JP5638015B2 - Negative electrode material for lithium ion secondary battery, negative electrode mixture, negative electrode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Description

  The present invention relates to a negative electrode material for a lithium ion secondary battery using a carbon powder, a negative electrode mixture, a lithium ion secondary battery negative electrode, and a lithium ion secondary battery.

Lithium ion batteries use an organic solvent with low conductivity for the electrolyte, so it is necessary to shorten the distance between the electrodes. Therefore, a thin electrode active material, binder, solvent, etc. were mixed on the current collector metal foil. An electrode is produced by applying the slurry. The problem in this manufacturing process is to improve the peel strength characteristics. This is because when the electrode is peeled off during the manufacturing process and particle detachment appears, not only will this part become defective, but the peeled particles will adhere to other parts as foreign matter, creating new defective parts. This is to induce a significant decrease in process yield. Therefore, the increase in peel strength has an important meaning in battery production.
The peel strength is important even when the produced battery is used. For example, lithium ion batteries are expected to be used more and more in automotive applications. However, if an electrode having a low peel strength is used in an in-vehicle application that is constantly exposed to vibration, peeling easily occurs inside the battery and the battery life is significantly shortened.

Increasing the binder amount is the easiest and most effective means for increasing the peel strength, and is a conventionally performed technique. However, this simultaneously induces deterioration of various battery characteristics due to an increase in non-conductive components. Therefore, an increase in peel strength by this means is not desirable, and it is an important issue to realize an increase in peel strength by improving the negative electrode material.
In addition to the increase in the amount of the binder, an example in which various surface coatings are previously applied to the negative electrode material has been proposed. However, they are all ineffective and the peel strength remains at most about 10 N / m.

  Another problem is that when graphite or highly crystalline carbon is used as the negative electrode active material, a non-aqueous electrolyte containing propylene carbonate, one of the most common non-aqueous solvents, is used. In addition, there is a problem that propylene carbonate decomposes during charging. As a solution to this, Patent Document 1 proposes coating the surface of carbon, which is a negative electrode active material, with a water-soluble polymer.

  Patent Document 2 describes a dehydration condensate of polyacrylic acid and a polyacrylamide component as a novel binder capable of suppressing the decomposition reaction of the propylene carbonate electrolyte.

  Patent Document 3 describes a negative electrode material for a lithium ion secondary battery in which a polymer compound having a carboxyl group and a metal oxide are attached to the surface of a negative electrode material.

JP-A-11-129992 JP 2007-287570 A JP 2010-129363 A

An object of the present invention is to obtain a negative electrode material in which peeling of the negative electrode material from the negative electrode current collector is suppressed, and the adhesiveness between the negative electrode material and the copper foil is particularly high when applied. The present inventors examined a negative electrode that has a low peel strength in an electrode peel test even when the powder of the negative electrode material is relatively firmly bonded by a binder.
As a result, in the coated electrode, it is estimated that the binder component moves from the vicinity of the copper foil to the surface side of the electrode accompanying the evaporation of the moisture as the solvent during drying after coating. As a result of insufficient adhesion between the foils, it is considered that the electrode peel test results in a low peel strength.
Moreover, in order to avoid the influence on battery performance, when adding 3rd components other than the negative electrode material and binder as a carbon component, it is necessary to keep to very small quantity. The present inventors have conducted intensive studies in consideration of these points.

  The present invention has been made in view of the above points, and when used as a negative electrode material for a lithium ion secondary battery, the adhesiveness with the copper foil is high, peeling of the negative electrode material is suppressed, and the adhesiveness is high. It aims at obtaining the negative electrode material which can comprise a negative electrode.

In the present invention, the carbon powder used for the negative electrode material is composed of carbon powder I having a small amount of SiO ₂ and Al ₂ O ₃ adhered on the surface thereof and carbon powder II having a coating treatment of a small amount of acrylic acid polymer. By using different types of carbon powder as the negative electrode material for the negative electrode mixture, the amount of the binder is not increased, and various performances as a lithium ion secondary battery are not affected. It was revealed that an increase in peel strength between the negative electrode current collector and the negative electrode mixture applied to the surface thereof can be realized. In addition, these have an effect of improving the peel strength particularly when carbon powder coated with SiO ₂ and / or Al ₂ O ₃ and carbon powder coated with acrylic acid polymer are separately manufactured and mixed. I found out.

As shown in JP 2010-129362 A (Patent Document 3), the SiO ₂ and Al ₂ O ₃ adhesion treatment on the surface of the carbon powder improves the wettability with the aqueous binder solution by imparting hydrophilicity to the surface of the carbon powder. However, it has been conventionally proposed that various battery characteristics are improved when an aqueous binder is used.
As for acrylic acid polymers, as shown in Japanese Patent Application Laid-Open No. 2007-287570 (Patent Document 2), the viscosity of the coating liquid in the aqueous binder increases due to the coating or addition effect of the acrylic acid polymer, and the coating electrode described above Since the phenomenon that the binder component moves from the vicinity of the copper foil to the surface side during drying is suppressed, it is said that the adhesion can be improved.
On the other hand, although the mechanism itself in which the peel strength has been dramatically improved in the present invention is not clear, it is not limited to the simple combination effect of the prior art, and the oxygen groups of SiO ₂ and Al ₂ O ₃ are made of acrylic acid. It is presumed that a high effect was obtained by adding a chemical affinity that acts as a hydrogen bond with the carboxyl group and an anchor effect in which SiO ₂ and Al ₂ O ₃ fine particles bite into the acrylic acid polymer layer. In order to realize the effects of the present invention by realizing these, carbon powder I to which SiO ₂ and Al ₂ O ₃ are adhered and carbon powder II having a coating portion of acrylic acid polymer are separately used as negative electrode materials, carbon It is important that the material I and the carbon material II exist separately in the negative electrode mixture, and each of them is highly dispersed.

