JP7315342B2 - Negative electrode mixture for secondary battery, negative electrode for secondary battery, and secondary battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to a negative electrode mixture for secondary batteries, a negative electrode for secondary batteries, and a secondary battery.

Non-aqueous electrolyte secondary batteries such as the lithium ion secondary battery disclosed in Patent Document 1 are widely used as power sources for portable devices such as notebook computers and mobile phones. Demand for non-aqueous electrolyte secondary batteries for xEVs, such as electric vehicles and hybrid vehicles, has been growing rapidly in recent years, and there are great expectations for further expansion of demand.

Lithium-ion secondary batteries for xEVs are required to have high capacity and long life characteristics in order to ensure the same performance as conventional gasoline engine vehicles.

Silicon (Si)-based negative electrode active materials have been proposed as a technique for increasing the capacity of lithium ion secondary batteries.
As Si-based negative electrode active materials, several types are known, such as those composed only of pure Si, SiO, which is a mixture of Si and SiO ₂ , and those compounded with carbon materials. Pure Si has a theoretical capacity of about 4200 mAh/g, which is more than four times the theoretical capacity of graphite, 372 mAh/g. However, since the Si-based active material undergoes a large volume change due to intercalation of lithium ions during charging and discharging, cracking of the surface of the active material and destruction of the electrode structure occur with repeated charging and discharging cycles. A crack on the surface of the active material produces an interface of the active material on which no film layer is newly formed, so that a decomposition reaction of the electrolytic solution (formation of a film layer) occurs. Destruction of the electrode structure cuts the physical contact between the active materials, resulting in an increase in the amount of the active material that does not participate in charging and discharging. Since both of them cause a decrease in capacity, there is a problem that the cycle characteristics of the Si-based active material are inferior to those of graphite.

In Patent Document 1, an attempt is made to suppress the expansion and contraction of the Si-based active material by using polyimide, which has excellent mechanical properties, as a binder.

In Patent Document 2, a study is made to improve the initial charge/discharge efficiency of a polyimide binder, but it is inferior and inadequate compared to the CMC/SBR system. Moreover, when polyimide is used, it is necessary to use an organic solvent, and in the production of the negative electrode, which is mainly water-based, there arises a problem that a large equipment change is required.

Patent Literatures 3 and 4 report examples of using a conventionally used CMC/SBR system in combination with polyacrylic acid to improve electrode adhesion and suppress breakage of the electrode structure. However, the total amount of CMC/SBR/polyacrylic acid added to the binder is often 7.0% or more and 7.5% or less, and there is a concern that the rate characteristics may deteriorate and the active material ratio in the negative electrode mixture cannot be increased. Therefore, there is a problem that the effect of increasing the capacity of the Si-based negative electrode is reduced.

Japanese Patent No. 5338924 JP 2009-245773 A Japanese Patent No. 6165261 Japanese Patent Application Laid-Open No. 6181590

Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to provide a negative electrode mixture for secondary batteries, a negative electrode for secondary batteries, and a secondary battery using the same that can suppress electrode swelling of the negative electrode even with a small addition amount and improve cycle characteristics.

In order to solve the above problems, according to one aspect of the present invention, there is provided a negative electrode mixture for a secondary battery comprising a water-soluble polymer binder (A) and a negative electrode active material (B), wherein the mass ratio of the water-soluble polymer binder (A) to the negative electrode active material (B) is 2.0:98.0 to 6.0:94.0, where the sum of these mass ratios is 100, and the water-soluble polymer binder (A) comprises a carboxyl group-containing acrylic monomer and a water-soluble acrylic acid derivative monomer. and wherein the negative electrode active material (B) contains 10.0% by mass or more and 20.0% by mass or less of the Si-based active material.

A negative electrode mixture for a secondary battery according to this viewpoint can suppress swelling of the negative electrode and improve cycle characteristics even with a small addition amount.

The water-soluble polymer binder (A) may contain 10% by mass or more and 40% by mass or less of structural units derived from the water-soluble acrylic acid derivative monomer.

From this point of view, it is possible to suppress the electrode swelling of the negative electrode and improve the cycle characteristics even with a small addition amount.

The shear viscosity at 25° C. of the 5.0% by mass aqueous solution of the copolymer may be 0.5 (Pa·s) or more and 5.0 (Pa·s) or less at a shear rate of 1.0 (1/s).

From this point of view, it is possible to reduce the negative electrode resistance while maintaining good adhesion.

Here, the carboxyl group-containing acrylic monomer may be at least one selected from the group consisting of acrylic acid, methacrylic acid, maleic acid, monomethylmaleic acid, 2-carboxyethyl acrylate, and 2-carboxyethyl methacrylate.

From this point of view, the characteristics of the secondary battery are further improved.

Also, the water-soluble acrylic acid derivative monomer may be at least one of an ethylene glycol chain-containing acrylic monomer and a hydroxyl group-containing acrylic monomer.

From this point of view, the characteristics of the secondary battery are further improved.

In addition, ethylene glycol chain-containing acrylic monomers include 2-methoxyethyl acrylate, 2-ethoxyethyl acrylate, 2-(2-methoxyethoxy)ethyl acrylate, 2-(2-ethoxyethoxy)ethyl acrylate, 2-(2-(2-methoxyethoxy)ethoxy)ethyl acrylate, 2-(2-(2-methoxyethoxy)ethoxy)ethyl acrylate, methoxypolyethylene glycol acrylate, 2-methoxyethyl methacrylate, 2-ethoxyethyl methacrylate, 2-(2-methoxyethyl acrylate). thoxy)ethyl methacrylate, 2-(2-ethoxyethoxy)ethyl methacrylate, 2-(2-(2-methoxyethoxy)ethoxy)ethyl methacrylate, 2-(2-(2-methoxyethoxy)ethoxy)ethyl methacrylate, and at least one selected from the group consisting of methoxypolyethylene glycol methacrylate.

From this point of view, the characteristics of the secondary battery are further improved.

Further, the hydroxyl group-containing acrylic monomer may be at least one selected from the group consisting of 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 3-hydroxypropyl acrylate, 2-hydroxybutyl acrylate, 4-hydroxybutyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl methacrylate, 2-hydroxybutyl methacrylate, and 4-hydroxybutyl methacrylate.

From this point of view, the characteristics of the secondary battery are further improved.

Also, at least part of the carboxyl group-containing acrylic monomer may be an alkali metal salt or an ammonium salt.

