JP2014001368A - Polymer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polymer which is used for adhesive, paint and binder for accumulation of electricity devices, is superior in adhesiveness and binding property and is soluble to organic solvent such as N-methyl-2-pyrrolidone (NMP).SOLUTION: In a star polymer consisting of a core unit having a four- or more-membered ring structure and arm units extending from three or more carbon atoms constituting the ring structure of the core unit, the arm units are composed of polymers of acid monomers having carboxyl group and polymers of styrene and the polymers of styrene are included in at least one part of the one arm unit.

Description

  The present invention relates to a polymer used for an adhesive, a paint, a storage battery binder, and the like.

  A lithium ion secondary battery is a secondary battery having a high charge / discharge capacity and capable of high output. Currently, it is mainly used as a power source for portable electronic devices, and further expected as a power source for electric vehicles that are expected to be widely used in the future. A lithium ion secondary battery has an active material capable of inserting and removing lithium (Li) in a positive electrode and a negative electrode, respectively. And it operates by moving lithium ions in the electrolyte provided between the two electrodes.

  In lithium ion secondary batteries, lithium-containing metal composite oxides such as lithium cobalt composite oxide are mainly used as the active material for the positive electrode, and carbon materials having a multilayer structure are mainly used as the active material for the negative electrode. Yes.

  The performance of the lithium ion secondary battery depends on the materials of the positive electrode, the negative electrode, and the electrolyte constituting the secondary battery. In particular, research and development of active material that forms an active material is being actively conducted. For example, silicon or silicon oxide having a higher capacity than carbon has been studied as a negative electrode active material.

  By using silicon as the negative electrode active material, a battery having a higher capacity than that using a carbon material can be obtained. However, silicon has a large volume change due to insertion and extraction of lithium during charging and discharging. Therefore, there is a problem that silicon is pulverized and falls off or peels from the current collector, and the charge / discharge cycle life of the battery is short. Therefore, by using silicon oxide as the negative electrode active material, volume change associated with insertion and extraction of lithium during charge and discharge can be suppressed more than silicon.

For example, the use of silicon oxide (SiO _x : x is about 0.5 ≦ x ≦ 1.5) as a negative electrode active material has been studied. It is known that SiO _x decomposes into Si and SiO ₂ when heat-treated. This is called disproportionation reaction, and it is separated into two phases of Si phase and SiO ₂ phase by solid internal reaction. The Si phase obtained by separation is very fine. Further, the SiO ₂ phase covering the Si phase has a function of suppressing the decomposition of the electrolytic solution. Therefore, the secondary battery using the negative electrode active material composed of SiO _x decomposed into Si and SiO ₂ has excellent cycle characteristics.

  The negative electrode containing the negative electrode active material described above is produced, for example, by applying a slurry containing a negative electrode active material and a binder to a current collector and drying it. For this reason, the performance of the binder responsible for the binding between the active material particles and the binding between the active material and the current collector greatly affects the performance of the negative electrode. When the binding force of the binder is low, the adhesiveness between the active material particles and the adhesiveness between the active material and the current collector are lowered, and the current collecting property is lowered.

  Even in the case of a negative electrode using the above-described negative electrode active material made of silicon oxide, a change in volume associated with insertion and extraction of lithium during charge / discharge reaction is inevitable. For this reason, since a big stress acts on the binder contained in the active material layer of a negative electrode, strong binding force is calculated | required by the binder.

  For example, Patent Document 1 below describes a negative electrode for a lithium ion secondary battery that contains a polymer selected from the group consisting of polyacrylic acid and polymethacrylic acid, and the polymer contains an acid anhydride group.

  Patent Document 2 below describes that a polymer obtained by copolymerizing acrylic acid and methacrylic acid is used as a negative electrode binder or a positive electrode binder.

  Further, Patent Document 3 below describes that a polymer obtained by copolymerizing acrylamide, acrylic acid and itaconic acid is used as a negative electrode binder or a positive electrode binder.

JP 2007-115671 Japanese Patent Laid-Open No. 2003-268053 JP 2006-513554 A

  Examples of conventionally used negative electrode binders include fluorine-containing polymers such as polyvinylidene fluoride (PVdF), water-soluble cellulose derivatives such as carboxymethyl cellulose (CMC), and water-soluble polymers such as polyacrylic acid. However, when these polymers are used as the binder for the negative electrode, the binding force of the active material to the current collector is still insufficient. There was a problem that it was gradually dropped from the stage and sufficient cycle characteristics could not be obtained.

