Binder composition for negative electrode, slurry for negative electrode, negative electrode, and lithium ion secondary battery

Abstract

Provided is a binder composition for negative electrode that has good binding property with an active material and a metal foil and is superior in reduction resistance. A binder composition for negative electrode comprising a graft copolymer obtained by graft copolymerizing, with polyvinyl alcohol, a monomer containing (meth)acrylonitrile as a main component, wherein the polyvinyl alcohol has an average degree of polymerization of 300 to 3000; the polyvinyl alcohol has a saponification degree of 70 to 100 mol %; the graft copolymer has a polyvinyl alcohol amount of 10 to 90 mass %; and the graft copolymer has a poly(meth)acrylonitrile amount of 90 to 10 mass %.

Claims

1. A slurry for negative electrode comprising: a binder composition for negative electrode comprising a graft copolymer obtained by graft copolymerizing, with polyvinyl alcohol, a monomer containing (meth)acrylonitrile as a main component; a negative electrode active material; and a conductive assistant, wherein: the polyvinyl alcohol has an average degree of polymerization of 300 to 3000; the polyvinyl alcohol has a saponification degree of 70 to 100 mol %; the graft copolymer has a polyvinyl alcohol amount of 10 to 90 mass %; the graft copolymer has a poly(meth)acrylonitrile amount of 90 to 10 mass %; and a homopolymer of poly(meth)acrylonitrile generated during the graft copolymerization has a mass average molecular weight of 30000 to 250000.

2. The slurry for negative electrode according to claim 1, wherein: the graft copolymer has a graft rate of 10 to 900%.

3. The slurry for negative electrode according to claim 1, wherein a solid content of the binder composition for negative electrode is 1 to 20 mass % of a total solid of the slurry for negative electrode.

4. The slurry for negative electrode according to claim 1, wherein the negative electrode active material comprises at least one selected from graphite and silicon compounds.

5. The slurry for negative electrode according to claim 1, wherein the conductive assistant comprises at least one selected from the group consisting of: (i) fibrous carbon; (ii) carbon black; and (iii) carbon composite in which fibrous carbon and carbon black are combined with each other.

6. The slurry for negative electrode according to claim 3, wherein the negative electrode active material comprises at least one selected from graphite and silicon compounds.

7. The slurry for negative electrode according to claim 3, wherein the conductive assistant comprises at least one member selected from the group consisting of: (i) fibrous carbon; (ii) carbon black; and (iii) carbon composite in which fibrous carbon and carbon black are combined with each other.

8. The slurry for negative electrode according to claim 4, wherein the conductive assistant comprises at least one member selected from the group consisting of: (i) fibrous carbon; (ii) carbon black; and (iii) carbon composite in which fibrous carbon and carbon black are combined with each other.

9. A negative electrode comprising: a metal foil; and a coating formed on the metal foil and comprising the slurry for negative electrode according to claim 1.

10. A lithium ion secondary battery comprising the negative electrode according to claim 9.

11. A negative electrode comprising: a metal foil; and a coating formed on the metal foil and comprising the slurry for negative electrode according to claim 3.

12. A negative electrode comprising: a metal foil; and a coating formed on the metal foil and comprising the slurry for negative electrode according to claim 4.

13. A negative electrode comprising: a metal foil; and a coating formed on the metal foil and comprising the slurry for negative electrode according to claim 5.

Description

EXAMPLES

(1) Hereinafter, the present invention will be described more specifically referring to examples and comparative examples. Here, the present invention shall not be limited to these.

Example 1

(2) <Preparation of PVA>

(3) Vinyl acetate (600 parts by mass) and methanol (400 parts by mass) were stocked, followed by deoxygenation by bubbling nitrogen gas. Subsequently, bis (4-t-butylcyclohexyl) peroxydicarbonate (0.3 parts by mass) as a polymerization initiator was stocked, and polymerization was carried out at 60 C. for 4 hours. The solid content concentration of the polymerization solution at the time of termination of the polymerization was 48%, and the polymerization rate of vinyl acetate determined from the solid content was 80%. Methanol vapor was blown into the obtained polymerization solution to remove unreacted vinyl acetate and then the resultant solution was diluted with methanol so that the concentration of polyvinyl acetate would be 40 mass %. To the diluted polyvinyl acetate solution (1200 parts by mass), 10 mass % methanol solution of sodium hydroxide (20 parts by mass) was added. Saponification reaction was carried out at 30 C. for 1.5 hours.

