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 capacity and cycle characteristics. [Solution] A negative electrode active material for a lithium-ion secondary battery containing a stacked disordered silicon carbide, an amorphous carbon material, and silicon, wherein the stacked disordered silicon carbide and the silicon are dispersed in the amorphous carbon material such that they have conductive paths with the amorphous carbon material, and the silicon content is 5.0 to 28.0% by mass, with a total amount of 100% by mass, and the stacked disordered silicon carbide content is 21.6 to 57.0% by mass.

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 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] However, according to our research, as described in Non-Patent Document 9, even when attempting to use a stacked disordered silicon carbide structure manufactured with a silicon:carbon ratio of 1:1 (molar ratio) as the negative electrode active material, it is difficult to insert and remove lithium ions during charging and discharging. Therefore, it has been found that it is not possible to manufacture lithium-ion secondary batteries with excellent capacity and 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 capacity and cycle characteristics. [Means for solving the problem]

[0007] As a result of diligent research, the inventors have discovered that by dispersing stacked disordered silicon carbide and silicon in an amorphous carbon material so that they have conductive paths with the amorphous carbon material, while adjusting the amount of silicon to be constant, the silicon carbide generated at the Si-C interface migrates to the excess conductive carbon matrix, allowing for reversible intercalation and deintercalation of lithium ions during charging and discharging. Furthermore, because silicon is present in the amorphous carbon material, the negative electrode active material for lithium-ion secondary batteries is not destroyed by the expansion and contraction of silicon, thus enabling the manufacture of lithium-ion secondary batteries with further improved capacity and cycle characteristics. This invention was completed based on these findings and further research. In other words, this invention encompasses the following configuration.

[0008] Item 1. A negative electrode active material for lithium-ion secondary batteries containing a stacked disordered structure silicon carbide, an amorphous carbon material, and silicon, The stacked disordered silicon carbide and the silicon are dispersed in the amorphous carbon material such that they have conductive paths with the amorphous carbon material, and The 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 type 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, with the total amount being 100% by mass and the content of the amorphous carbon material being 28.8 to 66.5% by mass.

[0010] Item 3. The negative electrode active material for a lithium ion secondary battery according to Item 1 or 2, where the average particle diameter of the laminated irregular structure type 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, where 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, where the laminated irregular structure type 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 the negative electrode active material for a lithium ion secondary battery according to any one of Items 1 to 5, comprising: A step of subjecting a raw material mixture containing a silicon-containing material and a carbon material to mechanochemical treatment until, with the total amount being 100% by mass, the silicon content 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. The manufacturing method comprising this step.

