Negative electrode material for lithium ion secondary battery and lithium ion secondary battery

Abstract

To provide a negative electrode material for a lithium ion secondary battery, which enables the production of a lithium ion secondary battery having a large battery capacity and excellent cycle characteristics.SOLUTION: A negative electrode material for a lithium ion secondary battery includes (A1) Si-based alloy particles containing 60.0 to 70.0 mass% of Si, 25.0 to 35.0 mass% of Sn, and 3.0 to 7.0 mass% of Cu, (A2) graphite particles; (B) a conductive additive, and (C) a binder.SELECTED DRAWING: None

Description

[Technical Field]

[0001] Embodiments of the present invention relate to a negative electrode material for a lithium ion secondary battery, a negative electrode material layer, a negative electrode, a lithium ion secondary battery, and a method for producing Si-based alloy particles. [Background technology]

[0002] Lithium-ion secondary batteries are widely used as power sources for electronic devices such as electric vehicles, mobile phones, smartphones, and laptop computers. With the spread of electric vehicles and the increasing sophistication of electronic devices, there are high expectations for improvements in the battery characteristics of lithium-ion secondary batteries.

[0003] The use of silicon alloys as negative electrode active materials for lithium ion secondary batteries has been investigated. For example, Patent Document 1 describes a negative electrode material for secondary batteries comprising alloy particles containing a transition metal and silicon. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-225143 Summary of the Invention [Problem to be solved by the invention]

[0005] Using silicon alloys as negative electrode active materials is expected to improve battery capacity, but the large volume change that silicon alloys undergo during charge and discharge leaves the cycle characteristics insufficient, and improvements are needed.

[0006] Therefore, an object of one embodiment of the present invention is to provide a negative electrode material for a lithium ion secondary battery, a negative electrode material layer, and a negative electrode that can provide a lithium ion secondary battery having a large battery capacity and excellent cycle characteristics.An object of another embodiment of the present invention is to provide a lithium ion secondary battery having a large battery capacity and excellent cycle characteristics.An object of yet another embodiment of the present invention is to provide a method for producing Si-based alloy particles for use in the negative electrode material for a lithium ion secondary battery. [Means for solving the problem]

[0007] The present invention includes the following embodiments, but is not limited to the following. (1) A negative electrode material for a lithium ion secondary battery, comprising (A1) Si-based alloy particles containing 60.0 to 70.0 mass% of Si, 25.0 to 35.0 mass% of Sn, and 3.0 to 7.0 mass% of Cu, (A2) graphite particles, (B) a conductive additive, and (C) a binder. (2) The negative electrode material for a lithium ion secondary battery according to (1) above, further containing (D) a solvent. (3) The negative electrode material for lithium ion secondary batteries according to (1) or (2), wherein the content of (A1) Si-based alloy particles is 5 to 15 mass%, the content of (A2) graphite particles is 75 to 85 mass%, the content of (B) conductive additive is 3 to 10 mass%, and the content of (C) binder is 3 to 10 mass%, based on the total mass of the solid contents of the negative electrode material for lithium ion secondary batteries. (4) The negative electrode material for a lithium ion secondary battery according to any one of (1) to (3) above, wherein the (A2) graphite particles include artificial graphite particles. (5) The negative electrode material for a lithium ion secondary battery according to any one of (1) to (4) above, wherein the (B) conductive additive contains carbon black. (6) The negative electrode material for a lithium ion secondary battery according to any one of (1) to (5) above, wherein the binder (C) contains a (meth)acrylic polymer. (7) A negative electrode material layer formed using the negative electrode material according to any one of (1) to (6) above. (8) A negative electrode having the negative electrode material layer according to (7) above and a current collector. (9) A lithium ion secondary battery having the negative electrode according to (8). (10) A method for producing (A1) Si-based alloy particles used in the negative electrode material for a lithium ion secondary battery according to any one of (1) to (6), the method comprising using Si, Sn, and Cu as raw materials and obtaining Si-based alloy particles by a gas atomization method. [Effects of the Invention]

