Anode material for lithium-ion secondary batteries and lithium-ion secondary batteries

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 lithium-ion secondary batteries, 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 rechargeable batteries are widely used as power sources for electronic devices such as electric vehicles, mobile phones, smartphones, and laptop computers. With the increasing popularity of electric vehicles and the advancement of electronic devices, improvements in the battery characteristics of lithium-ion rechargeable batteries are expected.

[0003] The use of silicon alloys as negative electrode active materials for lithium-ion secondary batteries is being considered. 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 Publication No. 2016-225143 [Overview of the project] [Problems that the invention aims to solve]

[0005] Using silicon alloys as the negative electrode active material is expected to improve battery capacity. On the other hand, silicon alloys undergo large volume changes during charging and discharging, resulting in less-than-ideal cycle characteristics, and improvements are desired.

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

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

[0008] According to embodiments of the present invention, it is possible to provide a negative electrode material, a negative electrode layer, and a negative electrode for a lithium-ion secondary battery that can be obtained with a large battery capacity and excellent cycle characteristics. Furthermore, according to another embodiment of the present invention, it is possible to provide a lithium-ion secondary battery with 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 the lithium-ion secondary battery. [Modes for carrying out the invention]

[0009] The embodiments of the present invention will be described in detail below. However, the present invention is not limited to the following embodiments, and variations of the following embodiments are also included, without altering the essence of the invention.

[0010] [Negative electrode material 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 including (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) negative electrode active material, which consists of (A1) Si-based alloy particles and (A2) graphite particles.

[0012] (A1)Si-based alloy particles (A1) The 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 Si, Sn, and Cu content is within the above range, a large battery capacity and good cycle characteristics are easily obtained. When the Si content is 60.0 mass% or higher, a high electrical capacity can be 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%. Including Sn can prevent volume expansion of the negative electrode, and can also suppress the decrease in electrical capacity due to adsorption and desorption of lithium ions by Sn. Including Sn makes it possible to alloy Si, Cu, and Sn by high-frequency melting. The Sn content may be 27.0-33.0% by mass, 28.0-32.0% by mass, or 29.0-31.0% by mass. The Cu content may be 3.5-6.5% by mass, 4.0-6.0% by mass, or 4.5-5.5% by mass.

[0013] (A1) Si alloy particles may contain unavoidable impurities. (A1) Si alloy particles may consist 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 unavoidable impurities.

[0014] (A1) The shape of the Si-based alloy particles is preferably spherical. When the shape of the particles is spherical, the packing density in the negative electrode material layer can be increased. From the viewpoint of improving the life characteristics, the average particle diameter of the (A1) Si-based alloy particles is preferably 0.1 to 3.0 μm, more preferably 0.3 to 2.0 μm, and still more preferably 0.5 to 1.5 μm. When the average particle diameter is 3.0 μm or less, the battery capacity is larger and the cycle characteristics tend to be better. 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 the laser diffraction method using the powder of the (A1) Si-based alloy particles. For the measurement, a laser diffraction type particle size distribution measuring device (for example, manufactured by CILAS) can be used.

[0015] (A1) The Si-based alloy particles can be obtained by using a production method including a gas atomization method, a water atomization method, a disk atomization method, a strip casting method, a liquid quenching method such as a roll quenching method, an arc melting method, a sputtering method, a chemical vapor deposition method, a firing method, etc. According to the gas atomization method or the water atomization method, spherical particles can be obtained, and it is easy to produce particles having a desired particle diameter depending on the conditions. According to the gas atomization method, oxidation of the obtained spherical particles can be suppressed.

[0016] (A1) The production method of the Si-based alloy particles includes, for example, using Si, Sn, and Cu as raw materials and obtaining the Si-based alloy particles by the gas atomization method. The production method of the (A1) Si-based alloy particles may further include steps such as pulverizing the Si-based alloy particles obtained by the gas atomization method, disintegrating the pulverized Si-based alloy particles, classifying the Si-based alloy particles, etc., as necessary.

[0017] In the gas atomization method, a raw material 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 is melted. The raw material may contain inevitable impurities. The melting may be high-frequency melting. In the gas atomization method, the melted raw material (molten metal) is cooled by spraying with a gas to obtain Si-based alloy particles (which may be described as "gas atomized particles" in this specification). The gas to be sprayed may be an inert gas such as argon, helium, or nitrogen. When the pressure of the gas sprayed onto the molten metal is high, the particle size of the gas atomized particles tends to be 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 for example, it is 10.0 MPa or less. The gas temperature is, for example, 0 to 100°C and may be 20 to 30°C. A higher cooling rate is preferred.

