Negative electrode for lithium-ion secondary battery, lithium-ion secondary battery, method for manufacturing CNT-Si paste, method for manufacturing a negative electrode for lithium-ion secondary battery, method for manufacturing a lithium-ion secondary battery

Abstract

This negative electrode for a lithium ion secondary battery is characterized by comprising a negative electrode mixture layer containing a carbon-based negative electrode active material, a Si-based negative electrode active material, and carbon nanotubes, wherein when 100 is the ratio at which the carbon nanotubes coat the surface of the Si-based negative electrode active material, the ratio at which the carbon nanotubes coat the surface of the carbon-based negative electrode active material is at least 20 but no higher than 50.

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 a lithium-ion secondary battery. [Background technology]

[0002] Carbon nanotubes are attracting attention as a conductive material to be included in the electrodes of lithium-ion secondary batteries. Compared to conventional conductive materials such as acetylene black, carbon nanotubes can significantly improve conductivity with a smaller content.

[0003] Furthermore, Si-based negative electrode active materials are attracting attention as negative electrode active materials for lithium-ion secondary batteries. Compared to carbon-based negative electrode active materials, Si-based materials can increase the capacity of batteries.

[0004] For example, Patent Documents 1 to 3 disclose a technique for improving the conductivity of a Si-based anode active material by coating its surface with carbon nanotubes using a dry method. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Special Publication No. 2016-533626 [Patent Document 2] Japanese Patent Publication No. 2011-198614 [Patent Document 3] Japanese Patent Publication No. 2004-356078 [Overview of the project] [Problems that the invention aims to solve]

[0006] Incidentally, lithium-ion secondary batteries using Si-based negative electrode active materials have a problem in that their charge-discharge cycle characteristics tend to deteriorate. Therefore, it is conceivable to use a negative electrode active material that is a mixture of Si-based negative electrode active material and carbon-based negative electrode active material, which has good charge-discharge cycle characteristics. However, in this mixture, the conductivity of the Si-based negative electrode active material and the carbon-based negative electrode active material is low, so the charge-discharge cycle characteristics will not improve unless a large amount of carbon nanotubes are added.

[0007] Therefore, the purpose of this disclosure is to provide a negative electrode for a lithium-ion secondary battery, a lithium-ion secondary battery, a method for producing a CNT-Si paste, a method for producing a negative electrode for a lithium-ion secondary battery, and a method for producing a lithium-ion secondary battery, which can suppress the deterioration of charge-discharge cycle characteristics by reducing the amount of carbon nanotubes added. [Means for solving the problem]

[0008] An anode for a lithium-ion secondary battery according to one aspect of the present disclosure has an anode composite layer comprising a carbon-based anode active material, a Si-based anode active material, and carbon nanotubes, characterized in that when the coverage rate of the carbon nanotubes on the surface of the Si-based anode active material is set to 100, the coverage rate of the carbon nanotubes on the surface of the carbon-based anode active material is 20 or more and 50 or less.

[0009] A lithium-ion secondary battery according to one aspect of this disclosure is characterized by comprising a negative electrode for the lithium-ion secondary battery.

[0010] Furthermore, a method for producing CNT-Si paste, according to one aspect of this disclosure, is characterized by comprising a dispersion step of dispersing carbon nanotubes in a mixed solution containing carbon nanotubes, a Si-based anode active material, a dispersant, and a dispersion medium, thereby coating the Si-based anode active material with carbon nanotubes.

[0011] Further, a method for manufacturing a negative electrode for a lithium-ion secondary battery according to an aspect of the present disclosure includes a negative electrode mixture paste preparation step of kneading a carbon-based negative electrode active material and a CNT-Si paste obtained by the method for manufacturing the CNT-Si paste to prepare a negative electrode mixture paste, and a coating step of coating the negative electrode mixture paste on a negative electrode current collector.

[0012] Further, a method for manufacturing a lithium-ion secondary battery according to an aspect of the present disclosure is characterized in that a lithium-ion secondary battery is manufactured using the negative electrode for a lithium-ion secondary battery obtained by the method for manufacturing the negative electrode for a lithium-ion secondary battery.

Advantages of the Invention

[0013] According to the present disclosure, it is possible to suppress a decrease in charge-discharge cycle characteristics by suppressing the addition amount of carbon nanotubes.

Brief Description of the Drawings

[0014] [Figure 1] It is a cross-sectional view of a lithium-ion secondary battery which is an example of an embodiment. [Figure 2] It is a cross-sectional view of a negative electrode according to an embodiment.

Modes for Carrying Out the Invention

[0015] (Lithium-Ion Secondary Battery) Figure 1 is a cross-sectional view of a lithium-ion secondary battery, which is an example of an embodiment. The lithium-ion secondary battery 10 shown in Figure 1 comprises a wound electrode body 14 in which a positive electrode 11 and a negative electrode 12 are wound around a separator 13, an electrolyte, insulating plates 18 and 19 arranged above and below the electrode body 14, respectively, and a battery case 15 that houses the above components. The battery case 15 is composed of a bottomed cylindrical case body 16 and a sealing body 17 that closes the opening of the case body 16. In addition, other forms of electrode bodies may be used instead of the wound electrode body 14, such as a laminated electrode body in which the positive electrode and negative electrode are alternately stacked with a separator. Examples of battery cases 15 include metal cases such as cylindrical, rectangular, coin-shaped, and button-shaped cases, and resin cases formed by laminating resin sheets (laminated batteries).

