JP5697906B2 - Electrode aqueous composition for secondary battery - Google Patents

Description

The present invention relates to an electrode aqueous composition for a secondary battery. More specifically, the present invention relates to a secondary battery electrode aqueous composition that can be suitably used as a composition for forming an electrode of a secondary battery such as a lithium ion battery.

Secondary batteries are rechargeable batteries that can be repeatedly charged and discharged. In recent years, due to the growing interest in environmental problems, not only electronic devices such as mobile phones and laptop computers, but also automobiles and airplanes. Is also in use. In response to the growing demand for secondary batteries, research is actively conducted. In particular, lithium-ion batteries that are light, small, and high in energy density are attracting attention from various industries. Development is actively underway.
A lithium ion battery is mainly composed of a positive electrode, an electrolyte, a negative electrode, and a separator, and an electrode in which an electrode composition is applied on a current collector is used. Among the electrode compositions, the positive electrode composition used for forming the positive electrode mainly comprises a positive electrode active material, a conductive additive, a binder, and a solvent. The binder is polyvinylidene fluoride (PVDF), and the solvent is N-methyl. -2-pyrrolidone (NMP) is commonly used. This is because PVDF is chemically and electrically stable, NMP is a solvent that is stable over time in which PVDF is dissolved, and lithium cobaltate that is commonly used as a positive electrode active material is hydrolyzed in water. This is because it is necessary to use an organic solvent. PVDF and NMP are also used in negative electrode compositions because of their chemical and electrical stability. However, PVDF does not have a high dissolved concentration, and is generally used at a concentration of 10% or less. When PVDF is used, it is difficult to increase the solid content concentration. Moreover, since NMP has a high boiling point, when NMP is used as a solvent, there is a problem that a large amount of energy is required for volatilization of the solvent when an electrode is formed. In addition, in recent years, an aqueous composition that does not use an organic solvent has been demanded for the electrode composition against the background of increasing interest in environmental problems. Under such circumstances, various researches and developments have been made on battery compositions and binders that can be used in battery compositions.

As a composition for forming an electrode of a non-aqueous secondary battery, a slurry composition for a non-aqueous electrolyte secondary battery positive electrode in which a water-insoluble polymer and an iron-containing compound are dispersed in water, and collecting this composition It is disclosed that an electrode for a nonaqueous electrolyte secondary battery positive electrode is produced by applying to a body and drying (for example, see Patent Document 1). Further, as a composition for forming an electrode for a non-aqueous electrolyte secondary battery, it is a coating composition for an active material layer containing at least an active material and a binder, and the binder has a solid content. An active material layer coating composition containing 2.5% by weight or less as a reference, and containing 20% by weight or less of an acrylic copolymer in the binder based on the total amount of the binder, and An electrode plate for a non-aqueous electrolyte secondary battery in which a layer of this composition is provided on the surface of a current collector is disclosed, and an acrylic copolymer and other polyvinylidene fluoride (PVDF) as a binder are disclosed. It is disclosed to use a mixture of resin (see, for example, Patent Document 2).

Japanese Patent Laying-Open No. 2005-63825 (pages 1-2, 4, 8-9) Japanese Patent Laying-Open No. 2006-107767 (pages 1-2 and 6)

Patent Document 1 describes non-fluorine polymers such as acrylic polymers, nitrile polymers, and diene polymers, and fluorine polymers such as PVDF and polytetrafluoroethylene (PTFE) as water-insoluble polymers. Among these, PVDF and PTFE have a low ability to bind a filler such as an electrode active material, and the film forming property of the resin itself is also low. Therefore, when there are many fillers, it is often impossible to form a film even after coating and drying. In Examples of Patent Document 1, an acrylic elastomer aqueous dispersion, a styrene / butadiene (SBR) copolymer aqueous dispersion, and an acrylonitrile / butadiene copolymer aqueous dispersion are used as the water-insoluble polymer. However, all of these have room for improvement in the chemical and electrical stability of the emulsion. Furthermore, in Patent Document 2, the coating film obtained when PVDF is used as a binder is hard, and the active material layer is dropped when the electrode plate is cut because the adhesion to the current collector is low. In order to solve the problem, it is disclosed that a mixture of an acrylic copolymer and another resin is used as a binder, and that a combination of an acrylic copolymer and PVDF is preferable. . However, since the polarity of the acrylic copolymer and PVDF is greatly different, it is generally incompatible only by mixing them, and the component that becomes a compatibilizer or the PVDF is added to the acrylic copolymer. The compatibility cannot be expressed unless a monomer having compatibility with is incorporated or melted and mixed in advance at a high temperature. In Patent Document 2, it is considered that the resin is present non-uniformly due to the incompatibility of these polymers. When an electrode film is formed using this resin, a problem remains in the uniformity of the film. In Patent Document 2, the resin is dissolved in an organic solvent such as N-methyl-pyrrolidone. However, in the case of an aqueous dispersion using an aqueous solvent, since the polymer exists in the form of particles, the resin is further compatible. There is a problem that it becomes difficult to form a uniform film.
The present invention has been made in view of the above-described situation, and is a chemically and electrically stable aqueous composition that has a good binding property to a filler and can form a uniform film while being aqueous. A secondary battery electrode aqueous composition that can be suitably used as a composition for forming an electrode for a secondary battery because the obtained film is excellent in adhesion to a substrate and flexibility. It is intended.

