JP6679142B2 - Lithium-ion secondary battery electrode forming composition, lithium-ion secondary battery electrode and lithium-ion secondary battery, and method for producing lithium-ion secondary battery electrode-forming composition - Google Patents

Description

  The present invention relates to a composition for forming a lithium ion secondary battery electrode, an electrode for a lithium ion secondary battery and a lithium ion secondary battery, and a method for producing a composition for forming a lithium ion secondary battery electrode.

  2. Description of the Related Art In recent years, lithium ion secondary batteries have been receiving attention from the viewpoints of downsizing and weight reduction of notebook computers, mobile phones, power tools, electronic devices, communication devices and the like. Further, recently, from the viewpoint of environmental application, a lithium ion secondary battery for electric vehicles and hybrid vehicles, particularly a lithium ion secondary battery having high voltage, high capacity and high energy density has been strongly demanded.

A lithium ion secondary battery is composed of a positive electrode using a metal oxide such as lithium cobalt oxide as an active material, a negative electrode using a carbon material such as graphite as an active material, and an electrolytic solution centering on carbonates. It is a secondary battery that is charged and discharged by moving between a positive electrode and a negative electrode.
More specifically, the positive electrode can be obtained by forming a positive electrode layer on the surface of a positive electrode current collector such as an aluminum foil from a composition containing a positive electrode active material such as a metal oxide and a binder, and the negative electrode is formed of copper. It can be obtained by forming a negative electrode layer on the surface of a negative electrode current collector such as a foil from a composition containing a negative electrode active material such as graphite and a binder. Therefore, each binder has a role of binding the active material and the binder and preventing cohesive failure of the positive electrode layer and the negative electrode layer.

  Conventionally, as the binder for the positive electrode layer and the negative electrode layer, polyvinylidene fluoride (PVDF) using N-methylpyrrolidone (NMP), which is an organic solvent, as a solvent is used from the viewpoint of the swelling resistance of the resin itself to the electrolytic solution. , Has been industrially used for many models. However, this binder has a low binding property to the active material and requires a large amount of binder for practical use, resulting in a drawback that the capacity and energy density of the lithium ion secondary battery are lowered. In addition, since NMP, which is an expensive organic solvent, is used as the binder, there are problems in the price of the final product and the work environment protection at the time of producing the slurry or the current collector.

  As a solution to these problems, in Patent Document 1, a specific amount of styrene, an ethylenically unsaturated carboxylic acid ester, an ethylenically unsaturated carboxylic acid, and an internal cross-linking agent are added to all ethylenically unsaturated monomers. A binder for a lithium ion secondary battery electrode having a glass transition temperature of 30 ° C. or lower, which is obtained by emulsion polymerization of an ethylenically unsaturated monomer contained as an essential component in the presence of an emulsifier, has been proposed.

JP, 2011-243464, A

However, although the binder of Patent Document 1 can improve the binding property of natural graphite, which is often used as a negative electrode active material, it cannot necessarily improve the binding property of other active materials such as artificial graphite. It was Among other active materials, although artificial graphite is expensive, it has few impurities and can reduce electric resistance and can improve battery performance. Therefore, a binder suitable for artificial graphite has been recently demanded.
The present invention is a composition for forming a lithium ion secondary battery electrode, which can improve the binding property between the active material and a binder resin, an electrode for a lithium ion secondary battery, and a lithium ion, even if it is an active material other than natural graphite. It is intended to provide a secondary battery. Further, the present invention provides a method for efficiently producing a composition for forming a lithium ion secondary battery electrode, which has a good binding property between an active material and a binder resin, even if it is an active material other than natural graphite. The purpose is to

The inventors of the present invention have conducted extensive studies to solve the above problems, and first estimated that the surface property of the active material has a great influence on the binding property between the active material and the binder resin. However, the surface properties of the active material have been difficult to analyze. In particular, artificial graphite loses the functional groups on the surface by firing, and it is extremely difficult to analyze the properties of the surface. Therefore, the quality of the binding property cannot be determined unless the active material layer is actually formed. Therefore, the inventors of the present invention have further earnestly studied, and as a means for specifying the surface property of the active material, paid attention to the lateral relaxation time of water protons by the pulse NMR method, and came to solve the above problems.
That is, the present invention relates to the following [1] to [7] composition for forming a lithium ion secondary battery electrode, an electrode for a lithium ion secondary battery and a lithium ion secondary battery, and a composition for forming a lithium ion secondary battery electrode. A method of manufacturing a product is provided.

[1] An active material and a binder resin are contained, the active material has a transverse relaxation time of water protons of a dispersion liquid of 10 milliseconds or more, which is measured under the following condition 1, and the binder resin is A composition for forming a lithium ion secondary battery electrode, wherein the transverse relaxation time of water protons of an aqueous emulsion measured under the following condition 2 is 200 milliseconds or more.
Condition 1: An active material dispersion liquid obtained by mixing active material, carboxymethylcellulose having a weight average molecular weight of 3,000,000 and a substitution degree of 0.9, and water in a mass ratio of 52.8: 0.5: 46.6. Relaxation time of water protons measured by pulsed NMR method.
Condition 2: Transverse relaxation time of water protons measured by pulse NMR method for anionic aqueous emulsion containing 40% by mass of binder resin.
[2] The composition for forming a lithium ion secondary battery electrode according to the above [1], wherein the active material is artificial graphite.
[3] The composition for forming a lithium ion secondary battery electrode according to the above [1] or [2], wherein the binder resin has an acid value of 20 mgKOH / g or less.
[4] The composition for forming a lithium ion secondary battery electrode according to any one of the above [1] to [3], wherein the binder resin is a copolymer of styrene and an ethylenically unsaturated carboxylic acid ester.
[5] A lithium ion secondary battery electrode having an active material-containing layer formed on the current collector by the lithium ion secondary battery electrode forming composition according to any one of [1] to [4] above. .
[6] A lithium ion secondary battery using the lithium ion secondary battery electrode according to the above [5].
[7] A method for producing a composition for forming a lithium ion secondary battery electrode, comprising the step of mixing an active material and a binder resin,
When the active material has a transverse relaxation time of the water protons of the dispersion liquid of 10 milliseconds or more as measured by the following condition 1, it acts as a binder resin, and the transverse relaxation time of water protons of the aqueous emulsion as measured by the following condition 2 is used. Is 200 milliseconds or more, and a method for producing a composition for forming a lithium ion secondary battery electrode.
Condition 1: An active material dispersion liquid obtained by mixing active material, carboxymethylcellulose having a weight average molecular weight of 3,000,000 and a substitution degree of 0.9, and water in a mass ratio of 52.8: 0.5: 46.6. Relaxation time of water protons measured by pulsed NMR method.
Condition 2: Transverse relaxation time of water protons measured by pulse NMR method for anionic aqueous emulsion containing 40% by mass of binder resin.