That is, the present invention provides the following (1) to (6).
(1) A lithium ion secondary battery containing carbon powder I having SiO ₂ powder and / or Al ₂ O ₃ powder adhered to the surface of the carbon powder and carbon powder II having the surface of the carbon powder coated with an acrylic acid polymer. Negative electrode material for
The total content of the SiO ₂ powder and Al ₂ O ₃ powder is
Carbon powder, SiO ₂ powder and / or Al ₂ O ₃ powder, and 0.1 to 3.0% by mass with respect to the total amount of acrylic acid polymer,
Content of the acrylic acid polymer is
0.1 to 2.5% by mass with respect to the total amount of carbon powder, SiO ₂ powder and / or Al ₂ O ₃ powder, and acrylic polymer,
The ratio of the said carbon powder I and the said carbon powder II is carbon powder I / carbon powder II = 10 / 90-90 / 10 by mass ratio, The negative electrode material for lithium ion secondary batteries characterized by the above-mentioned.
(2) A negative electrode mixture for a lithium ion secondary battery containing a mixture of the negative electrode material according to (1) and a water-soluble binder.

(3) A negative electrode for a lithium ion secondary battery, comprising the negative electrode material as described in (1) above.
(4) A lithium ion secondary battery comprising the negative electrode according to (3) above.
(5) Carbon powder having SiO ₂ powder and / or Al ₂ O ₃ powder adhered to the surface of the carbon powder, and the total content of SiO ₂ powder and Al ₂ O ₃ powder is carbon powder, SiO ₂ powder and / or Al ₂ O ₃ powder, and carbon powder I that is 0.1 to 3.0% by mass with respect to the total amount of the acrylic acid polymer, and the surface of the carbon powder is coated with the acrylic acid polymer, Carbon powder II whose content is 0.1 to 2.5% by mass with respect to the total amount of carbon powder, SiO ₂ powder and / or Al ₂ O ₃ powder, and acrylic acid polymer,
A negative electrode mixture for a lithium ion secondary battery, which is obtained by mixing water and adding a water-soluble binder.
(6) A negative electrode for a lithium ion secondary battery obtained by applying the negative electrode mixture according to (5) above to the surface of the negative electrode current collector and vacuum drying.

  According to the present invention, when used as a negative electrode material for a lithium ion secondary battery, it is possible to obtain a negative electrode material that constitutes a negative electrode mixture with high adhesion that suppresses peeling of the negative electrode material from the negative electrode current collector. . Moreover, if the negative electrode material of the present invention is used, a negative electrode mixture having high adhesion to the negative electrode current collector can be obtained. Furthermore, if the negative electrode mixture of the present invention is used, a lithium ion secondary battery having a negative electrode with high peel strength can be obtained.

It is sectional drawing which shows the coin-type secondary battery for evaluation.

<1> Negative electrode material The negative electrode material for lithium ion secondary batteries of the present invention is a combination material of carbon powder I and carbon powder II described below. Carbon powder I and carbon powder II are produced separately, and then a water-soluble binder is added to obtain a negative electrode mixture. The negative electrode mixture of the present invention has carbon powder I, carbon powder II, and a water-soluble binder, and has a characteristic of high peel strength when applied to a negative electrode current collector.
[Carbon material]
The object of the present invention is to improve the peel strength between the negative electrode mixture using a water-soluble binder and the negative electrode current collector. As the carbon base material of the negative electrode material, there are a graphite type and a non-graphite type, but a non-graphite type material typified by hard carbon uses a binder such as PVdF (PolyVinylidene Fluoride) that dissolves in an organic solvent. Therefore, graphite-based materials are mainly used for the carbon base material of the present invention using a water-soluble binder. As this graphite-based material, any of natural graphite, artificial graphite such as mesophase-based, coke-based and electrode end materials can be used, and a mixture thereof may be used.

Among these, natural graphite includes scaly, spherical shapes, and natural graphite composite materials prepared by processing organic compounds such as resins and pitches into natural graphite by physical techniques or heat mixing. Either can be used. In particular, it is desirable that the lattice spacing d002 in X-ray diffraction is 0.34 nm or less.
For artificial graphite, non-granulated carbonaceous particles such as mesophase spherules and bulk mesophase obtained by heat treatment of petroleum or coal-based pitch, or massive coke that has been further carbonized, etc. are about 3000 ° C. Although artificially graphitized at a high temperature can be used, those having a lattice plane distance d002 of 0.34 nm or less in X-ray diffraction are also desirable.
The graphite-based carbon powder is not limited to single use, and two or more types can be used.
The average particle diameter of the graphite-based carbon powder is preferably 1 to 50 μm, more preferably 2 to 30 μm, and still more preferably 5 to 20 μm. If the average particle size is too small, the irreversible capacity at the first charge is large, the bulk density is small, and the volume capacity is lowered. If the average particle size is too large, an electrode having a thickness equal to or smaller than the particle size cannot be produced. There is also a problem that the capacity at a large current is greatly reduced. Man-made graphite-based carbon materials are generally pulverized to adjust the particle size, and natural graphite is generally adjusted to a particle size when spheroidized by a shearing process. The particle size may be adjusted.