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] [Figure 1] The X-ray diffraction pattern of the negative electrode active material obtained in Example 1 is shown. [Figure 2] The X-ray diffraction pattern of the negative electrode active material obtained in Example 2 is shown. [Figure 3] The X-ray diffraction spectrum of laminated irregular structure type silicon carbide (SD-SiC) is shown. [Figure 4] The X-ray diffraction pattern of highly crystalline cubic β-SiC is shown. [Figure 5] The X-ray diffraction spectrum of amorphous carbon is shown. [Figure 6] In the reaction of graphite and silicon, the high-resolution transmission electron microscope (TEM) image of the negative electrode active material obtained with a pulverization time of 4 hours is shown. [[ID=�0]] [Figure 7] In Test Example 3, in the lithium ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 1, the charge-discharge curves when charging and discharging at 100 mA / g to Si-C (Si / C = 1 / 1 in mol%) in the first cycle and 1000 mA / g to Si-C (Si / C = 1 / 1 in mol%) in the second to eleventh cycles are shown. The charge-discharge rate is calculated based on the total weight of the generated laminated irregular structure type SiC and unreacted Si and equimolar C. The discharge capacity in each cycle is calculated based on the total weight of SiC and Si obtained by the calculation method shown in (1-4). [Figure 8]In Test Example 3, the charge-discharge curves are shown for a lithium-ion secondary battery (half-cell) manufactured using the negative electrode obtained in Example 2, where the charge-discharge rate was 100 mA / g ~ Si-C (Si / C = 1 / 1 in mol%) for the first cycle and 1000 mA / g ~ Si-C (Si / C = 1 / 1 in mol%) for cycles 2 to 11. The charge-discharge rate was calculated based on the total weight of equimolar C, unreacted SiC, and the generated stacked disordered structure SiC. The discharge capacity in each cycle was calculated based on the total weight of SiC and Si, whose content was determined by the calculation method shown in (1-4). [Figure 9] In Test Example 3, the changes in discharge capacity are shown for a lithium-ion secondary battery (half-cell) manufactured using the negative electrode obtained in Example 1, when charged and discharged at 100 mA / g ~ Si-C (Si / C = 1 / 1 in mol%) for the first cycle and 1000 mA / g ~ Si-C (Si / C = 1 / 1 in mol%) for cycles 2 to 11. The charge and discharge rates were calculated based on the total weight of equimolar C, unreacted SiC, and the generated stacked disordered structure SiC. The discharge capacity in each cycle was calculated based on the total weight of SiC and Si, whose content was determined by the calculation method shown in (1-4). [Figure 10] In Test Example 3, the changes in discharge capacity are shown for a lithium-ion secondary battery (half-cell) manufactured using the negative electrode obtained in Example 2, when charged and discharged at 100 mA / g ~ Si-C (Si / C = 1 / 1 in mol%) for the first cycle and 1000 mA / g ~ Si-C (Si / C = 1 / 1 in mol%) for cycles 2 to 11. The charge and discharge rates were calculated based on the total weight of equimolar C, unreacted SiC, and the generated stacked disordered structure SiC. The discharge capacity in each cycle was calculated based on the total weight of SiC and Si, whose content was determined by the calculation method shown in (1-4). [Figure 11]In Test Example 4, the charge-discharge curves are shown for a lithium-ion secondary battery (half-cell) manufactured using the negative electrode obtained in Example 1, where the charge-discharge rate was 100 mA / g ~ Si-C (Si / C = 1 / 1 in mol%) for cycles 1-5 and 1000 mA / g ~ Si-C (Si / C = 1 / 1 in mol%) for cycles 6-10. The charge-discharge rate was calculated based on the total weight of equimolar C, unreacted SiC, and the generated stacked disordered structure SiC. The discharge capacity in each cycle was calculated based on the total weight of SiC and Si, whose content was determined by the calculation method shown in (1-4). [Figure 12] In Test Example 4, the changes in discharge capacity are shown for a lithium-ion secondary battery (half-cell) manufactured using the negative electrode obtained in Example 1, when charged and discharged at 100 mA / g ~ Si-C (Si / C = 1 / 1 in mol%) for cycles 1 to 5, and at 1000 mA / g ~ Si-C (Si / C = 1 / 1 in mol%) for cycles 6 to 10. The charge and discharge rates were calculated based on the total weight of equimolar C, unreacted SiC, and the generated stacked disordered structure SiC. The discharge capacity in each cycle was calculated based on the total weight of SiC and Si, whose content was determined by the calculation method shown in (1-4). [Figure 13] The charge-discharge curves are shown for a lithium-ion secondary battery (half-cell) manufactured using the negative electrode obtained in Example 3, where the charge-discharge rate was 100 mA / g ~ Si-C (Si / C = 1 / 1 in mol%) for the first cycle and 1000 mA / g ~ Si-C (Si / C = 1 / 1 in mol%) for the second to fifth cycles. The charge-discharge rate was calculated based on the total weight of equimolar C, unreacted SiC, and the generated stacked disordered structure SiC. The discharge capacity in each cycle was calculated based on the total weight of SiC and Si, whose content was determined by the calculation method shown in (1-4). [Figure 14]The discharge capacity changes when a lithium-ion secondary battery (half-cell) manufactured using the negative electrode obtained in Example 3 is charged and discharged at 100 mA / g ~ Si-C (Si / C = 1 / 1 in mol%) for the first cycle and 1000 mA / g ~ Si-C (Si / C = 1 / 1 in mol%) for cycles 2 to 5. The charge and discharge rates were calculated based on the total weight of equimolar C, unreacted SiC, and the generated stacked disordered structure SiC. The discharge capacity in each cycle was calculated based on the total weight of SiC and Si, whose content was determined by the calculation method shown in (1-4). [Modes for carrying out the invention]

[0018] In this specification, "contains" is a concept that encompasses all of the following: "contains," "consist essentially of," and "consist of."

[0019] Furthermore, in this specification, when a numerical range is expressed as A to B, it means A or greater and B or less.

[0020] 1. Negative electrode active material for lithium-ion secondary batteries The present invention relates to a negative electrode active material for a lithium-ion secondary battery, which contains a stacked disordered structure silicon carbide, an amorphous carbon material, and silicon, wherein the stacked disordered structure silicon carbide and the silicon are dispersed in the amorphous carbon material such that they have conductive paths with the amorphous carbon material, and the silicon content is 5.0 to 28.0% by mass, with a total amount of 100% by mass.

[0021] 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.

[0022] 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. A structure is expected in which the layered disordered structure silicon carbide and unreacted silicon, which react at the carbon-silicon interface, are dispersed within the carbon aggregate that acts as a conductive material by the reaction of carbon and silicon, 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.

