JP2021002518A - Manufacturing method of electrode paste for secondary battery - Google Patents

Abstract

To provide a manufacturing method of an electrode paste for a secondary battery which can suppress a decrease in the conductivity due to repeated use.SOLUTION: A manufacturing method of an electrode paste for a secondary battery including active material particles for a secondary battery, a binder, graphene, and a carbon nanotube includes at least a step A of preparing a dispersion by mixing active material particles for a secondary battery and a binder in a solvent, and a step B of mixing graphene and carbon nanotubes with the dispersion obtained in the step A.SELECTED DRAWING: None

Description

The present invention relates to a method for producing an electrode paste for a secondary battery and a method for producing an electrode for a secondary battery using the same.

Lithium-ion batteries, which are a type of secondary battery, are used for portable devices such as smartphones, mobile phones, and laptop computers; for automobiles such as hybrid vehicles and electric vehicles; for storage of renewable energy such as solar batteries and wind power. It is used for various purposes. Lithium-ion batteries applied to these applications are strongly required to improve the energy density per volume. For example, in mobile device applications, it is required to be able to be used for a long time while maintaining a compact shape, and even in automobile applications, it is necessary to realize a long cruising range of several hundred kilometers in a limited battery storage space. Is required.

One of the major issues of the secondary battery is to improve the life, that is, to maintain the battery capacity when charging and discharging are repeated. One of the causes of the decrease in battery capacity due to repeated use is the decrease in electron conductivity. Initially, the electrodes are densely clogged and therefore have high electron conductivity. However, repeated charging and discharging causes the active material to expand and contract, creating voids in the electrodes, and insulation by decomposition products with the electrolytic solution. Due to the formation of a film on the body, the electron conductivity is extremely reduced, and as a result, the battery capacity is reduced. In order to prevent such a decrease in battery capacity, it is considered important to initially form a conductive network that firmly connects the active materials and maintain this network against the expansion and contraction of the active materials. ..

It is known that nanocarbon is effective for forming a conductive network in a positive electrode. For example, it is a graphene powder to which a surface treatment agent is attached and has an oxidation potential of 4.3 V or more based on metallic lithium. Composite carbon obtained by combining graphene powder and carbon nanotubes having a voltage of .9 V or less, an electrode containing electrode active material particles and a binder (see, for example, Patent Document 1), carbon black, or at least one of graphene and carbon nanotubes. A positive electrode containing a first binder capable of dispersing composite carbon, a conductive composition containing a solvent, a positive electrode active material, and a second binder (see, for example, Patent Document 2), lithium-containing composite oxidation. A positive electrode active material for a lithium ion secondary battery consisting of carbon nanotubes, graphene, and particles coated with at least one carbon material selected from carbon black having an average dispersed particle diameter of 0.2 μm or less on the surface of the object ( For example, Patent Document 3) and the like have been proposed.

JP-A-2019-052083 JP-A-2015-115106 Japanese Unexamined Patent Publication No. 2012-169217

However, according to the production method specifically disclosed in Patent Document 1, since graphene, carbon nanotubes, active material and the binder are mixed at the same time, the graphene or carbon nanotubes and the binder aggregate to form a conductive path. Since it is difficult, there is a problem that the conductivity is lowered by repeated use. The technique disclosed in Patent Document 2 also has a problem that the conductivity is lowered by repeated use because graphene and carbon nanotubes are composited, so that they easily aggregate with the binder and it is difficult to form a conductive path. there were. In the technique disclosed in Patent Document 3, since the carbon material to be coated is unevenly present on the surface of the positive electrode active material, there is a problem that the conductivity is lowered by repeated use.

Therefore, an object of the present invention is to provide a method for producing an electrode paste for a secondary battery, which can suppress a decrease in conductivity due to repeated use.

In order to solve the above problems, the present invention mainly has the following configurations.
A method for producing an electrode paste for a secondary battery containing active material particles for a secondary battery, a binder, graphene and carbon nanotubes. At least Step A: Active material particles for a secondary battery and a binder are mixed and dispersed in a solvent. Step of preparing the liquid and step B: A method for producing an electrode paste for a secondary battery, which comprises a step of mixing graphene and carbon nanotubes in the dispersion liquid obtained in the step A in this order.

According to the present invention, it is possible to obtain an electrode for a secondary battery in which a decrease in conductivity due to repeated use is suppressed.

<Active material particles for secondary batteries>
Examples of the active material particles for a secondary battery used in the present invention (hereinafter, may be simply referred to as “active material particles”) include lithium manganate (LiMn ₂ O ₄ ) having a spinel type structure and manganese having a rock salt type structure. Lithium oxide (LiMnO ₂ ), lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), ternary system in which nickel is partially replaced with manganese / cobalt (LiNi _x Mn _y Co _1-x-y O ₂ ) some nickel with cobalt-aluminum-substituted ternary _{(LiNi x Co y Al 1-} x-y O 2), or a metal oxide active material such as _{V 2 O 5, TiS 2,} MoS 2, NbSe 2 Metal compound active materials such as Lithium iron oxide (LiFePO ₄ ) with olivine structure, lithium manganese phosphate (LiMnPO ₄ ), and Li ₂ MnO _3- LiMO ₂ (M is Ni, Co, Particles such as Mn) can be used. Two or more of these may be used.

Among these, those having many surface functional groups are preferable. Active material particles with many surface functional groups include lithium nickelate (LiNiO ₂ ), a ternary system in which nickel is partially replaced with manganese and cobalt (LiNi _x Mn _y Co _1-x-y O ₂ ), and nickel is cobalt. such as aluminum part substituted ternary _{(LiNi x Co y Al 1-} x-y O 2) layered oxide containing nickel and the like. In such an active material particle having many surface functional groups, since the binder is easily attached to the active material particle in the step A described later, it is easy to maintain the electrode structure, and the decrease in conductivity due to repeated use can be further suppressed. .. Since such active material particles interact more strongly in combination with a binder having an oxygen atom ratio of 0.2 atom% or more and 8.0 atom% or less, which will be described later, the decrease in conductivity due to repeated use is further suppressed. be able to.

