Negative electrode material for secondary battery, nonaqueous electrolyte secondary battery, and method for manufacturing negative electrode material for secondary battery

Abstract

The present disclosure addresses the problem of providing: a nonaqueous electrolyte secondary battery which can sufficiently maintain cycle characteristics; a negative electrode material for a secondary battery which is used in a nonaqueous electrolyte secondary battery; and a method for manufacturing a negative electrode material for a secondary battery. This negative electrode material for a secondary battery contains particles of an alloy-based active material that are alloyed with lithium, and a crosslinked polymer. The crosslinked polymer covers the outer surface of the particles of the alloy-based active material in a shell shape.　

Description

Negative electrode material for secondary battery, non-aqueous electrolyte secondary battery, and method for manufacturing negative electrode material for secondary battery

[0001] The present invention relates to a negative electrode material for a secondary battery, a non-aqueous electrolyte secondary battery, and a method for producing a negative electrode material for a secondary battery.

[0002] In recent years, in non-aqueous electrolyte secondary batteries that include a positive electrode, a negative electrode facing the positive electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte solution, there has been a demand for improving the cycle characteristics by coating the outer surface of the negative electrode active material contained in the negative electrode with a polymer.

[0003] Patent Document 1 below describes coating the outer surface of a negative electrode active material with polyacrylic acid containing carboxyl groups and acid anhydride groups (hereinafter referred to as polyacrylic resin A). Specifically, the document describes preparing a slurry in which the negative electrode active material and polyacrylic acid A are uniformly dispersed, and then spray-drying the prepared slurry using a spray dryer or the like to coat the outer surface of the negative electrode active material with polyacrylic resin A. Patent Document 1 below also describes that the use of such a negative electrode active material improves the high-temperature cycle characteristics of a non-aqueous electrolyte secondary battery.

[0004] In the following Patent Document 2, Si, SiO ｘ (where 1≦x≦2) or a silicon alloy anode active material, and a coating layer that coats the outer surface of the anode active material. Patent Document 2 below also describes that the coating layer contains a polymer obtained by crosslinking a polymer precursor having a specific structural formula to form a network structure. Patent Document 2 below also describes that the outer surface of the anode active material is crosslinked by adhering a polymer precursor to the outer surface of the anode active material by spray drying, and then crosslinking the polymer precursor on the outer surface of the anode active material. Patent Document 2 below also describes that the electrochemical cycle performance of a non-aqueous electrolyte secondary battery is improved by crosslinking the outer surface of the anode active material.

[0005] JP 2017-152122 A JP 2015-50188 A

[0006] When the negative electrode active material is a material that alloys with lithium, such as silicon (Si) and tin (Sn) (hereinafter referred to as an alloy-based active material), the alloy-based active material repeatedly expands and contracts to a greater extent during charging and discharging than when the negative electrode active material is a carbon-based material. If the outer surface of the alloy-based active material is coated with a polymer or the like as described above, the polymer coating layer may be unable to flexibly accommodate the repeated large expansion and contraction during charging and discharging, resulting in defects (cracks) in the polymer coating layer. When defects (cracks) occur in the polymer coating layer, the nonaqueous electrolyte may come into contact with the outer surface of the alloy-based active material at the defective portion, causing decomposition of the nonaqueous electrolyte. This decomposition of the nonaqueous electrolyte may result in the inability to adequately maintain the cycle characteristics of the nonaqueous electrolyte secondary battery. However, it is difficult to say that sufficient research has yet been conducted into maintaining the cycle characteristics of a nonaqueous electrolyte secondary battery when using an alloy-based active material coated with a polymer or the like.

[0007] Therefore, an object of the present disclosure is to provide a non-aqueous electrolyte secondary battery that can sufficiently maintain cycle characteristics, a negative electrode material for a secondary battery used in a non-aqueous electrolyte secondary battery, and a method for manufacturing a negative electrode material for a secondary battery.

[0008] One aspect of the present invention relates to a negative electrode material for a secondary battery, comprising particles of an alloy-based active material that is alloyed with lithium, and a crosslinked polymer, wherein the crosslinked polymer coats the outer surfaces of the particles of the alloy-based active material in the form of a shell.

[0009] Another aspect of the present invention relates to a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode facing the positive electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein the negative electrode contains the above-described negative electrode material for secondary batteries.

[0010] Yet another aspect of the present invention relates to a method for producing a negative electrode material for a secondary battery, the method comprising: a first step of obtaining a mixed solution containing a monomer, a crosslinker for the monomer, and particles of an alloy-system active material that is to be alloyed with lithium; a second step of adding a poor solvent for the monomer to the mixed solution after the first step to obtain droplets containing the monomer and the crosslinker; and a third step of polymerizing the monomer and the crosslinker contained in the droplets after the second step to coat the outer surfaces of the particles of the alloy-system active material with a shell-like crosslinked polymer.

[0011] According to the present disclosure, it is possible to provide a non-aqueous electrolyte secondary battery capable of sufficiently maintaining cycle characteristics, a negative electrode material for a secondary battery used in a non-aqueous electrolyte secondary battery, and a method for manufacturing a negative electrode material for a secondary battery.

[0012] 1 is a longitudinal sectional view schematically showing a nonaqueous electrolyte secondary battery according to a first embodiment.

[0013] Below, embodiments of the present disclosure will be described using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values, materials, etc. may be applied as long as the effects of the present disclosure are obtained. Note that known components may be applied to components characteristic of the present disclosure. In this specification, when a "range from numerical value A to numerical value B" is mentioned, the range includes numerical value A and numerical value B.

[0014] In the following description, when lower and upper limits of numerical values ​​relating to specific physical properties, conditions, etc. are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of them can be selected and used alone, or two or more can be used in combination, unless otherwise specified.

[0015] The present disclosure encompasses any combination of two or more claims arbitrarily selected from the appended claims, i.e., any combination of two or more claims arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0016] [Negative Electrode Material for Secondary Batteries] The negative electrode material for secondary batteries according to the embodiment of the present disclosure includes particles of an alloy-based active material that alloys with lithium and a crosslinked polymer. In the negative electrode material for secondary batteries according to the embodiment of the present disclosure, the crosslinked polymer coats the outer surfaces of the particles of the alloy-based active material in the form of a shell. Hereinafter, the crosslinked polymer coating the outer surfaces of the particles of the alloy-based active material is also referred to as a "polymer shell." The thickness T of the polymer shell is sufficiently smaller than the particle diameter d of the particles of the alloy-based active material, and may be 50% or less of the particle diameter d. The polymer shell may be a monolayer or a thin multilayer (e.g., a multilayer of 10 or less layers). One polymer shell may contain multiple particles of the alloy-based active material or a single particle of the alloy-based active material. However, it is preferable that one particle of the alloy-based active material is contained within the polymer shell. The polymer shell may have a seamless capsule-like structure. In addition, in the negative electrode material for a secondary battery according to an embodiment of the present disclosure, the crosslinked polymer may form substantially a single continuous body and cover the outer surface of the alloy-based active material.

[0017] In the negative electrode material for a secondary battery according to an embodiment of the present disclosure, as explained above, it is important that the crosslinked polymer covers the outer surfaces of the particles of the alloy-based active material in a shell-like manner. The reason for this will be explained below.

[0018] When the outer surfaces of particles in an alloy-based active material layer are coated with a polymer by spray-drying a mixed solution containing a polymerized polymer and an alloy-based active material, the outer surfaces of the particles of the alloy-based active material are coated with multiple polymers. In such a case, it is thought that the ends of the multiple polymers overlap each other on the outer surfaces of the particles of the alloy-based active material. Here, since the monomers are chemically bonded to each other, the individual polymers have sufficient bond strength, but since the ends of the multiple polymers are physically bonded to each other, the bond strength is not necessarily sufficient.

[0019] Therefore, when the alloy-based active material particles repeatedly absorb and release lithium ions during charging and discharging of a non-aqueous electrolyte secondary battery, the alloy-based active material particles undergo large expansion and contraction, which is thought to cause peeling at the overlapping portions of the ends of multiple polymers that do not have sufficient bonding strength. In other words, it is thought that defects (cracks) caused by peeling occur in the polymer-coated portion. If defects (cracks) occur in the polymer-coated portion, the non-aqueous electrolyte contained in the non-aqueous electrolyte secondary battery may come into contact with the outer surface of the alloy-based active material particles at these defective portions, resulting in decomposition of the non-aqueous electrolyte. This decomposition of the non-aqueous electrolyte may make it difficult to maintain the cycle characteristics of the non-aqueous electrolyte secondary battery.

[0020] However, in the negative electrode material for secondary batteries according to this embodiment, the crosslinked polymer coats the outer surfaces of the particles of the alloy-based active material in a shell-like manner, so there are no overlapping portions of the ends of multiple polymers. Therefore, even if the particles of the alloy-based active material undergo significant expansion and contraction during charging and discharging of the nonaqueous electrolyte secondary battery, peeling of the polymer-coated portions is suppressed. This suppresses decomposition of the nonaqueous electrolyte, allowing the cycle characteristics of the nonaqueous electrolyte secondary battery to be adequately maintained. The above-described effects are even more favorably achieved when the crosslinked polymer forms a substantially continuous body that coats the outer surfaces of the particles of the alloy-based active material, or when the polymer shell has a seamless capsule-like structure.

[0021] Whether or not the crosslinked polymer coats the outer surface of the alloy-based active material in a substantially continuous state can be determined by stirring particles of a composite material (hereinafter simply referred to as composite material) in water, in which the outer surfaces of the particles of the alloy-based active material are coated with a polymer, allowing the mixed solution to stand, and then visually observing the mixed solution after standing. Specifically, this can be determined by observing whether or not floating polymer particles that have peeled off from the outer surfaces of the particles of the alloy-based active material are found in the mixed solution after standing. In the above-mentioned determination method, stirring is performed by dispersing 15 g of composite material particles in 50 g of water and stirring at a speed of 200 rpm for 60 minutes. Allowing to stand means leaving the mixed solution after stirring for 10 minutes or more.

