Negative electrode for secondary batteries and secondary batteries

Abstract

A secondary battery negative electrode according to the present invention is provided with a negative electrode mixture layer comprising: a negative electrode active material that contains an Si compound; and a binding material that includes cellulose nanofibers having a fiber diameter of 9 nm or less and a conductive material containing single-walled carbon nanotubes having a fiber diameter of less than 4 nm. The content of the cellulose nanofibers is greater than or equal to 0.005 mass% and less than 0.2 mass% in relation to the mass of the negative electrode active material.

Description

[Technical Field]

[0001] This disclosure relates to a negative electrode for a secondary battery and a secondary battery. [Background technology]

[0002] Si compounds are alloying materials that combine with lithium and are known to be able to absorb more lithium ions per unit volume compared to carbon-based active materials such as graphite. Therefore, they are expected to be used as negative electrode active materials in secondary batteries.

[0003] However, because Si compounds undergo large volume changes (expansion and contraction) during charging and discharging, repeated charging and discharging can break the conductive network between the negative electrode active materials. As a result, there is a problem of reduced charge-discharge cycle characteristics.

[0004] To address these issues, Patent Document 1 discloses a technique in which carbon nanotubes are added to a negative electrode composite layer containing a Si compound to suppress the disruption of the conductive network between negative electrode active materials due to the expansion and contraction of the Si compound, thereby suppressing the deterioration of charge-discharge cycle characteristics. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2016-110876 [Patent Document 2] Japanese Patent Publication No. 2017-228456 [Overview of the project]

[0006] In secondary batteries using negative electrode active materials containing Si compounds, further improvement in charge-discharge cycle characteristics is desired.

[0007] The negative electrode for a secondary battery according to one aspect of the present disclosure includes a negative electrode active material containing a Si compound, a conductive material containing single-walled carbon nanotubes with a fiber diameter of less than 4 nm, and a binder containing cellulose nanofibers with a fiber diameter of 9 nm or less, and includes a negative electrode composite material layer having the same. The content of the cellulose nanofibers is 0.005% by mass or more and less than 0.2% by mass with respect to the mass of the negative electrode active material.

[0008] Moreover, a secondary battery according to one aspect of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, and the negative electrode is the negative electrode for the secondary battery described above.

[0009] According to one aspect of the present disclosure, it is possible to improve the charge-discharge cycle characteristics of a secondary battery using a negative electrode active material containing a Si compound.

Brief Description of the Drawings

[0010] [Figure 1] FIG. 1 is a cross-sectional view of a secondary battery which is an example of an embodiment.

Modes for Carrying Out the Invention

[0011] The negative electrode for a secondary battery according to one aspect of the present disclosure includes a negative electrode active material containing a Si compound, a conductive material containing single-walled carbon nanotubes with a fiber diameter of less than 4 nm, and a binder containing cellulose nanofibers with a fiber diameter of 9 nm or less, and includes a negative electrode composite material layer having the same. The content of the cellulose nanofibers is 0.005% by mass or more and less than 0.2% by mass with respect to the mass of the negative electrode active material. And according to the negative electrode for a secondary battery which is one aspect of the present disclosure, it is possible to improve the charge-discharge cycle characteristics of a secondary battery. The mechanism for achieving the above effect is considered to be as follows.

[0012] Cellulose nanofibers are thought to adsorb to the surface of carbon nanotubes, thereby enhancing their dispersibility. Specifically, while single-walled carbon nanotubes with a fiber diameter of less than 4 nm tend to aggregate in bundles, the cellulose nanofibers adsorbed on the surface act as steric hindrance to these bundled carbon nanotubes, thus improving their dispersibility. In other words, during anode manufacturing, by adding a predetermined amount of cellulose nanofibers with a fiber diameter of 9 nm or less to a slurry containing the anode active material and carbon nanotubes with a fiber diameter of less than 4 nm, for example, single-walled carbon nanotubes can be uniformly dispersed. However, in the case of carbon nanotubes with a fiber diameter of 4 nm or more, they tend to clump together and aggregate like dust, making it difficult for cellulose nanofibers to act as steric hindrance, and thus preventing uniform dispersion of the carbon nanotubes. Therefore, as in the negative electrode of this embodiment, the presence of cellulose nanofibers with a fiber diameter of 9 nm or less in a predetermined amount suppresses the aggregation of single-walled carbon nanotubes (for example, by uniform dispersion), and a sufficiently conductive network is formed between the negative electrode active material containing the Si compound and the single-walled carbon nanotubes, thus improving the charge-discharge cycle characteristics.