According to the composition for forming a lithium ion secondary battery electrode, the electrode for a lithium ion secondary battery, and the lithium ion secondary battery of the present invention, even if an active material other than natural graphite is used, the active material and the binder resin are bound together. Wearability can be improved.
Further, conventionally, it was not possible to determine the binding property between the active material and the binder resin unless the active material-containing layer was actually formed, and thus the method for producing the lithium ion secondary battery electrode-forming composition of the present invention was used. According to this, it is possible to select an active material and a binder resin having excellent binding properties without actually forming the active material-containing layer, and to enhance the production efficiency of the composition for forming a lithium ion secondary battery electrode. You can

[Lithium-ion secondary battery electrode forming composition]
The composition for forming a lithium ion secondary battery electrode of the present invention (hereinafter, sometimes referred to as the “composition of the present invention”) contains an active material and a binder resin, and the active material comprises: The transverse relaxation time of the water protons of the dispersion measured by the following condition 1 is 10 milliseconds or more, and the binder resin is 200 milliseconds or more of the transverse relaxation time of the water protons of the aqueous emulsion measured by the following condition 2. Is what is.
Condition 1: An active material dispersion liquid obtained by mixing active material, carboxymethylcellulose having a weight average molecular weight of 3,000,000 and a substitution degree of 0.9, and water in a mass ratio of 52.8: 0.5: 46.6. Relaxation time of water protons measured by pulsed NMR method.
Condition 2: Transverse relaxation time of water protons measured by pulse NMR method for anionic aqueous emulsion containing 40% by mass of binder resin.

The composition of the present invention uses an active material having a transverse relaxation time of water protons (hydrogen nuclei) of 10 msec or more in the dispersion liquid measured under the condition 1, and measured as a binder resin under the condition 2. A water-based emulsion having a transverse relaxation time of water protons of 200 milliseconds or more is used.
In the present invention, attention is paid to the fact that the transverse relaxation time of water protons (hereinafter sometimes referred to as “transverse relaxation time”) measured by the pulsed NMR method indicates the wettability of the active material and the binder resin to water. did. That is, it means that if the lateral relaxation time is long, it is hard to be wet with water (low hydrophilicity), and if the lateral relaxation time is short, it is easy to wet with water (high hydrophilicity).
Conventionally, it has been difficult to analyze the surface properties of active materials, and in particular, artificial graphite has a surface functional group that disappears upon firing, making it difficult to analyze the surface properties. In the present invention, as an index for judging the hydrophilicity of the active material and the binder resin, attention is paid to the lateral relaxation time of water protons measured by the pulsed NMR method, and further, the lateral relaxation time of the active material and the lateral relaxation time of the binder resin. By making the relaxation time have a specific relationship, it is possible to improve the binding property between the active material and the binder resin even for active materials other than natural graphite. Further, the composition of the present invention satisfying the conditions 1 and 2 is good in that it can easily improve the binding property with the current collector described later.
The composition of the present invention can be used both as a positive electrode forming composition and a negative electrode forming composition of a lithium ion secondary battery electrode, but when it is used as a negative electrode forming composition, it is particularly effective.

(Active material)
The active material used has a transverse relaxation time of water protons of 10 milliseconds or more, which is measured under the following condition 1.
Condition 1: An active material dispersion liquid obtained by mixing active material, carboxymethylcellulose having a weight average molecular weight of 3,000,000 and a substitution degree of 0.9, and water in a mass ratio of 52.8: 0.5: 46.6. Relaxation time of water protons measured by pulsed NMR method.

The measurement of Condition 1 is performed by the pulsed NMR method. More specifically, a pulse nuclear magnetic resonance apparatus is used, a measurement nucleus is a hydrogen nucleus, a measurement temperature is 25 ° C., a frequency is 13 MHz, and a 90 ° pulse width is 2 μs under a measurement condition of a CPMG method (Carr-Purcell Meiboom-Gill method). The lateral relaxation time (spin-spin relaxation time) of water protons is measured by. When measuring the condition 1, the pH of the active material dispersion liquid is preferably adjusted to 7.0.
In the measurement under Condition 1, the average of the two components of the lateral relaxation time of water protons in contact with the surface of the active material and the lateral relaxation time of water protons in the free state not in contact with the surface of the active material Is calculated as the transverse relaxation time of water protons.
The lateral relaxation time of Condition 1 is preferably 11 milliseconds or longer, and more preferably 12 milliseconds or longer. The upper limit of the lateral relaxation time of Condition 1 is not particularly limited, but is preferably 50 milliseconds or less, more preferably 40 milliseconds or less, and further preferably 30 milliseconds or less.
Carboxymethyl cellulose (CMC) plays a role of improving the dispersion stability of the active material dispersion liquid by virtue of its thickening effect.

