Patent Publication Number: US-2023138888-A1

Title: Binder for secondary battery

Description:
TECHNICAL FIELD 
     The present invention relates to a binder for a secondary battery, a mixture for a secondary battery electrode, an electrode for a secondary battery, and a secondary battery. 
     BACKGROUND ART 
     In recent years, with a growing interest in solving environmental issues, and realizing a sustainable recycling-based society, research on nonaqueous electrolyte secondary batteries, such as lithium-ion secondary batteries, has been actively made. Lithium-ion secondary batteries are used as power sources for notebook computers, mobile phones, electric vehicles, and the like, because of their high operating voltage and high energy density. In these applications, the lithium-ion secondary batteries need to be repeatedly charged and discharged, and reused, and thus, are required to have a longer battery life. 
     An electrode for a lithium-ion secondary battery is typically produced as follows: A mixture slurry for a battery electrode (hereinafter sometimes simply referred to as “the slurry”), obtained by mixing an active material (electrode active material), a conductive assistant, and the like into a solution or a slurry in which a binder for a battery electrode is dissolved in a solvent or dispersed in a dispersion medium, is applied to a current collector. Then, the solvent or the dispersion medium is removed using a method such as drying to bind between the active material and the current collector, and between particles of the active material. 
     For example, a positive electrode is obtained by coating an aluminum foil current collector with a positive electrode mixture slurry in which an active material (such as lithium cobaltate (LiCoO 2 )), a binder (such as polyvinylidene fluoride (PVDF)), a conductive assistant (such as carbon black), and the like are dispersed in a dispersion medium, and by drying the slurry. 
     A negative electrode is obtained by coating a copper foil current collector with a negative electrode mixture slurry in which an active material (such as graphite), a binder (such as carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), PVDF, or polyimide), a conductive assistant (such as carbon black), and the like are dispersed in water or an organic solvent, and by drying the slurry. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: JP-A-7-240201 
         Patent Literature 2: JP-A-10-294112 
         Patent Literature 3: WO 2004/049475 
         Patent Literature 4: JP-A-10-302799 
       
    
     NON PATENT LITERATURE 
     
         
         Non Patent Literature 1: “Lithium Secondary Batteries”, p, 132 (published by Ohmsha Ltd. on Mar. 20, 2008) 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     With increasing use of lithium-ion secondary batteries, the use of various types of carbon materials (such as graphite) as negative electrode active materials that directly contribute to the electrode reaction has been studied, mainly for the purpose of increasing the battery capacity. 
     For the purpose of further increasing the capacity of lithium-ion secondary batteries, various compounds have been proposed as electrode active materials that directly contribute to the electrode reaction. Silicon (Si), tin (Sn), and germanium (Ge) that can be alloyed with lithium, or oxides and alloys thereof, for example, have been studied as negative electrode active materials. These negative electrode active materials have higher theoretical capacity density than that of carbon materials. In particular, silicon-containing particles, such as silicon particles or silicon oxide particles, are inexpensive, and thus have been widely studied (see Patent Literatures 1 and 2 and Non Patent Literature 1). 
     However, it is known that when silicon-containing particles, such as silicon particles or silicon oxide particles, are used as a negative electrode active material, the volume of the negative electrode active material varies significantly because of the intercalation and deintercalation reactions of lithium ions during charge/discharge, and thus, the negative electrode mixture is peeled from the negative electrode current collector, and the negative electrode active material is easily removed. 
     Polyvinylidene fluoride (PVDF), which has heretofore been used as a binder, needs to be used in large amounts because of its low binding force and flexibility. Furthermore, because PVDF is soluble only in an organic solvent, there has been a need for a binder that can reduce the environmental burden (see Patent Literatures 3 and 4). 
     Study has been made on the use of a rubbery polymer, styrene-butadiene rubber (SBR), as an aqueous binder that is expected to provide the effect of reducing the environmental burden without reducing the binding force. However, SBR has a problem of insufficient binding force when an active material with high expansion and shrinkage, such as a negative electrode formed using silicon-containing particles, is used. 
     Under such circumstances, it is a main object of the present invention to provide a binder for a secondary battery having excellent binding force. It is also an object of the present invention to provide a mixture for a secondary battery electrode, an electrode for a secondary battery, and a secondary battery, each obtained using the binder for a secondary battery. 
     Solution to Problem 
     The present inventors have conducted extensive research to solve the above-described problem. As a result, they have found that a binder for a secondary battery comprising a polymer compound, wherein the polymer compound contains an acrylic repeating unit, and a 3% by mass aqueous solution of the polymer compound has a yellowness index of not more than a predetermined value, exhibits excellent binding force. The present invention has been completed after further research based on this finding. 
     In summary, the present invention provides aspects of the invention comprising the following features: 
     Item 1. A binder for a secondary battery comprising a polymer compound, 
     wherein the polymer compound contains an acrylic repeating unit, and 
     a 3% by mass aqueous solution of the polymer compound has a yellowness index of 14 or less. 
     Item 2. The binder for a secondary battery according to item 1, wherein the polymer compound contains a repeating unit represented by formula (1): 
     