[0008] According to an embodiment of the present invention, it is possible to provide a negative electrode material for a lithium ion secondary battery, a negative electrode material layer, and a negative electrode, which enable a lithium ion secondary battery having a large battery capacity and excellent cycle characteristics to be obtained. According to another embodiment of the present invention, it is possible to provide a lithium ion secondary battery having a large battery capacity and excellent cycle characteristics. Furthermore, according to yet another embodiment of the present invention, it is possible to provide a method for producing Si-based alloy particles for use in the negative electrode material for a lithium ion secondary battery. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present invention will be described in detail. Note that the present invention is not limited to the following embodiments, and the present invention also includes modifications of the following embodiments within the scope of the present invention.

[0010] [Anode materials for lithium-ion secondary batteries] The negative electrode material for lithium-ion secondary batteries (hereinafter sometimes referred to as "negative electrode material") contains (A) a negative electrode active material containing (A1) Si-based alloy particles and (A2) graphite particles, (B) a conductive additive, and (C) a binder. The (A1) Si-based alloy particles contain 60.0 to 70.0 mass% Si, 25.0 to 35.0 mass% Sn, and 3.0 to 7.0 mass% Cu.

[0011] (A) Negative electrode active material The negative electrode material contains (A) a negative electrode active material, which contains (A1) Si-based alloy particles and (A2) graphite particles.

[0012] (A1)Si-based alloy particles The (A1) Si-based alloy particles contain 60.0 to 70.0 mass% Si, 25.0 to 35.0 mass% Sn, and 3.0 to 7.0 mass% Cu, based on the mass of the (A1) Si-based alloy particles. When the contents of Si, Sn, and Cu are within the above ranges, a large battery capacity and good cycle characteristics are likely to be obtained. When the Si content is 60.0 mass% or more, a high electric capacity is obtained. The Si content may be 62.0 to 68.0 mass%, 63.0 to 67.0 mass%, or 64.0 to 66.0 mass%. When Sn is contained, volume expansion of the negative electrode can be prevented, and a decrease in electric capacity due to absorption and desorption of lithium ions by Sn can be suppressed. When Sn is contained, Si, Cu, and Sn can be alloyed by high-frequency melting. The Sn content may be 27.0 to 33.0 mass%, 28.0 to 32.0 mass%, or 29.0 to 31.0 mass%, and the Cu content may be 3.5 to 6.5 mass%, 4.0 to 6.0 mass%, or 4.5 to 5.5 mass%.

[0013] The (A1) Si-based alloy particles may contain inevitable impurities. The (A1) Si-based alloy particles may be particles consisting of, for example, 60.0 to 70.0 mass % Si, 25.0 to 35.0 mass % Sn, 3.0 to 7.0 mass % Cu, and inevitable impurities.

[0014] The (A1) Si-based alloy particles are preferably spherical in shape. A spherical particle shape can increase the packing density in the negative electrode material layer. From the viewpoint of improving life characteristics, the (A1) Si-based alloy particles preferably have an average particle diameter of 0.1 to 3.0 μm, more preferably 0.3 to 2.0 μm, and even more preferably 0.5 to 1.5 μm. An average particle diameter of 3.0 μm or less tends to increase the battery capacity and improve the cycle characteristics. The average particle diameter of the (A1) Si-based alloy particles is the median diameter (D50) in the volume-based particle size distribution, and is a value measured by laser diffraction using a powder of the (A1) Si-based alloy particles. A laser diffraction particle size distribution analyzer (for example, manufactured by CILAS) can be used for the measurement.

[0015] (A1) Si-based alloy particles can be obtained using a production method including gas atomization, water atomization, disk atomization, strip casting, liquid quenching methods such as roll quenching, arc melting, sputtering, chemical vapor deposition, and sintering. Gas atomization and water atomization can produce spherical particles, and depending on the conditions, can easily produce particles having a desired particle size. Gas atomization can prevent oxidation of the resulting spherical particles.