  The active material can be used without particular limitation as long as it satisfies the condition 1. As the active material, those capable of inserting and extracting lithium ions are preferably used. The active material includes a positive electrode active material and a negative electrode active material. The present invention is more likely to exert an effect when a negative electrode active material is used as an active material, and more easily exerts an effect when a carbonaceous material or artificial graphite is used among negative electrode active materials, and in particular, analysis of surface properties is performed. When artificial graphite, which is difficult to use, is used, a very remarkable effect can be exhibited.

The shape of the active material is not particularly limited, and a spherical shape, a scaly shape, or the like can be used. Among these, spherical ones are preferable from the viewpoint of electronic conductivity.
From the viewpoint of dispersibility of the active material, the average particle diameter of the active material is preferably 5 to 100 μm, more preferably 10 to 50 μm, and further preferably 15 to 30 μm. The average particle diameter can be calculated by the laser diffraction method.
Average specific surface area of the active material, from the viewpoint of the dispersibility of the active material is preferably 0.1 to 100 m ² / g, more preferably 0.1 to 50 m ² / g, 0.1 to More preferably, it is 30 m ² / g. The average specific surface area can be obtained by measuring the specific surface area by the BET nitrogen adsorption method (according to JIS Z8830).

Examples of the positive electrode active material include metal composite oxides, particularly metal composite oxides containing at least one metal of lithium and iron, cobalt, nickel, manganese, and the like, and preferably Li _{x My 1} O ₂ ( However, M is one or more transition metals, preferably represents at least one of Co, Mn or Ni, 1.10>x> 0.05,1 ≧ y1> 0), _or, Li _{x M y2} O ₄ (provided that M represents one or more transition metals, preferably Mn or Ni, and 1.10>x> 0.05, 2 ≧ y2> 0), or Li _{x My 1} PO ₄ (provided that , M represents one or more transition metals, preferably at least one of Fe, Co, Mn or Ni, and 1.10>x> 0.05, 1 ≧ y1> 0). It is, for example LiCoO _2, L _{NiO 2, Li x Ni y3 Mn} z Co a O 2 ( wherein, 1.10>x>0.05,1> y3 >0,1>z>0,1>a> is 0.), LiMn Examples thereof include composite oxides represented by ₂ O ₄ and LiFePO ₄ .

Examples of the negative electrode active material include various silicon oxides (SiO _{2 and the} like); carbonaceous materials; metal composite oxides such as Li ₄ Ti ₅ O _{12 and the} like, and artificial graphite is preferable.
Artificial graphite is obtained by firing a carbonaceous material such as amorphous carbon, graphite, natural graphite, pitch-based carbon fiber, and polyacetylene at a temperature of about 3000 ° C., and has a different crystal structure from the carbonaceous material. In addition, the artificial graphite is a hexagonal plate-like crystal having an atomic bond shape and a turtle shell-like layered substance as a structure.

(Binder resin)
As the binder resin, one having a transverse relaxation time of 200 milliseconds or more of water protons (hydrogen atomic nuclei) of an aqueous emulsion measured under the following condition 2 is used.
Condition 2: Transverse relaxation time of water protons measured by pulse NMR method for anionic aqueous emulsion containing 40% by mass of binder resin.

The measurement of the condition 2 is performed by the pulse NMR method. More specifically, a pulse nuclear magnetic resonance apparatus is used, a measurement nucleus is a hydrogen nucleus, a measurement temperature is 25 ° C., a frequency is 13 MHz, and a 90 ° pulse width is 2 μs under a measurement condition of a CPMG method (Carr-Purcell Meiboom-Gill method). The lateral relaxation time (spin-spin relaxation time) of water protons was measured by. When measuring the condition 2, it is preferable to adjust the pH of the anionic aqueous emulsion to 7.0.
In the measurement of Condition 2, there are two components: a lateral relaxation time of water protons in contact with the surface of the binder resin particles and a lateral relaxation time of water protons in a free state not in contact with the surface of the binder resin particles. Is calculated as the transverse relaxation time of water protons.
The lateral relaxation time of Condition 2 is preferably 220 milliseconds or longer, and more preferably 300 milliseconds or longer. Although the upper limit of the lateral relaxation time of Condition 2 is not particularly limited, it is preferably 750 milliseconds or less, more preferably 600 milliseconds or less, and further preferably 450 milliseconds or less.

The anionic aqueous emulsion used for the measurement of Condition 2 can be prepared by the following methods (1) and (2), for example.
(1) An anionic emulsifier is used to prepare an aqueous emulsion containing 40% by mass of a binder resin.
(2) A reactive anionic emulsifier is used as a polymerizable monomer for forming a binder resin to prepare an aqueous emulsion containing 40% by mass of the polymerized binder resin.
The average particle size of the binder resin particles in the aqueous emulsion does not particularly affect the transverse relaxation time of water protons, but is preferably 50 to 500 nm, more preferably 100 to 250 nm. The average particle size of the binder resin particles in the aqueous emulsion can be measured by a laser diffraction method.

  The binder resin can be used without particular limitation as long as it satisfies the condition 2. As the binder resin satisfying the condition 2, one having a low acid value is preferable, and specifically, one having an acid value of 20 mgKOH / g or less is preferable, one having an acid value of 15 mgKOH / g or less is more preferable, and 10 mgKOH / g or less. Are more preferable.

The binder resin preferably has a glass transition temperature of 30 ° C. or lower, more preferably 20 ° C. or lower, and further preferably 15 ° C. or lower, from the viewpoint of making the electrode formed from the composition of the present invention hard to break. More preferable. From the viewpoint of handleability, the glass transition temperature of the binder resin is preferably −20 ° C. or higher.
The glass transition temperature of the binder resin is the glass transition temperature Tgi (i = i) of each homopolymer of the ethylenically unsaturated monomer Mi (i = 1, 2, ..., I) used for emulsion polymerization of the binder resin. 1,2, ..., i) and each weight fraction Xi (i = 1,2, ..., i) of the ethylenically unsaturated monomer Mi, the theoretical value is calculated by the following formula (I). Can be calculated as
1 / Tg = Σ (Xi / Tgi) (I)

  Examples of the binder resin include styrene-butadiene rubber; copolymer of styrene and ethylenically unsaturated carboxylic acid ester; ethylene-vinyl acetate copolymer, ethylene-vinyl versatic acid copolymer, ethylene-acrylic acid ester copolymer. Examples thereof include ethylene-ethylenically unsaturated carboxylic acid ester copolymers such as polymers. Among these, a copolymer of styrene and an ethylenically unsaturated carboxylic acid ester can improve the binding property between the active material and the binder resin, has excellent swelling resistance to an electrolytic solution, and has excellent charge-discharge cycle characteristics. It is preferable because it is excellent. A copolymer of styrene and an ethylenically unsaturated carboxylic acid ester is also good in that it has excellent binding properties with the current collector.

