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

Abstract

To provide a material that can serve as a negative electrode active material for manufacturing a lithium ion secondary battery having excellent capacity and cycle characteristics.SOLUTION: A negative electrode active material for a lithium ion secondary battery contains stacked irregular-structure silicon carbide and an amorphous carbon material, wherein the content of the stacked irregular-structure silicon carbide is 30 to 60 mass% and the content of the amorphous carbon material is 40 to 70 mass% relative to 100 mass% of the total amount.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Currently, the charge / discharge capacity of lithium-ion secondary batteries depends largely on the active materials of the positive and negative electrodes. It is known that lithium iron phosphate is the main positive electrode active material, and its future industrialization will advance due to its stability, cost, and availability. Meanwhile, the next generation of negative electrode active materials has yet to be fully determined. Currently, graphite is used as the negative electrode active material, but its theoretical charge / discharge capacity is 372 mAh / g, and the development of a higher-capacity negative electrode active material is awaited. Meanwhile, silicon has a theoretical charge / discharge capacity of over 3500 mAh / g, approximately 10 times that of graphite. However, during charging, lithium ions inserted into and extracted from the negative electrode react with silicon to form a compound, causing the silicon volume to expand by approximately 3 to 4 times. When the lithium ions dissociate from the compound during discharge, the silicon volume decreases by approximately 1 / 4 to 1 / 3. This disrupts contact between the active material and the electrode during charge-discharge cycles, significantly reducing cycle performance. For example, capacity drops to approximately 35% after just five cycles. 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 all use highly crystalline cubic β-SiC. Furthermore, due to poor reproducibility and unsuitability for mass production, a decisive active material has yet to be identified.

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

[0001] direction (see, for example, Non-Patent Document 9). The stacked disordered silicon carbide reported in Non-Patent Document 9 was produced with a silicon:carbon ratio of 1:1 (molar ratio). [Prior art documents] [Non-patent 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). Summary of the Invention [Problem to be solved by the invention]

[0005] However, according to the research of the present inventors, even if an attempt is made to use stacked disordered silicon carbide manufactured with a silicon:carbon ratio of 1:1 (molar ratio) as a negative electrode active material as in Non-Patent Document 9, it is difficult to insert and extract lithium ions during charge and discharge, and therefore it has been found that it is not possible to manufacture a lithium ion secondary battery with excellent capacity and cycle characteristics.

[0006] Therefore, an object of the present invention is to provide a material that can be used as a negative electrode active material for producing a lithium ion secondary battery having excellent capacity and cycle characteristics. [Means for solving the problem]

[0007] As a result of extensive research, the present inventors have found that when stacking disordered structure silicon carbide is produced under carbon-rich conditions, stacking disordered structure silicon carbide containing silicon carbide and an amorphous carbon material is obtained, and that the silicon carbide produced at the Si-C interface migrates to the conductive excess carbon matrix phase, which can reversibly absorb and release lithium ions as the lithium ions are charged and discharged, thereby enabling the production of a lithium ion secondary battery with excellent capacity and cycle characteristics. The present invention was completed based on this finding and through further research. That is, the present invention includes the following features.

[0008] Item 1. A negative electrode active material for a lithium ion secondary battery containing stacked irregular structure silicon carbide and an amorphous carbon material, A negative electrode active material for a lithium ion secondary battery, wherein the content of the stacked irregular structure silicon carbide is 30 to 60 mass % and the content of the amorphous carbon material is 40 to 70 mass %, with the total amount being 100 mass %.

[0009] Item 2. The negative electrode active material for a lithium ion secondary battery according to Item 1, wherein the stacked disordered structure silicon carbide is dispersed in the amorphous carbon material so as to have a conductive path between the stacked disordered structure silicon carbide and the amorphous carbon material.

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

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

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

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

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

[0015] Item 8. A method for producing a negative electrode active material for a lithium ion secondary battery according to any one of items 1 to 5, A step of subjecting a raw material mixture containing a silicon-containing material and a carbon material to mechanochemical treatment. Equipped with The manufacturing method, wherein the content of the silicon-containing material is 20 to 45 mass % and the content of the carbon material is 50 to 80 mass % relative to the total amount of the raw material mixture, which is 100 mass %. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a material that can serve as a negative electrode active material for producing a lithium ion secondary battery having excellent capacity and cycle characteristics. [Brief explanation of the drawings]