According to another aspect of the present invention, there is provided a negative electrode for a secondary battery containing the negative electrode mixture for a secondary battery.

In the secondary battery negative electrode from this viewpoint, the secondary battery negative electrode mixture can suppress swelling of the negative electrode even with a small addition amount. In addition, the constituent elements of the negative electrode are adhered to each other satisfactorily by the negative electrode mixture for the secondary battery.

In addition, it may further contain a styrene-butadiene copolymer (SBR).

From this point of view, the characteristics of the secondary battery are further improved.

According to another aspect of the present invention, there is provided a secondary battery comprising the above negative electrode for secondary battery.

In the secondary battery from this point of view, the negative electrode mixture for the secondary battery can suppress the swelling of the negative electrode and improve the cycle characteristics even with a small addition amount. In addition, the constituent elements of the negative electrode are adhered to each other satisfactorily by the negative electrode mixture for the secondary battery. Furthermore, negative electrode resistance is reduced.

ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to suppress the electrode swelling of a negative electrode, and to improve cycling characteristics, even if the addition amount of the negative mix for secondary batteries is small.

1 is a side sectional view schematically showing the configuration of a lithium ion secondary battery; FIG. 1 is a plan view schematically showing a positive electrode and a negative electrode produced in Examples. FIG. It is a perspective view which shows roughly the positive electrode and negative electrode produced in the Example. FIG. 3 is a plan view schematically showing a separator produced in Examples. 1 is a perspective view schematically showing a state in which a positive electrode and a negative electrode face each other with a separator interposed therebetween and are packaged with a laminate film in an example. FIG. 1 is a perspective view schematically showing a state in which a positive electrode and a negative electrode face each other with a separator interposed therebetween and are packaged with a laminate film in an example. FIG. 1 is a perspective view schematically showing a laminated secondary battery produced in an example; FIG.

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.
In the present specification and drawings, constituent elements having substantially the same functional configuration are denoted by the same reference numerals, thereby omitting redundant description.

<1. Negative electrode mixture for secondary battery>
First, a negative electrode mixture for a secondary battery according to one embodiment of the present invention will be described. The secondary battery negative electrode mixture according to the present embodiment is used to form a secondary battery negative electrode. A negative electrode mixture for a secondary battery according to this embodiment includes a water-soluble polymer binder (A) and a negative electrode active material (B). The mass ratio of the water-soluble polymer binder (A) and the negative electrode active material (B) is 2.0:98.0 to 6.0:94.0, with the sum of these mass ratios being 100. The water-soluble polymer binder (A) contains a copolymer of a carboxyl group-containing acrylic monomer and a water-soluble acrylic acid derivative monomer. The negative electrode active material (B) contains 10.0% by mass or more and 20.0% by mass or less of the Si-based active material.

The mass ratio of the water-soluble polymer binder (A) and the negative electrode active material (B) is 2.0:98.0 to 6.0:94.0, preferably 2.5 to 97.5:5.0 to 95.0, more preferably 3.0 to 97.0:5.0 to 95.0, with the sum of these mass ratios being 100.
If the mass ratio of the water-soluble polymer binder (A) and the negative electrode active material (B) is within the above range, the secondary battery negative electrode mixture can suppress the electrode swelling of the negative electrode and improve the cycle characteristics even with a small addition amount.

The shear viscosity of the 5.0% by mass aqueous solution of the copolymer at 25° C. is preferably 0.5 (Pa s) or more and 5.0 (Pa s) or less at a shear rate of 1.0 (1/s), more preferably 0.7 (Pa s) or more and 3.5 (Pa s) or less.
If the shear viscosity of the copolymer is within the above range, the negative electrode mixture for secondary batteries can reduce the negative electrode resistance while maintaining better adhesion.

Since the negative electrode mixture for secondary batteries has the above structure, it has high ion conductivity. Therefore, the internal resistance of the lithium ion secondary battery, specifically the negative electrode resistance, is reduced. Furthermore, the negative electrode mixture for secondary batteries has sufficiently good adhesion even when the amount is small. Therefore, even if it is a small amount, it is possible to prepare the slurry for manufacturing the negative electrode, and to stably bind the constituent elements in the negative electrode active material layer. In this respect as well, the internal resistance of the lithium ion secondary battery, specifically the negative electrode resistance, is reduced. Furthermore, since deterioration in electronic conductivity due to peeling and structural destruction of the negative electrode is suppressed, the life of the lithium ion secondary battery is improved.

Furthermore, since the viscosity is high in the low shear rate region, the dispersion stability of the negative electrode preparation slurry is improved. Specifically, the viscosity of the negative electrode preparation slurry can be increased appropriately, and the negative electrode preparation slurry can be easily and stably applied onto the current collector. Furthermore, sedimentation of the negative electrode active material in the negative electrode preparation slurry can be suppressed. When the shear viscosity is less than 0.5 (Pa·s), the viscosity of the negative electrode preparation slurry is too low, and it becomes difficult to retain a sufficient amount of the negative electrode preparation slurry on the current collector. When the shear viscosity exceeds 5.0 (Pa·s), the viscosity of the copolymer is too high and it becomes difficult to stir the negative electrode preparation slurry. That is, it is not possible to prepare a slurry for preparing a negative electrode in which the negative electrode active material and the like are evenly dispersed. A preferable upper limit of the shear viscosity is 5.0 (Pa·s) or less.

Here, in order for the shear viscosity of the 5.0% by mass aqueous solution of the copolymer at 25° C. to be 0.5 (Pa s) or more and 5.0 (Pa s) or less at a shear rate of 1.0 (1/s), it is necessary to mix the carboxyl group-containing acrylic monomer and the water-soluble acrylic acid derivative monomer at the mass ratio described later, and copolymerize these monomers without neutralizing the carboxyl group of the carboxyl group-containing acrylic monomer. This makes it possible to synthesize a high-molecular-weight copolymer while preventing an excessive increase in the viscosity of the reaction solution. Specifically, the copolymer gradually precipitates out of the reaction solution as the molecular weight of the copolymer increases (that is, the progress of the copolymerization reaction), resulting in a white slurry, so that the viscosity of the reaction solution does not excessively increase, and stirring can be continued until the reaction is completed. Such a copolymer can achieve the shear viscosity described above. On the other hand, when the above monomers are copolymerized in a state in which some or all of the carboxyl group-containing acrylic monomers are neutralized, the shear viscosity of a 5.0% by mass aqueous solution of the copolymer at 25° C. tends to decrease, and may become less than 0.5 (Pa s) at a shear rate of 1.0 (1/s).

