Negative electrode for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

Abstract

A negative electrode (12), which is an example of an embodiment, is provided with: a negative electrode core (40); and a negative electrode mixture layer (41) that contains a negative electrode active material (50) and a water-soluble polymer (51) and is disposed on the negative electrode core (40). The water-soluble polymer (51) is adsorbed to the negative electrode active material (50) in an amount of 0.50 mass% or more relative to the negative electrode active material (50), and is dissociated from the negative electrode active material (50) in an amount of 1.05 mass% or less relative to the negative electrode active material (50). The negative electrode active material (50) contains a carbon material and a silicon-containing material.

Description

Technical Field

[0001] The present disclosure relates to a negative electrode for a nonaqueous electrolyte secondary battery and a nonaqueous electrolyte secondary battery using the negative electrode. Background Art

[0002] The negative electrode constituting a non-aqueous electrolyte secondary battery such as a lithium ion battery is usually manufactured by applying a negative electrode mixture slurry containing a negative electrode active material and a binder etc. on a core such as a metal foil, and drying and compressing the coating to form a negative electrode mixture layer on the core.

[0003] Patent document 1 discloses a negative electrode mixture slurry for a lithium ion secondary battery, which is prepared by dispersing a negative electrode active material, carboxymethyl cellulose, and an SBR emulsion containing styrene-butadiene rubber particles in water. Patent document 1 records that carboxymethyl cellulose in an amount equivalent to 0.10 to 0.40 wt % of the negative electrode active material is adsorbed on the negative electrode active material.

[0004] In addition, Patent Document 2 discloses a negative electrode for a lithium ion secondary battery comprising two negative electrode active materials. Patent Document 2 states that a resin (A) is formed on the particle surface of the negative electrode active material (A), and a resin (B) is formed on the particle surface of a negative electrode active material (B) that is smaller than the negative electrode active material (A), and the coverage amount of the resin (B) relative to the negative electrode active material (B) is greater than the coverage amount of the resin (A) relative to the negative electrode active material (A).

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Publication No. 2013-257978

[0008] Patent Document 2: Japanese Patent Application Publication No. 2013-187033 Summary of the invention

[0009] In non-aqueous electrolyte secondary batteries, it is an important issue to improve cycle characteristics while ensuring high capacity. The research results of the present inventors show that the dispersion state of the negative electrode active material in the negative electrode mixture layer, especially the dispersion state of the silicon-containing material, has a great influence on the cycle characteristics. In the prior art including Patent Documents 1 and 2, the dispersion state of the silicon-containing material in the negative electrode mixture layer is not considered, and there is still a lot of room for improvement in improving the cycle characteristics.

[0010] The negative electrode for a non-aqueous electrolyte secondary battery disclosed in the present invention comprises: a negative electrode core; and a negative electrode mixture layer comprising a negative electrode active material and a water-soluble polymer and arranged on the negative electrode core, wherein the water-soluble polymer is adsorbed on the negative electrode active material in an amount of more than 0.50 mass % relative to the negative electrode active material, and exists free from the negative electrode active material in an amount of less than 1.05 mass % relative to the negative electrode active material, and the negative electrode active material comprises carbon material and silicon-containing material.

[0011] The nonaqueous electrolyte secondary battery disclosed herein includes the above-mentioned negative electrode, a positive electrode, and a nonaqueous electrolyte.

[0012] The nonaqueous electrolyte secondary battery disclosed herein has a high capacity and excellent cycle characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a cross-sectional view of a nonaqueous electrolyte secondary battery as an example of the embodiment.

[0014] Figure 2 It is a diagram showing the state of the negative electrode active material and the water-soluble polymer in the negative electrode mixture layer.

[0015] Figure 3 This is a diagram for explaining a method for measuring the amount of water-soluble polymer adsorbed to and released from a negative electrode active material. DETAILED DESCRIPTION

[0016] By using silicon-containing materials as negative electrode active materials, it is possible to achieve high capacity of the battery, but as mentioned above, the research results of the present inventors show that the dispersion state of the negative electrode active material, especially the silicon-containing material, in the negative electrode mixture layer has a great influence on the cycle characteristics of the battery. In addition, it was found that when a predetermined amount of water-soluble polymer is adsorbed on the particle surface of the negative electrode active material and a predetermined amount of free water-soluble polymer that is not adsorbed on the negative electrode active material exists, the dispersibility of the negative electrode active material (especially the silicon-containing material) becomes good, and the cycle characteristics are specifically improved.

[0017] The adsorbed components and free components of the water-soluble polymer relative to the negative electrode active material are believed to exist in the same amount in the negative electrode mixture slurry used to manufacture the negative electrode. If there is an adsorbed component of the water-soluble polymer above a specified amount, the affinity for the dispersion medium (water) is improved, and aggregation of the negative electrode active material becomes less likely to occur. That is, a good dispersion state of the negative electrode active material can be ensured in the negative electrode mixture layer. As a result, it is believed that: the uniformity of the battery reaction is promoted, and the capacity reduction caused by the deterioration of the active material accompanying charging and discharging is effectively suppressed. In addition, it is believed that the specified amount of free components contributes to the stabilization of the slurry.

[0018] Hereinafter, an example of an embodiment of a negative electrode for a non-aqueous electrolyte secondary battery disclosed in the present invention and a non-aqueous electrolyte secondary battery using the negative electrode will be described in detail with reference to the accompanying drawings. It should be noted that a configuration formed by selectively combining the constituent elements of the multiple embodiments and modified examples described below is included in the scope of the present invention.