The median diameter of the active material particles used in the present invention is preferably 5 μm or more and 20 μm or less. In the present specification, the median diameter of the active material particles is the median value obtained by magnifying and observing the active material particles using a scanning electron microscope (SEM) and measuring the diameters of 100 randomly selected active material particles. Can be obtained by calculating. Here, the diameter of the active material particle is an arithmetic mean value of the longest diameter and the shortest diameter of the observed individual active material particles.

The median diameter of the active material particles does not change in the electrode paste for the secondary battery and in the electrode in the general manufacturing method. When directly measuring the median diameter of the active material particles contained in the electrode, the surface of the electrode is observed by SEM, and the diameters of 100 active material particles randomly selected from the active material particles appearing on the surface are measured. , The median diameter can be obtained by calculating the median value.

The active material particles are preferably granules. The granulated material can be obtained, for example, by granulating a slurry in which powder is dispersed into a spherical shape by spray drying or the like. Since the primary particles of the active material particles, which are granules, are aggregated to form secondary particles, the surface tends to have an uneven shape. In the production method of the present invention, the binder adheres to the surface of the active material particles in step A described later, and graphene adheres to the surface of the active material particles and the binder in step B. In particular, since graphene has a sheet-like two-dimensional shape, the active material particles and graphene can come into contact with each other on the surface, and tend to easily follow the uneven shape of the surface. When the substance particles are used, the effect of the present invention is more remarkable. As the positive electrode active material preferably used as the granule, such as ternary _{(LiNi x Mn y Co 1-} x-y O 2) and _{LiNi x Co y Al 1-x} -y O 2 and the like.

<Binder>
Examples of the binder include polysaccharides such as carboxymethyl cellulose, hydroxypropyl cellulose, regenerated cellulose and diacetyl cellulose, thermoplastic resins such as polyvinyl chloride, polyvinylpyrrolidone, polyvinyl alcohol, polyethylene and polypropylene, polytetrafluoroethylene (PTFE) and polyphenyl. Fluorine-based polymers such as vinylidene (PVDF), ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, butadiene rubber, polymers with rubber elasticity such as fluororubber, polyimide precursors and / or polyimide resins, Examples thereof include polyamideimide resin, polyamide resin, polyacrylic acid, sodium polyacrylate, acrylic resin, polyacrylonitrile, and polyether such as polyethylene oxide. Two or more of these may be used. Among these, polyvinylidene fluoride (PVDF) is preferable.

The number average molecular weight of the binder is likely to adhere to the active material surface by appropriate viscosity, from a more suppressing the conductivity decreases due to repeated use, 2.0 × 10 ⁵ or more preferably, 3.0 × 10 ⁵ The above is more preferable. On the other hand, the number average molecular weight of the binder is to suppress aggregation, the more suppressing the conductivity decreases due to repeated use, preferably 2.0 × 10 ⁶ or less, more preferably 1.0 × 10 ⁶ or less. The number average molecular weight of the binder refers to a polystyrene-equivalent value and can be analyzed by gel permeation chromatography.

The binder preferably contains an oxygen atom, and easily interacts with graphene to maintain the electrode structure. The oxygen atom ratio of the binder is preferably 0.2 atom% or more, more preferably 0.5 atom% or more, still more preferably 1.0 atom% or more, from the viewpoint of interaction with graphene. On the other hand, the oxygen atom ratio of the binder is preferably 8.0 atomic% or less, more preferably 6.0 atomic% or less, still more preferably 4.0 atomic% or less, from the viewpoint of suppressing electrochemical decomposition. The oxygen atom ratio of the binder can be measured by X-ray photoelectron spectroscopy. As the excitation X-ray in the X-ray photoelectron spectroscopy measurement, the oxygen atom ratio is a value measured using monochromatic Al Kα1 and 2 rays (1486.6 eV) under the conditions of an X-ray diameter of 200 μm and a photoelectron escape angle of 45 °.

<Graphene>
Graphene, in a narrow sense, refers to a sheet of sp ^2- bonded carbon atoms having a thickness of one atom (single-layer graphene), but in the present specification, it also includes those having a flaky form in which single-layer graphene is laminated. It is called "graphene". Similarly, "graphene oxide" is also referred to as a name including those having a laminated flaky form. In the production method of the present invention, graphene adheres to a part of the surface of the active material particles or the binder in the step B described later, thereby improving the conductivity of the surface of the active material particles and reducing the conductivity due to repeated use. It can be suppressed.

The thickness of graphene used in the present invention is not particularly limited, but is preferably 100 nm or less, more preferably 50 nm or less, still more preferably, from the viewpoint of improving flexibility and making it easier to follow the uneven shape of the surface of the active material particles. It is 20 nm or less. The thickness of graphene can be determined as follows. First, graphene is diluted to 0.002% by weight with N-methylpyrrolidone (NMP), dropped onto a glass substrate, and dried. Then, the graphene on the substrate is observed using a laser microscope capable of measuring the three-dimensional shape, and the thickness of each graphene is measured. If there are variations in the thickness of individual graphene, calculate the area average. The thickness of 50 randomly selected graphenes is calculated in this way, and the arithmetic mean value is used as the graphene thickness.

In the present invention, the size of graphene in the plane direction is preferably 0.5 μm or more, more preferably 1 μm or more, from the viewpoint of improving conductivity by forming a conductive path. On the other hand, the size of graphene in the plane direction is preferably 10 μm or less, more preferably 5 μm or less, from the viewpoint of easily adhering to the surface of the active material particles and further suppressing a decrease in conductivity due to repeated use. The magnitude of graphene in the surface direction here refers to the average of the longest diameter and the shortest diameter of the graphene surface, and specifically means the average value measured according to Measurement Example 1 described later.

