Negative electrode for lithium-ion secondary battery, lithium-ion secondary battery, method for producing cnt-si paste, method for producing negative electrode for lithium-ion secondary battery, method for producing lithium-ion secondary battery

Abstract

The negative electrode for a lithium-ion secondary battery is characterized by having a negative electrode composite material layer containing a carbon-based negative electrode active material, an Si-based negative electrode active material, and carbon nanotubes, and when the coverage of the carbon nanotubes with respect to the surface of the Si-based negative electrode active material is taken as 100, the coverage of the carbon nanotubes with respect to the surface of the carbon-based negative electrode active material is 20 or greater but 50 or less.

Description

Technical Field

[0001] This disclosure relates to a negative electrode for lithium-ion secondary batteries, a lithium-ion secondary battery, a method for manufacturing CNT-Si paste, a method for manufacturing a negative electrode for lithium-ion secondary batteries, and a method for manufacturing lithium-ion secondary batteries. Background Technology

[0002] Carbon nanotubes have attracted attention as conductive materials used in the electrodes of lithium-ion secondary batteries. Compared with conventional conductive materials such as acetylene black, carbon nanotubes can significantly improve conductivity with a smaller content.

[0003] In addition, Si-based anode active materials have attracted attention as negative electrode active materials for lithium-ion secondary batteries. Compared with carbon-based anode active materials, Si-based anode active materials can achieve higher battery capacity.

[0004] For example, patent documents 1 to 3 disclose a technique for improving the conductivity of Si-based negative electrode active materials by dry coating carbon nanotubes on the surface of the materials.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Publication No. 2016-533626

[0008] Patent Document 2: Japanese Patent Application Publication No. 2011-198614

[0009] Patent Document 3: Japanese Patent Application Publication No. 2004-356078 Summary of the Invention

[0010] The problem the invention aims to solve

[0011] However, lithium-ion secondary batteries using Si-based anode active materials suffer from a tendency for their charge-discharge cycle characteristics to degrade. Therefore, a hybrid anode active material combining Si-based and carbon-based anode active materials with good charge-discharge cycle characteristics can be considered. However, in this mixture, due to the low conductivity between the Si-based and carbon-based anode active materials, the charge-discharge cycle characteristics will not improve without the addition of a large amount of carbon nanotubes.

[0012] Therefore, the purpose of this disclosure is to provide a negative electrode for lithium-ion secondary batteries, a lithium-ion secondary battery, a method for manufacturing CNT-Si paste, a method for manufacturing a negative electrode for lithium-ion secondary batteries, and a method for manufacturing lithium-ion secondary batteries that can suppress the amount of carbon nanotubes added thereby suppressing the decline in charge-discharge cycle characteristics.

[0013] Solution for solving the problem

[0014] The negative electrode for a lithium-ion secondary battery, as one aspect of this disclosure, is characterized by having a negative electrode composite material layer comprising a carbon-based negative electrode active material, a Si-based negative electrode active material, and carbon nanotubes, wherein when the coverage of the carbon nanotubes on the surface of the Si-based negative electrode active material is set to 100%, the coverage of the carbon nanotubes on the surface of the carbon-based negative electrode active material is 20 or more and 50 or less.

[0015] The lithium-ion secondary battery of this disclosure is characterized by having a negative electrode for the lithium-ion secondary battery.

[0016] Furthermore, the method for manufacturing CNT-Si paste as one aspect of this disclosure is characterized by having a dispersion step, wherein a mixture comprising carbon nanotubes, a Si-based negative electrode active material, a dispersion material, and a dispersion medium is dispersed, and the carbon nanotubes are coated onto the Si-based negative electrode active material.

[0017] Furthermore, the method for manufacturing a negative electrode for a lithium-ion secondary battery, as one aspect of this disclosure, is characterized by comprising: a negative electrode composite material paste preparation step, wherein a carbon-based negative electrode active material and a CNT-Si paste obtained by the CNT-Si paste manufacturing method are mixed to prepare a negative electrode composite material paste; and a coating step, wherein the negative electrode composite material paste is coated onto a negative electrode current collector.

[0018] Furthermore, the method for manufacturing a lithium-ion secondary battery as one aspect of this disclosure is characterized in that it uses a lithium-ion secondary battery negative electrode obtained by the method for manufacturing a lithium-ion secondary battery negative electrode to manufacture the lithium-ion secondary battery.

[0019] The effects of the invention

[0020] According to this disclosure, the amount of carbon nanotubes added can be suppressed, thereby suppressing the decline in charge-discharge cycle characteristics. Attached Figure Description

[0021] Figure 1 This is a cross-sectional view of a lithium-ion secondary battery as an example of an implementation method.

[0022] Figure 2 This is a cross-sectional view of the negative electrode involved in the implementation method. Detailed Implementation

[0023] (Lithium-ion secondary battery)

[0024] Figure 1 This is a cross-sectional view of a lithium-ion secondary battery as an example of an implementation method. Figure 1The lithium-ion secondary battery 10 shown includes: a wound electrode body 14 formed by winding a positive electrode 11 and a negative electrode 12 with a separator 13 between them; an electrolyte; insulating plates 18 and 19 respectively disposed above and below the electrode body 14; and a battery casing 15 that houses the above components. The battery casing 15 is composed of a bottomed cylindrical casing body 16 and a sealing body 17 that blocks the opening of the casing body 16. It should be noted that other forms of electrode bodies, such as a stacked electrode body in which the positive and negative electrodes are alternately stacked with a separator, can be used instead of the wound electrode body 14. In addition, examples of battery casing 15 include cylindrical, square, coin-shaped, button-shaped, and other metal casings, as well as resin casings formed by laminating resin sheets (laminated batteries).

