JP2022155939A - Method for manufacturing slurry for positive electrode of secondary battery - Google Patents

Abstract

To improve peel strength of a positive electrode active material layer manufactured by using slurry for a positive electrode of a secondary battery.SOLUTION: A method for manufacturing a slurry for a positive electrode of a secondary battery includes a step (I) and a step (II). The step (I) is a step of preparing an intermediate slurry by mixing (A) an olivine-type positive electrode active material, (B) carboxyalkyl-group-containing cellulose ether, (C) carbon nanotubes, and (D) an aqueous solvent. The step (II) is a step of further adding (C) carbon nanotubes to the intermediate slurry and mixing them.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a method for producing slurry for a secondary battery positive electrode.

The slurry composition for forming the electrode active material layer of the positive electrode of the secondary battery includes, for example, an electrode active material such as a lithium composite oxide, a conductive aid such as a carbon nanotube, and a carboxyalkyl group-containing cellulose such as carboxymethyl cellulose. It contains an ether or a salt thereof and cellulose fibers.

Patent Document 1 discloses, as a method for producing the slurry composition, a production method in which an electrode active material, a conductive aid, and a carboxyalkyl group-containing cellulose ether or a salt thereof are mixed in advance, and then cellulose fibers are mixed. there is Further, Patent Document 1 discloses a production method in which an electrode active material and a carboxyalkyl group-containing cellulose ether or a salt thereof are mixed in advance, then cellulose fibers are mixed, and then a conductive aid is mixed. .

JP 2018-116820 A

The inventors conducted intensive research on slurry for a secondary battery positive electrode, which is a mixture of a specific olivine-type polyanionic compound, which is a type of lithium composite oxide, a carboxyalkyl group-containing cellulose ether, carbon nanotubes, and water as an electrode active material. gone. As a result, in the procedure for preparing the slurry for the secondary battery positive electrode, by dividing the carbon nanotubes into a plurality of times and mixing them, the peel strength of the positive electrode active material layer produced using the slurry for the secondary battery positive electrode is improved. found to do.

A method for producing a slurry for a secondary battery positive electrode that achieves the above objects is a polyanionic compound having an olivine structure represented by the general formula LiM _h PO ₄ (M is Mn, Fe, Co, Ni, Cu, Mg , Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, Te and Mo, and h is a numerical value satisfying 0<h<2. ), mixing a carboxyalkyl group-containing cellulose ether, carbon nanotubes, and an aqueous solvent to prepare an intermediate slurry; and further adding the carbon nanotubes to the intermediate slurry and mixing.

At least part of the carbon nanotubes are preferably single-walled carbon nanotubes.
When the total amount of the carbon nanotubes contained in the secondary battery positive electrode slurry is 100 parts by mass, the amount of the carbon nanotubes mixed in the step of preparing the intermediate slurry is 50 to 70 parts by mass. is preferred.

Preferably, the polyanionic compound is _LiFePO4 .

ADVANTAGE OF THE INVENTION According to this invention, the peel strength of the positive electrode active material layer manufactured using the slurry for secondary battery positive electrodes can be improved.

An embodiment of the method for producing a slurry for a secondary battery positive electrode according to the present invention will be described below.
First, the secondary battery positive electrode slurry produced by the method for producing the secondary battery positive electrode slurry of the present embodiment will be described.

<Slurry for secondary battery positive electrode>
The secondary battery positive electrode slurry is a slurry used for producing a positive electrode for a secondary battery such as a nickel-hydrogen secondary battery or a lithium-ion secondary battery. In this embodiment, a case where the secondary battery is a lithium-ion secondary battery is exemplified.

The secondary battery positive electrode slurry is a slurry containing (A) an olivine structure positive electrode active material, (B) a carboxyalkyl group-containing cellulose ether, (C) a carbon nanotube, and (D) an aqueous solvent. Moreover, it is preferable that the slurry for secondary battery positive electrodes contains (E) water-based binder. Moreover, the slurry for secondary battery positive electrodes can contain other components as needed.

(A) Olivine type positive electrode active material The olivine type positive electrode active material is a polyanionic compound having an olivine type structure represented by the general formula LiM _h PO ₄ . M in the general formula LiM _h PO ₄ is at least selected from the group consisting of Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, Te, Mo It is a kind of element, and h is a numerical value that satisfies 0<h<2. Examples of the olivine-type positive electrode active material include olivine-type lithium iron phosphate (LiFePO ₄ ) and olivine-type lithium manganese iron phosphate (LiMnFePO ₄ ). These olivine structure positive electrode active materials can be used alone or in combination of two or more. The olivine structure positive electrode active material may be coated with a carbonaceous material on the particle surface in order to improve electrical conductivity.

