JP5617663B2 - Cathode active material for lithium ion secondary battery and method for producing the same - Google Patents

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 secondary battery using the positive electrode active material.

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, miniaturization and weight reduction have been demanded for portable electronic devices and in-vehicle lithium ion secondary batteries, and further improvements in performance such as discharge capacity per unit mass, cycle characteristics, and stability of the battery are desired. Yes.

For the purpose of improving the performance, composite oxides (hereinafter sometimes referred to as “Li-rich positive electrode materials”) in which the ratio of the Li element to the transition metal element such as Ni, Co, and Mn is increased have been proposed. .
As an example of the Li-rich positive electrode material, a solid solution of LiMO ₂ (M is at least one transition metal element selected from Ni, Co, and Mn) and Li ₂ MnO ₃ has been proposed. In order to use a lithium ion secondary battery using the solid solution as a positive electrode active material at a high capacity, it is necessary to charge at a high voltage of 4.5 V or more at the first charge.

In the positive electrode active material at the time of the first charge, it is considered that the reaction shown in the following formula (1) is progressing. _That, Li 2 MnO ₃ from Li ₂ O are eliminated, is deposited further Li ₂ O is Li on the negative electrode, to generate the positive electrode _{O 2} (oxygen gas). Li deposited on the negative electrode cannot return to the positive electrode side even by discharge, causing irreversible capacity, and the generation of oxygen gas can cause the battery to burst.
Li ₂ MnO ₃ → MnO ₂ + Li ₂ O
→ LiMnO ₂ + Li (irreversible capacity) + 0.5O ₂ (1)

Non-Patent Document 1 discloses that surface treatment with VOx is performed to suppress gas generation during charging of a positive electrode material containing 0.5Li ₂ MnO ₃ —0.5LiNi _0.4 Co _0.2 Mn _0.4 O _2. The technology to do is described.
FIG. 1 of Patent Document 1 describes a positive electrode active material such as LiCo _1/3 Mn _1/3 Ni _1/3 O ₂ treated with NH ₃ at 250 ° C. and 350 ° C. No change is observed in the peak of the X-ray diffraction of the positive electrode active material treated with NH ₃ .

Patent Document 2 describes a method for producing a positive electrode active material in which a raw material lithium composite metal oxide composed of Li ₂ MnO ₃ is heat-treated at 300 ° C. or 310 ° C. in the presence of a hydride such as CaH ₂ .

US Pat. No. 7,314,682 JP 2010-198989 A

Chemical Communications, 2010, 46, 4090-4092

In the surface treatment using VOx, LiV ₃ O ₈ is generated due to Li conversion of VOx. However, since the LiV ₃ O ₈ average discharge voltage is as low as 3 V or less, the average discharge voltage is lowered.
As a method for reducing the oxygen gas generated during the initial charge, it is conceivable to heat-treat the Li-rich positive electrode material in an atmosphere containing ammonia. However, in the process of the NH _3, the reduction force with respect to low Li-rich based positive electrode material of the NH _3.
The treatment with a hydride such as CaH ₂ also has a low reducing power, and no peak at 2θ = 42 to 44 is observed in the X-ray diffraction diagram.

  The present invention provides a positive electrode active material for a lithium ion secondary battery that is excellent in discharge capacity and cycle characteristics and can suppress gas generation in a battery using the same as a positive electrode, positive electrode, lithium ion secondary battery, and lithium ion A method for producing a positive electrode active material for a secondary battery is provided.

The present invention relates to 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-containing composite oxide and a metal oxide represented by the formula MO (where M is a transition metal element corresponding to the transition metal element); The ratio (H1 / H2) between the maximum peak height (H1) of 42 to 43.8 ° and the maximum peak height (H2) of 2θ = 44 to 46 ° is 0.03 to 0.5, and 2θ = Lithium ions characterized in that the ratio (H3 / H4) of the maximum peak height (H3) of 14-16 ° to the maximum peak height (H4) of 2θ = 18-20 ° is 0-0.1 Positive electrode active material for secondary battery.

[2] A positive electrode for a lithium ion secondary battery comprising the positive electrode active material for a lithium ion secondary battery according to [1], a conductive material, and a binder.
[3] A lithium ion secondary battery comprising a positive electrode, a negative electrode, and a nonaqueous electrolyte, wherein the positive electrode is the positive electrode according to [2].

