JP2018170218A - Positive electrode paste for battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode paste for a battery, which enables formation of a positive electrode less in elution of a dispersant into electrolyte solution.SOLUTION: A positive electrode paste for a battery according to one embodiment of the present invention comprises a positive electrode active material, a conducting agent, a binding agent, a copolymer and a dispersant. The copolymer contains a constituting unit (a) represented by a formula (1), a constituting unit (b) represented by a formula (2), and a constituting unit (c) represented by a formula (3). In the copolymer, the content of the constituting unit (c) is 15 mass% or more and 45 mass% or less. In the formulas (1) to (3), R, R, R, R, R, R, R, R, Rand Rindividually and independently represent a hydrogen atom, a methyl group or an ethyl group; Ris a monovalent carbon hydride group with C8-30; Ris a straight or branched chain alkylene group with C2-4; and p is a number of 1-50.SELECTED DRAWING: None

Description

  The present invention relates to a positive electrode paste for a battery.

  A positive electrode for a non-aqueous electrolyte battery is usually manufactured using a positive electrode paste in which a positive electrode active material, a conductive agent, and a binder are dispersed in a dispersion medium. Specifically, the positive electrode paste is applied to a conductive base material such as an aluminum foil and passed through a drying furnace to volatilize and remove the dispersion medium, thereby obtaining a positive electrode.

  In order to increase the productivity of the positive electrode, it has been studied to increase the line speed in the coating process. However, when the line speed is increased, the residence time in the drying furnace is shortened, resulting in insufficient drying. One of the methods that enables drying in a short time is to increase the solid content concentration in the positive electrode paste, that is, to reduce the content of the dispersion medium. However, in this case, since the viscosity of the positive electrode paste is increased, there is a disadvantage that the coating property is deteriorated, for example, fading occurs during the coating.

  Therefore, in order to suppress an increase in the viscosity of the positive electrode paste, a positive electrode paste containing a copolymer having a specific structure has been proposed (see Patent Document 1). According to this positive electrode paste, the dispersibility of the conductive agent and the like is increased by containing the copolymer, and as a result, the viscosity can be reduced.

International Publication No. 2013/151062

  However, the battery using the positive electrode obtained from the positive electrode paste containing the copolymer has a disadvantage that the rate of increase in resistance due to repeated charge / discharge is large. The inventors have found that this is because of the high solubility of the copolymer contained in the positive electrode paste in the electrolytic solution. That is, it is presumed that the resistance is increased by the copolymer in which the copolymer is eluted from the positive electrode in the electrolytic solution causing clogging of the separator existing between the positive electrode and the negative electrode. On the other hand, it is considered that the solubility of the copolymer in the electrolytic solution is lowered by increasing the molecular weight of the copolymer. However, simply increasing the molecular weight of the copolymer is not preferable because the viscosity of the paste is difficult to decrease.

  The present invention has been made based on the circumstances as described above, and an object thereof is to provide a positive electrode paste for a battery that can form a positive electrode with less copolymer elution into an electrolytic solution. .

（式（１）〜（３）中、Ｒ^１、Ｒ^２、Ｒ^３、Ｒ^５、Ｒ^６、Ｒ^７、Ｒ^９、Ｒ^１０、Ｒ^１１、及びＲ^１２は、それぞれ独立して、水素原子、メチル基又はエチル基である。Ｒ^４は、炭素数８〜３０の１価の炭化水素基である。Ｒ^８は、炭素数２〜４の直鎖状又は分岐状のアルキレン基である。ｐは、１〜５０の数である。） One embodiment of the present invention made to solve the above problems includes a positive electrode active material, a conductive agent, a binder, a copolymer, and a dispersion medium, and the copolymer is represented by the following formula (1). The structural unit (a), the structural unit (b) represented by the following formula (2) and the structural unit (c) represented by the following formula (3), The battery positive electrode paste has a content of c) of 15% by mass or more and 45% by mass or less.

(In the formulas (1) to (3), R ¹ , R ² , R ³ , R ⁵ , R ⁶ , R ⁷ , R ⁹ , R ¹⁰ , R ¹¹ , and R ¹² are each independently a hydrogen atom. R ⁴ is a monovalent hydrocarbon group having 8 to 30 carbon atoms, and R ⁸ is a linear or branched alkylene group having 2 to 4 carbon atoms. p is a number from 1 to 50.)

  ADVANTAGE OF THE INVENTION According to this invention, the positive electrode paste for batteries which can form a positive electrode with little elution of the copolymer to electrolyte solution can be provided.

  A positive electrode paste for a battery according to an embodiment of the present invention (hereinafter sometimes simply referred to as “paste” or “positive electrode paste”) includes a positive electrode active material, a conductive agent, a binder, a copolymer, and a dispersion medium. And the copolymer is a structural unit (a) represented by the following formula (1), a structural unit (b) represented by the following formula (2), and a structural unit represented by the following formula (3) ( a positive electrode paste for a battery containing c), wherein the content of the structural unit (c) in the copolymer is 15% by mass or more and 45% by mass or less.

（式（１）〜（３）中、Ｒ^１、Ｒ^２、Ｒ^３、Ｒ^５、Ｒ^６、Ｒ^７、Ｒ^９、Ｒ^１０、Ｒ^１１、及びＲ^１２は、それぞれ独立して、水素原子、メチル基又はエチル基である。Ｒ^４は、炭素数８〜３０の１価の炭化水素基である。Ｒ^８は、炭素数２〜４の直鎖状又は分岐状のアルキレン基である。ｐは、１〜５０の数である。）

(In the formulas (1) to (3), R ¹ , R ² , R ³ , R ⁵ , R ⁶ , R ⁷ , R ⁹ , R ¹⁰ , R ¹¹ , and R ¹² are each independently a hydrogen atom. R ⁴ is a monovalent hydrocarbon group having 8 to 30 carbon atoms, and R ⁸ is a linear or branched alkylene group having 2 to 4 carbon atoms. p is a number from 1 to 50.)