[0018] (A1) In the method for producing Si-based alloy particles, the obtained gas atomized particles may be pulverized. Pulverization can refine the particles and homogenize the alloy. Examples of the pulverization method include dry pulverization and wet pulverization, and dry pulverization is preferred. Examples of the pulverization apparatus that can be used for dry pulverization include a vibration mill, a ball mill, a pin mill, an attritor, etc., and a vibration mill is preferred. The Si-based alloy particles obtained after pulverization (which may be described as "pulverized particles" in this specification) may be disintegrated. When the pulverized particles contain pulverized particles with aggregated powder, the aggregated pulverized particles can be dispersed by disintegration. For disintegration, for example, a jet mill can be used. It is preferable to adjust the conditions of pulverization and disintegration so that (A1) Si-based alloy particles with an average particle size of 0.1 to 3.0 μm are obtained.

[0019] The content of the (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% with respect to the total mass of the solid content of the negative electrode material. When the content of the (A1) Si-based alloy particles is 5 mass% or more, the effect of improving the discharge capacity is easily obtained. When the content of the (A1) Si-based alloy particles is 15 mass% or less, good cycle characteristics are easily obtained.

[0020] (A2) Graphite particles (A2) The graphite particles may be natural graphite particles and artificial graphite particles, or both. Preferably, they are artificial graphite particles. (A2) The shape of the graphite particles may be lumpy, scaly, flakey, spherical, rod-shaped, needle-shaped, or fibrous.

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

[0022] (A) The negative electrode active material may further contain any other negative electrode active material. Examples of any other negative electrode active material include metals such as Si, Sn, Pb, Al, Zn, Bi, In, Mg, and Ga that can be alloyed with lithium; metal oxides, etc.

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

[0024] (B) Conductive additive (B) Examples of conductive additives 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. (B) Conductive additives can be used individually or in combination of two or more. (B) Conductive additives preferably contain carbon black, and more preferably contain acetylene black.

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

[0026] (C) Binder Examples of (C) binders include polyvinylidene fluoride (PVDF), polyvinylidene chloride, polybenzimidazole (PBI), polyethersulfone (PES), polyimide, polyamide, polyamideimide, polyvinyl acetate, polyacrylonitrile, polyvinyl alcohol, carboxymethylcellulose (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, polybutylene terephthalate, etc. (C) Binders can be used individually or in combination of two or more. (C) Binders preferably contain (meth)acrylic polymer.

[0027] The content of (C) binder in the negative electrode material is, for example, 3-10% by mass, 4-8% by mass, or 4-6% by mass, relative to the total mass of solids in the negative electrode material. When the (C) binder content is 3% by mass or more, good cycle characteristics are more 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 a slurry. Examples of solvents (D) include nitrogen-containing solvents such as water, N-methyl-2-pyrrolidone, and N,N-dimethylacetamide; aromatic solvents such as toluene and xylene; alcoholic 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 etheric solvents such as tetrahydrofuran and dioxane. Solvents (D) can be used individually or in combination of two or more.

[0029] The content of solvent (D) in the negative electrode material is, for example, 10-90% by mass, 20-80% by mass, or 30-70% by mass, relative to the total mass of the solid content of the negative electrode material and solvent (D). When the content of solvent (D) is within the above range, the negative electrode material layer is easily formed.

[0030] (E) Any component The negative electrode material may contain any components such as thickeners, dispersants, and surfactants.

[0031] The method for manufacturing the negative electrode material is not particularly limited. The negative electrode material can be manufactured by mixing (A1) Si alloy particles, (A2) graphite particles, (B) a conductive additive, and (C) a binder, usually with (D) a solvent, and further with (E) any other component used as needed. Mixing equipment such as a kneader, mixer, roll mill, ball mill, homogenizer, planetary mixer, paint shaker, and sand mill can be used for mixing.

[0032] [Negative electrode material layer] The negative electrode material layer is a layer formed using the negative electrode material for lithium-ion secondary batteries described above. The negative electrode material layer can be formed by coating the negative electrode material and drying the coated layer. Coating methods include the doctor blade method, die coating method, and dipping method. The layer may be compressed after drying to improve its density.

[0033] [Negative electrode] The negative electrode comprises the negative electrode material layer and a current collector, and is obtained by forming the negative electrode material layer on the current collector using the lithium-ion secondary battery negative electrode material described above. 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 metal foils for the current collector include aluminum, nickel, stainless steel, iron, and copper, with copper foil being preferred.

