COMPOSITION FOR SECONDARY BATTERY ELECTRODE

Abstract

The present invention aims to provide a composition for a secondary battery electrode which is excellent in dispersibility of an active material and adhesiveness, capable of preventing battery deterioration and failure due to moisture absorption, and capable of providing a high-capacity secondary battery. Provided is a composition for a secondary battery electrode, containing: an active material; a binder; and an organic solvent, the binder containing a polyvinyl acetal resin, the polyvinyl acetal resin having an electron-donating group that has an acid dissociation constant in water of less than 16 and an electron-withdrawing group, the polyvinyl acetal resin having a degree of polymerization of 250 to 800 and a hydroxy group content of 35 to 70 mol %.

Claims

1. A composition for a secondary battery electrode, comprising: an active material; a binder; and an organic solvent, the binder containing a polyvinyl acetal resin, the polyvinyl acetal resin having an electron-donating group that has an acid dissociation constant in water of less than 16 and an electron-withdrawing group, the polyvinyl acetal resin having a degree of polymerization of 250 to 800 and a hydroxy group content of 35 to 70 mol %.

2. The composition for a secondary battery electrode according to claim 1, wherein the electron-donating group is a functional group having a lactam structure.

3. The composition for a secondary battery electrode according to claim 2, wherein the lactam structure is at least one cyclic structure selected from the group consisting of a 3-membered ring, a 4-membered ring, a 5-membered ring, and a 6-membered ring.

4. The composition for a secondary battery electrode according to claim 2, wherein the lactam structure is a structure represented by the following formula (1): ##STR00003## wherein n is an integer of 1 to 5 and R.sup.1 is a single bond or a C1-C10 saturated or unsaturated hydrocarbon.

5. The composition for a secondary battery electrode according to claim 3, wherein a hydrocarbon constituting the cyclic structure of the lactam structure is unsubstituted, or a hydrogen atom of the hydrocarbon is substituted with a C1-C10 saturated or unsaturated hydrocarbon.

6. The composition for a secondary battery electrode according to claim 1, wherein in the polyvinyl acetal resin, the amount of a structural unit having the electron-donating group is 0.01 to 10 mol %.

7. The composition for a secondary battery electrode according to claim 1, wherein the electron-withdrawing group is a carboxy group.

8. The composition for a secondary battery electrode according to claim 1, wherein in the polyvinyl acetal resin, the amount of a structural unit having the electron-withdrawing group is 0.01 to 10 mol %.

9. The composition for a secondary battery electrode according to claim 1, wherein the polyvinyl acetal resin has a degree of acetalization of 20 to 70 mol %.

10. The composition for a secondary battery electrode according to claim 1, wherein the polyvinyl acetal resin has an acetyl group content of 15 mol % or less.

11. The composition for a secondary battery electrode according to claim 1, containing 0.01 to 20 parts by weight of the polyvinyl acetal resin relative to 100 parts by weight of the active material.

12. The composition for a secondary battery electrode according to claim 1, further comprising a polyvinylidene fluoride resin.

13. A secondary battery comprising the composition for a secondary battery electrode according to claim 1.

Description

EXAMPLE 1

(Preparation of Composition for Secondary Battery Electrode)

[0135] To 20 parts by weight of a resin solution containing the obtained polyvinyl acetal resin A (polyvinyl acetal resin: 2.5 parts by weight) were added 50 parts by weight of lithium cobaltate (CELLSEED C-5H available from Nippon Chemical Industrial Co., Ltd.) as an active material, 5 parts by weight of acetylene black (DENKA BLACK available from Denki Kagaku Kogyo Kabushiki Kaisha) as a conductive aid, and 26 parts by weight of N-methylpyrrolidone, and they were mixed using a THINKY MIXER available from Thinky Corporation to prepare a composition for a secondary battery electrode.

EXAMPLES 2 to 18 AND COMPARATIVE EXAMPLES 1 TO 3 AND 5 TO 12

[0136] Compositions for a secondary battery electrode were obtained in the same manner as in Example 1 except that the polyvinyl acetal resins (resin types, amounts) as shown in Table 2 were used.

COMPARATIVE EXAMPLE 4

[0137] A composition for a secondary battery electrode was obtained as in Example 1 except that a mixture of the polyvinyl acetal resins T1 and T2 shown in Table 1 was used.

