WO2024024552A1

WO2024024552A1 - Nonaqueous secondary battery negative electrode and nonaqueous secondary battery

Info

Publication number: WO2024024552A1
Application number: PCT/JP2023/026133
Authority: WO
Inventors: 弘樹大島; 晃平居城
Original assignee: 日本ゼオン株式会社
Priority date: 2022-07-29
Filing date: 2023-07-14
Publication date: 2024-02-01

Abstract

The purpose of the present invention is to provide a nonaqueous secondary battery negative electrode capable of exhibiting excellent cycle characteristics in a nonaqueous second battery even when using a silicon-based active material. A negative electrode according to the present invention comprises a negative electrode mixture layer containing a negative electrode active material and carbon nanotubes. The negative electrode active material contains a carbon-based active material and a silicon-based active material. The silicon-based active material has an average particle diameter of 2-10 μm. The carbon nanotubes have an average diameter of 1.2-30 nm. The proportion of an area S1 where the carbon nanotubes are present on the surface of the silicon-based active material with respect to the total area of the area S1 and an area S2 where the carbon nanotubes are present on the surface of the carbon-based active material is 55-98%.

Description

Negative electrode for non-aqueous secondary batteries and non-aqueous secondary batteries

The present invention relates to a negative electrode for a non-aqueous secondary battery and a non-aqueous secondary battery.

Non-aqueous secondary batteries (hereinafter sometimes abbreviated as "secondary batteries") such as lithium-ion secondary batteries have the characteristics of being small, lightweight, high energy density, and capable of being repeatedly charged and discharged. Yes, and used for a wide range of purposes. Therefore, in recent years, improvements in battery components such as electrodes have been studied with the aim of further improving the performance of lithium ion secondary batteries.
Specifically, it is being considered to increase the capacity of a secondary battery by using a negative electrode that uses a combination of a carbon-based active material and a silicon-based active material having a high theoretical capacity as the negative electrode active material. However, silicon-based active materials expand and contract significantly during charging and discharging. Therefore, negative electrodes containing silicon-based active materials have the problem that the conductive paths formed in the negative electrode composite layer are likely to be cut due to the expansion and contraction of the silicon-based active material, resulting in a decrease in the cycle characteristics of the secondary battery. was there.

In order to suppress the cutting of conductive paths due to charging and discharging, a conventional method has been used in which carbon nanotubes (hereinafter sometimes abbreviated as "CNT") are added as a conductive material to a negative electrode composite layer containing a silicon-based active material. (For example, see Patent Documents 1 to 3).

Japanese Patent Application Publication No. 2016-110876 International Publication No. 2017/007013 Special Publication No. 2015-534240

However, the above-mentioned conventional technology requires that the secondary battery exhibit even more excellent cycle characteristics.

Therefore, the present invention provides a negative electrode for a non-aqueous secondary battery that allows a non-aqueous secondary battery to exhibit excellent cycle characteristics even when using a silicon-based active material, and a non-aqueous secondary battery that has excellent cycle characteristics. The purpose is to provide.

The present inventor conducted extensive studies with the aim of solving the above problems. The present inventor has determined that, in a negative electrode containing a carbon-based active material, a silicon-based active material, and a CNT in the negative electrode composite layer, the average particle diameter of the silicon-based active material and the average diameter of the CNT are set within predetermined ranges. However, the present inventors have newly discovered that the cycle characteristics of a secondary battery can be improved by arranging carbon-based active materials, silicon-based active materials, and CNTs so as to satisfy predetermined properties, and have completed the present invention.

That is, the present invention aims to advantageously solve the above problems, and according to the present invention, the following negative electrodes for non-aqueous secondary batteries [1] to [6], and [7] A non-aqueous secondary battery is provided.

[1] A negative electrode for a non-aqueous secondary battery comprising a negative electrode composite layer containing a negative electrode active material and carbon nanotubes, wherein the negative electrode active material contains a carbon-based active material and a silicon-based active material, and the silicon-based active material contains a carbon-based active material and a silicon-based active material. The average particle diameter of the substance is 2 μm or more and 10 μm or less, the average diameter of the carbon nanotubes is 1.2 nm or more and 30 nm or less, and the existing area S1 of the carbon nanotubes on the surface of the silicon-based active material is A negative electrode for a non-aqueous secondary battery, wherein the ratio of the existing area S1 to the total existing area S2 of the carbon nanotubes on the surface of the active material is 55% or more and 98% or less.
The average particle diameter of the silicon-based active material and the average diameter of the CNTs are each within the above-mentioned ranges, and within the sum of the existing area S1 of CNTs on the surface of the silicon-based active material and the existing area S2 of CNTs on the surface of the carbon-based active material. A negative electrode including a negative electrode composite layer in which the ratio of the existing area S1 (hereinafter sometimes referred to as "CNT adhesion ratio to silicon-based active material") is within the above-mentioned range provides excellent secondary battery performance. It is possible to exhibit excellent cycle characteristics.
In this specification, the "average particle diameter" of the silicon-based active material, the "average diameter" of the CNTs, and the "CNT adhesion ratio to the silicon-based active material" in the negative electrode composite layer are all as described in Examples. It can be measured using a method.

[2] [1] above, wherein the ratio of the mass of the silicon-based active material to the total mass of the silicon-based active material and the mass of the carbon nanotubes is 96.5% by mass or more and 99.95% by mass or less A negative electrode for a non-aqueous secondary battery as described in .
In the negative electrode composite material layer, if the ratio of the mass of the silicon-based active material to the total mass of the silicon-based active material and CNT is within the above range, the cycle characteristics of the secondary battery can be further improved.

[3] The silicon-based active material has the formula: Li _y SiO _z [where y is greater than 0 and less than or equal to 4, and z is greater than or equal to 0.5 and less than or equal to 4. ] The negative electrode for a non-aqueous secondary battery according to [1] or [2] above, comprising an active material represented by [1] or [2].
If the active material represented by the above formula is used as the silicon-based active material, the initial efficiency of the secondary battery can be increased.

[4] The silicon-based active material includes a composite of a Si-containing material and conductive carbon, and the ratio of the G-band peak intensity to the D-band peak intensity in the Raman spectrum of the conductive carbon is 4 or less. The negative electrode for a non-aqueous secondary battery according to any one of [1] to [3].
The above composite is used as a silicon-based active material, and the ratio of the G band peak intensity to the D band peak intensity in the Raman spectrum of the conductive carbon constituting the composite (hereinafter sometimes referred to as "G/D ratio") ) is below the above value, it is possible to further improve the cycle characteristics while increasing the initial efficiency of the secondary battery.
In this specification, a "composite of a Si-containing material and conductive carbon" is not classified as a carbon-based active material but as a silicon-based active material.
In this specification, "G/D ratio" refers to the Raman spectrum of conductive carbon contained in the composite using a microlaser Raman spectrophotometer (Nicolet Almega XR manufactured by Thermo Fisher Scientific Co., Ltd.). Then, for the obtained Raman spectrum, the intensity of the G band peak observed near 1590 cm ^-1 and the intensity of the D band peak observed near 1340 cm ^-1 are determined, and calculation can be made from these values. .

[5] The negative electrode for a non-aqueous secondary battery according to any one of [1] to [4] above, wherein the negative electrode composite layer further contains a dispersant.
If the negative electrode mixture layer contains a dispersant, the cycle characteristics of the secondary battery can be further improved.

[6] The negative electrode for a non-aqueous secondary battery according to any one of [1] to [4] above, wherein the negative electrode composite layer contains at least one of carboxymethyl cellulose and a salt thereof.
If the negative electrode composite layer includes at least one of carboxymethylcellulose and its salt (hereinafter, these may be collectively referred to as "carboxymethylcellulose (salt)"), the cycle characteristics of the secondary battery can be further improved. Can be done.

[7] A non-aqueous secondary battery comprising the negative electrode for a non-aqueous secondary battery according to any one of [1] to [6] above.
A secondary battery equipped with any of the negative electrodes described above has a high capacity because the negative electrode contains a silicon-based active material, and also has excellent cycle characteristics.

According to the present invention, there is provided a negative electrode for a non-aqueous secondary battery that allows a non-aqueous secondary battery to exhibit excellent cycle characteristics even when a silicon-based active material is used, and a non-aqueous secondary battery that has excellent cycle characteristics. can be provided.

Embodiments of the present invention will be described in detail below.
Here, the negative electrode for a nonaqueous secondary battery of the present invention is used as a negative electrode of a nonaqueous secondary battery such as a lithium ion secondary battery. Moreover, the non-aqueous secondary battery of the present invention is provided with the negative electrode for non-aqueous secondary batteries of the present invention.