[0016] The case body 16 is, for example, a metal container in the shape of a bottomed cylinder. A gasket 28 is provided between the case body 16 and the sealing body 17 to ensure airtightness inside the battery. The case body 16 has, for example, a protruding portion 22 that supports the sealing body 17, which is a part of the side surface that protrudes inward. The protruding portion 22 is preferably formed in an annular shape along the circumferential direction of the case body 16, and its upper surface supports the sealing body 17.

[0017] The sealing body 17 has a structure in which a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective centers, with the insulating member 25 interposed between their respective peripheries. When the internal pressure of the secondary battery 10 rises due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 towards the cap 27, thus interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0018] In the lithium-ion secondary battery 10 shown in Figure 1, the positive electrode lead 20 attached to the positive electrode 11 extends through a through-hole in the insulating plate 18 towards the sealing body 17, and the negative electrode lead 21 attached to the negative electrode 12 extends outside the insulating plate 19 towards the bottom of the case body 16. The positive electrode lead 20 is connected by welding or the like to the lower surface of the filter 23, which is the bottom plate of the sealing body 17, and the cap 27, which is the top plate of the sealing body 17 and is electrically connected to the filter 23, becomes the positive electrode terminal. The negative electrode lead 21 is connected by welding or the like to the inner surface of the bottom of the case body 16, and the case body 16 becomes the negative electrode terminal.

[0019] The positive electrode 11, negative electrode 12, separator 13, and electrolyte will be described in detail below.

[0020] The positive electrode 11 includes, for example, a positive electrode current collector and a positive electrode composite layer disposed on the positive electrode current collector. The positive electrode current collector can be one commonly used in the field of lithium-ion secondary batteries, such as sheets or foils containing stainless steel, aluminum, aluminum alloy, or titanium. The sheet may be a porous material. Porous materials include, for example, foams, woven fabrics, and nonwoven fabrics. The thickness of the sheet and foil is not particularly limited, but is, for example, 1 to 500 μm.

[0021] The positive electrode composite layer can contain conventionally known positive electrode active materials, conductive materials, binders, etc.

[0022] Examples of positive electrode active materials include olivine-type lithium salts such as LiFePO4, chalcogen compounds 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 in the metal oxide is substituted with a heterogeneous element. Examples of heterogeneous elements include 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. There may be one heterogeneous element or two or more heterogeneous elements.

[0023] Specific examples of the lithium-containing composite metal oxide include, for example, Li 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 (in each formula, 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).) and the like can be mentioned.

[0024] Examples of the conductive material include, for example, carbon black, graphite, carbon fiber, metal fiber, and the like. Examples of carbon black include acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and the like. The conductive material can be used alone or in combination of two or more.

[0025] Examples of the binder include, for example, polyethylene, polypropylene, fluorine-based binder, rubber particles, acrylic polymer, vinyl polymer, and the like. Examples of the fluorine-based binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), vinylidene fluoride-hexafluoropropylene copolymer, and the like. The binder can be used alone or in combination of two or more.

[0026] The separator 13 may be a microporous film 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 (polyethylene oxide or polypropylene oxide), cellulose (carboxymethylcellulose or hydroxypropylcellulose), poly(meth)acrylic acid, and poly(meth)acrylic acid esters. These polymer materials can be used individually or in combination of two or more. Multilayer films made by layering these microporous films can also be used. The thickness of the microporous film is, for example, 15 μm to 30 μm.

[0027] The electrolyte includes, for example, a non-aqueous solvent and an electrolyte salt. The electrolyte is not limited to a liquid electrolyte, but may also be a solid electrolyte using a gel-like polymer or the like.

[0028] Examples of non-aqueous solvents include cyclic carbonates and linear carbonates. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, and butylene carbonate. Examples of linear carbonates include dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0029] Examples of electrolyte salts include LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiN(SO2CF3)2, LiN(SO2C2F5)2, and LiC(SO2CF3)3. These electrolyte salts may be used individually or in combination of two or more types.

[0030] (Negative electrode) Figure 2 is a cross-sectional view of the negative electrode according to the embodiment. As shown in Figure 2, the negative electrode 12 has a negative electrode current collector 12a and a negative electrode composite layer 12b disposed on the negative electrode current collector 12a. As shown in Figure 2, the negative electrode composite layer 12b may be disposed on both sides of the negative electrode current collector 12a or on only one side.

[0031] As the negative electrode current collector 12a, those commonly used in the field of lithium-ion secondary batteries can be used, such as sheets or foils containing copper, nickel, or precious metals. The sheet may be a porous material. Porous materials include, for example, foams, woven fabrics, and nonwoven fabrics. The thickness of the sheet and foil is not particularly limited, but is, for example, 1 to 100 μm.

[0032] The negative electrode composite layer 12b comprises a carbon-based negative electrode active material, a Si-based negative electrode active material, and carbon nanotubes.