[0022] Examples of alloy-based active materials that can be alloyed with lithium include silicon (Si)-containing materials, tin (Sn)-containing materials, etc. From the viewpoint of increasing capacity, it is preferable to use a silicon (Si)-containing material as the alloy-based active material that can be alloyed with lithium.

[0023] The silicon-containing material may be silicon particles. The silicon particles are not particularly limited, but preferably have a particle size of 1 nm or more and 10 μm or less. In particular, nano-sized silicon particles are preferred from the viewpoint of reducing the non-uniformity of the lithium ion concentration and reducing particle collapse. That is, the silicon particles are preferably nanosilicon particles. The particle size of the silicon particles is preferably, for example, 500 nm or less. By having the particle size within the above range, the difference in lithium ion concentration between the surface and interior of the silicon particles is reduced, making it difficult for non-uniform volume expansion to occur in the silicon particles. This significantly reduces particle collapse and significantly suppresses the capacity decrease of the lithium ion battery. The smaller the particle size of the silicon particles, the more preferable, but it is not necessary to reduce it to less than 1 nm; 1 nm or more is sufficient, and it may be 3 nm or more.

[0024] As described above, in the negative electrode material for secondary batteries of the present disclosure, the particles of the alloy-based active material that alloys with lithium have their outer surfaces coated with a crosslinked polymer. That is, the negative electrode material for secondary batteries of the present disclosure is configured as a composite material of an alloy-based active material that alloys with lithium and a crosslinked polymer. Therefore, the particle size of the silicon particles can be determined by determining the particle sizes of the silicon particle portions of any 100 composite materials observed in high-resolution transmission electron microscope (HR-TEM) images and arithmetically averaging the particle sizes. The composite materials observed in HR-TEM images may be primary particles or secondary particles. Furthermore, it is not necessary to distinguish between primary particles and secondary particles. The particle size of the silicon particle portion can be calculated as the diameter of an equivalent circle having an area equal to the area of ​​the silicon particle portion obtained in the HR-TEM image.

[0025] The silicon-containing material may be a composite material containing a silicon phase and a matrix phase in which the silicon phase is dispersed. The matrix phase may be composed of a material having lithium ion conductivity. The matrix phase may include, for example, at least one selected from the group consisting of a silicon oxide phase and a carbon phase. In such a composite material, the silicon contained in the silicon phase reversibly forms an alloy with lithium. Therefore, such a composite material can also reversibly store and release lithium ions.

[0026] The silicon oxide phase may contain, in addition to Si and O, elements other than Si and O. The silicon oxide phase may contain silicon dioxide (SiO ２ ) phase, or may be constituted as a lithium silicate phase, or may be constituted as both of these phases.

[0027] When the silicon-containing material is the above-mentioned composite material, the composite material may be composed of any one of the following (a) to (c): (a) a silicon phase and silicon dioxide (SiO ) in which the silicon phase is dispersed; ２(b) a structure including a silicon phase and a lithium silicate phase in which the silicon phase is dispersed (second composite material); and (c) a structure including a silicon phase and a carbon phase in which the silicon phase is dispersed (third composite material).

[0028] When the silicon-containing material is the first composite material (case (a) above), there is an advantage that the volume change accompanying the absorption and desorption of lithium ions is small. It is presumed that one of the reasons for this advantage is that the silicon dioxide phase has a relatively large number of sites that irreversibly trap lithium ions, making it difficult for the first composite material to shrink in volume accompanying the desorption of lithium ions.

[0029] The first composite material can be synthesized, for example, by heating silicon oxide, which is a raw material, in a non-oxidizing atmosphere (inert atmosphere) to cause a disproportionation reaction.

[0030] When the silicon-containing material is the second composite material (case (b) above), the advantage of being able to reduce the irreversible capacity is obtained. Therefore, when the second composite material is used as the silicon-containing material, excellent charge / discharge efficiency can be obtained. This effect is particularly noticeable in the early stages of charge / discharge.

[0031] The lithium silicate phase contained in the second composite material may contain elements other than Si, O, and Li. Such elements may be at least one selected from the group consisting of Group 1 elements (other than Li) and Group 2 elements of the long form periodic table. The Group 1 elements and Group 2 elements may be, for example, K, Na, Mg, Ca, Sr, Ba, etc. The lithium silicate phase may also contain Al, B, La, P, Zr, Ti, Fe, Cr, Ni, Mn, Cu, Mo, Zn, etc.

[0032] The ratio of the number of O atoms to the number of Si atoms in the lithium silicate phase (O / Si) is, for example, greater than 2 and less than 4. In this case, in addition to being advantageous in terms of the stability of the lithium silicate phase, it is also advantageous in terms of lithium ion conductivity. The O / Si ratio may be greater than 2 and less than 3. The ratio of the number of Li atoms to the number of Si atoms in the lithium silicate phase (Li / Si) is, for example, greater than 0 and less than 4.

[0033] The lithium silicate, which is the raw material for obtaining the lithium silicate phase, has the formula Li ２ｚ SiO ２＋ｚ (0<z<2). It is preferable that z satisfies 0<z<1. When z is in this range, the stability of the lithium silicate is increased and the lithium silicate is easily produced. Furthermore, when the lithium silicate is made into a lithium silicate phase, the lithium ion conductivity can be increased. It is more preferable that z is 1 / 2.

[0034] The second composite material can be obtained, for example, by mixing and stirring the raw materials, lithium silicate and silicon, while crushing them in a mixer such as a ball mill to obtain a mixture, and then firing the mixture under pressure in an inert atmosphere. Note that the second composite material may also be obtained by heating the mixture to a predetermined temperature, necking at least one of the lithium silicate and silicon in the mixture to obtain a sintered body, and then pulverizing the sintered body.

[0035] Even when the silicon-containing material is the third composite material (case (c) above), the advantage of being able to reduce the irreversible capacity is obtained. In addition, the carbon phase exhibits capacity through a Faraday reaction with lithium ions, which is advantageous in realizing a high capacity.

[0036] The carbon phase may contain crystalline carbon (graphite) or amorphous carbon with low crystallinity (i.e., amorphous carbon). The amorphous carbon may be, for example, non-graphitizable carbon, easily graphitizable carbon, or other.

[0037] The third composite material can be obtained in the same manner as the second composite material, except that a carbon source and silicon are used as raw materials.

[0038] Examples of carbon sources that can be used include sugars, water-soluble resins, etc. Examples of carbon sources that can be used include carboxymethyl cellulose (CMC), polyvinylpyrrolidone, cellulose, sucrose, etc. The mixture may be obtained by dispersing the carbon source and silicon in an organic solvent such as alcohol.

[0039] The alloy-based active material may be aggregated with its outer surface coated with a crosslinked polymer. Such aggregated particles prevent the alloy-based active material from being over-dispersed in the negative electrode composite. This prevents an increase in the resistance of the negative electrode composite.

[0040] The crosslinked polymer preferably has at least a carboxyl group, more preferably has at least a structural unit derived from an ethylenically unsaturated carboxylic acid monomer, and more preferably is composed of a structural unit derived from an ethylenically unsaturated carboxylic acid monomer.

[0041] The crosslinked polymer may contain 50% by mass or more and 100% by mass or less of structural units derived from an ethylenically unsaturated carboxylic acid, based on the total structural units of the crosslinked polymer. The content may be 60% by mass or more, 70% by mass or more, or 80% by mass or more. The content may be 99.5% by mass or less, or 99.0% by mass or less.

[0042] The crosslinked polymer preferably has a carboxyl group neutralized with a lithium compound. Specifically, the crosslinked polymer preferably has a lithium carboxylate (COOLi). The lithium carboxylate is prepared by neutralizing the carboxyl group of the crosslinked polymer with lithium hydroxide monohydrate (LiOH·H ２ Lithium carboxylate can be obtained by neutralizing the negative electrode material for a secondary battery according to an embodiment of the present disclosure with a lithium compound such as lithium hydroxide monohydrate (LiOH.H O) in acetonitrile, for example.２ The crosslinked polymer can be obtained by stirring and mixing the lithium carboxylate and the lithium carboxylate. When the crosslinked polymer contains lithium carboxylate, the resistance of the negative electrode mixture can be reduced.

[0043] The structural unit derived from an ethylenically unsaturated carboxylic acid monomer can be obtained by polymerizing a monomer containing an ethylenically unsaturated carboxylic acid monomer. It can also be obtained by (co)polymerizing a (meth)acrylic acid ester monomer and then hydrolyzing it, by polymerizing (meth)acrylamide and (meth)acrylonitrile, etc., and then treating them with a strong alkali, or by reacting a polymer having a hydroxyl group with an acid anhydride. In this specification, "(meth)acrylic" means at least one of acrylic and methacrylic, and "(meth)acrylo" means at least one of acrylo and methacrylo.

[0044] Examples of ethylenically unsaturated carboxylic acid monomers include (meth)acrylic acid, (meth)acrylamidoalkylcarboxylic acids, ethylenically unsaturated monomers having a carboxyl group, or (partially) alkali-neutralized products thereof. Examples of (meth)acrylamidoalkylcarboxylic acids include (meth)acrylamidohexylcarboxylic acid and (meth)acrylamidodecanoic acid. Examples of ethylenically unsaturated monomers having a carboxyl group include monohydroxyethyl succinate (meth)acrylate, ω-carboxy-caprolactone mono(meth)acrylate, and β-carboxyethyl (meth)acrylate. The above monomers may be used alone or in combination of two or more. Among these, compounds having an acryloyl group are preferred due to their high polymerization rate, and among compounds having an acryloyl group, acrylic acid is particularly preferred. When acrylic acid is used as the ethylenically unsaturated carboxylic acid monomer, a polymer with a high carboxyl group content can be obtained. In this specification, "(meth)acrylate" refers to at least one of acrylate and methacrylate.

