Composition for lithium secondary battery electrodes

Abstract

The present invention aims to provide a composition for a lithium secondary battery electrode which is excellent in dispersibility of an active material and adhesiveness, capable of maintaining an appropriate viscosity for a long period of time, and capable of providing a high-capacity lithium secondary battery even when the amount of a binder is small. Provided is a composition for a lithium secondary battery electrode including: an active material; a polyvinyl acetal resin; and an organic solvent, the polyvinyl acetal resin having a structural unit having a hydroxyl group represented by the following formula (1), a structural unit having an acetal group represented by the following formula (2), and a structural unit having a carboxyl group, the polyvinyl acetal resin containing 45 to 95 mol % of the structural unit having a hydroxyl group represented by the following formula (1): ##STR00001##
where R.sup.1 represents a hydrogen atom or a C1-C20 alkyl group.

Claims

1. A composition for a lithium secondary battery electrode comprising: an active material; a polyvinyl acetal resin; and an organic solvent, the polyvinyl acetal resin having a structural unit having a hydroxyl group of the following formula (1), a structural unit having an acetal group of the following formula (2), and a structural unit having a carboxyl group, the polyvinyl acetal resin containing 0.01 to 5 mol % of the structural unit having the carboxyl group and 45 to 95 mol % of the structural unit having the hydroxyl group of the following formula (1): ##STR00005## where R.sup.1 represents a hydrogen atom or a C1-C20 alkyl group.

2. The composition for a lithium secondary battery electrode according to claim 1, wherein the structural unit having the carboxyl group includes at least one member selected from the group consisting of a structural unit having a carboxyl group of the following formulae formula (3), a structural unit having a carboxyl group of the following formula (4), and a structural unit having a carboxyl group of the following formula (5): ##STR00006## where R.sup.2, R.sup.3, R.sup.4, R.sup.5, and R.sup.6 each represent a single bond or a C1-C10 saturated or unsaturated hydrocarbon, and X represents hydrogen, sodium, or potassium.

3. The composition for a lithium secondary battery electrode according to claim 2, wherein the polyvinyl acetal resin has a degree of polymerization of 250 to 4,000.

4. The composition for a lithium secondary battery electrode according to claim 2, wherein the polyvinyl acetal resin is contained in an amount of 0.01 to 12 parts by weight relative to 100 parts by weight of the active material.

5. The composition for a lithium secondary battery electrode according to claim 2, further comprising a conductivity imparting agent.

6. A lithium secondary battery comprising the composition for a lithium secondary battery electrode according to claim 2.

7. The composition for a lithium secondary battery electrode according to claim 1, wherein the polyvinyl acetal resin has a degree of polymerization of 250 to 4,000.

8. The composition for a lithium secondary battery electrode according to claim 7, wherein the polyvinyl acetal resin is contained in an amount of 0.01 to 12 parts by weight relative to 100 parts by weight of the active material.

9. The composition for a lithium secondary battery electrode according to claim 7, further comprising a conductivity imparting agent.

10. A lithium secondary battery comprising the composition for a lithium secondary battery electrode according to claim 7.

11. The composition for a lithium secondary battery electrode according to claim 1, wherein the polyvinyl acetal resin is contained in an amount of 0.01 to 12 parts by weight relative to 100 parts by weight of the active material.

12. The composition for a lithium secondary battery electrode according to claim 11, further comprising a conductivity imparting agent.

13. A lithium secondary battery comprising the composition for a lithium secondary battery electrode according to claim 11.

14. The composition for a lithium secondary battery electrode according to claim 1, further comprising a conductivity imparting agent.

15. A lithium secondary battery comprising the composition for a lithium secondary battery electrode according to claim 14.

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

Description

DESCRIPTION OF EMBODIMENTS

(1) The present invention is more specifically described in the following with reference to, but not limited to, examples.