  A copolymer of styrene and an ethylenically unsaturated carboxylic acid ester (hereinafter sometimes simply referred to as a "copolymer") has the above-mentioned effects by using styrene and an ethylenically unsaturated carboxylic acid ester in combination. Is played. The copolymer can be obtained, for example, by emulsion-polymerizing a raw material composition containing styrene, an ethylenically unsaturated carboxylic acid ester and an internal crosslinking agent in an aqueous medium in the presence of an emulsifier.

Styrene mainly serves to improve the binding property between the active material and the binder resin and the binding property between the active material-containing layer and the current collector. In particular, when artificial graphite is used as the active material, the effect can be further enhanced.
The amount of styrene used is preferably 15 to 70% by mass, more preferably 30 to 60% by mass, and further preferably 35 to 55% by mass of the total monomer components forming the copolymer. preferable.
When the amount of styrene used is 15% by mass or more, the binding property between the active material and the binder resin and the binding property between the active material-containing layer and the current collector can be easily improved. Further, when the amount of styrene used is 70% by mass or less, the electrode formed from the composition of the present invention can be made difficult to break.

The ethylenically unsaturated carboxylic acid ester can be divided into those having no functional group and those having a functional group.
Examples of the ethylenically unsaturated carboxylic acid ester having no functional group include methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, isopropyl (meth) acrylate, and (meth) acrylate. ) N-butyl acrylate, iso-butyl (meth) acrylate, tert-butyl (meth) acrylate, n-hexyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, lauryl (meth) acrylate, Examples thereof include stearyl (meth) acrylate, cyclohexyl (meth) acrylate, isononyl (meth) acrylate, isobornyl (meth) acrylate, and benzyl (meth) acrylate. Among these, n-butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, lauryl (meth) acrylate, and isobornyl (meth) acrylate are preferable from the viewpoint of ease and durability of emulsion polymerization.

The amount of the ethylenically unsaturated carboxylic acid ester having no functional group to be used is preferably 25 to 85% by mass, and more preferably 30 to 65% by mass, based on all the monomer components forming the copolymer. More preferably, it is more preferably 40 to 55 mass%.
By adjusting the amount of the ethylenically unsaturated carboxylic acid ester having no functional group to 25% by mass or more, the flexibility and heat resistance of the formed electrode can be easily improved, and by adjusting the amount to 85% by mass or less, The binding property between the active material and the binder resin and the binding property between the active material-containing layer and the current collector can be easily improved.

  Examples of the ethylenically unsaturated carboxylic acid ester having a functional group include an ethylenically unsaturated carboxylic acid ester having a hydroxy group, a glycidyl group and the like. Examples thereof include 2-hydroxyethyl (meth) acrylate, 2-hydroxyalkyl (meth) acrylate such as 2-hydroxypropyl (meth) acrylate, and glycidyl acrylate. Among these, 2-hydroxyethyl (meth) acrylate is preferable.

The amount of the ethylenically unsaturated carboxylic acid ester having a functional group used is preferably 0.1 to 10% by mass, and preferably 0.5 to 5% by mass, based on all the monomer components forming the copolymer. It is more preferable, and it is still more preferable that the amount is 1 to 3% by mass.
By setting the amount of the ethylenically unsaturated carboxylic acid ester having a functional group to be 0.1% by mass or more, it is possible to easily improve the emulsion polymerization stability and mechanical stability, and to improve the dry film of the electrolytic solution. The swelling resistance can be easily improved. Further, when the amount of the ethylenically unsaturated carboxylic acid ester having a functional group used is large, the lateral relaxation time tends to be short, and conversely, when the amount used is small, the lateral relaxation time tends to be long. Therefore, by setting the amount of the ethylenically unsaturated carboxylic acid ester having a functional group to 10% by mass or less, the lateral relaxation time can be easily set within the range of the present invention, and the binding property between the active material and the binder resin, Also, the binding property between the active material-containing layer and the current collector can be easily improved.

An ethylenically unsaturated carboxylic acid may be further used as a monomer forming the above copolymer.
Examples of the ethylenically unsaturated carboxylic acid include acrylic acid, methacrylic acid, unsaturated monocarboxylic acids such as crotonic acid, maleic acid, fumaric acid, unsaturated dicarboxylic acids such as itaconic acid, and half esters of these unsaturated dicarboxylic acids. Among them, acrylic acid and itaconic acid are preferable. These ethylenically unsaturated carboxylic acids may be used alone or in combination of two or more.

  Addition of a small amount of ethylenically unsaturated carboxylic acid may contribute to the improvement of emulsion polymerization stability and mechanical stability, but addition of a large amount of ethylenically unsaturated carboxylic acid tends to shorten the transverse relaxation time, resulting in a binding between the active material and the binder resin. The adhesiveness and the binding property between the active material-containing layer and the current collector tend to decrease. Therefore, the amount of the ethylenically unsaturated carboxylic acid used is preferably 3% by mass or less, more preferably 2% by mass or less, and more preferably 1% by mass, based on all the monomer components forming the copolymer. The following is more preferable.