[0025] The casing body 16 is, for example, a bottomed cylindrical metal container. A sealing gasket 28 is provided between the casing body 16 and the sealing body 17 to ensure the airtightness of the battery interior. The casing body 16 has, for example, a portion of its side surface that bulges inward to support the sealing body 17. The bulge 22 is preferably formed in a ring shape along the circumference of the casing body 16, and its upper surface supports the sealing body 17.

[0026] The sealing body 17 has a structure in which a perforated metal plate 23, a lower valve body 24, an insulating component 25, an upper valve body 26, and a cover 27 are stacked sequentially from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a circular or annular shape, and all components except the insulating component 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective central portions, and the insulating component 25 is located between their respective peripheral portions. When the internal pressure of the secondary battery 10 rises due to heat generated by internal short circuits, for example, the lower valve body 24 deforms and breaks by pushing the upper valve body 26 towards the cover 27, 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 gas is discharged from the opening of the cover 27.

[0027] exist Figure 1 In the lithium-ion secondary battery 10 shown, the positive electrode lead 20 mounted on the positive electrode 11 extends towards the sealing body 17 through the through hole in the insulating plate 18, and the negative electrode lead 21 mounted on the negative electrode 12 extends towards the bottom of the housing body 16 through the outer side of the insulating plate 19. The positive electrode lead 20 is connected to the lower surface of the perforated metal plate 23, which serves as the bottom plate of the sealing body 17, by welding or the like, and the cover 27, which is electrically connected to the perforated metal plate 23, serves as the positive terminal. The negative electrode lead 21 is connected to the inner bottom surface of the housing body 16 by welding or the like, and the housing body 16 serves as the negative terminal.

[0028] The following is a detailed description of the positive electrode 11, the negative electrode 12, the separator 13, and the electrolyte.

[0029] The positive electrode 11, for example, has a positive current collector and a positive electrode composite material layer disposed on the positive current collector. As the positive current collector, current collectors commonly used in the field of lithium-ion secondary batteries can be used, such as sheets or foils containing stainless steel, aluminum, aluminum alloys, titanium, etc. The sheet can be a porous material. Porous materials include, for example, foamed materials, woven fabrics, non-woven fabrics, etc. The thickness of the sheet and foil is not particularly limited, for example, from 1 to 500 μm.

[0030] The positive electrode composite material layer may contain previously known positive electrode active materials, conductive materials, binders, etc.

[0031] Examples of positive electrode active materials include, for example, olivine-type lithium salts such as LiFePO4, chalcogenides such as titanium disulfide and molybdenum disulfide, manganese dioxide, and conventional lithium-containing composite metal oxides. Conventional lithium-containing composite metal oxides are, for example, metal oxides containing lithium and a transition metal, or metal oxides in which a portion of the transition metal is replaced by a different element. Examples of different elements include, for example, Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B, with Mn, Al, Co, Ni, and Mg being preferred. One or more different elements may be used.

[0032] As a specific example of lithium-containing composite metal oxides, Li can be cited as an example. x CoO2, Li x NiO2, Li x MnO2, Li x Co y Ni 1-y O2, Li x Co y M 1-y O z Li x Ni 1-y M y O z Li x Mn2O4, Li x Mn 2-y M y O4, LiMPO4, Li2MPO4F, etc., in which M represents at least one element selected from the group consisting of, for example, Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, V, and B. x = 0 to 1.2, y = 0 to 0.9, z = 2.0 to 2.3.

[0033] Examples of conductive materials include carbon black, graphite, carbon fiber, and metal fiber. Examples of carbon black include acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermally cracked carbon black. Conductive materials can be used alone or in combination of two or more.

[0034] Examples of adhesives that can be used include polyethylene, polypropylene, fluorinated adhesives, rubber granules, acrylic polymers, and vinyl polymers. Fluorinated adhesives include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and vinylidene fluoride-hexafluoropropylene copolymer. One type of adhesive can be used alone, or two or more can be used in combination.

[0035] The separator 13 may be a microporous membrane made of a polymer material, for example. Examples of polymer materials include polyethylene, polypropylene, polyvinylidene fluoride, polyvinylidene chloride, polyacrylonitrile, polyacrylamide, polytetrafluoroethylene, polysulfone, polyethersulfone, polycarbonate, polyamide, polyimide, polyether (ethylene oxide or polypropylene oxide), cellulose (carboxymethyl cellulose or hydroxypropyl cellulose), poly(meth)acrylic acid, and poly(meth)acrylate. One of these polymer materials may be used alone, or two or more may be used in combination. Alternatively, a multilayer membrane formed by stacking these microporous membranes may also be used. The thickness of the microporous membrane is, for example, 15 μm to 30 μm.

[0036] Electrolytes include, for example, non-aqueous solvents and electrolyte salts. Electrolytes are not limited to liquid electrolytes; they can be solid electrolytes using gel polymers, etc.

[0037] Examples of non-aqueous solvents include cyclic carbonates and chain carbonates. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, and butyl carbonate. Examples of chain carbonates include dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.

[0038] Examples of electrolyte salts include LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiN(SO2CF3)2, LiN(SO2C2F5)2, and LiC(SO2CF3)3. One or more of these electrolyte salts can be used.

[0039] (negative electrode)

[0040] Figure 2 This is a cross-sectional view of the negative electrode involved in the implementation method. For example... Figure 2 As shown, the negative electrode 12 has a negative electrode current collector 12a and a negative electrode composite material layer 12b disposed on the negative electrode current collector 12a. Figure 2As shown, the negative electrode composite material layer 12b can be disposed on both sides of the negative electrode current collector 12a, or it can be disposed on only one side.