(Negative electrode for non-aqueous secondary batteries)
The negative electrode of the present invention includes at least a negative electrode composite material layer, and optionally includes a current collector. That is, in one embodiment of the present invention, the negative electrode of the present invention includes a negative electrode composite material layer and a current collector. In addition, when the negative electrode of the present invention is provided with a current collector, the negative electrode may be provided with a negative electrode composite material layer on only one side of the current collector, or may be provided with negative electrode composite material layers on both sides of the current collector. good.
In addition, when the negative electrode of the present invention includes negative electrode composite material layers on both sides of the current collector, if at least one negative electrode composite material layer is a predetermined negative electrode composite material layer described below, the cycle characteristics of the secondary battery described above can be maintained. It can be improved sufficiently.

<Negative electrode composite material layer>
The negative electrode composite material layer contains at least a carbon-based active material, a silicon-based active material, and CNTs as negative electrode active materials, and optionally contains components other than the carbon-based active material, silicon-based active material, and CNTs (hereinafter referred to as "other components"). ).

<<Carbon-based active material>>
A carbon-based active material refers to an active material that has carbon as its main skeleton and into which lithium can be inserted (also referred to as "doping"). Examples of carbon-based active materials include carbonaceous materials and graphite materials. It will be done.
Note that one type of carbon-based active material may be used alone, or two or more types may be used in combination.

A carbonaceous material is a material with a low degree of graphitization (ie, low crystallinity) obtained by heat-treating a carbon precursor at 2000° C. or lower to carbonize it. Note that the lower limit of the heat treatment temperature during carbonization is not particularly limited, but may be, for example, 500° C. or higher.
Examples of carbonaceous materials include graphitizable carbon, which easily changes its carbon structure depending on the heat treatment temperature, and non-graphitic carbon, which has a structure similar to an amorphous structure such as glassy carbon. .
Here, examples of graphitizable carbon include carbon materials made from tar pitch obtained from petroleum or coal. Specific examples include coke, mesocarbon microbeads (MCMB), mesophase pitch carbon fibers, and pyrolytic vapor growth carbon fibers.
Furthermore, examples of the non-graphitic carbon include phenolic resin fired products, polyacrylonitrile carbon fibers, pseudo-isotropic carbon, furfuryl alcohol resin fired products (PFA), and hard carbon.

The graphitic material is a material having high crystallinity similar to graphite, which is obtained by heat-treating graphitizable carbon at 2000° C. or higher. Note that the upper limit of the heat treatment temperature is not particularly limited, but may be, for example, 5000° C. or lower.
Examples of the graphite material include natural graphite and artificial graphite.
Here, the artificial graphite includes, for example, artificial graphite obtained by heat-treating carbon containing graphitizable carbon mainly at 2800°C or higher, graphitized MCMB obtained by heat-treating MCMB at 2000°C or higher, mesophase pitch carbon fiber at 2000°C or higher. Examples include graphitized mesophase pitch carbon fibers heat-treated as described above.
Further, in the present invention, natural graphite whose surface is at least partially coated with amorphous carbon (amorphous coated natural graphite) may be used as the carbon-based negative electrode active material.

Here, the average particle diameter and specific surface area of the carbon-based active material are not particularly limited and can be the same as those of conventionally used carbon-based active materials.
Further, as the carbon-based active material, a graphite material (graphite active material) is preferable from the viewpoint of further improving the cycle characteristics of the secondary battery while increasing the initial efficiency of the secondary battery.

The proportion of the carbon-based active material contained in the negative electrode composite layer is preferably 60% by mass or more, more preferably 70% by mass or more, with the entire negative electrode composite layer being 100% by mass. It is more preferably 80% by mass or more, preferably 97% by mass or less, and more preferably 95% by mass or less. If the proportion of the carbon-based active material in the negative electrode composite layer is within the above-mentioned range, the cycle characteristics can be further improved while ensuring sufficient capacity and initial efficiency of the secondary battery.

<<Silicon-based active material>>
Examples of silicon-based active materials include silicon (Si), silicon-containing alloys, SiO, SiO _x , and Li _y SiO _z , and composites of these Si-containing materials and conductive carbon.
Note that one type of silicon-based active material may be used alone, or two or more types may be used in combination.

Examples of alloys containing silicon include alloy compositions containing silicon, aluminum, transition metals such as iron, and further containing rare earth elements such as tin and yttrium.

SiO _x is a compound containing at least one of SiO and SiO ₂ and Si, and x is usually 0.01 or more and less than 2. And SiO _x can be formed using, for example, a disproportionation reaction of silicon monoxide (SiO). Specifically, SiO _x can be prepared by heat treating SiO, optionally in the presence of a polymer such as polyvinyl alcohol, to produce silicon and silicon dioxide. Note that the heat treatment can be performed at a temperature of 900° C. or higher, preferably 1000° C. or higher in an atmosphere containing organic gas and/or steam after pulverizing and mixing SiO and optionally a polymer.

Li _y SiO _z is a compound composed of the elements Li, Si, and O, where y is greater than 0 and less than or equal to 4, and z is greater than or equal to 0.5 and less than or equal to 4. Li _y SiO _z can be obtained by chemically doping lithium by mixing a lithium compound and heat-treating the SiO _x described above, or by electrochemically doping using lithium foil as a counter electrode. It can be produced by inserting lithium using a known method.

Among the silicon (Si), silicon-containing alloys, SiO, SiO _x , and Li _{y SiO z described above, SiO x and Li y} _{SiO z} _are _preferred _from the viewpoint of increasing the initial efficiency while increasing the capacity of the secondary battery. From the viewpoint of increasing the initial efficiency of the secondary battery, Li _y SiO _z is more preferable.

Furthermore, at least one Si-containing material selected from the group consisting of silicon (Si), silicon-containing alloys, SiO, SiO _x , and Li _y SiO _z is coated with conductive carbon and/or composited with conductive carbon. It is preferable that a composite be formed by combining the two. By using a composite of a Si-containing material and conductive carbon, it is possible to further improve the cycle characteristics while increasing the initial efficiency of the secondary battery.
As a composite of a Si-containing material and conductive carbon, for example, a pulverized mixture of a Si-containing material, a polymer such as polyvinyl alcohol, and optionally a carbon material is prepared, for example, in an atmosphere containing organic gas and/or steam. Examples include compounds obtained by heat treatment. In addition, such composites can be produced by coating the surface of particles of a Si-containing material by chemical vapor deposition using an organic gas, or by granulating particles of a Si-containing material and graphite or artificial graphite by a mechanochemical method. It can also be obtained by a known method such as a method of converting.

Here, the conductive carbon constituting the composite has a G/D ratio of preferably 0.3 or more, more preferably 0.5 or more, preferably 4 or less, and 2 or less. It is more preferable that If the G/D ratio of the conductive carbon is within the above range, it is possible to further improve the cycle characteristics while increasing the initial efficiency of the secondary battery.

Here, the silicon-based active material in the negative electrode composite layer needs to have an average particle diameter of 2 μm or more and 10 μm or less, preferably 3 μm or more, more preferably 4 μm or more, and 5 μm or more. It is more preferably at most 9 μm, more preferably at most 8 μm, even more preferably at most 7 μm. If the average particle diameter of the silicon-based active material is less than 2 μm, the amount of side reactions will increase during the charging and discharging process, resulting in a decrease in the charging and discharging efficiency of the secondary battery, and if it exceeds 10 μm, the cycle characteristics of the secondary battery will deteriorate due to aggregation. is damaged.

The proportion of the silicon-based active material contained in the negative electrode composite layer is preferably 1% by mass or more, more preferably 4% by mass or more, with the entire negative electrode composite layer being 100% by mass. It is more preferably 7% by mass or more, particularly preferably 9% by mass or more, preferably 30% by mass or less, more preferably 25% by mass or less, and 20% by mass or less. is more preferable, and particularly preferably 15% by mass or less. If the proportion of silicon-based active material in the negative electrode composite layer is 1% by mass or more, the capacity of the secondary battery can be increased, and if it is 30% by mass or less, the cycle identification of the secondary battery can be further improved. can be done.

<<Carbon nanotubes>>
CNTs may be single-wall carbon nanotubes or multi-wall carbon nanotubes. Further, as the CNT, a combination of single-walled CNT and multi-walled CNT may be used. Note that from the viewpoint of further improving the cycle characteristics of the secondary battery, it is preferable to use single-walled CNTs as the CNTs.

Here, the CNTs in the negative electrode composite layer need to have an average diameter of 1.2 nm or more and 30 nm or less, preferably 2.0 nm or more, more preferably 2.5 nm or more, It is more preferably 3.0 nm or more, particularly preferably 3.5 nm or more, preferably 20 nm or less, more preferably 15 nm or less, even more preferably 11 nm or less, and even more preferably 7 nm or less. It is particularly preferable that CNTs with an average diameter of less than 1.2 nm are difficult to manufacture, and if the average diameter of CNTs exceeds 30 nm, the initial efficiency and cycle characteristics of the secondary battery will be impaired.

