JP7724148B2 - Conductive slurry for power storage devices - Google Patents

Description

The present invention relates to a conductive material slurry, a positive electrode paste for an electricity storage device, and a positive electrode for an electricity storage device.

In recent years, there has been active development of electric vehicles that do not emit carbon dioxide in order to curb global warming. However, electric vehicles have the drawback of having a shorter driving range than gasoline-powered vehicles and taking longer to charge their batteries. To shorten charging times, it is necessary to increase the speed at which electrons move within the positive electrode. Currently, carbon black is used as a conductive material in the positive electrodes of non-aqueous electrolyte batteries, but there is a demand for conductive materials with lower electronic resistance than carbon black.

Carbon nanotubes (hereinafter sometimes referred to as "CNTs") are a conductive material with lower electronic resistance than carbon black, and their physical and chemical properties make them promising for application in a variety of fields. CNTs have a high aspect ratio and can form conductive paths with just a small amount. CNTs are essentially nanocarbon materials formed by rolling a single sheet of graphite into a cylindrical shape; those rolled in a single layer are called single-walled CNTs, and those rolled in multiple coaxial layers are called multi-walled CNTs.

In order to reduce the coating resistance of the positive electrode, it is necessary to uniformly disperse the carbon material in an organic solvent. Patent Document 1 discloses a conductive paste for lithium-ion battery positive electrodes that uses a polyvinyl alcohol resin containing 10% by mass of structural units derived from 2-hydroxyethyl methacrylate (HEMA) and having a degree of saponification of 90 mol% as a dispersant for acetylene black. Patent Document 2 discloses the use of polyvinyl alcohol as a dispersant for CNTs. Patent Document 3 discloses a slurry composition for secondary battery electrodes that contains CNTs as the carbon material and a polyvinyl alcohol resin with a degree of saponification of 98 mol% as a dispersant.

JP 2017-130443 A Japanese Patent Application Laid-Open No. 2021-50106 WO2020/137403 publication

However, when a polyvinyl alcohol-based resin has a high degree of saponification (high polarity), the adsorption of the dispersant to the carbon material improves, but the solubility in the solvent deteriorates, resulting in poor productivity of the conductive material slurry. The solubility of the dispersant in the solvent is a factor that also affects the dispersibility of the carbon material, and poor solubility of the dispersant in the solvent is disadvantageous in improving the dispersibility of the carbon material. Patent Document 1 discloses that by including a polymerizable unsaturated group-containing monomer (A1-1) such as 2-hydroxyethyl methacrylate (HEMA) as a constituent component of a polymer dispersant, it is possible to achieve both solubility in the solvent and adsorption to the carbon material. However, there is still room for improvement in terms of improving the dispersibility of the carbon material in the conductive material slurry.
On the other hand, since an electricity storage device is repeatedly charged and discharged, it is desirable that the degree of deterioration of the electrodes due to the charge and discharge be small. The durability of an electrode against charge and discharge can be evaluated, for example, by high-temperature cycle characteristics, but Patent Documents 1 to 3 do not consider the relationship between the dispersant and high-temperature cycle characteristics.

The present disclosure therefore provides a conductive material slurry for an electricity storage device that exhibits good CNT dispersibility and can contribute to improved high-temperature cycle characteristics. The present disclosure also provides a positive electrode paste for an electricity storage device prepared using the conductive material slurry for an electricity storage device. The present disclosure also provides a positive electrode for an electricity storage device formed using the conductive material slurry for an electricity storage device.

In one aspect, the present disclosure provides a conductive material slurry for an electrical storage device, the conductive material slurry comprising a carbon material-based conductive material, a dispersant, and an organic solvent,
the carbonaceous conductive material includes carbon nanotubes,
The dispersant is a polymer containing a vinyl alcohol unit, a vinyl acetate unit, and a structural unit represented by the following general formula (1):
The polymer contains, in one molecule, 62 to 98% by mass of the vinyl alcohol unit and 1 to 25% by mass of the constitutional unit represented by general formula (1),
a ratio of the mass of the vinyl alcohol unit to the total mass of the vinyl alcohol unit and the vinyl acetate unit in one molecule of the polymer is 83 to 99%;
The conductive material slurry for an electricity storage device relates to a conductive material slurry in which the content of the vinyl alcohol unit in the conductive material slurry is 20 to 100 parts by mass per 100 parts by mass of the carbon nanotubes.
In the above general formula (1), R ¹ is hydrogen or a methyl group, p is an integer of 1 or 2, and q is an integer of 2 or 3.

In one aspect, the present disclosure provides a positive electrode paste for an electricity storage device, comprising a positive electrode active material, a binder, a carbonaceous conductive material, a dispersant, and an organic solvent,
the carbonaceous conductive material includes carbon nanotubes,
The dispersant is a polymer containing a vinyl alcohol unit, a vinyl acetate unit, and a structural unit represented by the following general formula (1):
The polymer contains, in one molecule, 62 to 98% by mass of the vinyl alcohol unit and 1 to 25% by mass of the constitutional unit represented by general formula (1),
a ratio of the mass of the vinyl alcohol unit to the total mass of the vinyl alcohol unit and the vinyl acetate unit in one molecule of the polymer is 83 to 99%;
The present invention relates to a positive electrode paste for an electricity storage device, wherein the content of the vinyl alcohol units in the positive electrode paste is 20 to 100 parts by mass per 100 parts by mass of the carbon nanotubes.
In the above general formula (1), R ¹ is hydrogen or a methyl group, p is an integer of 1 or 2, and q is an integer of 2 or 3.

In one aspect, the present disclosure provides a positive electrode for an electricity storage device, including a positive electrode mixture layer,
the positive electrode mixture layer includes a positive electrode active material, a carbon material-based conductive material, a dispersant, and a binder,
the carbonaceous conductive material includes carbon nanotubes,
The dispersant is a polymer containing a vinyl alcohol unit, a vinyl acetate unit, and a structural unit represented by the following general formula (1):
The polymer contains, in one molecule, 62 to 98% by mass of the vinyl alcohol unit and 1 to 25% by mass of the constitutional unit represented by general formula (1),
a ratio of the mass of the vinyl alcohol unit to the total mass of the vinyl alcohol unit and the vinyl acetate unit in one molecule of the polymer is 83 to 99%;
The present invention relates to a positive electrode for an electricity storage device, wherein the content of the vinyl alcohol units in the positive electrode mixture layer is 20 to 100 parts by mass per 100 parts by mass of the carbon nanotubes.
In the above general formula (1), R ¹ is hydrogen or a methyl group, p is an integer of 1 or 2, and q is an integer of 2 or 3.

The conductive material slurry for an electricity storage device according to the present disclosure contains the above polymer as a dispersant, and the content of vinyl alcohol units in the conductive material slurry is 20 to 100 parts by mass per 100 parts by mass of carbon nanotubes. This allows for good dispersion of CNTs in the conductive material slurry for an electricity storage device, and can contribute to improving high-temperature cycle characteristics.
The positive electrode paste for an electricity storage device of the present disclosure is prepared using the conductive material slurry for an electricity storage device of the present disclosure, and therefore it is possible to reduce the coating resistance of the positive electrode mixture layer and improve the high-temperature cycle characteristics.
The positive electrode for an electricity storage device according to the present disclosure has a positive electrode composite layer formed using the positive electrode paste for a battery according to the present disclosure prepared using the conductive material slurry for an electricity storage device according to the present disclosure, and therefore has good high-temperature cycle characteristics and low coating resistance of the positive electrode composite layer.