[0041] As the negative electrode current collector 12a, current collectors commonly used in the field of lithium-ion secondary batteries can be used, such as sheets or foils containing copper, nickel, or other precious metals. The sheet can be a porous material. Porous materials include, for example, foamed materials, woven fabrics, and nonwoven fabrics. The thickness of the sheet and foil is not particularly limited, for example, from 1 to 100 μm.

[0042] The negative electrode composite layer 12b contains carbon-based negative electrode active material, Si-based negative electrode active material, and carbon nanotubes.

[0043] Carbon nanotubes are coated on the surfaces of both the carbon-based and Si-based anode active materials. Specifically, carbon nanotubes are more abundant on the Si-based anode active material than on the carbon-based anode active material. Since the carbon-based anode active material has higher conductivity than the Si-based anode active material, even a small amount of carbon nanotubes adhering to the carbon-based anode active material ensures conductivity between the two materials, thus suppressing the degradation of the battery's charge-discharge cycle characteristics. Specifically, when the coverage rate of carbon nanotubes on the surface of the Si-based anode active material is set to 100%, the coverage rate is preferably 20% or more and 50% or less.

[0044] The coverage was determined by using SEM-EDX (Energy Dispersive X-ray spectroscopy) to distinguish region A on the support surface (the surface of the carbon-based negative electrode active material or the surface of the Si-based negative electrode active material) and region B covering the carbon nanotubes on the support surface, and by calculating the ratio of region B to the total area of ​​region A and region B.

[0045] As a carbon-based negative electrode active material, materials commonly used in the field of lithium-ion secondary batteries can be used, such as blocky artificial graphite (MAG), graphitized mesophase carbon microspheres (MCMB) and other artificial graphite, flake graphite, block graphite, earthy graphite and other natural graphite, hard carbon, soft carbon, activated carbon, etc.

[0046] As a Si-based negative electrode active material, any material capable of reversibly absorbing and releasing lithium ions is acceptable and is not particularly limited. Examples include Si particles, Si-containing alloy particles, and Si-containing composite particles. These can be used alone or in combination of two or more types. Examples of Si-containing alloy particles include alloys containing Si and metals selected from alkali metals, alkaline earth metals, transition metals, rare earth metals, or combinations thereof. Examples of Si-containing composite particles include lithium-ion conductive phases and Si particles dispersed within these phases. The lithium-ion conductive phase is, for example, at least one selected from silicon oxide, silicate, and carbon phases.

[0047] From the perspective of high lithium-ion conductivity, the silicate phase preferably contains at least one element selected from, for example, lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, and radium. Among these, from the perspective of high lithium-ion conductivity, the silicate phase is preferably a silicate phase containing lithium (hereinafter, sometimes referred to as lithium silicate phase).

[0048] Lithium silicate phase, for example, is given by formula: Li 2z SiO 2+z (0 < z < 2) indicates that, from the perspectives of stability, ease of manufacture, and lithium-ion conductivity, z preferably satisfies the relationship 0 < z < 1, and more preferably z = 1 / 2.

[0049] Si particles dispersed in a silica phase, for example, composite particles of the general formula SiO x (Preferably in the range of 0 < x < 2, more preferably in the range of 0.5 ≤ x ≤ 1.6) represents the composite particles of Si particles dispersed in the carbon phase, for example, those of the general formula Si. x C1 y (Preferably, the ranges of 0 < x ≤ 1 and 0 < y ≤ 1 are preferred, and more preferably the ranges of 0.3 ≤ x ≤ 0.45 and 0.7 ≤ y ≤ 0.55 are preferred.)

[0050] Carbon nanotubes can be categorized into single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes. Among these, single-walled carbon nanotubes are preferred, for example, from the perspective of further suppressing the degradation of battery charge-discharge cycle characteristics. It should be noted that a single-walled carbon nanotube (SWCNT) is a cylindrical carbon nanostructure composed of a single layer of graphene sheets; a double-walled carbon nanotube is a cylindrical carbon nanostructure composed of two concentrically stacked graphene sheets; and a multi-walled carbon nanotube is a cylindrical carbon nanostructure composed of three or more concentrically stacked graphene sheets. Furthermore, a graphene sheet refers to a layer in which the carbon atoms of the sp2 hybrid orbitals constituting the graphite (black lead) crystal are located at the vertices of a regular hexagon. The shape of carbon nanotubes is not limited. Examples of possible shapes include needle-like, cylindrical, fishbone-like (fishbone-shaped or cup-shaped stacked type), sheet-like, and rolled forms.

[0051] From the perspective of conductivity, the average length of carbon nanotubes is, for example, 5.0 μm or more and 10 μm or less. Furthermore, from the perspective of conductivity, the average bundle diameter of carbon nanotubes is, for example, 0.1 μm or more and 1 μm or less. Here, the average length of the carbon nanotubes is calculated by measuring the length of 10 carbon nanotubes using a scanning electron microscope (SEM) and then averaging them. The average bundle diameter of the carbon nanotubes is calculated by measuring the bundle diameter of 10 carbon nanotubes using a SEM or a transmission electron microscope (TEM) and then averaging them.

[0052] From the perspective of suppressing the decline in charge-discharge cycle characteristics, the content of carbon nanotubes is, for example, more than 0.004% by mass and less than 0.008% by mass relative to the total amount of the negative electrode composite layer 12b.

[0053] The negative electrode composite layer 12b may also contain various additives such as dispersants and binders.