(4) The solution after saponification was neutralized with acetic acid, filtered and dried at 100 C. for 2 hours to obtain PVA. The average degree of polymerization of the obtained PVA was 320 and the saponification degree was 86.3 mol %.

(5) <Polymerization Degree and Saponification Degree>

(6) The average degree of polymerization and the saponification degree of PVA was measured in accordance with the method provided in JIS K 6726.

(7) <Preparation of Binder A>

(8) Hereinafter, the preparation method of binder A is described. In this embodiment, the binder means a graft copolymer according to the present invention.

(9) PVA obtained (8.00 parts by mass) was added to dimethylsulfoxide (265.1 parts by mass), and was allowed to dissolve by stirring at 60 C. for 2 hours. Subsequently, ammonium peroxodisulfate (0.03 parts by mass) dissolved in acrylonitrile (30.3 parts by mass) and dimethylsulfoxide (3 parts by mass) were added at 60 C., followed by graft copolymerization with agitation at 60 C. Four hours after the initiation of polymerization, the polymerization was terminated by cooling the reaction mixture to room temperature.

(10) <Precipitation and Drying>

(11) The obtained reaction solution containing binder A (297 parts by mass) was added dropwise to methanol (2970 parts by mass) to precipitate binder A. The polymer was separated by filtration, vacuum dried for 2 hours at room temperature, and further vacuum dried at 80 C. for 2 hours. The solid content was 8.87 mass %, and the polymerization rate of acrylonitrile was 24.5% when calculated from the solid content.

(12) The mass of PAN in the obtained binder A was 44.5 mass % of the total polymer, the graft rate was 78%, and the mass average molecular weight of the PAN homopolymer was 105100. Measurement methods of these values will be described in the following <Composition Ratio>, <Graft Rate> and <Mass Average Molecular Weight>.

(13) <Composition Ratio>

(14) Composition ratio of binder A was calculated from the reaction rate (polymerization rate) of acrylonitrile and the composition of each of the stocked component used for polymerization. The mass % of PAN formed at the time of copolymerization (mass % of PAN in the graft copolymer) was calculated from the polymerization rate (%) of acrylonitrile, mass of acrylonitrile used for graft copolymerization (amount stocked), and mass of PVA used for graft copolymerization (amount stocked), using the afore-mentioned formula (2). Here, the mass ratio in the following table is the mass ratio in the resin component including the graft copolymer itself, and the PVA homopolymer and the PAN homopolymer formed during the copolymerization.

(15) <Graft Rate>

(16) Binder A (1.00 g) was precisely weighed and added to special grade DMF (50 cc, manufactured by KOKUSAN CHEMICAL Co., Ltd.), and the mixture was stirred at 80 C. for 24 hours. Subsequently, the mixture was centrifuged at 10000 rpm for 30 minutes with a centrifugal separator (model: H2000B, rotor: H, manufactured by KOKUSAN Co. Ltd.). After carefully separating the filtrate (DMF soluble matter), the matter insoluble in the pure water was vacuum dried at 100 C. for 24 hours. The graft rate was calculated using the formula (1) described above.

(17) <Mass Average Molecular Weight>

(18) The filtrate (DMF soluble matter) obtained after centrifugation was poured into methanol (1000 ml) to give a precipitate. The precipitate was vacuum dried at 80 C. for 24 hours, and mass average molecular weight expressed in terms of standard polystyrene was measured by GPC. GPC measurement was carried out under the following conditions.

(19) Column: two columns (GPC LF-804, 8.0300 mm, manufactured by Showa Denko K.K.) were connected in series and used.

(20) Column temperature: 40 C.

(21) Solvent: 20 mM-LiBr/DMF

(22) <Reductive Degradation Potential>

(23) Binder A (10 parts by mass) was dissolved in N-methylpyrrolidone (90 parts by mass) to obtain a polymer solution. Subsequently, acetylene black (1 part by mass, Denka Black (registered trademark) HS-100 manufactured by Denka Company Limited) was added to the obtained polymer solution (100 parts by mass), and the mixture was stirred. The obtained solution was applied to a copper foil so as to provide a dry thickness of 20 m, preliminarily dried at 80 C. for 10 minutes, and then dried at 105 C. for 1 hour to give a test piece.

