JP2011076916A - Binder for secondary battery electrode, secondary battery electrode, and secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a binder for secondary battery electrode in which a slurry containing the binder is excellent in temporal stability without causing a phase separation and a secondary battery having the binder shows high cycle characteristics and rate characteristics. <P>SOLUTION: The binder for secondary battery electrode consists of a copolymer of (meth) acrylonitrile and (meth) acrylic acid ester, and the copolymer is made of a component (A) in which the content of (meth) acrylonitrile unit is 60 wt.% or more and 95 wt.% or less and a component (B) in which the content of (meth) acrylonitrile unit is 5 wt.% or more and 40 wt.% or less, and the component (A) and the component (B) are mutually dissolved. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a binder for a secondary battery electrode, a slurry for a secondary battery electrode using the binder, an electrode for a secondary battery, and a secondary battery.

 The spread of portable terminals such as notebook personal computers, mobile phones and PDAs is remarkable. Lithium ion secondary batteries are often used as power sources for these portable terminals. Mobile terminals are required to be more convenient and are being made smaller, thinner, lighter, and higher in performance. To improve the performance of secondary batteries, improvements to electrodes, electrolytes, and other battery members are being studied. For particularly important electrodes, electrode active materials and current collectors are examined, as well as electrode active materials. The key is to study polymers that serve as binders for binding substances to current collectors. The electrode is usually a secondary battery electrode in which a polymer serving as a binder is dispersed or dissolved in a liquid medium such as water or an organic solvent, and an electrode active material and a conductive agent such as conductive carbon are mixed with the polymer. The slurry composition is obtained, and the slurry composition is applied to the current collector and dried to bind the mixed layer to the current collector.

 In order to improve the performance of the secondary battery, it is necessary to improve the battery capacity, life (cycle characteristics) and charge / discharge capacity maintenance rate (rate characteristics) at a high rate. The battery capacity is strongly influenced by the filling amount of the electrode active material. Further, since the rate characteristic is affected by the ease of electron movement, an increase in the conductivity-imparting agent is effective for improving the rate characteristic. In order to increase the electrode active material and the conductivity-imparting agent in a limited battery space, it is necessary to reduce the amount of binder. However, when the amount of the binder is reduced, the binding property of the electrode active material is impaired, and the electrode active material is peeled off from the current collector by repeated charge and discharge, thereby deteriorating cycle characteristics. For this reason, there is a demand for a binder that can strongly bind the electrode active material even if the amount used is small.

 Conventionally, fluorine-containing polymers such as polyvinylidene fluoride have been widely used as positive electrode binders for lithium ion secondary batteries. When a fluorine-containing polymer is used, the production of the electrode is easy, but since the binding force and flexibility are insufficient, it is difficult to increase the capacity of the battery and improve the rate characteristics. As a method for improving the drawbacks of the fluorine-containing polymer, it has been proposed to use a rubber-based polymer binder (Patent Document 1: Japanese Patent Laid-Open No. 4-255670). However, when an electrode is formed using a rubber polymer, the binding force and flexibility can be improved, but there is a problem that the battery reaction is restricted because the electrode active material is covered with the rubber polymer. Therefore, an attempt to use a fluorine-based polymer and a rubber-based polymer together, for example, a combination with a butadiene-based polymer (Patent Document 2: JP-A-6-215761), an acrylonitrile-butadiene copolymer rubber (NBR) Combined use (Patent Document 3: JP-A-9-63590), combined use with acrylic polymer (Patent Document 4: JP-A-9-199132), combined use of hydrogenated NBR and acrylonitrile / acrylic ester copolymer (Patent Document 5: Japanese Patent Laid-Open No. 2003-282061) has been proposed.

 Patent Document 6 (WO 2004/095613) describes an ethylenically unsaturated monomer that gives a polymer soluble in N-methylpyrrolidone (NMP) and an ethylenically unsaturated monomer that gives a polymer insoluble in NMP. A binder for a lithium ion secondary battery electrode comprising a polymer obtained by multistage polymerization of a monomer is disclosed. Examples of the ethylenically unsaturated monomer that gives a homopolymer soluble in NMP include acrylonitrile, methacrylonitrile, and an ethylenically unsaturated carboxylic acid ester having an alkyl group bonded to a non-carbonyl oxygen atom having 6 or less carbon atoms. Examples of the ethylenically unsaturated monomer that gives a homopolymer insoluble in NMP include ethylenically unsaturated carboxylic acid esters in which the alkyl group bonded to the non-carbonyl oxygen atom has 7 or more carbon atoms. Is illustrated.

JP-A-4-255670 JP-A-6-215761 Japanese Patent Laid-Open No. 9-63590 JP-A-9-199132 JP 2003-282061 A WO2004 / 095613

  However, according to the studies by the inventors, the binders described in Patent Documents 2 to 5 are obtained by mechanically mixing two different types of polymers. It was found that (compatibility) is not always sufficient. And the slurry obtained using such a binder is inferior in stability over time, phase separation occurs by standing for about 1 day, precipitate is generated, and long-term storage of the slurry is difficult, battery manufacturing process It is necessary to prepare the slurry sequentially in accordance with the progress of the process, hindering the smoothing of the process, and furthermore, the compatibility of the polymer constituting the binder is not sufficient, so that the binder causes phase separation in the electrode, and the polymer I understood that it segregates. As a result, the flexibility of the electrode active material layer is impaired, and in secondary batteries using this binder, battery characteristics such as rate characteristics and cycle characteristics may deteriorate over time due to polymer segregation. It was revealed.

  In addition, the binder described in Patent Document 6 can control the swellability with respect to the electrolytic solution and can improve the long-term cycle characteristics, but the rate characteristics are not sufficiently improved. In addition, since the binder has an NMP-soluble component and an insoluble component, and two components having greatly different characteristics are subjected to multistage polymerization, the uniform dispersibility (compatibility) between the polymers is not necessarily sufficient. It was found that the same problem occurs.

  Accordingly, the present invention provides a binder for a secondary battery electrode in which the slurry containing the binder is excellent in stability over time without causing phase separation, and further exhibits high cycle characteristics and rate characteristics in a secondary battery having the binder. The purpose is that.

  The present inventors diligently studied to provide a binder having excellent stability over time. As a result of copolymerization of (meth) acrylonitrile and (meth) acrylic acid ester by multistage polymerization, two types having different compositions were obtained. A copolymer of (meth) acrylonitrile and (meth) acrylic acid ester composed of the above components, wherein a binder in which the above components are compatible with each other is obtained, and the composition of the copolymer is set appropriately. It was found that the stability with time of the slurry was remarkably improved by controlling to the range. Furthermore, since the copolymer has at least a component containing a high content of (meth) acrylonitrile, the component is excellent in dispersibility of the conductive agent, and the conductive agent is uniformly distributed in the electrode. It has been found that the secondary battery including the battery has high rate characteristics, and the present invention has been completed based on these findings.

That is, this invention which solves the said subject contains the following matter as a summary.
(1) A binder for a secondary battery electrode comprising a copolymer of (meth) acrylonitrile and (meth) acrylic acid ester, wherein the copolymer has a (meth) acrylonitrile unit content of 60% by weight or more. 95% by weight or less of component (A) and (meth) acrylonitrile unit content of 5% by weight or more and 40% by weight or less of component (B), wherein component (A) and component (B) are in phase A binder for secondary battery electrodes formed by melting.

