Negative electrode active material for lithium-ion secondary batteries and method for manufacturing the same

Abstract

This invention provides a material that can serve as a negative electrode active material for manufacturing lithium-ion secondary batteries with excellent cycle characteristics. [Solution] A negative electrode active material for a lithium-ion secondary battery, comprising a stacked disordered silicon carbide and an amorphous carbon material, wherein the stacked disordered silicon carbide content is 50.0 to 97.0% by mass, with a total amount of 100% by mass, and the stacked disordered silicon carbide is embedded or dispersed in the amorphous carbon material such that it has conductive paths with the amorphous carbon material.

Description

[Technical Field]

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

[0002] Currently, the charge and discharge capacity of lithium-ion secondary batteries largely depends on the active materials of the positive and negative electrodes. For positive electrode active materials, lithium iron phosphate is known to be the primary material for future industrialization due to its stability, cost-effectiveness, and ease of supply. On the other hand, the next-generation negative electrode active material is still not fully established. Currently, graphite is used as the negative electrode active material, but its theoretical charge and discharge capacity is 372 mAh / g, and the development of even higher-capacity negative electrode active materials is awaited. Meanwhile, silicon has a theoretical charge and discharge capacity exceeding 3500 mAh / g, approximately 10 times that of graphite. However, during charging, lithium ions inserted into and removed from the negative electrode react with silicon to form compounds, causing the silicon volume to expand by approximately 3 to 4 times. Conversely, when lithium ions dissociate from the compound during discharge, the silicon volume decreases to approximately 1 / 4 to 1 / 3 of its original volume. As a result, contact between the active material and the electrode is not maintained during the charge-discharge cycle, leading to a significant decrease in cycle performance. For example, even after only 5 cycles, the capacity can drop to about 35%. For this reason, SiO (Si + SiO2) and cubic β-SiC, which exhibit minimal volume change, have been studied as negative electrode active materials. For example, the latter has been reported in various papers (see, for example, Non-Patent Documents 1 to 8), but in all cases, highly crystalline cubic β-SiC is used. Furthermore, due to poor reproducibility and unsuitability for mass production, a definitive active material has not yet been established.

[0003] Incidentally, the present inventors have developed not only highly crystalline cubic β-SiC silicon carbide, but also a layered disordered structure silicon carbide in which the closest packed layers of silicon or carbon in the silicon carbide are irregularly stacked in one dimension in the

[0001] direction (see, for example, Non-Patent Document 9). The layered disordered structure silicon carbide reported in Non-Patent Document 9 was manufactured by a mechanochemical treatment for 24 hours with a silicon:carbon ratio of 1:1 (molar ratio). [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] RSC Advances, 2013, 3, 15028. [Non-Patent Document 2] Mater. Res. Soc. Symp. Proc. Vol. 1678, 2014 Materials Research Society. [Non-Patent Document 3] Solid State Ionics, 263 (2014) 23-26. [Non-Patent Document 4] Materials. Front. Chem. Vol. 6 (2018) 166. [Non-Patent Document 5] ACS Appl. Energy Mater. 2020, 3, 12613-12626. [Non-Patent Document 6] New J. Chem., 2021, 45, 19105-19117. [Non-Patent Document 7] Nanomaterials 2022, 12, 659. [Non-Patent Document 8] J. Mater. Chem. A, 2022, 10, 5230-5243. [Non-Patent Document 9] J. Am. Chem. Soc., 98, 50-56 (2015). [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] The present inventors, as described in Non-Patent Document 9, use a silicon:carbon ratio of 1:1 (molar ratio). long We found that even when attempting to use a layered disordered silicon carbide structure manufactured by mechanochemical processing over time as a negative electrode active material, it is difficult to insert and remove lithium ions during charging and discharging, making it impossible to manufacture lithium-ion secondary batteries with particularly excellent cycle characteristics.

[0006] Therefore, the present invention aims to provide a material that can serve as a negative electrode active material for manufacturing lithium-ion secondary batteries with excellent cycle characteristics. [Means for solving the problem]

[0007] As a result of diligent research, the inventors have discovered that in a negative electrode active material for lithium-ion secondary batteries containing stacked disordered silicon carbide and amorphous carbon material, silicon carbide generated at the Si-C interface migrates to the conductive excess carbon matrix, allowing for reversible intercalation and deintercalation of lithium ions during charging and discharging. Furthermore, by increasing the silicon carbide content, the cycle characteristics can be dramatically improved despite a relatively small amount of carbon, which is also a conductive material. This invention was completed based on these findings and further research. In other words, this invention encompasses the following configuration. Item 1. Containing layered disordered structure silicon carbide and amorphous carbon material, With the total amount being 100% by mass, the content of the layered irregular structure silicon carbide is 50.0 to 97.0% by mass. The layered disordered silicon carbide is embedded or dispersed in the amorphous carbon material such that it has conductive paths with the amorphous carbon material. Anode active material for lithium-ion secondary batteries. Item 2. Furthermore, containing silicon, The negative electrode active material for a lithium-ion secondary battery according to item 1, wherein, with a total amount of 100% by mass, the silicon content is 2.0 to 35.0% by mass, and the layered disordered structure silicon carbide content is 50.0 to 97.0% by mass. Item 3. The negative electrode active material for lithium-ion secondary batteries according to Item 2, wherein the content of the amorphous carbon material is 0.5 to 20.0% by mass, with the total amount being 100% by mass. Item 4. The negative electrode active material for a lithium-ion secondary battery according to item 2 or 3, wherein the stacked disordered silicon carbide and the silicon are embedded or dispersed in the amorphous carbon material such that they have conductive paths with the amorphous carbon material. Item 5. The negative electrode active material for a lithium-ion secondary battery according to any one of items 1 to 4, wherein the average particle size of the stacked disordered structure silicon carbide is 5 to 20 nm. Item 6. The negative electrode active material for a lithium-ion secondary battery according to any one of items 1 to 5, wherein the average particle size of the ultra-high quality carbon material is 5 to 25 nm. Item 7. The stacked disordered silicon carbide is a negative electrode active material for a lithium-ion secondary battery according to any one of items 1 to 6, wherein, in X-ray diffraction measurements using CuKα rays, the full width at half maximum of the peak at 2θ = 36.0° is 2.0° or more within a tolerance of ±0.5°. Item 8. A negative electrode for a lithium-ion secondary battery containing the negative electrode active material for a lithium-ion secondary battery described in any one of items 1 to 7. Item 9. A lithium-ion secondary battery containing the negative electrode for lithium-ion secondary batteries described in Item 8. Item 10. A method for producing a negative electrode active material for a lithium-ion secondary battery according to any one of items 1 to 7, The process includes a step of subjecting a raw material mixture containing silicon-containing material and carbon material to mechanochemical treatment. With the total amount of the raw material mixture being 100% by mass, the content of the silicon-containing material is 40-60% by mass, and the content of the carbon material is 40-60% by mass. The mechanochemical treatment is carried out until the content of the laminated irregular structure type silicon carbide becomes 50.0 to 97.0% by mass with the total amount being 100% by mass. Production method.

Effect of the Invention

[0008] 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 having excellent cycle characteristics.