(24) The obtained test piece was used as the working electrode, lithium was used as the counter electrode and reference electrode, and a solution of ethylene carbonate/diethyl carbonate (= (volume ratio), concentration of 1 mol/L) in which LiPF.sub.6 was used as an electrolyte salt was used as the electrolytic solution, thereby assembling a three-pole cell (manufactured by TOYO SYSTEM CO., LTD.). Linear sweep voltammetry (hereinafter abbreviated as LSV) was performed at 25 C. with a scanning speed of 10 mV/sec using a Potentiostat/Galvanostat (1287 type, manufactured by Solartron Analytical). The reductive degradation potential was defined as the potential when the current reached 0.1 mA/cm.sup.2. The lower the reductive degradation potential, the more difficult for the reductive degradation to occur, and thus it can be considered that reduction resistance is high.

(25) TABLE-US-00001 TABLE 1 Example Example Example Example Example Example Example Example Example 1 2 3 4 5 6 7 8 9 Binder A B C D E F G H I Polymerization degree of PVA 320 1650 2990 1740 1610 1820 2230 1980 2240 Saponification degree of PVA 86.3 95.2 88.6 71.4 98.2 83.1 84.5 86.3 81.8 (mol %) Mass ratio PVA 55.5 60.0 41.5 58.4 59.6 11.2 79.8 70.6 11.6 (%) PAN 44.5 40.0 58.5 41.6 40.4 88.8 20.2 29.4 88.4 Graft rate (%) 78 65 137 69 66 769 24 40 739 Mass average molecular weight 105100 124500 156200 135600 128400 131200 146400 141200 156200 of PAN homopolymer Reductive degradation 0.010 0.015 0.005 0.015 0.015 0.004 0.030 0.030 0.004 potential (V)

Example 2

(26) The amount of bis (4-t-butylcyclohexyl) peroxydicarbonate in Example 1 was altered to 0.15 parts by mass, the amount stocked of vinyl acetate was altered to 630 parts by mass, and polymerization was carried out at 60 C. for 5 hours. The polymerization rate was 80%. Unreacted vinyl acetate was removed in a similar manner as in Example 1, and then the resultant solution was diluted with methanol so that the concentration of polyvinyl acetate would be 30 mass %. To this polyvinyl acetate solution (2000 parts by mass), methanol solution of sodium hydroxide (concentration of 10 mass %, 20 parts by mass) was added, followed by saponification reaction at 30 C. for 2.5 hours.

(27) Neutralization, filtration and drying were carried out in a similar manner as in Example 1 to give PVA having an average degree of polymerization of 1650 and a saponification degree of 95.2 mol %. Polymerization of PAN was carried out in a similar manner as in Example 1 using the obtained PVA to prepare binder B. The mass ratio of PVA and PAN in the binder B was 60.0:40.0, the graft rate was 65%, and the average molecular weight of PAN homopolymer was 124500. Composition ratio, graft rate, and mass average molecular weight of the PAN homopolymer were measured in a similar manner as in Example 1. The same applies to the following Examples 3 to 9.

Example 3

(28) The amount of vinyl acetate in Example 1 was altered to 930 parts by mass, the amount of polymerization initiator bis (4-t-butylcyclohexyl) peroxydicarbonate in Example 1 was altered to 0.15 parts by mass, and polymerization was carried out at 60 C. for 5 hours. The polymerization rate was 70%. Dilution with methanol was conducted so that the concentration of polyvinyl acetate would be 30 mass %. To this polyvinyl acetate solution (2000 parts by mass), methanol solution of sodium hydroxide (concentration of 10 mass %, 20 parts by mass) was added, followed by saponification reaction at 30 C. for 1.5 hours.

(29) Neutralization, filtration and drying were carried out in a similar manner as in Example 1 to give PVA having an average degree of polymerization of 2990 and a saponification degree of 88.6 mol %. Polymerization of PAN was carried out in a similar manner as in Example 1 using the obtained PVA to prepare binder C. The mass ratio of PVA and PAN in the binder C was 41.5:58.5, the graft rate was 137%, and the mass average molecular weight of PAN homopolymer was 156200.

Example 4

(30) The amount of vinyl acetate in Example 1 was altered to 630 parts by mass, the amount of polymerization initiator bis (4-t-butylcyclohexyl) peroxydicarbonate in Example 1 was altered to 0.15 parts by mass, and polymerization was carried out at 60 C. for 5 hours. The polymerization rate was 75%. Dilution with methanol was conducted so that the concentration of polyvinyl acetate would be 30 mass %. To this polyvinyl acetate solution (2000 parts by mass), methanol solution of sodium hydroxide (concentration of 10 mass %, 20 parts by mass) was added, followed by saponification reaction at 30 C. for 0.5 hours.