[0017] [Figure 1] 1 shows an X-ray diffraction pattern of the negative electrode active material obtained in Example 1. [Figure 2] The X-ray diffraction spectrum of stacked disordered silicon carbide (SD-SiC) is shown. [Figure 3] The X-ray diffraction pattern of highly crystalline cubic β-SiC is shown. [Figure 4] 1 shows an X-ray diffraction spectrum of amorphous carbon. [Figure 5] 1 shows an X-ray diffraction pattern of the negative electrode active material obtained in Example 3. [Figure 6] 1 shows an X-ray diffraction pattern of the negative electrode active material obtained in Example 4. [Figure 7] This shows a high-resolution transmission electron microscope (TEM) image of the negative electrode active material obtained by reacting graphite with silicon and grinding for 4 hours. [Figure 8] The results are shown for a lithium ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 1, where the charge / discharge current was 420 mA / g (0.1 C) for the first to third cycles, 2100 mA / g (0.5 C) for the fourth to eighth cycles, and 420 mA / g (0.1 C) for the ninth to eleventh cycles. The discharge capacity for each cycle was calculated relative to the weight of silicon. [Figure 9] The results are shown for a lithium ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 2, where the charge / discharge current was 420 mA / g (0.1 C) for the first to third cycles, 2100 mA / g (0.5 C) for the fourth to eighth cycles, and 420 mA / g (0.1 C) for the ninth to eleventh cycles. The discharge capacity for each cycle was calculated relative to the weight of silicon. [Figure 10]The lithium ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 2 was charged and discharged at 420 mA / g (0.1 C) for the first to third cycles, 2100 mA / g (0.5 C) for the fourth to eighth cycles, and 420 mA / g (0.1 C) for the ninth to eleventh cycles. The initial charge and discharge capacity at each rate was shown. The charge and discharge capacity was calculated relative to the weight of silicon. [Figure 11] The lithium-ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 1 was charged and discharged at 420 mA / g (0.1 C) for the first to third cycles, 2100 mA / g (0.5 C) for the fourth to eighth cycles, and 420 mA / g (0.1 C) for the ninth to eleventh cycles, and then at 50 mA / g (0.012 C). The charge and discharge capacities were calculated relative to the weight of silicon. [Figure 12] The lithium-ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 1 was charged and discharged at 420 mA / g (0.1 C) for the first to third cycles, 2100 mA / g (0.5 C) for the fourth to eighth cycles, and 420 mA / g (0.1 C) for the ninth to eleventh cycles, and then at 50 mA / g (0.012 C). The charge-discharge capacity was calculated relative to the weight of silicon carbide. DETAILED DESCRIPTION OF THE INVENTION

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

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

[0020] 1. Negative electrode active material for lithium-ion secondary batteries The negative electrode active material for a lithium ion secondary battery of the present invention is a negative electrode active material for a lithium ion secondary battery containing stacked irregular structure silicon carbide and an amorphous carbon material, in which the content of the stacked irregular structure silicon carbide is 40 to 60 mass % and the content of the amorphous carbon material is 40 to 60 mass % when the total amount is 100 mass %.

[0021] (1-1) Stacked irregular structure silicon carbide The stacked irregular structure silicon carbide contained in the negative electrode active material for a lithium ion secondary battery of the present invention is preferably, for example, SiCx (0.8≦x≦1.5). In particular, it is preferable to use SiC where x≈1 from the viewpoints of charge / discharge capacity, cycle characteristics, etc.

[0022] The stacked disordered silicon carbide contained in the negative electrode active material for a lithium ion secondary battery of the present invention can have broad peaks at 2θ=36.0°, 60.0°, and 72.0°, which are attributed to silicon carbide, within a tolerance range of ±0.5° in X-ray diffraction measurement using CuKα radiation.

[0023] Although highly crystalline β-SiC is usually used as silicon carbide for lithium ion secondary batteries, the stacking disordered silicon carbide contained in the negative electrode active material for lithium ion secondary batteries of the present invention is preferably low-crystalline silicon carbide from the viewpoints of charge / discharge capacity, cycle characteristics, etc. Specifically, the stacking disordered silicon carbide preferably has a full width at half maximum of a broad peak at 2θ=36.0° within a tolerance of ±0.5° in X-ray diffraction measurement using CuKα radiation of 2.0° or more, more preferably 2.5 to 10.0°.

[0024] The shape of the layered irregular structure silicon carbide is not particularly limited, and any shape such as powder, plate, granule, sphere, fiber, or block can be used.

