Negative electrode active material for lithium ion secondary battery and method for producing the same

Abstract

Provided is a material that can be used as a negative electrode active material capable of manufacturing a lithium ion secondary battery excellent in capacity and cycle characteristics. 【Solution means】A negative electrode active material for a lithium ion secondary battery containing a laminated irregular structure type silicon carbide, an amorphous carbon material, and silicon, wherein the laminated irregular structure type silicon carbide and the silicon are dispersed in the amorphous carbon material so as to have a conductive path with the amorphous carbon material, and with the total amount being 100% by mass, the content of the silicon is 5.0 to 28.0% by mass, and the content of the laminated irregular structure type silicon carbide is 21.6 to 57.0% by mass.

Description

Technical Field

[0001] The present invention relates to a negative electrode active material for a lithium ion secondary battery and a method for producing the same.

Background Art

[0002] Currently, the charge and discharge capacity of a lithium ion secondary battery largely depends on the active materials of the positive and negative electrodes. As the positive electrode active material, it is known that the industrialization will proceed mainly with lithium iron phosphate in the future due to its stability, cost, and ease of supply. On the other hand, for the next-generation negative electrode active material, it has not yet been fully determined. As the negative electrode active material, graphite is currently used, but the theoretical capacity of charge and discharge is 372 mAh / g, and the development of a higher-capacity negative electrode active material is awaited. On the other hand, silicon has a theoretical capacity of charge and discharge exceeding 3500 mAh / g and has a theoretical capacity about 10 times that of graphite. However, when lithium ions inserted and desorbed into the negative electrode during charging react with silicon to form a compound, the volume of silicon expands about 3 to 4 times. When lithium ions dissociate from the compound during discharge, the volume of silicon becomes about 1 / 4 to 1 / 3 conversely. During the charge and discharge cycle, the contact between the active material and the electrode cannot be maintained, and the cycle characteristics are significantly deteriorated. For example, even in only 5 cycles, the capacity decreases to about 35%. For this reason, as the negative electrode active material, SiO (Si + SiO 2 ) and cubic β-SiC with little volume change are being studied as active materials. For example, the latter has been reported in various papers (for example, see Non-Patent Documents 1 to 8), but in each case, highly crystalline cubic β-SiC has been used, and due to poor reproducibility and unsuitability for mass production, a decisive active material has not been determined.

[0003] Incidentally, the inventors of the present invention have developed not only highly crystalline cubic β-SiC for silicon carbide, but also a laminated irregular structure type silicon carbide in which the closest-packed layers of silicon or carbon in silicon carbide are laminated one-dimensionally and irregularly in the

[0001] direction (see, for example, Non-Patent Document 9). The laminated irregular structure type silicon carbide reported in Non-Patent Document 9 was produced with a silicon:carbon ratio of 1:1 (molar ratio).

Prior Art Documents

Non-Patent Documents

[0004]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Non-Patent Document 5

Non-Patent Document 6

Non-Patent Document 7

Non-Patent Document 8

Non-Patent Document 9

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, according to the research of the present inventors, even if a laminated irregular structure type silicon carbide produced with a silicon:carbon ratio of 1:1 (molar ratio) is used as a negative electrode active material as in Non-Patent Document 9, it is difficult to insert and desorb lithium ions during charge and discharge. Therefore, it has been found that a lithium ion secondary battery excellent in capacity and cycle characteristics cannot be manufactured.

[0006] For this reason, an object of the present invention is to provide a material that can be a negative electrode active material capable of manufacturing a lithium ion secondary battery excellent in capacity and cycle characteristics.

Means for Solving the Problems

[0007] As a result of intensive research, the present inventors adjusted the amount of silicon to be a certain amount while dispersing a laminated irregular structure type silicon carbide and silicon in an amorphous carbon material so that they have a conductive path with the amorphous carbon material. Thus, the silicon carbide generated at the Si-C interface moves to the conductive excess carbon matrix, which can reversibly occlude and release lithium ions with the charge and discharge of lithium ions. Also, since silicon is present in the amorphous carbon material, the negative electrode active material for a lithium ion secondary battery is not destroyed by the expansion and contraction of silicon, and it has been found that a lithium ion secondary battery with further improved capacity and cycle characteristics can be manufactured. The present invention has been completed through further research based on such findings. That is, the present invention includes the following configurations.

[0008] Item 1. A negative electrode active material for a lithium ion secondary battery containing a laminated irregular structure type silicon carbide, an amorphous carbon material, and silicon, the laminated irregular structure type silicon carbide and the silicon are dispersed in the amorphous carbon material so as to have a conductive path with the amorphous carbon material, and A negative electrode active material for a lithium-ion secondary battery, with the total amount being 100% by mass, the silicon content being 5.0 to 28.0% by mass, and the content of the laminated irregular structure silicon carbide being 21.6 to 57.0% by mass.

[0009] Item 2. The negative electrode active material for a lithium-ion secondary battery according to Item 1, wherein the content of the amorphous carbon material is 28.8 to 66.5% by mass with the total amount being 100% by mass.

[0010] Item 3. The negative electrode active material for a lithium-ion secondary battery according to Item 1 or 2, wherein the average particle diameter of the laminated irregular structure silicon carbide is 5 to 20 nm.

[0011] Item 4. The negative electrode active material for a lithium-ion secondary battery according to any one of Items 1 to 3, wherein the average particle diameter of the amorphous carbon material is 5 to 25 nm.

[0012] Item 5. The negative electrode active material for a lithium-ion secondary battery according to any one of Items 1 to 4, wherein the laminated irregular structure silicon carbide has a full width at half maximum of the peak at 2θ = 36.0° of 2.0° or more within an allowable range of ±0.5° in X-ray diffraction measurement using CuKα radiation.

[0013] Item 6. A negative electrode for a lithium-ion secondary battery containing the negative electrode active material for a lithium-ion secondary battery according to any one of Items 1 to 5.

[0014] Item 7. A lithium-ion secondary battery containing the negative electrode for a lithium-ion secondary battery according to Item 6.

[0015] Item 8. A method for manufacturing a negative electrode active material for a lithium-ion secondary battery according to any one of Items 1 to 5, a step of subjecting a raw material mixture containing a silicon-containing material and a carbon material to mechanochemical treatment until the silicon content is 5.0 to 28.0% by mass and the content of the laminated irregular structure silicon carbide is 21.6 to 57.0% by mass with the total amount being 100% by mass The manufacturing method comprising this.

Advantages of the Invention

[0016] According to the present invention, it is possible to provide a material that can be a negative electrode active material capable of manufacturing a lithium ion secondary battery excellent in capacity and cycle characteristics.

Brief Description of the Drawings

[0017]

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Mode for Carrying Out the Invention

[0018] In this specification, "containing" is a concept that encompasses any of "comprise", "consist essentially of", and "consist of".

[0019] Also, in this specification, when a numerical range is expressed as A to B, it indicates A or more and B or less.

[0020] 1. Anode active material for lithium-ion secondary battery The negative electrode active material for a lithium-ion secondary battery of the present invention is a negative electrode active material for a lithium-ion secondary battery containing laminated irregular structure type silicon carbide, an amorphous carbon material, and silicon, wherein the laminated irregular structure type silicon carbide and the silicon are dispersed in the amorphous carbon material so as to have a conductive path with the amorphous carbon material, and with the total amount being 100% by mass, the content of the silicon is 5.0 to 28.0% by mass.