As the monomer forming the above-mentioned copolymer, a monomer other than those described above having at least one polymerizable ethylenically unsaturated group may be used. Examples of such a monomer include (meth) acrylamide, N-methylol (meth) acrylamide, (meth) acrylonitrile, vinyl acetate, vinyl propionate, and other amide groups, and ethylenically unsaturated carboxylic acids having a functional group such as a nitrile group. Compounds other than ester, sodium p-styrenesulfonate, etc. may be mentioned.
Further, as a monomer forming the above-mentioned copolymer, mercaptan, thioglycolic acid and its ester, β-mercaptopropionic acid and its ester, etc. may be used in order to adjust the molecular weight.
Further, as a monomer forming the above copolymer, a reactive emulsifier described later may be further used.

The raw material composition of the copolymer of styrene and ethylenically unsaturated carboxylic acid ester further contains an internal cross-linking agent (internal cross-linking monomer) in order to further improve the swelling resistance of the dry film to the electrolytic solution. It is preferable.
The internal cross-linking agent has at least one ethylenically unsaturated bond and has a reactive group reactive with the functional group of the above-mentioned monomer, or has two or more ethylenically unsaturated bonds. Any thing can be used.
As such an internal crosslinking agent, a crosslinkable polyfunctional monomer having two or more unsaturated groups such as divinylbenzene, ethylene glycol di (meth) acrylate, trimethylolpropane tri (meth) acrylate and triallyl cyanurate. , Silane coupling agents such as vinyltrimethoxysilane, vinyltriethoxysilane, γ-methacryloxypropyltrimethoxysilane, γ-methacryloxypropyltriethoxysilane, and the like. Among these, divinylbenzene and trimethylolpropanetri (Meth) acrylate and γ-methacryloxypropyltrimethoxysilane are preferred. These internal cross-linking agents may be used alone or in combination of two or more.

  The amount of the internal cross-linking agent used is preferably 0.1 to 5 mass%, more preferably 0.1 to 3 mass%, and more preferably 0.2 to 5 mass% of all the monomer components forming the copolymer. It is more preferable that the content is ˜2% by mass. When the amount of the internal cross-linking agent used is 0.1% by mass or more, the swelling resistance of the dry film with respect to the electrolytic solution can be easily improved. it can.

  In order to set the transverse relaxation time of the copolymer of styrene and an ethylenically unsaturated carboxylic acid ester within the range of Condition 2, among the monomers forming the copolymer, styrene and an ethylenic ester having no functional group are used. It is preferable to use an internal crosslinking agent after increasing the total amount with the unsaturated carboxylic acid ester. Specifically, the total amount of styrene and an ethylenically unsaturated carboxylic acid ester having no functional group is preferably 90 mass% or more, more preferably 93 mass% or more, and 94 mass% or more. More preferably,

As the emulsifier used in the emulsion polymerization, usual anionic emulsifiers and nonionic emulsifiers are used.
Examples of the anionic emulsifier include alkylbenzene sulfonate, alkyl sulfate ester salt, polyoxyethylene alkyl ether sulfate ester salt, fatty acid salt and the like. Examples of the nonionic emulsifier include polyoxyethylene alkyl ether, polyoxyethylene alkylphenyl ether, polyoxyethylene polycyclic finyl ether, polyoxyalkylene alkyl ether, sorbitan fatty acid ester, and polyoxyethylene sorbitan fatty acid astel. These may be used alone or in combination of two or more.
Further, when a reactive emulsifier is used as the emulsifier, bleed-out of the emulsifier is prevented, and the mechanical stability of the electrode formed from the composition of the present invention can be improved, which is preferable. Examples of the reactive emulsifier include those represented by the following general formulas (1) to (5).

In the formula, R represents an alkyl group and m represents an integer of 10-40.

In the formula, n represents an integer of 10 to 12 and m represents an integer of 10 to 40.

In the formula, R represents an alkyl group and M represents NH ₄ or Na.

In the formula, R represents an alkyl group.

In the formula, AO represents an alkylene oxide having 2 or 3 carbon atoms, and m represents an integer of 10 to 40.

  In the case of a non-reactive emulsifier, a suitable amount of the emulsifier is preferably 0.1 to 3 parts by mass, based on 100 parts by mass of all the monomer components forming the copolymer, and 0.1 to 3 parts by mass. The amount is more preferably 2 parts by mass, further preferably 0.2 to 1 part by mass. In the case of a reactive emulsifier, it is preferably 0.3 to 5% by mass, more preferably 0.5 to 4% by mass, and 0.5 to 2% by mass based on all the monomer components forming the copolymer. More preferably, it is parts by mass. The non-reactive emulsifier and the reactive emulsifier may be used alone, but are preferably used as a mixture.

  As the radical polymerization initiator used in the emulsion polymerization, known and conventional ones can be used, and examples thereof include ammonium persulfate, potassium persulfate, hydrogen peroxide, t-butyl hydroperoxide and the like. Further, if necessary, these polymerization initiators may be used in combination with a reducing agent such as sodium bisulfite, Rongalit (sodium hydroxymethanesulfinate), and ascorbic acid to carry out redox polymerization.

  As the emulsion polymerization method, a polymerization method of charging all at once, a method of continuously supplying each component, or the like is applied. The polymerization is usually carried out at a temperature of 30 to 90 ° C with stirring. The polymerization stability, mechanical stability and chemical stability during emulsion polymerization can be improved by adding a basic substance to adjust the pH during or after the polymerization of the copolymer. As the basic substance used in this case, ammonia, triethylamine, ethanolamine, caustic soda, or the like can be used. These may be used alone or in combination of two or more.

The composition of the present invention is preferably used by dispersing or dissolving the active material and the binder resin in water or a mixture of water and a highly hydrophilic solvent. The composition of the present invention may be prepared, for example, by dispersing, dissolving or kneading a binder resin in a solvent, then adding an active material and additives used as necessary, and further dispersing, dissolving or kneading. To be
The content ratio of the active material and the binder resin in the composition of the present invention is 95.0 to 99.5 mass% of the active material and 0.5 to 5.0 mass% of the binder resin, based on the solid content. Is preferable, the active material is more preferably 98.0 to 99.5 mass%, the binder resin is more preferably 0.5 to 2.0 mass%, the active material is 99.0 to 99.5 mass%, the binder resin Is more preferably 0.5 to 1.0% by mass.