The present inventor has a high binding property between fillers, can form a coating film even if the filler content is increased, and the resulting coating film is excellent in adhesion to a substrate and flexibility. Various water-based compositions suitable as electrode materials were studied, and attention was focused on the binder contained in the composition. And, as a binder, using an aqueous dispersion of a modified polymer having a (meth) acryl-modified structure of a fluorine-containing polymer, while having chemically and electrically stable properties possessed by the fluorine-containing polymer, It has been found that low binding properties, low adhesion of the resulting coating film, and hard brittleness of the coating film, which are disadvantages of the fluorine-containing polymer, can be improved. That is, such a binder is chemically and electrically stable, and an excellent filler binding capable of forming a coating film even if the amount of filler such as an active material or a conductive aid is increased. It has been found that the obtained coating film is excellent in adhesion and flexibility to the base material as well as having a binder. Furthermore, the present inventor can form a uniform film even as an aqueous dispersion using an aqueous solvent, and this binder, when the fluorine-containing polymer is a polymer having a (meth) acryl-modified structure. It has also been found that the film-forming temperature of the coating film can be lowered and the ease of forming the coating film is improved by using this, and further, this binder is not only a positive electrode but also a negative electrode binder, unlike SBR. Also found that it can be used. Accordingly, the inventors have found that a composition using this binder is suitable as a composition for forming an electrode for a secondary battery, and have conceived that the above problems can be solved brilliantly, and have reached the present invention. Is.

That is, the present invention is an electrode aqueous composition that includes an electrode active material, a binder, and water as essential components, and forms an electrode of a secondary battery, wherein the binder contains a fluorine-containing heavy polymer having a (meth) acryl-modified structure. A secondary battery electrode aqueous composition comprising a combined aqueous dispersion.
The present invention is described in detail below.

The electrode aqueous composition for a secondary battery of the present invention contains an electrode active material, a binder and water as essential components, but each of these essential components may be included, or two or more thereof may be included. Also good.
Moreover, the binder in this invention may contain 1 type of the aqueous dispersion of the fluorine-containing polymer which has a (meth) acryl modified | denatured structure, and may contain 2 or more types.
Hereinafter, the fluorine-containing polymer before the (meth) acryl modification is referred to as a fluorine-containing polymer. Further, a fluorine-containing polymer having a (meth) acryl-modified structure is also referred to as a (meth) acryl-modified fluorine-containing polymer.

The binder contained in the aqueous electrode composition for secondary batteries of the present invention contains an aqueous dispersion of a fluorine-containing polymer having a (meth) acryl-modified structure. An aqueous dispersion of a fluorine-containing polymer having a (meth) acryl-modified structure is one in which a fluorine-containing polymer and a (meth) acrylic polymer are entangled with each other in dispersed particles. For example, particles having a structure such as an “IPN structure” in which acrylic enters a skeleton centered on fluorine. Therefore, a fluorine-containing polymer and an acrylic copolymer produced separately and then mixed are not applicable to the fluorine-containing polymer having a (meth) acryl-modified structure in the present invention.

The fluorine-containing polymer having the (meth) acryl-modified structure preferably has a mass ratio of 40/60 to 95/5 between the fluorine-containing polymer-derived structure portion and the (meth) acryl-modified structure portion. . With such a ratio, the chemical and electrical stability of the fluorine-containing polymer and the excellent binding properties and flexibility obtained by (meth) acrylic modification, the effect of lowering the film forming temperature Will be displayed in a well-balanced manner. More preferably, the mass ratio of the structural portion of the fluorine-containing polymer to the structural portion modified with (meth) acryl is 50/50 to 95/5, and more preferably 60/40 to 90/10. is there.

The fluorine-containing polymer is preferably a vinylidene fluoride polymer. The vinylidene fluoride-based polymer is a polymer having crystallinity, but crystallinity can be lowered by modifying this with (meth) acryl, and the binding property and flexibility of the resin are improved. A great effect can be obtained in terms of lowering the film forming temperature. Therefore, by using a (meth) acrylic modified vinylidene fluoride polymer, the chemical and electrical stability of the fluorine-containing polymer as a binder for secondary batteries and (meth) acrylic modification Thus, the excellent binding property and flexibility obtained, and the effect of lowering the film forming temperature can be more fully exhibited. The vinylidene fluoride polymer may be produced using only vinylidene fluoride as a raw material, or may be obtained by copolymerization of vinylidene fluoride and other monomers, It is preferably obtained by copolymerization of vinylidene fluoride and another monomer. By copolymerizing with other monomers, the crystallinity of the vinylidene fluoride polymer can be lowered and acrylic modification can be facilitated.

Other monomers to be copolymerized with the vinylidene fluoride (VDF) are not particularly limited. For example, perfluorovinyl ethers such as tetrafluoroethylene (TFE), hexafluoropropylene (HFP), and perfluoropropyl vinyl ether. And chlorotrifluoroethylene (CTFE) and the like, and one or more of these can be used. Among these, hexafluoropropylene (HFP) and perfluoroalkyl vinyl ethers are preferable.

When the above-mentioned vinylidene fluoride polymer is a copolymer of vinylidene fluoride and another monomer, the structure derived from vinylidene fluoride and other components are used in that the crystallinity of the vinylidene fluoride polymer is lowered. The ratio of the monomer-derived structure is preferably 60/40 to 97/3 by mass ratio.

The (meth) acryl-modified structural part is formed of one or more (meth) acrylic compounds described later, or a polymer obtained by polymerizing them. Preferably, the (meth) acryl-modified structural part is formed by a polymer of a (meth) acrylic compound. Moreover, among these, it is preferable that it is what is formed with the polymer of 1 type, 2 or more types of an acrylic compound, and those acrylic compounds. By these, the polymer of the (meth) acrylic-type compound obtained exhibits the effect of lowering the excellent binding property, flexibility, and film-forming temperature due to entanglement with the base fluorine-containing polymer. Can do.