[0021] In the case of a stacked irregular structure type silicon carbide, the closest packed layers of silicon are stacked irregularly, and carbon atoms are inserted into every other (half) of the tetrahedral sites, while the other half of the tetrahedral sites are empty sites. Also, in the stacked irregular structure type silicon carbide, all of the octahedral sites equal in number to silicon are empty. When the active material of this stacked irregular structure type silicon carbide is used as the negative electrode of a lithium ion secondary battery, upon charge and discharge, Li + ions are inserted into all of these empty half tetrahedral sites and empty octahedral sites, receive electrons from the counter electrode, and occupy the empty sites as lithium atoms. This is well known in the zinc blende structure CuSn and InSb where the closest packed layers are regularly stacked. For example, in the case of CuSn, when represented by a chemical formula, it becomes Li 2 CuSn and has a structure similar to a Whistler compound. Therefore, during the charge and discharge process, the expansion and contraction of the active material are significantly suppressed (J.T. Vaughey, K.D. Kepler, R. Benedek, M.M. Thackeray, Electrochem. Commun. 1 (1999) 517 - 521., J.T. Vaughey, J. O’Hara, M.M. Thackeray, Electrochem. Solid-State Lett. 3 (2000) 13 - 16.). In the case of the stacked irregular structure type silicon carbide, when represented by a chemical formula on purpose, it becomes Li 2 SiCx(SD)(Li 2 SiC(SD), etc.). Under the concept presented in the above two papers, when calculating the theoretical discharge capacity, it becomes 1336 mAh / g, which is almost the same as the discharge capacity of the stacked irregular structure type silicon carbide obtained experimentally. Also, since it can be expected that the expansion and contraction accompanying charge and discharge are suppressed, the cycle characteristics are good.

[0022] This time, considering the case where the pulverization time (reaction time) in the mechanochemical treatment in the production method of the present invention is shortened, and the laminated irregular structure type silicon carbide and unreacted silicon act as active materials. When carbon and silicon react, it is expected to have a structure in which the laminated irregular structure type silicon carbide reacting at the interface between carbon and silicon and unreacted silicon are dispersed so as to successfully secure a conductive path in the aggregate of carbon acting as a conductive material. Silicon + reacts with Li 4.4 ions to form Li 4.4 Si, and the theoretical discharge capacity is as large as 4198 mAh / g. However, since silicon expands by 3 to 4 times and contracts when releasing lithium ions, the conductive path is lost or the active material dissociates from the coating material, deteriorating the cycle characteristics of the lithium ion secondary battery. However, according to the present invention, it can be expected that the amorphous carbon material absorbs the expansion and contraction of silicon due to the reaction with lithium in the laminated irregular structure type silicon carbide and silicon generated in the amorphous carbon material.

[0023] (1-1) Laminated irregular structure type silicon carbide The laminated irregular structure type silicon carbide contained in the negative electrode active material for a lithium ion secondary battery of the present invention is preferably, for example, SiCx (0.8 ≤ x ≤ 1.5). In particular, using SiC with x ≈ 1 is preferable from the viewpoints of charge-discharge capacity, cycle characteristics, etc.

[0024] The laminated irregular structure type silicon carbide contained in the negative electrode active material for a lithium ion secondary battery of the present invention can have broad peaks at 2θ = 36.0°, 60.0° and 72.0° attributed to silicon carbide within an allowable range of ±0.5° in X-ray diffraction measurement using CuKα rays.

[0025] The silicon carbide used in a lithium-ion secondary battery is usually highly crystalline β-SiC. However, the laminated irregular structure type silicon carbide contained in the negative electrode active material for a lithium-ion secondary battery of the present invention is preferably low-crystalline silicon carbide from the viewpoints of charge-discharge capacity, cycle characteristics, etc. Specifically, the laminated irregular structure type silicon carbide preferably has a full width at half maximum of a broad peak at 2θ = 36.0° of 2.0° or more, more preferably 2.5 to 10.0°, within an allowable range of ±0.5° in X-ray diffraction measurement using CuKα rays.

[0026] The shape of the laminated irregular structure type silicon carbide is not particularly limited, and for example, powders, plates, granules, spheres, fibers, lumps, etc. of any shape can be used.

[0027] In the negative electrode active material for a lithium-ion secondary battery of the present invention, from the viewpoint of easy occlusion and desorption of lithium ions, it is preferable that small silicon carbide is dispersed in the amorphous carbon material. Therefore, the average particle diameter of the laminated irregular structure type silicon carbide is preferably small. For this reason, the average particle diameter of the laminated irregular structure type silicon carbide is preferably 5 to 20 nm, more preferably 5 to 10 nm. The average particle diameter of the laminated irregular structure type silicon carbide is measured by high-resolution transmission electron microscope observation.

[0028] In the negative electrode active material for a lithium-ion secondary battery of the present invention, the content of the laminated irregular structure type silicon carbide is not particularly limited. However, from the viewpoint of easy occlusion and desorption of lithium ions, since it is preferable that small silicon carbide is dispersed in the amorphous carbon material without aggregation so as to have a conductive path with the amorphous carbon material, based on the total amount of the negative electrode active material for a lithium-ion secondary battery of the present invention being 100% by mass, 21.6 to 57.0% by mass is preferable, and 28.5 to 46.2% by mass is more preferable.

[0029] (1-2) Amorphous carbon material In the negative electrode active material for a lithium ion secondary battery of the present invention, by containing an amorphous carbon material, the conductivity is improved and it is possible to conduct electricity.

[0030] The amorphous carbon material contained in the negative electrode active material for a lithium ion secondary battery of the present invention can have broad peaks at 2θ = 22.5° and optionally 42.0° depending on the amorphous carbon material within an allowable range of ±0.5° in X-ray diffraction measurement using CuKα rays.

[0031] From the viewpoints of charge-discharge capacity, cycle characteristics, etc., the amorphous carbon material contained in the negative electrode active material for a lithium ion secondary battery of the present invention preferably has a full width at half maximum of the broad peak at 2θ = 22.5° of 2.0° or more, more preferably 3.0 to 10.0°, within an allowable range of ±0.5° in X-ray diffraction measurement using CuKα rays.

[0032] The shape of the amorphous carbon material is not particularly limited, and for example, materials of any shape such as powdery, plate-like, granular, spherical, fibrous, and massive can be used, but usually it can be spherical.

[0033] In the negative electrode active material for a lithium ion secondary battery of the present invention, from the viewpoint of easily occluding and desorbing lithium ions, it is preferable that small silicon carbide is dispersed in the amorphous carbon material, so the average particle diameter of the amorphous carbon material is also preferably small. Since the amorphous carbon material is an amorphous material, it may be difficult to clearly determine its average particle diameter in some cases, but the average particle diameter of the amorphous carbon material is preferably 5 to 25 nm, more preferably 10 to 20 nm. The average particle diameter of the amorphous carbon material is measured by high-resolution transmission electron microscope observation.

[0034] In the negative electrode active material for a lithium ion secondary battery of the present invention, the content of the amorphous carbon material is not particularly limited, but from the viewpoints of easily improving conductivity and charge-discharge capacity, cycle characteristics, etc., based on the total amount of the negative electrode active material for a lithium ion secondary battery being 100% by mass, 28.8 to 66.5% by mass is preferable, and 38.0 to 53.9% by mass is more preferable.

[0035] (1-3) Silicon In the negative electrode active material for a lithium ion secondary battery of the present invention, by containing an amorphous carbon material, the capacity can be improved, and since silicon is dispersed in the amorphous carbon material so as to have a conductive path with the amorphous carbon material, the expansion and contraction of silicon can be suppressed. Therefore, the destruction of the negative electrode active material for a lithium ion secondary battery due to the expansion and contraction of silicon can be suppressed, and the cycle characteristics can also be improved.

[0036] The silicon contained in the negative electrode active material for a lithium ion secondary battery of the present invention can have peaks at 2θ = 28.0°, 47.0° and 56.0° depending on silicon within an allowable range of ±0.5° in X-ray diffraction measurement using CuKα rays.

[0037] The shape of silicon is not particularly limited, and for example, powders, plates, granules, spheres, fibers, lumps, etc. of any shape can be used, but usually it can be spherical.