  As the graphite-based carbon powder, a composite graphite-based carbon powder in which coal tar pitch is attached to and impregnated on the surface and inside of natural graphite or artificial graphite may be used. A method for producing a composite graphite-based carbon powder is, for example, putting a solution in which coal tar pitch is dissolved in tar oil into graphite-based carbon powder, stirring, and then removing the tar oil component by distillation under reduced pressure so that the pitch is on the surface and inside. A precursor of a composite graphite-based carbon powder adhered and impregnated with is obtained. This is filled in a crucible and heat-treated at, for example, 800 ° C. to 1300 ° C. to obtain a composite graphite carbon powder.

When these graphite-based negative electrode materials are used as negative electrode materials for lithium ion secondary batteries, particularly in negative electrode mixtures using a water-soluble binder, the peel strength from the negative electrode current collector is low, so the production yield of the coated electrode is low, In addition, there is a problem that a battery manufactured using these as a negative electrode has low cycle characteristics. In order to solve this problem, in the present invention, a negative electrode material having carbon powder I having SiO ₂ and Al ₂ O ₃ attached on the surface of carbon powder and carbon powder II having a coating of an acrylic acid polymer separately is used.

[Carbon powder I]
Carbon powder I is a composite carbon powder in which SiO ₂ powder and / or Al ₂ O ₃ powder adheres to the surface of the graphite-based carbon powder, and the total content of SiO ₂ powder and Al ₂ O ₃ powder is a negative electrode material. total carbon powder amount used in a 0.1 to 3.0 wt% based on the total weight 100 weight% of an acrylic acid polymer, and SiO ₂ powder and _Al 2 _{O 3} powder.

In the present invention, carbon powder I to which only SiO ₂ and Al ₂ O ₃ are attached and carbon powder II separately coated with an acrylic acid-based polymer are separately produced and mixed. Moreover, you may mix the carbon powder which does not perform either process (3rd carbon powder III which does not perform an adhesion | attachment process) in the range by which the effect of this invention is not lost. The effects similar to those described above can be obtained if the adhesion amount of SiO ₂ and Al ₂ O ₃ and the coating amount of the acrylic acid polymer with respect to the total mass of the carbon material after mixing are within the ranges specified in the present invention. On the other hand, when the adhesion treatment of SiO ₂ and Al ₂ O _{3 and} the coating with the acrylic acid polymer are performed on the same carbon powder particles, the surface of the carbon powder coated with the polymer has SiO ₂ , Al ₂ It has been confirmed that sufficient peel strength cannot be obtained because the O ₃ fine particles do not adhere directly (see Comparative Example 4 described later).

(Adhesion treatment of SiO ₂ and Al ₂ O ₃ to carbon powder)
The SiO ₂ and Al ₂ O ₃ may be attached to the carbon powder by attaching SiO ₂ or Al ₂ O ₃ alone or both to the same carbon powder. The total amount of SiO ₂ and Al ₂ O ₃ attached to the carbon powder is suitably in the range of 0.1 to 3.0% by mass, preferably 0.3 to 2.0% by mass, more preferably 0.00. It is the range of 5-1.0 mass%. When the adhesion amount of SiO ₂ and Al ₂ O ₃ is 0.1% by mass or less, the effect of improving the peel strength is hardly obtained. On the other hand, if it exceeds 3.0% by mass, the effect of addition is saturated, and if it is excessively added, the peel strength decreases. In addition, the capacity of the lithium ion secondary battery is reduced.

The method of adhesion treatment is not limited. For example, by adding fine particles of SiO ₂ and / or Al ₂ O ₃ to graphite-based carbon powder and applying a shearing force with a mixer or the like, a mixing process (hereinafter, a process of applying a shearing force is referred to as a mechanochemical process) To produce carbon powder I.
As the method of mechanochemical treatment, a kneading apparatus of a type in which shearing is applied to the powder, such as a mixing apparatus with a stirring blade or a grinding apparatus, is selected. Using these devices, the desired adhesion treatment is performed by mixing while applying shear so that the fine particles of SiO ₂ and Al ₂ O ₃ are uniformly applied to the surface of the carbon powder of the negative electrode material.

  The apparatus used for the mechanochemical treatment is not particularly limited as long as it can simultaneously apply a compressive force and a shearing force. For example, a Henschel mixer, a Nauter mixer, a pin mill type or ball mill type pulverizer, or a pressurizing machine. A kneader such as a kneader or a two-roller, a rotating ball mill, a hybridization system (manufactured by Nara Machinery Co., Ltd.), a mechanomicros (manufactured by Nara Machinery Co., Ltd.), a mechanofusion system (manufactured by Hosokawa Micron), etc. can be used. .