       
         
         
             
             
         
       
     
     wherein each R 1  is independently a hydrogen atom or a methyl group, and each R 2  is independently an NH 2  group, an OH group, an ONa group, or an OLi group. 
     Item 3. A mixture for a secondary battery electrode comprising the binder for a secondary battery according to item 1 or 2, and an active material. 
     Item 4. The mixture for a secondary battery electrode according to item 3, wherein the active material contains a carbon material. 
     Item 5. The mixture for a secondary battery electrode according to item 3 or 4, wherein the active material contains at least one of silicon and silicon oxide. 
     Item 6. An electrode for a secondary battery comprising the mixture for a secondary battery electrode according to any one of items 3 to 5. 
     Item 7. A secondary battery comprising the electrode for a secondary battery according to item 6. 
     Item 8. A lithium-ion secondary battery comprising the electrode for a secondary battery according to item 6. 
     Advantageous Effects of Invention 
     The present invention can provide a binder for a secondary battery having excellent binding force. The present invention also aims to provide a mixture for a secondary battery electrode, an electrode for a secondary battery, and a secondary battery (such as a lithium-ion secondary battery), each obtained using the binder for a secondary battery. 
    
    
     DESCRIPTION OF EMBODIMENTS 
     A binder for a secondary battery according to the present invention is a binder for a secondary battery comprising a polymer compound, wherein the polymer compound contains an acrylic repeating unit, and a 3% by mass aqueous solution of the polymer compound has a yellowness index of 14 or less. Because of these features, the binder for a secondary battery of the present invention (hereinafter sometimes referred to as “the binder”) exhibits excellent binding properties. The following describes in detail the binder for a secondary battery of the present invention, and a mixture for a secondary battery electrode, an electrode for a secondary battery, and a secondary battery (such as a lithium-ion secondary battery) obtained using the binder for a secondary battery. 
     As used herein, the term “comprising” includes “consisting essentially or” and “consisting of”. As used herein, the term “(meth)acrylic” refers to “acrylic or methacrylic”, and the term “(meth)acrylate” refers to “acrylate or methacrylate”, 
     As used herein, values connected with “to” refer to the numerical range including the values before and after “to” as the lower and upper limits. When a plurality of lower limits and a plurality of upper limits are mentioned separately, any lower limit and any upper limit may be selected and connected with “to”. 
     1. Binder for Secondary Battery 
     The binder for a secondary battery of the present invention is a binder for a secondary battery comprising a polymer compound. The polymer compound contains an acrylic repeating unit. A 3% by mass aqueous solution of the polymer compound has a yellowness index of 14 or less. The binder for a secondary battery of the present invention exhibits excellent binding properties because the polymer compound satisfies these features. The relationship between yellowness index and binding force is believed to be as follows: The presence of impurities as a coloring component between the binder and the substrate inhibits binding of the binder. When a polymer compound containing an acrylic repeating unit is obtained by copolymerizing the monomers for forming the repeating units, the resulting polymer compound tends to become colored due to unreacted monomers. While the ratio of unreacted monomers is desirably as low as possible, the unreacted monomers are difficult to eliminate completely, and affect the magnitude of the yellowness index. The degree of the coloration varies depending on the amount or type of the unreacted monomers, and the yellowness index also varies accordingly. The yellowness index also varies depending on the saponification conditions (such as the saponification temperature and the mixing speed of the components) during the production of the polymer compound. Furthermore, the addition of hydrazine or the like that reacts with the unreacted monomers in the production process also affects the yellowness index, and can adjust the magnitude of the yellowness index. Thus, in the binder for a secondary battery of the present invention, the yellowness index of the 3% by mass aqueous solution of the polymer compound can be easily adjusted to 14 or less, by lowering the saponification temperature, or mixing the components at once for saponification, or using hydrazine or the like. 
     The yellowness index is the YI value as measured using a spectrophotometer. The measurement method is specifically as follows: 0.6 g of the polymer compound is measured out first, 19.4 g of water is added thereto, and the polymer compound is dissolved by heating at 80° C. for 3 hours to obtain a 3% by mass aqueous solution of the copolymer. The obtained 3% by mass aqueous solution is used as a measurement sample, and the yellowness index (YI value) is measured using a spectrophotometer. In the measurement, the measurement sample is heated in a water bath at 93° C., and taken into a quartz cell (optical path length: 10 mm) immediately after the inside temperature reaches 90° C. The YI value is measured according to JIS K 7373-2006, as follows: Using a spectrophotometer (for example, UH-4150 (Hitachi, Ltd.)), transmittances are measured by spectrophotometry at 1 nm intervals in the range of 250 nm or more and 800 nm or less, using supplementary illuminant C and a 2-degree field of view. Based on the measured transmittances, tristimulus values X, Y, and Z in the XYZ color system are obtained, and the YI value is calculated from the X, Y, and Z values using the following equation: 
       yellowness index (YI)=100(1,2769 X− 1.0592 Z )/ Y    
     In the binder for a secondary battery of the present invention, the yellowness index of the 3% by mass aqueous solution of the polymer compound may be any value of 14 or less; however, from the viewpoint of further increasing the binding force, it is preferably 12 or less, more preferably 8 or less, and still more preferably 6 or less. The lower limit of the yellowness index of the 3% by mass aqueous solution of the polymer compound is 0. 
     The acrylic repeating unit is, for example, a repeating unit represented by formula (1): 
     