  Therefore, the inventors of the present application are polymers having a core part and an arm part made of a polymer chain extending from the core part, the core part has a ring structure of four or more members, and the arm part has a carboxyl group. It consists of a polymer of an acid monomer, and the arm part extends from three or more carbon atoms constituting the ring structure of the core part, and one end of each arm part is a single bond or a carbon atom constituting the ring structure of the core part. We have developed polymers linked via ether groups, ester groups, carbonyl groups, alkylene groups, or divalent groups that combine these. According to this polymer, rigidity is expressed by the core part having a ring structure, and adhesiveness and flexibility are expressed by the arm part having many carboxyl groups. Therefore, it has excellent adhesion to various substances, and is extremely useful as an adhesive, paint, and storage battery binder.

  However, the polymer is soluble in water but not in N-methyl-2-pyrrolidone (NMP) or the like. When forming a negative electrode of a non-aqueous secondary battery, a slurry composed of a negative electrode active material, a conductive additive, a binder, and an organic solvent such as N-methyl-2-pyrrolidone (NMP) is used. If the polymer is used as a binder, water must be used as a solvent, and there is a problem that the types of conductive assistants and negative electrode active materials are limited.

  The present invention has been made in view of the above circumstances, and provides a polymer that is excellent in adhesiveness and binding properties and is soluble in an organic solvent such as N-methyl-2-pyrrolidone (NMP). Objective.

A feature of the polymer of the present invention that solves the above problems is a polymer having a core part and an arm part composed of a polymer chain extending from the core part, the core part having a ring structure of four or more members, The number of the parts is 3 or more and extends from 3 or more carbon atoms constituting the ring structure of the core part, and one end of each arm part is a single bond or an ether group or an ester to the carbon atom constituting the ring structure of the core part. Bonded through a group, a carbonyl group, an alkylene group or a divalent group obtained by combining these,
The arm portion consists of a polymer of an acid monomer having a carboxyl group and a styrene polymer, and the styrene polymer is contained in at least a part of at least one arm portion.

  According to the polymer of the present invention, rigidity is expressed by the core portion having a ring structure, and adhesiveness and flexibility are expressed by the arm portion having many carboxyl groups. Therefore, it has excellent adhesion to various substances, and is extremely useful as an adhesive, paint, and storage battery binder. The compatibility with the organic solvent is improved by the styrene polymer contained in the arm portion, so that it becomes soluble in N-methyl-2-pyrrolidone (NMP) or the like.

  Furthermore, if the polymer of this invention is used for electrodes, such as a lithium ion secondary battery, dispersibility, such as acetylene black used as a conductive support agent, and ketjen black, will improve significantly. Therefore, the dispersing agent generally used for the conductive auxiliary agent can be made unnecessary, and the problem of resistance increase and capacity reduction due to the dispersing agent can be avoided, so that the performance of the power storage device is improved.

The structural formula of the polymer which concerns on an Example is shown. It is a particle size distribution map of acetylene black in the polymer solution concerning an example. It is a particle size distribution map of acetylene black in a polymer solution concerning a comparative example. It is a particle size distribution map of the ketjen black in the polymer solution which concerns on an Example. It is a particle size distribution figure of ketjen black in a polymer solution concerning a comparative example. It is a TEM photograph which shows the dispersibility of acetylene black in the polymer solution which concerns on an Example. It is a TEM photograph which shows the dispersibility of ketjen black in the polymer solution which concerns on an Example. It is a TEM photograph which shows the dispersibility of acetylene black in a polyacrylic acid solution. It is a TEM photograph which shows the dispersibility of ketjen black in a polyacrylic acid solution. It is a graph which shows the discharge capacity of the lithium ion secondary battery which has a negative electrode which contains the polymer which concerns on an Example as a binder.

  The polymer of the present invention comprises a core part and an arm part. The core part has a ring structure of four or more members, and may be derived from a monocyclic compound composed only of carbon or derived from a heterocyclic compound containing an element other than carbon. It may be. Examples of the four-membered monocyclic compound include cyclobutane, cyclobutene, and cyclobutadiene, and examples of the four-membered heterocyclic compound include azetidine, oxetane, azeto, and trimethylene sulfide. A four-membered ring structure must have at least three carbon atoms.

  Cyclopentane is a typical example of a five-membered monocyclic compound, and examples of a five-membered heterocyclic compound include azolidine, azole, imidazole, pyrazole, imidazoline, pyrrole, which contain nitrogen as a hetero atom. Examples include oxolane and oxygen containing oxygen as a hetero atom, thiol containing nitrogen as a hetero atom, oxazole containing nitrogen and oxygen as a hetero atom, and thiazole containing nitrogen and sulfur as a hetero atom. A five-membered ring structure requires at least three carbon atoms.