Brief Description of the Drawings

[0009] [Figure 1] A schematic diagram of a structure in which laminated irregular structure type silicon carbide and unreacted silicon are embedded or dispersed in an amorphous carbon material so as to secure a conductive path is shown. [Figure 2] X-ray diffraction diagrams of the negative electrode active materials obtained in Examples 1 to 3 and the negative electrode active materials obtained when the pulverization time was 10 hours as in Examples 1 to 3 are shown. [Figure 3] An X-ray diffraction spectrum of laminated irregular structure type silicon carbide (SD-SiC) is shown. [Figure 4] An X-ray diffraction diagram of highly crystalline cubic β-SiC is shown. [Figure 5] An X-ray diffraction spectrum of amorphous carbon is shown. [Figure 6] X-ray diffraction diagrams measured by mixing separately prepared SD-SiC and Si are shown in order to estimate the mixing ratio of SD-SiC and Si in the negative electrode active materials for lithium ion secondary batteries obtained in Examples 1 to 3. [Figure 7] A high-resolution transmission electron microscope (TEM) image of the negative electrode active material obtained when the pulverization time was 4 hours in the reaction of graphite and silicon is shown. [Figure 8] A photograph published in J. Am. Chem. Soc., 98, 50-56 (2015) observed by high-resolution TEM of SD-SiC after synthesis is shown. [Figure 9]The discharge capacity for each cycle of a lithium-ion secondary battery (half-cell) manufactured using the negative electrode obtained in Comparative Example 1, when charged and discharged at a charge / discharge rate of 100 mA / g ~ Si-C (Si / C = 1 / 1 in mol%), is shown. The discharge capacity for each cycle was calculated relative to the weight of SD-SiC. [Figure 10] The discharge capacity for each cycle of a lithium-ion secondary battery (half-cell) manufactured using the negative electrode obtained in Example 1 is shown, when the charge-discharge rate is set to 100 mA / g ~ Si-C (Si / C = 1 / 1 in mol%). The discharge capacity for each cycle was calculated relative to the weight of the active material (containing SD-SiC, unreacted Si, and carbon). [Figure 11] The discharge capacity for each cycle of lithium-ion secondary batteries (half-cells) manufactured using the negative electrodes obtained in Examples 2 and 3, when charged and discharged at a charge / discharge rate of 100 mA / g ~ Si-C (Si / C = 1 / 1 in mol%), is shown. The discharge capacity for each cycle was calculated relative to the weight of the active material (containing SD-SiC, unreacted Si, and carbon). For reference, the results of investigating the discharge capacity as an active material by nano-sizing only Si by mechanical grinding, as in Comparative Example 2, are also shown. [Figure 12] The charge-discharge curves for the first and fifth cycles of a lithium-ion secondary battery (half-cell) manufactured using the negative electrodes obtained in Examples 1 and 2, when charged and discharged at a charge-discharge rate of 100 mA / g ~ Si-C (Si / C = 1 / 1 in mol%), are shown. The discharge capacity in each cycle was calculated relative to the weight of the active material (containing SD-SiC, unreacted Si, and carbon). [Figure 13] The charge-discharge curves for the first and second cycles of a lithium-ion secondary battery (half-cell) manufactured using the negative electrode obtained in Example 3, when charged and discharged at a charge-discharge rate of 100 mA / g ~ Si-C (Si / C = 1 / 1 in mol%), are shown. The discharge capacity in each cycle was calculated relative to the weight of the active material (containing SD-SiC, unreacted Si, and carbon). [Modes for carrying out the invention]

[0010] In this specification, "contains" is a concept that encompasses all of the following: "contains," "consist essentially of," and "consist of."

[0011] Furthermore, in this specification, when a numerical range is expressed as A to B, it means A or greater and B or less.

[0012] 1. Negative electrode active material for lithium-ion secondary batteries The negative electrode active material for lithium-ion secondary batteries of the present invention contains a stacked disordered silicon carbide and an amorphous carbon material, with the total amount being 100% by mass, and the content of the stacked disordered silicon carbide being 50.0 to 97.0% by mass. The layered disordered silicon carbide is embedded or dispersed in the amorphous carbon material such that it has conductive paths with the amorphous carbon material.

[0013] In the manufacturing method of the lithium-ion secondary battery described later, the negative electrode active material of this invention may exhibit improved cycle characteristics by not increasing the amount of carbon in the raw materials too much and by using a mechanochemical processing time that leaves a small amount of silicon remaining in the mixture. Furthermore, it contains silicon, A negative electrode active material for lithium-ion secondary batteries containing a stacked disordered structure silicon carbide, an amorphous carbon material, and silicon, With the total amount being 100% by mass, the silicon content is 2.0 to 35.0% by mass, and the layered disordered structure silicon carbide content is 50.0 to 97.0% by mass. A negative electrode active material for a lithium-ion secondary battery, wherein the stacked disordered silicon carbide is embedded or dispersed in the amorphous carbon material such that it has conductive paths with the amorphous carbon material. It is preferable to do so.

[0014] In stacked disordered silicon carbide, the closest-packed silicon layers are irregularly stacked, with carbon atoms inserted in every other tetrahedral site (half), while the other half of the tetrahedral sites are empty. Furthermore, in stacked disordered silicon carbide, all octahedral sites, equal in number to the silicon, are empty. When this stacked disordered silicon carbide is used as the active material for the negative electrode of a lithium-ion secondary battery, charging and discharging will cause Li + Ions are inserted into all of these empty tetrahedral and octahedral sites, receiving electrons from the counter electrode and occupying the empty sites as lithium atoms. This is well known in the zincblende structure of CuSn and InSb, where the close-packed layers are stacked regularly. For example, in the case of CuSn, the chemical formula is Li2CuSn, resulting in a structure similar to a Heusler compound, which significantly suppresses the expansion and contraction of the active material during the charge-discharge process (JT Vaughey, KD Kepler, R. Benedek, MM Thackeray, Electrochem. Commun. 1 (1999) 517-521., JT Vaughey, J. O'Hara, MM Thackeray, Electrochem. Solid-State Lett. 3 (2000) 13-16.). In the case of stacked disordered silicon carbide, if we were to express it using a chemical formula, it would be Li2SiCx(SD) (Li2SiC(SD) etc.). Based on the ideas presented in the two papers mentioned above, the theoretical discharge capacity calculated is 1336 mAh / g, which is in close agreement with the discharge capacity of stacked disordered silicon carbide obtained experimentally. Furthermore, since expansion and contraction associated with charging and discharging are expected to be suppressed, the cycle characteristics are good.

[0015] This time, we consider the case where the grinding time (reaction time) in the mechanochemical treatment in the manufacturing method of the present invention is shortened, and a layered disordered structure silicon carbide and unreacted silicon act as the active material. As shown in the schematic diagram of Figure 1, carbon and silicon react, and a structure can be expected in which layered disordered structure silicon carbide and unreacted silicon react at the carbon-silicon interface and are embedded or dispersed within the carbon aggregate that acts as a conductive material, thereby ensuring a good conductive path. Silicon is Li + Reacting with ions, Li 4.4 While forming Si results in a high theoretical discharge capacity of 4198 mAh / g, silicon expands 3 to 4 times and contracts upon releasing lithium ions, leading to loss of conductive paths and detachment of the active material from the coating, thus degrading the cycle characteristics of the lithium-ion secondary battery. However, according to the present invention, with silicon and layered disordered silicon carbide formed in an amorphous carbon material, the amorphous carbon material is expected to absorb the expansion and contraction of silicon due to its reaction with lithium.

[0016] Layered irregular structure silicon carbide In the present invention, the stacked disordered silicon carbide contained in the negative electrode active material for lithium-ion secondary batteries is preferably, for example, SiCx (0.8 ≤ x ≤ 1.5). In particular, using SiC with x ≈ 1 is preferable from the viewpoint of charge / discharge capacity, cycle characteristics, etc.

[0017] The stacked disordered silicon carbide contained in the negative electrode active material for lithium-ion secondary batteries in the present invention can have broad peaks at 2θ = 36.0°, 60.0°, and 72.0°, which are attributed to silicon carbide, within a tolerance of ±0.5° in X-ray diffraction measurements using CuKα rays.