The conductive material slurry for electricity storage devices disclosed herein (hereinafter also referred to as "conductive material slurry") contains a polymer as a dispersant containing vinyl alcohol units, vinyl acetate units, and units represented by general formula (1) in a specific mass ratio per molecule, and is based on the new finding that by specifying the amount of vinyl alcohol units per 100 parts by mass of CNTs in the conductive material slurry, the dispersibility of CNTs in the conductive material slurry is improved, and the high-temperature cycle characteristics of a positive electrode for electricity storage devices formed using the conductive material slurry are improved.

In the present disclosure, the details of the mechanism by which the dispersibility of CNTs in the conductive material slurry and the high-temperature cycle characteristics are improved are not clear, but are presumed to be as follows.
In polymers (dispersants) containing vinyl alcohol units and vinyl acetate units, it is known that the vinyl alcohol units exhibit adsorption properties toward CNTs, and the vinyl acetate units are the components responsible for solubility in solvents. However, when the dispersant becomes highly polar, the solubility of the dispersant in organic solvents such as NMP decreases. If the solubility of the dispersant is low, it takes a long time to dissolve in the solvent, resulting in poor productivity, and the dispersibility also decreases due to the poor stability of the dispersant in the solvent. On the other hand, if the solubility of the dispersant in organic solvents is too high, the vinyl alcohol units become less likely to adsorb to CNTs, resulting in poor dispersibility of CNTs.
In the present disclosure, the content of the dispersant in the conductive material slurry is set to 20 to 100 parts by mass of the vinyl alcohol units per 100 parts by mass of CNTs, and the content of the vinyl alcohol units in one polymer molecule is set to 62 to 98% by mass, thereby ensuring the necessary adsorptive power for CNTs for high CNT dispersibility. The ratio of the mass of the vinyl alcohol units to the total mass of the vinyl alcohol units and the vinyl acetate units in one polymer molecule is set to 83 to 99% by mass, and further, one polymer molecule contains 1 to 25% by mass of a structural unit represented by the following general formula (1), thereby improving the solubility of the polymer containing the vinyl alcohol units and the vinyl acetate units to a range that does not impair the necessary adsorptive power for CNTs, thereby increasing productivity and ensuring the stability of the dispersant in the solvent.
Furthermore, in the present disclosure, the polymer has a structure in which 1 to 25 mass% of the following general formula (1) is contained in one molecule, and therefore the glass transition temperature (Tg) of the dispersant is lower than, for example, a single-component polymer composed of vinyl alcohol units or a binary copolymer composed of vinyl alcohol units and vinyl acetate units. Therefore, the composite layer formed using the conductive slurry of the present disclosure has high adhesion to the current collector, and the composite layer becomes moderately soft while maintaining appropriate strength, which is thought to mitigate the adverse effects on the positive electrode of expansion and contraction associated with charging and discharging of the power storage device and improve high-temperature cycle characteristics.
However, the present invention is not limited to these mechanisms.
The decrease in glass transition temperature (Tg) caused by introducing a constituent unit represented by the following general formula (1) into a polymer containing vinyl alcohol units and vinyl acetate units can be confirmed by the Fox equation.

<Conductive material slurry>
In one embodiment, the conductive material slurry of the present disclosure contains a carbon material-based conductive material (hereinafter sometimes abbreviated as "conductive material"), a dispersant, and an organic solvent, and the conductive material contains CNTs.

[Conductive material]
In the conductive material slurry of the present disclosure, the conductive material preferably comprises 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, still more preferably 98% by mass or more, still more preferably substantially 100% by mass, and still more preferably 100% by mass. Examples of conductive materials other than CNT contained in the conductive material slurry of the present disclosure include carbon material-based conductive materials such as acetylene black, ketjen black, and graphite, and these can be used alone or in combination of two or more.

In this disclosure, CNT refers to an entity that includes multiple CNTs. The form of the CNTs used in preparing the conductive material slurry of this disclosure is not particularly limited; for example, multiple CNTs may be independent, multiple CNTs may be in a bundled or entangled form, or a mixture of these forms. CNTs may have various numbers of walls or diameters. CNTs may contain impurities (e.g., catalysts and amorphous carbon) derived from the CNT manufacturing process.

In one or more embodiments, the CNTs have a cylindrical shape formed by rolling one surface of graphite. Those rolled into one layer are called single-walled CNTs (SWCNTs), those rolled into two layers are called double-walled CNTs (DWCNTs), and those rolled into three or more layers are called multi-walled CNTs (MWCNTs). Depending on the properties required for the positive electrode composite layer formed using a positive electrode paste for an electricity storage device (hereinafter sometimes abbreviated as "positive electrode paste") containing the conductive material slurry of the present disclosure, single-walled, double-walled, or multi-walled CNTs, or mixtures thereof, can be used. Multi-walled CNTs are preferred for obtaining a positive electrode composite layer with good CNT dispersion and low resistance.

The average diameter of CNTs is measured using a scanning electron microscope (SEM) or an atomic force microscope (AFM). In the present disclosure, the average diameter is not particularly limited, but from the viewpoint of improving CNT dispersibility, it is preferably 3 nm or more, more preferably 5 nm or more, and even more preferably 8 nm or more. From the viewpoint of improving conductivity, it is preferably 100 nm or less, more preferably 50 nm or less, even more preferably 30 nm or less, even more preferably 20 nm or less, and even more preferably 15 nm or less. Specifically, the average diameter of CNTs is preferably 3 to 100 nm, more preferably 3 to 50 nm, even more preferably 3 to 30 nm, even more preferably 5 to 20 nm, even more preferably 8 to 20 nm, and even more preferably 8 to 15 nm.

The average length of CNTs is measured using a scanning electron microscope (SEM) or an atomic force microscope (AFM). In the present disclosure, the average length is not particularly limited, but from the viewpoint of improving conductivity, it is preferably 2 μm or more, more preferably 5 μm or more, even more preferably 10 μm or more, and even more preferably 30 μm or more. From the viewpoint of improving dispersibility, it is preferably 500 μm or less, more preferably 300 μm or less, even more preferably 200 μm or less, and even more preferably 120 μm or less. Specifically, the average length of CNTs is preferably 2 to 500 μm, more preferably 5 to 300 μm, even more preferably 10 to 200 μm, and even more preferably 30 to 120 μm.

The impurity content in CNT is measured by methods such as thermogravimetric analysis, and in this disclosure, the lower the content, the better. From the perspective of achieving a high concentration of useful CNT components, the impurity content in CNT is preferably 40% by mass or less, more preferably 30% by mass or less, even more preferably 20% by mass or less, even more preferably 10% by mass or less, and even more preferably substantially 0% by mass.

The content of the conductive material in the conductive material slurry of the present disclosure is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, and even more preferably 1.5% by mass or more, from the viewpoint of improving the convenience of adjusting the concentration of the positive electrode paste. Furthermore, from the viewpoint of providing the conductive material slurry with a viscosity that is easy to handle, the content is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less. Specifically, the content of the conductive material in the conductive material slurry of the present disclosure is preferably 0.5 to 10% by mass, more preferably 1.0 to 5% by mass, and even more preferably 1.5 to 3% by mass.