(31) Neutralization, filtration and drying were carried out in a similar manner as in Example 1 to give PVA having an average degree of polymerization of 1740 and a saponification degree of 71.4 mol %. Polymerization of PAN was carried out in a similar manner as in Example 1 using the obtained PVA to prepare binder D. The mass ratio of PVA and PAN in the binder D was 58.4:41.6, the graft rate was 69%, and the mass average molecular weight of PAN homopolymer was 135600.

Example 5

(32) The saponification reaction as in Example 2 was performed at 30 C. for 3 hours, and the rest of the conditions were carried out in a similar manner as in Example 2 to prepare binder E. The PVA obtained had an average degree of polymerization of 1610 and a saponification degree of 98.2%. The mass ratio of PVA and PAN in the binder E was 59.6:40.4, the graft rate was 66%, and the mass average molecular weight of PAN homopolymer was 128400.

Example 6

(33) The polymerization reaction of polyvinyl acetate as in Example 2 was performed for 6 hours, the duration of saponification was altered to 1 hour, and the rest of the conditions were carried out in a similar manner as in Example 2 to prepare PVA. The PVA obtained had an average degree of polymerization of 1820 and a saponification degree of 83.1%. Preparation of binder F was carried out by altering the amount of PVA to 1.40 parts by mass, and the rest of the conditions were carried out in a similar manner as in Example 1. The mass ratio of PVA and PAN in the obtained binder F was 11.2:88.8, the graft rate was 769%, and the mass average molecular weight of PAN homopolymer was 131200.

Example 7

(34) The amount of vinyl acetate in Example 2 was altered to 800 parts by mass, the duration of polymerization was altered to 4 hours, the duration of saponification was altered to 1 hour, and the rest of the conditions were carried out in a similar manner as in Example 2 to prepare PVA. The PVA obtained had an average degree of polymerization of 2230 and a saponification degree of 84.5%. The PVA thus obtained was used to prepare binder G. Preparation of binder G was carried out by altering the amount of PVA in Example 1 to 10.0 parts by mass, and the rest of the conditions were carried out in a similar manner as in Example 1. The mass ratio of PVA and PAN in the obtained binder G was 79.8:20.2, the graft rate was 24%, and the mass average molecular weight of PAN homopolymer was 146400.

Example 8

(35) The amount of vinyl acetate in Example 2 was altered to 700 parts by mass, the duration of polymerization was altered to 5 hours, and the rest of the conditions were carried out in a similar manner as in Example 2 to prepare PVA. The PVA obtained had an average degree of polymerization of 1980 and a saponification degree of 86.3%. The PVA thus obtained was used to prepare binder H. Preparation of binder H was carried out by altering the amount of PVA in Example 1 to 9.0 parts by mass, and the rest of the conditions were carried out in a similar manner as in Example 1. The mass ratio of PVA and PAN in the obtained binder H was 70.6:29.4, the graft rate was 40%, and the mass average molecular weight of PAN homopolymer was 141200.

Example 9

(36) The amount of vinyl acetate in Example 2 was altered to 800 parts by mass, the duration of reaction was altered to 6 hours, the duration of saponification was altered to 1 hour, and the rest of the conditions were carried out in a similar manner as in Example 2 to prepare PVA. The PVA obtained had an average degree of polymerization of 2240 and a saponification degree of 81.8%. The PVA thus obtained was used to prepare binder I. Preparation of binder I was carried out by altering the amount of PVA in Example 1 to 1.50 parts by mass, and the rest of the conditions were carried out in a similar manner as in Example 1. The mass ratio of PVA and PAN in the obtained binder I was 11.6:88.4, the graft rate was 739%, and the mass average molecular weight of PAN homopolymer was 156200.

Comparative Example 1

(37) The amount of vinyl acetate stocked during polyvinyl acetate preparation in Example 1 was altered to 550 parts by mass, the amount of methanol was altered to 500 parts by mass, the amount of bis (4-t-butylcyclohexyl) peroxydicarbonate was altered to 0.3 parts by mass, and the rest of the conditions were carried out in a similar manner as in Example 1 to prepare PVA. The obtained PVA had an average degree of polymerization of 250 and a saponification degree of 87.8 mol %. Preparation of binder J was carried out in a similar manner as in Example 1. The mass ratio of PVA and PAN in the obtained binder J was 55.5:44.5, the graft rate was 78%, and the mass average molecular weight of PAN homopolymer was 99800.