[0025] In the negative electrode active material for a lithium ion secondary battery of the present invention, it is preferable that small silicon carbide particles are dispersed in an amorphous carbon material from the viewpoint of facilitating the absorption and desorption of lithium ions, and therefore it is preferable that the average particle diameter of the stacking irregular structure silicon carbide is small. Therefore, the average particle diameter of the stacking irregular structure silicon carbide is preferably 5 to 20 nm, more preferably 5 to 10 nm. The average particle diameter of the stacking irregular structure silicon carbide is measured by observation with a high-resolution transmission electron microscope.

[0026] In the negative electrode active material for a lithium ion secondary battery of the present invention, the content of the stacked irregular structure silicon carbide is not particularly limited. However, from the viewpoint of facilitating occlusion and desorption of lithium ions, it is preferable that the small silicon carbide particles are dispersed in the amorphous carbon material, preferably without agglomeration, so as to have a conductive path between the small silicon carbide particles and the amorphous carbon material. Therefore, the content is preferably 30 to 60 mass%, more preferably 35 to 55 mass%, based on 100 mass% of the total amount of the negative electrode active material for a lithium ion secondary battery of the present invention.

[0027] (1-2) Amorphous carbon materials In the negative electrode active material for a lithium ion secondary battery of the present invention, the amorphous carbon material is contained, thereby improving the conductivity and enabling the flow of electricity.

[0028] The amorphous carbon material contained in the negative electrode active material for a lithium ion secondary battery of the present invention can have a broad peak at 2θ=22.5° and, if necessary, 42.0° due to the amorphous carbon material, within an allowable range of ±0.5° in X-ray diffraction measurement using CuKα radiation.

[0029] From the viewpoints of charge / discharge capacity, cycle characteristics, etc., the amorphous carbon material contained in the negative electrode active material for a lithium ion secondary battery of the present invention preferably has a full width at half maximum of a broad peak at 2θ=22.5° of 2.0° or more, more preferably 3.0 to 10.0°, within a tolerance of ±0.5°, in X-ray diffraction measurement using CuKα radiation.

[0030] The shape of the amorphous carbon material is not particularly limited, and any shape such as powder, plate, granule, sphere, fiber, or block can be used, but the amorphous carbon material is usually spherical.

[0031] In the negative electrode active material for a lithium ion secondary battery of the present invention, small silicon carbide particles are preferably dispersed in the amorphous carbon material from the viewpoint of facilitating the absorption and desorption of lithium ions, and therefore the average particle size of the amorphous carbon material is also preferably small. Because the amorphous carbon material is an amorphous material, 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, 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.

[0032] In the negative electrode active material for a lithium ion secondary battery of the present invention, the content of the amorphous carbon material is not particularly limited, but from the viewpoints of easily improving the conductivity, charge / discharge capacity, cycle characteristics, etc., it is preferably 40 to 70 mass %, more preferably 45 to 65 mass %, where the total amount of the negative electrode active material for a lithium ion secondary battery is 100 mass %.

[0033] (1-3) Negative electrode active material for lithium ion secondary batteries As described above, the negative electrode active material for a lithium ion secondary battery of the present invention contains stacked disordered structure silicon carbide and an amorphous carbon material.

[0034] In the negative electrode active material for a lithium ion secondary battery of the present invention, the stacking disordered silicon carbide formed at the Si-C interface migrates to the conductive excess carbon matrix phase, which can reversibly absorb and release lithium ions as the lithium ions are charged and discharged. Therefore, it is preferable that the stacking disordered silicon carbide is dispersed in the amorphous carbon material, preferably without agglomeration, so as to have a conductive path between the stacking disordered silicon carbide and the amorphous carbon material.

[0035] In addition to the above-described stacked irregular silicon carbide and amorphous carbon material, the negative electrode active material for a lithium ion secondary battery of the present invention may contain, within the range that does not impair the effects of the present invention, a silicon-containing raw material (silicon, silicon nitride, silicon oxide, etc.), a crystalline carbon material (graphite, etc.), crystalline silicon carbide (cubic β-SiC), and a third component such as a metal component (iron, etc.) or a metal-nonmetal compound unavoidable during synthesis. 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 a lithium ion secondary battery of the present invention.

[0036] In the negative electrode active material for lithium ion secondary batteries of the present invention, the stacking irregular structure silicon carbide formed at the Si-C interface migrates to the conductive excess carbon matrix, where it can reversibly absorb and release lithium ions during charging and discharging. Furthermore, although the stacking irregular structure silicon carbide in the negative electrode active material for lithium ion secondary batteries of the present invention is a silicon-containing material, it does not expand or contract as much as silicon during charging and discharging. Furthermore, in the negative electrode active material for lithium ion secondary batteries of the present invention, the stacking irregular structure silicon carbide is dispersed in the amorphous carbon material, preferably without agglomeration, so as to form a conductive path between the stacking irregular structure silicon carbide and the amorphous carbon material. This allows the amorphous carbon material adjacent to the stacking irregular structure silicon carbide to follow the expansion and contraction of the stacking irregular structure silicon carbide. Therefore, by using the negative electrode active material for lithium ion secondary batteries of the present invention, lithium ion secondary batteries with particularly high capacity and excellent cycle characteristics can be manufactured. In the present invention, the negative electrode active material for a lithium ion secondary battery is a concept that also includes a negative electrode active material for a metal lithium secondary battery in which lithium metal is used as the positive electrode.