[0019] In the embodiment described below, a non-aqueous electrolyte secondary battery 10 is exemplified as a cylindrical battery in which a wound electrode body 14 is housed in an outer can 16 with a bottomed cylindrical shape, but the outer shell of the battery is not limited to a cylindrical outer can. As other embodiments of the non-aqueous electrolyte secondary battery disclosed herein, a square battery with a square outer can, a coin-shaped battery with a coin-shaped outer can, and a bag-type battery with an outer shell composed of a laminate sheet including a metal layer and a resin layer can be listed. In addition, the electrode body is not limited to a wound type, and may also be a stacked electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked with a separator.

[0020] Figure 1 Schematic diagram of an axial cross section of a nonaqueous electrolyte secondary battery 10 as an example of an embodiment. Figure 1 As shown, the nonaqueous electrolyte secondary battery 10 includes a wound electrode body 14, a nonaqueous electrolyte, and an outer can 16 for storing the electrode body 14 and the nonaqueous electrolyte. The electrode body 14 includes a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are wound in a spiral shape with the separator 13 interposed therebetween. The outer can 16 is a metal container in a bottomed cylindrical shape with one end side open in the axial direction, and the opening of the outer can 16 is blocked by a sealing body 17. In the following, for convenience of description, the sealing body 17 side of the battery is set as the upper side, and the bottom side of the outer can 16 is set as the lower side.

[0021] The non-aqueous electrolyte has ion conductivity (for example, lithium ion conductivity) and may be a liquid electrolyte (electrolyte) or a solid electrolyte.

[0022] The liquid electrolyte (electrolyte) contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. As the non-aqueous solvent, for example, esters, ethers, nitriles, amides, and mixed solvents of two or more of these can be used. As an example of a non-aqueous solvent, ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents thereof can be cited. The non-aqueous solvent may also contain a halogen-substituted product (for example, fluoroethylene carbonate, etc.) in which at least a portion of the hydrogen in these solvents is replaced with a halogen atom such as fluorine. As the electrolyte salt, for example, LiPF 6 And other lithium salts.

[0023] As a solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc. are used. The polymer electrolyte, for example, contains a lithium salt and a matrix polymer, or contains a non-aqueous solvent, a lithium salt, and a matrix polymer. As a matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. As a polymer material, for example, a fluororesin, an acrylic resin, a polyether resin, etc. are used. As an inorganic solid electrolyte, for example, a well-known material in an all-solid-state lithium-ion secondary battery, etc. (for example, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halide-based solid electrolyte, etc.) is used.

[0024] The positive electrode 11, the negative electrode 12, and the separator 13 constituting the electrode body 14 are all strip-shaped long bodies, and are alternately stacked in the radial direction of the electrode body 14 by being wound into a spiral shape. The negative electrode 12 is formed to be one circle larger than the positive electrode 11 to prevent the precipitation of lithium. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the length direction and the width direction. The separator 13 is formed to be at least one circle larger than the positive electrode 11, for example, two sheets are arranged in a manner of clamping the positive electrode 11. The electrode body 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0025] Insulating plates 18 and 19 are disposed above and below the electrode body 14 , respectively. Figure 1 In the example shown, the positive electrode lead 20 extends to the sealing body 17 side through the through hole of the insulating plate 18, and the negative electrode lead 21 extends to the bottom side of the outer can 16 through the outer side of the insulating plate 19. The positive electrode lead 20 is connected to the lower surface of the internal terminal plate 23 of the sealing body 17 by welding or the like, and the top plate of the sealing body 17, that is, the cover 27 electrically connected to the internal terminal plate 23 serves as the positive terminal. The negative electrode lead 21 is connected to the bottom inner surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative terminal.

[0026] A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure the airtightness inside the battery. A groove portion 22 is formed on the outer can 16, in which a part of the side portion protrudes inward and supports the sealing body 17. The groove portion 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and the sealing body 17 is supported by its upper surface. The sealing body 17 is fixed to the upper part of the outer can 16 through the groove portion 22 and the open end of the outer can 16 fastened to the sealing body 17.

[0027] The sealing body 17 has a structure in which an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26 and a cover 27 are stacked in order from the electrode body 14 side. The components constituting the sealing body 17 have, for example, a disc shape or a ring shape, and the components except the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected at their respective central portions, and an insulating member 25 is sandwiched between their respective peripheral portions. When the internal pressure of the battery rises due to abnormal heat release, the lower valve body 24 is deformed and broken in a manner that pushes the upper valve body 26 toward the cover 27 side, thereby cutting off the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure rises further, the upper valve body 26 breaks, and the gas is discharged from the opening of the cover 27.

[0028] Hereinafter, the positive electrode 11 , the negative electrode 12 , and the separator 13 constituting the electrode body 14 , particularly the negative electrode 12 , will be described in detail.

[0029] [positive electrode]

[0030] The positive electrode 11 has a positive electrode core 30 and a positive electrode mixture layer 31 arranged on the positive electrode core 30. The positive electrode core 30 can use a foil of a metal that is stable within the potential range of the positive electrode 11, such as aluminum, aluminum alloy, stainless steel, titanium, or a thin film of the metal arranged on the surface. The positive electrode mixture layer 31 contains a positive electrode active material, a conductive agent, and a binder, and is preferably provided on both sides of the positive electrode core 30 except for the portion connected to the positive electrode lead 20. The positive electrode 11 can be manufactured, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, and a binder to the surface of the positive electrode core 30, drying the coating, and then compressing to form the positive electrode mixture layer 31 on both sides of the positive electrode core 30.

[0031] The positive electrode active material uses a lithium transition metal composite oxide containing transition metal elements such as Ni, Co, and Mn. As the metal elements contained in the lithium transition metal composite oxide, Ni, Co, Mn, Al, Be, B, Na, Mg, Si, K, Ca, Sc, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, Ta, W, Pb, Bi, etc. can be listed. Among them, it is preferred to contain at least one of Ni, Co, and Mn, and the composite oxide may further contain Al.