[0018] While highly crystalline β-SiC is commonly used as silicon carbide in lithium-ion secondary batteries, the stacked disordered structure silicon carbide contained in the negative electrode active material for lithium-ion secondary batteries of the present invention is preferably a low-crystallinity silicon carbide from the viewpoint of charge / discharge capacity, cycle characteristics, etc. Specifically, in X-ray diffraction measurements using CuKα rays, the stacked disordered structure silicon carbide preferably has a full width at half maximum of 2.0° or more, and more preferably 2.5 to 10.0°, within a tolerance range of ±0.5° for the broad peak at 2θ = 36.0°.

[0019] The shape of the layered irregular structure silicon carbide is not particularly limited; for example, any shape can be used, such as powder, plate, granule, sphere, fibrous, or lump.

[0020] In the negative electrode active material for lithium-ion secondary batteries of the present invention, it is preferable that small silicon carbides are embedded or dispersed in an amorphous carbon material from the viewpoint of easily intercalating and deintercalating lithium ions. Therefore, it is preferable that the average particle size of the stacked disordered structure silicon carbide is small. For this reason, the average particle size of the stacked disordered structure silicon carbide is preferably 5 to 20 nm, and more preferably 5 to 10 nm. The average particle size of the stacked disordered structure silicon carbide is measured by observation with a high-resolution transmission electron microscope.

[0021] In the negative electrode active material for lithium-ion secondary batteries of the present invention, the content of stacked disordered silicon carbide is not particularly limited. However, from the viewpoint of easily intercalating and deintercalating lithium ions, it is preferable that the small silicon carbide particles are embedded or dispersed in the amorphous carbon material without aggregation, so that they have conductive paths with the amorphous carbon material. Therefore, the content is preferably 50.0 to 97.0% by mass, and more preferably 55.0 to 96.8% by mass, based on 100% by mass of the total amount of the negative electrode active material for lithium-ion secondary batteries of the present invention. It is also possible to further increase the content of stacked disordered silicon carbide to 58.0% to 97.0% by mass, and particularly 61.0 to 96.8% by mass, based on 100% by mass of the total amount of the negative electrode active material for lithium-ion secondary batteries of the present invention.

[0022] Amorphous carbon materials In the negative electrode active material for lithium-ion secondary batteries of the present invention, the inclusion of an amorphous carbon material improves conductivity and makes it possible to conduct electricity.

[0023] The amorphous carbon material contained in the negative electrode active material for lithium-ion secondary batteries in the present invention can have broad peaks at 2θ = 22.5° and optionally 42.0°, which are characteristic of amorphous carbon material, within a tolerance of ±0.5° in X-ray diffraction measurements using CuKα rays.

[0024] In the present invention, the amorphous carbon material contained in the negative electrode active material for lithium-ion secondary batteries preferably has a full width at half maximum of 2.0° or more, and more preferably 3.0 to 10.0°, within a tolerance range of ±0.5°, when measured by X-ray diffraction using CuKα rays, with a broad peak at 2θ = 22.5°.

[0025] The shape of amorphous carbon material is not particularly limited; for example, it can be used in any form, such as powder, plate, granule, sphere, fibrous, or lump, but it is usually spherical.

[0026] In the negative electrode active material for lithium-ion secondary batteries of the present invention, it is preferable that small silicon carbide particles are embedded or dispersed in the amorphous carbon material from the viewpoint of easily intercalating and deintercalating lithium ions. Therefore, it is preferable that the average particle size of the amorphous carbon material is also small. Since amorphous carbon material is amorphous, it may be difficult to clearly determine its average particle size, but the average particle size of the amorphous carbon material is preferably 5 to 25 nm, and more preferably 10 to 20 nm. The average particle size of the amorphous carbon material is measured by observation with a high-resolution transmission electron microscope.

[0027] In the negative electrode active material for lithium-ion secondary batteries of the present invention, the content of amorphous carbon material is not particularly limited, but from the viewpoint of improving conductivity while also favoring a lower carbon content in terms of cycle characteristics, it is preferable that the content be 0.5 to 20.0% by mass, and more preferably 0.8 to 15.0% by mass, based on the total amount of the negative electrode active material for lithium-ion secondary batteries being 100% by mass.

[0028] silicon In the negative electrode active material for lithium-ion secondary batteries according to the present invention, the capacity can be improved by including an amorphous carbon material, and since silicon is embedded or dispersed in the amorphous carbon material so as to have conductive paths with the amorphous carbon material, the expansion and contraction of silicon can be suppressed. Therefore, the destruction of the negative electrode active material for lithium-ion secondary batteries due to the expansion and contraction of silicon can be suppressed, and the cycle characteristics can also be improved.

[0029] The silicon contained in the negative electrode active material for lithium-ion secondary batteries of the present invention can have peaks at 2θ = 28.0°, 47.0°, and 56.0°, which are silicon-dependent, within a tolerance range of ±0.5° in X-ray diffraction measurements using CuKα rays.

[0030] The shape of the silicon is not particularly limited; for example, it can be used in any form, such as powder, plate, granules, spheres, fibers, or lumps, but it is usually used in a spherical shape.

[0031] In the negative electrode active material for lithium-ion secondary batteries of the present invention, it is preferable that small silicon particles are embedded or dispersed in the amorphous carbon material, from the viewpoint of easily improving capacity and easily suppressing the expansion and contraction of silicon with the amorphous carbon material, thereby improving cycle characteristics. For this reason, the average crystallite diameter of silicon is preferably 5 to 100 nm, and more preferably 6 to 50 nm. The average crystallite diameter of silicon is measured by observation with a high-resolution transmission electron microscope.

[0032] In the negative electrode active material for lithium-ion secondary batteries of the present invention, the silicon content is preferably such that small silicon particles have conductive paths with the amorphous carbon material, and is embedded or dispersed in the amorphous carbon material without aggregation, from the viewpoint of easily improving capacity and easily suppressing the expansion and contraction of silicon by the amorphous carbon material, thereby easily improving cycle characteristics. Therefore, with the total amount of the negative electrode active material for lithium-ion secondary batteries of the present invention being 100% by mass, the silicon content is preferably 2.0 to 35.0% by mass, more preferably 2.3 to 30.0% by mass.

[0033] Method for measuring content The content of each component in this invention is not particularly limited, but the ratio of the peak intensities of the X-ray diffraction pattern of a sample in which SD-SiC and MG-Si (nano-Si) are mixed in a predetermined mixing ratio is I Si(28°) / I SiC(36°) By plotting the Si content against the data (the vertical axis being the logarithm of the intensity ratio, where Log1=0 for 0 and the base of the logarithm is e), a calibration curve can be drawn, and the amounts of Si and SiC used in the examples can be estimated from there.

[0034] Negative electrode active material for lithium-ion secondary batteries The negative electrode active material for lithium-ion secondary batteries in the present invention contains, as described above, a layered disordered structure silicon carbide, an amorphous carbon material, and silicon.

[0035] In the negative electrode active material for lithium-ion secondary batteries of the present invention, the stacked disordered silicon carbide structure formed at the Si-C interface is covered with a conductive unreacted carbon layer, and as a result, the SD-SiC can reversibly intercept and deintercept lithium ions in conjunction with the charging and discharging of lithium ions. Therefore, the stacked disordered silicon carbide structure is preferably covered with an amorphous carbon material layer without aggregation so that it has a conductive path with the amorphous carbon material.