(2) The binder for secondary battery electrodes according to (1), wherein the ratio of the component (A) and the component (B) in the copolymer is 20:80 to 80:20 (weight ratio). .

(3) A monomer composition (A) having a (meth) acrylonitrile content of 60 wt% or more and 95 wt%, and a monomer having a (meth) acrylonitrile content of 5 wt% or more and 40 wt% The method for producing a binder for a secondary battery electrode according to (1), wherein the composition (B) is subjected to multistage polymerization.

(4) The secondary battery electrode according to (3), wherein the multistage polymerization includes a polymerization step in which the component (A) is a main product and a polymerization step in which the component (B) is a main product. Method for manufacturing binder.

(5) A secondary battery electrode slurry comprising the secondary battery electrode binder according to (1) or (2), an electrode active material, and a solvent.

(6) A secondary battery electrode in which an electrode active material layer comprising the secondary battery electrode binder and electrode active material according to (1) or (2) is laminated on a current collector.

(7) A secondary battery having a positive electrode, an electrolytic solution, a separator and a negative electrode,
The secondary battery whose said positive electrode or negative electrode is a secondary battery electrode of (6) description.

  The binder of the present invention is a copolymer of (meth) acrylonitrile and (meth) acrylic acid ester composed of two or more components having different compositions, wherein the respective components are compatible with each other, and Since the composition of each component of the copolymer is controlled within a specific range, the slurry containing the binder is excellent in stability over time.

 Therefore, an electrode slurry produced using such a binder is less likely to cause phase separation even when stored for a long period of time, and thus can be mass-produced and stored for a long period of time. The quality of the battery produced is also constant. In addition, since the compatibility state of the binder is stably maintained in the manufactured electrode, battery characteristics such as flexibility, binding property, rate characteristics, cycle characteristics, etc. of the electrode active material layer deteriorate with time. There is nothing.

 Furthermore, when the copolymer has at least a component containing a high content of (meth) acrylonitrile, the component is excellent in dispersibility of the conductive agent, and the conductive agent is uniformly distributed in the electrode. A secondary battery containing a binder has high rate characteristics.

The present invention is described in detail below.
1. Binder The binder for secondary battery electrodes of the present invention is a copolymer of (meth) acrylonitrile and (meth) acrylic acid ester comprising two or more components having different compositions, and the respective components are compatible with each other. Do it.

  Here, (meth) acrylonitrile is used in the meaning including both acrylonitrile and methacrylonitrile. That is, (meth) acrylonitrile may be either acrylonitrile or methacrylonitrile, or may contain both at the same time. Similarly, (meth) acrylic acid ester is used in the meaning including both acrylic acid ester and methacrylic acid ester. Similarly, the (meth) acrylic acid ester may be either an acrylic acid ester or a methacrylic acid ester copolymer, or may contain both at the same time.

 (Meth) acrylic acid ester includes (meth) acrylic acid alkyl ester and (meth) acrylic acid ester having a functional group in the side chain. Among them, (meth) acrylic acid alkyl ester is preferable, and (meth) acrylic acid alkyl ester having 1 to 18 carbon atoms of the alkyl group bonded to the non-carbonyl oxygen atom is preferable. When the carbon number of the alkyl group is within the above range, the conductive agent is excellent in dispersibility and exhibits high rate characteristics.

Examples of the alkyl (meth) acrylate alkyl ester having 1 to 18 carbon atoms bonded to a non-carbonyl oxygen atom include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, and acrylic acid n. Alkyl acrylate esters such as butyl, t-butyl acrylate, n-hexyl acrylate, 2-ethylhexyl acrylate, nonyl acrylate, lauryl acrylate, stearyl acrylate, cyclohexyl acrylate, and isobornyl acrylate;
Methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, n-hexyl methacrylate, 2-ethylhexyl methacrylate, Examples include methacrylic acid alkyl esters such as octyl methacrylate, isodecyl methacrylate, lauryl methacrylate, tridecyl methacrylate, stearyl methacrylate, and cyclohexyl methacrylate.

 Among these, (meth) acrylic acid alkyl ester having 1 to 18 carbon atoms is more preferable, (meth) acrylic acid alkyl ester having 2 to 12 carbon atoms is more preferable, and (meth) acrylic acid is particularly preferable. n-butyl, 2-ethylhexyl (meth) acrylate. By using these alkyl (meth) acrylates, the slurry stability and electrode flexibility are excellent, and high rate characteristics are exhibited.

 As the (meth) acrylic acid ester having a functional group in the side chain, (meth) acrylic acid ester having an alkoxyl group such as 2-methoxyethyl acrylate and 2-methoxyethyl methacrylate; 2-hydroxyethyl acrylate And (meth) acrylic acid ester having a hydroxyl group such as 2-hydroxyethyl methacrylate; (meth) acrylic acid ester having an epoxy group such as glycidyl acrylate and glycidyl methacrylate;

 The copolymer of (meth) acrylonitrile and (meth) acrylic acid ester constituting the binder of the present invention comprises two or more components having different compositions. That is, the (meth) acrylonitrile / (meth) acrylic acid ester copolymer has a (meth) acrylonitrile unit content of 60% by weight to 95% by weight of component (A) and a (meth) acrylonitrile unit content. Is 5% by weight or more and 40% by weight or less of the component (B).

  In the present invention, the component (A) contains 60% by weight or more and 95% by weight or less of (meth) acrylonitrile units. When the content of the (meth) acrylonitrile unit in the component (A) is in the above range, the conductive agent is excellent in dispersibility, and the conductive agent can be uniformly distributed in the electrode. High rate characteristics can be exhibited in the secondary battery including the same. The content of the (meth) acrylonitrile unit in the component (A) is preferably 65% by weight or more and 90% by weight or less, more preferably 70% by weight or more and 85% by weight or less. When the content of the (meth) acrylonitrile unit in the component (A) is in the above range, the slurry has excellent stability over time while exhibiting high rate characteristics.

  In addition to the (meth) acrylonitrile unit, the component (A) includes a monomer unit copolymerizable with the above-mentioned (meth) acrylic acid ester unit and the (meth) acrylonitrile and (meth) acrylic acid ester described later. Also good. Among these, it is preferable to comprise a (meth) acrylic acid ester unit.

  The content of the (meth) acrylic acid ester unit in the component (A) is preferably 5% by weight or more and 40% by weight or less, more preferably 10% by weight or more and 35% by weight or less with respect to the total amount of the component (A). Particularly preferably, it is 15% by weight or more and 30% by weight or less. When the content of the (meth) acrylic acid ester unit in the component (A) is included in the above range, the slurry has excellent stability over time while exhibiting high rate characteristics.

  The content of other copolymerizable monomer units other than (meth) acrylic acid ester in component (A) is preferably 20% by weight or less, more preferably 1 to 10%, based on the total amount of component (A). % By weight. When the content of other copolymerizable monomer units other than the (meth) acrylic acid ester in the component (A) is included in the above range, the above-described effects of each monomer unit can be exhibited. In addition, the stability of the binder and the slurry, which is the effect of the present invention, is excellent, and the cycle characteristics and rate characteristics of the obtained binder are excellent.

