JPH09180725A - Binder solution for battery and electrode mix - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide binder solution, which can form a stable battery electrode, by dissolving vinylidene fluorite polymer in the organic solvent, and sampling a part thereof, and adding acid so as to adjust the pH of the liquid, which is obtained by diluting the sampled solvent with ion exchanged water. SOLUTION: At the time of obtaining the vinylidene fluorite polymer binder solution, vinylidene fluorite polymer at about 0.1-20 parts by weight in relation to the organic solvent at 100 parts by weight is dissolved in the organic solvent. A part of the solvent is sampled, and pH is adjusted by adding the acid so that the pH of the liquid, which is obtained by diluting the sampled solvent with the ion exchanged water at 10 times, becomes 9 or less. As the acid to be added, organic acid such as acrylic acid, formic acid and citric acid is desirable. Powder electrode material is dispersed for mixing in the vinylidene fluorite polymer binder solution so as to obtain the electrode mix. Single polymer, co-polymer and the denaturated material of vinylidene fluorite is contained in the vinylidene fluorite group polymer. As the organic solvent, the nitrogen contained organic solvent is desirable.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is intended to stably fix a powder electrode material (mainly an electrode active material and a conductive auxiliary agent added as necessary) of a non-aqueous battery, particularly a lithium ion battery, to an electrode. The present invention relates to a solution of a binder made of a vinylidene fluoride-based polymer, that is, a vinylidene fluoride-based polymer binder solution, and an electrode mixture prepared by dispersing a powder electrode material therein.

[0002]

2. Description of the Related Art In recent years, the development of electronic technology has been remarkable, and various devices have been reduced in size and weight. Along with the miniaturization and weight reduction of this electronic device, the demand for the miniaturization and weight reduction of a battery serving as a power source for the electronic device has also become very large. In order to obtain more energy with a small volume and weight, it is necessary to increase the voltage per battery. From this viewpoint, recently, a carbonaceous material capable of storing lithium or lithium ions is used as a negative electrode active material, and a positive electrode active material is used. For example, a battery using a non-aqueous electrolyte using lithium cobalt oxide has attracted attention.

[0003] However, in such a non-aqueous battery, the ionic conductivity of the aqueous electrolyte is usually 10 ^-1 S / S.
cm, whereas the ionic conductivity of the non-aqueous electrolyte is 1
Since it is as low as about 0 ⁻² to 10 ⁻⁴ S / cm, it is essential to use a thin and large-area electrode having a thickness of several μm to several hundred μm as a practical battery. Such a thin film and as a method of industrially inexpensively obtaining a large area electrode, as a binder solution obtained by dissolving an organic polymer serving as a binder of an electrode active material in a solvent, after dispersing the powder electrode material,
A method is known in which an electrode is molded by coating and drying it on a metal electrode foil or a metal net. Further, as such a binder solution for a non-aqueous battery, various vinylidene fluoride-based polymers may be used as N-methyl as described in JP-A-6-93025 and JP-A-6-172452. −
It is known to be dissolved in a polar solvent such as 2-pyrrolidone, dimethylformamide, N, N-dimethylacetamide. This is because the vinylidene fluoride polymer is excellent in chemical resistance, weather resistance, stain resistance and the like and dissolves in the polar solvent as described above, but with respect to the non-aqueous electrolyte solution used in the non-aqueous battery. The reason is that although it swells to some extent, it is stable, and good adhesiveness can be maintained even on a substrate such as a metal by copolymerization or modification treatment.

[0004]

When the above vinylidene fluoride polymer is used as a binder and dissolved in a polar organic solvent, the solution viscosity may increase more than usual depending on the lot of the solvent. Was found. When the viscosity of the solution increases, not only it becomes difficult to obtain a uniform film thickness in the electrode coating step, but also gelation occurs when kneading with the active material, which sometimes makes film formation itself difficult. Further, even if film formation is possible, a phenomenon has been found in which the binder after coating and drying has a large degree of swelling in the non-aqueous electrolyte solution. When the swelling of the binder in the non-aqueous electrolyte becomes severe, the contact resistance between the powder electrode material (especially active material) and the metal electrode foil or metal net or the contact resistance between the active materials increases, increasing the internal resistance of the battery. To do. Further, in the case of a secondary battery that can be repeatedly charged and discharged, an increase in internal resistance leads to deterioration of charge and discharge cycle characteristics, which may eventually shorten the battery life.

