JPH0574461A - Secondary battery negative electrode - Google Patents

Abstract

PURPOSE:To provide a secondary battery negative electrode which is excellent in cycle characteristic and preservation characteristic, especially preservation characteristic at high temperatures and safety. CONSTITUTION:A secondary battery negative electrode using a carbonaceous material as negative electrode active material. In the negative electrode the negative electrode active material is bonded by a binder composed mainly of styrene-butadiene latex having a butadiene content of 40 to 95wt.% and a gel content of 75 to 100%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention has a cycle property, a storage property,
The present invention relates to a novel secondary battery with excellent safety.

[0002]

2. Description of the Related Art In recent years, electronic devices have been remarkably reduced in size and weight, and accordingly, there has been a great demand for reduction in size and weight of batteries as power sources. A non-aqueous battery has been receiving attention because it is impossible to satisfy such a requirement with a battery using a conventional general aqueous electrolyte solution. Such non-aqueous batteries have excellent performance in terms of size and weight reduction, and primary batteries typified by lithium batteries and further lithium / titanium disulfide secondary batteries have been proposed, some of which have been proposed. Has already been put to practical use.

However, although such a non-aqueous battery is excellent in performances such as high energy density and small size and light weight, it has a drawback in output characteristics as compared with the aqueous battery and has not been widely used in general. In particular, in the field of secondary batteries where output characteristics are required, this defect is one of the factors that hinder practical use. The reason why the non-aqueous battery is inferior in the output characteristics is that the ionic conductivity is high in the case of the aqueous electrolyte, which is usually 1
It has a value of the order of 0 ^-1 Ω ^-1 CM ^-1 , whereas in the case of a non-aqueous system, it usually has a low ionic conductivity of 10 ^-2 to 10 ^-4 Ω ^-1 CM ^-1. ..

As one method for solving such a problem, it is considered to increase the electrode area, that is, to use a thin film or a large area electrode.

As a conventional method of molding an electrode, a method of mixing an electrode active material and an organic polymer and compression-molding is generally used. In the case of such a method, there is an advantage that the binder, which is an insulating material, has a relatively small influence on the electrode active material, and the kind and shape of the binder used are also limited, but on the other hand, it is not possible to manufacture a thin film / large area electrode. It's extremely difficult.

On the other hand, as a method for producing a thin film / large area electrode, after dispersing an electrode active material in a solvent solution of an organic polymer,
A method of forming an electrode by coating and drying is known. According to this method, a thin film / large area electrode can be easily obtained, which is very convenient, but the influence of the binder, which is an insulating material, on the electrode active material is extremely large, and when the electrode is assembled into a battery, for example, it is remarkable. This was not a practical method because the overvoltage increased.

From this point of view, the present inventors have used a slurry containing an aqueous dispersion latex, a water-soluble thickener and an electrode active material, instead of using a conventional slurry containing a binder dissolved in an organic solvent and an electrode active material. It has been found that the use of the above-mentioned material can perform coating drying, which is suitable for the production of thin film / large area electrodes, without causing any practical problems, and that the secondary battery has excellent cycleability with almost no increase in conductive pressure. It was possible to provide (Japanese Patent Application No. 2-1
57556).

[0008]

As mentioned above, although a great improvement has been made, some problems still remain in terms of reduction in discharge capacity under high temperature environmental conditions. That is, although the characteristics at room temperature are sufficient, there remains a problem to be solved in that when a battery in a charged state is stored under high temperature conditions, the discharge capacity is lower than that before storage.

[0009]

Means and Actions for Solving the Problems The present inventors have
As a result of various studies on the aqueous dispersion latex, it was found that when a styrene / butadiene latex having a specific structure is used as a binder for the negative electrode active material, a secondary battery having good charge storage performance under high temperature conditions can be obtained. The present invention has been completed and the present invention has been completed.

That is, the present invention is a secondary battery negative electrode using a carbonaceous material as a negative electrode active material, wherein the negative electrode has a butadiene content of 40% to 95% and a gel content of 75% to 10%.
The negative electrode active material is bound by a binder containing 0% of styrene / butadiene latex as a main component.

The present invention will be described in detail below.

The styrene / butadiene latex of the present invention is industrially produced by a usual emulsion polymerization method and has a butadiene content of 40% by weight to 95% by weight, and the styrene / butadiene latex is further dried. The polymer obtained has a gel content of 75% to 100%,
It is preferably 90% to 100%. Here, the gel content refers to the insoluble content of the polymer in toluene.

If the butadiene content is less than 40% by weight,
The electrode lacks adhesive strength and flexibility. If it exceeds 95% by weight, the adhesive strength is insufficient.

When the gel content is less than 75%, the adhesive strength of the electrode and the swelling resistance to the electrolytic solution used in the non-aqueous battery described later are insufficient, and the charge storage performance under high temperature conditions deteriorates.