The proportion of CNTs contained in the negative electrode composite layer is preferably 0.001% by mass or more, more preferably 0.005% by mass or more, with the entire negative electrode composite layer being 100% by mass. , more preferably 0.008% by mass or more, preferably 0.5% by mass or less, more preferably 0.4% by mass or less, and even more preferably 0.2% by mass or less. preferable. If the proportion of CNTs in the negative electrode composite layer is within the above-mentioned range, it is possible to further improve the cycle characteristics while increasing the initial efficiency of the secondary battery.

In addition, in the negative electrode composite material layer, the ratio of the mass of the silicon-based active material to the total mass of the silicon-based active material and the mass of CNT is 96% by mass, assuming that the total mass of the silicon-based active material and CNT is 100% by mass. It is preferably at least .5% by mass, more preferably at least 97.0% by mass, even more preferably at least 97.5% by mass, particularly preferably at least 98.0% by mass, It is preferably 99.95% by mass or less, more preferably 99.92% by mass or less. If the ratio of the mass of CNT to the total mass of silicon-based active material and CNT is within the above range, the cycle characteristics of the secondary battery can be further improved.

<<Other ingredients>>
Other components that the negative electrode composite layer may optionally include include negative electrode active materials other than carbon-based active materials and silicon-based active materials (hereinafter referred to as "other negative electrode active materials"), conductive materials other than CNT, Examples include polymer components. Note that the negative electrode composite material layer may contain only one type of other components, or may contain two or more types of other components.

[Other negative electrode active materials]
Other negative electrode active materials include, but are not particularly limited to, lithium metal, single metals other than Si that can form lithium alloys (for example, Ag, Al, Ba, Bi, Cu, Ga, Ge, In, Ni, P, Pb, Sb, Sn, Sr, Zn, Ti, etc.) and their alloys, as well as their oxides, sulfides, nitrides, silicides, carbides, and phosphides. These may be used alone or in combination of two or more.

[Conductive materials other than CNT]
The conductive material other than CNT is not particularly limited, and examples thereof include carbon black (acetylene black, Ketjen Black (registered trademark), furnace black, etc.), carbon flakes, carbon nanofibers, and the like. These may be used alone or in combination of two or more.

[Polymer component]
The polymer components that can be optionally included in the negative electrode composite layer are not particularly limited, and include dispersants and thickeners used as manufacturing aids when forming the negative electrode composite layer, and polymer components that bind each component in the negative electrode composite layer. Examples include binding materials used for attaching the adhesive. Note that one type of polymer component may be used alone, or two or more types may be used in combination.
In one embodiment, the negative electrode composite material layer included in the negative electrode of the present invention preferably contains at least one selected from the group consisting of a dispersant, a thickener, and a binder as a polymer component.

-Dispersant-
The dispersant is a polymer that can favorably disperse CNTs and the like during the process of forming the negative electrode composite material layer. In particular, when manufacturing the negative electrode of the present invention using the "method for manufacturing negative electrodes for secondary batteries" described later, the dispersant adsorbs to CNTs and disperses the CNTs well, while at the same time dispersing the CNTs onto the surface of the silicon-based active material. It may also serve to assist in adhesion. Due to the action of such a dispersant, CNTs can be easily arranged around the silicon-based active material in the negative electrode composite material layer, and the CNT adhesion ratio to the silicon-based active material can be improved.

Here, the dispersant preferably has an acidic group from the viewpoint of increasing the CNT adhesion rate to the silicon-based active material and further improving the cycle characteristics of the secondary battery.
The acidic group that the dispersant has is not particularly limited, but from the viewpoint of further improving the cycle characteristics of the secondary battery, carboxylic acid groups, sulfonic acid groups, and phosphoric acid groups are preferable, and carboxylic acid groups are particularly preferable. Note that the dispersant may have only one type of acidic group, or may have two or more types of acidic groups.

Specific examples of the dispersant include, but are not particularly limited to, carboxymethyl cellulose, polyacrylic acid, polymethacrylic acid, salts thereof (such as sodium salt), and polyvinylpyrrolidone. Among these, preferred are dispersants (polymers) having acidic groups such as carboxymethyl cellulose, polyacrylic acid, polymethacrylic acid, and salts thereof, from the viewpoint of sufficiently obtaining the above-mentioned effect of improving cycle characteristics of the secondary battery. Carboxymethyl cellulose (salt) is more preferred.
In addition, one type of dispersant may be used alone, or two or more types may be used in combination.

Here, the weight average molecular weight of the dispersant is preferably 1,000 or more, more preferably 3,000 or more, even more preferably 30,000 or more, and preferably 60,000 or more. Particularly preferred is 200,000 or less, more preferably 100,000 or less. If the weight average molecular weight of the dispersant is within the above-mentioned range, the dispersant can satisfactorily exhibit its function, and can further improve the cycle characteristics while increasing the capacity and initial efficiency of the secondary battery.
In this specification, the "weight average molecular weight" of a polymer component can be measured by the following method.
<Weight average molecular weight>
Measurement is performed by gel permeation chromatography (GPC) using a LiBr-NMP solution with a concentration of 10 mM under the following measurement conditions.
・Separation column: Shodex KD-806M (manufactured by Showa Denko Co., Ltd.)
・Detector: Differential refractometer detector RID-10A (manufactured by Shimadzu Corporation)
・Flow rate of eluent: 0.3 mL/min ・Column temperature: 40°C
・Standard polymer: TSK standard polystyrene (manufactured by Tosoh Corporation)

Further, the dispersant is preferably water-soluble. If the dispersant is water-soluble, the dispersant can exhibit its function well, and can further improve the cycle characteristics while increasing the capacity and initial efficiency of the secondary battery.
In this specification, various components such as polymer components are "water-soluble" when 0.5 g (in terms of solid content) of the component is dissolved in 100 g of water at a temperature of 25°C, the amount of insoluble matter is 1 It means less than .0% by mass.

In one embodiment of the present invention, the proportion of the dispersant contained in the negative electrode composite layer is preferably 0.001% by mass or more, and 0.005% by mass, with the entire negative electrode composite layer being 100% by mass. It is more preferably at least 0.008% by mass, even more preferably at least 0.9% by mass, more preferably at most 0.5% by mass, and even more preferably at most 0.2% by mass. % or less is more preferable. If the proportion of the dispersant in the negative electrode composite layer is within the above range, it is possible to further improve the cycle characteristics while increasing the capacity and initial efficiency of the secondary battery.

-Thickener-
The thickener is a polymer that is added in the process of forming the negative electrode composite material layer for the purpose of increasing the viscosity of the negative electrode slurry to ensure coating properties.
Specific examples of the thickener are not particularly limited, and include the same components as the above-mentioned "dispersant", but carboxymethyl cellulose (salt) is preferred.
In addition, one type of thickener may be used alone, or two or more types may be used in combination.

Here, the weight average molecular weight of the thickener is preferably more than 200,000, more preferably 250,000 or more, preferably 2,000,000 or less, and 1,000,000 or less. It is more preferable that If the weight average molecular weight of the thickener is within the above-mentioned range, the thickener can perform its function well, and can further improve the cycle characteristics while increasing the capacity and initial efficiency of the secondary battery. .

Furthermore, the thickener is preferably water-soluble. If the thickener is water-soluble, the thickener can perform its function well, and can further improve the cycle characteristics while increasing the capacity and initial efficiency of the secondary battery.

In one embodiment of the present invention, the proportion of the thickener contained in the negative electrode composite layer is preferably 0.1% by mass or more, and 0.5% by mass, with the entire negative electrode composite layer being 100% by mass. % or more, further preferably 0.8% by mass or more, preferably 5% by mass or less, more preferably 4% by mass or less, and 3% by mass or less. is even more preferable. If the proportion of the thickener in the negative electrode composite layer is within the above range, it is possible to further improve the cycle characteristics while increasing the capacity and initial efficiency of the secondary battery.

-Binding material-
As described above, the binder is a polymer having adhesive properties that can bind the negative electrode active material, CNT, etc. in the negative electrode composite material layer.
A specific example of the binder is not particularly limited and any known binder can be used, but a polymer containing an aliphatic conjugated diene monomer unit such as a 1,3-butadiene unit or an isoprene unit is used. It is preferable.
As used herein, the expression "contains a monomer unit" in a polymer means that "a repeating unit derived from the monomer is contained in the polymer obtained using the monomer". .

Examples of polymers containing aliphatic conjugated diene monomer units include aliphatic conjugated diene polymers such as polybutadiene and polyisoprene; aromatic polymers such as styrene-butadiene polymers and styrene-butadiene-styrene block copolymers; Examples include vinyl-aliphatic conjugated diene copolymers; vinyl cyanide-aliphatic conjugated diene copolymers such as acrylonitrile-butadiene polymers. The polymer containing these aliphatic conjugated diene monomer units may have the above-mentioned acidic group.
Note that one type of binder may be used alone, or two or more types may be used in combination.

Furthermore, it is preferable that the binder does not fall under the above-mentioned "water-soluble" category, that is, it is water-insoluble. If the binder is water-insoluble, it can exhibit good adhesion in the negative electrode composite layer, and can further improve the cycle characteristics while increasing the capacity and initial efficiency of the secondary battery.