  In this invention, a component (B) contains 5 to 40 weight% of (meth) acrylonitrile units. When the content of the (meth) acrylonitrile unit in the component (B) is in the above range, the compatibility with the component (A) is excellent, and the flexibility of the secondary battery electrode containing the binder of the present invention is excellent. . The content of the (meth) acrylonitrile unit in the component (B) is preferably 10% by weight to 30% by weight, and more preferably 15% by weight to 25% by weight. When the content of the (meth) acrylonitrile unit in the component (B) is in the above range, the electrolytic solution resistance is excellent in the battery while exhibiting high compatibility of the binder and stability with time of the slurry.

  In addition to the (meth) acrylonitrile unit, the component (B) contains a monomer unit that can be copolymerized with the above-mentioned (meth) acrylic acid ester unit or the (meth) acrylonitrile and (meth) acrylic acid ester described later. Also good. Among these, it is preferable to comprise a (meth) acrylic acid ester unit.

 The content of the (meth) acrylic acid ester unit in the component (B) is preferably 60% to 95% by weight, more preferably 70% to 90% by weight, based on the total amount of the component (B). Most preferably, it is 75 wt% or more and 85 wt% or less. When the content of the (meth) acrylic acid ester unit in the component (B) is within the above range, the slurry has excellent stability over time while exhibiting high rate characteristics.

 The content of other copolymerizable monomer units other than the (meth) acrylic acid ester unit in the component (B) is preferably 20% by weight or less, more preferably 1% by weight based on the total amount of the component (B). % To 10% by weight. When the content of other copolymerizable monomer units other than (meth) acrylic acid ester is included in the above range, the above-described effects of each monomer unit can be exhibited and the effect of the present invention. The binder and the slurry are excellent in stability over time, and the obtained binder has excellent cycle characteristics and rate characteristics.

  In the binder for a secondary battery electrode of the present invention, the ratio of the component (A) and the component (B) in the copolymer is preferably 20:80 to 80:20 by weight, Preferably it is 30: 70-70: 30, Most preferably, it is 40: 60-60: 40. When the ratio of the component (A) and the component (B) in the copolymer is within the above range, the component (A) and the component (B) exhibit high compatibility, and the aging stability of the slurry is also improved. Excellent and excellent in both cycle characteristics and rate characteristics of the obtained secondary battery.

  The content of monomer units derived from (meth) acrylonitrile with respect to the total amount of copolymer of (meth) acrylonitrile and (meth) acrylic acid ester constituting the binder is preferably 30 to 70% by weight, more preferably 40 to 40%. The content of the monomer unit derived from (meth) acrylic acid ester is preferably 70 to 30% by weight, and more preferably 60 to 40% by weight. When the content of the monomer unit derived from (meth) acrylonitrile and the monomer unit derived from (meth) acrylic acid ester in the binder of the present invention is in the above range, the electrolyte solution resistance is excellent, and the binder Excellent output characteristics of secondary batteries.

  The copolymer of (meth) acrylonitrile and (meth) acrylic acid ester may contain other monomer units in addition to the (meth) acrylonitrile unit and (meth) acrylic acid ester unit. Good.

  Other copolymerizable monomer units include monomer units derived from conjugated dienes, monomer units derived from polyfunctional unsaturated carboxylic acid esters, monomer units derived from unsaturated carboxylic acids, and aromatic vinyl. The monomer unit derived from a compound is mentioned.

  Conjugated dienes used for constituting monomer units derived from conjugated dienes include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2- Examples include ethyl-1,3-butadiene, 2-chloro-1,3-butadiene, 1,3-pentadiene, 1,3-hexadiene, and 2,4-hexadiene. Of these conjugated dienes, 1,3-butadiene is preferred. By containing a conjugated diene unit, the flexibility of the binder can be controlled.

  The polyfunctional unsaturated carboxylic acid ester used for constituting the monomer unit derived from the polyfunctional unsaturated carboxylic acid ester is a carboxylic acid ester having two or more carbon-carbon double bonds. Specific examples of the polyfunctional unsaturated carboxylic acid ester include diethylene glycol diacrylate, 1,3-butylene glycol diacrylate, tripropylene glycol acrylate, 1,4-butanediol acrylate, neopentyl glycol diacrylate, and 1,6-hexane. Diacrylates such as diol acrylate, 1,9-nonanediol acrylate, and polyethylene glycol diacrylate; Triacrylates such as trimethylolpropane triacrylate; ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, 1,3-butylene glycol Dimethacrylates such as dimethacrylate and polyethylene glycol dimethacrylate; trimethylolpro Tri methacrylates such as emissions trimethacrylate; and the like. By containing such a monomer unit, swelling of the binder with respect to the electrolytic solution may be suppressed, and the long-term storage characteristics of the obtained battery may be improved.

  The unsaturated carboxylic acid used to constitute the monomer unit derived from the unsaturated carboxylic acid includes unsaturated monocarboxylic acids such as acrylic acid, methacrylic acid, crotonic acid and isocrotonic acid; maleic acid, fumaric acid, And unsaturated dicarboxylic acids such as citraconic acid, mesaconic acid, glutaconic acid, and itaconic acid. Of these, unsaturated monocarboxylic acids are preferred. By containing an unsaturated carboxylic acid unit, the binding property of the binder may be improved.

  Examples of the aromatic vinyl compound used for constituting the monomer unit derived from the aromatic vinyl compound include styrene, α-methylstyrene, and β-methylstyrene, and styrene is preferable. By including an aromatic vinyl unit, the binding property of the binder may be improved.

  The content of other copolymerizable monomer units with respect to the total amount of the copolymer of (meth) acrylonitrile and (meth) acrylic acid ester constituting the binder is preferably 20% by weight or less, more preferably 1 to 10%. % By weight. When the content of other copolymerizable monomer units in the copolymer of (meth) acrylonitrile and (meth) acrylic acid ester is included in the above range, each monomer unit exhibits the above-described content. In addition to exhibiting the effects, the binder and slurry, which are the effects of the present invention, are excellent in stability over time, and the cycle characteristics and rate characteristics of the obtained secondary battery are excellent.

 In the present invention, it is preferable that a component having a glass transition temperature of 25 ° C. or less is contained in the copolymer of (meth) acrylonitrile and (meth) acrylic acid ester. Specifically, it is preferable that the glass transition temperature of at least one of the component (A) and the component (B) is 25 ° C. or lower because an electrode having high flexibility can be obtained. The glass transition temperature is preferably 25 ° C. or lower, more preferably 15 ° C. or lower, and particularly preferably −5 ° C. or lower.

  In addition, the glass transition temperature of the component (A) and the component (B) in the copolymer of (meth) acrylonitrile and (meth) acrylic acid ester is the copolymer weight of (meth) acrylonitrile and (meth) acrylic acid ester. It can be determined by measuring the glass transition temperature of the coalescence. When the glass transition temperatures of the two components are significantly different, two different glass transition temperatures can be measured. When the glass transition temperatures of the two components are close, it is measured as one glass transition temperature, indicating that both the component (A) and the component (B) have substantially the same glass transition temperature.

  In addition, the glass transition temperature of each component in the copolymer of (meth) acrylonitrile and (meth) acrylic acid ester can be adjusted by appropriately combining the monomers exemplified above and their contents.

  In the present invention, the component (A) constituting the binder is a component insoluble in tetrahydrofuran, and the component (B) is a component soluble in tetrahydrofuran.

Therefore, the content of (meth) acrylonitrile units in each component in the binder of the present invention can be measured using the above characteristics. Specifically, the binder is dissolved in THF and separated into a THF soluble part and a THF insoluble part, and each of the THF insoluble part and the THF soluble part is shared by ¹ H-NMR, ¹³ C-NMR, etc. It is determined by analyzing the polymerization composition.