       
         
         
             
             
         
       
     
     In formula (1), each R 1  is independently a hydrogen atom or a methyl group, and each R 2  is independently an NH 2  group, an OH group, an ONa group, or an OLi group. The polymer compound may contain at least one of the repeating unit in which R 2  is an NH 2 , group, the repeating unit in which R 2  is an OH group, the repeating unit in which R 2  is an ONa group, and the repeating unit in which R 2  is an OLi group. That is, the repeating unit of formula (1) contained in the polymer compound may be only the repeating unit in which R 2  is an NH 2  group, or only the repeating unit in which R 2  is an OH group, or only the repeating unit in which R 2  is an ONa group, or only the repeating unit in which R 2  is an OLi group. Alternatively, the polymer compound may contain the repeating unit of formula (1) in which R 2  is an ONa group and the repeating unit of formula (1) in which R 2  is an OH group; or may contain the repeating unit of formula (1) in which R 2  is an OLi group and the repeating unit of formula (1) in which R 2  is an OH group; or may contain the repeating unit of formula (1) in which R 2  is an NH 2  group and the repeating unit of formula (1) in which R 2  is an OH group; or may contain the repeating unit of formula (1) in which R 2  is an NH 2  group, the repeating unit of formula (1) in which R 2  is an ON group, and the repeating unit of formula (1) in which R 2  is an ONa group; or may contain the repeating unit of formula (1) in which R 2  is an NH 2  group, the repeating unit of formula (1) in which R 2  is an OH group, the repeating unit of formula (1) in which R 2  is an ONa group, and the repeating unit of formula (1) in which R 2  is an OLi group; or may contain the repeating unit of formula (1) in which R 2  is an OH group, the repeating unit of formula (1) in which R 2  is an OLi group, and the repeating unit of formula (1) in which R 2  is an NH 2  group. 
     In the polymer compound, the ratio of the repeating unit represented by formula (1) as an acrylic repeating unit, for example, is not limited. From the viewpoint of further increasing the binding force of the binder for a secondary battery of the present invention, when the total ratio of repeating units constituting the polymer compound is taken as 100 mol %, the total ratio of the repeating unit represented by formula (1) is preferably 5 to 95 mol %, more preferably 10 to 90 mol %, and still more preferably 10 to 80 mol %. 
     The polymer compound preferably contains a vinyl alcohol repeating unit. Specifically, the polymer compound more preferably contains a repeating unit represented by formula (2): 
     
       
         
         
             
             
         
       
     