[Dispersant]
The dispersant contained in the conductive material slurry of the present disclosure (hereinafter, sometimes referred to as the "dispersant of the present disclosure" or the "polymer of the present disclosure") is a polymer containing, in one molecule, a vinyl alcohol unit, a vinyl acetate unit, and a constitutional unit represented by the following general formula (1). The vinyl alcohol unit is represented by _-CH2 -CH(OH)-. The vinyl acetate unit is represented, for example, by -( _CH3COO )CH- _CH2- . The vinyl alcohol unit is a component that contributes to adsorption to CNTs, and the vinyl acetate unit is a component that contributes to dissolution in organic solvents such as NMP. The constitutional unit represented by the following general formula (1) is a component that contributes to solubility in organic solvents and improvement of high-temperature cycle characteristics.
In the above general formula (1), R ¹ is hydrogen or a methyl group, p is an integer of 1 or 2, and q is an integer of 2 or 3.

[Vinyl alcohol unit, vinyl acetate unit]
The content of vinyl alcohol units per molecule of the polymer of the present disclosure is 62% by mass or more from the viewpoint of improving CNT dispersibility. It is presumed that when the content of vinyl alcohol units per molecule is 62% by mass or more, the vinyl alcohol units adsorb to the CNTs and hold the CNTs, and the polyvinyl structure that is the main chain of the polymer spreads over the CNT surfaces. The steric repulsion of the polymer creates a strong steric repulsion between the CNTs, making the CNTs highly dispersible in the solvent. The content of vinyl alcohol units per molecule of the polymer of the present disclosure is preferably 69% by mass or more, more preferably 75% by mass or more, from the viewpoint of improving CNT dispersibility. On the other hand, from the viewpoints of productivity and the stability of the conductive material slurry, it is preferably 98% by mass or less, more preferably 95% by mass or less.

In the polymer of the present disclosure, the ratio (100×a/(a+b)) of the mass (a) of vinyl alcohol units to the sum of the mass (a) of vinyl alcohol units and the mass (b) of vinyl acetate units in one molecule is 83 to 99% by mass. When the ratio (100×a/(a+b)) is 83% by mass or more, excessive dissolution of the polymer in organic solvents such as N-methyl-2-pyrrolidone (NMP) is suppressed, while sufficient adsorption of the polymer to CNTs is ensured, which is thought to contribute to improved dispersibility. When the ratio (100×a/(a+b)) is 99% by mass or less, the polymer is moderately soluble in organic solvents, in combination with the effect of the structural unit represented by general formula (1). Therefore, preparation of the conductive material slurry does not take an excessively long time, and the conductive material slurry is also stable.
The mass a of the vinyl alcohol unit and the mass b of the vinyl acetate unit can be calculated from the amount of vinyl acetate used in the synthesis of the polymer of the present disclosure and the amount of sodium hydroxide used in the saponification reaction, respectively.

The above-mentioned ratio 100×a/(a+b) within one molecule of the polymer of the present disclosure is 83% by mass or more, preferably 90% by mass or more, and more preferably 94% by mass or more, from the viewpoint of improving the dispersibility of CNTs, and is 99% by mass or less, preferably 98% by mass or less, and more preferably 95% by mass or less, from the viewpoint of productivity and the stability of the conductive material slurry.

[Constituent unit represented by general formula (1)]
The structural unit represented by the general formula (1) is as described above, but from the viewpoint of high-temperature cycle properties, in the general formula (1), ^R1 is preferably a methyl group, p is preferably 2, and q is preferably 2. Examples of monomers that provide the structural unit represented by the general formula (1) include 2-hydroxyethyl (meth)acrylate and 2-hydroxypropyl (meth)acrylate, and of these, 2-hydroxyethyl (meth)acrylate is preferred, and 2-hydroxyethyl methacrylate is more preferred.

The content of the structural unit represented by general formula (1) in one molecule of the polymer of the present disclosure is 1% by mass or more, preferably 3% by mass or more, and more preferably 5% by mass or more, from the viewpoints of improving productivity by improving the solubility of the polymer and improving high-temperature cycle characteristics. On the other hand, from the viewpoints of suppressing a decrease in the adsorption force of the polymer to CNTs due to excessive dissolution of the polymer and suppressing a decrease in the strength of the composite layer, the content is 25% by mass or less, preferably 15% by mass or less, and more preferably 10% by mass or less. In the present disclosure, the content of the structural unit represented by general formula (1) in one molecule of the polymer of the present disclosure is specifically 1 to 25% by mass, preferably 3 to 15% by mass, and more preferably 5 to 10% by mass.
The content (mass %) of the constitutional unit represented by general formula (1) in one molecule of the polymer of the present disclosure can be considered to be the mass ratio of the monomer that provides the constitutional unit represented by general formula (1) relative to all reactive monomers used in the synthesis of the polymer of the present disclosure.

[Other building blocks]
The polymer of the present disclosure may contain other structural units in addition to vinyl alcohol units, vinyl acetate units, and the structural units represented by the general formula (1). Examples of the other structural units include one or more structural units selected from vinyl ester units other than vinyl acetate, vinyl butyral units, styrene units, ethylene units, and butadiene units. The vinyl ester units other than vinyl acetate are derived from vinyl ester monomers used as raw material monomers for the polymer. Examples of vinyl ester monomers include vinyl propionate, vinyl butyrate, and vinyl laurate.

The degree of polymerization of the polymer disclosed herein is preferably 20 or more, more preferably 50 or more, even more preferably 200 or more, and even more preferably 300 or more, from the viewpoint of improving the dispersibility of CNTs by favorably coating the CNTs with a polyvinyl structure; and from the viewpoint of improving the dispersibility of CNTs by improving the dispersibility of the polymer itself, it is preferably 5000 or less, more preferably 3000 or less, even more preferably 2000 or less, and even more preferably 1000 or less. In this disclosure, the degree of polymerization of the polymer is specifically preferably 20 to 5000, more preferably 50 to 3000, even more preferably 200 to 2000, and even more preferably 300 to 1000.

If the content of the vinyl alcohol units in the conductive material slurry of the present disclosure is too low, the CNTs will aggregate, and the stability of the conductive material slurry will not be maintained. If the content is too high, the aggregation effect of the vinyl alcohol units during solvent drying will hinder contact between the CNTs, resulting in high coating resistance of the positive electrode coating film. Here, "the content of the vinyl alcohol units in the conductive material slurry" means the total amount of vinyl alcohol units contained in the conductive material slurry.

The content of vinyl alcohol units in the conductive material slurry of the present disclosure is 20 parts by mass or more, preferably 23 parts by mass or more, per 100 parts by mass of CNT, from the viewpoint of high CNT dispersibility and conductive material slurry stability; and 100 parts by mass or less, preferably 30 parts by mass or less, per 100 parts by mass of CNT, from the viewpoint of coating film resistance. Specifically, the content of vinyl alcohol units in the conductive material slurry of the present disclosure is 20 to 100 parts by mass, preferably 23 to 30 parts by mass, per 100 parts by mass of CNT. The content of dispersant in the conductive material slurry is an amount such that the amount of vinyl alcohol units is 20 to 100 parts by mass, preferably 23 to 30 parts by mass, per 100 parts by mass of CNT.