[0033] Carbon nanotubes are coated on the surfaces of both the carbon-based and Si-based anode active materials. However, carbon nanotubes are more abundant on the Si-based anode active material than on the carbon-based one. Since the carbon-based anode active material has higher conductivity than the Si-based anode active material, even if the amount of carbon nanotubes attached to the carbon-based anode active material is small, conductivity between the Si-based and carbon-based anode active materials can be ensured, thereby suppressing a decrease in the battery's charge-discharge cycle characteristics. Specifically, when the coating rate of carbon nanotubes on the surface of the Si-based anode active material is set to 100, it is preferable that the coating rate of carbon nanotubes on the surface of the carbon-based anode active material be between 20 and 50.

[0034] The coverage rate is determined by using SEM-EDX (Energy Dispersive X-ray spectrometry) to distinguish between region A, which is the surface of the support material (the surface of the carbon-based anode active material or the surface of the Si-based anode active material), and region B, which is the carbon nanotube covering the support surface, and then calculating the ratio of the area of ​​region B to the total area of ​​region A and region B.

[0035] As the carbon-based negative electrode active material, those commonly used in the field of lithium-ion secondary batteries can be used. For example, massive artificial graphite (MAG), artificial graphite such as graphitized mesophase carbon microbeads (MCMB), natural graphite such as flaky graphite, massive graphite, and earthy graphite, hard carbon, soft carbon, activated carbon, etc. can be mentioned.

[0036] As the Si-based negative electrode active material, there is no particular limitation as long as it can reversibly occlude and release lithium ions. For example, Si particles, alloy particles containing Si, and composite particles containing Si can be mentioned. These may be used alone or in combination of two or more. The alloy particles containing Si include, for example, an alloy containing Si and a metal selected from an alkali metal, an alkaline earth metal, a transition metal, a rare earth metal, or a combination thereof. The composite particles containing Si include, for example, a lithium ion conductive phase and Si particles dispersed in the lithium ion conductive phase. The lithium ion conductive phase is, for example, at least one selected from a silicon oxide phase, a silicate phase, and a carbon phase.

[0037] The silicate phase preferably contains at least one element selected from lithium, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, and radium, for example, from the viewpoint of high lithium ion conductivity. Among them, the silicate phase is preferably a lithium-containing silicate phase (hereinafter sometimes referred to as a lithium silicate phase) from the viewpoint of high lithium ion conductivity.

[0038] The lithium silicate phase is represented by, for example, the formula: Li 2z SiO 2+z (0 < z < 2). From the viewpoints of stability, ease of production, lithium ion conductivity, etc., z preferably satisfies the relationship of 0 < z < 1, and z = 1 / 2 is more preferable.

[0039] The composite particles in which Si particles are dispersed in the silicon oxide phase are represented by, for example, the general formula SiO x(The range of 0 < x < 2 is preferred, and the range of 0.5 ≤ x ≤ 1.6 is more preferred). The composite particles in which Si particles are dispersed in the carbon phase are represented by, for example, the general formula SixC1y (the range of 0 < x ≤ 1 and 0 < y ≤ 1 is preferred, and the range of 0.3 ≤ x ≤ 0.45 and 0.7 ≤ y ≤ 0.55 is more preferred).

[0040] The carbon nanotubes include single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes. Among these, for example, single-walled carbon nanotubes are preferred in terms of more suppressing the deterioration of the charge-discharge cycle characteristics of the battery. Note that a single-walled carbon nanotube (SWCNT) is a carbon nanostructure in which a graphene sheet forms a single cylindrical shape with one layer, a double-walled carbon nanotube is a carbon nanostructure in which two graphene sheets are stacked concentrically to form a single cylindrical shape, and a multi-walled carbon nanotube is a carbon nanostructure in which three or more graphene sheets are stacked concentrically to form a single cylindrical shape. Further, the graphene sheet refers to a layer in which carbon atoms in sp2 hybrid orbitals constituting the crystal of graphite (carbon) are located at the vertices of a regular hexagon. The shape of the carbon nanotubes is not limited. Such shapes include various forms including needle-like, cylindrical tube-like, fishbone-like (fishbone or cup stacking type), platelet, and coil-like.

[0041] The average length of the carbon nanotubes may be, for example, 5.0 μm or more and 10 μm or less in terms of conductivity and the like. Also, the average bundle diameter of the carbon nanotubes may be, for example, 0.1 μm or more and 1 μm or less in terms of conductivity and the like. Here, the average length of the carbon nanotubes is calculated from the average value obtained by measuring the lengths of 10 carbon nanotubes using a scanning electron microscope (SEM). The average bundle diameter of the carbon nanotubes is calculated from the average value obtained by measuring the bundle diameters of 10 carbon nanotubes using SEM or a transmission electron microscope (TEM).

[0042] The carbon nanotube content may be between 0.004% by mass and 0.008% by mass relative to the total amount of the negative electrode composite layer 12b, for example, in terms of suppressing a decrease in charge-discharge cycle characteristics.

[0043] The negative electrode composite layer 12b may contain various additives such as dispersants and binders.

