Secondary battery

Abstract

The present invention improves cycle characteristics. This secondary battery comprises: a positive electrode; a negative electrode; a separator disposed between the positive and negative electrodes; and an electrolyte. The negative electrode has a negative electrode current collector and a negative electrode active material layer in contact with the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, a first binder, and a second binder. The negative electrode active material includes a first negative electrode active material that is a material containing silicon. The first binder is an N-vinylacetamide polymer which is a polymer having a monomer represented by formula (1). The second binder is an emulsion-based binder. The negative electrode active material layer has a first surface that is a surface in contact with the negative electrode current collector, and a second surface that is a surface opposite to the first surface. The negative electrode active material layer on the first surface contains more of the second binder than on the second surface. (In formula (1), R1 and R2 each independently represent hydrogen or an alkyl group which may have a substituent.

Description

secondary battery

[0001] The present invention relates to a secondary battery.

[0002] Patent Document 1 discloses a negative electrode for a secondary battery containing an N-vinylacetamide polymer containing N-vinylacetamide as a monomer.

[0003] JP 2013-149395 A

[0004] However, the secondary battery disclosed in Patent Document 1 may have a reduced cycle characteristic.

[0005] The present invention has been made in view of the above problems, and an object of the present invention is to improve cycle characteristics.

[0006] A secondary battery according to one aspect of the present invention includes a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte. The negative electrode has a negative electrode current collector and a negative electrode active material layer in contact with the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, a first binder, and a second binder. The negative electrode active material includes the first negative electrode active material, which is a silicon-containing substance. The first binder is an N-vinylacetamide-based polymer that is a polymer having a monomer represented by formula (1). The second binder is an emulsion-based binder. The negative electrode active material layer has a first surface that is in contact with the negative electrode current collector and a second surface that is a surface opposite to the first surface of the separator. The negative electrode active material layer includes a larger amount of the second binder in the first surface than in the second surface. (In formula (1), R １ and R ２ each independently represents hydrogen or an alkyl group which may have a substituent.

[0007] According to the present invention, cycle characteristics can be improved.

[0008] Fig. 1 is a cross-sectional view showing an example of a secondary battery according to a first embodiment. Fig. 2 is an enlarged cross-sectional view showing a part of a cross section of the electrode body according to Fig. 1. Fig. 3 is a cutaway view showing another example of the secondary battery according to the first embodiment. Fig. 4 is a schematic view of a cross section taken along line IV-IV in Fig. 3.

[0009] Hereinafter, an embodiment of the present invention will be described, but the present invention is not limited to this embodiment.

[0010] (Secondary battery) Fig. 1 is a cross-sectional view showing an example of a secondary battery according to the first embodiment. The secondary battery 1 shown in Fig. 1 is a laminated lithium-ion secondary battery. As shown in Fig. 1, the secondary battery 1 includes a battery element 20, an exterior member 30, and an adhesive 32.

[0011] The battery element 20 is provided inside an exterior member 30. As shown in FIG. 1 , the battery element 20 includes an electrode body 200, a positive electrode lead 21, and a negative electrode lead 22. The positive electrode lead 21 is a terminal drawn from a positive electrode 210 (described later) to the outside of the exterior member 30. That is, the positive electrode lead 21 is a terminal that serves as a positive electrode of the secondary battery 1. In FIG. 1 , the positive electrode lead 21 is provided on an end surface of the electrode body 200. The negative electrode lead 22 is a terminal drawn from the inside of a negative electrode 220 (described later) to the outside of the exterior member 30. That is, the negative electrode lead 22 is a terminal that serves as a negative electrode of the secondary battery 1. In FIG. 1 , the negative electrode lead 22 is provided on an end surface of the electrode body 200. Details of the electrode body 200 will be described later.

[0012] The exterior member 30 is a case in which the battery element 20 is housed. The exterior member 30 includes two exterior sheets 30a and 30b. The exterior sheets 30a and 30b each include an insulating layer, a metal layer, and an outermost layer. In the example of FIG. 1 , the exterior sheet 30a has a recess 31. As a result, the battery element 20 is housed in the exterior member 30 by housing the battery element 20 in the recess 31 and bonding the peripheral edges of the exterior sheets 30a and 30b.

[0013] The exterior sheets 30a, 30b are constructed by laminating an insulating layer, a metal layer, and an outermost layer in this order from the inside, i.e., the side where the battery element 20 is provided, and then bonding them together by lamination or other processing. The insulating layers of the exterior sheets 30a, 30b are made of a resin such as polyethylene, polypropylene, modified polyethylene, modified polypropylene, or a polyolefin resin containing ethylene or propylene as a monomer. This allows the exterior sheets 30a, 30b to reduce the moisture permeability of the secondary battery 1 and improve its airtightness. The metal layers of the exterior sheets 30a, 30b are made of a metal plate or foil material such as aluminum, stainless steel, nickel, or iron. The outermost layer may be made of any material, but is preferably made of the same resin as the insulating layer or a material with high resistance to tearing, punctures, etc., such as nylon.

[0014] The adhesive 32 is a member for making the exterior member 30 airtight. The adhesive 32 is provided between the exterior member 30 and the positive electrode lead 21 and the negative electrode lead 22. The material of the adhesive 32 preferably has adhesion to the positive electrode lead 21 and the negative electrode lead 22. For example, when the positive electrode lead 21 and the negative electrode lead 22 are made of a metal material, the adhesive 32 is made of a polyolefin resin such as polyethylene, polypropylene, modified polyethylene, or modified polypropylene. This allows the adhesive 32 to seal the gap between the exterior member 30 and the positive electrode lead 21 and the negative electrode lead 22, thereby making the interior of the exterior member 30 airtight.

[0015] Fig. 2 is an enlarged cross-sectional view showing a portion of the cross section of the electrode assembly in Fig. 1. More specifically, Fig. 2 is a cross-sectional view showing a portion of one layer of a positive electrode 210 and one layer of a negative electrode 220 of the electrode assembly 200. As shown in Fig. 2, the electrode assembly 200 includes a positive electrode 210, a negative electrode 220, and a separator 230. In the secondary battery 1, the electrode assembly 200 has a structure in which the positive electrode 210 and the negative electrode 220 are stacked in the thickness direction with the separator 230 interposed therebetween. The positive electrode 210 and the negative electrode 220 included in the electrode assembly 200 are layered members for the charge / discharge reaction of the secondary battery according to the first embodiment.

[0016] The positive electrode 210 includes a positive electrode current collector 211 and a positive electrode active material layer 212. In the positive electrode 210, the positive electrode current collector 211 is laminated between the positive electrode active material layers 212.

[0017] The positive electrode current collector 211 is a conductive layer, and may be made of, for example, aluminum foil, stainless steel foil, etc. In the example of Fig. 1 , the positive electrode current collector 211 has a rectangular shape in plan view in the thickness direction, with protrusions on the positive electrode lead 21 side. The protrusions of the positive electrode current collector 211 are connected to the positive electrode lead 21.

[0018] The positive electrode active material layer 212 is a layer containing a positive electrode active material. The positive electrode active material layer 212 contains a positive electrode active material and a conductive agent. The positive electrode active material layer 212 is not limited to the materials listed above, and may further contain, for example, a binder and a dispersant.