[0036] Furthermore, in the negative electrode active material for lithium-ion secondary batteries of the present invention, the silicon is covered with SD-SiC formed at the silicon-carbon interface, unreacted carbon, and the conductive material acetylene black (AB) mixed during the production of the half-cell, which absorbs the expansion and contraction of the silicon and prevents the destruction of the negative electrode active material for lithium-ion secondary batteries of the present invention. Therefore, it is preferable that the silicon also has conductive paths with the amorphous carbon material, preferably without aggregation, by embedding or dispersing the amorphous carbon material layer between the silicon and SD-SiC.

[0037] In addition to the stacked disordered structure silicon carbide, amorphous carbon material, and silicon described above, the negative electrode active material for lithium-ion secondary batteries in the present invention may also include, to the extent that it does not impair the effects of the present invention, silicon-containing raw materials (silicon nitride, silicon oxide, etc.), crystalline carbon material (graphite, etc.), crystalline silicon carbide (cubic β-SiC), metal components (iron, etc.) that are unavoidable during synthesis, and third components such as metal-nonmetal compounds. The content of these third components can be 0 to 10% by mass, particularly 0.01 to 5% by mass, based on 100% by mass of the total amount of the negative electrode active material for lithium-ion secondary batteries of the present invention.

[0038] In the negative electrode active material for lithium-ion secondary batteries according to the present invention as described above, the stacked disordered silicon carbide generated at the Si-C interface migrates to the conductive excess carbon matrix, and this can reversibly intercept and deintercept lithium ions in conjunction with the charging and discharging of lithium ions. Furthermore, although the stacked disordered silicon carbide in the negative electrode active material for lithium-ion secondary batteries according to the present invention is a silicon-containing material, it does not expand and contract as much during charging and discharging as silicon. In addition, in the negative electrode active material for lithium-ion secondary batteries according to the present invention, the stacked disordered silicon carbide and silicon are preferably embedded or dispersed in the amorphous carbon material without aggregation, so as to have conductive paths between them and the amorphous carbon material. This allows the amorphous carbon material adjacent to the stacked disordered silicon carbide and silicon to follow the expansion and contraction of the stacked disordered silicon carbide and the small amount of silicon. Therefore, by using the negative electrode active material for lithium-ion secondary batteries according to the present invention, it is possible to manufacture lithium-ion secondary batteries with high capacity and excellent cycle characteristics, preferably lithium-ion secondary batteries with particularly excellent cycle characteristics. In this invention, the term "negative electrode active material for lithium-ion secondary batteries" is a concept that also includes negative electrode active materials for metallic lithium secondary batteries in which lithium metal is used as the positive electrode.

[0039] 2. Negative electrode for lithium-ion secondary batteries The negative electrode for lithium-ion secondary batteries of the present invention contains the negative electrode active material for lithium-ion secondary batteries of the present invention. More specifically, the negative electrode for lithium-ion secondary batteries of the present invention may comprise a negative electrode active material layer containing the negative electrode active material for lithium-ion secondary batteries of the present invention.

[0040] The negative electrode active material layer may consist solely of the negative electrode active material for lithium-ion secondary batteries of the present invention as described above, but it may also contain conductive agents such as carbon black (acetylene black, furnace black, Ketjen black, etc.), flaky graphite, graphene, or amorphous carbon obtained by heat treatment of organic materials, as needed. In particular, since the negative electrode active material for lithium-ion secondary batteries of the present invention contains a large amount of layered irregular structure silicon carbide, the use of conductive agents is especially effective. These conductive agents can be used individually or in combination of two or more types.

[0041] Furthermore, the negative electrode active material layer may also contain, as necessary, binders, thickeners, or dispersants such as fluorinated polymers (polyvinylidene fluoride resin, polytetrafluoroethylene resin, vinylidene fluoride-hexafluoropropylene copolymer, etc.), polyolefin resins (styrene-butadiene copolymer resin, ethylene vinyl alcohol copolymer resin, etc.), synthetic rubbers (styrene-butadiene rubber, acrylonitrile-butadiene rubber, ethylene propylene diene rubber, etc.), polyacrylonitrile, polyamide, polyimide, polyacrylic acid, polyacrylic acid ester, polyvinyl ether, carboxymethylcellulose, carboxymethylcellulose sodium salt, carboxymethylcellulose ammonium, polyurethane, hydroxypropylcellulose, hydroxyethylcellulose, methylcellulose, etc. These binders, thickeners, or dispersants may be used individually or in combination of two or more.

[0042] The content of the negative electrode active material for the lithium-ion secondary battery of the present invention can be 60 to 90% by mass, particularly 65 to 85% by mass, based on 100% by mass of the total amount of the negative electrode active material layer, from the viewpoint of charge / discharge capacity, cycle characteristics, etc. Furthermore, the content of the conductive agent can be 2 to 30% by mass, particularly 3 to 25% by mass, based on 100% by mass of the total amount of the negative electrode active material layer, from the viewpoint of charge / discharge capacity, cycle characteristics, etc. Furthermore, the content of the binder, thickener, or dispersant can be 5 to 30% by mass, particularly 10 to 20% by mass, based on 100% by mass of the total amount of the negative electrode active material layer, from the viewpoint of charge / discharge capacity, cycle characteristics, etc.

[0043] The thickness of the negative electrode active material layer is not particularly limited, but from the viewpoint 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.

[0044] Such a negative electrode active material layer can be manufactured by forming a negative electrode mixture containing the negative electrode active material for lithium-ion secondary batteries of the present invention and, if necessary, a conductive agent, binder, thickener, dispersant, etc., into a layer. For example, the negative electrode mixture can be dried by a conventional method and formed into a layer.

[0045] As described above, the negative electrode for the lithium-ion secondary battery of the present invention preferably comprises the negative electrode active material layer described above, but more specifically, it preferably comprises a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

[0046] The negative electrode current collector is preferably made of a material that is electrochemically stable at the potential used and has high electronic conductivity, such as copper, stainless steel, nickel, or carbon material. This negative electrode current collector can be in the form of a foil, mesh, or other similar component.

[0047] When manufacturing the negative electrode for the lithium-ion secondary battery of the present invention as described above, it can be manufactured by forming the above-described negative electrode mixture in layers 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 in layers to manufacture the negative electrode for the lithium-ion secondary battery of the present invention.

[0048] 3. Lithium-ion rechargeable batteries The lithium-ion secondary battery of the present invention includes the negative electrode for the 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 the 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.

[0049] 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.

[0050] Examples of the positive electrode current collector constituting the positive electrode include materials that are electrochemically stable at the potential at which, for example, aluminum, stainless steel, carbon materials, etc. are used and have high electronic conductivity.

[0051] 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, lithium transition metal composite oxides having an α-NaFeO2-type crystal structure, lithium transition metal oxides having a spinel-type crystal structure, polyanion compounds, chalcogen compounds, sulfur, etc. can be mentioned. As the lithium transition metal composite oxide having an α-NaFeO2-type crystal structure, for example, Li[Li x1 Ni γ1 Mn β1 Co (1-x1-γ1-β1) O2(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)Examples include ]O2 (0≦x2<0.5, 0≦γ2≦1, 0≦β2≦1, 0≦γ2+β2≦1), etc. As a lithium transition metal oxide having a spinel-type crystal structure, Li x3 Mn2O4 (0.9 ≤ x 3 < 1.5), Li x4 Ni γ4 Mn (2-γ4) Examples include O4 (0.9 ≤ x4 < 1.5, 0 ≤ γ4 ≤ 2). Examples of polyanion compounds include LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, Li2CoPO4F, etc. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, molybdenum dioxide, etc. Some atoms or polyanions in these materials may be substituted with atoms or anions of other elements. These positive electrode active materials can be used individually or in combination of two or more types.

[0052] Other than the positive electrode active material that constitutes the positive electrode, the same materials as those used for the negative electrode other than the negative electrode active material can be used for the positive electrode constituent materials, and their content can also be the same as that of the negative electrode constituent materials other than the negative electrode active material.