Graphene may be surface-treated, and as the surface-treating agent, a molecule containing a nitrogen atom is preferable. Molecules containing a nitrogen atom have a high affinity for NMP, which is widely used in battery fabrication, and are therefore preferably used for the dispersion of graphene. The molecule containing a nitrogen atom preferably has an aromatic ring from the viewpoint of improving the adsorptivity to the graphene surface. Examples of the compound containing a nitrogen atom include pyrazolone compounds, tertiary amines, secondary amines, and primary amines. More specifically, antipyrine, aminopyrine, 4-aminoantipyrine, 1-phenyl-3-methyl-5-pyrazolone, 4-benzoyl-3-methyl-1-phenyl-2-pyrazolin-5-one, 1-( 2-Chlorophenyl) -3-methyl-2-pyrazolin-5-one, 5-oxo-1-phenyl-2-pyrazolin-3-carboxylic acid, 1- (2-chloro-5-sulfophenyl) -3-methyl -5-Pyrazolone, 1- (4-chlorophenyl) -3-methyl-2-pyrazolin-5-one, 1- (4-sulfophenyl) -3-methyl-5-pyrazolone, 3-chloroaniline, benzylamine, Examples thereof include 2-phenylethylamine, 1-naphthylamine, dopamine hydrochloride, dopamine and dopa. Two or more of these may be used.

The specific surface area measured by the BET measurement method of graphene is preferably 80 m ² / g or more and 400 m ² / g or less, more preferably 100 m ² / g or more and 300 m ² / g or less, and 130 m ² / g or more and 200 m ² / g or less. Is even more preferable. The specific surface area of graphene reflects the thickness of graphene and the degree of exfoliation of graphene. The larger the specific surface area of graphene, the thinner the graphene and the higher the degree of exfoliation. When the specific surface area of graphene is 80 m ² / g or more, a conductive network of electrodes can be formed more easily. On the other hand, when the specific surface area of graphene is 400 m ² / g or less, the handleability can be improved. The BET measurement method is the method described in JIS Z8830: 2013, the adsorption gas amount is measured by the carrier gas method, and the adsorption data is analyzed by the one-point method. Specifically, measurement example 3 described later. Can be measured according to.

On the surface of graphene, hydroxy group (-OH), carboxyl group (-COOH), ester bond (-C (= O) -O-), ether bond (-C-O-C-), carbonyl group (-C) (= O)-), It is preferable to have an oxygen atom derived from a highly polar functional group such as an epoxy group. By having these functional groups on the graphene surface, it is possible to interact with the functional groups contained in the binder and the hydroxyl groups existing on the surface of the active material, and improve the binding property between the active material surface and the binder and graphene. The element ratio (O / C ratio) of oxygen to carbon measured by X-ray photoelectron spectroscopy of graphene is preferably 0.06 or more, more preferably 0.07 or more, and even more preferably 0.10 or more. On the other hand, the O / C ratio of graphene is preferably 0.30 or less, more preferably 0.20 or less, still more preferably 0.15 or less, from the viewpoint of improving the conductivity of graphene itself. Although a surface treatment agent may be added to graphene, not only the functional group of graphene itself but also the oxygen atom derived from the functional group of such a surface treatment agent is included in the "oxygen atom on the surface of graphene". It shall be. That is, in graphene to which the surface treatment agent is applied, the O / C ratio of the surface after the surface treatment agent treatment is preferably in the above range.

In X-ray photoelectron spectroscopy, the surface of a sample placed in an ultra-high vacuum is irradiated with soft X-rays, and the photoelectrons emitted from the surface are detected by an analyzer. Elemental information on the surface of a substance can be obtained by measuring the photoelectrons with a wide scan and obtaining the binding energy value of the bound electrons in the substance. Furthermore, the element ratio can be quantified using the peak area ratio. Specifically, the O / C ratio shall be measured according to Measurement Example 2 described later.

The O / C ratio can be adjusted, for example, when the chemical stripping method is used, by changing the degree of oxidation of graphene oxide as a raw material or changing the amount of the surface treatment agent. The higher the degree of oxidation of graphene oxide, the larger the amount of oxygen remaining after reduction, and the lower the degree of oxidation, the lower the amount of oxygen after reduction. Further, the larger the amount of the surface treatment agent having an acidic group attached, the larger the amount of oxygen can be.

Examples of the method for producing graphene used in the present invention include a method of directly peeling from graphite and a method of peeling graphite to be oxidized graphite to obtain graphene oxide and further reducing it. The latter method is preferable in that the size of graphene can be easily adjusted and graphene having a high specific surface area can be easily produced. As a method for producing graphene oxide, for example, any method such as the Hammers method can be used. Moreover, you may use commercially available graphene oxide.

<Carbon nanotube>
The electrode paste for a secondary battery in the present invention contains carbon nanotubes as a conductive auxiliary agent. By containing the carbon nanotubes, it is possible to form a conductive path between the active material particles and improve the conductivity of the entire electrode. In addition, by coexisting with graphene, the carbon nanotubes can be coated with graphene to further improve the conductivity between the active material particles, and a strong three-dimensional conductive network can be formed over the entire electrode. Become.

Examples of carbon nanotubes include single-walled carbon nanotubes, two-walled carbon nanotubes, multi-walled carbon nanotubes having three or more layers, and carbon nanotubes synthesized by the CVD method, which are also called vapor phase carbon fibers (for example, "VGCF"). (Registered trademark) (manufactured by Showa Denko Co., Ltd.) and the like. Two or more of these may be included.

The carbon nanotubes preferably have a shape that is flexible and easy to disperse. From this point of view, the outer diameter of the carbon nanotubes is preferably 3.0 nm or more and 50 nm or less, and more preferably 5.0 nm or more and 20 nm or less. When the outer diameter of the carbon nanotube is 3.0 nm or more, the bundle generated by the agglutination due to the van der Waals force can be easily unwound, the dispersibility can be improved, and the decrease in conductivity due to repeated use can be further suppressed. .. On the other hand, when the diameter of the carbon nanotube is 50 nm or less, the flexibility is high, and a conductive network between the active material particles and the conductive auxiliary agent can be more easily formed. In addition, the stress generated during electrode production can be reduced, and the decrease in conductivity due to repeated use can be further suppressed. The diameter of the carbon nanotube is the fiber diameter (outer diameter) of the carbon nanotube, and the carbon nanotube is magnified and observed at a magnification of 100,000 times using a field emission scanning microscope (FE-SEM), and is randomly selected. It can be obtained by measuring the outer diameter of 50 carbon nanotubes selected in 1 and calculating the arithmetic average value thereof.