[0019] The positive electrode active material is preferably a lithium-containing compound such as a lithium-containing composite oxide or a lithium-containing phosphate compound. The lithium-containing composite oxide is an oxide containing lithium and one or more elements other than lithium as constituent elements. The lithium-containing composite oxide has, for example, a layered rock salt type or a spinel type crystal structure. The lithium-containing phosphate compound is a phosphate compound containing lithium and one or more elements other than lithium as constituent elements. The lithium-containing phosphate compound has, for example, an olivine type crystal structure. A specific example of the lithium-containing composite oxide is LiNiO ２ , LiCoO ２ , LiCo ０．９８ Al ０．０１ Mg ０．０１ O ２ , LiNi ０．５ Co ０．２ Mn ０．３ O ２ , LiNi ０．８ Co ０．１５ Al ０．０５ O ２ , LiNi ０．３３ Co ０．３３ Mn ０．３３ O ２ , Li １．２ Mn ０．５２ Co ０．１７５ Ni ０．１ O ２, Li １．１５ (Mn ０．６５ Ni ０．２２ Co ０．１３ ) O ２ , LiMn ２ O ４ Specific examples of lithium-containing phosphate compounds include LiFePO ４ , LiMnPO ４ , LiFe ０．５ Mn ０．５ P.O. ４ , LiFe ０．３ Mn ０．７ P.O. ４ And so on.

[0020] The binder contained in the positive electrode active material layer 212 may be any material, and may include, for example, one or more of synthetic rubber and polymer compounds. Examples of synthetic rubber include styrene-butadiene rubber, fluorine-based rubber, and ethylene propylene diene. Examples of polymer compounds include polyvinylidene fluoride (PVdF) and polyimide.

[0021] The conductive agent contained in the positive electrode active material layer 212 may be any material, and may include, for example, carbon. Examples of carbon include graphite, carbon black, acetylene black, and ketjen black. However, the conductive agent contained in the positive electrode active material layer 212 is not limited to these materials as long as it is a conductive material, and may also be a metal material, a conductive polymer, or the like.

[0022] The negative electrode 220 includes a negative electrode current collector 221 and a negative electrode active material layer 222. In the negative electrode 220, the negative electrode current collector 221 is laminated between the negative electrode active material layers 222.

[0023] The negative electrode current collector 221 is a conductor, and for example, copper foil or the like can be used. In the example of Fig. 1 , the shape of the negative electrode current collector 221 is a rectangular sheet having protrusions on the negative electrode lead 22 side when viewed in a plan view in the thickness direction. The protrusions of the negative electrode current collector 221 are connected to the negative electrode lead 22.

[0024] The negative electrode active material layer 222 is a layer containing a negative electrode active material, a first binder, and a second binder.

[0025] The negative electrode active material includes a first negative electrode active material. Preferably, the negative electrode active material further includes a second negative electrode active material. Here, the negative electrode active material refers to a reducing agent that undergoes a reduction reaction upon charging of the secondary battery 1 and an oxidation reaction upon discharging of the secondary battery 1, and that allows these oxidation-reduction reactions to occur reversibly, and is capable of reacting to absorb and desorb charge carriers in the secondary battery 1. Therefore, for example, fibrous carbon such as carbon nanotubes and carbon fine particles such as carbon black do not substantially absorb or desorb lithium ions, which are charge carriers in lithium-ion secondary batteries, and are therefore not included in the negative electrode active material of the present disclosure.

[0026] The first negative electrode active material is a material containing silicon. Materials containing silicon include elemental silicon, silicon alloys, and silicon compounds. Examples of silicon alloys that can be used as the first negative electrode active material include those containing at least one element selected from the group consisting of tin (Sn), nickel (Ni), copper (Cu), iron (Fe), cobalt (Co), manganese (Mn), zinc (Zn), indium (In), silver (Ag), titanium (Ti), germanium (Ge), bismuth (Bi), antimony (Sb), and chromium (Cr) as the second constituent element other than silicon. Examples of silicon compounds that can be used as the first negative electrode active material include silicon oxide (SiO ｘ Examples of the first negative electrode active material include those containing oxygen (O) or carbon (C), such as silicon carbide (SiC), and may contain the second constituent element described above in addition to silicon. The first negative electrode active material may be doped with Li. ｘ In this case, it is preferable that the negative electrode is doped with Li during the negative electrode production process. ｘ The irreversible capacity of the first negative electrode active material can be reduced. The first negative electrode active material may be a composite of Si and another material such as carbon, or a composite of a Si alloy and another material such as carbon. This allows the crystal structure to be more stable than that of elemental Si. The particle surfaces of the first negative electrode active material are preferably partially or entirely coated with carbon. This improves the electronic conductivity of the particle surfaces of the first negative electrode active material.

[0027] The second negative electrode active material is a negative electrode active material containing carbon as a constituent element. Examples of materials that can be used as the second negative electrode active material include MCMB (MesoCarbon MicroBeads), artificial graphite, natural graphite, non-graphitizable carbon, and graphitizable carbon. More specifically, examples of materials that can be used as the second negative electrode active material include pyrolytic carbons, cokes, glassy carbon fiber, organic polymer compound sintered bodies, activated carbon, and carbon blacks. Examples of cokes include pitch coke, needle coke, and petroleum coke. The organic polymer compound sintered bodies are obtained by sintering a polymer compound such as a phenolic resin or a furan resin at an appropriate temperature and carbonizing it.

[0028] The negative electrode active material is not limited to the first negative electrode active material and the second negative electrode active material, and may include other negative electrode active materials, such as metals, semimetal alloys or compounds, and tin (Sn) alloys or compounds, which can absorb and release lithium. Examples of metals and semimetals that can be used as the negative electrode active material include tin (Sn), lead (Pb), aluminum (Al), indium (In), zinc (Zn), antimony (Sb), bismuth (Bi), cadmium (Cd), magnesium (Mg), boron (B), gallium (Ga), germanium (Ge), arsenic (As), silver (Ag), zirconium (Zr), yttrium (Y), and hafnium (Hf). Among these, germanium, tin, and lead are preferred. Tin is more preferred because it has a high ability to absorb and release lithium and can achieve a high energy density.

[0029] The mass ratio of the first negative electrode active material to the negative electrode active material is preferably 10% or more, more preferably greater than 10%, and even more preferably 30% or more. This improves the negative electrode capacity. In the present disclosure, the mass ratio of the first negative electrode active material to the negative electrode active material refers to the mass of the first negative electrode active material contained in the negative electrode active material layer 222 relative to the mass of the negative electrode active material contained in the negative electrode active material layer 222. That is, for example, when the negative electrode active material contained in the negative electrode active material layer 222 is composed of the first negative electrode active material and the second negative electrode active material, and the negative electrode active material layer 222 contains A mass % of the first negative electrode active material and B mass % of the second active material, the mass ratio R of the first negative electrode active material to the negative electrode active material can be calculated as R = A / (A + B).

[0030] The mass ratio of the first negative electrode active material to the negative electrode active material can be measured by observing a cross section cut along the thickness direction of the negative electrode active material layer 222 with a scanning electron microscope (SEM) and by mapping using energy dispersive X-ray spectroscopy (EDX). The measurement method will be described in detail below.

[0031] First, the exposed cross-section of the anode active material layer 222 is identified using an SEM, and then EDX mapping is performed on the exposed portion to identify the region occupied by particles containing Si (particles of the first anode active material) and the region occupied by particles containing only C (particles of the second anode active material).

