JPH11111268A - Negative electrode for lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve adhesive strength of mutual carbon materials and the carbon material and a current collecting body in a negative electrode, and remarkably improve a highly efficient discharge characteristic and a cycle characteristic by using hydrogenated carboxy modified rubber as a binding agent to bind a negative electrode active material composed of a powdery carbon material to store/release a lithium ion. SOLUTION: A hydrogenation rate of carboxy modified rubber is 30 to 90%, and it is carboxy modified butadiene rubber containing butadiene, and a copolymer of one or more kinds selected from a group composed of ethylene, isoprene and styrene and butadiene is used as a basic skeleton, and the copolymer is carboxilically modified. The carboxy modified butadiene rubber is desirable to be carboxilically modified by using a copolymer of acrylonitrile and butadiene as a basic skeleton. It is ideal to set the butadiene content of this binding agent not less than 20 wt.% and the gel content not less than 60%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a negative electrode active material comprising a powdery carbon material capable of inserting and extracting lithium ions, and a binder for binding the negative electrode active materials to each other. The present invention relates to a provided negative electrode for a lithium secondary battery.

[0002]

2. Description of the Related Art In recent years, as a negative electrode active material of a lithium secondary battery, carbon dioxide such as coke or graphite, which can occlude and release lithium ions, has been used because dendritic lithium metal is not deposited and the cycle life is long. Materials are being used. And as a binder for a negative electrode using such a carbon material, from the viewpoint of chemical stability and the like,
Fluorine-based resins such as polyvinylidene fluoride are used.

[0003] However, since the fluorine-based resin has insufficient binding force to the carbon material and the current collector, the adhesion between the negative electrode active materials and between the negative electrode active material and the current collector is insufficient. Battery characteristics such as rate discharge characteristics and cycle characteristics are apt to deteriorate. In particular, when graphite having a d value (d ₀₀₂ ) of less than 3.40 ° on the lattice plane (002) plane is used as the carbon material, the adhesiveness is likely to be deteriorated.
This tendency becomes even more pronounced when natural graphite having a ₀₀₂ ) of less than 3.36 ° is used.

Therefore, Japanese Patent Application Laid-Open No. 8-287915 proposes a technique using a carboxy-modified vinyl rubber as a binder. However, the binder used in this technique has not yet obtained sufficient cycle characteristics and high-rate discharge characteristics due to insufficient chemical stability against a non-aqueous electrolyte.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems, and has been made by improving the adhesion between carbon materials in a negative electrode and between a carbon material and a current collector. It is another object of the present invention to provide a negative electrode for a lithium secondary battery that can dramatically improve high-rate discharge characteristics and cycle characteristics of a battery using the negative electrode.

[0006]

In order to achieve the above object, the present invention is directed to a negative electrode active material made of a powdery carbon material that occludes and releases lithium ions; a negative electrode current collector; In a negative electrode for a lithium secondary battery including a binder for binding a negative electrode active material, as the binder,
It is characterized by using a hydrogenated carboxy-modified rubber.

The carboxy-modified rubber has a viscoelastic adhesive force and an excellent binding force to a current collector made of metal, but has insufficient chemical stability. Other components in the battery tend to deteriorate the binding force. Here, in the above configuration, a carboxy-modified rubber in which the number of unsaturated bonds is reduced by hydrogenation is used as a binder. Therefore, the chemical stability of the carboxy-modified rubber in the battery is improved, and as a result, the cycle characteristics and the high-rate discharge characteristics of the negative electrode are improved.

According to a second aspect of the present invention, in the negative electrode for a lithium secondary battery according to the first aspect, the carboxy-modified rubber has a hydrogenation rate of 30 to 90%. The reason for restricting the hydrogenation rate in this way is as follows. That is, when the hydrogenation rate is less than 30%, the chemical stability of the binder decreases. On the other hand, the hydrogenation rate is 90%
If it exceeds 300, the elasticity of the binder is remarkably reduced, the adhesion between the active materials or between the active material layer and the current collector is reduced, and the internal resistance of the negative electrode is increased. For this reason, if the ratio is outside the above range, the cycle characteristics and the high-rate discharge characteristics are likely to deteriorate.

According to a third aspect of the present invention, in the negative electrode for a lithium secondary battery according to the first or second aspect, a carboxy-modified butadiene rubber containing butadiene is used as the carboxy-modified rubber.