[0037] 2. Negative electrodes for lithium-ion secondary batteries The negative electrode for a lithium ion secondary battery of the present invention contains the negative electrode active material for a lithium ion secondary battery of the present invention. More specifically, the negative electrode for a lithium ion secondary battery of the present invention can include a negative electrode active material layer containing the negative electrode active material for a lithium ion secondary battery of the present invention.

[0038] The negative electrode active material layer can be composed solely of the negative electrode active material for lithium ion secondary batteries of the present invention described above. However, if necessary, a conductive agent such as carbon black (e.g., acetylene black, furnace black, or ketjen black); flake graphite; graphene; or amorphous carbon obtained by heat-treating an organic material can be added. In particular, when the content of the stacked irregular silicon carbide in the negative electrode active material for lithium ion secondary batteries of the present invention is high, the use of a conductive agent is particularly effective. These conductive agents can be used alone or in combination of two or more.

[0039] Furthermore, the negative electrode active material layer may further contain, as necessary, a binder, thickener, or dispersant, such as a fluorine-based polymer (polyvinylidene fluoride resin, polytetrafluoroethylene resin, vinylidene fluoride-hexafluoropropylene copolymer, etc.), a polyolefin-based resin (styrene-butadiene copolymer resin, ethylene-vinyl alcohol copolymer resin, etc.), a synthetic rubber (styrene-butadiene rubber, acrylonitrile-butadiene rubber, ethylene-propylene-diene rubber, etc.), polyacrylonitrile, polyamide, polyimide, polyacrylic acid, polyacrylic acid ester, polyvinyl ether, carboxymethyl cellulose, carboxymethyl cellulose sodium salt, carboxymethyl cellulose ammonium, polyurethane, hydroxypropyl cellulose, hydroxyethyl cellulose, methyl cellulose, etc. These binders, thickeners, or dispersants may be used alone or in combination of two or more.

[0040] The content of the negative electrode for a lithium ion secondary battery of the present invention can be 70 to 95 mass%, particularly 80 to 90 mass%, based on 100 mass% of the total amount of the negative electrode active material layer, from the viewpoints of charge / discharge capacity, cycle characteristics, etc. Furthermore, the content of the binder, thickener, or dispersant can be 5 to 30 mass%, particularly 10 to 20 mass%, based on 100 mass% of the total amount of the negative electrode active material layer, from the viewpoints of charge / discharge capacity, cycle characteristics, etc.

[0041] 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.

[0042] Such a negative electrode active material layer can be produced by forming a negative electrode mixture containing the negative electrode active material for a lithium ion secondary battery of the present invention and, if necessary, a binder, a thickener, a dispersant, etc. into a layer. For example, the negative electrode mixture can be dried by a conventional method and formed into a layer.

[0043] As described above, the negative electrode for a lithium ion secondary battery of the present invention preferably includes the above-described negative electrode active material layer, and specifically, it preferably includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

[0044] 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 a carbon material. This negative electrode current collector can be, for example, a foil-shaped or mesh-shaped member.

[0045] The negative electrode for a lithium ion secondary battery of the present invention can be produced by forming the above-described negative electrode mixture into a layer on a negative electrode current collector. For example, the negative electrode for a lithium ion secondary battery of the present invention can be produced by drying the negative electrode mixture on the negative electrode current collector by a conventional method and forming it into a layer.

[0046] 3. Lithium-ion secondary batteries The lithium ion secondary battery of the present invention includes the above-described negative electrode for a lithium ion secondary battery of the present invention. The lithium ion secondary battery of the present invention may also include a positive electrode, an electrolyte, and a container for storing these, which are applicable to known lithium ion secondary batteries, in addition to the negative electrode for a lithium ion secondary battery of the present invention.

[0047] The positive electrode may be any electrode capable of supplying lithium ions to the negative electrode, and any known positive electrode may be used.

[0048] Examples of the positive electrode current collector that constitutes the positive electrode include materials that are electrochemically stable at the potential used and have high electronic conductivity, such as aluminum, stainless steel, and carbon materials.