[0016] The method for producing (A1) Si-based alloy particles includes, for example, using Si, Sn, and Cu as raw materials to obtain Si-based alloy particles by a gas atomization method. The method for producing (A1) Si-based alloy particles may further include, as necessary, steps such as pulverizing the Si-based alloy particles obtained by the gas atomization method, crushing the pulverized Si-based alloy particles, and classifying the Si-based alloy particles.

[0017] In the gas atomization method, raw materials containing 60.0 to 70.0 mass% Si, 25.0 to 35.0 mass% Sn, and 3.0 to 7.0 mass% Cu are melted. The raw materials may contain unavoidable impurities. The melting may be high-frequency melting. In the gas atomization method, the molten raw materials (molten metal) are cooled by gas atomization to obtain Si-based alloy particles (sometimes referred to as "gas atomized particles" in this specification). The gas to be atomized may be an inert gas such as argon, helium, or nitrogen. If the pressure of the gas sprayed onto the molten metal is high, the particle size of the gas atomized particles tends to become small. From this perspective, the gas pressure is preferably 3.0 MPa or more. The upper limit of the gas pressure is not particularly limited, but is, for example, 10.0 MPa or less. The gas temperature is, for example, 0 to 100°C, and may be 20 to 30°C. A faster cooling rate is preferable.

[0018] In the method for producing (A1) Si-based alloy particles, the obtained gas atomized particles may be pulverized. Pulverization can reduce the particle size and homogenize the alloy. Examples of pulverization methods include dry pulverization and wet pulverization, with dry pulverization being preferred. Pulverization devices that can be used for dry pulverization include a vibration mill, a ball mill, a pin mill, an attritor, and the like, with a vibration mill being preferred. The Si-based alloy particles obtained after pulverization (sometimes referred to as "pulverized particles" in this specification) may be pulverized. When the powder of pulverized particles contains agglomerated pulverized particles, the agglomerated pulverized particles can be dispersed by pulverization. For example, a jet mill can be used for pulverization. It is preferable to adjust the pulverization and pulverization conditions so as to obtain (A1) Si-based alloy particles having an average particle size of 0.1 to 3.0 μm.

[0019] The content of (A1) Si-based alloy particles in the negative electrode material is, for example, 5 to 15 mass%, 7 to 11 mass%, or 8 to 10 mass% relative to the total mass of the solid content of the negative electrode material. When the content of (A1) Si-based alloy particles is 5 mass% or more, the effect of improving discharge capacity is likely to be obtained. When the content of (A1) Si-based alloy particles is 15 mass% or less, good cycle characteristics are likely to be obtained.

[0020] (A2) Graphite particles The (A2) graphite particles may be either or both of natural graphite particles and artificial graphite particles. Artificial graphite particles are preferred. The (A2) graphite particles may have a block, scale, flake, spherical, rod, needle, or fibrous shape.

[0021] The content of (A2) graphite particles in the negative electrode material is, for example, 75 to 85 mass%, 79 to 83 mass%, or 80 to 82 mass% relative to the total mass of the solid content of the negative electrode material. When the content of (A2) graphite particles is 75 mass% or more, good cycle characteristics are likely to be obtained. When the content of (A2) graphite particles is 85 mass% or less, the content of (A1) Si-based alloy particles can be made sufficient, and the effect of improving discharge capacity is likely to be obtained.

[0022] The (A) negative electrode active material may further contain any negative electrode active material, such as a metal capable of alloying with lithium, such as Si, Sn, Pb, Al, Zn, Bi, In, Mg, or Ga; or a metal oxide.

[0023] The content of the (A) negative electrode active material in the negative electrode material is, for example, 80 to 99 mass%, 86 to 94 mass%, or 88 to 92 mass%, based on the total mass of the solid contents of the negative electrode material. The mass ratio of the (A1) Si-based alloy particles to the (A2) graphite particles in the (A) negative electrode active material is, for example, 5 to 15:85 to 95, 8 to 12:88 to 92, or 9 to 11:89 to 91.