  Examples of the six-membered monocyclic compound include benzene and cyclohexane, and the six-membered heterocyclic compound includes piperidine containing nitrogen as a hetero atom, pyridine, pyrazine, tetrahydropyran containing oxygen as a hetero atom, Examples include thiane and thiapyran containing sulfur as a hetero atom, morpholine containing nitrogen and oxygen as a hetero atom, and thiazine containing nitrogen and sulfur as a hetero atom. A six-membered ring structure requires at least three carbon atoms.

  Examples of the seven-membered monocyclic compounds include cycloheptane and cycloheptene. Examples of the seven-membered heterocyclic compounds include hexamethyleneimine (azeban), azatropylidene (azepine), heterozygote containing nitrogen as a hetero atom. Examples include hexamethylene oxide (oxeban) containing oxygen as an atom, oxycycloheptatriene (oxepin), and tiotropylidene (thiepin) containing sulfur as a heteroatom. A seven-membered ring structure must have at least three carbon atoms.

  Examples of the eight-membered monocyclic compound include cyclooctane and cyclooctene. It may be a core portion derived from a monocyclic compound having at least eight members or a heterocyclic compound. The ring structure of an eight-membered ring needs to have at least 3 carbon atoms.

  The core part may be only one ring or may have a polycyclic structure composed of a plurality of rings. For example, as a combination of a plurality of six-membered monocyclic compounds, there are anthracene, naphthacene, pentacene, benzopyrene, chrysene, pyrene, triphenylene, corannulene, coronene, obalene, and the like.

  The arm part is a polymer chain extending from the core part, and consists of a polymer of an acid monomer having a carboxyl group and a polymer of styrene. Examples of the acid monomer include acrylic acid, methacrylic acid, itaconic acid, fumaric acid, (anhydrous) maleic acid, and vinyl acetate. The arm part may be a homopolymer of one kind of monomer selected from these acid monomers, or may be a copolymer of a plurality of monomers. For example, polyacrylic acid, polymethacrylic acid, polymaleic acid, polyvinyl acetate, acrylic acid-methacrylic acid copolymer, acrylic acid-maleic acid copolymer, methacrylic acid-maleic acid copolymer, acrylic acid-fumaric acid copolymer Polymer, methacrylic acid-fumaric acid copolymer, acrylic acid-itaconic acid copolymer, methacrylic acid-itaconic acid copolymer, acrylic acid-vinyl acetate copolymer, methacrylic acid-vinyl acetate copolymer, acrylic acid- Examples thereof include a methacrylic acid-maleic acid copolymer, an acrylic acid-methacrylic acid-fumaric acid copolymer, and an acrylic acid-methacrylic acid-itaconic acid copolymer.

  A copolymer obtained by copolymerizing a part of the acid monomer in place of another monomer such as styrene, a styrene derivative, butylene, isobutylene, an acrylic ester, a methacrylic ester, or acrylonitrile may be used.

  The arm part extends from three or more carbon atoms constituting the ring structure of the core part, and one end of each arm part is bonded to the carbon atom constituting the ring structure of the core part with a single bond or an ether group, an ester group, and a carbonyl group. They are bonded via a group, an alkylene group or a divalent group obtained by combining these groups. The polymers constituting each arm part may be the same or different.

  The arm portion is composed of a polymer of an acid monomer having a carboxyl group and a polymer of styrene. For example, when the number of arm portions is 6, some of them can be formed from a polymer of acid monomers having a carboxyl group, and the remaining arm portions can be formed from a styrene polymer. Alternatively, one arm portion may include a polymer block of an acid monomer having a carboxyl group and a polymer block of styrene. From the viewpoint of compatibility with an organic solvent, it is preferable that a polymer block of styrene exists on the terminal side of the arm portion.

  The composition ratio (polymer of styrene / polymer of acid monomer) of the polymer of the acid monomer having a carboxyl group and the polymer of styrene is desirably in the range of 3 to 30 in terms of mass ratio. If the styrene polymer is less than this range, the compatibility with the organic solvent is reduced. If the styrene polymer is more than this range, the binding property as a binder is lowered, and the lithium transport ability is also lowered. Become.

  Further, it is desirable that at least one arm part includes a polyacrylic acid skeleton represented by Formula 1. By including a polyacrylic acid skeleton, the binding force is further increased, and it is useful as a binder for electrodes of power storage devices such as lithium ion secondary batteries.

