JP2024058944A - Anode mixture, method of manufacturing anode, anode and secondary battery - Google Patents

Abstract

[Problem] To provide a negative electrode mixture that can increase the capacity of a secondary battery and improve cycle durability. [Solution] The negative electrode mixture includes a negative electrode active material, a conductive agent, and a binder, wherein the negative electrode active material includes a metal-based active material, and the conductive agent includes carbon black having a BET specific surface area of 700 to 1500 m2/g and a DBP oil absorption of 300 to 600 ml/100 g, and single-walled carbon nanotubes. [Selected Figures] None

Description

The present invention relates to a negative electrode mixture, a method for manufacturing a negative electrode, a negative electrode, and a secondary battery.

To reduce the burden on the environment, hybrid and electric vehicles that use secondary batteries as power sources are being developed. There is a demand for improved driving range for electric vehicles, and secondary batteries with even higher capacities are being demanded.

As a secondary battery, lithium-ion secondary batteries are mainly used because they are easy to obtain high capacity and high durability. Normally, lithium-ion secondary batteries are composed of a positive electrode, a negative electrode, and an electrolyte, and the positive electrode and the negative electrode contain an active material, a conductive agent, and a binder. The active material is an important factor related to the capacity of a lithium-ion secondary battery, and conventionally, graphite has mainly been used as the negative electrode active material.

In recent years, metal-based active materials such as silicon-based active materials have been attracting attention as negative electrode active materials that can realize higher-capacity lithium-ion secondary batteries.
Patent Document 1 describes an anode composition that contains a silicon-based active material such as silicon (Si), a silicon alloy, silicon oxide, a silicon-carbon composite, or a silicon oxide-carbon composite; a carbon material such as carbon black, acetylene black, ketjen black, graphite, graphene, carbon nanotubes, or vapor-grown carbon fiber; and a specific binder.

JP 2019-522317 A

As disclosed in Patent Document 1, when a silicon-based active material is used as the negative electrode active material, the capacity of the secondary battery is improved compared to when a graphite active material is used. On the other hand, the silicon-based active material undergoes a large volume change accompanying the absorption and release of lithium during charging and discharging, and is prone to detachment from the electrode, peeling from the current collector, and disconnection of the conductive path of the electrode due to miniaturization, thereby reducing the cycle durability of the secondary battery.
Therefore, an object of the present invention is to provide a negative electrode mixture that can increase the capacity of a secondary battery and improve the cycle durability.

The present invention has the following aspects.
[1] A negative electrode mixture comprising a negative electrode active material, a conductive agent, and a binder, wherein the negative electrode active material comprises a metal-based active material, and the conductive agent comprises carbon black having a BET specific surface area of 700 to 1500 m ² /g and a DBP oil absorption of 300 to 600 ml/100 g, and single-walled carbon nanotubes.
[2] The negative electrode mixture according to [1], wherein the content of the single-walled carbon nanotubes is 1 to 20 mass% with respect to the total mass of the conductive agent.
[3] The negative electrode mixture according to [1] or [2], wherein the content of the conductive agent is 1 to 30 mass% with respect to the total mass of the negative electrode active material.
[4] The negative electrode mixture according to any one of [1] to [3], wherein the metal-based active material contains a Si-based active material.
[5] A method for producing a negative electrode, comprising a step of applying the negative electrode mixture according to any one of [1] to [4] above onto a current collector.
[6] A negative electrode comprising a negative electrode current collector and a negative electrode mixture layer present on the negative electrode current collector, the negative electrode mixture layer including a negative electrode active material, a conductive agent, and a binder, the negative electrode active material including a metal-based active material, and the conductive agent including carbon black having a BET specific surface area of 700 to 1500 m ² /g and a DBP oil absorption of 300 to 600 ml/100 g, and single-walled carbon nanotubes.
[7] A secondary battery comprising the negative electrode according to [6] above, a positive electrode, and an electrolyte present between the negative electrode and the positive electrode.

The present invention provides a negative electrode composite that can increase the capacity of a secondary battery and improve its cycle durability.