The binder contained in the aqueous electrode composition for secondary batteries of the present invention may contain a component that acts as another binder as long as it contains an aqueous dispersion of a fluorine-containing polymer having a (meth) acryl-modified structure. Other components that act as binders include water-soluble resins such as poly (meth) acrylates and unsaturated polyalkylene glycol ether polymers, fluorine-containing polymers not modified with (meth) acryl, SBR latex, carboxy Celluloses such as methylcellulose can be used.

The binder of the present invention preferably has a minimum film forming temperature of 40 ° C. or lower. By reducing the film forming temperature, the formation of the coating film is facilitated, and the productivity of the coating film can be improved.
The minimum film forming temperature of the binder is more preferably 35 ° C. or lower.
As the binder of the present invention, it is preferable to use a binder having such a preferable minimum film-forming temperature as an aqueous dispersion of a fluorine-containing polymer having a (meth) acryl-modified structure.
The minimum film forming temperature of the binder can be measured by a thermal gradient tester.
Moreover, it is preferable that the film | membrane at the time of film forming is transparent. That the film is transparent means that the resin is compatible due to the entanglement between the fluorine-containing polymer and the acrylic polymer, and the film is uniform. The film being transparent means that, for example, when a film having a dry film thickness of 40 μm is formed at 40 ° C., turbidity cannot be confirmed when the film is visually observed.

The binder of the present invention is preferably such that the crystallization temperature is not observed after drying at a temperature equal to or higher than the crystallization temperature of the fluorine-containing polymer. When the fluorine-containing polymer having a (meth) acryl-modified structure can be sufficiently homogenized, the fluorine-containing polymer and (meth) acryl are dried by drying at a temperature higher than the crystallization temperature of the fluorine-containing polymer. The crystallization temperature is no longer observed when the modified structural part is sufficiently mixed and homogenized to be amorphous. When the binder includes an aqueous dispersion of a fluorine-containing polymer having such a (meth) acryl-modified structure, the above-described effects of the present invention are more remarkably exhibited.

It is preferable that the ratio of solid content of the binder of this invention is 20-70 mass% with respect to 100 mass% of the whole binder. When the amount is less than 20% by mass, a large amount of water is contained, so that it becomes easy to limit the range of the solid content of the paint when blended with the paint. When it exceeds 70 mass%, the handling property of resin will fall. The solid content of the binder is a component remaining after volatilizing water as a solvent from the binder, that is, a component other than the solvent containing water as a main component in the binder.

The fluorine-containing polymer having the (meth) acryl-modified structure can be produced by producing a fluorine-containing polymer and then reacting the (meth) acrylic compound in the presence of the fluorine-containing polymer.
The aqueous dispersion of the fluorine-containing polymer can be synthesized by a usual means for synthesizing the polymer using suspension polymerization or emulsion polymerization. For production of an aqueous dispersion of a fluorine-containing polymer having a (meth) acryl-modified structure, an aqueous dispersion of a fluorine-containing polymer was produced, and then a (meth) acrylic compound was added and dispersed in water. A method of incorporating a (meth) acrylic compound into the fluorine-containing polymer particles and polymerizing the (meth) acrylic compound can be used.
In addition, a fluorine-containing polymer having a (meth) acryl-modified structure is prepared by mixing an acrylic polymer with a fluorine-containing polymer at a high temperature, homogenizing and compatibilizing the polymer, and then adding a dispersant or the like to the obtained polymer. It can also be obtained by a method of using and dispersing in water.

Examples of the (meth) acrylic compound to be reacted with the fluorine-containing polymer include (meth) acrylic acid and (meth) acrylic acid salts; methyl (meth) acrylate, ethyl (meth) acrylate, and (meth) acrylic acid. (Meth) acrylic acid alkyl esters such as propyl and butyl (meth) acrylate; hydroxyalkyl (meth) acrylates such as hydroxyethyl (meth) acrylate and hydroxybutyl (meth) acrylate; (meth) acrylonitrile and glycidyl (meth) acrylate , Alkylene glycol-containing (meth) acrylates, amino acrylates such as diethylaminoethyl acrylate, and the like, and one or more of these can be used. Among these, (meth) acrylic acid alkyl ester is preferable.

The aqueous electrode composition for secondary batteries of the present invention includes a binder containing an aqueous dispersion of a fluorine-containing polymer having a (meth) acryl-modified structure, so that the chemical and electrical properties of the fluorine-containing polymer are included. Both stability and excellent binding properties and flexibility obtained by (meth) acryl modification can be exhibited, and the effect of lowering the film forming temperature can be exhibited. By using it, both the positive electrode and the negative electrode can be formed.
A binder for use in the aqueous electrode composition for a secondary battery according to the present invention, wherein the binder includes an aqueous dispersion of a fluorine-containing polymer having a (meth) acryl-modified structure. The binder for aqueous compositions is also one aspect of the present invention.

In the aqueous electrode composition for secondary batteries of the present invention, the binder content is preferably 1 to 10% by mass with respect to 100% by mass of the electrode active material. When the content of the binder is within this range, it is possible to bind the electrode active material and a filler such as a conductive additive described later, and to suppress the decrease in the active material content in the electrode.