[0045] The crosslinked polymer may contain a structural unit derived from another ethylenically unsaturated monomer copolymerizable with the ethylenically unsaturated carboxylic acid monomer. Examples of structural units derived from other ethylenically unsaturated monomers include structural units derived from ethylenically unsaturated monomer compounds having anionic groups other than carboxyl groups, such as sulfonic acid groups and phosphoric acid groups, or nonionic ethylenically unsaturated monomers. These structural units can be introduced by copolymerizing an ethylenically unsaturated monomer compound having anionic groups other than carboxyl groups, such as sulfonic acid groups and phosphoric acid groups, or a monomer containing a nonionic ethylenically unsaturated monomer. Among these, structural units derived from other ethylenically unsaturated monomers are preferably structural units derived from nonionic ethylenically unsaturated monomers, from the viewpoint of flex resistance.

[0046] The crosslinked polymer may contain structural units derived from other ethylenically unsaturated monomers in an amount of 1% by mass to 50% by mass, based on the total structural units of the crosslinked polymer, and the content may be 5% by mass to 40% by mass or 10% by mass to 30% by mass.

[0047] Examples of nonionic ethylenically unsaturated monomers include (meth)acrylamide and its derivatives. Examples of (meth)acrylamide derivatives include N-alkyl(meth)acrylamide compounds and N,N-dialkyl(meth)acrylamide compounds. Examples of N-alkyl(meth)acrylamide compounds include isopropyl(meth)acrylamide, t-butyl(meth)acrylamide, N-n-butoxymethyl(meth)acrylamide, and N-isobutoxymethyl(meth)acryl. Examples of N,N-dialkyl(meth)acrylamide compounds include dimethyl(meth)acrylamide and diethyl(meth)acrylamide. These monomers may be used alone or in combination of two or more.

[0048] Examples of nonionic ethylenically unsaturated monomers include (meth)acrylic acid esters. Examples of (meth)acrylic acid esters include (meth)acrylic acid alkyl ester compounds, (meth)acrylic acid cycloalkyl ester compounds, (meth)acrylic acid alkoxyalkyl ester compounds, (meth)acrylic acid hydroxyalkyl ester compounds, and (meth)acrylic acid aralkyl ester compounds. Examples of (meth)acrylic acid alkyl ester compounds include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. Examples of (meth)acrylic acid cycloalkyl ester compounds include cyclohexyl (meth)acrylate and methylcyclohexyl (meth)acrylate. Examples of (meth)acrylic acid alkoxyalkyl ester compounds include 2-methoxyethyl (meth)acrylate and ethoxyethyl (meth)acrylate. Examples of (meth)acrylic acid hydroxyalkyl ester compounds include hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, and hydroxybutyl (meth)acrylate. Examples of (meth)acrylic acid aralkyl ester compounds include phenyl (meth)acrylate, phenylmethyl (meth)acrylate, and phenylethyl (meth)acrylate. These monomers may be used alone or in combination of two or more.

[0049] In the crosslinked polymer, the mode of crosslinking is not particularly limited. Examples of the mode of crosslinking include copolymerization of a crosslinkable monomer, utilization of chain transfer to a polymer chain during radical polymerization, and post-crosslinking by adding a crosslinking agent as needed after synthesizing a copolymer having a reactive functional group. Among these, crosslinking with a crosslinkable monomer is preferred from the viewpoints of simple operation and easy control of the degree of crosslinking.

[0050] As described above, the crosslinked polymer preferably has a structure derived from a crosslinkable monomer. The crosslinked polymer having a structure derived from a crosslinkable monomer can further improve the elasticity of the crosslinked polymer. This can further prevent peeling of the polymer-coated portion, even if significant expansion and contraction occurs in the particles of the alloy-based active material during charging and discharging of a nonaqueous electrolyte secondary battery. The crosslinked polymer preferably has a structure in which one polymer having an alkenyl structure in its molecule and another polymer having an alkenyl structure in its molecule are crosslinked by a crosslinkable monomer. In such a structure, the one polymer and the other polymer may have the same structure or different structures. The one polymer having an alkenyl structure and the other polymer may be obtained, for example, by polymerizing the ethylenically unsaturated carboxylic acid monomer described above. Furthermore, the crosslinked polymer described above may have a partial sodium salt structure in which a portion of the polymer is substituted with sodium.

[0051] When the crosslinked polymer is crosslinked by a crosslinkable monomer, the amount of the crosslinkable monomer used may be, for example, 0.05% by mass or more and 8% by mass or less, 0.1% by mass or more and 7% by mass or less, 0.2% by mass or more and 6% by mass or less, or 0.3% by mass or more and 5% by mass or less, based on all the constituent monomers of the crosslinked polymer.

[0052] Examples of the crosslinkable monomer include polyfunctional polymerizable monomers having two or more polymerizable unsaturated groups, and monomers having a self-crosslinkable crosslinkable functional group such as a hydrolyzable silyl group.

[0053] The polyfunctional polymerizable monomer is a compound having two or more polymerizable functional groups, such as (meth)acryloyl groups and alkenyl groups, in the molecule. As the polyfunctional polymerizable monomer, a polyfunctional (meth)acrylate compound, a polyfunctional alkenyl compound, or a compound having both a (meth)acryloyl group and an alkenyl group is preferably used. That is, in the negative electrode material for a secondary battery according to the embodiment of the present disclosure, the crosslinkable polymer preferably contains a structural unit derived from either a polyfunctional (meth)acrylate compound, a polyfunctional alkenyl compound, or a compound having both a (meth)acryloyl group and an alkenyl group. Among these, polyfunctional alkenyl compounds are preferred in terms of the ease of obtaining a uniform crosslinked structure, and polyfunctional allyl ether compounds having multiple allyl ether groups in the molecule are more preferred. In this specification, the term "(meth)acryloyl group" refers to at least one of an acryloyl group and a methacryloyl group.

[0054] Examples of polyfunctional (meth)acrylate compounds include di(meth)acrylates of dihydric alcohols, poly(meth)acrylates of trihydric or higher polyhydric alcohols, and bisamides. Examples of di(meth)acrylates include ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, polyethylene glycol di(meth)acrylate, and polypropylene glycol di(meth)acrylate. Examples of poly(meth)acrylates include trimethylolpropane tri(meth)acrylate, trimethylolpropane ethylene oxide modified tri(meth)acrylate, glycerin tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and pentaerythritol tetra(meth)acrylate. Examples of bisamides include methylenebisacrylamide and hydroxyethylbisacrylamide.

[0055] Examples of polyfunctional alkenyl compounds include polyfunctional allyl ether compounds, polyfunctional allyl compounds, and polyfunctional vinyl compounds. Examples of polyfunctional allyl ether compounds include trimethylolpropane diallyl ether, trimethylolpropane triallyl ether, pentaerythritol diallyl ether, pentaerythritol triallyl ether, tetraallyloxyethane, polyallylsucrose, and 1,3-diallyloxy-2-propanol. Examples of polyfunctional allyl compounds include diallyl phthalate. Examples of polyfunctional vinyl compounds include divinylbenzene.

[0056] Examples of compounds having both a (meth)acryloyl group and an alkenyl group include allyl (meth)acrylate, isopropenyl (meth)acrylate, butenyl (meth)acrylate, pentenyl (meth)acrylate, and 2-(2-vinyloxyethoxy)ethyl (meth)acrylate.

[0057] Specific examples of the monomer having a self-crosslinkable crosslinkable functional group include, for example, a hydrolyzable silyl group-containing vinyl monomer, N-methylol(meth)acrylamide, and N-methoxyalkyl(meth)acrylate, etc. These compounds may be used alone or in combination of two or more.

[0058] The hydrolyzable silyl group-containing vinyl monomer is not particularly limited as long as it is a vinyl monomer having at least one hydrolyzable silyl group. Examples include vinylsilanes, silyl group-containing acrylic esters, silyl group-containing methacrylic esters, silyl group-containing vinyl ethers, and silyl group-containing esters. Examples of vinylsilane groups include vinyltrimethoxysilane, vinyltriethoxysilane, vinylmethyldimethoxysilane, and vinyldimethylmethoxysilane. Examples of silyl group-containing acrylic esters include trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, and methyldimethoxysilylpropyl acrylate. Examples of silyl group-containing methacrylic esters include trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyldimethoxysilylpropyl methacrylate, and dimethylmethoxysilylpropyl methacrylate. Examples of silyl group-containing vinyl ethers include dimethylmethoxysilylpropyl methacrylate. Examples of silyl group-containing vinyl esters include vinyl trimethoxysilylundecanoate.

[0059] The crosslinked polymer can be obtained, for example, by suspension polymerization, reversed-phase suspension polymerization, emulsion polymerization, etc. As described below, the negative electrode material for a secondary battery according to an embodiment of the present disclosure may be obtained by encapsulating particles of an alloy-based active material that can be alloyed with lithium in droplets of a polymerization component containing a monomer and a crosslinker for the monomer, polymerizing the monomer and the crosslinker contained in the droplets, and coating the outer surfaces of the particles of the alloy-based active material with a shell-like coating of the polymerization component.

[0060] [Method for manufacturing negative electrode material for secondary battery] A method for manufacturing a negative electrode material for secondary battery according to an embodiment of the present disclosure includes: a first step of obtaining a mixed solution containing a monomer, a crosslinker for the monomer, and particles of an alloy-based active material that is alloyed with lithium; a second step of adding a poor solvent for the monomer to the mixed solution obtained after the first step to obtain droplets containing the monomer and the crosslinker; and a third step of polymerizing the monomer and crosslinker contained in the droplets obtained after the second step to coat the outer surfaces of the particles of the alloy-based active material with a shell-like crosslinked polymer.