     When the polymer compound contains the repeating unit represented by formula (2), the total ratio thereof is not limited. From the viewpoint of further increasing the binding force of the binder for a secondary battery of the present invention, when the total ratio of repeating units constituting the polymer compound is taken as 100 mol %, the total ratio of the repeating unit represented by formula (2) is preferably 5 to 95 mol %, more preferably 20 to 90 mol %, and still more preferably 50 to 90 mol %. 
     When the polymer compound contains an acrylic repeating unit (for example, the repeating unit represented by formula (1)) and a vinyl alcohol repeating unit (for example, the repeating unit represented by formula (2)), these repeating units may be arranged either randomly or as blocks, preferably randomly from the viewpoint of further increasing the binding force. 
     When the polymer compound contains an acrylic repeating unit (for example, the repeating unit represented by formula (1)) and a vinyl alcohol repeating unit (for example, the repeating unit represented by formula (2)), the molar ratio of the monomer forming the repeating unit represented by formula (1) to the monomer forming the repeating unit represented by formula (2) is preferably 5/95 to 95/5, more preferably 10/90 to 70/30, and still more preferably 15/85 to 65/35. If the molar ratio of the monomer forming the repeating unit represented by formula (1) is less than 5/95, the polymer compound has insufficient binding force when used as binder; whereas if the molar ratio is above 95/5, the polymer compound may become brittle when used as a binder, which is undesirable. 
     When the total ratio of repeating units constituting the polymer compound is taken as 100 mol %, the ratio of the sum of the total ratio of the acrylic repeating unit and the total ratio of the vinyl alcohol repeating unit in the polymer compound is preferably 80 mot % or more, more preferably 90 mol % or more, still more preferably 95 mol % or more, particularly preferably 95 mol % or more, and may even be 100 mol % (that is, the repeating units constituting the polymer compound may contain only the acrylic repeating unit and the vinyl alcohol repeating unit), from the viewpoint of favorably increasing the binding force. 
     The repeating units constituting the polymer compound may contain another repeating unit. Such another repeating unit may be a repeating unit formed by a monomer copolymerizable with the acrylic repeating unit. Examples of such copolymerizable monomers include monomers with ethylenically unsaturated bonds. Specific examples of monomers with ethylenically unsaturated bonds include styrene, vinyl chloride, ethylene, vinylsulfonic acid, and maleic acid. 
     As described above, the yellowness index of the 3% by mass aqueous solution of the polymer compound can be favorably adjusted by adjusting the saponification conditions (such as the saponification temperature and the mixing speed of the components) of the polymer compound. 
     The number average molecular weight of the polymer compound is, for example, about 10,000 to 8,000,000, preferably about 30,000 to 1,000,000, although not limited thereto. The number average molecular weight of the polymer compound is the value as determined based on polyethylene glycol/polyethylene oxide standards by gel permeation chromatography (GPC). 
     The method for producing the polymer compound containing an acrylic repeating unit may be any known method for producing a copolymer, without limitation. One example of known methods for producing copolymers is the method for producing a copolymer of a vinyl alcohol and an alkali metal-neutralized product of ethylenically unsaturated carboxylic acid disclosed in WO 2017/168947. In the production of the copolymer, the saponification conditions (such as the saponification temperature and the mixing speed of the components) may be adjusted, and the polymer compound may be crosslinked with a crosslinking agent, to adjust the yellowness index of the 3% by mass aqueous solution of the polymer compound to 14 or less. When the polymer compound contains another repeating unit, the monomer component of the repeating unit may be mixed during polymerization. 
     In the binder of the present invention, the ratio of the polymer compound is preferably 80% by mass or more, more preferably 90% by mass or more, and still more preferably 95% by mass or more, and may even be 100% by mass (that is, the binder of the present invention may contain only the polymer compound), as long as the binder exhibits excellent binding force. 
     The binder of the present invention may contain another binder material different from the polymer compound. Examples of other binder materials include aqueous binders soluble or dispersible in water. Specific examples of other binder materials include carboxymethylcellulose (CMC), acrylic resin, sodium polyacrylate, sodium alginate, polyimide (PI), polyamide, polyamideimide, polyacryl, styrene-butadiene rubber (SBR), styrene-ethylene-butylene-styrene copolymer (SEBS), polyvinyl alcohol (PVA), and ethylene vinyl acetate copolymer (EVA). These other binder materials may be contained alone or in combinations of two or more, in the binder of the present invention. When another binder material is contained in the binder of the present invention, the content can be adjusted appropriately in the range of 0 to 100 parts by mass per 100 parts by mass of the polymer compound. 
     The binder of the present invention is suitable for use as a binder for a secondary battery, particularly as a binder contained in a mixture for a secondary battery electrode. For a secondary battery electrode, the binder can be applied to both positive and negative electrodes. 
     2. Mixture for Secondary Battery Electrode 
     The mixture for a secondary battery electrode of the present invention (hereinafter sometimes referred to as “the electrode mixture”) comprises the binder for a secondary battery or the present invention and an active material. As described above, the binder of the present invention, which has excellent binding force, is suitable for use as a mixture for a secondary battery electrode, together with the active material. 