[0023] (1-1) Layered irregular structure silicon carbide The stacked disordered silicon carbide contained in the negative electrode active material for lithium-ion secondary batteries of the present invention is preferably of the form 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.

[0024] The stacked disordered silicon carbide contained in the negative electrode active material for lithium-ion secondary batteries of 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.

[0025] 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°.

[0026] 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.

[0027] In the negative electrode active material for lithium-ion secondary batteries of the present invention, it is preferable that small silicon carbides are 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.

[0028] In the negative electrode active material for lithium-ion secondary batteries of the present invention, the content of stacked disordered structure silicon carbide is not particularly limited, but from the viewpoint of easily intercalating and deintercalating lithium ions, it is preferable that small silicon carbide particles are dispersed in the amorphous carbon material without aggregation so that they have conductive paths with the amorphous carbon material. Therefore, with the total amount of the negative electrode active material for lithium-ion secondary batteries of the present invention as 100% by mass, the content is preferably 21.6 to 57.0% by mass, and more preferably 28.5 to 46.2% by mass.

[0029] (1-2) 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.

[0030] The amorphous carbon material contained in the negative electrode active material for lithium-ion secondary batteries of 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.

[0031] The amorphous carbon material contained in the negative electrode active material for lithium-ion secondary batteries of the present invention 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°.

[0032] 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.

[0033] In the negative electrode active material for lithium-ion secondary batteries of the present invention, it is preferable that small silicon carbide particles are 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.

[0034] 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 easily improving conductivity, charge / discharge capacity, cycle characteristics, etc., it is preferably 28.8 to 66.5% by mass, and more preferably 38.0 to 53.9% by mass, based on the total amount of the negative electrode active material for lithium-ion secondary batteries as 100% by mass.

[0035] (1-3) Silicone In the negative electrode active material for lithium-ion secondary batteries of the present invention, the capacity can be improved by including an amorphous carbon material, and since silicon is 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.

[0036] 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.

[0037] 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.

[0038] In the negative electrode active material for lithium-ion secondary batteries of the present invention, it is preferable that small silicon particles are 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, it is preferable that the average crystallite diameter of silicon be small. 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.

[0039] In the negative electrode active material for lithium-ion secondary batteries of the present invention, the silicon content is preferably dispersed in the amorphous carbon material without aggregation, so as to facilitate the improvement of capacity and the improvement of cycle characteristics by suppressing the expansion and contraction of silicon with the amorphous carbon material, so as to have conductive paths between small silicon particles and the amorphous carbon material. 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 5.0 to 28.0% by mass, more preferably 10.0 to 20.0% by mass.

[0040] (1-4) Method for measuring the content We will examine electrodes fabricated by coating copper foil with a conductive material. In this case, we will assume that the copper foil is coated with 85% by mass of carbon and active material as conductive materials, 7.5% by mass of polyacrylic acid (PAA) as a binder, and 7.5% by mass of carboxymethylcellulose (CMC) as a binder.

[0041] Thus, the conductive material, carbon, and the active material account for 85% by mass. When the conductive material and active material are in equal amounts, the silicon is weighed and synthesized so that 100% of the silicon reacts with the carbon to produce a layered disordered silicon carbide structure. Typically, reacting the mixture in a high-energy ball mill for 24 hours converts all of the mixed silicon into layered disordered silicon carbide. In this case, 85% by mass of the material coating the current collector copper foil is silicon and carbon, of which half is layered disordered silicon carbide and the remainder is carbon.

[0042] Here, carbon may also insert and deinsert lithium ions, but unlike graphite, which has a layered structure, the theoretical charge-discharge capacity of amorphous carbon materials cannot be calculated. Furthermore, the amount of lithium ions inserted and deinserted by carbon varies depending on the grinding conditions of the mechanochemical treatment, making it difficult to specify. For convenience, in this specification, we will assume that carbon does not insert or deinsert lithium ions, that is, that only silicon carbide and silicon insert and deinsert lithium ions, and will perform calculations accordingly.

[0043] Considering an electrode containing m(g) of layered disordered silicon carbide, the theoretical discharge capacity is 1336mAh / g, therefore: m × 1336mAh / g = Di(mAh) Di: initial discharge capacity The discharge capacity can be calculated in this way.

