydroxide (LiOH: mp 462 ° C.), lithium nitrate (LiNO _3: mp 261 ° C.), lithium chloride (LiCl: melting point 613 ° C.), lithium sulfate (Li ₂ SO ₄ : melting point 859 ° C.), and the like. Among these, lithium carbonate or lithium hydroxide is preferable from the viewpoint of easy handling and quality stability.

2.3 Mixing step and firing step In the mixing step, the fine powder lithium compound is mixed with the dry powder of the metal composite hydroxide. When these powders are mixed, the ratio of the lithium atom number Li of the lithium compound to the sum Me of nickel, cobalt, and aluminum atoms in the metal composite hydroxide (ie, Li / Me) is 1.00 to 1.20. The blending ratio of the finely powdered lithium compound and the dry powder of the metal composite hydroxide is adjusted so as to fall within the above range. When this ratio is smaller than 1.00, lithium atoms are less likely to be taken into the 3a site, which is the lithium site. Therefore, when the lithium metal composite oxide finally produced is incorporated into the battery as the positive electrode active material, Battery characteristics may not be obtained. Conversely, if the ratio is greater than 1.20, sintering is likely to be promoted, and the particle size and crystallite size may become too large, leading to deterioration of cycle characteristics.

Note that the above-mentioned site means a crystallographically equivalent lattice position. A state in which a certain atom exists at a lattice position is referred to as “site is occupied”, and the site is referred to as an occupied site. For example, LiCoO ₂ has three occupied sites, which are called lithium site, cobalt site, and oxygen site, respectively, and these occupied sites are also called 3a site, 3b site, and 6c site.

  In the firing step, a mixture of the fine powder lithium compound obtained in the mixing step and the dry powder of the metal composite hydroxide (hereinafter also referred to as a lithium metal mixture) is preferably 800 to 950 ° C. in an air atmosphere. By firing within the temperature range, a lithium metal composite oxide is produced. By firing within this temperature, the metal composite hydroxide changes into a metal composite oxide while maintaining the form of secondary particles having a substantially solid structure, and the molten lithium compound has a slight void in the secondary particles. Go inside. That is, even if this calcination treatment is performed, the characteristics of the form such as the solid structure of the metal composite hydroxide particles as the precursor are almost inherited to the lithium metal composite oxide.

  When the calcination temperature is less than 800 ° C., the reactivity decreases, the content of unreacted surplus lithium in the lithium metal composite oxide obtained by this calcination increases, the crystal structure is not sufficiently adjusted, the crystallinity deteriorates, etc. May occur and necessary lithium may be easily eluted from the positive electrode active material itself, so that desired battery characteristics may not be obtained. Conversely, when the firing temperature is higher than 950 ° C., intense sintering is likely to occur between the lithium metal composite oxide particles, which may cause abnormal particle growth.

  In addition, you may calcine a lithium metal mixture within the temperature range of 600-780 degreeC before performing this baking. By calcination at this temperature, the reaction in the subsequent baking proceeds more gently, so that the unreacted excess lithium can be reduced and the crystallinity can be improved in the resulting lithium metal composite oxide. In this calcination, the above calcination temperature is preferably held for 0.5 to 10 hours, more preferably 2 to 4 hours. The atmosphere at the time of calcination may be an air atmosphere or an oxidizing atmosphere, but the oxygen concentration of the atmosphere gas is preferably 18 to 100% by volume. In the case of performing calcination, it is not always necessary to cool to room temperature between the calcination and the subsequent baking, and the calcination may be performed by raising the temperature from the calcination temperature.

2.4 Deagglomeration / Crushing Process The lithium metal composite oxide powder obtained in the above firing process may have agglomerated secondary particles or mild sintering. Therefore, it is preferable to disintegrate and / or disintegrate these aggregates and sintered bodies of the lithium metal composite oxide by performing a disaggregation / disintegration step after the firing step. Thereby, the particle size and particle size distribution of the lithium metal composite oxide particles can be adjusted within a range suitable for the positive electrode active material. It should be noted that known agglomeration and crushing means such as a pin mill and a hammer mill can be used for the above-described agglomeration and crushing method of the aggregate and sintered body. In addition, it is preferable to appropriately adjust the pulverization / disintegration force to such an extent that the secondary particles are not broken at the time of the above pulverization / disintegration. Thus, it becomes possible by controlling the rotational speed of the pulverization / disintegration apparatus while confirming the particle diameter and particle size distribution.

2.5 Water Washing Process The lithium metal composite oxide powder treated in the above-mentioned deaggregation / cracking process is then washed and solid-liquid separated in the water washing process. The lithium metal composite oxide powder obtained in the above-described firing step can be used as a positive electrode active material as it is, but can be brought into contact with the electrolyte solution by removing excess lithium and fragile portions on the particle surface. The surface area increases and the charge / discharge capacity can be improved. In addition, since excess lithium can cause side reactions in the lithium ion secondary battery and may cause expansion of the battery due to gas generation, it is preferable to wash with water, but the necessary lithium present in the crystal lattice near the surface It is preferable to optimize the washing conditions so that even ions are not washed away during washing. Specifically, the slurry concentration at the time of washing with water is preferably such that the amount of the lithium metal composite oxide is 100 to 2000 g with respect to 1 L of water. If the slurry concentration exceeds 2000 g / L, the viscosity becomes very high and stirring becomes difficult, and the alkali concentration in the slurry becomes high, so that the removal rate of impurities becomes slow due to ion equilibrium. On the other hand, if the slurry concentration is less than 100 g / L, the amount of surplus lithium on the particle surface decreases, but since the necessary lithium desorption also occurs from the crystal lattice in the vicinity of the surface, the crystal structure tends to collapse, In the subsequent process, carbon dioxide in the atmosphere is absorbed and lithium carbonate is likely to be precipitated, which may deteriorate the weather resistance.

