Secondary battery

Abstract

A secondary battery according to the present disclosure comprises a positive electrode, a negative electrode, and a nonaqueous electrolyte. At least one selected from the group consisting of the positive electrode, the negative electrode, and the nonaqueous electrolyte contains a polymer of an ethylenically unsaturated carboxylic acid compound and a polyalkylene glycol compound having three or more hydroxyl groups.

Description

secondary battery

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

[0002] Conventionally, secondary batteries have been known that include a positive electrode, a negative electrode, and a non-aqueous electrolyte. In such secondary batteries, the negative electrode typically includes a negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector. The negative electrode mixture layer typically includes a negative electrode active material and a binder.

[0003] Patent Document 1 below describes a binder for secondary battery electrodes containing a crosslinked polymer, the crosslinked polymer being obtained by polymerizing a monomer composition containing a non-crosslinkable monomer and a crosslinkable monomer, the non-crosslinkable monomer containing 50% by mass to 100% by mass of an ethylenically unsaturated carboxylic acid monomer relative to the total amount of the non-crosslinkable monomer, the crosslinkable monomer containing a monomer having at least one polymerizable unsaturated group other than an allyl group, and the crosslinked polymer having a volume-based median particle size measured in an aqueous medium after neutralization to a degree of neutralization of 80 to 100 mol % of 0.1 μm to 10 μm. Patent Document 1 also describes that such a binder for secondary batteries exhibits high binding strength in an electrode mixture layer even after repeated charge and discharge.

[0004] Patent Document 2 below describes a slurry for secondary battery electrodes containing a binder resin composition for secondary battery electrodes having a polymer (A) having a specific amide structural unit and a polyhydric alcohol (B), an active material, and a solvent. Patent Document 2 below also describes that in an electrode layer formed from the above-mentioned slurry for secondary battery electrodes, excellent binding properties are exhibited by the polymer (A) and the polyhydric alcohol (B), that the electrode layer has excellent flexibility, and that a lithium ion secondary battery including the electrode layer exhibits excellent long-term cycle characteristics.

[0005] International Publication No. 2019 / 230714 Japanese Patent Application Laid-Open No. 2015-76225

[0006] When a secondary battery is charged, the negative electrode active material expands significantly due to the absorption of lithium ions, and the expansion of the negative electrode active material may also cause the negative electrode mixture layer to expand significantly. In other words, the negative electrode may expand significantly. When the negative electrode active material is a material that alloys with lithium, such as silicon (Si) or tin (Sn) (hereinafter referred to as an alloy-based active material), the expansion of the negative electrode mixture layer becomes particularly significant. In other words, the expansion of the negative electrode becomes particularly significant.

[0007] However, there has not yet been sufficient research into how to suppress the expansion of the negative electrode caused by charging.

[0008] Therefore, an object of the present disclosure is to provide a secondary battery that can suppress expansion of the negative electrode caused by charging.

[0009] One aspect of the present invention relates to a secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein at least one selected from the group consisting of the positive electrode, the negative electrode, and the non-aqueous electrolyte contains a polymer of an ethylenically unsaturated carboxylic acid compound and a polyalkylene glycol compound having three or more hydroxyl groups.

[0010] According to the present disclosure, a secondary battery can be provided that can suppress expansion of the negative electrode caused by charging.

[0011] 1 is a schematic perspective view of a secondary battery according to a first embodiment, with a portion thereof cut away;

[0012] 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.

[0013] 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.

[0014] The present disclosure encompasses any combination of two or more features arbitrarily selected from the appended claims, i.e., any combination of two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0015] [Secondary Battery] A secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. In the secondary battery according to an embodiment of the present disclosure, at least one selected from the group consisting of the positive electrode, the negative electrode, and the non-aqueous electrolyte contains a polymer of an ethylenically unsaturated carboxylic acid compound (hereinafter also referred to as a "first binder") and a polyalkylene glycol compound having three or more hydroxyl groups (hereinafter also referred to as a "binder additive").

[0016] The polymer of an ethylenically unsaturated carboxylic acid compound is a polymer composed of monomer units (ethylenically unsaturated carboxylic acid compound units) derived from at least one selected from the group consisting of ethylenically unsaturated carboxylic acids and ethylenically unsaturated carboxylic acid salts. However, it is sufficient that 80 mol % or more of the monomer units constituting the polymer are ethylenically unsaturated carboxylic acid compound units. The ethylenically unsaturated carboxylic acid salt may or may not be ionically dissociated.

[0017] In the secondary battery according to the embodiment of the present disclosure, as described above, it is important that at least one selected from the group consisting of the positive electrode, the negative electrode, and the non-aqueous electrolyte contains a polymer of an ethylenically unsaturated carboxylic acid compound as the first binder and a polyalkylene glycol compound having three or more hydroxyl groups as the binder additive. The reason for this will be explained below.