[0044] The dispersant adjusts the dispersibility of solid components such as carbon nanotubes contained in the paste described later. Examples include carboxymethylcellulose or its salts (hereinafter sometimes referred to as CMC or CMC salt), conventionally known thickeners such as polyethylene glycol and polyethylene oxide, and anionic, cationic, nonionic, or amphoteric surfactants. As a dispersant, CMC or CMC salt is preferred because it functions as a binder. Examples of CMC salts include ammonium salts, sodium salts, potassium salts, and lithium salts.

[0045] Examples of binders that can be used include polyethylene, polypropylene, fluorine-based binders, rubber particles, acrylic polymers, and vinyl polymers. Examples of fluorine-based binders include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and vinylidene fluoride-hexafluoropropylene copolymer. Examples of rubber particles include acrylic rubber particles, styrene-butadiene rubber (SBR) particles, and acrylonitrile rubber particles.

[0046] The negative electrode composite layer 12b may contain carbon materials such as carbon black (CB), acetylene black (AB), or Ketjenblack as a conductive material.

[0047] [Method for manufacturing CNT-Si paste] A method for producing CNT-Si paste includes a dispersion step in which carbon nanotubes are dispersed in a mixed solution containing a Si-based anode active material, a dispersant, and a dispersion medium, thereby coating the Si-based anode active material with carbon nanotubes. Preferably, the method for producing CNT-Si paste includes a preliminary step before the dispersion step in which carbon nanotubes, a dispersant, and a dispersion medium are mixed to obtain a CNT-containing solution.

[0048] (Preliminary process) Before the dispersion process, carbon nanotubes, a dispersant, and a dispersion medium are mixed to obtain a paste of CNT-containing liquid in which carbon nanotubes are dispersed in the dispersion medium. In the subsequent dispersion process, the mixture containing the pre-prepared CNT-containing liquid and the Si-based anode active material is dispersed. Performing the dispersion using the pre-prepared CNT-containing liquid in this way allows for more efficient coating of the surface of the Si-based anode active material with carbon nanotubes than when carbon nanotubes and the Si-based anode active material are added to the dispersion medium and then dispersed.

[0049] As the dispersion medium, water is preferable from the standpoint of ease of recovery during drying and environmental compatibility, but organic solvents may also be used. The water is not particularly limited to ultrapure water, pure water, or industrial water, but it is common to use Japanese Industrial Standard A1 class water when selecting a grade that is not restricted by processing costs or usage volume. The carbon nanotubes and dispersion materials are as described above.

[0050] In the preliminary step, it is preferable to mix the carbon nanotubes, dispersant, and dispersion medium using, for example, an in-line mixer. For example, the magicLAB manufactured by IKA Corporation can be used as the in-line mixer.

[0051] (Dispersion process) The carbon nanotubes are dispersed in a mixture of the CNT-containing liquid obtained in the preliminary step and the Si-based anode active material, thereby coating the Si-based anode active material. If the preliminary step is not available, the carbon nanotubes are dispersed in a mixture of the dispersion medium, the dispersion agent, and the Si-based anode active material.

[0052] In the dispersion process, the dispersion treatment is preferably selected from at least one of the following: dispersion by shear stirring, dispersion by bead milling, and dispersion by ultrasound. Among these treatments, dispersion by shear stirring is preferred because it can loosen bundles of multiple carbon nanotubes that are intertwined, thereby enabling high dispersion of carbon nanotubes. This allows for efficient coating of the surface of the Si-based anode active material with carbon nanotubes. In order to efficiently loosen the bundles of carbon nanotubes, it is preferable to shear stir the mixture with a shear force (1 / s) of 100,000 (1 / s) or more, calculated from the flow velocity and clearance of the paste. It is preferable to use a high-pressure homogenizer for shear stirring. That is, it is preferable to pass the mixture through a high-pressure homogenizer and shear stir the mixture. In a high-pressure homogenizer, the average bundle diameter of the carbon nanotubes can be adjusted by the number of times the mixture is passed through, and consequently, the dispersibility of the carbon nanotubes and the coating rate of carbon nanotubes on the Si-based anode active material can be adjusted. To improve the dispersibility of carbon nanotubes and increase the coating rate of carbon nanotubes on the Si-based negative electrode active material, it is preferable that the number of times the mixed solution is passed through the high-pressure homogenizer (number of passes) is between 1 and 50.

[0053] In this way, a CNT-Si dispersion paste is obtained in which a dispersant and a Si-based anode active material coated with carbon nanotubes are dispersed in a dispersion medium. In this embodiment, the adhesion between the Si-based anode active material and the carbon nanotubes is lower compared to the conventional dry method. Therefore, by the kneading process performed in the method for manufacturing anodes for lithium-ion secondary batteries described later, some of the carbon nanotubes attached to the surface of the Si-based anode active material can be peeled off and attached to the surface of the carbon-based anode active material.

[0054] [Manufacturing method for negative electrodes in lithium-ion secondary batteries] A method for manufacturing a negative electrode for a lithium-ion secondary battery comprises a negative electrode mixture 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 kneaded together to prepare a negative electrode mixture paste, and a coating step, in which the negative electrode mixture paste is applied to a negative electrode current collector. Furthermore, it is preferable that the method for manufacturing a negative electrode for a lithium-ion secondary battery includes a pre-kneading step before the negative electrode mixture paste preparation step, in which a carbon-based negative electrode active material, a binder, and a dispersion medium are kneaded together to prepare a carbon-based negative electrode active material paste.