[0010] The hydrogenated carboxy-modified butadiene rubber has the excellent viscoelasticity and adhesiveness of butadiene rubber and also has excellent chemical stability. Therefore, even if the charge and discharge cycle is performed over a long period of time, the suitable binding property is maintained, and thus the composition is excellent as a binder for a negative electrode for a lithium secondary battery.

The term carboxy-modified butadiene rubber in the present specification is used as a term of a lower concept of carboxy-modified rubber.

According to a fourth aspect of the present invention, there is provided the negative electrode for a lithium secondary battery according to the third aspect, wherein the carboxy-modified butadiene rubber is at least one selected from the group consisting of ethylene, isoprene, styrene, and vinylpyridine. And a butadiene as a basic skeleton, and using a carboxy-modified butadiene rubber in which the copolymer is carboxy-modified.

When butadiene is copolymerized with ethylene or isoprene, the flexibility of the rubber is increased as compared to butadiene alone. Therefore, in the case of the carboxy-modified butadiene rubber having the above composition, the active materials can be bonded to each other and the active material layer with the current collector flexibly and strongly, and poor adhesion hardly occurs.

According to a fifth aspect of the present invention, in the negative electrode for a lithium secondary battery according to the third aspect, a copolymer of acrylonitrile and butadiene serves as a basic skeleton as the carboxy-modified butadiene rubber, and the copolymer has It is characterized by using a carboxy-modified carboxy-modified butadiene rubber.

Since acrylonitrile is a substance having a large polarity, a carboxy-modified butadiene rubber composed of a copolymer of butadiene and acrylonitrile has excellent affinity with an active material and a current collector. Therefore, the active materials can be strongly bonded to each other and the active material layer and the current collector, and as a result, the high-rate discharge characteristics and the cycle characteristics of the negative electrode are significantly improved.

The invention according to claim 6 is the invention according to claim 4 or 5.
The negative electrode for a rechargeable lithium battery according to the above, wherein the carboxy-modified butadiene rubber has a butadiene content of 2
0% by weight or more and the gel content is 60% or more. With this configuration, the cycle characteristics and high-rate discharge characteristics of the negative electrode can be significantly improved.

The invention according to claim 7 is the invention according to claim 4 or 5.
The negative electrode for a rechargeable lithium battery according to the above, wherein the carboxy-modified butadiene rubber has a butadiene content of 2
0 to 80% by weight, and the gel content is 60% or more. With this configuration, in addition to the cycle characteristics and high-rate discharge characteristics of the negative electrode of the negative electrode, the negative electrode peel strength can be further improved.

The invention described in claim 8 is the third or seventh invention.
In the negative electrode for a lithium secondary battery described above, as the carboxy-modified butadiene rubber, a carboxy-modified one using an ethylenically unsaturated dicarboxylic acid monomer and / or a derivative of an ethylenically unsaturated dicarboxylic acid monomer is used.

A carboxy-modified butadiene rubber using an ethylenically unsaturated dicarboxylic acid monomer or the like is easily compatible with an active material and a current collector, so that the binding properties between the active materials and between the active material layer and the current collector are improved. .

According to a ninth aspect of the present invention, in the negative electrode for a lithium secondary battery according to the eighth aspect, the ethylenically unsaturated dicarboxylic acid monomer comprises acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and maleic acid. It is characterized by using one or more acids selected from the group.
With this configuration, the operation and effect described in the seventh aspect are further exhibited.

According to a tenth aspect of the present invention, in the negative electrode for a lithium secondary battery according to any one of the third to ninth aspects, the content of the carboxy-modified butadiene rubber with respect to the total weight of the negative electrode excluding the current collector is 0.1 to 5. It is characterized by being 0% by weight.

The content of the carboxy-modified butadiene rubber relative to the total weight of the negative electrode excluding the current collector is too large or too small, so that it is difficult to obtain a sufficient action and effect, and sufficient adhesion is obtained in the above range. The details will be described later.

The invention according to claim 11 is the negative electrode for a lithium secondary battery according to any one of claims 3 to 9, wherein the content of the carboxy-modified butadiene rubber with respect to the total weight of the negative electrode excluding the current collector is 0.3 to 3%. 0.0% by weight. At a content in this range, particularly remarkable effects can be obtained.