The fiber length of the carbon nanotubes is preferably 5.0 μm or more and 50 μm or less, and more preferably 10 μm or more and 30 μm or less. When the fiber length is 5.0 μm or more, it is less affected by the contact resistance and the conductive network can be formed more easily, so that the decrease in conductivity due to repeated use can be further suppressed. On the other hand, when the fiber length is 50 μm or less, the dispersibility can be improved and the conductive network can be formed more easily, so that the decrease in conductivity due to repeated use can be further suppressed. The fiber length of the carbon nanotubes was measured by observing the carbon nanotubes at a magnification of 50,000 times using a field emission scanning microscope (FE-SEM) and measuring the lengths of 50 randomly selected carbon nanotubes. Then, it can be obtained by calculating the arithmetic average value.

The specific surface area of the carbon nanotubes is preferably 90 m ² / g or more and 350 m ² / g or less. When the specific surface area is 90 m ² / g or more, the conductive network can be formed more easily, so that the decrease in conductivity due to repeated use can be further suppressed. On the other hand, when the specific surface area is 350 m ² / g or less, the amount of the solvent when the carbon nanotube dispersion liquid is prepared by dispersing in a solvent can be reduced, the adhesion at the time of electrode formation is improved, and repeated use is used. The decrease in conductivity can be further suppressed. The BET measurement method shall be performed on carbon nanotubes by the method described in JIS Z8830: 2013, the adsorbed gas amount shall be measured by the carrier gas method, and the adsorption data shall be analyzed by the one-point method.

<Electrode paste for secondary batteries>
In the electrode paste for a secondary battery (hereinafter, may be simply referred to as "paste") in the present invention, the total content of graphene and carbon nanotubes as conductive aids is based on 100 parts by mass of active material particle content. , 0.3 parts by mass or more and 2 parts by mass or less is preferable. When the total content of graphene and carbon nanotubes is 0.3 parts by mass or more, it is easy to form a conductive path, and the conductivity can be further improved. The total content of graphene and carbon nanotubes is more preferably 0.5 parts by mass or more, and further preferably 0.7 parts by mass or more. On the other hand, when the total content of graphene and carbon nanotubes is 2 parts by mass or less, the electrode density can be improved and the energy per volume can be improved. The total content of graphene and carbon nanotubes is more preferably 1.5 parts by mass or less, and even more preferably 1.3 parts by mass or less.

In the electrode paste for a secondary battery in the present invention, the contents of graphene and carbon nanotubes are preferably about the same from the viewpoint of further improving the respective effects described above. Specifically, the content of graphene is preferably 10% by weight or more and 90% by weight or less, more preferably 20% by weight or more and 80% by weight or less, and 30% by weight or more and 70% by weight with respect to the total content of graphene and carbon nanotubes. % Or less is more preferable.

<Manufacturing method of electrode paste for secondary batteries>
The method for producing the electrode paste for a secondary battery of the present invention is at least
Step A: A step of mixing active material particles for a secondary battery and a binder in a solvent to prepare a dispersion, and Step B: A step of mixing graphene and carbon nanotubes with the dispersion obtained in Step A in this order. Have. Graphene has a large specific surface area and easily adheres to the binder, and graphene and carbon nanotubes easily adhere due to π-π interaction. Therefore, in the conventionally known method of mixing graphene, carbon nanotubes, active material particles, and a binder at the same time, graphene, carbon nanotubes, and a binder are mutually gathered and aggregated to adhere to the surface of the active material particles with sufficient bonding force. Was difficult. Therefore, when the produced electrode is used repeatedly, there is a problem that the conductive material is easily peeled off from the active material particles and the conductivity is lowered. In the present invention, after sufficiently adhering the binder to the surface of the active material particles in step A, graphene and carbon nanotubes are mixed in step B, so that the active material particles to which the binder is attached are coated with graphene, and the graphene and the binder are used. By interacting with each other, graphene can be fixed on the surface of the active material particles, and aggregation of carbon nanotubes is also suppressed. Therefore, peeling of the conductive material can be suppressed even after repeated use, and a decrease in conductivity can be suppressed. In step B, graphene and carbon nanotubes may be added in sequence and mixed, or may be added and mixed at the same time. Each step will be described in detail below.

First, in step A, the active material particles and the binder are mixed in a solvent to prepare a dispersion. By this step, the binder can be attached to the surface of the active material particles, and the aggregation of the electrode paste for the secondary battery can be suppressed. It is not necessary to attach the binder to the entire surface of the active material particles.

Examples of the solvent include water, dimethylacetamide, dimethylformamide, N-methylpyrrolidone and the like. Two or more of these may be used. Of these, N-methylpyrrolidone is preferable.

As a mixing device for mixing the active material particles and the binder, a device capable of applying a shearing force is preferable. For example, a planetary mixer, "Fillmix" (registered trademark) (Primix Corporation), a self-revolving mixer, a wet planetary ball mill, etc. Can be mentioned.

It is preferable to dissolve the binder to be mixed in the step A in the above-mentioned solvent in advance and use it as a binder solution, so that the binder can be uniformly adhered to the surface of the active material particles. In this case, the binder concentration of the binder solution used in step A is preferably 2% by mass or more and 20% by mass or less. When the binder concentration is 2% by mass or more, the viscosity of the binder solution can be appropriately increased and the dispersibility of the paste can be improved. The binder concentration is preferably 4% by mass or more. On the other hand, when the binder concentration is 20% by mass or less, the dispersibility of the binder in the binder solution can be improved. The binder concentration is more preferably 15% by mass or less, further preferably 10% by mass or less.

