JP2012169217A - Positive electrode active material for lithium ion secondary battery, and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material for a lithium ion secondary battery showing excellent cycle characteristics and rate characteristics even if charged at a high voltage, and a method for manufacturing the same.SOLUTION: There is provided a positive electrode active material for a lithium ion secondary battery comprising particles (III) where a carbon material (I), or the carbon material (I) and an oxide (II) cover the surface of a lithium-containing composite oxide containing Li element and at least one kind of transition metal element selected from Ni, Co, and Mn, where the molar amount of Li element is more than 1.2 times of the total molar amount of the transition metal elements. The carbon material (I) is at least one kind of carbon material selected from carbon nanotubes, graphene, and carbon black having an average dispersed-particle size of 0.2 μm or smaller. The oxide (II) is an oxide of at least one kind of metal element selected from Zr, Ti, and Al.

Description

  The present invention relates to a positive electrode active material for a lithium ion secondary battery and a method for producing the same. The present invention also relates to a positive electrode and a lithium ion secondary battery using the positive electrode active material of the present invention.

Lithium ion secondary batteries are widely used in portable electronic devices such as mobile phones and notebook computers. As a positive electrode active material for a lithium ion secondary battery, a composite oxide of lithium and a transition metal or the like such as LiCoO ₂ , LiNiO ₂ , LiNi _0.8 Co _0.2 O ₂ , LiMnO ₄ is used. In recent years, there has been a demand for downsizing and weight reduction as portable electronic devices and lithium ion secondary batteries for vehicles, and the discharge capacity per unit mass or the characteristic that the discharge capacity does not decrease after repeated charge / discharge cycles (hereinafter referred to as cycle) Further improvement is also desired. In particular, in a vehicle-mounted application, it is desired to further improve characteristics (hereinafter, also referred to as rate characteristics) in which the discharge capacity does not decrease when discharged at a high discharge rate.

In Patent Document 1, in order to improve rate characteristics, as a positive electrode of a lithium ion secondary battery, LiMn ₂ O ₄ as a positive electrode active material, a carbon nanotube having a length of 1 to 10 μm and a specific surface area of 250 m ² / g as a conductive material, and The use of a binder is described. Since the lithium ion secondary battery has a small discharge capacity and is manufactured by a method of adding carbon nanotubes together with a binder in a paste forming step for producing a positive electrode, it is difficult to uniformly disperse the carbon nanotubes.

Patent Document 2, the molar amount of Li element is 0.9-1.1 mol per mol of the total molar amount of the transition metal elements formula _{Li p N x M y O z} F a (0.9 ≦ p ≦ 1.1, 0.965 ≦ x <1.00, 0 <y ≦ 0.035, 1.9 ≦ z ≦ 2.1, x + y = 1, 0 ≦ a ≦ 0.02) A method of obtaining a positive electrode active material in which zirconium oxide is coated on a surface layer of a lithium-containing composite oxide by stirring and mixing a composite oxide and an aqueous solution containing zirconium and firing at a high temperature of 450 ° C. or higher in an oxygen atmosphere. Is described. Since the surface of the positive electrode active material is coated with electrochemically inactive zirconium oxide, the conductivity of the surface layer is low, which causes a reduction in rate characteristics.

In Patent Document 3, in order to improve the rate characteristics, LiNi _0.5 Co _0.2 Mn _0.3 O ₂ and a thermoplastic resin such as polypropylene are coated and fired at 700 ° C. for 4 hours, whereby positive electrode active A positive electrode active material in which a conductive carbon material is formed on the material surface is described.

Japanese Patent Laid-Open No. 11-283629 International Publication No. 2007/102407 International Publication No. 2010/027119

  The present invention provides a positive electrode active material for a lithium ion secondary battery that is excellent in cycle characteristics and rate characteristics even when charged at a high voltage, and a method for producing the same. In addition, a positive electrode and a lithium ion secondary battery using the positive electrode active material are provided.

The present invention is the following inventions.
[1] Including Li element and at least one transition metal element selected from Ni, Co, and Mn (provided that the molar amount of Li element is more than 1.2 times the total molar amount of the transition metal element) A lithium ion secondary battery comprising the following carbon material (I), or the following carbon material (I) and particles (III) covered by the following, on the surface of the lithium-containing composite oxide: Positive electrode active material.
Carbon material (I): At least one carbon material selected from carbon nanotubes, graphene, and carbon black having an average dispersed particle size of 0.2 μm or less.
Oxide (II): An oxide of at least one metal element selected from Zr, Ti, and Al.
[2] The positive electrode active material according to [1], wherein the mass of the carbon material (I) is 0.0001 to 0.05 times the mass of the lithium-containing composite oxide.
[3] The molar amount of at least one metal element selected from Zr, Ti, and Al is 0.0001 to 0.05 times the molar amount of the transition metal element of the lithium-containing composite oxide. 1] The positive electrode active material described in 1].
[4] A positive electrode for a lithium ion secondary battery comprising the positive electrode active material for a lithium ion secondary battery according to any one of [1] to [3], a conductive material, and a binder.
[5] A lithium ion secondary battery including the positive electrode, the negative electrode, and a nonaqueous electrolyte according to [4].
[6] Including Li element and at least one transition metal element selected from Ni, Co, and Mn (however, the molar amount of Li element is more than 1.2 times the total molar amount of the transition metal element) The following composition (1), or the following composition (1) and the following composition (2) are brought into contact with the lithium-containing composite oxide and heated at 50 to 500 ° C. A method for producing a positive electrode active material for an ion secondary battery.
Composition (1): A composition in which at least one carbon material (I) selected from carbon nanotubes, graphene, and carbon black having an average dispersed particle size of 0.2 μm or less is dispersed in a solvent.
Composition (2): A composition in which a compound containing at least one metal element selected from Zr, Ti, and Al is dissolved or dispersed in a solvent.
[7] The lithium-containing composite oxide and the composition are prepared by adding and stirring the composition (1) or the composition (1) and the composition (2) to the lithium-containing composite oxide. (1) or the production method according to [6], wherein the composition (1) and the composition (2) are contacted.
[8] The composition (1) or the composition (1) and the composition (2) are sprayed onto the lithium-containing composite oxide by a spray coating method, so that the lithium-containing composite oxide and the composition The production method according to [6], wherein the product (1) or the composition (1) and the composition (2) are contacted.

  The positive electrode active material of the present invention is excellent in cycle characteristics and rate characteristics even when charged at a high voltage. The positive electrode and the lithium ion secondary battery of the present invention are excellent in cycle characteristics and rate characteristics even when charged at a high voltage. Furthermore, the production method of the present invention can produce a positive electrode active material that is excellent in cycle characteristics and rate characteristics even when charged at a high voltage.

<Positive electrode active material>
The positive electrode active material of the present invention contains Li element and at least one transition metal element selected from Ni, Co, and Mn (provided that the molar amount of Li element is relative to the total molar amount of the transition metal element). It is more than 1.2 times.) It is characterized by comprising carbon material (I) or particles (III) coated with carbon material (I) and oxide (II) on the surface of the lithium-containing composite oxide. And

(Lithium-containing composite oxide)
The lithium-containing composite oxide in the present invention contains Li element and at least one transition metal element selected from Ni, Co, and Mn (provided that the molar amount of Li element is the total molar amount of the transition metal element). On the other hand, it is more than 1.2 times {(molar amount of Li element / total molar amount of transition metal element)> 1.2}. The composition ratio (molar ratio) of Li element to the total molar amount of the transition metal element is preferably 1.25 to 1.75, and more preferably 1.25 to 1.65. By setting the composition ratio, the discharge capacity per unit mass of the lithium ion secondary battery can be further increased.