According to a twelfth aspect of the present invention, in the negative electrode for a lithium secondary battery according to any one of the first to eleventh aspects, the carbon material has a d value (d ₀₀₂ ) of less than 3.40 ° on a lattice (002) plane. Characterized by using graphite. Since such graphite has a property that it is difficult to bind, the operation and effect of the present invention are further exhibited.

The invention of claim 13 is the first invention.
The negative electrode for a lithium secondary battery according to 1 or 12, wherein
As the above carbon material, d value in lattice (002) plane
(D ₀₀₂) Uses less than 3.36 kg of natural graphite.
And features. Such natural graphite is self-lubricating and
Since it has a strong cleavage property, it has a property that it is more difficult to bind. Yo
Therefore, the function and effect of the present invention are more remarkably exhibited.

The invention according to claim 14 is the first invention.
The negative electrode for a lithium secondary battery according to 2 or 13, wherein
The negative electrode for a lithium secondary battery is characterized in that it is used as a negative electrode of a spiral power generator in which a negative electrode and a positive electrode are overlapped and wound with a separator interposed therebetween.

In the spiral power generator, a large stress acts upon winding, and a stress always acts toward the outside of the winding axis even in the battery, so that the negative electrode active material and the current collector are not connected to each other. Is easily hindered. Therefore, in the negative electrode used for such a spiral power generator, the operation and effect of the present invention are more remarkably exhibited.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION A negative electrode for a lithium secondary battery according to the present invention comprises a negative electrode active material composed of a powdery carbon material capable of inserting and extracting lithium ions, a current collector, negative electrode active materials, and A rubber binder for binding the negative electrode active material to the current collector.

Examples of the carbon material of the negative electrode according to the present invention include a graphitic carbon material (artificial graphite and natural graphite) capable of inserting and extracting lithium ions and a carbonaceous material partially having a graphite structure. Either can be used. Also,
In addition to the carbon material as the negative electrode active material, for example, carboxymethyl cellulose, polyvinylpyrrolidone, or the like can be added to the negative electrode according to the present invention as a binding aid.

The current collector is not particularly limited as long as the material is conductive. For example, a material such as a copper foil or a nickel foil can be used.

Examples of the carboxy-modified rubber which is a main component of the negative electrode according to the present invention include carboxy-modified butadiene rubber, ethylene butadiene rubber, butyl rubber, chloroprene rubber, isoprene rubber, isobutylene-isoprene rubber, vinylpyridine butadiene rubber and the like. These carboxy-modified rubbers are those in which a part or all of the unsaturated bond portion in the chemical structure is saturated by hydrogenation. And preferably, this hydrogenation allows 30 to 90% of the unsaturated bond portion
Is used. The reason for this is that, as is apparent from FIG. 1 described later, when the hydrogenation rate is in this range, the cycle characteristics of the negative electrode can be significantly improved. When the hydrogenation rate is less than 30%, the chemical stability to the electrolyte or the like is reduced, resulting in poor cycle characteristics. On the other hand, when the hydrogenation rate exceeds 90%, the rubber elasticity is greatly reduced. As a result, the adhesion between the active material and the current collector becomes insufficient, and the long-term cycle characteristics and high-rate discharge characteristics deteriorate.

As the hydrogenated carboxy-modified rubber, a butadiene rubber (including a butadiene rubber consisting of butadiene alone) is used from the viewpoint of adhesive strength and viscoelasticity, and ethylene, isoprene, styrene, and One or more selected from the group consisting of vinylpyridine and a copolymer of butadiene is used as a basic skeleton, and a carboxy-modified copolymer is used.More preferably, acrylonitrile and butadiene are copolymerized. It is preferable to use carboxy-modified butadiene rubber.

[0033] Butadiene is replaced with 1 such as ethylene or isoprene.
Those obtained by copolymerizing at least one kind are excellent in the adhesion between the active materials and between the active material and the current collector, and particularly, butadiene obtained by copolymerizing acrylonitrile. Acrylonitrile rubber has excellent affinity with the current collector. Therefore, when such a hydrogenated carboxy-modified butadiene rubber is used as a binder for a negative electrode using a carbon material as an active material, the carbon materials and the carbon material are strongly bonded to the current collector. Moreover, since the chemical stability to the components in the battery is increased by hydrogenation, good binding is maintained for a long time.
As a result, the high-rate discharge characteristics and cycle characteristics of the negative electrode are significantly improved.