  The copolymer of (meth) acrylonitrile and (meth) acrylic acid ester constituting the binder for a secondary battery electrode of the present invention comprises two or more components having different compositions as described above, and each of the components is a phase. It is melted. The term "compatible" as used herein means that a copolymer of (meth) acrylonitrile and (meth) acrylic acid ester composed of two or more components having different compositions as described above is uniformly mixed in a polymer solution state. When a film is produced from a solution, it means taking a microphase separation state. Here, the compatible state in the polymer solution in the (meth) acrylonitrile / (meth) acrylic acid ester copolymer is described as N-methylpyrrolidone (hereinafter referred to as “NMP”) used in the electrode slurry described later. It can be confirmed by the haze value of the polymer solution when it is dissolved. Further, the microphase separation state of the polymer film prepared from the polymer solution can be confirmed by a transmission electron microscope (hereinafter sometimes referred to as “TEM”). In the present invention, the haze value of a polymer solution prepared by dissolving a copolymer of (meth) acrylonitrile and (meth) acrylic acid ester in NMP is 5% or less, and the domain is 200 nm in the TEM image of the film prepared from the polymer solution. When it is below, it is judged that the polymer component is compatible.

  As described above, the binder of the present invention comprises a copolymer of two or more types of (meth) acrylonitrile and (meth) acrylic acid ester having different compositions, and the composition of the copolymer is specified. Since it is controlled within the range, the stability over time is excellent. Therefore, the electrode-forming slurry produced using such a binder is unlikely to cause phase separation even when stored for a long period of time, so that mass production and long-term storage are possible, and the battery manufacturing process proceeds smoothly. The quality of the battery obtained is also constant. In addition, since the compatibility state of the binder is stably maintained in the manufactured electrode, battery characteristics such as flexibility, binding property, rate characteristics, cycle characteristics, etc. of the electrode layer may deteriorate over time. Absent.

  As long as the binder for secondary battery electrodes of the present invention has the above composition and properties, its production method is not particularly limited, but in order to obtain a binder in which the polymer components are uniformly mixed, a multistage polymerization method is particularly used. It is advantageous to adopt.

  Specifically, the monomer composition (A) having a (meth) acrylonitrile content of 60 wt% or more and 95 wt% and the (meth) acrylonitrile content of 5 wt% or more and 40 wt% or less. Multi-stage polymerization is performed with the monomer composition (B). In the present invention, the multistage polymerization means that a part of the monomer composition is polymerized first, and then monomer compositions having different types and / or mixing ratios are added and polymerized. “Subsequently” polymerization means that the monomer is left in the previous polymerization step, that is, the polymerization of the next stage is performed when the polymerization conversion rate does not reach 100%.

  Multi-stage polymerization includes two or more stages of polymerization processes in which the monomer composition and polymerization conditions in the polymerization system are different. In the present invention, it is preferably performed in two or three stages, and more preferably in two stages. If it is carried out in four or more stages, the process becomes complicated and the productivity may be lowered. Hereinafter, a process for preparing a binder by performing multistage polymerization in two stages will be described.

  In the case of carrying out the polymerization in two stages, it is preferably a multistage polymerization including a polymerization step in which the component (A) is a main product and a polymerization step in which the component (B) is a main product. The co-polymer finally obtained is a multi-stage polymerization having a first-stage polymerization step with A) as the main product and a second-stage polymerization step with component (B) as the main product. It is more preferable because the reaction rate of coalescence is high. In the following description, a case where the component (A) is mainly produced by the first stage polymerization and the component (B) is mainly produced by the second stage polymerization will be described.

  In the polymerization in the first stage, (meth) acrylonitrile, (meth) acrylic acid ester and other monomer components as required are supplied to the polymerization system to mainly produce component (A). The monomer composition (monomer composition (A)) supplied to the first stage polymerization system is selected from the monomer compositions contained in the component (A). At the same time, a polymerization initiator such as potassium persulfate is supplied and heated to about 60 ° C. to polymerize the monomer composition (A) to mainly produce the component (A). The polymerization conversion rate of the monomer composition (A) in the first polymerization step is preferably 60 to 97% by weight, more preferably 65 to 97% by weight, and further preferably 70 to 95% by weight. When the polymerization conversion rate in the first stage polymerization process reaches a predetermined value, the monomer composition (B) substantially equal to the monomer composition of the component (B) is supplied to the polymerization system, and the polymerization is continued. The monomer composition (B) is selected from the monomer compositions contained in the component (A) described above. The polymerization conversion rate of the monomer composition (B) in the second polymerization step is preferably 90% by weight or more, more preferably 95% by weight or more.

  The weight ratio (A / B) between the monomer composition (A) and the monomer composition (B) supplied into the polymerization system is the weight ratio between the component (A) and the component (B). The monomer composition (A) / monomer composition (B) is 20/80 wt% to 80/20 wt%, preferably 30/70 wt% to 70/30 wt%, Preferably, it is 40/60 wt% to 60/40 wt%.

 Examples of the polymerization initiator include conventional components such as an azo polymerization initiator [for example, 2,2′-azobisbutyric acid such as azobisnitrile (for example, 2,2′-azobisisobutyronitrile (AIBN)). Ronitriles; 2,2′-azobisvaleronitriles such as 2,2′-azobis (2,4-dimethylvaleronitrile); 2 such as 2,2′-azobis (2-hydroxymethylpropionitrile) 2,1'-azobispropionitriles; 1,1'-azobis-1-cycloalkanenitriles such as 1,1'-azobiscyclohexane-1-carbonitrile; 4,4'-azobis (4-cyano Azobiscyanocarboxylic acids such as valeric acid), azobutyronitriles such as azonitrile (eg 2- (carbamoylazo) isobutyronitrile; 2-phenyl Azovaleronitriles such as zo-4-methoxy-2,4-dimethylvaleronitrile)]; peroxide-based polymerization initiators [for example, cyclohexanone peroxide, 3,3,5-trimethylcyclohexanone peroxide, methylcyclohexanone Ketone peroxides such as peroxide, 1,1-bis (t-butylperoxy) -3,3,5-trimethylcyclohexane, 1,1-bis (t-butylperoxy) cyclohexane, n-butyl-4 , 4-bis (t-butylperoxy) valate and other peroxyketals, cumene hydroperoxide, diisopropylbenzene hydroperoxide, hydroperoxide such as 2,5-dimethylhexane-2,5-dihydroperoxide Ji-t-B Ruperoxide, dicumyl peroxide, t-butylcumyl peroxide, α, α′-bis (t-butylperoxy-m-isopropyl) benzene, 2,5-dimethyl-2,5-di (t-butylperoxide) Dialkyl peroxides such as oxy) hexyne-3, diacyl peroxides such as benzoyl peroxide, decanoyl peroxide, lauroyl peroxide, 2,4-dichlorobenzoyl peroxide, bis (t-butylcyclohexyl) peroxydi Peroxycarbonates such as carbonates, organic peroxides such as peroxyesters such as t-butylperoxybenzoate and 2,5-dimethyl-2,5-di (benzoylperoxy) hexane; hydrogen peroxide, persulfuric acid Salt (potassium persulfate, ammonium persulfate) Beam, etc.) inorganic peroxides etc.] can be exemplified. Such polymerization initiators can be used alone or in combination.