Therefore, the main object of the present invention is to enable formation of a battery electrode which is stable without causing an increase in viscosity itself and is stable without causing excessive swelling in a non-aqueous electrolyte solution. It is intended to provide a vinylidene fluoride-based polymer binder solution for forming a non-aqueous battery electrode, and an electrode mixture having stable characteristics in which a powder electrode material is dispersed.

[0006]

The vinylidene fluoride-based polymer binder solution for forming a non-aqueous battery electrode of the present invention comprises:
It was developed in order to achieve the above-mentioned object, and it is a solution of a vinylidene fluoride polymer dissolved in an organic solvent, a part of which is collected and diluted 10 times with ion-exchanged water to obtain a pH of a liquid. And a vinylidene fluoride polymer binder solution for forming a non-aqueous battery electrode, which is prepared by adding an acid so that the ratio becomes 9 or less.

The operation and effect of the present invention will be slightly added. As a result of intensive studies by the present inventors, it was found that the cause of the above-mentioned abnormal increase in viscosity of the vinylidene fluoride-based polymer binder solution is related to the acid-alkalinity of the system. That is, it is difficult to directly determine the acid-alkalinity of the above-mentioned vinylidene fluoride-based polymer solution in an organic solvent, but a part of the acid-alkalinity is sampled and deionized with 10
The pH of the solution obtained by doubling the dilution is one measure. It was found that the pH of the liquid thus measured exceeded 9 in the system in which the viscosity increased. This is related to the dehydrofluoric acid reaction of vinylidene fluoride-based polymer in an alkaline medium, which is a known phenomenon per se, and is also directly related to the binder performance. Most of polar solvents having good solubility in the base polymer are N-methylpyrrolidone,
It is a nitrogen-containing organic solvent such as dimethylformamide, and the presence or absence of the thickening phenomenon depending on the lot of the solvent and the degree of the difference are different from those of alkaline substances such as amines during the production or subsequent storage of the nitrogen-containing organic solvent. It is understood to be associated with an increase in the alkalinity of the solvent due to residual, formation or liberation. Note that the solvent resistance (swelling resistance) of the binder decreases due to dehydrofluoric acid from the vinylidene fluoride-based polymer because oxygen is easily added to the defluorination site, and as a result, the affinity for the non-aqueous electrolyte solution is increased. It is considered that the crystallinity of the polymer is lowered due to an increase in the polymer property or an increase in the number of different kinds of bonds as a powerful factor for providing good solvent resistance. Based on such analysis, the present invention effectively suppresses the above-mentioned inconvenience associated with the thickening phenomenon of the vinylidene fluoride polymer binder solution by adding an acid to lower the pH of the system to 9 or less. Based on the knowledge of things.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The vinylidene fluoride polymer used in the present invention includes vinylidene fluoride homopolymers, copolymers and modified products thereof. A vinylidene fluoride homopolymer is preferable from the viewpoint of durability against a non-aqueous electrolyte, particularly swelling resistance. However, since the adhesion to an electrode substrate such as a metal is slightly insufficient, it is more preferable to use a copolymer with another monomer, particularly a monoester of an unsaturated dibasic acid, vinylene carbonate or an epoxy-containing vinyl monomer. A copolymer in which a polar group such as a carboxyl group, a carbonate group or an epoxy group is introduced by copolymerization with a polymer or the like (JP-A-6-172452, etc.) is preferably used. Also, in a solvent that dissolves or swells a vinylidene fluoride homopolymer or a copolymer, a silane coupling agent having both a vinylidene fluoride polymer such as an amino group or a mercapto group, a reactive group and a hydrolyzable group. Alternatively, a modified vinylidene fluoride-based polymer obtained by treatment in a titanate-based coupling agent (JP-A-6-93).
No. 025) is also preferably used. However, in order to maintain good swelling resistance to the non-aqueous electrolyte as a whole,
It is preferable to maintain the untreated vinylidene fluoride unit in the range of 90 mol% or more, particularly 95 mol% or more.

The vinylidene fluoride polymer has an intrinsic viscosity (logarithmic viscosity at 30 ° C. of a solution of 4 g of resin dissolved in 1 liter of N, N-dimethylformamide)
0.5 or more, more preferably 0.5 to 2.0, especially 0.8 to 1.
It is preferable to have a value within the range of 5.