  The transition metal element contained in the lithium-containing composite oxide is at least one selected from Ni, Co, and Mn, more preferably Mn is essential, including all elements of Ni, Co, and Mn. It is particularly preferable. As a transition metal element, metal elements other than Ni, Co, Mn, and Li (hereinafter referred to as other metal elements) may be included. Examples of the other metal elements include at least one selected from Cr, Fe, Al, Ti, Zr, Mo, Nb, V, and Mg. The proportion of the other metal element is preferably 0.001 to 0.50 mol, and more preferably 0.005 to 0.05 mol in the total amount (1 mol) of the transition metal element.

The lithium-containing composite oxide in the present invention is preferably a compound represented by the following formula (1). The compound represented by Formula (1) in this invention is a composition before passing through the process of charging / discharging or activation. Here, activation means removing lithium oxide (Li ₂ O) or lithium and lithium oxide from the lithium-containing composite oxide. As a normal activation method, there is an electrochemical activation method in which a voltage larger than 4.4 V or 4.6 V (expressed as a potential difference from the oxidation / reduction potential of Li ⁺ / Li) is applied. Moreover, a chemical activation method is mentioned by performing chemical reaction using acids, such as a sulfuric acid, hydrochloric acid, or nitric acid.

_{Li (Li x Mn y Me z} ) O p F q (1)
In the formula (1), Me is at least one element selected from Co, Ni, Cr, Fe, Al, Ti, Zr, Mo, Nb, V, and Mg. In the formula (1), 0.09 <x <0.3, y> 0, z> 0, 0.4 ≦ y / (y + z) ≦ 0.8, x + y + z = 1, 1.2 <(1 + x) / (Y + z), 1.9 <p <2.1, 0 ≦ q ≦ 0.1. As Me, Co, Ni, and Cr are preferable, and Co and Ni are particularly preferable. In the formula (1), 0.1 <x <0.25 is preferable, 0.11 <x <0.22 is more preferable, 0.5 ≦ y / (y + z) ≦ 0.8 is preferable, and 55 ≦ y / (y + z) ≦ 0.75 is more preferable.

Examples of the lithium-containing composite oxide include Li (Li _0.13 Ni _0.26 Co _0.09 Mn _0.52 ) O ₂ and Li (Li _0.13 Ni _0.22 Co _0.09 Mn _0.56 ) O. ₂ , Li (Li _0.13 Ni _0.17 Co _0.17 Mn _0.53 ) O ₂ , Li (Li _0.15 Ni _0.17 Co _0.13 Mn _0.55 ) O ₂ , Li (Li _{0 .16} Ni _0.17 Co _0.08 Mn _0.59 ) O ₂ , Li (Li _0.17 Ni _0.17 Co _0.17 Mn _0.49 ) O ₂ , Li (Li _0.17 Ni _0.21 Co _0.08 Mn _0.54 ) O ₂ , Li (Li _0.17 Ni _0.14 Co _0.14 Mn _0.55 ) O ₂ , Li (Li _0.18 Ni _0.12 Co _0.12 Mn _{0 .58} ) O ₂ , Li (Li _{0.1 8} Ni _0.16 Co _0.12 Mn _0.54 ) O ₂ , Li (Li _0.20 Ni _0.12 Co _0.08 Mn _0.60 ) O ₂ , Li (Li _0.20 Ni _0.16 Co _0.08 Mn _0.56 ) O ₂ , Li (Li _0.20 Ni _0.13 Co _0.13 Mn _0.54 ) O ₂ , Li (Li _0.22 Ni _0.12 Co _0.12 Mn _{0. 54} ) O ₂ and Li (Li _0.23 Ni _0.12 Co _0.08 Mn _0.57 ) O ₂ are preferred.

Still lithium-containing complex _{oxide, Li (Li 0.16 Ni 0.17 Co} 0.08 Mn 0.59) O 2, Li (Li 0.17 Ni 0.17 Co 0.17 Mn 0.49) O ₂ , Li (Li _0.17 Ni _0.21 Co _0.08 Mn _0.54 ) O ₂ , Li (Li _0.17 Ni _0.14 Co _0.14 Mn _0.55 ) O ₂ , Li (Li _0.18 Ni _0.12 Co _0.12 Mn _0.58 ) O ₂ , Li (Li _0.18 Ni _0.16 Co _0.12 Mn _0.54 ) O ₂ , Li (Li _0.20 Ni _{0. 12} Co _0.08 Mn _0.60 ) O ₂ , Li (Li _0.20 Ni _0.16 Co _0.08 Mn _0.56 ) O ₂ , Li (Li _0.20 Ni _0.13 Co _0.13 Mn _{0.54) O 2,} it is particularly preferred There.

  When the lithium-containing composite oxide in the present invention is a compound represented by the formula (1), the composition ratio of Li element to the total molar amount of the transition metal element is 1.2 <( 1 + x) / (y + z), preferably 1.25 ≦ (1 + x) / (y + z) ≦ 1.75, and more preferably 1.25 ≦ (1 + x) / (y + z) ≦ 1.65. When the composition ratio is in the above range, a positive electrode material having a high discharge capacity per unit mass can be obtained.

  The shape of the lithium-containing composite oxide is preferably particulate. 3-30 micrometers is preferable, as for the average particle diameter (D50) of lithium containing complex oxide, 4-25 micrometers is more preferable, and 5-20 micrometers is especially preferable. Here, the average particle size (D50) is a particle size distribution at a point where the cumulative curve is 50% in a cumulative curve obtained by obtaining a particle size distribution on a volume basis and setting the total volume to 100%. It means% diameter. The particle size distribution is obtained from a frequency distribution and a cumulative volume distribution curve measured with a laser scattering particle size distribution measuring apparatus. The particle size is measured by sufficiently dispersing the powder in an aqueous medium by ultrasonic treatment or the like and measuring the particle size distribution (for example, a laser diffraction / scattering particle size distribution measuring device Partica LA-950VII manufactured by HORIBA, etc.). Used).

The specific surface area of the lithium-containing composite oxide is preferably ^{0.3~10m 2 / g, 0.5~5m 2 /} g is particularly preferred. When the specific surface area is 0.3 to 10 m ² / g, the capacity is high and a dense positive electrode layer can be formed.

The lithium-containing composite oxide in the present invention preferably has a layered rock salt type crystal structure (space group R-3m). In addition, since the lithium-containing composite oxide in the present invention has a high ratio of Li element to transition metal element, XRD (X-ray diffraction) measurement shows a peak in the range of 2θ = 20 to 25 ° as in the case of layered Li ₂ MnO _3. Is observed.

(Carbon material (I))
The carbon material (I) in the present invention is at least one carbon material selected from carbon nanotubes, graphene, and carbon black having an average dispersed particle size of 0.2 μm or less. The carbon material (I) preferably has high conductivity and at least one dimension of the shape is nano-sized. Here, nano size means less than 1 micrometer. By increasing the conductivity of the carbon material (I), it is possible to improve the conductivity of the positive electrode active material and improve the initial capacity and rate characteristics. Further, when the carbon material (I) is nano-sized, the surface of the lithium-containing composite oxide can be uniformly coated. The shape of the carbon material (I) can be evaluated using an electron microscope such as SEM (scanning electron microscope) or TEM (transmission electron microscope).

  Since carbon nanotubes are in the form of fine fibers, high conductivity can be obtained with a small amount of coating due to the entanglement of the fibers. The average diameter of the carbon nanotube is preferably 0.4 to 200 nm, more preferably 1 to 100 nm, and particularly preferably 5 to 50 nm. The average length of the carbon nanotubes is preferably 0.5 to 30 μm, more preferably 1 to 20 μm, and particularly preferably 1 to 10 μm. When the average diameter and the average length are within the above ranges, the conductivity, dispersibility, and adhesion to the lithium-containing composite oxide surface are good.