[Method of producing polymer]
The method for synthesizing the polymer of the present disclosure is not particularly limited, and the polymer can be obtained by a known method, for example, by polymerizing vinyl acetate and a monomer that provides a structural unit represented by general formula (1) to obtain a copolymer, and then saponifying the copolymer.

[Organic solvent]
The solvent contained in the conductive material slurry of the present disclosure is preferably an organic solvent capable of dissolving the binder contained in the positive electrode paste. Examples of the organic solvent include amide polar organic solvents such as dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), and N-methyl-2-pyrrolidone (NMP); alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, and octanol; glycols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, and hexylene glycol; glycerin, trimethylolpropane, pentanediol, and the like. Examples of suitable organic solvents include polyhydric alcohols such as diethyl ether, pentane, sorbitol, and sorbitol; glycol ethers such as ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, tetraethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, triethylene glycol monoethyl ether, tetraethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, and tetraethylene glycol monobutyl ether; ketones such as acetone, methyl ethyl ketone, methyl propyl ketone, and cyclopentanone; and esters such as ethyl acetate, γ-butyl lactone, and ε-propiolactone. The organic solvent contained in the conductive material slurry may be a single type or a combination of two or more types. When the binder is PVDF (polyvinylidene fluoride resin), which is contained in conventional battery positive electrode pastes, the organic solvent is generally N-methyl-2-pyrrolidone (NMP). NMP is particularly preferred as the organic solvent from the viewpoint of improving the dispersibility of CNTs.

(Binder)
The binder contained in the positive electrode paste of the present disclosure, which will be described later, may be added to the conductive material slurry or the positive electrode active material of the present disclosure when preparing the positive electrode paste. In one embodiment, the binder may be contained in the conductive material slurry of the present disclosure in advance.

There are no particular restrictions on the binder, and any known binder used in the production of battery electrodes can be used. Examples include vinylidene fluoride polymers such as polyvinylidene fluoride (PVDF), vinylidene fluoride copolymers with hexafluoropropylene or monochlorotrifluoroethylene, and carboxylic acid-modified vinylidene fluoride. There are no particular restrictions on the weight-average molecular weight of these vinylidene fluoride polymers, but vinylidene fluoride polymers with a molecular weight of 100,000 to 1,500,000 are commonly used. Styrene-butadiene rubber, polyacrylonitrile, etc. may also be mixed with vinylidene fluoride polymers.

The amount of binder contained in the conductive material slurry, positive electrode paste, or positive electrode composite layer of a positive electrode for an electrical storage device according to the present disclosure is preferably 150 parts by mass or more, more preferably 300 parts by mass or more, and even more preferably 500 parts by mass or more, per 100 parts by mass of vinyl alcohol units in the conductive material slurry, positive electrode paste, or positive electrode composite layer of a positive electrode for an electrical storage device, from the viewpoints of ensuring the integrity of the positive electrode composite layer and the adhesion between the positive electrode composite layer and the current collector. From the viewpoints of dispersibility of CNTs and preventing a loss of dispersion stability of the conductive material slurry or positive electrode paste due to an increase in the viscosity of the conductive material slurry or positive electrode paste resulting from the addition of the binder, the amount of binder is preferably 800 parts by mass or less, more preferably 700 parts by mass or less, and even more preferably 660 parts by mass or less, per 100 parts by mass of vinyl alcohol units in the conductive material slurry, positive electrode paste, or positive electrode composite layer of a positive electrode for an electrical storage device. Specifically, the amount of binder contained in the conductive material slurry, positive electrode paste, or positive electrode mixture layer of the positive electrode for an electrical storage device is preferably 150 to 800 parts by mass, more preferably 300 to 700 parts by mass, and even more preferably 500 to 660 parts by mass per 100 parts of vinyl alcohol units in the conductive material slurry, positive electrode paste, or positive electrode mixture layer of the positive electrode for an electrical storage device.

[Nitrogen-containing organic compound]
In one embodiment, the positive electrode paste and the positive electrode composite layer of the positive electrode of the present disclosure, which will be described later, may further contain a nitrogen-containing organic compound. The nitrogen-containing organic compound may be added to the conductive material slurry and other components of the present disclosure when preparing the positive electrode paste. In one embodiment, the nitrogen-containing organic compound may be pre-contained in the conductive material slurry of the present disclosure. The addition of the nitrogen-containing organic compound allows the preparation of a conductive material slurry and a positive electrode paste that have low viscosity and good handleability.

While the details of the mechanism by which viscosity is reduced by adding a nitrogen-containing organic compound are unclear, the polymer disclosed herein does not cover the entire surface of the carbonaceous conductive material; instead, exposed portions exist on the surface of the carbonaceous conductive material. Therefore, adjacent carbonaceous conductive materials aggregate in organic solvents due to π-π interactions and hydrogen bonding between polar groups present on the surface of the carbonaceous conductive material. However, when a nitrogen-containing organic compound is included, the nitrogen-containing organic compound interacts with the polar groups (neutralization reaction or dipole-dipole interaction) to suppress hydrogen bonding between the carbonaceous conductive materials. Furthermore, the amine (cation) interacts with the π electrons on the carbonaceous conductive material to suppress π-π interactions between the carbonaceous conductive materials. It is believed that the suppression of hydrogen bonding and π-π interactions improves the dispersibility of the carbonaceous conductive material, resulting in a lower viscosity compared to when the nitrogen-containing organic compound is not added. However, the present disclosure should not be interpreted as being limited to these mechanisms.

Preferred nitrogen-containing organic compounds are compounds having at least one primary, secondary, or tertiary amino group in the molecule (hereinafter referred to as a primary, secondary, or tertiary amine). In this application, a primary, secondary, or tertiary amino group refers to a structure in which all three bonds to the central nitrogen atom are single bonds. As long as a nitrogen-containing organic compound has one such primary, secondary, or tertiary amino group, it may also have other nitrogen atoms in the molecule, and these other nitrogen atoms may be primary, secondary, or tertiary amino groups, or may form imino groups with double bonds. If other nitrogen atoms are present in the molecule, it is preferable that they are not adjacent to the nitrogen atom of the amino group.

The primary, secondary, and tertiary amines are preferably aliphatic amines, and may be linear (branched or linear) or cyclic. In the case of cyclic amines, saturated rings are preferred. Non-cyclic amines may have an unsaturated group such as an imino group along with the amino group. The hydrocarbon group portion of the aliphatic amine may be substituted with OH, an amino group, COOH, or the like. In addition, —CH ₂ — in the aliphatic group may be replaced with O (oxygen atom), and in this case, it is preferable that the O (oxygen atom) is not adjacent to the nitrogen atom of the amino group. Examples of the primary, secondary, and tertiary amines include alkylamines, amino group-containing alcohols, carboxyl-substituted alkylamines, imidazoles, piperazines, guanidines, piperidines, and pyrrolidines.

Preferred alkylamines are primary, secondary, or tertiary amines in which the alkyl groups present are, independently of one another, branched or linear alkyl groups, and more preferably have alkyl groups such that the total number of carbon atoms in the molecule is 15 or less. Specific examples include hexylamine, octylamine, diethylamine, dibutylamine, trimethylamine, triethylamine, tributylamine, N-propylethylamine, N-butylethylamine, and N,N-dimethylcyclohexylamine. The alkyl group may also be substituted with an amino group, in which case the amine will contain two or more primary, secondary, or tertiary amino groups. Examples include di- or triamines such as ethylenediamine and diethylenediaminetriamine.