Next, in step B, graphene and carbon nanotubes are mixed with the dispersion liquid obtained in step A. Graphene has the property of easily adhering to the surface and easily interacts with the binder. On the other hand, carbon nanotubes have less interaction with the surface of active material particles than graphene, and have less interaction with binders. Therefore, graphene preferentially adheres to the surface of the active material particles, and the carbon nanotubes are dispersed in the paste. Therefore, graphene adheres to the surface of the active material particles in the electrode formed after the paste is applied and dried, and the carbon nanotubes are present between the active material particles. As a result, graphene can improve the surface conductivity of the active material particles, and carbon nanotubes can improve the conductivity between the active material particles. Further, since graphene firmly adheres to the surface of the active material particles via the binder, it is possible to suppress a decrease in conductivity due to repeated use.

It is preferable that the graphene to be mixed in the step B is dispersed in the above-mentioned solvent in advance and used as the graphene dispersion liquid, and the graphene is dispersed in a thin shape and easily adheres to the surface of the active material particles. In this case, the graphene concentration of the graphene dispersion is preferably 1% by mass or more and 20% by mass or less. When the graphene concentration is 1% by mass or more, the viscosity of the graphene dispersion can be appropriately increased and the dispersibility of the paste can be improved. The graphene concentration is more preferably 2% by mass or more, and further preferably 2.5% by mass or more. On the other hand, when the graphene concentration is 20% by mass or less, the dispersibility of graphene in the graphene dispersion can be improved. The graphene concentration is more preferably 10% by mass or less, further preferably 5% by mass or less.

It is preferable that the carbon nanotubes to be mixed in the step B are dispersed in the above-mentioned solvent in advance and used as the carbon nanotube dispersion liquid, and the dispersibility in the paste can be improved. In this case, the carbon nanotube concentration of the carbon nanotube dispersion liquid is preferably 1% by mass or more and 20% by mass or less. When the carbon nanotube concentration is 1% by mass or more, the viscosity of the carbon nanotube dispersion liquid can be appropriately increased, and the dispersibility of the paste can be improved. The carbon nanotube concentration is more preferably 2% by mass or more, and further preferably 2.5% by mass or more. On the other hand, when the carbon nanotube concentration is 20% by mass or less, the dispersibility of the carbon nanotubes in the carbon nanotube dispersion liquid can be improved. The carbon nanotube concentration is more preferably 10% by mass or less, and further preferably 5% by mass or less.

In step B, it is preferable to prepare a graphene carbon nanotube mixed dispersion by mixing the graphene dispersion and the carbon nanotube dispersion having the above-mentioned concentrations in advance, and mix it with the dispersion obtained in step A, and disperse in the paste. The sex can be improved.

Further, in step B, it is preferable to mix the graphene dispersion liquid and the dispersion liquid obtained in step A, and then mix the carbon nanotube dispersion liquid. By such a method, graphene can be easily formed on the positive electrode active material particles to which the binder is attached, and carbon nanotubes can be easily formed on the graphene particles, so that the decrease in conductivity due to repeated use can be further suppressed. it can.

Next, the method for manufacturing the electrode for the secondary battery of the present invention will be described. The method for manufacturing an electrode for a secondary battery of the present invention includes a step of applying the electrode paste for a secondary battery obtained by the above method to a current collector and drying it. Examples of the current collector include metal foil and metal mesh, and aluminum foil is preferable.

By using the electrode paste for a secondary battery obtained by the production method of the present invention, a binder is sufficiently adhered to the surface of the active material particles in the electrode, graphene partially covers the surface of the active material particles, and carbon nanotubes are used. Is distributed over the entire electrode except the surface of the active material particles. By having such a configuration, the mechanical strength improving effect of the binder, the surface conductivity improving effect of graphene, and the in-electrode conductivity improving effect of carbon nanotubes are exhibited, and when used in a secondary battery, it is repeatedly used. It is possible to suppress the deterioration of conductivity.

<Measurement example>
[Measurement example 1: Size of graphene in the plane direction]
The graphene dispersion 1 obtained in Synthesis Example 2 was diluted to 0.002% by mass with an NMP solvent, dropped onto a glass substrate, dried, and adhered onto the substrate. The graphenes on the substrate were magnified and observed with a 50x objective lens using a Keyence laser microscope VK-X250, and for each of the 50 randomly selected graphenes, the length of the longest part of the small piece (major axis, μm) and the length of the shortest part (minor axis, μm) were measured. (Major diameter + minor diameter) / 2 was calculated, and the average value was taken as the size (μm) in the plane direction of graphene.

[Measurement example 2: Graphene O / C ratio]
The graphene dispersion 1 obtained in Synthesis Example 2 was vacuum dried to obtain a graphene powder. The obtained graphene powder was subjected to X-ray photoelectron spectroscopy measurement using an X-ray photoelectron spectrometer Quantera SXM (manufactured by PHI), and the O / C ratio was determined from the oxygen atom ratio and the carbon atom ratio. The excited X-rays were monochromatic Al Kα1 and 2 rays (1486.6 eV), the X-ray diameter was 200 μm, and the photoelectron escape angle was 45 °.

[Measurement example 3: Specific surface area of graphene]
The reduced graphene obtained in Synthesis Example 2 was freeze-dried to obtain graphene powder. The specific surface area of the obtained graphene powder was measured using a fully automatic specific surface area measuring device HM Model-1210 (manufactured by Macsorb). The measurement principle was the BET flow method (one-point method), and the degassing conditions were 100 ° C. x 180 minutes.

[Measurement example 4: Oxygen atom ratio of binder]
The PVDF powder used in each Example and Comparative Example was subjected to X-ray photoelectron spectroscopy measurement using an X-ray photoelectron spectrometer Quantera SXM (manufactured by PHI) to determine the oxygen atom ratio. However, in the examples using the PVDF dispersion, the PVDF powder obtained by vacuum-drying the PVDF dispersion was measured. The excited X-rays were monochromatic Al Kα1 and 2 rays (1486.6 eV), the X-ray diameter was 200 μm, and the photoelectron escape angle was 45 °.

[Measurement Example 5: Number Average Molecular Weight of Binder]
For PVDF used in each Example and Comparative Example, polystyrene-equivalent number average molecular weight was measured using gel permeation chromatography (GPC) using dimethylformamide as a solvent and polystyrene as a standard substance.