[0055] (Preliminary mixing process) Before the negative electrode composite paste preparation step, a carbon-based negative electrode active material, binder, and dispersion medium are kneaded to prepare a carbon-based negative electrode active material paste. In the subsequent negative electrode composite paste preparation step, the pre-prepared carbon-based negative electrode active material paste and CNT-Si paste are kneaded together. Using a pre-prepared carbon-based negative electrode active material paste in this way improves the dispersibility of the carbon-based negative electrode active material and Si-based negative electrode active material compared to adding carbon-based negative electrode active material powder to the CNT-Si paste and kneading it together.

[0056] As mentioned above, water is preferred as the dispersion medium. The dispersion medium may be added in multiple stages during the preliminary mixing process. The binder is as described above.

[0057] The mixing in the preliminary mixing process and the negative electrode mixture paste preparation process described later can be carried out using known mixers. For example, batch mixers such as Banbury mixers or pressure kneaders in which two rotor blades rotate inside a container, or twin-screw planetary mixers in which two blades perform orbital and rotational motions simultaneously can be used. Alternatively, continuous screw mixers such as single-screw or twin-screw extruders, spiral mixers using rotors with pins, or film mixers that use centrifugal force to trap the slurry in a high-speed swirling thin film for mixing may also be used.

[0058] (Negative electrode composite paste preparation process) The carbon-based anode active material paste obtained in the preliminary mixing step and the CNT-Si paste obtained by the CNT-Si paste manufacturing method are mixed in a kneader to prepare the anode composite paste. Preferably, the carbon-based anode active material paste obtained in the preliminary mixing step, the CNT-Si paste obtained by the CNT-Si paste manufacturing method, and an emulsion binder are mixed to prepare the anode composite paste. If there is no preliminary mixing step, the carbon-based anode composite, CNT-Si paste, an arbitrary binder, and an arbitrary dispersion medium are mixed in a kneader to prepare the anode composite paste.

[0059] During mixing, some of the carbon nanotubes attached to the surface of the Si-based anode active material peel off and adhere to the surface of the carbon-based anode active material. Since no large shear force is applied during paste mixing, the amount of carbon nanotubes that migrate from the Si-based anode active material to the carbon-based anode active material is limited. Depending on the mixing conditions such as mixing time and mixing temperature, it is preferable to mix the materials so that, when the carbon nanotube coverage rate on the surface of the Si-based anode active material is set to 100, the carbon nanotube coverage rate on the surface of the carbon-based anode active material is between 20 and 50.

[0060] By fabricating a negative electrode using such a negative electrode composite paste, conductivity between the Si-based negative electrode active material and the carbon-based negative electrode active material can be ensured even with a low amount of carbon nanotube addition. This allows for suppression of the deterioration of charge-discharge cycle characteristics while reducing the amount of carbon nanotube added. In the case of Si-based negative electrode active material coated with carbon nanotubes obtained by the conventional dry method, very few carbon nanotubes migrate to the carbon-based negative electrode active material during kneading. Therefore, unless carbon nanotubes are added during kneading, conductivity between the Si-based negative electrode active material and the carbon-based negative electrode active material cannot be ensured. Furthermore, when carbon nanotubes (or carbon nanotube-containing paste), Si-based negative electrode active material, and carbon-based negative electrode active material are kneaded together, both the Si-based and carbon-based negative electrode active materials are uniformly coated with carbon nanotubes. Therefore, a large amount of carbon nanotubes must be added to ensure conductivity between the Si-based and carbon-based negative electrode active materials.

[0061] As an emulsion binder, in addition to SBR, for example, polyacrylic acid, polyvinylpyrrolidone, polyvinyl alcohol, etc., can be used.

[0062] (Coating process) The negative electrode composite paste obtained in the negative electrode composite paste preparation process is applied to 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, reverse roll coater, lip coater, blade coater, knife coater, gravure coater, and dip coater. The negative electrode composite paste applied to the negative electrode current collector may be dried by near-natural drying, but considering productivity, it is desirable to dry it at a temperature of 100°C to 200°C for 10 minutes to 1 hour. The coating obtained by drying the negative electrode composite paste may be rolled. Rolling is performed, for example, several times at a predetermined linear pressure using a roll press machine until a predetermined thickness is reached.

[0063] Through this series of processes, a negative electrode for a lithium-ion secondary battery is obtained, in which a negative electrode composite layer is formed on a negative electrode current collector. The obtained negative electrode for a lithium-ion secondary battery may be cut and processed to a predetermined size according to the battery size. Then, the lithium-ion secondary battery described above can be manufactured using the negative electrode for the lithium-ion secondary battery. [Examples]

[0064] The present disclosure will be further illustrated by the following examples, but the present disclosure is not limited to these examples.