TABLE-US-00001 TABLE 1 Polyvinyl alcohol Electron-donating Electron-withdrawing Aldehyde Degree of group (A) group (B) Amount Resin Degree of Mixing saponification Functional Amount Functional Amount (parts by type polymerization ratio (mol %) group type pKa (mol %) group type pKa (mol %) weight) A 250 98.3 a 0.3 0.01 p 4.8 10 53.1 B 250 98.3 a 0.3 0.1 p 4.8 0.1 62.7 C 250 98.3 a 0.3 10 p 4.8 0.01 53.5 D 250 98.3 a 0.3 0.1 p 4.8 0.1 48.3 E 250 98.4 a 0.3 0.1 p 4.8 0.1 27.1 F 400 98.5 a 0.3 0.1 p 4.8 0.1 62.4 G 400 98.4 a 0.3 0.01 p 4.8 10 37.2 H 400 98.3 a 0.3 0.1 p 4.8 0.1 49.2 I 400 98.3 a 0.3 10 p 4.8 0.01 38.1 J1 400 98.3 b 9.2 0.1 q 1.9 0.1 47.5 J2 400 98.1 c 0.4 0.1 p 4.8 0.1 63.1 J3 800 98.4 a 0.3 0.1 d 4.8 0.1 49.1 K 600 1:1 97.8 a 0.3 0.2 27.4 200 97.3 p 4.8 0.2 L 800 98.4 a 0.3 0.1 p 4.8 0.1 63.5 M 800 98.2 a 0.3 0.1 p 4.8 0.1 48.8 N 800 98.5 a 0.3 0.01 p 4.8 10 18.0 O 800 98.3 a 0.3 0.1 p 4.8 0.1 28.2 P 400 1:1 98.3 a 0.3 0.2 48.4 400 98.2 p 4.8 0.2 Q 200 97.3 a 0.3 0.1 p 4.8 0.1 47.0 R 250 98.2 a 0.3 0.1 47.5 S 250 98.3 p 4.8 0.1 48.0 T1 250 98.5 a 0.3 0.2 47.9 T2 250 98.4 p 4.8 0.2 47.7 U 250 98.1 48.2 V 400 98.2 a 0.3 0.1 p 4.8 0.1 68.1 W 400 98.4 e 49 0.1 48.7 X 600 1:1 98.3 e 49 0.2 47.2 200 98.2 p 4.8 0.2 Y1 900 98.1 a 0.3 0.1 p 4.8 0.1 48.0 Y2 400 98.1 a 0.3 0.1 p 4.8 0.1 22.6 Y3 400 97.6 a 0.3 0.1 p 4.8 0.1 13.5 Y4 1500 97.3 a 0.3 0.1 p 4.8 0.1 51.2 Polyvinyl acetal Hydroxy Degree Acetyl Electron-donating Electron-withdrawing group of group group (A) group (B) A/B Resin Degree of content acetalization content Amount Amount content type polymerization (mol %) (mol %) (mol %) pKa (mol %) pKa (mol %) ratio A 250 35.2 53.1 1.7 0.3 0.01 4.8 10 0.001 B 250 35.4 62.7 1.7 0.3 0.1 4.8 0.1 1 C 250 34.8 53.5 1.7 0.3 10 4.8 0.01 1000 D 250 49.8 48.3 1.7 0.3 0.1 4.8 0.1 1 E 250 71.1 27.1 1.6 0.3 0.1 4.8 0.1 1 F 400 35.9 62.4 1.5 0.3 0.1 4.8 0.1 1 G 400 51.2 37.2 1.6 0.3 0.01 4.8 10 0.001 H 400 48.9 49.2 1.7 0.3 0.1 4.8 0.1 1 I 400 50.20 38.1 1.7 0.3 10.00 4.8 0.01 1000 J1 400 50.6 47.5 1.7 9.2 0.1 1.9 0.1 1 J2 400 34.8 63.1 1.9 0.3 0.1 4.8 0.1 1 J3 800 49.1 49.1 1.6 0.3 0.1 4.8 0.1 1 K 400 69.8 27.4 2.5 0.3 0.1 4.8 0.1 1 L 800 34.7 63.5 1.6 0.3 0.1 4.8 0.1 1 M 800 49.2 48.8 1.8 0.3 0.1 4.8 0.1 1 N 800 70.5 18.0 1.5 0.3 0.01 4.8 10 0.001 O 800 69.9 28.2 1.7 0.3 0.1 4.8 0.1 1 P 800 49.5 48.4 1.8 0.3 0.1 4.8 0.1 1 Q 200 50.1 47.0 2.7 0.3 0.1 4.8 0.1 1 R 250 50.6 47.5 1.8 0.3 0.1 S 250 50.2 48.0 1.7 4.8 0.1 T1 250 50.4 47.9 1.5 0.3 0.2 T2 250 50.5 47.7 1.6 4.8 0.2 U 250 49.9 48.2 1.9 V 400 29.9 68.1 1.8 0.3 0.1 4.8 0.1 1 W 400 49.6 48.7 1.6 49.0 0.1 X 400 50.7 47.2 1.8 49.0 0.1 4.8 0.1 1 Y1 900 49.9 48.0 1.9 0.3 0.1 4.8 0.1 1 Y2 400 75.3 22.6 1.9 0.3 0.1 4.8 0.1 1 Y3 400 83.9 13.5 2.4 0.3 0.1 4.8 0.1 1 Y4 1500 45.9 51.2 2.7 0.3 0.1 4.8 0.1 1