[0018] Polymers of ethylenically unsaturated carboxylic acid compounds (e.g., polyacrylic acid) exhibit good binding properties for negative electrode active materials, particularly alloy-based active materials, and are therefore often contained in the negative electrode mixture layer as binders. However, polymers of ethylenically unsaturated carboxylic acid compounds do not necessarily have excellent elasticity and toughness, and therefore cannot adequately absorb the force generated by the expansion of the negative electrode active material when the negative electrode active material expands significantly in the negative electrode mixture layer due to charging. In such cases, the expansion of the negative electrode active material causes the negative electrode mixture layer to expand significantly. In other words, the negative electrode expands significantly.

[0019] However, in the secondary battery according to the embodiment of the present disclosure, the negative electrode contains a polymer of an ethylenically unsaturated carboxylic acid compound as the first binder and a polyalkylene glycol compound having three or more hydroxyl groups as the binder additive. Therefore, it is believed that the ethylenically unsaturated carboxylic acid compound and the polyalkylene glycol compound having three or more hydroxyl groups form a three-dimensional network structure in the negative electrode mixture layer through hydrogen bonds. This three-dimensional network structure is believed to provide excellent elasticity and toughness. Therefore, even if the negative electrode active material expands significantly in the negative electrode mixture layer due to charging, the three-dimensional network structure can sufficiently absorb the force generated by the expansion of the negative electrode active material. This prevents the negative electrode mixture layer from expanding due to the expansion of the negative electrode active material. In other words, the negative electrode can be prevented from expanding. Furthermore, even when an ethylenically unsaturated carboxylic acid compound and a polyalkylene glycol compound having three or more hydroxyl groups are contained in at least one of the electrolyte solution and the positive electrode, by transferring at least a portion of these compounds to the negative electrode, the formation of a three-dimensional network structure in the negative electrode mixture layer can be expected. Note that the formation of the three-dimensional network structure is more pronounced when the polyalkylene glycol compound having three or more hydroxyl groups has a branched structure. Furthermore, it is believed that the three-dimensional network structure is not sufficiently formed when a compound having three or more hydroxyl groups but a small molecular weight (e.g., glucose) is used.

[0020] Examples of ethylenically unsaturated carboxylic acids include (meth)acrylic acid; (meth)acrylamidoalkylcarboxylic acids such as (meth)acrylamidohexanoic acid and (meth)acrylamidododecanoic acid; monohydroxyethyl succinate (meth)acrylate, ω-carboxy-caprolactone mono(meth)acrylate, and β-carboxyethyl (meth)acrylate. Examples of salts of ethylenically unsaturated carboxylic acids include (partially) alkali-neutralized products of ethylenically unsaturated carboxylic acids. Examples of cations constituting the salts include lithium ions, sodium ions, potassium ions, ammonium ions, imidazolium ions, and pyridinium ions. Ethylenically unsaturated carboxylic acids may be used alone or in combination of two or more. Among ethylenically unsaturated carboxylic acids, ethylenically unsaturated carboxylic acids having an acryloyl group are preferred, and acrylic acid is more preferred, because they have a high polymerization rate, making it easy to obtain polymers with long primary chain lengths and exhibit excellent binding properties. In other words, the polymer of an ethylenically unsaturated carboxylic acid compound is preferably polyacrylic acid or a salt thereof. In the case of acrylic acid, a polymer containing many carboxyl groups is easily obtained.

[0021] In this specification, "(meth)acrylic" means at least one of acrylic and methacrylic, and "(meth)acrylate" means at least one of acrylate and methacrylate.

[0022] The polyalkylene glycol compound having three or more hydroxyl groups may be a polyalkylene glycol or a polyalkylene glycol derivative. Examples of polyalkylene glycols include polyethylene glycol (PEG), polyethylene glycol glyceryl ether, polyethylene glycol diglyceryl ether, polyethylene glycol sorbitol ether, polypropylene glycol (PPG), polypropylene glycol glyceryl ether, polypropylene glycol diglyceryl ether, polypropylene glycol sorbitol ether, and polybutylene glycol. The polyalkylene glycol compound preferably has three or four hydroxyl groups.

[0023] The polyalkylene glycol compound preferably contains at least one of polyethylene glycol (PEG) and polypropylene glycol (PPG). The polyalkylene glycol compound preferably contains a branched polyalkylene glycol compound. By containing a branched polyalkylene glycol compound, the above-mentioned three-dimensional structure can be more significantly formed, as explained above. Examples of branched polyalkylene glycol compounds include a four-branched polyalkylene glycol represented by the following formula (1) and a three-branched polyalkylene glycol represented by the following formula (2). An example of the four-branched polyalkylene glycol represented by the following formula (1a) is a four-branched polyethylene glycol represented by the following formula (1a), and an example of the three-branched polyalkylene glycol represented by the following formula (2) is a three-branched polypropylene glycol represented by the following formula (2a). However, in formulas (1) and (2), R1 to R4 may each independently be, for example, an ethylene group or a propylene group. In addition, in formula (1) and formula (1a), a, b, c, and d are each an integer of 1 or more and 1000 or less, more preferably an integer of 1 or more and 100 or less, and even more preferably an integer of 1 or more and 50 or less. In formula (2) and formula (2a), a, b, and c are each an integer of 1 or more and 500 or less, more preferably an integer of 1 or more and 100 or less, and even more preferably an integer of 1 or more and 50 or less. Note that if the values ​​of a, b, c, and d are too large, for example, in a slurry containing the components of the negative electrode mixture layer and the dispersion medium, the solubility of the branched polyalkylene glycol in the dispersion medium becomes insufficient. In such cases, it becomes difficult to uniformly disperse the branched polyalkylene glycol in the negative electrode mixture layer. As a result, the expansion of the negative electrode may not be sufficiently suppressed.