(1-1. Water-soluble polymer binder)
Carboxyl group-containing acrylic monomers include acrylic acid, methacrylic acid, maleic acid, monomethyl maleic acid, 2-carboxyethyl acrylate, and 2-carboxyethyl methacrylate. It is preferably at least one selected from the group consisting of 2-carboxyethyl methacrylate). In this case, the characteristics of the lithium ion secondary battery are further improved.

The water-soluble acrylic acid derivative monomer is preferably at least one of an ethylene glycol chain-containing acrylic monomer and a hydroxyl group-containing acrylic monomer. In this case, the characteristics of the lithium ion secondary battery are further improved.

The water-soluble polymer binder (A) preferably contains 10% by mass or more and 40% by mass or less of structural units derived from a water-soluble acrylic acid derivative monomer. In this case, the characteristics of the lithium ion secondary battery are further improved.
The water-soluble polymer binder (A) contains structural units derived from carboxyl group-containing acrylic monomers in addition to structural units derived from water-soluble acrylic acid derivative monomers.

Ethylene glycol chain-containing acrylic monomers include 2-methoxyethyl acrylate, 2-ethoxyethyl acrylate, 2-(2-methoxyethoxy)ethyl acrylate, 2-(2-ethoxyethoxy)ethyl acrylate, 2-(2-(2-methoxyethoxy)ethoxy)ethyl acrylate, 2-(2-(2-methoxyethoxy)ethoxy)ethyl acrylate, methoxypolyethylene glycol acrylate, 2-methoxyethyl methacrylate, 2-ethoxyethyl methacrylate, 2-(2-methoxyethoxy) 2-(2-(2-methoxyethoxy)ethoxy)ethyl methacrylate, and 2-(2-(2-methoxyethoxy)ethoxy)ethyl methacrylate, 2-methoxyethyl acrylate, 2-ethoxyethyl acrylate, 2-(2-methoxyethoxy)ethyl acrylate te, 2-(2-ethoxyethoxy)ethyl acrylate, 2-(2-(2-methoxyethoxy)ethoxy)ethyl acrylate, methoxypolyethylene glycol acrylate, 2-methoxyethyl methacrylate te, 2-ethoxyethyl methacrylate, 2-(2-methoxyethoxy) ethyl methacrylate, 2-(2-ethoxyethoxy) ethyl methacrylate, 2-(2-(2-methoxyethoxy) ethoxy) ethy It is preferably at least one selected from the group consisting of l methacrylate, and methoxypolyethylene glycol methacrylate). In this case, the characteristics of the lithium ion secondary battery are further improved.

The hydroxyl group-containing acrylic monomer is 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 3-hydroxypropyl acrylate, 2-hydroxybutyl acrylate, 4-hydroxybutyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl methacrylate, 2-hydroxybutyl methacrylate, 4-hydroxybutyl methacrylate (2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 3 - hydroxypropyl acrylate, 2-hydroxybutyl acrylate, 4-hydroxybutyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 3-hydroxyp It is preferably at least one selected from the group consisting of ropyl methacrylate, 2-hydroxybutyl methacrylate, and 4-hydroxybutyl methacrylate. In this case, the characteristics of the lithium ion secondary battery are further improved.

At least part of the carboxyl group-containing acrylic monomer is preferably an alkali metal salt or an ammonium salt. In this case, the characteristics of the lithium ion secondary battery are further improved.

(1-2. Negative electrode active material)
Examples of the negative electrode active material (B) include graphite active materials (artificial graphite, natural graphite, a mixture of artificial graphite and natural graphite, natural graphite coated with artificial graphite, etc.), silicon or tin or a mixture of fine particles of their oxides and a graphite active material, fine particles of silicon or tin, alloys based on silicon or tin, titanium oxide compounds such as Li ₄ Ti ₅ O ₁₂ , lithium nitrides, and the like. Silicon oxide is represented by SiO _x (0≦x≦2). In addition to these, examples of the negative electrode active material include metallic lithium and the like. In the negative electrode mixture for a secondary battery of the present embodiment, the negative electrode active material (B) contains 10.0% by mass or more and 20.0% by mass or less of the Si-based active material, preferably 12.5% by mass or more and 17.5% by mass or less. In this case, the characteristics of the lithium ion secondary battery are improved. Examples of Si-based active materials include fine particles of silicon, oxides of silicon (SiO _x (0≦x≦2)), alloys based on silicon (e.g., Mg—Si alloys, Si—Sn alloys, etc.), and Si—C composite materials.

In the negative electrode mixture for a secondary battery according to the present embodiment described above, the mass ratio of the water-soluble polymer binder (A) and the negative electrode active material (B) is 2.0:98.0 to 6.0:94.0, where the sum of these mass ratios is 100, the water-soluble polymer binder (A) contains a copolymer of a carboxyl group-containing acrylic monomer and a water-soluble acrylic acid derivative monomer, and the negative electrode active material (B) contains 10.0% by mass of a Si-based active material. More than 20.0% by mass or less is included. Therefore, it is possible to suppress electrode swelling of the negative electrode and improve cycle characteristics even with a small addition amount.

<2. Secondary battery>
A specific configuration of the lithium-ion secondary battery 10 according to one embodiment of the present invention described above will be described below with reference to FIG. FIG. 1 is an explanatory diagram illustrating the configuration of a lithium ion secondary battery according to one embodiment of the present invention. Moreover, the lithium ion secondary battery 10 has a negative electrode 30 as a secondary battery negative electrode according to one embodiment of the present invention.

A lithium ion secondary battery 10 shown in FIG. 1 is an example of a secondary battery according to the present embodiment. As shown in FIG. 1, the lithium ion secondary battery 10 includes a positive electrode 20, a negative electrode 30, a separator 40, and a non-aqueous electrolyte. The charging ultimate voltage (oxidation-reduction potential) of the lithium ion secondary battery 10 is, for example, 4.0 V (vs. Li/Li ⁺ ) or more and 5.0 V or less, particularly 4.2 V or more and 5.0 V or less. The shape of the lithium-ion secondary battery 10 is not particularly limited, but may be, for example, cylindrical, prismatic, laminate, button, or the like.