[0065] <Example 1> [Preparation of CNT-Si paste] A carbon nanotube with a particle size distribution diameter (D50) of 700 μm measured by laser diffraction method (Microtrac MT3000), carboxymethyl cellulose (CMC) as a dispersant, and water as a dispersion medium were mixed at a mass ratio of 99:0.6:0.4 for 5 minutes using an in-line mixer (IKA magicLAB) to prepare a CNT-containing solution. The particle size distribution diameter (D50) of the carbon nanotubes in the obtained CNT-containing solution measured by laser diffraction method (Microtrac MT3000) was 145 μm.

[0066] Next, Li 2z SiO 2+z An Si-based negative electrode active material in which fine particles of Si are dispersed in a lithium silicate phase represented by Li

[0067] [Fabrication of negative electrode] Using a kneader (manufactured by Primix Corporation, High Bismix 2P-1 type), graphite, water as a dispersion medium, and CMC as a binder were kneaded for 120 minutes at a mass ratio of 100:100:0.98 to prepare a graphite paste. Then, with respect to 100 of the graphite paste, the above CNT-Si paste and SBR as an emulsion binder were added so that the mass ratio was 10:1.3, and then kneaded for 5 minutes to prepare a negative electrode slurry.

[0068] A negative electrode slurry was coated onto both sides of a negative electrode core made of copper foil. After drying the coating, the material was rolled using a rolling mill and cut to a predetermined electrode size to produce the negative electrode. The carbon nanotube content was 0.004% by mass relative to the total amount of the negative electrode composite layer. Furthermore, when the carbon nanotube coverage rate of the Si-based negative electrode active material was set to 100, the carbon nanotube coverage rate of the graphite was 20. The average bundle diameter of the carbon nanotubes was 0.9 μm.

[0069] [Fabrication of the positive electrode] A lithium transition metal composite oxide of the NCA (Ni-Al-Co) type containing 88% by mass of Ni was used as the positive electrode active material. The positive electrode active material, carbon nanotubes, and polyvinylidene fluoride (PVdF) were mixed in a mass ratio of 100:0.4:0.8, and then an appropriate amount of NMP was added to prepare a positive electrode composite paste. Next, the positive electrode composite paste was coated onto both sides of a positive electrode core made of aluminum foil, the coating was dried, and then rolled using a rolling mill, cut to a predetermined electrode size, and a positive electrode was manufactured.

[0070] [Preparation of non-aqueous electrolytes] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 3:3:4. Lithium hexafluoride phosphate (LiPF6) was dissolved in this mixed solvent at a concentration of 1.2 mol / liter to prepare a non-aqueous electrolyte.

[0071] [Preparation of test cells] A flat, wound electrode body was fabricated by spirally winding the positive and negative electrodes via a polyolefin separator, followed by radial press molding. This electrode body was then housed in an outer casing made of aluminum laminate sheet, the non-aqueous electrolyte was injected, and the opening of the casing was sealed to obtain a test cell.

[0072] [Cycle Test] The test cell was charged at a constant current of 0.5C under a temperature of 25°C until the battery voltage reached 4.2V. Then, it was charged at a constant voltage of 0.05C until the current value was 0.05C at 4.2V, and finally discharged at a constant current of 0.7C until the battery voltage reached 2.5V. This constituted one cycle. A 10-minute break was taken between each cycle, and this was repeated 200 times. The discharge capacity after the first cycle and the discharge capacity after the 200th cycle were determined, and the capacity retention rate was calculated using the following formula. Capacity retention rate (%) = (Discharge capacity at 200 cycles ÷ Discharge capacity at 1 cycle) × 100

[0073] <Example 2> The test cell was prepared in the same manner as in Example 1, except that the number of passes in the dispersion process was set to 20, and the above cycle test was performed. When the coverage rate of the Si-based negative electrode active material carbon nanotubes was set to 100, the coverage rate of the graphite carbon nanotubes was 20. The average bundle diameter of the carbon nanotubes was 0.6 μm.

[0074] <Example 3> The test cell was prepared in the same manner as in Example 1, except that the number of passes in the dispersion process was set to 50, and the above cycle test was performed. When the coverage rate of the Si-based negative electrode active material carbon nanotubes was set to 100, the coverage rate of the graphite carbon nanotubes was 20. The average bundle diameter of the carbon nanotubes was 0.1 μm.

[0075] <Example 4> In preparing the CNT-containing solution, a test cell was prepared in the same manner as in Example 1, except that the mixing time using an in-line mixer was set to 20 minutes, and the above cycle test was performed. The average diameter of the particle size distribution of carbon nanotubes in the CNT-containing solution, as measured by laser diffraction, was 100 μm. When the coverage rate of the Si-based negative electrode active material carbon nanotubes was set to 100, the coverage rate of the graphite carbon nanotubes was 20. The average bundle diameter of the carbon nanotubes was 0.7 μm.

[0076] <Example 5> In preparing the CNT-containing solution, a test cell was prepared in the same manner as in Example 1, except that the mixing time using an in-line mixer was set to 60 minutes, and the above cycle test was performed. The average diameter of the particle size distribution of carbon nanotubes in the CNT-containing solution, as measured by laser diffraction, was 70 μm. Furthermore, when the coverage rate of the Si-based negative electrode active material carbon nanotubes was set to 100, the coverage rate of the graphite carbon nanotubes was 20. The average bundle diameter of the carbon nanotubes was 0.4 μm.