[0038] In the negative electrode active material for a lithium ion secondary battery of the present invention, from the viewpoint of easily improving the capacity and easily suppressing the expansion and contraction of silicon by the amorphous carbon material, thus easily improving the cycle characteristics, it is preferable that small silicon is dispersed in the amorphous carbon material. Therefore, the average crystallite size of silicon is preferably small. For this reason, the average crystallite size of silicon is preferably 5 to 100 nm, more preferably 6 to 50 nm. The average crystallite size of silicon is measured by high-resolution transmission electron microscope observation.

[0039] In the negative electrode active material for a lithium ion secondary battery of the present invention, the silicon content is preferably small from the viewpoint of being particularly likely to improve the capacity and being likely to improve the cycle characteristics because the expansion and contraction of silicon are easily suppressed by the amorphous carbon material, so that small silicon is preferably dispersed in the amorphous carbon material without aggregation so as to have a conductive path with the amorphous carbon material. Therefore, based on the total amount of the negative electrode active material for a lithium ion secondary battery of the present invention being 100% by mass, 5.0 to 28.0% by mass, preferably 10.0 to 20.0% by mass is more preferable.

[0040] (1-4) Measurement method of content Examination is carried out on the electrode prepared by coating a current collector copper foil. At this time, this time, it is assumed that the current collector copper foil of the electrode is coated with 85% by mass of carbon as a conductive material and an active material, 7.5% by mass of a binder polyacrylic acid (PAA), and 7.5% by mass of a binder carboxymethyl cellulose (CMC).

[0041] In this way, when carbon as a conductive material and an active material account for 85% by mass and the conductive material and the active material are in the same amount, silicon is weighed and synthesized so as to react with carbon by 100% to generate a laminated irregular structure type silicon carbide. Usually, when reacting for 24 hours with a high energy ball mill, all the mixed silicon is converted into a laminated irregular structure type silicon carbide. In this case, 85% by mass of the substances coated on the current collector copper foil is silicon and carbon, and half of them is laminated irregular structure type silicon carbide and the rest is carbon.

[0042] Here, although carbon may also insert and desorb lithium ions, unlike graphite having a layered structure, the theoretical charge-discharge capacity of the amorphous carbon material cannot be calculated, and the amount of carbon inserting and desorbing lithium ions also varies depending on the pulverization conditions of the mechanochemical treatment and the like, and it is difficult to specify. Therefore, in this specification, for convenience, it is assumed that carbon does not insert and desorb lithium ions, that is, only silicon carbide and silicon insert and desorb lithium ions, and the calculation is made accordingly.

[0043] Considering an electrode containing m(g) of the laminated irregular structure type silicon carbide, since the theoretical discharge capacity is 1336 mAh / g, as follows: m × 1336 mAh / g = Di (mAh) Di: Initial discharge capacity The discharge capacity can be calculated as above.

[0044] When m = 0.0030 g, Di = 4.008 mAh. However, when the synthesis time of the laminated irregular structure type silicon carbide (grinding time in mechanochemical treatment) is 12 hours, the measured discharge capacity is 6.564 mAh, which increases significantly. This is considered to be because silicon with a large theoretical discharge capacity (4198 mAh / g) remains unreacted. Let the weight fraction of the generated laminated irregular structure type silicon carbide be x, and the total carbon equimolar to the silicon that did not form the laminated irregular structure type silicon carbide be (1 - x). Then, the weight fraction and weight, that is, the content of each active material (laminated irregular structure type silicon carbide and silicon) can be calculated from the following formula. m × 1336 mAh / g × x + m × 4198 mAh / g × (1 - x) × 28.08 / 40.09 = 6.564 mAh When m = 0.0030 g, x = 0.4689 and (1 - x) = 0.5311 Therefore, the weight of the laminated irregular structure type silicon carbide is 0.0030 × 0.4689 (g), and the weight of silicon is 0.0030 × 0.5311 × 28.08 / 40.09 (g).

[0045] (1 - 5) Negative electrode active material for lithium ion secondary battery As described above, the negative electrode active material for a lithium ion secondary battery of the present invention contains a laminated irregular structure type silicon carbide, an amorphous carbon material, and silicon.

[0046] In the negative electrode active material for a lithium-ion secondary battery of the present invention, the laminated irregular structure type silicon carbide generated at the Si-C interface moves to the conductive excess carbon matrix, and since it can reversibly occlude and release lithium ions with the charge and discharge of lithium ions, it is preferable that the laminated irregular structure type silicon carbide is dispersed in the amorphous carbon material without preferably aggregating so as to have a conductive path with the amorphous carbon material.

[0047] Also, in the negative electrode active material for a lithium-ion secondary battery of the present invention, since silicon is covered with the excess carbon matrix to absorb the expansion and contraction of silicon and prevent the destruction of the negative electrode active material for a lithium-ion secondary battery of the present invention, it is preferable that silicon is also dispersed in the amorphous carbon material without preferably aggregating so as to have a conductive path with the amorphous carbon material.

[0048] The negative electrode active material for a lithium-ion secondary battery of the present invention includes, in addition to the above-described laminated irregular structure type silicon carbide, amorphous carbon material, and silicon, as long as the effects of the present invention are not impaired, silicon-containing materials (such as silicon nitride, silicon oxide) as raw materials, crystalline carbon materials (such as graphite), crystalline silicon carbide (cubic β-SiC), metal components (such as iron) and metal-nonmetal compounds that are inevitable during synthesis. It does not exclude including a third component such as these. The content of these third components can be 0 to 10% by mass, particularly 0.01 to 5% by mass, with the total amount of the negative electrode active material for a lithium-ion secondary battery of the present invention being 100% by mass.

[0049] In the negative electrode active material for a lithium-ion secondary battery of the present invention as described above, the laminated irregular structure type silicon carbide generated at the Si-C interface moves to the conductive excess carbon matrix, and this can reversibly occlude and release lithium ions with the charge and discharge of lithium ions. Further, although the laminated irregular structure type silicon carbide included in the negative electrode active material for a lithium-ion secondary battery of the present invention is a material containing silicon, it is a material that does not expand and contract as much as silicon during charge and discharge. Further, in the negative electrode active material for a lithium-ion secondary battery of the present invention, the laminated irregular structure type silicon carbide and silicon are preferably dispersed without aggregation so as to have a conductive path with the amorphous carbon material in the amorphous carbon material. As a result, the amorphous carbon material adjacent to the laminated irregular structure type silicon carbide and silicon can follow the expansion and contraction of the laminated irregular structure type silicon carbide and a small amount of silicon. Therefore, by using the negative electrode active material for a lithium-ion secondary battery of the present invention, in particular, a lithium-ion secondary battery having a high capacity and excellent cycle characteristics can be manufactured. In the present invention, the negative electrode active material for a lithium-ion secondary battery is a concept that also includes a negative electrode active material for a metal lithium secondary battery using lithium metal as a positive electrode.

[0050] 2. Anode for lithium-ion secondary battery The negative electrode for a lithium-ion secondary battery of the present invention contains the negative electrode active material for a lithium-ion secondary battery of the present invention. More specifically, the negative electrode for a lithium-ion secondary battery of the present invention can include a negative electrode active material layer containing the negative electrode active material for a lithium-ion secondary battery of the present invention.

[0051] The negative electrode active material layer can be composed only of the negative electrode active material for the lithium-ion secondary battery of the present invention described above. However, if necessary, as a conductive agent, carbon black such as acetylene black, furnace black, ketjen black, etc.; flaky graphite; graphene; amorphous carbon obtained by heat-treating an organic substance, etc. can also be included. In particular, in the negative electrode active material for the lithium-ion secondary battery of the present invention, when the content of the laminated irregular structure type silicon carbide is large, it is particularly effective to use a conductive agent. These conductive agents can be used alone or in combination of two or more.