[4] Including Li element and at least one transition metal element selected from Ni, Co, and Mn (provided that the molar amount of Li element exceeds 1.2 times the total molar amount of the transition metal element) The lithium-containing composite oxide is heat-treated at 400 to 470 ° C. in an atmosphere containing hydrogen, whereby the lithium-containing composite oxide and the metal oxide represented by the formula MO (where M is the transition metal) A method for producing a positive electrode active material for a lithium ion secondary battery, comprising obtaining a positive electrode active material for a lithium ion secondary battery containing a transition metal element corresponding to the element.
[5] Ratio of the maximum peak height (H1) of 2θ = 42 to 43.8 ° and the maximum peak height (H2) of 2θ = 44 to 46 ° in the X-ray diffraction diagram of the positive electrode active material (H1 / H2) is 0.03 to 0.5, and the ratio (H3 / H4) of the maximum peak height (H3) of 2θ = 14 to 16 ° and the maximum peak height (H4) of 2θ = 18 to 20 ° The manufacturing method of the positive electrode active material for lithium ion secondary batteries as described in [4] whose is 0-0.1.
[6] The method for producing a positive electrode active material according to [4] or [5], wherein the positive electrode active material is washed with water or a polar organic solvent after the heat treatment.
[7] The production method according to any one of [4] to [6], wherein the hydrogen concentration in an atmosphere containing hydrogen is 0.3 to 20% by mass.

  The lithium ion secondary battery provided with the positive electrode active material positive electrode of the present invention exhibits excellent discharge capacity.

In addition, the positive electrode active material of the present invention contains a metal oxide (MO) from the analysis result in the X-ray diffraction measurement, and the layered crystal structure is not broken or even if the crystal structure is broken. Is. Therefore, the positive electrode active material of the present invention is one in which LiMO ₂ is generated by first charging a lithium ion secondary battery having a positive electrode using the positive electrode active material, and has excellent discharge capacity. The generation of Li and oxygen gas is suppressed.

  Further, according to the method for producing a positive electrode active material of the present invention, a uniformly reduced lithium-containing composite oxide can be obtained, and a sufficient amount of metal oxide (MO) can be generated.

FIG. 1 is a graph showing the results of TG-DTA measurement in an N ₂ gas atmosphere containing 3% by mass of H ₂ gas of the lithium-containing composite oxide of the example. FIG. 2 is a graph showing the results of XRD measurement of the positive electrode active materials of Examples and Comparative Examples 1, 2, and 4.

<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). More than 1.2 times.) Lithium-containing composite oxide and metal oxide (MO) (where M is a transition metal element corresponding to the transition metal element). The ratio of the element is a value in the lithium-containing composite oxide before the first charge (also referred to as activation) described later.

  In a lithium ion secondary battery including a positive electrode using the lithium-containing composite oxide, when the molar amount of Li element in the lithium-containing composite oxide is more than 1.2 times the total molar amount of the transition metal element, The discharge capacity per unit mass after activation (first charge) can be improved. The ratio of the molar amount of the Li element to the total molar amount of the transition metal element is preferably 1.25 to 1.75 in order to further increase the discharge capacity per unit mass of the lithium ion secondary battery. More preferably, it is from 25 to 1.65.

The transition metal element contains at least one element selected from Ni, Co, and Mn, preferably contains Mn, and particularly preferably contains Ni, Co, and Mn. The transition metal element may consist only of Ni, Co, and Mn, and may contain a transition metal element other than Ni, Co, and Mn (hereinafter referred to as other transition metal elements) as necessary. Good. As other transition metal elements, transition metal elements selected from Cr, Fe, Al, Ti, Zr, Mg, and the like are preferable. When other transition metal elements are included, the amount is preferably 0.001 to 0.50 times mole, more preferably 0.003 to 0.10 times mole, and more preferably 0.005 to the total amount (molar amount) of the transition metal. -0.05 times mole is particularly preferable.
A preferred composition of the lithium-containing composite oxide will be described later.