Although it is not clear why the gel content of the styrene / butadiene latex polymer of the present invention affects the high temperature storage performance, the degree of crosslinking of the latex polymer, that is, the gel content affects the flow of the polymer at high temperature. It is presumed that the more difficult the polymer is to flow, the more the discharge capacity after storage at high temperature is suppressed.

Further, a copolymerizable monomer other than styrene and butadiene can be used in the styrene / butadiene latex of the present invention, for example, methyl (meth).
Ethylenically unsaturated carboxylic acid esters such as acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, (meth) acrylonitrile, hydroxyethyl (meth) acrylate, acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid Ethylenically unsaturated carboxylic acids such as acids can be used. In particular,
As the ethylenically unsaturated carboxylic acid, itaconic acid, fumaric acid, it is possible to use a dicarboxylic acid such as maleic acid,
It is preferable in terms of the adhesive strength of the electrodes. To adjust the gel content,
A general method such as adjustment of the polymerization temperature, adjustment of the amount of polymerization initiator, adjustment of the amount of chain transfer agent, etc. is used.

Although not particularly limited, the particle size of the styrene / butadiene latex is preferably 0.01.
˜0.5 μ, and more preferably 0.01 to 0.3 μ.

The amount of the latex compounded is not particularly limited, but is usually 0.1% with respect to 100 parts by weight of the negative electrode active material.
1 to 20 parts by weight, preferably 0.5 to 10 parts by weight.

If the amount is less than 0.1 parts by weight, good adhesion cannot be obtained, and if the amount exceeds 20 parts by weight, the overvoltage is remarkably increased and the battery characteristics are adversely affected.

The solid content concentration of the coating liquid is not particularly limited, but is usually 30% by weight to 65% by weight, preferably 40% by weight to 65% by weight.

Furthermore, in the secondary battery negative electrode of the present invention, a water-soluble thickener may be used as an additive in an amount of 2 to 60 parts by weight based on 100 parts by weight of the styrene / butadiene latex solid content of the present invention.

Examples of the water-soluble thickener include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, polyacrylic acid (salt), oxidized starch, phosphorylated starch, casein and the like.

The secondary battery negative electrode of the present invention comprises a negative electrode active material, a styrene / butadiene latex and, if necessary, a water-soluble thickener, but does not necessarily exclude other components. For example, dispersants such as sodium hexametaphosphate, sodium tripolyphosphate, sodium pyrophosphate, sodium polyacrylate, etc., and nonionic agents as stabilizers for latex, and those to which additives such as anionic surfactants are added are also included. .. The average particle size of the carbonaceous material used in the present invention is a problem such as a decrease in current efficiency, a decrease in stability of the slurry, and an increase in interparticle resistance in the coating film of the obtained electrode.
The range of 0.1 to 50 μ, preferably 3 to 25 μ, and more preferably 5 to 15 μ is suitable.

The slurry is applied as a coating liquid onto a substrate and dried to form a battery negative electrode. At this time, if necessary, it may be molded together with the current collector material. Alternatively, aluminum foil,
A current collector such as a copper foil can also be used as the base material.

As the coating method, an arbitrary coater head such as a reverse roll method, a comma bar method, a gravure method or an air knife method can be used.

The secondary battery negative electrode of the present invention can be used in both water-based batteries and non-aqueous batteries, but when used as a negative electrode in non-aqueous batteries, particularly excellent battery performance can be obtained.

In the case of assembling a non-aqueous battery using the secondary battery negative electrode of the present invention, the electrolyte of the non-aqueous electrolyte is not particularly limited, but if one example is given, LiClO ₄ , LiBF _4
4 , LiAsF ₆ , CF ₃ SO ₃ Li, LiPF ₆ , L
iI, LiAlCl ₄ , NaClO ₄ , NaBF ₄ , N
aI, (n-Bu) ₄ NClO ₄ , (n-Bu) ₄ NBF
₄ , KPF _{6 and the} like. Examples of the organic solvent of the electrolytic solution used include ethers, ketones, lactones, nitriles, amines, amides, sulfur compounds, chlorinated hydrocarbons, esters, carbonates,
Nitro compounds, phosphoric acid ester compounds, sulfolane compounds, and the like can be used. Among them, ethers, ketones, nitriles, chlorinated hydrocarbons,
Carbonates and sulfolane compounds are preferable.

As typical examples of these, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, anisole, monoglyme, acetonitrile, propionitrile, 4-methyl-2-pentanone, butyronitrile, valeronitrile, benzonitrile, 1 , 2-
Dichloroethane, γ-butyrolactone, dimethoxyethane, methyl formate, propylene carbonate, ethylene carbonate, dimethylformamide, dimethylsulfoxide, dimethylthioformamide, sulfolane, 3-methyl-sulfolane, trimethyl phosphate, triethyl phosphate and mixed solvents thereof. However, the present invention is not limited to these.