Preferred amino group-containing alcohols are the above-mentioned alkylamines in which the hydrogen atom of the alkyl group has been replaced with OH. Examples include monoethanolamine, diethanolamine, triethanolamine, N-butyldiethanolamine, N,N-dimethylaminoethanol, N-n-butylethanolamine, 2-(methylamino)ethanol, N-methylethanolamine, N-ethylethanolamine, 2-amino-1-propanol, 2-amino-2-methyl-1-propanol, 1-amino-2-propanol, and 2-amino-1,3-propanediol. Of these, 2-amino-1,3-propanediol is preferred.

Carboxyl-substituted alkylamines include the above-mentioned alkylamines in which the hydrogen atom of the alkyl group is substituted with COOH, such as ethylenediaminetetraacetic acid, 1,3-propanediaminetetraacetic acid, 1,2-propanediaminetetraacetic acid, 1,3-diamino-2-hydroxypropanetetraacetic acid, glycol ether diaminetetraacetic acid, trans-1,2-cyclohexanediaminetetraacetic acid, hexamethylenediaminetetraacetic acid, dicarboxymethylglutamic acid, dicarboxymethylaspartic acid, S,S-ethylenediaminedisuccinic acid, ethylenediaminedi(o-hydroxyphenyl)acetic acid, hydroxyethyliminodiacetic acid, ethylenediaminediacetic acid, iminodiacetic acid, ethylenediaminedipropionic acid, nitrilotriacetic acid, hydroxyethylenediaminetriacetic acid, nitrilotripropionic acid, methylglycinediacetic acid, diethylenetriaminepentaacetic acid, and triethylenetetraminehexaacetic acid. In addition, some or all of the carboxyl groups may form a salt with an alkali metal such as sodium.

Specific examples of imidazoles include 1,2-dimethylimidazole, 2-methylimidazole, 2-ethyl-4-methylimidazole, 4-ethyl-2-methylimidazole, and 1-methyl-4-ethylimidazole.

Preferred piperazines are unsubstituted or alkyl-substituted piperazines, where the alkyl group may further contain an amino group. The alkyl group may be substituted at any position on the piperazine ring, either on a nitrogen atom or a carbon atom. Specific examples include piperazine, 1-methylpiperazine, 1-ethylpiperazine, 1-propylpiperazine, 1,4-dimethylpiperazine, 1,4-diethylpiperazine, 1,4-dipropylpiperazine, 2-methylpiperazine, 2-ethylpiperazine, 3-propylpiperazine, 2,6-dimethylpiperazine, 2,6-diethylpiperazine, 2,6-dipropylpiperazine, 2,5-dimethylpiperazine, 2,5-diethylpiperazine, and 2,5-dipropylpiperazine. Also preferred are piperazines substituted with an aminoalkyl group, such as 1-aminoethylpiperazine.

Other examples of the nitrogen-containing organic compound include guanidines, piperidines, and pyrrolidines. Specific examples include guanidine and guanidine salts, piperidine, 1-methylpiperidine, 1-ethylpiperidine, 1-propylpiperidine, 2,3, or 4-methylpiperidine, 2,3, or 4-ethylpiperidine, 2,6-dimethylpiperidine, 2,6-diethylpiperidine, 2,6-dipropylpiperidine, 2,4-dimethylpiperidine, 2,4-diethylpiperidine, 1-aminoethylpiperidine, morpholine, pyrrolidine, 1-methylpyrrolidine, 1-ethylpyrrolidine, 1-propylpyrrolidine, 2 or 3-methylpyrrolidine, 2 or 3-ethylpyrrolidine, 2,5-dimethylpyrrolidine, 2,5-diethylpyrrolidine, 2,5-dipropylpyrrolidine, 2,4-dimethylpiperidine, 2,4-diethylpiperidine, and 1-aminoethylpyrrolidine.

The content of the nitrogen-containing organic compound contained in the conductive material slurry, positive electrode paste, or positive electrode composite layer of a positive electrode for an electrical storage device is preferably 0.5 parts by mass or more, more preferably 1 part by mass or more, and even more preferably 5 parts by mass or more, per 100 parts by mass of the conductive material, from the viewpoint of improving the dispersibility of the conductive material. From the viewpoint of high conductivity, it is preferably 2000 parts by mass or less, more preferably 1000 parts by mass or less, and even more preferably 500 parts by mass or less. Specifically, the content of the nitrogen-containing organic compound contained in the conductive material slurry, positive electrode paste, or positive electrode composite layer of a positive electrode for an electrical storage device is preferably 0.5 to 2000 parts by mass, more preferably 1 to 1000 parts by mass, and even more preferably 5 to 500 parts by mass, per 100 parts by mass of the conductive material.

The conductive material slurries of the present disclosure may further contain other components, provided that the effects of the present disclosure are not impaired. Examples of other components include antioxidants, neutralizing agents, antifoaming agents, preservatives, dehydrating agents, rust inhibitors, plasticizers, etc. (hereinafter also referred to as additives).

[Method for producing conductive material slurry]
In one or more embodiments, the conductive material slurry of the present disclosure can be prepared by mixing a conductive material, a dispersant of the present disclosure, an organic solvent, and, as necessary, components such as a binder, a nitrogen-containing organic compound, or an additive using a mixer/disperser, and performing a dispersion treatment until a uniform composition is achieved. In one aspect, a method for producing a conductive material slurry of the present disclosure includes the steps of: dissolving a dispersant of the present disclosure in an organic solvent to obtain a dispersant solution; adding a conductive material to the dispersant solution; and performing a dispersion treatment using a mixer/disperser on the dispersant solution to which the conductive material has been added.

The mixing and dispersing machine may be at least one selected from the group consisting of an ultrasonic homogenizer, a vibration mill, a jet mill, a ball mill, a bead mill, a sand mill, a roll mill, a homogenizer, a high-pressure homogenizer, an ultrasonic device, an attritor, a dissolver, and a paint shaker.

The dispersion treatment is preferably carried out while circulating the crude dispersion by, for example, discharging it from the mixer-disperser and then injecting it back into the mixer-disperser. This circulation is preferably carried out 2 to 40 times, and the rate of discharging and injecting the dispersion is preferably 10 to 60 g/min.

In the method for producing a conductive material slurry disclosed herein, some of the components constituting the conductive material slurry may be mixed first and then mixed with the remainder, or each component may be added in multiple batches rather than all at once. The conductive material may be mixed with the other components in a dry state, or may be mixed with a solvent and then mixed with the other components. Examples of such solvents include the same organic solvents as described above. It is preferable that the binder is dissolved in a solvent, i.e., the binder solution is mixed with the other components. Examples of such solvents include the same organic solvents as described above.

When preparing a conductive material slurry of the present disclosure containing a binder, it is preferable to mix a binder solution with a crude dispersion containing a conductive material, a dispersant of the present disclosure, an organic solvent, and, as necessary, a nitrogen-containing organic compound or additives, etc., and it is preferable to agitate the mixture containing the crude dispersion and binder solution using a mixer or the like before supplying it to a mixer/disperser.