[0013] An example of an embodiment will be described in detail below with reference to the drawings. Note that the non-aqueous electrolyte secondary battery of this disclosure is not limited to the embodiments described below. Also, the drawings referenced in the description of the embodiments are schematic.

[0014] Figure 1 is a cross-sectional view of a secondary battery, which is an example of an embodiment. The secondary battery 10 shown in Figure 1 comprises a wound electrode body 14 in which a positive electrode 11 and a negative electrode 12 are wound around a separator 13, a non-aqueous electrolyte, insulating plates 18 and 19 arranged above and below the electrode body 14, respectively, and a battery case 15 that houses the above components. The battery case 15 is composed of a bottomed cylindrical case body 16 and a sealing body 17 that closes the opening of the case body 16. In addition, other forms of electrode bodies may be used instead of the wound electrode body 14, such as a laminated electrode body in which the positive electrode and negative electrode are alternately stacked with a separator. Examples of battery cases 15 include cylindrical, rectangular, coin-shaped, button-shaped metal outer casings, and pouch outer casings formed by laminating a resin sheet and a metal sheet.

[0015] The case body 16 is, for example, a bottomed cylindrical metal outer casing. A gasket 28 is provided between the case body 16 and the sealing body 17 to ensure airtightness inside the battery. The case body 16 has, for example, a protruding portion 22 that supports the sealing body 17, which is a part of the side surface that protrudes inward. The protruding portion 22 is preferably formed in an annular shape along the circumferential direction of the case body 16, and its upper surface supports the sealing body 17.

[0016] The sealing body 17 has a structure in which a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective centers, with the insulating member 25 interposed between their respective peripheries. When the internal pressure of the secondary battery 10 rises due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 towards the cap 27, thus interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0017] In the secondary battery 10 shown in Figure 1, the positive electrode lead 20 attached to the positive electrode 11 extends through a through-hole in the insulating plate 18 towards the sealing body 17, and the negative electrode lead 21 attached to the negative electrode 12 extends outside the insulating plate 19 towards the bottom of the case body 16. The positive electrode lead 20 is connected by welding or the like to the lower surface of the filter 23, which is the bottom plate of the sealing body 17, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the filter 23, becomes the positive electrode terminal. The negative electrode lead 21 is connected by welding or the like to the inner surface of the bottom of the case body 16, and the case body 16 becomes the negative electrode terminal.

[0018] The following provides a detailed explanation of each component of the secondary battery 10.

[0019] [Negative electrode] The negative electrode 12 comprises a negative electrode current collector made of, for example, a metal foil, and a negative electrode composite layer formed on the current collector. The negative electrode current collector can be made of, for example, a metal foil that is stable in the negative electrode potential range, such as copper, or a film with the metal arranged on its surface. The negative electrode composite layer includes a negative electrode active material, a binder, and a conductive material.

[0020] The negative electrode 12 can be manufactured, for example, by preparing a negative electrode composite slurry containing a negative electrode active material, a binder, a conductive material, etc., applying this negative electrode composite slurry onto a negative electrode current collector, drying it to form a negative electrode composite layer, and then performing a compression step to compress the negative electrode composite layer using a rolling roller or the like.

[0021] The negative electrode active material contains a Si compound. The Si compound can be any material that can intercept and release lithium ions, but from the viewpoint of increasing the capacity of secondary batteries, it is preferable that it contains a lithium ion conduction phase and Si particles dispersed in the lithium ion conduction phase, and the lithium ion conduction phase is preferably at least one selected from a silicate phase, a silicon oxide phase and a carbon phase.