The average particle diameter of the fine particles of SiO ₂ and / or Al ₂ O ₃ is preferably 1 μm or less, and more preferably 0.1 μm or less. As the fine particles of SiO ₂ and / or Al ₂ O ₃ , a sol or colloid dispersed in a powdery product, water, alcohol, organic solvent or the like produced by a gas phase method is used. The shape is not particularly limited, but may be any of granular, lump, spherical, plate-like, fibrous, film-like, and scale-like shapes, and a shape close to a spherical shape such as granular, lump-like, and spherical is preferable.

(Carbon powder II)
The carbon powder II coats the surface of the graphite-based carbon powder with an acrylic acid-based polymer. The content of acrylic acid polymer is 0.1 to 2.5 mass with respect to total mass 100% by weight of the total carbon powder content, acrylic acid polymer, and SiO ₂ powder and _Al 2 _{O 3} powder used in the negative electrode material %. The content of the acrylic acid polymer is preferably 0.2 to 1.0% by mass, more preferably 0.3 to 0.8% by mass.
The polymer to be coated is not limited as long as it is an acrylic acid polymer. Any of polyacrylic acid, sodium polyacrylate, acrylic acid / maleic acid copolymer salt and the like can be used. In addition, polyacrylic acid having a molecular weight of 5000 to 1000000 is commercially available as a reagent. However, any of them will have an effect when an appropriate amount is added.
As the graphite-based carbon material used, the above-mentioned natural graphite, coke-based or mesophase-based artificial graphite, and composite graphite-based carbon powder can also be used, and graphite is not limited to single use, and may be two or more types. If the total amount of the polymer in the total amount of the graphite-based carbon material mixture is within the range of 0.1 to 2.5% by mass, the effect of increasing the peel strength is exhibited.

(Coating treatment of polyacrylic acid on carbon powder)
The method of coating treatment is not limited. The acrylic polymer coating on the carbon powder may be performed by using a solid, for example, a method such as the above-mentioned mechanochemical treatment, or by mixing the aqueous solution using a kneading apparatus such as a planetary mixer. It is possible to coat the surface. In any treatment, the function of increasing the peel strength of the electrode can be exhibited. The carbon powder to be adhered may be a graphite-based carbon powder that has not been subjected to a special treatment, or a composite graphite-based carbon powder having the coal tar pitch previously described in the mechanochemical treatment adhered to the surface and inside. Also good.
Specifically, after adding polyacrylic acid powder and ion-exchanged water to graphite-based carbon powder and kneading with a planetary mixer (hereinafter, the mixing process is referred to as a mixer mixing process), followed by vacuum drying. To completely remove moisture. Also, mechanochemical treatment is performed using a graphite carbon powder and / or a composite graphite carbon powder with a coal tar pitch attached to the surface and inside and a solid of an acrylic acid polymer while applying a shearing force with a Henschel mixer or the like. Carbon powder II may be produced.

<2. Negative electrode mix>
The lithium ion secondary battery negative electrode mixture of the present invention uses carbon powder I and carbon powder II, which are the carbon materials of the present invention. You may mix the 3rd carbon powder III which does not perform an adhesion | attachment process with said graphite type carbon material in the range by which the effect of this invention is not lost. Also when carbon powder III is mixed, the content of SiO ₂ powder and / or Al ₂ O ₃ powder is 0.1 to 3.0% by mass, and the content of acrylic acid polymer is 0.1 to 3.0% by mass. 2.5% by mass.
In carrying out the present invention, carbon powder I that has been subjected to adhesion treatment of SiO ₂ and Al ₂ O ₃ and carbon powder II that has been subjected to coating treatment of an acrylic acid polymer are separately prepared in advance and mixed with a binder. To produce a negative electrode mixture. A production method in which carbon powder I and carbon powder II are mixed in advance and a water-soluble binder is added to the resulting mixture is preferred. The method of mixing the whole after mixing the carbon powder I and the binder and the carbon powder II and the binder increases the viscosity of the mixture, but may be performed and the production method of the present invention is not limited.
(Water-soluble binder)
As the binder, those having chemical stability and electrochemical stability to the electrolyte are preferably used. For example, polyethylene, polyvinyl alcohol, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), and the like are used. Used. Moreover, these can also be used together. In general, the binder is preferably used in an amount of about 1 to 20% by mass in solid content in the total amount of the negative electrode mixture. More preferably, it is 1-7 mass%. The reason is that if there is too little binder, the peel strength will be low. If there is too much binder, the binder becomes an electric resistance and the battery characteristics are deteriorated.
As a specific example of the preparation of the negative electrode mixture, the negative electrode mixture is prepared by mixing particles of the above-described mixed negative electrode material with a binder, and this negative electrode mixture is usually applied to one or both sides of the current collector. The method of forming a negative mix layer is mentioned.
〔solvent〕
For production of the negative electrode, a normal solvent for producing a negative electrode can be used. When the negative electrode mixture is dispersed in a solvent and made into a paste, and then applied to the current collector and dried, the negative electrode mixture layer is uniformly and firmly adhered to the current collector. The solvent can be selected from known solvents. More specifically, for example, after the particles of the negative electrode material and an aqueous dispersion binder such as styrene butadiene rubber and a water-soluble binder such as carboxymethyl cellulose are mixed with a solvent such as water and alcohol to form a slurry, A paste is prepared by kneading with a kneader or a mixer. If this paste is applied to one or both sides of the current collector and dried, a negative electrode in which the negative electrode mixture layer is uniformly bonded can be obtained. The film thickness of the negative electrode mixture layer is preferably 10 to 200 μm, more preferably 20 to 100 μm.