[0044] When m = 0.0030 g, Di = 4.008 mAh. However, when the time for synthesizing the layered disordered silicon carbide (grinding time in mechanochemical processing) is set to 12 hours, the measured discharge capacity becomes 6.564 mAh, a significant increase. This is thought to be because silicon with a large theoretical discharge capacity (4198 mAh / g) remains unreacted. Let x be the weight fraction of the layered disordered silicon carbide produced, and let (1-x) be the sum of the silicon that did not form the layered disordered silicon carbide and equimolar carbon. Then, the weight fraction and weight of each active material (layered disordered silicon carbide and silicon), i.e., the content, can be calculated from the following formula. m×1336mAh / g×x+m×4198mAh / g×(1-x)×28.08 / 40.09=6.564mAh If m = 0.0030g, then x = 0.4689, and (1-x) = 0.5311. Therefore, the weight of the layered disordered 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 batteries The negative electrode active material for lithium-ion secondary batteries of the present invention contains, as described above, a stacked disordered structure silicon carbide, an amorphous carbon material, and silicon.

[0046] In the negative electrode active material for lithium-ion secondary batteries of the present invention, 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. Therefore, it is preferable that the stacked disordered silicon carbide is dispersed in the amorphous carbon material without aggregation, so as to have conductive paths with the amorphous carbon material.

[0047] Furthermore, in the negative electrode active material for lithium-ion secondary batteries of the present invention, silicon is covered by an excess carbon matrix, which absorbs the expansion and contraction of 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 is dispersed in the amorphous carbon material without aggregation, so as to have conductive paths with the amorphous carbon material.

[0048] 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 of 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.

[0049] In the lithium-ion secondary battery anode active material of 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 allows for reversible intercalation and deintercalation of lithium ions in conjunction with the charging and discharging of lithium ions. Furthermore, although the stacked disordered silicon carbide in the lithium-ion secondary battery anode active material of 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 lithium-ion secondary battery anode active material of the present invention, the stacked disordered silicon carbide and silicon are preferably dispersed without aggregation in the amorphous carbon material so as to have conductive paths between them and the amorphous carbon material. As a result, the amorphous carbon material adjacent to the stacked disordered silicon carbide and silicon can follow the expansion and contraction of the stacked disordered silicon carbide and the small amount of silicon. Therefore, by using the lithium-ion secondary battery anode active material of the present invention, it is possible to manufacture a lithium-ion secondary battery with particularly high capacity and 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 where lithium metal is used as the positive electrode.

[0050] 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.

[0051] 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, when 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.

[0052] 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.

[0053] 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, 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 above-mentioned 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.

[0054] 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.

[0055] 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 binder, thickener, dispersant, etc., into layers. For example, the negative electrode mixture can be dried by a conventional method and formed into layers.

[0056] 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.

[0057] 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.

[0058] When manufacturing the negative electrode for a lithium-ion secondary battery according to the present invention, it can be manufactured by forming the above-mentioned negative electrode mixture in layers on a negative electrode current collector. For example, the negative electrode for a lithium-ion secondary battery according to the present invention can be manufactured by drying the negative electrode mixture on a negative electrode current collector by a conventional method and forming it in layers.

[0059] 3. Lithium-ion rechargeable batteries The lithium-ion secondary battery of the present invention is equipped with the negative electrode for lithium-ion secondary batteries of the present invention described above. In addition to the negative electrode for lithium-ion secondary batteries of the present invention, the lithium-ion secondary battery of the present invention may also be equipped with a positive electrode, an electrolyte, and a container for housing these, which are applicable to known lithium-ion secondary batteries.

[0060] Any positive electrode capable of supplying lithium ions to the negative electrode will suffice, and well-known positive electrodes can be used.

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

[0062] In addition, as the positive electrode active material constituting the positive electrode, usually, a material that can occlude and release lithium ions is 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) O2 (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 Mn2O4 (0.9 ≦ x3 < 1.5), Li x4 Ni γ4 Mn (2-γ4) O4 (0.9 ≦ x4 < 1.5, 0 ≦ γ4 ≦ 2), etc. can be mentioned. As the polyanion compound, LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, Li2CoPO4F, etc. can be mentioned. As the chalcogen compound, titanium disulfide, molybdenum disulfide, molybdenum dioxide, etc. can be mentioned. Atoms or polyanions in these materials may be partially substituted with atoms or anion species composed of other elements. These positive electrode active materials can be used alone or in combination of two or more.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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 for example, it involves a process of subjecting a raw material mixture containing a silicon-containing material and a carbon material to mechanochemical treatment until, with a total amount of 100% by mass, the silicon content is 5.0 to 28.0% by mass and the layered disordered structure silicon carbide content is 21.6 to 57.0% by mass. It is equipped with.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] Regarding the mixing ratio of the raw materials, as described later, in the manufacturing method of the present invention, the grinding time in the mechanochemical treatment is shortened to produce a layered irregular structure silicon carbide, and silicon, which is a raw material, is intentionally left in place and dispersed in the amorphous carbon material so that it has conductive paths with the amorphous carbon material. However, even in this case, the ratio of the raw materials used is almost the same as the ratio of each element in the product, so the ratio of silicon and carbon in the negative electrode active material for lithium-ion secondary batteries of the present invention can be the same as the target ratio. 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, and 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, and more preferably 55 to 75% by mass.