[0061] As the monomer, various types of monomers described above can be used, such as ethylenically unsaturated carboxylic acid monomers and other ethylenically unsaturated monomers copolymerizable with the ethylenically unsaturated carboxylic acid monomers.

[0062] The crosslinking agent for the monomer may be any of the various crosslinking monomers described above, such as a polyfunctional polymerizable monomer having two or more polymerizable unsaturated groups, or a monomer having a self-crosslinkable crosslinking functional group such as a hydrolyzable silyl group.

[0063] In the method for producing a negative electrode material for a secondary battery according to an embodiment of the present disclosure, the polymerization of the monomer and the crosslinking agent is preferably carried out using a polymerization initiator, which may be any of various known initiators such as azo compounds, organic peroxides, and inorganic peroxides.

[0064] Examples of azo compounds include 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(N-butyl-2-methylpropionamide), 2-(tert-butylazo)-2-cyanopropane, 2,2'-azobis(2,4,4-trimethylpentane), 2,2'-azobis(2-methylpropane), and 4,4'-azobis(4-cyanovaleric acid). These may be used alone or in combination of two or more.

[0065] Examples of organic peroxides include 2,2-bis(4,4-di-t-butylperoxycyclohexyl)propane, 1,1-di(t-hexylperoxy(cyclohexane), 1,1-di(t-butylperoxy)cyclohexane, n-butyl-4,4-di(t-butylperoxy)valerate, 2,2-di(t-butylperoxy)butane, t-butyl hydroperoxide, cumene hydroperoxide, and 1,1,3,3-tetramethylbutyl halides. Hydroperoxide, t-butylcumyl peroxide, di-t-butyl peroxide, di-t-hexyl peroxide, di(3,5,5-trimethylhexanoyl) peroxide, dilauroyl peroxide, bis(4-t-butylcyclohexyl) peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, di-sec-butyl peroxydicarbonate, cumyl peroxyneodecanoate, 1,1,3,3-tetramethylhexanoate Examples of the peroxyalkylene compounds include tetramethylbutylperoxyneodecanoate, t-hexylperoxyneodecanoate, t-butylperoxyneodecanoate, t-butylperoxyneoheptanoate, t-hexylperoxypivalate, t-butylperoxypivalate, 2,5-dimethyl-2,5-di(2-ethylhexanoyl)hexane, 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate, t-butylperoxy-2-ethylhexanoate, t-butylperoxylaurate, t-butylperoxy-3,5,5-trimethylhexanoate, t-hexylperoxyisopropyl monocarbonate, t-butylperoxyisopropyl monocarbonate, t-butylperoxyacetate, t-hexylperoxybenzoate, and t-butylperoxybenzoate. These compounds may be used alone or in combination of two or more.

[0066] Examples of inorganic peroxides include potassium persulfate, sodium persulfate, and ammonium persulfate. When the monomer and the crosslinking agent are polymerized by oxidation-reduction polymerization (redox polymerization), a reducing agent may be used in addition to the inorganic peroxide. Examples of reducing agents include sodium sulfite, sodium thiosulfate, sodium formaldehyde sulfoxylate, ascorbic acid, sulfurous acid gas (SO ). ２ ), and ferrous sulfate.

[0067] As described above, when a polymerization initiator is used, the polymerization initiator is preferably incorporated into the droplets containing the monomer and the crosslinking agent, so that the polymerization reaction between the monomer and the crosslinking agent can be initiated quickly.

[0068] The content of the polymerization initiator may be 0.001 parts by mass or more and 2 parts by mass or less, 0.005 parts by mass or more and 1.5 parts by mass or less, or 0.01 parts by mass or more and 1.0 part by mass or less, when the total amount of the monomer components (the monomer and the crosslinking agent) is 100 parts by mass.

[0069] The concentration of the monomer component at the start of polymerization is usually 2% by mass or more and 30% by mass or less, and may be 5% by mass or more and 30% by mass or less. The polymerization temperature may be, for example, 0°C or more and 100°C or less, or 20°C or more and 80°C or less. The polymerization temperature may be constant during the polymerization reaction, or may vary during the polymerization reaction. The polymerization time may be 1 minute or more and 20 hours or less, or 1 hour or more and 10 hours or less.

[0070] In the method for producing a negative electrode material for a secondary battery according to an embodiment of the present disclosure, the monomer and the crosslinker may be polymerized while particles of the alloy-based active material are encapsulated in the droplets. In the polymerization using droplets as described above, the particle size D1 of the droplets is typically in the range of several tens of nanometers to 1,000 μm. Furthermore, the particle size D2 of the alloy-based active material particles is typically in the nano-size range or in the range of submicrons to several microns. Therefore, in order to ensure that the particles of the alloy-based active material are sufficiently incorporated into the droplets, the ratio of the particle size D1 of the droplets to the particle size D2 of the alloy-based active material particles (D1 / D2) is preferably 1.02 or greater, more preferably 1.10 or greater, and even more preferably 1.20 or greater. Furthermore, D1 / D2 may be 100 or less, 10 or less, or 5 or less. The monomer and the crosslinker may be polymerized in a state where a plurality of particles of the alloy-based active material are encapsulated in one droplet, or in a state where a single particle of the alloy-based active material is encapsulated in one droplet. The alloy-based active material may be nanoparticles having a nano-sized particle size. Nanosilicon particles are preferably used as the nanoparticles.

[0071] As explained above, the particle diameter D2 of the alloy-based active material particles can be determined using high-resolution transmission electron microscope (HR-TEM) images. The particle diameter D1 of the droplets can be determined by observing a liquid fraction collected from the poor solvent of the monomer using a SEM (transmission electron microscope). Specifically, it can be determined as the arithmetic mean value of the particle diameters of any 100 droplets observed in the SEM image.

[0072] In the second step of the method for producing a negative electrode material for a secondary battery according to an embodiment of the present disclosure, droplets containing the monomer and the crosslinking agent may be obtained by suspending the monomer and the crosslinking agent in a poor solvent for the monomer. That is, in the method for producing a negative electrode material for a secondary battery according to an embodiment of the present disclosure, the polymerization of the monomer and the crosslinking agent may be carried out by suspension polymerization or reversed-phase suspension polymerization. Note that if the monomer is a water-soluble monomer and the poor solvent for the monomer is a non-aqueous solvent, the polymerization of the monomer and the crosslinking agent is carried out by reversed-phase suspension polymerization. If the monomer is a water-insoluble monomer and the poor solvent for the monomer is water, the polymerization of the monomer and the crosslinking agent is carried out by suspension polymerization. Polymerizing the monomer and the crosslinking agent by suspension polymerization or reversed-phase suspension polymerization is simple and facilitates the formation of a shell-like coating of the crosslinked polymer on the outer surfaces of the particles of the alloy-based active material. Furthermore, in suspension polymerization and reversed-phase suspension polymerization, the particle size D1 of the droplets is often approximately 10 μm to 1000 μm. Therefore, suspension polymerization and reversed-phase suspension polymerization can be suitably employed when particles of an alloy-based active material having a particle size of submicrons to several microns are used. In reversed-phase suspension polymerization, liquid paraffin or the like can be used as a poor solvent for the monomer.

[0073] A method for producing a negative electrode material for a secondary battery according to an embodiment of the present disclosure may include using a water-insoluble monomer as the monomer and water as a poor solvent for the monomer. In a second step, a surfactant is further added to the poor solvent for the monomer to form micelles of the surfactant. The alloy-based active material, the monomer, and the crosslinker are introduced into the surfactant micelles to obtain droplets containing the monomer and the crosslinker. In a third step, the monomer and the crosslinker contained in the droplets are polymerized inside the surfactant micelles. That is, the method for producing a negative electrode material for a secondary battery according to an embodiment of the present disclosure may be carried out by emulsion polymerization. Even when the polymerization of the monomer and the crosslinker is carried out by emulsion polymerization, it is simple and easy to coat the outer surfaces of the particles of the alloy-based active material with a shell-like crosslinked polymer. Furthermore, in emulsion polymerization, the droplet diameter D1 is often on the order of several tens to several hundreds of nanometers. Therefore, emulsion polymerization can be suitably employed when using particles of an alloy-based active material having a nano-sized particle size.

[0074] [Non-aqueous electrolyte secondary battery] A non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode facing the positive electrode, a separator disposed between the positive electrode and the negative electrode, a non-aqueous electrolyte, and a battery case that accommodates the positive electrode, the negative electrode, the separator, and the non-aqueous electrolyte.

[0075] (Positive Electrode) The positive electrode includes a positive electrode current collector and a positive electrode mixture layer formed on the positive electrode current collector. The positive electrode mixture layer includes a positive electrode active material, a binder, and a conductive additive.

[0076] The positive electrode current collector preferably has a strip shape (long shape) in a plan view. As the positive electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet) is used. Examples of materials for the positive electrode current collector include metal materials such as Al, Al alloys, Ti, Ti alloys, and Fe alloys. The Fe alloy may be stainless steel. The thickness of the positive electrode current collector is not particularly limited, but is preferably 1 to 50 μm, more preferably 5 to 20 μm, and even more preferably 10 to 20 μm.

[0077] The positive electrode mixture layer may be formed on both main surfaces of the positive electrode current collector, or may be formed on only one main surface. When the positive electrode current collector is a porous conductive substrate as described above, the positive electrode mixture layer may be formed in a state where at least a portion of the positive electrode mixture layer is embedded in the pores of the porous substrate.