[0032] Examples of the conductive agent contained in the positive electrode mixture layer 31 include carbon black such as acetylene black and Ketjen black, graphite, carbon nanotubes (CNTs), carbon nanofibers, graphene, metal fibers, metal powders, conductive whiskers, etc. The conductive agent may be used alone or in combination of two or more.

[0033] As the binder contained in the positive electrode mixture layer 31, fluorine-containing resins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylene, polypropylene, ethylene-propylene-isoprene copolymer, ethylene-propylene-butadiene copolymer, and other olefin resins, polyacrylonitrile (PAN), polyimide, polyamide, ethylene-acrylic acid copolymer, and other acrylic resins can be exemplified. In addition, these resins can also be used in combination with carboxymethyl cellulose (CMC) or its salt, polyethylene oxide (PEO), etc. The binder can be used alone or in combination. The content of the conductive agent and the binder relative to the mass of the positive electrode mixture layer 31 is, for example, 0.1% by mass or more and 5% by mass or less.

[0034] [negative electrode]

[0035] The negative electrode 12 has a negative electrode core 40 and a negative electrode mixture layer 41 arranged on the negative electrode core 40. The negative electrode core 40 can use a foil of a metal that is stable within the potential range of the negative electrode 12, such as copper, copper alloy, stainless steel, nickel, nickel alloy, or a thin film of the metal arranged on the surface. The negative electrode mixture layer 41 contains a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode core 40 except for the portion connected to the negative electrode lead 21. The negative electrode 12 can be made, for example, by applying a negative electrode mixture slurry containing a negative electrode active material and a binder on the surface of the negative electrode core 40, drying the coating, and then compressing it to form the negative electrode mixture layer 41 on both sides of the negative electrode core 40. It should be noted that the negative electrode mixture layer 41 may also contain a conductive agent such as CNT.

[0036] The negative electrode 12 contains a carbon material and a silicon-containing material as negative electrode active materials. By using a combination of carbon materials and silicon-containing materials, it is easy to take into account both high capacity and excellent cycle characteristics. The negative electrode 12 may also contain a silicon-containing material other than a silicon-containing material, or a material containing other elements that can form an alloy with Li as a negative electrode active material, but in this embodiment, it essentially only contains carbon materials and silicon-containing materials. In addition, the negative electrode mixture layer 41 contains a negative electrode active material and a water-soluble polymer. The water-soluble polymer makes the dispersion state of the negative electrode active material in the negative electrode mixture layer 41, especially the dispersion state of the silicon-containing material, better.

[0037] The content of carbon material is preferably higher than the content of silicon-containing material. From the viewpoint of improving cycle characteristics, the content of silicon-containing material is preferably less than 40% by mass of the total mass of the negative electrode active material, more preferably less than 35% by mass, and particularly preferably less than 30% by mass. From the viewpoint of high capacity and improved cycle characteristics, the content of silicon-containing material is preferably more than 5% by mass of the total mass of the negative electrode active material. An example of a suitable range of the content of silicon-containing material is more than 5% by mass and less than 35% by mass of the total mass of the negative electrode active material, more preferably more than 5% by mass and less than 30% by mass, or more than 5% by mass and less than 25% by mass.

[0038] The carbon material acting as the negative electrode active material is, for example, at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon and hard carbon. Among them, as the carbon material, it is preferred to use at least artificial graphite such as blocky artificial graphite (MAG), graphitized mesophase carbon microbeads (MCMB), natural graphite such as flaky graphite, blocky graphite, and earthy graphite, or a mixture thereof. The volume-based D50 of the carbon material is, for example, 1 μm or more and 30 μm or less, preferably 5 μm or more and 25 μm or less, or 10 μm or more and 20 μm or less.

[0039] Soft carbon and hard carbon are classified as amorphous carbon with an underdeveloped graphite crystal structure. More specifically, it refers to a carbon component with a d(002) interplanar spacing of 0.342 nm or more based on X-ray diffraction. Soft carbon is also called graphitizable carbon, which is carbon that is easily graphitized by high temperature treatment compared to hard carbon. Hard carbon is also called difficult to graphitize carbon. It should be noted that in the composition of the present invention, there is no need to clearly distinguish between soft carbon and hard carbon. As a negative electrode active material, graphite and at least one amorphous carbon of soft carbon and hard carbon can also be used in combination.

[0040] The silicon-containing material is a material containing Si, and as an example, silicon alloys, silicon compounds and composite materials containing Si can be listed. Among them, composite materials containing Si are preferred. The D50 of the silicon-containing material is usually smaller than the D50 of graphite. The volume-based D50 of the silicon-containing material is, for example, greater than 1 μm and less than 20 μm, or greater than 1 μm and less than 15 μm. It should be noted that the silicon-containing material can be used alone or in combination of two or more.

[0041] Suitable silicon-containing materials (composite materials) are composite particles containing an ion-conducting phase and a Si phase dispersed in the ion-conducting phase. The ion-conducting phase is, for example, at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. The silicide phase is a phase of a compound composed of Si and an element more electropositive than Si, and as an example, NiSi, Mg 2 Si、TiSi 2etc. The Si phase is formed by dispersing Si into fine particulate form. The ion conduction phase is a continuous phase composed of an aggregate of finer particles than the Si phase.

[0042] The average value of the size of the Si phase is preferably 1 nm or more and 200 nm or less, more preferably 1 nm or more and 100 nm or less. The average size of the Si phase is calculated by photographing a scanning electron microscope (SEM) image of the cross-section of the particles of the silicon-containing material and averaging the diameters of the circumscribed circles of the Si phase extracted by image analysis. The average size of the Si phase can be, for example, 1 nm or more and 50 nm or less. By reducing the size of the Si phase, it is possible to maintain a high capacity and suppress the particle expansion rate accompanying charge and discharge.