Comparative Example 2

(38) The amount of vinyl acetate stocked during polyvinyl acetate preparation in Example 1 was altered to 3000 parts by mass, the amount of methanol was altered to 500 parts by mass, the amount of bis (4-t-butylcyclohexyl) peroxydicarbonate was altered to 0.15 parts by mass, the duration of the reaction was altered to 12 hours, the duration of saponification was altered to 2.5 hours, and the rest of the conditions were carried out in a similar manner as in Example 1 to prepare PVA. The obtained PVA had an average degree of polymerization of 3620 and a saponification degree of 93.2 mol %. Preparation of binder K was carried out in a similar manner as in Example 1, except that the amount of PVA in Example 1 was altered to 9.00 parts by mass. The mass ratio of PVA and PAN in the obtained binder K was 62.6:37.4, the graft rate was 58%, and the mass average molecular weight of PAN homopolymer was 212500. Attempts were made to prepare slurry for electrode by using the binder K. Aggregation of conductive material due to insoluble binder component was observed, and it was difficult to apply the slurry for electrode.

Comparative Example 3

(39) The amount of vinyl acetate stocked during polyvinyl acetate preparation in Example 1 was altered to 600 parts by mass, the duration of the reaction was altered to 6 hours, the duration of saponification was altered to 0.5 hour, and the rest of the conditions were carried out in a similar manner as in Example 1 to prepare PVA. The obtained PVA had an average degree of polymerization of 630 and a saponification degree of 65.1 mol %.

(40) Preparation of binder L was carried out in a similar manner as in Example 1, except that the amount of PVA in Example 1 was altered to 6.20 parts by mass, and the duration of reaction was altered to 10 hours. The mass ratio of PVA and PAN in the obtained binder L was 75.5:24.5, the graft rate was 31%, and the mass average molecular weight of PAN homopolymer was 138200.

Comparative Example 4

(41) The amount of vinyl acetate stocked during polyvinyl acetate preparation in Example 1 was altered to 630 parts by mass, the duration of the reaction was altered to 5 hours, the duration of saponification was altered to 0.5 hour, and the rest of the conditions were carried out in a similar manner as in Example 1 to prepare PVA. The obtained PVA had an average degree of polymerization of 1710 and a saponification degree of 54.9 mol %. Preparation of binder M was carried out in a similar manner as in Example 1, except that the amount of PVA in Example 1 was altered to 6.5 parts by mass. The mass ratio of PVA and PAN in the obtained binder M was 41.8:58.2, the graft rate was 135%, and the mass average molecular weight of PAN homopolymer was 128800.

Comparative Example 5

(42) The amount of vinyl acetate stocked during polyvinyl acetate preparation in Example 1 was altered to 950 parts by mass, and the rest of the conditions were carried out in a similar manner as in Example 1 to prepare PVA. The obtained PVA had an average degree of polymerization of 2940 and a saponification degree of 88.1 mol %.

(43) Preparation of binder N was carried out in a similar manner as in Example 1, except that the amount of PVA in Example 1 was altered to 0.80 parts by mass. The mass ratio of PVA and PAN in the obtained binder N was 7.6:92.4, the graft rate was 1179%, and the mass average molecular weight of PAN homopolymer was 175300.

Comparative Example 6

(44) The amount of vinyl acetate stocked during polyvinyl acetate preparation in Example 1 was altered to 1000 parts by mass, the amount of bis (4-t-butylcyclohexyl) peroxydicarbonate was altered to 0.15 parts by mass, the duration of the reaction was altered to 6 hours, the duration of saponification was altered to 2.5 hours, and the rest of the conditions were carried out in a similar manner as in Example 1 to prepare PVA. The obtained PVA had an average degree of polymerization of 3380 and a saponification degree of 95.1 mol %.

(45) Preparation of binder 0 was carried out in a similar manner as in Example 1, except that the amount of PVA in Example 1 was altered to 12.00 parts by mass. The mass ratio of PVA and PAN in the obtained binder P was 92.5:7.5, the graft rate was 8%, and the mass average molecular weight of PAN homopolymer was 186100. Attempts were made to prepare slurry for electrode by using the binder O. Aggregation of conductive material due to insoluble binder component was observed, and it was difficult to apply the slurry for electrode.