  Strictly speaking, graphene refers to a single graphite layer, but the graphene in the present invention refers to a carbon material composed of one to multiple graphite layers. Since graphene is a thin film, high conductivity can be obtained with a small amount of coating by efficiently covering the lithium-containing composite oxide. The average thickness of graphene is preferably 0.3 to 20 nm, more preferably 0.5 to 10 nm, and particularly preferably 1 to 5 nm. The average equivalent circle diameter of graphene is preferably 0.2 to 20 μm, and more preferably 0.5 to 10 μm. When the average thickness and the average equivalent circle diameter are within the above ranges, the conductivity, dispersibility, and adhesion to the lithium-containing composite oxide surface are good.

  The average dispersed particle size of carbon black is 0.2 μm or less, preferably 0.005 to 0.2 μm, more preferably 0.01 to 0.15 μm, and particularly preferably 0.02 to 0.10 μm. Since carbon black having an average dispersed particle size of 0.2 μm or less has a small particle size and good dispersibility, the surface of the lithium-containing composite oxide can be uniformly coated. When the average dispersed particle size is in the above range, the conductivity, dispersibility, and adhesion to the lithium-containing composite oxide surface are improved. The average dispersed particle size of carbon black can be determined by a dynamic light scattering method.

  Carbon nanotubes are particularly preferred as the carbon material (I) in the present invention. Since carbon nanotubes can provide higher conductivity in a smaller amount than carbon black, it is considered that the effect of improving the rate characteristics can be obtained in a smaller amount. In addition, carbon nanotubes are considered to have a higher capacity because they are less likely to inhibit the diffusion of Li ions than graphene.

(Oxide (II))
The oxide (II) is an oxide of at least one metal element selected from Zr, Ti, and Al. The oxide _{(II), ZrO 2, TiO} 2, and _Al 2 _{O 3} are preferable, ZrO ₂ are more preferable. By covering with these compounds, elution of transition metal elements such as manganese from the surface of the lithium-containing composite oxide into the electrolyte is suppressed, and cycle characteristics are improved. The oxide (II) in the present invention is preferably a compound that is inert to the decomposition product in order to prevent contact with the decomposition product of the electrolyte caused by charging (oxidation reaction) at a high voltage.

  When oxide (II) is particulate, the average particle size of oxide (II) is preferably 0.1 to 100 nm, more preferably 0.1 to 50 nm, and particularly preferably 0.1 to 30 nm. The average particle size of the oxide (II) is a particle size observed with an electron microscope such as SEM or TEM. The average particle diameter is expressed as an average particle diameter of particles covering the surface of the lithium-containing composite oxide.

(Particle (III))
The particles (III) in the present invention are particles in which the surface of a lithium-containing composite oxide is coated with the carbon material (I) or the carbon material (I) and the oxide (II). Here, the covering means that the carbon material (I) or the carbon material (I) and the oxide (II) are partially or entirely deposited on the surface of the lithium-containing composite oxide by chemical adsorption or physical adsorption. It means the attached state.

  In the particle (III), the carbon material (I) or the oxide (II) is coated on the surface of the lithium-containing composite oxide. For example, after cutting the particle (III), the cross section is polished, It can be evaluated by performing elemental mapping with a line microanalyzer analysis method (EPMA). According to the evaluation method, the carbon material (I) or the oxide (II) is the center of the lithium-containing composite oxide (here, the center refers to a portion not in contact with the surface of the lithium-containing composite oxide, from the surface). It is preferable that the average distance is the portion with the longest average distance).

  The proportion of the carbon material (I) in the particles (III) can be measured by a carbon-in-solid analyzer. A solid carbon analyzer is an apparatus that measures the amount of carbon in a sample by quantitatively analyzing a gas generated by burning the sample at a high temperature.

  When the carbon material (I) is coated on the surface of the particle (III) and the oxide (II) is not coated, the ratio of the carbon material (I) in the particle (III) is determined by a solid-state carbon analyzer. It is determined from the difference between the amount of carbon contained in the measured particle (III) and the amount of carbon contained in the lithium-containing composite oxide measured by a solid carbon analyzer.

When the surface of the particles (III) is coated with the carbon material (I) and the oxide (II), the ratio of the carbon material (I) in the particles (III) is determined by the particle ( It is determined from the difference between the amount of carbon contained in III) and the amount of carbon contained in particles in which only the oxide (II) is coated on the surface of the lithium-containing composite oxide measured with a solid-in-carbon analyzer.
In addition, when the ratio of carbon material (I) cannot be calculated | required with a carbon analyzer, you may calculate based on the preparation amount of lithium containing composite particle and carbon material (I).

  When the carbon material (I) is a carbon nanotube and / or graphene, high conductivity can be obtained even in a small amount. Therefore, the ratio of the carbon material (I) is 0.0001 to 0 with respect to the mass of the particles (II). 0.02 times is preferable, 0.0003 to 0.01 times is more preferable, and 0.0005 to 0.005 times is particularly preferable.

  When the carbon material (I) is carbon black having an average dispersed particle size of 0.2 μm or less, the proportion of the carbon material (I) is preferably 0.001 to 0.05 times the mass of the particles (III), 0.003 to 0.04 times is more preferable, and 0.005 to 0.03 times is particularly preferable. By setting the ratio of the carbon material (I) within the above range, both high capacity and high rate characteristics can be achieved.

When the coverage of the carbon material (I) on the lithium-containing composite oxide is carbon nanotubes and / or graphene, high conductivity can be obtained even with a small amount. Therefore, the coverage of the carbon material (I) is 0.00002. ˜0.02 g / m ² is preferable, 0.0006 to 0.01 g / m ² is more preferable, and 0.001 to 0.005 g / m ² is particularly preferable. The carbon coverage can be determined from the ratio of the carbon material (I) in the particles (III) and the specific surface area of the lithium-containing composite oxide.

When the carbon material (I) is carbon black having an average dispersed particle size of 0.2 μm or less, the coverage of the carbon (I) is preferably 0.0002 to 0.05 g / m ² , and 0.0006 to 0.04 g. / M ² is more preferable, and 0.001 to 0.03 g / m ² is particularly preferable.

  The ratio of the oxide (II) in the particles (III) is such that the molar amount of at least one metal element selected from Zr, Ti, and Al in the oxide (II) is the mole of the transition metal element in the lithium-containing composite oxide. 0.0001-0.05 times is preferable with respect to quantity, 0.0003-0.04 times are more preferable, 0.0005-0.03 times are especially preferable.

  The ratio of the oxide (II) in the particles (III) can be measured by dissolving the positive electrode active material in an acid and performing ICP (high frequency inductively coupled plasma) measurement. In addition, when the ratio of oxide (II) cannot be calculated | required by ICP measurement, you may calculate based on the preparation amount of lithium containing composite particle and oxide (II).

In the positive electrode active material of the present invention, the shape of the carbon material (I) and the oxide (II) coated on the surface of the lithium-containing composite oxide can be evaluated by an electron microscope such as SEM or TEM. The shape of the carbon material (I) and the oxide (II) may be in the form of particles, film, fiber, lump or the like.
The carbon material (I) and the oxide (II) may be at least partially coated on the surface of the lithium-containing composite oxide.
The positive electrode active material of the present invention has a large discharge capacity and is excellent in rate characteristics and cycle characteristics.

<Method for producing positive electrode active material>
The production method of the present invention includes Li element and at least one transition metal element selected from Ni, Co, and Mn (provided that the molar amount of Li element is 1 with respect to the total molar amount of the transition metal element). .. More than 2 times.) Lithium-containing composite oxide and the following composition (1) or the following composition (1) and the following composition (2) are brought into contact and heated at 50 to 500 ° C. A lithium ion secondary battery comprising a particle (III) coated with carbon material (I) or carbon material (I) and oxide (II) on the surface of the lithium-containing composite oxide This is a method for obtaining a positive electrode active material.
Composition (1): A composition in which at least one carbon material (I) selected from carbon nanotubes, graphene, and carbon black having an average dispersed particle size of 0.2 μm or less is dispersed in a solvent.
Composition (2): A composition in which a compound containing at least one metal element selected from Zr, Ti, and Al is dissolved or dispersed in a solvent.