[0024]

[0025]

[0026]

[0027]

[0028] The polyalkylene glycol compound preferably has a number average molecular weight Mn of 500 to 10,000. The number average molecular weight Mn may be 600 or more, or 700 or more. The number average molecular weight Mn may be 8,000 or less, 5,000 or less, or 4,000 or less. When the number average molecular weight Mn is within the above numerical range, the above three-dimensional network structure is more easily formed. The number average molecular weight Mn is a polystyrene-equivalent value measured using gel permeation chromatography (GPC). Note that GPC is usually measured using a polystyrene gel column and water / methanol (volume ratio 8 / 2) as the mobile phase.

[0029] The polyalkylene glycol derivative may be a compound in which one of the hydrogen atoms in the polyalkylene glycols exemplified above is substituted with a substituent. The substituent is preferably a hydroxyl group, a carboxyl group, an oxycarbonyl group (e.g., an alkoxycarbonyl group), a halogen atom, an amide group, a hydrocarbon group, or an oxyhydrocarbon group (e.g., an alkoxy group). When the substituent is a hydrocarbon group or an oxyhydrocarbon group (e.g., an alkoxy group), at least one hydrogen atom of the substituent may be substituted with a hydroxyl group, a carboxyl group, an oxycarbonyl group (e.g., an alkoxycarbonyl group), an amide group, or a halogen atom. The number of carbon atoms in the hydrocarbon group and the oxyhydrocarbon group may be 1 to 30 or 1 to 8.

[0030] In the secondary battery according to the embodiment of the present disclosure, at least one selected from the group consisting of the positive electrode, the negative electrode, and the non-aqueous electrolyte preferably contains 1 to 30 parts by mass of a binder additive per 100 parts by mass of the first binder. At least one selected from the group consisting of the positive electrode, the negative electrode, and the non-aqueous electrolyte may contain 3 or more masses of a binder additive per 100 parts by mass of the first binder. At least one selected from the group consisting of the positive electrode, the negative electrode, and the non-aqueous electrolyte may contain 20 parts by mass or less of a binder additive per 100 parts by mass of the first binder, or may contain 10 parts by mass or less. By containing the binder additive in the above numerical range, the three-dimensional network structure can be more suitably formed. In the secondary battery according to the embodiment of the present disclosure, it is particularly preferable that the negative electrode contains the binder additive in the above numerical range.

[0031] When the positive electrode or negative electrode contains the first binder and the binder additive, the amount of the first binder may be, for example, 0.1 to 20 parts by mass, and the amount of the binder additive may be, for example, 0.3 to 10 parts by mass, relative to 100 parts by mass of the positive electrode active material or the negative electrode active material.

[0032] In the secondary battery according to the embodiment of the present disclosure, the positive electrode and the negative electrode face each other. In the secondary battery according to the embodiment of the present disclosure, it is preferable that a separator be disposed between the positive electrode and the negative electrode. In the secondary battery according to the embodiment of the present disclosure, it is preferable that the positive electrode, the negative electrode, the non-aqueous electrolyte, and the separator are housed in a battery case. The positive electrode, the negative electrode, the non-aqueous electrolyte, the separator, and the battery case will be described below.

[0033] (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.

[0034] 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.

[0035] The positive electrode mixture layer may be formed on both main surfaces of the positive electrode current collector, or 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.

[0036] 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.

[0037] 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 , Liａ Mn ２－ｂ M ｂ O ４ , LiMPO ４ , Li ２ MPO ４ Examples 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.

[0038] 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.

[0039] 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.

[0040] 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 mixture layer.

[0041] 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 mixture 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 mixture 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 materials are observed. Then, the circular equivalent diameters of the cross sections of 10 or more positive electrode active materials are determined by image processing, 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 (the area of ​​the positive electrode active material observed in the cross section of the positive electrode mixture 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 determined from the cross section.

[0042] 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 resins such as polyimide and polyamideimide; vinyl resins such as polyacrylonitrile, polyvinylpyrrolidone and polyvinyl acetate; polyethersulfone, nitrile rubber, etc. That is, it is preferable to use a copolymer as the resin material. Note that the vinyl resin is a resin containing a vinyl group (CH ２=CH-). Furthermore, as described above, the positive electrode mixture layer may contain a polymer of the above-mentioned ethylenically unsaturated carboxylic acid compound as a binder. This polymer is also included in the resin material. Examples of polymers of ethylenically unsaturated carboxylic acid compounds include acrylic resins such as polyacrylic acid, polymethyl acrylate, and ethylene-acrylic acid copolymers. When the positive electrode mixture layer contains a polymer of an ethylenically unsaturated carboxylic acid compound as a binder, it is preferable to contain a polyalkylene compound having three or more hydroxyl groups as a binder additive. When the positive electrode mixture layer contains a polymer of an ethylenically unsaturated carboxylic acid compound and a polyalkylene compound having three or more hydroxyl groups, at least a portion of these may be transferred to the negative electrode.