[Measurement example 6: Output characteristics / cycle characteristics]
The positive electrode plate produced in each Example and Comparative Example was punched out with a punch of φ15.9 mm to form a working electrode, and a lithium foil cut out to a diameter of 16.1 mm × a thickness of 0.2 mm was used as a counter electrode and cut out to a diameter of 17 mm. A 2032 type coin battery was produced using "SELGARD" (registered trademark) # 2400 (manufactured by Celgard) as a separator and a solvent of ethylene carbonate: diethyl carbonate = 7: 3 containing 1 M of LiPF6 as an electrolytic solution, and a charge / discharge tester was prepared. Charge / discharge measurement was performed by (TOSCAT-3100 manufactured by Toyo System Co., Ltd.).

In the charge / discharge measurement, the charging voltage was 4.3 V and the discharge voltage was 3.0 V, constant current charging was performed at 1 C up to the charging voltage, and then constant voltage charging was performed at the charging voltage. The end current of constant voltage charging was 0.01C. The discharge rate was set to 1C, and charging and discharging were repeated three times to determine the third discharge capacity. After that, the charging conditions were the same, the discharge rate was set to 5C, charging and discharging were repeated three times, the third discharge capacity was obtained, and the output characteristics were evaluated.

After the measurement of the above output measurement, the cycle characteristic evaluation was continued. Similarly, with the discharge rate set to 1C, charging and discharging were performed 500 times, the discharge capacity at the 500th time was obtained, and the cycle characteristics were evaluated.

<Synthesis example>
(Synthesis Example 1: Synthesis of graphene oxide)
Using 1500 mesh natural graphite powder (Shanghai Ichiho Ishikumi Co., Ltd.) as a raw material, 220 ml of 98% concentrated sulfuric acid, 5 g of sodium nitrate, and 30 g of potassium permanganate were added to 10 g of natural graphite powder in an ice bath. The mixture was mechanically stirred for 1 hour, and the temperature of the mixed solution was maintained at 20 ° C. or lower. This mixed solution is taken out from the ice bath, stirred in a water bath at 35 ° C. for 4 hours, and then the suspension obtained by adding 500 ml of ion-exchanged water is stirred at 90 ° C. for another 15 minutes using a hot plate stirrer. went. Finally, 600 ml of ion-exchanged water and 50 ml of hydrogen peroxide were added, heating was stopped, and the mixture was stirred with a stirrer for 5 minutes to obtain a graphene oxide dispersion. This was filtered, metal ions were washed with a dilute hydrochloric acid solution, acid was washed with ion-exchanged water, and washing was repeated until the pH reached 7, to prepare a graphene oxide gel.

(Synthesis Example 2: Preparation of Graphene Dispersion Solution 1)
The graphene oxide gel prepared in Synthesis Example 1 was diluted to a concentration of 30 mg / ml with ion-exchanged water and treated with an ultrasonic cleaner for 30 minutes to obtain a uniform graphene oxide dispersion.

20 ml of the graphene oxide dispersion and 0.3 g of dopamine hydrochloride as a surface treatment agent were mixed and treated with Homo Disper 2.5 type (manufactured by Primix Corporation) at a rotation speed of 3000 rpm for 60 minutes.

The graphene oxide dispersion was applied with ultrasonic waves for 40 minutes under the condition of an output of 300 W using an ultrasonic device UP400S (manufactured by hielscher).

The graphene oxide dispersion after sonication is diluted to 5 mg / ml with ion-exchanged water, 0.3 g of sodium dithionite is added to 20 ml of the diluted dispersion, and a reduction reaction is carried out at 40 ° C. for 1 hour. went. Then, the washing step of filtering using a vacuum suction filter, further diluting to 0.5% by mass with ion-exchanged water and suction filtration was repeated 5 times to obtain reduced graphene.

After cleaning, dilute to 0.5% by mass using NMP, and use "Filmix" (registered trademark) 30-30 type (manufactured by Primix Corporation) at a rotation speed of 40 m / s (shear velocity: 20000 per second) to 60. It was treated for seconds and suction filtered. After filtration, dilute to 0.5% by mass using NMP, treat with Homo Disper 2.5 type (manufactured by Primix Corporation) at a rotation speed of 3000 rpm for 30 minutes, and repeat suction filtration twice to disperse graphene. Liquid 1 (concentration: 3.0% by mass) was obtained.

When the size of the obtained graphene dispersion 1 in the plane direction of graphene was measured based on Measurement Example 1, it was 3 μm. Moreover, when the O / C ratio of graphene was measured based on Measurement Example 2, it was 0.12.

The specific surface area of the reduced graphene washed with water before the addition of NMP was measured according to Measurement Example 3 and found to be 180 m ² / g.

(Synthesis Example 3: Preparation of Carbon Nanotube Dispersion Solution 1)
Add 1.0 part by mass of polyvinylpyrrolidone to 4.0 parts by mass of FLoTube7010 (carbon nanotube manufactured by Canon, average diameter 7 to 11 nm, average length 10 to 20 μm), and further add 95 parts of NMP (manufactured by Mitsubishi Chemical Corporation). Fibrous carbon dispersion liquid by adding 0 parts by mass and treating with "Fillmix" (registered trademark) 30-30 type (Primix) at a peripheral speed of 40 m / s (shear velocity: 40,000 per second) for 60 seconds. 1 (concentration 4.0% by mass) was obtained.

(Synthesis Example 4: Preparation of Carbon Nanotube Dispersion Solution 2)
"VGCF" (registered trademark) -X (Showa Denko KK, vapor-grown carbon fiber, average diameter 150 nm, average length 6 μm) 1.0 part by mass of polyvinylpyrrolidone was added to 4.0 parts by mass, and NMP (NMP) Add 95.0 parts by mass of Mitsubishi Chemical Co., Ltd., and use "Filmix" (registered trademark) 30-30 type (Primix) at a peripheral speed of 40 m / s (shear speed: 40,000 per second) to 60. Fibrous carbon dispersion 2 (concentration 4.0% by mass) was obtained by treating for seconds.