The viscosity of the conductive material slurry of the present disclosure at 25°C is preferably low, and from the viewpoint of improving handleability during preparation of the positive electrode paste, it is preferably 90 Pa·s or less, more preferably 70 Pa·s or less, and even more preferably 60 Pa·s or less.

<Positive electrode paste for energy storage devices>
In one aspect, the present disclosure relates to a positive electrode paste for an electrical storage device, comprising a conductive material slurry of the present disclosure and a positive electrode active material. The positive electrode paste of the present disclosure prepared using the conductive material slurry of the present disclosure has good dispersibility of CNTs, thereby reducing the coating resistance of the positive electrode mixture layer and improving high-temperature cycle characteristics.

In one aspect, the present disclosure relates to a positive electrode paste for an electricity storage device, comprising a positive electrode active material, a binder, a conductive material, the dispersant of the present disclosure, and an organic solvent, wherein the carbonaceous conductive material contains CNTs, and the content of the vinyl alcohol units in the positive electrode paste of the present disclosure is 20 to 100 parts by mass per 100 parts by mass of CNTs. "The content of the vinyl alcohol units in the positive electrode paste" refers to the total amount of vinyl alcohol units contained in the positive electrode paste of the present disclosure.

The positive electrode paste of the present disclosure contains the dispersant of the present disclosure in an amount such that the content of the vinyl alcohol units in the positive electrode paste of the present disclosure is 20 to 200 parts by mass per 100 parts by mass of CNT. This results in good dispersion of CNT in the positive electrode paste, reduced coating resistance of the positive electrode composite layer, and improved high-temperature cycle characteristics.

[Cathode active material]
The positive electrode active material may be any active material capable of absorbing and releasing lithium and capable of undergoing a charge-discharge reaction, and examples thereof include lithium metal composite oxides such as _LiCoO2 , _LiNiO2 , _LiMn2O4 , _ternary (NMC) LiNi _x Mn _y Co _{z O2} , Li-rich (Li(Li _x Me _1-x ) _O2 (Me = Co, Ni, Mn, etc.) and Ni-rich (LiNi _x Co _y Al _{z O2} ). The positive electrode active material is used as a granular material. The average particle size thereof may be, for example, 1 μm or more and 40 μm or less.

(Content of positive electrode active material in positive electrode paste)
The content of the positive electrode active material in the positive electrode paste of the present disclosure is, in terms of solid content, preferably 80% by mass or more, more preferably 90% by mass or more, from the viewpoint of increasing the energy density and capacity, and is preferably 99.5% by mass or less, more preferably 99% by mass or less, from the viewpoint of improving the binding strength of the positive electrode mixture layer to the current collector.

From the viewpoints of preventing aggregation of the positive electrode active material particles and maintaining the stability of the positive electrode paste, the content of the positive electrode active material in the positive electrode paste or the positive electrode composite layer of the energy storage device described below, expressed as the ratio of the mass of the positive electrode active material to the total mass of the vinyl alcohol units in the positive electrode paste or the positive electrode composite layer (positive electrode active material/vinyl alcohol units), is preferably 150 or more, more preferably 300 or more, and even more preferably 500 or more. Furthermore, from the viewpoints of preventing aggregation of the positive electrode active material particles in the positive electrode composite layer after solvent removal due to the positive electrode active material being covered with an excess amount of dispersant and preventing an increase in the coating resistance of the positive electrode composite layer, the content of the positive electrode active material in the positive electrode paste or the positive electrode composite layer, expressed as the mass ratio (positive electrode active material/vinyl alcohol units), is preferably 800 or less, more preferably 750 or less, and even more preferably 700 or less. The content of the positive electrode active material in the positive electrode paste, expressed as the mass ratio (positive electrode active material/vinyl alcohol unit), is preferably 150 to 800, more preferably 300 to 750, and even more preferably 500 to 700.

(Dispersant content in positive electrode paste)
The content of the dispersant (polymer) in the positive electrode paste of the present disclosure is, in terms of solid content, preferably 0.05% by mass or more, more preferably 0.10% by mass or more, from the viewpoint of reducing the resistance of the positive electrode mixture layer, and is preferably 1% by mass or less, more preferably 0.5% by mass or less, from the viewpoint of maintaining a high energy density of the power storage device. Specifically, the content of the dispersant (polymer) in the positive electrode paste of the present disclosure is preferably 0.05 to 1% by mass, more preferably 0.10 to 0.5% by mass.

(Conductive material content in positive electrode paste)
The content of the conductive material in the positive electrode paste of the present disclosure is preferably 0.3 mass % or more, more preferably 0.5 mass % or more, calculated as solid content, from the viewpoint of the conductivity of the positive electrode mixture layer, and is preferably 2 mass % or less, more preferably 1 mass % or less, from the viewpoint of maintaining a high energy density of the power storage device. Specifically, the content of the conductive material in the positive electrode paste of the present disclosure is preferably 0.3 to 2 mass %, more preferably 0.5 to 1 mass %.

(Binder content in positive electrode paste)
The content of the binder in the positive electrode paste of the present disclosure is preferably 0.5 mass% or more, more preferably 0.8 mass% or more, calculated as solids, from the viewpoints of the coating properties of the positive electrode mixture layer and the binding property with the current collector, and is preferably 1.5 mass% or less, more preferably 1.2 mass% or less, from the viewpoint of maintaining a high energy density of the electricity storage device. Specifically, the content of the binder in the positive electrode paste of the present disclosure is preferably 0.5 to 1.5 mass%, more preferably 0.8 to 1.2 mass%.

The positive electrode pastes of the present disclosure may further contain other components, as long as the effects of the present disclosure are not impaired. Examples of other components include antioxidants, neutralizing agents, antifoaming agents, preservatives, dehydrating agents, rust inhibitors, and plasticizers.

(Method for manufacturing positive electrode paste)
In one or more embodiments, the positive electrode paste of the present disclosure can be produced by mixing and stirring a positive electrode active material, a conductive material slurry of the present disclosure, a solvent (additional solvent) for adjusting the solid content, and, if necessary, a binder. The components may be mixed in any order. Examples of the solvent (additional solvent) include the same organic solvents as those described above, with NMP being preferred. A planetary mixer, a bead mill, a jet mill, or the like can be used for mixing and stirring, or these can be used in combination.

The positive electrode paste of the present disclosure can be prepared by premixing some of the components used in producing the positive electrode paste and then mixing the premixed components with the remainder. Alternatively, each component may be added in multiple batches rather than all at once. This reduces the mechanical load on the stirring device. The solids concentration of the positive electrode paste of the present disclosure can be adjusted according to the viscosity suitable for applying the positive electrode paste to the current collector. Alternatively, components other than the positive electrode active material may be mixed and dispersed until homogeneous, after which the positive electrode active material is added and stirred until further homogeneous. In one embodiment, the method for producing a positive electrode paste of the present disclosure includes a step of adding and dispersing a positive electrode active material in the conductive slurry of the present disclosure. In another embodiment, the method for producing a positive electrode paste of the present disclosure includes a step of adding a binder solution, in which a binder is dissolved in an organic solvent, to the conductive slurry of the present disclosure, mixing the two, and then adding and dispersing the positive electrode active material.