[0073] 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.

[0074] 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.

[0075] 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 thorough stirring, enabling the efficient production of the negative electrode active material for lithium-ion secondary batteries of the present invention.

[0076] In this case, assuming the use of 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 negative electrode active material for lithium-ion secondary batteries 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 negative electrode active material for lithium-ion secondary batteries of the present invention, 250 to 400 rpm is preferred, and 300 to 350 rpm is more preferred.

[0077] 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.

[0078] In this invention, a layered disordered silicon carbide is produced by intentionally reacting only a portion of the silicon-containing material and the carbon material, while intentionally leaving some silicon as a raw material and dispersing it in the amorphous carbon material so that it has conductive paths with the amorphous carbon material. For this reason, the grinding time (processing time) of the mechanochemical treatment is intentionally set to a short time. In a previous report (J. Am. Chem. Soc., 98, 50-56 (2015)), it was found that 24 hours are required for Si and C to completely react and synthesize a layered disordered silicon carbide at a molar ratio of 1:1. For example, when performing the reaction under the same conditions, the grinding time (processing time) of the mechanochemical treatment should be shorter, preferably 6 to 18 hours, and particularly preferably 8 to 15 hours. Furthermore, this mechanochemical treatment can be performed in multiple stages with pauses in between as needed.

[0079] Furthermore, if mechanochemical treatment is repeated multiple times, the above conditions can be applied to the mechanochemical treatment in each step.

[0080] 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.

[0081] Unlike the manufacturing method of the present invention described above, when a material obtained by dispersing a layered disordered silicon carbide structure, obtained by long-term mechanochemical treatment, in an amorphous carbon material so as to have conductive paths with the amorphous carbon material is mixed with silicon, it is not possible to 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]

[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 (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).

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

[0085] Powder synthesis Using a high-energy ball mill apparatus (Fritsch P5; Si3N4 pot: 250 mL; Si3N4 balls: 10 mm in diameter), 2.6267 g of silicon, 3.9907 g of natural graphite, and 0.8830 g of acetylene black were added to the pot. Mechanochemical treatment was performed at 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.

[0086] 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.

[0087] First, 0.2630 g of polyacrylic acid (PAA) and 2.2 mL of distilled water were added to a container, and the mixture was stirred and degassed twice to obtain an aqueous binder solution. Next, 2.9765 g of the lithium-ion secondary battery negative electrode active material obtained above (Example 1) was added to the aqueous binder solution, mixed with a spatula, and 2 mL of distilled water was added in installments while checking the consistency, and the mixture was stirred and degassed twice.

[0088] Furthermore, 0.2633 g of carboxymethylcellulose (CMC) was added to the obtained aqueous solution, and after stirring and degassing, 1 mL of distilled water was added, and stirring and degassing was 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 1.

[0090] [Example 2: 10 hours] Mixing ratio The raw material mixing ratio was 3.5009 g of sample and 5.4 g of distilled water. The sample was weighed to contain 29.8% by mass of silicon, 45.2% by mass of natural graphite, 10.0% 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), 2.6261 g of silicon, 3.9911 g of natural graphite, and 0.8835 g of acetylene black were added to the pot. 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 for lithium-ion secondary batteries of Example 2.

[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, 0.2630 g of polyacrylic acid (PAA) and 2.2 mL of distilled water were added to a container, and the mixture was stirred and degassed twice to obtain an aqueous binder solution. Next, 2.9748 g of the lithium-ion secondary battery negative electrode active material obtained above (Example 1) was added to the aqueous binder solution, mixed with a spatula, and 2 mL of distilled water was added in installments while checking the consistency, and the mixture was stirred and degassed twice.

[0094] Furthermore, 0.2631 g of carboxymethylcellulose (CMC) was added to the obtained aqueous solution, and after stirring and degassing, 1 mL of distilled water was added, and stirring and degassing was performed twice to obtain the negative electrode mixture.

[0095] 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 1.

[0096] [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°.

[0097] Figures 1 and 2 show the X-ray diffraction spectra of the negative electrode active materials for lithium-ion secondary batteries obtained in Examples 1 and 2. From Figures 1 and 2, broad peaks at 2θ = 36.0°, 60.0°, and 72.0°, which are attributed to layered disordered silicon carbide (SD-SiC); a broad peak at 2θ = 22.5°, which is attributed to amorphous carbon; and peaks at 2θ = 28.0°, 47.0°, and 56.0°, which are attributed to silicon. Therefore, it can be understood that the negative electrode active materials obtained in Examples 1 and 2 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 and 2 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)).