[0024] (B) Conductive additive Examples of the (B) conductive additive include carbon black such as acetylene black, furnace black, ketjen black, channel black, lamp black, thermal black, and hollow carbon black; fibrous carbon materials such as carbon nanotubes and carbon nanofibers; metal particles such as Cu, Ni, Al, Au, and Ag; and conductive metal oxides such as ITO and titanium oxide. The (B) conductive additive may be used alone or in combination of two or more. The (B) conductive additive preferably contains carbon black, and more preferably contains acetylene black.

[0025] The content of the (B) conductive additive in the negative electrode material is, for example, 3 to 10 mass%, 4 to 8 mass%, or 4 to 6 mass% relative to the total mass of the solid content of the negative electrode material. When the content of the (B) conductive additive is 3 mass% or more, the resistance value can be kept low.

[0026] (C) Binder Examples of the (C) binder include polyvinylidene fluoride (PVDF), polyvinylidene chloride, polybenzimidazole (PBI), polyethersulfone (PES), polyimide, polyamide, polyamideimide, polyvinyl acetate, polyacrylonitrile, polyvinyl alcohol, carboxymethyl cellulose (CMC), polyvinylpyrrolidone, polytetrafluoroethylene (PTFE), polyethylene, polypropylene, polystyrene, (meth)acrylic polymer, polyacrylamide, acrylonitrile-butadiene-styrene polymer, styrene-butadiene polymer, phenolic resin, epoxy resin, polyethylene terephthalate, polyphenyl sulfide, polyetherimide, polyacetal, polyphenylene oxide, and polybutylene terephthalate. The (C) binder can be used alone or in combination of two or more. It is preferable that the (C) binder contains a (meth)acrylic polymer.

[0027] The content of the binder (C) in the negative electrode material is, for example, 3 to 10 mass%, 4 to 8 mass%, or 4 to 6 mass% relative to the total mass of the solid content of the negative electrode material. When the content of the binder (C) is 3 mass% or more, good cycle characteristics are likely to be obtained.

[0028] (D) Solvent The negative electrode material may further contain a solvent. The negative electrode material containing a solvent may be in the form of a slurry. Examples of the (D) solvent include water, nitrogen-containing solvents such as N-methyl-2-pyrrolidone and N,N-dimethylacetamide, aromatic solvents such as toluene and xylene, alcohol solvents such as methanol, ethanol, n-propanol, isopropyl alcohol and isobutyl alcohol, ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone, ester solvents such as ethyl acetate and butyl acetate, and ether solvents such as tetrahydrofuran and dioxane. The (D) solvent may be used alone or in combination of two or more.

[0029] The content of the (D) solvent in the negative electrode material is, for example, 10 to 90 mass %, 20 to 80 mass %, or 30 to 70 mass % relative to the total mass of the solid content of the negative electrode material and the (D) solvent. When the content of the (D) solvent is within this range, it is easy to form a negative electrode material layer.

[0030] (E) Optional Ingredients The negative electrode material may contain optional components such as a thickener, a dispersant, and a surfactant.

[0031] The method for producing the negative electrode material is not particularly limited. The negative electrode material can be produced by mixing (A1) Si-based alloy particles, (A2) graphite particles, (B) conductive additive, and (C) binder, usually with (D) solvent, and further with (E) optional components, which are used as needed. For mixing, a mixer such as a kneader, mixer, roll mill, ball mill, homogenizer, planetary mixer, paint shaker, or sand mill can be used.

[0032] [Negative electrode material layer] The negative electrode material layer is a layer formed using the above-mentioned negative electrode material for lithium ion secondary batteries. The negative electrode material layer can be formed by applying the negative electrode material and drying the applied layer. Examples of application methods include a doctor blade method, a die coating method, and a dipping method. The layer after drying may be compressed to increase its density.