(2-1. Positive electrode 20)
The positive electrode 20 includes a current collector 21 and a positive electrode active material layer 22 . The current collector 21 may be any conductive material such as aluminum, stainless steel, and nickel coated steel.

The positive electrode active material layer 22 contains at least a positive electrode active material, and may further contain a conductive agent and a positive electrode binder. The positive electrode active material is, for example, a solid solution oxide containing lithium, but is not particularly limited as long as it can electrochemically intercalate and deintercalate lithium ions. _Solid solution oxides include, _for example, _{LiaMnxCoyNizO2} ( _{1.150≤a≤1.430} , 0.45≤x≤0.6 _, 0.10≤y≤0.15 _, 0.20≤z≤0.28 ₎ , LiMnxCoyNizO2 (0.3≤x≤0.85, 0.10≤y≤0.3, 0.30≤y≤0.3 _{) . 10≤z≤0.3} ) _{LiMn1.5Ni0.5O4 .}

The conductive agent includes, for example, carbon black such as ketjen black and acetylene black, natural graphite, artificial graphite, etc., but is not particularly limited as long as it increases the conductivity of the positive electrode.

Positive electrode binders include, for example, polyvinylidene fluoride, ethylene-propylene-diene terpolymer, styrene-butadiene rubber, and acrylonitrile-butadiene rubber. butadiene rubber, fluororubber, polyvinyl acetate, polymethylmethacrylate, polyethylene, nitrocellulose, etc., and the positive electrode active material and the conductive agent are bound on the current collector 21. It is not particularly limited as long as it can be used.

The positive electrode active material layer 22 is produced, for example, by the following method. That is, first, a positive electrode mixture is produced by dry-mixing a positive electrode active material, a conductive agent, and a positive electrode binder. Next, the positive electrode mixture slurry is prepared by dispersing the positive electrode mixture in an appropriate organic solvent, and the positive electrode mixture slurry is applied onto the current collector 21, dried, and rolled to prepare a positive electrode active material layer.

(2-2. Anode 30)
The negative electrode 30 includes a current collector 31 and a negative electrode active material layer 32 . The current collector 31 may be any conductor as long as it is made of, for example, aluminum, stainless steel, nickel-plated steel, or the like. The negative electrode active material layer 32 contains at least the negative electrode mixture for a secondary battery described above.

The negative electrode active material layer 32 may further contain, for example, a styrene-butadiene copolymer (SBR) in addition to the negative electrode mixture for secondary batteries described above. In this case, the characteristics of the lithium ion secondary battery are further improved. Although the mass ratio of the styrene-butadiene copolymer is not particularly limited, it is preferable that the mass ratio to the copolymer of the carboxyl group-containing acrylic monomer and the water-soluble acrylic acid derivative monomer constituting the water-soluble polymer binder (A) is 1.0 or more and 3.0 or less. The lower limit of the mass ratio is preferably 1.5 or more, and the upper limit is more preferably 2.0 or less. Although the shape of the styrene-butadiene copolymer is not particularly limited, it is preferably particulate. This is because, in this case, the styrene-butadiene copolymer functions as a binder and is less likely to inhibit the high ionic conductivity of the copolymer that constitutes the water-soluble polymer binder (A).

(2-3. Separator)
The separator 40 is not particularly limited, and may be of any type as long as it is used as a separator for lithium ion secondary batteries. As the separator, it is preferable to use, alone or in combination, a porous membrane, a non-woven fabric, or the like, which exhibits excellent high-rate discharge performance. Examples of the resin constituting the separator include polyolefin-based resins such as polyethylene and polypropylene, and polyester-based resins such as polyethylene terephthalate and polybutylene terephthalate. , polyvinylidene difluoride, vinylidene difluoride-hexafluoropropylene copolymer, vinylidene difluoride-perfluorovinyl ether copolymer oronylether copolymer), vinylidene difluoride-tetrafluoroethylene copolymer, vinylidene difluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, vinylidene difluoride-hexafluoroacetone copolymer, vinylidene fluoride-ethylene copolymer vinylide-ethylene copolymer, vinylidene difluoride-propylene copolymer, vinylidene difluoride-trifluoropropylene copolymer ), vinylidene difluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene difluoride-ethylene-tetrafluoroethylene copolymer, etc. can be mentioned. The porosity of the separator is not particularly limited, and any porosity of a conventional lithium ion secondary battery separator can be applied.

(2-4. Non-aqueous electrolyte)
As the non-aqueous electrolyte, the same non-aqueous electrolyte as conventionally used in lithium ion secondary batteries can be used without particular limitation. The non-aqueous electrolyte has a composition in which an electrolyte salt is contained in a non-aqueous solvent. Examples of non-aqueous solvents include cyclic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, and vinylene carbonate. cyclic esters such as γ-butyrolactone and γ-valerolactone; chain carbonates such as dimethyl carbonate, diethyl carbonate and ethylmethyl carbonate; Chain esters such as methylformate, methylacetate, methylbutyrate, tetrahydrofuran or its derivatives, 1,3-dioxane, 1,4-dioxane, 1,2-dimethoxyethane (1,2 Ethers such as 1,4-dibutoxyethane or methyldiglyme, nitriles such as acetonitrile and benzonitrile, dioxolane or derivatives thereof, ethylene sulfide (ethylene sulfide), sulfolane, sultone, derivatives thereof and the like can be used alone or in combination of two or more thereof. When two or more solvents are mixed and used, the mixing ratio of each solvent can be the mixing ratio used in conventional lithium-ion secondary batteries.

