JPH10208731A - Electrode for lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode for a lithium secondary battery having a small irreversible capacity in a charging and discharging initial period, high charging and discharging efficiency, and a long cycle life. SOLUTION: This electrode for a lithium secondary battery is one produced by coating the surface of a collector body with an active material layer containing a carbon material as a main component. The active material layer is formed by a process of producing a slurry by dissolving substrate containing 70-99.5wt.% of the carbon material and 0.5-30wt.% of a cellulose ether type substance in water and a process of forming the active material layer by applying the resultant slurry to the collector body. The cellulose ether type substance is cellulose ether or one or more of its salts.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode for a lithium battery in which an active material layer mainly composed of a carbon material is coated on a current collector.

[0002]

2. Description of the Related Art Conventionally, carbon materials such as graphite and coke have been used as active materials for negative electrodes of lithium secondary batteries. Since the carbon material can hold and release Li ⁺ (lithium ion) between layers or on the surface, it can be a high-capacity negative electrode active material. In manufacturing a lithium secondary battery using a carbon material, a carbon material is applied to a current collector, and the current collector is fixed with a binder. When the electrolyte of the battery is a non-aqueous solution, polytetrafluoroethylene (PT) is used as a binder.
FE), polyvinylidene fluoride, fluorine rubber and the like have been used. For example, in JP-A-6-243896, a crosslinked polymer is used as a binder.

[0003]

However, the above-mentioned conventional binder has a low electric discharge capacity due to too high electric resistance, and also has a decrease in binding force due to reaction with lithium, thereby deteriorating the cycle characteristics of the battery. Thus, the performance of the battery deteriorates.

Therefore, Japanese Patent Application Laid-Open No. 5-101829 proposes to use carboxymethylcellulose (CMC), which is considered to have reactivity with lithium ions, mixed with fluororubber as a binder. . By using this, it is possible to obtain a negative electrode having high charge / discharge efficiency of a battery and a long cycle life.

However, even with this method, the characteristics of the negative electrode were not sufficiently satisfactory. That is, CMC
Alternatively, when a binder obtained by adding a fluororubber to a salt thereof is used, the charge / discharge capacity is reduced and the irreversible capacity is increased as compared with the case where CMC or a salt thereof alone is used as the binder. The cycle characteristics also deteriorate. Further, when the fluororubber alone is used as the binder, the charge / discharge capacity is reduced, the irreversible capacity is increased, and the cycle characteristics are worse than when CMC or its salt carrier is used as the binder.

When fluororubber is used as a binder, it is necessary to use an organic solvent to dissolve the fluororubber. However, many organic solvents are harmful to the human body and the environment. For example, N-methylpyrrolidone, which is often used as a solvent, is harmful to humans and the environment.

The present invention has been made in view of the above problems, and has as its object to provide an electrode for a lithium secondary battery having a small irreversible capacity, a high charge / discharge efficiency, and a long cycle life.

[0008]

According to a first aspect of the present invention, there is provided an electrode for a lithium secondary battery in which a current collector has a surface coated with an active material layer containing a carbon material as a main component. 70-99.5% by weight of raw material and cellulose ether-based substance
5 to 30% by weight of a base material dissolved in water to form a slurry; and applying the slurry to a current collector to form an active material layer. Further, the cellulose ether-based material is an electrode for a lithium secondary battery, characterized in that the material is at least one of cellulose ether and a salt thereof.

In the electrode for a lithium secondary battery of the present invention, a cellulose ether-based material is used as a binder for a carbon material. Since the functional group of the cellulose ether-based material reacts and bonds with the surface of the current collector and the surface of the carbon material, the binding power of the cellulose ether-based material that binds the carbon material is strong. Further, the cellulose ether-based material is insoluble in a non-aqueous electrolyte used in a lithium secondary battery, such as diethyl carbonate. Therefore, even during use of the battery, the binding force of the cellulose ether-based material does not decrease. Further, there is no possibility that the carbon material is separated from the current collector.

[0010] Further, the cellulose ether-based material has low reactivity with lithium. Therefore, the irreversible capacity of the battery can be reduced. Therefore, as in the past,
Unlike the FE and the fluoro rubber, the active material layer is not decomposed and deteriorated by the reaction with lithium, and has excellent cycle characteristics. Further, there is no need to use a harmful organic solvent (N-methylpyrrolidone or the like) which has been used for developing the fluororubber as in the related art. Therefore, according to the electrode of the present invention, a safe lithium secondary battery can be manufactured.

[0011] The above-mentioned cellulose ether-based substance is
Substrate (1) comprising a carbon material and a cellulose ether-based material
00% by weight).
As a result, the cellulose ether-based material sufficiently plays a role as a binder for the carbon material. Therefore, the carbon material is strongly bound to the cellulose ether-based material, and the cycle characteristics of the battery can be improved. Further, the electric resistance of the electrode can be reduced, and the charge / discharge capacity can be increased.