[0043] The above composite material may also have a conductive layer covering the surface of the ion conduction phase. The conductive layer is composed of a material having a higher conductivity than the ion conductor, and forms a good conductive path in the negative electrode mixture layer 41. The conductive layer is, for example, a carbon coating film composed of a conductive carbon material. As the conductive carbon material, carbon black such as acetylene black and Ketjen black, graphite, and amorphous carbon with low crystallinity can be used. Considering the ensuring of conductivity and the diffusibility of Li ions into the particles, the thickness of the conductive layer is preferably 1 nm or more and 200 nm or less, or 5 nm or more and 100 nm or less. The thickness of the conductive layer can be measured by observing the cross-section of the composite material using SEM or transmission electron microscope (TEM).

[0044] The ion conduction phase may also contain at least one selected from the group consisting of elements of Group 1 and Group 2 of the periodic table. The ion conductive layer may also be a silica phase doped with Li. In addition, the ion conduction phase may also contain at least one selected from the group consisting of B, Al, Zr, Nb, Ta, V, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, W, and lanthanide elements.

[0045] An example of a suitable composite material containing Si is a composite particle having an island structure in which fine Si is dispersed substantially uniformly in an amorphous silica phase and having the general formula SiO x (0 < x ≤ 2). The main component of the silica may also be silicon dioxide. In addition, the silica phase may also be doped with Li. The content ratio (x) of oxygen to Si is, for example, 0.5 ≤ x < 2.0, preferably 0.8 ≤ x ≤ 1.5.

[0046] Another example of a suitable composite material containing Si is a composite particle having an island structure in which fine Si is dispersed substantially uniformly in an amorphous silicate phase. A suitable silicate phase is a lithium silicate phase containing Li. The lithium silicate phase is, for example, represented by the general formula Li 2z SiO(2+z) (0 < z < 2) represents the phase of the composite oxide. Preferably, the lithium silicate phase does not contain Li. 4 SiO 4 (Z = 2). Li 4 SiO 4 is an unstable compound, reacts with water and shows alkalinity. Therefore, it sometimes deteriorates Si and causes a decrease in charge-discharge capacity. From the viewpoints of stability, productivity, Li ion conductivity, etc., the lithium silicate phase is preferably Li 2 SiO 3 (Z = 1) or Li 2 Si 2 O 5 (Z = 1 / 2) as the main component.

[0047] Another example of a suitable composite material containing Si is composite particles having an island structure in which fine Si is dispersed substantially uniformly in a carbon phase. The carbon phase is preferably an amorphous carbon phase. The carbon phase may also contain a crystalline phase component, but preferably has a large amount of amorphous phase component. The amorphous carbon phase is composed of a carbon material having an average crystal plane spacing of the (002) plane exceeding 0.34 nm measured by, for example, X-ray diffraction method. It should be noted that the composite material containing the carbon phase may or may not have a conductive layer different from the carbon phase.

[0048] The oil absorption of the silicon-containing material is, for example, 45 ml / 100 g or more and 50 ml / 100 g or less, preferably 48 ml / 100 g or more and 46 ml / 100 g or less. The oil absorption refers to the amount of oily component that can be absorbed by 100 g of the silicon-containing material. In this specification, it refers to the oil absorption of linseed oil measured according to JIS K 5101.

[0049] The oil absorption of the silicon-containing material has a great influence on the adsorption amount and free amount of the water-soluble polymer 51 described later. Therefore, it is necessary to control the manufacturing conditions of the negative electrode 12 according to the oil absorption of the silicon-containing material used so that the adsorption amount and free amount are within a suitable range. The silicon-containing material can be obtained, for example, by synthesizing SiO 2 nanoparticles using alkoxysilane as a raw material and then forming an ion-conducting phase encapsulating the SiO 2 nanoparticles.

[0050] Among the binders contained in the negative electrode mixture layer 41, similar to the case of the positive electrode 11, a fluororesin, an olefin resin, PAN, polyimide, polyamide, an acrylic resin, etc. can be used, but polyvinyl acetate, styrene-butadiene rubber (SBR), etc. can also be used. Among them, SBR is preferably used. The binder can be used alone or in combination of multiple kinds. The content rate of the binder is, for example, 0.1 mass% or more and 5 mass% or less with respect to the mass of the negative electrode mixture layer 41.

[0051] The negative electrode mixture layer 41 further includes a water-soluble polymer. The water-soluble polymer functions as a binder, for example, similarly to SBR, and functions as a thickener in the negative electrode mixture slurry. One type of water-soluble polymer can be used alone, or multiple types can be used in combination. The content of the water-soluble polymer in the negative electrode mixture layer 41 is, for example, 0.1% by mass or more and 5.0% by mass or less relative to the negative electrode active material, preferably 0.5% by mass or more and 3.0% by mass or less, and more preferably 1.0% by mass or more and 2.0% by mass or less. If the content of the water-soluble polymer is within this range, the dispersion state of the negative electrode active material becomes better.

[0052] Figure 2 4 is a diagram showing the state of the negative electrode active material and the water-soluble polymer in the negative electrode mixture layer 41. The negative electrode active material contains a carbon material such as graphite and a silicon-containing material. Figure 2 In FIG. 5 , only the silicon-containing material 50 is shown. Figure 2 As shown, a silicon-containing material 50 as a negative electrode active material constituting a negative electrode mixture layer 41 and a water-soluble polymer 51 functioning as a binder are present on the negative electrode core 40 . Figure 2 (a) shows a negative electrode 12 as an example of an embodiment of the present disclosure, Figure 2 (b) shows a negative electrode in which the dispersion state of the silicon-containing material 50 is poor as a comparative example.