  The positive electrode active material of the present invention has a maximum peak height (H1) of 2θ = 42 to 43.8 ° and a maximum of 2θ = 44 to 46 ° in X-ray diffraction (XRD) measurement using CuKα rays as an X-ray source. The ratio (H1 / H2) to the peak height (H2) is 0.03 to 0.5, the maximum peak height (H3) of 2θ = 14 to 16 ° and the maximum peak height of 2θ = 18 to 20 ° The ratio (H3 / H4) to (H4) is 0 to 0.1. H1 / H2 is more preferably 0.04 to 0.4, and particularly preferably 0.05 to 0.3. H3 / H4 is preferably 0 to 0.05, particularly preferably 0 to 0.03.

In the X-ray diffraction measurement, the H1 is a cubic (space group Fm-3m) CoO or NiO <200> plane, H2 is a layered rock salt crystal structure (space group R-3m) <104> plane, and H3 is The <010> plane of LiMnO ₂ of orthorhombic crystal (space group Pm2m) and H4 are the heights of the peaks attributed to the <003> plane of the layered rock salt type crystal structure (space group R-3m).
In order to exhibit the target performance of the positive electrode active material of the present invention, H1 / H2 serving as an index indicating that a sufficient amount of metal oxide (MO) is contained is 0.03 to 0.5, and further a layered crystal H3 / H4 whose index is maintaining the structure is 0 to 0.1. That is, the positive electrode active material of the present invention contains a metal oxide (MO), and even if the layered crystal structure is not broken or the crystal structure is broken, there is little. In the conventional positive electrode active material, since H1 / H2 is 0.00 to 0.01, the peak ratio in the X-ray diffraction diagram does not fall within the scope of the present invention.

<Method for producing positive electrode active material>
The positive electrode active material for a lithium ion secondary battery of the present invention can be produced by the following production method. That is, it 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 more than 1.2 times the total molar amount of the transition metal element). A lithium-containing composite oxide and a metal oxide (MO) (where M corresponds to the transition metal element ) by heat-treating the lithium-containing composite oxide at 400 to 470 ° C. in an atmosphere containing hydrogen. A transition metal element).

  The lithium-containing composite oxide 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 relative to the total molar amount of the transition metal element). It is sufficient that it is more than 1.2 times), and is not particularly limited, but a compound represented by the following formula (3) is preferable.

_{Li (Li x Mn y Me z} ) O p F q (3)
(However, Me is at least one element selected from Co, Ni, Cr, Fe, Al, Ti, Zr, and Mg. X is 0.09 <x <0.3, and y and z are positive integers. 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. x is preferably 0.1 <x <0.25, and more preferably 0.11 <x <0.22. y and z are preferably 0.5 ≦ y / (y + z) ≦ 0.8, and more preferably 0.55 ≦ y / (y + z) ≦ 0.75.

As a compound represented by the above formula (3), Li (Li _0.13 Ni _0.26 Co _0.09 Mn _0.52 ) O ₂ , 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 .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 , 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.

Further, examples of the compound represented by the formula _{(3), 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,} are particularly Masui.

  For example, when the lithium-containing composite oxide is a compound represented by the formula (3), the ratio of the molar amount of the Li element to the total molar amount of the transition metal element is 1.2 <(1 + x) / (y + z). Yes, 1.25 ≦ (1 + x) / (y + z) ≦ 1.75 is preferable, and 1.25 ≦ (1 + x) / (y + z) ≦ 1.65 is more preferable. When the composition ratio is in the above range, the discharge capacity per unit mass can be increased.

The lithium-containing composite oxide is preferably particulate, and the average particle diameter (D50) is preferably 3 to 30 μm, more preferably 4 to 25 μm, and particularly preferably 5 to 20 μm. In the present invention, the average particle size (D50) is a volume-based cumulative particle size distribution at which the particle size distribution is obtained on a volume basis and the cumulative curve is 50% in a cumulative curve with the total volume being 100%. 50% diameter is meant.
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.

  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, Examples include ion exchange. In addition, since the discharge capacity is improved when the transition metal element is uniformly contained in the lithium-containing composite oxide, the precursor (co-precipitation composition) of the lithium-containing composite oxide obtained by the coprecipitation method and the lithium compound It is preferable to use a method of mixing and baking.