Further, as the carboxy-modified butadiene rubber, it is preferable to use a carboxy-modified rubber using an ethylenically unsaturated dicarboxylic acid monomer and / or a derivative of an ethylenically unsaturated dicarboxylic acid monomer. Butadiene rubber modified by carboxy with an acid such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and maleic acid is particularly preferred in that the adhesiveness to the current collector can be improved. The degree of carboxy modification may be about 1 to 5%.

The carboxy-modified butadiene rubber preferably has a butadiene content of 20% by weight or more and a gel content of 60% or more, more preferably a butadiene content of 40 to 80% by weight and a gel content of 40 to 80% by weight. Quantity is 60
% Or more. The reason is as follows.

First, as shown in FIG. 3 described below, when the butadiene content is 20% or more, excellent cycle characteristics can be obtained. Further, from FIG. 4 described later, when the gel content is set to 60%, the high-rate discharge characteristics can be improved. Therefore, it is preferable that the butadiene content be 20% by weight or more and the gel content be 60% or more. Further, from FIG. 2 described later, when the butadiene content is 20 to 80%, the peel strength of the negative electrode can be significantly increased. In general, when the peel strength is high, the active material is prevented from dropping during battery assembly or charge / discharge, and a decrease in energy density due to the drop of the active material is prevented. Therefore, when considering that the peel strength can be further increased, it is more preferable that the butadiene content be 20 to 80% and the gel content be 60% or more. The details of the experimental conditions and the like in each figure will be described later (the same applies to other figures).

Here, the above-mentioned gel content is a ratio of an insoluble component to toluene relative to the total weight, and specifically, a component (dry weight) which is not extracted even by the Soxhlet circulation extraction method using toluene is a gel. The gel component is expressed as a percentage, divided by the total weight (dry weight). The large gel content means that the proportion of the rubber component having a large molecular weight is high. And
Components having a high molecular weight are chemically more stable than components having a low molecular weight, and are less likely to swell in a lithium electrolyte. Then, according to the results shown in FIG.
It has been confirmed that when the gel content is 60% or more, high-rate discharge characteristics can be significantly improved.

The addition amount (content) of the hydrogenated carboxy-modified rubber rubber to the negative electrode according to the present invention is preferably 0.1 to 0.1 with respect to the weight of the negative electrode excluding the current collector.
5.0 wt%, more preferably 0.3 to 3.0 wt%. As shown in FIG. 5 described later, when the content of the rubber binder is 0.1 to 5.0% by weight, 70% or more of the initial capacity can be secured even after 500 cycles. On the other hand, the content is less than 0.1% by weight and the content is 5.0
If the amount exceeds 10% by weight, the cycle characteristics deteriorate. The reason is that the binder content is 0.1 to 5.0% by weight.
Thus, it is considered that a sufficient binding force cannot be obtained, and when the content exceeds 5.0% by weight, the internal resistance of the electrode increases. Therefore, the content is preferably in the range of 0.1 to 5.0% by weight, but from the viewpoint of the internal resistance, the smaller the amount of the binder, the better the balance between the binding force and the internal resistance. In this case, the content is more preferably 0.3 to 3.0% by weight.
It is good to do.

The weight of the negative electrode excluding the current collector refers to the total amount (collection amount) including the active material main body (carbon material) and the hydrogenated carboxy-modified rubber, binding aid, or other additional components according to the present invention. Excluding electrical conductors).

The hydrogenated carboxy-modified rubber according to the present invention is usually used in a state of being dissolved in a solvent or as an emulsion (latex) which is colloidally dispersed in water with an emulsifier. Is sufficiently kneaded with an active material and the like, applied to a current collector, and further dried to exert its effect.

By the way, the hydrogenated carboxy-modified rubber according to the present invention described above exerts its excellent effects regardless of the kind of the carbon material as the active material.
In particular, the plane distance d ₀₀₂ in the lattice plane (002) plane is 3.4.
A remarkable effect is obtained in combination with graphite of less than 0 °, and a more remarkable effect is obtained in combination with natural graphite of less than 3.36 ° (see Tables 1 and 2 below). The reason for this is that graphite has self-lubricating properties and cleavage properties, so that the binding force is less apt to act than coke. This is because it is difficult for the adhesive to bind sufficiently.