<3. Method for producing negative electrode mixture>
It is preferable to mix in advance the carbon powder I and the carbon powder II described above, and if necessary, the carbon powder that is not subjected to any treatment (the third carbon powder III that is not subjected to the adhesion treatment).
The ratio of the carbon powder I and the carbon powder II is preferably carbon powder I / carbon powder II = 10/90 to 90/10 in mass ratio. More preferably, the carbon powder I / carbon powder II is 20/80 to 80/20 by mass ratio, and more preferably, the carbon powder I / carbon powder II is 40/60 to 60/40 by mass ratio. Most preferably, carbon powder I / carbon powder II = 40/60 to 20/80 in terms of mass ratio.
Within this range, the SiO ₂ and Al ₂ O ₃ adhering portions of the carbon powder I can work uniformly on the acrylic polymer coating portion of the carbon powder II, which is preferable.
In the kneading method, carbon powders I and II and, if necessary, carbon powder III are mixed by adding water to a solvent with a planetary mixer. The resulting mixture is optionally vacuum dried. An example is a method in which a water-soluble binder is added to the obtained mixture and further mixed to obtain a negative electrode mixture.

  Next, a lithium ion secondary battery (hereinafter also referred to as “the lithium ion secondary battery of the present invention”) using the negative electrode material of the present invention will be described.

[Lithium ion secondary battery]
In general, a lithium ion secondary battery includes a negative electrode, a positive electrode, and a nonaqueous electrolyte as main battery components. The positive and negative electrodes are each composed of a lithium ion carrier, and lithium ions are input and output between layers. In essence, this is a battery mechanism in which lithium ions are doped into the negative electrode during charging and are dedoped from the negative electrode during discharging.
The lithium ion secondary battery of the present invention is not particularly limited except that the negative electrode material of the present invention is used, and other battery components conform to the elements of a general lithium ion secondary battery.

[Negative electrode]
Specifically, the negative electrode mixture layer is formed by applying the negative electrode mixture of the present invention to one or both sides of the current collector. At this time, a normal solvent can be used.
(Current collector)
The shape of the current collector used for the negative electrode is not particularly limited, and examples thereof include a foil shape; and a net shape such as a mesh and an expanded metal. Examples of the current collector include copper, stainless steel, and nickel.

[Positive electrode]
As a material for the positive electrode (positive electrode active material), it is preferable to select a material that can be doped / dedoped with a sufficient amount of lithium ions. Examples of such a positive electrode active material include transition metal oxides, transition metal chalcogenides, vanadium oxides and lithium-containing compounds thereof, and general formula M _X Mo ₆ S _8-y (where X is 0 ≦ X ≦ 4, Y is a numerical value in a range of 0 ≦ Y ≦ 1, and M represents a metal such as a transition metal), a lithium chelate phosphate compound, lithium iron phosphate, activated carbon, activated carbon fiber, and the like. May be used alone or in combination of two or more. Examples of the vanadium oxide include V ₂ O ₅ , V ₆ O ₁₃ , V ₂ O ₄ , and V ₃ O ₈ .

The lithium-containing transition metal oxide is a composite oxide of lithium and a transition metal, and may be a solid solution of lithium and two or more transition metals. Specifically, the lithium-containing transition metal oxide is LiM (1) _1-p M (2) _p O ₂ (wherein P is a numerical value in the range of 0 ≦ P ≦ 1, M (1), M (2) is composed of at least one transition metal element) or LiM (1) _2-q M (2) _q O ₄ (wherein Q is a numerical value in the range of 0 ≦ Q ≦ 1 and M (1 And M (2) is composed of at least one transition metal element). Here, examples of the transition metal element represented by M include Co, Ni, Mn, Cr, Ti, V, Fe, Zn, Al, In, and Sn. Co, Fe, Mn, Ti, Cr, and V Al is preferred. Preferred transition metal oxides include LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiNi _0.9 Co _0.1 O ₂ , LiNi _0.5 Co _0.5 O ₂ and the like.
Such lithium-containing transition metal oxides are, for example, Li, transition metal oxides or salts as starting materials, these starting materials are mixed according to the composition, and fired in a temperature range of 600 to 1000 ° C. in an oxygen atmosphere. Can be obtained. Note that the starting materials are not limited to oxides or salts, and can be synthesized from hydroxides or the like.

  The positive electrode active material may be used alone or in combination of two or more. For example, a carbon salt such as lithium carbonate can be added to the positive electrode. Moreover, when forming a positive electrode, conventionally well-known various additives, such as a electrically conductive agent, can be used suitably.

  As a method of forming a positive electrode using such a positive electrode material, for example, a positive electrode mixture layer is formed by applying a positive electrode mixture composed of a positive electrode material, a binder and a conductive agent to both surfaces of a current collector. As the binder, those exemplified for the negative electrode can be used. As the conductive agent, for example, a carbon material, graphite, carbon black, or VGCF can be used. The shape of the current collector is not particularly limited, and the same shape as the negative electrode is used.