[0022] It is preferable that a conductive coating composed of a highly conductive material is formed on the particle surface of the Si compound. Examples of constituent materials for the conductive coating include at least one selected from carbon materials, metals, and metal compounds. Among these, carbon materials such as amorphous carbon are preferred. The carbon coating can be formed, for example, by a CVD method using acetylene, methane, etc., or by mixing coal pitch, petroleum pitch, phenolic resin, etc. with a silicon-based active material and performing heat treatment. Alternatively, a conductive coating may be formed by fixing a conductive filler such as carbon black to the particle surface of the Si compound using a binder.

[0023] Specific examples of Si compounds include Si compound A, which contains a silicate phase and Si particles dispersed in the silicate phase; Si compound B, which contains a silicon oxide phase and Si particles dispersed in the silicon oxide phase; and Si compound C, which contains a carbon phase and Si particles dispersed in the carbon phase. These can be used individually or in combination of two or more.

[0024] The silicate phase of Si compound A preferably contains at least one element selected from lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, radium, etc., due to its high lithium ion conductivity. Among these, a silicate phase containing lithium (hereinafter sometimes referred to as the lithium silicate phase) is preferred.

[0025] The silicon particle content in Si compound A is preferably 30% by mass or more and 80% by mass or less, preferably 35% by mass or more and 75% by mass or less, and more preferably 55% by mass or more and 70% by mass or less, in terms of increasing capacity and improving charge-discharge cycle characteristics.

[0026] The average particle size of the Si particles is preferably 500 nm or less, more preferably 200 nm or less, and even more preferably 50 nm or less before the first charge, from the viewpoint of suppressing cracks in the Si particles themselves, etc. After the first charge, the average particle size of the Si particles is preferably 400 nm or less, and more preferably 100 nm or less. The average particle size of the Si particles is measured by observing a cross-sectional SEM (scanning electron microscope) photograph of the Si compound. Specifically, the average particle size of the Si particles is obtained by averaging the maximum diameters of any 100 Si particles.

[0027] The lithium silicate phase is represented by, for example, the formula: Li 2z SiO 2+z (0 < z < 2). From the viewpoints of stability, ease of production, lithium ion conductivity, etc., it is preferable that z satisfies the relationship 0 < z < 1, and more preferably z = 1 / 2.

[0028] The Si compound B in which Si particles are dispersed in the silicon oxide phase is represented by, for example, the general formula SiO x (a range of 0 < x < 2 is preferable, and a range of 0.5 ≦ x ≦ 1.6 is more preferable). The Si compound C in which Si particles are dispersed in the carbon phase is represented by, for example, the general formula Si x C y (a range of 0 < x ≦ 1 and 0 < y ≦ 1 is preferable, and a range of 0.3 ≦ x ≦ 0.45 and 0.7 ≦ y ≦ 0.55 is more preferable). The carbon phase contains, for example, at least one of amorphous carbon and crystalline carbon. The content and average particle size of the Si particles in the Si compounds B and C may be the same as those in the Si compound A.

[0029] The content of the Si compound in the negative electrode active material is preferably 1% by mass or more and 10% by mass or less with respect to the mass of the negative electrode active material, from the viewpoints of increasing the capacity of the secondary battery, improving the charge-discharge cycle characteristics, etc.

[0030] The negative electrode active material preferably contains graphite particles from the viewpoint of improving the charge-discharge cycle characteristics of the secondary battery, etc. The graphite particles are not particularly limited, such as natural graphite and artificial graphite. The interplanar spacing (d of the (002) plane of the graphite particles by wide-angle X-ray diffraction002 The plane spacing (d 002 When the ) and crystallite size (Lc(002)) satisfy the above range, the battery capacity of the secondary battery tends to be larger compared to when the above range is not satisfied.

[0031] The content of graphite particles in the negative electrode active material is preferably 80% by mass or more and 90% by mass or less, relative to the mass of the negative electrode active material, for example, in terms of increasing the capacity of the secondary battery and improving the charge-discharge cycle characteristics.

[0032] The content of the negative electrode active material in the negative electrode composite layer is preferably 85% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more, relative to the mass of the negative electrode composite layer.