Furthermore, the hydrogenated carboxy-modified rubber exhibits a more remarkable effect when used for a negative electrode constituting a spiral power generator in which a negative electrode and a positive electrode are superposed and wound via a separator. . The reason for this is that, when a spiral type is used, a large stress acts on the electrode when producing the power generator, and the stress in the direction in which the negative electrode active material layer is peeled off from the current collector even after being housed in the battery can. Is constantly acting, the conventional binder causes the active material to fall off or poor current collection, resulting in a decrease in battery capacity and deterioration in cycle characteristics and high-rate discharge characteristics. If it is a modified rubber, the binding agent has elasticity, and the binding between the active material and the current collector is achieved with a strong binding force capable of maintaining the binding against these stresses. Because it is.

[0043]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the contents of the present invention will be described more specifically based on experiments.

(Preparation of Various Negative Electrodes) First, artificial graphite (d ₀₀₂ = 3.36 ° to 3.40 °) having a particle diameter of 1 to 30 μm was prepared as a carbon material as a negative electrode active material. Various butadiene rubbers were prepared as the carboxy-modified rubber as a binder. Then, using each of them, a carbon negative electrode was produced according to the following production method.

Preparation of carbon negative electrode 98 parts by weight of natural graphite powder, 1 part by weight of a binder consisting of butadiene rubber (used as latex type) as dry weight, and 1 part by weight of carboxymethyl cellulose as a slurry stabilizer And an appropriate amount of water, and kneaded to prepare a negative electrode active material slurry.
It was applied on both sides of an 8 μm copper foil and dried under reduced pressure at 110 ° C. for 3 hours to form a negative electrode. Basically, the following experiments were performed using the carbon negative electrode produced in this manner (detailed conditions are described in each experimental section).

[Experiment 1] In Experiment 1, the relationship between the hydrogenation rate and the cycle characteristics was examined by changing the hydrogenation rate for acrylonitrile butadiene rubber. Specifically, the procedure was performed as follows. First, 10 kinds of acrylonitrile butadiene rubbers having different hydrogenation rates were prepared, and these rubbers were used to produce a negative electrode according to the above-mentioned method. Next, a lithium secondary battery having the following structure was formed by combining this negative electrode with a known positive electrode, and a cycle characteristic test was performed using this battery.

Battery Structure First, the overall structure of the lithium secondary battery used in Experiment 1 (the same applies to other applicable experiments) will be described with reference to FIG. FIG. 6 is a schematic cross-sectional view of a battery, where 1 is a known positive electrode made of LiCoO ₂ . Reference numeral 2 denotes a carbon negative electrode manufactured using the butadiene-based rubber latex as a binder according to the above-described method. Further, 3 is a separator for separating the positive and negative electrodes, 4 is a positive electrode lead, 5 is a negative electrode lead,
Reference numeral 6 denotes a positive electrode external terminal, 7 denotes a battery can, 8 denotes a sealing plate, and 9 denotes an insulating packing.

The positive electrode 1 and the negative electrode 2 are accommodated in a battery can 7 while being spirally wound via a separator 3, and a lithium electrolyte is injected into the battery can 7.
The positive electrode 1 is connected to a positive electrode external terminal 6 via a positive electrode lead 4, and the negative electrode 2 is connected to a battery can 7 also serving as a negative electrode external terminal via a negative electrode lead 5.

The positive electrode 1 was manufactured as follows. 8
Lithium-containing cobalt dioxide LiC heat-treated at 00 ° C
oO ₂ as a cathode material, this cathode material LiCoO ₂ ,
A carbon powder as a conductive agent and a fluororesin powder as a binder are mixed at a weight ratio of 85: 10: 5, and this mixture is applied to both surfaces of a positive electrode current collector made of aluminum foil, A heat treatment was performed at ° C. to form a positive electrode.

As the non-aqueous electrolyte, ethylene carbonate and 1,2-dimethoxyethane were mixed at a volume ratio of 1: 1 and mixed with lithium hexafluorophosphate LiPF.
₆ was dissolved at a rate of 1 MOL / L and used.

As the separator 3, a 30 μm-thick lithium-ion-permeable polypropylene microporous membrane (Cell Card manufactured by Hoechst Celanese Corporation) was used.