As the lithium-containing composite oxide in the present invention, the lithium-containing composite oxide can be used, and the preferred embodiments are also the same.
As a method for producing a lithium-containing composite oxide, a method of mixing and baking a precursor of a lithium-containing composite oxide obtained by a coprecipitation method and a lithium compound, a hydrothermal synthesis method, a sol-gel method, a dry mixing method, an ion The exchange method and the like can be mentioned, but since the contained transition metal elements are uniformly mixed, the discharge capacity is excellent, so the precursor (coprecipitation composition) of the lithium-containing composite oxide obtained by the coprecipitation method and the lithium compound The method of mixing and baking is preferable.

  The composition (1) is a composition in which at least one carbon material (I) selected from carbon nanotubes, graphene, and carbon black having an average dispersed particle size of 0.2 μm or less is dispersed in a solvent.

  As the carbon material (I) in the present invention, the carbon material (I) can be used, and preferred embodiments are also the same.

  As the solvent used in the composition (1), a solvent containing water is preferable in terms of stability and reactivity of the compound containing a metal element, and a mixed solvent of water and a water-soluble alcohol and / or polyol is more preferable. Only water is particularly preferred. Examples of the water-soluble alcohol include methanol, ethanol, 1-propanol, and 2-propanol. Examples of the polyol include ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, polyethylene glycol, butanediol, and glycerin. The total content of the water-soluble alcohol and the polyol contained in the solvent is preferably 0 to 20% by mass, more preferably 0 to 10% by mass with respect to the total amount of each solvent (total amount of solvent). When the solvent is only water, it is particularly preferable because it is excellent in terms of safety, environment, handling, and cost.

  As pH of composition (1), 3-12 are preferable, 3.5-12 are more preferable, and 4-10 are especially preferable. When the pH is in the above range, there is little elution of Li element from the lithium-containing composite oxide when the lithium-containing composite oxide is brought into contact with the composition (1).

  In order to improve the dispersibility of the carbon material (I) in the solvent, the composition (1) may contain a polymer dispersant or a surfactant. However, if the polymer dispersant or the surfactant remains in the positive electrode material, the battery characteristics are adversely affected. Therefore, the total content of the polymer dispersant and the surfactant in the composition (1) is the carbon material (I). Is preferably 3% by mass or less, more preferably 1% by mass or less, and particularly preferably 0 to 0.1% by mass.

  The concentration of the carbon material (I) contained in the composition (1) is preferably high because it is necessary to remove the solvent by heating in a later step. However, if the concentration is too high, the viscosity increases, and the uniform mixing property of the composition (1) with another element source that forms the positive electrode active material is reduced. Therefore, the carbon material contained in the composition (1) ( The concentration of I) is preferably 0.5 to 25% by mass, particularly preferably 2 to 15% by mass.

  The composition (2) is a composition in which a compound containing at least one metal element selected from Zr, Ti, and Al (hereinafter also simply referred to as compound (3)) is dissolved or dispersed in a solvent.

  When the composition (2) is a composition obtained by dissolving the compound (3) in a solvent, examples of the compound (3) include ammonium zirconium carbonate, zirconium ammonium halide, zirconium acetate, titanium lactate ammonium salt, titanium lactate, titanium Diisopropoxybis (triethanolaminate), peroxotitanium, titanium peroxocitrate complex, aluminum acetate, aluminum oxalate, aluminum citrate, aluminum lactate, basic aluminum lactate, aluminum maleate are preferred, ammonium zirconium carbonate, titanium Lactate, titanium lactate ammonium salt, aluminum lactate, and basic aluminum lactate are more preferred.

  When the solvent in the composition (2) is water, the compound (3) is preferably zirconium ammonium carbonate, ammonium zirconium halide, titanium lactate, titanium lactate ammonium salt, aluminum lactate, or basic aluminum lactate. Since the solubility in water is good by using the above compound, the metal element concentration in the composition (2) can be increased, and precipitation does not occur even when contacted with the lithium-containing composite oxide in the present invention. It is considered that the metal element oxide (II) can be uniformly coated on the surface of the lithium-containing composite oxide.

  When the composition (2) is a composition in which the compound (3) is dispersed in a solvent, the compound (3) includes zirconium oxide, titanium oxide, aluminum oxide, zirconium hydroxide, titanium hydroxide, and aluminum hydroxide. preferable. The compound (3) to be dispersed is preferably fine particles. The average particle diameter of the fine particles is preferably 1 to 100 nm, more preferably 2 to 50 nm, and particularly preferably 3 to 30 nm. The average particle diameter can be determined by a dynamic light scattering method. After applying the composition (2) by using the above compound, the oxide (II) can be formed without decomposing the organic matter. If there is no decomposition of the organic matter, the heating temperature in the present invention can be lowered and the heating can be performed in an inert atmosphere.

  When the composition (2) in the present invention is brought into contact with a Li-excess lithium-containing composite oxide such that the molar amount of Li element exceeds 1.2 with respect to the total molar amount of the transition metal element, The pH of 2) will increase. As the compound (3), a compound that does not generate a precipitate even when the pH is increased is preferable, and a compound that does not generate a precipitate even when the pH is 11 or more is more preferable.

As a solvent used by the composition (2), the solvent used by the said composition (1) can be used similarly, and its preferable aspect is also the same.
The solvents for the composition (1) and the composition (2) may be the same or different.

  In the production method of the present invention, the composition (1), or the composition (1) and the composition (2) are brought into contact with the lithium-containing composite oxide. When the composition (1) and the composition (2) are brought into contact with each other, they may be brought into contact with each other independently, or may be brought into contact after the composition (1) and the composition (2) are mixed. When contacting independently, the order which a composition contacts is not limited, and the frequency | count of contact is also not limited.

  The composition (1) and the composition (2) can each independently contain a pH adjuster. As the pH adjuster, those that volatilize or decompose upon heating are preferable. Specific examples include organic acids such as acetic acid, citric acid, lactic acid, and formic acid, and ammonia.

  As pH of a composition (2), 3-12 are preferable, 3.5-12 are more preferable, and 4-10 are especially preferable. If the pH is in the above range, the lithium element is less eluted from the lithium-containing composite oxide when brought into contact with the lithium-containing composite oxide, and good battery characteristics are obtained because there are few impurities such as a pH adjuster. .

  When preparing the composition (1), it is preferable to perform a dispersion treatment as necessary. As a dispersion treatment method, a known method such as a ball mill, a bead mill, a high-pressure homogenizer, a high-speed homogenizer, or an ultrasonic dispersion apparatus can be used. By the dispersion treatment, the carbon material (I) can be stably dispersed in the solvent.

  The concentration of the compound (3) in the composition (2) is preferably high since it is necessary to remove the solvent by heating in the subsequent step. However, when the concentration is too high, the viscosity increases, and the uniform mixing property with the positive electrode active material and the composition (1) decreases. The concentration of the compound (3) in the composition (2) is preferably 0.5 to 30% by mass and particularly preferably 4 to 20% by mass in terms of metal element oxide (II).

  As a contact method between the lithium-containing composite oxide and the composition (1), or the composition (1) and the composition (2), a spray coating method, a wet method, or the like can be applied. The method of spraying on objects is particularly preferred. In the case of the wet method, since it is necessary to remove the solvent by filtration or evaporation after contact, the process may be complicated. In the case of the spray coating method, the process is simple, and the surface of the lithium-containing composite oxide can be uniformly coated with the carbon material (I) or the carbon material (I) and the oxide (II).