  The water used for the water washing is not particularly limited, but pure water with controlled impurities such as ion exchange water is preferable. By using pure water, it is possible to prevent a decrease in battery performance due to residual impurities. For the solid-liquid separation of the slurry performed after the water washing treatment, a commonly used solid-liquid separation apparatus can be used, and for example, a suction filter, a filter press, a centrifuge, or the like can be used. In addition, after solid-liquid separation of the slurry using a solid-liquid separation device, it is preferable that the amount of adhering water remaining on the surface of the separated particles is as small as possible. When the amount of adhering water increases, the load in the next drying step becomes too high, and the water present on the surface of the lithium metal composite oxide particles may exceed an allowable level.

2.6 Drying Step The wet cake of lithium metal composite oxide obtained by solid-liquid separation after water washing in the water washing step is then dried in the drying step. Although the drying temperature in that case does not have limitation in particular, 80-350 degreeC is preferable. If this temperature is less than 80 ° C., the drying rate becomes too slow, and a gradient of lithium concentration occurs between the particle surface and inside the particle, which may deteriorate the battery characteristics. On the other hand, near the particle surface, it is expected to be very close to the stoichiometric ratio or slightly desorbed lithium and close to the charged state. It may cause the structure to collapse and the battery characteristics may deteriorate.

2.7 Coating process [Ion conductive polymer solution]
The lithium metal composite oxide powder dried in the drying step is then subjected to a coating treatment with a particle coating slurry in the coating step. This particle coating slurry is composed of an ion conductive polymer solution and electron conductive particles. The former ion conductive polymer solution is obtained by dissolving an ion conductive polymer, which is a raw material for forming a coating layer, in an organic solvent. In the ion conductive polymer solution, the concentration of the ion conductive polymer is preferably 0.5 to 30% by mass, and more preferably 5 to 15% by mass. By setting this concentration to 0.5 to 30% by mass, the coating layer thickness can be accurately adjusted.

  As described above, the ion conductive polymer is preferably at least one selected from polyethylene glycol (PEG), polyethylene oxide (PEO), derivatives or salts thereof. Organic solvents used to dissolve this ion-conducting polymer include acetonitrile, ethylene chloride, chloroform, trichloroethylene, methylene chloride, 1,4-dioxane, aromatic hydrocarbons, ethanol, and other solvents such as methanol in aliphatic hydrocarbons. What added the adjuvant can be used. The organic solvent described above preferably has a moisture content of 0.005% by mass or less, and more preferably 0.001% by mass or less, in order to eliminate as much as possible moisture from being mixed into the coated particles. .

[Slurry for particle coating]
A particle coating slurry can be prepared by adding electronically conductive particles to the above ion conductive polymer solution and uniformly dispersing the particles. There is no particular limitation on the dispersion method of the electronic conductive particles, and it is preferable to ultrasonically disperse using an ultrasonic generator such as a homogenizer. As described above, the electron conductive particles are preferably carbon particles such as Denka black, Ketjen black, acetylene black, and carbon black. These carbon particles preferably have a volume average particle size (MV) of 0.01 to 0.06 μm. Moreover, it is preferable that 1-50 mass parts of electronic conductive particles are contained with respect to 100 mass parts of ion conductive polymers, and it is more preferable that 5-30 mass parts is contained. When the content is less than 1 part by mass, sufficient electronic conductivity is difficult to be obtained. Conversely, when it exceeds 50 parts by mass, the dispersibility in producing the particle coating slurry may deteriorate.

[Coating treatment]
As described above, the fluidized bed fine particle coating method is preferably used for the coating treatment. In the fluidized bed fine particle coating method, a coating material is dissolved in a predetermined solvent, and a slurry in which fine particles are dispersed in the solution is made into a slurry. A coating layer containing fine particles can be formed. In this fluidized bed fine particle coating method, it is preferable to use a multi-flow type spray nozzle. Thus, by repeating solution spraying and drying on the target particles, it is possible to maintain a uniform layer thickness even when a thin layer of several nm level is coated. It is also possible to coat only the fine particles on the target particles by making the fine particles dispersed in a predetermined solvent, spraying the fine particles onto the target particles, and passing the hot air to dry and volatilize the solvent.

  In the embodiment of the present invention described above, the slurry sprayed and dried by the fluidized bed fine particle coating method is a particle coating slurry, and the can body is used to make the lithium metal composite oxide particles that are the target particles into a fluid state. An air flow is generated in the container, and a high dispersion rotor such as a blade type or a flat type is rotated. There are no particular limitations on the airflow generation conditions and the rotation conditions of the highly dispersed rotor in this case, and they are adjusted as appropriate according to the coating conditions. In addition, the airflow temperature in the can container is preferably 50 ° C. or less, more preferably 40 ° C. or less in consideration of the melting point of the ion conductive polymer. Examples of the apparatus for performing the fluidized bed fine particle coating method include a rolling fluidized coating apparatus MP-01 (manufactured by POWREC Co., Ltd.) and a FLOW-COATER (flow coater) series (manufactured by Freund Sangyo Co., Ltd.). Can do.