  In forming the above-described negative electrode and positive electrode, various conventionally known additives such as a conductive agent and a binder can be appropriately used.

[Non-aqueous electrolyte]
The non-aqueous electrolyte is an electrolyte salt used in a normal non-aqueous electrolyte. For example, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiB (C ₆ H ₅ ), LiCl, LiBr, LiCF ₃ SO ₃ , LiCH ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ CH ₂ OSO ₂ ) ₂ , LiN (CF ₃ CF ₂ OSO ₂ ) ₂ , LiN (HCF ₂ CF ₂ CH ₂ OSO ₂ ) ₂ , LiN [(CF ₃ ) ₂ CHOSO ₂ ] ₂ LiB [C ₆ H ₃ (CF ₃ ) ₂ ] ₄ , LiAlCl ₄ , LiSiF ₆ and other lithium salts can be used. In particular, LiPF ₆ and LiBF ₄ are preferably used from the viewpoint of oxidation stability.
The electrolyte salt concentration in the non-aqueous electrolyte is preferably 0.1 to 5 mol / L, and more preferably 0.5 to 3.0 mol / L. The non-aqueous electrolyte may be a liquid non-aqueous electrolyte or a polymer electrolyte such as a solid electrolyte or a gel electrolyte.

  In the case of a liquid nonaqueous electrolyte solution, an aprotic organic solvent such as ethylene carbonate, propylene carbonate, dimethyl carbonate or the like can be used as the nonaqueous solvent.

In the case of a polymer electrolyte, a matrix polymer gelled with a plasticizer (non-aqueous electrolyte) is included. Examples of the matrix polymer include ether-based polymers such as polyethylene oxide and cross-linked products thereof, fluorine-based polymers such as polymethacrylate-based, polyacrylate-based, polyvinylidene fluoride, and vinylidene fluoride-hexafluoropropylene copolymer. Can be used alone or as a mixture, and among them, a fluorine-based polymer is preferable from the viewpoint of redox stability and the like.
As the electrolyte salt and non-aqueous solvent constituting the plasticizer (non-aqueous electrolyte solution) contained in the polymer electrolyte, those described above can be used.

In the lithium ion secondary battery of the present invention, a separator can be used. For example, a negative electrode containing the carbon materials I and II of the present invention, a gel electrolyte, and a positive electrode are laminated in this order using a gel electrolyte. However, it can also be configured by being housed in a battery exterior material.
The structure of the lithium ion secondary battery of the present invention is arbitrary, and is not particularly limited with respect to its shape and form. For example, it may be a stacked type, a wound type, a cylindrical type, a square type, or a coin type Can be selected arbitrarily.

  Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not limited to these.

<Evaluation method>
(1) Method of measuring peel strength of negative electrode The peel strength of the obtained negative electrode is determined according to the 180 ° peel strength measurement method shown in JIS K 6854-2 in a state where no press molding pressure is applied to the electrode. Measure and calculate the load using the graph.

(2) Measuring method of battery characteristics (discharge capacity, initial charge / discharge efficiency, and cycle characteristics) of the negative electrode Using the obtained negative electrode, a coin-type secondary battery was produced in the following manner, and battery evaluation was performed.
As the evaluation battery, a coin-type secondary battery having the structure shown in FIG. 1 was produced.
The separator 5 impregnated with the non-electrolyte solution was sandwiched between the working electrode (negative electrode) 2 in close contact with the current collector 7b and the counter electrode (positive electrode) 4 in close contact with the current collector 7a. Thereafter, the exterior cup 1 and the exterior can 3 were combined so that the working electrode current collector 7 b side was accommodated in the exterior cup 1 and the counter electrode 4 current collector 7 a side was accommodated in the exterior can 3. At that time, the outer peripheral cup 1 and the outer can 3 were prepared by interposing an insulating gasket 6 between the outer peripheral cup 1 and the outer can 3 and caulking both peripheral portions. A separator impregnated with an electrolytic solution was sandwiched between the negative electrode and the positive electrode as a counter electrode, and then the negative electrode and the positive electrode were accommodated in the outer cup and the outer can, and then the outer cup and the outer can were combined. At that time, an insulating gasket was interposed between the peripheral portions of the outer cup and the outer can, and both peripheral portions were caulked and sealed.
The measured battery characteristics are values of discharge capacity, initial charge / discharge efficiency, and cycle characteristics, which were calculated from the results of a charge / discharge test at a temperature of 25 ° C.
(3) Charging capacity and discharging capacity After performing 0.9 mA constant current charging until the circuit voltage reached 0 mV, switching to constant voltage charging was continued until the current value reached 20 μA. The charging capacity was determined from the amount of electricity applied during that time.
After measuring the charge capacity, it was paused for 120 minutes. Next, constant current discharge was performed at a current value of 0.9 mA until the circuit voltage reached 1.5 V, and the discharge capacity was obtained from the energization amount during this period. This was the first cycle.
(4) Initial charge / discharge efficiency The initial charge / discharge efficiency was calculated from the following equation.
Initial charge / discharge efficiency (%) = (first cycle discharge capacity / first cycle charge capacity) × 100
In this test, the process of occluding lithium ions in the negative electrode material was charged, and the process of separating from the negative electrode material was discharged.