(Negative electrode mixture)
The negative electrode mixture of this embodiment is a composition containing a negative electrode active material, a conductive agent (hereinafter also referred to as "negative electrode conductive agent"), and a binder (hereinafter also referred to as "negative electrode binder"). The negative electrode mixture may contain a solvent (hereinafter also referred to as "negative electrode solvent") as necessary. Hereinafter, the negative electrode mixture containing a solvent is also referred to as "negative electrode mixture slurry".

<Negative Electrode Active Material>
In this embodiment, the negative electrode active material includes a metal-based active material, which is a material that is easier to increase the capacity of compared to carbon-based active materials such as graphite.
The metal-based active material is preferably a metal or metalloid or a compound thereof capable of alloying and dealloying with lithium. The metal or metalloid may be one or more selected from group (I) consisting of Si, Sn, Al, Sb, Bi, As, Ge and Pb. The metal-based active material may be one or more selected from group (I), a mixture of two or more selected from group (I), or an alloy of two or more selected from group (I). The compound may be an oxide of the metal, an oxide of the metalloid, a complex of the metal and the metalloid, or the like.
The surface of the metal-based active material may be coated with a carbon material.

From the viewpoint of facilitating the attainment of a higher capacity, it is preferable that the metal-based active material contains a Si-based active material.
Specific examples of the Si-based active material include silicon (Si), a mixture of silicon and one or more selected from group (I) (excluding silicon), an alloy of silicon and one or more selected from group (I) (excluding silicon), silicon oxide, silicon coated with a carbon material, and silicon oxide coated with a carbon material.

The negative electrode active material may include a metal-based active material and a carbon-based active material.
As the carbon-based active material, low-crystalline carbon, high-crystalline carbon, etc. can be used. Examples of low-crystalline carbon include softened carbon and hardened carbon. Examples of high-crystalline carbon include natural graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitch, petroleum coke, coal-based coke, etc.

The amount of the metal-based active material is preferably 5% by mass or more, more preferably 10% by mass or more, and may be 100% by mass, based on the total mass of the negative electrode active material.
The amount of the Si-based active material is preferably 50% by mass or more, more preferably 80% by mass or more, based on the total mass of the metal-based active material, and may be 100% by mass.

<Negative electrode conductive material>
In this embodiment, the negative electrode conductive agent includes carbon black and carbon nanotubes.

Carbon black
The type of carbon black is not particularly limited, and examples thereof include furnace black, channel black, thermal black, acetylene black, etc. Carbon black that has been subjected to a graphitization treatment or an oxidation treatment can also be used.
When producing the negative electrode mixture slurry, carbon black slurry in which carbon black is dispersed in a solvent using a dispersant can also be used.

The average primary particle size of the carbon black is preferably from 20 to 55 nm, and more preferably from 30 to 50 nm.
In this specification, the average primary particle size of carbon black is measured by the following method.
That is, the carbon black to be measured is added to chloroform, and ultrasonically treated for 10 minutes at 150 kHz and 0.4 kW using an ultrasonic disperser to prepare a dispersion sample, which is then sprinkled on and fixed onto a carbon-reinforced support film. The dispersion sample on the support film is photographed using a transmission electron microscope, and the particle sizes of 1,000 or more carbon black particles are measured randomly using an Ender device from the image magnified 50,000 to 200,000 times, and the average value is taken as the average primary particle size.

In this embodiment, the negative electrode conductive agent contains carbon black having a BET specific surface area of 700 to 1500 m ² /g and a DBP oil absorption of 300 to 600 ml/100 g (hereinafter also referred to as "carbon black (Z)").

The BET specific surface area of the carbon black (Z) is 700 m ² /g to 1500 m ² /g, preferably 1200 m ² /g to 1500 m ² /g. When the BET specific surface area of the carbon black is equal to or greater than the lower limit of the above range, a conductive network of the carbon black is sufficiently formed in the negative electrode mixture, making it easy to obtain good conductivity. When the BET specific surface area is equal to or less than the upper limit of the above range, an increase in the viscosity of the negative electrode mixture slurry is suppressed, the dispersibility of the carbon black is increased, and good conductivity is easily obtained.
In this specification, the BET specific surface area of carbon black is a value measured by the BET method using nitrogen adsorption, and is measured under conditions in accordance with ASTM D3037.