[0054] The dispersing material is a material that adjusts the dispersibility of solid components such as carbon nanotubes contained in the paste described below. Examples include carboxymethyl cellulose or its salts (hereinafter sometimes referred to as CMC or CMC salts), polyethylene glycol, polyethylene oxide, and other conventionally known thickeners; anionic, cationic, nonionic, or amphoteric surfactants, etc. As a dispersing material, CMC or CMC salts are preferred, for example, from the perspective of having the function of a binder. Examples of CMC salts include ammonium salts, sodium salts, potassium salts, lithium salts, etc.

[0055] Examples of adhesives that can be used include polyethylene, polypropylene, fluorinated adhesives, rubber granules, acrylic polymers, and vinyl polymers. Examples of fluorinated adhesives include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and vinylidene fluoride-hexafluoropropylene copolymer. Examples of rubber granules include acrylic rubber granules, styrene-butadiene rubber (SBR) granules, and acrylonitrile rubber granules.

[0056] The negative electrode composite layer 12b, as a conductive material, can contain carbon materials such as carbon black (CB), acetylene black (AB), and Ketjen black.

[0057] [Manufacturing method of CNT-Si paste]

[0058] The manufacturing method of CNT-Si paste includes a dispersion step, in which a mixture containing carbon nanotubes, a Si-based negative electrode active material, a dispersion material, and a dispersion medium is dispersed, and carbon nanotubes are coated onto the Si-based negative electrode active material. Preferably, the manufacturing method of CNT-Si paste includes a preparatory step prior to the dispersion step, in which carbon nanotubes, a dispersion material, and a dispersion medium are mixed to obtain a CNT-containing liquid.

[0059] (Preparatory steps)

[0060] Prior to the dispersion step, carbon nanotubes, a dispersion material, and a dispersion medium are mixed to obtain a paste containing a CNT-containing liquid in which carbon nanotubes are dispersed. In the subsequent dispersion step, the mixture containing the pre-prepared CNT-containing liquid and the Si-based anode active material is dispersed. Thus, compared to dispersing the Si-based anode active material by adding carbon nanotubes and the dispersion medium, using the pre-prepared CNT-containing liquid allows for more efficient coating of carbon nanotubes onto the surface of the Si-based anode active material.

[0061] Water is preferred as the dispersion medium, considering factors such as ease of recovery during drying and environmental adaptability, but organic solvents can also be used. Water is not specifically limited to ultrapure water, pure water, or industrial water, but if a grade that is not limited by treatment costs and usage volume is selected, Japanese Industrial Standard A1 grade is typically used. Carbon nanotubes and dispersion materials are as described above.

[0062] In the preparatory steps, for example, an online mixer is preferably used to mix carbon nanotubes, dispersion materials, and dispersion media. An online mixer such as the magicLAB manufactured by IKA can be used.

[0063] (Distributed processes)

[0064] The mixture containing CNTs and Si-based anode active material obtained in the preparative step is dispersed, and the carbon nanotubes are coated onto the Si-based anode active material. It should be noted that, without the preparative step, the mixture containing carbon nanotubes, a dispersing material, and the Si-based anode active material in the dispersion medium is dispersed.

[0065] The dispersion process in the dispersion step is preferably selected from at least one of shear-stirring dispersion, bead milling dispersion, and ultrasonic dispersion. Among these processes, shear-stirring dispersion is preferred from the perspective of unwinding the bundles of multiple carbon nanotubes to achieve high dispersion of carbon nanotubes. This allows for efficient coating of carbon nanotubes onto the surface of the Si-based anode active material. From the perspective of efficiently unwinding the carbon nanotube bundles, shear stirring of the mixture with a shear force (1 / s) of 100,000 or more, calculated from the flow rate and gap of the paste, is preferred. Shear stirring is preferably performed using a high-pressure homogenizer. That is, the mixture is preferably passed through a high-pressure homogenizer to perform shear stirring. In the high-pressure homogenizer, the average bundle diameter of the carbon nanotubes can be adjusted according to the number of passes of the mixture, thereby adjusting the dispersion of the carbon nanotubes and the coverage of the carbon nanotubes on the Si-based anode active material. From the perspectives of improving the dispersibility of carbon nanotubes and increasing the coverage of carbon nanotubes on Si-based anode active materials, the number of times the mixture passes through the high-pressure homogenizer (pass count) is preferably more than 1 and less than 50.

[0066] In this way, a CNT-Si dispersion paste can be obtained in which a dispersion material and a Si-based negative electrode active material coated with carbon nanotubes are dispersed in a dispersion medium. Compared with the conventional dry process, the Si-based negative electrode active material coated with carbon nanotubes in this embodiment has low adhesion to the carbon nanotubes. Therefore, through the mixing process performed in the manufacturing method of the negative electrode for lithium-ion secondary batteries described later, a portion of the carbon nanotubes attached to the surface of the Si-based negative electrode active material can be peeled off and can adhere to the surface of the carbon-based negative electrode active material.

[0067] [Manufacturing method for negative electrode of lithium-ion secondary battery]

[0068] A method for manufacturing a negative electrode for lithium-ion secondary batteries includes: a negative electrode composite material paste preparation step, in which a carbon-based negative electrode active material and a CNT-Si paste obtained by a CNT-Si paste manufacturing method are mixed to prepare a negative electrode composite material paste; and a coating step, in which the negative electrode composite material paste is coated onto a negative electrode current collector. Furthermore, the method for manufacturing a negative electrode for lithium-ion secondary batteries preferably includes a pre-mixing step prior to the negative electrode composite material paste preparation step, in which a carbon-based negative electrode active material, a binder, and a dispersion medium are mixed to prepare a carbon-based negative electrode active material paste.