  As described above, the coated positive electrode active material obtained by coating the lithium metal composite oxide particles with the coating layer of the ion conductive polymer containing the electron conductive particles is carbon dioxide gas or moisture when handled in the atmosphere. The problem of weather resistance that occurs due to the influence of, such as lithium ion deficiency inside the crystal of the particle, and the initial discharge capacity when incorporated in a lithium ion secondary battery is less likely to occur, and in an atmospheric environment A positive electrode active material that can be stored for a long time and has excellent durability can be provided.

  In addition, the ion conductive polymer constituting the coating layer is readily soluble in water, the organic solvent contained in the positive electrode mixture paste, and the electrolyte contained in the lithium ion secondary battery. In the process of manufacturing a lithium ion secondary battery, the electrical characteristics of the positive electrode active material are not impaired because it is dissolved in an organic solvent or an electrolytic solution. Furthermore, since the electron conductive particles constituting the coating layer are originally used as a conductive material and the ion conductive polymer is used as a polymer electrolyte, even if the coating layer is dissolved, the constituent material does not adversely affect the battery characteristics. . For example, polyethylene glycol 20000, which is an ion conductive polymer, not only dissolves in water without causing pH fluctuation at the time of dissolution, but also N-methyl-2-pyrrolidone, which is a polar organic solvent often used as a material for positive electrode mixture paste. It is also soluble in (NMP) and dimethyl carbonate (DMC), which is one of esters used as a non-aqueous electrolyte. Furthermore, when the coated positive electrode active material is sampled and subjected to various analyzes such as composition, lithium hydroxide content, lithium carbonate content, etc., the pretreatment of the analysis is hindered because the coating layer is easily dissolved in water. Absent.

  As described above, in the embodiment of the present invention, in the process of manufacturing the lithium ion secondary battery through the production of the positive electrode mixture paste, a material that is easily dissolved in an organic solvent or an electrolytic solution and that can be dissolved is used. The lithium metal composite oxide is coated. Thereby, when the positive electrode active material is used as a lithium ion secondary battery through the improvement of the weather resistance and durability when in the form of powder before being produced as a positive electrode composite paste, and the production of the positive electrode mixture paste To improve the electrical characteristics of In addition, it is desirable that the above coating layer is almost completely dissolved in an organic solvent or an electrolytic solution during the manufacturing process of the lithium ion secondary battery. Since the conductive particles are uniformly dispersed, there is almost no problem with respect to both insertion / extraction of lithium ions and electronic conductivity. Furthermore, since the production method of the present invention can produce the above positive electrode active material easily and efficiently, and is particularly suitable for mass production on an industrial scale, its industrial significance is extremely large.

3. Lithium ion secondary battery The above-described coated positive electrode active material for a lithium ion secondary battery according to an embodiment of the present invention is a general lithium ion secondary composed of a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. A lithium ion secondary battery can be produced in the same manner as when producing a battery. Hereinafter, each component of the lithium ion secondary battery will be described.

3.1 Positive electrode In the production of the positive electrode, the above-described coated positive electrode active material for a lithium ion secondary battery, a conductive material, and a binder are mixed to form a positive electrode mixture, and the capacity of the electric double layer is further increased. Therefore, after adding activated carbon as needed, an organic solvent for viscosity adjustment and the like is added and further kneaded to prepare a positive electrode mixture paste. Even when the coated positive electrode active material of the embodiment of the present invention is used for the positive electrode, the mixing ratio of each material constituting the positive electrode mixture is the performance of the lithium ion secondary battery as in the case of the positive electrode of a general lithium secondary battery. It becomes an important factor to decide. The mixing ratio of each material in the positive electrode mixture is, for example, 60 to 95% by mass of the positive electrode active material and 1 to 20% of the conductive material with respect to the total mass of 100% by mass in the solid content of the positive electrode mixture excluding the organic solvent. It is preferable to blend so as to contain 1% by mass to 1% by mass of a binder and a binder (binder).

  The viscosity of the positive electrode mixture paste can be measured using a vibration viscometer. In this vibration type viscometer, when a vibrator is resonated in a fluid with a certain amplitude and frequency, there is a correlation between the excitation force required for the vibrator to move and the viscous resistance of the fluid. As can be seen, this is a device for measuring the viscosity using this correlation, and can measure a wide range of fluids from a low-viscosity fluid such as water to a high-viscosity fluid of the 10,000 mPa · s level.

  In an embodiment of the present invention, the solid content of the above-described coated positive electrode active material, conductive material, and binder and the organic solvent are expressed as “solid content (g) / organic solvent, which is the mass ratio of the solid content to these organic solvents. A positive electrode mixture paste was prepared by blending so that the ratio of (g) ”was 1.875, and 10 g or more of this positive electrode mixture paste was sampled in a predetermined container, and the fluid temperature was 20 ° C. in a water bath. The viscosity is measured with the vibration viscometer described above. The vibration type viscometer was calibrated using JS-200 (viscosity at 20 ° C .: 170 mPa · s) and JS-2000 (viscosity standard solutions defined in JIS-Z-8809). A two-point calibration using a viscosity at 20 ° C .: 1800 mPa · s is desirable. As described above, by using the positive electrode mixture paste having a mass ratio of “1.875”, the secondary battery is produced through the positive electrode mixture paste using the coated positive electrode active material of the embodiment of the present invention. The presence or absence of the gelation problem to be obtained can be effectively evaluated.

  The obtained positive electrode mixture paste is applied to the surface of a current collector made of aluminum foil and dried to volatilize (evaporate) the organic solvent. In order to increase the electrode density of the sheet-like positive electrode thus produced, after pressurizing with a roll press or the like as necessary, the sheet-like positive electrode has a desired shape by further cutting to an appropriate size as necessary. It can be incorporated in a secondary battery as a positive electrode. Note that the method for manufacturing the positive electrode is not limited to the above method, and other methods may be used.