The DBP oil absorption of carbon black (Z) is 300 ml/100 g to 600 ml/100 g, preferably 350 ml/100 g to 550 ml/100 g. When the DBP oil absorption of carbon black is equal to or greater than the lower limit of the above range, a conductive network of carbon black is sufficiently formed in the negative electrode mixture, making it easier to obtain good conductivity. When the DBP oil absorption of carbon black is equal to or less than the upper limit of the above range, the viscosity increase of the negative electrode mixture slurry is suppressed, the dispersibility of carbon black is increased, and good conductivity is easily obtained.
In this specification, the DBP oil absorption of carbon black is a value measured under conditions in accordance with ASTM D 2414.

Examples of carbon black (Z) include, but are not limited to, Lion Specialty Chemicals Co., Ltd. product names "Ketjen Black EC300J", "Ketjen Black EC600JD", "Carbon ECP", and "Carbon ECP600JD".

In this embodiment, the negative electrode conductive agent may contain carbon black other than carbon black (Z) as long as the effects of the present invention are not impaired.

Carbon nanotubes
Carbon nanotubes are generally classified into single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes.
When producing the negative electrode mixture slurry, a carbon nanotube slurry (hereinafter, also referred to as "CNT slurry") in which carbon nanotubes are dispersed in a solvent using a dispersant can also be used.

In this embodiment, the negative electrode conductive agent includes a single-walled carbon nanotube. The single-walled carbon nanotube has a cylindrical shape of planar graphite rolled up, and has a structure in which one layer of graphite is rolled up.
The average outer diameter of the single-walled carbon nanotubes is preferably 0.1 to 10 nm, and more preferably 0.5 to 5 nm.
The average outer diameter of the single-walled carbon nanotubes in this specification can be determined, for example, by measuring the outer diameters of approximately 100 random carbon nanotubes using a transmission electron microscope and calculating the average value.

The BET specific surface area of the single-walled carbon nanotubes is preferably 200 m ² /g to 1500 m ² /g, and more preferably 400 m ² /g to 1200 m ² /g. When the BET specific surface area of the single-walled carbon nanotubes is equal to or greater than the lower limit, a conductive network of the single-walled carbon nanotubes is sufficiently formed in the negative electrode mixture, and good conductivity is easily obtained. When the BET specific surface area of the single-walled carbon nanotubes is equal to or less than the upper limit, the single-walled carbon nanotubes are easily disentangled in the dispersion step when preparing the negative electrode mixture slurry, and the structure of the single-walled carbon nanotubes is less likely to be destroyed, and good conductivity is easily obtained.
In this specification, the BET specific surface area of the carbon nanotubes is a value measured by the BET method using nitrogen adsorption, and is measured under conditions in accordance with ASTM D3037.

The carbon purity of the single-walled carbon nanotubes is preferably 70% by mass or more, and more preferably 80% by mass or more, relative to 100% by mass of the single-walled carbon nanotubes. By keeping the carbon purity within the above range, it is possible to prevent problems such as short circuits caused by the formation of dendrites due to impurities.

Examples of single-walled carbon nanotubes that satisfy the above average outer diameter, BET specific surface area, and carbon purity include, but are not limited to, Zeon Corporation's product name "ZEONANOS (registered trademark) SG101" and OCSiAl's product name "TUBALL (registered trademark)."

In this embodiment, the negative electrode conductive agent may contain carbon nanotubes other than single-walled carbon nanotubes as long as the effects of the present invention are not impaired.

<Anode conductive agent content>
The ratio of the negative electrode conductive agent added to the negative electrode mixture can be changed according to the ratio of the metal-based active material contained in the negative electrode mixture. The ratio of the negative electrode conductive agent added may be increased as the ratio of the metal-based active material added increases, and the ratio of the negative electrode conductive agent added may be decreased as the ratio of the metal-based active material added decreases.

The content of the negative electrode conductive agent is preferably 0.1% by mass to 30% by mass, and more preferably 1% by mass to 20% by mass, relative to the total mass of the negative electrode active material including the metal-based active material. By specifying the amount of the negative electrode conductive agent to be added within the above range, sufficient conductivity can be obtained without inhibiting the electrode reaction. In addition, excellent cycle durability can be obtained.