(2) (Synthesis of Polyvinyl Acetal Resin A)

(3) An amount of 350 parts by weight of carboxyl group-containing a polyvinyl alcohol A having a structural unit having a carboxyl group represented by the formula (3) (degree of polymerization: 800, degree of saponification: 98 mol %, amount of structural unit having a carboxyl group represented by the formula (3) (carboxyl-containing group content): 5 mol %, R.sup.2: CH.sub.2, X: H) was added to 3000 parts by weight of pure water, and dissolved with stirring at 90 C. for about two hours. After cooling to 40 C., to the resulting solution was added 230 parts by weight of hydrochloric acid having a concentration of 35% by weight. The temperature of the solution was further lowered to 5 C., and the solution was blended with 50 parts by weight of n-butyraldehyde. The resulting solution was subjected to acetalization while the temperature was maintained at 5 C. for precipitation of a reaction product. Then, the solution was maintained to have a temperature of 30 C. for three hours for completion of the reaction. The solution was then subjected to neutralization by a known method, washing with water, and drying to give polyvinyl acetal resin A in the form of white powder.

(4) The obtained polyvinyl acetal resin A was dissolved in DMSO-d.sub.6 (dimethyl sulfoxide) and the following parameters were measured by .sup.13C-NMR (nuclear magnetic resonance spectrum): the amount of a structural unit represented by the formula (1) [hydroxyl group content]; the amount of a structural unit represented by the formula (2) [degree of acetalization]; the amount of a structural unit having a carboxyl group represented by the formula (3) [carboxyl-containing group content]; and the amount of the structural unit represented by the formula (6) [acetyl group content]. The hydroxyl group content was 45 mol %, the degree of acetalization (degree of butyralization) was 48 mol %, the carboxyl-containing group content was 5 mol %, and the acetyl group content was 2 mol %.

(5) The glass transition temperature (Tg) of the obtained polyvinyl acetal resin A was measured with a differential scanning calorimeter (DSC6220 available from Seiko Instruments Inc.).

(6) (Synthesis of Polyvinyl Acetal Resins B to K, P, and Q)

(7) Polyvinyl acetal resins B to K, P, and Q were synthesized in the same manner as in the case of the polyvinyl acetal resin A, except that a polyvinyl alcohol (type) and an aldehyde (type, amount) as shown in Table 1 were used. A polyvinyl alcohol not having a carboxyl group was used as a polyvinyl alcohol K.

(8) (Synthesis of Polyvinyl Acetal Resin L).

(9) A polyvinyl acetal resin L was synthesized in the same manner as in the case of the polyvinyl acetal resin A, except that the carboxyl group-containing polyvinyl alcohol A having a structural unit having a carboxyl group represented by the formula (3) was changed to a carboxyl group-containing polyvinyl alcohol L having a structural unit having a carboxyl group represented by the formula (4) (degree of polymerization: 400, degree of saponification: 98 mol %, amount of the structural unit having a carboxyl group represented by formula (4) [carboxyl-containing group content]: 5 mol %, R.sup.3: CH.sub.2, R.sup.4: single bond) and an aldehyde (type, amount) as shown in Table 1 was used.

(10) (Synthesis of Polyvinyl Acetal Resin M)

(11) A polyvinyl acetal resin M was synthesized in the same manner as in the case of the polyvinyl acetal resin A, except that the carboxyl group-containing polyvinyl alcohol A having a structural unit having a carboxyl group represented by the formula (3) was changed to a carboxyl group-containing polyvinyl alcohol M having a structural unit having a carboxyl group represented by the formula (4) (degree of polymerization: 600, degree of saponification: 95 mol %, amount of the structural unit having a carboxyl group represented by formula (4) [carboxyl-containing group content]: 3 mol %, R.sup.3: CH.sub.2, R.sup.4: C.sub.6H.sub.4) and an aldehyde (type, amount) as shown in Table 1 was used.

(12) (Synthesis of Polyvinyl Acetal Resin N)

(13) A polyvinyl acetal resin N was synthesized in the same manner as in the case of the polyvinyl acetal resin A, except that the carboxyl group-containing polyvinyl alcohol A having a structural unit having a carboxyl group represented by the formula (3) was changed to a carboxyl group-containing polyvinyl alcohol N having a structural unit having a carboxyl group represented by the formula (5) (degree of polymerization: 400, degree of saponification: 95 mol %, amount of the structural unit having a carboxyl group represented by formula (5) [carboxyl-containing group content]: 3 mol %, R.sup.5: CH.sub.2, R.sup.6: single bond) and an aldehyde (type, amount) as shown in Table 1 was used.