  As the conductive material constituting the positive electrode mixture, any of graphite such as natural graphite, artificial graphite, and expanded graphite, and carbon black materials such as acetylene black and ketjen black can be used. . In addition, the binder that plays a role in binding the particles of the positive electrode active material includes fluororesins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), ethylene propylene diene rubber (EPDM), fluorine A thermoplastic resin such as rubber, styrene butadiene, cellulose resin, polyacrylic acid, polypropylene (PP), or polyethylene (PE) can be used. Use N-methyl-2-pyrrolidone (NMP) or the like as an organic solvent to disperse the positive electrode active material, conductive material, and activated carbon and add the binder to form the positive electrode mixture paste. Can do.

3.2 Negative electrode The negative electrode facing the positive electrode is made into a paste by adding an appropriate organic solvent to the negative electrode mixture obtained by mixing a negative electrode active material capable of occluding and desorbing lithium ions with a binder. This can be produced by applying the material to a surface of a current collector of a metal foil such as copper and drying it, followed by press forming with a roll press or the like as necessary to increase the electrode density. As the negative electrode active material, in addition to lithium metal and lithium alloy, an organic compound fired body such as natural graphite, artificial graphite, and phenol resin, and a powdery body of carbon material such as coke can be used. Further, as the negative electrode binder, a fluorine-containing resin such as polyvinylidene fluoride (PVDF) is used in the same manner as the positive electrode. As the organic solvent for dispersing the negative electrode active material and the binder, N- Methyl-2-pyrrolidone (NMP) or the like can be used.

3.3 Separator The separator interposed between the positive electrode and the negative electrode plays a role of separating the positive electrode and the negative electrode to hold the electrolyte, and is made of a thin film made of polyethylene or polypropylene. In addition, those having innumerable fine holes are preferably used.

3.4 Nonaqueous Electrolytic Solution A nonaqueous electrolytic solution in which a lithium salt as a supporting salt is dissolved in an organic solvent is preferably used. Examples of the organic solvent include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and trifluoropropylene carbonate (TFPC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl In addition to chain carbonates such as methyl carbonate (EMC) and dipropyl carbonate (DPC), ether compounds such as tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), and dimethoxyethane (DME), ethyl methyl sulfone And a mixture of one or more selected from the group consisting of sulfur compounds such as butane sultone, phosphorus compounds such as triethyl phosphate, and trioctyl phosphate can be used.

In addition to lithium hexafluorophosphate (LiPF ₆ ), the above supporting salts include lithium tetrafluoroborate (LiBF ₄ ), lithium perchlorate (LiClO ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), lithium bis Lithium salts such as (trifluoromethanesulfonyl) imide (LiN (CF ₃ SO ₂ ) ₂ ) and complex salts thereof can be used. Note that the non-aqueous electrolyte may contain a radical scavenger, a surfactant, a flame retardant, and the like.

3.5 Battery Shape and Configuration Lithium ion secondary batteries having various shapes such as a cylindrical type and a stacked type are accommodated in a sealed battery case containing the positive electrode, negative electrode, separator, and non-aqueous electrolyte described above. Can be produced. In any case, the electrode body has a basic structure in which a non-aqueous electrolyte solution is impregnated into an electrode body having a laminated structure in which a positive electrode and a negative electrode face each other with a separator interposed therebetween. The current collector on the positive electrode side and the positive electrode terminal communicating with the outside, and the current collector on the negative electrode side and the negative electrode terminal communicating with the outside are electrically connected by a current collecting lead or the like, respectively. EXAMPLES Next, although an Example and a comparative example are given and this invention is demonstrated more concretely, this invention is not limited to the following Example at all.

[Example 1]
Lithium metal composite oxidation by the crystallization process and washing process of the metal composite hydroxide, the pulverization process of the lithium compound, the mixing process and calcining / firing process of the lithium metal mixture, and the decoagulation / disintegration process After the product was manufactured and evaluated from the viewpoint of composition and the like, a lithium ion secondary battery was manufactured from the lithium metal composite oxide through the coated positive electrode active material, and the initial discharge capacity was evaluated.

1. Preparation and evaluation of lithium metal composite oxide [crystallization process of metal composite hydroxide]
First, a raw material solution in which nickel sulfate hexahydrate and cobalt sulfate heptahydrate are dissolved in water, and an aluminum solution prepared by adding sodium hydroxide to a solution in which sodium aluminate is dissolved. The mixture was prepared so that the molar ratio of nickel, cobalt, and aluminum when Ni was mixed was Ni: Co: Al = 82: 15: 3. On the other hand, 900 ml of water is put into a 6 L reaction tank up to the overflow, the temperature of the liquid is heated to 40 ° C. using a water bath, and nitrogen gas is introduced into the reaction tank at a rate of 3 L / min. Was adjusted to a non-oxidizing atmosphere having an oxygen concentration of 0.1% by volume.

  Next, while stirring the water in the reaction vessel, the raw material solution adjusted above, aluminum solution, ammonia water having a concentration of 25% by mass, and sodium hydroxide aqueous solution having a concentration of 25% by mass are continuously supplied to form a metal. A crystallization reaction of the composite hydroxide was performed. During the crystallization reaction, the supply amount of the sodium hydroxide aqueous solution is adjusted so that the pH of the reaction solution is 11.7 on the basis of 25 ° C., and the ammonium ion concentration of the reaction solution is 10.0 g / L. The supply amount of the ammonia water was adjusted so as to be maintained.