[0033] [Negative electrode] The negative electrode has the above-mentioned negative electrode material layer and a current collector, and is obtained by forming the negative electrode material layer on the current collector using the above-mentioned negative electrode material for lithium ion secondary batteries. The current collector may be a conductive substrate, such as a film, sheet, or nonwoven fabric. The thickness of the current collector is, for example, 1 to 100 μm. Examples of the current collector include metal foils such as aluminum, nickel, stainless steel, iron, and copper, with copper foil being preferred.

[0034] [Lithium-ion secondary battery] The lithium ion secondary battery has the above-mentioned negative electrode and positive electrode, and contains an electrolyte between the negative electrode and the positive electrode. The lithium ion secondary battery may have a battery container for accommodating the negative electrode, the positive electrode, and the electrolyte, a separator, etc. Positive electrode active materials used for the positive electrode include cobalt-based (LiCoO2), nickel-based (LiNiO2), manganese-based (LiMn2O4), NMC-based (LiCo 1-x-y Ni x Mn y O2), NCA type (LiNi x Co 1-x-y Al y O2) and the like. The separator is, for example, a woven or nonwoven fabric, and examples of the material include polytetrafluoroethylene (PTFE), polyethylene, polypropylene, polyester, and glass fiber. The lithium ion secondary battery may have a separator between the negative electrode and the positive electrode. Examples of the shape of the lithium ion secondary battery include a cylindrical shape, a prismatic shape, a sheet shape, and a coin shape.

[0035] The electrolyte may be a non-aqueous electrolyte containing an electrolyte and a non-aqueous solvent. Examples of the electrolyte include LiPF6, LiClO4, LiBF4, LiAsF6, LiCF3SO3, LiC2F5SO3, LiC(SO2CF3)3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(CF3SO2)(C4F9SO2), LiC(CF3SO2)3, LiN(SO2CF3)(SO2C4F9), LiN(COC2F5)2, LiBC4O8, LiAsF6, LiClO4, Li2B 10 Cl 10 , Li2B 12 Cl 12 and mixtures thereof. Examples of non-aqueous solvents include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, fluoroethylene carbonate, difluoroethylene carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, acetonitrile, methyl acetate, ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, γ-butyrolactone, γ-valerolactone, and mixed solvents containing two or more of these. The non-aqueous solvent may be, for example, a mixed solvent containing ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate, or a mixed solvent containing ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and fluoroethylene carbonate. [Example]

[0036] The present invention will be specifically described below using examples and comparative examples, but the scope of the present invention should not be construed as being limited to the following examples.

[0037] [Example 1] (A1) Preparation of Si-based alloy particles Metallic Si, metallic Sn, and metallic Cu were prepared as materials for the Si-based alloy particles, and mixed to a mass ratio of 65.0 mass% Si, 30.0 mass% Sn, and 5.0 mass% Cu. The mixture was then high-frequency melted in a melting furnace to produce a molten metal. A powder of Si-based alloy particles (gas atomized particles) was obtained from the molten metal by gas atomization. Next, the Si-based alloy particles (gas atomized particles) were pulverized using a vibration mill to obtain fine Si-based alloy particles (pulverized particles). After that, the Si-based alloy particles (pulverized particles) were disintegrated using a jet mill to disperse the agglomerates contained in the pulverized particles, thereby obtaining Si-based alloy particles with a D50 of 0.93 μm. The D50 of the Si-based alloy particles was measured using a laser diffraction particle size distribution analyzer (manufactured by CILAS).

[0038] (Production of negative electrode materials for lithium-ion secondary batteries) The above-mentioned Si-based alloy particles and artificial graphite particles as the negative electrode active material, acetylene black (Li-400 manufactured by Denka Co., Ltd.) as a conductive additive, a (meth)acrylic polymer as a binder, and a solvent were mixed so that the solid content was 90 mass % of the negative electrode active material (10.0 mass % of the Si-based alloy particles and 90.0 mass % of the artificial graphite particles in the negative electrode active material), 5.0 mass % of the conductive additive, and 5.0 mass % of the binder, to prepare a negative electrode slurry (negative electrode material for lithium ion secondary batteries).