(14) (Synthesis of Polyvinyl Acetal Resin 0)

(15) A polyvinyl acetal resin 0 was synthesized in the same manner as in the case of the polyvinyl acetal resin A, except that the carboxyl group-containing polyvinyl alcohol A having a structural unit having a carboxyl group represented by the formula (3) was changed to a carboxyl group-containing polyvinyl alcohol 0 having a structural unit having a carboxyl group represented by the formula (5) (degree of polymerization: 800, degree of saponification: 95 mol %, amount of structural unit having a carboxyl group represented by formula (5) [carboxyl-containing group content]: 1 mol %, R.sup.5: CH.sub.2, R.sup.6: CHCH) and an aldehyde (type, amount) as shown in Table 1 was used.

(16) (Synthesis of Polyvinyl Acetal Resin R)

(17) A polyvinyl acetal resin R was synthesized in the same manner as in the case of the polyvinyl acetal resin A, except that the carboxyl group-containing polyvinyl alcohol A having a structural unit having a carboxyl group represented by the formula (3) was changed to a sulfonic acid group-containing polyvinyl alcohol R having a structural unit having a sulfonic acid group (degree of polymerization: 800, degree of saponification: 97 mol %, amount of the structural unit having a sulfonic acid group [sulfonic acid group content]: 3 mol %, structural unit represented by formula (3) in which a part corresponding to a carboxyl group is replaced with sulfonic acid group, R.sup.2: CH.sub.2) and an aldehyde (type, amount) as shown in Table 1 was used.

(18) TABLE-US-00001 TABLE 1 Polyvinyl alcohol Degree of Type of Resin Degree of saponification R.sup.2 R.sup.3 R.sup.4 R.sup.5 R.sup.6 acid-modified type polymerization (mol %) structure structure structure structure structure group A 800 98 CH.sub.2 Carboxyl B 600 99 CH.sub.2 Carboxyl C 1400 95 CH.sub.2 Carboxyl D 1700 95 CHCH Carboxyl E 3300 95 C.sub.6H.sub.4 Carboxyl F 250 95 CH.sub.2 Carboxyl G 4000 70 CH.sub.2 Carboxyl H 900 95 CH.sub.2 Carboxyl I 1300 95 CH.sub.2 Carboxyl J 800 95 CH.sub.2 Carboxyl K 800 95 L 400 98 CH.sub.2 Single bond Carboxyl M 600 95 CH.sub.2 C.sub.6H.sub.4 Carboxyl N 400 95 CH.sub.2 Single bond Carboxyl O 800 95 CH.sub.2 CHCH Carboxyl P 800 99 CH.sub.2 Carboxyl Q 800 98 CH.sub.2 Carboxyl R 800 97 CH.sub.2 Sulfonic acid Polyvinyl Polyvinyl acetal alcohol Carboxyl- Acid-modified Aldehyde Hydroxyl containing group Amount group Degree of Acetyl group group Resin content*.sup.1 (parts by content acetalization content content*.sup.1 Tg type (mol %) Type weight) (mol %) (mol %) (mol %) (mol %) ( C.) A 5 Butylaldehyde 50 45 48 2 5 74 B 3 Butylaldehyde 4 94 2 1 3 C 0.1 Butylaldehyde 47 50 44.9 5 0.1 72 D 3 Butylaldehyde 39 55 37 5 3 76 E 3 Butylaldehyde 34 60 32 5 3 78 F 3 Benzaldehyde 24 70 22 5 3 G 3 Propionaldehyde 24 45 22 30 3 H 0.01 Acetaldehyde 12 85 9.99 5 0.01 I 3 Vinylaldehyde 14 80 12 5 3 J 3 Butylaldehyde 54 40 52 5 3 70 K 0 Butylaldehyde 37 60 35 5 0 76 L 5 Butylaldehyde 45 50 43 2 5 77 M 3 Butylaldehyde 39 55 37 5 3 75 N 3 Butylaldehyde 34 60 32 5 3 76 O 1 Butylaldehyde 21 75 19 5 1 67 P 2 Butylaldehyde 98 96 1 1 2 Q 3 Butylaldehyde 45 43 52 2 3 75 R 3 Butylaldehyde 62 60 34 3 0 *.sup.1The acid-modified group content and thecarboxyl-containing group content each refer to the amount of a structural unit having its modified group

Example 1

(19) (Preparation of Composition for Lithium Secondary Battery Electrode)

(20) 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 from Denki Kagaku Kogyo Kabushiki Kaisha) as a conductivity imparting agent, 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 lithium secondary battery electrode.