[0098] [Test Example 2: Electron Microscope Observation] To investigate the size of the stacked disordered SiC structure 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 6.

[0099] 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 the reaction between silicon and carbon generates a layered disordered silicon carbide (SD-SiC) (average particle size of about 5-10 nm) while silicon remains, and amorphous carbon material (average particle size of about 10-20 nm) is present around it. The layered disordered silicon carbide (SD-SiC) (average particle size of about 5-10 nm) and silicon are dispersed in the amorphous carbon material (average particle size of about 10-20 nm) without agglomerating, so that conductive paths exist between them. Furthermore, the silicon particles become smaller as the reaction time increases, and in cases where the reaction time is about 10-12 hours, as in Examples 1 and 2, the average crystallite size was about 8 nm.

[0100] [Manufacturing example: Manufacturing of lithium-ion secondary batteries (half-cells)] The negative electrodes obtained in Examples 1 and 2 were used as the negative electrodes.

[0101] Furthermore, lithium metal is used as the positive electrode.

[0102] 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.

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

[0104] [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.

[0105] Figures 7-10 show the charge-discharge curves and discharge capacity changes when a lithium-ion secondary battery (half-cell) manufactured using the negative electrodes obtained in Examples 1-2 is charged and discharged at 100 mA / g ~ Si-C (Si / C = 1 / 1 in mol%) for the first cycle and 1000 mA / g ~ Si-C (Si / C = 1 / 1 in mol%) for cycles 2-11. The subsequent charge-discharge rates are calculated based on the total weight of equimolar amounts of C in the generated stacked disordered structure SiC and unreacted Si. In Figures 7-10, the discharge capacity in each cycle is calculated based on the total weight of SiC and Si, whose content is determined by the calculation method shown in (1-4). From these results, it can be seen that using the negative electrode active material for lithium-ion secondary batteries of the present invention results in superior capacity, cycle characteristics, and rate characteristics.

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

[0107] Here, carbon may also insert and deinsert lithium ions, but unlike graphite, which has a layered structure, the theoretical charge-discharge capacity of amorphous carbon materials cannot be calculated. Furthermore, the amount of lithium ions inserted and deinserted by carbon varies depending on the grinding conditions of the mechanochemical treatment, making it difficult to specify. For convenience, in this specification, we will assume that carbon does not insert or deinsert lithium ions, that is, that only silicon carbide and silicon insert and deinsert lithium ions, and will perform calculations accordingly.

[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, the ratio of SiC:(Si+C) = 0.6299:0.3701 (by weight).

[0109] Note that x is the weight fraction of SiC, and 1-x represents the weight fraction of Si+C. Also, 6.667mAh is the measured value, and 0.003455g is the sum of the weight of SiC and the weight of unreacted Si and an equimolar amount of C. Of the negative electrode layer (coated to the current collector), C+SiC accounts for 85 mass%, and half of that (50 mass%) consists of C and SiC, respectively. Therefore, the raw materials are mixed so that when the raw material C and raw material Si react completely, C and Si-C (equomoles) each account for 50 mass.

[0110] (Calculation of specific volume in 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 0.003455g × 0.6299 (weight fraction of SiC). The weight of Si (unreacted Si) is 0.003455g × 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 0.003455g × 0.3701 (weight fraction of Si + C) × 12.01 / 40.09 plus 0.003455g of surplus C, since the raw materials were originally mixed to be 50% by mass of C and 50% by mass of SiC (Si powder and C powder).

[0112] From these results, the ratio of SiC weight:Si weight:C weight, when divided by 0.003455, 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.

[0113] From the above, the ratio of C:SiC:Si is 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, the ratio of SiC:(Si+C) = 0.3784:0.6216 (by weight).

[0115] Note that x is the weight fraction of SiC, and 1-x represents the weight fraction of Si+C. Also, 10.33mAh is the measured value, and 0.004426g is the sum of the weight of SiC and the weight of unreacted Si and an equimolar amount of C. Of the negative electrode layer (coated to the current collector), C+SiC accounts for 85 mass%, and half of that (50 mass%) consists of C and SiC, respectively. Therefore, the raw materials are mixed so that when the raw material C and raw material Si react completely, C and Si-C (equomoles) each account for 50 mass.

[0116] (Calculation of specific volume 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 become SiC + surplus 50% by mass of C) is 0.004426 g × 0.6216 (weight fraction of Si + C) × 12.01 / 40.09 plus the surplus C of 0.004426 g, since the raw materials were originally mixed to be 50% by mass of C and 50% by mass of SiC (Si powder and C powder).