[0138] In the Functional group type column in Table 1, a represents a functional group having a lactam structure (aziridine group), b represents an amino group, c represents a functional group having a lactam structure (pyrrolidone group), d represents an amide group, e represents a methyl group, p represents a carboxy group, and q represents a sulfo group. All of the b, d, e, p, and q have a structure in which the functional group is directly bonded to carbon constituting the main chain.

<Evaluation>

[0139] The compositions for a secondary battery electrode obtained in the examples and the comparative examples were evaluated as follows. Table 2 shows the results. In the examples and comparative examples, the rating o (Good) was given when the white powder of polyvinyl acetal resin was appropriately obtained, and the rating x (Poor) was given when the white powder of the polyvinyl acetal resin was not appropriately obtained due to, for example, particles sticking together (resin powderization evaluation).

(1) Adhesiveness (Peeling Force)

[0140] Evaluation of the adhesion to aluminum foil was performed on the compositions for a secondary battery electrode obtained in the examples and comparative examples.

[0141] Each of the compositions for an electrode was applied to aluminum foil (thickness: 20 m) to the thickness after drying of 20 m, and dried to prepare a test sample in which a sheet-like electrode was formed on aluminum foil.

[0142] A piece in a size of 1 cm in length and 2 cm in width was cut out from the sample. The sample piece was immobilized using an AUTOGRAPH (AGS-J available from Shimadzu Corporation) and the electrode sheet was pulled up for measurement of the peeling force (N) needed for completely peeling the electrode sheet from the aluminum foil. The adhesiveness of each composition was evaluated based on the following criteria. [0143] o (Good): Peeling force of higher than 8.0 N [0144] (Average): Peeling force of 5.0 to 8.0 N [0145] x (Poor): Peeling force of lower than 5.0 N

(2) Dispersibility (Surface Roughness)

[0146] Using the test sample in (1) Adhesiveness, the surface roughness Ra was measured in conformity with JIS B 0601 (1994). The surface roughness of the electrode was evaluated based on the following criteria. Commonly, when the dispersibility of the active material is higher, the surface roughness is said to be smaller. [0147] o (Good): Ra of less than 3 m [0148] (Average): Ra of 3 m or more but less than 4 m [0149] x (Poor): Ra of 4 m or more [0150] (3) Resistance against electrolyte solution (solvent solubility)

(Production of Electrode Sheet)

[0151] Onto a polyethylene terephthalate (PET) film preliminarily subjected to release treatment was applied each of the compositions for a secondary battery electrode obtained in the examples and comparative examples to the thickness after drying of 20 m, and dried to give an electrode sheet.

[0152] A 2-cm-square piece was cut out from the electrode sheet to prepare an electrode sheet sample.

(Evaluation of Dissolution Rate)

[0153] The obtained sample was accurately weighed, and the weight of the resin contained in the sample was calculated based on the weight ratio of the components contained in the sheet. Then, the sample was placed in a mesh bag, and the total weight of the mesh bag and the sample was accurately measured.

[0154] The mesh bag containing the sample was immersed in a solvent mixture (diethyl carbonate:ethylene carbonate=1:1) which is a solvent of an electrolyte solution, and left to stand at 60 C. for five hours. After the standing, the mesh bag was taken out and dried under the conditions of 150 C. and eight hours, thereby completely vaporizing the solvent.

[0155] The mesh bag was taken out from the dryer, left to stand at room temperature for one hour, and weighed. The amount of the resin dissolved in the solvent mixture was calculated based on the weight change before and after the test, and the dissolution rate of the resin was calculated based on the ratio between the amount of the resin dissolved and the preliminarily calculated weight of the resin. The obtained dissolution rate was evaluated based on the following criteria. [0156] o (Good): Dissolution rate of lower than 1% [0157] (Average): Dissolution rate of 1% or higher but lower than 2% [0158] x (Poor): Dissolution rate of 2% or higher

(4) Moisture Absorption

[0159] The sample obtained in the (3) Resistance against electrolyte solution was accurately weighed.