[0069] (Premixing process)

[0070] Before the anode composite paste preparation process, carbon-based anode active material, binder, and dispersion medium are mixed to prepare a carbon-based anode active material paste. In the subsequent anode composite paste preparation process, the pre-prepared carbon-based anode active material paste and CNT-Si paste are mixed. This method, using the pre-prepared carbon-based anode active material paste, compared to adding carbon-based anode active material powder to the CNT-Si paste for mixing, improves the dispersibility of either the carbon-based or Si-based anode active material.

[0071] As described above, water is preferred as the dispersion medium. The dispersion medium can be added in multiple stages during the premixing process. The binder is as described above.

[0072] The mixing in the pre-mixing process and the subsequent negative electrode composite paste preparation process can be performed using a known mixing machine. For example, a Banbury internal mixer or a pressure kneader with two rotating rotor blades inside a container, or a bi-axial planetary mixer / mixer with two blades simultaneously performing revolution and rotation, or other batch mixing machines, can be used. Alternatively, a continuous screw mixer such as a single-screw mixing extruder or a bi-screw mixing extruder, a helical mixer using a rotor with pins, or a filled mixer that uses centrifugal force to enclose the slurry in a high-speed rotating film for mixing can also be used.

[0073] (Preparation process of negative electrode composite paste)

[0074] The negative electrode composite paste is prepared by mixing the carbon-based negative electrode active material paste obtained in the premixing process and the CNT-Si paste obtained by the CNT-Si paste manufacturing method using a mixing mill. Preferably, the negative electrode composite paste is prepared by mixing the carbon-based negative electrode active material paste obtained in the premixing process, the CNT-Si paste obtained by the CNT-Si paste manufacturing method, and an emulsion binder. It should be noted that, without a premixing process, the negative electrode composite paste can be prepared by mixing the carbon-based negative electrode composite material, CNT-Si paste, any binder, and any dispersion medium using a mixing mill.

[0075] Through mixing, a portion of the carbon nanotubes attached to the surface of the Si-based negative electrode active material are peeled off and attached to the surface of the carbon-based negative electrode active material. Since the mixing of the paste does not apply large shear forces, the amount of carbon nanotubes transferred from the Si-based negative electrode active material to the carbon-based negative electrode active material is limited. Although it also depends on mixing conditions such as mixing time and mixing temperature, it is preferable to mix such that the carbon nanotube coverage of the surface of the Si-based negative electrode active material is 20% or more and 50% or less, when the coverage of the carbon nanotubes on the surface of the Si-based negative electrode active material is set to 100%.

[0076] By using this negative electrode composite paste to fabricate the negative electrode, conductivity between the Si-based and carbon-based negative electrode active materials can be ensured even with low carbon nanotube addition. Therefore, the amount of carbon nanotubes added can be suppressed, while also preventing a decrease in charge-discharge cycle characteristics. It should be noted that when the Si-based negative electrode active material is coated with carbon nanotubes obtained through conventional dry processes, almost no carbon nanotubes are transferred to the carbon-based negative electrode active material during mixing. Therefore, if carbon nanotubes are not added during mixing, conductivity between the Si-based and carbon-based negative electrode active materials cannot be ensured. Furthermore, when carbon nanotubes (or carbon nanotube-containing paste), Si-based negative electrode active materials, and carbon-based negative electrode active materials are mixed, the carbon nanotubes uniformly coat both the Si-based and carbon-based negative electrode active materials. Therefore, a large amount of carbon nanotubes needs to be added to ensure conductivity between the Si-based and carbon-based negative electrode active materials.

[0077] In addition to SBR, other materials such as polyacrylic acid, polyvinylpyrrolidone, and polyvinyl alcohol can also be used as emulsion binders.

[0078] (Coating process)

[0079] The negative electrode composite paste obtained through the negative electrode composite paste preparation process is coated onto the negative electrode current collector. The negative electrode composite paste can be applied to the surface of the negative electrode current collector using, for example, a slit die coater, a reverse roller coater, a lip coater, a paddle coater, a doctor blade coater, a gravure coater, and a dip coater. The negative electrode composite paste coated onto the negative electrode current collector can be dried near-naturally, but for productivity reasons, drying at a temperature of 100°C to 200°C for 10 minutes to 1 hour is preferred. The coating film obtained by drying the negative electrode composite paste can also be calendered. Calendering can be performed several times under a specified linear pressure using a roller press, for example, until a specified thickness is achieved.

[0080] Through this series of processes, a negative electrode for lithium-ion secondary batteries with a negative electrode composite material layer formed on the negative electrode current collector can be obtained. The obtained negative electrode for lithium-ion secondary batteries can also be cut and processed to a specified size according to the battery dimensions. Furthermore, the aforementioned lithium-ion secondary batteries can be manufactured using this negative electrode for lithium-ion secondary batteries.

[0081] Example

[0082] The present disclosure will be further illustrated below by way of examples, but the present disclosure is not limited to these examples.

[0083] <Example 1>

[0084] [Preparation of CNT-Si paste]

[0085] A CNT-containing solution was prepared by mixing carbon nanotubes with a diameter (D50) of 700 μm based on the particle size distribution obtained by laser diffraction (MicrotracMT3000), carboxymethyl cellulose (CMC) as the dispersing material, and water as the dispersion medium at a mass ratio of 99:0.6:0.4 for 5 minutes using an online mixer (IKA magicLAB). The resulting CNT-containing solution had a carbon nanotube diameter (D50) of 145 μm based on the particle size distribution obtained by laser diffraction (MicrotracMT3000).