  Since the copolymer contained in the battery positive electrode paste has the structural unit (c) having an amide group in the side chain at a predetermined ratio, the solubility in an electrolyte solution is compared when compared with the same molecular weight. Is low. Therefore, according to the positive electrode paste for a battery, it is possible to form a positive electrode with less copolymer elution into the electrolytic solution, and as a result, to obtain a battery in which the rate of increase in resistance due to repeated charge and discharge is reduced. Can do. Moreover, since the said copolymer has the said structural unit (a) and a structural unit (b), the said positive electrode paste for batteries can suppress the raise of the viscosity at the time of making solid content concentration high, and is favorable. Has applicability and the like. This is because the structural unit (a) having a hydrophobic group is firmly adsorbed on the particle surface of the conductive agent in the paste, and the structural unit (b) having a polyoxyalkylene group provides a strong steric repulsion between the particles. Thus, it is presumed that the effect of suppressing the aggregation of particles in the paste is produced and the viscosity of the paste is lowered. As described above, the above copolymer can lower the solubility in the electrolyte without increasing the molecular weight, and thus achieves both reduction in the viscosity of the positive electrode paste and suppression of elution from the formed positive electrode into the electrolyte. Can be made.

  The mass ratio (structural unit (a) / structural unit (b)) between the structural unit (a) and the structural unit (b) of the copolymer is preferably 0.5 or more and 5 or less. By setting the mass ratio of the structural unit (a) to the structural unit (b) in the above range, the viscosity reduction effect of the positive electrode paste can be further enhanced.

  The copolymer preferably has a weight average molecular weight of 15,000 or more and 100,000 or less. Thereby, an elution suppression effect and a viscosity reduction effect can be improved with good balance.

  The content of the copolymer is preferably 0.5 parts by mass or more and 30 parts by mass or less with respect to 100 parts by mass of the conductive agent. By setting the content of the copolymer within the above range, it is possible to exert a favorable viscosity reduction effect while suppressing the amount of elution into the electrolyte solution in the positive electrode obtained.

  The dispersion medium is preferably a non-aqueous dispersion medium. Thereby, a viscosity reduction effect etc. can be heightened more.

  Hereinafter, the positive electrode paste for a battery according to an embodiment of the present invention will be described in detail.

<Positive electrode paste for batteries>
The positive electrode paste for a battery includes a positive electrode active material, a conductive agent, a binder, a copolymer, and a dispersion medium. The battery positive electrode paste may further contain other components (other functional materials other than the copolymer).

[Positive electrode active material]
The positive electrode active material is a substance that can occlude and release ions such as lithium. As the positive electrode active material, for example, a composite oxide represented by Li _x MO _y (M represents at least one transition metal) (Li _x CoO ₂ having a layered α-NaFeO ₂ type crystal structure, Li _x NiO _2). Li _x MnO ₃ , Li _x Ni _α Co _(1-α) O ₂ , Li _x Ni _α Mn _β Co _(1-α-β) O ₂ , Li _{1 + w} Ni _α Mn _β Co _{(1-α-β- w) O 2,} etc., _Li x _Mn 2 _O 4 having a spinel crystal _{structure, Li x Ni α Mn (2} -α) O 4 , _{etc.), Li w Me x (AO} y) z (Me is at least one transition Represents a metal, and A represents, for example, P, Si, B, V, etc.) polyanion compound (LiFePO ₄ , LiMnPO ₄ , LiNiPO ₄ , LiCoPO ₄ , Li ₃ V ₂ (PO ₄ ) ₃ , Li ₂ Mn SiO ₄ , Li ₂ CoPO ₄ F, etc.). The elements or polyanions in these compounds may be partially substituted with other elements or anion species. As the positive electrode active material, one kind of these compounds may be used alone, or two or more kinds may be mixed and used.

  The lower limit of the content of the positive electrode active material in the solid content of the battery positive electrode mixture paste can be, for example, 50% by mass, 60% by mass, or 80% by mass. 90 mass% is preferable. On the other hand, the upper limit of the content can be 99% by mass or 98% by mass. In addition, solid content means components other than a dispersion medium.

[Conductive agent]
The conductive agent is not particularly limited as long as it is a conductive material that does not adversely affect battery performance. Examples of such a conductive agent include natural or artificial graphite, furnace black, acetylene black, carbon black such as ketjen black, metal, conductive ceramics, and the like, and acetylene black is preferable.

Further, when a conductive agent having a large specific surface area is used, it is preferable because the charge / discharge performance (output, etc.) of the positive electrode can be improved. However, when such a conductive agent is used, the paste viscosity tends to increase. is there. However, the above copolymer is preferable because the effect of reducing the paste viscosity becomes larger as the conductive agent having a large specific surface area is used. Specifically, the lower limit of the specific surface area of the conductive agent is ^{1 m} 2 / g are preferred, more preferably ^{10 m} 2 / g, more preferably ^{30 m} 2 / g. On the other hand, the upper limit is preferably 200 meters ² / g, more preferably ^{100m 2 / g, 80m 2 /} g is more preferable. Moreover, the said specific surface area points out the BET specific surface area calculated | required by BET method.

  The lower limit of the content of the conductive agent in the solid content in the battery positive electrode paste is, for example, preferably 1% by mass and more preferably 2% by mass. By setting the content of the conductive agent to the above lower limit or more, good conductivity can be exhibited. On the other hand, the upper limit of the content may be, for example, 20% by mass, preferably 10% by mass, and more preferably 5% by mass. By making content of a electrically conductive agent below the said upper limit, adhesiveness with the base material etc. of the positive electrode compound material layer obtained can be improved.

[Binder]
The binder (binder) is a component that fixes a positive electrode active material, a conductive agent, and the like. As the binder, thermoplastic resins such as vinylidene fluoride resins (polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer, etc.), polytetrafluoroethylene (PTFE), polyethylene, polypropylene, polyimide; Examples thereof include elastomers such as ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), and fluorine rubber; polysaccharide polymers.

  As the binder, it is particularly preferable to use a vinylidene fluoride resin because of its good compatibility with the copolymer in the positive electrode paste.