On the other hand, when the content of the cellulose ether-based material is less than 0.5% by weight, the binding force of the cellulose ether-based material is reduced, and the cycle characteristics of the battery are deteriorated.
If the content of the cellulose ether-based material exceeds 30% by weight, the electric resistance of the active material layer increases, and the discharge capacity of the battery decreases.

[0013] The carbon material is 70 to 9 in the base material.
9.5% by weight. If the content is less than 70% by weight, the electric resistance of the active material layer increases, and the discharge capacity of the battery decreases. If it exceeds 99.5% by weight, the binding strength of the carbon material is low, and the cycle characteristics of the battery deteriorate.

The cellulose ether-based material is at least one of cellulose ether and a salt thereof. As the cellulose ether, for example, it is preferable to use one or more selected from the group consisting of methylcellulose, ethylcellulose, benzylcellulose, tritylcellulose, cyanethylcellulose, carboxymethylcellulose, carboxyethylcellulose, aminoethylcellulose, and oxyethylcellulose. Among them, the degree of etherification is 0.5 to 2.5, the average degree of polymerization is 100 to 2000, and the average molecular weight is 25,000 to 4000.
00 is preferred. As a result, the carbon material is firmly bound, and peeling from the current collector can be further suppressed.

As the above-mentioned cellulose ether salt, for example, a sodium salt, a potassium salt, an ammonium salt and a mixture thereof can be used, and it is particularly preferable to use an ammonium salt. As a result, the carbon material is firmly bound, and peeling from the current collector can be further suppressed even during use.

As the carbon material, it is preferable to use, for example, petroleum raw coke, pitch coke, resin fired carbon material, carbon black, mesophase carbon black, vapor grown carbon fiber, and carbon fiber. This gives
The effect of obtaining a large charge / discharge capacity can be expected.
The carbon material is preferably subjected to a heat treatment.
Thereby, it can be used as a negative electrode for a lithium secondary battery. In order to further exert this effect, the temperature of the heat treatment is preferably from 600 to 1500 ° C.

It is preferable to use, for example, a copper foil, an aluminum foil, or a stainless steel foil as the current collector.
As a result, a large current collection effect can be expected.

Next, the active material layer is preferably dried and pressed. Thereby, the active material layer can be easily formed. The drying temperature is 60-20
Preferably it is 0 ° C. If the temperature is lower than 60 ° C., it may take a long time to dry the slurry. Also, 20
If the temperature exceeds 0 ° C., the current collector may be oxidized.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1 Embodiment 1 of the present invention is a carbonaceous substrate electrode for a lithium secondary battery. The electrode of this example is formed by coating the surface of a current collector with an active material layer mainly composed of a carbon material. The active material layer is obtained by dissolving a base material composed of 96% by weight of a carbon material and 4% by weight of a cellulose ether-based material in water to form a slurry, applying the slurry to a current collector, and drying the slurry.

Next, a method for manufacturing the above electrode will be described in detail. First, the carbon material was heat-treated at 900 ° C. for 1 hour under an argon stream (flow rate 200 cc / min.).
As the carbon material, petroleum raw coke having an average particle size of 2.68 μm was used.

Next, a cellulose ether-based substance dissolved in water was added to the carbon material to form a slurry. Sodium carboxymethylcellulose (hereinafter referred to as CMCNa) was used as the cellulose ether-based substance. The slurry was made of a base material composed of 96% by weight of petroleum raw coke and 4% by weight of CMCNa (a total of 100%).
%) Is 100 parts by weight and 100 parts by weight of water.

Next, a slurry was applied on the current collector. A copper foil having a diameter of 15 mm and a thickness of 20 μm was used as a current collector. Next, this slurry was dried at 100 ° C. for 1 hour. The active material layer after drying was 20 mg. Next, at 170 ° C. while the active material layer was pressed against the current collector.
For 3 minutes. Thus, a tablet-shaped electrode of this example was obtained.

Embodiment 2 This embodiment is different from Embodiment 1 in that the heat treatment condition for the raw petroleum coke (carbon material) is 1400 ° C.
Other configurations are the same as those of the first embodiment.

(Comparative Example 1) In this example, fluorocarbon rubber was used instead of the cellulose ether-based substance.
This is different from the first embodiment. That is, the raw petroleum coke (average particle size: 2.68 μm) is subjected to a heat treatment (900 ° C., under an argon gas flow) under the same conditions as in Embodiment 1, and then n-
A slurry was prepared by adding 10% by weight of a fluorine rubber (manufactured by Daikin, trade name: G-751) dissolved in methylpyrrolidone. The slurry was applied to a current collector copper foil (thickness: 20).
μm), and then an electrode was manufactured in the same manner as in Embodiment 1.