[Lithium-ion secondary battery electrode]
The lithium-ion secondary battery electrode of the present invention (hereinafter, may be referred to as “the electrode of the present invention”) is formed on the current collector from the above-mentioned composition for forming a lithium-ion secondary battery electrode of the present invention. It has an active material-containing layer obtained by
The electrode of the present invention can be used as both a positive electrode and a negative electrode of a lithium ion secondary battery, but when it is used as a negative electrode, it can exert a particular effect.

The collector is not particularly limited as long as it is a metallic collector such as iron, copper, aluminum, nickel, and stainless. Among these, aluminum is preferable as the current collector for the positive electrode, and copper is preferable as the current collector for the negative electrode.
The shape of the current collector is also not particularly limited, but it is usually preferable to use a sheet having a thickness of 0.001 to 0.5 mm.

The electrode of the present invention can be obtained, for example, by applying the above-mentioned composition for forming an electrode of the lithium ion secondary battery of the present invention onto a current collector and drying it.
As a coating method, a general method can be used, for example, a reverse roll method, a direct roll method, a doctor blade method, a knife method, an extrusion method, a curtain method, a gravure method, a bar method, a dip method and a squeeze method. I can give you. Among these, by selecting a coating method in accordance with various physical properties such as viscosity of the composition of the present invention and drying property, from the viewpoint that the surface state of the active material-containing layer can be improved, the doctor blade method, the knife method Or, the extrusion method is preferable.
Further, the electrode of the present invention can be pressed as necessary after the formation of the active material-containing layer. As a pressing method, a general method can be used, but a die pressing method and a calendar pressing method are particularly preferable. The pressing pressure is not particularly limited, but 0.2 to 3 t / cm ² is preferable.

  The electrode of the present invention has a good binding property between the active material and the binder resin and can prevent cohesive failure of the active material-containing layer. Further, the electrode of the present invention can improve the binding property between the active material-containing layer and the current collector. The effect can be made extremely excellent particularly when copper is used as the current collector.

[Lithium-ion secondary battery]
The lithium-ion secondary battery of the present invention (hereinafter sometimes referred to as the “battery of the present invention”) comprises the above-described lithium-ion secondary battery electrode of the present invention.

The battery of the present invention can be manufactured according to a known method using a positive electrode and / or a negative electrode, an electrolytic solution, and if necessary, components such as a separator. As the electrode, both the positive electrode and the negative electrode may use the above-described electrode of the present invention, and the positive electrode or the negative electrode may use the above-described electrode of the present invention, but the negative electrode uses the above-described electrode of the present invention. It can be particularly effective in this case.
As a battery outer package, a metal outer package or an aluminum laminate outer package can be used. The battery may have any shape such as a coin shape, a button shape, a sheet shape, a cylinder shape, a square shape, and a flat shape. Any known lithium salt can be used as the electrolyte in the electrolytic solution of the battery, and may be selected according to the type of the active material. For _{example, LiClO 4, LiBF 6, LiPF} 6, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF 6, LiSbF 6, LiB 10 Cl 10, LiAlCl 4, LiCl, LiBr, LiB (C 2 H 5) 4, CF 3 _{SO 3 Li, CH 3 SO 3} Li, LiCF 3 SO 3, LiC 4 F 9 SO 3, Li (CF 3 SO 2) 2 N, lithium and the like lower fatty acid.

  The solvent for dissolving the electrolyte is not particularly limited as long as it is usually used as a liquid for dissolving the electrolyte, and ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate ( DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), vinylene carbonate (VC) and other carbonates; γ-butyrolactone, γ-valerolactone and other lactones; trimethoxymethane, 1,2-dimethoxyethane, Ethers such as diethyl ether, 2-ethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran; Sulfoxides such as dimethyl sulfoxide; Oxo such as 1,3-dioxolane, 4-methyl-1,3-dioxolane Orchids; nitrogen-containing compounds such as acetonitrile, nitromethane, formamide, dimethylformamide; organic acid esters such as methyl formate, methyl acetate, ethyl acetate, butyl acetate, methyl propionate, ethyl propionate; phosphoric acid triesters and diglymes Triglymes; sulfolanes such as sulfolane and methylsulfolane; oxazolidinones such as 3-methyl-2-oxazolidinone; sultones such as 1,3-propane sultone, 1,4-butane sultone and naphtha sultone; and the like. These may be used alone or in combination of two or more.

[Method for producing composition for forming lithium-ion secondary battery electrode]
The method for producing a lithium ion secondary battery electrode-forming composition of the present invention is a method for producing a lithium ion secondary battery electrode-forming composition including a step of mixing an active material and a binder resin,
When the active material has a transverse relaxation time of the water protons of the dispersion liquid of 10 milliseconds or more as measured by the following condition 1, it acts as a binder resin, and the transverse relaxation time of water protons of the aqueous emulsion as measured by the following condition 2 is used. Is 200 milliseconds or more.
Condition 1: An active material dispersion liquid obtained by mixing active material, carboxymethylcellulose having a weight average molecular weight of 3,000,000 and a substitution degree of 0.9, and water in a mass ratio of 52.8: 0.5: 46.6. Relaxation time of water protons measured by pulsed NMR method.
Condition 2: Transverse relaxation time of water protons measured by pulse NMR method for anionic aqueous emulsion containing 40% by mass of binder resin.