     In the electrode mixture of the present invention, the content of the binder of the present invention is preferably 0.5 to 40% by mass, more preferably 1 to 25% by mass, and still more preferably 1.5 to 10% by mass. When the content of the binder of the present invention is 0.5% by mass or more, deterioration of cycle life characteristics due to insufficient binding force, agglomeration due to an insufficient viscosity of the slurry, and the like can be prevented. On the other hand, when the binder content is 40% by mass or less, a high capacity tends to be obtained upon charge/discharge of the battery. 
     The electrode mixture of the present invention can be produced by using the binder of the present invention, using known methods. For example, the electrode mixture can be produced by mixing the active material, the binder of the present invention, water, and optionally a conductive assistant, a dispersion assistant, and the like, to form a pasty slurry. The timing of adding water is not limited. The binder of the present invention may be previously dissolved in water and then mixed with the active material and the like to form a slurry. Alternatively, the active material, the binder of the present invention, and optionally a conductive assistant, a dispersion assistant, and the like may be mixed together in a solid state, and then water may be added to form a pasty slurry. 
     In the electrode mixture of the present invention, the ratio of water is preferably 40 to 2,000 parts by mass, and more preferably 50 to 1,000 parts by mass, per 100 parts by mass of solids in the electrode mixture. When the ratio of water is in the above-defined range, handleability of the electrode mixture (slurry) of the present invention tends to be further improved, 
     [Active Material] 
     The active material is an electrode active material, including a negative electrode active material and a positive electrode active material. When, for example, the active material is a negative electrode active material, it may contain, for example, a carbon material, and may also contain, for example, at least one of silicon and silicon oxide. Specific materials of the negative electrode active material and the positive electrode active material are described below. 
     (Negative Electrode Active Material) 
     Negative electrode active materials used in the art may be used without limitation as the negative electrode active material, for example, carbon materials, such as crystalline carbon or amorphous carbon. Examples of crystalline carbon include graphite such as natural or artificial graphite in an amorphous, plate-like, flake, spherical or fibrous form. Examples of amorphous carbon include soft carbon (graphitizable carbon) or hard carbon (non-graphitizable carbon), mesophase pitch carbide, and calcined coke. Moreover, a material capable of intercalation and deintercalation of a large number of lithium ions, such as silicon (Si), tin (Sn), or Ti (titanium), may also be used as the negative electrode active material. Any such materials, which may be in the form of any of a single material, an alloy, a compound, a solid solution, and a composite active material containing a silicon-containing material, a tin-containing material, and a titanium-containing material, can exhibit the effects of the present invention. The silicon-containing material may be Si, SiOx (0.05&lt;x&lt;1.95), or an alloy, a compound, or a solid solution thereof obtained by partially substituting Si with at least one element selected from the group consisting of B, Mg Ni, Ti, Mo, Co, Ca, Cr, Cu, Fe, Mn, Nb, Ta, V, W, Zn, C, N, and Sn. These materials may be referred to as silicon or silicon oxide. The tin-containing material may be Ni 2 Sn 4 , Mg 2 Sn, SnO x  (0&lt;x&lt;2), SnO 2 , SnSiO 3 , or LiSnO, for example. The titanium-containing material may be a lithium titanate, such as Li 2 TiO 3  or Li 4 Ti 5 O 12 , or a titanium-niobium composite compound, for example. These materials may be used alone or in combinations of two or more. Preferred among these is silicon or silicon oxide, such as Si alone or silicon oxide, 
     More preferred as the negative electrode active material is a composite obtained by mixing first and second negative electrode active materials, using silicon or silicon oxide as the first negative electrode active material, and a carbon material as the second negative electrode active material. In this case, the mixture ratio of the first and second negative electrode active materials is preferably 5/95 to 95/5 in terms of mass ratio. Any carbon materials commonly used in nonaqueous electrolyte secondary batteries may be used as the carbon material, and representative examples include crystalline carbon, amorphous carbon, or a combination thereof. Examples of crystalline carbon include the same as those mentioned above. 
     The method for producing the negative electrode active material is not limited. To produce the active material composite formed of the mixture of the first and second negative electrode active materials, the method is not limited as long as it can homogeneously disperse these active materials. An example of methods for producing the negative electrode active material is a method in which the first and second negative electrode active materials are mixed in a ball mill. Another example is a method in which a precursor of the second negative electrode active material is deposited on the surface of the particles of the first negative electrode active material, and then carbonized by a heat-treatment method. The precursor of the second negative electrode active material may be any carbon precursor that can be formed into a carbon material by heat treatment, and examples include glucose, citric acid, pitch, tar, and binder materials (such as polyvinylidene fluoride, carboxymethylcellulose, acrylic resin, sodium polyacrylate, sodium alginate, polyimide, polytetrafluoroethylene, polyamide, polyamideimide, polyacryl, styrene-butadiene rubber, polyvinyl alcohol, and ethylene-vinyl acetate copolymer). 
     The heat-treatment method is a method in which the carbon precursor is subjected to heat treatment at 600 to 4,000° C. in a non-oxidizing atmosphere (an atmosphere that prevents oxidation, such as a reducing atmosphere, an inert atmosphere, or a reduced pressure atmosphere) and carbonized to have conductivity. 
     (Positive Electrode Active Material) 