[0043] The binder may be a resin material, and may be used alone or in combination of two or more kinds.

[0044] The binder may have an average particle size of 10 μm to 150 μm or less.

[0045] 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 mixture layer.

[0046] 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).

[0047] As the conductive aid, for example, a conductive carbonaceous material can be used. Examples of the conductive carbonaceous material include carbon black, carbon nanotubes (CNT), 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.

[0048] 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.

[0049] (Negative Electrode) The negative electrode includes a negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector. The negative electrode mixture layer includes a negative electrode active material and a binder.

[0050] 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.

[0051] 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.

[0052] The secondary battery according to the embodiment of the present disclosure preferably includes an alloy-based active material as the negative electrode active material. The alloy-based active material is an active material that alloys with lithium. Examples of the alloy-based active material include silicon (Si)-containing materials and tin (Sn)-containing materials. From the viewpoint of achieving high capacity, it is preferable to use a silicon (Si)-containing material as the alloy-based active material.

[0053] 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.

[0054] The particle size of silicon particles can be determined by determining the particle sizes of any 100 silicon particles observed in a high-resolution transmission electron microscope (HR-TEM) image and arithmetically averaging them. The silicon particles observed in the HR-TEM image may be primary particles or secondary particles. There is no need to distinguish between primary particles and secondary particles. The particle size of a silicon particle can be calculated as the diameter of an equivalent circle having an area equal to the area of ​​the silicon particle obtained in the HR-TEM image.

[0055] 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.

[0056] 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.

[0057] When the silicon-containing material is the above 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).

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] The negative electrode mixture layer may contain a carbon-based material in addition to the alloy-based active material. 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.

[0070] 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.

[0071] When an alloy-based active material and graphite are used in combination as the negative electrode active material, the proportion of the alloy-based active material in the negative electrode active material is, for example, 5% by mass or more and 70% by mass or less. This proportion may be 10% by mass or more and 60% by mass or less, 20% by mass or more and 50% by mass or less, or 25% by mass or more and 40% by mass or less. By having this proportion in the above range, it is possible to achieve a good balance between improved cycle characteristics and high capacity.

[0072] 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 resins such as polyimide and polyamideimide; vinyl resins such as polyacrylonitrile, polyvinylpyrrolidone, and polyvinyl acetate; polyethersulfone; and rubber-like materials such as styrene-butadiene copolymer rubber (SBR). Polymers of the above-mentioned ethylenically unsaturated carboxylic acid compounds are also included as binders. These polymers are also included in the resin materials. Examples of polymers of ethylenically unsaturated carboxylic acid compounds include acrylic resins such as polyacrylic acid, polymethyl acrylate, and ethylene-acrylic acid copolymer. Furthermore, cellulose derivatives such as cellulose ether can be used as binders. Examples of cellulose derivatives include carboxymethyl cellulose (CMC), its modified products, and methyl cellulose. Examples of modified CMC products include CMC salts. Examples of the salt include alkali metal salts (for example, sodium salts), ammonium salts, etc. The binder may be used alone or in combination of two or more kinds.

[0073] Among the various binders described above, the negative electrode mixture layer preferably contains a polymer of an ethylenically unsaturated carboxylic acid compound as the first binder. Furthermore, when the negative electrode mixture layer contains this polymer, it is preferable that the negative electrode mixture layer further contains a polyalkylene glycol compound having three or more hydroxyl groups as a binder additive. By including the above two compounds in the negative electrode mixture layer, expansion of the negative electrode can be particularly effectively suppressed. Furthermore, at least a portion of the above two compounds may migrate to at least one of the positive electrode and the non-aqueous electrolyte.

[0074] The negative electrode mixture layer preferably contains, in addition to the first binder, at least one selected from the group consisting of carboxymethyl cellulose (CMC) and styrene-butadiene copolymer rubber (SBR) as a second binder. By including CMC, adhesiveness can be further improved. By including SBR, more suitable elasticity can be exhibited. The negative electrode mixture layer may contain 0.1 to 10 parts by mass of CMC, or 0.3 to 3 parts by mass, per 100 parts by mass of the negative electrode active material. The negative electrode mixture layer may contain 0.1 to 10 parts by mass of SBR, or 1 to 10 parts by mass, per 100 parts by mass of the negative electrode active material.

[0075] The negative electrode mixture layer may contain a conductive additive in addition to the negative electrode active material and binder. As the conductive additive, a conductive carbonaceous material can be used, as described for the positive electrode. 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.

[0076] 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.

[0077] (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.

[0078] 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.

[0079] Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC).

[0080] Examples of the chain carbonate ester include diethylene carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0081] Examples of the cyclic carboxylic acid ester include γ-butyrolactone (GBL), γ-valerolactone (GVL), and the like.