(Synthesis Example 5: Preparation of Carbon Nanotube Dispersion Solution 3)
"VGCF" (registered trademark) -H (Showa Denko KK, vapor-grown carbon fiber, average diameter 150 nm, average length 6 μm) 1.0 part by mass of polyvinylpyrrolidone was added to 4.0 parts by mass, and NMP (NMP) Add 95.0 parts by mass of Mitsubishi Chemical Co., Ltd., and use "Filmix" (registered trademark) 30-30 type (Primix) at a peripheral speed of 40 m / s (shear speed: 40,000 per second) to 60. Fibrous carbon dispersion 3 (concentration 4.0% by mass) was obtained by the second treatment.

[Example 1]
100 parts by mass of LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (NCM) (Yumicoa, granulated product with a median diameter of 10 μm) as active material particles, and polyvinylidene polyvinylfluoride (number average molecular weight 7) as a binder solution. .4 × 10 ^5, added 2.0 parts by mass solution (5.0 mass%) in terms of the solid content of the oxygen atomic ratio 3.0%), to obtain a dispersion by mixing rotational speed 30rpm30 minutes using a planetary mixer (Step A).

To the obtained dispersion, 0.5 parts by mass of graphene dispersion 1 (3.0%) prepared in Synthesis Example 2 as a conductive auxiliary agent was added in terms of solid content, and then carbon nanotubes prepared in Synthesis Example 3 were added. 0.5 parts by mass of the dispersion liquid 1 was added in terms of solid content (step B), and NMP was additionally added as a solvent so as to have a total amount of 50 parts by mass. Mixing for minutes gave an electrode paste.

This electrode paste is applied to an aluminum foil (thickness 18 μm) using a doctor blade so as to have a texture of 2.0 mg / cm ² , dried at 80 ° C. for 30 minutes, pressed by a roll press machine, and vacuumed. Electrodes were obtained by vacuum drying at 120 ° C. for 5 hours using a dryer. The thickness of the electrode mixture layer (excluding aluminum foil) was 40 μm.

[Example 2]
In step B, an electrode paste and an electrode were obtained in the same manner as in Example 1 except that 0.5 parts by mass of carbon nanotube powder FLoTube7010 was added instead of the carbon nanotube dispersion liquid 1 prepared in Synthesis Example 3.

[Example 3]
In step A, except that 2.0 parts by mass of polyvinylidene fluoride powder (3.0% oxygen atom ratio) and 18 parts by mass of NMP were added instead of the solution of polyvinylidene fluoride (oxygen atom ratio 3.0%). An electrode paste and an electrode were obtained in the same manner as in Example 1.

[Examples 4 to 5]
In step B, the electrode paste and the electrode were obtained in the same manner as in Example 1 except that the addition amounts of the graphene dispersion liquid 1 and the carbon nanotube dispersion liquid 1 were changed so as to have the weights shown in Table 1 in terms of solid content. Got

[Example 6]
An electrode paste and an electrode were obtained in the same manner as in Example 1 except that polyvinylidene fluoride (oxygen atom ratio 0.2%) was used instead of polyvinylidene fluoride (oxygen atom ratio 3.0%) in step A. It was.

[Example 7]
In step B, an electrode paste and an electrode were obtained in the same manner as in Example 1 except that the carbon nanotube dispersion liquid 2 obtained in Synthesis Example 4 was added instead of the carbon nanotube dispersion liquid 1 prepared in Synthesis Example 3. ..

[Example 8]
Examples except that "VGCF" (registered trademark) -X (showa Denko Corporation vapor-grown carbon fiber, average diameter 150 nm, average length 6 μm) was added in place of carbon nanotube powder FLoTube7010 in step B. An electrode paste and an electrode were obtained in the same manner as in 2.

[Example 9]
Examples except that "VGCF" (registered trademark) -H (vapor-grown carbon fiber manufactured by Showa Denko KK, average diameter 150 nm, average length 6 μm) was added in place of carbon nanotube powder FLoTube7010 in step B. An electrode paste and an electrode were obtained in the same manner as in 2.

[Example 10]
Step A was carried out in the same manner as in Example 1 to obtain a dispersion liquid.

To the obtained dispersion, 0.5 parts by mass of the carbon nanotube dispersion 1 prepared in Synthesis Example 3 as a conductive auxiliary agent was added in terms of solid content, and then the graphene dispersion 1 prepared in Synthesis Example 2 (3. (0%) was added in an amount of 0.5 parts by mass in terms of solid content (step B), and NMP was additionally added as a solvent so as to have a total amount of 50 parts by mass, and then the rotation speed was 30 rpm using a planetary mixer. Mixing for 30 minutes gave an electrode paste. Using the obtained electrode paste, an electrode was obtained in the same manner as in Example 1.

[Example 11]
100 parts by weight of LiMn2O4 (the median diameter 7 [mu] m) as the active material particles, polyvinylidene fluoride as a binder solution (the number average molecular weight 7.4 × 10 ^5, the oxygen atomic ratio 3.0%) solution of the (5.0 wt%) 2.0 parts by mass was added in terms of solid content and mixed using a planetary mixer at a rotation speed of 30 rpm for 30 minutes to obtain a dispersion solution (step A).

To the obtained dispersion, 0.8 parts by mass of graphene dispersion 1 (3.0%) prepared in Synthesis Example 2 as a conductive auxiliary agent was added in terms of solid content, and then carbon nanotubes prepared in Synthesis Example 3 were added. 0.8 parts by mass of the dispersion liquid 1 was added in terms of solid content (step B), and 10 parts by mass of NMP was additionally added as a solvent, and then mixed for 30 minutes at a rotation speed of 30 rpm using a planetary mixer to make an electrode paste. Got Using the obtained electrode paste, an electrode was obtained in the same manner as in Example 1.