     Any positive electrode active materials used in the art may be used without limitation as the positive electrode active material. The positive electrode active material may be a lithium-containing composite oxide, for example. Examples of lithium-containing composite oxides include LiMnO 2 , LiFeO 2 , LiCoO 2 , LiMn 2 O 4 , Li 2 FeSiO 4 , LiNi 1/3 Co 1/3 Mn 1/3 O 2 , LiNi 0.5 Co 0.2 Mn 0.3 O 2 , LiNi 0.6 Co 0.2 Mn 0.2 O 2 , LiNi 0.8 Co 0.1 Mn 0.1 O 2 , LiNi x Co y M z O 2  (wherein 0.01&lt;x&lt;1, 0≤y≤1, 0≤z≤1, x+y+z=1, and M is at least one element selected from the group consisting of Mn, V, Mg, Mo, Nb, Fe, Cu, and Al), and LiFePO 4 . 
     [Conductive Assistant] 
     Any conductive assistants used in the art may be used without limitation as the conductive assistant. While the conductive assistant is not limited as long as it has conductivity, the conductive assistant is preferably carbon powder. Examples of carbon powder include commonly used carbon materials, such as acetylene black (AB), Ketjen black (KB), graphite, carbon fibers, carbon tubes, graphene, amorphous carbon, hard carbon, soft carbon, glassy carbon, carbon nanofibers, and carbon nanotubes. These materials may be used alone or in combinations of two or more. 
     While the ratio of the conductive assistant is not limited, it is preferably 0.1 to 30% by mass, more preferably 0.3 to 10% by mass, and still more preferably 2 to 5% by mass, relative to total 100% by mass of the active material, the binder, and the conductive assistant. If the ratio of the conductive assistant is less than 0.1% by mass, the conductivity of the electrode may not be sufficiently improved. If the ratio of the conductive assistant is above 30% by mass, this is undesirable in that: the ratio of the active material relatively decreases, which makes it difficult to obtain a high capacity upon charge/discharge of the battery; carbon repels water, which makes it difficult to homogeneously disperse the active material, leading to agglomeration of the active material; and the amount of the binder to be used increases because the conductive assistant is smaller in size, and thus, is larger in surface area than the active material. 
     [Dispersion Assistant] 
     The electrode mixture of the present invention may further contain a dispersion assistant. While the dispersion assistant is not limited, it is preferably a humic acid or an organic acid containing a carboxy group and at least one substituent selected from the group consisting of a hydroxyl group, an amino group, and an imino group. Examples of organic acids having a hydroxyl group and a carboxy group include lactic acid, tartaric acid, citric acid, malic acid, glycolic acid, tartronic acid, glucuronic acid, and humic acid. Examples of organic acids having an amino group and a carboxy group include glycine, alanine, phenylalanine, 4-aminobutyric acid, leucine, isoleucine, lysine, glutamic acid, aspartic acid, glutamine, asparagine, histidine, tryptophan, cysteine, and polymers thereof. Examples of organic acids having an imino group and a carboxy group include proline, 3-hydroxyproline, 4-hydroxyproline, and pipecolic acid. Preferred among these are glucuronic acid, humic acid, glycine, polyglycine, aspartic acid, and glutamic acid, because they are readily available. 
     The ratio of the dispersion assistant may be 0.01 part by mass or more, relative to total 100 parts by mass of the active material, the binder, and the conductive assistant, in order to finely disperse the active material and the like efficiently and effectively during the preparation of an active material dispersion. To maintain the fine dispersibility and dispersion stability, a sufficient amount of the dispersion assistant to be added is 5.0 parts by mass or less. 
     The electrode mixture of the present invention may contain other conventional additives and the like. 
     In the electrode mixture of the present invention, the binder of the present invention is used for the purpose of bonding particles of the active material, bonding the active material and the conductive assistant, and bonding the active material or the conductive assistant and a current collector. That is, the binder of the present invention is used to form a satisfactory active material layer when the slurry is applied onto the current collectors of both electrodes, and dried. 
     3. Electrode for Secondary Battery 
     The electrode for a secondary battery of the present invention (hereinafter sometimes referred to as “the electrode”) comprises the above-described mixture for a secondary battery electrode of the present invention. The electrode of the present invention is produced by using the mixture for a secondary battery electrode of the present invention (i.e., using the binder of the present invention), according to methods employed in the art. That is, the electrode of the present invention can be produced by, for example, applying the electrode mixture of the present invention onto a current collector, and drying. 
     When the electrode of the present invention is a negative electrode, the material constituting the current collector may be, for example, a conductive material such as C. Cu, Ni, Fe, V, Nb, Ti, Cr, Mo, Ru, Rh, Ta, W, Os, Ir, Pt, Au, or Al, or an alloy containing two or more of these conductive materials (such as stainless steel) Alternatively, the current collector may be Fe plated with Cu. The material constituting the current collector of the negative electrode is preferably Cu, Ni, or stainless steel, for example, in that they have high electrical conductivity, and have excellent oxidation resistance and stability in an electrolytic solution. Cu or Ni is preferred in terms of material cost. 
     When the electrode of the present invention is a positive electrode, the material constituting the current collector may be, for example, a conductive material such as C, Ti, Cr. Mo, Ru, Rh, Ta, W, Os, Ir, Pt, Au, or Al, or an alloy containing two or more of these conductive materials (such as stainless steel). The material constituting the current collector of the positive electrode is preferably C, Al, or stainless steel, for example, in that they have high electrical conductivity, and have excellent oxidation resistance and stability in an electrolytic solution. Al is preferred in terms of material cost. 