The mass ratio of (A) the olivine-type structure positive electrode active material in the secondary battery positive electrode slurry is: (A) the olivine-type structure positive electrode active material in the secondary battery positive electrode slurry, (B) the carboxyalkyl group-containing cellulose ether, (C) It is preferable to set based on the total mass of the carbon nanotubes (hereinafter referred to as total main component). The mass ratio of (A) the olivine structure positive electrode active material in the secondary battery positive electrode slurry is, for example, 94.0 parts by mass or more and 99.5 parts by mass or less when the total of the main components is 100 parts by mass. It is preferably 96.0 parts by mass or more and 99.3 parts by mass or less, and even more preferably 98.0 parts by mass or more and 99.0 parts by mass or less.

(B) Carboxyalkyl Group-Containing Cellulose Ether Examples of carboxyalkyl group-containing cellulose ethers include carboxyalkyl cellulose, alkyl carboxymethyl cellulose, and hydroxyalkyl carboxymethyl cellulose. Examples of carboxyalkyl cellulose include carboxymethyl cellulose. Alkyl carboxymethyl cellulose includes, for example, methyl carboxymethyl cellulose. Hydroxyalkyl carboxymethyl cellulose includes, for example, hydroxyethyl carboxymethyl cellulose and hydroxypropyl carboxymethyl cellulose. These carboxyalkyl group-containing cellulose ethers may be salts thereof. Moreover, these carboxyalkyl group-containing cellulose ethers can be used alone or in combination of two or more.

It is preferable to set the mass ratio of (B) the carboxyalkyl group-containing cellulose ether in the secondary battery positive electrode slurry based on the total of the main components. The mass ratio of (B) the carboxyalkyl group-containing cellulose ether in the secondary battery positive electrode slurry is, for example, preferably 0.1 parts by mass or more and 5 parts by mass or less when the total amount of the main components is 100 parts by mass. It is more preferably 0.2 parts by mass or more and 4 parts by mass or less, and further preferably 0.3 parts by mass or more and 3 parts by mass or less.

(C) Carbon nanotubes Carbon nanotubes may be multi-walled carbon nanotubes (MWCNT) or single-walled carbon nanotubes (SWCNT). These carbon nanotubes can be used alone or in combination of two or more. At least part of the (C) carbon nanotubes contained in the secondary battery positive electrode slurry is preferably single-walled carbon nanotubes. In particular, it is preferable that all of (C) the carbon nanotubes are single-walled carbon nanotubes.

When single-walled carbon nanotubes and multi-walled carbon nanotubes are used in combination, (C) the mass ratio of single-walled carbon nanotubes to the total carbon nanotubes is, for example, preferably 1% or more and 99% or less, and 10% or more and 90%. The following are more preferable. Single-walled carbon nanotubes have better electrical conductivity than multi-walled carbon nanotubes. Therefore, by increasing the mass ratio of the single-walled carbon nanotubes in the entire carbon nanotubes (C), the conductivity of the positive electrode active material layer produced using the secondary battery positive electrode slurry is improved.

It is preferable to set the mass ratio of (C) carbon nanotubes in the secondary battery positive electrode slurry based on the total amount of the main components. The mass ratio of (C) carbon nanotubes in the secondary battery positive electrode slurry is, for example, preferably 0.01 parts by mass or more and 5 parts by mass or less when the total of the main components is 100 parts by mass, and 0.03 mass parts. It is more preferably 0.05 parts by mass or more and 2 parts by mass or less, more preferably 0.05 parts by mass or more and 2 parts by mass or less.

(D) Aqueous solvent The aqueous solvent is water or a mixed solvent of water and a non-aqueous solvent. Although water is not particularly limited, for example, ion-exchanged water, which is water treated with an ion-exchange resin, and ultrapure water, which is water treated with a reverse osmosis membrane water purification system, are preferable. Non-aqueous solvents constituting the mixed solvent include, for example, solvents miscible with water such as lower alcohols, acetone, tetrahydrofuran, ethylene glycol, N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, acetonitrile and dimethylsulfoxide. be done. These non-aqueous solvents can be used alone or in combination of two or more. The volume ratio of water in the mixed solvent is, for example, preferably 50% by volume or more and 99.9% by volume or less, more preferably 60% by volume or more and 99% by volume or less.