The pH and ammonium ion concentration of the above reaction solution are the Orion-Star manufactured by Thermo Fisher Scientific Co., Ltd., which is a pH / ammonia measurement kit (combined machine of pH meter and ammonium ion meter) installed in the reaction tank in advance. -A214 was used for each measurement. The crystallization treatment was performed for 4 hours while maintaining the above reaction conditions, and then the resulting slurry after crystallization treatment was subjected to solid-liquid separation to obtain a nickel / cobalt / aluminum composite hydroxide (metal composite hydroxide). Ni _0.82 Co _0.15 Al _0.03 (OH) ₂ was obtained.

[Cleaning process of metal composite hydroxide]
The metal composite hydroxide obtained in the crystallization step is re-slurried by adding ion-exchange water so that the slurry concentration becomes 100 g / L, and further added with a sodium hydroxide solution and stirred for 30 minutes. The composite hydroxide was alkali washed. After the alkali cleaning, the metal composite hydroxide slurry was subjected to solid-liquid separation with a suction filter, and the resulting metal composite hydroxide wet cake was rinsed with pure water, followed by solid-liquid separation by filtration. . The washed wet cake obtained was dried for 24 hours at 120 ° C. using an air dryer to obtain a dry powder of metal composite hydroxide.

[Lithium compound grinding process]
A lithium compound as a lithium source of the lithium metal composite oxide is charged into a jet mill (manufactured by Seishin Enterprise Co., Ltd.), and becomes a fine powder having a maximum particle size of 10.0 μm or less and a volume average particle size of 5.0 μm or less. Until pulverization. For measurement of the particle size distribution of the fine powder lithium compound, Microtrac MT3300EX-II (manufactured by Microtrac Bell Co., Ltd.), which is a laser diffraction / scattering particle size distribution measuring apparatus, was used.

[Mixing process of lithium metal mixture]
Weigh and mix each so that Li / Me, which is the ratio of the lithium atom number Li of the fine powder lithium compound to the total atom number Me of nickel, cobalt, and aluminum of the above metal composite hydroxide, becomes 1.01. A lithium metal mixture was obtained. In this mixing, a TURBULA-Type T2C (manufactured by Willy et Bacofen (WAB)), which is a shaker mixer device, was used.

[Calcination and firing process]
The lithium metal mixture obtained in the above mixing step was calcined in an air stream at 760 ° C. for 4 hours and then calcined in an air stream at 900 ° C. for 10 hours. After firing, particles of lithium metal composite oxide were obtained by cooling to room temperature.

[Deaggregation and crushing process of lithium metal composite oxide]
The cooled lithium metal composite oxide particles obtained in the calcination / firing step described above include aggregates and sintered products formed by secondary particles agglomerating or slightly sintered. Therefore, crushing and crushing treatment was performed by a jet mill (manufactured by Seishin Enterprise Co., Ltd.). The composition, porosity, volume average particle diameter, specific surface area, lithium hydroxide content and lithium carbonate content, and water content of the lithium metal composite oxide particles obtained above were evaluated by the following methods.

(1) Composition The lithium metal composite oxide particles obtained above are sampled and thermally decomposed with an inorganic acid to obtain an analytical sample liquid. This analytical sample liquid is ICPE-9000, which is a multi-type ICP emission spectroscopic analyzer. The composition of the lithium metal composite oxide was determined by measurement using (manufactured by Shimadzu Corporation).

(2) Porosity The lithium metal composite oxide particles obtained above are sampled and embedded in a resin, and this is cut using a cross-section polisher IB-19530CP (manufactured by JEOL Ltd.), which is a cross-sectional sample preparation device. did. For the imaging of the cut surface, a JSM-7001F (manufactured by JEOL Ltd.), which is a Schottky field emission scanning electron microscope SEM-EDS, was used. Using the analysis / measurement software (WinRoof 6.1.1), the void in the cross section of the particle is measured as a black region, and the dense portion of the particle is measured as a white region. Area / (area of the black region + area of the white region) × 100 ”and arithmetically averaged them to obtain the porosity.

(3) Volume average particle diameter The volume average particle diameter (MV) was obtained from a volume reference distribution measured by a laser diffraction / scattering method for lithium metal composite oxide particles. As a measuring device, Microtrac MT3300EX-II (manufactured by Microtrac Bell Co., Ltd.), which is a laser diffraction / scattering type and a built-in ultrasonic generator particle size distribution measuring device, was used.

(4) Specific surface area The specific surface area of the lithium metal composite oxide particles was analyzed by a nitrogen gas adsorption / desorption method based on the BET single point method. The measuring device used was a Macsorb 1200 series (manufactured by Mountec Co., Ltd.) which is a gas flow type specific surface area measuring device.

(5) Lithium hydroxide content and lithium carbonate content The lithium hydroxide content and lithium carbonate content of the lithium metal composite oxide particles were analyzed by neutralization titration (sequential titration by RB Warder method). . As an analyzer, an automatic titrator COM-1750 (manufactured by Hiranuma Sangyo Co., Ltd.) was used, and the end point (potential difference) was determined by a pH composite electrode.

(6) Water The water content of the lithium metal composite oxide particles was analyzed by a dry weight method. Specifically, 50 g of lithium metal composite oxide particles are precisely weighed on a magnetic flat plate, dried with a constant temperature dryer set to 50 ° C. until constant weight is obtained over 48 hours, and then the lithium metal composite oxide before and after drying. Moisture content was determined from the weight difference of the particles. An analytical electronic balance GR-202 (manufactured by A & D Co., Ltd.) was used for the weight measurement of the tare and the analysis sample.