[0032] Next, the volume of each of the particles (measurement target particles) of the first negative electrode active material or the second negative electrode active material is measured. Hereinafter, the method for measuring the volume of the measurement target particles will be described in detail using two shape models, a first model and a second model. Here, the first model is a sphere, and the second model is a cube. First, the area occupied by the cross-section of the measurement target particle is extracted from the SEM observation image, and a circle and a square are fitted to the area. The areas of the circle and square used in the fitting are set to be the same as the cross-section area of ​​the particle. Then, the area of ​​the cross-section of the measurement target particle protruding from the circle and the area of ​​the circle protruding from the cross-section of the measurement target particle are added together to calculate a first model evaluation value. Similarly, the area of ​​the cross-section of the measurement target particle protruding from the square and the area of ​​the square protruding from the cross-section of the measurement target particle are added together to calculate a second model evaluation value. Here, if the first model evaluation value is equal to or less than the second model evaluation value, the first model is applied to the measurement target particle. In this case, the volume of the particle to be measured is calculated by regarding the particle to be measured as a sphere with a radius equal to the radius of the circle. On the other hand, if the second model evaluation value is less than the first model evaluation value, the second model is applied to the particle to be measured. In this case, the volume of the particle to be measured is calculated by regarding the particle to be measured as a cube with one side equal to one side of the cube.

[0033] The volume of each particle of the first negative electrode active material calculated by the above method is then added together to obtain the volume V of the first negative electrode active material. Ａ Similarly, the volume of each particle of the second negative electrode active material calculated by the above method is added together to calculate the volume V of the second negative electrode active material. Ｂ As a result, the density of the first negative electrode active material is calculated as ρ Ａ , the density of the second negative electrode active material is ρ Ｂ The mass ratio R of the first negative electrode active material to the negative electrode active material is R = V Ａ ρ Ａ / (V Ａ ρ Ａ +V Ｂ ρ Ｂ ) can be calculated as follows.

[0034] Examples of tin alloys that can be used as the negative electrode active material include those containing at least one of nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, and chromium as a second constituent element other than tin. Examples of tin compounds that can be used as the negative electrode active material include those containing oxygen or carbon, and may contain the above-mentioned second constituent element in addition to tin.

[0035] The first binder contained in the negative electrode active material layer 222 is an N-vinylacetamide-based polymer. An example of the N-vinylacetamide-based polymer is poly-N-vinylacetamide (PNVA). In the present disclosure, the N-vinylacetamide-based polymer refers to a polymer containing a monomer represented by formula (1). This can suppress a decrease in electronic conductivity between negative electrode active material particles due to expansion and contraction of the negative electrode active material layer 222 with charge / discharge cycles, thereby improving the cycle retention rate. In the following description, the monomer represented by formula (1) will be described as an N-vinylacetamide-based monomer. (In formula (1), R １ and R ２ each independently represents hydrogen or an alkyl group which may have a substituent.) In the present disclosure, "may have a substituent" means that there is no substituent, or that a hydrogen group is substituted with one or more substituents. Examples of the substituent include a hydrocarbon group and a halogen group. Here, examples of the halogen group include a fluorine group, a chlorine group, and a bromine group. The number of carbon atoms in the alkyl group is preferably 1 or more and 5 or less.

[0036] The N-vinylacetamide polymer may be a copolymer containing an N-vinylacetamide monomer and a monomer other than the N-vinylacetamide monomer. Here, the monomer other than the N-vinylacetamide monomer is, for example, at least one of an alkali metal acrylate, an alkali metal methacrylate, and derivatives of an alkali metal acrylate and an alkali metal methacrylate. Here, the alkali metal salt refers to a salt containing an alkali metal such as lithium (Li), sodium (Na), or potassium (K).

[0037] When a copolymer containing an N-vinylacetamide monomer and a monomer other than the N-vinylacetamide monomer is used, the content of the N-vinylacetamide monomer in the N-vinylacetamide polymer is preferably 1 mol% to 100 mol%, more preferably 30 mol% to 70 mol%, and most preferably 40 mol% to 60 mol%, assuming that the total of all monomers constituting the N-vinylacetamide polymer is 100 mol%. This improves the binding strength between the negative electrode active material particles. In particular, when the content of the N-vinylacetamide monomer is 40 mol% to 60 mol%, the water solubility and viscosity of the resulting N-vinylacetamide polymer are improved, thereby sufficiently improving the binding strength between the negative electrode active material particles.

[0038] Whether or not the negative electrode active material layer 222 contains an N-vinylacetamide-based polymer as the first binder can be determined by pyrolysis GC-MS of the negative electrode active material layer 222. More specifically, if a mass spectrum derived from an N-vinylacetamide-based monomer can be observed when mass analysis is performed by pyrolysis GC-MS on a sample scraped from the negative electrode active material layer 222, it can be said that the negative electrode active material layer 222 contains an N-vinylacetamide-based polymer.

[0039] The second binder contained in the negative electrode active material layer 222 is an emulsion-based binder. In the present disclosure, an emulsion-based binder refers to a binder that can become a liquid (i.e., an emulsion) that maintains a particulate state when dispersed in a solvent. By including an emulsion-based binder in the negative electrode active material layer 222, the second binder can be unevenly distributed at the interface between the negative electrode current collector 221 and the negative electrode active material layer 222 during the formation of the negative electrode active material layer 222, thereby improving adhesion to the negative electrode current collector 221. Examples of emulsion-based binders that can be used as the second binder include at least one of styrene-butadiene polymer, acrylonitrile-butadiene polymer, polyurethane, methyl methacrylate polymer, and acrylic-modified butadiene rubber.

[0040] An example of a styrene-butadiene polymer is styrene-butadiene rubber (SBR). In the present disclosure, the styrene-butadiene polymer refers to a copolymer containing a styrene-based monomer, which is at least one of styrene and a styrene derivative, and a butadiene-based monomer, which is at least one of butadiene and a butadiene derivative. In the present disclosure, the derivative of styrene refers to a substance in which a hydrogen atom of styrene is substituted with another functional group. Here, the derivative of butadiene refers to a substance in which a hydrogen atom of butadiene is substituted with another functional group. The styrene-butadiene polymer may be a copolymer of a styrene-based monomer, a butadiene-based monomer, and a monomer other than a styrene-based monomer and a butadiene-based monomer.

[0041] When the total of all monomers constituting the styrene-butadiene polymer is taken as 100 mol%, the total content of the styrene-based monomer and the butadiene-based monomer in the styrene-butadiene polymer is preferably 10 mol% or more and 90 mol% or less, and more preferably 30 mol% or more and 70 mol% or less. This can improve the binding strength of the anode active material layer 222 to the anode current collector 221. In particular, if the total content of the styrene-based monomer and the butadiene-based monomer is 30 mol% or more and 70 mol% or less, the binding strength of the anode active material layer 222 to the anode current collector 221 can be sufficiently improved.

[0042] An example of the acrylonitrile-butadiene polymer is acrylonitrile-butadiene rubber (NBR). In the present disclosure, the acrylonitrile-butadiene polymer refers to a copolymer containing an acrylonitrile-based monomer, which is at least one of acrylonitrile and an acrylonitrile derivative, and a butadiene-based monomer.