The positive electrode material to be used is not particularly limited, but for example, MnO ₂ , MoO ₃ , V
₂ O ₅ , V ₆ O ₁₃ , Fe ₂ O ₃ , Fe ₃ O ₄ , Li
_(1-x) CoO ₂ , Li _(1-x) .NiO ₂ , Li _X Co _y S
_{n Z O 2, TiS 2,} TiS 3, MoS 3, FeS 2,
Inorganic compounds such as CuF ₂ and NiF ₂ , carbon fluoride,
Carbon materials such as graphite, vapor grown carbon fibers and / or pulverized products thereof, PAN-based carbon fibers and / or pulverized products thereof, pitch-based carbon fibers and / or pulverized products thereof, and conductivity of polyacetylene, poly-p-phenylene, etc. And the like.

In particular Li_(1-x) CoO₂ , Li_(1-x) Ni
O₂, Li_XCo_ySn_ZO₂， Li_(1-x) Co_(1-y) Ni
_y O₂ When using a lithium ion-containing composite oxide such as
In this case, it is possible to assemble both the positive and negative electrodes in the discharged state, which is preferable.
It will be a new combination.

Further, if necessary, a battery is constructed by using components such as a separator, a current collector, a terminal and an insulating plate. The structure of the battery is not particularly limited, but a positive electrode, a negative electrode, if necessary, a paper-type battery in which the separator is a single layer or multiple layers, or a positive electrode, a negative electrode, if necessary, a separator roll. An example is a form of a cylindrical battery or the like that is wound into a shape.

[0032]

EXAMPLES The present invention will be described in more detail with reference to Examples and Comparative Examples.

Method for measuring gel content; latex at 120 ° C.
After drying for 1 hour to form a film, the polymer weight of 10
Soak in 0 parts by weight of toluene and shake for 3 hours, then 200
Insoluble matter is collected by filtering with a mesh filter.
It was dried at 0 ° C. for 1 hour, the weight of the insoluble matter was measured, and the gel content was determined by the following formula.

Gel content = (weight of toluene insoluble matter / weight before immersion) × 1
00 (%) Examples 1 to 5 Commercial needle coke (KOA-SJC manufactured by Koa Oil Co., Ltd.)
ake) was crushed to an average particle size of 10 μm. This crushed product 1
10 parts by weight of styrene / butadiene latex (solid content 50% by weight) prepared with the composition shown in Table 1 with respect to 00 parts by weight, 100 parts by weight of an aqueous carboxymethylcellulose solution (solid content 1% by weight) as a thickener, 1/10 1 part by weight of normal ammonia water was added and mixed to obtain a coating liquid. 10μ
Using Ni foil as a base material, this coating liquid was applied and dried at 160 g / m ² to obtain a negative electrode having a thickness of 150 μm.

On the other hand, Li _1.03 Co _0.95 Sn having an average particle size of 2 μ
_0.042 O ₂ 100 parts by weight, graphite powder 7.5 parts by weight and acetylene black 2.5 parts by weight are mixed, and 50 parts by weight of fluororubber methyl isobutyl ketone solution (concentration 4% by weight) is added, mixed and stirred to obtain a coating liquid. And 290 g / m 2 of this coating liquid using a commercially available Al foil (thickness 15μ) as a base material
^It was applied and dried in ² to obtain a positive electrode having a thickness of 110 μm.

The above positive electrode and negative electrode were cut into 1 cm × 5 cm, and the battery shown in FIG. 1 was assembled. This battery was charged up to 4.2 V and discharged at 10 mA ( ² mA / cm ² ) up to 2.7 V. The cycle was repeated. Table 2 shows the overvoltage, charge / discharge efficiency, cycle retention rate, and discharge capacity reduction rate after charge storage at 60 ° C. in the charge / discharge cycle of these batteries.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

EFFECT OF THE INVENTION A battery using the negative electrode of the present invention has excellent current efficiency, excellent capacity retention (cycle property), low overvoltage, and excellent high temperature retention performance. It is extremely useful for obtaining excellent batteries of.

[Brief description of drawings]

FIG. 1 is a sectional view of a constitutional example of a battery of the present invention.

[Explanation of Codes] 1 Positive electrode 2 Negative electrode 3,3 'Current collector 4,4' SUS net 5,5 'External electrode terminal 6 Battery case 7 Separator 8 Electrolyte or solid electrolyte

Claims (2)

[Claims] 1. A secondary battery negative electrode using a carbonaceous material as a negative electrode active material, wherein the negative electrode has a butadiene content of 40% by weight or more.
A negative electrode for a secondary battery, wherein the negative electrode active material is bound by a binder containing styrene / butadiene latex as a main component having a gel content of 95% by weight and a gel content of 75% to 100%. 2. The secondary battery negative electrode according to claim 1, wherein a water-soluble thickener is used as an additive in an amount of 2 to 60 parts by weight based on 100 parts by weight of the latex solid content.