  The amount of the composition (1) in which the composition (1) is brought into contact with the lithium-containing composite oxide is preferably 1 to 50% by mass, more preferably 2 to 40% by mass with respect to the lithium-containing composite oxide. 30% by mass is particularly preferred.

  When the composition (1) and the composition (2) are brought into contact with the lithium-containing composite oxide, the total amount of the composition (1) and the composition (2) is 1 to 50 mass with respect to the lithium-containing composite oxide. % Is preferable, 2 to 40% by mass is more preferable, and 3 to 30% by mass is particularly preferable. When the ratio of the total amount of the composition (1) and the composition (2) is in the above range, the surface of the lithium-containing composite oxide can be easily coated with the carbon material (I) and the oxide (II). Further, there is an advantage that the lithium-containing composite oxide is not agglomerated and easily stirred when contacted by a spray coating method.

  In either case of the spray coating method or the wet method, lithium is added to the lithium-containing composite oxide by adding the composition (1) or the composition (1) and the composition (2) and stirring. It is preferable to bring the composite oxide in contact with the composition (1), or the composition (1) and the composition (2). As the stirring device, a drum mixer or a solid-air low shear stirring device can be used. By contacting the composition (1), or the composition (1) and the composition (2) with the lithium-containing composite oxide with stirring, the carbon material (I), or the carbon material (I) and the oxidation are more uniformly obtained. Particles (III) in which the product (II) is coated on the surface of the lithium-containing composite oxide can be obtained.

  Next, heating is performed in the production method of the present invention. The heating temperature is 50 ° C to 500 ° C, more preferably 80 ° C to 450 ° C, and particularly preferably 100 to 400 ° C. By heating, oxide (II) can be efficiently formed from a compound containing at least one metal element selected from Zr, Ti, and Al, and volatile solvents such as water and organic components And impurities can be removed.

  When the lithium-containing composite oxide is brought into contact with the composition (1), or when the composition (1) and the composition (2) in which the compound (3) is dispersed are brought into contact, heating is inert such as nitrogen or argon. It is preferable to carry out in an atmosphere. Oxidative decomposition of the carbon material (I) can be suppressed by heating in an inert atmosphere. By setting the heating temperature to 50 ° C. or higher, the solvent can be efficiently removed. By setting the heating temperature to 500 ° C. or lower, the transition metal in the lithium-containing composite oxide is not reduced.

  When using the composition (2) in which the compound (3) is dispersed, the heating is preferably performed in an oxygen-containing atmosphere. The heating temperature is preferably 200 to 500 ° C, more preferably 250 to 450 ° C, and particularly preferably 300 to 450 ° C. By setting the heating temperature to 200 ° C. or higher, the oxide (II) can be easily formed from the compound (3), volatile impurities such as residual moisture can be reduced, and deterioration of cycle characteristics can be suppressed. By setting the heating temperature to 500 ° C. or less, the oxidation thermal decomposition reaction of the carbon material (I) is difficult to proceed, and the rate characteristics are improved.

  The heating time is preferably 0.1 to 24 hours, more preferably 0.5 to 18 hours, and particularly preferably 1 to 12 hours.

  The pressure at the time of heating is not specifically limited, Normal pressure or pressurization is preferable, and normal pressure is particularly preferable.

  In the production method of the present invention, the step of bringing the lithium-containing composite oxide into contact with the composition (1) or the composition (1) and the composition (2) and the step of heating may be performed after the contact. Well, you may heat while making it contact.

<Positive electrode>
The positive electrode for a lithium ion secondary battery of the present invention comprises a positive electrode active material layer containing the positive electrode active material, a conductive material, and a binder formed on a positive electrode current collector (positive electrode surface). The positive electrode for a lithium ion secondary battery is prepared by, for example, preparing a slurry or kneaded material by dissolving the positive electrode active material, the conductive material and the binder of the present invention in a solvent, dispersing in a dispersion medium, or kneading with a solvent. Then, the prepared slurry or kneaded material can be produced by carrying it on the positive electrode current collector plate by coating or the like.

  Examples of the conductive material include carbon black such as acetylene black, graphite, and ketjen black.

  As binders, fluorine-containing resins such as polyvinylidene fluoride and polytetrafluoroethylene, polyolefins such as polyethylene and polypropylene, polymers having unsaturated bonds such as styrene / butadiene rubber, isoprene rubber and butadiene rubber, and copolymers thereof, Examples thereof include acrylic acid-based polymers such as acrylic acid copolymers and methacrylic acid copolymers, and copolymers thereof.

<Lithium ion secondary battery>
The lithium ion secondary battery of this invention contains the said positive electrode for lithium ion secondary batteries, a negative electrode, and a nonaqueous electrolyte.

  The negative electrode is formed by forming a negative electrode active material layer containing a negative electrode active material on a negative electrode current collector. For example, it can be produced by preparing a slurry by kneading a negative electrode active material with an organic solvent, and applying, drying, and pressing the prepared slurry to a negative electrode current collector.

  As the negative electrode current collector plate, for example, a metal foil such as a nickel foil or a copper foil can be used.

  The negative electrode active material may be any material that can occlude and release lithium ions at a relatively low potential, such as lithium metal, lithium alloy, lithium compound, carbon material, periodic table 14 and group 15 metal. Oxides, carbon compounds, silicon carbide compounds, silicon oxide compounds, titanium sulfide, boron carbide compounds, and the like can be used.

  As the lithium alloy and the lithium compound, lithium alloys and lithium compounds composed of lithium and a metal capable of forming an alloy or compound with lithium can be used.

Examples of the carbon material for the negative electrode active material include non-graphitizable carbon, artificial graphite, natural graphite, pyrolytic carbon, coke such as pitch coke, needle coke, petroleum coke, graphite, glassy carbon, phenol Organic polymer compound fired bodies, carbon fibers, activated carbon, carbon blacks, etc., obtained by firing and carbonizing a resin, furan resin or the like at an appropriate temperature can be used.
The group 14 metal of the periodic table is silicon or tin, with silicon being preferred.
Other materials that can be used as the negative electrode active material include oxides such as iron oxide, ruthenium oxide, molybdenum oxide, tungsten oxide, titanium oxide, and tin oxide, and nitrides such as Li _2.6 Co _0.4 N. It is done.

As the nonaqueous electrolyte, it is preferable to use a nonaqueous electrolytic solution in which an electrolyte salt is dissolved in a nonaqueous solvent.
As the non-aqueous electrolyte, one prepared by appropriately combining an organic solvent and an electrolyte can be used. As the organic solvent, those known as an organic solvent for an electrolytic solution can be used, and propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, Diethyl ether, sulfolane, methyl sulfolane, acetonitrile, acetic acid ester, butyric acid ester, propionic acid ester and the like can be used. In particular, from the viewpoint of voltage stability, it is preferable to use cyclic carbonates such as propylene carbonate and chain carbonates such as dimethyl carbonate and diethyl carbonate. An organic solvent may be used individually by 1 type, and may mix and use 2 or more types.
As the nonaqueous electrolyte, a solid electrolyte containing an electrolyte salt, a polymer electrolyte, a solid electrolyte or a gel electrolyte in which an electrolyte is mixed or dissolved, and the like can be used.

The solid electrolyte may be any material having lithium ion conductivity, and an inorganic solid electrolyte and a polymer solid electrolyte can be used.
As the inorganic solid electrolyte, lithium nitride, lithium iodide, or the like can be used.
As the polymer solid electrolyte, an electrolyte salt and a polymer compound that dissolves the electrolyte salt can be used. Examples of the polymer compound include polyethylene oxide, polypropylene oxide, polyphosphazene, polyaziridine, polyethylene sulfide, polyvinyl alcohol, polyvinylidene fluoride, polyhexafluoropropylene, and their derivatives, mixtures, and composites.