The ratio of the (D) aqueous solvent in the secondary battery positive electrode slurry is not particularly limited, but for example, the solid content concentration of the secondary battery positive electrode slurry is 30% by mass or more and 80% by mass or less. is preferable, and the ratio is more preferably 40% by mass or more and 70% by mass or less. In addition, the said solid content concentration means the density|concentration of the component (D) except the aqueous solvent in the slurry for secondary battery positive electrodes.

(E) Aqueous Binder The aqueous binder is a binder that can be dissolved or dispersed in an aqueous solvent. The water-based binder is not particularly limited, and a conventionally known material can be used as a water-based binder contained in the positive electrode active material layer of a lithium ion secondary battery. Examples of water-based binders include fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluororubber, thermoplastic resins such as polypropylene and polyethylene, imide resins such as polyimide and polyamideimide, and alkoxysilyl group-containing resins. , acrylic resins such as poly(meth)acrylic acid, styrene-butadiene rubber, alginates such as sodium alginate and ammonium alginate, water-soluble cellulose ester crosslinked products, and starch-acrylic acid graft polymers. Further, the water-based binder may be a material obtained by emulsifying the above-described resin that can be used as a water-based binder, for example, an acrylic emulsion obtained by emulsifying an acrylic resin. These water-based binders can be used alone or in combination of two or more.

It is preferable to set the mass ratio of (E) the water-based binder in the slurry for the secondary battery positive electrode based on the total of the main components. The mass ratio of (E) the water-based binder in the secondary battery positive electrode slurry is preferably 0.1 parts by mass or more and 9 parts by mass or less, for example, when the total of the main components is 100 parts by mass. It is more preferably 5 parts by mass or more and 7 parts by mass or less, and even more preferably 1 part by mass or more and 5 parts by mass or less.

(Other ingredients)
The slurry for the secondary battery positive electrode is not limited to the configuration containing only the above (A) olivine structure positive electrode active material, (B) carboxyalkyl group-containing cellulose ether, (C) carbon nanotubes, and (D) aqueous solvent, and any Other components may be included as components. Other components include, for example, (A) a positive electrode active material other than the olivine structure positive electrode active material (hereinafter referred to as other positive electrode active material).

Other positive electrode active materials may be those that can be used as a positive electrode active material for a lithium ion secondary battery that can occlude and release charge carriers such as lithium ions. Other positive electrode active materials include, for example, lithium composite metal oxides having a layered rock salt structure, metal oxides having a spinel structure, and the like. These other positive electrode active materials may have their particle surfaces coated with a carbonaceous material in order to improve electrical conductivity. Moreover, these positive electrode active materials can be used individually or in combination of two or more. When the other positive electrode active material is included, the mass ratio of the other positive electrode active material to the entire positive electrode active material contained in the secondary battery positive electrode slurry is, for example, 10% or more and 90% or less.

As described above, the secondary battery positive electrode slurry contains the above components (A) to (D), and preferably further contains component (E). Combinations of the above components (A) to (E) contained in the secondary battery positive electrode slurry include, for example, (A) LiFePO ₄ having a particle surface coated with a carbonaceous substance, (B) carboxymethyl cellulose, ( Combinations of C) single-walled carbon nanotubes, (D) water, and (E) acrylic emulsions are included.

<Method for producing slurry for secondary battery positive electrode>
A method for producing a secondary battery positive electrode slurry comprises mixing (A) an olivine-type structure positive electrode active material, (B) a carboxyalkyl group-containing cellulose ether, (C) a carbon nanotube, and (D) an aqueous solvent to prepare an intermediate slurry. and step (II) of preparing a secondary slurry by further adding (C) carbon nanotubes to the primary slurry and mixing. That is, in the method for producing the secondary battery positive electrode slurry, (C) the carbon nanotubes are divided into steps (I) and (II) and mixed. Moreover, the method for producing the slurry for a secondary battery positive electrode includes the step (III) of adding (E) an aqueous binder to the secondary slurry and mixing.