In the manufacturing method of the positive electrode active material for lithium ion secondary batteries of this invention, said lithium containing complex oxide is heat-processed at 400-470 degreeC in the atmosphere containing hydrogen.
The atmosphere containing hydrogen refers to the presence of hydrogen gas. The hydrogen gas can be used as it is or diluted with an inert gas, the latter being preferred. Examples of the hydrogen gas atmosphere diluted with an inert gas, and N ₂ gas atmosphere containing H ₂ gas, such as Ar gas atmosphere containing H ₂ gas are mentioned, N ₂ gas containing H ₂ gas in terms of cost An atmosphere is preferred. The hydrogen concentration is preferably 0.3 to 20% by mass, more preferably 0.5 to 5.5% by mass, and particularly preferably 1 to 3.5% by mass. A sufficient reduction reaction can be carried out by setting the hydrogen concentration to 0.3 to 20% by mass.

In the method for producing a positive electrode active material for a lithium ion secondary battery of the present invention, when the above lithium-containing composite oxide is used, the following formulas (4) and (5) are obtained by heat treatment in an atmosphere containing hydrogen. A reaction is thought to occur.
2Li ₂ MnO ₃ + H ₂ → 2LiMnO ₂ + 2LiOH (4)
2LiMO ₂ + H ₂ → 2MO + 2LiOH (5)

When a compound represented by Li ₂ MnO ₃ and LiMO ₂ is used as the lithium-containing composite oxide, the above formula (4) is obtained between the compound represented by Li ₂ MnO ₃ and LiMO ₂ and hydrogen by heat treatment. ) And (5) proceed, and a metal oxide represented by the formula MO can be generated on the surface of the lithium-containing composite oxide. Here, if the amount of H ₂ gas is too small, the amount of metal oxide (MO) produced is reduced. Further, if the H ₂ gas contained in the atmosphere containing hydrogen exceeds the above range, the metal oxide (MO) is further reduced by hydrogen to M (metal), and battery characteristics such as capacity reduction may be deteriorated. There is.

  When the lithium-containing composite oxide is heat-treated in the presence of a solid reducing agent such as a metal hydride, it is difficult to perform a uniform reduction reaction, and the product has an effect equivalent to that of the present invention. I can't get it. On the other hand, in the production method of the present invention, by using hydrogen and reacting under specific temperature conditions, a lithium-containing composite oxide whose surface is uniformly reduced can be obtained, and a sufficient amount of metal can be uniformly obtained. It is thought that an oxide (MO) can be generated.

The positive electrode active material of the present invention shows a specific pattern in an X-ray diffraction diagram. That is, the maximum peak height (H1) of 2θ = 42 to 43.8 ° in the X-ray diffraction measurement indicates the amount of NiO or CoO that is a metal oxide (MO) generated, and the maximum of 2θ = 14 to 16 °. The peak height (H3) indicates the amount of orthorhombic LiMnO ₂ produced by breaking the layered crystal structure.
Therefore, the lithium metal composite oxide having a peak height ratio (H1 / H2) of 0.03 to 0.5 and a peak height ratio (H3 / H4) of 0 to 0.1 is a metal An oxide (MO) is included, and the crystal structure of the lithium metal composite oxide is a layered structure or a slight structure even when a part of the layered structure is broken. That is, since the lithium-containing composite oxide in the present invention maintains a layered crystal structure, a high discharge capacity can be maintained.

  The heat treatment temperature in an atmosphere containing hydrogen is 400 to 470 ° C, preferably 430 to 465 ° C, and more preferably 450 to 460 ° C. By setting the heat treatment temperature to 400 to 470 ° C., a sufficient amount of metal oxide (MO) can be generated. When the heat treatment temperature is less than 400 ° C., metal oxide (MO) cannot be generated, and the effect of performing the heat treatment cannot be obtained. Further, when the heat treatment temperature exceeds 470 ° C., the metal oxide (MO) is reduced by hydrogen to become M (metal), and a sufficient amount of metal oxide (MO) may not be obtained. The layered crystal structure may collapse.

  The heat treatment time is preferably 0.1 to 10 hours, more preferably 0.5 to 5 hours, and particularly preferably 1 to 3 hours. By setting the heat treatment time in the range of 0.1 to 10 hours, a sufficient amount of metal oxide (MO) can be efficiently generated.