[0033] The conductive material contained in the negative electrode composite layer includes single-walled carbon nanotubes (SWCNTs). Single-walled carbon nanotubes (SWCNTs) are carbon nanostructures in which a single layer of graphene sheet forms a single cylindrical shape. A graphene sheet refers to a layer in which carbon atoms with sp2 hybrid orbitals that make up the crystal of graphite are located at the vertices of a regular hexagon. The shape of single-walled carbon nanotubes is not limited, but examples of such shapes include needle-shaped, cylindrical tube-shaped, fishbone-shaped (fishbone or cup stacked type), playing card-shaped (platelet), and coil-shaped.

[0034] The outermost diameter (i.e., fiber diameter) of a single-walled carbon nanotube may be less than 4 nm, for example, because it facilitates the formation of a conductive network with the negative electrode active material. However, it is preferably between 1 nm and 3 nm. The outermost diameter of a single-walled carbon nanotube can be determined by measuring the outer diameters of 50 arbitrary carbon nanotubes using a field emission scanning microscope (FE-SEM) or a transmission electron microscope (TEM) and calculating the arithmetic mean.

[0035] The fiber length of single-walled carbon nanotubes is preferably 500 nm or more and 200 μm or less, and preferably 1 μm or more and 100 μm or less, for example, in terms of efficiently forming a conductive network with the negative electrode active material. The fiber length of single-walled carbon nanotubes can be determined by measuring the lengths of 50 arbitrary single-walled carbon nanotubes using a field emission scanning microscope (FE-SEM) and calculating the arithmetic mean.

[0036] The content of single-walled carbon nanotubes is preferably 0.001% by mass or more and 0.1% by mass or less, and more preferably 0.01% by mass or more and 0.1% by mass or less, relative to the mass of the negative electrode active material, for example, in terms of efficiently forming a conductive network with the negative electrode active material.

[0037] The conductive material may include not only single-walled carbon nanotubes but also multi-walled carbon nanotubes, to the extent that it does not impair the effects of the present disclosure. Multi-walled carbon nanotubes are carbon nanostructures in which two or more graphene sheets are stacked concentrically to form a single cylindrical shape.

[0038] The conductive material may include particulate conductive material as needed. Examples of particulate conductive material include carbon materials such as carbon black, acetylene black, Ketjen black, and graphite. When using particulate conductive material, it is preferable that the primary particle size is between 5 nm and 100 nm, and the aspect ratio is less than 10.

[0039] The binder contains cellulose nanofibers. Cellulose nanofibers are fine fibers containing cellulose, which forms the backbone of plant cell walls. The fiber diameter of such cellulose nanofibers may be 9 nm or less, but it is preferable that it be between 1 nm and 8 nm, and more preferably between 1 nm and 3 nm, in order to further improve the dispersibility of single-walled carbon nanotubes. The method for measuring the fiber diameter is the same as for carbon nanotubes.

[0040] The fiber length of cellulose nanofibers is preferably 1 μm to 20 μm, and more preferably 5 μm to 10 μm, in order to further improve the dispersibility of single-walled carbon nanotubes. The method for measuring the fiber length is the same as for carbon nanotubes.

[0041] The cellulose nanofiber content may be 0.005% by mass or more and 0.2% by mass or less relative to the mass of the negative electrode active material, but it is preferable to have a content of 0.005% by mass or more and 0.1% by mass or less, and more preferably 0.01% by mass or more and 0.1% by mass or less, in order to further improve the charge-discharge cycle characteristics.

[0042] The binder may contain, in addition to cellulose nanofibers, fluororesins, PAN, polyimide resins, acrylic resins, polyolefin resins, styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts, polyvinyl alcohol (PVA), etc. These may be used individually or in combination of two or more types.

[0043] [Positive electrode] The positive electrode 11 is composed of a positive electrode current collector, such as a metal foil, and a positive electrode composite layer formed on the positive electrode current collector. The positive electrode current collector can be a metal foil that is stable in the positive electrode potential range, such as aluminum, or a film with the metal arranged on its surface. The positive electrode composite layer includes, for example, a positive electrode active material, a binder, a conductive material, etc.