  The molecular weight of the polymer constituting at least one arm part is preferably in the range of 1,000 to 100,000, more preferably 1,000 to 50,000, and 1,000 to 10,000 in terms of number average molecular weight (Mn). When the molecular weight of the arm part is less than 1,000, flexibility and adhesion are insufficient, and when the molecular weight of the arm part exceeds 100,000, it becomes difficult to dissolve in the solvent. When the molecular weight of the arm part is 50,000 to 100,000, there is a possibility of gelation, and when used as a binder, there is network-like adhesion. In addition, when the molecular weight of the arm part is 1,000 to 50,000, the distribution of the chain is stable, so that the dispersibility is high. The molecular weight of each arm part may be the same or different.

  Of the carbon atoms that make up the ring structure of the core, not only hydrogen but also various substituents such as alkyl groups such as methyl and ethyl groups, carboxyl groups, and hydroxyl groups are bonded to carbon atoms that are not bonded to the arm. You may do it.

  In order to synthesize the polymer of the present invention, a method of polymerizing a monomer using a polyfunctional initiator, a coupling method of a living polymer and a polyfunctional reagent, and a method of adding an appropriate amount of a polymerizable divinyl compound to the living polymer A homomonomer homopolymerization method or the like can be employed. Among them, the number of arm portions can be easily controlled by using a method of polymerizing a monomer using a polyfunctional initiator or a coupling method of a living polymer and a polyfunctional reagent. A method of polymerizing a monomer using a polyfunctional initiator is suitable for the synthesis of a star polymer having a long-chain arm portion.

  Alternatively, a star polymer may be synthesized using an ester of an acid monomer as an arm portion monomer, and then the ester group may be hydrolyzed to generate a carboxyl group.

  Hereinafter, embodiments of the present invention will be specifically described with reference to Examples and Comparative Examples.

  FIG. 1 shows a typical structural formula of a polymer according to this example. This star polymer is composed of a core part 1 composed of a benzene ring and six arm parts 2 bonded to all six carbon atoms constituting the ring structure of the core part 1. The arm portion 2 of the star polymer has a polyacrylic acid block 20 and a polystyrene block 21, and one end of the polyacrylic acid block 20 is bonded to a carbon atom constituting the core portion 1 through a methylene group. One end of a polystyrene block 21 is bonded to the other end of the polyacrylic acid block 20, and a hydroxyl group 3 is bonded to the other end of the polystyrene block 21.

  Some cores 1 have a carbon atom to which the arm part 2 is not bonded, and a hydroxyl group is bonded to the carbon atom via a methylene group.

  Hereinafter, a method for synthesizing this star polymer will be described.

  First, methyl acrylate was vacuum distilled at room temperature to remove the contained polymerization inhibitor. 160 ml of this methyl acrylate, 0.5 g of hexakis (bromomethyl) benzene as a base skeleton compound shown in the chemical formula 2, 20 ml of 2-propanol, and 2 g of tris 2-dimethylaminoethylamine as an amine-based ligand (ligand) The mixture was placed in an eggplant-shaped flask, stirred well, and allowed to stand.

  Furthermore, after adding 0.74 g of copper (I) bromide as a metal halide (activator), the solution in the flask was heated to 50 ° C. to 52 ° C. with stirring in a nitrogen gas atmosphere, and the solution turned green. After confirming this, the mixture was further heated and stirred for 6 hours. The reaction atmosphere may be a non-oxidizing atmosphere such as an argon gas atmosphere or a reduced pressure atmosphere as well as a nitrogen gas atmosphere. At this time, copper bromide (I) deposits bromine from the base skeleton compound, and a carbon radical is generated in the base skeleton compound, and polymerization of methyl acrylate existing in the system starts. The growing radical is again bonded to bromine and becomes a polymer having a C-Br bond at the terminal. The bromine atom of this C—Br bond moves again to copper (I) bromide, and the growth reaction continues, so the molecular weight increases with time. The solution turned initially purplish gray and a trace amount of precipitate formed.

  After completion of heating, the solution was cooled to room temperature, the system was opened, 10 ml of acetic acid was added to neutralize the amine ligand, and the mixture was stirred well. The solution in the flask was poured into a 6 to 8-fold volume of methanol / acetic acid solution (acetic acid concentration: about 1% by weight) and stirred well to precipitate the polymer. At this time, the solution is colored blue by copper ions.