[0082] 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).

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] The above-mentioned various solvents (non-aqueous solvents) may be used alone or in combination of two or more.

[0088] 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.

[0089] The concentration of the lithium salt in the liquid nonaqueous electrolyte (nonaqueous electrolytic 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 liquid nonaqueous electrolyte (nonaqueous electrolytic solution) having excellent ionic conductivity and appropriate viscosity can be obtained.

[0090] 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.

[0091] Furthermore, as described above, the liquid non-aqueous electrolyte (nonaqueous electrolyte solution) may contain a polymer of an ethylenically unsaturated carboxylic acid compound as the first binder and a polyalkylene compound having three or more hydroxyl groups as the binder additive. When the liquid non-aqueous electrolyte (nonaqueous electrolyte solution) contains a polymer of an ethylenically unsaturated carboxylic acid compound and a polyalkylene compound having three or more hydroxyl groups, at least a portion of these may be transferred to the negative electrode. When the liquid non-aqueous electrolyte (nonaqueous electrolyte solution) contains the first binder and the binder additive, the content thereof may be, for example, 0.1 to 10 mass%.

[0092] Examples of solid electrolytes that can be used include solid or gel polymer electrolytes and inorganic solid electrolytes. Examples of inorganic solid electrolytes that can be used include materials known in all-solid-state lithium ion secondary batteries (e.g., oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.). Polymer electrolytes include, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. Examples of matrix polymers include polymer materials that absorb non-aqueous solvents and gel. Examples of polymer materials include fluororesins, acrylic resins, and polyether resins. The solid electrolyte may include an ethylenically unsaturated carboxylic acid (first binder) and a polyalkylene glycol (binder additive) having three or more hydroxyl groups. Even in such cases, at least a portion of the first binder and binder additive can be transferred to the negative electrode.

[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] (Battery Case) The battery case includes, for example, a cylindrical case body with a bottom and a sealing body that seals the opening of the case body. The case body may be made of metal. A gasket may be disposed between the case body and the sealing body. Disposing the gasket can ensure the hermeticity of the battery case.

[0096] A specific configuration of a secondary battery according to an embodiment of the present disclosure will be described below with reference to Fig. 1. Note that, hereinafter, the secondary battery according to an embodiment of the present disclosure will be simply referred to as a secondary battery according to a first embodiment.

[0097] As shown in FIG. 1 , a secondary battery 10 includes a bottomed, rectangular battery case 4, and an electrode group 11 and an electrolyte (not shown) housed within the battery case 4. The electrode group 11 includes a strip-shaped negative electrode, a strip-shaped positive electrode, and a separator interposed therebetween. The negative electrode current collector is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. In other words, the positive electrode is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The periphery of the sealing plate 5 fits into the open end of the battery case 4, and the fitting is laser-welded. The sealing plate 5 has an electrolyte injection hole, which is closed with a seal plug 8 after electrolyte injection.

[0098] In the above example, a secondary battery configured by housing a wound electrode group in a rectangular battery case has been described, but the configuration of the secondary battery is not limited to this. The secondary battery may be configured by housing a wound electrode group in a cylindrical battery case. Furthermore, the secondary battery may be configured by housing a stacked electrode group in a battery case such as a film exterior (e.g., a pouch).

[0099] (Additional Notes) The above descriptions disclose the following technologies. (Technology 1) A secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein at least one selected from the group consisting of the positive electrode, the negative electrode, and the non-aqueous electrolyte contains a polymer of an ethylenically unsaturated carboxylic acid compound and a polyalkylene glycol compound having three or more hydroxyl groups. (Technology 2) The secondary battery according to Technology 1, wherein the polyalkylene glycol compound includes a branched polyalkylene glycol compound. (Technology 3) The secondary battery according to Technology 1 or 2, wherein the polyalkylene glycol compound is a polyalkylene glycol or a polyalkylene glycol derivative. (Technology 4) The secondary battery according to Technology 3, wherein the polyalkylene glycol derivative is a compound in which one hydrogen atom in the polyalkylene glycol is substituted with a substituent, and the substituent is a hydroxyl group, a carboxyl group, an oxycarbonyl group, a halogen atom, an amide group, a hydrocarbon group, or an oxyhydrocarbon group. (Technology 5) The secondary battery according to Technology 4, wherein at least one hydrogen atom of the hydrocarbon group or the oxyhydrocarbon group is substituted with a hydroxyl group, a carboxyl group, an oxycarbonyl group, an amide group, or a halogen atom. (Technology 6) The secondary battery according to any one of Technology 1 to 5, wherein the polyalkylene glycol compound includes at least one of polyethylene glycol and polypropylene glycol. (Technology 7) The secondary battery according to any one of Technology 1 to 6, wherein the polyalkylene glycol compound has a number average molecular weight Mn of 500 to 10,000. (Technology 8) The secondary battery according to any one of Technology 1 to 7, wherein the polyalkylene glycol compound has three or four hydroxyl groups. (Technology 9) The secondary battery according to any one of Technology 1 to 8, wherein the polymer of the ethylenically unsaturated carboxylic acid compound is polyacrylic acid or a salt thereof. (Technology 10) The secondary battery according to any one of Technology 1 to 9, wherein the secondary battery includes at least one selected from the group consisting of carboxymethyl cellulose and styrene-butadiene copolymer rubber.(Technology 11) The secondary battery according to any one of Technologies 1 to 10, wherein the negative electrode contains a polymer of the ethylenically unsaturated carboxylic acid compound and the polyalkylene glycol compound. (Technology 12) The secondary battery according to any one of Technologies 1 to 11, wherein the amount of the polyalkylene glycol compound is 1 part by mass or more and 30 parts by mass or less per 100 parts by mass of the polymer of the ethylenically unsaturated carboxylic acid compound. (Technology 13) The secondary battery according to any one of Technologies 1 to 12, wherein the negative electrode contains an alloy-based active material.