[0039] (Preparation of negative electrode) A Cu foil was prepared as a current collector, and the negative electrode slurry was applied to one side of the Cu foil using a doctor blade and dried to prepare a negative electrode (negative electrode material layer thickness after drying: 20 μm).

[0040] (Preparation of half-cell for negative electrode evaluation) A coin-shaped half-cell for evaluating the negative electrode was fabricated using the above negative electrode and Li metal as the counter electrode. -3), a mixed solvent (ethylene carbonate (EC):dimethyl carbonate (DMC):ethyl methyl carbonate (EMC) = 1:1:1 (volume ratio)), and an electrolyte solution containing fluoroethylene carbonate (FEC) (5 mass%) was used.

[0041] (evaluation) The negative electrode evaluation half cell was charged to 1.5 V at a current of 0.1 C and discharged to 0.01 V at a current of 0.1 C under a room temperature (25°C) atmosphere. This cycle was repeated 50 times. The initial discharge capacity and cycle characteristics are shown in Table 1. The initial discharge capacity (mAh / g) is the discharge capacity at the first cycle. The cycle characteristics (%) were calculated using the following formula from the discharge capacity at the first cycle and the discharge capacity at the 50th cycle. Cycle characteristics (%) = (discharge capacity at 50th cycle / discharge capacity at 1st cycle) x 100

[0042] [Comparative Example 1] A half cell for evaluating the negative electrode was prepared and evaluated in the same manner as in Example 1, except that the negative electrode active material was changed to artificial graphite particles (100.0 mass %).

[0043] Comparative Example 2 A half cell for evaluating the negative electrode was prepared and evaluated in the same manner as in Example 1, except that the negative electrode active material was changed to artificial graphite particles (10.0 mass %) and SiOx particles (90.0 mass %).

[0044] The evaluation results of the initial discharge capacity and cycle characteristics are shown in Table 1. The negative electrode formed using the negative electrode material containing Si-based alloy particles had a large discharge capacity and showed excellent cycle characteristics, with the decrease in discharge capacity being suppressed even after 50 cycles.

[0045] [Table 1]

Claims

1. A negative electrode material for a lithium ion secondary battery, comprising: (A1) Si-based alloy particles containing 60.0 to 70.0 mass% of Si, 25.0 to 35.0 mass% of Sn, and 3.0 to 7.0 mass% of Cu; (A2) graphite particles; (B) a conductive additive; and (C) a binder.

2. The negative electrode material for a lithium ion secondary battery according to claim 1 , further comprising (D) a solvent.

3. 2. The negative electrode material for lithium ion secondary batteries according to claim 1, wherein, relative to the total mass of the solid contents of the negative electrode material for lithium ion secondary batteries, the content of (A1) Si-based alloy particles is 5 to 15 mass%, the content of (A2) graphite particles is 75 to 85 mass%, the content of (B) conductive additive is 3 to 10 mass%, and the content of (C) binder is 3 to 10 mass%.

4. 2. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the (A2) graphite particles include artificial graphite particles.

5. The negative electrode material for a lithium ion secondary battery according to claim 1 , wherein the conductive additive (B) contains carbon black.

6. The negative electrode material for a lithium ion secondary battery according to claim 1 , wherein the binder (C) contains a (meth)acrylic polymer.

7. A negative electrode material layer formed using the negative electrode material according to any one of claims 1 to 6.

8. A negative electrode comprising the negative electrode material layer according to claim 7 and a current collector.

9. A lithium ion secondary battery comprising the negative electrode according to claim 8.

10. A method for producing (A1) Si-based alloy particles used in the negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 6, comprising using Si, Sn, and Cu as raw materials and obtaining Si-based alloy particles by a gas atomization method.