  The resulting precipitate was collected by filtration and dissolved in approximately 5 times the volume of acetone as the precipitate. This solution was poured into a 6-8 volume methanol / acetic acid solution (acetic acid concentration: about 1% by weight) and stirred well to precipitate the polymer. This operation was repeated 3 to 5 times to wash the precipitate. When the color of the methanol / acetic acid solution disappeared, the polymer precipitate was collected by filtration and dried in vacuum at room temperature.

  Theoretically, this polymer is a star polymer having the structure shown in Chemical Formula 3, the arm portion is polymethyl acrylate, and the end of the arm portion is a -C-Br group. This star polymer had a number average molecular weight (Mn) = 71,300 and PDI = 1.60 in terms of polystyrene.

  This polymer was analyzed by 1H-NMR (JOEL GSX, 400 MHz, deuterated chloroform, 19.7 ° C.). When the -C-Br group is bonded to the core, the peak appears in the vicinity of 4.56 ppm (document value). On the other hand, the peak of the -C-Br group bonded to the end of the arm part appears at a position slightly lower than 4.56 ppm. Therefore, the number of arm portions can be calculated from the area ratio of these two peaks.

  Since the peak area of -C-Br group bonded to the core part was 1.00 and the peak area of -C-Br group bonded to the end of the arm part was 9.56, the number of arm parts was [ 9.56 / (9.56 + 1.00)] × 6 = 5.43. In other words, the average number of arm parts extending from one core part is 5.43, and the number of arm parts extending from one core part is 6, and the star polymer with the structure shown in Formula 3 must be included. I understood. Each arm part had a number average molecular weight (Mn) of 79,900.

  However, as shown in the chemical formula 4, there are those having a carbon atom to which the arm portion is not bonded to the core portion, and a bromine group is bonded to the carbon atom via a methylene group.

  Next, 37 g of xylene, 3.6 g of 2-propanol, 20 g of the above star polymer, 30 g of styrene monomer, and 0.25 g of tris 2-dimethylaminoethylamine as an amine-based ligand are placed in an eggplant type flask. After stirring, it was allowed to stand. Further, 0.14 g of copper (I) bromide was added, and the solution in the flask was heated to 110 ° C. with stirring in a reduced-pressure atmosphere. After confirming that the solution had turned green, the mixture was further heated and stirred for 2 hours. Note that the reaction atmosphere is not limited to a reduced pressure atmosphere, but may be a non-oxidizing atmosphere such as a nitrogen gas atmosphere or an argon gas atmosphere.

  At this time, copper (I) bromide deposits bromine from the end of the arm part, carbon radicals are generated in the arm part, and polymerization of the styrene monomer present in the system begins. The growing radical is again bonded to bromine and becomes a polymer having a C-Br bond at the terminal. The bromine atom of this C—Br bond moves again to copper (I) bromide, and the growth reaction continues, so the molecular weight increases with time.

  After completion of heating, the solution was cooled to room temperature, the system was opened, 10 ml of acetic acid was added to neutralize the amine ligand, and the mixture was stirred well. The solution in the flask was poured into a 6 to 8-fold volume of methanol / acetic acid solution (acetic acid concentration: about 1% by weight) and stirred well to precipitate the polymer. At this time, the solution is colored blue by copper ions.

  The resulting precipitate was collected by filtration and dissolved in approximately 5 times the volume of acetone as the precipitate. This solution was poured into a 6-8 volume methanol / acetic acid solution (acetic acid concentration: about 1% by weight) and stirred well to precipitate the polymer. This operation was repeated 3 to 5 times to wash the precipitate. When the color of the methanol / acetic acid solution disappeared, the polymer precipitate was collected by filtration and dried in vacuum at room temperature.

  When this polymer was analyzed by 1H-NMR (JOEL GSX, 400 MHz, deuterated chloroform, 19.7 ° C.), a peak identified as a carboxymethyl group and a peak identified as polystyrene were observed. Therefore, theoretically, this polymer is a star polymer mainly having the structure shown in Formula 5, the arm part is composed of a polymethyl acrylate block and a polystyrene block, and the end of the arm part is a -C-Br group. Yes.

  A bromine group bonded to the benzene ring via a methylene group also contributes to the reaction, and a polystyrene block may be formed on the benzene ring via the methylene group.

  This star polymer had a number average molecular weight (Mn) of 1,240,000 and PDI = 1.95 in terms of polystyrene, and the arm portions each had a number average molecular weight (Mn) of 526,000. The number average molecular weight (Mn) of the polymethyl acrylate block in each arm part was 79,900, and the number average molecular weight (Mn) of the polystyrene block was 22,500. The polystyrene in this star polymer determined by NMR was 18.6 mol%.