Comparative Example 7

(46) The amount of vinyl acetate stocked during polyvinyl acetate preparation in Example 1 was altered to 1100 parts by mass, the amount of bis (4-t-butylcyclohexyl) peroxydicarbonate was altered to 0.15 parts by mass, the duration of the reaction was altered to 12 hours, the duration of saponification was altered to 0.5 hours, and the rest of the conditions were carried out in a similar manner as in Example 1 to prepare PVA. The obtained PVA had an average degree of polymerization of 3510 and a saponification degree of 75.4 mol %. Preparation of binder P was carried out in a similar manner as in Example 1, except that the amount of PVA in Example 1 was altered to 13.00 parts by mass. The mass ratio of PVA and PAN in the obtained binder P was 94.5:5.5, the graft rate was 6%, and the mass average molecular weight of PAN homopolymer was 191200. Attempts were made to prepare slurry for electrode by using the binder P. Aggregation of conductive material due to insoluble binder component was observed, and it was difficult to apply the slurry for electrode.

Comparative Example 8

(47) The amount of vinyl acetate stocked during polyvinyl acetate preparation in Example 1 was altered to 930 parts by mass, the amount of bis (4-t-butylcyclohexyl) peroxydicarbonate was altered to 0.15 parts by mass, the duration of the reaction was altered to 12 hours, the duration of saponification was altered to 2.5 hours, and the rest of the conditions were carried out in a similar manner as in Example 1 to prepare PVA. The obtained PVA had an average degree of polymerization of 2980 and a saponification degree of 95.0 mol %. Preparation of binder Q was carried out in a similar manner as in Example 1, except that the amount of PVA in Example 1 was altered to 13.00 parts by mass. The mass ratio of PVA and PAN in the obtained binder Q was 95.0:5.0, the graft rate was 5%, and the mass average molecular weight of PAN homopolymer was 189100.

Comparative Example 9

(48) Polyvinylidene fluoride (KF Polymer (registered trademark) #1120 manufactured by KUREHA CORPORATION) was used as binder R.

(49) TABLE-US-00002 TABLE 2 Comparative Comparative Comparative Comparative Comparative Example Example Example Example Example 1 2 3 4 5 Binder J K L M N Polymerization 250 3620 630 1710 2490 degree of PVA Saponification degree 87.8 93.2 65.1 54.9 88.1 of PVA (mol %) Mass PVA 55.5 62.6 75.5 41.8 7.6 ratio PAN 44.5 37.4 24.5 58.2 92.4 (%) polyvinylidene fluoride Graft rate (%) 78 58 31 135 1179 Mass average 99800 212500 138200 128800 175300 molecular weight of PAN homopolymer Reductive degradation 0.023 Difficult 0.042 0.045 0.010 potential (V) to make an electrode Comparative Comparative Comparative Comparative Example Example Example Example 6 7 8 9 Binder O P Q R Polymerization 3380 3510 2980 degree of PVA Saponification degree 95.1 75.4 95.0 of PVA (mol %) Mass PVA 92.5 94.5 95.0 ratio PAN 7.5 5.5 5.0 (%) polyvinylidene 100 fluoride Graft rate (%) 8 6 5 Mass average 186100 191200 189100 molecular weight of PAN homopolymer Reductive degradation Difficult to 0.038 0.040 potential (V) make an electrode

Example 10

(50) Binder A was used to prepare a slurry for positive electrode in accordance with the following procedure. Peel strength of the slurry for positive electrode was measured. Further, negative electrode and lithium ion secondary battery were made using the slurry for negative electrode, and the peel strength, discharge rate characteristics, and cycle characteristics of the electrode were evaluated. Results are shown in Table 3.