  In the past, since it was difficult to analyze the surface properties of the active material, the binding property between the active material and the binder resin could not be judged unless the active material-containing layer was actually formed. On the other hand, according to the method for producing the composition for forming a lithium ion secondary battery electrode of the present invention, by using the concept of the lateral relaxation time of water protons, the binding property can be achieved without actually forming the active material-containing layer. It is possible to select an excellent active material and a binder resin, and it is possible to enhance the production efficiency of the composition for forming a lithium ion secondary battery electrode.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited thereto. In the examples and comparative examples, “parts” and “%” represent parts by mass and% by mass, respectively, unless otherwise specified.
In addition, the materials used in Examples and Comparative Examples, and the lithium-ion secondary battery electrode-forming composition, the lithium-ion secondary battery electrode and the lithium-ion secondary battery obtained in Examples and Comparative Examples are as follows. Was measured and evaluated. The results are shown in Table 1 or 2.

(Measurement of lateral relaxation time)
A pulse nuclear magnetic resonance apparatus (XIGO, Acronare) is used, the measurement nucleus is a hydrogen nucleus, the measurement temperature is 25 ° C., the frequency is 13 MHz, and the pulse width is 2 μs. The CPMG method (Carr-Purcell Meiboom-Gill method) ), The transverse relaxation time of water protons of the anionic aqueous emulsions A to F of the binder resin and the transverse relaxation time of water protons of the active material dispersions i to iii were measured.
The active material dispersions i to iii were the following active materials i to iii, carboxymethyl cellulose having a weight average molecular weight of 3,000,000 and a substitution degree of 0.9, and water, 52.8: 0.5: 46. A mixture obtained by mixing at a mass ratio of 6 was used.
Active material i; natural graphite (spherical, average particle size 17 μm, average specific surface area 5.9 m ² / g)
Active material ii; artificial graphite (spherical, average particle diameter 18 μm, average specific surface area 1.3 m ² / g)
Active material iii; artificial graphite (scaly, average particle size 22 μm, average specific surface area 1.3 m ² / g)

(Cohesion)
A composition for forming a lithium ion secondary battery electrode (negative electrode) was applied on a copper foil serving as a current collector so that the wet thickness would be 150 μm, and heated and dried at 60 ° C. for 30 minutes. Then, it was vacuum-dried at 120 ° C. for 1 h and left at 23 ° C. and 50% RH for 24 hours to obtain a test piece. The coated surface of the test piece and the SUS plate were bonded together using a double-sided tape, and 180 ° peeling was performed (peeling width 25 mm, peeling speed 100 mm / min), and the peel strength was measured. A material having a small peel strength means that the active material-containing layer is likely to undergo cohesive failure and the binding property between the active material and the binder resin is low.

(Charge / discharge high temperature cycle characteristics)
The battery cycle test was performed at 45 ° C. with CC-CV charging (upper limit voltage 4.2V, current 1C, CV time 1.5 hours, CC discharge (lower limit voltage 3.0V, current 1C)). The maintenance ratio was the ratio of the discharge capacity at the 200th cycle to the discharge capacity at the first cycle.

(Synthesis of Binder Resin Anionic Aqueous Emulsion A)
In a separable flask having a cooling tube, a thermometer, a stirrer, and a dropping funnel, 40 parts of ion-exchanged water and a reactive anionic emulsifier represented by the above general formula (4) (manufactured by Sanyo Chemical Industry Co., Ltd., trade name Eleminol JS- 20 parts, active ingredient 40%) 0.2 parts were charged and the temperature was raised to 75 ° C.
Next, 2.3 parts of a reactive anionic emulsifier (manufactured by Sanyo Kasei Co., Ltd., trade name Eleminol JS-20, active ingredient 40%) represented by the general formula (4), a non-reactive anionic emulsifier ( Manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., trade name Hitenol 08E, polyoxyethylene alkyl ether sulfate ester salt) 0.4 parts, styrene 50.4 parts, 2-ethylhexyl acrylate 44.1 parts, 2-hydroxyethyl methacrylate. A monomer emulsion prepared by previously mixing 2.0 parts, 1.6 parts of acrylic acid, 0.4 part of sodium p-styrenesulfonate, 0.5 part of trimethylolpropane methacrylate and 85 parts of ion-exchanged water is used for 3 hours. Dropped. At the same time, 0.43 parts of potassium persulfate dissolved in 20 parts of ion-exchanged water as a polymerization initiator was added dropwise at 80 ° C. for 3 hours for polymerization. After completion of dropping, the mixture was aged for 2 hours, cooled, and 1.8 parts of aqueous ammonia was added to obtain an anionic aqueous emulsion A. The ratio of the binder resin in the obtained anionic aqueous emulsion A was 40%, the viscosity was 60 mPa · s, the average particle size of the resin particles in the emulsion was 130 nm, and the pH was 7.0.
The viscosity was measured using a Brookfield rotary viscometer at a liquid temperature of 23 ° C., a rotation speed of 60 rpm, and a No. 2 or No. It measured with 3 rotors.

(Synthesis of Binder Resin Anionic Aqueous Emulsions B to F)
Anionic aqueous emulsions B to F were obtained in the same manner as above, except that the composition of the monomer was changed to the composition shown in Table 1.

(Anionic aqueous emulsion G of binder resin)
As the anionic water-based emulsion G of the binder resin, an anionic water-based emulsion of styrene-butadiene rubber (glass transition temperature −7 ° C. (measured value by DSC)) (binder resin ratio 40%, viscosity 11 mPa · s, in emulsion) The average particle diameter of the resin particles of (190 nm, pH 7.0) was adjusted.

(Example 1)
1. Preparation of Positive Electrode Forming Composition and Positive Electrode A mixture of 90 parts of LiNi _1/3 Mn _1/3 Co _1/3 O ₂ , 5 parts of acetylene black as a conduction aid, and 5 parts of polyvinylidene fluoride as a binder resin was mixed. , N-methylpyrrolidone were added and further mixed to prepare a positive electrode-forming composition.
Then, by a doctor blade method, the composition was applied on one surface of an aluminum foil having a thickness of 20 μm to be a current collector so that the thickness after the roll press treatment was 60 μm, followed by drying at 120 ° C. for 5 minutes and a pressing step. After that, a positive electrode active material-containing layer was formed.