[0053] The water-soluble polymer 51 includes an adsorbed polymer 51A adsorbed on the particle surface of the silicon-containing material 50, and a free polymer 51B that is not adsorbed on the particle surface of the silicon-containing material 50 and is separated from the silicon-containing material 50 and free. It should be noted that the adsorbed polymer 51A and the free polymer 51B are the same type of polymer. Figure 2 In the example shown in (a), the aggregation of the silicon-containing material 50 is suppressed, and a good dispersion state of the silicon-containing material 50 is ensured. Figure 2 In the example shown in (b), the silicon-containing material 50 is aggregated, and Figure 2 The dispersion state of the silicon-containing material 50 is worse than that of the case (a).

[0054] The water-soluble polymer 51 is adsorbed not only on the silicon-containing material 50, but also on carbon materials such as graphite. The water-soluble polymer 51 exists in a state of being adsorbed on the negative electrode active material in an amount of 0.50 mass % or more relative to the negative electrode active material, and in a state of being free from the negative electrode active material in an amount of 1.05 mass % or less relative to the negative electrode active material. That is, the amount equivalent to more than 0.50 mass % of the mass of the negative electrode active material is the adsorbed polymer 51A, and the amount equivalent to less than 1.05 mass % of the mass of the negative electrode active material is the free polymer 51B. In this case, the dispersibility of the negative electrode active material (especially the silicon-containing material 50) becomes good, and the cycle characteristics are greatly improved. If the adsorption amount of the water-soluble polymer 51 is less than 0.50 mass %, it becomes easy to occur. Figure 2 Aggregation of the silicon-containing material 50 as shown in (b) does not provide an effect of improving the cycle characteristics.

[0055] The adsorbed polymer 51A is a polymer adsorbed on the surface of the particles of the negative electrode active material. In the quantitative method of the water-soluble polymer 51 described later, it is present in the precipitate 61 (described later). Figure 3 The adsorbed polymer 51A is adsorbed on the particle surface in a manner that covers at least a portion of the particle surface of the negative electrode active material. In contrast, the free polymer 51B, for example, may be in contact with the negative electrode active material, but in the quantitative method of the water-soluble polymer 51, it is present in the supernatant 62 (reference Figure 3 )middle.

[0056] The water-soluble polymer 51 may be any polymer that dissolves in water, but is preferably at least one selected from the group consisting of polysaccharides, polyacrylic acid, polyvinyl alcohol, water-soluble acrylic resins, water-soluble epoxy resins, water-soluble polyesters, water-soluble polyamides, derivatives thereof, and salts thereof. The water-soluble polymer 51 may be a polymer derived from a natural substance or a synthetic polymer.

[0057] Polysaccharides are C 6 H 10 O 5 Examples of the polymer having the basic structure include starch such as acetic starch, phosphate starch, carboxymethyl starch, and hydroxyethyl starch; cellulose such as carboxymethyl cellulose, methyl cellulose, hydroxyethyl cellulose, and hydroxypropyl cellulose; polysaccharide uronic acid glycosides such as pectin acid and alginic acid; pullulan; and dextrin.

[0058] Among them, it is preferred to use at least one selected from the group consisting of carboxymethyl cellulose (CMC), polyacrylic acid (PAA) and their salts. The water-soluble polymer 51 may be substantially composed of only CMC salt or only PAA salt, but is preferably CMC salt (e.g., sodium salt) and PAA salt (e.g., sodium salt, ammonium salt, lithium salt). The weight average molecular weight of CMC is, for example, 160,000 or more and 380,000 or less. The weight average molecular weight of PAA is, for example, 5,000 or more and 1,000,000 or less.

[0059] The mass ratio of CMC salt to PAA salt is not particularly limited, and it is preferred that PAA salt is contained in a larger amount than CMC salt. An example of a suitable content of CMC salt is 0.3% by mass or more and 0.9% by mass or less relative to the negative electrode active material. An example of a suitable content of PAA salt is 0.6% by mass or more and 1.2% by mass or less relative to the negative electrode active material. In addition, the content of SBR is preferably lower than the content of the water-soluble polymer 51, and may also be lower than the content of CMC salt.

[0060] The amount of the water-soluble polymer 51 (adsorbed polymer 51A) adsorbed on the negative electrode active material may be 0.50% by mass or more of the amount equivalent to the mass of the negative electrode active material, but is preferably 0.70% by mass or less. That is, the amount of the adsorbed polymer 51A is preferably 0.50% by mass or more and 0.70% by mass or less relative to the negative electrode active material. In addition, the amount of the water-soluble polymer 51 (free polymer 51B) released from the negative electrode active material may be 1.05% by mass or less of the amount equivalent to the mass of the negative electrode active material, but is preferably 0.85% by mass or more. That is, the amount of the free polymer 51B is preferably 0.85% by mass or more and 1.05% by mass or less relative to the negative electrode active material. In this case, the effect of improving the cycle characteristics becomes more significant. In addition, the free amount of the free polymer 51B has a great contribution to the stability of the negative electrode mixture slurry, and therefore, as a result, has a great influence on the dispersion state of the silicon-containing material 50.

[0061] Figure 3 51A and 51B are diagrams for explaining the method of measuring the adsorbed polymer 51A and the free polymer 51B. Figure 3 As shown in (a) of FIG. 1 , the negative electrode mixture layer 41 is dispersed in water to prepare a slurry 60. Next, as Figure 3 As shown in (b), the slurry 60 is centrifuged to separate into a precipitate 61 and a supernatant 62. In this case, the adsorbed polymer 51A is present in the precipitate 61, and the free polymer 51B is present in the supernatant 62. It should be noted that solid-liquid separation means such as filtration may be used instead of centrifugal separation. In addition, Figure 3, only the silicon-containing material 50 is shown as the negative electrode active material, but the carbon material is also precipitated by centrifugal separation, and the adsorption polymer 51A adsorbed on the carbon material is also included in the precipitate 61 .

