JP2003142082A - Manufacturing method of electrode for battery and lithium battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance electrode strength by increasing dispersibility without using a viscosity improver such as conventionally used carboxymethyl cellulose when paste using a polar solvent is manufactured from an active material having a non-polar surface in manufacturing a negative electrode for a secondary battery. SOLUTION: The paste is manufactured by this manufacturing method having a first process for dispersing an active material capable of inserting and desorbing lithium into a water-soluble organic solvent, a second process for adding and dispersing water in the dispersed solution, a third process for adding a binder to the dispersed solution obtained from the second process, and a fourth process for applying the dispersed solution obtained from the third process to metal foil and thereafter drying it with hot air.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for manufacturing a battery electrode and a lithium battery using the same.

[0002]

2. Description of the Related Art In recent years, portable electronic devices for consumer use,
Cordless is advancing rapidly. Along with this, there is an increasing demand for a small-sized, lightweight secondary battery having a high energy density, which serves as a driving power source. From this point of view, non-aqueous secondary batteries, especially lithium secondary batteries, have great expectations as batteries having high voltage and high energy density, and their development is urgently needed.

Heretofore, manganese dioxide, vanadium pentoxide, titanium disulfide and the like have been used as positive electrode active materials for lithium secondary batteries. A battery was constituted by these positive electrodes, a metallic lithium negative electrode and an organic electrolytic solution, and charging and discharging were repeated. However, generally, in a secondary battery using lithium metal for the negative electrode, problems such as an internal short circuit due to dendrite-like lithium generated during charging and a side reaction between the active material and the electrolytic solution are major obstacles. Further, no one has been found to be satisfactory in high-rate charge / discharge characteristics and over-discharge characteristics. Furthermore, recently, the safety of lithium batteries has been strictly required, and it is important to ensure the safety in battery systems using lithium metal or lithium alloy for the negative electrode.

Therefore, an electrode active material utilizing the intercalation reaction of a layered compound has been attracting attention, and an intercalation compound is used as an electrode material for secondary batteries. In particular, a carbon material capable of intercalating / deintercalating Li ions is promising as a negative electrode material for a lithium secondary battery, and its development is being actively conducted. On the other hand, with the use of a carbon material for the negative electrode, LiCoO ₂ which has a higher voltage and is a compound containing Li as a positive electrode active material.
It has been proposed to use LiNiO ₂ or LiNiO ₂ , or a composite oxide in which some of Co and Ni are replaced with other elements.

However, although such a non-aqueous secondary battery can obtain a high energy density, it is difficult to obtain a high output density as compared with an aqueous battery. This is largely due to the ionic conductivity of the electrolytic solution, and the ionic conductivity of the non-aqueous electrolytic solution is 100 times lower than that of the aqueous solution. As a method for solving these problems, it is conceivable to increase the electrode area, that is, to use a thin, large-area negative electrode. In particular, a manufacturing method in which a metal foil that is a current collector is coated with a paste in which an electrode active material is dispersed in a solution and dried is known, and a thin electrode and a large-area negative electrode can be relatively easily obtained. It is possible.

In such an electrode manufacturing method, when a negative electrode is manufactured by a method in which a paste in which a powder of graphite or the like is dispersed in a solution is applied on both surfaces of a copper foil as a current collector and dried, a thin type, large size It is easy to obtain a negative electrode having an area, but the dispersibility of the paste and the binding force of the binder have a great influence on the negative electrode performance. The binder is classified into the case of using non-aqueous polyvinylidene fluoride (PVDF) and the case of using a water-based styrene-butadiene copolymer. The former has a good binding force between the active materials, but has a weak binding force with a copper foil generally used as a negative electrode current collector. Therefore, it is necessary to add about 5% to 10% by weight to the active material, which is disadvantageous for increasing the capacity. On the other hand, the styrene-butadiene copolymer has a stronger binding force to the active material and the copper foil than PVDF. Therefore, a weight ratio of 0.5 to 3% by weight to the active material is sufficient, which is advantageous for increasing the capacity.

[0007]

Styrene-butadiene copolymers are generally used in the form of being dispersed in water (emulsion). However, it is very difficult to disperse an active material having a non-polar surface represented by graphite in water which is a polar solvent. Therefore, in the past, a paste was used in which graphite was dispersed using an aqueous solution of carboxymethyl cellulose (CMC), and then a water emulsion such as a styrene / butadiene copolymer was mixed as a binder. However, even if such a method is used, it is difficult to disperse graphite or the like in water, and there are many graphite particles that have undergone secondary agglomeration in the paste, resulting in deterioration of battery characteristics.