  Next, 25.0 g of the obtained polymer was dissolved in 100 ml of benzene, an aqueous potassium hydroxide solution (KOH: 78.2 g, water 150 ml) was gradually added, and the solution was allowed to stand at room temperature for 3 days with stirring. The reaction was judged to be complete when both the benzene phase and the aqueous phase became transparent when allowed to stand. After completion of the reaction, the aqueous phase was separated and recovered, and an aqueous nitric acid solution was added to adjust the pH to 3 or less. The obtained acidic aqueous solution was dialyzed with distilled water for 3 days to 1 week using a cellulose tube. The aqueous solution after dialysis was dried by freeze drying to obtain a polymer powder.

  In the obtained polymer, ester groups of polymethyl acrylate blocks are hydrolyzed to become carboxyl groups. Therefore, the arm part is composed of a polyacrylic acid block and a polystyrene block, and it is considered that the end of the arm part contains many star polymers having a structure shown in FIG.

[Comparative example]
First, methyl acrylate was vacuum distilled at room temperature to remove the contained polymerization inhibitor. 20.0 ml of this methyl acrylate, 0.225 g of hexakis (bromomethyl) benzene as a base skeleton compound shown in Chemical Formula 2, 5.00 ml of 2-propanol, and 0.57 g of tris 2-dimethylaminoethylamine as an amine-based ligand Was placed in an eggplant-shaped flask, stirred well, and allowed to stand. Further, 0.182 g of 99.9% pure copper (I) bromide was added as a metal halide, degassed with a vacuum pump, and sealed.

  The solution in the flask was heated to 50 ° C. to 52 ° C. while stirring, and after confirming that the solution had turned green, the mixture was further heated and stirred for 8 hours. At this time, copper (I) bromide deposits bromine from the base skeleton compound, carbon radicals are generated in the initiator, and polymerization of methyl acrylate present in the system begins. The growing radical is again bonded to bromine and becomes a polymer having a C-Br bond at the terminal. The bromine atom of this C—Br bond moves again to copper (I) bromide, and the growth reaction continues, so the molecular weight increases with time. The solution turned initially purplish gray and a trace amount of precipitate formed.

  After completion of heating, the solution was cooled to room temperature, the system was opened, 1.0 ml of acetic acid was added to neutralize the amine-based ligand, and the mixture was stirred well. The solution in the flask was poured into a 6 to 8-fold volume of methanol / acetic acid solution (acetic acid concentration: about 1% by weight) and stirred well to precipitate the polymer precursor. At this time, the solution is colored blue by copper ions.

  The resulting precipitate was collected by filtration and dissolved in approximately 5 times the volume of acetone as the precipitate. This solution was poured into a methanol / acetic acid solution of 6 to 8 times its volume (acetic acid concentration: about 1% by weight) and stirred well to precipitate the polymer precursor. This operation was repeated 3 to 5 times to wash the precipitate, and when the coloring of the methanol / acetic acid solution disappeared, the precipitate of the polymer precursor was collected by filtration and vacuum dried at room temperature.

  Theoretically, this polymer precursor is considered to be a star polymer having the structure shown in Formula 3, wherein the arm portion is polymethyl acrylate, and the end of the arm portion is a -C-Br group. This star polymer had a number average molecular weight (Mn) = 35,500 and PDI = 1.60 in terms of polystyrene, and the arm portions each had a number average molecular weight (Mn) of 8,200.

  This polymer precursor was analyzed by 1H-NMR (JOEL GSX, 400 MHz, deuterated chloroform, 19.7 ° C.). When the -C-Br group is bonded to the core, the peak appears in the vicinity of 4.56 ppm (document value). On the other hand, the peak of the -C-Br group bonded to the end of the arm part appears at a position slightly lower than 4.56 ppm. Therefore, the number of arm portions can be calculated from the area ratio of these two peaks.

  Since the peak area of -C-Br group bonded to the core part was 0.05 and the peak area of -C-Br group bonded to the end of the arm part was 0.21, the number of arm parts was [ 0.21 / (0.21 + 0.05)] × 6 = 4.85. In other words, it was found that the number of arm parts is five, and the structure shown in the chemical formula 4 is mainly used.