[0052] In addition, if necessary, the negative electrode active material layer may further contain, as a binder, thickener or dispersant, a fluorine-based polymer (polyvinylidene fluoride resin, polytetrafluoroethylene resin, vinylidene fluoride-hexafluoropropylene copolymer, etc.), a polyolefin-based resin (styrene-butadiene copolymer resin, ethylene-vinyl alcohol copolymer resin, etc.), synthetic rubber (styrene-butadiene rubber, acrylonitrile-butadiene rubber, ethylene-propylene-diene rubber, etc.), polyacrylonitrile, polyamide, polyimide, polyacrylic acid, polyacrylate, polyvinyl ether, carboxymethyl cellulose, sodium carboxymethyl cellulose, ammonium carboxymethyl cellulose, polyurethane, hydroxypropyl cellulose, hydroxyethyl cellulose, methyl cellulose, etc. These binders, thickeners or dispersants can be used alone or in combination of two or more.

[0053] From the viewpoints of charge-discharge capacity, cycle characteristics, etc., the content of the negative electrode for the lithium-ion secondary battery of the present invention can be 70 to 95% by mass, particularly 80 to 90% by mass, with the total amount of the negative electrode active material layer being 100% by mass. Also, from the viewpoints of charge-discharge capacity, cycle characteristics, etc., the content of the above binder, thickener or dispersant can be 5 to 30% by mass, particularly 10 to 20% by mass, with the total amount of the negative electrode active material layer being 100% by mass.

[0054] The thickness of the negative electrode active material layer is not particularly limited, but from the viewpoints of charge-discharge capacity, cycle characteristics, etc., it is preferably 1 to 300 μm, more preferably 10 to 250 μm, and even more preferably 50 to 200 μm.

[0055] Such a negative electrode active material layer can be produced by forming a negative electrode mixture containing the negative electrode active material for a lithium ion secondary battery of the present invention and, if necessary, a binder, a thickener, a dispersant, etc. into a layer. For example, the negative electrode mixture can be dried by a conventional method and formed into a layer.

[0056] As described above, the negative electrode for a lithium ion secondary battery of the present invention preferably includes the above-described negative electrode active material layer. Specifically, it preferably includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

[0057] The negative electrode current collector is preferably made of a material that is electrochemically stable at the potential used, such as copper, stainless steel, nickel, a carbon material, etc., and has high electronic conductivity. This negative electrode current collector can be, for example, a member in the form of a foil, a mesh, etc.

[0058] When manufacturing such a negative electrode for a lithium ion secondary battery of the present invention, it can be produced by forming the above-described negative electrode mixture into a layer on the negative electrode current collector. For example, on the negative electrode current collector, the negative electrode mixture can be dried by a conventional method and formed into a layer to produce the negative electrode for a lithium ion secondary battery of the present invention.

[0059] 3. Lithium-ion secondary battery The lithium ion secondary battery of the present invention includes the negative electrode for a lithium ion secondary battery of the present invention described above. Further, the lithium ion secondary battery of the present invention can include, in addition to the negative electrode for a lithium ion secondary battery of the present invention, a positive electrode, an electrolytic solution, and a container for housing these, which are applied to known lithium ion secondary batteries.

[0060] As the positive electrode, any material that can supply lithium ions to the negative electrode may be used, and a well-known positive electrode can be used.

[0061] As the positive electrode current collector constituting the positive electrode, for example, materials that are electrochemically stable at the potentials at which aluminum, stainless steel, carbon materials, etc. are used and have high electronic conductivity can be exemplified.

[0062] Also, as the positive electrode active material constituting the positive electrode, a material that can usually occlude and release lithium ions is generally used. For example, α-NaFeO 2 Lithium transition metal composite oxides having a rock salt-type crystal structure, lithium transition metal oxides having a spinel-type crystal structure, polyanion compounds, chalcogen compounds, sulfur, etc. can be mentioned. α-NaFeO 2 As the lithium transition metal composite oxide having a rock salt-type crystal structure, for example, Li[Li x1 Ni γ1 Mn β1 Co (1-x1-γ1-β1) O 2 (0 ≦ x1 < 0.5, 0 ≦ γ1 ≦ 1, 0 ≦ β1 ≦ 1, 0 ≦ γ1 + β1 ≦ 1), Li[Li x2 Ni γ2 Co β2 Al (1-x2-γ2-β2) O 2 (0 ≦ x2 < 0.5, 0 ≦ γ2 ≦ 1, 0 ≦ β2 ≦ 1, 0 ≦ γ2 + β2 ≦ 1), etc. can be mentioned. As the lithium transition metal oxide having a spinel-type crystal structure, Li x3 Mn 2 O 4 (0.9 ≦ x3 < 1.5), Li x4 Ni γ4 Mn (2-γ4) O 4 (0.9 ≦ x4 < 1.5, 0 ≦ γ4 ≦ 2), etc. can be mentioned. As the polyanion compound, LiFePO 4 , LiMnPO 4 , LiNiPO 4 , LiCoPO 4 , Li 3 V 2 (PO 4 ) 3 , Li 2 MnSiO 4 , Li 2 CoPO 4Examples include F. Examples of the chalcogen compound include titanium disulfide, molybdenum disulfide, molybdenum dioxide, etc. Atoms or polyanions in these materials may be partially substituted with atoms or anion species composed of other elements. These cathode active materials can be used alone or in combination of two or more kinds.

[0063] As the cathode constituent material other than the cathode active material constituting the cathode, the same materials as those of the anode constituent material other than the anode active material in the above-described anode can be used, and the content thereof can also be the same as that of the anode constituent material other than the anode active material in the anode.

[0064] The electrolytic solution is an electrolytic solution in which a salt is dissolved in an aprotic organic solvent, and is disposed between the cathode and the anode. For example, it is preferably impregnated and held in a separator made of a non-woven fabric or the like for preventing short circuit between the cathode and the anode.

[0065] Examples of the aprotic organic solvent constituting the above-described electrolytic solution include esters such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, methyl formate, and methyl acetate; furans such as tetrahydrofuran and 2-methyltetrahydrofuran; ethers such as dioxolane, diethyl ether, dimethoxyethane, diethoxyethane, and methoxyethoxyethane; dimethyl sulfoxide; sulfolanes such as sulfolane and methyl sulfolane; and acetonitrile. These aprotic organic solvents may be used alone or in combination of two or more kinds.

[0066] On the one hand, salts dissolved in such aprotic organic solvents include, for example, lithium salts such as lithium perchlorate, lithium borofluoride, lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium trifluoromethanesulfonate, lithium halide, lithium chloroaluminate, and lithium bis(fluorosulfonyl)imide). These salts may be used alone or in combination of two or more.

[0067] 4. Method for manufacturing anode active material for lithium-ion secondary battery The method for producing the negative electrode active material for a lithium ion secondary battery of the present invention is not particularly limited. For example, with respect to a raw material mixture containing a silicon-containing material and a carbon material, with the total amount being 100% by mass, the content of the silicon is 5.0 to 28.0% by mass, and the content of the laminated irregular structure type silicon carbide is subjected to a mechanochemical treatment until it becomes 21.6 to 57.0% by mass. It includes.

[0068] The silicon-containing material used as a raw material is not particularly limited, and in addition to silicon, silicon-containing compounds and the like can be used. These silicon-containing materials can be used alone or in combination of two or more. Further, since the silicon-containing material is mixed and pulverized by mechanochemical treatment, there is no limitation on the particle size of the silicon-containing material to be used, and usually, a commercially available powdery silicon-containing material can be used.

[0069] The carbon material used as a raw material is not particularly limited, and carbon blacks such as acetylene black, furnace black, and ketjen black; graphite; graphene; amorphous carbon, etc. can be used. These carbon materials can be used alone or in combination of two or more. Further, since the carbon material is mixed and pulverized by mechanochemical treatment, there is no limitation on the particle size of the carbon material to be used, and usually, a commercially available powdery carbon material can be used.