電解質塩としては、例えば、ＬｉＣｌＯ _４ 、ＬｉＢＦ _４ 、ＬｉＡｓＦ _６ 、ＬｉＰＦ _６ 、ＬＩＰＦ _６－ｘ （Ｃ _ｎ Ｆ _２ｎ＋１ ） _ｘ ［但し、１＜ｘ＜６、ｎ＝１ｏｒ２］、ＬｉＳＣＮ、ＬｉＢｒ、ＬｉＩ、Ｌｉ _２ ＳＯ _４ 、Ｌｉ _２ Ｂ _１０ Ｃｌ _１０ 、ＮａＣｌＯ _４ 、ＮａＩ、ＮａＳＣＮ、ＮａＢｒ、ＫＣｌＯ _４ 、ＫＳＣＮ等のリチウム（Ｌｉ）、ナトリウム（Ｎａ）またはカリウム（Ｋ）の１種を含む無機イオン塩、ＬｉＣＦ _３ ＳＯ _３ 、ＬｉＮ（ＣＦ _３ ＳＯ _２ ） _２ 、ＬｉＮ（Ｃ _２ Ｆ _５ ＳＯ _２ ） _２ 、ＬｉＮ（ＣＦ _３ ＳＯ _２ ）（Ｃ _４ Ｆ _９ ＳＯ _２ ）、ＬｉＣ（ＣＦ _３ ＳＯ _２ ） _３ 、ＬｉＣ（Ｃ _２ Ｆ _５ ＳＯ _２ ） _３ 、（ＣＨ _３ ） _４ ＮＢＦ _４ 、（ＣＨ _３ ） _４ ＮＢｒ、（Ｃ _２ Ｈ _５ ） _４ ＮＣｌＯ _４ 、（Ｃ _２ Ｈ _５ ） _４ ＮＩ、（Ｃ _３ Ｈ _７ ） _４ ＮＢｒ、（ｎ－Ｃ _４ Ｈ _９ ） _４ ＮＣｌＯ _４ 、（ｎ－Ｃ _４ Ｈ _９ ） _４ ＮＩ、（Ｃ _２ Ｈ _５ ） _４ Ｎ－ｍａｌｅａｔｅ、（Ｃ _２ Ｈ _５ ） _４ Ｎ－ｂｅｎｚｏａｔｅ、（Ｃ _２ Ｈ _５ ） _４ Ｎ－ｐｈｔａｌａｔｅ、ステアリルスルホン酸リチウム（ｓｔｅａｒｙｌ ｓｕｌｆｏｎｉｃ ａｃｉｄ ｌｉｔｈｉｕｍ）、オクチルスルホン酸リチウム（ｏｃｔｙｌ ｓｕｌｆｏｎｉｃ ａｃｉｄ ｌｉｔｈｉｕｍ）、ドデシルベンゼンスルホン酸リチウム（ｄｏｄｅｃｙｌ ｂｅｎｚｅｎｅｕｌｆｏｎｉｃ ａｃｉｄ ｌｉｔｈｉｕｍ）等の有機イオン塩等が挙げられ、これらのイオン性化合物を単独、あるいは２種類以上混合して用いることが可能である。 The concentration of the electrolyte salt may be the same as that of the non-aqueous electrolyte used in conventional lithium ion secondary batteries, and is not particularly limited. In this embodiment, a non-aqueous electrolytic solution containing an appropriate lithium compound (electrolyte salt) at a concentration of about 0.8 mol/L or more and 1.5 mol/L or less can be used.

Various additives may be added to the non-aqueous electrolyte. Examples of such additives include negative-electrode-acting additives, positive-acting additives, ester-based additives, carbonate-based additives, sulfate-based additives, phosphate-based additives, borate-based additives, acid-anhydride-based additives, electrolyte-based additives, and the like. Any one of these may be added to the non-aqueous electrolyte, or a plurality of additives may be added to the non-aqueous electrolyte.

The lithium ion secondary battery 10 according to the present embodiment described above uses the negative electrode mixture for a secondary battery according to the present embodiment when manufacturing the negative electrode 30 . Therefore, an unintended increase in the layer thickness of the negative electrode active material layer 32 is suppressed even when the additive amount is small.

<3. Method for producing lithium ion secondary battery>
Next, a method for manufacturing the lithium ion secondary battery 10 will be described. The positive electrode 20 is produced as follows. First, a mixture of the positive electrode active material, the conductive agent, and the positive electrode binder in the above ratio is dispersed in a solvent (for example, N-methyl-2-pyrrolidone) to form a slurry. Next, the slurry is applied onto the current collector 21 and dried to form the positive electrode active material layer 22 . In addition, the method of application is not particularly limited. Examples of coating methods include a knife coater method, a gravure coater method, and the like. The following coating steps are also performed by the same method. Then, the positive electrode active material layer 22 is pressed with a press so that the density falls within the above range. Thereby, the positive electrode 20 is produced.

The negative electrode 30 is also manufactured in the same manner as the positive electrode 20 . First, a slurry is formed by dispersing the negative electrode mixture for a secondary battery described above in a solvent (for example, water). Next, the slurry is applied onto the current collector 31 and dried to form the negative electrode active material layer 32 . The drying temperature is preferably 150° C. or higher. Next, the negative electrode active material layer 32 is pressed with a pressing machine so as to have a density within the above range. Thus, the negative electrode 30 is produced.

Next, an electrode structure is produced by sandwiching the separator 40 between the positive electrode 20 and the negative electrode 30 . Next, the electrode structure is processed into a desired shape (for example, cylindrical, rectangular, laminated, button-shaped, etc.) and inserted into a container of that shape. Next, by injecting a non-aqueous electrolyte into the container, each pore in the separator 40 is impregnated with the electrolyte. Thereby, a lithium ion secondary battery is produced.

As described above, according to the present embodiment, as a negative electrode mixture for a secondary battery, the water-soluble polymer binder (A) contains a water-soluble polymer binder (A) containing a copolymer of a carboxyl group-containing acrylic monomer and a water-soluble acrylic acid derivative monomer, and the negative electrode active material (B). This negative electrode mixture for a secondary battery can suppress swelling of the negative electrode even when added in a small amount, and can improve the cycle characteristics of the lithium ion secondary battery.

Hereinafter, the present invention will be described in more detail based on specific examples. However, the following examples are merely examples of the present invention, and the present invention is not limited to the following examples.

<1. Water-soluble polymer binder (A) synthesis>
Next, an example of this embodiment will be described. First, a synthesis example of the water-soluble polymer binder (A) will be described. The compounding ratios of the monomers and polymer initiators described below represent mass ratios unless otherwise specified.

The feature of this synthesis method is that it is possible to synthesize a high-molecular-weight water-soluble polymer binder (A) while preventing an excessive increase in the viscosity of the reaction solution by using acrylic acid without neutralization. If the water-soluble polymer binder (A) is synthesized in a state in which acrylic acid is partially or wholly neutralized, the viscosity of the reaction solution will excessively increase as the molecular weight increases, making stirring difficult. On the other hand, when neutralization is not performed, the water-soluble polymer binder (A) gradually precipitates as the molecular weight increases, and a white slurry is obtained. Therefore, the viscosity of the reaction solution does not excessively increase, and stirring can be continued until the reaction is completed.