  6.0 g of this star polymer was dissolved in 12.0 ml of toluene, an aqueous potassium hydroxide solution (KOH: 9.5 g, 20 ml of water) was gradually added, and the solution was allowed to stand at room temperature for 3 days with stirring. When both the toluene phase and the aqueous phase became transparent when allowed to stand, the reaction was judged to be complete. After completion of the reaction, the aqueous phase was separated and recovered, and an aqueous nitric acid solution was added to adjust the pH to 3 or less. The obtained acidic aqueous solution was dialyzed with distilled water for 3 days to 1 week using a cellulose tube. The aqueous solution after dialysis was dried by freeze drying to obtain a polymer powder.

  In the obtained polymer, the ester group of the arm part is hydrolyzed to form a carboxyl group, so that the arm part becomes a polyacrylic acid skeleton, and the end of the arm part has a hydroxyl group bonded thereto. Contains a lot of polymer.

<Test Example 1>
The star polymers of Examples and Comparative Examples were mixed with N-methyl-2-pyrrolidone (NMP) or distilled water so as to have a concentration of 50% by mass, and the solubility was visually determined. The results are shown in Table 1. In addition, what was completely melt | dissolved and was a transparent solution was set as (circle), and the thing in which white turbidity or precipitation produced was evaluated as x.

  From the table, it is clear that the star polymers of the examples are excellent in solubility in N-methyl-2-pyrrolidone (NMP), which is caused by including a polystyrene block in the arm portion.

<Test Example 2>
The star polymers of Examples and Comparative Examples and polyacrylic acid having a number average molecular weight of 39,800 were used as samples, and 1.00 parts by mass was dissolved in 50.0 parts by mass of N-methyl-2-pyrrolidone. 1.00 parts by mass of acetylene black powder not contained was gradually added.

  In addition, the star polymers of Examples and Comparative Examples and polyacrylic acid having a number average molecular weight of 39,800 were used as samples, and 1.00 parts by mass was dissolved in 50.0 parts by mass of N-methyl-2-pyrrolidone, and dispersed while stirring. 1.00 parts by mass of ketjen black powder containing no agent was gradually added.

  The particle size distribution of acetylene black and ketjen black in each suspension obtained was measured using a laser zeta electrometer (“ELS-8000” manufactured by Otsuka Electronics Co., Ltd.), and TEM observation was performed. The particle size distribution is shown in FIGS. 2 to 5, and the TEM images are shown in FIGS.

  As shown in FIGS. 6 to 9, both acetylene black and ketjen black are aggregated in the polyacrylic acid solution, but the degree of aggregation is extremely small in the polymer solutions of the examples. From the comparison between FIG. 4 and FIG. 5, in the example, a peak appears on the small particle size side, and the maximum peak position is also shifted to the small particle size side from the comparative example. Therefore, the dispersibility of ketjen black is improved by using the star polymer of the example.

  On the other hand, in the case of acetylene black shown in FIGS. 2 and 3, since a large peak appears on the small particle size side as shown in FIG. 2 in the example, acetylene is obtained by using the star polymer of the example. It is clear that the dispersibility of black is particularly improved, which is the effect of introducing polystyrene blocks.

  Therefore, if the star polymer of the embodiment is used as a binder of the power storage device, a conductive additive that does not contain a dispersant can be used, so that problems such as an increase in resistance and a decrease in capacity due to the dispersant can be avoided.

<Preparation of negative electrode for lithium ion secondary battery>
The star polymer powder of the example was dissolved in N-methyl-2-pyrrolidone (NMP) or distilled water to a concentration of 10% by mass to prepare a binder solution.

SiO powder (manufactured by Sigma-Aldrich Japan, average particle size 5 μm) was heat-treated at 900 ° C. for 2 hours to prepare SiO _x powder having an average particle size of 5 μm. With this heat treatment, if silicon monoxide SiO is a homogeneous solid having a ratio of Si to O of approximately 1: 1, it is separated into two phases of Si phase and SiO ₂ phase by solid internal reaction. The Si phase obtained by separation is very fine.

Slurries were prepared by mixing 50 parts by mass of the obtained SiO _x powder, 37 parts by mass of natural graphite powder, 3 parts by mass of ketjen black, and 100 parts by mass of a binder solution (10 parts by mass as a star polymer). .

  This slurry was applied to the surface of an electrolytic copper foil (current collector) having a thickness of 20 μm using a doctor blade to form a negative electrode active material layer on the copper foil. Thereafter, the current collector and the negative electrode active material layer were firmly and closely joined by a roll press. This was vacuum-dried at 100 ° C. for 3 hours to form a negative electrode having a negative electrode active material layer thickness of 16 μm.