The present invention pays attention to this point and provides means for dispersing an active material having a non-polar surface in a polar solvent in a short time without causing secondary aggregation.

[0009]

In order to achieve the above object, the method for producing a battery electrode according to the present invention comprises a first method in which an active material for inserting and releasing lithium ions is dispersed in a water-soluble organic solvent. A first step of creating a dispersion, a second step of creating a second dispersion by adding water to the first dispersion to disperse it, and a binder is added to the second dispersion. The method is characterized by including a third step of forming the third dispersion liquid thus prepared, and a fourth step of applying the third dispersion liquid to a metal foil and drying it.

At this time, the water-soluble organic solvent is alcohol,
It is advantageous to contain at least one of N-methylpyrrolidone or acetone. Further, it is effective that the binder contains at least one of a styrene / butadiene copolymer and polyvinyl alcohol. Further, it is effective that the active material contains at least one of graphite, low crystalline carbon and alloy.

The amount of the water-soluble organic solvent mixed is preferably 1% by weight or more and 40% by weight or less with respect to water.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION A non-aqueous electrolyte secondary battery comprising a chargeable / dischargeable positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the surface of an active material capable of inserting and releasing lithium ions is generally It is non-polar. Therefore, dispersion in an organic solvent having a non-polar portion is easy. On the other hand, the styrene-butadiene copolymer used as the binder is generally provided in the form of an emulsion using water. It is possible to add this binder dispersion to an organic solvent in which the active material is dispersed, but in general, the binder dispersion causes agglomeration, so that it is possible to disperse between the active material particles. The binding force between the active materials is reduced.

Therefore, in the present invention, since the styrene / butadiene copolymer dispersion is used for the active material having a non-polar surface, the active material is dispersed in a water-soluble organic solvent in the first step. Since the water-soluble organic solvent has a polar group and a nonpolar group at the same time, an active material having a nonpolar surface such as graphite can be easily dispersed. Next, in the second step, water is added to the dispersion liquid in which the active material is dispersed in the water-soluble organic solvent. Further, in the third step, a binder is added to the dispersion liquid obtained in the second step. The amount of addition depends on the active material, but is about 0.5 to 3% by weight of the binder solid component relative to the active material. In the fourth step, the dispersion obtained in the third step is applied to a metal foil by a method such as a doctor blade and then dried with hot air to obtain a negative electrode.

Although the respective steps have been described separately in the above description, the first step and the second step are continuously performed such that the water-soluble organic solvent and water are mixed in advance and then graphite is added. You can do it in. Further, the first step to the third step may be continuously performed.

As a substance having both a non-polar portion and a polar portion, there are surface active agents such as fatty acid soap and alkylbenzene sulfonate. However, many of them remain in the negative electrode even after the fourth step. In the non-aqueous electrolyte secondary battery, the above-mentioned residual surfactant is not desirable because it is decomposed by an electrochemical reaction and causes irreversible capacity and gas generation. For this purpose, alcohol, N-methylpyrrolidone, and acetone are used as the water-soluble organic solvent in which the active material into which lithium can be inserted is dispersed.
These materials are desirable because they do not remain in the negative electrode after the fourth step.

Examples of the binder usable in the present invention include diacetyl cellulose, hydroxypropyl cellulose, ethylene glycol, ethylene / propylene / diene copolymers and acrylonitrile / butadiene copolymers. However, in the present invention, a styrene-butadiene copolymer and polyvinyl alcohol are used because they have a strong binding force with the current collector and can be easily dispersed in water. The styrene / butadiene copolymer is a synthetic rubber obtained by radical copolymerization of butadiene and styrene, and has a structural unit as shown in Chemical formula 1. Generally, m = 75 and n = 25, which were also used in the present application. Further, polyvinyl alcohol is referred to as Poval or PVA, and is generally obtained by transesterification of polyvinyl acetate and has a structure as shown in Chemical formula 2.

[0017]

[Chemical 1]

[0018]

[Chemical 2]

Further, it is necessary that the active material capable of inserting and releasing lithium ions is at least one of graphite, low crystalline carbon and alloy. Other substances are not desirable because the production method according to the present invention does not disperse well or water causes deterioration of the active material.