[0043] When the total of all monomers constituting the acrylonitrile-butadiene polymer is taken as 100 mol%, the total content of the acrylonitrile-based monomer and the butadiene-based monomer in the acrylonitrile-butadiene polymer is preferably 10 mol% or more and 90 mol% or less, and more preferably 30 mol% or more and 70 mol% or less. This can improve the binding strength of the anode active material layer 222 to the anode current collector 221. In particular, if the total content of the acrylonitrile-based monomer and the butadiene-based monomer is 30 mol% or more and 70 mol% or less, the binding strength of the anode active material layer 222 to the anode current collector 221 can be sufficiently improved.

[0044] In this disclosure, polyurethane (PU) refers to a copolymer containing a urethane bond. That is, polyurethane is a copolymer containing a monomer having an isocyanate group and a monomer having a hydroxyl group as monomers that form urethane bonds, and the isocyanate group and the hydroxyl group are bonded to form the urethane bond. The monomer having an isocyanate group is, for example, a diisocyanate, but it may also be a polyisocyanate with three or more functional groups in which the isocyanate group is protected with a blocking agent such as a phenol, alcohol, oxime, or lactam. The monomer having a hydroxyl group is, for example, polyvinyl alcohol, carboxymethyl cellulose, or methyl cellulose. The monomer having a hydroxyl group may be used alone or in combination of two or more types.

[0045] An example of a methyl methacrylate polymer is polymethyl methacrylate (PMMA). In the present disclosure, the methyl methacrylate polymer refers to a polymer containing a methyl methacrylate monomer, which is at least one of methyl methacrylate and a derivative of methyl methacrylate, and examples thereof include a styrene-methyl methacrylate copolymer and a methyl methacrylate-butadiene-styrene copolymer (MBS).

[0046] In the present disclosure, the acrylic-modified butadiene rubber is a polybutadiene polymer in which (meth)acrylic acid is ester-bonded.

[0047] Whether or not the negative electrode active material layer 222 contains at least one of a styrene-butadiene polymer, an acrylonitrile-butadiene polymer, a polyurethane, a methyl methacrylate-based polymer, and an acrylic-modified butadiene rubber as the second binder can be determined by pyrolysis GC-MS of the negative electrode active material layer 222. More specifically, if mass analysis is performed by pyrolysis GC-MS on a sample scraped from the negative electrode active material layer 222, and a mass spectrum derived from the structure contained in the styrene-butadiene polymer, the acrylonitrile-butadiene polymer, the polyurethane, the methyl methacrylate-based polymer, and the acrylic-modified butadiene rubber can be observed, it can be said that the negative electrode active material layer 222 contains at least one of a styrene-butadiene polymer, an acrylonitrile-butadiene polymer, a polyurethane, a methyl methacrylate-based polymer, and an acrylic-modified butadiene rubber.

[0048] The negative electrode active material layer 222 is not limited to containing only the first negative electrode active material, the second negative electrode active material, the first binder, and the second binder.

[0049] For example, the anode active material layer 222 may further contain an anode conductive agent. The anode conductive agent includes at least one of a carbon material, a metal material, and a conductive polymer compound. Specific examples of carbon materials used as the anode conductive agent include particulate carbon materials such as carbon black, acetylene black, and ketjen black, and fibrous carbon materials such as carbon nanotubes. The carbon nanotubes are, for example, single-wall carbon nanotubes (SWCNTs). This can improve the electronic conductivity of the particle surfaces of the first anode active material. The mass ratio of the anode conductive agent to the anode active material layer 222 is preferably 5% or less, more preferably 2% or less. This can improve the paintability of the anode slurry.

[0050] For example, the negative electrode active material layer 222 may further contain a binder other than the first binder and the second binder.

[0051] The negative electrode active material layer 222 has a first surface 222a in contact with the negative electrode current collector 221 and a second surface 222b opposite to the first surface 222a. In the first embodiment, the second surface 222b is in contact with the separator 230.

[0052] The negative electrode active material layer 222 contains a larger amount of the second binder on the first surface 222a than on the second surface 222b. In other words, the mass ratio of the second binder to the material constituting the negative electrode active material layer 222 on the first surface 222a is greater than the mass ratio of the second binder to the material constituting the negative electrode active material layer 222 on the second surface 222b. This improves the adhesion between the negative electrode current collector 221 and the negative electrode active material layer 222, thereby improving cycle characteristics. The mass ratio of the second binder to the material constituting the negative electrode active material layer 222 can be measured by pyrolysis GC-MS of the negative electrode active material layer 222. More specifically, the mass ratio can be measured by performing mass analysis by pyrolysis GC-MS on a sample scraped from the negative electrode active material layer 222 on the first surface 222a and the second surface 222b, and calculating the mass ratio of the second binder to the sample. If the mass ratio on the first surface 222a is greater than the mass ratio on the second surface 222b, it can be said that the negative electrode active material layer 222 contains more second binder on the first surface 222a than on the second surface 222b.

[0053] The negative electrode active material layer 222 contains a larger amount of the first binder on the second surface 222b than on the first surface 222a. In other words, the mass ratio of the first binder to the materials constituting the negative electrode active material layer 222 on the second surface 222b is greater than the mass ratio of the first binder to the materials constituting the negative electrode active material layer 222 on the first surface 222a. This improves the adhesion between the separator 230 and the negative electrode active material layer 222, thereby further improving the cycle characteristics. The mass ratio of the first binder to the materials constituting the negative electrode active material layer 222 can be measured by pyrolysis GC-MS of the negative electrode active material layer 222. More specifically, the mass ratio can be measured by performing mass analysis by pyrolysis GC-MS on a sample scraped from the negative electrode active material layer 222 on the first surface 222a and the second surface 222b, and calculating the mass ratio of the first binder to the sample. If the mass ratio on the second surface 222b is greater than the mass ratio on the first surface 222a, it can be said that the negative electrode active material layer 222 contains more first binder on the second surface 222b than on the first surface 222a.

[0054] In the first embodiment, the negative electrode 220 is fabricated by sequentially applying a first negative electrode mixture containing a large amount of the second binder and a second negative electrode mixture containing a large amount of the first binder to the negative electrode current collector 221. This allows the first surface 222a to contain more of the second binder than the second surface 222b, and the second surface 222b to contain more of the first binder than the first surface 222a. Note that the fabrication method of the negative electrode 220 is not limited to the above method. For example, the first negative electrode mixture may be applied and dried, followed by the application and drying of the second negative electrode mixture. Alternatively, one type of negative electrode mixture may be used, and the amounts of the first binder and the second binder may be made different between the first surface 222a and the second surface 222b by changing process conditions such as the drying rate.

[0055] The separator 230 is a film that insulates the positive electrode 210 from the negative electrode 220. The separator 230 is provided between the main surface of the positive electrode 210 and the main surface of the negative electrode 220 so that the positive electrode 210 and the negative electrode 220 do not come into direct contact with each other. In the example of FIG. 1 , the separator 230 has a rectangular sheet shape when viewed in a plan view in the thickness direction. The separator 230 includes a substrate 231 and a negative electrode-side coating layer 233.

[0056] The material of the substrate 231 is preferably electrically stable, chemically stable with respect to the positive electrode active material, the negative electrode active material, and the electrolyte, and has insulating properties. The substrate 231 can be, for example, a polymer nonwoven fabric, a porous film, or a layer made of glass or ceramic fibers. The material of the substrate 231 more preferably includes a porous polyolefin film. This can improve the safety of the battery by preventing short circuits and providing a shutdown effect.