(Comparative Example 2) This example is different from Comparative Example 1 in that the heat treatment condition of the petroleum raw coke was set to 1400 ° C. Others are the same as Comparative Example 1.

Comparative Example 3 The present example is different from Example 1 in that a mixture of CMCNa and fluororubber was used instead of the cellulose ether-based material. That is, the raw petroleum coke (average particle size: 2.68 μm) was subjected to a heat treatment (900 ° C., under an argon stream) under the same conditions as in the first embodiment. CMCNa was dissolved in distilled water in advance, and the fluororubber was dispersed in an ester solvent. In this dispersion, the ratio between CMCNa and fluororubber was adjusted to be 1: 2 by weight.

Next, the heat-treated petroleum coke was dispersed in this dispersion to obtain a slurry. The heat-treated petroleum coke and the mixture of CMCNa and fluororubber were adjusted to have a weight ratio of 96: 4. Next, the slurry is applied to a copper foil (thickness: 20 μm) as a current collector.
Then, an electrode was manufactured by the same method as in the first embodiment.

(Comparative Example 4) This example is different from Comparative Example 3 in that the heat treatment condition of the petroleum raw coke was set to 1400 ° C. Others are the same as Comparative Example 3.

(Experimental Example) Next, as shown in FIG. 1, the electrodes of the first and second embodiments and the comparative examples 1 to 4 were used.
A test cell was prepared. The test cell 7 has the separator 3
Have electrodes 1 and 2 arranged with an electrolyte 4 interposed therebetween. The electrode 1 is a test electrode, and includes a current collector 11 and an active material layer 12 obtained by the above-described manufacturing method. On the other hand, the electrode 2 is a counter electrode and includes a current collector 21 and an active material layer 22 described below. The current collectors 11 and 21 are connected to the charging / discharging device 9.
It is connected to the.

As the separator 3, a porous polyethylene having a diameter of 20 mm and a thickness of 75 μm was used. Electrolyte 4
Was prepared by dissolving LiPF ₆ at a rate of 1 mol / L in a solution in which ethylene carbonate and diethyl carbonate were mixed in the same volume. Current collector 21
Is made of copper foil (thickness: 20 μm). The active material layer 22
It is made of a tablet-shaped lithium metal having a diameter of 15 mm and a thickness of 0.8 mm. The current collector 12 is made of SUS304 stainless steel.

Next, a charge / discharge test of the test cell was performed as follows. That is, constant current (CC) measurement was performed for 10 cycles. In addition, constant current constant voltage (CCCV) measurement
0 cycles were performed. In performing the CC measurement, the current density at the time of discharging was set to 1 mA / cm ^2, and the current density at the time of charging was set to 0.5 mA / cm ² . In carrying out the CCCV measurement, the current density was set to 1 mA / cm ^{2 for} both charging and discharging, and charging and discharging were performed between the electrode potentials of 0 to 1.5 V. Charging time is 5
Time.

The amount of electricity discharged in the first charge / discharge cycle was defined as the initial discharge capacity. The amount of electricity that could not be obtained in the next discharge from the initially charged amount of electricity was determined and defined as the irreversible capacity. When charge / discharge was performed for 10 cycles, the discharge capacity at the 10th cycle was shown as the discharge capacity. These initial discharge capacity, irreversible capacity and discharge capacity are indicated by values per gram of active material, that is, carbon material (petroleum coke), and the measurement results are shown in Table 1.

[0033]

[Table 1]

As can be seen from Table 1, Embodiment 1 and Embodiment 1
Sample No. 2 had higher initial discharge capacity and discharge capacity and smaller irreversible capacity as compared with Comparative Examples 1 to 4.

[0035]

According to the present invention, the irreversible capacity is small,
Further, an electrode for a lithium secondary battery having high charge / discharge efficiency and a long cycle life can be provided.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a test cell in an experimental example.

[Description of reference numerals] 1,2. . . Electrodes, 11, 21. . . Active material layer, 12, 22. . . Current collector, 3. . . 3. separator, . . Electrolyte, 7. . . 8. test cell, . . Charging and discharging equipment,

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yoshihiro Shimizu 41-cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside of Toyota Central R & D Laboratories Co., Ltd. 41, Yokomichi, Toyota Central Research Institute, Inc.

Claims (1)

[Claims] 1. An electrode for a lithium secondary battery comprising a current collector and a surface coated with an active material layer mainly composed of a carbon material, wherein the active material layer comprises 70 to 99.5% by weight of a carbon material. Forming a slurry by dissolving a base material composed of water and a cellulose ether-based material in 0.5 to 30% by weight in water; and applying the slurry to a current collector to form an active material layer. An electrode for a lithium secondary battery produced by a method, wherein the cellulose ether-based material is at least one of cellulose ether and a salt thereof.