The water-soluble polymer binder (A) having a high molecular weight in this manner enables preparation of slurry for electrode preparation in a smaller amount than a general acrylic acid-based binder resin composition. Furthermore, since the viscosity is high in the low shear rate range, the stability of the electrode-producing slurry is improved, and sedimentation of the active material can be suppressed.

(Synthesis example 1 of water-soluble polymer binder (A): synthesis example of ammonium polyacrylate/2-(2-ethoxyethoxy)ethyl polyacrylate = 90/10)
1200 g of distilled water, acrylic acid (63 g, 0.874 mol), and 2-(2-ethoxyethoxy)ethyl acrylate (7.0 g, 0.037 mol) were added to a 2000 mL five-necked separable flask equipped with a mechanical stirrer, stirring rod, thermometer, and cooling tube. The operation to return to pressure was repeated three times. Heating was started, and when the temperature of the reaction solution reached 65° C., ammonium persulfate (0.312 g, 0.00137 mol) as an initiator was dissolved in 0.3 ml of distilled water and added. The heating temperature was set to 80° C. for 1 hour, and the temperature was further raised to 90° C. and reacted for 2 hours to obtain a polymer composition as a white slurry-like solid.

After cooling to room temperature, the polymer composition was transferred to a 5 L container. While stirring with a mechanical stirrer, 25% aqueous ammonia (53.5 g, 0.9 equivalent to acrylic acid) was added and stirred until the polymer composition was completely dissolved and uniform. Thereafter, 25% aqueous ammonia was added little by little while measuring with a pH meter to adjust the pH within the range of 7.0 to 8.0.

About 5 mL of the reaction solution was taken, and the non-volatile content (NV) was measured to be 5.2% (theoretical value 5.3%). The remainder of the reaction solution was diluted with distilled water in such an amount that the NV would be 5.0% by back calculation from that value, and the binder aqueous solution (NV 5.0%) of Example 1 was obtained.

Moreover, when the shear viscosity of this aqueous binder solution was measured at 25° C., it was 2.5 (Pa·s) at a shear rate of 1.0 (1/s).

(Synthesis example 2 of water-soluble polymer binder (A): synthesis example of ammonium polyacrylate/2-(2-ethoxyethoxy)ethyl polyacrylate = 80/20)
Acrylic acid (56 g, 0.777 mol), 2-(2-ethoxyethoxy)ethyl acrylate (14 g, 0.074 mol), ammonium persulfate (0.291 g, 0.00128 mol), 25% ammonia water (47.56 g, 0.9 equivalents to acrylic acid) were used, all were synthesized in the same manner as in Synthesis Example 1. The non-volatile content (NV) of the reaction solution was measured and found to be 5.2% (theoretical value 5.3%). Moreover, the shear viscosity at 25° C. of the aqueous binder solution (NV 5.0%) was 2.3 (Pa·s) at a shear rate of 1.0 (1/s).

(Synthesis example 3 of water-soluble polymer binder (A): synthesis example of ammonium polyacrylate/2-(2-ethoxyethoxy)ethyl polyacrylate = 70/30)
Acrylic acid (49 g, 0.680 mol), 2-(2-ethoxyethoxy)ethyl acrylate (21 g, 0.112 mol), ammonium persulfate (0.271 g, 0.00119 mol), 25% aqueous ammonia (41.62 g, 0.9 equivalents to acrylic acid) were synthesized in the same manner as in Synthesis Example 1. The non-volatile content (NV) of the reaction solution was measured and found to be 5.3% (theoretical value 5.3%). Moreover, the shear viscosity at 25° C. of the binder aqueous solution (NV 5.0%) was 2.0 (Pa·s) at a shear rate of 1.0 (1/s).

(Synthesis Example 4 of water-soluble polymer binder (A): Synthesis example of lithium polyacrylate/4-hydroxybutyl polyacrylate = 90/10)
Acrylic acid (63 g, 0.874 mol), 4-hydroxybutyl acrylate (7 g, 0.049 mol), ammonium persulfate (0.316 g, 0.00138 mol), 25% aqueous ammonia (53.5 g, 0.9 equivalents relative to acrylic acid) were synthesized in the same manner as in Synthesis Example 1. The non-volatile content (NV) of the reaction solution was measured and found to be 5.2% (theoretical value 5.3%). Moreover, the shear viscosity at 25° C. of the binder aqueous solution (NV 5.0%) was 2.2 (Pa·s) at a shear rate of 1.0 (1/s).

(Synthesis Example 5 of water-soluble polymer binder (A): Synthesis example of lithium polyacrylate/4-hydroxybutyl polyacrylate = 80/20)
Acrylic acid (56 g, 0.777 mol), 4-hydroxybutyl acrylate (14 g, 0.097 mol), ammonium persulfate (0.299 g, 0.00131 mol), 25% aqueous ammonia (47.6 g, 0.9 equivalents relative to acrylic acid) were synthesized in the same manner as in Synthesis Example 1. The non-volatile content (NV) of the reaction solution was measured and found to be 5.2% (theoretical value 5.3%). Further, the shear viscosity at 25° C. of the aqueous binder solution (NV 5.0%) was 2.1 (Pa·s) at a shear rate of 1.0 (1/s).

(Synthesis Example 6 of water-soluble polymer binder (A): Synthesis example of lithium polyacrylate/4-hydroxybutyl polyacrylate = 70/30)
Acrylic acid (49 g, 0.680 mol), 4-hydroxybutyl acrylate (21 g, 0.146 mol), ammonium persulfate (0.283 g, 0.00124 mol), 25% aqueous ammonia (41.6 g, 0.9 equivalent to acrylic acid), was synthesized in the same manner as in Synthesis Example 1. The non-volatile content (NV) of the reaction solution was measured and found to be 5.1% (theoretical value 5.3%). Moreover, the shear viscosity at 25° C. of the aqueous binder solution (NV 5.0%) was 1.9 (Pa·s) at a shear rate of 1.0 (1/s).

<2. Preparation of negative electrode slurry>
(Comparative example 1)
A graphite-silicon composite negative electrode (silicon content 60%) 15.0% by mass, artificial graphite 82.0% by mass, carboxymethyl cellulose (CMC) 1.5% by mass, styrene-butadiene copolymer (SBR) 1.5% by mass. The non-volatile content in the negative electrode mixture slurry was 50% by mass with respect to the total mass of the slurry.