<Adhesion test>
A grid adhesion test (JIS K5400-8.5) was performed in which 100 grids were cut into the negative electrode active material layer at intervals of 1 mm using a cutter knife, and cellophane tape was applied and peeled off. As a result, no delamination of the negative electrode active material layer was observed at all 100 masses, and the star polymers of Examples and Examples were excellent in adhesion to copper foil, and binding of SiO _x powder, natural graphite powder, and ketjen black Excellent in properties.

<Preparation of positive electrode>
Li [Mn _1/3 Ni _1/3 Co _1/3 ] O ₂ as a positive electrode active material, acetylene black (AB) as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder resin are mixed. A slurry-like positive electrode mixture was prepared. The composition ratio of each component (solid content) in the slurry was Li [Mn _1/3 Ni _1/3 Co _1/3 ] O ₂ : AB: PVdF = 93: 3: 4 (mass ratio). This slurry was applied to a current collector, and a positive electrode mixture layer was laminated on the current collector. Specifically, this slurry was applied to the surface of an aluminum foil (current collector) having a thickness of 20 μm using a doctor blade.

  Thereafter, drying was performed at 80 ° C. for 20 minutes, and the organic solvent was volatilized and removed from the positive electrode mixture. After drying, the electrode density was adjusted with a roll press. This was heat-cured at 120 ° C. for 6 hours in a vacuum drying furnace to obtain a positive electrode in which a positive electrode mixture layer having a thickness of about 50 μm was laminated on the upper layer of the current collector.

<Production of lithium ion secondary battery>
The positive electrode was cut into 30 mm × 25 mm and the negative electrode was cut into 31 mm × 26 mm, and accommodated with a laminate film. A rectangular sheet (40 mm × 40 mm square, thickness 30 μm) made of polypropylene resin as a separator was sandwiched between the positive electrode and the negative electrode to form an electrode plate group. The electrode plate group was covered with a set of two laminated films, and the three sides were sealed. Then, the following electrolytic solution was injected into the bag-like laminated film. Thereafter, the remaining one side was sealed to obtain a laminate cell in which the four sides were hermetically sealed and the electrode plate group and the electrolyte were sealed. The electrolyte solution is a solution of EC (ethylene carbonate), MEC (methyl ethyl carbonate), DMC (dimethyl carbonate) = 3: 3: 4 (volume ratio) dissolved in LiPF ₆ at a concentration of 1 mol / L. Was used. The positive electrode and the negative electrode were provided with a tab that could be electrically connected to the outside, and a part of this tab extended to the outside of the laminate cell. Through the above steps, a lithium ion secondary battery of a single-layer laminate cell was obtained. Three lithium ion secondary batteries were made of the same.

<Evaluation test>
Using the lithium ion secondary battery of the example, the battery was charged at 4.2 V under the conditions of CCCV charging (constant current constant voltage charging) at a measurement temperature of 25 ° C. and 0.2 C, and the CC discharge capacity of 1/3 C was measured. The results are shown in FIG. It can be seen from FIG. 10 that the lithium ion battery using the star polymer of the example as the negative electrode binder functions as a battery.

The following technical idea (invention) can be understood from the embodiment.
(1) The binder according to claim 6, which is used for a power storage device including a conductive additive.

  The polymer of the present invention can be used not only for a binder for a negative electrode of a non-aqueous secondary battery but also for a binder for a positive electrode of a non-aqueous secondary battery, a coating material, an adhesive, and a binder for an electrode of a power storage device.

1: Core part 2: Arm part 3: Hydroxyl group
20: Polyacrylic acid block 21: Polystyrene block

Claims (6)

A polymer having a core part and an arm part made of a polymer chain extending from the core part,
The core portion has a ring structure of four or more members, the arm portions are three or more, and extend from three or more carbon atoms constituting the ring structure of the core portion. One end is bonded to the carbon atom constituting the ring structure of the core part through a single bond or an ether group, an ester group, a carbonyl group, an alkylene group, or a divalent group combining these,
The arm part is composed of a polymer of an acid monomer having a carboxyl group and a polymer of styrene, and the polymer is characterized in that a polymer of styrene is contained in at least a part of at least one of the arm parts.   2. The polymer according to claim 1, wherein the ring structure includes a benzene ring.   3. The polymer according to claim 1, wherein at least one of the arm portions includes a polyacrylic acid skeleton.   4. The polymer according to claim 3, wherein the number average molecular weight (Mn) of at least one arm part is in the range of 1,000 to 100,000.   5. The polymer according to claim 1, wherein the styrene polymer is formed on each terminal side of the arm portion.   A binder for a power storage device, comprising the polymer according to any one of claims 1 to 5.