[0118] From these results, the ratio of SiC weight:Si weight:C weight, when divided 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, the ratio of C:SiC:Si is 59.0 mass%:18.8 mass%:22.2 mass%.

[0120] [Test Example 4: Charge / Discharge Measurement (Part 2)] 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] Figures 11-12 show the charge-discharge curves and discharge capacity changes when a lithium-ion secondary battery (half-cell) manufactured using the negative electrode obtained in Example 1 is charged and discharged at 100 mA / g Si-C (Si / C = 1 / 1 in mol%) for cycles 1-5 and 1000 mA / g Si-C (Si / C = 1 / 1 in mol%) for cycles 6-10. As before, the charge-discharge rate is calculated based on the total weight of equimolar amounts of C from the generated stacked disordered structure SiC and unreacted Si. In Figures 11-12, the discharge capacity for each cycle is calculated based on the total weight of SiC and Si obtained by the calculation method shown in (1-4). From these results, it can be seen that using the negative electrode active material for lithium-ion secondary batteries of the present invention results in superior capacity, cycle characteristics, and rate characteristics.

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

[0123] Although carbon may also insert and remove lithium ions, unlike graphite which has a layered structure, the theoretical charge-discharge capacity of amorphous carbon materials cannot be calculated. Furthermore, the amount of lithium ions inserted and removed by carbon varies depending on the grinding conditions of the mechanochemical treatment, making it difficult to specify. For convenience, this specification assumes that carbon does not insert or remove lithium ions, meaning that only silicon carbide and silicon insert and remove lithium ions.

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

[0125] Note that x is the weight fraction of SiC, and 1-x represents the weight fraction of Si+C. Also, 6.667mAh is the measured value, and 0.003455g is the sum of the weight of SiC and the weight of unreacted Si and an equimolar amount of C. Of the negative electrode layer (coated to the current collector), C+SiC accounts for 85 mass%, and half of that (50 mass%) consists of C and SiC, respectively. Therefore, the raw materials are mixed so that when the raw material C and raw material Si react completely, C and Si-C (equomoles) each account for 50 mass.

[0126] (Calculation of specific volume 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 0.003455g × 0.6485 (weight fraction of SiC). The weight of Si (unreacted Si) is 0.003455g × 0.3515 (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 0.003455g × 0.3515 (weight fraction of Si + C) × 12.01 / 40.09 plus 0.003455g of surplus C, since the raw materials were originally mixed to be 50% by mass of C and 50% by mass of SiC (Si powder and C powder).

[0128] From these results, the ratio of SiC weight:Si weight:C weight, when divided by 0.003455, 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] From the above, the ratio of C:SiC:Si is 55.3 mass%:32.4 mass%:12.3 mass%.

[0130] From these results, a content ratio equivalent to that calculated from the results of Test Example 3 was obtained, confirming the reliability of the method for calculating the content ratio.

[0131] Comparative Example 1 A mixture of 60% by mass of carbon and 40% by mass of layered disordered silicon carbide (SD-SiC) as the active material made up 80% by mass of the total electrode coating material. 5% by mass of pulverized nano-Si with an average crystallite size of approximately 8 nm was added, and 7.5% by mass of polyacrylic acid (PAA) and 7.5% by mass of carboxymethylcellulose (CMC) were added to make a total of 100% by mass. The slurry, prepared using a stirring and defoaming machine, was coated onto a Cu foil current collector to fabricate the negative electrode. In other words, carbon, SD-SiC, and Si were simply mixed, and the SD-SiC and silicon were not dispersed in the amorphous carbon material in such a way that they would have conductive paths with the amorphous carbon material. In this charge-discharge measurement, at a charge-discharge rate of 100 mA / g, the discharge amount after the first charge was below the theoretical charge-discharge amount of SD-SiC, which is 1336 mAh / g. Furthermore, the discharge amount after the second charge was even lower, decreasing to approximately 990-1080 mAh / g, indicating no significant effect from the Si addition.

[0132] Comparative Example 2 Similar to Examples 1 and 2, a mixture of 50% by mass of carbon and 50% by mass of silicon carbide was prepared as the active material, making up 85% by mass of the total coating amount. This mixture was then combined with 7.5% by mass of polyacrylic acid (PAA) and 7.5% by mass of carboxymethylcellulose (CMC), and the slurry was prepared using a stirring and defoaming machine. This slurry was then coated onto a Cu foil current collector to produce a negative electrode. In this case, the powder synthesized was obtained by reducing the mechanochemical processing time of the grinding conditions to 5 hours, using the same conditions as in Examples 1 and 2. X-ray diffraction results showed that at 5 hours, most of the diffraction peaks were Si, and the diffraction peak of layered disordered structure silicon carbide (SD-SiC) was hidden by the Si diffraction peak, indicating that almost no SD-SiC was generated. Using this electrode, charging and discharging were performed initially at 100 mA / g ~ Si-C (Si / C = 1 / 1 in mol%), as in the charge-discharge experiment in Test Example 3. Subsequently, from the 2nd to the 10th charge-discharge cycle, the rate was increased to 1000 mA / g. As a result, the discharge capacity decreased monotonically without being retained.