[0077] <Example 6> The test cell was prepared in the same manner as in Example 1, except that the mixing time for preparing the graphite paste was set to 30 minutes, and the amount of carbon nanotubes added was changed so that the carbon nanotube content was 0.008% by mass relative to the total amount of the negative electrode composite layer. The above cycle test was then 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.

[0078] <Example 7> The test cell was prepared in the same manner as in Example 1, except that the mixing time for preparing the graphite paste was set to 60 minutes, and the amount of carbon nanotubes added was changed so that the carbon nanotube content was 0.01% by mass relative to the total amount of the negative electrode composite layer. The above cycle test was then 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.

[0079] <Comparative Example 1> [Preparation of CNT paste] A CNT-containing solution was prepared by mixing carbon nanotubes with a particle size distribution diameter of 700 μm, measured by laser diffraction, carboxymethylcellulose (CMC) as a dispersant, and water as a dispersion medium in a mass ratio of 99:0.6:0.4 using an in-line mixer (IKA magicLAB) for 5 minutes. The particle size distribution diameter of the carbon nanotubes in the CNT-containing solution, measured by laser diffraction, was 145 μm.

[0080] Next, the CNT-containing solution was subjected to dispersion treatment by passing it through a valve-type high-pressure homogenizer (Sanmaru Machinery Industry Econizer Lab 02) 20 times at a flow rate of 14 L / h and a pressure of 80 Pa (number of passes: 20) to prepare a CNT paste.

[0081] [Fabrication of the negative electrode] Using a kneader (Primix, Hibismix 2P-1 model), CNT paste, graphite, Si-based anode active material, water as a dispersion medium, and CMC as a binder were kneaded for 90 minutes in a mass ratio of 10:100:10:100:0.97 to prepare a paste. Then, SBR as an emulsion binder was added to 100 parts of the paste in a mass ratio of 2.5, and the mixture was kneaded for 10 minutes to prepare an anode slurry.

[0082] A negative electrode slurry was coated onto both sides of a negative electrode core made of copper foil. After drying the coating, the material was rolled using a rolling mill and cut to a predetermined electrode size to produce the negative electrode. The carbon nanotube content was 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 was set to 100%, the carbon nanotube coverage of the graphite was also 100%. The average bundle diameter of the carbon nanotubes was 0.05 μm.

[0083] Then, a test cell was prepared in the same manner as in Example 1, except that the above-mentioned negative electrode was used, and the above-mentioned cycle test was performed.

[0084] <Comparative Example 2> A solution was prepared by adding carbon nanotubes and Si-based anode active material to water in a mass ratio of 0.2:100. Media balls were then added to the solution, mixed, and dried. The resulting powder was pulverized to obtain Si-CNT powder, in which carbon nanotubes were coated on the surface of the Si-based anode active material.

[0085] Using a kneader (Primix, Hibismix 2P-1 model), Si-CNT powder, graphite, water as a dispersion medium, and CMC as a binder were kneaded for 120 minutes in a mass ratio of 10:100:100:0.97 to prepare a paste. Then, SBR as an emulsion binder was added to 100 parts of the paste in a mass ratio of 1.3, and the mixture was kneaded for 10 minutes to prepare a negative electrode slurry.

[0086] A negative electrode slurry was coated onto both sides of a negative electrode core made of copper foil. After drying the coating, the material was rolled using a rolling mill and cut to a predetermined electrode size to produce the negative electrode. The carbon nanotube content was 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 was set to 100%, the carbon nanotube coverage of the graphite was 0. The average carbon nanotube bundle diameter was 0.02 μm.

[0087] Then, a test cell was prepared in the same manner as in Example 1, except that the above-mentioned negative electrode was used, and the above-mentioned cycle test was performed.

[0088] <Comparative Example 3> Using a hybridization dry mixer, carbon nanotubes and a Si-based anode active material were mixed in a mass ratio of 0.2:100 for 120 minutes to obtain Si-CNT powder in which carbon nanotubes were coated on the surface of the Si-based anode active material.

[0089] Using a kneader (Primix, Hibismix 2P-1 model), Si-CNT powder, graphite, water as a dispersion medium, and CMC as a binder were kneaded for 120 minutes in a mass ratio of 10:100:100:0.97 to prepare a paste. Then, SBR as an emulsion binder was added to 100 parts of the paste in a mass ratio of 1.3, and the mixture was kneaded for 10 minutes to prepare a negative electrode slurry.

[0090] A negative electrode slurry was coated onto both sides of a negative electrode core made of copper foil. After drying the coating, the material was rolled using a rolling mill and cut to a predetermined electrode size to produce the negative electrode. The carbon nanotube content was 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 was set to 100%, the carbon nanotube coverage of the graphite was 0. The average carbon nanotube bundle diameter was 0.02.

[0091] Then, a test cell was prepared in the same manner as in Example 1, except that the above-mentioned negative electrode was used, and the above-mentioned cycle test was performed.