The proportion of the binder contained in the negative electrode composite layer is preferably 0.1% by mass or more, and preferably 0.3% by mass or more, with the entire negative electrode composite layer being 100% by mass. It is more preferably 0.7% by mass or more, still more preferably 4% by mass or less, more preferably 3% by mass or less, and even more preferably 2% by mass or less. If the proportion of the binder in the negative electrode composite layer is within the above range, it is possible to further improve the cycle characteristics while increasing the capacity and initial efficiency of the secondary battery.

-Carboxymethylcellulose and its salts-
Carboxymethylcellulose (salt) will be explained in more detail. Carboxymethylcellulose (salt) is a polymeric component that can be used both as a dispersant and as a thickener, as described above. That is, the negative electrode composite layer may contain only carboxymethylcellulose (salt) as a dispersant, only carboxymethylcellulose (salt) as a thickener, or may contain only carboxymethylcellulose (salt) as a dispersant. Both carboxymethylcellulose (salt) as a thickener and carboxymethylcellulose (salt) as a thickener may be included. In this way, carboxymethylcellulose (salt) can be used in multiple ways in the formation stage of the negative electrode composite material layer, so for example, when it is used only as a dispersant, when it is used only as a thickener, The amount of carboxymethyl cellulose (salt) contained in the finally obtained negative electrode composite material layer differs between the cases where it is used as both a dispersant and a dispersant. That is, regarding the amount of carboxymethyl cellulose (salt) contained in the negative electrode composite material layer, a plurality of aspects are assumed depending on the use of carboxymethyl cellulose (salt) and other circumstances.

In one embodiment, carboxymethyl cellulose (salt) is used as both a dispersant and a thickener when forming the negative electrode composite layer. Although not limited to such embodiments, as an example, the proportion of carboxymethyl cellulose (salt) contained in the negative electrode composite layer is 0.1% by mass, with the entire negative electrode composite layer being 100% by mass. It is preferably at least 0.5% by mass, more preferably at least 1.5% by mass, even more preferably at most 5% by mass, and at most 4% by mass. is more preferable, and even more preferably 3% by mass or less. If the proportion of carboxymethyl cellulose (salt) in the negative electrode composite layer is within the above range, it is possible to further improve the cycle characteristics while increasing the capacity and initial efficiency of the secondary battery.
In another embodiment, carboxymethylcellulose (salt) is used as a dispersant and not as a thickener when forming the negative electrode composite material layer. Although not limited to such an embodiment, as an example, the proportion of carboxymethyl cellulose (salt) contained in the negative electrode composite layer is 0.005% by mass, with the entire negative electrode composite layer being 100% by mass. It is preferably at least 0.009% by mass, more preferably at least 0.01% by mass, even more preferably at most 1% by mass, and at most 0.7% by mass. It is more preferable that the amount is at most 0.4% by mass, even more preferably at most 0.15% by mass. If the proportion of carboxymethyl cellulose (salt) in the negative electrode composite layer is within the above range, it is possible to further improve the cycle characteristics while increasing the capacity and initial efficiency of the secondary battery.

<<CNT adhesion ratio to silicon-based active material>>
The negative electrode composite material layer included in the negative electrode of the present invention needs to have a CNT adhesion ratio to the silicon-based active material described above of 55% or more and 98% or less. When the CNT adhesion ratio to the silicon-based active material is within the above range, the cycle characteristics of the secondary battery can be improved.
Here, the reason why the secondary battery can exhibit excellent cycle characteristics when the CNT adhesion ratio to the silicon-based active material is within the above range is not clear, but it is presumed to be as follows.

First, in the negative electrode composite material layer formed using a negative electrode slurry containing a carbon-based active material, a silicon-based active material, and CNTs, contact occurs only with the carbon-based active material, and conductive path formation involving the silicon-based active material occurs. The inventor's studies have revealed that there may be a large amount of non-contributing CNTs. On the other hand, in the negative electrode of the present invention, since the CNT adhesion ratio to the silicon-based active material is 55% or more, it can be said that a sufficient amount of CNTs exist around the silicon-based active material. Therefore, even after the silicon-based active material repeatedly expands and contracts due to charging and discharging of the secondary battery, conductive paths between adjacent silicon-based active materials and between adjacent silicon-based active materials and carbon-based active materials can be maintained. On the other hand, as the proportion of CNTs attached to the silicon-based active material increases, the amount of CNTs present on the surface of the carbon-based active material decreases. This is presumed to be because conductive paths between the carbon-based active materials are not sufficiently formed due to a decrease in the amount of CNTs present on the surface of the carbon-based active material, but the proportion of CNTs attached to the silicon-based active material is excessive. It became clear that as the temperature increased, the cycle characteristics actually deteriorated. On the other hand, in the negative electrode of the present invention, since the CNT adhesion ratio to the silicon-based active material is 98% or less, a sufficient conductive path between the carbon-based active materials can be ensured.
For the above reasons, in the negative electrode of the present invention, the CNT adhesion ratio to the silicon-based active material is 55% or more and 98% or less, so the conductive path including the two types of negative electrode active materials is maintained well even after charging and discharging. Therefore, it is thought that the negative electrode can improve the cycle characteristics of a secondary battery.

The CNT adhesion ratio to the silicon-based active material is preferably 60% or more, more preferably 70% or more, from the viewpoint of further improving the cycle characteristics while increasing the initial efficiency of the secondary battery. , more preferably 75% or more, preferably 94% or less, and more preferably 92% or less.
Note that the negative electrode of the present invention in which the CNT adhesion ratio to the silicon-based active material is within the above-mentioned predetermined range can be manufactured using the "method for manufacturing a negative electrode for secondary battery" described below. The CNT adhesion ratio to the silicon-based active material can be adjusted by changing various conditions in the procedure.

<Current collector>
As the current collector that is optionally included in the negative electrode of the present invention, a material that has electrical conductivity and is electrochemically durable is used. Specifically, as the current collector, for example, a current collector made of iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, platinum, etc. can be used. Among these, copper foil (current collector made of copper) is particularly preferred as the negative electrode. Note that the above-mentioned materials may be used alone or in combination of two or more in any ratio.

<Method for manufacturing negative electrode for secondary battery>
The negative electrode of the present invention, in which the adhesion ratio of CNTs to the silicon-based active material in the negative electrode composite layer is controlled within a predetermined range, is obtained by granulating a composition for composite particles containing a silicon-based active material, CNTs, and a dispersant. A step of obtaining particles (granulation step), a step of preparing a negative electrode slurry containing composite particles, a carbon-based active material, and a solvent (slurry preparation step), and drying the negative electrode slurry to obtain a negative electrode composite layer. It is preferable to manufacture by a manufacturing method including a step (composite material layer forming step). Note that the manufacturing method may include steps other than the above-mentioned granulation step, slurry preparation step, and composite layer forming step.

<<Granulation process>>
In the granulation step, the composition for composite particles is granulated to obtain composite particles containing at least a silicon-based active material, CNTs, and a dispersant.

[Composition for composite particles]
The composition for composite particles includes a silicon-based active material, CNTs, and a dispersant, and optionally includes a dispersion medium. The dispersion medium is not particularly limited and both water and organic solvents can be used, but water is preferred. Note that the composition for composite particles preferably does not contain a binder, that is, it is preferable that the composite particles also do not contain a binder.
Although the method for preparing the composition for composite particles is not particularly limited, it is preferable to prepare a CNT paste by mixing CNTs and a dispersant in a dispersion medium, and then add a silicon-based active material to the obtained CNT paste. By preparing a composite particle composition using this procedure, it is possible to easily increase the proportion of CNTs attached to the silicon-based active material in the resulting negative electrode composite layer. Although the reason for this is not clear, it is presumed that by pre-dispersing the CNTs using a dispersant in the dispersion medium, the CNTs can be uniformly attached to the surface of the silicon-based active material via the dispersant.
Note that a known mixer such as a disper or a bead mill can be used to mix each component.

Here, the amount of each component other than the dispersion medium in the composition for composite particles may be appropriately determined depending on the amount ratio of each component in the intended negative electrode composite layer.
On the other hand, in the composition for composite particles and the resulting composite particles, the proportion of the mass of carboxymethyl cellulose (salt) in the total mass of the silicon-based active material and the mass of carboxymethyl cellulose (salt) is larger than that of the silicon-based active material. and carboxymethylcellulose (salt) as 100% by mass, it is preferably 0.01% by mass or more, more preferably 0.03% by mass or more, and preferably 0.06% by mass or more. More preferably, it is 2% by mass or less, more preferably 1.6% by mass or less, even more preferably 1% by mass or less, even more preferably 0.8% by mass or less. , 0.6% by mass or less is particularly preferred. If the proportion of the mass of carboxymethylcellulose (salt) in the total mass of the silicon-based active material and the mass of carboxymethylcellulose (salt) is within the above range, carboxymethylcellulose (salt) is present in the resulting composite particles. This is presumed to be because the CNTs adhere to the silicon-based active material with appropriate adhesion force and adhesion amount, but in the resulting negative electrode composite layer, the proportion of CNTs adhering to the silicon-based active material increases excessively. It can be easily controlled within the desired range without any problems.