  In the production method of the present invention, the product after the heat treatment is preferably washed with water or a polar organic solvent. Examples of the polar organic solvent include methanol, ethanol, 2-propanol and the like. The positive electrode active material is preferably washed only with water. Washing with water is excellent in terms of safety, environment, handling, and cost.

When the positive electrode active material is washed with water or a polar organic solvent, LiOH in the product can be removed.
The amount of water or the polar organic solvent used for washing is not particularly limited as long as LiOH can be removed. Specifically, it is preferably 0.5 to 1000 times, more preferably 5 to 100 times the mass of the positive electrode active material.

LiOH causes the electrolytic solution to decompose and reduce the discharge capacity. For example, when a mixed solution of ethylene carbonate (EC) and diethyl carbonate (DEC) in a mass ratio (EC: DEC = 1: 1) having LiPF ₆ as a solute is used as the electrolytic solution, the electrolytic solution is decomposed. This will cause the discharge capacity to decrease.
In addition, LiOH makes the slurry easy to aggregate when forming the slurry containing the positive electrode active material in the step of manufacturing the positive electrode, and makes it difficult to manufacture the positive electrode. For this reason, it is preferable to remove LiOH from the positive electrode active material.

  In the positive electrode active material obtained by the production method of the present invention, a metal oxide (MO) is generated on the surface of the lithium-containing composite oxide, and the generation of Li and oxygen gas generated during the initial charge can be suppressed. it is conceivable that. In addition, since a lithium-containing composite oxide having a high lithium ratio is included, it is considered that a high discharge capacity can be maintained in a lithium ion secondary battery including a positive electrode using the same.

<Positive electrode>
The positive electrode for lithium ion secondary batteries of this invention contains said positive electrode active material, a electrically conductive material, and a binder.
The positive electrode for a lithium ion secondary battery is formed by forming a positive electrode active material layer containing the positive electrode active material of the present invention on a positive electrode current collector (positive electrode surface). The positive electrode for a lithium ion secondary battery is, for example, a slurry or kneaded product 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. And the prepared slurry or kneaded material is supported 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 the present invention includes a positive electrode, a negative electrode, and a nonaqueous electrolyte, and a positive electrode material used for the positive electrode before activation includes the above lithium composite metal compound.
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. For example, 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 include non-graphitizable carbon, artificial graphite, natural graphite, pyrolytic carbon, pitch coke, needle coke, petroleum coke, and other cokes, graphite, glassy carbon, phenolic resin, and furan resin. An organic polymer compound fired body, carbon fiber, activated carbon, carbon black or the like obtained by firing and carbonizing the above at an appropriate temperature can be used.
The group 14 metal of the periodic table is, for example, silicon or tin, and most preferably silicon. In addition, materials that can occlude and release lithium ions at a relatively low potential include, for example, oxides such as iron oxide, ruthenium oxide, molybdenum oxide, tungsten oxide, titanium oxide, and tin oxide, and other nitrides. It can be used similarly.

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. Any organic solvent can be used as long as it is used in this type of battery. For example, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxy. Ethane, 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. Moreover, such an organic solvent may be used individually by 1 type, and may be used in mixture of 2 or more types.

In addition, as the nonaqueous electrolyte, a solid electrolyte containing an electrolyte salt, a polymer electrolyte, a solid or 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. For example, any of 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. As this polymer compound, an ether polymer such as poly (ethylene oxide) or a crosslinked product thereof, a poly (methacrylate) ester, an acrylate, or the like is used alone or in a molecule, or a mixture thereof is used. be able to.

The matrix of the gel electrolyte may be any matrix that absorbs the non-aqueous electrolyte and gels, and various polymers can be used. In addition, as the polymer material used for the gel electrolyte, for example, fluorine-based polymers such as poly (vinylidene fluoride) and poly (vinylidene fluoride-co-hexafluoropropylene) can be used. Moreover, as a polymer material used for the gel electrolyte, for example, polyacrylonitrile and a copolymer of polyacrylonitrile can be used. Moreover, as a polymer material used for the gel electrolyte, for example, an ether polymer such as polyethylene oxide, a copolymer of polyethylene oxide, and a crosslinked product thereof can be used. Examples of the copolymerization monomer include polypropylene oxide, methyl methacrylate, butyl methacrylate, methyl acrylate, and butyl acrylate.
In addition, from the viewpoint of stability against the redox reaction, it is particularly preferable to use a fluorine-based polymer among the above-described polymers.