[0062] The procedure for quantifying the adsorbed polymer 51A and the free polymer 51B is as follows.

[0063] (1) The negative electrode mixture layer is dispersed in a solvent (water) at a predetermined ratio to prepare a slurry 60. At this time, the free polymer 51B dissolves in water, while the adsorbed polymer 51A does not dissolve in water while being adsorbed on the negative electrode active material (carbon material and silicon-containing material 50).

[0064] (2) The slurry 60 is centrifuged to purify the supernatant 62 from which the negative electrode active material is removed.

[0065] (3) The total carbon content of the supernatant 62 is measured, and the organic carbon content (the amount of free polymer 51B) is calculated from the difference between the total carbon content in the liquid and the inorganic carbon (fine powder of active material that is not removed by centrifugal separation).

[0066] (4) The amount of organic carbon adsorbed to the negative electrode active material (the amount of the adsorption polymer 51A) is calculated by subtracting the amount of organic carbon from the amount of the resin component per unit volume contained in the negative electrode mixture layer.

[0067] The total carbon content of the supernatant 62 can be measured using a non-dispersive infrared gas analysis method. First, the amount of carbon dioxide produced by thermally decomposing the supernatant 62 is measured according to the non-dispersive infrared gas analysis method. Next, the amount of carbon dioxide produced by adding hydrochloric acid to the supernatant 62 is similarly measured. The amount of carbon dioxide produced when hydrochloric acid is added is the amount of inorganic carbon from the fine powder of the active material that has not fallen off by centrifugal separation. It should be noted that when SBR is contained in the negative electrode mixture layer 41, substantially all of the SBR is present in the supernatant 62. Therefore, the amount of free polymer 51B is calculated by subtracting the amount of SBR from the amount of organic carbon calculated from the difference between the total carbon content and the inorganic carbon in the liquid.

[0068] It is considered that the mass ratio of the adsorbed polymer 51A to the free polymer 51B in the negative electrode mixture layer 41 is the same in the negative electrode mixture slurry. That is, the mass ratio of the adsorbed polymer 51A to the free polymer 51B in the negative electrode mixture layer 41 is determined during the preparation of the negative electrode mixture slurry. The negative electrode mixture slurry and its preparation method are described below.

[0069] The negative electrode mixture slurry includes, for example, a negative electrode active material having a carbon material and a silicon-containing material 50, a water-soluble polymer 51, SBR, and a dispersion medium. Water is used as a dispersion medium, but a water-soluble solvent that dissolves in water may also be used in combination. In the negative electrode mixture slurry, the water-soluble polymer 51 is adsorbed on the negative electrode active material in an amount of, for example, 0.50% by mass or more, preferably 0.50% by mass or more and 0.70% by mass or less relative to the negative electrode active material, and is free without being adsorbed on the negative electrode active material in an amount of 1.05% by mass or less, preferably 0.85% by mass or more and 1.05% by mass or less. The total amount of the water-soluble polymer 51 is preferably 0.5% by mass or more and 3.0% by mass or less relative to the negative electrode active material.

[0070] The negative electrode mixture slurry is prepared, for example, by mixing and kneading the negative electrode active material, the water-soluble polymer 51 and water, and then adding a dispersant of SBR and water to the slurry obtained by the kneading and kneading them in two stages. In the first kneading step, the viscosity of the slurry is high, so the water-soluble polymer 51 can be efficiently adsorbed on the particle surface of the negative electrode active material. Kneading can use existing known devices such as planetary mixers. Generally, the faster the rotation speed of the kneading device and the longer the kneading treatment time, the more the adsorption amount of the water-soluble polymer 51 increases and the more the free amount decreases.

[0071] The adsorption amount and free amount of the water-soluble polymer 51 are greatly dependent on the oil absorption amount of the silicon-containing material 50. Under the same mixing conditions, when the oil absorption amount of the silicon-containing material 50 becomes smaller, there is a tendency for the adsorption amount to increase and the free amount to decrease; when the oil absorption amount of the silicon-containing material 50 becomes larger, there is a tendency for the adsorption amount to decrease and the free amount to increase. The mixing conditions such as the rotation speed and processing time of the mixing device are adjusted to: the adsorption amount of the water-soluble polymer 51 relative to the negative electrode active material is greater than 0.50 mass% and less than 0.70 mass%, and the free amount of the water-soluble polymer 51 is greater than 0.85 mass% and less than 1.05 mass%. In addition, it is preferred to adjust the adsorption amount and free amount of the water-soluble polymer 51 in consideration of the oil absorption amount of the silicon-containing material 50.

[0072] [Separator]

[0073] The separator 13 uses a porous sheet with ion permeability and insulation. As specific examples of the porous sheet, microporous films, woven fabrics, non-woven fabrics, etc. can be cited. As the material of the separator 13, suitable materials are polyolefins such as polyethylene and polypropylene, cellulose, etc. The separator 13 can be a single-layer structure or a multi-layer structure. For example, the separator 13 can have a multi-layer structure including a thermoplastic resin layer such as polyolefin and a cellulose fiber layer, a two-layer structure of polyethylene (PE) / polypropylene (PP), or a three-layer structure of PE / PP / PE.

[0074] A filler layer containing an inorganic filler may also be arranged at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of the inorganic filler include oxides containing metal elements such as Ti, Al, Si, and Mg, phosphate compounds, and the like. The filler layer may be formed by applying a slurry containing the filler to the surface of the positive electrode 11, the negative electrode 12, or the separator 13. In addition, a resin layer (heat-resistant layer) having high heat resistance such as an aromatic polyamide resin may also be arranged on the surface of the separator 13. The separator 13 may also have, for example, a substrate formed of a porous sheet and a filler layer or a heat-resistant layer arranged on the substrate.