[0057] The negative electrode side coating layer 233 is a layer that coats the negative electrode 220 side of the substrate 231. The negative electrode side coating layer 233 contains a first binder, which further improves the adhesion between the separator 230 and the negative electrode active material layer 222, thereby further improving the cycle characteristics.

[0058] The negative electrode side coating layer 233 preferably further contains particles made of an inorganic substance. Examples of the inorganic substance include metals, semiconductors, oxides of metals and semiconductors, and nitrides of metals and semiconductors. Examples of metals used as the inorganic substance include aluminum (Al) and titanium (Ti). Examples of semiconductors used as the inorganic substance include silicon (Si) and boron (B). Examples of oxides and nitrides used as the inorganic substance include alumina (Al). ２ O ３ ), boron nitride (BN), aluminum nitride (AlN), titanium dioxide (TiO ２ ), silicon dioxide (SiO ２ ) and the like. In addition, it is preferable that the inorganic material has insulating properties, is easily available, and has a large heat capacity. The particle size of the inorganic material particles is not particularly limited, and for example, when alumina is used as the inorganic material, an inorganic material having an average particle size of 0.5 μm can be used. In the present disclosure, the average particle size refers to the median diameter.

[0059] A positive electrode-side coating layer containing a binder may be coated on the positive electrode 210 side of the substrate 231. The positive electrode-side coating layer may be made of the same material as the negative electrode-side coating layer 233, or may be made of a different material from the negative electrode-side coating layer 233.

[0060] The electrolyte solution is impregnated into the separator 230. In the example of Fig. 1, the electrolyte solution fills the space inside the exterior member 30. The electrolyte solution is a non-aqueous electrolyte solution containing an electrolyte salt and a solvent that dissolves the electrolyte salt.

[0061] The electrolyte salt is, for example, lithium perchlorate (LiClO ４ ), lithium hexafluorophosphate (LiPF ６ ), lithium tetrafluoroborate (LiBF ４ ), lithium bis(trifluoromethanesulfonyl)imide (LiN(SO ２ CF ３ ) ２ ), lithium bis(pentafluoroethanesulfonyl)imide (LiN(SO ２ C ２ F ５ ) ２ ), lithium hexafluoroarsenate (LiAsF ６ ) and other lithium salts.

[0062] Examples of the solvent include lactone-based solvents such as γ-butyrolactone, γ-valerolactone, δ-valerolactone, and ε-caprolactone; carbonate-based solvents such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; ether-based solvents such as 1,2-dimethoxyethane, 1-ethoxy-2-methoxyethane, 1,2-diethoxyethane, tetrahydrofuran, and 2-methyltetrahydrofuran; nitrile-based solvents such as acetonitrile; sulfolane-based solvents; phosphoric acids; phosphate ester solvents; and pyrrolidones.

[0063] The electrolyte preferably contains at least one additive selected from the group consisting of fluorinated carboxylic acid esters, sulfonic acid esters, sulfonic acid anhydrides, and carboxylic acid anhydrides. This promotes the formation of a low-resistance solid electrolyte interphase (SEI), thereby improving the charge load characteristics. Examples of fluorinated carboxylic acid esters include fluoroethylene carbonate (FEC). Examples of sulfonic acid anhydrides include propanedisulfonic acid anhydride (PSAH). Examples of sulfonic acid esters include 1,3-propane sultone. Examples of carboxylic acid anhydrides include 1,4-dioxane-2,6-dione.

[0064] The battery according to the first embodiment has been described above, but the secondary battery according to the first embodiment is not limited to that shown in Fig. 1. Other examples will be described below using the drawings, but the same components as those in Figs. 1 and 2 will be designated by reference numerals and will not be described again.

[0065] Fig. 3 is a cutaway view showing a different example of the secondary battery according to the first embodiment. Fig. 4 is a schematic view of a cross section taken along line IV-IV in Fig. 3. The secondary battery 1A shown in Figs. 3 and 4 differs from the example shown in Fig. 1 in that an electrode body 200A is wound around a positive electrode lead 21A and a negative electrode lead 22A.

[0066] The battery element 20A is provided inside the exterior member 30. As shown in FIG. 4 , the battery element 20A includes an electrode body 200A, a positive electrode lead 21A, a negative electrode lead 22A, and a protective material 23. The positive electrode lead 21A is a terminal drawn from inside the battery element 20A to the outside of the exterior member 30, and the positive electrode lead 21A is provided near the center of the battery element 20A. The negative electrode lead 22A is a terminal drawn from inside the battery element 20A to the outside of the exterior member 30, and the negative electrode lead 22A is provided near the center of the battery element 20A. The protective material 23 is a member that protects the outside of the battery element 20A. The protective material 23 is provided so as to be wrapped around the electrode body 200A. The protective material 23 is, for example, an insulating tape.

[0067] 4, the electrode assembly 200A is a laminate for the charge / discharge reaction of the secondary battery according to the first embodiment. The electrode assembly 200A includes a positive electrode 210A including a positive electrode current collector 211A and a positive electrode active material layer 212A, a negative electrode 220A including a negative electrode current collector 221A and a negative electrode active material layer 222A, and a separator 230A. The electrode assembly 200A has a structure in which the positive electrode lead 21A and the negative electrode lead 22A are wound around the center, and is laminated in the following order from the outside, i.e., from the protective material 23 side: the negative electrode current collector 221A, the negative electrode active material layer 222A, the separator 230A, the positive electrode active material layer 212A, the positive electrode current collector 211A, the positive electrode active material layer 212A, the separator 230A, and the negative electrode active material layer 222A. In the electrode body 200A, no layers other than the negative electrode current collector 221A, separator 230A, and positive electrode current collector 211A are provided near the positive electrode lead 21A and the negative electrode lead 22A. With this structure, the positive electrode current collector 211A is connected to the positive electrode lead 21A, and the negative electrode current collector 221A is connected to the negative electrode lead 22A.

[0068] As described above, the secondary battery 1 according to the first embodiment includes a positive electrode 210, a negative electrode 220, a separator 230 between the positive electrode 210 and the negative electrode 220, and an electrolyte. The negative electrode 220 includes a negative electrode current collector 221 and a negative electrode active material layer 222 in contact with the negative electrode current collector 221. The negative electrode active material layer 222 includes a negative electrode active material, a first binder, and a second binder. The negative electrode active material includes a first negative electrode active material that is a silicon-containing substance. The first binder is an N-vinylacetamide-based polymer that is a polymer having a monomer represented by formula (1). The second binder is an emulsion-based binder. The negative electrode active material layer 222 includes a first surface 222a that is in contact with the negative electrode current collector 221 and a second surface 222b that is the surface opposite to the first surface 222a. The negative electrode active material layer 222 contains a larger amount of the second binder on the first surface 222a than on the second surface 222b. (In formula (1), R １ and R ２ each independently represents an alkyl group having 1 to 5 carbon atoms which may have a substituent.

[0069] This allows the adhesion between the negative electrode current collector 221 and the negative electrode active material layer 222 to be maintained during the charge / discharge cycle, and also prevents the negative electrode active material layer 222 from expanding and then shrinking during the charge / discharge cycle, thereby improving the cycle characteristics.

[0070] In a preferred embodiment, the mass ratio of the first negative electrode active material to the negative electrode active material is 10% or more, which can improve the negative electrode capacity while maintaining the bond between the negative electrode current collector 221 and the negative electrode active material layer 222.