[0053] Furthermore, the electrolyte is preferably an electrolyte obtained by dissolving a salt in an aprotic organic solvent, and is placed between the positive electrode and the negative electrode. It is preferable that the electrolyte is impregnated and held in a separator made of, for example, a nonwoven fabric to prevent short circuits between the positive and negative electrodes.

[0054] Examples of aprotic organic solvents that constitute the electrolyte mentioned above 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; sulforanes such as sulfolane and methylsulfolane; and acetonitrile. These aprotic organic solvents may be used individually or in combination of two or more.

[0055] On the other hand, examples of lithium salts that dissolve in such aprotic organic solvents include lithium perchlorate, lithium borofluoride, lithium hexafluoride phosphate, lithium hexafluoride arsenate, lithium trifluoromethanesulfonate, lithium halides, lithium aluminate chloride, and lithium bis(fluorosulfonyl)imide. These salts may be used individually or in combination of two or more.

[0056] 4. Method for producing negative electrode active material for lithium-ion secondary batteries The method for producing the negative electrode active material for lithium-ion secondary batteries of the present invention is not particularly limited, but includes a step of subjecting a raw material mixture containing a silicon-containing material and a carbon material to a mechanochemical treatment. Equipped with, With the total amount of the raw material mixture being 100% by mass, the content of the silicon-containing material is 40-60% by mass, and the content of the carbon material is 40-60% by mass. The mechanochemical treatment is carried out until the content of the layered irregular structure silicon carbide is 50.0 to 97.0% by mass, with the total amount being 100% by mass.

[0057] Specifically, a raw material mixture containing silicon-containing material and carbon material can be subjected to mechanochemical treatment until, with a total amount of 100% by mass, the silicon content is 2.0 to 35.0% by mass and the layered disordered structure silicon carbide content is 50.0 to 97.0% by mass.

[0058] There are no particular restrictions on the silicon-containing materials used as raw materials; in addition to silicon, silicon-containing compounds and the like can be used. These silicon-containing materials can be used individually or in combination of two or more types. Furthermore, since the silicon-containing materials are mixed and pulverized by mechanochemical processing, there are no restrictions on the particle size of the silicon-containing materials used, and commercially available powdered silicon-containing materials can usually be used.

[0059] There are no particular restrictions on the carbon materials used as raw materials; carbon black such as acetylene black, furnace black, and Ketjen black; graphite; graphene; amorphous carbon, etc., can be used. These carbon materials can be used individually or in combination of two or more. Furthermore, since the carbon materials are mixed and pulverized by mechanochemical processing, there are no restrictions on the particle size of the carbon materials used, and commercially available powdered carbon materials can usually be used.

[0060] Mechanochemical processing is a method of grinding and mixing raw materials while applying mechanical energy. This method involves applying mechanical impact and friction to the raw materials, causing them to come into intense contact with silicon-containing materials and carbon materials, resulting in refinement and a reaction between the materials. In other words, mixing, grinding, and reaction occur simultaneously. Therefore, it is possible to ensure a more reliable reaction of the raw materials without heating them to high temperatures. Mechanochemical processing can sometimes yield metastable crystalline structures that cannot be obtained through conventional heat treatment.

[0061] These raw materials can be mixed together simultaneously and subjected to mechanochemical treatment, or some of the raw materials can be subjected to mechanochemical treatment first, and then the remaining raw materials can be added and subjected to mechanochemical treatment.

[0062] Regarding the mixing ratio of raw materials, the elemental ratio of silicon and carbon in the negative electrode active material for lithium-ion secondary batteries of the present invention can be considered. In particular, from the viewpoint of improving cycle characteristics, the content of stacked disordered structure silicon carbide is increased, that is, the content of amorphous carbon material is decreased. Assuming the total amount of the raw material mixture is 100% by mass, the content of silicon-containing material is preferably 40-60% by mass, more preferably 45-55% by mass, and even more preferably 46-54% by mass. Also, assuming the total amount of the raw material mixture is 100% by mass, the content of carbon material is preferably 40-60% by mass, more preferably 45-55% by mass, and even more preferably 46-54% by mass.

[0063] In the present invention, the energy input in the mechanochemical treatment is preferably 10 to 110 kWh / 1 kg of raw material mixture, and more preferably 25 to 80 kWh / 1 kg of raw material mixture, from the viewpoint of easily obtaining the layered disordered structure silicon carbide of the present invention.

[0064] The energy input for mechanochemical treatment is described in the following reference: Burgio, N., Lasonna, A., Magini, M., Martelii, S. and Padella, F., Il Nuovo Cimento, Vol. 13, pp. 459-476 (1991). It is calculated using the formula shown.

[0065] In mechanochemical processing, when rotation and revolution are applied to the raw material mixture, a strong combined centrifugal force can be exerted, generating convective motion and vortices due to rotation. These flows are effectively combined to achieve dense stirring, enabling the efficient production of the layered irregular structure silicon carbide of the present invention.

[0066] In this case, assuming the use of the P-5 manufactured by Fritzke, the rotation-orbit ratio is 2, and the upper limit of the orbital speed is set to 400 rpm. Therefore, although the rotational speed is not particularly limited, from the viewpoint of easily obtaining the layered disordered structure silicon carbide of the present invention, 500 to 800 rpm is preferred, and 600 to 700 rpm is more preferred. Similarly, although the orbital speed is not particularly limited, from the viewpoint of easily obtaining the layered disordered structure silicon carbide of the present invention, 250 to 400 rpm is preferred, and 300 to 350 rpm is more preferred.

[0067] There are no particular restrictions on the temperature during the mechanochemical treatment, and it can be adjusted as appropriate from the viewpoint of easily obtaining the negative electrode active material for lithium-ion secondary batteries of the present invention. For example, it can be set to room temperature.

[0068] Regarding the mechanochemical treatment time, it is preferable to increase the mechanochemical treatment time, especially from the viewpoint of improving the cycle characteristics. However, in a previous report (J. Am. Chem. Soc., 98, 50-56 (2015)), a 24-hour reaction was performed for Si and C to completely react and synthesize a stacked disordered structure silicon carbide at a molar ratio of 1:1. However, if the reaction is complete and the structure consists only of a stacked disordered structure silicon carbide, the cycle characteristics deteriorate, so it is preferable to use a shorter time than this.

[0069] In this invention, since we aim to produce a negative electrode active material for lithium-ion secondary batteries that intentionally contains free silicon, we intentionally react only a portion of the silicon-containing material and the carbon material to produce a layered disordered structure silicon carbide, while intentionally leaving some silicon as a raw material. These silicon and carbon materials are then embedded or dispersed in the amorphous carbon material to create conductive paths between them. For this reason, the grinding time (processing time) for the mechanochemical treatment is intentionally kept short, but if it is too short, the cycle characteristics will deteriorate. For example, the mechanochemical treatment can be carried out for 7 to 15 hours (especially 8 to 13 hours). Furthermore, this mechanochemical treatment can be carried out in multiple stages with pauses in between as needed.

[0070] Furthermore, since the reaction time tends to be shorter at higher rotation speeds, it is preferable to adjust the rotation speed as needed. This mechanochemical treatment can also be performed in multiple stages, with pauses in between as necessary.

[0071] Furthermore, if mechanochemical treatment is repeated multiple times, the above conditions can be applied to the mechanochemical treatment in each step.

[0072] When performing the mechanochemical treatment described above, specifically, mixing and grinding can be carried out using mechanical grinding equipment such as ball mills, planetary ball mills, bead mills, rod mills, vibratory mills, disc mills, hammer mills, jet mills, surface modification and grinding equipment, and high-pressure gas pulverizers.