2. Preparation of Negative Electrode Forming Composition and Negative Electrode 100 parts of the active material ii, 3.75 parts of the emulsion A, and 50 parts of 2% CMC (weight average molecular weight 3,000,000, substitution degree 0.9) aqueous solution were mixed. Then, 28 parts of water was further added to obtain a composition for forming a lithium ion secondary battery electrode (negative electrode) of Example 1.
Then, the composition was applied onto one surface of a copper foil having a thickness of 10 μm to be a current collector so that the thickness after the roll press treatment would be 60 μm, dried at 80 ° C. for 5 minutes, and subjected to a pressing step to produce a negative electrode active material. A substance-containing layer was formed.

3. Preparation of Battery A positive electrode active material-containing layer and a negative electrode active material-containing layer were provided with conductive tabs, and a separator made of a polyolefin-based porous film was interposed between the positive electrode and the negative electrode to activate the positive electrode and the negative electrode. The materials were housed in an aluminum laminate outer casing (battery pack) so that they face each other. An electrolyte of 1.0 mol / L (liter) ethylene carbonate (EC) / diethyl carbonate (EMC) = 40/60 (volume ratio) of LiPF ₆ was injected into this outer package, and vacuum impregnation was performed. The lithium ion secondary battery of Example 1 was obtained by heat fusion.

(Examples 2 to 10) (Comparative examples 1 to 9)
A composition for forming a lithium ion secondary battery electrode, a lithium ion secondary battery electrode, and a lithium ion secondary battery were prepared in the same manner as in Example 1 except that the active material and emulsion used for preparing the negative electrode were changed to the emulsions shown in Table 2. I got a battery.

As is clear from Table 1, the compositions for forming the lithium ion secondary battery electrode, the lithium ion secondary battery electrode and the lithium ion secondary battery of Examples 1 to 10 have high peel strength and the active material and the binder resin. It can be confirmed that the binding property of is good. Furthermore, in Examples 1 to 4 and Examples 6 to 9, since the binder resin was a copolymer of styrene and an ethylenically unsaturated carboxylic acid ester, the charge and discharge high temperature recycling characteristics were excellent. .
On the other hand, in Comparative Examples 1 to 9, since either the lateral relaxation time of the active material or the lateral relaxation time of the binder resin does not satisfy the conditions of the present invention, the binding property between the active material and the binder resin is good. However, the coating film was cohesively destroyed, resulting in poor peel strength.

Claims (4)

An artificial graphite active material and a binder resin are contained, and the active material has a transverse relaxation time of water protons of the dispersion liquid of 10 milliseconds or more and 50 milliseconds or less as measured by the following condition 1.
The binder resin has an acid value of 20 mgKOH / g or less, is a copolymer of styrene and an ethylenically unsaturated carboxylic acid ester, and the monomer component forming the copolymer is 30 to 60% by mass of styrene. no group ethylenically unsaturated carboxylic acid ester 30 to 65% by weight, ethylenically having a functional group unsaturated carboxylic acid ester 0.5 to 5% by weight, ethylenically unsaturated carboxylic acid 0-2% by weight, and At least 0.1 % by mass of an internal crosslinking agent is contained, and among the monomer components, the total amount of the styrene and the ethylenically unsaturated carboxylic acid ester having no functional group is 90% by mass or more, and A lithium ion ion whose transverse relaxation time of water protons of the aqueous emulsion measured under the following condition 2 is 200 milliseconds or more and 750 milliseconds or less. Composition for forming secondary battery electrode.
Condition 1: An active material dispersion liquid obtained by mixing active material, carboxymethylcellulose having a weight average molecular weight of 3,000,000 and a substitution degree of 0.9, and water in a mass ratio of 52.8: 0.5: 46.6. Relaxation time of water protons measured by pulsed NMR method.
Condition 2: Transverse relaxation time of water protons measured by pulse NMR method for anionic aqueous emulsion containing 40% by mass of binder resin.   A lithium ion secondary battery electrode having an active material-containing layer formed of the composition for forming a lithium ion secondary battery electrode according to claim 1 on a current collector.   A lithium ion secondary battery comprising the lithium ion secondary battery electrode according to claim 2. A method for producing a composition for forming a lithium ion secondary battery electrode, which comprises a step of mixing an active material that is artificial graphite and a binder resin,
When the active material has a transverse relaxation time of water protons of the dispersion liquid of 10 milliseconds or more and 50 milliseconds or less as measured by the following condition 1, the binder resin has an acid value of 20 mgKOH / g or less and styrene. And an ethylenically unsaturated carboxylic acid ester, wherein the monomer component forming the copolymer is 30 to 60% by mass of styrene, and the ethylenically unsaturated carboxylic acid ester having no functional group is 30 to 65. %, At least 0.5 to 5% by mass of an ethylenically unsaturated carboxylic acid ester having a functional group, 0 to 2 % by mass of an ethylenically unsaturated carboxylic acid , and 0.1 to 3 % by mass of an internal crosslinking agent, Of the monomer components, the total amount of the styrene and the ethylenically unsaturated carboxylic acid ester having no functional group is 90% by mass or more, and the following conditions: A method for producing a lithium ion secondary battery electrode-forming composition, wherein the transverse relaxation time of water protons of the water-based emulsion measured by 2 is 200 milliseconds or more and 750 milliseconds or less .
Condition 1: An active material dispersion liquid obtained by mixing active material, carboxymethylcellulose having a weight average molecular weight of 3,000,000 and a substitution degree of 0.9, and water in a mass ratio of 52.8: 0.5: 46.6. Relaxation time of water protons measured by pulsed NMR method.
Condition 2: Transverse relaxation time of water protons measured by pulse NMR method for anionic aqueous emulsion containing 40% by mass of binder resin.