[0078] The positive electrode active material is a material that electrochemically absorbs and releases lithium ions. The positive electrode active material may be, for example, a lithium-containing transition metal oxide. Representative examples of lithium-containing transition metal oxides include lithium cobalt oxide and lithium nickel oxide, which have a layered crystal structure and are classified as rock salt type.

[0079] As the positive electrode active material, for example, a composite oxide containing lithium and a transition metal such as Ni, Co, or Mn can be used. ａ CoO ２ , Li ａ NiO ２ , Li ａ MnO ２ , Li ａ Co ｂ Ni １－ｂ O ２ , Li ａ Co ｂ M １－ｂ O ｃ , Li ａ Ni １－ｂ M ｂ O ｃ , Li ａ Mn ２ O ４ , Li ａ Mn ２－ｂ M ｂ O ４ , LiMPO ４ , Li ２ MPO ４Examples of the metals include F. In the above composite oxide, M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. In the above composite oxide, a, b, and c satisfy 0<a≦1.2, 0<b≦0.9, and 2.0≦c≦2.3. The value of a, which represents the molar ratio of lithium, increases or decreases with charge and discharge.

[0080] As the positive electrode active material, it is preferable to use a lithium nickel composite oxide. The lithium nickel composite oxide is, for example, represented by the formula (1): Li ａ Ni ｂ M １－ｂ O ２ In formula (1), M is at least one selected from the group consisting of Mn, Co, and Al, and a and b satisfy 0<a≦1.2 and 0.3≦b<1, respectively. From the viewpoint of increasing capacity, b preferably satisfies 0.85≦b<1. From the viewpoint of stabilizing the crystal structure, the lithium nickel composite oxide contains Co and Al as M and can be represented by formula (2): Li ａ Ni ｂ Co ｃ Al ｄ O ２ In formula (2), a, b, c, and d satisfy the following conditions: 0<a≦1.2, 0.85≦b<1, 0<c<0.15, 0<d≦0.1, and b+c+d=1.

[0081] The positive electrode active material may have an average particle size of 5 μm or more and 30 μm or less, or 10 μm or more, or 20 μm or less, or 15 μm or less.

[0082] The average particle size of the positive electrode active material is the cumulative 50% particle size (median diameter) in a volume-based particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer. As the laser diffraction / scattering particle size distribution analyzer, for example, the Microtrac Series MT3300 manufactured by Nikkiso Co., Ltd. is used. The measurement using the particle size distribution analyzer can be performed before the positive electrode active material is incorporated into the positive electrode composite layer.

[0083] The average particle diameter of the positive electrode active material may be measured from a cross section obtained by cutting the laminate of the positive electrode composite layer and the positive electrode current collector in the thickness direction. The cross section may be formed using a cross-section polisher (CP). In this case, the positive electrode composite layer may be embedded in a thermosetting resin (e.g., epoxy resin). The average particle diameter from the cross section can be measured using a scanning electron microscope (SEM) image of the cross section. An SEM image can be used in which 10 or more positive electrode active material particles are observed. Then, image processing is performed to determine the circular equivalent diameters of the cross sections of 10 or more positive electrode active material particles, and the average value is calculated as the average particle diameter. Here, the circular equivalent diameter refers to the diameter of a circle having the same area as the cross section of the positive electrode active material particle (the area of ​​the positive electrode active material particle observed in the cross section of the positive electrode composite layer). Note that the average particle diameter of the positive electrode active material measured using a particle size distribution analyzer is equivalent to the average particle diameter calculated from the cross section.

[0084] Examples of the binder include resin materials. Examples of the resin material include fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resin; polyimide and polyamideimide; acrylic resins such as polyacrylic acid, polymethyl acrylate, and ethylene-acrylic acid copolymer; vinyl resins such as polyacrylonitrile, polyvinylpyrrolidone, and polyvinyl acetate; polyethersulfone, and nitrile rubber. That is, it is preferable to use a copolymer as the resin material. Note that vinyl resins are resins containing a vinyl group (CH ２ It is a resin obtained by polymerizing a monomer having the formula (=CH-).

[0085] The binder may be a resin material, and may be used alone or in combination of two or more kinds.

[0086] The binder may have an average particle size of 10 μm to 150 μm or less.

[0087] Like the average particle diameter of the positive electrode active material, the average particle diameter of the binder is the cumulative 50% particle diameter (median diameter) in a volume-based particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer. As the laser diffraction / scattering particle size distribution analyzer, for example, the Microtrac Series MT3300 manufactured by Nikkiso Co., Ltd. is used. The measurement using the particle size distribution analyzer can be performed before the binder is incorporated into the positive electrode composite layer.

[0088] When the binder is a resin material, the binder can be obtained by various known polymerization methods such as suspension polymerization, emulsion polymerization, solution polymerization, bulk polymerization, etc. From the viewpoints of easy adjustment of the polymerization temperature and easy increase of the degree of polymerization, the polymerization method is preferably suspension polymerization or emulsion polymerization. In other words, the binder is preferably a resin material obtained by suspension polymerization or emulsion polymerization.

[0089] From the viewpoint of increasing the voltage resistance, the binder preferably contains a fluororesin as a resin material, and among fluororesins, it is preferable that the binder contains polyvinylidene fluoride (PVDF).

[0090] The resin material contained in the binder may have a weight average molecular weight Mw of 300,000 or more, a weight average molecular weight Mw of 500,000 or more, or a weight average molecular weight Mw of 1,000,000 or more. The weight average molecular weight Mw of the resin material contained in the binder is a polystyrene-equivalent value measured by gel permeation chromatography (GPC). GPC is typically measured using a polystyrene gel column and water / methanol (volume ratio 8 / 2) as the mobile phase.

[0091] As the conductive aid, for example, a conductive carbonaceous material can be used. Examples of the conductive carbonaceous material include carbon black, carbon nanotubes, graphite, etc. Examples of carbon black include acetylene black, ketjen black, etc. The conductive aid may be used alone or in combination of two or more.

[0092] The positive electrode can be obtained, for example, by applying a slurry containing the components of the positive electrode mixture layer and a dispersion medium onto a positive electrode current collector to form a coating film, and then drying and compressing the coating film. The dispersion medium can be at least one selected from the group consisting of water and organic solvents (e.g., N-methyl-2-pyrrolidone). The components of the positive electrode mixture layer include a positive electrode active material, a binder, and a conductive additive.

[0093] (Separator) A porous sheet having ion permeability and insulating properties is used for the separator. Examples of the form of the porous sheet include a microporous film, a woven fabric, and a nonwoven fabric. The separator may be made of a polymer material. Examples of the polymer material include an olefin resin, a polyamide resin, and cellulose. Examples of the olefin resin include polyethylene, polypropylene, and a copolymer of ethylene and propylene. The separator may contain an additive as needed. Examples of the additive include an inorganic filler.

[0094] The separator may include multiple layers differing in at least one of form and composition, such as a laminate of a polyethylene microporous film and a polypropylene microporous film, or a laminate of a nonwoven fabric containing cellulose fibers and a nonwoven fabric containing thermoplastic resin fibers.

[0095] (Negative electrode) The negative electrode includes a negative electrode current collector and a negative electrode composite layer formed on the negative electrode current collector. The negative electrode composite layer includes the negative electrode material for a secondary battery according to an embodiment of the present disclosure and a binder. In the nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure, the negative electrode includes the negative electrode material for a secondary battery according to an embodiment of the present disclosure by including the above-described negative electrode composite layer.

[0096] The negative electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of materials for the negative electrode current collector include metal materials such as Ni, Ni alloys, Cu, Cu alloys, and Fe alloys. The Fe alloy may be stainless steel. The thickness of the negative electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm.

[0097] The negative electrode mixture layer may be formed on both main surfaces of the negative electrode current collector, or may be formed on only one main surface. When the negative electrode current collector is a porous conductive substrate as described above, the negative electrode mixture layer may be formed in a state where at least a portion of the layer is embedded in the pores of the porous conductive substrate.

[0098] As described above, the negative electrode material for a secondary battery includes a negative electrode active material that is alloyed with lithium and a crosslinked polymer. In the negative electrode material for a secondary battery, the crosslinked polymer is substantially composed of one molecule and coats the outer surface of the negative electrode active material that is alloyed with lithium.

[0099] Examples of binders include resin materials. Examples of resin materials include fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resin; polyimide and polyamideimide; acrylic resins such as polyacrylic acid, polymethyl acrylate, and ethylene-acrylic acid copolymer; vinyl resins such as polyacrylonitrile, polyvinylpyrrolidone, and polyvinyl acetate; polyethersulfone; and rubber-like materials such as styrene-butadiene copolymer rubber (SBR). One type of binder may be used alone, or two or more types may be used in combination.

[0100] The negative electrode mixture layer may contain a carbon-based material in addition to the negative electrode material for a secondary battery according to an embodiment of the present disclosure. Examples of the carbon-based material include graphite, easily graphitized carbon (soft carbon), and hardly graphitized carbon (hard carbon). Among the above carbon materials, graphite is preferred because it has excellent charge / discharge stability and can reduce irreversible capacity.

[0101] Graphite is a carbon material having a (002) plane spacing d002 of, for example, 0.340 nm or less as measured by X-ray diffraction. The crystallite size Lc(002) of graphite as measured by X-ray diffraction may be, for example, 5 nm or more, 5 nm or more to 300 nm or less, or 10 nm or more to 200 nm or less.

[0102] When graphite and the negative electrode material for a secondary battery according to an embodiment of the present disclosure are used in combination, the proportion of the negative electrode material for a secondary battery according to an embodiment of the present disclosure in the negative electrode active material is, for example, 1% by mass or more and 20% by mass or less. The proportion may be 3% by mass or more and 15% by mass or less, or 3% by mass or more and 10% by mass or less. When the proportion is in the above range, it is possible to obtain a balanced improvement in cycle characteristics and a high capacity.