(5) Cycle characteristics An evaluation battery different from the evaluation battery that evaluated the discharge capacity and the initial charge / discharge efficiency was produced in the same manner as the coin-type battery, and the following evaluation was performed.
After 4.0 mA constant current charging was performed until the circuit voltage reached 0 mV, switching to constant voltage charging was continued until the current value reached 20 μA, and then rested for 120 minutes. Next, constant current discharge was performed at a current value of 4.0 mA until the circuit voltage reached 1.5V. The charge / discharge was repeated 50 times, and the cycle characteristics were calculated from the obtained discharge capacity using the following formula.
Cycle characteristics = (discharge capacity at the 50th cycle / discharge capacity at the first cycle) × 100

Example 1
1) Manufacture of composite graphite powder In an autoclave, natural graphite (average particle size: 10 μm) was added as a core material, and a solution in which coal tar pitch was dissolved in tar oil was added as an organic compound, and heated to 140 ° C. with stirring. . After continuing the heating, the tar oil component was removed by distillation under reduced pressure to obtain a precursor of composite graphite powder having pitch adhered and impregnated on the surface and inside. This was filled in a crucible and heat treated at 1000 ° C. to obtain composite graphite powder “A” having an average particle size of 13 μm.

(Method for producing mesophase materials)
Mesophase spherules obtained by heat treatment of coal tar pitch were filled in a crucible and graphitized at 3000 ° C. to obtain mesophase-based artificial graphite “B” having an average particle size of 15 μm.

2) Production of carbon powder I (attachment of SiO ₂ and Al ₂ O ₃ to composite graphite powder “A”)
50 parts by mass of the above composite graphite powder “A”, 1 part by mass of SiO ₂ fine particles having an average particle diameter of 50 nm are added, and a mixing process is performed for 30 minutes while applying shear at a peripheral speed of 10 m / sec with a Henschel mixer. 51 parts by mass of a SiO ₂ adhering product (carbon powder I) to the composite graphite powder “A” as the powder I was obtained. This method is described as “Mechanochemical” in the table.

3) Production of carbon powder II (artificial graphite “B” coated with acrylic polymer)
1 part by weight of polyacrylic acid powder having a molecular weight of 5,000 and 200 parts by weight of ion-exchanged water are added to 50 parts by weight of the above-described artificial mesophase graphite “B”, and kneaded with a planetary mixer for 30 minutes. Then, vacuum drying was performed at 90 ° C. to completely remove moisture, and 51 parts by mass of a polyacrylic acid-coated product (carbon powder II) on artificial graphite “B”, which was carbon powder II, was obtained. This method is described as “mixer mixing” in the table. In the table, PAA is polyacrylic acid, and PMMA is polymethyl methacrylate.

4) Production of negative electrode The carbon powder I and the carbon powder II obtained above were mixed at a mass ratio of 1 shown in Table 1. The obtained negative electrode material for a lithium ion secondary battery was a mixture of two types of graphite-based negative electrode materials and was 102 parts by mass. The amount of SiO ₂ powder was 0.5% by mass, and the amount of polyacrylic acid was 0.5% by mass.
1 part by mass of carboxymethyl cellulose ammonium and 2 parts by mass of carboxy-modified styrene butadiene rubber are added to the obtained negative electrode material for a lithium ion secondary battery, and mixed using a planetary mixer using water as a solvent. The negative electrode mixture paste was obtained by stirring. The obtained paste was applied onto a 15 μm thick copper foil and vacuum dried at a temperature of 110 ° C. to obtain a negative electrode.

5) Measurement of peel strength For the obtained negative electrode, the molding pressure was not applied by a press or the like, and the load was measured and calculated using an autograph according to the 180 ° peel strength measurement method shown in JIS K 6854-2. did. The results are shown in Table 2. The peel strength was 35 N / m, which exceeded the target value of 20 N / m.

6) Production of counter electrode (positive electrode) Lithium metal foil (thickness 0.5 mm) was pressed onto a nickel net and punched into a circular shape with a diameter of 15.5 mm, and the current collector made of nickel net was in close contact with the current collector. A counter electrode (positive electrode) made of lithium metal foil was produced.

7) Electrolyte / Separator LiPF ₆ was dissolved at a concentration of 1 mol / dm ^{3 in} a mixed solvent of ethylene carbonate 33 vol% -methyl ethyl carbonate 67 vol% to prepare a non-aqueous electrolyte. The obtained nonaqueous electrolytic solution was impregnated into a polypropylene porous body (thickness 20 μm) to produce a separator impregnated with the electrolytic solution.

8) Characteristics of Battery Negative Electrode Performance A coin-type battery having the above-described configuration was prototyped for the obtained negative electrode, the battery characteristics were evaluated, and the results are shown in Table 2.
The discharge capacity was 355 mAh / g, and the initial charge / discharge efficiency was 93.8%, each showing a normal value of the target level.
The cycle characteristic was 94.2%, which was a good value exceeding 93% of the target level.