The electrode active material used in the aqueous electrode composition for secondary batteries of the present invention is preferably an electrode active material capable of occluding and releasing lithium ions. By using such an electrode active material, it can be suitably used as an electrode of a lithium ion battery.
Examples of the compound capable of inserting and extracting lithium ions include lithium-containing metal oxides. Examples of such metal oxides include lithium cobaltate, lithium iron phosphate, lithium manganese phosphate, and lithium manganate. It is done.

When the aqueous electrode composition for secondary batteries of the present invention is used as a material for forming a positive electrode, the electrode active material preferably contains a compound having an olivine structure.
That is, it is one of the preferred embodiments of the present invention that the electrode active material is a positive electrode active material containing a compound having an olivine structure. Below, the electrode aqueous composition for secondary batteries of this invention used as a material which forms a positive electrode is also called positive electrode aqueous composition of this invention.
The compound having an olivine structure is represented by the following formula:
Li _x A _y D _z PO ₄
(However, A is one or more selected from the group consisting of Cr, Mn, Fe, Co, Ni and Cu, and D is Mg, Ca, Sr, Ba, Ti, Zn, B, It is one or more selected from the group consisting of Al, Ga, In, Si, Ge, Sc, Y and rare earth elements, where x, y and z are 0 <x <2, 0 <y <1.5. And a number satisfying 0 ≦ z <1.5.). This compound forms a (PO ₄ ) ³ -polyanion by bonding an oxygen atom in the structure to phosphorus, and in principle, no combustion reaction occurs because oxygen is immobilized in the crystal structure. For this reason, since the electrode active material containing this compound will be excellent in safety | security, it will be able to be used suitably especially for the use to a medium sized power supply. The A component is preferably Fe, Mn, or Ni, and particularly preferably Fe. The D component is preferably Mg, Ca, Ti, or Al.

Examples of the compound having the olivine structure include lithium iron phosphate and lithium manganese phosphate among the compounds capable of inserting and extracting lithium ions. Further, as the positive electrode active material, it is preferable to use a material whose surface is partially or entirely covered with carbon in order to supplement conductivity. The coating amount of carbon is preferably 20 parts by weight or less, and more preferably 10 parts by weight or less with respect to 100 parts by weight of the positive electrode active material.
In the positive electrode aqueous composition of the present invention, the compound having an olivine structure is preferably 70% by mass or more with respect to 100% by mass of the entire positive electrode active material. More preferably, it is 90% by mass or more, and most preferably, the positive electrode active material is composed of only a compound having an olivine structure.

In the positive electrode aqueous composition of the present invention, the compound having the olivine structure preferably has an average particle size of 1 μm or less. The smaller the average particle size, the larger the surface area, and it is necessary to bind with a small amount of the binder, so that the flexibility of the binder is required. The aqueous electrode composition for secondary batteries of the present invention is preferably applied to particles having a small particle diameter because the binding property and flexibility are improved by the acrylic modification of the fluorine-containing polymer used as a binder. It becomes a form. By using a positive electrode active material containing a compound having an olivine structure with an average particle size of 1 μm or less, electrical characteristics such as output characteristics when a positive electrode composition for a secondary battery is used as a positive electrode of a battery are excellent. It becomes possible. The average particle size of the compound having an olivine structure is more preferably 0.01 to 0.8 μm.
The average particle diameter of the positive electrode active material can be measured by a dynamic light scattering particle size distribution meter (1.7 in the case of LiFePO ₄ ).

The positive electrode active material is preferably a positive electrode active material containing lithium iron phosphate as a main component. The compound contained in the positive electrode active material is more preferably lithium iron phosphate among the compounds having the olivine structure, and is preferably the main component of the positive electrode active material. Lithium iron phosphate has high stability against overcharge and uses abundant resources such as iron and phosphorus, so it is inexpensive and preferable in terms of manufacturing cost.
The phrase “containing lithium iron phosphate as a main component” means that the content of lithium iron phosphate is 100% by mass or more with respect to 100% by mass of the whole positive electrode active material, preferably 80% by mass or more, More preferably, it is 90 mass% or more. Most preferably, the positive electrode active material consists only of lithium iron phosphate.

In the positive electrode aqueous composition of the present invention, the content of the conductive assistant is preferably 2 to 25% by mass with respect to 100% by mass of the positive electrode active material. When there is less content of a conductive support agent than 2 mass%, there exists a possibility that electrode resistance cannot be reduced and the effect of mixing a conductive support agent may not be obtained. On the other hand, when the amount is more than 25% by mass, the amount of the active material contained in the positive electrode is reduced, so that it is difficult to ensure the capacity. More preferably, it is 5-15 mass% with respect to 100 mass% of positive electrode active materials.

The conductive auxiliary agent preferably has an average particle size of 1 μm or less. By using a positive electrode active material having an average particle diameter of 1 μm or less, it is possible to improve electrical characteristics such as output characteristics when the positive electrode formed from the positive electrode aqueous composition of the present invention is used as a positive electrode of a battery. It becomes possible. The average particle diameter is more preferably 0.01 to 0.8 μm, and still more preferably 0.03 to 0.5 μm.
The average particle diameter of the conductive assistant can be measured by a dynamic light scattering particle size distribution meter (conducting assistant refractive index is set to 2.0).

The conductive assistant is used to increase the output of the lithium ion battery, and conductive carbon is mainly used.
Examples of the conductive carbon include carbon black, fiber carbon, and graphite. Of these, ketjen black and acetylene black are preferred.
Ketjen Black has a hollow shell structure and easily forms a conductive network. Therefore, it is preferable because the equivalent performance is exhibited with an addition amount about half that of conventional carbon black. Further, acetylene black is preferred because carbon black produced by using a high purity acetylene gas has very few impurities and surface crystallites are developed.