[0070] Mechanochemical treatment is a method of grinding and mixing raw materials while applying mechanical energy. According to this method, by applying mechanical impact and friction to the raw materials to grind and mix them, the silicon-containing material and the carbon material come into intense contact and are refined, and a reaction of the raw materials occurs. That is, at this time, mixing, pulverization, and reaction occur simultaneously. Therefore, it is possible to more surely react the raw materials without heating the raw materials to a high temperature. By using mechanochemical treatment, a metastable crystal structure that cannot be obtained by normal heat treatment may be obtained.

[0071] Regarding these raw materials, all of them can be mixed simultaneously and subjected to mechanochemical treatment, or some of the raw materials can first be subjected to mechanochemical treatment and then the remaining raw materials can be added and subjected to mechanochemical treatment.

[0072] Regarding the mixing ratio of the raw materials, as described later, in the production method of the present invention, in the mechanochemical treatment, the pulverization time is shortened to produce a laminated irregular structure type silicon carbide, and silicon as a raw material is intentionally left remaining, and these are dispersed in the amorphous carbon material so as to have a conductive path between them. However, even in this case, since the charging ratio of the raw materials almost directly becomes the ratio of each element of the product, it can be made the same ratio as the element ratio of silicon and carbon in the negative electrode active material for a lithium ion secondary battery of the present invention aimed at. Specifically, with the total amount of the raw material mixture being 100% by mass, the content of the silicon-containing material is preferably 20 to 60% by mass, more preferably 25 to 50% by mass. Also, with the total amount of the raw material mixture being 100% by mass, the content of the carbon material is preferably 40 to 80% by mass, more preferably 55 to 75% by mass.

[0073] In the present invention, from the viewpoint of easily obtaining the laminated irregular structure type silicon carbide of the present invention, the energy input amount in the mechanochemical treatment is preferably 10 to 110 kWh / kg of the raw material mixture, more preferably 25 to 80 kWh / kg of the raw material mixture.

[0074] Incidentally, the energy input for the mechanochemical treatment is calculated using the formula described in the following literature: Burgio, N., Lasonna, A., Magini, M., Martelii, S. and Padella, F., Il Nuovo Cimento, Vol. 13, pp. 459-476 (1991). It is calculated by the formula published in this reference.

[0075] In the mechanochemical treatment, when rotation and revolution are applied to the raw material mixture, a strong synthetic centrifugal force can be applied, causing convective motion and generating vortices due to rotation. These flows can be effectively combined to perform dense stirring, and the negative electrode active material for the lithium-ion secondary battery of the present invention can be efficiently produced.

[0076] In this case, assuming the use of P-5 manufactured by Fritsch, the rotation-revolution ratio is 2, and the upper limit of the revolution speed is set at 400 rpm. Therefore, the rotation speed is not particularly limited, but from the viewpoint of easily obtaining the negative electrode active material for the lithium-ion secondary battery of the present invention, 500 to 800 rpm is preferable, and 600 to 700 rpm is more preferable. Also, the revolution speed is not particularly limited, but from the viewpoint of easily obtaining the negative electrode active material for the lithium-ion secondary battery of the present invention, 250 to 400 rpm is preferable, and 300 to 350 rpm is more preferable.

[0077] Regarding the temperature during the mechanochemical treatment, there is no particular limitation, and it can be appropriately adjusted from the viewpoint of easily obtaining the negative electrode active material for the lithium-ion secondary battery of the present invention. For example, it can be set at room temperature.

[0078] In the present invention, by deliberately reacting only a part of the silicon-containing material and the carbon material, a laminated irregular structure type silicon carbide is produced, and the raw material silicon is deliberately left remaining, and these are dispersed in the amorphous carbon material so as to have a conductive path between them. For this reason, the pulverization time (treatment time) of the mechanochemical treatment is deliberately set to a short time. From a previous report (J. Am. Chem. Soc., 98, 50-56 (2015).), it can be seen that it takes 24 hours for Si and C to completely react and synthesize a laminated irregular structure type silicon carbide at a molar ratio of 1:1. Therefore, as an example, when carrying out the reaction under the same conditions, the pulverization time (treatment time) of the mechanochemical treatment is preferably 6 to 18 hours, particularly 8 to 15 hours, which is shorter than this. Further, this mechanochemical treatment can also be carried out in multiple stages with breaks in between as necessary.

[0079] In addition, when repeating the mechanochemical treatment multiple times, in the mechanochemical treatment of each step, the above conditions can be adopted.

[0080] When carrying out the mechanochemical treatment as described above, specifically, for example, mechanical pulverization devices such as a ball mill, a planetary ball mill, a bead mill, a rod mill, a vibration mill, a disk mill, a hammer mill, a jet mill, a surface modification / pulverization device, and a high-pressure gas micronizer can be used for mixing and pulverization.

[0081] Note that, different from the manufacturing method of the present invention described above, when a material in which a laminated irregular structure type silicon carbide obtained by a long-time mechanochemical treatment is dispersed in an amorphous carbon material so as to have a conductive path between them and silicon are mixed, silicon cannot be dispersed in the amorphous carbon material so as to have a conductive path between them. As a result, the expansion and contraction of silicon cannot be suppressed, the destruction of the negative electrode active material for a lithium ion secondary battery due to the expansion and contraction of silicon cannot be suppressed, and the cycle characteristics cannot be improved either.

Example

[0082] The present invention will be specifically described based on examples, but the present invention is not limited to these examples.

[0083] The raw material powders used were silicon (Silicon; manufactured by Kojundo Chemical Laboratory Co., Ltd.; average particle size 45 μm, 99.99%), natural graphite (manufactured by Shanghai Shanshan New Materials Co., Ltd.; average particle size 17 μm, ash 0.02%), and acetylene black (Li-100 manufactured by Denka Co., Ltd.; average particle size 35 nm).

[0084] [Example 1: 12 hours] Mixing ratio The mixing ratio of the raw materials was 3.5028 g of sample and 5.4 g of distilled water, and the sample was weighed out to be 29.8 mass% silicon, 45.2 mass% natural graphite, 10.0 mass% acetylene black, 7.5 mass% polyacrylic acid (PAA), and 7.5 mass% carboxymethyl cellulose (CMC).

[0085] Powder synthesis High energy ball mill (Fritsch P5; Si 3 N 4 Pot: 250mL; Si 3 N 4 A metal ball (diameter: 10 mm) was used, and 2.6267 g of silicon, 3.9907 g of natural graphite, and 0.8830 g of acetylene black were added to the pot. A mechanochemical treatment was performed under the conditions of a revolution speed of 300 rpm, a rotation speed of 600 rpm, a grinding time of 12 hours, and a ball weight:powder weight=40:1, to obtain a negative electrode active material for a lithium ion secondary battery of Example 1.

[0086] Anode fabrication The stirring and defoaming conditions for the negative electrode were as follows: stirring was performed at a rotation speed of 800 rpm and a revolution speed of 2000 rpm for 60 seconds, and defoaming was performed at a rotation speed of 60 rpm and a revolution speed of 2200 rpm for 30 seconds.

[0087] First, 0.2630 g of polyacrylic acid (PAA) and 2.2 mL of distilled water were added to a container, and stirring and defoaming were performed twice to obtain an aqueous binder solution. Next, 2.9765 g of the negative electrode active material for a lithium-ion secondary battery obtained in Example 1 above was added to the aqueous binder solution, mixed with a spatula, and 2 mL of distilled water was added separately while checking the condition, followed by performing stirring and defoaming twice.

[0088] Furthermore, 0.2633 g of carboxymethyl cellulose (CMC) was added to the obtained aqueous solution, 1 mL of distilled water was added after stirring and defoaming, and stirring and defoaming were performed twice to obtain a negative electrode mixture.

[0089] Thereafter, the obtained negative electrode mixture was applied to a copper foil with a thickness of 150 μm using a doctor blade, and vacuum drying was performed at 80 °C for 12 hours to obtain a negative electrode for a lithium-ion secondary battery of Example 1.