2. Production and evaluation of coated positive electrode active material for lithium ion secondary battery Lithium ion secondary oxide is coated with an ion conductive polymer containing electron conductive particles on the lithium metal composite oxide particles produced above. A coated positive electrode active material for a battery was produced. Specifically, polyethylene glycol 20000 (manufactured by Wako Pure Chemical Industries, Ltd.) is used as an ion conductive polymer for an ion conductive polymer solution that is a raw material of a particle coating slurry to be applied to lithium metal composite oxide particles. ) Was prepared and dissolved in 900 parts by mass of ultra-dehydrated chloroform (manufactured by Wako Pure Chemical Industries, Ltd.) having a water content of 10 ppm or less to prepare an ion conductive polymer solution.

  Acetylene black (trade name: DENKA BLACK (powdered product), volume average particle size (primary): 35 nm) manufactured by Denki Kagaku Kogyo Co., Ltd. is prepared as an electronic conductive particle to be added to the above-described ion conductive polymer solution. After adding 10 g to 990 g of the above ion conductive polymer solution and stirring with a stirrer, ultrasonic dispersion treatment was performed with an ultrasonic homogenizer US-300T (manufactured by Nippon Seiki Seisakusho Co., Ltd.), and 1000 g of particle coating slurry was obtained. Produced.

A coated positive electrode active material for a lithium ion secondary battery was prepared from the particle coating slurry prepared above and the above lithium metal composite oxide particles. Specifically, 500 g of the above lithium metal composite oxide particles are put into a can body container of MP-01 (manufactured by POWREC Co., Ltd.) which is a rolling fluidized bed coating apparatus, and the airflow temperature is 50 ° C. and the airflow volume is 0. The particles were fluidized in an air stream of .3 m ³ / hour. In this state, the particle coating slurry was sprayed at a spraying speed of 2 g / min for 125 minutes while rotating the blade rotor at 500 rpm. As a result, droplets of the particle coating slurry were applied to the surfaces of the particles. After the above 125 minutes had elapsed, spraying of the particle coating slurry was stopped, followed by drying for 5 hours using the same apparatus to prepare a coated positive electrode active material for a lithium ion secondary battery.

  After analyzing the coated positive electrode active material obtained above with the same analysis method as in the case of the lithium metal composite oxide particles described above, the specific surface area, lithium hydroxide content, lithium carbonate content, and moisture were further analyzed. The average thickness of the coating layer on the particle surface and gelation during paste preparation were evaluated by the following methods. For the evaluation of lithium hydroxide content, lithium carbonate content, moisture, and gelation during paste preparation, the coated positive electrode active material after 90 days storage in the atmosphere was used as an analysis sample.

(1) Surface coating layer average thickness After pre-processing the above-mentioned coated positive electrode active material by the focused ion beam (FIB) method, the surface coating layer average is obtained by imaging the cross section of the particles of the analysis sample with a transmission electron microscope. The thickness was measured. Note that a focused ion beam processing apparatus (FB-2000A) manufactured by Hitachi High-Technologies Corporation is used for the pretreatment, and a transmission electron microscope (JEM-ARM200F) manufactured by JEOL Ltd. is used for cross-sectional imaging. It was. The “average thickness” of the coating layer is determined for each of the 20 measurement target particles arbitrarily selected by the image analysis / measurement software (WinRoof 6.1.1) manufactured by Mitani Corporation for the obtained cross-sectional image. One particle to be measured by measuring the thickness of the four coating layers where two straight lines orthogonal to each other at the approximate center of the particle cross section intersect the coating layer, and averaging the thicknesses of the four coating layers obtained. After determining the thickness of the coating layer, the thickness of the coating layer of the 20 particles to be measured was obtained by arithmetic averaging.

(2) Gelation determination and viscosity measurement at the time of preparing the positive electrode mixture paste With respect to 20 g of the coated positive electrode active material for the lithium ion secondary battery, 2.2 g of acetylene black as a conductive material, 12 manufactured by Kureha Co., Ltd. Weigh so that the mass% polyvinylidene fluoride NMP solution (model number KF polymer # 1120) is 2.2 g and N-methyl-2-pyrrolidone (manufactured by Kanto Chemical Co., Ltd.) is 9.6 ml. They were put in a container and mixed for 10 minutes at a rotational speed of 2000 rpm using a non-bubbling kneader (model number NBK-1) manufactured by Nippon Seiki Seisakusho, which is a kneader mixer. The paste obtained was transferred to a glass bottle and sealed, then stored in a dry box at a temperature of 25 ° C. and a dew point of −40 ° C. and left for 24 hours. The fluidity of the paste after being left for 24 hours was evaluated by the flow condition when the glass bottle that was almost full was tilted and transferred to a petri dish, and the glass bottle was tilted by about 30 ° with respect to the vertical direction for 24 hours. As in the case before leaving, the paste flowed out well, “○”, the paste was less likely to flow than in the case of the above “○”, and the paste flowed out when the glass bottle was tilted about 45 ° with respect to the vertical direction. The paste was evaluated as “x” when the paste was more difficult to flow out than in the case of “Δ”, and the paste was solidified by gelation.