[0160] The sample was then left to stand at a relative humidity of 95% and 30 C. for 24 hours. The sample was taken out and then accurately weighed. The moisture content was calculated from the weight change before and after the test, and evaluated according to the following criteria. [0161] o (Good): Moisture content of less than 5%. [0162] (Average): Moisture content of 5% or more but less than 7%. [0163] x (Poor): Moisture content of 7% or more

(5) Electrode Resistance Measurement

[0164] The electrode resistance of the electrode sheet obtained in the (1) Adhesiveness was measured with an electrode resistance meter (available from Hioki E.E. Corp.), and evaluated according to the following criteria. [0165] o (Good): Electrode resistance of less than 1,000 /sq [0166] x (Poor): Electrode resistance of 1,000 /sq or more

TABLE-US-00002 TABLE 2 Composition for secondary battery electrode Binder Amount relative Evaluation Active to 100 parts Dispersibility material Amount by weight of Adhesiveness Surface (parts by Resin (parts by active material Resin Peeling roughness weight) type weight) (parts by weight) powerization force (N) Rating Ra (m) Example 1 50 A 2.5 5 6.6 2.0 Example 2 50 B 0.005 0.01 6.6 1.9 Example 3 50 C 10 20 6.6 1.8 Example 4 50 D 2.5 5 7.8 2.3 Example 5 50 E 2.5 5 9.0 2.7 Example 6 50 F 2.5 5 10.8 1.5 Example 7 50 G 2.5 5 12.0 2.0 Example 8 50 H 2.5 5 12.6 1.9 Example 9 50 I 2.5 5 12.0 1.8 Example 10 50 J1 2.5 5 10.8 1.5 Example 11 50 J2 2.5 5 12.3 1.9 Example 12 50 J3 2.5 5 16.7 1.2 Example 13 50 K 2.5 5 13.2 2.3 Example 14 50 L 2.5 5 15.0 1.1 Example 15 50 M 2.5 5 16.2 1.5 Example 16 50 N 2.5 5 17.4 2.0 Example 17 50 O 2.5 5 17.4 1.9 Example 18 50 P 2.5 5 12.5 1.8 Comparative 50 Q 2.5 5 4.8 X 2.5 Example 1 Comparative 50 R 2.5 5 10.2 1.3 Example 2 Comparative 50 S 2.5 5 11.4 1.6 Example 3 Comparative 50 T1 + 2.5 5 11.4 1.8 Example 4 T2 Comparative 50 U 2.5 5 12 1.9 Example 5 Comparative 50 V 2.5 5 10.8 1.5 Example 6 Comparative 50 W 2.5 5 11.2 1.8 Example 7 Comparative 50 X 2.5 5 10.8 2.1 Example 8 Comparative 50 Y1 2.5 5 18.3 1.3 Example 9 Comparative 50 Y2 2.5 5 X 13.8 2.5 Example 10 Comparative 50 Y3 2.5 5 X 10.1 2.1 Example 11 Comparative 50 Y4 2.5 5 20.1 4.1 Example 12 Evaluation Moisture Resistance against absorption Electrode electrolyte solution Moisture resistance Dispersibility Dissolution content Resistance Rating rate (%) Rating (%) Rating (/sq) Rating Example 1 0.8 4.7 761 Example 2 0.7 4.2 483 Example 3 0.6 4.7 866 Example 4 0.9 4.0 448 Example 5 1.1 2.8 419 Example 6 0.6 5.2 502 Example 7 0.9 4.5 717 Example 8 0.8 4.0 465 Example 9 0.7 3.5 810 Example 10 0.5 2.6 465 Example 11 1.2 5.1 624 Example 12 0.8 4.3 446 Example 13 1.0 2.8 433 Example 14 0.5 5.2 523 Example 15 0.7 4.0 483 Example 16 1.0 3.3 679 Example 17 0.9 2.8 448 Example 18 0.9 4.2 512 Comparative 0.9 4.0 492 Example 1 Comparative 0.5 2.4 1026 X Example 2 Comparative 0.6 3.8 1054 X Example 3 Comparative 0.8 4.3 1196 X Example 4 Comparative 0.8 3 1652 X Example 5 Comparative 2.5 X 2.6 465 Example 6 Comparative 2.4 X 2.1 1543 X Example 7 Comparative 2.2 X 4.6 1351 X Example 8 Comparative 2.1 X 4 441 Example 9 Comparative 1.1 5.4 419 Example 10 Comparative 0.4 7.1 X 548 Example 11 Comparative X 1.2 3.5 1240 X Example 12

INDUSTRIAL APPLICABILITY

[0167] The present invention can provide a composition for a secondary battery electrode which is excellent in dispersibility of an active material and adhesiveness, capable of preventing battery deterioration and failure due to moisture absorption, and capable of providing a high-capacity secondary battery.