[0073] In this invention, we aim to produce a negative electrode active material for lithium-ion secondary batteries that intentionally contains free silicon. However, unlike the manufacturing method of this invention described above, when a material is mixed with silicon that has been obtained by long-term mechanochemical treatment to form a stacked disordered structure silicon carbide embedded or dispersed in an amorphous carbon material so as to have conductive paths with the amorphous carbon material, it is not possible to embed or disperse the silicon in the amorphous carbon material so as to have conductive paths with the amorphous carbon material. As a result, the expansion and contraction of silicon cannot be suppressed, the destruction of the negative electrode active material for lithium-ion secondary batteries due to the expansion and contraction of silicon cannot be suppressed, and the cycle characteristics cannot be improved. [Examples]

[0074] The present invention will be described in detail based on the examples provided, but the present invention is not limited to these examples.

[0075] The raw material powders used were silicon (manufactured by High Purity 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).

[0076] [Example 1: 12 hours] Mixing ratio The raw material mixing ratio was 2.80226 g of sample and 6.95 g of distilled water. The sample was weighed to consist of 49.0% by mass of silicon, 21.0% by mass of natural graphite, 15.0% by mass of acetylene black, 7.5% by mass of polyacrylic acid (PAA), and 7.5% by mass of carboxymethylcellulose (CMC).

[0077] Powder synthesis Using a high-energy ball mill apparatus (Fritsch P5; Si3N4 pot: 250 mL; Si3N4 balls: 10 mm in diameter), 5.2534 g of silicon and 2.2478 g of natural graphite were added to the pot, and mechanochemical treatment was performed with an orbital speed of 300 rpm, a rotational speed of 600 rpm, a grinding time of 12 hours, and a ball weight:powder weight ratio of 40:1 to obtain the negative electrode active material for lithium-ion secondary batteries of Example 1.

[0078] Negative electrode fabrication The conditions for the agitator and defoamer used in the negative electrode preparation were as follows: for agitation, the rotation speed was 800 rpm and the revolution speed was 2000 rpm for 60 seconds; and for defoaming, the rotation speed was 60 rpm and the revolution speed was 2200 rpm for 30 seconds.

[0079] First, 0.21062 g of polyacrylic acid (PAA) and 1.7 mL of distilled water were added to a container, and the mixture was stirred and degassed twice to obtain an aqueous binder solution. Next, 0.42020 g of acetylene black and 3.05 mL of distilled water were added to the aqueous binder solution, and the mixture was stirred and degassed twice to obtain an aqueous binder solution containing a conductive agent.

[0080] Then, 1.9612 g of the lithium-ion secondary battery negative electrode active material obtained in Example 1 was added to the aqueous solution of the conductive binder, mixed with a spatula, and 0.5 mL of distilled water was added in portions while checking the consistency, and stirring and degassing were performed twice.

[0081] Furthermore, 0.21024 g of carboxymethylcellulose (CMC) was added to the obtained aqueous solution, and after stirring and degassing, 1.7 mL of distilled water was added, and stirring and degassing were performed twice to obtain the negative electrode mixture.

[0082] Subsequently, the obtained negative electrode mixture was applied to the copper foil with a doctor blade to a thickness of 175 μm, and vacuum drying was performed at 80°C for 12 hours to obtain the negative electrode for lithium-ion secondary batteries of Example 1.

[0083] [Example 2: 9 hours] Mixing ratio The raw material mixing ratio was 2.80340 g of sample and 6.3 g of distilled water. The sample was weighed to contain 49.0% by mass of silicon, 21.0 parts by mass of natural graphite, 15.0% by mass of acetylene black, 7.5% by mass of polyacrylic acid (PAA), and 7.5% by mass of carboxymethylcellulose (CMC).

[0084] Powder synthesis Using a high-energy ball mill apparatus (Fritsch P5; Si3N4 pot: 250 mL; Si3N4 balls: 10 mm in diameter), 5.2522 g of silicon and 2.2501 g of natural graphite were added to the pot, and mechanochemical treatment was performed with an orbital speed of 300 rpm, a rotational speed of 600 rpm, a grinding time of 9 hours, and a ball weight:powder weight ratio of 40:1 to obtain the negative electrode active material for lithium-ion secondary batteries of Example 2.

[0085] Negative electrode fabrication The conditions for the agitator and defoamer used in the negative electrode preparation were as follows: for agitation, the rotation speed was 800 rpm and the revolution speed was 2000 rpm for 60 seconds; and for defoaming, the rotation speed was 60 rpm and the revolution speed was 2200 rpm for 30 seconds.

[0086] First, 0.21046 g of polyacrylic acid (PAA) and 1.7 mL of distilled water were added to a container, and the mixture was stirred and degassed three times to obtain an aqueous binder solution. Next, 0.42049 g of acetylene black and 3.05 mL of distilled water were added to the aqueous binder solution, and the mixture was stirred and degassed twice to obtain an aqueous binder solution containing a conductive agent.

[0087] Then, 1.9624 g of the lithium-ion secondary battery negative electrode active material obtained in Example 2 above was added to the aqueous solution of the conductive agent-containing binder, mixed with a spatula, and 3 mL of distilled water was added in portions while checking the consistency, and stirring and degassing were performed twice.

[0088] Furthermore, 0.21005 g of carboxymethylcellulose (CMC) was added to the obtained aqueous solution, and after stirring and degassing, 1.4 mL of distilled water was added, and stirring and degassing were performed twice to obtain the negative electrode mixture.

[0089] Subsequently, the obtained negative electrode mixture was applied to the copper foil with a doctor blade to a thickness of 150 μm, and vacuum drying was performed at 80°C for 12 hours to obtain the negative electrode for lithium-ion secondary batteries of Example 2.

[0090] [Example 3: 7 hours] Mixing ratio The raw material mixing ratio was 2.80119 g of sample and 6.2 g of distilled water. The sample was weighed to consist of 49.0% by mass of silicon, 21.0% by mass of natural graphite, 15.0% by mass of acetylene black, 7.5% by mass of polyacrylic acid (PAA), and 7.5% by mass of carboxymethylcellulose (CMC).

[0091] Powder synthesis Using a high-energy ball mill apparatus (Fritsch P5; Si3N4 pot: 250 mL; Si3N4 balls: 10 mm in diameter), 5.2536 g of silicon and 2.2469 g of natural graphite were added to the pot, and mechanochemical treatment was performed with an orbital speed of 300 rpm, a rotational speed of 600 rpm, a grinding time of 12 hours, and a ball weight:powder weight ratio of 40:1 to obtain the negative electrode active material for lithium-ion secondary batteries of Example 3.

[0092] Negative electrode fabrication The conditions for the agitator and defoamer used in the negative electrode preparation were as follows: for agitation, the rotation speed was 800 rpm and the revolution speed was 2000 rpm for 60 seconds; and for defoaming, the rotation speed was 60 rpm and the revolution speed was 2200 rpm for 30 seconds.

[0093] First, 0.21021 g of polyacrylic acid (PAA) and 1.7 mL of distilled water were added to a container, and the mixture was stirred and degassed twice to obtain an aqueous binder solution. Next, 0.42058 g of acetylene black and 3.00 mL of distilled water were added to the aqueous binder solution, and the mixture was stirred and degassed twice to obtain an aqueous binder solution containing a conductive agent.

[0094] Then, 1.9602 g of the lithium-ion secondary battery negative electrode active material obtained in Example 3 above was added to an aqueous solution of a conductive binder, mixed with a spatula, and 0.2 mL of distilled water was added in installments while checking the consistency, and stirring and degassing were performed twice.