  Here, since the copolymer is unsuitable as a binder for the positive electrode paste, the copolymer is not included in the binder here.

  As a minimum of content of the above-mentioned binder which accounts for solid content in the positive electrode compound paste for batteries, for example, 1 mass% is preferred, 2 mass% is more preferred, and 2.5 mass% is still more preferred. Adhesiveness with the base material etc. of the positive electrode compound material layer obtained by making content of a binder into the said minimum or more can be improved. On the other hand, as an upper limit of this content, it is 20 mass%, for example, 10 mass% is preferable and 6 mass% is more preferable.

[Copolymer]
The copolymer includes a structural unit (a) represented by the following formula (1), a structural unit (b) represented by the following formula (2), and a structural unit (c) represented by the following formula (3). Containing. The copolymer may further contain other structural unit (d). The said copolymer can raise the dispersibility of a electrically conductive agent and can reduce the viscosity of a paste.

In formulas (1) to (3), R ¹ , R ² , R ³ , R ⁵ , R ⁶ , R ⁷ , R ⁹ , R ¹⁰ , R ¹¹ , and R ¹² are each independently a hydrogen atom, A methyl group or an ethyl group. R ⁴ is a monovalent hydrocarbon group having 8 to 30 carbon atoms. R ⁸ is a linear or branched alkylene group having 2 to 4 carbon atoms. p is a number from 1 to 50.

(Structural unit (a))
The copolymer, by including the structural unit having R ⁴ is a hydrophobic group (a), it is possible to satisfactorily adsorbed on the particle surfaces of the conductive agent in the paste.

As said R < ¹ > and R < ² >, a hydrogen atom is preferable. As said R < ³ >, a hydrogen atom and a methyl group are preferable and a methyl group is more preferable. By using R ^{1 to} R ³ as these atoms or groups, the adsorptivity to the conductive agent, the viscosity reducing effect of the positive electrode paste, the ease of introduction during copolymerization, and the like can be enhanced.

Examples of the monovalent hydrocarbon group having 8 to 30 carbon atoms of R ⁴ include aliphatic hydrocarbon groups such as alkyl groups, alkenyl groups, and alkynyl groups, and aromatic hydrocarbon groups. Among these, an aliphatic hydrocarbon group is preferable, and an alkyl group and an alkenyl group are preferable. The lower limit of the carbon number of ^{R 4,} preferably 10, more preferably from 12, more preferably 13, 18 is more preferred. On the other hand, the upper limit of this carbon number is preferably 26, more preferably 22, and even more preferably 20. By limiting R ⁴ to these, the adsorptivity to a conductive agent or the like, the viscosity reduction effect of the positive electrode paste, and the like can be enhanced. Specific examples of the group represented by R ⁴ include octyl group, 2-ethylhexyl group, decyl group, lauryl group, myristyl group, cetyl group, stearyl group, oleyl group, and behenyl group.

  Monomers that give the structural unit (a) include 2-ethylhexyl (meth) acrylate, octyl (meth) acrylate, lauryl (meth) acrylate, stearyl (meth) acrylate, isostearyl (meth) acrylate, and behenyl (meth) acrylate. Etc. Among these, lauryl (meth) acrylate, stearyl (meth) acrylate, and behenyl (meth) acrylate are preferable from the viewpoint of adsorptivity to a conductive agent and the like and a viscosity reduction effect of the positive electrode paste. 1 type may be used for these monomers and 2 or more types may be used for them.

  As a minimum of content of the said structural unit (a) in the said copolymer, 10 mass% is preferable, 20 mass% is more preferable, and 30 mass% is further more preferable. On the other hand, as an upper limit of this content, 80 mass% is preferable, 70 mass% is more preferable, and 60 mass% is further more preferable. By making content of a structural unit (a) into the said range, the adsorptivity to a electrically conductive agent, the viscosity reduction effect of a positive electrode paste, etc. can be improved.

(Structural unit (b))
By containing the structural unit (b) having a polyoxyalkylene group, the copolymer can bring about a strong steric repulsion between particles adsorbed by the copolymer. Thereby, the aggregation suppression effect of the particle | grains in a paste arises, and the viscosity raise of a paste can be suppressed.

As the ^{R 5} and ^{R 6,} a hydrogen atom is preferred. R ⁷ is preferably a hydrogen atom and a methyl group, and more preferably a methyl group. R ⁹ is preferably a methyl group or an ethyl group, and more preferably a methyl group. By using R ^{5 to} R ⁷ and R ⁹ as these atoms or groups, the viscosity reduction effect of the positive electrode paste, the ease of introduction during copolymerization, and the like can be enhanced.

Examples of the linear or branched alkylene group having 2 to 4 carbon atoms in the R ^8, an ethylene group and propylene group are preferred, and more preferably an ethylene group. By using R ⁸ as these groups, the viscosity reduction effect of the positive electrode paste, the ease of introduction during copolymerization, and the like can be enhanced. In R ⁸ , a plurality of types of alkylene groups may be contained.

The above p is the average number of added moles of alkylene oxide (—R ⁸ —O—) and is not limited to an integer. As an upper limit of p, 40 is preferable, 25 is more preferable, 10 is more preferable, 7 is more preferable, and 4 is more preferable. By making p below the above upper limit, the solubility in the electrolytic solution can be further reduced.

  Monomers that give the structural unit (b) include methoxypoly (ethylene glycol / propylene glycol) (meth) acrylate, ethoxypoly (ethylene glycol / propylene glycol) (meth) acrylate, and poly (ethylene glycol / propylene glycol) mono (meth). An acrylate etc. can be mentioned. Among these, methoxypolyethylene glycol methacrylate is preferable. 1 type may be used for these monomers and 2 or more types may be used for them.

  As a minimum of content of the above-mentioned structural unit (b) in the above-mentioned copolymer, 5 mass% is preferred, 10 mass% is more preferred, and 15 mass% is still more preferred. On the other hand, as an upper limit of this content, 60 mass% is preferable, 50 mass% is more preferable, and 40 mass% is further more preferable. By making content of a structural unit (b) into the said range, the viscosity reduction effect of a positive electrode paste and the elution suppression effect to electrolyte solution can be improved more.