In this embodiment, the content of the single-walled carbon nanotubes is preferably 1 to 20 mass %, and more preferably 1 to 10 mass %, relative to the total mass of the negative electrode conductive agent.
In the present embodiment, the content of carbon black (Z) is preferably 80 to 99 mass %, and more preferably 90 to 99 mass %, relative to the total mass of the negative electrode conductive agent.

In this embodiment, the ratio of single-walled carbon nanotubes to the total mass of carbon black (Z) and single-walled carbon nanotubes ((mass of single-walled carbon nanotubes/(mass of carbon black (Z)+mass of single-walled carbon nanotubes)×100)) is not particularly limited, but is preferably 1 to 20 mass%, and more preferably 1 to 10 mass%. Within the above range, carbon black (Z) is suitable for sufficiently filling the gaps between the active material particles, and the single-walled carbon nanotubes are suitable for effectively reinforcing the surroundings. Therefore, the electronic conductivity in the negative electrode mixture layer is further improved, and the capacity expression rate is increased. In addition, the strength of the negative electrode mixture layer is further increased, and thus better cycle durability is obtained.

In the present embodiment, the negative electrode conductive agent may contain other conductive agents in addition to the carbon black (Z) and the single-walled carbon nanotubes, as long as the effects of the present invention are not impaired.
The content of the other conductive agent is preferably 50% by mass or less, more preferably 30% by mass or less, based on the total mass of the negative electrode conductive agent. It may be zero.
It is preferable that the negative electrode conductive agent does not contain any carbon nanotubes (double-walled or multi-walled carbon nanotubes) other than single-walled carbon nanotubes, or if it does contain any other carbon nanotubes, the amount is small.
For example, the content of the other carbon nanotubes relative to the single-walled carbon nanotubes is preferably 50% by mass or less, more preferably 30% by mass or less, or may be zero. When the content of the other carbon nanotubes is equal to or less than the upper limit, a high capacity expression rate and excellent cycle durability can be obtained.

<Negative electrode binder>
The negative electrode binder is not particularly limited, but preferred examples thereof include polyacrylic acid (PAA), lithium polyacrylate (PAALi), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), styrene butadiene rubber (SBR), polyvinyl alcohol (PVA), polyethylene oxide (PEO), polyethylene glycol (PEG), carboxymethyl cellulose (CMC), polyacrylonitrile (PAN), polyimide (PI), and polyethyleneimine (PEI).
The negative electrode binder may be used alone or in combination of two or more. When two or more types are used in combination, the combination and ratio may be appropriately selected depending on the purpose.

The proportion of the negative electrode binder added to the negative electrode mixture can vary depending on the proportion of the metal-based active material contained in the negative electrode mixture. The proportion of the binder added may be increased as the proportion of the metal-based active material added increases, and the proportion of the binder added may be decreased as the proportion of the metal-based active material added decreases.

The content of the negative electrode binder is preferably 1% by mass to 30% by mass, and more preferably 1% by mass to 20% by mass, relative to the total mass of the negative electrode active material and the negative electrode conductive agent. If the content of the negative electrode binder is equal to or greater than the lower limit of the above range, the adhesion between the negative electrode mixture layer and the current collector can be improved, and if it is equal to or less than the upper limit, the electronic conductivity in the negative electrode mixture layer can be improved.

<Negative electrode solvent>
The negative electrode solvent is a solvent for dissolving or dispersing the components constituting the negative electrode mixture, and any known solvent can be used as appropriate.
Specific examples of the negative electrode solvent include water, N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, and N-methylformamide. The negative electrode solvent may be used alone or in combination of two or more. When two or more types are used in combination, the combination and ratio may be appropriately selected depending on the purpose.

The amount of the negative electrode solvent added is preferably set so that the negative electrode mixture slurry has a desired viscosity.
For example, when the negative electrode mixture slurry is applied onto a negative electrode current collector and then heated and dried to remove the solvent to form a negative electrode mixture layer, it is preferable to set the content of the negative electrode solvent so as to obtain a viscosity suitable for coating.