[0075] Example

[0076] Hereinafter, the present disclosure will be further described based on examples, but the present disclosure is not limited to these examples.

[0077] <Example 1>

[0078] [Production of positive electrode]

[0079] As the positive electrode active material, a lithium-containing transition metal composite oxide is used. The positive electrode active material, acetylene black and polyvinylidene fluoride are mixed in a mass ratio of 98:1:1, and N-methyl-2-pyrrolidone (NMP) is used as a dispersion medium to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry is applied on a positive electrode core formed by aluminum foil, and after the coating film is dried and compressed, the positive electrode core is cut into a specified electrode size to obtain a positive electrode having a positive electrode mixture layer formed on both sides of the positive electrode core. It should be noted that an exposed portion where the surface of the positive electrode core is exposed is provided on a part of the positive electrode.

[0080] [Synthesis of Silicon-Containing Materials]

[0081] In a mixture of ethanol / water / ammonia, tetraethoxysilane (TEOS) and hexadecyltrimethylammonium bromide (CTAB) were mixed to obtain SiO modified with CTAB. 2 Then, by adding resorcinol and formaldehyde to polymerize, SiO encapsulated nanoparticles are obtained. 2 Nanoparticle polymer particles. At this time, the ratio of resorcinol to TEOS was set to about 0.5:1. The obtained polymer particles were dried, heated to 800°C in a nitrogen atmosphere to carbonize the polymer, and then mixed with magnesium powder and heated at 650°C in an argon atmosphere to perform magnesium thermal reduction reaction. 2 MgO was dissolved from the particles after the reaction in an O / ethanol solution, and the particles were washed with ethanol and then dried to obtain a mesoporous silicon-containing material in which a Si phase was dispersed in an amorphous carbon phase.

[0082] The oil absorption of the obtained silicon-containing material was measured by the above method (the same applies to the following Examples and Comparative Examples). The oil absorption of the material was 45 ml / 100 g.

[0083] [Preparation of negative electrode mixture slurry]

[0084] As the negative electrode active material, a mixture of the above-mentioned silicon-containing material and artificial graphite in a mass ratio of 10:90 is used. The negative electrode active material, sodium carboxymethyl cellulose (CMC-Na) and lithium polyacrylate are mixed in a solid content mass ratio of 100:0.65:0.90, and water is used as a dispersion medium to obtain a first slurry. In the first kneading step of kneading the raw materials of the first slurry, a planetary mixer is used and the process is carried out under the condition that the solid content is 55% or more. Next, a dispersant of SBR is added to the first slurry in such a manner that the SBR becomes 0.4% by mass relative to the negative electrode active material, and water is added for kneading, thereby obtaining a negative electrode mixture slurry. In the second kneading step, a planetary mixer is used and the process is carried out under the condition that the solid content is 47% or more.

[0085] [Production of negative electrode]

[0086] The negative electrode mixture slurry is applied to both sides of the negative electrode core formed of copper foil, and after the coating is dried, the coating is rolled using a roller and cut into a predetermined electrode size to obtain a negative electrode having negative electrode mixture layers formed on both sides of the negative electrode core. It should be noted that an exposed portion where the surface of the negative electrode core is exposed is provided on a portion of the negative electrode.

[0087] [Preparation of non-aqueous electrolyte]

[0088] LiPF was dissolved in a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (MEC) and dimethyl carbonate (DMC) at a volume ratio of 3:3:4 (25°C) at a concentration of 1.2 mol / L. 6 A non-aqueous electrolyte was prepared.

[0089] [Fabrication of test battery cell (non-aqueous electrolyte secondary battery)]

[0090] An aluminum lead was installed on the exposed portion of the positive electrode, and a nickel lead was installed on the exposed portion of the negative electrode. The positive electrode and the negative electrode were spirally wound with a polyolefin separator interposed therebetween, and then press-formed in the radial direction to produce a flat wound electrode body. The electrode body was housed in an outer shell composed of an aluminum laminate sheet, and after the non-aqueous electrolyte was injected, the opening of the outer shell was sealed to obtain a test battery cell.

[0091] <Example 2>

[0092] A test cell was prepared in the same manner as in Example 1 except that the silicon-containing material X2 having an oil absorption of 49.5 ml / 100 g was used instead of the silicon-containing material X1. In the synthesis of the silicon-containing materials of Examples 2 to 4, the oil absorption was adjusted by appropriately changing the pulverization time and the amount of fine powder removed.

[0093] <Example 3>

[0094] A test cell was prepared in the same manner as in Example 1 except that the silicon-containing material X3 having an oil absorption of 48.8 ml / 100 g was used instead of the silicon-containing material X1.

[0095] <Example 4>

[0096] A test cell was prepared in the same manner as in Example 1 except that the silicon-containing material X4 having an oil absorption of 45.6 ml / 100 g was used instead of the silicon-containing material X1.

[0097] <Comparative Example 1>

[0098] A test cell was prepared in the same manner as in Example 1 except that the silicon-containing material X5 having an oil absorption of 51.5 ml / 100 g was used instead of the silicon-containing material X1.

[0099] The cycle characteristics (capacity retention rate) of each test battery cell of the embodiment and the comparative example were evaluated by the following method, and the evaluation results are shown in Table 1. Figure 3 The amount of water-soluble polymer adsorbed on the negative electrode active material and the amount released from the negative electrode active material in the negative electrode mixture layer of each test battery cell were measured by the method shown in the figure. The results of the measurement of the amount of water-soluble polymer adsorbed and released are shown in Table 1 together with the evaluation results of the capacity retention rate.