Cycle Characteristics Test Method The battery was charged at 1.25 A until the battery voltage reached 4.1 V, and further charged while gradually reducing the charging current value to 20 mA while maintaining the battery voltage at 4.1 V. A cycle characteristic test was performed under the condition that a cycle of discharging at a current value of 1.25 A until the temperature reached 2.75 V was repeated 500 times at 25 ° C. The ratio of the discharge capacity after 500 cycles to the initial discharge capacity in this charge / discharge cycle was taken as the cycle characteristic value.

(Results of Experiment 1) FIG. 1 shows the hydrogenation rate (%).
And the relationship between cycle characteristics. In FIG. 1, it was recognized that good cycle characteristics were obtained when the hydrogenation rate was in the range of 30% or more and 90% or less. Here, the hydrogenation rate (%) is defined as a percentage in which the hydrogenation rate when hydrogen is added to all of the unsaturated bond portions in the butadiene rubber is 100, and the actual hydrogenation rate is expressed as a percentage. It is.

[Experiment 2] In experiment 2, various butadiene rubbers having different butadiene contents were prepared for various butadiene rubbers composed of a copolymer of butadiene and other components, and the kind of butadiene copolymer and butadiene content were prepared. The effect of this difference on the negative electrode peel strength (Kg / cm ² ) was examined. Specifically, five kinds of ethylene butadiene copolymers having different butadiene contents, isoprene butadiene copolymers, styrene butadiene copolymers, vinylpyridine butadiene copolymers, and acrylonitrile butadiene copolymers are prepared. A negative electrode was prepared according to the above-mentioned method using rubber latex as a binder. And about these negative electrodes, the peeling strength test with respect to the negative electrode was performed by the following method.

Peel Strength Test Method for Negative Electrode A pressure-sensitive adhesive tape was attached to the surface (active material layer) of the negative electrode, and the end of the tape was attached to a spring balance to apply a tensile force. Was measured. The adhesive tape used had a sufficient adhesive strength. Therefore, peeling of the pressure-sensitive adhesive tape from the negative electrode surface means that the active material layer was peeled from the current collector.

(Results of Experiment 2) The results of the experiment are shown in FIG. In FIG. 2, all butadiene copolymers exhibited higher negative electrode peel strengths when the butadiene content was in the range of 20 to 80%. Among these, the peel strength of the acrylonitrile-butadiene copolymer (acrylonitrile-butadiene rubber) was particularly high.

[Experiment 3] In Experiment 3, based on the above experiment, an acrylonitrile-butadiene copolymer was used as a binder, and the effect of a difference in butadiene content in the acrylonitrile-butadiene copolymer on cycle characteristics was examined. The experimental method and the method for measuring the cycle characteristics are the same as those in the above-described experiment 1. The result is shown in FIG. FIG. 3 shows that when the butadiene content of the acrylonitrile-butadiene copolymer exceeds 20% by weight, the cycle characteristics are significantly improved.

[Experiment 4] In Experiment 4, an acrylonitrile-butadiene copolymer was used, and the relationship between the gel content of the copolymer and high-rate discharge characteristics was examined. The measurement of the high-rate discharge characteristics was performed according to the following method, and the method for manufacturing the negative electrode was the same as described above.

[0059]High-rate discharge characteristics test method Charged to 4.1 V at a current value of 1.25 A (1 C) (20
0.25A (0.2C)
At a current value of 2.75 V until the battery voltage reaches 2.75 V
Discharge capacity C₁And a battery with a current value of 2.5A (2C)
Discharge capacity C when discharging until the voltage reaches 2.75 V
_TwoWas measured. And C₁C for _TwoFind the ratio of
This value (C_Two/ C₁) Is the high-rate discharge characteristic value.

(Experimental Results) FIG. 4 shows the relationship between the gel content and the high-rate discharge characteristics. In FIG. 4, when the gel content exceeds 50%, the degree of improvement in the high-rate discharge characteristics increases, and at 60% or more, a high-rate discharge characteristic value of about 0.97 is obtained. With this result,
It was demonstrated that when the gel content (%) of the rubber-based binder is 60% or more, high high-rate discharge characteristics can be obtained.

As described above, the rubber binder having a low gel content has a higher proportion of low molecular weight components than the binder having a high gel content. Therefore, in the negative electrode using the rubber-based binder having a low gel content, the active material layer swells due to the non-aqueous electrolyte, and the adhesion between the negative electrode active materials and between the negative electrode active material layer and the current collector deteriorates. . Then, as a result, it is considered that the current collection efficiency decreases and the high-rate discharge characteristics deteriorate.