The positive electrode aqueous composition of the present invention preferably further uses a dispersant. When a dispersant is used, the dispersant is not particularly limited, and various dispersants such as an anionic, nonionic or cationic surfactant, or a polymer dispersant can be used. A more stable conductivity of the positive electrode film can be achieved by promoting the formation of fine particles of the positive electrode active material and the conductive auxiliary agent and improving the dispersibility by the dispersant.

It is preferable that the positive electrode aqueous composition of this invention contains a dispersing agent 5-20 mass% with respect to 100 mass% of conductive support agents. When the content of the dispersant is in such a range, the positive electrode active material and the conductive additive can be sufficiently uniformly dispersed. More preferably, it is 6-15 mass% with respect to 100 mass% of conductive support agents.

The positive electrode aqueous composition of the present invention is an aqueous positive electrode composition using water as a solvent, and the content of water in the composition is 50 to 300% by mass with respect to 100% by mass of the positive electrode active material. It is preferable. When the water content is within this range, a slurry having an appropriate viscosity can be produced. More preferably, it is 70-200 mass%.

The positive electrode aqueous composition of the present invention may contain components other than those described above. When components other than those described above are included, the content is preferably such that the proportion of the solid content of the positive electrode aqueous composition falls within the following preferred range.

The positive electrode aqueous composition of the present invention preferably has a solid content of 35 to 70% by mass with respect to 100% by mass of the entire positive electrode aqueous composition. More preferably, it is 40-60 mass%. The solid content of the positive electrode aqueous composition is a component remaining after the solvent is volatilized from the positive electrode aqueous composition, that is, a component other than the solvent in the positive electrode aqueous composition.

When the positive electrode aqueous composition of the present invention contains a conductive additive, the content of the positive electrode active material, conductive additive, binder, and other components other than these three components in the solid content of the positive electrode aqueous composition is positive electrode Active material / conductive aid / binder / others = 70-95 / 2-20 / 2-10 / 0-5 is preferable. With such a content ratio, it is possible to improve electrical characteristics such as output characteristics when an electrode formed from a positive electrode aqueous composition is used as a positive electrode of a battery. More preferably, they are 80-93 / 3-3-15 / 4-10 / 0-3. In addition, the other component here means the component contained in solid content other than a positive electrode active material, a conductive support agent, and a binder, and a dispersing agent is contained.

When using the aqueous electrode composition for secondary batteries of the present invention as a material for forming a negative electrode, those generally used as a negative electrode active material can be used. Examples of the negative electrode active material include carbon-based materials such as graphite, natural graphite, and artificial graphite, polyacene-based conductive polymers, composite metal oxides such as lithium titanate, and lithium alloys.
Furthermore, it is one of the preferred embodiments of the present invention that the electrode active material is a negative electrode active material made of a carbon material.

The negative electrode composition for forming a negative electrode for a secondary battery can contain a conductive additive, a dispersant, a thickener and the like as required in addition to the negative electrode active material, the binder, and water. As a conductive support agent, a dispersing agent, and a thickener, the thing similar to the thing in the positive electrode composition mentioned above can be used.

When the electrode aqueous composition for a non-aqueous secondary battery of the present invention is used as a material for forming a negative electrode, the content ratio of the negative electrode active material, conductive additive, binder and other components in the solid content of the composition is 85 to 99. It is preferably / 0 to 10/1 to 10/0 to 5. In addition, other components here mean components other than a negative electrode active material, a conductive support agent, and a binder, and a dispersing agent, a thickener, etc. are contained.

The aqueous electrode composition for secondary batteries of the present invention preferably has a viscosity of 1 to 20 Pa · s. When the viscosity of the aqueous electrode composition for secondary batteries is in such a range, it is possible to ensure appropriate fluidity during coating. More preferably, it is 3-15 Pa * s, More preferably, it is 4-10 Pa * s.
Moreover, it is preferable that the electrode aqueous composition for secondary batteries of this invention has a thixo value of 2.5-8. When it is 2.5 or less, the coating liquid flows and becomes easy to repel, and when it is 8 or more, the coating liquid does not have fluidity and is difficult to apply. More preferably, it is 3.5-7.
The viscosity of the aqueous battery electrode composition can be measured with a B-type viscometer.
The thixo value can be obtained as a value obtained by measuring the viscosity of 6 rpm and 60 rpm at 25 ± 1 ° C. with a B-type viscometer and dividing the viscosity of 6 rpm by the viscosity of 60 rpm.

It is preferable that pH of the electrode aqueous composition for secondary batteries of this invention is 6-9. When the pH is in such a range, the aluminum current collector is hardly corroded, and the battery performance of the material can be sufficiently exhibited.
The pH can be measured at 25 ° C. using a glass electrode type hydrogen ion thermometer F-21 (Horiba, Ltd.).

When the positive electrode aqueous composition for a secondary battery of the present invention uses LiFePO ₄ as the positive electrode active material, the refractive index of the filler component is 1.7 using a particle size measuring device for dynamic light scattering. It is preferable that an average particle diameter is 0.1-1 micrometer. When the average particle diameter when the electrode aqueous composition for secondary batteries is in a slurry state is in such a range, it can be confirmed that the filler component is sufficiently refined and mixed.