<Optional components of negative electrode>
The negative electrode mixture may contain any optional components other than the above-mentioned negative electrode active material, negative electrode conductive agent, negative electrode binder, and negative electrode solvent.
Specific examples of optional components include dispersants and viscosity modifiers.
For example, when a CNT slurry containing a dispersant is used in producing the negative electrode composite slurry, the negative electrode composite may contain the dispersant derived from the CNT slurry.
Furthermore, when a carbon black slurry containing a dispersant is used in producing the negative electrode mixture slurry, the negative electrode mixture may contain the dispersant derived from the carbon black slurry.

The negative electrode mixture slurry may contain a viscosity modifier. Examples of the viscosity modifier include water-soluble polymers such as polyvinyl alcohol, carboxymethyl cellulose and its salts, methyl cellulose and its salts, and polymethacrylic acid and its salts. Specific examples of salts include alkali metals such as sodium and potassium.

The content of optional components is preferably 5% by mass or less, and more preferably 3% by mass or less, based on the total mass of the negative electrode active material, conductive agent, and negative electrode binder. It may be zero.

(Method of manufacturing negative electrode mixture)
The negative electrode mixture can be produced by uniformly mixing a negative electrode active material including a metal-based active material, a negative electrode conductive agent including carbon black (Z) and single-walled carbon nanotubes, a negative electrode binder, a solvent as required, and optional components as required.
The mixing method may be a known method, and since the components of the negative electrode mixture can be uniformly mixed, for example, a method of mixing using a ball mill, a sand mill, a twin-screw kneader, a rotation-revolution type mixer, a planetary mixer, a disperser mixer, a thin film swirling mixer, or the like can be used.

(Method of Manufacturing Negative Electrode)
The method for producing the negative electrode of this embodiment includes a step of applying the above-described negative electrode mixture of this embodiment onto a current collector to form a negative electrode mixture layer.
The negative electrode mixture to be applied onto the current collector is preferably a negative electrode mixture slurry containing a solvent. When using the negative electrode mixture slurry, it is preferable to form a negative electrode mixture layer by applying the negative electrode mixture slurry onto the negative electrode current collector to form a negative electrode mixture slurry layer, and then heat and dry the layer to remove the solvent contained in the negative electrode mixture slurry layer, thereby forming the negative electrode mixture layer.
After forming the negative electrode mixture layer, it is preferable to apply pressure using a roll press or the like in a direction in which the negative electrode mixture layer and the negative electrode current collector approach each other, thereby closely adhering the negative electrode mixture layer and the negative electrode current collector to form a negative electrode.
The negative electrode current collector can be formed using a known conductive material. For example, a metal foil such as a copper foil can be used as the negative electrode current collector.
The negative electrode mixture layer may be provided on one surface or both surfaces of the negative electrode current collector.

(Negative electrode)
The negative electrode of this embodiment includes a negative electrode current collector and a negative electrode mixture layer formed using the negative electrode mixture of this embodiment.
The negative electrode mixture layer includes a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder. It may further include optional components. The composition of the negative electrode mixture layer is the same as the composition of the solid content of the negative electrode mixture other than the solvent, except for the modification of the components that may occur during the manufacturing process of the negative electrode.
In the negative electrode mixture layer, the negative electrode active material and the negative electrode conductive agent are bound to the surface of the negative electrode current collector via a negative electrode binder. The negative electrode mixture layer is preferably a porous body.

(Secondary battery)
The secondary battery of this embodiment includes the negative electrode of this embodiment described above. Specifically, the secondary battery includes a negative electrode, a positive electrode, and an electrolyte present between the negative electrode and the positive electrode.
The secondary battery is preferably a non-aqueous electrolyte secondary battery in which the electrolyte is a non-aqueous electrolyte (electrolytic solution, gel electrolyte, etc.), and is preferably a lithium ion secondary battery.
The non-aqueous electrolyte secondary battery includes, for example, the negative electrode, a positive electrode, and a non-aqueous electrolyte (electrolytic solution, gel electrolyte, etc.), and may further include a separator between the negative electrode and the positive electrode, if necessary.