[0086] Next, preparations will be made by Li 2z SiO 2+z (0<z<2) represents a Si-based negative electrode active material in which Si particles are dispersed in the lithium silicate phase. Then, the CNT-containing liquid, the above-mentioned Si-based negative electrode active material and water are mixed at a mass ratio of 1:40 with a solid component concentration of 50% by mass to obtain a mixture. The mixture is then dispersed by passing it through a valve-type high-pressure homogenizer (Sanmaru Machinery Industry econizerlabo02) at a flow rate of 14 L / h and a pressure of 30 Pa twice (passes: 2) to prepare a CNT-Si paste.

[0087] [Making the negative electrode]

[0088] A graphite paste was prepared by mixing graphite, water as a dispersion medium, and CMC as a binder at a mass ratio of 100:100:0.98 for 120 minutes using a mixer (HIVIS MIX Model 2P-1 manufactured by Primix). Then, the above CNT-Si paste and SBR as an emulsion binder were added to the graphite paste at a mass ratio of 10:1.3 and mixed for 5 minutes to prepare the negative electrode slurry.

[0089] The negative electrode slurry is coated on both sides of a negative electrode core material made of copper foil. After the coating is dried, it is calendered using calendering rolls and cut to the specified electrode size to fabricate the negative electrode. The carbon nanotube content is 0.004% by mass relative to the total amount of the negative electrode composite layer. Furthermore, when the carbon nanotube coverage of the Si-based negative electrode active material is set to 100%, the carbon nanotube coverage of the graphite is 20%. Additionally, the average bundle diameter of the carbon nanotubes is 0.9 μm.

[0090] [The production of the positive electrode]

[0091] An NCA (Ni-Al-Co) lithium transition metal composite oxide containing 88% by mass of Ni was used as the positive electrode active material. A positive electrode composite paste was prepared by mixing the positive electrode active material, carbon nanotubes, and polyvinylidene fluoride (PVdF) in a mass ratio of 100:0.4:0.8, followed by the addition of an appropriate amount of NMP. This positive electrode composite paste was then coated onto both sides of a positive electrode core material made of aluminum foil. After the coating was dried, it was calendered using calendering rollers and cut to the specified electrode size to fabricate the positive electrode.

[0092] [Preparation of non-aqueous electrolytes]

[0093] Ethyl carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 3:3:4. A non-aqueous electrolyte was prepared by dissolving lithium hexafluoride phosphate (LiPF6) in this mixed solvent at a concentration of 1.2 mol / L.

[0094] [Preparation of the experimental battery]

[0095] The positive and negative electrodes are wound into a spiral shape with a polyolefin separator and then pressed radially to form a flat, wound electrode body. This electrode body is then housed in a shell made of aluminum laminate, and the non-aqueous electrolyte is injected. The opening of the shell is then sealed to obtain the test battery.

[0096] [Cyclic Test]

[0097] The test battery was charged at a constant current of 0.5C at 25°C until the battery voltage reached 4.2V. Then, it was charged at 4.2V until the current reached 0.05C, followed by constant current discharge at 0.7C until the battery voltage reached 2.5V. This constituted one cycle. A 10-minute rest period was added after each cycle, and this process was repeated 200 times. The discharge capacity of the first cycle and the discharge capacity of the 200th cycle were calculated, and the capacity retention rate was calculated using the following formula.

[0098] Capacity retention (%) = (Discharge capacity in the 200th cycle ÷ Discharge capacity in the 1st cycle) × 100

[0099] <Example 2>

[0100] Except that the number of passes in the dispersion treatment was set to 20, the test cells were fabricated in the same manner as in Example 1, and the above-described cycle test was performed. It should be noted that when the carbon nanotube coverage of the Si-based negative electrode active material was set to 100%, the carbon nanotube coverage of the graphite was 20%. Furthermore, the average bundle diameter of the carbon nanotubes was 0.6 μm.

[0101] <Example 3>

[0102] Except that the number of passes in the dispersion treatment was set to 50, the test cells were fabricated in the same manner as in Example 1, and the above-described cycle test was performed. It should be noted that when the carbon nanotube coverage of the Si-based negative electrode active material was set to 100%, the carbon nanotube coverage of the graphite was 20%. Furthermore, the average bundle diameter of the carbon nanotubes was 0.1 μm.

[0103] <Example 4>

[0104] Except for setting the mixing time using an online mixer to 20 minutes in the preparation of the CNT-containing solution, the test cell was fabricated in the same manner as in Example 1, and the above-described cycling test was performed. The average diameter of the carbon nanotubes in the CNT-containing solution, based on laser diffraction, was 100 μm. When the carbon nanotube coverage of the Si-based negative electrode active material was set to 100%, the carbon nanotube coverage of the graphite was 20%. Furthermore, the average bundle diameter of the carbon nanotubes was 0.7 μm.

[0105] <Example 5>

[0106] Except for setting the mixing time using an online mixer to 60 minutes in the preparation of the CNT-containing solution, the test cell was fabricated in the same manner as in Example 1, and the above-described cycling test was performed. The average diameter of the carbon nanotubes in the CNT-containing solution, based on laser diffraction, was 70 μm. Furthermore, when the carbon nanotube coverage of the Si-based negative electrode active material was set to 100%, the carbon nanotube coverage of the graphite was 20%. The average bundle diameter of the carbon nanotubes was 0.4 μm.