[0103] The negative electrode mixture layer may contain a conductive additive in addition to the negative electrode active material and binder. As described for the positive electrode, a conductive carbonaceous material can be used as the conductive additive. Furthermore, in the negative electrode, in addition to the conductive carbonaceous material, metal fibers, metal powder such as aluminum, and the like can also be used. The conductive additive may be used alone or in combination of two or more.

[0104] The negative electrode mixture layer may contain a thickener as needed. Examples of the thickener include cellulose derivatives such as cellulose ether. Examples of cellulose derivatives include carboxymethyl cellulose (CMC) and its modifications, methyl cellulose, and the like. Examples of modified CMC include salts of CMC. Examples of salts include alkali metal salts (e.g., sodium salts) and ammonium salts.

[0105] The negative electrode can be obtained, for example, by applying a slurry containing the components of the negative electrode mixture layer and a dispersion medium onto a negative electrode current collector to form a coating film, and then drying and compressing the coating film. The dispersion medium can be at least one selected from the group consisting of water and organic solvents (e.g., N-methyl-2-pyrrolidone). The components of the negative electrode mixture layer include a negative electrode active material, a binder, a conductive additive, and a thickener.

[0106] (Non-aqueous electrolyte) The non-aqueous electrolyte has ion conductivity (for example, lithium ion conductivity). The non-aqueous electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte. The liquid non-aqueous electrolyte (nonaqueous electrolyte solution) contains a solvent (nonaqueous solvent) and a solute dissolved in the solvent. Examples of the solute include lithium salts. Various additives may be added to the non-aqueous electrolyte.

[0107] As the solvent, various known organic solvents can be used, such as cyclic carbonate esters, chain carbonate esters, cyclic carboxylic acid esters, chain carboxylic acid esters, chain ethers, cyclic ethers, fluorinated chain ethers, and fluorinated cyclic ethers.

[0108] Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC).

[0109] Examples of the chain carbonate ester include diethylene carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0110] Examples of the cyclic carboxylic acid ester include γ-butyrolactone (GBL), γ-valerolactone (GVL), and the like.

[0111] Examples of the chain carboxylic acid ester include non-aqueous solvents such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP).

[0112] Examples of chain ethers include dimethyl ether, ethyl methyl ether, diethyl ether, ethyl propyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, and o-dimethoxybenzene. The chain ether may be a chain ether having two or more ether bonds. Examples of such chain ethers include 1,1-dimethoxymethane, 1,1-diethoxyethane, 1,2-dimethoxyethane (DME), 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol ethyl methyl ether, diethylene glycol dibutyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol ethyl methyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, and tetraethylene glycol ethyl methyl ether.

[0113] Examples of cyclic 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, and crown ethers.

[0114] The fluorinated chain ether has a structure in which one or more hydrogen atoms of the chain ethers described above are substituted with fluorine atoms. Examples of the fluorinated chain ether include bis(2,2,2-trifluoroethyl) ether, 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether.

[0115] Fluorinated cyclic ethers are those in which one or more hydrogen atoms of the above-mentioned cyclic ethers have been substituted with fluorine atoms. Examples of fluorinated cyclic ethers include 3,3,4,4-tetrafluorotetrahydrofuran.

[0116] The above-mentioned various solvents (non-aqueous solvents) may be used alone or in combination of two or more.

[0117] Examples of the lithium salt include lithium salts of chlorine-containing acids, lithium salts of fluorine-containing acids, lithium salts of fluorine-containing acid imides, lithium halides, and lithium salts containing oxalate complexes. Examples of the lithium salts of chlorine-containing acids include LiClO ４ , LiAlCl ４ , LiB １０ Cl １０ Examples of lithium salts of fluorine-containing acids include LiPF ６ , LiPF ２ O ２ , LiBF ４ , LiSbF ６ , LiAsF ６ , LiCF ３ SO ３ , LiCF ３ CO ２ Examples of lithium salts of fluorine-containing acid imides include LiN(FSO ２ ) ２ , LiN(CF ３ SO ２ ) ２ , LiN(CF ３ SO ２ ) (FSO ２ ), LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ ), LiN(C ２ F ５ SO ２ ) ２ Examples of lithium halides include LiCl, LiBr, and LiI. Examples of lithium salts containing oxalate complexes include LiB(C ２ O ４ ) ２, LiBF ２ (C ２ O ４ ), LiPF ４ (C ２ O ４ ), LIPF ２ (C ２ O ４ ) ２ The above lithium salts may be used alone or in combination of two or more.

[0118] The concentration of the lithium salt in the liquid nonaqueous electrolyte (nonaqueous electrolyte solution) may be 1 mol / L or more and 5 mol / L or less, or 1 mol / L or more and 3 mol / L or less. By setting the lithium salt concentration within the above range, a nonaqueous electrolyte (nonaqueous electrolyte solution) having excellent ionic conductivity and appropriate viscosity can be obtained.

[0119] The liquid non-aqueous electrolyte (nonaqueous electrolyte solution) may contain various known additives. Examples of such additives include 1,3-propane sultone, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, fluorobenzene, ethylene sulfite (ES), etc. Note that cyclic carbonates such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC), which are exemplified as solvents, may also function as additives.

[0120] Examples of the solid electrolyte include solid or gel polymer electrolytes and inorganic solid electrolytes. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. The matrix polymer includes, for example, a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, and polyether resin. Examples of the inorganic solid electrolyte include materials known in all-solid-state lithium ion secondary batteries (e.g., oxide-based solid electrolytes, sulfide-based solid electrolytes, halide-based solid electrolytes, etc.).

[0121] A specific configuration of a nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure will be described below with reference to Fig. 1. Note that, hereinafter, the nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure will be simply referred to as a nonaqueous electrolyte secondary battery according to a first embodiment.

[0122] FIG. 1 is a longitudinal cross-sectional view schematically illustrating a nonaqueous electrolyte secondary battery 10 according to a first embodiment. The nonaqueous electrolyte secondary battery 10 is a cylindrical battery. The nonaqueous electrolyte secondary battery 10 includes a cylindrical battery case (battery can), a wound electrode group 14 housed in the battery case, and a nonaqueous electrolyte (not shown). The battery case includes a cylindrical case body 15 with a bottom and a sealing body 16 that seals the opening of the case body 15. The case body 15 is made of metal. A gasket 27 is disposed between the case body 15 and the sealing body 16. The gasket 27 ensures the hermeticity of the battery case. The case body 15, the sealing body 16, and the gasket 27 form an exterior body. Within the case body 15, insulating plates 17 and 18 are disposed at both ends of the electrode group 14 in the direction of the winding axis.

[0123] The case body 15 has a step portion 21. The step portion 21 supports the sealing body 16. The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26. The above-mentioned components constituting the sealing body 16 are electrically connected to each other except for the insulating member 24. The cap 26 functions as a positive electrode terminal. The case body 15 functions as a negative electrode terminal.

[0124] The electrode group 14 is a wound electrode group composed of a positive electrode 11, a negative electrode 12, and a separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 are all strip-shaped. The nonaqueous electrolyte secondary battery 10 according to the first embodiment includes, as the negative electrode 12, a negative electrode containing the negative electrode material for a secondary battery according to the embodiment of the present disclosure described above. The positive electrode 11, the separator 13, and the nonaqueous electrolyte can be configured as described above.

[0125] The positive electrode 11 is electrically connected to the cap 26 via a positive electrode lead 19. One end of the positive electrode lead 19 is connected to the positive electrode 11. The other end of the positive electrode lead 19 is connected to the sealing body 16 (filter 22). The negative electrode 12 is electrically connected to the case body 15 via a negative electrode lead 20. One end of the negative electrode lead 20 is connected to the negative electrode 12. The other end of the negative electrode lead 20 is connected to the case body 15.

[0126] In the above example, a nonaqueous electrolyte secondary battery configured by housing a wound electrode group in a cylindrical battery case (battery can) has been described, but the configuration of the nonaqueous electrolyte secondary battery is not limited to this. The nonaqueous electrolyte secondary battery may also be configured by housing a wound electrode group in a prismatic battery case (battery can). Furthermore, the nonaqueous electrolyte secondary battery may also be configured by housing a stacked electrode group in a battery case such as a film exterior body (e.g., a pouch).