The ratio of water-soluble organic solvent to water is 1% by weight.
% Or more and 40% by weight or less is required. When the content of the water-soluble organic solvent is more than 40% by weight, the binder dispersion liquid causes agglomeration to reduce the binding force, which is not desirable.
Further, if the water-soluble organic solvent is less than 1% by weight, the active material cannot be dispersed and the powder causes secondary aggregation. For this reason, even if the binder is added after the addition of water, the secondary agglomerated active material particle binder is concentrated and the binding force between the active materials is reduced, which is not desirable.

[0021]

The present invention will be described below with reference to specific examples.
The dispersibility was evaluated by the average roughness of the negative electrode surface and the peel strength of the active material. The average roughness of the negative electrode surface was measured by tracing the negative electrode surface using a Taly step. As for the peel strength of the active material, a stainless steel scratch bar having a width of 4 mm was perpendicular to the negative electrode, and the vertical load applied to the scratch bar was scanned while scratching in the horizontal direction with respect to the negative electrode. It was determined by measuring the vertical load when peeling from the.

Example 1 As a first step, 4 g of NMP was added to 10 g of natural graphite powder having an average particle size of 10 μm to disperse graphite. Next, in the second step, 6 g of water was added to the above dispersion liquid so that the entire graphite was immersed in the liquid, and then stirred for 10 minutes using a stirrer. As a third step, 0.62 g of an aqueous dispersion of styrene / butadiene copolymer (solid content: 50% by weight) was added to the dispersion thus obtained, and the mixture was further stirred for 5 minutes. As a fourth step for the dispersion obtained in this way, a thickness of 200 μm was applied on a copper foil having a thickness of 20 μm using a doctor blade method, and 60 ° C., 10
A negative electrode was obtained by drying in minutes. The average surface roughness of the obtained negative electrode was 10 μm, and the peel strength was 540 g. Even when alcohol and acetone were used instead of MNP, the measurement results of surface roughness and peel strength were almost the same as above.

(Comparative Example 1) An aqueous solution containing 1% by weight of CMC dissolved therein.
To 10 g, 10 g of the graphite powder used in Example 1 was added and dispersed by stirring with a stirrer for 10 minutes. After that, a negative electrode was obtained by the same method as in Example 1. When the surface roughness and the peel strength were measured by the same method as in Example 1, the average roughness was 30 μm and the peel strength was 380 g.

Thus, comparing Example 1 with Comparative Example 1, the dispersibility of the active material was improved by the method of the present invention even with the same stirring time, and as a result, the average surface roughness and the peel strength were improved. .

Example 2 A low crystalline carbon material having an average particle size of 15 μm was obtained by using the method disclosed in JP-A-07-069611.
An electrode was prepared using this carbonaceous material by the same method as in Example 1, and the surface roughness and peel strength were measured. As a result, the average roughness was 15 μm and the peel strength was 530 g.

Comparative Example 2 A negative electrode was prepared using the above carbonaceous material in the same manner as in Comparative Example 1 and the surface roughness and peel strength were measured. The average roughness was 35 μm and the peel strength was 420 g. Met.

Example 3 An alloy material having an average particle size of 20 μm was obtained by using the method disclosed in Japanese Patent Laid-Open No. 13-00666. Using this alloy material, an electrode was produced in the same manner as in Example 1,
When the surface roughness and peel strength were measured, the average roughness was 20μ.
m, and the peel strength was 510 g.

Comparative Example 3 A negative electrode was prepared using the above carbonaceous material in the same manner as in Comparative Example 1, and the surface roughness and peel strength were measured. The average roughness was 45 μm and the peel strength was 300 g. Met.

As described above, from Examples 2 and 3 and Comparative Examples 2 and 3, the effects of the present invention were shown also in the low crystalline carbon materials and alloy materials.

Example 4 A negative electrode was prepared in the same manner as in Example 1 except that polyvinyl alcohol was used as the binder, and the surface roughness and peel strength were measured.
The average roughness was 10 μm and the peel strength was 500 g.

Comparative Example 4 A negative electrode was prepared in the same manner as in Example 1 except that an acrylonitrile-butadiene copolymer was used as the binder, and the surface roughness and peel strength were measured. The average roughness was 10 μm. Yes, peel strength is 4
It was 40 g.

(Example 5) Using the method of Example 1, the proportion of NMP used is 0.5% by weight, 1.0% by weight, and
Negative electrodes were produced by changing the amounts to 5.0% by weight, 10.0% by weight, 20.0% by weight, 40.0% by weight and 50.0% by weight. The peel strength of each negative electrode was measured by the same method as in Example 1. The results are shown in Fig. 1. As can be seen from this Figure 1, 1% by weight to 4
When it deviated from 0% by weight, the peel strength was extremely lowered, showing the effect of the present invention.