In step (I), (A) an olivine-type positive electrode active material, (B) a carboxyalkyl group-containing cellulose ether, (C) a portion of carbon nanotubes, and (D) an aqueous solvent are mixed and kneaded. to prepare a primary slurry. The amount of (C) carbon nanotubes mixed in step (I) is, for example, 30 parts by mass when the total amount of (C) carbon nanotubes contained in the produced secondary battery positive electrode slurry is 100 parts by mass. It is preferably from 50 parts by mass to 80 parts by mass, and more preferably from 50 parts by mass to 70 parts by mass. Further, the blending amount of (C) carbon nanotubes mixed in step (I) is, for example, 0.01 mass parts when the total of the main components of the secondary battery positive electrode slurry to be produced is 100 mass parts. It is preferably 4 parts by mass or more and 4 parts by mass or less.

The specific kneading method in step (I) is not particularly limited as long as it is a method that can uniformly knead each component contained in the primary slurry. You can apply the smelting method. Examples of the kneading method include manual kneading using a stirring rod, planetary mixer, homomixer, homodisper, Henschel mixer, Banbury mixer, ribbon mixer, V-type mixer, and rotation-revolution mixer. kneading by mechanical stirring using a conventional mixer, an ultrasonic disperser, or the like.

In step (II), the remaining (C) carbon nanotubes are further mixed with the primary slurry and kneaded to prepare a secondary slurry. The amount of (C) carbon nanotubes mixed in step (II) is the total amount of (C) carbon nanotubes mixed in step (I) from the total amount of (C) carbon nanotubes in the secondary battery positive electrode slurry to be produced. This is the amount obtained by subtracting the amount of nanotubes. Further, the amount of (C) carbon nanotubes mixed in step (II) is, for example, 0.01 parts by mass when the total of the main components of the secondary battery positive electrode slurry to be produced is 100 parts by mass. More than 3.5 parts by mass or less is preferable.

A specific kneading method in step (II) is not particularly limited, and a conventionally known kneading method used for producing slurry for a secondary battery positive electrode can be applied similarly to the kneading method in step (I).

In step (III), the secondary slurry is mixed with (E) an aqueous binder and kneaded to prepare a secondary battery positive electrode slurry.
A mass ratio ((E)/(C) ) is, for example, preferably 1 or more and 50 or less, more preferably 2 or more and 45 or less.

A specific kneading method in step (III) is not particularly limited, and a conventionally known kneading method used for producing slurry for a secondary battery positive electrode can be applied similarly to the kneading method in step (I).

When producing a secondary battery positive electrode slurry containing other components, the other components may be mixed in any of steps (I) to (III). Moreover, after step (III), a step (IV) of mixing a part or all of other components with the slurry prepared in step (III) may be provided. In the case of a method for producing a secondary battery positive electrode slurry that does not include the above step (IV), the slurry prepared in step (III) becomes the secondary battery positive electrode slurry. Moreover, when the manufacturing method of the slurry for secondary battery positive electrodes includes the said step (IV), the slurry prepared in step (IV) turns into the slurry for secondary battery positive electrodes.

(A) the olivine-type positive electrode active material and (B) the carboxyalkyl group-containing cellulose ether may also be divided into step (I) and step (II) and mixed. In this case, in each of the primary slurry and the secondary slurry, the mass ratio of (C) carbon nanotubes to the sum of (A) the olivine structure positive electrode active material and (B) the carboxyalkyl group-containing cellulose ether is higher than that of the primary slurry. The formulation of each material is adjusted so that the secondary slurry is higher.

Moreover, the form of each component to be blended in steps (I) to (IV) is not particularly limited, and can be appropriately selected based on the properties of each component. For example, it may be blended in the form of a single substance, or in the form of a mixed liquid such as a solution or a dispersion. As the solvent constituting the mixed liquid, the same solvents as those exemplified for (D) aqueous solvent can be used.

As an example, (C) carbon nanotubes have the property of having a strong cohesive force, so it is preferable to mix them in the form of a paste-like dispersion. By blending in the form of a dispersion, aggregation of (C) carbon nanotubes in the primary slurry or secondary slurry can be suppressed. When the (C) carbon nanotubes are blended in the form of a dispersion, the amount of the (C) carbon nanotubes is the mass of the solid content of the dispersion.

<Electrode for secondary battery positive electrode>
A secondary battery positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on at least one surface of the positive electrode current collector. The positive electrode active material layer is a layer formed from the slurry for a secondary battery positive electrode of the present invention. Therefore, the positive electrode active material layer includes (A) an olivine-type structure positive electrode active material, (B) a carboxyalkyl group-containing cellulose ether, and (C) carbon nanotubes, which are solids contained in the secondary battery positive electrode slurry. ing.