<Positive electrode for electricity storage device>
In one aspect, the present disclosure relates to a positive electrode for an electricity storage device, including a current collector and a positive electrode composite layer bonded to the current collector, the positive electrode composite layer being formed using the positive electrode paste of the present disclosure. Also, in one aspect, the present disclosure relates to a positive electrode for an electricity storage device, including a positive electrode composite layer, the positive electrode composite layer including a positive electrode active material, a carbon material conductive material, a dispersant of the present disclosure, and a binder, the carbon material conductive material including CNTs, and the content of the vinyl alcohol units in the positive electrode composite layer is 20 to 100 parts by mass per 100 parts by mass of CNTs. The "content of the vinyl alcohol units in the positive electrode composite layer" refers to the total amount of vinyl alcohol units contained in the positive electrode composite layer, and can be considered as the total amount of vinyl alcohol units contained in the positive electrode paste used to form the positive electrode composite layer.

The positive electrode for an electricity storage device of the present disclosure contains the dispersant of the present disclosure in an amount such that the content of the vinyl alcohol units in the positive electrode mixture layer is 20 to 100 parts by mass per 100 parts by mass of CNT. This results in good dispersibility of CNT in the positive electrode mixture layer, reduced coating resistance of the positive electrode mixture layer, and improved high-temperature cycle characteristics.

In the manufacturing method of the positive electrode for an electricity storage device disclosed herein, the positive electrode paste disclosed herein is applied to a current collector, such as aluminum foil, and then dried to form a positive electrode composite layer. To increase the density of the positive electrode, compaction can be performed using a press. A die head, cone reverse roll, direct roll, gravure roll, etc. can be used to apply the positive electrode paste. Drying after application can be performed using heating, air flow, infrared irradiation, etc., alone or in combination. The positive electrode can be pressed using a roll press, etc.

The present invention will be explained in more detail below through manufacturing examples, working examples, and comparative examples.

<Production of Dispersant>
(Dispersant a1)
A copolymerization reaction was carried out at 60°C in a reaction vessel equipped with a thermometer, a reflux condenser, a nitrogen gas inlet tube, and a stirrer using 95 parts by mass of vinyl acetate and 5 parts by mass of 2-hydroxyethyl methacrylate (monomer (C)) as polymerizable monomers, methanol as a solvent, and azobisisobutyronitrile as a polymerization initiator, to obtain a resin solution. Next, a methanol solution of sodium hydroxide was added to the resin solution to carry out a saponification reaction, and after thorough washing, the solution was dried in a hot air dryer to obtain a dispersant a1 having a degree of polymerization of 300 and the composition shown in Table 1.
6 parts of the obtained dispersant a1 (6 parts solid content) were added to 962 parts of N-methyl-2-pyrrolidone (NMP) (solvent) heated to 80°C and mixed to completely dissolve the dispersant a1. The time required for complete dissolution was 4 hours.
The time required for complete dissolution was measured by mixing 6 parts of dispersant a1 with 962 parts of N-methyl-2-pyrrolidone (NMP), heating the mixture to 80°C, and measuring the time required for the presence of aggregates, which are undissolved matter, to disappear when visually observed.

Dispersants a2 to a9 listed in Table 1 were prepared by controlling the amounts (parts by mass) of vinyl acetate and monomer (C) used and the saponification reaction described above (Dispersant a1). The time required for complete dissolution of the resulting dispersants in NMP is shown in Table 2.

<Preparation of CNT Slurry>
As examples of conductive material slurries, CNT slurries A1 to A12 in Table 2 were prepared by the following method.

(CNT Slurry A1)
To 100 g of an NMP solution of dispersant a1 (solid content 0.624 g), 2 g of multi-walled carbon nanotubes (MWCNT, average diameter 12 nm, average length 40 μm) as fibrous carbon nanostructures were added to obtain a crude dispersion. The crude dispersion containing MWCNT and dispersant a1 was then filled into a high-pressure homogenizer (manufactured by Biryu Co., Ltd., product name "BERYU MINI") equipped with a multistage pressure control device (multistage step-down device) that applies back pressure during dispersion, and the crude dispersion was subjected to a dispersion treatment at a pressure of 100 MPa. Specifically, while applying back pressure, shear force was applied to the crude dispersion to disperse the MWCNT, and CNT slurry A1 was obtained as a fibrous carbon nanostructure dispersion. The dispersion treatment was performed while circulating the dispersion by discharging it from the high-pressure homogenizer and injecting it back into the high-pressure homogenizer. This circulation was performed 20 times, and the discharge and injection rate of the dispersion was 30 g/min. The content (total amount) of vinyl alcohol units in the CNT slurry A1 was 28.1 parts by mass relative to 100 parts by mass of the CNT content.

(CNT Slurries A2 to A12)
CNT slurries A2 to A12 were each prepared using the same procedure as for preparing CNT slurry A1, so as to have the composition shown in Table 2. The nitrogen-containing organic compound used to prepare CNT slurry A7 was 2-amino-1,3-propanediol.

[Viscosity measurement]
The viscosity (25°C) of the conductive material slurry was measured as follows: An Anton Paar MCR302 rheometer equipped with a parallel plate PP50 was used, and the shear rate was increased from 0.1 ^s to 1000 ^s (forward run), and then the shear rate was decreased from 1000 ^s to 0.1 ^s (return run). The viscosity at a shear rate of 1 ^s on the return run was measured, and the results are shown in Table 2.

[Particle size measurement]
The CNT slurries A1 to A12 prepared as described above were each diluted 500 times with NMP. These were then placed in glass cells as measurement targets, and the cells were attached to a Malvern Panalytical Zetasizer Nano-S to measure the particle size of the CNTs at a temperature of 20°C.

[Preparation of positive electrode paste]
(Preparation of Positive Electrode Pastes B1 to B12)
7.5 g of CNT slurries A1 to A12, 4.3 g of NMP, and 3.0 g of PVDF (8%) NMP solution (KF Polymer L#7208, manufactured by Kureha Corporation) were weighed into a 50 ml sample bottle and uniformly stirred with a spatula. Then, 24.0 g of LCO (lithium cobalt oxide, manufactured by Nippon Chemical Industry Co., Ltd., CellSeed C-8hV) was added as the positive electrode active material, and the mixture was again stirred with a spatula until uniform. Further stirring was performed for 5 minutes using a planetary centrifugal mixer (AR-100, manufactured by Thinky Corporation), and positive electrode pastes B1 to B12 were obtained, respectively.

The contents (mass %) of the positive electrode active material, binder (PVDF), dispersant (polymer), and conductive material (CNT) in positive electrode pastes B1 to B12, converted to solid content, are as follows:

(mass ratio)
Positive electrode paste B1 to B4, B7, B8, B10 to B12 (solid content concentration: 62% by mass)
Positive electrode active material: binder: dispersant: conductive material=98.23:0.98:0.19:0.60
Positive electrode pastes B5, B6, and B9 (solid content: 62% by mass)
Positive electrode active material: binder: dispersant: conductive material=97.82:0.98:0.60:0.60

[Measurement of Coating Resistance Value of Positive Electrode Composite Layer]
The positive electrode paste prepared according to the above [Preparation of Positive Electrode Paste] was dropped onto a polyester film and uniformly coated with a 100 μm applicator. The coated polyester film was dried at 100 ° C for 1 hour to obtain a positive electrode composite layer with a thickness of 40 μm. The coating resistance was measured at a limit voltage of 10 V using a Loresta-GP (manufactured by Mitsubishi Chemical Analytical Co., Ltd.) equipped with a PSP probe. The results are shown in Table 2.