  In addition, for the viscosity measurement, 6 g of acetylene black as a conductive material, 3 g of polyvinylidene fluoride (model number KF polymer # 1100) manufactured by Kureha Co., Ltd., N-methyl-2-pyrrolidone (21 g of the above coated positive electrode active material) A positive electrode mixture paste was separately prepared by kneading 16 g of Kanto Chemical Co., Ltd.). This positive electrode mixture paste has a ratio of solid content (g) / organic solvent (g) "of 1.875. For the viscosity measurement of this positive electrode mixture paste, a vibration type viscometer Viscomate VM-100A manufactured by Seconic Co., Ltd. was used, and a viscometer calibration standard solution (JS-200, JS) defined in JIS-Z-8809. -2000) was made by Nippon Grease Co., Ltd.

3. Preparation of Lithium Ion Secondary Battery and Evaluation of Initial Discharge Capacity 24.5 g of Covered Positive Electrode Active Material for Lithium Ion Secondary Battery After Storage in the Air for 90 Days, 7.0 g of Acetylene Black, Polytetrafluoro 3.5 g of ethylene (PTFE) was weighed out and put into a rotating / revolving mixer together with 10 g of N-methyl-2-pyrrolidone (NMP) and kneaded for 30 minutes to prepare a positive electrode mixture paste. The obtained positive electrode mixture paste was applied to the surface of a current collector made of aluminum foil, and dried in a vacuum dryer at an ambient temperature of 120 ° C. for 12 hours. After the drying treatment, it was press-molded with a pressure of 100 MPa to a diameter of 11 mm and a thickness of 100 μm, and further punched out to the size of the positive electrode film to produce a positive electrode. In addition, the desired weight (solid content is about 150 mg) was ensured after punching by finely adjusting the thickness at the time of the application. Furthermore, lithium metal was prepared as a negative electrode, and an equivalent mixed solution (manufactured by Toyama Pharmaceutical Co., Ltd.) of ethylene carbonate (EC) and diethyl carbonate (DEC) using 1 mol / L LiClO ₄ as a supporting salt as an electrolyte was prepared. . Using these, a 2032 type coin cell for evaluation was produced in a glove box in an argon gas atmosphere controlled at a dew point of −80 ° C.

After leaving the evaluation coin cell for about 24 hours to stabilize the open circuit voltage OCV, the current density with respect to the positive electrode is set to 0.5 mA / cm ² and the cut-off voltage is charged to 4.3 V to obtain the charge capacity. The capacity when discharged to a cutoff voltage of 3.0 V after resting for 1 hour was measured as the initial discharge capacity.

[Example 2]
In preparing the metal composite hydroxide, the oxygen concentration in the gas phase in the reaction vessel is adjusted to 0.5% by volume instead of 0.1% by volume, and the spray time of the particle coating slurry is prepared in preparing the coated positive electrode active material. A lithium secondary battery was fabricated through the lithium metal composite oxide and the coated positive electrode active material in the same manner as in Example 1 except that the time was changed to 25 minutes instead of 125 minutes, and evaluated in the same manner.

[Example 3]
In preparing the metal composite hydroxide, the oxygen concentration in the gas phase in the reaction vessel is adjusted to 1.0% by volume instead of 0.1% by volume, and the spray time of the particle coating slurry is prepared in preparing the coated positive electrode active material. A lithium secondary battery was fabricated through the lithium metal composite oxide and the coated positive electrode active material in the same manner as in Example 1 except that the time was changed to 95 minutes instead of 125 minutes, and evaluated in the same manner.

[Example 4]
In preparing the metal composite hydroxide, the oxygen concentration in the gas phase in the reaction vessel is adjusted to 1.5% by volume instead of 0.1% by volume, and the spray time of the particle coating slurry is prepared in preparing the coated positive electrode active material. A lithium secondary battery was fabricated through the lithium metal composite oxide and the coated positive electrode active material in the same manner as in Example 1 except that the time was changed to 10 minutes instead of 125 minutes, and evaluated in the same manner.

[Example 5]
When preparing the metal composite hydroxide, the oxygen concentration in the gas phase in the reaction vessel is adjusted to 2.0% by volume instead of 0.1% by volume, and the spray time of the particle coating slurry is prepared when preparing the coated positive electrode active material. A lithium secondary battery was fabricated through the lithium metal composite oxide and the coated positive electrode active material in the same manner as in Example 1 except that the time was changed to 55 minutes instead of 125 minutes, and evaluated in the same manner.

[Comparative Example 1]
A lithium metal composite oxide and a lithium secondary battery were produced in the same manner as in Example 2 except that the coating treatment was not performed, and were similarly evaluated.

[Comparative Example 2]
In preparing the metal composite hydroxide, the oxygen concentration in the gas phase in the reaction vessel is adjusted to 1.0% by volume instead of 0.1% by volume, and the spray time of the particle coating slurry is prepared in preparing the coated positive electrode active material. A lithium secondary battery was fabricated through the lithium metal composite oxide and the coated positive electrode active material in the same manner as in Example 1 except that the time was changed to 5 minutes instead of 125 minutes, and evaluated in the same manner.

[Comparative Example 3]
When preparing the metal composite hydroxide, the oxygen concentration in the gas phase in the reaction vessel is adjusted to 1.8% by volume instead of 0.1% by volume, and the spray time of the particle coating slurry is prepared when preparing the coated positive electrode active material. A lithium secondary battery was fabricated through the lithium metal composite oxide and the coated positive electrode active material in the same manner as in Example 1 except that the time was changed to 240 minutes instead of 125 minutes, and evaluated in the same manner. The evaluation results of Examples 1 to 5 and Comparative Examples 1 to 3 are shown in Table 1 below.