(Structural unit (c))
The said copolymer can exhibit the elution suppression effect to the electrolyte solution in the positive electrode obtained by having the structural unit (c) which has an amide group.

As the ^{R 10} and ^{R 11,} a hydrogen atom is preferable. R ¹² is preferably a hydrogen atom and a methyl group, and more preferably a methyl group. By using R ^{10 to} R ¹² as these atoms or groups, it is possible to enhance the elution suppression effect into the electrolyte, ease of introduction during copolymerization, and the like.

  Examples of the monomer that gives the structural unit (c) include (meth) acrylamide and 2-butenoic acid amide, and (meth) acrylamide is preferable. 1 type may be used for these monomers and 2 or more types may be used for them.

  The lower limit of the content of the structural unit (c) in the copolymer is 15% by mass, preferably 17.5% by mass, more preferably 20% by mass, and even more preferably 26% by mass. By making content of a structural unit (c) more than the said minimum, the elution inhibitory effect to electrolyte solution can be heightened. On the other hand, the upper limit of this content is 45 mass%, 40 mass% is preferable, 37.5 mass% is more preferable, and 35 mass% is further more preferable. By making content of a structural unit (c) below the said upper limit, the viscosity reduction effect of a positive electrode paste can be improved more.

(Structural unit (d))
The monomer that provides the other structural unit (d) is not particularly limited as long as it is copolymerizable with the monomer that provides the structural unit (a), the structural unit (b), and the structural unit (c). .

  Monomers that give structural unit (d) include acid monomers such as (meth) acrylic acid; methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, isobutyl (meth) acrylate, ( (Meth) acrylic acid esters such as tert-butyl (meth) acrylate, benzyl (meth) acrylate, cyclohexyl (meth) acrylate, isobornyl (meth) acrylate, dimethylaminoethyl (meth) acrylate; N, N -(Meth) acrylamides such as dimethyl (meth) acrylamide, tert-butyl (meth) acrylamide, N-isopropyl (meth) acrylamide; styrenes such as styrene and p-methylstyrene; vinyl esters such as vinyl acetate; 2 -Vinylpyridines such as vinylpyridine; vinylpyrrolidone, etc. And vinyl pyrrolidones and the like. 1 type may be used for these monomers and 2 or more types may be used for them.

  As an upper limit of content of the said structural unit (d) in the said copolymer, 60 mass% is preferable, 40 mass% is more preferable, 20 mass% is further more preferable, 5 mass% is still more preferable, 1 mass %. By making content of a structural unit (d) below the said upper limit, the elution suppression effect to electrolyte solution and the viscosity reduction effect of positive electrode paste can be improved more.

  In the copolymer, it is preferable that substantially no structural unit derived from a monomer (crosslinkable monomer) having two or more polymerizable groups among the structural units (d). Here, from the viewpoint of the effect of reducing the viscosity of the positive electrode paste, in the positive electrode paste, the copolymer is preferably dissolved in the dispersion medium. Moreover, it can also be used in the state which one part melt | dissolved. When the copolymer contains a structural unit derived from a crosslinkable monomer, the solubility of the copolymer in the dispersion medium is lowered, and the viscosity reduction effect of the positive electrode paste tends to be lowered. The upper limit of the content of the structural unit derived from the crosslinkable monomer in the copolymer is preferably 5% by mass, more preferably 1% by mass, and still more preferably 0.1% by mass. By making content of the structural unit derived from a crosslinkable monomer below the said upper limit, the viscosity reduction effect of a positive electrode paste can be heightened.

(Unit ratio)
In the copolymer, the lower limit of the mass ratio of the structural unit (a) to the structural unit (b) (structural unit (a) / structural unit (b)) is preferably 0.5, more preferably 0.75. 1 is more preferable. On the other hand, the upper limit of this mass ratio is preferably 5, more preferably 4, and even more preferably 3. By setting the mass ratio of the structural unit (a) and the structural unit (b) within the above range, the effect of reducing the viscosity of the positive electrode paste can be further enhanced.

  In the copolymer, the lower limit of the mass ratio of the structural unit (b) to the structural unit (c) (structural unit (b) / structural unit (c)) is preferably 0.2, more preferably 0.4. 0.6 is more preferable. On the other hand, the upper limit of this mass ratio is preferably 5, more preferably 4, more preferably 3, and even more preferably 1. By setting the mass ratio of the structural unit (b) and the structural unit (c) within the above range, the effect of suppressing elution into the electrolyte solution and the effect of reducing the viscosity of the positive electrode paste when compared with the same molecular weight are further enhanced. be able to.

  In the copolymer, the lower limit of the mass ratio of the structural unit (a) to the structural unit (c) (structural unit (a) / structural unit (c)) is preferably 0.2, more preferably 0.5. 0.8 is more preferable. On the other hand, the upper limit of this mass ratio is preferably 5, more preferably 4, and even more preferably 3. By setting the mass ratio of the structural unit (a) and the structural unit (c) within the above range, it is possible to further enhance the effect of suppressing elution into the electrolytic solution and the effect of reducing the viscosity of the positive electrode paste.

(Molecular weight)
The lower limit of the weight average molecular weight measured by gel permeation chromatography (GPC) of the copolymer is preferably 5,000, more preferably 7,500, still more preferably 10,000, and even more preferably 15,000. Preferably, 18,000 is particularly preferable. By making a weight average molecular weight more than the said minimum, the solubility to electrolyte solution can be reduced more and the viscosity reduction effect can fully be exhibited. On the other hand, the upper limit of the weight average molecular weight is preferably 500,000, more preferably 250,000, still more preferably 100,000, and particularly preferably 70,000. By making a weight average molecular weight below the said upper limit, the viscosity reduction effect of a positive electrode paste can be improved more, and a synthesis | combination becomes easy.