     The shape of the current collector may be, for example, a foil-like substrate or a three-dimensional substrate, although not limited thereto. The use of a three-dimensional substrate (such as a metal foam, a mesh, a woven fabric, a nonwoven fabric, or an expanded metal) provides an electrode having a high capacity density, even if the binder has poor adhesion to the current collector. Additionally, satisfactory high-rate charge/discharge characteristics are achieved. 
     4. Secondary Battery 
     The secondary battery of the present invention comprises the above-described electrode for a secondary battery of the present invention. The secondary battery of the present invention may comprise the electrode for a secondary battery of the present invention as either one of or both a positive electrode and a negative electrode. The secondary battery of the present invention is produced by using the electrode for a secondary battery of the present invention (i.e., using the binder of the present invention), according to methods employed in the art. 
     The secondary battery of the present invention is preferably a nonaqueous electrolyte secondary battery, particularly a lithium-ion secondary battery. Because the lithium-ion secondary battery must contain lithium ions, the electrolyte is preferably a lithium salt. Examples of the lithium salt include lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium trifluoromethanesulfonate, and lithium trifluoromethanesulfonimide. These electrolytes may be used alone or in combinations of two or more. 
     Examples of electrolytic solutions include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, and γ-butyrolactone. These electrolytic solutions may be used alone or in combinations of two or more. Particularly preferred is propylene carbonate alone, a mixture of ethylene carbonate and diethyl carbonate, or γ-butyrolactone alone. In the mixture of ethylene carbonate and diethyl carbonate, the mixture ratio can be adjusted as desired such that the ratio of one component falls within the range of 10 to 90% by volume. 
     Known secondary battery structures can be similarly employed for other secondary batteries. 
     EXAMPLES 
     The present invention will be hereinafter described in detail with reference to examples and comparative examples, although the present invention is not limited to the examples. 
     [Synthesis of Binder for Secondary Battery] 
     Production Example 1 
     In a reaction vessel equipped with a stirrer, a thermometer, a N 2  gas inlet tube, a reflux condenser, and a dropping funnel, 768 parts by mass of water and 12 parts by mass of anhydrous sodium sulfate were placed, and N 2  gas was blown into the reaction vessel to deoxidize the system. Subsequently, 1 part by mass of partially saponified polyvinyl alcohol (degree of saponification; 88%) and 1 part by mass of lauryl peroxide were placed in the reaction vessel, the inside temperature was increased to 60° C., and then 51.8 parts by mass of methyl acrylate and 208 parts by mass of vinyl acetate were added dropwise through the dropping funnel over 4 hours. Then, the inside temperature was maintained at 65° C. for 2 hours. Then, the solids were filtered off. In the same reaction vessel as above, 450 parts by mass of methanol, 420 parts by mass of water, 132 parts by mass of sodium hydroxide, and 0.52 part by mass of hydrazine were placed, and the solids divided in 10 portions were added in divided portions over 10 hours in an environment at 30° C. After the addition of all of the solids, the components were mixed with stirring at 30° C. for 3 hours. After completion of stirring, the solid was filtered off and then washed with methanol, and dried at 70° C. for 8 hours to obtain a vinyl alcohol/acrylic acid ester copolymer (binder for a secondary battery).  1 H-NMR (BRUKER) measurement of the obtained copolymer under the following conditions confirmed that the copolymer contains structures derived from the repeating units represented by formulae (1) and (2), 
     ( 1 H-NMR Measurement) 
     2 g of the obtained copolymer was measured out, 33 g of heavy water was added thereto, and the copolymer was dissolved by heating at 90° C. for 3 hours. The obtained heavy water solution was subjected to NMR measurement under the following conditions. 
     Apparatus: AVANCE IIIHD 400 (AVANCE III 400), manufactured by Bruker 
     Measurement method.  1 H-NMR 
     Measurement nucleus frequency: bf1 (400.1300000) 
     Spectral width: Sw (20.5524) 
     Observation point: td (65536) 
     Data point: Si (65536) 
     Measurement temperature: 27° C. 
     (Measurement of Yellowness Index) 
     0.6 g of the obtained copolymer was measured out, 19.4 g of water was added thereto, and the copolymer was dissolved by heating at 80° C. for 3 hours to obtain a 3% by mass aqueous solution of the copolymer. The obtained 3% by mass aqueous solution was used as a measurement sample, and the yellowness index (YI value) was measured using a spectrophotometer. The measurement sample was heated in a water bath at 93° C., and taken into a quartz cell (optical path length: 10 mm) immediately after the inside temperature reached 90° C. The YI value was measured according to JIS K 7373-2006, as follows: Using the spectrophotometer UH-4150 (Hitachi, Ltd.), transmittances were measured by spectrophotometry at 1 nm intervals in the range of 250 nm or more and 800 nm or less, using supplementary illuminant C and a 2-degree field of view. Based on the measured transmittances, tristimulus values X, Y, and Z in the XYZ color system were obtained, and the YI value was calculated from the X, Y, and Z values using the equation shown below. The result is shown in Table 1. 
       yellowness index (YI)=100(1.2769 X− 1.0592 Z )/ Y    
     Production Example 2 
     A vinyl alcoholsacrylic acid ester copolymer (binder for a secondary battery) was obtained as in Production Example 1, except that the solids, 450 parts by mass of methanol, 420 pans by mass of water, 132 parts by mass of sodium hydroxide, and 0.52 part by mass of hydrazine were mixed at once while being maintained at 30° C., and stirred at the same temperature for 3 hours.  1 H-NMR (BRUKER) measurement of the obtained copolymer in the same manner as above confirmed that the copolymer contains structures derived from the repeating units represented by formulae (1) and (2). The yellowness index of a 3% by mass aqueous solution of the obtained copolymer was also measured as in Production Example 1. The result is shown in Table 1. 