The organic solvent used to obtain the binder solution of the present invention by dissolving the above vinylidene fluoride polymer is preferably a polar solvent such as N-methyl-2-pyrrolidone or dimethylformamide. , N,
N-dimethylacetamide, N, N-dimethylsulfoxide, hexamethylphosphamide, dioxane,
Examples include tetrahydrofuran, tetramethylurea, triethyl phosphate, trimethyl phosphate, and the like. These organic solvents may be used alone or in admixture of two or more. As described above, most of good solvents for vinylidene fluoride-based polymers are nitrogen-containing solvents, and those that easily generate an alkaline substance that causes an increase in viscosity of the binder solution are also nitrogen-containing organic solvents. Therefore, the organic solvent preferably used in the present invention is a nitrogen-containing organic solvent, but for a non-nitrogen-based organic solvent, it is necessary to add an acid to adjust the pH according to the present invention. This is preferable because it is possible to avoid the inconvenience associated with the increase in pH. Examples of such a non-nitrogen-based good solvent include the above-mentioned dioxane, tetrahydrofuran, trialkyl phosphate and the like, which are preferably used alone or mixed with a nitrogen-containing organic solvent.

To obtain the binder solution of the present invention, 0.1 to 20 parts by weight, especially 1 to 15 parts by weight of the above vinylidene fluoride polymer is added to 100 parts by weight of these organic solvents.
It is preferable to dissolve in the ratio of If it is less than 0.1 part by weight, the proportion of the polymer in the solution is too small, and the effect as a binder cannot be obtained. On the other hand, if it exceeds 20 parts by weight, the viscosity of the solution becomes too high and it becomes difficult to adjust the electrode mixture.

According to the present invention, with respect to the binder solution after addition of the acid, a part of the solution is sampled and adjusted so that the pH of the 10-fold diluted solution with ion-exchanged water is 9 or less.

The acid to be added is not particularly limited, but it is preferable that the acid is scattered as a result of decomposition or evaporation during the step of applying as an electrode mixture and drying, and does not remain in the molded electrode. Those that are difficult to react with the substance are preferable. In this respect, inorganic acids such as hydrochloric acid and sulfuric acid are not always preferable because they easily react with the electrode active material. Particularly when an electrode active material that easily forms an intercalation compound with an inorganic substance such as graphite is used, it is preferable to use an organic acid having a large molecular diameter that makes it difficult to form an intercalation compound. It is preferable that it has a high vapor pressure or decomposes and scatters in the temperature range of the drying step (usually 175 ° C. or lower which is the melting point of PVDF). Examples of organic acids that are preferably used from this viewpoint include acrylic acid, formic acid, citric acid, acetic acid, oxalic acid, lactic acid, pyrupic acid, malonic acid, propionic acid, maleic acid, butyric acid and the like.

As a method of adding an acid to the vinylidene fluoride polymer solution, a method of previously adding an acid to an organic solvent to adjust the pH of the solvent and then dissolving the polymer is preferable. A method of adding an acid at the same time to dissolve the polymer to dissolve it, or a method of adding the acid to the solution after the polymer is completely dissolved may be used.

The amount of acid added is such that the pH of a 10-fold diluted solution of the binder solution after addition with ion-exchanged water (substantially equal to the diluted solution pH when an acid is previously added to an organic solvent according to a preferred embodiment) is 9 or less, It is preferably added in an amount of 7.5 or less. The lower limit is not particularly limited, but is generally up to about 3, and addition of more than this cannot be expected to increase the effect. However, in the case of the organic acid, which is the preferred embodiment described above, since it decomposes and volatilizes in the molding process by coating and drying the electrode, it is not harmful even if added in excess. Even if the pH of the organic solvent or binder solution before adding the acid is 9 or less in advance, expect a buffering effect against the pH increase due to some reason such as the subsequent decomposition of the solvent,
It is preferred to add a fixed amount of acid. In this case, the addition amount of acid is 100 pp in 1 liter of binder solution.
It is preferable to add at least m, especially 300 to 10,000 ppm. In such a mode, it is particularly preferable to add an organic acid which causes little harm even if it remains.

The powdery electrode material (active material and conductive material added as necessary, other auxiliaries) is dispersed and mixed in the vinylidene fluoride polymer binder solution of the present invention obtained as described above. By this, an electrode mixture is obtained.

As the active material, in the case of the positive electrode, the general formula L is used.
A complex metal chalcogen compound represented by iMY ₂ (M is at least one kind of transition metal such as Co, Ni, Fe, Mn, Cr and V: Y is a chalcogen element such as O and S), particularly LiCoO _2. Complex metal oxides and LiMn
_A composite metal oxide having a spinel structure such as ₂ O ₄ is preferable. In the case of the negative electrode, graphite, activated carbon, or a carbonaceous material such as one obtained by firing and carbonizing phenol resin or pitch is preferable as the active material.