[0071] In a more preferable embodiment, the mass ratio of the first negative electrode active material to the negative electrode active material is 30% or more, which can further improve the negative electrode capacity while maintaining the bond between the negative electrode current collector 221 and the negative electrode active material layer 222.

[0072] In a preferred embodiment, the negative electrode active material layer 222 contains a larger amount of the first binder on the second surface 222b than on the first surface 222a, thereby maintaining the binding between the separator 230 and the negative electrode active material layer 222 during charge-discharge cycles and suppressing a decrease in electronic conductivity between the negative electrode active material particles due to expansion and then contraction of the negative electrode active material layer 222 during charge-discharge cycles, thereby improving cycle characteristics.

[0073] In a more desirable embodiment, the second surface 222b is in contact with the separator 230. The separator 230 has a substrate 231 and an anode-side coating layer 233 on the anode 220 side of the substrate 231. The anode-side coating layer 233 contains a first binder. This makes it possible to maintain stronger binding between the separator 230 and the anode active material layer 222 during charge / discharge cycles, thereby further improving cycle characteristics.

[0074] Examples will be described below. Table 1 shows examples and comparative examples. Note that the present invention is not limited to these examples.

[0075]

[0076] Example 1 A positive electrode according to Example 1 was fabricated by the following method. Lithium cobalt oxide (LiCoO２ ) and amorphous carbon powder (Ketjen black) and polyvinylidene fluoride (PVdF) were mixed in a mass ratio of 95:2:3 to prepare a positive electrode mixture. This positive electrode mixture was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode mixture slurry, and then a strip-shaped aluminum foil with a thickness of 10 μm was prepared as a positive electrode current collector, and the prepared positive electrode mixture slurry was uniformly applied to both sides of the aluminum foil. The obtained coating was dried with hot air and then compression-molded using a roll press to form a positive electrode sheet. The formed positive electrode sheet was cut into a 70 mm x 800 mm strip to prepare a positive electrode. A positive electrode lead was attached to the exposed portion of the positive electrode current collector layer of the prepared positive electrode.

[0077] The negative electrode according to Example 1 was fabricated by the following method. The first negative electrode mixture contained Li-predoped silicon oxide (SiO ｘ 46.5% by mass of ZnO as the first negative electrode active material, 46.5% by mass of artificial graphite as the second negative electrode active material, 3% by mass of PNVA as the first binder, 2% by mass of SBR as the second negative electrode binder, 1.7% by mass of carbon black as the negative electrode conductive material, and 0.3% by mass of SWCNT were mixed with an appropriate amount of ion-exchanged water and kneaded and stirred in a planetary mixer. Similarly, the second negative electrode mixture was prepared by mixing 46.5% by mass of Li-predoped silicon oxide (SiO ) as the first negative electrode active material, 46.5% by mass of artificial graphite as the second negative electrode active material, 3% by mass of PNVA as the first binder, 2% by mass of SBR as the second negative electrode binder, 1.7% by mass of carbon black as the negative electrode conductive material, and 0.3% by mass of SWCNT with an appropriate amount of ion-exchanged water, and kneading and stirring the mixture in a planetary mixer. ｘ ) 47.0 mass% as the second negative electrode active material, 47.0 mass% artificial graphite as the second negative electrode active material, 4 mass% PVNA as the first negative electrode binder, and 2.0 mass% SWCNT as the negative electrode conductive agent were mixed with an appropriate amount of ion-exchanged water and kneaded and stirred in a centrifugal mixer. Then, an 8 μm thick copper foil was prepared as a negative electrode current collector, and the first negative electrode mixture and the second negative electrode mixture paint were uniformly applied to both sides of the copper foil so as to be laminated in order. The resulting coating was dried with hot air and then compression-molded using a roll press to form a negative electrode sheet. This negative electrode sheet was cut into a 72 mm x 810 mm strip to prepare a negative electrode. A negative electrode lead was attached to the exposed portion of the negative electrode current collector of the prepared negative electrode.

[0078] In the following description, it is assumed that the first negative electrode active material is A1 mass % and the second negative electrode active material is B1 mass % in the first negative electrode mixture, i.e., the first surface. In this case, the mass ratio R1 (%) of the first negative electrode active material to the negative electrode active material in the first surface satisfies the relationship of the following formula (2). Furthermore, it is assumed that the first negative electrode active material is A2 mass % and the second negative electrode active material is B2 mass % in the second negative electrode mixture, i.e., the second surface. In this case, it is assumed that the mass ratio R2 (%) of the first negative electrode active material to the negative electrode active material in the second surface satisfies the relationship of the following formula (3). R1 ​​= A1 / (A1 + B1) × 100 (2) R2 = A2 / (A2 + B2) × 100 (3)

[0079] In Example 1, the first negative electrode active material in the first layer and the second layer was silicon oxide (SiO ｘ ), and the second negative electrode active material is artificial graphite. As shown in Table 1, the mass ratio R1 in Example 1 is 50% (= 46.5 mass% / (46.5 mass% + 46.5 mass%)). The mass ratio R2 in Example 1 is 50% (= 47.5 mass% / (47.5 mass% + 47.5 mass%)).

[0080] The separator according to Example 1 was produced by the following method. PVdF having an average molecular weight of 150,000 as a binder and NMP as a solvent were mixed and dissolved sufficiently in a mass ratio of 10:90, and alumina (Al 2 O 3 ) having an average particle size of 0.5 μm as inorganic particles was added to the solution. ２ O ３ ) fine powder was added in an amount twice the mass of PVdF and thoroughly stirred to prepare a coating slurry for the negative electrode side sheet layer. The prepared coating slurry for the negative electrode side sheet layer was applied to the negative electrode side surface of a 15 μm thick microporous polyethylene film substrate, and a positive electrode side sheet layer and a negative electrode side sheet layer were formed to a thickness of 3 μm to prepare a separator sheet. The separator sheet was cut into a 74 mm × 860 mm strip to prepare the separator according to Example 1.

[0081] The electrolyte solution according to Example 1 was prepared by dissolving lithium hexafluorophosphate (LiPF ) as an electrolyte salt in a solvent obtained by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a mass ratio of 5:5.６ ) was dissolved to a molar concentration of 1 mol / L.

[0082] The secondary battery according to Example 1 was fabricated by the following method. The positive and negative electrodes were stacked with the separator interposed therebetween, tightly adhered, and wound longitudinally. A protective tape was then applied to the outermost periphery of the wound laminate to fabricate an electrode assembly. The fabricated electrode assembly was then loaded into a housing member, and three sides of the housing member were heat-sealed, with one side remaining unsealed to form an opening. The housing member was a laminate film consisting of a 25 μm-thick nylon film as the outermost layer, a 40 μm-thick aluminum foil as the metal layer, and a 30 μm-thick polypropylene film as the insulating layer. The electrolyte solution prepared above was then injected through the opening in the housing member, and the remaining side of the housing member was heat-sealed in a reduced-pressure environment to seal the housing member, thereby fabricating the secondary battery according to Example 1.

[0083] In Example 1, the negative electrode capacity was calculated by the following method. Here, the negative electrode capacity refers to the capacity of the negative electrode active material.