(51) TABLE-US-00003 TABLE 3 Example Comparative Example 10 11 12 13 14 15 16 17 18 10 11 12 Slurry Binder Binder A B C D E F G H I J K L composition solution Amount of 7 7 7 7 7 7 7 7 7 7 Difficult 7 for negative binder to make electrode Negative Amount of 10 10 10 10 10 10 10 10 10 10 an 10 electrode silicon electrode active powder material Amount of 80 80 80 80 80 80 80 80 80 80 80 artificial graphite Conductive Amount of 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 assistant acetylene black Amount 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 of carbon nano fiber Evaluation Peel strength (mN/mm) 190 210 260 220 215 160 255 230 155 120 185 High rate discharge 89 88 91 85 85 90 73 73 92 80 68 capacity retention rate (%) Cycle capacity 85 86 90 86 83 90 70 69 91 77 65 retention rate (%) Comparative Example 13 14 15 16 17 18 Slurry Binder Binder M N O P Q R composition solution Amount of 7 7 Difficult Difficult 7 7 for negative binder to make to make electrode Negative Amount of 10 10 an an 10 10 electrode silicon electrode electrode active powder material Amount of 80 80 80 80 artificial graphite Conductive Amount of 1.5 1.5 1.5 1.5 assistant acetylene black Amount 1.5 1.5 1.5 1.5 of carbon nano fiber Evaluation Peel strength (mN/mm) 220 120 230 150 High rate discharge 64 81 66 79 capacity retention rate (%) Cycle capacity 63 75 62 70 retention rate (%) The amount of binder solution, negative electrode active material, and conductive assistant is in terms of solids in a slurry for negative electrode. The unit is mass %.
<Preparation of Slurry for Negative Electrode>

(52) Binder A obtained (8 parts by mass) was dissolved in N-methylpyrrolidone (92 parts by mass, hereinafter abbreviated as NMP) to give a binder solution.

(53) Further, acetylene black (1.5 parts by mass by solids in slurry for negative electrode, Denka Black (registered trademark) HS-100 manufactured by Denka Company Limited) as a conductive assistant, NMP dispersion of carbon nanofiber Flotube 9000 (1.5 parts by mass by solids in slurry for negative electrode, manufactured by CNano Technology, Ltd.) as a fibrous carbon as a conductive assistant, and the binder solution (7 parts by mass by solids in slurry for negative electrode) were mixed by agitation. After mixing, artificial graphite (80 parts by mass by solids in slurry for negative electrode, KS-6 (name of product) manufactured by TIMCAL Ltd.) as a negative electrode active material and silicon powder (10 parts by mass by solids in slurry for negative electrode, 350 mesh, purity: 99.9%, manufactured by Nilaco Corporation) as a negative electrode active material were mixed by agitation to obtain the slurry for negative electrode. The slurry for negative electrode mentioned here comprises a binder solution, a negative electrode active material, and a conductive assistant.

(54) <Binding Property (Peel Strength)>

(55) The obtained slurry for negative electrode was applied on a copper foil so that the film thickness after drying would be 100 m, followed by preliminary drying at 80 C. for 10 minutes. Subsequently, the slurry was dried at 105 C. for 1 hour to give a negative electrode plate.

(56) The obtained negative electrode plate was pressed with a roll press machine at a linear pressure of 0.2 to 3.0 ton/cm, and adjustment was made so that the average thickness of the negative electrode plate would be 90 m. The obtained negative electrode plate was cut into a width of 1.5 cm. The surface of the negative electrode active material (the surface on the side coated with the slurry for negative electrode) of the negative electrode plate was attached to a stainless steel plate using a double-sided tape. Subsequently, an adhesive tape was attached to the copper foil on the surface (the surface opposite to the side coated with the slurry for negative electrode) of the negative electrode to prepare a test piece. The stress when the adhesive tape attached to the copper foil was peeled off at 23 C. and relative humidity of 50%, with a peeling direction of 180 and a peeling speed of 50 mm/min was measured. This measurement was repeated 5 times to obtain an average value, and was taken as peel strength.

(57) <Preparation of Negative Electrode>

(58) The prepared slurry for negative electrode was applied on both sides of the 10 m thick copper foil using an automatic coating machine by an amount of 70 g/m.sup.2, followed by preliminarily drying at 80 C. for 10 minutes. Subsequently, pressing was performed with a roll press machine at a linear pressure of 0.2 to 3 ton/cm, and the thickness of the negative electrode plates were adjusted to 90 m including both sides. Further, the negative electrode plate was cut into a width of 55 mm to prepare a rectangular negative electrode plate. Nickel current collector tab was ultrasonically welded to the end portion of the negative electrode plate, followed by drying at 105 C. for 1 hour in order to completely remove volatile components such as residual solvent and adsorbed moisture, thereby giving a negative electrode.