[0092] <Comparative Example 4> A test cell was prepared in the same manner as in Comparative Example 1, except that the carbon nanotube content was set to 0.004% by mass relative to the total amount of the negative electrode composite layer, and the above cycle test was performed.

[0093] Table 1 shows the evaluation results of the volume retention rate in the examples and comparative examples. The volume retention rates for Examples 2-7 and Comparative Examples 1-4 are shown as relative values ​​with the volume retention rate of Example 1 set to 100.

[0094] [Table 1]

[0095] In all of the test cells prepared using the methods of Examples 1 to 7, the amount of carbon nanotubes added was lower compared to the test cells prepared using the conventional method in Comparative Examples 2 and 3, and the deterioration of the battery's charge-discharge cycle characteristics was also suppressed. Comparative Examples 1 and 4 were prepared using the same method, but when the amount of carbon nanotubes added was increased as in Comparative Example 1, the results were similar to those of Example 1. However, when the amount of carbon nanotubes added was the same as in Example 1, as in Comparative Example 4, the effect of suppressing the deterioration of charge-discharge cycle characteristics was inferior to that of the Examples. [Explanation of symbols]

[0096] 10 Lithium-ion secondary battery, 11 Positive electrode, 12 Negative electrode, 12a Negative electrode current collector, 12b Negative electrode composite layer, 13 Separator, 14 Electrode body, 15 Battery case, 16 Case body, 17 Sealing body, 18,19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Protruding part, 23 Filter, 24 Lower valve body, 25 Insulating material, 26 Upper valve body, 27 Cap, 28 Gasket.

Claims

1. The negative electrode composite layer contains a carbon-based negative electrode active material, a Si-based negative electrode active material, and carbon nanotubes. A negative electrode for a lithium-ion secondary battery, wherein, 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 20 or more and 50 or less.

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

3. The carbon nanotube content is 0.004% by mass or more and 0.01% by mass or less with respect to the total amount of the negative electrode composite layer, as described in claim 1 or 2.

4. A lithium-ion secondary battery comprising a negative electrode for a lithium-ion secondary battery according to any one of claims 1 to 3.

5. A dispersion step comprising: dispersing carbon nanotubes, Si-based anode active material, a dispersant, and a dispersion medium in a mixed solution that does not contain carbon-based anode active material, thereby coating the Si-based anode active material with carbon nanotubes; A negative electrode composite paste preparation step involves kneading the carbon-based negative electrode active material and the CNT-Si paste obtained by the dispersion step to prepare a negative electrode composite paste. The process includes a coating step of applying the negative electrode composite paste to the negative electrode current collector, A method for manufacturing a negative electrode for a lithium-ion secondary battery, comprising the negative electrode composite paste preparation step, in which the carbon-based negative electrode active material and the CNT-Si paste are kneaded together such that, when the coating rate of the carbon nanotubes on the surface of the Si-based negative electrode active material is set to 100, the coating rate of the carbon nanotubes on the surface of the carbon-based negative electrode active material is 20 or more and 50 or less.

6. Prior to the dispersion step, there is a preliminary step of mixing the carbon nanotubes, the dispersion material, and the dispersion medium to obtain a CNT-containing liquid. The method for manufacturing a negative electrode for a lithium-ion secondary battery according to claim 5, wherein the dispersion step involves performing a dispersion treatment on the mixed liquid containing the CNT-containing liquid and the Si-based negative electrode active material.

7. The method for manufacturing a negative electrode for a lithium-ion secondary battery according to claim 5 or 6, wherein the dispersion treatment includes at least one of the following: dispersion treatment by shear stirring, dispersion treatment by a bead mill, and dispersion treatment by ultrasonic treatment.

8. The method for manufacturing a negative electrode for a lithium-ion secondary battery according to claim 7, wherein the dispersion treatment is a dispersion treatment by shear stirring, and the dispersion treatment by shear stirring is performed by shear stirring the mixture at a shear rate of 100,000 (1 / s) or more.

9. The method for manufacturing a negative electrode for a lithium-ion secondary battery according to claim 7 or 8, wherein the dispersion treatment by shear stirring is performed by passing the mixture through a high-pressure homogenizer.

10. The method for manufacturing a negative electrode for a lithium-ion secondary battery according to any one of claims 7 to 9, wherein the number of times the mixture is passed through a high-pressure homogenizer (number of passes) is 1 or more and 50 or less.

11. Prior to the negative electrode composite paste preparation step, there is a preliminary kneading step in which the carbon-based negative electrode active material, binder, and dispersion medium are kneaded together to prepare the carbon-based negative electrode active material paste. The method for manufacturing a negative electrode for a lithium-ion secondary battery according to claim 10, wherein the negative electrode composite paste preparation step involves kneading the carbon-based negative electrode active material paste and the CNT-Si paste.

12. The method for manufacturing a negative electrode for a lithium-ion secondary battery according to claim 11, wherein in the negative electrode composite paste preparation step, the carbon-based negative electrode active material paste, the CNT-Si paste, and the emulsion binder are kneaded together.

13. A method for manufacturing a lithium-ion secondary battery, comprising manufacturing a lithium-ion secondary battery using a negative electrode for a lithium-ion secondary battery obtained by the method described in any one of claims 5 to 12.

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