(Content)
The lower limit of the content of the copolymer in the solid content in the battery positive electrode paste may be 0.1 parts by mass with respect to 100 parts by mass of the conductive agent, but 0.5 parts by mass is Preferably, 1.5 parts by mass is more preferred, 3 parts by mass is even more preferred, and 6 parts by mass is even more preferred. By making content of a copolymer more than the said minimum, the dispersibility of a electrically conductive agent can improve more and the viscosity reduction effect of a positive electrode paste can be improved more. On the other hand, the upper limit of the content may be 40 parts by mass, preferably 30 parts by mass, more preferably 25 parts by mass, even more preferably 20 parts by mass, and 15 parts by mass. It may be more preferable. By setting the content of the copolymer to be equal to or less than the above upper limit, the battery output can be maintained in a good state, and the amount of elution into the electrolytic solution can be suppressed.

  The lower limit of the content of the copolymer in the solid content of the battery positive electrode paste is, for example, 0.01% by mass. By setting the content of the copolymer to the above lower limit or more, a sufficient viscosity reducing effect can be exhibited. On the other hand, the upper limit of the content may be, for example, 3% by mass, but is preferably 2% by mass, and more preferably 1.5% by mass. By making the content of the copolymer not more than the above upper limit, the amount of elution into the electrolytic solution can be further reduced, and the adhesion of the resulting positive electrode mixture layer to the base material can be increased.

(Method of synthesizing copolymer)
The method for synthesizing the copolymer is not particularly limited, and a method used for polymerization of ordinary (meth) acrylic acid esters is used. The copolymer can be synthesized by, for example, a free radical polymerization method, a living radical polymerization method, an anionic polymerization method, a living anion polymerization method, or the like. For example, when the free radical polymerization method is used, the monomer that gives the structural unit (a), the structural unit (b), the structural unit (c), and, if necessary, the structural unit (d) is polymerized by a solution polymerization method. Can be performed. In addition, it is preferable to synthesize | combine by solution polymerization from a viewpoint of the ease of a synthesis | combination and the solubility to a solvent.

  Solvents used for solution polymerization include, for example, aliphatic hydrocarbons (hexane, heptane, etc.), aromatic hydrocarbons (toluene, xylene, etc.), lower alcohols (ethanol, isopropanol, etc.), ketones (acetone, methyl ethyl ketone, etc.) , Ethers (tetrahydrofuran, diethylene glycol dimethyl ether, etc.), and other organic solvents such as dimethyl sulfoxide, dimethylformamide, N-methylpyrrolidone can be used. The amount of solvent is a mass ratio with respect to the total amount of monomers, and is preferably 0.5 to 10 times.

  As a polymerization initiator used for the synthesis of the copolymer, a known radical polymerization initiator can be used. Examples of the radical polymerization initiator include azo polymerization initiators, hydroperoxides, dialkyl peroxides, diacyl peroxides, and ketone peroxides. The lower limit of the polymerization initiator amount is preferably 0.01 mol%, more preferably 0.1 mol%, still more preferably 0.2 mol%, based on the total amount of monomer components. On the other hand, as this upper limit, 5 mol% is preferable, 3 mol% is more preferable, and 2 mol% is further more preferable. The polymerization reaction is preferably performed in a temperature range of 60 ° C. to 180 ° C. under a nitrogen stream, and the reaction time is preferably 0.5 hours or more and 20 hours or less.

  Moreover, a well-known chain transfer agent for adjusting molecular weight can be used. Examples of the chain transfer agent include mercapto compounds such as isopropyl alcohol and mercaptoethanol.

  In the above copolymer, the arrangement of the structural unit (a), the structural unit (b), the structural unit (c), and the structural unit (d) contained as necessary may be any of random, block, gradient, graft, etc. But you can.

[Dispersion medium]
The dispersion medium is a component that uniformly disperses the positive electrode active material, the conductive agent, the binder, the copolymer, and the like. The copolymer is at least partially dissolved in the dispersion medium in order to further enhance the viscosity reduction effect.

  As said dispersion medium, what can melt | dissolve a copolymer can be used. Use of a non-aqueous dispersion medium such as an aprotic polar solvent such as N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMA) dimethylsulfoxide (DMSO), water, etc. it can. The dispersion medium is preferably a non-aqueous dispersion medium, more preferably an aprotic polar solvent, from the viewpoint of the solubility of the copolymer and preventing excessive moisture from being brought into the battery. NMP is particularly preferred because of the high solubility of the copolymer. The non-aqueous dispersion medium refers to a dispersion medium other than water, and is a dispersion medium that does not substantially contain water.

  As solid content concentration (content of components other than a dispersion medium) of the said positive electrode paste for batteries, it can be 40 mass% or more and 70 mass% or less, for example.

[Method for preparing positive electrode paste for battery]
The positive electrode paste for a battery can be prepared by mixing and stirring a positive electrode active material, a conductive agent, a binder, a copolymer, a dispersion medium, and the like. For this mixing and stirring, a known stirring device such as a planetary mixer, a bead mill, or a jet mill can be used. When each component is charged into the stirring device, it may be charged while rotating the stirring blade. Thereby, the mechanical load of the stirring device can be suppressed, the bulk of the components in the stirring container can be suppressed, and the preliminary mixing of each component can be performed. Alternatively, the entire amount may be divided into a plurality of times without being charged at once. Thereby, the mechanical load of the stirring device can be suppressed.

[How to use the positive electrode paste for batteries]
The positive electrode paste for a battery is used as a positive electrode preparation material for a battery, preferably a nonaqueous electrolyte secondary battery. The positive electrode can be produced by applying the positive electrode paste for a battery to a conductive base material such as an aluminum foil and drying it. In order to increase the density of the positive electrode, consolidation can be performed by a press machine. A die head, a comma reverse roll, a direct roll, a gravure roll, or the like can be used for coating the positive electrode paste. Drying after coating can be performed alone or in combination with heating, airflow, infrared irradiation and the like. The positive electrode can be pressed by a roll press or the like.