[0100] 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.

[0101] 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.

[0102] [Example 1] (1) Preparation of Working Electrode (Negative Electrode) A negative electrode slurry was prepared by mixing a negative electrode active material, a first binder (polyacrylic acid (PAA)), a binder additive (tri-branched polypropylene glycol (PPG)), a second binder (carboxymethyl cellulose (CMC) and styrene-butadiene copolymer rubber (SBR), a conductive additive (carbon nanotubes (CNT)), and an appropriate amount of water. A mixture of graphite (Gr) and a silicon-containing third composite material (SiC) was used as the negative electrode active material. The mass ratio of Gr to SiC was Gr:SiC = 70:30. As described above, the third composite material refers to a silicon-containing material having a structure including a silicon phase and a carbon phase in which the silicon phase is dispersed. The tri-branched PPG had three hydroxyl groups and a number-average molecular weight Mn of 3,000. The number average molecular weight Mn was measured according to the method described in the embodiment section above. Furthermore, in the negative electrode slurry, 5 parts by mass of PAA, 0.25 parts by mass of PPG, 5 parts by mass of CMC, 5 parts by mass of SBR, and 0.5 parts by mass of CNT were added per 100 parts by mass of the negative electrode active material. That is, in the negative electrode slurry, 5 parts by mass of PPG as a binder additive was added per 100 parts by mass of PAA as a first binder. Table 1 below shows the type of first binder, the type of binder additive, the number of hydroxyl groups of the binder additive (also simply referred to as the hydroxyl group number), the number average molecular weight Mn of the binder additive, and the content of the binder additive based on 100 parts by mass of the first binder (also simply referred to as the binder additive content).

[0103] The negative electrode slurry was applied to one side of an electrolytic copper foil as a current collector to form a coating film, and the electrolytic copper foil on which the coating film was formed was punched out to a size of 2 cm x 2 cm, and then the coating film on the punched electrolytic copper foil was dried. In this way, a negative electrode having a negative electrode mixture layer formed on the electrolytic copper foil was obtained. The initial thickness T(0) of the negative electrode mixture layer was measured at any 9 points of the produced negative electrode. １ ~T(0) ９ was measured.

[0104] (2) Preparation of Counter Electrode A lithium metal foil was attached to one side of an electrolytic copper foil (current collector) to obtain an electrolytic copper foil with a lithium metal foil attached thereto, and then the electrolytic copper foil with the lithium metal foil attached thereto was punched out to a size of 2.5 cm × 2.5 cm to prepare a counter electrode.

[0105] (3) Preparation of Electrolyte Solution Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of EC:EMC:DMC=4:1:15 to obtain a mixed solvent. ６ was dissolved at a concentration of 1.3 mol / L to prepare an electrolyte solution.

[0106] (4) Assembling the Test Cell An electrode assembly was constructed by arranging a negative electrode and a counter electrode facing each other with a separator interposed therebetween. A microporous film made of polyolefin was used as the separator. Leads were attached to the negative electrode and counter electrode, respectively. The electrode assembly was housed in an exterior body made of an aluminum laminate sheet, and after the electrolyte solution was poured into this exterior body, the opening of the exterior body was sealed. At this time, a portion of the leads attached to the negative electrode and counter electrode was exposed from the exterior body. In this way, a test cell according to Example 1 was obtained.

[0107] [Example 2] A test cell according to Example 2 was obtained in the same manner as in Example 1, except that the binder additive in the negative electrode slurry was changed to a 4-branched polyethylene glycol (PEG). In Example 2, the 4-branched PEG had four hydroxyl groups and a number-average molecular weight Mn of 800. The number-average molecular weight Mn was measured according to the method described in the above embodiment. Table 1 below shows the type of first binder, the type of binder additive, the number of hydroxyl groups in the binder additive (also simply referred to as the hydroxyl group number), the number-average molecular weight Mn of the binder additive, and the content of the binder additive based on 100 parts by mass of the first binder (also simply referred to as the binder additive content).