[0044] The positive electrode 11 can be manufactured by, for example, applying a positive electrode composite material slurry containing a positive electrode active material, a binder, a conductive material, etc. onto a positive electrode current collector, drying to form a positive electrode composite material layer, and then performing a compression process of compressing this positive electrode composite material layer using a rolling roller or the like.

[0045] Examples of the positive electrode active material include lithium transition metal oxides containing transition metal elements such as Co, Mn, and Ni. The lithium transition metal oxide is, for example, Li x CoO2, Li x NiO2, Li x MnO2, Li x Co y Ni 1-y O2, Li x Co y M 1-y O z 、Li x Ni 1-y M y O z 、Li x Mn2O4, Li x Mn 2-y M y O4, LiMPO4, Li2MPO4F (M; at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3). These may be used alone or in combination of multiple kinds. In terms of achieving high capacity of the secondary battery, the positive electrode active material preferably contains lithium nickel composite oxides such as Li x NiO2, Li x Co y Ni 1-y O2, Li x Ni 1-y M y O z (M; at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3), etc.

[0046] Examples of conductive materials include carbon black (CB), acetylene black (AB), Ketjenblack, and carbon-based particles such as graphite. These may be used individually or in combination of two or more types.

[0047] Examples of binders include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, and polyolefin resins. These may be used individually or in combination of two or more types.

[0048] [Separator] For the separator 13, for example, a porous sheet having ion permeability and insulating properties can be used. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator include polyethylene, olefin resins such as polypropylene, and cellulose. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin resin. Alternatively, it may be a multilayer separator containing a polyethylene layer and a polypropylene layer, or a separator with a material such as aramid resin or ceramic coated on its surface may be used.

[0049] [Non-aqueous electrolytes] A non-aqueous electrolyte comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The non-aqueous electrolyte is not limited to a liquid electrolyte (electrolyte solution) but may also be a solid electrolyte using a gel-like polymer or the like. Examples of non-aqueous solvents include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may also contain halogen-substituted solvents in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine.

[0050] Examples of the above esters include cyclic carbonate esters such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; linear carbonate esters such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone and γ-valerolactone; and linear carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate, and γ-butyrolactone.

[0051] Examples of the above ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methylphenyl ether Examples include chain ethers such as ethylphenyl ether, butylphenyl ether, pentylphenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl.

[0052] As the halogen-substituted product, it is preferable to use fluorinated cyclic carbonate esters such as fluoroethylene carbonate (FEC), fluorinated linear carbonate esters, or fluorinated linear carboxylic acid esters such as methyl fluoropropionate (FMP).

[0053] The electrolyte salt is preferably a lithium salt. Examples of lithium salts include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), LiPF 6-x (C n F 2n+1 ) x (1 < x < 6, n is 1 or 2), LiB 10 Cl 10 , LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylate, borate salts such as Li2B4O7, Li(B(C2O4)F2), imide salts such as LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 SO2){l, m are integers of 1 or more}, etc. The lithium salt may be used alone or in combination of multiple kinds. Among these, from the viewpoints of ionic conductivity, electrochemical stability, etc., it is preferable to use LiPF6. The concentration of the lithium salt is preferably 0.8 to 1.8 mol per 1 L of the solvent.

[0054] <Example> Hereinafter, the present disclosure will be further described by way of examples, but the present disclosure is not limited to these examples.

[0055] <Example 1> [Preparation of negative electrode] Graphite particles and a Si compound were mixed in a mass ratio of 90:10. This mixture was used as the negative electrode active material. Single-walled carbon nanotubes (SWCNTs) with a fiber diameter of 1 nm to 3 nm and cellulose nanofibers (CNFs) with a fiber diameter of 3 nm to 4 nm were also prepared. These were then mixed in a mass ratio of negative electrode active material:SWCNTs:CNFs:carboxymethylcellulose (CMC):styrene-butadiene copolymer rubber (SBR) of 100:0.05:0.01:1:2 to prepare a negative electrode mixture slurry. This slurry was applied to both sides of a copper foil current collector using the doctor blade method, and after the coating film was dried, the coating film was compressed with a rolling roller to produce a negative electrode in which negative electrode mixture layers were formed on both sides of the negative electrode current collector.