Any electrolyte salt can be used as long as it is used in this type of battery. As the electrolyte salt, for example, LiClO ₄ , LiPF ₆ , LiBF ₄ , CH ₃ SO ₃ Li, LiCl, LiBr, or the like can be used.
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. .

The lithium ion secondary battery of the present invention is used after being activated. In the lithium ion secondary battery of the present invention, Li ₂ MnO ₃ and MO contained in the positive electrode undergo a reaction shown in the following formula (2) by performing initial charge as activation to generate LiMO ₂ . Let The produced LiMO ₂ functions as a positive electrode material because it releases Li during charging and absorbs it during discharging together with LiMO ₂ contained in the positive electrode before activation.
Li ₂ MnO ₃ + MO → MnO ₂ + Li ₂ O + MO
→ LiMnO ₂ + LiMO ₂ (2)

In the lithium ion secondary battery of the present invention, LiMO ₂ generated by activation is charged and discharged together with LiMO ₂ contained in the positive electrode before activation, and Li is moved between the positive electrode and the negative electrode. Thus, the lithium ion secondary battery of the present invention is excellent in discharge capacity and cycle characteristics.

<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 (high frequency inductively coupled plasma), they were 11.6 mass%, 10.5 mass%, and 42.3 mass%, respectively (molar ratio of nickel). : Cobalt: Manganese = 0.172: 0.156: 0.672).

The precursor (20 g) and lithium carbonate (12.6 g) having a lithium content of 26.9 mol / kg were mixed and calcined at 800 ° C. for 12 hours in an oxygen-containing atmosphere. Got. 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-DTA (thermogravimetric-differential thermal analysis) measurement>
Using a TG-DTA measuring device (trade name: TG-DTA2000SA, manufactured by Bruker AXS), TG-DTA measurement of the lithium-containing composite oxides of the examples was performed. As the atmosphere gas in TG-DTA measurement using a _{N 2} gas containing _{H 2} gas 3 wt%. Moreover, in TG-DTA measurement, it heated up from room temperature to 600 degreeC with the temperature increase rate of 5 degree-C / min, and measured. The result is shown in FIG.

FIG. 1 is a graph showing the results of TG-DTA measurement of the lithium-containing composite oxide of the example. As shown in FIG. 1, in the lithium containing complex oxide of an Example, it turns out that TG increases little by little with temperature rise in the range of room temperature-350 degreeC.
This is because the lithium-containing composite oxide occludes H ₂ or, as shown in the following formulas (4) and (5), the reaction for generating the metal oxide (MO) proceeds and the weight (solid weight) ) Is estimated to have increased.
2Li ₂ MnO ₃ + H ₂ → 2LiMnO ₂ + 2LiOH (4)
2LiMO ₂ + H ₂ → 2MO + 2LiOH (5)

Moreover, as shown in FIG. 1, it turns out that TG is decreasing rapidly with the temperature rise from the temperature of about 450 degreeC.
As shown in the following formulas (6), (7), and (8), the lithium-containing composite oxide is reduced by H ₂ to generate metal and water, and the water is volatilized to reduce the weight (solid weight). ) Decreased.
Li ₂ MnO ₃ + 2H ₂ → Mn + 2LiOH + H ₂ O (6)
LiMO ₂ + 1.5H ₂ → M + LiOH + H ₂ O (7)
MO + H ₂ → M + H ₂ O (8)

Further, from the result of the DTA measurement shown in FIG. 1, it was confirmed that the reduction reaction rapidly progressed as the temperature increased from a temperature of about 450 ° C.
As described above, from the result of the TG-DTA measurement shown in FIG. 1, the temperature range of the heat treatment in the atmosphere containing hydrogen is set to 400 to 470 ° C., and the reduction reaction by H ₂ is controlled, whereby the metal oxide ( It is estimated that a positive electrode active material containing MO) can be generated.