     Production Example 3 
     A vinyl alcohol/acrylic acid ester copolymer (binder for a secondary battery) was obtained as in Production Example 1, except that 1 part by mass of lauryl peroxide was replaced by 0.5 part by mass of dimethyl 2,2-azobis(2-methyl propionate).  1 H-NMR (BRUKER) measurement of the obtained copolymer in the same manner as above confirmed that the copolymer contains structures derived from the repeating units represented by formulae (1) and (2). The yellowness index of a 3% by mass aqueous solution of the obtained copolymer was also measured as in Production Example 1. The result is shown in Table 1. 
     Production Example 4 
     A vinyl alcoholtacrylic acid ester copolymer (binder for a secondary battery) was obtained as in Production Example 1, except that the solids were mixed with stirring over 10 hours while being maintained at 40° C.  1 H-NMR (BRUKER) measurement of the obtained copolymer in the same manner as above confirmed the presence of the repeating units represented by formulae (1) and (2). The yellowness index of a 3% by mass aqueous solution of the obtained copolymer was also measured as in Production Example 1. The result is shown in Table 1. 
     Production Example 5 
     A vinyl alcohol/acrylic acid ester copolymer (binder for a secondary battery) was obtained as in Production Example 1, except that the solids were mixed with stirring over 10 hours while being maintained at 50° C.  1 H-NMR (BRUKER) measurement of the obtained copolymer in the same manner as above confirmed that the copolymer contains structures derived from the repeating units represented by formulae (1) and (2). The yellowness index of a 3% by mass aqueous solution of the obtained copolymer was also measured as in Production Example 1. The result is shown in Table 1. 
     [Production of Mixture for Secondary Battery Electrode and Electrode] 
     Example 1 
     4 parts by mass of the copolymer obtained in Production Example 1 was dissolved in 96 parts by mass of water to obtain an aqueous solution of a binder (binder composition). Next, 90.2 parts by mass of artificial graphite (MAG-D manufactured by Hitachi Chemical Co., Ltd.) and 6.8 parts by mass of silicon monoxide (OSAKA Titanium technologies) as electrode active materials were added to 75 parts by mass of the aqueous binder solution, and the mixture was kneaded. Additionally, 96 parts by mass of water for adjusting the viscosity was added, and the mixture was kneaded to prepare a negative electrode mixture in slurry form. The negative electrode mixture was applied onto a rolled copper foil having a thickness of 18 μm and dried: thereafter, the rolled copper foil and the coating were tightly bonded together using a roll press (manufactured by Oono-Roll Corporation) and then subjected to heat treatment (under reduced pressure at 120° C. for 12 hours or more) to produce a negative electrode. The thickness of the active material layer in the negative electrode was 42 μm, and the capacity density of the negative electrode was 3.24 mAh/cm 2 . 
     Example 2 
     A negative electrode was produced as in Example 1, except that the copolymer obtained in Production Example 2 was used as the binder. 
     Example 3 
     A negative electrode was produced as in Example 1, except that the copolymer obtained in Production Example 3 was used as the binder. 
     Comparative Example 1 
     A negative electrode was produced as in Example 1, except that the copolymer obtained in Production Example 4 was used as the binder. 
     Comparative Example 2 
     A negative electrode was produced as in Example 1, except that the copolymer obtained in Production Example 5 was used as the binder. 
     [Binding Force] 
     For each of the negative electrodes obtained in Examples 1 to 3 and Comparative Examples 1 and 2, the peel strength (N/15 mm) upon peeling of the active material layer from the copper foil serving as a collecting electrode was measured as the binding force. The specific method was as follows: The negative electrode was cut into a width of 80 mm×15 mm, and adhesive tape was applied to a surface (negative electrode active material layer-side) of the negative electrode. Then, the negative electrode (current collector-side) was fixed to a stainless steel plate by attaching it with double-faced adhesive tape, and used as an evaluation sample. The evaluation sample was subjected to a 90 degree peel test of the negative electrode with respect to the stainless steel plate (90 degree peel test of the adhesive tape with respect to the negative electrode fixed to the stainless steel plate), using a tensile testing machine (compact tabletop tester EZ-SX manufactured by Shimadzu Corporation), and the peel strength between the active material layer and the current collector in the negative electrode was measured. Table 1 shows the evaluation results of the peel test (peel strength). 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Binding Force 
               
               
                   
                 Yellowness Index 
                 N/15 mm 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Example 1 
                 11.3 
                 0.27 
               
               
                   
                 Example 2 
                 5.5 
                 0.27 
               
               
                   
                 Example 3 
                 0.9 
                 0.27 
               
               
                   
                 Comparative Example 1 
                 14.6 
                 0.24 
               
               
                   
                 Comparative Example 2 
                 15.3 
                 0.22 
               
               
                   
                   
               
            
           
         
       
     
     Each of the electrodes (negative electrodes) of Examples 1 to 3 is an electrode for a secondary battery obtained using an electrode mixture comprising each of the binders produced in Production Examples 1 to 3 (a binder for a secondary battery comprising a copolymer (polymer compound), wherein the copolymer contains an acrylic repeating unit, and a 3% by mass aqueous solution of the copolymer has a yellowness index of 14 or less); and an active material. It is observed that the binders in the electrodes of Examples 1 to 3 have higher binding force.