[0100] [Evaluation of cycle characteristics (capacity retention rate after cycle test)]

[0101] The test battery cell was charged at a constant current of 0.3 It until the battery voltage reached 4.2 V under a temperature environment of 25°C, and then charged at a constant voltage of 4.2 V until the current value reached 0.02 It. Then, the battery was discharged at a constant current of 0.5 It until the battery voltage reached 2.5 V. This charge and discharge was regarded as one cycle, and 400 cycles were performed. The discharge capacity of the first cycle and the discharge capacity of the 400th cycle were obtained, and the capacity retention rate was calculated according to the following formula.

[0102] Capacity retention rate (%) = (400th cycle discharge capacity ÷ 1st cycle discharge capacity) × 100

[0103] [Table 1]

[0104]

[0105] As shown in Table 1, the test battery cells of the embodiments all had higher capacity retention rates after the cycle test and better cycle characteristics than the test battery cells of Comparative Example 1. From the results, it can be understood that the capacity retention rate is greatly improved when the water-soluble polymer is adsorbed on the negative electrode active material in an amount of 0.50% by mass or more relative to the negative electrode active material and when the water-soluble polymer is free from the negative electrode active material in an amount of 1.05% by mass or less relative to the negative electrode active material.

[0106] The present disclosure is further illustrated by the following embodiments.

[0107] Configuration 1: A negative electrode for a non-aqueous electrolyte secondary battery, comprising: a negative electrode core; and a negative electrode mixture layer comprising a negative electrode active material and a water-soluble polymer and arranged on the aforementioned negative electrode core, wherein the aforementioned water-soluble polymer is adsorbed on the aforementioned negative electrode active material in an amount of 0.50 mass % or more relative to the aforementioned negative electrode active material, and exists in a free state from the aforementioned negative electrode active material in an amount of 1.05 mass % or less relative to the aforementioned negative electrode active material, and the aforementioned negative electrode active material contains carbon material and silicon-containing material.

[0108] Configuration 2: The negative electrode for a non-aqueous electrolyte secondary battery according to Configuration 1, wherein the silicon-containing material includes an ion conductive phase and a Si phase dispersed in the ion conductive phase.

[0109] Configuration 3: The negative electrode for a nonaqueous electrolyte secondary battery according to Configuration 2, wherein the ion conductive phase is at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase.

[0110] Configuration 4: A negative electrode for a non-aqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the water-soluble polymer is at least one selected from the group consisting of polysaccharides, polyacrylic acid, water-soluble acrylic resins, water-soluble epoxy resins, water-soluble polyesters, water-soluble polyamides, their derivatives and their salts.

[0111] Configuration 5: The negative electrode for a nonaqueous electrolyte secondary battery according to any one of 1 to 4, wherein a content of the water-soluble polymer in the negative electrode mixture layer is 1.0% by mass or more and 3.0% by mass or less relative to the negative electrode active material.

[0112] Configuration 6: A negative electrode for a non-aqueous electrolyte secondary battery according to Configuration 5, wherein the amount of the water-soluble polymer adsorbed on the negative electrode active material is greater than 0.50 mass % and less than 0.70 mass % relative to the negative electrode active material, and the amount of the water-soluble polymer free from the negative electrode active material is greater than 0.85 mass % and less than 1.05 mass % relative to the negative electrode active material.

[0113] Configuration 7: A nonaqueous electrolyte secondary battery comprising: the negative electrode for a nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 6, a positive electrode, and a nonaqueous electrolyte.

[0114] Description of Reference Numerals

[0115] 10 non-aqueous electrolyte secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 16 outer can, 17 sealing body, 18, 19 insulating plate, 20 positive electrode lead, 21 negative electrode lead, 22 groove portion, 23 internal terminal plate, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cover, 28 gasket, 30 positive electrode core, 31 positive electrode mixture layer, 40 negative electrode core, 41 negative electrode mixture layer, 50 negative electrode active material, 51 water-soluble polymer, 51A adsorbed polymer, 51B free polymer, 60 slurry, 61 precipitate, 62 supernatant.

Claims

1. A negative electrode for a non-aqueous electrolyte secondary battery, comprising: A negative electrode core; and A negative electrode mixture layer comprising a negative electrode active material and a water-soluble polymer and disposed on the negative electrode core, The water-soluble polymer is present in a state of being adsorbed on the negative electrode active material in an amount of 0.50 mass % or more relative to the negative electrode active material, and is present in a state of being free from the negative electrode active material in an amount of 1.05 mass % or less relative to the negative electrode active material, The negative electrode active material includes a carbon material and a silicon-containing material.

2. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, in, The silicon-containing material includes an ion-conducting phase and a Si phase dispersed in the ion-conducting phase.

3. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 2, in, The ion-conducting phase is at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase.

4. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, in, The water-soluble polymer is at least one selected from the group consisting of polysaccharides, polyacrylic acid, polyvinyl alcohol, water-soluble acrylic resins, water-soluble epoxy resins, water-soluble polyesters, water-soluble polyamides, derivatives thereof, and salts thereof.

5. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, in, The content of the water-soluble polymer in the negative electrode mixture layer is 0.5 mass % or more and 3.0 mass % or less relative to the negative electrode active material.

6. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 5, in, The amount of the water-soluble polymer adsorbed on the negative electrode active material is 0.50 mass % or more and 0.70 mass % or less relative to the negative electrode active material, The amount of the water-soluble polymer released from the negative electrode active material is 0.85 mass % or more and 1.05 mass % or less relative to the negative electrode active material.

7. A non-aqueous electrolyte secondary battery comprising: A negative electrode for a non-aqueous electrolyte secondary battery, a positive electrode, and a non-aqueous electrolyte according to any one of claims 1 to 6.

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