[0084] A counter electrode Li coin cell was separately prepared using a positive electrode in which the positive electrode active material layer according to Example 1 was formed on only one side of the positive electrode current collector, and charging was carried out under the following conditions to measure the electrical capacity and determine the charge capacity per thickness of the positive electrode active material layer: Charge rate: 0.1 C, Charging method: CCCV, Charge control voltage: 4.45 V, Charge cut-off current: 0.01 C

[0085] Similarly, a counter electrode Li coin cell was prepared using a negative electrode in which a negative electrode active material layer according to Example 1 was formed on only one side of a negative electrode current collector using the components according to Example 1, and charging was carried out under the following conditions to measure the electrical capacity and determine the charge capacity per thickness of the negative electrode active material: Charge rate: 0.1 C Charging method: CCCV Charge control voltage: 0 V Charge cut-off current: 0.01 C

[0086] Using the charge capacity per thickness of the positive electrode active material and the charge capacity per thickness of the negative electrode active material measured above, a test battery was fabricated by adjusting the thickness of the positive electrode active material layer and the negative electrode active material layer by changing the solid content ratio of the positive electrode mixture slurry and the negative electrode mixture slurry, the application speed, etc., so that the ratio of the charge capacity of the positive electrode to the charge capacity of the negative electrode was 0.9. The test battery fabricated was discharged under the following conditions to measure the capacity and calculate the negative electrode capacity. Here, the negative electrode capacity was measured per 1 g of the first negative electrode active material and the second negative electrode active material. The negative electrode capacity ratio was calculated as a relative value to the negative electrode capacity in Example 6 described below. Discharge rate: 0.1 C Discharge method: CC Discharge end voltage: 1.0 V

[0087] In Example 1, a cycle characteristic test was carried out in the following manner: The cycle characteristic test was carried out at 23°C.

[0088] The battery prepared above was subjected to the first charge / discharge cycle under the following conditions: charge rate: 0.2 C, charge method: CCCV, charge control voltage: 4.40 V, charge cut-off current: 0.025 C, discharge rate: 0.2 C, discharge method: CC, discharge cut-off voltage: 3.0 V.

[0089] Thereafter, the second to 100th cycles were performed under the following conditions, and the discharge capacities at the second and 100th cycles were measured. The ratio of the discharge capacity at the 100th cycle to the discharge capacity at the second cycle was calculated as the cycle retention rate: Charge rate: 1.0 C, Charging method: CCCV, Charge control voltage: 4.40 V, Charge cut-off current: 0.025 C, Discharge rate: 1.0 C, Discharge method: CC, Discharge cut-off voltage: 3.0 V.

[0090] The post-cycle peel strength ratio was measured by the following method. After 100 cycles of discharge in the above-mentioned cycle characteristic test, the secondary battery was disassembled in a dry room, the negative electrode was removed, and the negative electrode was washed with dimethyl carbonate (DMC) to prepare a measurement sample. In the peel test, a 180° peel test was performed based on JIS Z 0237 (2009) to measure the peel force between the negative electrode current collector and the negative electrode active material layer. Here, the post-cycle peel strength ratio was calculated as a relative value to the value indicating the peel strength in Comparative Example 1.

[0091] Example 2 In Example 2, as shown in Table 1, a battery was fabricated in the same manner as in Example 1 except that the second binder was changed to NBR, and measurements were carried out.

[0092] Example 3 In Example 3, as shown in Table 1, a battery was fabricated in the same manner as in Example 1 except that the second binder was changed to PU, and measurements were carried out.

[0093] Example 4 In Example 4, as shown in Table 1, a battery was fabricated in the same manner as in Example 1, except that the second binder was changed to PMMA, and measurements were carried out.

[0094] (Example 5) In Example 5, as shown in Table 1, a battery was produced in the same manner as in Example 1, except that the content of the first binder (PNVA) in the second negative electrode mixture was changed to 3 mass % and 1% of SBR was further added as the second binder, and measurements were performed.

[0095] Example 6 In Example 6, as shown in Table 1, a battery was fabricated and measured in the same manner as in Example 1, except that the mass ratio R1 of the first negative electrode active material (silicon oxide) to the negative electrode active material contained in the first negative electrode mixture was changed to 10% (= 9.3 mass % / (9.3 mass % + 83.7 mass %)) and the mass ratio R2 of the first negative electrode active material (silicon oxide) to the negative electrode active material contained in the second negative electrode mixture was changed to 10% (= 9.4 mass % / (9.3 mass % + 84.6 mass %)).

[0096] Example 7 In Example 7, as shown in Table 1, a battery was fabricated and measured in the same manner as in Example 1, except that the mass ratio R1 of the first negative electrode active material (silicon oxide) to the negative electrode active material contained in the first negative electrode mixture was changed to 30% (= 27.9 mass % / (27.9 mass % + 65.1 mass %) and the mass ratio R2 of the first negative electrode active material (silicon oxide) to the negative electrode active material contained in the second negative electrode mixture was changed to 30% (= 28.2 mass % / (28.2 mass % + 65.8 mass %)).

[0097] Example 8 In Example 8, as shown in Table 1, a battery was fabricated and measured in the same manner as in Example 1, except that the mass ratio R1 of the first anode active material (silicon oxide) to the anode active material contained in the first anode mixture was changed to 30% (= 65.1 mass % / (65.1 mass % + 27.9 mass %) and the mass ratio R2 of the first anode active material (silicon oxide) to the anode active material contained in the second anode mixture was changed to 30% (= 65.8 mass % / (65.8 mass % + 28.2 mass %)).

[0098] (Example 9) In Example 9, as shown in Table 1, for the first anode mixture, the content of the first anode active material (silicon oxide) was changed to 90% by mass, the second anode active material was not included, the content of the first binder (PNVA) was changed to 6%, the content of carbon black was 2.55% by mass, the content of SWCNT was 0.45% by mass, and the content of the anode conductor was changed to 3.0% by mass. Also, in Example 9, as shown in Table 1, for the second anode mixture, the content of the first anode active material (silicon oxide) was changed to 91% by mass, the second anode active material was not included, the content of the first binder (PNVA) was changed to 8%, the content of carbon black was 2.55% by mass, the content of SWCNT was 0.45% by mass, and the content of the anode conductor was changed to 3.0% by mass. Except for changing the components of the first anode mixture and the second anode mixture as described above, batteries were fabricated and measured in the same manner as in Example 1. That is, in Example 9, the mass ratios R1 and R2 of the first anode active material (silicon oxide) to the anode active material contained in the first anode mixture and the second anode mixture were set to 100%.

[0099] (Example 10) In Example 10, as shown in Table 1, a battery was fabricated and measured in the same manner as in Example 1, except that the first negative electrode active material of the first negative electrode mixture and the second negative electrode mixture was changed to elemental silicon (Si).

[0100] (Example 11) In Example 11, as shown in Table 1, a battery was fabricated and measured in the same manner as in Example 1, except that the first negative electrode active material of the first negative electrode mixture and the second negative electrode mixture was changed to silicon carbide (SiC).

[0101] Example 12 In Example 12, as shown in Table 1, the first negative electrode active material of the first negative electrode mixture and the second negative electrode mixture was a silicon-containing alloy (SiTi ０．０１ A battery was fabricated in the same manner as in Example 1 except that the above-mentioned battery was changed to the above-mentioned battery, and measurements were carried out.