[Experiment 5] In Experiment 5, an acrylonitrile-butadiene copolymer was used to produce a negative electrode in which the amount of butadiene-based rubber used was changed (the content relative to the weight of the negative electrode excluding the current collector is expressed in% by weight). The relationship between cycle characteristics was investigated. The measurement method and measurement conditions are the same as those in Experiment 1.

(Experimental Results) FIG. 5 shows the relationship between the binder content (%) in the negative electrode excluding the current collector and the cycle characteristics. As is clear from FIG. 5, when the content of the acrylonitrile-butadiene copolymer was in the range of 0.3% by weight to 5.0% by weight, the cycle characteristics were significantly improved.

Here, the relationship between the binder content (%) and the high-rate discharge characteristics is not shown, but in general, when the content of the rubber-based binder increases, the internal resistance of the electrode increases. High-rate discharge characteristics deteriorate. Therefore, it is preferable to reduce the binder content as long as a sufficient binding force can be obtained. However, in FIG. 5, even if the content (%) with respect to the negative electrode weight excluding the current collector exceeds 3.0%, It is recognized that there is no improvement in the cycle characteristics. Therefore, the content of the rubber binder according to the present invention is more preferably 0.3 to 3.0% by weight.

[Experiment 6] In Experiment 6, natural graphite, artificial graphite and coke having a particle diameter of 1 to 30 μm were prepared as a negative electrode active material (carbon material). On the other hand, itaconic acid, fumaric acid, each acid-modified using maleic acid as an acid denaturing agent,
The acrylonitrile butadiene copolymer of the present invention (butadiene content 60
%, Gel content 95%). Then, using the carbon material and the acrylonitrile-butadiene copolymer (acrylonitrile-butadiene rubber), various negative electrodes were produced according to the above-described method, and the peel strength of each negative electrode was measured according to the method of Experiment 2, and the type of carbon material and The relationship between the type of acid modifier and the peel strength of the negative electrode was examined.

The d value (d ₀₀₂ ) of each carbon material on the lattice plane (002) plane is less than 3.36 ° for natural graphite,
Artificial graphite is 3.36Å3.40Å, coke is 3.4
0 ° to 3.60 °. In the acid modification, 10% of the acid modifier was used based on the total weight of the acrylonitrile butadiene copolymer and the acid modifier.

On the other hand, various carbon anodes were prepared by using 5% by weight of polyvinylidene fluoride (PVdF) instead of unmodified, acid-modified with acrylic acid or methacrylic acid, and acrylonitrile-butadiene rubber for comparison. It was produced in the same manner as described above. Then, a similar peeling test was performed on the carbon negative electrode. In addition, polyvinylidene fluoride was used by dissolving it in N-methyl-2-pyrrolidone.

(Experimental Results) Tables 1 and 2 show the experimental results. As is clear from Tables 1 and 2, the various negative electrodes using polyvinylidene fluoride as the binder had a significantly lower peel strength than the other negative electrodes. Further, all of the various negative electrodes using the acid-modified acrylonitrile butadiene rubber as the binder had higher peeling strength than the negative electrode using polyvinylidene fluoride. Furthermore, the negative electrode of the present invention using acrylonitrile butadiene rubber acid-modified with an unsaturated dicarboxylic acid such as itaconic acid, fumaric acid, and maleic acid is a natural graphite, artificial graphite, and coke, both of which are comparative examples using polyvinylidene fluoride. The peel strength is remarkably large compared to acrylic acid, acrylonitrile butadiene rubber acid-modified with methacrylic acid,
Significant improvement in peel strength was observed.

In the case of a natural graphite negative electrode, a particularly remarkable improvement in peel strength (2869% improvement over PVdF)
Was observed. From these results, the excellent effects of the present invention were confirmed.

[0070]

[Table 1]

[0071]

[Table 2]

[0072]

As described above, a carbon material as an active material used for a negative electrode for a lithium secondary battery is hardly affected by a binder, and natural graphite is particularly advantageous for lithium ions to be inserted and desorbed. Although it has an interlayer structure, it has strong self-lubricating properties and cleavage properties, so that the current collecting efficiency is likely to be reduced due to insufficient binding force.