As the gel electrolyte or the like, various polymer compounds that absorb the nonaqueous electrolyte and gelate can be used. As the polymer compound used for the gel electrolyte, fluorine-based polymers such as poly (vinylidene fluoride) and poly (vinylidene fluoride-co-hexafluoropropylene) can be used. As the polymer compound used in the gel electrolyte, an ether polymer such as polyacrylonitrile, a polyacrylonitrile copolymer, polyethylene oxide, a polyethylene oxide copolymer, and a crosslinked product thereof can be used. Examples of the monomer used in the copolymer include polypropylene oxide, methyl methacrylate, butyl methacrylate, methyl acrylate, and butyl acrylate.
As the matrix of the gel electrolyte, a fluorine-based polymer is particularly preferable from the viewpoint of stability against redox reaction.

Examples of the electrolyte salt used in the electrolyte include LiClO ₄ , LiPF ₆ , LiBF ₄ , CH ₃ SO ₃ Li, LiCl, and LiBr.

  The shape of the lithium ion secondary battery of the present invention can be appropriately selected from coin shapes, sheet shapes (film shapes), folded shapes, wound bottomed cylindrical shapes, button shapes, and the like according to applications. .

<Synthesis of lithium-containing composite oxide>
Nickel (II) sulfate hexahydrate (140.6 g), cobalt sulfate (II) heptahydrate (131.4 g), manganese (II) sulfate pentahydrate (482.2 g) and distilled water (1245. 9 g) was added and dissolved uniformly to obtain a raw material solution. Distilled water (320.8 g) was added to ammonium sulfate (79.2 g) and dissolved uniformly to obtain an ammonia source solution. Distilled water (1920.8 g) was added to ammonium sulfate (79.2 g) and dissolved uniformly to obtain a mother liquor. Distilled water (600 g) was added to sodium hydroxide (400 g) and dissolved uniformly to obtain a pH adjusting solution.
The mother liquor was placed in a 2 L baffled glass reaction vessel, heated to 50 ° C. with a mantle heater, and a pH adjusting solution was added so that the pH was 11.0. While stirring the solution in the reaction vessel with an anchor type stirring blade, the raw material solution was added at a rate of 5.0 g / min, and the ammonia source solution was added at a rate of 1.0 g / min, and a composite hydroxide of nickel, cobalt, and manganese was added. Precipitated. During the addition of the raw material solution, the pH adjusting solution was added so as to keep the pH in the reaction vessel at 11.0. Moreover, nitrogen gas was flowed at a flow rate of 0.5 L / min in the reaction tank so that the precipitated hydroxide was not oxidized. Further, the liquid was continuously extracted so that the amount of the liquid in the reaction tank did not exceed 2 L.

In order to remove impurity ions from the obtained composite hydroxide of nickel, cobalt, and manganese, washing was repeated by pressure filtration and dispersion in distilled water. When the electrical conductivity of the filtrate reached 25 μS / cm, the washing was finished and dried at 120 ° C. for 15 hours to obtain a precursor.
When the contents of the precursors nickel, cobalt, and manganese were measured by ICP, they were 11.6% by mass, 10.5% by mass, and 42.3% by mass, respectively (in terms of molar ratio, nickel: cobalt: manganese = 0). 172: 0.156: 0.672).
A precursor (20 g) and lithium carbonate (12.6 g) having a lithium content of 26.9 mol / kg were mixed and baked at 800 ° C. for 12 hours in an oxygen-containing atmosphere to obtain a lithium-containing composite oxide of the example. . The composition of the lithium-containing composite oxide of the obtained example is Li (Li _0.2 Ni _0.137 Co _0.125 Mn _0.538 ) O ₂ . The average particle diameter D50 of the lithium-containing composite oxide of the example was 5.3 μm, and the specific surface area measured using the BET (Brunauer, Emmett, Teller) method was 4.4 m ² / g.

<TG (
Thermogravimetric) measurement>
Using a TG measuring device (trade name: TG-DTA2000SA, manufactured by Bruker AXS), TG measurement of carbon nanotubes was performed in an air atmosphere. Moreover, in TG measurement, it heated up from room temperature to 800 degreeC with the temperature increase rate of 10 degree-C / min, and measured.
The weight loss of the carbon nanotubes was 1.5% at 400 ° C, 4.4% at 500 ° C, 16.1% at 550 ° C, and 70.2% at 600 ° C. The carbon nanotubes were less likely to undergo thermal decomposition at 500 ° C. or less in the air atmosphere, and the weight loss was small. However, it was confirmed that the weight reduction rapidly progressed above 500 ° C. TG measurement is also performed on graphene and carbon black. It can be confirmed that thermal decomposition hardly occurs at 500 ° C. or lower.
Therefore, at a heating temperature of 500 ° C. or less, the charging ratio of the carbon material (I) is set as the ratio of the carbon material (I) in the particles (III).

(Example 1) <Carbon nanotube coating on lithium-containing composite oxide>
A pH 8.2 aqueous dispersion (composition (1)) containing 1% by mass of carbon nanotubes (average diameter 10 nm, average length 5 μm) is prepared.
Next, the prepared composition (1) (3 g) was sprayed and added to the stirred lithium-containing composite oxide (15 g), and the lithium-containing composite oxide and composition of the example were added. (1) is brought into contact with mixing. Subsequently, the obtained mixture was heated at 300 ° C. for 1 hour in a nitrogen atmosphere, and the positive electrode active material (A) of Example 1 comprising particles (III) covered with the carbon material (I) on the surface of the lithium-containing composite oxide. )

(Example 2) <Carbon nanotube coating on lithium-containing composite oxide>
The positive electrode of Example 2 was prepared in the same manner as in Example 1 except that an aqueous dispersion (pH 8.0) containing 0.1% by mass of carbon nanotubes (average diameter 10 nm, average length 5 μm) was used as composition (1). An active material (B) is obtained.

(Example 3) <Graphene coating on lithium-containing composite oxide>
The same procedure as in Example 1 was carried out except that an aqueous dispersion (pH 8.5) containing 1% by mass of graphene (average film thickness 3 nm, average equivalent circle diameter 3 μm) was used as the composition (1). Material (C) is obtained.

(Example 4) <Carbon black coating on lithium-containing composite oxide>
The positive electrode active material (D) of Example 4 was prepared in the same manner as in Example 1 except that an aqueous dispersion (pH 6.5) containing 5% by mass of carbon black (average dispersed particle size 100 nm) was used as the composition (1). Get.

(Example 5) <Carbon black coating on lithium-containing composite oxide>
The positive electrode active material (E) of Example 5 was prepared in the same manner as in Example 1 except that an aqueous dispersion (pH 7.0) containing 20% by mass of carbon black (average dispersed particle size 100 nm) was used as the composition (1). Get.

(Example 6) <Carbon nanotube and Zr coating on lithium-containing composite oxide>
A pH 8.1 aqueous dispersion (composition (1)) containing 1.5% by mass of carbon nanotubes (average diameter 10 nm, average length 5 μm) is prepared. Subsequently, zirconium ammonium carbonate (chemical formula: (NH ₄ ) ₂ [Zr (CO ₃ ) ₂ (OH) ₂ ]) aqueous solution (2.18 g) having a zirconium content of 20.7 mass% in terms of ZrO ₂ was added to distilled water (22 .82) g is added to prepare a pH 6.0 aqueous solution of Zr (composition (2)).
Next, the prepared composition (1) (2 g) was sprayed and added to the stirred lithium-containing composite oxide (15 g), and the lithium-containing composite oxide and carbon nanotube of the example were added. Contact with the dispersion while mixing. The prepared composition (2) (1.85 g) is then added by spraying and brought into contact with mixing. The obtained mixture was heated at 400 ° C. for 1 hour in an air atmosphere, and the surface of the lithium-containing composite oxide was composed of particles (III) covered with the carbon material (I) and the oxide (II) containing Zr. 6 positive electrode active material (F) is obtained.