A positive electrode current collector is a chemically inert electrical conductor for continuing current flow to the positive electrode active material layer during discharging or charging of a lithium ion secondary battery. The positive electrode current collector is foil-shaped. The thickness of the foil-shaped positive electrode current collector is, for example, 1 μm or more and 100 μm or less, preferably 10 μm or more and 60 μm or less. As a material constituting the positive electrode current collector, for example, a metal material, a conductive resin material, a conductive inorganic material, or the like can be used. Examples of the metal material include copper, aluminum, nickel, titanium, and stainless steel. Examples of the conductive resin material include a resin obtained by adding a conductive filler to a conductive polymer material or a non-conductive polymer material as necessary.

The positive electrode current collector may comprise multiple layers including one or more layers containing the aforementioned metal material or conductive resin material. One or both surfaces of the positive electrode current collector may be coated with a known protective layer. One or both surfaces of the positive electrode current collector may be surface-treated by a known method such as plating. Examples of the surface treatment include chromate treatment and chromate phosphoric acid treatment. In this embodiment, the positive electrode current collector is made of aluminum foil.

The thickness of the positive electrode active material layer is, for example, 50 μm or more and 1000 μm or less, preferably 70 μm or more and 500 μm or less.
<Method for producing secondary battery positive electrode>
A method for manufacturing a secondary battery positive electrode has an active material layer forming step of applying the secondary battery positive electrode slurry of the present invention to at least one surface of a positive electrode current collector to form a positive electrode active material layer. In the active material layer forming step, a drying step of drying the coating layer of the slurry for the secondary battery positive electrode is normally performed after the coating step of applying the slurry for the secondary battery positive electrode.

Examples of the method for applying the secondary battery positive electrode slurry to the positive electrode current collector include a roll method, a die coating method, a reverse roll method, a doctor blade method, a knife method, a gravure method, a dip method, and a squeeze method. The secondary battery positive electrode slurry can be applied to one side or both sides of the positive electrode current collector. When applying to both surfaces of the positive electrode current collector, it may be applied one by one successively, or both surfaces may be applied simultaneously. Alternatively, the surface of the positive electrode current collector may be coated continuously or intermittently. The thickness, length and width of the coating layer of the secondary battery positive electrode slurry are appropriately set according to the size of the lithium ion secondary battery.

Examples of drying methods for the secondary battery positive electrode slurry include natural drying, low-temperature air, hot air, vacuum, infrared rays, far-infrared rays, electron beams, and microwaves. Two or more of these drying methods may be combined. The drying temperature is 20°C or higher and 120°C or lower, preferably 40°C or higher and 100°C or lower.

Furthermore, in order to increase the electrode density, a compression step of compressing the positive electrode active material layer may be performed after the drying step. Examples of the method for compressing the positive electrode active material layer include a mold press method and a calender press method. The pressing pressure is preferably 0.1 t/cm ² or more and 10 t/cm ² or less, more preferably 0.5 t/cm ² or more and 5.0 t/cm ² or less.

Between each step of the coating step, the drying step, and the compression step, and at least part of the timing after the compression step, a winding step of winding the electrode for a secondary battery positive electrode into a roll shape may be performed. Moreover, you may perform a drying process again after a compression process.

<Lithium ion secondary battery>
As a lithium ion secondary battery using the secondary battery positive electrode manufactured by the above-described manufacturing method, for example, a stacked power storage device in which a plurality of power storage cells are stacked is exemplified. The basic configuration of the power storage device will be described below.

A storage cell of the power storage device includes a negative electrode in which a negative electrode active material layer is formed on a foil-shaped negative electrode current collector, a positive electrode as a secondary battery positive electrode, a separator, a spacer, and an electrolyte. The negative electrode and the positive electrode are arranged such that the negative electrode active material layer and the positive electrode active material layer face each other with the separator interposed therebetween. The spacer is arranged so as to surround the positive electrode active material layer and the negative electrode active material layer, and is adhered to the positive electrode current collector and the negative electrode current collector. A closed space surrounded by a spacer, a positive electrode, and a negative electrode is formed inside the storage cell. The sealed space accommodates the separator and the electrolyte.