[0095] Furthermore, 0.21020 g of carboxymethylcellulose (CMC) was added to the obtained aqueous solution, and after stirring and degassing, 1.3 mL of distilled water was added, and stirring and degassing were performed twice to obtain the negative electrode mixture.

[0096] Subsequently, the obtained negative electrode mixture was applied to the copper foil with a doctor blade to a thickness of 150 μm, and vacuum drying was performed at 80°C for 12 hours to obtain the negative electrode for lithium-ion secondary batteries of Example 3.

[0097] [Comparative Example 1: 24 hours] The process was carried out in the same manner as in Example 1, except that the grinding time for the mechanochemical treatment in powder synthesis was set to 24 hours. 20% by mass of acetylene black was mixed with the resulting mixed powder, and mechanochemical treatment was performed at an orbital speed of 300 rpm, a rotational speed of 600 rpm, a grinding time of 10 hours, and a ball weight:powder weight ratio of 40:1 to obtain the negative electrode active material and negative electrode for lithium-ion secondary batteries of Comparative Example 1.

[0098] [Comparative Example 2: Silicone] Mixing ratio The raw material mixing ratio was 3.5682 g of sample and 5.7 g of distilled water. The sample was weighed to consist of 70.0% by mass of silicon, 15.0% by mass of acetylene black, 7.5% by mass of polyacrylic acid (PAA), and 7.5% by mass of carboxymethylcellulose (CMC).

[0099] Powder synthesis Using a high-energy ball mill apparatus (Fritsch P5; Si3N4 pot: 250 mL; Si3N4 balls: 10 mm in diameter), 7.5027 g of silicon was added to the pot, and mechanochemical treatment was performed with an orbital speed of 300 rpm, a rotational speed of 600 rpm, a grinding time of 12 hours, and a ball weight:powder weight ratio of 40:1 to obtain the negative electrode active material for lithium-ion secondary batteries of Comparative Example 2.

[0100] Negative electrode fabrication The conditions for the agitator and defoamer used in the negative electrode preparation were as follows: for agitation, the rotation speed was 800 rpm and the revolution speed was 2000 rpm for 60 seconds; and for defoaming, the rotation speed was 60 rpm and the revolution speed was 2200 rpm for 30 seconds.

[0101] First, 0.2720 g of polyacrylic acid (PAA) and 1.5 mL of distilled water were added to a container, and the mixture was stirred and degassed twice to obtain an aqueous binder solution. Next, 0.5370 g of acetylene black and 2.8 mL of distilled water were added to the aqueous binder solution, and the mixture was stirred and degassed twice to obtain an aqueous binder solution containing a conductive agent.

[0102] Then, 2.5008 g of the lithium-ion secondary battery negative electrode active material obtained above (Comparative Example 2) was added to an aqueous solution of a conductive binder, mixed with a spatula, and 0.4 mL of distilled water was added in portions while checking the consistency, followed by stirring and degassing twice.

[0103] Furthermore, 0.2584 g of carboxymethylcellulose (CMC) was added to the obtained aqueous solution, and after stirring and degassing, 1.0 mL of distilled water was added, and stirring and degassing were performed twice to obtain the negative electrode mixture.

[0104] Subsequently, the obtained negative electrode mixture was applied to copper foil with a doctor blade to a thickness of 150 μm, and vacuum drying was performed at 80°C for 12 hours to obtain the negative electrode for lithium-ion secondary batteries of Comparative Example 2.

[0105] [Test Example 1: X-ray diffraction measurement] Using CuKα as the X-ray source, X-ray diffraction measurements were performed in the range of 2θ = 20 to 80°.

[0106] Figure 2 shows the X-ray diffraction spectra of the negative electrode active materials for lithium-ion secondary batteries obtained in Examples 1 to 3. Figure 2 summarizes the X-ray diffraction patterns of the active materials in Example 1 (12 hours), Example 2 (9 hours, similar to Examples 1 to 3), and Example 3 (7 hours), with different grinding times. From the X-ray diffraction patterns in Figure 2, broad peaks at 2θ = 36.0°, 60.0°, and 72.0°, attributed to layered disordered silicon carbide (SD-SiC), a broad peak at 2θ = 22.5°, attributed to amorphous carbon, and peaks at 2θ = 28.0°, 47.0°, and 56.0°, attributed to silicon, indicating that the negative electrode active materials obtained in Examples 1 to 3 contain layered disordered silicon carbide (SD-SiC), amorphous carbon, and silicon. For reference, Figures 3 and 4 show the X-ray diffraction spectra of stacked disordered silicon carbide (SD-SiC) and highly crystalline cubic β-SiC, as previously reported (J. Am. Chem. Soc., 98, 50-56 (2015)), and Figure 5 shows the X-ray diffraction spectrum of amorphous carbon. From these, it can be understood that the silicon carbide contained in the negative electrode active material for lithium-ion secondary batteries obtained in Examples 1 to 3 is not highly crystalline cubic β-SiC, but rather stacked disordered silicon carbide (SD-SiC) as previously reported (J. Am. Chem. Soc., 98, 50-56 (2015)).

[0107] To estimate the mixing ratio of SD-SiC and Si in the negative electrode active materials for lithium-ion secondary batteries obtained in Examples 1-3, X-ray diffraction spectra measured by mixing separately prepared SD-SiC and Si are shown in Figure 6. However, to match the actual synthesis conditions, nanopowder was used for the Si, which was pulverized by mechanical grinding. From Figure 6, the peak ratio of SD-SiC and MG-Si, which changes with the grinding time in the mechanochemical treatment, can be reproduced. The peak intensity at 2θ=28.0° attributed to silicon changes, and the X-ray diffraction pattern in Figure 6, which shows that the silicon content decreases as the grinding time increases, as observed in Figure 2, can also be reproduced, indicating that the peak ratio of SD-SiC and MG-Si is useful for estimating the SD-SiC and Si content in the examples.

[0108] On the other hand, in Comparative Example 1, the peak attributed to silicon disappeared due to the grinding time in the mechanochemical treatment.

[0109] Furthermore, in the negative electrode active material obtained in Example 1, the content of layered disordered silicon carbide (SD-SiC) is estimated to be 96.5 mass%, the content of amorphous carbon is estimated to be 1.0 mass%, and the content of silicon is estimated to be 2.5 mass%. Similarly, in the negative electrode active material obtained in Example 2, the content of layered disordered silicon carbide (SD-SiC) is estimated to be 73.7 mass%, the content of amorphous carbon is estimated to be 7.9 mass%, and the content of silicon is estimated to be 18.4 mass%. Similarly, in the negative electrode active material obtained in Example 3, the content of layered disordered silicon carbide (SD-SiC) is estimated to be 53.8 mass%, the content of amorphous carbon is estimated to be 13.9 mass%, and the content of silicon is estimated to be 32.3 mass%.

[0110] [Test Example 2: Electron Microscope Observation (Part 2)] To investigate the size of the stacked disordered structure of SiC that forms at the interface between amorphous carbon and silicon, a negative electrode active material for lithium-ion secondary batteries, obtained by grinding an equimolar mixture of carbon and silicon in a high-energy ball mill for 4 hours, was observed using a high-resolution transmission electron microscope (TEM). The results are shown in Figure 7.

[0111] As a result, it can be understood that silicon carbide with an average particle size of about 5-10 nm is formed at the interface between the black silicon and the white amorphous carbon. Considering this together with the results of Test Example 1, it can be understood that the silicon carbide being formed is not highly crystalline cubic β-SiC, but rather a layered disordered structure silicon carbide (SD-SiC) as described in a previous report (J. Am. Chem. Soc., 98, 50-56 (2015)). From this, it can be understood that a layered disordered silicon carbide (SD-SiC) (average particle size of about 5-10 nm) is produced by the reaction of silicon and carbon, and amorphous carbon material (average particle size of about 10-20 nm) is present around it, and the layered disordered silicon carbide (SD-SiC) (average particle size of about 5-10 nm) is embedded or dispersed in the amorphous carbon material (average particle size of about 10-20 nm) without agglomerating, so as to have conductive paths with the amorphous carbon material.