Examples 2 to 14, Comparative Examples 1 to 5

(21) Compositions for a lithium secondary battery electrode were each obtained in the same manner as in Example 1, except that the polyvinyl acetal resin (resin type, amount) as shown in Table 2 was used.

(22) <Evaluation>

(23) The compositions for a lithium secondary battery electrode obtained in the examples and comparative examples were evaluated for the following parameters. Table 2 shows the results.

(24) (1) Adhesiveness (Peeling Force)

(25) Evaluation of the adhesion to aluminum foil was performed on the compositions for a lithium secondary battery electrode obtained in the examples and comparative examples.

(26) 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.

(27) 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.

(28) (Good): Peeling force of higher than 8.0 N.

(29) (Average): Peeling force of 5.0 to 8.0 N.

(30) x (Poor): Peeling force of lower than 5.0 N.

(31) (2) Dispersibility (Surface Roughness)

(32) 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.

(33) (Excellent): Ra of less than 2 m.

(34) (Good): Ra of 2 m or more but less than 5 m.

(35) (Average): Ra of 5 m or more but less than 8 m.

(36) x (Poor): Ra of 8 m or more.

(37) (3) Solvent Solubility (Production of Electrode Sheet)

(38) Onto a polyethylene terephthalate (PET) film preliminarily subjected to release treatment was applied each of the compositions for a lithium 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.

(39) A 2-cm-square piece was cut out from the electrode sheet to prepare an electrode sheet sample.

(40) (Evaluation of Elution)

(41) 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.

(42) The mesh bag containing the sample was immersed in a solvent mixture (diethyl carbonate:ethylene carbonate=1:1) which is an electrolyte solvent and left to stand at 60 C. for 5 hours. After the standing, the mesh bag was taken out and dried under the conditions of 150 C. and 8 hours, thereby completely vaporizing the solvent.

(43) The mesh bag was taken out from the dryer, left to stand at room temperature for one hour, and weighed. The elution amount of the resin was calculated based on the weight change before and after the test, and the elution rate of the resin was calculated based on the ratio between the elution amount and the preliminarily calculated weight of the resin. The obtained elution rate was evaluated based on the following criteria.

(44) (Good): Elution rate of lower than 1%

(45) (Average): Elution rate of 1% or higher but lower than 2%

(46) x (Poor): Elution rate of 2% or higher

(47) (4) Viscosity Stability Over Time

(48) The paste viscosity of each of the compositions for a lithium secondary battery electrode obtained in the examples and comparative examples was measured using a Brookfield viscometer. The viscosity was measured on the day the paste was produced and a week after that day. The change rate of the viscosity over time was evaluated based on the following criteria. Commonly, when the viscosity stability is higher, the change rate of the viscosity over time is said to be smaller.

(49) (Excellent): The change rate of viscosity over time of 30% or lower

(50) (Good): The change rate of viscosity over time of higher than 30% but not higher than 50%

(51) (Average): The change rate of viscosity over time of higher than 50% but not higher than 80%

(52) x (Poor): The change rate of viscosity over time of higher than 80%

(53) (5) Evaluation of Electrode Resistance

(54) The volume resistance rate and interface resistance of the test sample electrode obtained in (1) Adhesiveness were measured with an electrode resistance meter (Hioki E.E. Corp.).

(55) (6) Evaluation of Battery Performance

(56) (a) Production of Coin Battery

(57) The composition for a lithium secondary battery positive electrode obtained in Example 1 was applied to aluminum foil and dried to form a layer with a thickness of 0.2 mm. A 12-mm piece was punched out from the resulting layer to prepare a positive electrode layer.