The power storage device includes a cell stack in which a plurality of power storage cells are stacked in a stacking direction, and a pair of current-carrying bodies sandwiching the cell stack in the stacking direction of the cell stack. A cell stack has a structure in which a plurality of storage cells are directly or indirectly stacked such that a negative electrode current collector and a positive electrode current collector are electrically connected. As a result, the plurality of storage cells forming the cell stack are connected in series. The pair of current-carrying bodies includes a positive current-carrying plate electrically connected to the positive current collector and a negative current-carrying plate electrically connected to the negative current collector. In the power storage device, charging and discharging are performed through terminals provided on each of the positive electrode plate and the negative electrode plate.

According to this embodiment, the effects described below can be obtained.
(1) A method for producing a slurry for a secondary battery positive electrode includes step (I) and step (II). Step (I) is a step of preparing an intermediate slurry by mixing (A) an olivine-type positive electrode active material, (B) a carboxyalkyl group-containing cellulose ether, (C) carbon nanotubes, and (D) an aqueous solvent. be. Step (II) is a step of further adding (C) carbon nanotubes to the intermediate slurry and mixing.

According to the manufacturing method, the mixed state of each component in the slurry for the secondary battery positive electrode can be made suitable for forming a positive electrode active material layer having high peel strength against the positive electrode current collector. Therefore, the peel strength of the positive electrode active material layer formed from the secondary battery positive electrode slurry is improved.

(2) (C) At least part of the carbon nanotubes are preferably single-walled carbon nanotubes.
Single-walled carbon nanotubes have better conductivity per unit mass than multi-walled carbon nanotubes. Therefore, when single-walled carbon nanotubes are used, the conductivity of the positive electrode active material layer formed from the secondary battery positive electrode slurry is improved even with a small blending amount. Therefore, the secondary battery positive electrode slurry obtained by this production method contributes to the improvement of the battery characteristics of the secondary battery.

(3) When the total amount of (C) carbon nanotubes contained in the secondary battery positive electrode slurry is 100 parts by mass, the amount of (C) carbon nanotubes mixed in the step of preparing the intermediate slurry is 50 to 70. Parts by mass are preferred.

In this case, the peel strength of the positive electrode active material layer formed from the secondary battery positive electrode slurry is further improved.
(4) (A) The olivine structure cathode active material is preferably _LiFePO4 .

In this case, the effect of improving the peel strength of the positive electrode active material layer formed from the secondary battery positive electrode slurry can be obtained more remarkably.
In addition, this embodiment can be changed and implemented as follows. This embodiment and the following modified examples can be implemented in combination with each other within a technically consistent range.

o Instead of the step (III) of mixing (E) an aqueous binder with the secondary slurry, (E) aqueous binding in the step (II) of mixing the remaining (C) carbon nanotubes with the primary slurry agents may be mixed. Also, (E) a water-based binder may be mixed in both step (II) and step (III).

o Step (III) of mixing (E) a water-based binder with the secondary slurry may be omitted. In this case, the produced secondary battery positive electrode slurry may be (E) a secondary battery positive electrode slurry that does not contain an aqueous binder, or may be mixed with (E) an aqueous binder at the time of use. The secondary battery positive electrode slurry to be used may also be used.

○ In the above embodiment, (C) the carbon nanotube was divided into two steps of step (I) and step (II) and mixed, but it may be divided into three or more times and mixed. For example, (C) carbon nanotubes may be further mixed in one or both of step (III) and step (IV) performed subsequent to step (II).

Examples that further embody the above embodiment will be described below.
<Example 1>
(Preparation of positive electrode slurry)
A positive electrode slurry containing LiFePO ₄ (LFP), carboxymethyl cellulose (CMC), single-walled carbon nanotubes (CNT), and acrylic emulsion (acrylic Em) at the solid content shown in Table 1 was prepared. First, as step (I), the total amount of LFP, the total amount of CMC, and a portion of CNT were mixed. To this mixture, water was added in such an amount that the solid content ratio of the finally prepared positive electrode slurry was 60% by mass. Then, by kneading at 10 rpm for 45 minutes using a planetary mixer, a primary slurry was obtained as an intermediate slurry.

Next, as step (II), the remainder of the CNTs was added to the primary slurry and kneaded at 10 rpm for 10 minutes using a planetary mixer to obtain a secondary slurry. The amount of CNTs in step (I) is 60 parts by mass when the total amount of CNTs (solid content) is 100 parts by mass, and the amount of CNTs in step (II) is 40 parts by mass. did. The CNTs in steps (I) and (II) were pastes with the same solid content ratio. Next, as step (III), the total amount of acrylic Em was added to the secondary slurry, and kneaded at 10 rpm for 10 minutes using a planetary mixer to obtain the positive electrode slurry of Example 1.