 The usage-amount of a polymerization initiator is not restrict | limited in particular, For example, it is 0.1-10 weight part with respect to a total of 100 weight part of monomers, Preferably it is about 1-5 weight part.

 The polymerization temperature can usually be selected from the range of about 50 ° C to 120 ° C, preferably about 60 ° C to 90 ° C. The polymerization reaction can be performed in an inert gas (for example, nitrogen gas, helium gas) atmosphere.

 The polymerization solvent may be water or an organic solvent. The organic solvent is not limited to one type, and may be a mixed solvent of a plurality of types of organic solvents. Specifically, aliphatic hydrocarbons such as pentane, hexane, heptane, hydrocarbon alcohols such as methanol, ethanol, n-propanol, isopropanol, tert-butyl alcohol, acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, etc. Hydrocarbon ketones such as dimethyl ether, diethyl ether, methyl ethyl ether, methyl tert-butyl ether, hydrocarbon ethers such as diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether, and cyclic aliphatic hydrocarbons such as tetrahydrofuran and 1,4-dioxane Ethers, nitriles such as acetonitrile, methyl acetate, ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, tert-butyl acetate, propio Hydrocarbon esters such as methyl acid and ethyl propionate, aromatic hydrocarbons such as toluene and xylene, chlorinated hydrocarbons such as methylene chloride, chloroform and carbon tetrachloride, 1,1,2-trichloro-1, 2,2-trifluoroethane (R-113), 1,1,1-trichloro-2,2,2-trifluoroethane (R-113a), 1,1-dichloro-1-fluoroethane (R-141b) ), 3,3-dichloro-1,1,1,2,2-pentafluoropropane (R-225ca), 1,3-dichloro-1,1,2,2,3-pentafluoropropane (R-225cb) ), Etc., 1,1,1,2,2,3,3,4,4,5,5,6,6-tridecafluorohexane, 1,1,1,2,2 , 3,3,4,4-Nonafluorohexa Fluorinated hydrocarbons such as methyl-2,2,3,3-tetrafluoroethyl ether, fluorinated hydrocarbon ethers such as 2,2,2-trifluoroethanol, 1,1,1,3 Fluorinated hydrocarbon alcohols such as 3,3-hexafluoroisopropanol, 2,2,3,3-tetrafluoropropanol, 2,2,3,3,4,4,5,5-octafluoropentanol Although it is mentioned, it is not limited to these.

  By such multistage polymerization, the binder of the present invention in which the polymer components are uniformly mixed is obtained.

  The polymerization method of the binder of the present invention is not particularly limited, and a known polymerization method such as an emulsion polymerization method, a suspension polymerization method, a dispersion polymerization method, or a solution polymerization method can be employed. Of these, emulsion polymerization is preferred.

Moreover, the copolymer of (meth) acrylonitrile and (meth) acrylic acid ester forming the binder of the present invention can be dissolved in an organic solvent. Here, the organic solvent means a polymerization solvent when the polymer liquid obtained by solution polymerization is used as it is, and a substitution solvent when used after solvent substitution.
It is possible to use a conventional binder such as PVDF, PTFE or rubbery polymer in combination with the binder of the present invention as long as the object of the present invention is not impaired.

2. Secondary Battery Electrode Slurry The secondary battery electrode slurry of the present invention comprises the above secondary battery electrode binder, an electrode active material, and a solvent.

  An electrode active material (positive electrode active material) for a lithium ion secondary battery positive electrode uses an active material capable of occluding and releasing lithium ions, and is broadly classified into an inorganic compound and an organic compound.

Examples of the positive electrode active material made of an inorganic compound include transition metal oxides, transition metal sulfides, lithium-containing composite metal oxides of lithium and transition metals, and the like. Examples of the transition metal include Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Mo.
Examples of transition metal oxides include MnO, MnO ₂ , V ₂ O ₅ , V ₆ O ₁₃ , TiO ₂ , Cu ₂ V ₂ O ₃ , amorphous V ₂ O—P ₂ O ₅ , MoO ₃ , V ₂ O. ₅ , V ₆ O _{13 and the} like. Among them, MnO, V ₂ O ₅ , V ₆ O ₁₃ and TiO ₂ are preferable from the viewpoint of cycle stability and capacity.

The transition metal _sulfide, TiS _2, TiS _3, amorphous _MoS 2, FeS, and the like.

  Examples of the lithium-containing composite metal oxide include a lithium-containing composite metal oxide having a layered structure, a lithium-containing composite metal oxide having a spinel structure, and a lithium-containing composite metal oxide having an olivine structure.

Examples of the lithium-containing composite metal oxide having a layered structure include lithium-containing cobalt oxide (LiCoO ₂ ), lithium-containing nickel oxide (LiNiO ₂ ), Co—Ni—Mn lithium composite oxide, and Ni—Mn—Al lithium. Examples thereof include composite oxides and lithium composite oxides of Ni—Co—Al.

Examples of lithium-containing composite metal oxides having a spinel structure include lithium manganate (LiMn ₂ O ₄ ) and Li [Mn _3/2 M _1/2 ] O _{4 in} which a part of Mn is substituted with another transition metal (here M may be Cr, Fe, Co, Ni, Cu or the like.

LiXMPO ₄ (wherein, M is Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, LiXMPO ₄ as the lithium-containing composite metal oxide having an olivine structure) An olivine type lithium phosphate compound represented by at least one selected from B and Mo, 0 ≦ X ≦ 2).

Among the above active materials, lithium-containing nickel oxide (LiNiO ₂ ), Co—Ni—Mn lithium composite oxide, Ni—Mn—Al lithium composite oxide, Ni—Co—Al lithium composite oxide, etc. It is preferable to use a nickel-containing active material. The nickel-containing active material is particularly excellent in binding properties with the binder, and high rate characteristics are obtained.

  Examples of the positive electrode active material made of an organic compound include polyaniline, polypyrrole, polyacene, disulfide compounds, polysulfide compounds, and N-fluoropyridinium salts. The positive electrode active material may be a mixture of the above inorganic compound and organic compound. The particle diameter of the positive electrode active material used in the present invention is appropriately selected in consideration of other constituent elements of the battery. From the viewpoint of improving battery characteristics such as load characteristics and cycle characteristics, the 50% volume cumulative diameter is Usually, it is 0.1-50 micrometers, Preferably it is 1-20 micrometers. When the 50% volume cumulative diameter is within this range, a secondary battery having a large charge / discharge capacity can be obtained, and handling of the slurry for electrodes and the electrodes is easy. The 50% volume cumulative diameter can be determined by measuring the particle size distribution by laser diffraction.

  Examples of the electrode active material for the negative electrode (negative electrode active material) include carbon allotropes such as graphite and coke. The active material composed of the allotrope of carbon can be used in the form of a mixture with a metal, a metal salt, an oxide, or the like or a cover. Moreover, as a negative electrode active material, lithium alloys, such as oxides and sulfates, such as silicon, tin, zinc, manganese, iron, nickel, lithium metal, Li-Al, Li-Bi-Cd, Li-Sn-Cd, Lithium transition metal nitride, silicon, etc. can be used. The particle diameter of the negative electrode active material is appropriately selected in consideration of other constituent elements of the battery. From the viewpoint of improving battery characteristics such as initial efficiency, load characteristics, and cycle characteristics, a 50% volume cumulative diameter is usually The thickness is 1 to 50 μm, preferably 15 to 30 μm.