(58) Separately, a 12-mm piece was punched out from a purchased negative electrode sheet for a lithium secondary battery (A100 available from Hohsen Corp.) to prepare a negative electrode layer. The electrolyte used was a mixed solution with ethylene carbonate containing LiPF.sub.6 (1M). The positive electrode layer was impregnated with the electrolyte and then placed on a positive electrode current collector. A porous PP film (separator) with a thickness of 25 mm impregnated with the electrolyte was further placed thereon.

(59) A lithium metal plate serving as a negative electrode layer was further placed thereon, and a negative electrode current collector covered with an insulating packing was placed on the top. The resulting laminate was pressurized using a caulking machine to provide a sealed coin battery.

(60) (b) Evaluation of Discharge Capacity and Charge/Discharge Cycle

(61) Evaluation of the discharge capacity and charge/discharge cycle was performed on the obtained coin batteries using a charge/discharge tester available from Hohsen Corp. The evaluation of the discharge capacity and charge/discharge cycle was performed under the conditions of the voltage range of 3.0 to 4.4 V and the evaluation temperature of 20 C.

(62) TABLE-US-00002 TABLE 2 Composition for lithium secondary battery electrode Polyviny acetal Evaluation Active Amount per 100 Dispersibility material Amount parts by weight of Adhesiveness Surface (Parts by Resin (parts by active material Peeling roughness weight) type weight) (parts by weight) force (N) Judgement Ra (m) Judgement Example 1 50 A 2.5 5 8.9 1.8 Example 2 50 B 0.005 0.01 6.4 2.6 Example 3 50 C 10 20 13.5 1.4 Example 4 50 D 2.5 5 9.6 2.0 Example 5 50 E 2.5 5 12.4 4.2 Example 6 50 F 2.5 5 7.6 2.2 Example 7 50 G 2.5 5 10.1 4.7 Example 8 50 H 2.5 5 8.7 1.7 Example 9 50 I 2.5 5 8.2 2.0 Example 10 50 A 6 12 11.9 1.6 Example 11 50 L 2.5 5 10.3 1.9 Example 12 50 M 2.5 5 9.8 2.4 Example 13 50 N 5 10 12.7 1.5 Example 14 50 O 4 8 10.9 1.7 Comparative 50 J 2.5 5 7.0 2.1 Example 1 Comparative 50 K 2.5 5 9.1 5.0 Example 2 Comparative 50 P 2.5 5 4.7 x 2.4 Example 3 Comparative 50 Q 2.5 5 11.0 1.8 Example 4 Comparative 50 R 2.5 5 8.2 5.9 Example 5 Evaluation Stability of viscosity over time Electrode resistance Battery performance Solvent Change rate of Volume Interface Discharge Charge/ solubility viscosity over resistance resistance capacity discharge Judgement time (%) Judgement (cm) (cm.sup.2) (mAh/g) cycle (%) Example 1 23 3.42 0.59 162 98 Example 2 30 3.24 0.42 140 98 Example 3 32 5.00 1.10 115 92 Example 4 29 3.35 1.27 131 95 Example 5 19 3.79 2.24 125 96 Example 6 39 4.21 0.66 147 94 Example 7 24 4.80 3.68 146 93 Example 8 42 4.62 0.70 153 92 Example 9 27 4.75 1.09 130 90 Example 10 19 4.97 0.62 108 94 Example 11 31 3.93 2.84 119 98 Example 12 35 3.84 2.93 114 91 Example 13 33 4.83 2.78 127 94 Example 14 39 436 1.84 121 93 Comparative x 20 5.99 3.21 62 73 Example 1 Comparative x 52 6.38 3.45 25 69 Example 2 Comparative 81 x 7.24 4.32 20 64 Example 3 Comparative x 28 4.02 3.04 87 80 Example 4 Comparative 86 x 3.87 4.24 100 89 Example 5

INDUSTRIAL APPLICABILITY

(63) The present invention provides a composition for a lithium secondary battery electrode which is excellent in dispersibility of an active material and adhesiveness, capable of maintaining an appropriate viscosity for a long period of time, and capable of providing a high-capacity lithium secondary battery even when the amount of a binder is small.