[Comprehensive evaluation]
As can be seen from Table 1 above, the coated positive electrode active materials for lithium ion secondary batteries of Examples 1 to 5 all have a coating layer average thickness in the range of 0.005 to 0.1 μm. Neither the content rate nor the lithium carbonate content rate was found to be significantly different from the state before coating even when stored in the air for 90 days after coating. In addition, the moisture content was less than 0.01% by weight of the lower limit of quantitative analysis even though it was stored in the air for 90 days after coating. Therefore, the coated positive electrode active material for a lithium ion secondary battery obtained by forming a coating layer on the lithium metal composite oxide is hardly affected by carbon dioxide gas or moisture in the atmosphere, It was found that long-term storage in an atmosphere is possible.

  Moreover, in Examples 1-5, the fluidity | liquidity change of the paste by gelatinization did not arise at the time of positive electrode compound material paste preparation, and the viscosity in 20 degreeC in solid content (g) / organic solvent (g) = 1.875. Was less than 5500 mPa · s. Furthermore, good results were obtained in the initial discharge capacity by the coin cell for evaluation. Thus, the coated positive electrode active material that satisfies the requirements of the present invention is excellent in weather resistance and long-term storage, and the positive electrode mixture paste produced using this material can suppress gelation during kneading and can be used as a positive electrode. The lithium ion secondary battery can maintain a high initial discharge capacity.

  On the other hand, the positive electrode active materials for lithium secondary batteries of Comparative Examples 1 to 3 do not have a coating layer and absorb a large amount of moisture in the atmosphere, or the coating layer thickness is too thin to suppress carbon dioxide and moisture. Inadequate, or the coating layer is too thick and the ion conductive polymer dissolved during the kneading of the positive electrode mixture paste causes a problem of increasing the viscosity. As a result, in the gelation evaluation at the time of preparing the positive electrode mixture paste, “Δ” or “×” was obtained, and the viscosity of the positive electrode mixture paste was not satisfactory. Moreover, the initial discharge capacity by the coin battery for evaluation was inferior to any of Examples 1-5.

DESCRIPTION OF SYMBOLS 1 Lithium metal complex oxide which has the form of a secondary particle 2 Cavity part 3 Coating layer 10 Coated positive electrode active material W Thickness of coating layer

Claims (11)

Lithium metal composite oxide particles mainly composed of secondary particles containing lithium, nickel, cobalt, and aluminum and having a porosity of less than 20%;
Lithium ions having an average thickness of 0.005 to 0.1 μm made of an ion conductive polymer including electron conductive particles formed on the surface of the lithium metal composite oxide particles A coated positive electrode active material for a secondary battery. ^2. The coated positive electrode for a lithium ion secondary battery according to claim 1, wherein the lithium metal composite oxide particles have a specific surface area of 1.0 to 2.0 m ² / g before coating. Active material. The lithium metal composite oxide is represented by the general _{formula: Li a (Ni 1-x} -y Co x Al y) O 2 (0.98 ≦ a ≦ 1.20,0.05 ≦ x ≦ 0.35,0. The coated positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the positive electrode active material is represented by 01 ≦ y ≦ 0.20, x + y <0.40).   The ion-conductive polymer is easily soluble in water, an organic solvent contained in the positive electrode mixture paste, and an electrolyte solution contained in the lithium ion secondary battery. The covering positive electrode active material for lithium ion secondary batteries as described in an item.   The lithium ion according to any one of claims 1 to 4, wherein the ion conductive polymer is at least one selected from polyethylene glycol, polyethylene oxide, derivatives or salts thereof. A coated positive electrode active material for a secondary battery.   The said positive electrode active material for lithium ion secondary batteries of any one of Claims 1-5 characterized by the above-mentioned.   The coated positive electrode active material for a lithium ion secondary battery according to claim 6, wherein the carbon particles have a volume average particle size of 0.01 to 0.06 µm.   The lithium ion secondary battery according to claim 1, wherein 1 to 50 parts by mass of the electroconductive particles are contained with respect to 100 parts by mass of the coating layer. Coated positive electrode active material.   A baking step of forming lithium metal composite oxide particles mainly composed of secondary particles containing lithium, nickel, cobalt, and aluminum and having a porosity of less than 20% by baking; and dissolving the ion conductive polymer in an organic solvent. A step of preparing a particle coating slurry by dispersing electron conductive particles in an ion conductive polymer solution obtained by spraying, and spraying the particle coating slurry while flowing the lithium metal composite oxide particles in an air stream The coating layer having an average thickness of 0.005 to 0.1 μm made of an ion conductive polymer containing electron conductive particles on the surface of the lithium metal composite oxide particles by air-drying after the droplets were applied. The manufacturing method of the covering positive electrode active material for lithium ion secondary batteries characterized by comprising the coating process of forming.   A positive electrode mixture paste comprising a solid content composed of the coated positive electrode active material according to claim 1, a conductive material and a binder and an organic solvent, wherein the solid content relative to the organic solvent A positive electrode mixture paste having a viscosity at 20 ° C. of less than 5500 mPa · s when the mass ratio is 1.875.   It is the evaluation method of the covering positive electrode active material of any one of Claims 1-8, Comprising: The mass of solid content which consists of the said covering positive electrode active material, a electrically conductive material, and a binder was remove | divided with the mass of the organic solvent. A positive electrode mixture paste is prepared by blending these solids and an organic solvent so that the value is 1.875, and whether or not the viscosity of the positive electrode mixture paste at 20 ° C. is less than 5500 mPa · s. A method for evaluating a coated positive electrode active material for a lithium ion secondary battery, characterized in that the coated positive electrode active material is evaluated on the basis of the above.