[0090] [Example 2: 10 hours] Mixing ratio The mixing ratio of the raw materials was 3.5009 g of the sample and 5.4 g of distilled water, and the sample was weighed so as to contain 29.8 mass% of silicon, 45.2 mass% of natural graphite, 10.0 mass% of acetylene black, 7.5 mass% of polyacrylic acid (PAA), and 7.5 mass% of carboxymethyl cellulose (CMC).

[0091] Powder synthesis Using a high-energy ball mill (P5 manufactured by Fritsch; Si 3 N 4 manufactured pot: 250 mL; Si 3 N 4 manufactured balls: diameter 10 mm), 2.6261 g of silicon, 3.9911 g of natural graphite, and 0.8835 g of acetylene black were added into the pot, and mechanochemical treatment was performed with a revolution speed of 300 rpm, a rotation speed of 600 rpm, a pulverization time of 10 hours, and a ball weight: powder weight = 40:1 to obtain a negative electrode active material for a lithium-ion secondary battery of Example 2.

[0092] Anode fabrication The conditions of the stirring and defoaming machine during the production of the negative electrode were as follows: for stirring, the rotation speed of the rotation was 800 rpm, the revolution speed was 2000 rpm, and it was carried out for 60 seconds. For defoaming, the rotation speed of the rotation was 60 rpm, the revolution speed was 2200 rpm, and it was carried out for 30 seconds.

[0093] First, 0.2630 g of polyacrylic acid (PAA) and 2.2 mL of distilled water were added to a container, and stirring and defoaming were carried out twice to obtain an aqueous binder solution. Next, 2.9748 g of the negative electrode active material for a lithium-ion secondary battery obtained in Example 1 above was added to the aqueous binder solution, mixed with a spatula, and 2 mL of distilled water was added separately while checking the condition, and stirring and defoaming were carried out twice.

[0094] Furthermore, 0.2631 g of carboxymethyl cellulose (CMC) was added to the obtained aqueous solution, 1 mL of distilled water was added after stirring and defoaming, and stirring and defoaming were carried out twice to obtain a negative electrode mixture.

[0095] Thereafter, the obtained negative electrode mixture was applied to a copper foil with a thickness of 150 μm using a doctor blade, and vacuum drying was carried out at 80 °C for 12 hours to obtain a negative electrode for a lithium-ion secondary battery of Example 1.

[0096] [Test Example 1: X-ray Diffraction Measurement] Using CuKα rays as the X-ray source, X-ray diffraction measurement was carried out in the range of 2θ = 20 to 80°.

[0097] The X-ray diffraction spectra of the negative electrode active materials for lithium-ion secondary batteries obtained in Examples 1 to 2 are shown in FIGS. 1 to 2. From FIGS. 1 to 2, at 2θ = 36.0°, 60.0° and 72.0°, broad peaks attributed to stacked irregular structured silicon carbide (SD-SiC), at 2θ = 22.5°, broad peaks attributed to amorphous carbon, and at 2θ = 28.0°, 47.0° and 56.0°, peaks attributed to silicon were obtained. Thus, it can be understood that the negative electrode active materials obtained in Examples 1 to 2 contain stacked irregular structured silicon carbide (SD-SiC), amorphous carbon and silicon. For reference, the X-ray diffraction spectra of stacked irregular structured silicon carbide (SD-SiC) and highly crystalline cubic β-SiC described in a previous report (J. Am. Chem. Soc., 98, 50-56 (2015).) are shown in FIGS. 3 to 4, and the X-ray diffraction spectrum of amorphous carbon is shown in FIG. 5. From these, it can be understood that the silicon carbide contained in the negative electrode active materials for lithium-ion secondary batteries obtained in Examples 1 to 2 is not highly crystalline cubic β-SiC, but stacked irregular structured silicon carbide (SD-SiC) described in a previous report (J. Am. Chem. Soc., 98, 50-56 (2015).).

[0098] [Test Example 2: Electron Microscopy Observation] In order to investigate the size of the stacked irregular structured SiC formed at the interface between amorphous carbon and silicon, a negative electrode active material for a lithium-ion secondary battery obtained by pulverizing an equimolar mixture of carbon and silicon with a high-energy ball mill for 4 hours was observed with a high-resolution transmission electron microscope (TEM). The results are shown in FIG. 6.

[0099] As a result, it can be understood that silicon carbide with an average particle size of about 5 to 10 nm is generated at the interface between black silicon and white amorphous carbon. Considering also the results of Test Example 1, it can be understood that the generated silicon carbide is not highly crystalline cubic β-SiC, but rather the stacked irregular structure type silicon carbide (SD-SiC) described in a previous report (J. Am. Chem. Soc., 98, 50-56 (2015)). From this, it can be understood that while the stacked irregular structure type silicon carbide (SD-SiC) (with an average particle size of about 5 to 10 nm) is being generated by the reaction between silicon and carbon, silicon remains, and an amorphous carbon material (with an average particle size of about 10 to 20 nm) exists around it, and the stacked irregular structure type silicon carbide (SD-SiC) (with an average particle size of about 5 to 10 nm) and silicon are dispersed in the amorphous carbon material (with an average particle size of about 10 to 20 nm) without aggregating so as to have a conductive path with the amorphous carbon material. Note that the silicon becomes smaller as the reaction time increases, and when the reaction time is about 10 to 12 hours as in Examples 1 to 2, the average crystallite size was about 8 nm.

[0100] [Production Example: Production of Lithium-Ion Secondary Battery (Half Cell)] As the negative electrode, the negative electrodes obtained in Examples 1 to 2 were used.

[0101] Also, as the positive electrode, lithium metal was used.

[0102] As the electrolyte, ethylene carbonate (EC) and diethyl carbonate (DEC) were used as solvents with EC / DEC = 50 / 50 (v / v), and 1 mol / L lithium hexafluorophosphate (LiPF 6 ) was used as the salt. This electrolyte was impregnated into a polypropylene porous film which is a separator.

[0103] A lithium-ion secondary battery composed of the above negative electrode, positive electrode, electrolyte, and separator was fabricated.

[0104] [Test Example 3: Charge and Discharge Measurement (Part 1)] The charge and discharge measurement was carried out using a two-electrode cell with a potentiostat / galvanostat analyzer ECstat-302. The cell was temperature-controlled in a thermostatic bath at 20°C.

[0105] In the lithium-ion secondary battery (half cell) manufactured using the negative electrodes obtained in Examples 1 to 2, in the first cycle, the charge and discharge were carried out at 100 mA / g to Si-C (Si / C = 1 / 1 in mol%), and in the second to eleventh cycles, at 1000 mA / g to Si-C (Si / C = 1 / 1 in mol%). The charge and discharge curves and the change in discharge capacity are shown in Figs. 7 to 10. Hereinafter, the charge and discharge rate was calculated based on the total weight of the generated laminated irregular structure type SiC and the unreacted Si and an equal molar amount of C. In Figs. 7 to 10, the discharge capacity in each cycle is calculated based on the total weight of SiC and Si obtained by the calculation method of the content shown in (1-4). From the above results, it can be understood that by using the negative electrode active material for a lithium-ion secondary battery of the present invention, excellent performance in terms of capacity, cycle characteristics, and rate characteristics can be achieved.

[0106] Based on these results, the content ratios of carbon, silicon carbide, and silicon in the negative electrode active material for a lithium-ion secondary battery of Examples 1 to 2 were evaluated by the method described in the above (1-4).

[0107] Here, although carbon may also insert and desorb lithium ions, unlike graphite having a layered structure, the theoretical charge and discharge capacity of an amorphous carbon material cannot be calculated, and the amount of carbon inserting and desorbing lithium ions also varies depending on the pulverization conditions of the mechanochemical treatment, etc., and it is difficult to specify. Therefore, for the sake of convenience in this specification, it is assumed that carbon does not insert and desorb lithium ions, that is, only silicon carbide and silicon insert and desorb lithium ions, and the calculation will be made on this assumption.