[Granulation]
Although the granulation method is not particularly limited, granulation by spray drying is preferred from the viewpoint of suppressing an excessive increase in the proportion of CNTs attached to the silicon-based active material in the resulting negative electrode composite layer.
The spray drying conditions are not particularly limited. For example, the drying temperature during spray granulation is preferably 80°C or more and 250°C or less, and preferably 90°C or more and 120°C or less. The drying temperature can be measured as the ambient temperature on the outlet side of the spray dryer.
Note that after spray drying, the obtained particles may be subjected to additional drying treatment such as vacuum drying, if necessary. The drying temperature when performing the additional drying treatment is not particularly limited, but is preferably 100°C or higher, more preferably 110°C or higher, preferably 160°C or lower, and 140°C or lower. It is more preferable.

[Composite particles]
It is presumed that the obtained composite particles have a structure in which CNTs are attached to the surface of a silicon-based active material via a dispersant.
Here, the volume average particle diameter of the composite particles is preferably 2 μm or more, more preferably 3 μm or more, preferably 10 μm or less, and more preferably 8 μm or less. If the volume average particle diameter of the composite particles is within the above range, the CNT adhesion ratio to the silicon-based active material in the negative electrode composite layer can be easily controlled within the desired range. Further, by having the composite particle diameter within the above range, it is possible to further improve the cycle characteristics while increasing the initial efficiency of the secondary battery.
The "volume average particle diameter" of composite particles is the particle diameter at 50% of the integrated value in the particle size distribution (volume basis) measured using a particle size distribution measuring device based on laser scattering/diffraction method, that is, the 50% volume average particle diameter. (D50). Further, in the present invention, the "volume average particle diameter" of the composite particles can be measured in accordance with JIS Z8825:2013, and specifically, can be measured using the method described in Examples.
The volume average particle diameter of the composite particles depends on the size (particle size, diameter, etc.) of the silicon-based active material and/or CNT, the amount of CNTs and dispersant added to the silicon-based active material, and the granulation process such as spray drying. It can be adjusted by changing the conditions.

<<Slurry preparation process>>
In the slurry preparation step, the composite particles obtained in the granulation step are used to obtain a negative electrode slurry containing at least a carbon-based active material, a silicon-based active material, CNT, and a solvent. More specifically, a carbon-based active material, composite particles, a solvent, a binder, a thickener, and the like used as necessary are mixed to obtain a slurry for a negative electrode.

Note that when a binder is used, the amount of the binder to be used can be determined depending on the amount of the binder in the desired negative electrode composite material layer. When using a thickener, the amount of the thickener used can be determined depending on the viscosity of the negative electrode slurry and the desired amount of the thickener in the negative electrode composite layer.
The solvent is not particularly limited and both water and organic solvents can be used, but water is preferred. Note that a known mixer such as a planetary mixer can be used to mix each component.

<<Composite material layer formation process>>
In the composite material layer forming step, the negative electrode slurry obtained in the slurry preparation step is dried to remove the solvent, and a negative electrode composite material layer is formed.
In one embodiment, the composite material layer forming step includes applying a negative electrode slurry to at least one surface of the current collector, and drying the negative electrode slurry applied to at least one surface of the current collector. This is carried out by forming a negative electrode composite material layer on top.

[Application]
The method for applying the negative electrode slurry onto the current collector is not particularly limited, and any known method can be used. Specifically, as a coating method, a doctor blade method, a dip method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, a brush coating method, etc. can be used. The thickness of the slurry film on the current collector after coating and before drying can be appropriately set depending on the thickness of the negative electrode composite material layer obtained by drying.

[Drying]
The method of drying the negative electrode slurry on the current collector is not particularly limited, and any known method can be used, such as drying with warm air, hot air, low humidity air, vacuum drying, irradiation with infrared rays, electron beams, etc. For example, a drying method can be mentioned. By drying the negative electrode slurry on the current collector in this way, it is possible to form a negative electrode composite material layer on the current collector and obtain a negative electrode for a secondary battery comprising the current collector and the negative electrode composite material layer. can.

Note that after the above drying, the negative electrode composite material layer may be subjected to pressure treatment using a mold press, a roll press, or the like. The pressure treatment can improve the adhesion between the negative electrode composite material layer and the current collector.

(Non-aqueous secondary battery)
The secondary battery of the present invention includes the negative electrode of the present invention described above. Since the secondary battery of the present invention includes the negative electrode of the present invention, it has excellent cycle characteristics. In addition, it is preferable that the non-aqueous secondary battery of this invention is a lithium ion secondary battery, for example.

Here, the configuration of a lithium ion secondary battery as an example of the secondary battery of the present invention will be described below. This lithium ion secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode is the negative electrode of the present invention described above.

<Positive electrode>
The positive electrode is not particularly limited, and any known positive electrode can be used.

<Electrolyte>
As the electrolytic solution, an organic electrolytic solution in which a supporting electrolyte is dissolved in an organic solvent is usually used. As the supporting electrolyte, for example, lithium salt is used. Examples of lithium salts include LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAlCl ₄ , LiClO ₄ , CF ₃ SO ₃ Li, C ₄ F ₉ SO ₃ Li, CF ₃ COOLi, (CF ₃ CO) ₂ NLi. , (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) NLi, and the like. Among these, LiPF ₆ , LiClO ₄ , and CF ₃ SO ₃ Li are preferred, and LiPF ₆ is particularly preferred since they are easily soluble in solvents and exhibit a high degree of dissociation. Note that one type of electrolyte may be used alone, or two or more types may be used in combination in any ratio. Usually, the lithium ion conductivity tends to increase as a supporting electrolyte with a higher degree of dissociation is used, so the lithium ion conductivity can be adjusted depending on the type of supporting electrolyte.

The organic solvent used in the electrolyte is not particularly limited as long as it can dissolve the supporting electrolyte, but examples include dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), Carbonates such as butylene carbonate (BC) and methyl ethyl carbonate (EMC); Esters such as γ-butyrolactone and methyl formate; Ethers such as 1,2-dimethoxyethane and tetrahydrofuran; Sulfur-containing compounds such as sulfolane and dimethyl sulfoxide etc. are preferably used. Alternatively, a mixture of these solvents may be used. Among them, it is preferable to use carbonates because they have a high dielectric constant and a wide stable potential range, and it is more preferable to use a mixture of ethylene carbonate and ethyl methyl carbonate.
The concentration of the electrolyte in the electrolytic solution can be adjusted as appropriate, for example, preferably 0.5 to 15% by mass, more preferably 2 to 13% by mass, and 5 to 10% by mass. is even more preferable. Additionally, known additives such as fluoroethylene carbonate and ethylmethylsulfone may be added to the electrolyte.

<Separator>
The separator is not particularly limited, and for example, those described in JP-A No. 2012-204303 can be used. Among these, polyolefins are preferred because they can reduce the overall film thickness of the separator, thereby increasing the ratio of the electrode active material in the lithium ion secondary battery and increasing the capacity per volume. A microporous membrane made of a resin of the type (polyethylene, polypropylene, polybutene, polyvinyl chloride) is preferred.

<Method for manufacturing lithium ion secondary battery>
The lithium ion secondary battery according to the present invention can be produced by, for example, stacking a positive electrode and a negative electrode with a separator interposed therebetween, rolling or folding them according to the battery shape as necessary, and placing them in a battery container. It can be manufactured by injecting an electrolyte into the container and sealing it. In order to prevent an increase in pressure inside the secondary battery, overcharging and discharging, etc., a fuse, an overcurrent prevention element such as a PTC element, an expanded metal, a lead plate, etc. may be provided as necessary. The shape of the secondary battery may be, for example, a coin shape, a button shape, a sheet shape, a cylindrical shape, a square shape, a flat shape, or the like.

EXAMPLES Hereinafter, the present invention will be specifically explained based on Examples, but the present invention is not limited to these Examples. In the following description, "%" and "part" representing amounts are based on mass unless otherwise specified.
In addition, in a polymer produced by copolymerizing multiple types of monomers, the proportion of monomer units formed by polymerizing a certain monomer in the polymer is usually , corresponds to the ratio of the certain monomer to the total monomers used for polymerization of the polymer (feeding ratio).
In Examples and Comparative Examples, the volume average particle diameter of the composite particles, the average particle diameter of the silicon-based active material in the negative electrode composite layer, the average diameter of CNTs, the CNT adhesion ratio to the silicon-based active material, and The initial efficiency, cycle characteristics, and resistance increase rate at low SOC (state of charge) of the lithium ion secondary battery were evaluated using the following methods.