(59) <Preparation of Positive Electrode>

(60) Li(Ni.sub.1/3Mn.sub.1/3Co.sub.1/3)O.sub.2 (87.42 parts by mass, CELLSEED (registered trademark) 111, manufactured by NIPPON CHEMICAL INDUSTRIAL CO., LTD.) as a positive electrode active material, polyvinylidene fluoride (7 parts by mass by solids, KF Polymer (registered trademark) #1120, manufactured by KUREHA CORPORATION) as a binder, acetylene black (3.72 parts by mass, Denka Black (registered trademark) HS-100 manufactured by Denka Company Limited), NMP dispersion of carbon nanofiber Flotube 9000 (1.86 parts by mass by solids, manufactured by CNano Technology, Ltd.) as a fibrous carbon, and NMP (appropriate amount so that the total solids would be 50 mass %) were added and mixed by agitation, thereby giving a slurry for positive electrode.

(61) Onto both sides of an aluminum foil having a thickness of 20 m, the slurry for positive electrode thus obtained was applied using an automatic coating machine by an amount of 140 g/m.sup.2, followed by preliminarily drying at 80 C. for 10 minutes. Subsequently, pressing was performed with a roll press machine at a linear pressure of 0.2 to 3 ton/cm, and the thickness of the positive electrode plates were adjusted to have 148 m including both sides. Further, the positive electrode plate was cut into a width of 54 mm to prepare a rectangular positive electrode plate. Aluminum current collector tab was ultrasonically welded to the end portion of the positive electrode plate, followed by drying at 105 C. for 1 hour in order to completely remove volatile components such as residual solvent and adsorbed moisture, thereby giving a positive electrode.

(62) <Preparation of Battery>

(63) The positive electrode and the negative electrode thus obtained were separated through a polyethylene microporous membrane separator having a thickness of 25 m and a width of 60 mm and wound together to provide a spiral wound body. The wound body was then inserted into a battery can. Subsequently, a nonaqueous electrolytic solution (5 ml, ethylene carbonate/methylethyl carbonate=30/70 (mass ratio) mixture) in which LiPF.sub.6 as an electrolyte was dissolved to have 1 mol/L concentration was injected into a battery container. Then the inlet was caulked and sealed, thereby obtaining a cylindrical lithium secondary battery having a diameter of 18 mm and a height of 65 mm. Performance of the lithium ion secondary battery thus obtained was evaluated in accordance with the procedure described below.

(64) <Discharge Rate Characteristics (High Rate Discharge Capacity Retention Rate)>

(65) The lithium ion secondary battery prepared was subjected to charging with constant current constant voltage charging profile (limited to 4.29 V and 0.2 ItA) at 25 C., and then the battery was discharged at a constant current of 0.2 ItA to 2.69 V. Subsequently, the discharge current was changed to 0.2 ItA and to 1 ItA, and the discharge capacity with respect to each discharge current was measured. The recovery charge in each measurement was conducted with constant current constant voltage charging profile (4.29 V (1 ItA cut)). The high rate discharge capacity retention rate was calculated as the rate of capacity measured in 1 ItA discharge to the capacity measured in the second 0.2 ItA discharge.

(66) <Cycle Characteristics (Cycle Capacity Retention Rate)>

(67) At an environmental temperature of 25 C., charging with constant current constant voltage charging profile (charging voltage of 4.29 V, 1 ItA) and discharging with constant current (discharge terminating voltage of 2.69 V, 1 ItA) were performed. Cycles of charging and discharging were repeated, and the rate of the discharge capacity measured in the 500th cycle to the discharge capacity measured in the 1st cycle was calculated as the cycle capacity retention rate.

Examples 11 to 18

(68) Binder A in Example 10 was altered to the binders shown in Table 3. The rest of the procedures were carried out in a similar manner as in Example 10, to conduct each of the evaluations. The results are shown in Table 3.

Comparative Examples 10 to 18

(69) A slurry for negative electrode, a negative electrode, a positive electrode, and a lithium ion secondary battery were prepared according to the method shown in Example 10, altering to the binders shown in Table 3. Subsequently, various evaluations were carried out. The results are shown in Table 3.

(70) From the results shown in Tables 1 and 2, the binder composition in the scope of the present invention showed superior reduction resistance. In addition, from the results shown in Table 3, binding property (peel strength) between the layer of negative electrode active material and the current collector was shown to be superior. Further, the lithium ion secondary battery manufactured using the binder composition in the scope of the present invention showed superior cycle characteristics and discharge rate characteristics.