  The ratio of the amount of the electrode active material and the binder is generally 0.1 to 30 parts by weight, preferably 0.2 to 20 parts by weight, more preferably 0.5 to 100 parts by weight with respect to 100 parts by weight of the electrode active material. 10 parts by weight. When the amount of the binder is within this range, the obtained electrode has a current collector, an electrode active material layer, and an excellent binding force inside the active material layer, and a battery with low internal resistance and excellent cycle characteristics can be obtained. Can do.

 In addition to the electrode active material and the binder, the slurry for the secondary battery electrode of the present invention has various functions such as a thickener, a conductive material, a reinforcing material, a dispersing agent, a leveling agent, and an electrolytic solution additive as necessary. The additive which expresses can be contained.

  Examples of thickeners include cellulosic polymers such as carboxymethylcellulose, methylcellulose, hydroxypropylcellulose, and ammonium salts and alkali metal salts thereof; (modified) poly (meth) acrylic acid and ammonium salts and alkali metal salts thereof; ) Polyvinyl alcohols such as polyvinyl alcohol, copolymers of acrylic acid or acrylate and vinyl alcohol, maleic anhydride or copolymers of maleic acid or fumaric acid and vinyl alcohol; polyethylene glycol, polyethylene oxide, polyvinyl pyrrolidone, modified Examples include polyacrylic acid, oxidized starch, phosphoric acid starch, casein, various modified starches, and preferably ammonium salts and alkali metal salts of carboxymethyl cellulose. Used. This is because the thickener is easy to cover the surface of the active material uniformly during slurry preparation, and thus has an effect of improving the adhesion between the active materials. As the thickener, acrylonitrile-butadiene copolymer hydride can also be used in addition to the above.

  As for the compounding quantity of a thickener, 0.5-2.0 weight part is preferable with respect to 100 weight part of electrode active materials. When the blending amount of the thickener is within this range, the coating property and the adhesion with the current collector are good.

  As the conductive material, conductive carbon such as acetylene black, ketjen black, carbon black, graphite, vapor-grown carbon fiber, and carbon nanotube can be used. By using the conductive material, the electrical contact between the active materials can be improved, and the discharge rate characteristics can be improved when used for a secondary battery. The compounding amount of the conductive material is usually 0 to 20 parts by weight, preferably 1 to 10 parts by weight with respect to 100 parts by weight of the electrode active material.

  As the reinforcing material, various inorganic and organic spherical, plate-like, rod-like or fibrous fillers can be used. By using a reinforcing material, a tough and flexible negative electrode can be obtained, and excellent long-term cycle characteristics can be exhibited. The compounding quantity of a reinforcing agent is 0.01-20 weight part normally with respect to 100 weight part of electrode active materials, Preferably it is 1-10 weight part. By being included in the said range, a high capacity | capacitance and a high load characteristic can be shown.

  Examples of the dispersant include an anionic compound, a cationic compound, a nonionic compound, and a polymer compound. A dispersing agent is selected according to the electrode active material and electrically conductive agent to be used. The content of the dispersing agent in the electrode slurry is preferably 0.01 to 10 parts by weight with respect to 100 parts by weight of the total amount of the electrode active material. When the amount of the dispersant is in the above range, the slurry has excellent stability, a smooth negative electrode can be obtained, and a high battery capacity can be exhibited.

  Examples of the leveling agent include surfactants such as alkyl surfactants, silicon surfactants, fluorine surfactants, and metal surfactants. By mixing the leveling agent, the repelling that occurs during coating can be prevented, and the smoothness of the electrode can be improved. The content ratio of the leveling agent in the electrode slurry is preferably 0.01 to 10 parts by weight with respect to 100 parts by weight of the total amount of the electrode active material. When the leveling agent is within the above range, the productivity, smoothness, and battery characteristics during electrode production are excellent.

  As the electrolytic solution additive, vinylene carbonate used in the electrode slurry and the electrolytic solution can be used. The content ratio of the electrolyte additive in the electrode slurry is preferably 0.01 to 10 parts by weight with respect to 100 parts by weight of the electrode active material. When the electrolytic solution additive is in the above range, the cycle characteristics and the high temperature characteristics are excellent. Other examples include nanoparticles such as fumed silica and fumed alumina. By mixing the nanoparticles, the thixotropy of the electrode slurry can be controlled, and the leveling property of the resulting electrode can be improved. The content ratio of the nanoparticles and the like in the electrode slurry is preferably 0.01 to 10 parts by weight with respect to 100 parts by weight of the electrode active material. When the nanoparticles are in the above range, the slurry stability and productivity are excellent, and high battery characteristics are exhibited.

  Furthermore, the secondary battery electrode slurry of the present invention includes trifluoropropylene carbonate, vinylene carbonate, catechol carbonate, 1,6-dioxaspiro [4,4] nonane-2,7 in order to increase the stability and life of the battery. -Dione, 12-crown-4-ether, etc. can be used. These may be used by being contained in an electrolyte solution described later.

  The amount of the organic solvent in the secondary battery electrode slurry is adjusted so as to have a viscosity suitable for coating depending on the type of electrode active material, binder, and the like. Specifically, the solid content concentration of the electrode active material, the binder, and other additives is preferably adjusted to 30 to 90% by weight, more preferably 40 to 80% by weight.

  The electrode slurry is obtained by mixing a secondary battery electrode binder, an electrode active material, an additive added as necessary, and other organic solvents using a mixer. Mixing may be performed by supplying the above components all at once to the mixer and mixing them. However, the conductive material and the thickener are mixed in an organic solvent to disperse the conductive material in the form of fine particles. It is preferable to add a battery electrode binder and an electrode active material and further mix them, because the dispersibility of the slurry is improved. As the mixer, a ball mill, sand mill, pigment disperser, crusher, ultrasonic disperser, homogenizer, planetary mixer, Hobart mixer, etc. can be used. Is preferable.

  The particle size of the slurry for secondary battery electrodes of the present invention is preferably 35 μm or less, more preferably 25 μm or less. When the particle size of the slurry is in the above range, the conductive material is highly dispersible and a homogeneous electrode can be obtained.

3. Secondary battery electrode The secondary battery electrode of the present invention is obtained by attaching the slurry for the secondary battery electrode of the present invention to a current collector such as a metal foil. Examples of a method for producing such a secondary battery electrode include a method in which the slurry is applied to a current collector and dried. The electrode active material is dispersed and fixed to the electrode in a binder formed on the current collector surface.

  The current collector is not limited as long as it has conductivity, but metal foil such as aluminum foil and copper foil is usually used. Although the thickness of metal foil is not specifically limited, Usually, 1-50 micrometers, Preferably it is 1-30 micrometers. If the current collector is too thin, the mechanical strength will be weakened, which may cause production problems such as breakage and wrinkling, and if it is too thick, the capacity of the battery as a whole will decrease. Tend to. In order to increase the adhesive strength with the electrode active material layer, the current collector is preferably one whose surface is roughened. Examples of the roughening method include a mechanical polishing method, an electrolytic polishing method, and a chemical polishing method. In the mechanical polishing method, an abrasive cloth paper with a fixed abrasive particle, a grindstone, an emery buff, a wire brush provided with a steel wire or the like is used. Further, an intermediate layer may be formed on the current collector surface in order to increase the adhesive strength and conductivity with the electrode active material layer.