  In the positive electrode thus obtained, dissolution / elution of the copolymer in the electrolytic solution is suppressed, and as a result, an increase in resistance due to repeated charge / discharge of the battery can be suppressed. Here, the electrolytic solution usually has an electrolyte salt and a solvent for dissolving the electrolyte salt. Examples of the solvent include non-aqueous solvents such as cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate, and chain carbonates such as diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. The non-aqueous solvent means a solvent other than water and is a solvent that does not substantially contain water.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples.

The abbreviations of the raw materials used in the examples are as follows.
SMA: stearyl methacrylate (manufactured by Shin-Nakamura Chemical Co., Ltd., product number: NK-ester S)
PEG (2) MA: methoxy polyethylene glycol methacrylate (manufactured by Shin-Nakamura Chemical Co., Ltd., product number: NK ester M-20G, average added mole number of ethylene oxide p = 2)
PEG (9) MA: methoxypolyethylene glycol methacrylate (manufactured by Shin-Nakamura Chemical Co., Ltd., product number: NK-ester M-90G, average added mole number of ethylene oxide p = 9)
MAAm: methacrylamide (manufactured by Wako Pure Chemical Industries)
MAA: methacrylic acid (manufactured by Wako Pure Chemical Industries)
DMAAm: Dimethylacrylamide (manufactured by Wako Pure Chemical Industries, Ltd.)
NMP: N-methyl-2-pyrrolidone (manufactured by Wako Pure Chemical Industries, Ltd.)
V-65B: 2,2′-azobis (2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd.)

[Synthesis Example 1] Synthesis of Copolymer H As a monomer solution for initial charging, a solution consisting of 4.8 g of SMA, 2.8 g of PEG (2) MA, 22.4 g of MAAm and 86.7 g of NMP, and A mixed solution consisting of 0.8 g of V-65B and 3.3 g of NMP was prepared as an initial charge initiator solution. Next, a mixed solution composed of 106.1 g of SMA, 61.9 g of PEG (2) MA, and 24.7 g of NMP as the dropping monomer solution 1, 67.6 g of MAAm as the dropping monomer solution 2, And a mixed solution consisting of 135.3 g of NMP, as a monomer solution 3 for dropping, a mixed solution consisting of 21.7 g of SMA, 12.7 g of PEG (2) MA, and 20.0 g of NMP, an initiator solution for dropping A mixed solution composed of 6.7 g of V-65B and 26.7 g of NMP as 1 and a mixed solution of 0.8 g of V-65B and 3.3 g of NMP as a dropping initiator solution 2 were prepared.
The monomer for initial charge was put into a separable flask equipped with a reflux tube, a stirrer, a thermometer, a nitrogen introduction tube, and a dropping funnel, and the inside of the reaction vessel was purged with nitrogen for 1 hour or more while stirring and heating to 65 ° C. After confirming that the bath temperature reached 65 ° C., the initial charge initiator solution was added while stirring. After stirring for 10 minutes, the dropping monomer solution 1, the dropping monomer solution 2 and the dropping initiator solution 1 were dropped into the tank over 160 minutes. Thereafter, the dropping monomer solution 3 and the dropping initiator solution 2 were dropped into the tank over 20 minutes. After completion of dropping, the mixture was further stirred at 65 ° C. for 1 hour. While continuing stirring, the temperature was raised to 80 ° C., and it was confirmed that the temperature in the tank had reached 80 ° C., and further stirred for 2 hours. Next, 600 g of NMP was added and stirred until a homogeneous solution was obtained. After dilution, the mixture was air-cooled to 40 ° C. or lower to obtain a copolymer H.

[Synthesis Example 2] Synthesis of Copolymer I As a monomer solution for initial charging, a solution consisting of 4.8 g of SMA, 2.8 g of PEG (2) MA, 22.4 g of MAAm, and 89.3 g of NMP, and A mixed solution composed of 0.2 g of V-65B and 0.7 g of NMP was prepared as an initial charge initiator solution. Next, as a monomer solution 1 for dropping, a mixed solution consisting of 106.1 g of SMA and 61.9 g of PEG (2) MA and 8.2 g of NMP, and 67.6 g of MAAm as a monomer solution 2 for dropping, And a mixed solution consisting of 135.3 g of NMP, as a monomer solution 3 for dropping, a mixed solution consisting of 21.7 g of SMA, 12.7 g of PEG (2) MA, and 17.9 g of NMP, an initiator solution for dropping A mixed solution consisting of 1.4 g of V-65B and 43.2 g of NMP as 1 and a mixed solution of 0.2 g of V-65B and 5.4 g of NMP were prepared as the dropping initiator solution 2.
The monomer for initial charge was put into a separable flask equipped with a reflux tube, a stirrer, a thermometer, a nitrogen introduction tube, and a dropping funnel, and the inside of the reaction vessel was purged with nitrogen for 1 hour or more while stirring and heating to 65 ° C. After confirming that the bath temperature reached 65 ° C., the initial charge initiator solution was added while stirring. After stirring for 10 minutes, the dropping monomer solution 1, the dropping monomer solution 2 and the dropping initiator solution 1 were dropped into the tank over 160 minutes. Thereafter, the dropping monomer solution 3 and the dropping initiator solution 2 were dropped into the tank over 20 minutes. After completion of dropping, the mixture was further stirred at 65 ° C. for 1 hour. Thereafter, 150 g of NMP was added while stirring was continued, and the temperature was raised to 80 ° C. After confirming that the temperature in the tank reached 80 ° C., the mixture was further stirred for 2 hours. Next, 450 g of NMP was added and stirred until a homogeneous solution was obtained. After dilution, the mixture was air-cooled to 40 ° C. or less to obtain a copolymer I.