<Volume average particle diameter of composite particles>
Composite particles were measured in accordance with JIS Z8825:2013 using a laser diffraction/scattering particle size distribution analyzer (Microtrack MT-3300EXII, manufactured by Microtrack Bell Co., Ltd.) without dispersing the particles with compressed air. The volume average particle diameter was measured by dry method. As the volume average particle diameter, the particle diameter (D50) at an integrated value of 50% in the particle size distribution in terms of volume was adopted as described above.
<Average particle diameter of silicon-based active material>
The negative electrode was cut in the thickness direction using a cross section sample preparation device (Cross Section Polisher (registered trademark), manufactured by JEOL Ltd.). Next, the cut cross section of the negative electrode was observed using a scanning electron microscope (SEM). The breadth and length of each silicon-based active material particle included in the observation field were measured, and the average value of the breadth and length was taken as the particle diameter of the silicon-based active material particle. The same operation was performed in an arbitrary number of observation fields, and the arithmetic mean value of the particle diameters of a total of 50 particles of the silicon-based active material was taken as the average particle diameter of the silicon-based active material.
It was confirmed that the particles in the observation field were silicon-based active materials by measuring the Si intensity using a scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX).
<Average diameter of CNT>
The negative electrode was cut in the thickness direction using a cross section sample preparation device (Cross Section Polisher, manufactured by JEOL Ltd.). The diameters of 50 CNTs included in an arbitrary number of observation fields were measured, and the arithmetic mean value of the diameters of these 50 diameters was taken as the average diameter of the CNTs.
<CNT adhesion ratio to silicon-based active material>
The surface of the negative electrode on the negative electrode composite layer side was observed using a scanning electron microscope (SEM). In the observation field, the diameter and length of each CNT present on the silicon-based active material particles were measured, and the area was determined from their product. These operations were performed on a total of 20 silicon-based active material particles, and the average value was calculated by adding up these areas and dividing by 20, which was taken as the existing area S1 of CNTs on the surface of the silicon-based active material.
Next, in the observation field, the diameter and length of each CNT present on the carbon-based active material particles were measured, and the area was determined from the product of these. These operations were performed on a total of 20 carbon-based active material particles, and the average value was calculated by adding up these areas and dividing by 20, which was taken as the existing area S2 of CNTs on the surface of the carbon-based active material.
From the values of S1 and S2 obtained as described above, the CNT adhesion ratio (%) to the silicon-based active material was calculated using the formula: {S1/(S1+S2)}×100.
<Initial efficiency>
After injecting the electrolyte, the lithium ion secondary battery was allowed to stand in an environment of 25° C. for 24 hours. Next, the cell was charged by a constant current and constant voltage method at 0.1C until the cell voltage was 4.35V and the cut current value was 0.02C to obtain an initial charging capacity. Thereafter, the initial discharge capacity was obtained by a constant current method at 0.1 C in an environment of 25°C. Based on these values, initial efficiency (%)=(initial discharge capacity)/(initial charge capacity)×100 was calculated and evaluated based on the following criteria. The higher the initial efficiency, the more effectively the active material of the lithium ion secondary battery can be charged and discharged.
A+: Initial efficiency is 88% or more A: Initial efficiency is 86% or more and less than 88% B: Initial efficiency is 84% or more and less than 86% C: Initial efficiency is less than 84% <Cycle characteristics>
After injecting the electrolyte, the lithium ion secondary battery was allowed to stand in an environment of 25° C. for 24 hours. Next, a charging/discharging operation was performed in which the battery was charged to a cell voltage of 4.35 V using a constant current method at 0.1 C and discharged to a cell voltage of 2.75 V, and the initial capacity C0 was measured. Furthermore, in an environment of 25°C, the cell voltage was charged to 4.35V using the constant current method at 1.0C, and the charging and discharging processes were repeated by charging the cell voltage to 4.35V using the constant current method and discharging until the cell voltage reached 2.75V. C1 was measured. Then, capacity retention rate (%)=C1/C0×100 was calculated and evaluated based on the following criteria. The higher the capacity retention rate, the better the cycle characteristics of the lithium ion secondary battery.
A: Capacity retention rate is 90% or more B: Capacity retention rate is 85% or more and less than 90% C: Capacity retention rate is 80% or more and less than 85% D: Capacity retention rate is less than 80% <Resistance increase rate at low SOC ＞
After injecting the electrolyte, the lithium ion secondary battery was allowed to stand in an environment of 25° C. for 24 hours. Next, a charging/discharging operation was performed in which the battery was charged to a cell voltage of 4.35 V using a constant current method at 0.1 C and discharged to a cell voltage of 2.75 V, and the initial capacity was measured. After that, after charging to SOC 20%, charging at 1.0C for 30 seconds and discharging for 30 seconds centering on SOC 20%, and dividing the amount of voltage change in 30 seconds during charging by the amount of current. IV resistance R1 was calculated.
Again, a high temperature storage test was performed by charging the cell voltage to 4.35V using the constant current method at 0.1C and storing it for one week in a constant temperature bath at 60℃. IV resistance was similarly measured to obtain IV resistance R2. The resistance increase rate was calculated by (R2/R1)×100 and evaluated according to the following criteria. The smaller the rate of increase in resistance, the less likely the conductive path is to be cut, indicating that the conductive path is well maintained in the negative electrode composite layer even after high-temperature storage or repeated charging and discharging.
A: Resistance increase rate is less than 110% B: Resistance increase rate is 110% or more and less than 120% C: Resistance increase rate is 120% or more and less than 140% D: Resistance increase rate is 140% or more

(Example 1)
<Preparation of composite particles>
Single-walled CNT (manufactured by Nippon Zeon Co., Ltd., product name "SG101"), sodium salt of carboxymethyl cellulose (a water-soluble dispersant. Weight average molecular weight: 80,000, hereinafter referred to as "CMC"), and as a dispersion medium. and an appropriate amount of ion-exchanged water were stirred with a disper (3000 rpm, 60 minutes), and then mixed for 30 minutes at a peripheral speed of 8 m/s using a bead mill using zirconia beads with a diameter of 1 mm. Thereafter, a CNT paste (solid content concentration: 1.0%) was manufactured by further mixing in a bead mill for 30 minutes. Note that the mixing ratio of single-walled CNT and CMC was set to single-walled CNT:CMC=1:1 on a mass basis.
SiO (composite compound coated with conductive carbon; G/D ratio of conductive carbon: 0.7) as a silicon-based active material was added to the prepared CNT paste, and mixed for 10 minutes at 2000 rpm using a disper. A composition for composite particles was obtained. The mixing ratio of CNT paste and SiO was adjusted to be SiO:single-layer CNT:CMC=1000:1:1 on a mass basis.
The obtained composition for composite particles was spray-dried and granulated using a spray dryer (manufactured by Okawara Kakoki Co., Ltd.) while controlling the outlet side temperature to 100°C. The particles obtained after spray drying were vacuum dried at 120° C. for 10 hours to obtain composite particles. The volume average particle diameter of this composite particle was measured. The results are shown in Table 1.
<Preparation of binder>
In a reactor, 180 parts of ion-exchanged water, 25 parts of an aqueous sodium dodecylbenzenesulfonate solution (concentration 10%) as an emulsifier, 63 parts of styrene, 4 parts of methacrylic acid, and 0.3 parts of t-dodecyl mercaptan as a molecular weight regulator. parts were added in this order. Next, the gas inside the reactor was replaced with nitrogen three times, and then 33 parts of 1,3-butadiene as an aliphatic conjugated diene monomer was charged. A polymerization reaction was started by adding 0.1 part of cumene hydroperoxide as a polymerization initiator to a reactor maintained at 10° C., and the polymerization reaction was continued for 16 hours with stirring. Next, 0.1 part of a hydroquinone aqueous solution (concentration 10%) as a polymerization terminator was added to terminate the polymerization reaction to obtain a mixture containing a polymer. A 5% aqueous sodium hydroxide solution was added to the mixture containing this polymer to adjust the pH to 8. Thereafter, unreacted monomers were removed by heating and vacuum distillation. Then, it was cooled to obtain an aqueous dispersion (solid content concentration: 40%) containing a styrene-butadiene polymer (a water-insoluble binder; hereinafter referred to as "SBR").
<Preparation of slurry for negative electrode>
In a planetary mixer equipped with a disperser, 87 parts of artificial graphite (volume average particle diameter: 24.5 μm, specific surface area: 3.5 m ² /g) as a carbon-based active material and 10 parts of the composite particles obtained above were added. 02 parts (10 parts of SiO, 0.01 part of CMC, 0.01 part of CNT), 1.98 parts of an aqueous solution of CMC (weight average molecular weight: 300,000) as a thickener (solid content of CMC) equivalent amount) was added, the solid content concentration was adjusted to 58% with ion-exchanged water, and the mixture was mixed for 60 minutes at room temperature. Next, the solid content concentration was adjusted to 50% with ion-exchanged water, and 1.0 part (corresponding to the solid content of SBR) of the above-mentioned aqueous dispersion containing SBR was added to obtain a mixed solution. The resulting mixed solution was defoamed under reduced pressure to obtain a slurry for a negative electrode with good fluidity.
<Preparation of negative electrode>
The slurry for the negative electrode obtained as above was coated with a comma coater on a copper foil (thickness: 16 μm) as a current collector so that the film thickness after drying was 105 μm and the coating amount was 10 mg/ ^cm2 . I applied it to make it look like this. The copper foil coated with this negative electrode slurry was transported at a speed of 0.5 m/min in an oven at a temperature of 100°C for 2 minutes, and then in an oven at a temperature of 120°C for 2 minutes. The negative electrode slurry was dried to obtain a negative electrode material. This negative electrode original fabric was rolled with a roll press to obtain a negative electrode in which the thickness of the negative electrode composite material layer was 80 μm.
Regarding the obtained negative electrode, the average particle diameter of the silicon-based active material, the average diameter of the CNTs, and the CNT adhesion ratio to the silicon-based active material were determined. The results are shown in Table 1.
<Preparation of positive electrode>
In a planetary mixer, 95 parts of LiCoO ₂ having a spinel structure as a positive electrode active material, 3 parts of PVDF (polyvinylidene fluoride) as a binder for the positive electrode in terms of solid content, 2 parts of acetylene black as a conductive material, and 20 parts of N-methylpyrrolidone as a solvent were added and mixed to obtain a positive electrode slurry.
The obtained positive electrode slurry was coated on an aluminum foil (thickness: 20 μm) as a current collector using a comma coater so that the film thickness after drying was about 100 μm. The aluminum foil coated with this positive electrode slurry was transported at a speed of 0.5 m/min in an oven at a temperature of 60°C for 2 minutes, and then in an oven at a temperature of 120°C for 2 minutes. The positive electrode slurry was dried to obtain a positive electrode material. This positive electrode original fabric was rolled with a roll press to obtain a positive electrode in which the thickness of the positive electrode composite layer was 70 μm.
<Preparation of separator>
A single-layer polypropylene separator (width 65 mm, length 500 mm, thickness 25 μm; manufactured by a dry method; porosity 55%) was prepared. This separator was cut into a 5 cm x 5 cm square and used for manufacturing the following lithium ion secondary battery.
<Manufacture of secondary batteries>
An aluminum packaging material exterior was prepared as the battery exterior. The above positive electrode was cut out into a square of 4 cm x 4 cm and placed so that the surface on the current collector side was in contact with the exterior of the aluminum packaging material. Next, the square separator was placed on the surface of the positive electrode composite layer of the positive electrode. Further, the above negative electrode was cut into a square of 4.2 cm x 4.2 cm, and this was placed on a separator so that the surface on the negative electrode composite layer side faced the separator. After that, a LiPF ₆ solution with a concentration of 1.0 M was used as an electrolyte (the solvent was a mixed solvent of ethylene carbonate/diethyl carbonate = 1/2 (volume ratio), and fluoroethylene carbonate and vinylene carbonate were added as additives at 2 volume % each (solvent ratio). ) containing). Furthermore, in order to seal the opening of the aluminum packaging material, heat sealing was performed at 150° C. to close the aluminum packaging material exterior, thereby manufacturing a laminate cell type lithium ion secondary battery. This lithium ion secondary battery was evaluated for initial efficiency, cycle characteristics, and rate of increase in resistance at low SOC. The results are shown in Table 1.