(Example 7) <Carbon nanotube and Al coating on lithium-containing composite oxide>
An aqueous Al solution having a pH of 5.5 was prepared by adding distilled water (22.80 g) to a basic aluminum lactate aqueous solution (2.20 g) having an aluminum content of 8.5% by mass in terms of Al ₂ O ₃ as the composition (2). The positive electrode active material (G) of Example 7 is obtained in the same manner as in Example 6 except that it is used.

(Example 8) <Carbon nanotube and Ti coating on lithium-containing composite oxide>
The composition (2) was carried out except that a titanium aqueous solution having a titanium content of 13.5% by mass in terms of TiO ₂ (3.34 g) and distilled water (21.66 g) added thereto and a pH 4.5 aqueous solution of Ti was used. In the same manner as in Example 6, the positive electrode active material (H) of Example 8 is obtained.

Example 9 <Carbon nanotube and Zr particle coating on lithium-containing composite oxide>
A pH 8.1 aqueous dispersion (composition (1)) containing 1.5% by mass of carbon nanotubes (average diameter 10 nm, average length 5 μm) is prepared. Next, an acidic zirconia dispersion having a zirconium content of 30% by mass in terms of ZrO ₂ (manufactured by Sakai Chemical Industry Co., Ltd., SZR zirconia aqueous dispersion, average particle size 3.9 nm) (1.5 g), distilled water (23.5 g) Is added to prepare a ZrO ₂ dispersion (composition (2)) having a pH of 3.8.
Next, the prepared carbon nanotube dispersion (2 g) is sprayed and added to the lithium-containing composite oxide (15 g) of the stirring example, and the lithium-containing composite oxide and carbon nanotube dispersion of the example are added. Contact with mixing with liquid. The prepared ZrO ₂ dispersion (1.85 g) is then added by spraying and contacted with mixing. The obtained mixture was heated at 300 ° C. for 1 hour in a nitrogen atmosphere, and the surface of the lithium-containing composite oxide was composed of particles (III) covered with the carbon material (I) and the oxide (II) containing the Zr element. The positive electrode active material (I) of Example 9 is obtained.

(Example 10) <Carbon nanotube and Zr particle coating on lithium-containing composite oxide>
The same procedure as in Example 9 was performed except that an aqueous dispersion (pH 8.6) containing 1.5% by mass of graphene (average film thickness 3 nm, average equivalent circle diameter 3 μm) was used as the composition (1). A positive electrode active material (J) is obtained.

(Example 11) <Carbon nanotube and Zr particle coating on lithium-containing composite oxide>
The same procedure as in Example 9 was used except that an aqueous dispersion (pH 6.5) containing 7.5% by mass of carbon black (average dispersed particle size 100 nm) was used as the composition (1). K) is obtained.

(Example 12) <Coating of carbon nanotubes to lithium-containing composite oxide>
The positive electrode active material (L) of Example 12 is obtained in the same manner as in Example 1 except that the heating temperature is 150 ° C.

(Example 13) <Carbon nanotube and Zr particle coating on lithium-containing composite oxide>
The positive electrode active material (M) of Example 12 is obtained in the same manner as in Example 9 except that the heating temperature is 150 ° C.

(Comparative Example 1)
The lithium-containing composite oxide of the example was not subjected to coating treatment, and the positive electrode active material (N) of Comparative Example 1 was used.

(Comparative Example 2) <Zr coating on lithium-containing composite oxide>
Zirconium ammonium carbonate (chemical formula: (NH ₄ ) ₂ [Zr (CO ₃ ) ₂ (OH) ₂ ]) aqueous solution (2.18 g) having a zirconium content of 20.7 mass% in terms of ZrO ₂ was added to distilled water (22. 82) g is added to prepare a pH 6.0 aqueous solution of Zr (composition (2)).
Next, the composition (2) (1.85 g) is sprayed and added to the lithium-containing composite oxide (15 g) of the stirring example, and brought into contact with mixing. The obtained mixture was heated at 400 ° C. for 1 hour in an air atmosphere, and the positive electrode active material (O) of Comparative Example 2 consisting of particles coated with oxide (II) containing Zr on the surface of the lithium-containing composite oxide was obtained. obtain.

(Comparative Example 3) <Carbon black coating on lithium-containing composite oxide>
The positive electrode active material (P) of Comparative Example 3 was prepared in the same manner as in Example 1 except that an aqueous dispersion (pH 6.2) containing 5% by mass of carbon black (average dispersed particle size 600 nm) was used as the composition (1). Get.

(Comparative Example 4)
LiMn ₂ O ₄ having a D50 of 5 μm and a specific surface area of 4 m ² / g is used as the positive electrode active material (Q) of Comparative Example 4.

(Comparative Example 5) <Carbon nanotube coating on LiMn ₂ O ₄ >
The positive electrode active material of Comparative Example 5 was prepared in the same manner as in Example 1 except that LiMn ₂ O ₄ having a D50 of 5 μm and a specific surface area of 4 m ² / g was used instead of the lithium-containing composite oxide of Example. R) is obtained.

(Comparative Example 6) <Carbon nanotube and Zr coating on LiMn ₂ O ₄ >
The positive electrode active material of Comparative Example 6 was prepared in the same manner as in Example 6 except that LiMn ₂ O ₄ having a D50 of 5 μm and a specific surface area of 4 m ² / g was used instead of the lithium-containing composite oxide of Example. S) is obtained.

<Preparation of positive electrode sheet>
As the positive electrode active material, the positive electrode active materials (A) to (S) of Examples 1 to 13 and Comparative Examples 1 to 6 were used, respectively, and the positive electrode active material, acetylene black (conductive material), and polyvinylidene fluoride ( A polyvinylidene fluoride solution (solvent N-methylpyrrolidone) containing 12.1% by mass of a binder is mixed, and N-methylpyrrolidone is further added to prepare a slurry. The positive electrode active material, acetylene black, and polyvinylidene fluoride are in a mass ratio of 82/10/8. One side of the slurry is applied to a 20 μm thick aluminum foil (positive electrode current collector) using a doctor blade. The positive electrode sheet of Examples 1 to 13 and Comparative Examples 1 to 6 to be a positive electrode for a lithium battery is prepared by drying at 120 ° C. and performing roll press rolling twice.

(Comparative Example 7)
As the positive electrode active material, the positive electrode active material (N) of Comparative Example 1 was used, and the positive electrode active material, carbon nanotubes (average diameter 10 nm, average length 5 μm), acetylene black (conductive material), and polyvinylidene fluoride (binder) 12 A polyvinylidene fluoride solution containing 1% by mass (solvent N-methylpyrrolidone) is mixed, and further N-methylpyrrolidone is added to prepare a slurry. The positive electrode active material, carbon nanotube, acetylene black, and polyvinylidene fluoride are 81.8 / 0.164 / 10/8 in mass ratio. One side of the slurry was applied to a 20 μm thick aluminum foil (positive electrode current collector) using a doctor blade. The positive electrode body sheet of the comparative example 7 used as the positive electrode for lithium batteries is produced by drying at 120 degreeC and performing roll press rolling twice.

(Comparative Example 8)
As the positive electrode active material, the positive electrode active material (N) of Comparative Example 1 was used, and the positive electrode active material, graphene (average film thickness 3 nm, average equivalent circle diameter 5 μm), acetylene black (conductive material), and polyvinylidene fluoride (binder) Is mixed with a polyvinylidene fluoride solution containing 12.1% by mass (solvent N-methylpyrrolidone), and N-methylpyrrolidone is further added to prepare a slurry. The positive electrode active material, graphene, acetylene black, and polyvinylidene fluoride have a mass ratio of 81.8 / 0.164 / 10/8. One side of the slurry is applied to a 20 μm thick aluminum foil (positive electrode current collector) using a doctor blade. The positive electrode sheet of Comparative Example 8 which becomes a positive electrode for a lithium battery is prepared by drying at 120 ° C. and performing roll press rolling twice.