(Example 1)
A negative electrode mixture slurry was prepared in the same manner as in Comparative Example 1, except that 3.0% by mass of the binder resin composition synthesized in Synthesis Example 1 of the water-soluble polymer binder (A) was used instead of 1.5% by mass of carboxymethylcellulose (CMC) and 1.5% by mass of styrene-butadiene copolymer (SBR).

(Example 2)
A negative electrode mixture slurry was prepared in the same manner as in Comparative Example 1, except that 3.0% by mass of the binder resin composition synthesized in Synthesis Example 2 of the water-soluble polymer binder (A) was used instead of 1.5% by mass of carboxymethyl cellulose (CMC) and 1.5% by mass of styrene-butadiene copolymer (SBR).

(Example 3)
A negative electrode mixture slurry was prepared in the same manner as in Comparative Example 1, except that 3.0% by mass of the binder resin composition synthesized in Synthesis Example 3 of the water-soluble polymer binder (A) was used instead of 1.5% by mass of carboxymethyl cellulose (CMC) and 1.5% by mass of styrene-butadiene copolymer (SBR).

(Example 4)
A negative electrode mixture slurry was prepared in the same manner as in Comparative Example 1, except that 3.0% by mass of the binder resin composition synthesized in Synthesis Example 4 of the water-soluble polymer binder (A) was used instead of 1.5% by mass of carboxymethylcellulose (CMC) and 1.5% by mass of styrene-butadiene copolymer (SBR).

(Example 5)
A negative electrode mixture slurry was prepared in the same manner as in Comparative Example 1, except that 3.0% by mass of the binder resin composition synthesized in Synthesis Example 5 of the water-soluble polymer binder (A) was used instead of 1.5% by mass of carboxymethyl cellulose (CMC) and 1.5% by mass of styrene-butadiene copolymer (SBR).

(Example 6)
A negative electrode mixture slurry was prepared in the same manner as in Comparative Example 1, except that 3.0% by mass of the binder resin composition synthesized in Synthesis Example 6 of the water-soluble polymer binder (A) was used instead of 1.5% by mass of carboxymethyl cellulose (CMC) and 1.5% by mass of styrene-butadiene copolymer (SBR).

<3. Negative electrode production>
(Comparative example 2)
The gap of the bar coater was adjusted so that the mixture coating amount (surface density) after drying was 10.1 mg/cm ² , and the negative electrode mixture slurry of Comparative Example 1 was uniformly coated onto a copper foil (current collector, thickness 10 μm) with this bar coater. Then, it was dried for 15 minutes in a blower type dryer set at 80°C. Then, it was pressed with a roll press so that the mixture density was 1.65 g/cm ³ . Subsequently, the negative electrode was produced by vacuum-drying at 150 degreeC for 6 hours.

(Example 7)
A negative electrode was produced in the same manner as in Comparative Example 2, except that the negative electrode mixture slurry prepared in Example 1 was used instead of Comparative Example 1.

(Example 8)
A negative electrode was produced in the same manner as in Comparative Example 2, except that the negative electrode mixture slurry prepared in Example 2 was used instead of Comparative Example 1.

(Example 9)
A negative electrode was produced in the same manner as in Comparative Example 2, except that the negative electrode mixture slurry prepared in Example 3 was used instead of Comparative Example 1.

(Example 10)
A negative electrode was produced in the same manner as in Comparative Example 2, except that the negative electrode mixture slurry prepared in Example 4 was used instead of Comparative Example 1.

(Example 11)
A negative electrode was produced in the same manner as in Comparative Example 2, except that the negative electrode mixture slurry prepared in Example 5 was used instead of Comparative Example 1.

(Example 12)
A negative electrode was produced in the same manner as in Comparative Example 2, except that the negative electrode mixture slurry prepared in Example 6 was used instead of Comparative Example 1.

<3. Positive electrode production>
(Preparation of positive electrode mixture slurry)
Solid solution oxide Li _1.20 Mn _0.55 Co _0.10 Ni _0.15 O ₂ 97.4% by mass, Ketjenblack 1.3% by mass, and polyvinylidene fluoride 1.3% by mass were dispersed in N-methyl-2-pyrrolidone to form a positive electrode mixture slurry. The non-volatile content in the positive electrode mixture slurry was 50% by mass with respect to the total mass of the slurry.

(Preparation of positive electrode)
Next, the gap of the bar coater was adjusted so that the coating amount (surface density) of the mixture after drying was 23.6 mg/cm ² , and the positive electrode mixture slurry was applied onto an aluminum current collector foil as a current collector with this bar coater. Next, the positive electrode material mixture slurry was dried for 15 minutes with a blower dryer set at 80°C. Next, the dried positive electrode mixture was pressed with a roll press so that the density of the mixture was 3.65 g/cm ³ . Then, the positive electrode mixture was vacuum-dried at 80° C. for 6 hours to prepare a sheet-like positive electrode comprising a positive electrode current collector and a positive electrode active material layer.

<4. Production of secondary battery>
(Comparative Example 3)
The negative electrode shown in Comparative Example 2 (indicated by reference numeral 130 in FIG. 2) and the positive electrode shown in the positive electrode preparation example (indicated by reference numeral 120 in FIG. 2) were each cut into a 3 cm×5 cm rectangle with tab portions 121 and 131 shown in FIG. 2 using a Thomson blade. As shown in FIG. 3 , a nickel tab lead 132 was welded to the tab portion 131 of the cut negative electrode 130 , and an aluminum tab lead 122 was welded to the tab portion 121 of the cut positive electrode 120 . Then, as shown in FIG. 4, the separator (polyethylene microporous film with a thickness of 25 microns) 140 was cut into a rectangle of 3.5 cm×5.5 cm using a Musson blade. Next, as shown in FIGS. 5 and 6, the positive electrode 120 and the negative electrode 130 were opposed to each other with the separator 140 interposed therebetween, wrapped with a laminate film 150, and thermocompression bonded. At this time, as shown in FIG. 6, two continuous sides other than the side where the tab portions 121 and 131 are arranged are thermocompression bonded to form a thermocompression bonded portion 151, and the laminate film 150 is formed into a bag shape. Next, as shown in FIG. 6 , 160 μL of an electrolytic solution (1.4 M LiPF ₆ ethylene carbonate/diethyl carbonate/fluoroethylene carbonate = 10/70/20 mixed solution (volume ratio)) 160 was added to the bag-shaped laminate film 150 , and then the laminate film 150 was completely sealed by vacuum lamination to fix the tab portions 121 and 131 , thereby producing the laminate type secondary battery 100 . (See FIG. 7). A total of two identical laminated secondary batteries were produced.