  The method for applying the slurry to the current collector is not particularly limited. For example, it is applied by a doctor blade method, a dip method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, a brush coating, or the like. The amount to be applied is not particularly limited, but is an amount such that the thickness of the electrode active material layer formed after removing the organic solvent is usually 0.005 to 5 mm, preferably 0.01 to 2 mm. The drying method is not particularly limited, and examples thereof include drying with warm air, hot air, low-humidity air, vacuum drying, and drying by irradiation with (far) infrared rays or electron beams. The drying conditions are adjusted so that the liquid medium volatilizes as soon as possible within a speed range that normally prevents stress concentration from causing cracks in the electrode active material layer or peeling of the electrode active material layer from the current collector. To do.

  Further, the electrode may be stabilized by pressing the current collector after drying. Examples of the pressing method include a mold press and a calendar press.

The secondary battery electrode of the present invention preferably has a high density. The higher the density, the higher the effect of the present invention. The density of the electrode is determined by the active material used and the mixing ratio thereof. For example, when LiCoO ₂ that is the positive electrode active material is used and the mixing ratio is 90 wt% or more, the density is 3.7 g / cc or more. A density is preferred. The rate characteristics tend to deteriorate as the electrode density increases, but high rate characteristics can be maintained by using the binder of the present invention.

4). Secondary battery The secondary battery of the present invention is a secondary battery having the above-described secondary battery electrode of the present invention. More specifically, it is obtained by combining the above-described secondary battery electrode and electrolytic solution with components such as conventionally known separators and battery containers. As a specific manufacturing method, for example, a positive electrode and a negative electrode are overlapped via a separator, and this is wound according to the shape of the battery, folded into a battery container, and an electrolyte is injected into the battery container to seal it. The method of doing is mentioned. If necessary, an expanded metal, an overcurrent prevention element such as a fuse or a PTC element, a lead plate, or the like can be inserted to prevent an increase in pressure inside the battery and overcharge / discharge. The shape of the battery may be any of a coin shape, a button shape, a sheet shape, a cylindrical shape, a square shape, a flat shape, and the like.

As the electrolytic solution, an organic electrolytic solution in which a supporting electrolyte is dissolved in an organic solvent is used. A lithium salt is used as the supporting electrolyte. The lithium salt is not particularly _{limited, LiPF 6, LiAsF 6, LiBF} 4, LiSbF 6, LiAlCl 4, LiClO 4, CF 3 SO 3 Li, C 4 F 9 SO 3 Li, CF 3 COOLi, (CF 3 CO) ₂ NLi, (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) NLi, and the like. Among these, LiPF ₆ , LiClO ₄ , and CF ₃ SO ₃ Li that are easily soluble in a solvent and exhibit a high degree of dissociation are preferable. Two or more of these may be used in combination. Since the lithium ion conductivity increases as the supporting electrolyte having a higher degree of dissociation is used, the lithium ion conductivity can be adjusted depending on the type of the supporting electrolyte.

  The organic solvent used in the electrolytic solution is not particularly limited as long as it can dissolve the supporting electrolyte, but dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), butylene carbonate. Carbonates such as (BC) and methyl ethyl carbonate (MEC); esters such as γ-butyrolactone and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; sulfur-containing compounds such as sulfolane and dimethyl sulfoxide; Are preferably used. Moreover, you may use the liquid mixture of these solvents. Among these, carbonates are preferable because they have a high dielectric constant and a wide stable potential region. Since the lithium ion conductivity increases as the viscosity of the solvent used decreases, the lithium ion conductivity can be adjusted depending on the type of the solvent.

  The concentration of the supporting electrolyte in the electrolytic solution is usually 1 to 30% by weight, preferably 5% to 20% by weight. Further, it is usually used at a concentration of 0.5 to 2.5 mol / L depending on the type of the supporting electrolyte. If the concentration of the supporting electrolyte is too low or too high, the ionic conductivity tends to decrease. Since the degree of swelling of the polymer particles increases as the concentration of the electrolytic solution used decreases, the lithium ion conductivity can be adjusted by the concentration of the electrolytic solution.

(Example)
Hereinafter, the present invention will be described with reference to examples, but the present invention is not limited thereto. In the examples, parts and% are based on weight unless otherwise specified.
Evaluation in Examples and Comparative Examples was performed under the following conditions.

<Binder properties: compatibility evaluation>
The haze value of the NMP solution of the obtained polymer was measured with a digital haze meter.
Further, the obtained NMP solution of the polymer was dried at 120 ° C. for 10 hours in a nitrogen atmosphere to produce a polymer film of the polymer, and the microphase separation state was observed with a transmission electron microscope. When the haze value of the NMP solution of the polymer was 5% or less and the domain size of the TEM image was 200 nm or less, it was judged that the polymer was compatible.

<Slurry characteristics: slurry stability>
The obtained electrode slurry is put in a test tube having a diameter of 1 cm to a height of 5 cm, and five test samples are used. The test sample is placed vertically on a desk. The state of the installed electrode slurry is observed for 10 days and determined according to the following criteria. It shows that the slurry stability is so excellent that two-phase separation is not observed.
A: Two-phase separation is not observed even after 10 days.
B: Two-phase separation is observed after 6 to 10 days.
C: Two-phase separation is observed after 2 to 5 days.
D: Two-phase separation is observed after 1 day.
E: Two-phase separation is observed within 3 hours.

<Electrode characteristics: flexibility>
The electrode is cut into a rectangle having a width of 1 cm and a length of 5 cm to form a test piece. Place the test specimen on the current collector side down and lay a stainless steel rod with a diameter of 1 mm on the current collector side in the center of the length (2.5 cm from the end). Install. The test piece is bent 180 degrees around the stainless bar so that the electrode active material layer is on the outside. Using stainless bars having different diameters, the bent portions of the electrode active material layer of the test piece are observed for cracking or peeling, and determined according to the following criteria. It shows that an electrode is excellent in flexibility so that there is no cracking when it is wound on a stainless steel rod having a small diameter.
A: No cracking by scratching to 1mm stainless steel rod B: No cracking by hammering to 1.5mm stainless steel rod C: Cracking by hammering to 1mm stainless steel rod C: By soldering to 2mm stainless steel rod No cracking by cracking to 1.5mm stainless steel rod D: No cracking by scratching to 2.5mm stainless steel rod E: Cracking by soldering to 2mm stainless steel rod E: To stainless steel rod of 3mm or more No cracks
Cracked by hammering on 2.5mm stainless steel rod

<Battery characteristics: cycle characteristics>
Using the obtained lithium ion secondary battery, a charge / discharge cycle in which the battery was charged to 4.3 V at a constant current of 0.1 C at 20 ° C. and discharged to 3.0 V at a constant current of 0.1 C was performed. The charge / discharge cycle was performed up to 100 cycles, and the ratio of the discharge capacity at the 100th cycle to the initial discharge capacity was defined as the capacity retention rate, and the following criteria were used. It shows that the capacity | capacitance reduction by repeated charging / discharging is so small that this value is large.
A: 80% or more B: 75% or more and less than 80% C: 70% or more and less than 75% D: 50% or more and less than 70% E: Less than 50%