The conductive material is added for the purpose of improving the conductivity of the electrode mixture layer when an active material having a low electron conductivity such as LiCoO ₂ is used, and carbon such as carbon black, graphite fine powder or fibers is used. Fine materials, fine metal powders such as nickel and aluminum, or fibers are used. When a carbonaceous material having high conductivity is used as the active material, it is not necessary to use these conductive materials.

In the electrode mixture of the present invention, it is preferable to mix 0.1 to 50 parts by weight, especially 1 to 20 parts by weight of the powder electrode material with 100 parts by weight of the vinylidene fluoride polymer.

The formed electrode mixture is made of a metal foil such as iron, stainless steel, steel, copper, aluminum, nickel, titanium, or a metal net, and has a thickness of 5 to 100 μm.
In the case of a small scale, it is applied on at least one side, preferably both sides, of a current collector having a thickness of, for example, 5 to 20 μm, and dried, for example, at 50 to 170 ° C. The electrode is formed by forming the electrode mixture layer.

The electrode formed by using the binder solution and the electrode mixture of the present invention is preferably used as an electrode of a non-aqueous battery, especially a lithium ion battery, and an electrolyte used in this case is, for example, a lithium salt. It is possible to use a solution obtained by dissolving the above electrolyte in a non-aqueous solvent (organic solvent).

Here, as the electrolyte, LiPF ₆ , Li
AsF ₆ , LiClO ₃ , LiBF ₄ , CH ₃ SO ₃ L
i, CF ₃ SO ₃ Li, LiCl, LiBr and the like.
Further, as the organic solvent of the electrolyte, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-putyl lactone, methyl propionate, propionic acid. Ethyl, a mixed solvent thereof, and the like are used, but not limited thereto.

[0023]

The present invention will be described more specifically with reference to examples and comparative examples.

(Preparation of Vinylidene Fluoride Polymer) According to Example 1 described in JP-A-6-172452, a vinylidene fluoride polymer containing a carboxyl group was obtained as follows.

In an autoclave having a content of 2 liters, 1040 g of ion-exchanged water, 0.8 g of methyl cellulose, 2.5 g of ethyl acetate, 4 g of diisopropyl peroxydicarbonate, 396 g of vinylidene fluoride and 4 g of maleic acid monomethyl ester were charged (vinylidene fluoride). : Maleic acid monomethyl ester = 100: 1.01), 2
Suspension polymerization was carried out at 8 ° C. for 47 hours. After completion of the polymerization, the polymer slurry was dehydrated, washed with water and dried at 80 ° C. for 20 hours to obtain a polymer powder.

The polymerization rate was 90% by weight, and the obtained polymer had an intrinsic viscosity of 1.1 and a carbonyl group content of 1.2 × 10.
^-4 mol / g.

(Organic solvent) A commercially available product of 20 lots of N-methyl-2-pyrrolidone stored for about 3 months at room temperature was used. Hereinafter, it is abbreviated as “NMP”.
The pH of the 10-fold diluted solution with ion-exchanged water is 6.5 to 1
It was in the range of 1.0.

Comparative Example 1 The vinylidene fluoride polymer was dissolved in NMP of a lot in which the pH of a 10-fold diluted solution with ion-exchanged water showed 9.5 at a temperature of 50 ° C. so that the concentration of the vinylidene fluoride polymer was 13% by weight. do it,
A binder solution was obtained.

Example 1 Maleic acid was added to the lot of NMP used in Comparative Example 1 above.
000 ppm was added, and the pH of the 10-fold diluted solution with ion-exchanged water was measured again, and it was 3.2. Next, the vinylidene fluoride polymer was dissolved in NMP containing maleic acid so as to have a concentration of 13% by weight as in Comparative Example 1 to obtain a binder solution.

(Measurement of Viscosity and Fluorine Ion Concentration) The viscosities of the binder solutions obtained in Comparative Example 1 and Example 1 were measured using a rotary viscometer in accordance with JIS K7117. The concentration of fluorine ion (F ⁻ ) was also measured.

(Film Formation and Swelling Test) The binder solutions of Comparative Example 1 and Example 1 were cast on glass plates and dried at 130 ° C. for 2 hours to obtain a film having a thickness of about 200 μm.