(Positive electrode and method for manufacturing the positive electrode)
The positive electrode includes a positive electrode current collector and a positive electrode mixture layer.
The positive electrode mixture layer can be manufactured by a known method using a positive electrode mixture. For example, the positive electrode mixture layer can be formed by applying a positive electrode mixture (hereinafter also referred to as "positive electrode mixture slurry") containing a solvent onto a positive electrode current collector to form a positive electrode mixture slurry layer, and then heating and drying to remove the solvent contained in the positive electrode mixture slurry layer. Furthermore, it is preferable to press the positive electrode mixture layer and the positive electrode current collector in a direction approaching each other using a roll press or the like, and to form a positive electrode by adhering the positive electrode mixture layer and the positive electrode current collector to each other.
The positive electrode current collector can be formed using a known conductive material. For example, a metal foil such as an aluminum foil can be used as the positive electrode current collector.
The positive electrode mixture layer may be provided on one surface or both surfaces of the positive electrode current collector.

(Positive electrode mixture)
The positive electrode mixture of the present embodiment is a composition containing a positive electrode active material, a conductive agent (hereinafter also referred to as a "positive electrode conductive agent"), and a binder (hereinafter also referred to as a "positive electrode binder") and may further contain a solvent (hereinafter also referred to as a "positive electrode solvent").
The composition of the positive electrode mixture layer and the composition of the solid content of the positive electrode mixture other than the solvent are the same, except for denaturation of the components that may occur during the manufacturing process of the positive electrode.
In the positive electrode mixture layer, the positive electrode active material and the positive electrode conductive agent are bound to the surface of the positive electrode current collector via a positive electrode binder. The positive electrode mixture layer is preferably a porous body.

<Positive electrode active material>
The positive electrode active material is not particularly limited, but may be a metal compound such as a metal oxide or metal sulfide that can reversibly dope or intercalate lithium ions.
For example, lithium and transition metal composite oxide powders such as lithium manganese composite oxide (e.g., Li _x Mn ₂ O ₄ or Li _x MnO ₂ ), lithium nickel composite oxide (e.g., Li _x NiO ₂ ), lithium cobalt composite oxide (Li x CoO ₂ ), lithium nickel cobalt composite oxide (e.g., Li _x Ni _1-y Co _y O ₂ ), lithium manganese cobalt composite oxide (e.g., Li _x Mn _y Co _1-y O ₂ ), lithium nickel manganese cobalt composite oxide (e.g., Li _x Ni _y Co _z Mn _1-y-z O ₂ ), and spinel-type lithium manganese nickel composite oxide (e.g., Li _x Mn _2-y Ni _y O ₄ ), lithium phosphate powders having an olivine structure (e.g., Li _x FePO ₄ , Li _x Fe _1-y Examples of the transition metal oxide powder _include powders of transition metal oxides _{such as manganese} oxide, iron oxide, copper oxide, _nickel oxide, _vanadium oxide (e.g., _V2O5 , _V6O13 ), and titanium oxide; and powders of transition metal sulfides such as iron sulfate ( _Fe2 ( _SO4 ) ₃ ), _TiS2 , and FeS.
Here, x, y, and z are numbers satisfying the following conditions: 0<x<1, 0<y<1, 0<z<1, and 0<y+z<1. These positive electrode active materials may be used alone or in combination.

<Positive electrode conductive agent>
The positive electrode conductive agent is not particularly limited, but any conductive material can be used.
For example, it is preferable to include at least one selected from the group consisting of channel black, thermal black, acetylene black, furnace black, carbon nanofiber, carbon nanotube, graphene, and the like.

<Positive electrode binder>
The positive electrode binder is not particularly limited, but any binder that is commonly used for positive electrodes can be used.
Examples of the binder include fluorine-based resins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), and polymers having unsaturated bonds such as styrene-butadiene rubber (SBR), isoprene rubber (IR), and butadiene rubber (BR). These binders can be used alone or in combination.

<Positive electrode solvent>
The positive electrode solvent is a solvent for dissolving or dispersing the components constituting the positive electrode mixture, and any known solvent can be used as appropriate.
Specific examples of the positive electrode solvent include N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, and N-methylformamide. The positive electrode solvent may be used alone or in combination of two or more. When two or more types are used in combination, the combination and ratio may be appropriately selected depending on the purpose.

The amount of the positive electrode solvent added is preferably set so that the positive electrode mixture slurry has a desired viscosity.
For example, when the positive electrode mixture slurry is applied onto a positive electrode current collector and then the solvent is removed to form a positive electrode mixture layer, it is preferable to set the content of the positive electrode solvent so as to obtain a viscosity suitable for coating.