<Battery characteristics: Rate characteristics>
The discharge capacity at each constant current amount was measured in the same manner as the measurement of the cycle characteristics except that the measurement condition was changed to a constant current amount of 2.0C. The ratio of the discharge capacity at 2.0 C to the battery capacity at 0.1 C was calculated as a percentage to obtain charge / discharge rate characteristics, and the following criteria were used. It shows that internal resistance is so small that this value is large, and high-speed charge / discharge is possible.
A: 65% or more B: 60% or more and less than 65% C: 55% or more and less than 60% D: 50% or more and less than 55% E: 40% or more and less than 50% F: Less than 40%

Example 1
<Preparation of polymer-1>
In an autoclave equipped with a stirrer, 70 parts of acrylonitrile, 100 parts of component (A) consisting of 27 parts of butyl acrylate and 3 parts of glycidyl methacrylate, 1200 parts of ion-exchanged water, 15 parts of sodium dodecylbenzenesulfonate and 0.9 parts of potassium persulfate The mixture was sufficiently stirred and heated to 60 ° C. to initiate the first stage polymerization. When the polymerization conversion rate reached 70%, the component (B) comprising 15 parts of acrylonitrile, 82 parts of butyl acrylate and 3 parts of glycidyl methacrylate as the second stage monomer was added to the monomer composition (A ) Was added in an amount equal to 100 parts, and the reaction was continued. When the second stage polymerization conversion reached 95% or more, the polymerization was stopped by cooling. Next, 3 times the total weight of N-methylpyrrolidone (NMP) was added, and the water was evaporated by an evaporator to obtain an NMP solution of polymer-1 having a solid content of 6%. The glass transition temperature and the compatible state on the low temperature side of polymer-1 were evaluated. The results are shown in Table 2.

<Preparation of slurry for positive electrode>
To 100 parts of lithium-containing cobalt oxide active material (product name: Cellseed C-10N, Nippon Chemical Industry), 1.2 parts of binder (corresponding to solid content) and 2 parts of acetylene black as a conductive agent are mixed, and N -A slurry for positive electrode was prepared by mixing methylpyrrolidone so as to have a solid content of 80% and mixing with a planetary mixer.

<Production of positive electrode>
The positive electrode slurry is applied on a 20 μm thick aluminum foil with a comma coater so that the film thickness after drying is about 120 μm, dried at 60 ° C. for 20 minutes, and then heat-treated at 150 ° C. for 2 hours to form an electrode. I got the original fabric. This electrode original fabric was rolled with a roll press to produce a positive electrode plate in which the density was 3.7 g / cm ³ and the total thickness of the copper foil and the positive electrode active material layer was controlled to 100 μm. The flexibility of the produced positive electrode plate was measured. The results are shown in Table 2.

<Production of battery>
The obtained positive electrode plate was cut out into a circular sheet having a diameter of 15 mm. A separator made of a circular polypropylene porous film having a diameter of 18 mm and a thickness of 25 μm, a metallic lithium used as a negative electrode, and an expanded metal are sequentially laminated on the positive electrode active material layer surface side of this positive electrode, and this is made of stainless steel provided with a polypropylene packing. It was stored in a coin-type outer container (diameter 20 mm, height 1.8 mm, stainless steel thickness 0.25 mm). The electrolyte is poured into the container so that no air remains, and the outer container is fixed with a 0.2 mm thick stainless steel cap through a polypropylene packing, and the battery can is sealed, and the diameter is A lithium ion secondary battery having a thickness of 20 mm and a thickness of about 2 mm was produced. The battery characteristics of the produced lithium ion secondary battery were evaluated. The results are shown in Table 2.

(Examples 2-7 and Comparative Example 1)
When preparing the polymer solution, the same operations as in Example 1 were performed except that the monomers constituting the component (A) and the component (B), and the weight ratio thereof were changed as shown in Table 1. went. The evaluation results are shown in Table 2.

(Example 8)
When producing the slurry for the positive electrode, the same as Example 1 except that a Co—Ni—Mn lithium composite oxide based active material (product name: Cellseed NMC613, manufactured by Nippon Chemical Industry Co., Ltd.) was used as the positive electrode active material. Was performed. The evaluation results are shown in Table 2.

(Comparative Example 2)
A polymer having a solid content concentration of 6% obtained by dissolving 50 parts of poly (n-butyl acrylate) (glass transition temperature of −55 ° C.) and 50 parts of polyvinylidene fluoride (glass transition temperature of −40 ° C.) in NMP. A solution was obtained. In Example 1, the same operation as in Example 1 was performed, except that a mixed solution of n-butyl polyacrylate and polyvinylidene fluoride was used as the polymer solution instead of polymer-1. The evaluation results are shown in Table 2.

 In Table 1, “AN” represents acrylonitrile, “BA” represents n-butyl acrylate, “2EHA” represents 2-ethylhexyl acrylate, and “GMA” represents glycidyl methacrylate.

 According to the present invention, as shown in Examples 1 to 8, in the component (A) and the component (B) of the copolymer of (meth) acrylonitrile and (meth) acrylic acid ester constituting the binder. When the content of each (meth) acrylonitrile contained is within the predetermined range of the present application, the stability of the slurry and the flexibility of the electrode are good, and a secondary battery excellent in battery characteristics such as rate characteristics and cycle characteristics is obtained. You can see that Further, in the examples, the content of (meth) acrylonitrile in the component (A) is 70 to 85% by weight, and the content of (meth) acrylonitrile in the component (B) is in the range of 15 to 25% by weight. And a ratio of component (A) to component (B) in a weight ratio of 40:60 to 60:40 is used, and a Co—Ni—Mn lithium composite oxide system is used as an electrode active material. Example 8 was the most excellent in all of slurry stability, electrode flexibility, and battery characteristics.

  On the other hand, in the case where the compatibility of the polymer components is inferior (Comparative Example 1) and in the case of mixing two kinds of polymers (Comparative Example 2), all of the slurry stability, electrode flexibility and battery characteristics are remarkably inferior.

Claims (7)

A binder for a secondary battery electrode comprising a copolymer of (meth) acrylonitrile and (meth) acrylic acid ester,
The copolymer is
A component (A) having a (meth) acrylonitrile unit content of 60% by weight to 95% by weight;
The content of the (meth) acrylonitrile unit is 5% by weight or more and 40% by weight or less of the component (B),
A binder for a secondary battery electrode, wherein the component (A) and the component (B) are compatible.  The binder for secondary battery electrodes according to claim 1, wherein a ratio of the component (A) to the component (B) in the copolymer is 20:80 to 80:20 by weight. A monomer composition (A) having a (meth) acrylonitrile content of 60 wt% or more and 95 wt%,
2. The method for producing a binder for a secondary battery electrode according to claim 1, wherein the monomer composition (B) having a (meth) acrylonitrile content of 5 wt% to 40 wt% is subjected to multistage polymerization. .   The binder for a secondary battery electrode according to claim 3, wherein the multistage polymerization includes a polymerization step in which the component (A) is a main product and a polymerization step in which the component (B) is a main product. Production method.  The slurry for secondary battery electrodes which has the binder for secondary battery electrodes of Claim 1 or 2, an electrode active material, and a solvent.   The secondary battery electrode by which the electrode active material layer which consists of a binder for secondary battery electrodes and an electrode active material of Claim 1 or 2 is laminated | stacked on the electrical power collector. A secondary battery having a positive electrode, an electrolyte, a separator and a negative electrode,
The secondary battery whose said positive electrode or negative electrode is the secondary battery electrode of Claim 6.