(Example 13)
A laminate type secondary battery was produced in the same manner as in Comparative Example 3, except that the negative electrode produced in Example 7 was used.

(Example 14)
A laminate type secondary battery was produced in the same manner as in Comparative Example 3, except that the negative electrode produced in Example 8 was used.

(Example 15)
A laminate type secondary battery was produced in the same manner as in Comparative Example 3, except that the negative electrode produced in Example 9 was used.

(Example 16)
A laminate type secondary battery was produced in the same manner as in Comparative Example 3, except that the negative electrode produced in Example 10 was used.

(Example 17)
A laminate type secondary battery was produced in the same manner as in Comparative Example 3, except that the negative electrode produced in Example 11 was used.

(Example 18)
A laminate type secondary battery was produced in the same manner as in Comparative Example 3, except that the negative electrode produced in Example 12 was used.

<5. Evaluation of negative electrode swelling>
One of the produced laminate type secondary batteries was attached to a charging/discharging device, left at 25° C. for 8 hours, charged at 0.1 C to 4.3 V (CC charging), then charged at 4.3 V to 0.01 C (CV charging), and stopped. The battery was disassembled in a dry room, and the negative electrode with 100% SOC was taken out. The thickness of the negative electrode was measured with a micrometer, and the swelling (%) of the negative electrode was obtained from the following formula (1).
{(thickness of negative electrode at 100% SOC−thickness of negative electrode before battery production)/(thickness of negative electrode before battery production−thickness of current collector (copper foil))}×100 (1)

<6. Evaluation of cycle characteristics>
The other laminated secondary battery thus prepared was attached to a charging/discharging device, left at 25° C. for 8 hours, charged at 0.2 C to 4.3 V (CC charge), charged at 4.3 V to 0.02 C (CV charge), and then discharged at 0.2 C to 2.5 V (CC discharge) and stopped.

Next, the battery was charged at 0.5 C to 4.3 V (CC charge), then charged at 4.3 V to 0.05 C (CV charge), and then discharged at 0.5 C to 2.5 V (CC discharge). Taking this as one cycle, 100 cycles of charging and discharging were repeated.

The capacity retention rate (%) after 100 cycles was obtained from the following formula (2) and defined as cycle characteristics.
(capacity after 100 cycles)/(initial capacity) x 100 (2)

Table 1 summarizes the evaluation results of electrode swelling and cycle characteristics of the negative electrode.

According to Table 1, it can be seen that the negative electrode of the secondary battery according to the present example has small electrode swelling and excellent cycle characteristics even when the content of the secondary battery negative electrode mixture is relatively small at 3%.

Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention belongs can conceive of various modifications or modifications within the scope of the technical idea described in the claims, and it is naturally understood that these also belong to the technical scope of the present invention.

10 lithium ion secondary battery 20 positive electrode 30 negative electrode 40 separator

Claims (10)

A negative electrode mixture for a secondary battery comprising a water-soluble polymer binder (A) and a negative electrode active material (B),
The mass ratio of the water-soluble polymer binder (A) and the negative electrode active material (B) is 2.0:98.0 to 6.0:94.0, with the sum of these mass ratios being 100,
The water-soluble polymer binder (A) contains a copolymer of a carboxyl group-containing acrylic monomer and a water-soluble acrylic acid derivative monomer,
The negative electrode active material (B) contains 10.0% by mass or more and 20.0% by mass or less of a Si-based active material,
A negative electrode mixture for a secondary battery, wherein the 5.0% by mass aqueous solution of the copolymer has a shear viscosity at 25° C. of 0.5 (Pa·s) or more and 5.0 (Pa·s) or less at a shear rate of 1.0 (1/s). The negative electrode mixture for a secondary battery according to claim 1, wherein the water-soluble polymer binder (A) contains 10% by mass or more and 40% by mass or less of structural units derived from the water-soluble acrylic acid derivative monomer. The carboxyl group-containing acrylic monomer is at least one selected from the group consisting of acrylic acid, methacrylic acid, maleic acid, monomethyl maleic acid, 2-carboxyethyl acrylate, and 2-carboxyethyl methacrylate. The secondary battery negative electrode mixture according to claim 1 or 2 . The negative electrode mixture for a secondary battery according to any one of claims 1 to 3 , wherein the water-soluble acrylic acid derivative monomer is at least one of an ethylene glycol chain-containing acrylic monomer and a hydroxyl group-containing acrylic monomer. The ethylene glycol chain-containing acrylic monomer is 2-methoxyethyl acrylate, 2-ethoxyethyl acrylate, 2-(2-methoxyethoxy)ethyl acrylate, 2-(2-ethoxyethoxy)ethyl acrylate, 2-(2-(2-methoxyethoxy)ethoxy)ethyl acrylate, 2-(2-(2-methoxyethoxy)ethoxy)ethyl acrylate, methoxypolyethylene glycol acrylate, 2-methoxyethyl methacrylate, 2-ethoxyethyl methacrylate, 2-(2-methoxyethoxylate). oxy)ethyl methacrylate, 2-(2-ethoxyethoxy)ethyl methacrylate, 2-(2-(2-methoxyethoxy)ethoxy)ethyl methacrylate, 2-(2-(2-methoxyethoxy)ethoxy)ethyl methacrylate, and at least one selected from the group consisting of methoxypolyethylene glycol methacrylate.4The negative electrode mixture for secondary batteries as described in . The hydroxyl group-containing acrylic monomer is at least one selected from the group consisting of 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 3-hydroxypropyl acrylate, 2-hydroxybutyl acrylate, 4-hydroxybutyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl methacrylate, 2-hydroxybutyl methacrylate, and 4-hydroxybutyl methacrylate. The secondary battery negative electrode mixture according to claim 4 . 7. The negative electrode mixture for a secondary battery according to claim 1 , wherein at least part of said carboxyl group-containing acrylic monomer is an alkali metal salt or an ammonium salt. A negative electrode for a secondary battery, comprising the negative electrode mixture for a secondary battery according to any one of claims 1 to 7 . The negative electrode for a secondary battery according to claim 8 , further comprising a styrene-butadiene copolymer (SBR). A secondary battery comprising the negative electrode for a secondary battery according to claim 8 or 9 .

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