[0107] <Example 6>

[0108] The mixing time for preparing the graphite paste was set to 30 minutes, and the amount of carbon nanotubes added was varied so that the carbon nanotube content relative to the total amount of the negative electrode composite layer was 0.008% by mass. Otherwise, the test cell was prepared in the same manner as in Example 1, and the above-described cycle test was performed. Furthermore, when the carbon nanotube coverage of the Si-based negative electrode active material was set to 100%, the carbon nanotube coverage of the graphite was 40%. The average bundle diameter of the carbon nanotubes was 0.9 μm.

[0109] <Example 7>

[0110] The mixing time for preparing the graphite paste was set to 60 minutes, and the amount of carbon nanotubes added was varied so that the carbon nanotube content relative to the total amount of the negative electrode composite layer was 0.01% by mass. Otherwise, the test cell was fabricated in the same manner as in Example 1, and the above-described cycle test was performed. Furthermore, when the carbon nanotube coverage of the Si-based negative electrode active material was set to 100%, the carbon nanotube coverage of the graphite was 50%. The average bundle diameter of the carbon nanotubes was 0.9 μm.

[0111] <Comparative Example 1>

[0112] [Preparation of CNT paste]

[0113] A CNT-containing solution was prepared by mixing carbon nanotubes with a diameter of 700 μm based on laser diffraction particle size distribution, carboxymethyl cellulose (CMC) as a dispersant, and water as a dispersion medium at a mass ratio of 99:0.6:0.4 for 5 minutes using an online mixer (IKA magicLAB). The diameter of the carbon nanotubes in the CNT-containing solution based on laser diffraction particle size distribution was 145 μm.

[0114] Next, a CNT paste was prepared by dispersing the CNT-containing liquid by passing it through a valve-type high-pressure homogenizer (Sanmaru Machinery Industry econizer labo02) at a flow rate of 14 L / h and a pressure of 80 Pa 20 times.

[0115] [Making the negative electrode]

[0116] A paste was prepared by mixing CNT paste, graphite, Si-based negative electrode active material, water as a dispersion medium, and CMC as a binder at a mass ratio of 10:100:10:100:0.97 for 90 minutes using a mixer (Primix HIVIS MIX Model 2P-1). Then, SBR as an emulsion binder was added to the paste at a mass ratio of 2.5, and the mixture was mixed for 10 minutes to prepare the negative electrode slurry.

[0117] The negative electrode slurry is coated on both sides of a negative electrode core material made of copper foil. After the coating is dried, it is calendered using calendering rollers and cut to the specified electrode size to fabricate the negative electrode. The carbon nanotube content is 0.02% by mass relative to the total amount of the negative electrode composite layer. Furthermore, when the carbon nanotube coverage of the Si-based negative electrode active material is set to 100%, the carbon nanotube coverage of the graphite is also 100%. The average bundle diameter of the carbon nanotubes is 0.05 μm.

[0118] Furthermore, except for using the aforementioned negative electrode, the test battery was fabricated in the same manner as in Example 1, and the aforementioned cycle test was conducted.

[0119] <Comparative Example 2>

[0120] Carbon nanotubes and Si-based anode active material were added to water at a mass ratio of 0.2:100. The dielectric spheres were then placed in the solution, mixed, and dried. The resulting powder was pulverized to obtain Si-CNT powder with carbon nanotubes coated on the surface of the Si-based anode active material.

[0121] A paste was prepared by mixing Si-CNT powder, graphite, water as a dispersion medium, and CMC as a binder at a mass ratio of 10:100:100:0.97 for 120 minutes using a mixer (HIVIS MIX Model 2P-1 manufactured by Primix). Then, SBR as an emulsion binder was added to the paste at a mass ratio of 1.3, and the mixture was mixed for 10 minutes to prepare the negative electrode slurry.

[0122] The negative electrode slurry is coated on both sides of a negative electrode core material made of copper foil. After the coating is dried, it is calendered using calendering rollers and cut to the specified electrode size to fabricate the negative electrode. The carbon nanotube content is 0.1% by mass relative to the total amount of the negative electrode composite layer. Furthermore, when the carbon nanotube coverage of the Si-based negative electrode active material is set to 100%, the carbon nanotube coverage of the graphite is 0. The average bundle diameter of the carbon nanotubes is 0.02 μm.

[0123] Furthermore, except for using the aforementioned negative electrode, the test battery was fabricated in the same manner as in Example 1, and the aforementioned cycle test was conducted.

[0124] <Comparative Example 3>

[0125] Carbon nanotubes and Si-based anode active material were mixed at a mass ratio of 0.2:100 for 120 minutes using a hybridization dry mixer to obtain Si-CNT powder with carbon nanotubes coated on the surface of the Si-based anode active material.

[0126] A paste was prepared by mixing Si-CNT powder, graphite, water as a dispersion medium, and CMC as a binder at a mass ratio of 10:100:100:0.97 for 120 minutes using a mixer (HIVIS MIX Model 2P-1 manufactured by Primix). Then, SBR as an emulsion binder was added to the paste at a mass ratio of 1.3, and the mixture was mixed for 10 minutes to prepare the negative electrode slurry.

[0127] The negative electrode slurry is coated on both sides of a negative electrode core material made of copper foil. After the coating is dried, it is calendered using calendering rollers and cut to the specified electrode size to fabricate the negative electrode. The carbon nanotube content is 0.1% by mass relative to the total amount of the negative electrode composite layer. Furthermore, when the carbon nanotube coverage of the Si-based negative electrode active material is set to 100%, the carbon nanotube coverage of the graphite is 0. The average bundle diameter of the carbon nanotubes is 0.02.

[0128] Furthermore, except for using the aforementioned negative electrode, the test battery was fabricated in the same manner as in Example 1, and the aforementioned cycle test was conducted.