For this reason, in the conventional carbon anode, the excellent ion insertion / desorption property of the carbon material could not be fully utilized. However, according to the present invention, the active material fell off and the current collection efficiency was prolonged even in a long cycle. Therefore, the power generation capability of the carbon material, in particular, the excellent power generation capability of the natural graphite can be sufficiently obtained.

That is, the present invention has an excellent effect that it is possible to provide a negative electrode for a lithium secondary battery having remarkably excellent negative electrode peel strength, high rate discharge characteristics and cycle characteristics.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the hydrogenation rate (%) for a butadiene rubber and cycle characteristics.

FIG. 2 is a graph showing the relationship between the butadiene content (%) of various butadiene copolymers and the negative electrode peel strength.

FIG. 3 is a graph showing a relationship between a butadiene content of a butadiene rubber and cycle characteristics.

FIG. 4 is a graph showing the relationship between the gel content (%) of butadiene rubber and high-rate discharge characteristics.

FIG. 5 is a graph showing the relationship between the content (%) of a rubber binder in the weight of a negative electrode excluding a current collector and cycle characteristics.

FIG. 6 is a schematic cross-sectional view of a test lithium secondary battery used in a cycle characteristic test and a high-rate discharge characteristic test.

Claims (14)

[Claims] 1. A lithium secondary battery comprising: a negative electrode active material made of a powdery carbon material that occludes and releases lithium ions; a negative electrode current collector; and a binder for binding the negative electrode active material. A negative electrode for a lithium secondary battery, wherein a hydrogenated carboxy-modified rubber is used as the binder. 2. The negative electrode for a lithium secondary battery according to claim 1, wherein the carboxy-modified rubber has a hydrogenation ratio of 30 to 90%. 3. The carboxy-modified rubber according to claim 1, wherein the carboxy-modified rubber is a carboxy-modified butadiene rubber containing butadiene.
Or the negative electrode for a lithium secondary battery according to 2. 4. The carboxy-modified butadiene rubber comprises a copolymer of butadiene and one or more members selected from the group consisting of ethylene, isoprene, styrene and vinylpyridine as a basic skeleton, and the copolymer is a carboxy-modified butadiene rubber. The negative electrode for a lithium secondary battery according to claim 3, wherein the negative electrode is modified. 5. The negative electrode for a lithium secondary battery according to claim 3, wherein the carboxy-modified butadiene rubber has a basic skeleton of a copolymer of acrylonitrile and butadiene, and the copolymer is carboxy-modified. 6. The negative electrode for a lithium secondary battery according to claim 4, wherein the butadiene content in the carboxy-modified butadiene rubber is 20% by weight or more, and the gel content is 60% or more. 7. The carboxy-modified butadiene rubber has a butadiene content of 20 to 80% by weight,
The negative electrode for a lithium secondary battery according to claim 4, wherein the gel content is 60% or more. 8. The lithium secondary according to claim 3, wherein the carboxy-modified butadiene rubber is carboxy-modified using an ethylenically unsaturated dicarboxylic acid monomer and / or a derivative of an ethylenically unsaturated dicarboxylic acid monomer. Negative electrode for battery. 9. The lithium secondary battery according to claim 8, wherein the ethylenically unsaturated dicarboxylic acid monomer is at least one acid selected from the group consisting of acrylic acid, methacrylic acid, itaconic acid, fumaric acid and maleic acid. Negative electrode for secondary battery. 10. The content of the carboxy-modified butadiene rubber relative to the total weight of the negative electrode excluding the current collector is 0.1 to 10%.
The negative electrode for a lithium secondary battery according to claim 3, wherein the amount is 5.0% by weight. 11. The content of the carboxy-modified butadiene rubber in the total weight of the negative electrode excluding the current collector is 0.3 to 0.3.
The negative electrode for a lithium secondary battery according to claim 3, wherein the amount is 3.0% by weight. 12. The negative electrode for a lithium secondary battery according to claim 1, wherein the carbon material is graphite having a d value (d _{00 2} ) of less than 3.40 ° on a lattice plane (002) plane. 13. The negative electrode for a lithium secondary battery according to claim 1, wherein the carbon material is a natural graphite having a d value (d _{00 2} ) of less than 3.36 ° on a lattice (002) plane. 14. The lithium secondary battery according to claim 12, wherein the negative electrode for a lithium secondary battery is used as a negative electrode of a spiral type power generator in which a negative electrode and a positive electrode are overlapped and wound with a separator interposed therebetween. Negative electrode for battery.