When the electrode aqueous composition for a secondary battery of the present invention includes a positive electrode active material and a conductive additive, the positive electrode active material and the conductive additive are uniformly dispersed as a method for producing the positive electrode aqueous composition. There is no particular limitation as long as it is, but a dispersant or a water-soluble resin is mixed with water as a solvent, and then a conductive additive is mixed and dispersed using beads, a ball mill, a stirring mixer, etc. Then, a positive electrode active material dissolved or dispersed in water is added, and similarly, a dispersion treatment is performed, and a fluorine-containing polymer emulsion having a (meth) acryl-modified structure is mixed to obtain a positive electrode composition for a secondary battery. It is preferable. When the composition is prepared by such a procedure, it is preferable that the positive electrode active material and the conductive additive are sufficiently uniformly dispersed.
The negative electrode aqueous composition is also preferably prepared by the same method except that the negative electrode active material is used instead of the positive electrode active material.

As described above, the electrode aqueous composition for a secondary battery of the present invention uses a binder containing an aqueous dispersion of a fluorine-containing polymer having a (meth) acryl-modified structure. It is excellent in the binding property of fillers such as active materials and conductive assistants, and the formed coating film is excellent in adhesion and flexibility. And the electrode formed from such an electrode aqueous composition can exhibit sufficient performance as an electrode for a secondary battery. Thus, the fluorine-containing polymer having a (meth) acryl-modified structure is useful as a material used for forming an electrode.
An electrode for a secondary battery formed using such an aqueous electrode composition for a secondary battery of the present invention, or an electrode for a secondary battery containing a fluorine-containing polymer having a (meth) acryl-modified structure, It is one of the present inventions.
Further, a secondary battery configured using such a secondary battery electrode is also one aspect of the present invention.

The secondary battery of the present invention preferably has an initial discharge capacity of 120 mAh / g or more when the positive electrode active material is LiFePO ₄ . More preferably, it is 130 mAh / g or more. When the negative electrode active material is graphite, the initial discharge capacity is preferably 250 mAh / g or more. More preferably, it is 300 mAh / g or more.
In addition, the secondary battery of the present invention preferably has an electric capacity maintenance rate of 85% or more after 100 cycles of 100 cycles of charge and discharge. More preferably, it is 90% or more. By confirming the maintenance rate of 100 cycles, it can be confirmed that the dispersant does not adversely affect the initial battery physical properties.
The electric capacity of the secondary battery can be measured by a charge / discharge evaluation apparatus.

The aqueous electrode composition for a secondary battery of the present invention has the above-described configuration, has a good binding property to a filler, and can form a uniform film while being aqueous, and is chemically and electrically stable. An aqueous composition containing a binder, and the resulting film has excellent adhesion to the substrate and flexibility, and can be used as either a positive electrode or negative electrode material. It is a composition which can be used suitably as a composition which forms.

The present invention will be described in more detail with reference to the following examples. However, the present invention is not limited to these examples. Note that “%” means “mass%” unless otherwise specified.

(1) Resin for Binder The minimum film-forming temperature and crystallization temperature of vinylidene fluoride (VDF) -acrylic modified emulsion used as the binder used in Examples and Comparative Examples are as follows. The minimum film forming temperature and the crystallization temperature were measured and evaluated as follows.
VDF type-acrylic modified emulsion content ratio VDF type polymer / acrylic = 70: 30
Minimum filming temperature 28 ℃
Crystallization temperature 1st time: 98 ° C. 2nd time: none VDF-based acrylic-modified emulsion was a MFT or higher transparent film without cracks.
1. Minimum film forming temperature (MFT)
Using a thermal gradient tester, the aqueous dispersion was coated on a glass plate with a 0.2 mm applicator and then dried, and the temperature at which cracks occurred was measured as the minimum film-forming temperature.
2. For the crystallization temperature measurement sample, water was dried at room temperature, and the polymer was pulverized using a mixer to obtain a polymer sample. Using a differential scanning calorimeter DSC-6200 (manufactured by Seiko Denshi Kogyo Co., Ltd.), the measurement was performed at 20 ° C./min in a nitrogen atmosphere.

Synthesis of Water-Soluble Resin (I) The water-soluble resin (I) used as a binder in the examples was synthesized as follows.
A reactor equipped with a thermometer, a stirrer, a dripping device, a nitrogen introduction tube and a reflux cooling device was charged with an average of 50 moles of ethylene oxide added to 41.14 g of ion-exchanged water and 3-methyl-3-buten-1-ol. After charging 49.37 g of a saturated polyalkylene glycol ether monomer (IPN50), the reactor was purged with nitrogen under stirring, and the temperature was raised to 60 ° C. in a nitrogen atmosphere. .86 g was added, and an aqueous solution composed of 5.90 g of acrylic acid (AA) and 1.48 g of ion-exchanged water was added dropwise over 3 hours.
Simultaneously with this dropping, an aqueous solution composed of 0.26 g of 3-mercaptopropionic acid, 0.10 g of L-ascorbic acid and 15.91 g of ion-exchanged water was dropped over 3.5 hours. Then, after maintaining the temperature of 60 degreeC continuously for 1 hour, it cooled and complete | finished superposition | polymerization.
Thereafter, a pH of about 6 and a solid content of 40 wt% were adjusted using a 48 wt% sodium hydroxide aqueous solution and ion exchange water at 30 ° C. to obtain an aqueous solution of the water-soluble resin (I).
The obtained water-soluble resin (I) was a copolymer of IPN50 / AA = 85.6 / 14.4, and the weight average molecular weight was 36300.
The weight average molecular weight was measured by GPC (gel permeation chromatography) under the following conditions.
Measuring instrument: Waters LCM1
Molecular weight column: G4000SWXL, G3000SWXL, 2000SWXL (all manufactured by Tosoh Corporation) connected in series Eluent: Water 10999 g, acetonitrile 6001 g dissolved sodium acetate dianhydride 115.6 Standard substance for calibration curve: polyethylene Glycol measurement method: A solution in which the solid content of the measurement object was dissolved in the eluent so as to be 0.3% by mass and filtered through a filter was measured.