[Comparative Synthesis Example 1] Synthesis of Copolymer O As an initial charging solution, a solution composed of 90 g of NMP was prepared. Next, as a monomer solution 1 for dropping, a mixed solution consisting of 132.6 g of SMA, 77.4 g of PEG (2) MA, 90 g of MAAm, and 300 g of NMP, and 3 g of V- A mixed solution consisting of 65B and 60 g of NMP was prepared.
The monomer for initial charge was put into a separable flask equipped with a reflux tube, a stirrer, a thermometer, a nitrogen introduction tube, and a dropping funnel, and the inside of the reaction vessel was purged with nitrogen for 1 hour or more while stirring and heating to 65 ° C. After confirming that the temperature in the tank reached 65 ° C., the dropping monomer solution 1 and the dropping initiator solution 1 were dropped into the tank over 180 minutes. After completion of dropping, the mixture was further stirred at 65 ° C. for 1 hour. Then, it heated up to 80 degreeC, it confirmed that the temperature in a tank reached | attained 80 degreeC, and also stirred for 2 hours. Air-cooled to 40 ° C. or lower to obtain a copolymer O.

[Synthesis Examples 3 to 12 and Comparative Synthesis Examples 2 to 3] Synthesis of Copolymers A to G and J to N Copolymers C to G are an initial monomer solution, an initial initiator solution, a dropped monomer solution 1, and a drop. Monomer solution 2, dripped monomer solution 3, dripping initiator solution 1 and dripping initiator solution 2 have the same composition as shown in Table 1, and the same as in Synthesis Example 1 except for the amount of solvent for dilution and the polymerization temperature. The respective methods were synthesized.
Copolymers A, B, J, K, and L are prepared as follows: initial monomer solution, initial initiator solution, dropped monomer solution 1, dropped monomer solution 2, dropped monomer solution 3, dropped initiator solution 1 and dropped initiator solution 2. Each composition was synthesized in the same manner as in Synthesis Example 2 except that the composition was as shown in Table 1, and the amount of solvent for dilution and the polymerization temperature.
Copolymers M and N had initial monomer solution, initial initiator solution, dropped monomer solution 1, dropped monomer solution 2, and dropped initiator solution 1 as described in Table 1, and other than the polymerization temperature. Was synthesized by the same method as in Comparative Synthesis Example 1 above.

[Measurement of weight average molecular weight of copolymer]
The weight average molecular weight of each obtained copolymer was calculated from gel permeation chromatography (GPC) measurement. Detailed conditions are as follows. All weight average molecular weights are polystyrene conversion molecular weights. As the sample, a 0.3 mass% N, N-dimethylformamide solution prepared and passed through a 0.2 μm PTFE filter was used. The measurement results are shown in Table 1.
Measuring device: HLC-8320GPC (manufactured by Tosoh Corporation)
Column: α-M + α-M (manufactured by Tosoh Corporation)
Detector: Differential refractive index detector (RI)
Eluent: DMF solution flow rate of 60 mmol / L H ₃ PO ₄ , 50 mmol / L LiBr: 1 mL / min
Column temperature: 40 ° C
Sample solution injection volume: 10 μL

[Measurement of viscosity of acetylene black (AB) paste]
Acetylene black (AB) (DENKA black powder product manufactured by Denka) as a conductive agent, polyvinylidene fluoride (a 12 mass% NMP solution of # 1100 manufactured by Kureha Co.) as a binder, and NMP as a non-aqueous solvent are used. Thus, an AB paste was prepared. The mass ratio of the conductive agent and the binder was 50:50 (solid content conversion). The AB paste was prepared through a kneading step using a multi-blender mill with a solid content of 15% by mass by adjusting the amount of NMP.
To 20 g of the AB paste, 4% by mass of each copolymer with respect to AB was added, and defoamed and stirred using a rotation / revolution mixer (Awatori Nertaro AR-100) (stirring: 2000 rpm × 4 min, demolding). Foam: 2200 rpm × 1 min). A rheometer was used for viscosity measurement. The measurement was carried out under the conditions shown below. The obtained AB paste viscosity is shown in Table 1. The viscosity of the AB paste to which no copolymer was added was 42000 mPa · s.
Measuring apparatus: Modular Compact Rheometer MCR 302 (manufactured by Anton Paar)
Jig: 25 mm Parallel plate gap width: 0.5 mm
Temperature: 20 ° C
Shear rate range: 0.1-400 s ⁻¹
Measuring point interval: 1s

[Measurement of dissolution rate]
In order to confirm the solubility of the copolymer in the electrolytic solution, the dissolution rate in the solvent used in the electrolytic solution was measured. The obtained copolymer solution was placed in a petri dish and dried under reduced pressure at 140 ° C. under a nitrogen stream for 12 hours or more. 9 g of a mixed solvent of ethylene carbonate and diethylene carbonate (volume ratio 50/50) was added to 1 g of the obtained copolymer to prepare a 10% suspension. The obtained suspension was allowed to stand at 40 ° C. for 1 hour. Then, it filtered with a 0.5 micrometer PTFE filter, and the undissolved copolymer was removed. The mass of the copolymer dissolved in the mixed solvent was measured by drying the filtered solution at 140 ° C. under reduced pressure and a nitrogen stream. The dissolution rate in the mixed solvent was determined from the following formula. The obtained dissolution rates are shown in Table 1.

[Evaluation of appearance]
A 27.5 mass% copolymer A / NMP solution 27.3g and NMP 22.7g were added to the sample bottle, and 50 g of 15 mass% NMP solutions of the copolymer A were prepared. The prepared solution was allowed to stand at 60 ° C. for 1 hour, and then the appearance was visually evaluated. Copolymers B to O were also evaluated in the same manner. The evaluation results are shown in Table 1. Those that are transparent indicate that the copolymer is sufficiently dissolved in NMP, and those that are cloudy indicate that at least a part of the copolymer is not dissolved in NMP.

  As shown in Table 1, it can be seen that the copolymers A to L have low solubility in the electrolytic solution. Therefore, it turns out that the elution to the electrolyte solution of a copolymer decreases from the positive electrode obtained from the positive electrode paste for batteries containing copolymer A-L. Further, as shown in Table 1, the AB paste containing the copolymers A to L has a sufficiently low viscosity, and the copolymers A to L have a sufficient solubility in NMP as a dispersion medium. It turns out that it has. The copolymer J having a large molecular weight has a relatively low solubility in NMP and a relatively high paste viscosity. On the other hand, it can be seen that the copolymer G having a small molecular weight is slightly dissolved in the electrolytic solution. It can also be seen that the copolymers I, J and L are not partly dissolved in NMP.