(Comparative Example 9)
The positive electrode active material (N) of Comparative Example 1 was used as the positive electrode active material, and 12.1 mass of the positive electrode active material, carbon black (average dispersed particle size 100 nm), acetylene black (conductive material), and polyvinylidene fluoride (binder). A polyvinylidene fluoride solution containing 1% (solvent N-methylpyrrolidone) is mixed, and N-methylpyrrolidone is further added to prepare a slurry. The positive electrode active material, carbon black, acetylene black, and polyvinylidene fluoride have a mass ratio of 81.2 / 0.82 / 10/8. One side of the slurry is applied to a 20 μm thick aluminum foil (positive electrode current collector) using a doctor blade. The positive electrode body sheet of the comparative example 9 used as the positive electrode for lithium batteries is produced by drying at 120 degreeC and performing roll press rolling twice.

<Battery assembly>
A punched positive electrode sheet of Examples 1 to 13 and Comparative Examples 1 to 9 is used for the positive electrode, a metal lithium foil having a thickness of 500 μm is used for the negative electrode, and the negative electrode current collector is 1 mm thick. The separator is made of 25 μm-thick porous polypropylene, and the electrolyte is LiPF ₆ / EC (ethylene carbonate) + DEC (diethyl carbonate) (1) with a concentration of 1 (mol / dm ³ ). : 1) solution (volume ratio of the LiPF ₆ EC and DEC to solute (EC: DEC = 1: 1). which means mixed solution) example 1 to the stainless steel simple closed cell type with The lithium batteries of Example 13 and Comparative Examples 1 to 9 are assembled in an argon glove box.

<Evaluation of initial capacity><Evaluation of rate characteristics><Evaluation of cycle characteristics>
The lithium batteries of Examples 1 to 13 and Comparative Examples 1 to 9 are evaluated at 25 ° C.
Charge to 4.8 V with a load current of 150 mA per 1 g of the positive electrode active material, and discharge to 2.5 V with a load current of 37.5 mA per 1 g of the positive electrode active material. Subsequently, the battery is charged to 4.3 V at a load current of 150 mA per 1 g of the positive electrode active material, and discharged to 2.5 V at a load current of 37.5 mA per 1 g of the positive electrode active material.
The lithium batteries of Examples 1 to 13 and Comparative Examples 1 to 9 that were charged and discharged in this manner were subsequently charged to 4.5 V with a load current of 200 mA per 1 g of the charge / discharge positive electrode active material, and the positive electrode active material Discharge to 2.5 V at a load current of 100 mA per gram. The discharge capacity of the positive electrode active material at 4.5 to 2.5 V is set to 4.5 V initial capacity.

Subsequently, it charges to 4.5V with a load current of 200 mA per 1 g of charge / discharge positive electrode active material, and discharges at a high rate to 2.5 V with a load current of 2000 mA per 1 g of positive electrode active material. The value obtained by dividing the discharge capacity of the positive electrode active material at 4.5 to 2.5 V in the high rate discharge by the 4.5 V initial capacity is defined as the rate maintenance rate.
Next, a charge / discharge cycle of charging to 4.5 V with a load current of 200 mA per 1 g of charge / discharge positive electrode active material and discharging at a high rate to 2.5 V with a load current of 100 mA per 1 g of positive electrode active material is repeated 100 times. The value obtained by dividing the discharge capacity at the 100th 4.5V charge / discharge cycle by the 4.5V initial capacity is defined as the cycle maintenance ratio.

Table 1 summarizes the 4.8V initial capacity, 4.5V initial capacity, rate maintenance rate, and cycle maintenance rate of the lithium batteries of Examples 1 to 13 and Comparative Examples 1 to 9.
In Table 1, a case where the 4.5 V initial capacity is larger than 200 mAh / g is evaluated as “◯”, a case where it is 180 to 200 mAh / g is evaluated as “Δ”, and a case where it is less than 180 mAh / g is evaluated as “X”. The rate maintenance rate is evaluated as ◯ when the rate is greater than 80%, Δ when the rate is 70 to 80%, and × when the rate is less than 70%. Moreover, the case where the cycle maintenance ratio is greater than 80% is evaluated as ◯, the case where it is 70 to 80% is evaluated as Δ, and the case where it is less than 70% is evaluated as ×.

In Examples 1 to 13, since the surface of the positive electrode active material is coated with the carbon material (I), the rate maintenance rate and the cycle maintenance rate are excellent as compared with Comparative Example 1 that is only the lithium-containing composite oxide.
Furthermore, since Examples 1-13 can coat | cover uniformly on the surface of lithium containing complex oxide since carbon material (I) is nano size, compared with the comparative example 3, the rate maintenance factor is excellent, Li By using a lithium-containing oxide having a high element ratio, the initial capacity is superior to Comparative Examples 4 to 6.
Since Examples 9 to 11, 12, and 13 have the carbon material (I) and the oxide (II), not only the cycle maintenance rate but also the rate maintenance rate are excellent as compared with Comparative Example 2.

  According to the present invention, a positive electrode active material for a lithium ion secondary battery having a high discharge capacity per unit mass and excellent rate characteristics and cycle characteristics can be obtained. The positive electrode active material can be used for lithium-ion secondary batteries for electronic devices such as small and light mobile phones, in-vehicle batteries, and the like.

Claims (8)

Li element and at least one transition metal element selected from Ni, Co, and Mn are included (however, the molar amount of Li element is more than 1.2 times the total molar amount of the transition metal element). ) The lithium ion-containing composite oxide is composed of the following carbon material (I) or particles (III) coated with the following carbon material (I) and the following oxide (II). Positive electrode active material for secondary batteries.
Carbon material (I): At least one carbon material selected from carbon nanotubes, graphene, and carbon black having an average dispersed particle size of 0.2 μm or less.
Oxide (II): An oxide of at least one metal element selected from Zr, Ti, and Al.   The positive electrode active material according to claim 1, wherein the mass of the carbon material (I) is 0.0001 to 0.05 times the mass of the lithium-containing composite oxide.   The molar amount of at least one metal element selected from Zr, Ti, and Al is 0.0001 to 0.05 times the molar amount of the transition metal element of the lithium-containing composite oxide. The positive electrode active material as described.   The positive electrode for lithium ion secondary batteries containing the positive electrode active material for lithium ion secondary batteries as described in any one of Claims 1-3, a electrically conductive material, and a binder.   The lithium ion secondary battery containing the positive electrode of Claim 4, a negative electrode, and a nonaqueous electrolyte. Li element and at least one transition metal element selected from Ni, Co, and Mn are included (however, the molar amount of Li element is more than 1.2 times the total molar amount of the transition metal element). Lithium ion secondary characterized in that the following composition (1) or the following composition (1) and the following composition (2) are brought into contact with the lithium-containing composite oxide and heated at 50 to 500 ° C. A method for producing a positive electrode active material for a battery.
Composition (1): A composition in which at least one carbon material (I) selected from carbon nanotubes, graphene, and carbon black having an average dispersed particle size of 0.2 μm or less is dispersed in a solvent.
Composition (2): A composition in which a compound containing at least one metal element selected from Zr, Ti, and Al is dissolved or dispersed in a solvent.   The composition (1) or the composition (1) and the composition (2) are added to the lithium-containing composite oxide and stirred to obtain a lithium-containing composite oxide and the composition (1). Or the method according to claim 6, wherein the composition (1) and the composition (2) are contacted.   The composition (1), or the composition (1) and the composition (2) are sprayed onto the lithium-containing composite oxide by a spray coating method, so that the lithium-containing composite oxide and the composition (1) are sprayed. ), Or the composition (1) and the composition (2) are brought into contact with each other.