(Embodiment 6) When an active material capable of inserting and removing lithium is dispersed using the manufacturing method of the present invention,
The manufacturing time was compared by the manufacturing time flow and the conventional manufacturing method using carboxymethyl cellulose (CMC). The outline of the process is as follows. → represents the change over time.

Present Invention: Cathode Active Material (Graphite) → NMP
→→ Paste conventional method: Cathode active material (graphite) → CMC →→→→→→
The paste NMP has a function of interfacial activity for the negative electrode active material,
As a result, the wettability was improved, and the time required to obtain the paste of the target was greatly shortened.

Example 7 Using the electrode plate obtained in Example 1 and the electrode plate obtained in Comparative Example 1, batteries shown in FIG. 2 were assembled. 1 is a positive electrode case made of stainless steel, 2 is a negative electrode case made of stainless steel, 3 is an insulating packing made of polypropylene resin, 4 is a positive electrode using LiCoO ₂ as an active material, 5 is a negative electrode obtained in Example 1 or Comparative Example 1. , 6 are separators made of a polypropylene resin porous film. As the electrolyte, ethylene carbonate (EC) and dimethyl carbonate (DMC) are used.
LiPF ₆ dissolved in a solvent mixed at a volume ratio of C: DMC = 1: 1 at a concentration of 1 mol / liter was used.

FIG. 3 shows a constant current of 0.2 mA from 4.2V to 3.V.
The electric capacity of the discharge of each cycle obtained by charging and discharging between 6 V is shown. Battery A is an example
The battery using the electrode plate obtained in 1 and Battery B are the batteries using the electrode plate obtained in Comparative Example 1. Battery A is battery B
The cycle characteristics are superior to the above, and the battery characteristics are improved by the method of the present invention.

[0037]

As described above, according to the present invention, the first step of dispersing an active material capable of inserting and desorbing lithium in a water-soluble organic solvent, the first step of dispersing water by adding water to the dispersion liquid A second step, a third step of adding a binder to the dispersion obtained in the second step, and a fourth step of applying the binder-added dispersion to a metal foil and then drying with hot air. A method for producing a negative electrode for a secondary battery having the above. By the method of the present invention, it becomes possible to manufacture a negative electrode for a secondary battery in which the negative electrode active material is at least one of graphite, low crystalline carbon and alloy in a shorter time than ever.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the weight ratio of MNP and the peel strength of electrodes in Example 5.

FIG. 2 is a cross-sectional view of the battery manufactured in Example 7.

FIG. 3 is a diagram comparing the cycle characteristics of the battery using the electrode plate according to the present invention and the battery using the electrode plate according to the comparative example in Example 7.

[Explanation of symbols]

1 Positive case 2 Negative electrode case 3 Insulating packing made of polypropylene resin 4 positive electrode 5 Negative electrode 6 separator

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 5H029 AJ14 AK03 AL06 AL12 AM03                       AM07 BJ03 BJ12 CJ02 CJ08                       CJ28 DJ08 DJ16 EJ12 HJ10                 5H050 AA19 BA17 CA08 CB07 CB12                       DA03 DA11 EA00 EA22 EA23                       FA02 FA17 GA00 GA02 GA10                       GA27 HA10

Claims (6)

[Claims] 1. A first step of preparing a first dispersion liquid in which an active material for inserting and releasing lithium ions is dispersed in a water-soluble organic solvent, and water is added to the first dispersion liquid. Second step of creating a second dispersion liquid dispersed by, a third step of creating a third dispersion liquid by adding a binder to the second dispersion liquid, the third dispersion liquid Is applied to a metal foil and dried, and a fourth step of producing a battery electrode. 2. The method for producing a battery electrode according to claim 1, wherein the water-soluble organic solvent contains at least one of alcohol, N-methylpyrrolidone and acetone. 3. The method for producing a battery electrode according to claim 1, wherein the binder contains at least one of a styrene / butadiene copolymer and polyvinyl alcohol. 4. The method for producing a battery electrode according to claim 1, wherein the active material contains at least one of graphite, low crystalline carbon and an alloy. 5. The mixing amount of the water-soluble organic solvent is 1 with respect to water.
The method for producing a battery electrode according to claim 1, 2, 3 or 4, wherein the content is not less than 40% by weight. 6. A lithium battery comprising an electrode prepared by the manufacturing method according to claim 1 and a non-aqueous electrolyte.