[0108] Example 1 (Derivation of SiC:(Si + C)) (0.003455x × 1336) + (0.003455(1 - x) × 28.08 / 40.09 × 4198) = 6.667 Since x = 0.6299, SiC:(Si + C) = 0.6299:0.3701 (by weight).

[0109] Note that x is the weight fraction of SiC, and 1 - x means the weight fraction of Si + C. Also, 6.667 mAh is the measured value, and 0.003455 g is the combined amount of the weight of SiC, unreacted Si, and C with an equal molar amount to the unreacted Si. Among the negative electrode layer (the one coated on the current collector), C + SiC is 85% by mass, and since half of it (50% by mass) is C and SiC respectively, when the raw material C and raw material Si react completely, the raw materials are mixed so that C and Si - C (equal molar) are each 50% by mass.

[0110] (Calculation of the specific capacity of the first cycle) 6.667 / (0.003455x + 0.003455(1 - x) × 28.08 / 40.09) = D D = 2170 mAh / g.

[0111] (Calculation of C:SiC:Si) The weight of SiC is 0.003455 g × 0.6299 (weight fraction of SiC). The weight of Si (unreacted Si) is 0.003455 g × 0.3701 (weight fraction of Si + C) × 28.08 / 40.09. The weight of C (unreacted C that did not become SiC + surplus 50% by mass of C) is in addition to 0.003455 g × 0.3701 (weight fraction of Si + C) × 12.01 / 40.09. Since the raw materials (Si powder and C powder) are mixed so that originally C is 50% by mass and SiC is 50% by mass, the surplus C is the amount added with 0.003455 g.

[0112] From these results, the weight ratio of SiC:Si:C is 0.6299:0.3701×28.08 / 40.09:1+0.3701×12.01 / 40.09 = 0.6299:0.2592:1.1087 = 31.5:13.0:55.5 when the whole is divided by 0.003455.

[0113] From the above, C:SiC:Si = 55.5 mass%:31.5 mass%:13.0 mass%.

[0114] Example 2 (Derivation of SiC:(Si + C)) (0.004426x×1336)+(0.004426(1 - x)×28.08 / 40.09×4198)=10.33 Since x = 0.3784, SiC:(Si + C)=0.3784:0.6216 (weight ratio).

[0115] Here, x is the weight fraction of SiC, and 1 - x means the weight fraction of Si + C. Also, 10.33 mAh is the measured value, and 0.004426 g is the combined amount of the weight of SiC, unreacted Si, and C with an equal molar amount to it. Among the negative electrode layer (coated on the current collector), C + SiC is 85 mass%, and since half of it (50 mass%) is C and SiC respectively, the raw materials C and Si are mixed so that C and Si - C (equal molar) are each 50 mass% when the raw materials react completely.

[0116] (Calculation of the specific capacity in the first cycle) 10.33 / (0.004426x + 0.004426(1 - x)×28.08 / 40.09)=D D = 2867 mAh / g.

[0117] (Calculation of C:SiC:Si) The weight of SiC is 0.004426 g × 0.3784 (weight fraction of SiC). The weight of Si (unreacted Si) is 0.004426 g × 0.6216 (weight fraction of Si + C) × 28.08 / 40.09. The weight of C (unreacted C that did not form SiC + excess 50 mass% of C) is in addition to 0.004426 g × 0.6216 (weight fraction of Si + C) × 12.01 / 40.09. Since the raw materials (Si powder and C powder) were originally mixed so that it would be 50 mass% C and 50 mass% SiC, the excess C is 0.004426 g added.

[0118] From these results, the weight ratio of SiC:Si:C, when dividing the whole by 0.004426, is 0.3784:0.6216×28.08 / 40.09:1 + 0.6216×12.01 / 40.09 = 0.3784:0.4465:1.186 = 18.8:22.2:59.0.

[0119] From the above, C:SiC:Si = 59.0 mass%:18.8 mass%:22.2 mass%.

[0120] [Test Example 4: Charge-Discharge Measurement (Part 2)] The charge-discharge measurement was performed using a two-electrode cell with a potentiostat / galvanostat analyzer ECstat-302. The cell was temperature-controlled in a thermostatic bath at 20°C.

[0121] In a lithium-ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 1, the charge-discharge curves and the transition of the discharge capacity when charging and discharging at 100 mA / g to Si-C (Si / C = 1 / 1 in mol%) for the 1st to 5th cycles and at 1000 mA / g to Si-C (Si / C = 1 / 1 in mol%) for the 6th to 10th cycles are shown in FIGS. 11 to 12. Note that, as in the above, the charge-discharge rate is calculated with respect to the total weight of the generated laminated irregular structure type SiC and the equimolar amount of C of unreacted Si. In FIGS. 11 to 12, the discharge capacity in each cycle is calculated with respect to the total weight of SiC and Si obtained by the calculation method of the content shown in (1-4). From the above results, it can be understood that by using the negative electrode active material for a lithium-ion secondary battery of the present invention, it is excellent in all of capacity, cycle characteristics, and rate characteristics.

[0122] Based on these results, the content ratios of carbon, silicon carbide, and silicon in the negative electrode active material for a lithium-ion secondary battery of Example 1 were evaluated by the method described in the above (1-4).

[0123] Although carbon also has the possibility of inserting and desorbing lithium ions, unlike graphite having a layered structure, the theoretical charge-discharge capacity of an amorphous carbon material cannot be calculated, and the amount of carbon inserting and desorbing lithium ions also varies depending on the pulverization conditions of the mechanochemical treatment and the like, and it is difficult to specify. Therefore, in this specification, for convenience, it is assumed that carbon does not insert and desorb lithium ions, that is, only silicon carbide and silicon insert and desorb lithium ions, and the calculation will be made on this assumption.

[0124] (Derivation of SiC:(Si + C)) (0.003455x × 1336) + (0.003455(1 - x) × 28.08 / 40.09 × 4198) = 6.564 Since x = 0.6485, SiC:(Si + C) = 0.6485:0.3515 (weight ratio).

[0125] Here, x is the weight fraction of SiC, and 1 - x represents the weight fraction of Si + C. Also, 6.667 mAh is the measured value, and 0.003455 g is the combined amount of the weight of SiC, unreacted Si, and C with an equimolar amount to the unreacted Si. Among the negative electrode layers (the ones coated on the current collector), C + SiC is 85% by mass, and since half of it (50% by mass) is C and SiC respectively, when the raw material C and raw material Si react completely, the raw materials are mixed so that C and Si - C (equimolar) are each 50% by mass.

[0126] (Calculation of the specific capacity in the first cycle) 6.564 / (0.003455x + 0.003455(1 - x)×28.08 / 40.09)=D D = 2123 mAh / g.

[0127] (Calculation of C:SiC:Si) The weight of SiC is 0.003455 g×0.6485 (weight fraction of SiC). The weight of Si (unreacted Si) is 0.003455 g×0.3515 (weight fraction of Si + C)×28.08 / 40.09. The weight of C (unreacted C that did not become SiC + the remaining 50% by mass of C) is in addition to 0.003455 g×0.3515 (weight fraction of Si + C)×12.01 / 40.09. Since the raw materials (Si powder and C powder) were originally mixed so that it would be 50% by mass of C and 50% by mass of SiC, the excess C is the amount with 0.003455 g added.

[0128] From these results, when the whole is divided by 0.003455, the weight of SiC: the weight of Si: the weight of C is 0.6485:0.3515×28.08 / 40.09:1 + 0.3515×12.01 / 40.09 = 0.6485:0.2462:1.1053 = 32.4:12.3:55.3.

[0129] Therefore, C:SiC:Si = 55.3% by mass:32.4% by mass:12.3% by mass.

[0130] From these results, a content ratio equivalent to that calculated from the results of Test Example 3 was calculated, and the reliability of the content ratio calculation method was confirmed.