（正極用電極の作製）
実施例１の正極用スラリーを用いて正極用電極を作製した。まず、正極集電体となる厚さ１２μｍのアルミニウム箔の片側の面上に、アプリケータを用いて正極用スラリーを塗工した。その後、５０℃のホットプレートを用いて４５分間、加熱し、塗工された正極用スラリーを乾燥させることにより正極活物質層を形成した。正極用スラリーは、乾燥後の正極活物質層の目付け量が７０ｇ/ｃｍ^２となるように塗工した。得られた電極を実施例１の正極用電極とした。

(Preparation of positive electrode)
Using the positive electrode slurry of Example 1, a positive electrode was produced. First, an applicator was used to apply a positive electrode slurry onto one surface of a 12 μm thick aluminum foil serving as a positive electrode current collector. Thereafter, the positive electrode active material layer was formed by heating for 45 minutes using a hot plate at 50° C. and drying the coated positive electrode slurry. The positive electrode slurry was applied so that the weight of the positive electrode active material layer after drying was 70 g/cm ² . The obtained electrode was used as the positive electrode of Example 1.

<Example 2>
A positive electrode slurry of Example 2 was obtained in the same manner as in Example 1, except that the amounts of CNTs in steps (I) and (II) were changed. The amount of CNTs in step (I) is 20 parts by mass when the total amount of CNTs (solid content) is 100 parts by mass, and the amount of CNTs in step (II) is 80 parts by mass. did. Next, using the positive electrode slurry of Example 2, a positive electrode of Example 2 was obtained in the same manner as in Example 1.

<Comparative Example 1>
A positive electrode slurry of Comparative Example 1 was obtained in the same manner as in Example 1, except that no CNTs were added in step (I) and the entire amount of CNTs was added in step (II). Next, using the positive electrode slurry of Comparative Example 1, a positive electrode of Comparative Example 1 was obtained in the same manner as in Example 1.

<Measurement of peel strength>
Measurement samples were obtained by cutting the positive electrodes of Examples 1 and 2 and Comparative Example 1 into 2.5 cm×4 cm. Using a peel tester (LTS-50N-S300, manufactured by MINEBEA), a 90-degree peel test in accordance with JIS K 6854-1 in which the positive electrode current collector and the positive electrode active material layer of the measurement sample are peeled off. did The peel strength between the positive electrode active material layer and the positive electrode current collector in the measurement sample was calculated by dividing the strength measured in the 90-degree peel test by the line width of 2.5 cm. Table 2 shows the results.

<Measurement of Blackness of Surface Color of Positive Electrode Active Material Layer>
The blackness of the surface of the positive electrode active material layer of the positive electrodes of Examples 1 and 2 and Comparative Example 1 was measured using a reflection densitometer (RD-19 manufactured by Sakata Inx Engineering Co., Ltd.). Table 2 shows the results.

<Measurement of Density of Positive Electrode Active Material Layer>
The densities of the positive electrode active material layers of the positive electrodes of Examples 1 and 2 and Comparative Example 1 were measured. Table 2 shows the results.

表２に示すように、正極用スラリーの製造時にＣＮＴの全量をステップ（ＩＩ）で混合した比較例１と比較して、ステップ（Ｉ）及びステップ（ＩＩ）の二回に分割してＣＮＴを混合した実施例１及び実施例２は、正極活物質層の剥離強度が向上した。特に、ＣＮＴの全量を１００質量部とした場合のうちの５０～７０質量部のＣＮＴをステップ（Ｉ）で混合した実施例１は、ステップ（Ｉ）におけるＣＮＴの配合量が上記の範囲外となる量である実施例２と比較して、正極活物質層の剥離強度が更に向上した。

As shown in Table 2, compared with Comparative Example 1 in which the entire amount of CNTs was mixed in step (II) during the production of the positive electrode slurry, CNTs were mixed in two steps, step (I) and step (II). In Example 1 and Example 2, which were mixed, the peel strength of the positive electrode active material layer was improved. In particular, in Example 1 in which 50 to 70 parts by mass of CNTs were mixed in step (I) when the total amount of CNTs was 100 parts by mass, the amount of CNTs in step (I) was outside the above range. The peel strength of the positive electrode active material layer was further improved as compared with Example 2 in which the amount was

From these results, it can be seen that the peel strength of the positive electrode active material layer is changed by mixing the CNTs in two steps, step (I) and step (II), during the production of the positive electrode slurry. That is, by mixing LFP, CMC, and CNTs to prepare a primary slurry and then further mixing CNTs into the primary slurry, the peel strength of the positive electrode active material layer is increased. The degree of increase in peel strength varies depending on the amount of CNT compounded in step (I) and the amount and ratio of CNT compounded in step (II).