[0127] (Additional Notes) The above description discloses the following technologies. (Technology 1) A negative electrode material for a secondary battery, comprising particles of an alloy-based active material that is alloyed with lithium and a crosslinked polymer, wherein the crosslinked polymer coats the outer surfaces of the particles of the alloy-based active material in a shell-like manner. (Technology 2) The negative electrode material for a secondary battery according to Technology 1, wherein the crosslinked polymer forms a substantially continuous body and coats the outer surfaces of the particles of the alloy-based active material. (Technology 3) The negative electrode material for a secondary battery according to Technology 1 or 2, wherein the crosslinked polymer has at least a carboxyl group. (Technology 4) The negative electrode material for a secondary battery according to any one of Technology 1 to 3, wherein the crosslinked polymer has at least a structural unit derived from an ethylenically unsaturated carboxylic acid monomer. (Technology 5) The negative electrode material for a secondary battery according to any one of Technology 1 to 4, wherein the crosslinked polymer is constituted by a structure derived from an ethylenically unsaturated carboxylic acid monomer. (Technology 6) The negative electrode material for a secondary battery according to Technology 4 or 5, wherein the ethylenically unsaturated carboxylic acid monomer is acrylic acid. (Technology 7) The negative electrode material for secondary batteries according to any one of Technologies 1 to 6, wherein the crosslinked polymer has a structure derived from a crosslinkable monomer. (Technology 8) The negative electrode material for secondary batteries according to Technology 7, wherein the crosslinked polymer has a structure in which one polymer having an alkenyl structure in the molecule and another polymer having an alkenyl structure in the molecule are crosslinked by a crosslinkable monomer. (Technology 9) The negative electrode material for secondary batteries according to Technology 7 or 8, wherein the structure derived from the crosslinkable monomer includes a structure derived from any one of a polyfunctional (meth)acrylate compound, a polyfunctional alkenyl compound, or a compound having both a (meth)acryloyl group and an alkenyl group. (Technology 10) The negative electrode material for secondary batteries according to any one of Technologies 1 to 9, wherein particles of the alloy-based active material contain silicon. (Technology 11) The negative electrode material for secondary batteries according to any one of Technologies 3 to 10, wherein the crosslinked polymer has a carboxyl group neutralized with a lithium compound. (Technology 12) The negative electrode material for a secondary battery according to any one of Technologies 1 to 11, wherein the particles of the alloy-based active material are aggregated in a state where their outer surfaces are covered with the crosslinked polymer.(Technology 13) A non-aqueous electrolyte secondary battery comprising: a positive electrode, a negative electrode facing the positive electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, wherein the negative electrode comprises the negative electrode material for secondary batteries according to any one of Technologies 1 to 11. (Technology 14) A method for producing a negative electrode material for secondary batteries comprising: a first step of obtaining a mixed solution containing a monomer, a crosslinking agent for the monomer, and particles of an alloy-system active material that can be alloyed with lithium; a second step of adding a poor solvent for the monomer to the mixed solution obtained after the first step to obtain droplets containing the monomer and the crosslinking agent; and a third step of polymerizing the monomer and the crosslinking agent contained in the droplets obtained after the second step to coat the outer surfaces of the particles of the alloy-system active material with a shell-like crosslinked polymer. (Technology 15) A method for producing an anode material for a secondary battery according to Technology 14, wherein in the second step, droplets containing the monomer and the crosslinking agent are obtained by suspending the monomer, the crosslinking agent, and particles of the alloy-based active material in a poor solvent for the monomer. (Technology 16) A method for producing an anode material for a secondary battery according to Technology 15, wherein the monomer is a water-soluble monomer, and the poor solvent for the monomer is a non-aqueous solvent. (Technology 17) A method for producing an anode material for a secondary battery according to Technology 14, wherein in the second step, a surfactant is further added to the poor solvent for the monomer to form micelles of the surfactant, and the particles of the alloy-based active material, the monomer, and the crosslinking agent are allowed to enter inside the micelles of the surfactant, thereby obtaining droplets containing the monomer and the crosslinking agent, and in the third step, the monomer and the crosslinking agent contained in the droplets are polymerized inside the micelles of the surfactant. (Technology 18) A negative electrode material for a secondary battery, obtained by encapsulating particles of an alloy-based active material that can be alloyed with lithium in droplets of a polymerization component containing a monomer and a crosslinking agent for the monomer, polymerizing the monomer and the crosslinking agent contained in the droplets, and coating the outer surfaces of the particles of the alloy-based active material with a shell-shaped coating of the polymerization component.

[0128] While the present invention has been described in terms of presently preferred embodiments, such disclosure should not be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0129] Hereinafter, the present disclosure will be specifically described based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0130] [Example 1] (1) Preparation of Positive Electrode Positive Electrode Active Material (LiNi ０．８８ Co ０．０９ Al ０．０３ O ２ A cathode mixture was obtained by mixing the cathode composite (a conductive additive (acetylene black)) and the binder (polyvinylidene fluoride (PVdF)) in a mass ratio of 100:0.8:0.7. An appropriate amount of N-methyl-2-pyrrolidone as a dispersion medium was added to the cathode composite and stirred to obtain a cathode mixture slurry.

[0131] The positive electrode composite slurry according to Example 1 was applied to both sides of an aluminum foil (thickness: 15 μm) serving as a positive electrode current collector to form a coating film. The coating film was then dried, and the dried coating film was compressed in the thickness direction using a roller to form a positive electrode composite layer on the positive electrode current collector. The laminate of the positive electrode current collector and the positive electrode composite layer was then cut to a predetermined size. In this manner, a positive electrode according to Example 1 was obtained.

[0132] (2) Negative Electrode A negative electrode active material (graphite and composite particles A), a binder (styrene-butadiene copolymer rubber (SBR)), and a thickener (carboxymethyl cellulose (CMC)) were mixed in a mass ratio of 100:1:1 to obtain a negative electrode mixture. An appropriate amount of water as a dispersion medium was added to the negative electrode mixture and stirred to obtain a negative electrode mixture slurry. In the negative electrode active material, the mass ratio of graphite to composite particles A was 95:5. Composite particles A were obtained through the following steps 1 and 2, and the outer surfaces of nanosilicon particles (average particle size 200 nm) were coated with a crosslinked polymer that essentially formed a single continuum. The fact that composite particles A were configured as described above was confirmed according to the method described in the above embodiment.

[0133] <Procedure 1> (1) Acrylic acid (ethylenically unsaturated carboxylic acid monomer), sorbitan monostearate (surfactant), 1,3-diallyloxy-2-propanol (crosslinkable monomer), and V-501 (polymerization initiator) were placed in a 200 mL four-neck flask, and the components were stirred and mixed using a stirrer tip to obtain a mixed solution. (2) Nanosilicon particles were added to the mixed solution, and the mixed solution containing the nanosilicon particles was stirred and mixed at room temperature (23±2°C) to obtain a nanosilicon dispersion solution. (3) Liquid paraffin (a poor solvent for the monomer) containing sorbitan monostearate (surfactant) was added to the nanosilicon dispersion solution and stirred and mixed to obtain a reaction solution. Note that the flask was degassed using argon gas during this stirring and mixing. (4) While introducing argon gas into the flask, the temperature of the reaction solution was raised to 65°C to initiate the polymerization reaction of acrylic acid. (5) After 16 hours from the start of the reaction, the reaction was terminated to obtain a reaction product, and 400 mL of tetrahydrofuran (THF) was added to the flask and stirred, followed by washing the reaction product. (6) After washing, the reaction product was subjected to suction filtration using filter paper (No. 2) to recover the solid content. (7) After washing the solid content with 400 mL of THF and recovering the solid content by suction filtration was repeated three times, the solid content after the third suction filtration was dried at 50°C for 12 hours. (8) The dried solid content was pulverized using a mortar to obtain composite particle intermediate a.

[0134] <Procedure 2> (1) Composite particle intermediate a and acetonitrile were placed in a 300 mL recovery flask and stirred with a stirrer tip to obtain a mixed solution. (2) Lithium hydroxide monohydrate (LiOH·H ２ 0) was added and stirred at room temperature (23±2°C) for 24 hours to obtain a reaction solution. (3) A portion of the reaction solution was sampled, and the sampled solution was subjected to IR analysis to confirm that the reaction had proceeded. (4) The reaction solution was subjected to suction filtration using filter paper (No. 2) to recover a first solid fraction. (5) 300 mL of acetonitrile was added to the first solid fraction, and the mixture was stirred and washed for 15 minutes. (6) The washing solution was subjected to suction filtration using filter paper (No. 2) to recover a second solid fraction, which was then dried under reduced pressure at 50°C for 24 hours. (7) The dried second solid fraction was recovered to obtain composite particles A.

[0135] The yield of composite particle A was 13.27 g, and the yield was 73.6%. Furthermore, composite particle A was a nanosilicon particle whose outer surface was coated with crosslinked polyacrylic acid (PAA), and the polymer ratio was 75% by mass. Furthermore, the above procedure 1 was carried out using the formulation in Table 1 below, and the above procedure 2 was carried out using the formulation in Table 2 below. The polymer ratio was calculated based on the amount of formulation shown in Formulation 1 in Table 1 below.

[0136]

[0137]

[0138] The negative electrode composite slurry according to Example 1 was applied to both sides of a copper foil (thickness: 15 μm) serving as a negative electrode current collector to form a coating film. The coating film was then dried, and the dried coating film was then compressed in the thickness direction using a roller to form a negative electrode composite layer on the negative electrode current collector. The laminate of the negative electrode current collector and the negative electrode composite layer was then cut to a predetermined size. In this manner, a negative electrode according to Example 1 was obtained.

[0139] (3) Nonaqueous Electrolyte (Nonaqueous Electrolyte Solution) Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 2:2:6 (EC:EMC:DMC), and vinylene carbonate was further added to obtain a nonaqueous mixed solvent. LiPF ６The VC content in the nonaqueous electrolyte was 2 mass %. ６ The concentration of VC was set to 1.3 mol / L. VC functions as a non-aqueous solvent and also functions as an additive for forming a good SEI coating on the outer surface of the graphite contained in the negative electrode active material.

[0140] (4) Fabrication of Secondary Battery One end of an aluminum positive electrode lead was attached to an aluminum foil positive electrode current collector by welding. One end of a nickel negative electrode lead was attached to a copper foil negative electrode current collector by welding. The positive and negative electrodes were then stacked in an inert gas atmosphere with a separator interposed therebetween to obtain a laminate, which was then wound to obtain a wound electrode group. A polyethylene microporous film was used as the separator.

[0141] Next, as shown in FIG. 1 , a secondary battery is completed by housing a wound electrode group (electrode group 14) and a non-aqueous electrolyte (nonaqueous electrolyte solution) in a cylindrical battery case. As shown in FIG. 1 , in the wound electrode group (electrode group 14), the positive electrode 11 is connected to a sealing body 16 via a positive electrode lead 19, and the negative electrode 12 is connected to a case body 15 via a negative electrode lead 20. The other end of the positive electrode lead 19 is connected to the sealing body 16, and the other end of the negative electrode lead 20 is connected to the case body 15. As described in the embodiment section, the cap 26 provided on the sealing body 16 functions as a positive electrode terminal, and the case body 15 functions as a negative electrode terminal.