[0056] [Fabrication of the positive electrode] As the positive electrode active material, aluminum-containing lithium nickel cobalt oxide (LiNi 0.88 Co 0.09 Al 0.03 O2 was used. A cathode composite slurry was prepared by mixing 100 parts by mass of the above cathode active material, 1 part by mass of acetylene black, and 0.9 parts by mass of polyvinylidene fluoride in an N-methyl-2-pyrrolidone (NMP) solvent. This slurry was applied to both sides of a 15 μm thick aluminum foil, and after the coating film was dried, the coating film was rolled with a rolling roller to produce a cathode in which cathode composite layers were formed on both sides of the cathode current collector.

[0057] [Preparation of non-aqueous electrolytes] LiPF6 was dissolved at a concentration of 1.4 mol / L in a non-aqueous solvent prepared by mixing ethylene carbonate (EC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC) in a volume ratio of 20:5:75. Furthermore, 3% by mass of vinylene carbonate and 0.5% by mass of 1,6-diisocyanate hexane were added. This was used as the non-aqueous electrolyte.

[0058] [Manufacturing of secondary batteries] (1) An aluminum positive electrode lead was attached to the positive electrode current collector, and a nickel-copper-nickel negative electrode lead was attached to the negative electrode current collector. Then, a polyethylene separator was placed between the positive and negative electrodes and the wires were wound around them to create a wound electrode body. (2) Insulating plates were placed above and below the electrode body, the negative electrode lead was welded to the case body, and the positive electrode lead was welded to the sealing body, thereby housing the electrode body inside the case body. (3) After injecting a non-aqueous electrolyte into the case body using a reduced pressure method, the open end of the case body was crimped to a sealing body via a gasket. This constituted a secondary battery.

[0059] <Example 2> A secondary battery was fabricated in the same manner as in Example 1, except that the mass ratio of the negative electrode active materials—SWCNT, CNF, CMC, and styrene-butadiene copolymer rubber (SBR)—was 100:0.05:0.03:1:2.

[0060] <Example 3> A secondary battery was prepared in the same manner as in Example 1, except that the mass ratio of the negative electrode active materials—SWCNT, CNF, CMC, and styrene-butadiene copolymer rubber (SBR)—was 100:0.05:0.1:1:2.

[0061] <Example 4> A secondary battery was fabricated in the same manner as in Example 1, except that cellulose nanofibers (CNF) with a fiber diameter of 8 nm to 9 nm were used, and the mass ratio of the negative electrode active material (SWCNT:CNF:CMC:styrene-butadiene copolymer rubber (SBR)) was mixed to be 100:0.05:0.1:1:2.

[0062] <Comparative Example 1> A secondary battery was fabricated in the same manner as in Example 1, except that cellulose nanofibers (CNF) with a fiber diameter of 3 nm to 4 nm were not used, and the mass ratio of the negative electrode active material, SWCNT, CMC, and styrene-butadiene copolymer rubber (SBR) was mixed to be 100:0.05:1:2.

[0063] <Comparative Example 2> A secondary battery was prepared in the same manner as in Example 1, except that the mass ratio of the negative electrode active materials—SWCNT, CNF, CMC, and styrene-butadiene copolymer rubber (SBR)—was 100:0.05:0.2:1:2.

[0064] <Comparative Example 3> A secondary battery was fabricated in the same manner as in Example 1, except that cellulose nanofibers (CNF) with a fiber diameter of 10 nm to 11 nm were used, and the mass ratio of the negative electrode active material (SWCNT:CNF:CMC:styrene-butadiene copolymer rubber (SBR)) was mixed to be 100:0.05:0.1:1:2.

[0065] <Comparative Example 4> A secondary battery was fabricated in the same manner as in Example 1, except that multi-walled carbon nanotubes (MWCNTs) with a fiber diameter of 7 nm to 10 nm were used, and the mass ratio of the negative electrode active material (MWCNTs: CNF: CMC: styrene-butadiene copolymer rubber (SBR)) was mixed to be 100:0.5:0.1:1:2.