(Examples 2-6, Comparative Examples 1-3)
In the same manner as in Example 1, the raw materials and production conditions shown in Table 1 were mixed with a composite graphite powder “A” and mesophase-type artificial graphite “B” as a base material, and SiO _{2 under} various conditions and procedures shown in Table 1. Coin-type batteries similar to those described above were manufactured as various sample batteries by applying Al ₂ O ₃ and applying an acrylic acid polymer. The peel strength and battery characteristics of the obtained battery were evaluated in the same manner as in Example 1. The results are shown in Table 2.

(Comparative Example 4)
After mixing artificial graphite “B”, polyacrylic acid (PAA) and SiO ₂ , a mechanochemical treatment was performed in the same manner as in Example 1 to obtain a carbon powder coated and adhered to polyacrylic acid and SiO _2. This was mixed with composite graphite powder “A”. Polyacrylic acid (PAA) and SiO ₂ were 0.5% by mass with respect to the mixture of the coated and adhered carbon powder and the composite graphite powder “A”, respectively. In the case of Comparative Example 4, the carbon powder I / carbon powder II shown in Table 1 indicates a mass ratio of the composite graphite powder “A” / artificial graphite “B”. A binder similar to that in Example 1 was added to the obtained mixture to obtain a negative electrode mixture as in Example 1. A negative electrode was prepared in the same manner as in Example 1, and the same coin-type battery was prepared using the same positive electrode, separator, and electrolyte as in Example 1, and the battery characteristics were evaluated.

When comparing Examples 1 to 7 and Comparative Examples 1 to 3, the example shows that the carbon powder I has a predetermined amount of SiO ₂ and / or Al ₂ O ₃ fine particles attached to the surface of the carbon powder, It can be seen that by using the carbon powder II coated with a predetermined amount of acrylic acid polymer, the 180 ° peel strength is high and the same excellent battery characteristics can be obtained. Moreover, since Comparative Example 1 has too little SiO ₂ and / or Al ₂ O _{3 and} too little acrylic polymer, Comparative Examples ₂ and ₃ have too much SiO ₂ and / or Al ₂ O _3, respectively. Since there are too many acid type polymers, it turns out that neither peel strength raises sufficiently. When Example 4 is compared with Comparative Example 4, it can be seen that 180 ° peel strength is low when SiO ₂ and / or Al ₂ O ₃ and an acrylic acid polymer are present on the surface of the same carbon powder.

The present invention relates to carbon powder I in which 0.1 to 3.0% by mass of SiO ₂ and / or Al ₂ O ₃ fine particles are attached to the surface of carbon powder that is a negative electrode material of a lithium ion secondary battery, By using carbon powder II coated with 1 to 2.5% by mass of acrylic acid polymer separately as a raw material, a peel strength of 20 N / m or more, which is an important characteristic in the battery manufacturing process and battery performance, is achieved. It is feasible, and in addition to capacity and initial efficiency, a negative electrode material that is particularly excellent in cycle characteristics can be obtained.

DESCRIPTION OF SYMBOLS 1 Exterior cup 2 Working electrode 3 Exterior can 4 Counter electrode 5 Separator 6 Insulating gasket 7a Current collector 7b Current collector

Claims (6)

A negative electrode material for a lithium ion secondary battery comprising carbon powder I having SiO ₂ powder and / or Al ₂ O ₃ powder adhered to the surface of carbon powder, and carbon powder II having a surface coated with acrylic acid polymer on the surface of carbon powder Because
The total content of the SiO ₂ powder and Al ₂ O ₃ powder is
Carbon powder, SiO ₂ powder and / or Al ₂ O ₃ powder, and 0.1 to 3.0% by mass with respect to the total amount of acrylic acid polymer,
Content of the acrylic acid polymer is
0.1 to 2.5% by mass with respect to the total amount of carbon powder, SiO ₂ powder and / or Al ₂ O ₃ powder, and acrylic polymer,
The ratio of the said carbon powder I and the said carbon powder II is carbon powder I / carbon powder II = 10 / 90-90 / 10 by mass ratio, The negative electrode material for lithium ion secondary batteries characterized by the above-mentioned.   A negative electrode mixture for a lithium ion secondary battery, comprising a mixture of the negative electrode material according to claim 1 and a water-soluble binder.   A negative electrode for a lithium ion secondary battery, comprising the negative electrode material according to claim 1.   A lithium ion secondary battery comprising the negative electrode according to claim 3. Carbon powder with SiO ₂ powder and / or Al ₂ O ₃ powder adhered to the surface of the carbon powder, and the total content of SiO ₂ powder and Al ₂ O ₃ powder is carbon powder, SiO ₂ powder and / or Al ₂ O ₃ powder and carbon powder I which is 0.1-3.0% by mass with respect to the total amount of acrylic acid polymer, and the surface of the carbon powder is coated with acrylic acid polymer, and the content of acrylic acid polymer is Carbon powder II that is 0.1 to 2.5% by mass with respect to the total amount of carbon powder, SiO ₂ powder and / or Al ₂ O ₃ powder, and acrylic acid polymer,
A negative electrode mixture for a lithium ion secondary battery, which is obtained by mixing water and adding a water-soluble binder.   The negative electrode for lithium ion secondary batteries obtained by apply | coating the negative mix of Claim 5 to the negative electrode collector surface, and vacuum-drying.

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