[0142] [Example 2] A negative electrode was produced using composite particles B instead of composite particles A in the negative electrode active material. A secondary battery according to Example 2 was obtained in the same manner as in Example 1 except for the negative electrode. Composite particles B were obtained in the same manner as composite particles A except that the mass of the nanosilicon particles in Formulation 1 was 13.5 g. That is, composite particles B were also obtained by coating the outer surfaces of nanosilicon particles with crosslinked polyacrylic acid (PAA) by reversed-phase suspension polymerization. The yield of composite particles B was 21.31 g, which was 78.8%. The polymer ratio in composite particles B was 50% by mass.

[0143] [Example 3] A negative electrode was produced using composite particles C instead of composite particles A in the negative electrode active material. A secondary battery according to Example 3 was obtained in the same manner as in Example 1 except for the negative electrode. Composite particles C were obtained in the same manner as composite particles A except that the mass of the nanosilicon particles in Formulation 1 was 40.5 g. That is, composite particles C were also obtained by coating the outer surfaces of nanosilicon particles with crosslinked polyacrylic acid (PAA) by reversed-phase suspension polymerization. The yield of composite particles C was 42.87 g, which was 79.4%. The polymer ratio in composite particles C was 25% by mass.

[0144] [Comparative Example 1] A negative electrode was produced using nanosilicon particles whose outer surfaces were not coated with a polymer instead of the composite particles A in the negative electrode active material. A secondary battery according to Comparative Example 1 was obtained in the same manner as in Example 1 except for the negative electrode.

[0145] Comparative Example 2 A negative electrode was produced using composite particles D instead of composite particles A in the negative electrode active material. A secondary battery according to Comparative Example 2 was obtained in the same manner as in Example 1, except for the negative electrode. Composite particles D were prepared by preparing a mixed aqueous solution in which nanosilicon particles and cross-linked sodium polyacrylate were mixed in a mass ratio of 80:20 (nanosilicon particles:cross-linked sodium polyacrylate), and then spray-drying this mixed aqueous solution at a hot air temperature of 150°C to coat the outer surfaces of the nanosilicon particles with a polymer. The polymer ratio in composite particles D was 20% by mass.

[0146] Comparative Example 3 A negative electrode was produced using composite particles E instead of composite particles A in the negative electrode active material. A secondary battery according to Comparative Example 3 was obtained in the same manner as in Example 1, except for the negative electrode. Composite particles E were prepared by preparing a mixed aqueous solution of nanosilicon particles and cross-linked sodium polyacrylate in a mass ratio of 95:5 (nanosilicon particles:cross-linked sodium acrylate), and spray-drying this mixed aqueous solution at a hot air temperature of 150°C, thereby coating the outer surfaces of the nanosilicon particles with a polymer. The polymer ratio in composite particles E was 5 mass%.

[0147] The alloy-based active material type, coating polymer type, coating method, and polymer ratio for the secondary battery negative electrode materials of each example (Examples 1 to 3 and Comparative Examples 1 to 4) are summarized in Table 3 below.

[0148]

[0149] <Evaluation> Cycle Retention A charge-discharge cycle was repeated until the capacity retention rate calculated by the following formula reached 80%, in which the battery voltage was charged at a constant current of 0.5 C to 4.2 V, the current value was reduced to 1 / 50 C at 4.2 V, and the battery voltage was then discharged at a constant current of 0.5 C to 2.5 V. The cycle retention rate was evaluated for each secondary battery (Examples 1 to 3 and Comparative Examples 1 to 3) based on the cycle retention rate. The results of the cycle retention rate evaluation (cycle number) are shown in Table 4 below. Note that the higher the cycle number, the better the evaluation of cycle retention rate. Capacity retention rate (%) = (discharge capacity at Nth cycle) / (discharge capacity at 1st cycle) × 100

[0150] The coating form of the battery negative electrode material for each example is also shown in Table 4. In Table 4, "continuous" means that the crosslinked polyacrylic acid coats the outer surface of the nanosilicon particle so as to form a single continuous body, and "discontinuous" means that the end portions of multiple crosslinked polyacrylic acids coat the outer surface of the nanosilicon particle so as to overlap each other.

[0151]

[0152] From Table 4, it was confirmed that in the secondary batteries according to Examples 1 to 3, the outer surfaces of the nanosilicon particles were coated with crosslinked polyacrylic acid to form a single continuous body, and the cycle numbers, which serve as an index for evaluating cycle maintenance, all exceeded 50 cycles. In contrast, in the secondary batteries according to Comparative Examples 2 and 3, it was confirmed that the outer surfaces of the nanosilicon particles were coated with terminal portions of multiple crosslinked polyacrylic acids that overlapped with each other, and the respective cycle numbers were low at 23 and 18 cycles. Furthermore, in Comparative Example 1, which used silicon particles whose outer surfaces were not coated with a polymer, the cycle number was also low at 11 cycles. This shows that by using a negative electrode material for a secondary battery in which the outer surface of an alloy-based active material that alloys with lithium is coated with a crosslinked polymer to form essentially a single continuous body, a secondary battery that can sufficiently maintain its cycle characteristics can be obtained.

[0153] The negative electrode material for secondary batteries according to the present disclosure can be used in non-aqueous electrolyte secondary batteries that are required to maintain sufficient cycle characteristics.

[0154] 10: Non-aqueous electrolyte secondary battery 11: Positive electrode 12: Negative electrode 13: Separator 14: Electrode group 15: Case body 16: Sealing body 27: Gasket

Claims

1. A negative electrode material for a secondary battery, comprising particles of an alloy-based active material that is alloyed with lithium, and a crosslinked polymer, wherein the crosslinked polymer coats the outer surfaces of the particles of the alloy-based active material in the form of a shell.

2. The negative electrode material for a secondary battery according to claim 1, wherein the crosslinked polymer forms substantially a single continuous body and coats the outer surfaces of the particles of the alloy-based active material.

3. The negative electrode material for a secondary battery according to claim 1, wherein the crosslinked polymer has at least a carboxyl group.

4. The negative electrode material for a secondary battery according to claim 1, wherein the crosslinked polymer has structural units derived from at least an ethylenically unsaturated carboxylic acid monomer.

5. The negative electrode material for a secondary battery according to claim 1, wherein the crosslinked polymer is composed of a structure derived from an ethylenically unsaturated carboxylic acid monomer.

6. The negative electrode material for a secondary battery according to claim 4 or 5, wherein the ethylenically unsaturated carboxylic acid monomer is acrylic acid.

7. The negative electrode material for a secondary battery according to any one of claims 1 to 5, wherein the crosslinked polymer has a structure derived from a crosslinkable monomer.

8. The negative electrode material for a secondary battery according to claim 7, wherein the crosslinked polymer has a structure in which one polymer having an alkenyl structure in the molecule and another polymer having an alkenyl structure in the molecule are crosslinked by a crosslinking monomer.

9. The negative electrode material for a secondary battery according to claim 7, wherein the structure derived from the crosslinkable monomer includes a structure derived from either a polyfunctional (meth)acrylate compound, a polyfunctional alkenyl compound, or a compound having both a (meth)acryloyl group and an alkenyl group.

10. The negative electrode material for a secondary battery according to any one of claims 1 to 5, wherein the particles of the alloy-based active material contain silicon.

11. The negative electrode material for a secondary battery according to any one of claims 3 to 5, wherein the crosslinked polymer has a carboxyl group neutralized with a lithium compound.

12. The negative electrode material for a secondary battery according to any one of claims 1 to 5, wherein the particles of the alloy-based active material are aggregated in a state in which their outer surfaces are coated with the crosslinked polymer.

13. A non-aqueous electrolyte secondary battery comprising: a positive electrode; a negative electrode facing said positive electrode; a separator disposed between said positive electrode and said negative electrode; and a non-aqueous electrolyte, wherein said negative electrode comprises the negative electrode material for secondary batteries according to any one of claims 1 to 5.

14. A method for producing a negative electrode material for a secondary battery, comprising: a first step of obtaining a mixed solution containing a monomer, a crosslinking agent for the monomer, and particles of an alloy-based active material that is to be alloyed with lithium; a second step of adding a poor solvent for the monomer to the mixed solution after the first step to obtain droplets containing the monomer and the crosslinking agent; and a third step of polymerizing the monomer and the crosslinking agent contained in the droplets after the second step to coat the outer surfaces of the particles of the alloy-based active material with a shell-like crosslinked polymer.

15. A method for producing a negative electrode material for a secondary battery as described in claim 14, wherein in the second step, droplets containing the monomer and the crosslinking agent are obtained by suspending particles of the monomer, the crosslinking agent, and the alloy-based active material in a poor solvent for the monomer.

16. The method for producing a negative electrode material for a secondary battery according to claim 15, wherein the monomer is a water-soluble monomer and the poor solvent for the monomer is a non-aqueous solvent.

17. A method for producing a negative electrode material for a secondary battery as described in claim 14, wherein the monomer is a water-insoluble monomer and the poor solvent for the monomer is water; in the second step, a surfactant is further added to the poor solvent for the monomer to form micelles of the surfactant, and droplets containing the monomer and the crosslinking agent are obtained by causing the particles of the alloy-based active material, the monomer, and the crosslinking agent to enter inside the micelles of the surfactant; and in the third step, the monomer and the crosslinking agent contained in the droplets are polymerized inside the micelles of the surfactant.

18. A negative electrode material for a secondary battery, obtained by encapsulating particles of an alloy-based active material that alloys with lithium in droplets of a polymerizable component containing a monomer and a crosslinker for the monomer, polymerizing the monomer and the crosslinker contained in the droplets, and coating the outer surfaces of the particles of the alloy-based active material with a shell-like coating of the polymerizable component.

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