(Example 2)
A binder, a negative electrode, a positive electrode, a separator, and a secondary battery were prepared in the same manner as in Example 1, except that the composite particles and negative electrode slurry prepared as follows were used, and various evaluations were performed. The results are shown in Table 1.
<Preparation of composite particles>
Single-walled CNTs (manufactured by Nippon Zeon Co., Ltd., product name "SG101"), CMC (water-soluble dispersant, weight average molecular weight: 80,000), and an appropriate amount of ion-exchanged water as a dispersion medium were stirred with a disper. (3000 rpm, 60 minutes), and then mixed for 30 minutes at a peripheral speed of 8 m/s using a bead mill using zirconia beads with a diameter of 1 mm. Thereafter, a CNT paste (solid content concentration: 1.0%) was manufactured by further mixing in a bead mill for 30 CMC minutes. Note that the mixing ratio of single-walled CNT and CMC was set to single-walled CNT:CMC=15:10 on a mass basis.
SiO (composite compound coated with conductive carbon; G/D ratio of conductive carbon: 0.7) as a silicon-based active material was added to the prepared CNT paste, and mixed for 10 minutes at 2000 rpm using a disper. A composition for composite particles was obtained. The mixing ratio of CNT paste and SiO was adjusted to be SiO:single-layer CNT:CMC=1000:15:10 on a mass basis.
The obtained composition for composite particles was spray-dried and granulated using a spray dryer (manufactured by Okawara Kakoki Co., Ltd.) while controlling the outlet side temperature to 100°C. The particles obtained after spray drying were vacuum dried at 120° C. for 10 hours to obtain composite particles.
<Preparation of slurry for negative electrode>
In a planetary mixer equipped with a disperser, 87 parts of artificial graphite (volume average particle diameter: 24.5 μm, specific surface area: 3.5 m ² /g) as a carbon-based active material and 10 parts of the composite particles obtained above were added. 25 parts (10 parts of SiO, 0.1 part of CMC, 0.15 parts of CNT), 1.75 parts of an aqueous solution of CMC (weight average molecular weight: 300,000) as a thickener (solid content of CMC) equivalent amount) was added, the solid content concentration was adjusted to 58% with ion-exchanged water, and the mixture was mixed for 60 minutes at room temperature. Next, the solid content concentration was adjusted to 50% with ion-exchanged water, and 1.0 part (corresponding to the solid content of SBR) of the above-mentioned aqueous dispersion containing SBR was added to obtain a mixed solution. The resulting mixed solution was defoamed under reduced pressure to obtain a slurry for a negative electrode with good fluidity.

(Example 3)
A binder, a negative electrode, a positive electrode, a separator, and a secondary battery were prepared in the same manner as in Example 1, except that the composite particles and negative electrode slurry prepared as follows were used, and various evaluations were performed. The results are shown in Table 1.
<Preparation of composite particles>
Single-walled CNTs (manufactured by Nippon Zeon Co., Ltd., product name "SG101"), CMC (water-soluble dispersant, weight average molecular weight: 80,000), and an appropriate amount of ion-exchanged water as a dispersion medium were stirred with a disper. (3000 rpm, 60 minutes), and then mixed for 30 minutes at a peripheral speed of 8 m/s using a bead mill using zirconia beads with a diameter of 1 mm. Thereafter, a CNT paste (solid content concentration: 1.0%) was manufactured by further mixing in a bead mill for 30 CMC minutes. Note that the mixing ratio of single-walled CNT and CMC was set to single-walled CNT:CMC=1:1 on a mass basis.
SiO (composite compound coated with conductive carbon; G/D ratio of conductive carbon: 0.7) as a silicon-based active material was added to the prepared CNT paste, and mixed for 10 minutes at 2000 rpm using a disper. A composition for composite particles was obtained. The mixing ratio of CNT paste and SiO was adjusted to be SiO:single-layer CNT:CMC=1000:5:5 on a mass basis.
The obtained composition for composite particles was spray-dried and granulated using a spray dryer (manufactured by Okawara Kakoki Co., Ltd.) while controlling the outlet side temperature to 100°C. The particles obtained after spray drying were vacuum dried at 120° C. for 10 hours to obtain composite particles.
<Preparation of slurry for negative electrode>
In a planetary mixer equipped with a disperser, 77 parts of artificial graphite (volume average particle diameter: 24.5 μm, specific surface area: 3.5 m ² /g) as a carbon-based active material and 20 parts of the composite particles obtained above were added. 2 parts (20 parts of SiO, 0.1 part of CMC, 0.1 part of CNT), 1.8 parts of an aqueous solution of CMC (weight average molecular weight: 300,000) as a thickener (solid content of CMC) equivalent amount) was added, the solid content concentration was adjusted to 58% with ion-exchanged water, and the mixture was mixed for 60 minutes at room temperature. Next, the solid content concentration was adjusted to 50% with ion-exchanged water, and 1.0 part (corresponding to the solid content of SBR) of the above-mentioned aqueous dispersion containing SBR was added to obtain a mixed solution. The resulting mixed solution was defoamed under reduced pressure to obtain a slurry for a negative electrode with good fluidity.

(Example 4)
Same as Example 1 except that CMC (water-soluble dispersant; weight average molecular weight: 50,000) was used instead of CMC (water-soluble dispersant; weight average molecular weight: 80,000) when preparing composite particles. Similarly, composite particles, a binder, a slurry for a negative electrode, a negative electrode, a positive electrode, a separator, and a secondary battery were prepared and various evaluations were performed. The results are shown in Table 1.

(Example 5)
When preparing composite particles, the composite particles, binder, negative electrode slurry, negative electrode, positive electrode, separator, and secondary battery were prepared in the same manner as in Example 1, except that the vacuum drying temperature was changed from 120°C to 140°C. We prepared and conducted various evaluations. The results are shown in Table 1.