As described below, the change in the peel strength of the positive electrode active material layer is caused by the interaction between the CNTs floating in the positive electrode slurry and the acrylic Em, which is a water-based binder contained in the positive electrode slurry. It is assumed that

In the case of the positive electrode slurries of Examples 1 and 2, most of the CNTs mixed in step (I) adhered to the surfaces of the integrated particles in which the LFP and CMC that were mixed together were integrated. or exists inside the integrated particles. Most of the CNTs mixed in step (II) are present in the positive electrode slurry in a floating state. On the other hand, in the case of the positive electrode slurry of Comparative Example 1, most of the CNTs are present in a floating state in the positive electrode slurry because all the CNTs are mixed in step (II).

As data showing this point, the blackness of the surface of the positive electrode active material layer of Comparative Example 1 is higher than that of Examples 1 and 2. The high blackness of the surface of the positive electrode active material layer means that a large amount of CNTs migrate to the surface when the coating film of the positive electrode slurry is dried to form the positive electrode active material layer, that is, the CNTs floating in the slurry indicates that there are many

As described above, in the case of Examples 1 and 2, the ratio of CNTs in a floating state to the total CNTs in the positive electrode slurry decreases. As a result, it is possible to suppress a decrease in the binding strength of the water-based binder due to adhesion of the CNTs in a floating state to the water-based binder. As a result, the peel strength of the positive electrode active material layer is improved.

However, since the particles of the water-based binder are basically light, if the amount of CNTs adhered to the water-based binder is too small, when the positive electrode slurry coating film is dried to form the positive electrode active material layer, Migration occurs in which most of the water-based binder migrates to the surface of the positive electrode active material layer. Although detailed data are not shown, a test was conducted in which the proportion of CNTs in a floating state was significantly reduced by mixing the entire amount of CNTs in step (I). Migration of the water-based binder occurred. Therefore, it is necessary to allow CNTs in a floating state to exist to some extent in the positive electrode slurry.

Therefore, in order to improve the peel strength of the positive electrode active material layer, it is important to use a positive electrode slurry that contains CNTs in a floating state and that has a suppressed ratio of CNTs in a floating state to all CNTs. Become. As a method for producing such a positive electrode slurry, it is effective to mix CNTs in two steps, step (I) and step (II), as in Examples 1 and 2.

In addition, the positive electrode active material layers of the positive electrodes of Examples 1 and 2 have lower electrode densities than the positive electrode active material layers of the positive electrode of Comparative Example 1. When the electrode density of the positive electrode active material layer after the coating step is low, cracks are less likely to occur in the positive electrode active material layer when the positive electrode is wound into a roll. Incidentally, the electrode density can be finally adjusted by pressing the electrodes. Therefore, when the positive electrode is wound into a roll after the coating step, it is particularly effective to use the positive electrode slurries of Examples 1 and 2.

Claims (4)

A polyanionic compound having an olivine structure represented by the general formula LiM _h PO ₄ (M is Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, is at least one element selected from the group consisting of B, Te, and Mo, and h is a numerical value that satisfies 0<h<2.), a carboxyalkyl group-containing cellulose ether, carbon nanotubes, and an aqueous solvent are mixed. preparing an intermediate slurry by
further adding and mixing the carbon nanotubes into the intermediate slurry;
A method for producing a secondary battery positive electrode slurry comprising: 2. The method for producing slurry for a secondary battery positive electrode according to claim 1, wherein at least part of said carbon nanotubes are single-walled carbon nanotubes. When the total amount of the carbon nanotubes contained in the secondary battery positive electrode slurry is 100 parts by mass, the amount of the carbon nanotubes mixed in the step of preparing the intermediate slurry is 50 to 70 parts by mass. The method for producing the slurry for a secondary battery positive electrode according to claim 1 or 2. The method for producing a slurry for a secondary battery positive electrode according to any one of claims 1 to 3, wherein the polyanionic compound is LiFePO ₄ .