<Optional components of positive electrode>
The positive electrode mixture may contain any optional components other than the above-mentioned positive electrode active material, positive electrode conductive agent, positive electrode binder, and positive electrode solvent.
Specific examples of optional components include dispersants and viscosity modifiers.

(Method of manufacturing positive electrode mixture)
The positive electrode mixture can be produced by uniformly mixing a positive electrode active material, a positive electrode conductive agent, a positive electrode binder, an optional solvent, and optional components.
The mixing method may be a known method, and the same method as the above-mentioned method for mixing the negative electrode mixture can be used.

(Separator)
The separator is placed between the positive electrode and the negative electrode. The material constituting the separator may be any material that has no electronic conductivity and has lithium ion conductivity. For example, nylon, cellulose, polysulfone, polyethylene, polypropylene, polybutene, polyacrylonitrile, polyimide, polyamide, and a composite material of two or more of these. The form of the separator may be a woven fabric, a nonwoven fabric, a microporous membrane, or the like. The separator may contain various plasticizers, antioxidants, flame retardants, or the like, or may be coated with a metal oxide or the like.

(Electrolyte)
The electrolytic solution is preferably a non-aqueous electrolytic solution in which an electrolyte is dissolved in an organic solvent.
As the organic solvent, it is possible to use an aprotic organic solvent, for example, one or more selected from propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. In addition, one or more selected from vinylene carbonate (VC), ethylene sulfite (ES), fluoroethylene carbonate (FEC), etc. may be added as an additive to the organic solvent.
The electrolyte contained in the electrolytic solution is not particularly limited, and _LiClO4 , _LiBF4 , _LiAsF6 , _{LiPF6 , LiCF3SO3} , LiN( _CF3SO2 ) ₂ , LiI _{, LiAlCl4} , and mixtures thereof can be used. A lithium salt containing one or a mixture of two or more of _LiBF4 and _LiPF6 is preferable.

(Secondary battery manufacturing method)
The secondary battery can be manufactured by a known method. For example, a non-aqueous electrolyte secondary battery can be manufactured by interposing a separator between a negative electrode and a positive electrode, sealing the battery with an outer casing, and injecting an electrolyte.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to the following description.

(Raw materials used)
<Negative electrode conductive material>
CB1: Carbon black, Lion Specialty Chemicals Co., Ltd. product name "Carbon ECP600JD", BET specific surface area 1387 m ² /g, DBP oil absorption amount 489 ml/100 g, average primary particle diameter 34 nm.
CB2: Carbon black, Imerys Graphite & Carbon product name "Super P (registered trademark) Li", BET specific surface area 62 m ² /g, DBP oil absorption amount 247 ml/100 g, average primary particle size 40 nm.
CB3: Carbon black, Denka Company Ltd. product name "Li-400", BET specific surface area 39 m ² /g, DBP oil absorption 140 ml/100 g, average primary particle diameter 48 nm.
Single-walled CNT1: single-walled carbon nanotube, OCSiAl product name "TUBALL (registered trademark) Batt H ₂ O", average outer diameter 1.6±0.4 nm, BET specific surface area 980 m ² /g, carbon purity >93 mass %.
Multi-walled CNT1: multi-walled carbon nanotube, Jiangsu Cnano Technology Co., Ltd. product name "LB217-54"), average outer diameter 7 to 11 nm, BET specific surface area 200 to 300 m ² /g, carbon purity >99 mass %.
Multi-walled CNT2: multi-walled carbon nanotubes, Jiangsu Cnano Technology Co., Ltd. product name "LB260-32", average outer diameter 5 to 11 nm, BET specific surface area 250 to 350 m ² /g, carbon purity > 95 mass %.

(Examples 1 to 4, Comparative Examples 1 to 6)
A negative electrode active material, a negative electrode conductive agent, a negative electrode binder, and a solvent as required were mixed to prepare a negative electrode mixture slurry.
The solid content composition of the negative electrode mixture slurry for each example is shown in Table 1.
The active material used was silicon particles (average particle size: 200 nm) and graphite particles (average particle size: 17 μm), and the binder used was polyacrylic acid.
Single-walled CNT1, multi-walled CNT1, and multi-walled CNT2 are CNT slurries dispersed in water using a dispersant, and the carbon nanotube content shown in the table is the mass of only the carbon nanotubes in the CNT slurry. The dispersant content shown in the table is the mass of only the dispersant in the CNT slurry.
The content of the solvent in the negative electrode mixture slurry was an appropriate amount for making the slurry suitable for coating.
In the table, "-" means that the component is not included.