(2) Preparation Example 1 of Positive Electrode Composition
Disperse by mixing 20.80 g of water, 1.11 g of styrene-maleic acid copolymer dispersant (concentration 27% product) and 3.00 g of acetylene black HS-100 (manufactured by Denka), and further dispersing phosphoric acid. Iron lithium (Chinese imported product) 25.50 g, 48 wt% VDF-acrylic modified emulsion (70:30) (manufactured by Arkema) 2.50 g was added and dispersed in order, filtered and positive electrode composition (1 )

Example 2
Disperse by mixing 20.50 g of water, 1.11 g of styrene-maleic acid copolymer dispersant, 2.25 g of water-soluble resin (I), and 2.40 g of acetylene black HS-100, and phosphoric acid. Iron lithium (Chinese imported product) 25.50 g, 48 wt% VDF-acrylic modified emulsion (70:30) 1.87 g was added and dispersed in order, and filtered to obtain a positive electrode composition (2).

Comparative Example 1
17.89 g of water, 1.11 g of styrene-maleic acid copolymer dispersant and 3.00 g of acetylene black HS-100 are mixed and dispersed, and further, 25.50 g of lithium iron phosphate (imported in China), 2.50 g of SBR latex (solid content 48 wt%) was added and dispersed in order, followed by filtration to obtain a comparative positive electrode composition (1).

Comparative Example 2
3.50 g of water, 1.11 g of a styrene-maleic acid copolymer dispersant, 30.00 g of carboxymethyl cellulose (manufactured by Daicel, CMC 1380, 1% concentration product), and 3. 3 g of acetylene black HS-100. 00 g mixed and dispersed, and further added 25.50 g of lithium iron phosphate (China imported product) and 1.88 g of SBR latex (solid content 48 wt%) in order and dispersed, followed by filtration to produce a comparative positive electrode composition (2 )

Comparative Example 3
Disperse by mixing 20.36 g of water, 1.11 g of styrene-maleic acid copolymer dispersant, 3.43 g of sodium polyacrylate (solid content 35 wt%), and 3.00 g of acetylene black HS-100. Furthermore, 25.50 g of lithium iron phosphate (imported from China) was added and dispersed, followed by filtration to obtain a comparative positive electrode composition (3).

Comparative Example 4
19.39 g of water, 1.11 g of styrene-maleic acid copolymer dispersant and 3.00 g of acetylene black HS-100 are mixed and dispersed, and further 25.50 g of lithium iron phosphate (China imported product) is mixed and dispersed. did. A dispersion obtained by adding 1.5 g of water to 3.0 g of PTFE NMP dispersion (product of Kitamura PTFE particles 0.2 μm, solid content 40%) was prepared, and the above-described active material and the like were dispersed in this adjusted dispersion. The mixture was dispersed in the dispersion and filtered to obtain a comparative positive electrode composition (4).

(3) Various evaluations of positive electrode composition Positive electrode compositions (1) and (2) obtained in Examples 1 and 2 and Comparative positive electrode compositions (1) to (4) obtained in Comparative Examples 1 to 4 ) Was evaluated. The evaluation method is as follows. The evaluation results are shown in Table 1. In Table 1, in the blending composition, in addition to the amount and total amount of each component, the amount of solid content therein is also shown. For example, the amount (2.50 / 1.20) of the 48 wt% VDF-based acrylic modified emulsion (70:30) of Example 1 is 48 wt% of the aqueous binder solution of the VDF-acrylic modified emulsion (70:30). 2.50 g is added, which means that the solid content of the binder is 1.20 g.
In Table 1, PAA-Na represents sodium polyacrylate.
1. The positive electrode composition 0.5g was put into the solid content aluminum pan, and it dried at 150 degreeC for 1 hour using the hot air dryer. The mass of the solid content was measured from the weight before and after drying.
2. A viscosity B type viscometer (manufactured by Tokyo Keiki Co., Ltd.) was used to measure the viscosity at 25 ± 1 ° C. and 30 rpm.
3. Using a thixo value B type viscometer (manufactured by Tokyo Keiki Co., Ltd.), the viscosity at 25 ± 1 ° C., 6 rpm and 60 rpm was measured, and the value obtained by dividing the viscosity at 6 rpm by the viscosity at 60 rpm was determined.
4). pH
A value of 25 ° C. was measured using a glass electrode type hydrogen ion thermometer F-21 (Horiba, Ltd.).

5. The positive electrode composition was coated on the film strength aluminum foil using an applicator. The film was dried at 100 ° C. for 10 minutes and 200 ° C. for 60 minutes, and further subjected to hot pressing at 200 ° C. for 30 minutes to produce a positive electrode film having a thickness of about 40 μm.
When the positive electrode film was punched with a 12 mm punch, it was judged from the ease of chipping or peeling of the cut surface.
○ ・ ・ ・ No chipping or peeling △ ・ ・ ・ Sometimes chipping or peeling at the cut end
×: When punched, the cut edge always has chipping and peeling. Battery evaluation was performed using a coin cell by using a charge / discharge test charge / discharge measuring apparatus ACD-001 (manufactured by Asuka Electronics Co., Ltd.).
Negative electrode: Li foil electrolyte: 1 mol% / L LiPF ₆ EC / EMC = 1/1 (manufactured by Kishida Chemical)
Charging / discharging conditions: 0.2C

(4) Preparation Example 3 of Negative Electrode Composition
6.28 g of water, 30.00 g of a carboxymethyl cellulose solution (manufactured by Daicel, CMC1380, 1% concentration product) and 29.40 g of CGB-10 (manufactured by Nippon Graphite) as a negative electrode active material are mixed and dispersed. Further, 0.62 g of a 48 wt% VDF-acrylic modified emulsion (70:30) was added and dispersed, followed by filtration to obtain a negative electrode composition (1).