[Examples 1 to 7, Comparative Example 1]
In order to confirm the adhesion of the positive electrode mixture layer using the positive electrode paste containing the copolymer, the following adhesion test was performed. The adhesion test 1 simulates the processing performance of the electrode plate (slit process, etc.), and the adhesion test 2 simulates the winding performance of the electrode plate.

[Preparation of positive electrode paste]
The pastes of Examples 1 to 7 and Comparative Example 1 were prepared using the copolymer shown in Table 2 synthesized in the above synthesis example, the positive electrode active material, the conductive agent, the binder, and the non-aqueous dispersion medium. The positive electrode active material is LiNi _1/3 Mn _1/3 Co _1/3 O ₂ , the conductive agent is acetylene black (BET specific surface area: 68 m ² / g), and the binder is PVDF (Kureha's “# 1100” 12 % NMP solution) and NMP was used as the non-aqueous dispersion medium. Further, the mass ratio (content in the solid content) of the positive electrode active material (active material), the binder, the conductive agent, and the copolymer was as shown in Table 2. The positive electrode paste was prepared through a kneading step using a multi-blender mill by adjusting the solid content by adjusting the amount of the non-aqueous solvent. In this example and comparative example, the solid content of the positive electrode paste refers to a material made of the positive electrode active material, a binder, a conductive agent, and a copolymer, that is, a material other than a non-aqueous dispersion medium. .

[Production of positive electrode plate]
Each of the obtained positive electrode pastes was applied to one side of a 20 μm-thick aluminum foil as a base material using a doctor blade, and sufficiently dried to form a composite material layer (positive electrode composite material layer made of positive electrode paste) The positive electrode plate which has) was produced. The coating mass of the positive electrode plate after drying was 17 mg / cm ² .

[Adhesion test 1]
The mixture layer coating portion of the positive electrode plate after the drying was cut into a length of 30 mm × 60 mm using a cutting machine, and the mixture layer of the cut portion was observed. In the cut part, it was judged that a material layer with a width of 1 mm or more dropped and peeled was poor in adhesion, and a positive electrode plate that did not have no problem. The results of adhesion are shown in Table 2 as “B” for the positive electrode plate having poor adhesion and “A” for the positive electrode plate having no problem.

[Adhesion test 2]
In the same manner as in the adhesion test 1, the mixture layer coating portion of the positive electrode plate after the drying was cut into a length of 30 mm × 60 mm using a cutting machine. The cut positive electrode plate was bent twice around an intermediate point in the long side direction. The first time, after bending 180 degrees so that the uncoated surfaces were in contact with each other, it was returned to the state before bending. The second time, after bending 180 ° so that the coated surfaces were in contact with each other, it was returned to the state before bending. At the end of the first bending (returned before bending) and at the end of the second bending (returned before bending), the falling state of the composite layer was confirmed. In the short side direction of the positive electrode plate at the bent portion (intermediate point in the long side direction), the width at which the composite material layer dropped and peeled was measured. And the composite-material layer drop-off rate was computed with the following formula | equation.
Compound layer drop-off rate (%) = {width (mm) / 30 (mm) from which the compound layer was dropped} × 100
Table 2 shows the drop rate of the composite material layer.

[result]
From the result of the adhesion test 1, it was found that in the case of Comparative Example 1 in which a binder is not used in the positive electrode paste separately from the copolymer, poor adhesion occurs. Thus, it can be said that the positive electrode paste containing a copolymer and not using a binder is not suitable for a battery such as a lithium battery.

  Further, from the adhesion test 2, it was found that in Example 7 in which the content of the copolymer in the solid content of the positive electrode paste exceeds 2% by mass, the mixture layer can be detached and peeled when the positive electrode plate is bent. It was. In the electrode body in which the positive electrode, the first separator, the negative electrode, and the second separator having a long strip shape are overlapped and wound in a flat shape around the winding axis, both ends in the direction orthogonal to the winding axis The electrode plate of the curved portion is bent at a relatively acute angle, and may be temporarily bent when inserted into the case. As in Example 7, when the electrode plate is bent, the composite material layer is dropped and peeled off. In the wound electrode body, the composite material layer may be dropped and peeled off. Therefore, the content of the copolymer in the solid content of the positive electrode paste is preferably 2% by mass or less.

  The present invention can be suitably used as a positive electrode paste for a battery such as a secondary battery.

Claims (5)

Including a positive electrode active material, a conductive agent, a binder, a copolymer and a dispersion medium,
The copolymer is a structural unit (a) represented by the following formula (1), a structural unit (b) represented by the following formula (2), and a structural unit (c) represented by the following formula (3). Containing
The battery positive electrode paste whose content of the said structural unit (c) in the said copolymer is 15 to 45 mass%.

(In the formulas (1) to (3), R ¹ , R ² , R ³ , R ⁵ , R ⁶ , R ⁷ , R ⁹ , R ¹⁰ , R ¹¹ , and R ¹² are each independently a hydrogen atom. R ⁴ is a monovalent hydrocarbon group having 8 to 30 carbon atoms, and R ⁸ is a linear or branched alkylene group having 2 to 4 carbon atoms. p is a number from 1 to 50.)   2. The battery according to claim 1, wherein the mass ratio of the structural unit (a) and the structural unit (b) of the copolymer (structural unit (a) / structural unit (b)) is 0.5 or more and 5 or less. Positive electrode paste.   The battery positive electrode paste according to claim 1 or 2, wherein the copolymer has a weight average molecular weight of 15,000 or more and 100,000 or less.   4. The positive electrode paste for a battery according to claim 1, wherein the content of the copolymer is 0.5 parts by mass or more and 30 parts by mass or less with respect to 100 parts by mass of the conductive agent.   The battery positive electrode paste according to any one of claims 1 to 4, wherein the dispersion medium is a non-aqueous dispersion medium.