<Hydrogen treatment>
15 g of the lithium-containing composite oxide of the example was placed on a ceramic boat and placed in an annular furnace having an inner diameter of φ10 cm. While flowing N ₂ gas containing 3% by mass of H ₂ gas at a flow rate of 0.3 L / min, heating was performed at the hydrogen treatment temperature shown in Table 1 for 3 hours, and Examples 1 to 3 and Comparative Examples 2 to 4 The positive electrode active material was obtained.
Further, the lithium-containing composite oxide of the example was not subjected to hydrogen treatment, and the positive electrode active material of Comparative Example 1 was obtained.

  With respect to the positive electrode active materials of Examples 1 to 3 and Comparative Examples 1 to 4 thus obtained, XRD measurement using CuKα rays as an X-ray source was performed. The product name RINT-TTR-III manufactured by Rigaku Corporation was used for XRD measurement. XRD measurement was performed at a voltage of 50 kV, a tube current of 300 mA, a scanning axis of 2θ / θ, a measurement range of 2θ = 10 to 80 °, a sampling width of 0.02 °, and a scanning speed of 4 ° / min. The background is removed from the obtained XRD spectrum using analysis software MDI-JADE, and the maximum peak height H1 of 2θ = 42 to 43.8 ° and the maximum peak height of 2θ = 44 to 46 ° are obtained from the result. The ratio H1 / H2 with H2, and the ratio H3 / H4 between the maximum peak height H3 of 2θ = 14 to 16 ° and the maximum peak height H4 of 2θ = 18 to 20 ° were calculated. The results are shown in Table 1.

As shown in Table 1, in the positive electrode active materials of Examples 1 to 3, H1 / H2 is in the range of 0.03 to 0.5, and H3 / H4 is in the range of 0 to 0.1. It was. Therefore, the positive electrode active material of the example contains a metal oxide (MO), generates a small amount of orthorhombic LiMnO ₂ that lowers the discharge capacity of the lithium ion secondary battery, and contains a large amount of layered crystal structure. It is a waste.

On the other hand, in Comparative Example 1 where no hydrogen treatment is performed and Comparative Example 2 where the hydrogen treatment temperature is low, the amount of metal oxide (MO) produced is insufficient because H1 / H2 is small.
In Comparative Example 3 and Comparative Example 4 where the hydrogen treatment temperature is high, the amount of orthorhombic LiMO ₂ produced is large because H3 / H4 is large.

Moreover, the result of the XRD measurement of the positive electrode active material of Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 4 is shown in FIG.
As shown in FIG. 2, in the positive electrode active material of Example 1, a peak due to the amount of NiO or CoO that is a metal oxide (MO) is observed at 2θ = 44 to 46 °.
On the other hand, in Comparative Example 1 where no hydrogen treatment is performed and Comparative Example 2 where the hydrogen treatment temperature is low, a peak having a sufficiently high height of 2θ = 44 to 46 ° is not observed. Further, in Comparative Example 4 where the hydrogen treatment temperature is high, as shown in FIG. 2, a peak due to the production amount of orthorhombic LiMnO ₂ is observed at 2θ = 14 to 16 °.

  Next, the positive electrode active materials of Examples 1 to 3 and Comparative Examples 1 to 4 were washed with distilled water to remove LiOH on the surface of the positive electrode active material, and then dried at 300 ° C.

<Preparation of positive electrode sheet>
As the positive electrode active material, the positive electrode active materials of Examples 1 to 3 and Comparative Examples 1 to 4 were used, respectively, and 12.1 mass of the positive electrode active material, acetylene black (conductive material), and polyvinylidene fluoride (binder). % Polyvinylidene fluoride solution (solvent N-methylpyrrolidone) was mixed, and N-methylpyrrolidone was further added to prepare a slurry. The positive electrode active material, acetylene black, and polyvinylidene fluoride were in a mass ratio of 80/12/8. One side of the slurry was applied to a 20 μm thick aluminum foil (positive electrode current collector) using a doctor blade. It dried at 120 degreeC and produced the positive electrode body sheet | seat of the Example used as a positive electrode for lithium batteries, and Comparative Example 1- Comparative Example 4 by performing roll press rolling twice.