Next, these films were treated with LiClO.
₄ 8.8 parts by weight was immersed in an electrolytic solution obtained by dissolving 53.6 parts by weight of propylene carbonate and 37.6 parts by weight of dimethoxyethane, respectively, and immersed at 70 ° C. for 72 hours, The swelling degree was calculated every 24 hours on the way by the weight increase rate of the cast film (increase weight / raw film weight × 100).

The above measurement results are summarized in Table 1 below.

[0034]

[Table 1]

The results in Table 1 above show that the vinylidene fluoride-based polymer binder solution obtained by adding 1000 ppm of maleic acid was compared to the vinylidene fluoride-based polymer binder solution of the comparative example in which NMP was used as it was. , Viscosity increase, low fluoride ion concentration, and stable,
It is also found that the swelling resistance to the electrolyte for non-aqueous batteries is remarkably improved.

Comparative Example 2 The above vinylidene fluoride polymer containing a carboxyl group was dissolved in NMP of a lot in which the pH of a 10-fold diluted solution with ion-exchanged water showed 10.9 to a concentration of 13% by weight. A binder solution was formed.

Example 2 NMP of the same lot as in Comparative Example 2 was added with 1000 ppm.
Malonic acid was added, and then the above vinylidene fluoride polymer containing a carboxyl group was dissolved to a concentration of 13% by weight to form a binder solution.

A film was formed from each of the binder solutions of Comparative Example 2 and Example 2 in the same manner as in Example 1 above, and the immersion swelling test in the electrolytic solution was conducted at 70 ° C. for 72 hours. The degree of swelling after 72 hours was as follows.

[0039]

[Table 2]

Comparative Example 3 13% by weight of vinylidene fluoride homopolymer (“KF1100”, manufactured by Kureha Chemical Industry Co., Ltd.) was added to NMP in a lot in which the pH of a 10-fold diluted solution with ion-exchanged water was 9.5. Was dissolved at a temperature of 50 ° C. to obtain a binder solution.

Example 3 Maleic acid was added to 1 lot of NMP used in Comparative Example 3 above.
000 ppm was added, and the pH of the 10-fold diluted solution with ion-exchanged water was measured again, and it was 3.2. Next, the vinylidene fluoride homopolymer used in Comparative Example 1 was dissolved in the NMP containing maleic acid so as to have a concentration of 13% by weight to obtain a binder solution.

The results of measuring the viscosity and the film swelling degree of the binder solutions obtained in Comparative Example 3 and Example 3 as in Example 1 are shown in Table 3 below.

[0043]

[Table 3]

Also in this case, the effect of preventing the viscosity increase of the binder solution and the effect of improving the swelling resistance of the film are confirmed by the addition of the acid.

Example 4, Comparative Example 4 The films obtained in Example 1 and Comparative Example 1 were respectively coated with 11.6% by weight of LiPF ₆ , 51.0% by weight of ethylene carbonate and 37.4% by weight of diethyl carbonate. It was dipped in an electrolytic solution consisting of at 70 ° C. for 72 hours and left to stand. The degree of swelling of the film after immersion (the rate of increase in weight due to swelling) was 18% by weight and 24% by weight, respectively.

[0046]

As described above, according to the present invention, when a vinylidene fluoride polymer is dissolved in an organic solvent to form a binder solution for forming a non-aqueous battery electrode, an acid, preferably an organic acid, is added. By adding it, it becomes possible to effectively prevent the increase in the viscosity of the binder solution, which has often been a problem in the past, and the reduction in the binder effect due to the swelling of the binder in the formed electrode by the electrolytic solution. .

Claims (6)

[Claims] 1. A solution of a vinylidene fluoride polymer dissolved in an organic solvent, a portion of which is sampled and diluted 10 times with ion-exchanged water to obtain an acid so that the pH of the resulting solution is 9 or less. A vinylidene fluoride polymer binder solution for forming a non-aqueous battery electrode, which is prepared by adding it. 2. The vinylidene fluoride-based polymer is a copolymer of 95 mol% or more of vinylidene fluoride and another monomer, or a vinylidene fluoride homopolymer or a modified product of the copolymer. Binder solution. 3. The binder solution according to claim 1, wherein the vinylidene fluoride-based polymer is a carboxyl group-containing vinylidene fluoride-based polymer. 4. The binder solution according to claim 1, wherein the acid is an organic acid. 5. The binder solution according to claim 1, wherein the organic solvent is a nitrogen-containing organic solvent. 6. An electrode mixture obtained by dispersing a powder electrode material in the binder solution according to claim 1.