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

[0035] The electrolyte may be a non-aqueous electrolyte, and the non-aqueous electrolyte contains an electrolyte and a non-aqueous solvent. As the electrolyte, 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, etc. are included. As the non-aqueous solvent, 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 are included. 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.

Examples

[0036] Hereinafter, the present invention will be specifically described using examples and comparative examples. However, the scope of the present invention is not construed as being limited to the following examples.

[0037] [Example 1] (Preparation of (A1) Si-based alloy particles) As materials for Si-based alloy particles, metallic Si, metallic Sn, and metallic Cu were prepared and mixed in a mass ratio of 65.0 mass% metallic Si, 30.0 mass% metallic Sn, and 5.0 mass% metallic Cu. This mixture was then high-frequency melted in a melting furnace to produce molten metal. Powdered Si-based alloy particles (gas-atomized particles) were obtained from the molten metal by gas atomization. Next, the Si-based alloy particles (gas atomized particles) were crushed using a vibratory mill to obtain finely milled Si-based alloy particles (crushed particles). Subsequently, the Si-based alloy particles (crushed particles) were broken down using a jet mill to disperse the aggregates contained in the crushed particles, and Si-based alloy particles with a D50 of 0.93 μm were obtained. The D50 of the Si-based alloy particles was measured using a laser diffraction particle size distribution analyzer (manufactured by CILAS).

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

[0039] (Fabrication of the negative electrode) A copper foil was prepared as a current collector, and a slurry for the negative electrode was applied to one side of the copper foil using a doctor blade. The mixture was then dried to create a negative electrode (with a negative electrode material layer thickness of 20 μm after drying).

[0040] (Preparation of half-cells for negative electrode evaluation) A coin-shaped half-cell for negative electrode evaluation was fabricated using the above negative electrode and Li metal as the counter electrode. LiPF6(1moldm) was used as the electrolyte. -3An electrolyte containing a mixed solvent (ethylene carbonate (EC):dimethyl carbonate (DMC):ethyl methyl carbonate (EMC) = 1:1:1 (volume ratio)) and fluoroethylene carbonate (FEC) (5% by mass) was used.

[0041] (evaluation) Under room temperature (25°C), a negative electrode evaluation half-cell was charged to 1.5V with a current of 0.1C and discharged to 0.01V with a current of 0.1C, repeating this operation 50 times. The initial discharge capacity and cycle characteristics are shown in Table 1. The initial discharge capacity (mAh / g) is the discharge capacity of the first cycle. The cycle characteristics (%) were calculated from the discharge capacity of the first cycle and the discharge capacity of the 50th cycle using the following formula. Cycle performance (%) = (Discharge capacity at 50 cycles / Discharge capacity at 1 cycle) × 100

[0042] [Comparative Example 1] A half-cell for negative electrode evaluation 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 negative electrode evaluation 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] Table 1 shows the evaluation results for initial discharge capacity and cycle characteristics. The negative electrode formed using a negative electrode material containing Si-based alloy particles exhibited a large discharge capacity and suppressed the decrease in discharge capacity even after 50 cycles, demonstrating excellent cycle characteristics.

[0045] [Table 1]

Claims

1. (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, and (A2) graphite particles, (A) a negative electrode active material, (B) a conductive additive, and (C) a binder, comprising: (A1) Si-based alloy particles containing 60.0 to 70.0 mass% of Si, (A2) Sn, and (C) a negative electrode material for lithium-ion secondary batteries. The average particle size of the (A1) Si alloy particles is 0.3 to 3.0 μm. The aforementioned (B) conductive additive contains carbon black, The negative electrode material for the lithium-ion secondary battery has the following characteristics based on the total mass of solids: (A1) Si-based alloy particles have a content of 8 to 10% by mass, (A2) graphite particles have a content of 80 to 82% by mass, (B) conductive additives have a content of 4 to 6% by mass, and (C) binders have a content of 4 to 6% by mass. Anode material for lithium-ion secondary batteries.

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

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

4. The anode material for a lithium-ion secondary battery according to claim 1, wherein the (C) binder comprises a (meth)acrylic polymer.

5. A negative electrode material layer formed using the negative electrode material described in any one of claims 1 to 4.

6. A negative electrode having a negative electrode material layer and a current collector as described in claim 5.

7. A lithium-ion secondary battery having the negative electrode described in claim 6.

8. A method for producing (A1) Si-based alloy particles for use as a negative electrode material for lithium-ion secondary batteries according to any one of claims 1 to 4, comprising using Si, Sn, and Cu as raw materials and obtaining Si-based alloy particles by a gas atomization method.

Images