[0108] Example 3 A test cell according to Example 3 was obtained in the same manner as in Example 2, except that the binder additive in the negative electrode slurry was changed to a three-branched PEG having a number-average molecular weight Mn of 1,000. The three-branched PEG had a structure represented by the following formula (3). In the following formula (3), n is 6 or 7. The number-average molecular weight Mn was measured according to the method described in the above embodiment. Table 1 below shows the type of first binder, the type of binder additive, the number of hydroxyl groups in the binder additive (also simply referred to as the hydroxyl group number), the number-average molecular weight Mn of the binder additive, and the content of the binder additive based on 100 parts by mass of the first binder (also simply referred to as the binder additive content).

[0109]

[0110] [Example 4] In the negative electrode slurry, the binder additive was changed to a three-branched PPG having a number average molecular weight Mn of 300, and the amount of binder additive added was 50 parts by mass based on 100 parts by mass of the first binder. Except for this, a test cell according to Example 4 was obtained in the same manner as in Example 1. Table 1 below shows the type of first binder, the type of binder additive, the number of hydroxyl groups in the binder additive (also simply referred to as the hydroxyl group number), the number average molecular weight Mn of the binder additive, and the content of the binder additive (also simply referred to as the binder additive content) based on 100 parts by mass of the first binder.

[0111] Comparative Example 1 A test cell according to Comparative Example 1 was obtained in the same manner as in Example 1, except that no binder additive was used in the negative electrode slurry. Table 1 below shows the type of the first binder.

[0112] Comparative Example 2 A test cell according to Comparative Example 2 was obtained in the same manner as in Example 1, except that the binder additive in the negative electrode slurry was changed to PEG having two hydroxyl groups. The number-average molecular weight Mn of the PEG having two hydroxyl groups was 400. The number-average molecular weight Mn was measured according to the method described in the above embodiment section. Table 1 below shows the type of first binder, the type of binder additive, the number of hydroxyl groups in the binder additive (also simply referred to as the hydroxyl group number), the number-average molecular weight Mn of the binder additive, and the content of the binder additive based on 100 parts by mass of the first binder (also simply referred to as the binder additive content).

[0113] Comparative Example 3 A test cell according to Comparative Example 3 was obtained in the same manner as in Example 1, except that the binder additive in the negative electrode slurry was changed to glutamic acid (GLU) represented by the following formula (4). As can be seen from the following formula (4), GLU has two hydroxyl groups and a molecular weight Mn of 147 g / mol. Table 1 below shows the type of first binder, the type of binder additive, the number of hydroxyl groups in the binder additive (also simply referred to as the hydroxyl group number), the molecular weight M of the binder additive, and the content of the binder additive based on 100 parts by mass of the first binder (also simply referred to as the binder additive content).

[0114]

[0115] Comparative Example 4 A test cell according to Comparative Example 4 was obtained in the same manner as in Example 1, except that the binder additive in the negative electrode slurry was changed to glucose (GLC) represented by the following formula (5). As can be seen from the following formula (5), GLC has five hydroxyl groups and a molecular weight M of 180 g / mol. Table 1 below shows the type of first binder, the type of binder additive, the number of hydroxyl groups in the binder additive (also simply referred to as the hydroxyl group number), the molecular weight M of the binder additive, and the content of the binder additive based on 100 parts by mass of the first binder (also simply referred to as the binder additive content).

[0116]

[0117] [Comparative Example 5] A test cell according to Comparative Example 5 was obtained in the same manner as in Example 1, except that PAA, which was the first binder, was not used in the negative electrode slurry. Table 1 below shows the type of binder additive, the number of hydroxyl groups of the binder additive (also simply referred to as the hydroxyl group number), the number average molecular weight Mn of the binder additive, and the content of the binder additive based on 100 parts by mass of the first binder (also simply referred to as the binder additive content).

[0118] Comparative Example 6 A test cell according to Comparative Example 6 was obtained in the same manner as in Example 1, except that the first binder in the negative electrode slurry was changed to polyvinylpyrrolidone K90 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) In addition, the following Table 1 shows the type of binder additive, the number of hydroxyl groups in the binder additive (also simply referred to as the hydroxyl group number), the number average molecular weight Mn of the binder additive, and the content of the binder additive based on 100 parts by mass of the first binder (also simply referred to as the binder additive content).

[0119]

[0120] <Charge / Discharge Test> The test cells according to each example (Examples 1 to 4 and Comparative Examples 1 to 6) were charged and discharged as follows in a thermostatic chamber at 25° C. The rest time between charge and discharge was 20 minutes.

[0121] Charging: Constant current charging was performed at 0.1 C (1 C is the current value required to discharge the design capacity in 1 hour) until the cell voltage reached 0.005 V, followed by a 20-minute pause. Next, constant current charging was performed at 0.01 C until the cell voltage reached 0.005 V, followed by a 20-minute pause. Further, constant current charging was performed at 0.001 C until the cell voltage reached 0.005 V.

[0122] Discharge: A constant current discharge was performed at 0.1 C until the cell voltage reached 1 V, followed by a 20-minute pause. Next, a constant current discharge was performed at 0.01 C until the cell voltage reached 1 V, followed by a 20-minute pause. Further, a constant current discharge was performed at 0.001 C until the cell voltage reached 1 V.