[0133] Example 3 Similar to Examples 1 and 2, a mixture of 50% by mass of carbon and 50% by mass of silicon carbide as the active material constituted 85% by mass of the total coating amount. This mixture was then combined with 7.5% by mass of polyacrylic acid (PAA) and 7.5% by mass of carboxymethylcellulose (CMC), and the slurry was prepared using a stirring and defoaming machine. This slurry was then coated onto a Cu foil current collector to produce a negative electrode. In this case, the powder used was synthesized under the same conditions as in Examples 1 and 2, but with the mechanochemical processing time of the grinding conditions reduced to 8 hours. As a result, similar to Examples 1 and 2, it was confirmed that SD-SiC and silicon were dispersed in the amorphous carbon material so as to have conductive paths with the amorphous carbon material. Using this electrode, charging and discharging were performed initially at 100 mA / g ~ Si-C (Si / C = 1 / 1 in mol%), as in the charge-discharge experiment of Test Example 3. Subsequently, the second to fifth charge-discharge cycles were performed at a rate of 1000 mA / g ~ Si-C (Si / C = 1 / 1 in mol%). As a result, as shown in Figure 13, the initial discharge amount after the initial charge at 100 mA / g ~ Si-C (Si / C = 1 / 1 in mol%) was 3422 mAh / g. At the subsequent charge-discharge rate of 1000 mA / g, the discharge amount decreased slightly from 2760 mAh / g (second cycle) to 2544 mAh / g (fifth cycle), but a high discharge amount was still maintained. This is shown in Figure 14.

[0134] At this time, using the calculation method (1-4), the weight fraction of the stacked disordered structure SiC calculated from the initial discharge capacity of 100 mA / g was 0.2065, and the sum of unreacted Si and equimolar C was 0.7935. Therefore, the respective content ratios of carbon, silicon carbide, and silicon in the negative electrode active material for lithium-ion secondary batteries were calculated as follows (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, the ratio of SiC:(Si+C) = 0.2065:0.7935 (by weight).

[0136] Note that x is the weight fraction of SiC, and 1-x represents the weight fraction of Si+C. Also, 12.10mAh is the measured value, and 0.004636g is the sum of the weight of SiC and the weight of unreacted Si and an equimolar amount of C. Of the negative electrode layer (coated to the current collector), C+SiC accounts for 85 mass%, and half of that (50 mass%) consists of C and SiC respectively. Therefore, the raw materials are mixed so that when the raw material C and raw material Si react completely, C and Si-C (equomoles) each account for 50 mass.

[0137] (Calculation of specific volume 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 + surplus 50% by mass of C) is 0.004636 g × 0.7935 (weight fraction of Si + C) × 12.01 / 40.09 plus 0.004636 g of surplus C, since the raw materials were originally mixed to be 50% by mass of C and 50% by mass of SiC (Si powder and C powder).

[0139] From these results, the ratio of SiC weight:Si weight:C weight, when divided 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, the ratio of C:SiC:Si is 61.9 mass%:10.3 mass%:27.8 mass%.

Claims

1. A negative electrode active material for lithium-ion secondary batteries containing a stacked disordered structure silicon carbide, an amorphous carbon material, and silicon, The stacked disordered silicon carbide and the silicon are dispersed in the amorphous carbon material such that they have conductive paths with the amorphous carbon material, and A negative electrode active material for a lithium-ion secondary battery, wherein, with a total amount of 100% by mass, the silicon content is 5.0 to 28.0% by mass, and the layered disordered structure silicon carbide content is 21.6 to 57.0% by mass.

2. The negative electrode active material for a lithium-ion secondary battery according to claim 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.

3. 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.

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

5. 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°.

6. 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 5.

7. A lithium-ion secondary battery comprising the negative electrode for a lithium-ion secondary battery as described in 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 process of mechanochemically treating a raw material mixture containing silicon-containing material and carbon material until, with a total amount of 100% by mass, the silicon content is 5.0 to 28.0% by mass and the layered irregular structure silicon carbide content is 21.6 to 57.0% by mass. A manufacturing method that includes the following features.

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