[0066] <Comparative Example 5> A secondary battery was fabricated in the same manner as in Example 1, except that multi-walled carbon nanotubes (MWCNTs) with a fiber diameter of 7 nm to 10 nm and cellulose nanofibers (CNFs) with a fiber diameter of 10 nm to 11 nm were used, and the mass ratio of the negative electrode active material:MWCNTs:CNFs:CMCs:styrene-butadiene copolymer rubber (SBR) was mixed to be 100:0.5:0.1:1:2.

[0067] <Comparative Example 6> A secondary battery was fabricated in the same manner as in Example 1, except that multi-walled carbon nanotubes (MWCNTs) with a fiber diameter of 7 nm to 10 nm were used, cellulose nanofibers (CNFs) with a fiber diameter of 3 nm to 4 nm were not used, and the mass ratio of the negative electrode active material:MWCNTs:CMC:styrene-butadiene copolymer rubber (SBR) was mixed to be 100:0.5:1:2.

[0068] [Charge-discharge cycle test] Each secondary battery in the examples and comparative examples was charged with a constant current of 0.5C at a temperature of 25°C until the battery voltage reached 4.2V, and then discharged with a constant current of 0.5C until the battery voltage reached 2.5V. This charge-discharge cycle was repeated 100 times, and the capacity retention rate was calculated using the following formula.

[0069] Capacity retention rate (%) = (Discharge capacity at 100th cycle ÷ Discharge capacity at 1st cycle) × 100 Table 1 summarizes the capacity retention rate results in the charge-discharge cycle tests for the examples and comparative examples. A higher capacity retention rate indicates improved charge-discharge cycle characteristics.

[0070] [Table 1]

[0071] As can be seen from Table 1, Examples 1 to 4 all showed higher capacity retention rates compared to Comparative Examples 1 to 6. Therefore, it can be said that by using single-walled carbon nanotubes with a fiber diameter of less than 4 nm as the conductive material, and using cellulose nanofibers with a fiber diameter of 9 nm or less in an amount of 0.005% or more by mass and less than 0.2% by mass relative to the mass of the negative electrode active material as a binder, the deterioration of charge-discharge cycle characteristics can be suppressed. [Explanation of Symbols]

[0072] 10 Secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode body 15 Battery case 16 Case body 17 Sealing body 18,19 Insulating board 20 Positive leads 21 Negative lead 22 Protruding section 23 Filters 24 Lower valve body 25 Insulating material 26 Upper valve body 27 caps 28 Gaskets

Claims

1. The negative electrode composite layer comprises a negative electrode active material containing a Si compound, a conductive material containing single-walled carbon nanotubes with a fiber diameter of less than 4 nm, and a binder containing cellulose nanofibers with a fiber diameter of 9 nm or less. The cellulose nanofiber content is 0.005% by mass or more and 0.1% by mass or less, relative to the mass of the negative electrode active material, in a negative electrode for a secondary battery.

2. The negative electrode for a secondary battery according to claim 1, wherein the content of the single-walled carbon nanotubes is 0.001% by mass or more and 0.1% by mass or less, relative to the mass of the negative electrode active material.

3. The negative electrode for a secondary battery according to claim 1 or 2, wherein the fiber diameter of the cellulose nanofiber is 1 nm or more and 8 nm or less.

4. The negative electrode for a secondary battery according to any one of claims 1 to 3, wherein the fiber diameter of the carbon nanotube is 1 nm or more and 3 nm or less.

5. The negative electrode for a secondary battery according to any one of claims 1 to 4, wherein the Si compound comprises a lithium ion conducting phase and silicon particles dispersed within the lithium ion conducting phase.

6. The negative electrode for a secondary battery according to claim 5, wherein the lithium ion conducting phase comprises at least one of silicate, silicon oxide, amorphous carbon, and crystalline carbon.

7. Equipped with a positive electrode, a negative electrode, and a non-aqueous electrolyte, A secondary battery wherein the negative electrode is the negative electrode for a secondary battery described in any one of claims 1 to 6.

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