[0131] Comparative Example 1 A mixture of active materials of 60% by mass of carbon and 40% by mass of laminated irregular structure type silicon carbide (SD-SiC) was added to 80% by mass of the total electrode coating material, and 5% by mass of pulverized nano-Si with an average crystallite diameter of about 8 nm was added thereto. Then, 7.5% by mass of polyacrylic acid (PAA) and 7.5% by mass of carboxymethyl cellulose (CMC) were combined to make 100% by mass, and the slurry adjusted by a stirring and defoaming machine was coated on a current collector Cu foil to fabricate a negative electrode. That is, carbon, SD-SiC, and Si were simply mixed, and SD-SiC and silicon were not dispersed in the amorphous carbon material so as to have a conductive path with the amorphous carbon material. At this time, in the charge-discharge measurement, at a charge-discharge rate of 100 mA / g, after charging, the discharge capacity of the first discharge was lower than the theoretical charge-discharge capacity of SD-SiC of 1336 mAh / g, and further decreased significantly to about 990 to 1080 mAh / g at the second time, and no significant effect of Si addition was observed.

[0132] Comparative Example 2 Similar to Examples 1 and 2, a mixture of active materials of 50% by mass of carbon and 50% by mass of silicon carbide was used as 85% by mass of the total coating amount, and a slurry prepared by combining 7.5% by mass of polyacrylic acid (PAA) and 7.5% by mass of carboxymethyl cellulose (CMC) and adjusting it with a stirring and defoaming machine was coated on a current collector Cu foil to fabricate a negative electrode. At this time, under the powder synthesis conditions of Examples 1 and 2, the powder synthesized by reducing the mechanochemical treatment time of the pulverization conditions to 5 hours was used. As a result of X-ray diffraction, at 5 hours, most of the diffraction peaks were Si, and since the diffraction peaks of the laminated irregular structure type silicon carbide (SD-SiC) were hidden in the diffraction peaks of Si, it can be understood that almost no SD-SiC was generated. Using this electrode, like the charge-discharge experiment of Test Example 3, charge-discharge was first performed at 100 mA / g to Si-C (Si / C = 1 / 1 in mol%), and subsequently, from the second to the tenth charge-discharge was performed at a rate of 1000 mA / g. As a result, the discharge capacity monotonically decreased without being retained.

[0133] Example 3 Similar to Examples 1 to 2, a mixture of active materials of 50% by mass of carbon and 50% by mass of silicon carbide was used as 85% by mass of the total coating amount, and a slurry prepared by combining 7.5% by mass of polyacrylic acid (PAA) and 7.5% by mass of carboxymethyl cellulose (CMC) and adjusting it with a stirring and defoaming machine was coated on a current collector Cu foil to fabricate a negative electrode. At this time, under the powder synthesis conditions of Examples 1 and 2, the powder synthesized by reducing the mechanochemical treatment time of the pulverization conditions to 8 hours was used. As a result, similar to Examples 1 to 2, it was confirmed that SD-SiC and silicon were dispersed in the amorphous carbon material so as to have a conductive path with the amorphous carbon material. Using this electrode, as in the charge-discharge experiment of Test Example 3, charging and discharging were first performed at 100 mA / g to Si-C (Si / C = 1 / 1 in mol%), and subsequently, from the second to the fifth charge-discharge cycles were performed at a rate of 1000 mA / g to Si-C (Si / C = 1 / 1 in mol%). As a result, as shown in FIG. 13, the initial discharge capacity after the initial charge at 100 mA / g to Si-C (Si / C = 1 / 1 in mol%) was 3422 mAh / g, and at the charge-discharge rate of 1000 mA / g after the second cycle, the discharge capacity slightly decreased from 2760 mAh / g (second cycle) to 2544 mAh / g (fifth cycle), but still maintained a high discharge capacity. The state is shown in FIG. 14.

[0134] At this time, according to the calculation method of (1-4), the weight fraction of laminated irregular structure type SiC calculated from the initial discharge capacity of 100 mA / g was 0.2065, and the total of unreacted Si and C equimolar to Si was 0.7935. Therefore, when calculating the content ratios of carbon, silicon carbide, and silicon in the negative electrode active material for a lithium-ion secondary battery, the following results were obtained (the calculation method is the same as before).

[0135] (Derivation of SiC:(Si + C)) (0.004636x×1336)+(0.004636(1 - x)×28.08 / 40.09×4198)=12.10 Since x = 0.2065, SiC:(Si + C)=0.2065:0.7935 (weight ratio).

[0136] Here, x is the weight fraction of SiC, and 1 - x represents the weight fraction of Si + C. Also, 12.10 mAh is the measured value, and 0.004636 g is the combined amount of the weight of SiC, unreacted Si, and C with an equimolar amount to the unreacted Si. Among the negative electrode layers (the ones coated on the current collector), C + SiC is 85% by mass, and since half of it (50% by mass) is C and SiC respectively, the raw materials C and raw Si are mixed such that when they react completely, C and Si - C (equimolar) are each 50% by mass.

[0137] (Calculation of the specific capacity in the first cycle) 12.10 / (0.004636x + 0.004636(1 - x)×28.08 / 40.09)=D D = 3422 mAh / g.

[0138] (Calculation of C:SiC:Si) The weight of SiC is 0.004636 g×0.2065 (weight fraction of SiC). The weight of Si (unreacted Si) is 0.004636 g×0.7935 (weight fraction of Si + C)×28.08 / 40.09. The weight of C (unreacted C that did not become SiC + the remaining 50% by mass of C) is in addition to 0.004636 g×0.7935 (weight fraction of Si + C)×12.01 / 40.09. Since the raw materials (Si powder and C powder) were mixed so that originally it would be 50% by mass of C and 50% by mass of SiC, the additional C is 0.004636 g.

[0139] From these results, the weight ratio of SiC:Si:C, when dividing the whole by 0.004636, is 0.2065:0.7935×28.08 / 40.09:1 + 0.7935×12.01 / 40.09 = 0.2065:0.5558:1.2377 = 10.3:27.8:61.9.

[0140] From the above, C:SiC:Si = 61.9% by mass:10.3% by mass:27.8% by mass.

Claims

1. A negative electrode active material for a lithium ion secondary battery, comprising: a laminated irregular structure type silicon carbide; an amorphous carbon material; and silicon, The stacked disordered silicon carbide and the silicon are dispersed in the amorphous carbon material so as to have a conductive path between the stacked disordered silicon carbide and the amorphous carbon material, and The negative electrode active material for a lithium ion secondary battery has a silicon content of 5.0 to 28.0 mass % and a stacked irregular structure type silicon carbide content of 21.6 to 57.0 mass %, based on a total amount of 100 mass %.

2. 2. The negative electrode active material for lithium ion secondary batteries according to claim 1, wherein the content of the amorphous carbon material is 28.8 to 66.5 mass%, based on a total amount being 100 mass%.

3. 2. The negative electrode active material for lithium ion secondary batteries according to claim 1, wherein the average particle size of the stacked irregular structure type silicon carbide is 5 to 20 nm.

4. 2. The negative electrode active material for lithium ion secondary batteries according to claim 1, wherein the amorphous carbon material has an average particle size of 5 to 25 nm.

5. 2. The negative electrode active material for a lithium ion secondary battery according to claim 1, wherein the stacked irregular structure type silicon carbide has a full width at half maximum of a peak at 2θ=36.0° of 2.0° or more within a tolerance range of ±0.5° in X-ray diffraction measurement using CuKα radiation.

6. A negative electrode for a lithium ion secondary battery, comprising the negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 5.

7. A lithium ion secondary battery comprising the negative electrode for lithium ion secondary batteries according to claim 6.

8. A method for producing a negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 5, A manufacturing method comprising a step of subjecting a raw material mixture containing a silicon-containing material and a carbon material to a mechanochemical treatment until the content of the silicon is 5.0 to 28.0 mass% and the content of the stacked irregular structure type silicon carbide is 21.6 to 57.0 mass%, with the total amount being 100 mass%.

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