<Battery assembly>
A punched positive electrode sheet of Examples 1 to 3 and Comparative Examples 1 to 4 is used as a positive electrode, a metal lithium foil having a thickness of 500 μm is used as a negative electrode, and a stainless steel having a thickness of 1 mm is used as a negative electrode current collector. A plate is used, a porous polypropylene having a thickness of 25 μm is used as a separator, and LiPF ₆ / EC (ethylene carbonate) + DEC (diethyl carbonate) (1: 1) having a concentration of 1 (mol / dm ³ ) as an electrolyte. ) Examples of stainless steel simple sealed cell type and Comparative Examples 1 to 3 using a solution (meaning a mixed solution of EC and DEC in a volume ratio of EC and DEC (EC: DEC = 1: 1) having LiPF ₆ as a solute) The lithium battery of Comparative Example 4 was assembled in an argon glove box.

<Evaluation of initial capacity><Evaluation of initial efficiency>
The lithium batteries of Examples 1 to 3 and Comparative Examples 1 to 4 thus obtained were evaluated at 25 ° C.
The positive electrode active material was charged to 4.8 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 discharge capacity of the positive electrode active material at 4.8 to 2.5 V is the initial capacity, and the discharge capacity / charge capacity is the initial efficiency.

Table 1 shows the initial capacities and the initial efficiencies of the lithium batteries of Examples 1 to 3 and Comparative Examples 1 to 4.
As shown in Table 1, it was confirmed that the lithium batteries of Examples 1 to 3 were excellent in initial capacity and initial efficiency. Moreover, since the lithium battery of Example 1- Example 3 has high initial efficiency, it is estimated that the amount of oxygen generation is also suppressed.
On the other hand, the lithium batteries of Comparative Examples 1 to 4 have insufficient initial efficiency when the initial capacity is excellent, and insufficient initial capacity when the initial capacity is excellent. There was no one with both excellent initial efficiency.

  According to the present invention, a positive electrode active material for a lithium ion secondary battery, a positive electrode, and a lithium ion that are small and light, have a high discharge capacity per unit mass, have excellent initial efficiency, and can suppress the generation of gas in the battery. A secondary battery can be obtained, and can be used for electronic devices such as mobile phones, in-vehicle batteries, and the like.

Claims (7)

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). ) A lithium-containing composite oxide and a metal oxide represented by the formula MO (where M is a transition metal element corresponding to the transition metal element),
In the X-ray diffraction diagram, the ratio (H1 / H2) of the maximum peak height (H1) of 2θ = 42 to 43.8 ° and the maximum peak height (H2) of 2θ = 44 to 46 ° is 0.03 to 0.03. 0.5, and the ratio (H3 / H4) of the maximum peak height (H3) of 2θ = 14 to 16 ° and the maximum peak height (H4) of 2θ = 18 to 20 ° is 0 to 0.1 A positive electrode active material for a lithium ion secondary battery.   The positive electrode for lithium ion secondary batteries containing the positive electrode active material for lithium ion secondary batteries of Claim 1, a electrically conductive material, and a binder.   A lithium ion secondary battery comprising a positive electrode, a negative electrode, and a nonaqueous electrolyte, wherein the positive electrode is the positive electrode according to claim 2. 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). ) By heat-treating the lithium-containing composite oxide at 400 to 470 ° C. in an atmosphere containing hydrogen, the lithium-containing composite oxide and the metal oxide represented by the formula MO (where M corresponds to the transition metal element) A positive electrode active material for a lithium ion secondary battery comprising a transition metal element).   The ratio (H1 / H2) between the maximum peak height (H1) of 2θ = 42 to 43.8 ° and the maximum peak height (H2) of 2θ = 44 to 46 ° in the X-ray diffraction pattern of the positive electrode active material is 0.03 to 0.5, and the ratio (H3 / H4) of the maximum peak height (H3) of 2θ = 14 to 16 ° and the maximum peak height (H4) of 2θ = 18 to 20 ° is 0 to The manufacturing method of the positive electrode active material for lithium ion secondary batteries of Claim 4 which is 0.1.   The manufacturing method of the positive electrode active material of Claim 4 or Claim 5 which wash | cleans a positive electrode active material with water or a polar organic solvent after performing the said heat processing. The manufacturing method according to any one of claims 4 to 6, wherein a hydrogen concentration in an atmosphere containing hydrogen is 0.3 to 20% by mass.

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