(Example 6)
When preparing composite particles, the composite particles, binder, negative electrode slurry, negative electrode, positive electrode, separator, and secondary battery were prepared in the same manner as in Example 1, except that the vacuum drying temperature was changed from 120°C to 160°C. We prepared and conducted various evaluations. The results are shown in Table 1.

(Example 7)
When preparing composite particles, Li _y SiO _z (where y is more than 0 and less than 4, and z is more than 0.5 and less than 4) is used instead of SiO as a silicon-based active material. Composite particles, binder, slurry for negative electrode, negative electrode, positive electrode, Separators and secondary batteries were prepared and various evaluations were performed. The results are shown in Table 1.

(Example 8)
A binder, a negative electrode, a positive electrode, a separator, and a secondary battery were prepared in the same manner as in Example 1, except that the composite particles and negative electrode slurry prepared as follows were used, and various evaluations were performed. The results are shown in Table 1.
<Preparation of composite particles>
Multi-walled CNTs, CMC (water-soluble dispersant, weight average molecular weight: 80,000) and an appropriate amount of ion-exchanged water as a dispersion medium were stirred in a disper (3000 rpm, 60 minutes), and then 1 mm diameter zirconia Mixing was carried out for 30 minutes at a peripheral speed of 8 m/s using a bead mill using beads. Thereafter, a CNT paste (solid content concentration: 1.0%) was manufactured by further mixing in a bead mill for 30 CMC minutes. In addition, the mixing ratio of multiwall CNT and CMC was set to multiwall CNT:CMC=1:1 on a mass basis.
SiO (composite compound coated with conductive carbon; G/D ratio of conductive carbon: 0.7) as a silicon-based active material was added to the prepared CNT paste, and mixed for 10 minutes at 2000 rpm using a disper. A composition for composite particles was obtained. The mixing ratio of CNT paste and SiO was adjusted to be SiO:multilayer CNT:CMC=1000:10:10 on a mass basis.
The obtained composition for composite particles was spray-dried and granulated using a spray dryer (manufactured by Okawara Kakoki Co., Ltd.) while controlling the outlet side temperature to 100°C. The particles obtained after spray drying were vacuum dried at 120° C. for 10 hours to obtain composite particles.
<Preparation of slurry for negative electrode>
In a planetary mixer equipped with a disperser, 87 parts of artificial graphite (volume average particle diameter: 24.5 μm, specific surface area: 3.5 m ² /g) as a carbon-based active material and 10 parts of the composite particles obtained above were added. 2 parts (10 parts of SiO, 0.1 part of CMC, 0.1 part of CNT), 1.8 parts of an aqueous solution of CMC (weight average molecular weight: 300,000) as a thickener (solid content of CMC) equivalent amount) was added, the solid content concentration was adjusted to 58% with ion-exchanged water, and the mixture was mixed for 60 minutes at room temperature. Next, the solid content concentration was adjusted to 50% with ion-exchanged water, and 1.0 part (corresponding to the solid content of SBR) of the above-mentioned aqueous dispersion containing SBR was added to obtain a mixed solution. The resulting mixed solution was defoamed under reduced pressure to obtain a slurry for a negative electrode with good fluidity.

(Example 9)
Composite particles were prepared in the same manner as in Example 1, except that polyvinylpyrrolidone (a water-soluble dispersant, weight average molecular weight: 65,000, hereinafter referred to as "PVP") was used instead of CMC. Particles, a binder, a slurry for a negative electrode, a negative electrode, a positive electrode, a separator, and a secondary battery were prepared and various evaluations were performed. The results are shown in Table 1.

(Comparative example 1)
A binder, a negative electrode, a positive electrode, a separator, and a secondary battery were prepared in the same manner as in Example 1, except that the slurry for a negative electrode prepared as follows was used, and various evaluations were performed. The results are shown in Table 1. Note that composite particles were not prepared.
<Preparation of slurry for negative electrode>
Single-walled CNTs (manufactured by Nippon Zeon Co., Ltd., product name "SG101"), CMC (water-soluble dispersant, weight average molecular weight: 80,000), and an appropriate amount of ion-exchanged water as a dispersion medium were stirred with a disper. (3000 rpm, 60 minutes), and then mixed for 30 minutes at a peripheral speed of 8 m/s using a bead mill using zirconia beads with a diameter of 1 mm. Thereafter, a CNT paste (solid content concentration: 1.0%) was manufactured by further mixing in a bead mill for 30 minutes. Note that the mixing ratio of single-walled CNT and CMC was set to single-walled CNT:CMC=1:1 on a mass basis.
In a planetary mixer equipped with a disperser, 87 parts of artificial graphite (volume average particle diameter: 24.5 μm, specific surface area: 3.5 m ² /g) as a carbon-based active material and 10 parts of SiO as a silicon-based active material were added. , 0.02 part of the CNT paste obtained above (equivalent to the solid content of CMC and CNT, 0.01 part of CMC, 0.01 part of CNT), CMC as a thickener (weight average molecular weight: 300 ,000) was added thereto, the solid content concentration was adjusted to 58% with ion-exchanged water, and the mixture was mixed for 60 minutes at room temperature. Next, the solid content concentration was adjusted to 50% with ion-exchanged water, and 1.0 part (equivalent to the solid content of SBR) of an aqueous dispersion containing SBR obtained in the same manner as in Example 1 was added to make a mixed solution. Obtained. The resulting mixed solution was defoamed under reduced pressure to obtain a slurry for a negative electrode with good fluidity.

(Comparative example 2)
When preparing composite particles, the composite particles, binder, negative electrode slurry, negative electrode, positive electrode, separator, and secondary battery were prepared in the same manner as in Example 1, except that the vacuum drying temperature was changed from 120°C to 200°C. We prepared and conducted various evaluations. The results are shown in Table 1.

(Comparative example 3)
Composite particles, binder, and negative electrode were prepared in the same manner as in Example 1, except that multi-wall CNTs (different from the multi-wall CNTs used in Example 8) were used instead of single-wall CNTs when preparing composite particles. A slurry, a negative electrode, a positive electrode, a separator, and a secondary battery were prepared and various evaluations were performed. The results are shown in Table 1.

(Comparative example 4)
When preparing composite particles, SiO (composite compound coated with conductive carbon; G/D ratio of conductive carbon: 0.7) having a particle size different from that used in Example 1 was used as a silicon-based active material. Composite particles, a binder, a slurry for a negative electrode, a negative electrode, a positive electrode, a separator, and a secondary battery were prepared in the same manner as in Example 1 except that they were used, and various evaluations were performed. The results are shown in Table 1.

In addition, in Table 1,
"Ratio of Si in CNT+Si" indicates the ratio of the mass of the silicon-based active material to the total mass of the silicon-based active material and the mass of the CNT.

From Table 1, in the negative electrode composite layer, the average particle diameter of the silicon-based active material and the average diameter of CNT are within the predetermined range, and in addition, the CNT adhesion ratio to the silicon-based active material is within the predetermined range. In Examples 1 to 9, it can be seen that the secondary batteries can exhibit excellent cycle characteristics.

Claims

A negative electrode for a non-aqueous secondary battery comprising a negative electrode composite layer containing a negative electrode active material and carbon nanotubes,
The negative electrode active material includes a carbon-based active material and a silicon-based active material,
The average particle diameter of the silicon-based active material is 2 μm or more and 10 μm or less,
The average diameter of the carbon nanotubes is 1.2 nm or more and 30 nm or less, and
The proportion of the existing area S1 in the total of the existing area S1 of the carbon nanotubes on the surface of the silicon-based active material and the existing area S2 of the carbon nanotubes on the surface of the carbon-based active material is 55% or more and 98% or less. A negative electrode for non-aqueous secondary batteries.
The non-containing material according to claim 1, wherein the proportion of the mass of the silicon-based active material in the total mass of the silicon-based active material and the mass of the carbon nanotubes is 96.5% by mass or more and 99.95% by mass or less. Negative electrode for water-based secondary batteries.
The silicon-based active material has the formula: Li y SiO z [where y is greater than 0 and less than or equal to 4, and z is greater than or equal to 0.5 and less than or equal to 4. ] The negative electrode for a non-aqueous secondary battery according to claim 1, comprising an active material represented by the following.
2. The silicon-based active material includes a composite of a Si-containing material and conductive carbon, and the ratio of the G-band peak intensity to the D-band peak intensity in the Raman spectrum of the conductive carbon is 4 or less. The negative electrode for non-aqueous secondary batteries described above.
The negative electrode for a non-aqueous secondary battery according to claim 1, wherein the negative electrode composite material layer further contains a dispersant.
The negative electrode for a non-aqueous secondary battery according to claim 1, wherein the negative electrode composite layer contains at least one of carboxymethyl cellulose and a salt thereof.
A non-aqueous secondary battery comprising the negative electrode for a non-aqueous secondary battery according to any one of claims 1 to 6.