Comparative Example 5
A comparative negative electrode composition (1) was obtained in the same manner as in Example 3 except that the binder was changed to 0.63 g of SBR latex.

(5) Various evaluations of negative electrode composition About the negative electrode composition (1) obtained in Example 3 and the comparative negative electrode composition (1) obtained in Comparative Example 5, the negative electrode film properties (peel strength), and The electrical characteristics were evaluated. The evaluation method is as follows.
7). Negative electrode film properties (peel strength)
A negative electrode composition (1) and a comparative negative electrode composition (1) were coated on a copper foil to obtain a negative electrode film.
The film was 1 cm wide, and a double-sided tape was attached to the negative electrode composition side.
Using dynamic viscoelastic device RSA III (manufactured by TI Instruments), holding copper foil and double-sided tape side (with release substrate) and using tensile mode (speed 5 cm / min) Was measured.
8). Battery evaluation was performed using a coin cell by using a charge / discharge test charge / discharge measuring apparatus ACD-001 (manufactured by Asuka Electronics Co., Ltd.).
Positive electrode: positive electrode composition and negative electrode composition described in Examples and Comparative Examples Negative electrode: Li foil electrolyte solution: 1 mol% / L LiPF ₆ EC / EMC = 1/1 (manufactured by Kishida Chemical)
Charging / discharging conditions: 0.2C

From the comparison between Examples 1 and 2 and Comparative Examples 1 to 4 in Table 1, by using a VDF-acrylic modified emulsion as a binder, a slurry having a good slurry characteristic value can be formed by using a small amount of binder. It was confirmed that the coating film obtained from the positive electrode composition was excellent in film strength and electrical characteristics. On the other hand, in Comparative Examples 1 and 2 using SBR latex as the binder, the cycle deterioration of the charge / discharge capacity in electrical characteristics was somewhat large, and in Comparative Example 3 using sodium polyacrylate as the binder, The viscosity increased, the flexibility of the film was low, and the workability was poor. Further, in Comparative Example 4 using a PTFE dispersion as a binder, the flexibility of the film could not be expressed, and it was difficult to use as a positive electrode film.
Furthermore, from the comparison between Example 3 and Comparative Example 5 in Table 2, a composition using a VDF-acrylic modified emulsion as a binder can also be used as a composition for forming a negative electrode. It was confirmed that the obtained coating film had excellent film strength and electrical characteristics equivalent to the coating film obtained from the composition using SBR latex as a binder. As described above, since the VDF-acrylic modified polymer can form a film by heating at 100 ° C., the secondary battery electrode aqueous composition of the present invention is used as a material for forming either the negative electrode or the positive electrode. In addition, it can be said that it is an aqueous composition suitable as a material for a secondary battery electrode that is excellent in terms of ease of forming a coating film.

Claims (8)

A positive electrode aqueous composition comprising an electrode active material, a binder and water as essential components, and forming a positive electrode of a secondary battery,
The binder includes an aqueous dispersion of a fluorine-containing polymer having a (meth) acryl-modified structure,
The fluorine-containing polymer having a structure modified with (meth) acryl has a structure part derived from the fluorine-containing polymer and a structure part modified with (meth) acryl,
The (meth) acryl-modified structural part is formed by a polymer of a (meth) acrylic compound,
The positive electrode aqueous composition for a secondary battery, wherein the fluorine-containing polymer is a vinylidene fluoride polymer. The positive electrode water-based composition for a secondary battery according to claim 1, wherein the binder has a minimum film forming temperature of 40 ° C. or lower. The fluorine-containing polymer having a structure modified with (meth) acryl is characterized in that a mass ratio of the structure part derived from the fluorine-containing polymer and the structure part modified with (meth) acryl is 40/60 to 95/5. The positive electrode aqueous composition for a secondary battery according to claim 1 or 2. The (meth) fluorine-containing polymer having an acrylic modified structure, after producing an aqueous dispersion of fluorine-containing polymer, prepared by polymerizing a (meth) acrylic compound in the presence of the fluorine-containing polymer The positive electrode aqueous composition for secondary battery according to any one of claims 1 to 3, wherein The positive electrode aqueous composition for secondary battery according to claim 1, wherein the electrode active material is an electrode active material capable of occluding and releasing lithium ions. The positive electrode aqueous composition for a secondary battery according to claim 5, wherein the electrode active material is a positive electrode active material containing a compound having an olivine structure. The positive electrode aqueous composition for a secondary battery according to claim 5 or 6, wherein the electrode active material is a positive electrode active material containing lithium iron phosphate as a main component. A binder used in the positive electrode aqueous composition for a secondary battery according to claim 1,
The binder includes an aqueous dispersion of a fluorine-containing polymer having a (meth) acryl-modified structure,
The fluorine-containing polymer having a structure modified with (meth) acryl has a structure part derived from the fluorine-containing polymer and a structure part modified with (meth) acryl,
The (meth) acryl-modified structural part is formed by a polymer of a (meth) acrylic compound,
The fluorine-containing polymer is a vinylidene fluoride polymer, a binder for a positive electrode aqueous composition for a secondary battery.