[0123] Using the above charge / discharge capacity results, the mass (g) of the negative electrode active material, and the density (g / mL) of the negative electrode active material in the negative electrode mixture layer during cell fabrication, the charge capacity per unit mass of the negative electrode active material Cmc (mAh / g), the charge capacity per unit mass of the negative electrode active material Cmd (mAh / g), and the charge capacity per unit volume of the negative electrode active material Cvc (mAh / mL) were determined.

[0124] (Initial Efficiency) For the test cells according to each example, the initial efficiency of the negative electrode was calculated using the following formula. The initial efficiency of the negative electrode is the ratio of the discharge capacity to the initial charge capacity of the negative electrode active material. Initial efficiency E (%) = [Cmd (mAh / g) / Cmc (mAh / g)] × 100

[0125] (Swelling Ratio) For the test cells according to each example, the swelling ratio of the negative electrode was determined according to the following procedure. The swelling ratios are shown in Table 2 below.

[0126] Procedure (1) After charging, the test cell was disassembled to remove the negative electrode, and the initial thickness T(0) of the negative electrode mixture layer was measured. １ ~T(0) ９ The thickness T(C) of the negative electrode mixture layer in a fully charged state was measured at the same nine points as those in １ ~T(C) ９ (2) Using the following formula, calculate the volume ratios X1 to X9 of the negative electrode when fully charged, and calculate the arithmetic mean of these to determine the average volume ratio X (%) of the negative electrode when fully charged. Volume ratio Xn (%) = [thickness T (C) ｎ / Thickness T(0) ｎ ] × 100 (3) Using the following formula, the swelling ratio Y of the negative electrode during charging per charge capacity is calculated. As mentioned above, the charge capacity Cvc is the charge capacity per unit volume of the negative electrode active material. Swelling ratio Y [% / (Ah / mL)] = [(average volume ratio X - 100) / (charge capacity Cvc (mAh / mL))] × 1000

[0127]

[0128] Table 2 shows that the test cells according to Examples 1 to 4 and Comparative Examples 1 to 6 all showed good results, with no significant difference in the charge capacity Cmd and initial efficiency E per unit mass of the negative electrode active material. On the other hand, it can be seen that the test cells according to Examples 1 to 4 all had an expansion ratio Y of 40% or less, whereas the test cells according to Comparative Examples 1 to 6 all had an expansion ratio Y greater than 40%. This result shows that the expansion ratio Y of the negative electrode can be suppressed in the test cells according to Examples 1 to 4.

[0129] The secondary battery according to the present disclosure can be used in applications where it is necessary to suppress expansion of the negative electrode caused by charging.

[0130] 1: Electrode group 2: Positive electrode lead 3: Negative electrode lead 4: Battery case 5: Sealing plate 6: Negative electrode terminal 7: Gasket 8: Seal 10: Secondary battery

Claims

1. A secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein at least one selected from the group consisting of the positive electrode, the negative electrode, and the non-aqueous electrolyte contains a polymer of an ethylenically unsaturated carboxylic acid compound and a polyalkylene glycol compound having three or more hydroxyl groups.

2. The secondary battery according to claim 1, wherein the polyalkylene glycol compound includes a branched polyalkylene glycol compound.

3. The secondary battery according to claim 1 or 2, wherein the polyalkylene glycol compound is a polyalkylene glycol or a polyalkylene glycol derivative.

4. The secondary battery according to claim 3, wherein the polyalkylene glycol derivative is a compound in which one of the hydrogen atoms in the polyalkylene glycol is substituted with a substituent, and the substituent is a hydroxyl group, a carboxyl group, an oxycarbonyl group, a halogen atom, an amide group, a hydrocarbon group, or an oxyhydrocarbon group.

5. The secondary battery according to claim 4, wherein at least one hydrogen atom of said hydrocarbon group or said oxyhydrocarbon group is substituted with a hydroxyl group, a carboxyl group, an oxycarbonyl group, an amide group, or a halogen atom.

6. The secondary battery according to claim 1 or 2, wherein the polyalkylene glycol compound contains at least one of polyethylene glycol and polypropylene glycol.

7. The secondary battery according to claim 1 or 2, wherein the polyalkylene glycol compound has a number average molecular weight Mn of 500 to 10,000.

8. The secondary battery according to claim 1 or 2, wherein the polyalkylene glycol compound has three or four hydroxyl groups.

9. The secondary battery according to claim 1 or 2, wherein the polymer of the ethylenically unsaturated carboxylic acid compound is polyacrylic acid or a salt thereof.

10. The secondary battery according to claim 1 or 2, which contains at least one selected from the group consisting of carboxymethyl cellulose and styrene-butadiene copolymer rubber.

11. The secondary battery according to claim 1 or 2, wherein the negative electrode contains a polymer of the ethylenically unsaturated carboxylic acid compound and the polyalkylene glycol compound.

12. The secondary battery according to claim 1 or 2, wherein the amount of the polyalkylene glycol compound is 1 part by mass or more and 30 parts by mass or less per 100 parts by mass of the polymer of the ethylenically unsaturated carboxylic acid compound.

13. The secondary battery according to claim 1 or 2, wherein the negative electrode contains an alloy-based active material.

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