Next, the obtained negative electrode mixture slurry was applied onto a copper foil (negative electrode current collector), heated and dried to remove the solvent, and then pressed to form a negative electrode mixture layer, thereby obtaining a negative electrode having an electrode density of 1.4 g/ ^cm3 .
Next, a polyethylene separator was placed between the obtained negative electrode and the positive electrode (lithium metal foil), and the resultant was sealed with an aluminum laminate film, and an electrolyte solution was injected to prepare an evaluation cell. The electrolyte solution used was a mixed solvent containing 1 mass % vinylene carbonate and having a volume ratio of ethylene carbonate/diethyl carbonate of 1/2, in which lithium hexafluorophosphate was dissolved to a concentration of 1.0 mol/L.

The obtained evaluation cell was subjected to the following battery characteristic test.
<Battery characteristic test>
The evaluation cell was charged to 0.01 V at a current density of 0.1 C in an environment at a temperature of 25° C., and then discharged to 1.5 V at a current density of 0.1 C. The discharge capacity at this time is shown in the table as the initial capacity. The initial charge/discharge efficiency (%) (= discharge capacity/charge capacity×100) was calculated and is shown in the table.
Next, in an environment at a temperature of 25° C., charging was performed at a current density of 0.5 C until the voltage reached 0.01 V, followed by a 10-minute pause, followed by discharging at a current density of 0.5 C until the voltage reached 1.5 V, followed by a 10-minute pause, and this cycle was repeated. The capacity retention rate (%) at the 10th cycle (= charge capacity after 10 cycles/charge capacity after 1 cycle×100) is shown in the table.

As shown by the results in Table 1, in Examples 1 to 4, lithium ion secondary batteries having high initial capacity, good initial charge/discharge efficiency, and excellent cycle durability were obtained.
On the other hand, Comparative Example 1 in which the conductive agent contained carbon black (Z) but did not contain single-walled carbon nanotubes was inferior in cycle durability.
Moreover, in Comparative Example 2 in which the conductive agent contained single-walled carbon nanotubes but did not contain carbon black (Z), the initial capacity and cycle durability were poor.
Moreover, Comparative Examples 3 and 4, in which multi-walled carbon nanotubes were used instead of single-walled carbon nanotubes, were inferior to Example 3 in cycle durability.
Furthermore, Comparative Examples 5 and 6, in which the BET specific surface area and DBP oil absorption of the carbon black were small, were inferior in initial capacity and cycle durability.

Claims (7)

A negative electrode mixture including a negative electrode active material, a conductive agent, and a binder,
The negative electrode active material includes a metal-based active material,
The conductive agent comprises carbon black having a BET specific surface area of 700 to 1500 m ² /g and a DBP oil absorption of 300 to 600 ml/100 g, and single-walled carbon nanotubes. The negative electrode mixture according to claim 1, wherein the content of the single-walled carbon nanotubes is 1 to 20 mass% relative to the total mass of the conductive agent. The negative electrode mixture according to claim 1, wherein the content of the conductive agent is 1 to 30 mass% relative to the total mass of the negative electrode active material. The negative electrode mixture according to claim 1, wherein the metal-based active material includes a Si-based active material. A method for manufacturing a negative electrode, comprising a step of applying the negative electrode mixture described in any one of claims 1 to 4 onto a current collector. A negative electrode current collector and a negative electrode mixture layer on the negative electrode current collector,
the negative electrode mixture layer contains a negative electrode active material, a conductive agent, and a binder,
The negative electrode active material includes a metal-based active material,
The negative electrode, wherein the conductive agent comprises carbon black having a BET specific surface area of 700 to 1500 m ² /g and a DBP oil absorption of 300 to 600 ml/100 g, and single-walled carbon nanotubes. A secondary battery comprising the negative electrode according to claim 6, a positive electrode, and an electrolyte present between the negative electrode and the positive electrode.