[0102] (Example 13) In Example 13, a battery was fabricated and measured in the same manner as in Example 1, except that the binder in the negative electrode side coating layer of the separator was changed to PNVA, as shown in Table 1. In Example 12, a solution was prepared by mixing PNVA having an average molecular weight of 1,000,000 as a binder and ion-exchanged water as a solvent in a mass ratio of 3:97, and alumina (Al 2 O 3 ) having an average particle size of 0.5 μm as an inorganic substance. ２ O ３ The fine powder was added in an amount twice the mass of the PNVA, and the mixture was thoroughly stirred to prepare a coating slurry for the negative electrode side sheet layer.

[0103] Comparative Example 1 In Comparative Example 1, as shown in Table 1, for the second negative electrode mixture, the content of the first binder (PNVA) was changed to 2 mass %, and the content of the second binder (SBR) was changed to 2 mass %, except that a battery was fabricated in the same manner as in Example 1, and measurements were performed.

[0104] Comparative Example 2 In Comparative Example 2, as shown in Table 1, for the first negative electrode mixture, the content of the first binder (PNVA) was changed to 4 mass % and the content of the second binder (SBR) was changed to 1 mass %, and for the second negative electrode mixture, the content of the first binder (PNVA) was changed to 1 mass % and the content of the second binder (SBR) was changed to 3 mass %, except that a battery was fabricated and measured in the same manner as in Example 1.

[0105] Comparative Example 3 In Comparative Example 3, as shown in Table 1, a battery was produced in the same manner as in Example 1 except that the first binder was changed to carboxymethyl cellulose sodium (CMC-Na), and measurements were performed.

[0106] As shown in Table 1, in Examples 1 to 13, by making the content of the second binder on the first surface larger than the content of the second binder on the second surface, the cycle retention rate and peel strength were improved compared to Comparative Examples 1 and 2, in which the content of the second binder on the first surface was equal to or less than the content of the second binder on the second surface.

[0107] As shown in Table 1, in Examples 1 to 13, by using an NVA polymer as the first binder, the cycle retention rate and peel strength were improved compared to Comparative Example 3, in which a substance other than an NVA polymer was used as the first binder.

[0108] As shown in Table 1, in Examples 1 and 7 to 9, sufficient negative electrode capacity could be obtained by setting the mass ratios R1 and R2 of the first negative electrode active material in the negative electrode active material to 10% or more.

[0109] As shown in Table 1, in Examples 1 and 7 to 9, the mass ratios R1, R2 of the first negative electrode active material in the negative electrode active material were greater than 10%, and therefore the negative electrode capacity was improved compared to Example 6, in which the mass ratios R1, R2 of the first negative electrode active material in the negative electrode active material were 10% or less.

[0110] As shown in Table 1, in Examples 1 and 7 to 9, the mass ratios R1, R2 of the first negative electrode active material in the negative electrode active material were 30% or more, and therefore the negative electrode capacity was improved compared to Example 6, in which the mass ratios R1, R2 of the first negative electrode active material in the negative electrode active material were less than 30%.

[0111] As shown in Table 1, in Examples 1 to 4, by making the content of the first binder on the second surface larger than the content of the first binder on the first surface, the cycle retention rate and peel strength were improved compared to Example 5 and Comparative Examples 1 and 2, in which the content of the first binder on the second surface was equal to or less than the content of the first binder on the first surface.

[0112] As shown in Table 1, in Example 13, the negative electrode coating layer contained the first binder (PNVA), and thus the cycle retention rate and peel strength were improved compared to Examples 1 to 12 and Comparative Examples 1 to 3 in which the negative electrode coating layer was made of a substance other than the first binder (PVdF).

[0113] The above-described embodiments are intended to facilitate understanding of the present invention and are not intended to limit the present invention. The present invention may be modified or improved without departing from the spirit thereof, and equivalents thereof are also included in the present invention.

[0114] The present invention may also be related to the following aspects.

[0115] (1) A secondary battery comprising: a positive electrode; a negative electrode; a separator between the positive electrode and the negative electrode; and an electrolyte; wherein the negative electrode has a negative electrode current collector; and a negative electrode active material layer in contact with the negative electrode current collector; the negative electrode active material layer includes a negative electrode active material, a first binder, and a second binder; the negative electrode active material includes the first negative electrode active material, which is a substance containing silicon; the first binder is an N-vinylacetamide-based polymer that is a polymer having a monomer represented by formula (1); and the second binder is an emulsion-based binder; the negative electrode active material layer has a first surface that is in contact with the negative electrode current collector and a second surface that is a surface opposite to the first surface; and the negative electrode active material layer includes a larger amount of the second binder in the first surface than in the second surface. (In formula (1), R １ and R ２ each independently represent hydrogen or an alkyl group which may have a substituent.) (2) The secondary battery according to (1), wherein a mass ratio of the first negative electrode active material to the negative electrode active material is 10% or more. (3) The secondary battery according to (2), wherein a mass ratio of the first negative electrode active material to the negative electrode active material is 30% or more. (4) The secondary battery according to any one of (1) to (3), wherein the negative electrode active material layer contains a larger amount of the first binder on the second surface than on the first surface. (5) The secondary battery according to any one of (1) to (4), wherein the second surface is in contact with the separator, and the separator has a substrate and a negative electrode-side coating layer on the negative electrode side of the substrate, and the negative electrode-side coating layer contains the N-vinylacetamide-based polymer.

[0116] REFERENCE SIGNS LIST 1, 1A Secondary battery 20, 20A Battery element 21, 21A Positive electrode lead 22, 22A Negative electrode lead 23 Protective material 30 Exterior member 30a, 30b Exterior sheet 31 Recess 32 Adhesive material 200, 200A Electrode body 210, 210A Positive electrode 211, 211A Positive electrode current collector 212, 212A Positive electrode active material layer 220, 220A Negative electrode 221, 221A Negative electrode current collector 222, 222A Negative electrode active material layer 222a First surface 222b Second surface 230, 230A Separator 231 Substrate 233 Negative electrode side coating layer

Claims

1. A secondary battery comprising: a positive electrode; a negative electrode; a separator between the positive electrode and the negative electrode; and an electrolyte; wherein the negative electrode has a negative electrode current collector; and a negative electrode active material layer in contact with the negative electrode current collector; the negative electrode active material layer includes a negative electrode active material, a first binder, and a second binder; the negative electrode active material includes a first negative electrode active material that is a substance containing silicon; the first binder is an N-vinylacetamide-based polymer that is a polymer having a monomer represented by formula (1); and the second binder is an emulsion-based binder; the negative electrode active material layer has a first surface that is in contact with the negative electrode current collector and a second surface that is a surface opposite to the first surface; and the negative electrode active material layer includes a larger amount of the second binder on the first surface than on the second surface. (In formula (1), R １ and R ２ each independently represents hydrogen or an alkyl group which may have a substituent.

2. The secondary battery according to claim 1, wherein the mass ratio of the first negative electrode active material to the negative electrode active material is 10% or more.

3. The secondary battery according to claim 2, wherein the mass ratio of the first negative electrode active material to the negative electrode active material is 30% or more.

4. The secondary battery according to any one of claims 1 to 3, wherein the negative electrode active material layer contains a larger amount of the first binder on the second surface than on the first surface.

5. A secondary battery according to any one of claims 1 to 4, wherein the second surface is in contact with the separator, the separator has a substrate and a negative electrode side coating layer on the negative electrode side of the substrate, and the negative electrode side coating layer contains the N-vinylacetamide-based polymer.

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