[Comparative Example 1]
After adding 0.5 parts by mass of the graphene dispersion prepared in Synthesis Example 2 in terms of graphene, 0.5 parts by mass of the carbon nanotube dispersion 1 prepared in Synthesis Example 3 was added as a binder solution. A solution (5.0% by mass) of vinylidene (oxygen atom ratio 3.0%) was added in an amount of 2.0 parts by mass in terms of solid content, and the mixture was mixed for 30 minutes at a rotation speed of 30 rpm using a planetary mixer. Then, 100 parts by mass of LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (NCM) (Yumicore, granules having a median diameter of 10 μm) was added as active material particles, and the total amount of NMP was added as a solvent. After additional addition to 50 parts by mass, the mixture was mixed with a planetary mixer at a rotation speed of 30 rpm for 30 minutes to obtain an electrode paste. Using the obtained electrode paste, an electrode was obtained in the same manner as in Example 1.

[Comparative Example 2]
An electrode paste and an electrode were obtained in the same manner as in Example 1 except that the graphene dispersion liquid 1 was not added and the carbon nanotube dispersion liquid 1 was added in an amount of 1 part by mass in terms of solid content.

[Comparative Example 3]
An electrode paste and an electrode were obtained in the same manner as in Example 1 except that the carbon nanotube dispersion liquid 1 was not added and the graphene dispersion liquid was added in an amount of 1 part by weight in terms of solid content.

[Comparative Example 4]
After adding 0.8 parts by mass of graphene dispersion 1 prepared in Synthesis Example 2 in terms of graphene, 0.8 parts by mass of carbon nanotube dispersion 1 prepared in Synthesis Example 3 was added as a binder solution. A solution (5.0% by mass) of vinylidene (oxygen atom ratio 3.0%) was added in an amount of 2.0 parts by mass in terms of solid content, and the mixture was mixed for 30 minutes at a rotation speed of 30 rpm using a planetary mixer. Then, 100 parts by mass of LiMn2O4 (median diameter 7 μm) was added as active material particles, and 10 parts by mass of NMP was additionally added as a solvent, and then mixed at a rotation speed of 30 rpm for 30 minutes using a planetary mixer to prepare an electrode paste. Obtained. Using the obtained electrode paste, an electrode was obtained in the same manner as in Example 1.

[Comparative Example 5]
The graphene dispersion 1 prepared in Synthesis Example 2 was added to 0.5 parts by mass in terms of graphene, and LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (NCM) (manufactured by Yumicore Co., Ltd., median diameter 10 μm) as active material particles. 100 parts by mass was added, and the mixture was mixed at a rotation speed of 30 rpm for 30 minutes using a planetary mixer. Next, 0.5 parts by mass of the carbon nanotube dispersion liquid 1 prepared in Synthesis Example 3 was added as a solid content, and a peripheral speed of 40 m / s (shear) was added using "Fillmix" (registered trademark) 30-30 type (Primix Corporation). Processing was performed at a speed of 40,000 per second for 60 seconds. Then, 2.0 parts by mass of polyvinylidene fluoride # 7200 (manufactured by Kureha Corporation) was added as a binder solution (5.0% by mass) as a solid content, and NMP was additionally added as a solvent to 10 parts by mass. , "Fillmix" (registered trademark) 30-30 type (Primix Corporation) was treated at a peripheral speed of 40 m / s (shear velocity: 40,000 per second) for 60 seconds to obtain an electrode paste. Using the obtained electrode paste, an electrode was obtained in the same manner as in Example 1.

Table 1 shows the main compositions and evaluation results of each Example and Comparative Example.

Claims (13)

A method for producing an electrode paste for a secondary battery containing active material particles for a secondary battery, a binder, graphene and carbon nanotubes. At least Step A: Active material particles for a secondary battery and a binder are mixed and dispersed in a solvent. Step of preparing the liquid and step B: A method for producing an electrode paste for a secondary battery, which comprises a step of mixing graphene and carbon nanotubes in the dispersion liquid obtained in the step A in this order. The method for producing an electrode paste for a secondary battery according to claim 1, wherein in the step B, a graphene dispersion having a concentration of 1% by mass or more and 20% by mass or less is mixed. The method for producing an electrode paste for a secondary battery according to claim 1 or 2, wherein in the step B, a carbon nanotube dispersion liquid having a concentration of 1% by mass or more and 20% by mass or less is mixed. In the step B, the graphene dispersion having a concentration of 1% by mass or more and 20% by mass or less and the dispersion obtained in the step A are mixed, and then the carbon nanotube dispersion having a concentration of 1% by mass or more and 20% by mass or less is mixed. The method for producing an electrode paste for a secondary battery according to any one of claims 1 to 3. The method for producing an electrode paste for a secondary battery according to any one of claims 1 to 4, wherein in the step A, a dispersion is prepared using a binder solution having a concentration of 2% by mass or more and 20% by mass or less prepared in advance. The method for producing an electrode paste for a secondary battery according to any one of claims 1 to 5, wherein the number average molecular weight of the binder is 2.0 × 10 ⁵ or more and 2.0 × 10 ⁶ or less. The method for producing an electrode paste for a secondary battery according to any one of claims 1 to 6, wherein the oxygen atom ratio of the binder is 0.2 atomic% or more and 8.0 atomic% or less. The method according to any one of claims 1 to 7, wherein the total content of graphene and carbon nanotubes is 0.3 parts by mass or more and 2 parts by mass or less with respect to 100 parts by mass of the active material particle content for the secondary battery. A method for producing electrode paste for a secondary battery. The method for producing an electrode paste for a secondary battery according to any one of claims 1 to 8, wherein the active material particles for the secondary battery contain particles of a layered oxide compound containing nickel. The method for producing an electrode paste for a secondary battery according to any one of claims 1 to 9, wherein the elemental ratio of oxygen to carbon measured by X-ray photoelectron spectroscopy of graphene is 0.05 or more and 0.30 or less. .. The method for producing an electrode paste for a secondary battery according to any one of claims 1 to 10, wherein the graphene has a size of 0.5 μm or more and 10 μm or less in the plane direction. The method for producing an electrode paste for a secondary battery according to any one of claims 1 to 11, wherein the graphene has a specific surface area of 80 m ² / g or more and 400 m ² / g or less. A method for manufacturing an electrode for a secondary battery, which comprises a step of applying the electrode paste for a secondary battery according to any one of claims 1 to 12 to a current collector and drying the paste.