[Preparation of electrode (positive electrode) and battery]
The positive electrode paste prepared according to the above [Preparation of Positive Electrode Paste] was applied to a 20 μm thick Al foil to a positive electrode capacity of 3 mAh/ ^cm² and then vacuum dried at 100°C for 12 hours using a vacuum dryer to produce an electrode material (positive electrode material) with a composite layer formed on a current collector. This positive electrode material was punched and pressed to obtain an electrode (positive electrode). A 19 mm diameter separator and a 15 mm diameter 0.5 mm thick coin-shaped lithium metal were placed on the positive electrode to produce a 2032-type coin cell (test half cell). The electrolyte used was 1M LiPF6 EC/DEC (volume ratio) = 3/7.

[High-temperature cycle characteristic measurement]
The battery was charged to 4.2 V at 0.2 C in a 25°C environment and discharged to 3.0 V, a cycle of which was repeated three times. Then, in a 60°C environment, the battery was charged to 4.2 V at 1 C and discharged to 3.0 V at 1 C, a cycle of which was repeated 200 times. The capacity retention rate ΔC = (C1/C0) × 100 (%) was calculated from the first discharge capacity (C0) and the 200th discharge capacity (C1), and the high-temperature cycle characteristics were evaluated according to the following criteria. A higher capacity retention rate indicates less discharge capacity loss and better high-temperature cycle characteristics.
A: ΔC is 90% or more B: ΔC is 80% or more but less than 90% C: ΔC is 60% or more but less than 80% D: ΔC is less than 60% (Charge/discharge conditions)
30℃, 0.2C, charge 4.45V CC/CV 1/10C cutoff, discharge CC 3.0V cutoff

As shown in Table 2, the Examples, in which the content of vinyl alcohol units per molecule of the dispersant is 62-98% by mass, the content of structural units derived from HEMA is 1-25% by mass, 100 x a/(a + b) is 83-99% by mass, and 20-100 parts by mass of vinyl alcohol units are contained per 100 parts by mass of CNT, are generally more preferable than the Comparative Examples in terms of productivity, CNT dispersibility, coating film resistance, and high-temperature cycle properties.

The conductive material slurry of the present disclosure has good CNT dispersibility, which allows for a low viscosity of the conductive material slurry. Furthermore, when the conductive material slurry of the present disclosure is used to prepare a positive electrode paste, the viscosity of the positive electrode paste is also low, which can contribute to lowering the resistance of the positive electrode coating film. It can also contribute to improving high-temperature cycle characteristics.

Claims (11)

A conductive material slurry for an electricity storage device, comprising a carbon material-based conductive material, a dispersant, and an organic solvent,
the carbonaceous conductive material includes carbon nanotubes,
The dispersant is a polymer containing a vinyl alcohol unit, a vinyl acetate unit, and a structural unit represented by the following general formula (1):
The polymer contains, in one molecule, 62 to 98% by mass of the vinyl alcohol unit and 1 to 25% by mass of the constitutional unit represented by general formula (1),
a ratio of the mass of the vinyl alcohol unit to the total mass of the vinyl alcohol unit and the vinyl acetate unit in one molecule of the polymer is 83 to 99%;
The conductive material slurry for an electricity storage device has a content of the vinyl alcohol unit in the conductive material slurry of 20 to 100 parts by mass per 100 parts by mass of the carbon nanotubes.
In the above general formula (1), R ¹ is hydrogen or a methyl group, p is an integer of 1 or 2, and q is an integer of 2 or 3. Further containing a binder,
2. The conductive material slurry for a power storage device according to claim 1, wherein a content of the binder in the conductive material slurry is 150 to 800 parts by mass per 100 parts by mass of the vinyl alcohol unit in the conductive material slurry. The conductive material slurry for an electrical storage device according to claim 1 or 2, further comprising a nitrogen-containing organic compound. A positive electrode paste for an electricity storage device, comprising a positive electrode active material, a binder, a carbon material-based conductive material, a dispersant, and an organic solvent,
the carbonaceous conductive material includes carbon nanotubes,
The dispersant is a polymer containing a vinyl alcohol unit, a vinyl acetate unit, and a structural unit represented by the following general formula (1):
The polymer contains, in one molecule, 62 to 98% by mass of the vinyl alcohol unit and 1 to 25% by mass of the constitutional unit represented by general formula (1),
a ratio of the mass of the vinyl alcohol unit to the total mass of the vinyl alcohol unit and the vinyl acetate unit in one molecule of the polymer is 83 to 99%;
The positive electrode paste for an electricity storage device has a content of the vinyl alcohol unit in the positive electrode paste of 20 to 100 parts by mass per 100 parts by mass of the carbon nanotubes.
In the above general formula (1), R ¹ is hydrogen or a methyl group, p is an integer of 1 or 2, and q is an integer of 2 or 3. The positive electrode paste for an electricity storage device according to claim 4, wherein the content of the binder in the positive electrode paste is 150 to 800 parts by mass per 100 parts by mass of the vinyl alcohol units in the positive electrode paste. The positive electrode paste for an electricity storage device according to claim 4 or 5, wherein the ratio of the mass of the positive electrode active material to the total mass of the vinyl alcohol units in the positive electrode paste (positive electrode active material/vinyl alcohol units) is 150 to 800. The positive electrode paste for an electricity storage device according to any one of claims 4 to 6, further comprising a nitrogen-containing organic compound. A positive electrode for an electricity storage device including a positive electrode mixture layer,
the positive electrode mixture layer includes a positive electrode active material, a carbon material-based conductive material, a polymer, and a binder,
the carbonaceous conductive material includes carbon nanotubes,
The dispersant is a polymer containing a vinyl alcohol unit, a vinyl acetate unit, and a structural unit represented by the following general formula (1):
The polymer contains, in one molecule, 62 to 98% by mass of the vinyl alcohol unit and 1 to 25% by mass of the constitutional unit represented by general formula (1),
a ratio of the mass of the vinyl alcohol unit to the total mass of the vinyl alcohol unit and the vinyl acetate unit in one molecule of the polymer is 83 to 99%;
The positive electrode for an electricity storage device, wherein the content of the vinyl alcohol unit in the positive electrode mixture layer is 20 to 100 parts by mass per 100 parts by mass of the carbon nanotubes.
In the above general formula (1), R ¹ is hydrogen or a methyl group, p is an integer of 1 or 2, and q is an integer of 2 or 3. The positive electrode for an electricity storage device according to claim 8, wherein the content of the binder in the positive electrode mixture layer is 150 to 800 parts by mass per 100 parts by mass of the vinyl alcohol units in the positive electrode mixture layer. The positive electrode for an electricity storage device according to claim 8 or 9, wherein the ratio of the mass of the positive electrode active material to the total mass of the vinyl alcohol units in the positive electrode mixture layer (positive electrode active material/vinyl alcohol units) is 150 to 800. The positive electrode for an electricity storage device according to any one of claims 8 to 10, wherein the positive electrode mixture layer further contains a nitrogen-containing organic compound.