[0129] <Comparative Example 4>

[0130] Except that the carbon nanotube content was set to 0.004% by mass relative to the total amount of the negative electrode composite layer, the test cell was made in the same manner as in Comparative Example 1, and the above-mentioned cycle test was performed.

[0131] The evaluation results of capacity retention rates in the examples and comparative examples are recorded in Table 1. The capacity retention rates of Examples 2-7 and Comparative Examples 1-4 are expressed as relative values ​​when the capacity retention rate of Example 1 is set to 100.

[0132] [Table 1]

[0133]

[0134] The test batteries prepared by the methods of Examples 1-7, compared with the test batteries prepared by the conventional methods of Comparative Examples 2-3, showed a lower amount of carbon nanotubes added and were able to suppress the decline in charge-discharge cycle characteristics. Comparative Examples 1 and 4 were prepared by the same methods, but if the amount of carbon nanotubes added was increased as in Comparative Example 1, the same charge-discharge cycle characteristics as in Example 1 were observed. However, when the amount of carbon nanotubes added was set to the same level as in Example 1, as in Comparative Example 4, the effect of suppressing the decline in charge-discharge cycle characteristics was worse than in the examples.

[0135] Explanation of reference numerals in the attached figures

[0136] 10 Lithium-ion secondary battery, 11 Positive electrode, 12 Negative electrode, 12a Negative electrode current collector, 12b Negative electrode composite material layer, 13 Separator, 14 Electrode body, 15 Battery casing, 16 Casing body, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Drum protrusion, 23 Perforated metal plate, 24 Lower valve body, 25 Insulating component, 26 Upper valve body, 27 Cover, 28 Sealing gasket.

Claims

1. A negative electrode for a lithium-ion secondary battery, comprising a negative electrode composite material layer including a carbon-based negative electrode active material, a Si-based negative electrode active material, and carbon nanotubes. When the coverage rate of the carbon nanotubes on the surface of the Si-based negative electrode active material is set to 100%, the coverage rate of the carbon nanotubes on the surface of the carbon-based negative electrode active material is more than 20% and less than 50%. The content of the carbon nanotubes is more than 0.004% by mass and less than 0.008% by mass relative to the total amount of the negative electrode composite material layer.

2. The negative electrode for a lithium-ion secondary battery according to claim 1, wherein, The average length of the carbon nanotubes is greater than 5 μm and less than 10 μm, and the average bundle diameter of the carbon nanotubes is greater than 0.1 μm and less than 1 μm.

3. A lithium-ion secondary battery comprising the negative electrode for a lithium-ion secondary battery as described in claim 1 or 2.

4. A method for manufacturing a negative electrode for a lithium-ion secondary battery according to claim 1 or 2, comprising: a negative electrode composite material paste preparation step, wherein a carbon-based negative electrode active material and a CNT-Si paste are mixed to prepare a negative electrode composite material paste; and In the coating process, the negative electrode composite material paste is applied to the negative electrode current collector. The method for manufacturing the CNT-Si paste includes: a dispersion step, wherein a mixture comprising carbon nanotubes, a Si-based negative electrode active material, a dispersion material, and a dispersion medium is dispersed, and the carbon nanotubes are coated onto the Si-based negative electrode active material.

5. The method for manufacturing a negative electrode for a lithium-ion secondary battery according to claim 4, wherein before the preparation step of the negative electrode composite material paste, a pre-mixing step is performed, in which the carbon-based negative electrode active material, binder, and dispersion medium are mixed to prepare a carbon-based negative electrode active material paste. In the preparation process of the negative electrode composite paste, the carbon-based negative electrode active material paste and the CNT-Si paste are mixed together.

6. The method for manufacturing a negative electrode for a lithium-ion secondary battery according to claim 5, wherein, In the preparation process of the negative electrode composite paste, the carbon-based negative electrode active material paste, the CNT-Si paste, and the emulsion binder are mixed together.

7. The method for manufacturing a negative electrode for a lithium-ion secondary battery according to claim 4, wherein the method for manufacturing the CNT-Si paste comprises, before the dispersion step: a preparatory step, wherein the carbon nanotubes, the dispersion material, and the dispersion medium are mixed to obtain a CNT-containing liquid. In the dispersion process, the mixture containing the CNT-containing liquid and the Si-based negative electrode active material is dispersed.

8. The method for manufacturing a negative electrode for a lithium-ion secondary battery according to claim 4 or 7, wherein the dispersion treatment in the method for manufacturing the CNT-Si paste includes at least one of shear-stirring dispersion treatment, bead milling dispersion treatment, and ultrasonic treatment dispersion treatment.

9. The method for manufacturing a negative electrode for a lithium-ion secondary battery according to claim 8, wherein in the method for manufacturing the CNT-Si paste, the dispersion treatment is the shear-stirring dispersion treatment, wherein the shear-stirring dispersion treatment shears and stirs the mixture with a shear force of 100,000 or more, wherein the unit of the shear force is 1 / s.

10. The method for manufacturing a negative electrode for a lithium-ion secondary battery according to claim 8, wherein in the method for manufacturing the CNT-Si paste, the shear-stirring-based dispersion treatment is performed by passing the mixture through a high-pressure homogenizer.

11. The method for manufacturing a negative electrode for a lithium-ion secondary battery according to claim 8, wherein in the method for manufacturing the CNT-Si paste, the mixture is passed through a high-pressure homogenizer 1 to 50 times.

12. A method for manufacturing a lithium-ion secondary battery, wherein the lithium-ion secondary battery is manufactured using a negative electrode of a lithium-ion secondary battery obtained by any one of claims 4 to 11.

Images