[0112] on the other hand, Journal of Nanoparticle Research (2007) 9:797-806 “Dispersion of nano-silicon carbide (SiC) powder in aqueous suspensions” DOI 10.1007 / s11051-006-9121-6 As described in the introduction, embedding or dispersing nano-sized particles in aqueous and non-aqueous solutions within primary particles is known to be thermodynamically unstable due to the large surface energy in the fine dispersion system. This paper describes that SiC nanoparticles are also difficult to embed or disperse without special treatment. Therefore, when both SiC and carbon are nanoparticles, homogeneous embedding or dispersion is not easy, and it is difficult to embed or disperse stacked disordered silicon carbide in such a way that it has conductive paths with amorphous carbon material using conventional methods.

[0113] Figure 8 shows a high-resolution TEM observation of synthesized SD-SiC, as published in J. Am. Chem. Soc., 98, 50-56 (2015). Since particles of approximately 30-100 nm are aggregated and firmly in contact, it is extremely difficult to break them down into primary particles, as also shown in J. Am. Chem. Soc., 98, 50-56 (2015).

[0114] Figure 8 shows that when a layered disordered silicon carbide is first manufactured and then mixed with a carbonaceous material, the layered disordered silicon carbide aggregates, preventing it from being embedded or dispersed. Therefore, it is difficult to embed or disperse the layered disordered silicon carbide in the carbonaceous material.

[0115] In contrast, according to the present invention, as described above, it is possible to adopt a structure in which the layered disordered silicon carbide is embedded or dispersed in an amorphous carbon material such that it has conductive paths with the amorphous carbon material.

[0116] [Manufacturing Example 1: Manufacturing of Lithium-ion Secondary Batteries (Half-Cells)] The negative electrodes obtained in Examples 1 and 2 were used as the negative electrodes.

[0117] Furthermore, lithium metal is used as the positive electrode.

[0118] The electrolyte consisted of ethylene carbonate (EC) and diethyl carbonate (DEC) as solvents at an EC / DEC ratio of 50 / 50 (v / v), with 1 mol / L lithium hexafluorophosphate (LiPF6) as the salt. This electrolyte was impregnated into a porous polypropylene film, which served as the separator.

[0119] A lithium-ion secondary battery was fabricated using the negative electrode, positive electrode, electrolyte, and separator described above.

[0120] [Test Example 3: Charge / Discharge Measurement (Part 1)] Charge and discharge measurements were performed using a two-electrode cell and a potentiometer / galvanostat analyzer ECstat-302. The cell temperature was controlled in a constant temperature bath at 20°C.

[0121] Figure 9 shows the charge-discharge curve and the change in discharge capacity when a lithium-ion secondary battery (half-cell) manufactured using the negative electrodes obtained in Examples 1-2 and Comparative Example 1 is charged and discharged at a charge-discharge rate of 100 mA / g to Si-C (Si / C = 1 / 1 in mol%).

[0122] As shown in Figure 9, when the carbon content in the raw material is not very high, and mechanochemical treatment is performed until no free silicon remains, as in Comparative Example 1, the discharge capacity itself can be maintained to a certain extent, but a decrease in discharge capacity is observed in the initial cycles (1 to 10 cycles), and it cannot be said that a lithium-ion secondary battery with particularly excellent cycle characteristics has been obtained. In this respect, it shows different behavior from Patent No. 7650014, which was tested under carbon-rich conditions.

[0123] On the other hand, Figure 10 shows that even when the amount of carbon in the raw material is not very high, as in Example 1, by deliberately leaving a certain amount of free silicon present, although the discharge capacity in higher cycles (approximately 20-42 cycles) is not significantly different from Comparative Example 1, in the initial cycles (approximately 1-10 cycles), not only is there no decrease in discharge capacity, but it is actually improved, resulting in a lithium-ion secondary battery with exceptionally superior cycle characteristics. Compared to Comparative Example 1, it is an unexpected result that deliberately leaving in free silicon, which is a concern due to potential electrode damage from expansion and contraction, actually improves cycle characteristics.

[0124] Furthermore, as can be seen from Figure 11, even when the amount of carbon in the raw material is not very high, by intentionally including a certain amount of free silicon, as in Examples 2 and 3, no decrease in discharge capacity is observed in the initial cycle, thus obtaining a lithium-ion secondary battery with exceptionally superior cycle characteristics. Moreover, in Examples 2 and 3, the discharge capacity itself is significantly improved compared to Example 1 and Comparative Example 1, so when about 10 to 30 mass% of free silicon is included, a lithium-ion secondary battery is obtained that not only has exceptionally superior cycle characteristics but also significantly improved capacity. On the other hand, when only Si was nano-sized by mechanical grinding and the discharge capacity was examined as an active material, as shown in Figure 11, the cycle characteristics decreased significantly from cycle 1 to 5, whereas in Examples 2 and 3, the capacity was almost maintained, indicating that the expansion and contraction due to charging and discharging was significantly improved by embedding the free silicon with SD-SiC and carbon. Note that the cycle characteristics are superior in Example 1, so the amount of free silicon can be appropriately selected according to the required characteristics. For reference, Figure 12 shows the charge-discharge curves for the first and fifth cycles of Examples 1 and 2. Figure 13 shows the charge-discharge curves for the first and second cycles of Example 3.

Claims

1. It contains layered irregular structure silicon carbide, amorphous carbon material and silicon, With the total amount being 100% by mass, the silicon content is 2.0 to 35.0% by mass, the layered disordered structure silicon carbide content is 50.0 to 97.0% by mass, and the amorphous carbon material content is 0.5 to 20.0% by mass. The stacked disordered silicon carbide and the silicon are embedded or dispersed in the amorphous carbon material such that they have conductive paths with the amorphous carbon material. Anode active material for lithium-ion secondary batteries.

2. The negative electrode active material for a lithium-ion secondary battery according to claim 1, wherein the average particle size of the stacked disordered structure silicon carbide is 5 to 20 nm.

3. The negative electrode active material for a lithium-ion secondary battery according to claim 1, wherein the average particle size of the amorphous carbon material is 5 to 25 nm.

4. The stacked disordered silicon carbide is a negative electrode active material for a lithium-ion secondary battery according to claim 1, wherein, in X-ray diffraction measurements using CuKα rays, the full width at half maximum of the peak at 2θ = 36.0° is 2.0° or more within an acceptable range of ±0.5°.

5. A negative electrode for a lithium-ion secondary battery, comprising the negative electrode active material for a lithium-ion secondary battery described in any one of claims 1 to 4.

6. A lithium-ion secondary battery comprising the negative electrode for a lithium-ion secondary battery described in claim 5.

7. A method for producing a negative electrode active material for a lithium-ion secondary battery according to any one of claims 1 to 4, The process includes a step of subjecting a raw material mixture containing silicon-containing material and carbon material to mechanochemical treatment. With the total amount of the raw material mixture being 100% by mass, the content of the silicon-containing material is 40 to 60% by mass, and the content of the carbon material is 40 to 60% by mass. The manufacturing method for the mechanochemical treatment involves performing the mechanochemical treatment until the silicon content is 2.0 to 35.0% by mass, and the layered disordered structure silicon carbide content is 50.0 to 97.0% by mass, with the total amount being 100% by mass.

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