[0049] As the positive electrode active material constituting the positive electrode, a material capable of absorbing and releasing lithium ions is usually used. Examples include lithium transition metal composite oxides having an α-NaFeO2 crystal structure, lithium transition metal oxides having a spinel crystal structure, polyanion compounds, chalcogen compounds, sulfur, etc. As the lithium transition metal composite oxides having an α-NaFeO2 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). Examples of lithium transition metal oxides with a spinel crystal structure include Li x3 Mn2O4(0.9≦x3<1.5), Li x4 Ni γ4 Mn (2-γ4)O4 (0.9≦x4<1.5, 0≦γ4≦2). Examples of polyanion compounds include LiFePO4, LiMnPO4, LiNiPO4, LiCoPO4, Li3V2(PO4)3, Li2MnSiO4, and Li2CoPO4F. Examples of chalcogen compounds include titanium disulfide, molybdenum disulfide, and molybdenum dioxide. Atoms or polyanions in these materials may be partially substituted with atoms or anion species of other elements. These positive electrode active materials may be used alone or in combination of two or more.

[0050] As the positive electrode constituent material other than the positive electrode active material that constitutes the positive electrode, the same material as the negative electrode constituent material other than the negative electrode active material in the negative electrode described above can be used, and the content thereof can also be the same as the negative electrode constituent material other than the negative electrode active material in the negative electrode.

[0051] The electrolyte solution is an electrolyte solution in which a salt is dissolved in an aprotic organic solvent, and is placed between the positive electrode and the negative electrode. For example, it is preferable that the electrolyte solution is impregnated and held in a separator made of nonwoven fabric or the like to prevent short-circuiting between the positive electrode and the negative electrode.

[0052] Examples of the aprotic organic solvent that constitutes the above-mentioned electrolytic solution include esters such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, methyl formate, and methyl acetate; furans such as tetrahydrofuran and 2-methyltetrahydrofuran; ethers such as dioxolane, diethyl ether, dimethoxyethane, diethoxyethane, and methoxyethoxyethane; dimethyl sulfoxide; sulfolanes such as sulfolane and methylsulfolane; acetonitrile, etc. These aprotic organic solvents may be used alone or in combination of two or more.

[0053] On the other hand, examples of salts that can be dissolved in such aprotic organic solvents include lithium salts such as lithium perchlorate, lithium fluoroborate, lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium trifluoromethanesulfonate, lithium halides, lithium chloroaluminate, and lithium bis(fluorosulfonyl)imide. These salts may be used alone or in combination of two or more.

[0054] 4.Method for producing negative electrode active material for lithium ion secondary battery The method for producing a negative electrode active material for a lithium ion secondary battery of the present invention is not particularly limited, but may include, for example, a step of subjecting a raw material mixture containing a silicon-containing material and a carbon material to a mechanochemical treatment. Equipped with The content of the carbon material is 50 to 70 mass % with the total amount of the raw material mixture being 100 mass %.

[0055] The silicon-containing material used as a raw material is not particularly limited, and in addition to silicon, silicon nitride, silicon oxide, silicon boride, metal silicides such as silicon titanate and iron silicide, etc. can be used. These silicon-containing materials can be used alone or in combination of two or more. Furthermore, since the silicon-containing material is mixed and pulverized by mechanochemical treatment, there is no limitation on the particle size of the silicon-containing material used, and commercially available powdered silicon-containing material can usually be used.

[0056] The carbon material used as the raw material is not particularly limited, and examples thereof include carbon blacks such as acetylene black, furnace black, and ketjen black; graphite; graphene; and amorphous carbon. These carbon materials can be used alone or in combination of two or more. Furthermore, since the carbon materials are mixed and pulverized by mechanochemical treatment, there are no limitations on the particle size of the carbon material used, and commercially available powdered carbon materials can usually be used.

[0057] Mechanochemical processing is a method of grinding and mixing raw materials while applying mechanical energy. According to this method, the raw materials are ground and mixed by applying mechanical impact and friction, which causes the silicon-containing material and the carbon material to come into vigorous contact with each other and become finer, resulting in a reaction of the raw materials. In other words, mixing, grinding, and reaction occur simultaneously. This makes it possible to more reliably react the raw materials without heating them to high temperatures. By using mechanochemical processing, a metastable crystalline structure that cannot be obtained by ordinary heat treatment can sometimes be obtained.

[0058] These raw materials can be mixed all at once 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.

[0059] The mixing ratio of the raw materials is almost the same as the elemental ratio of silicon and carbon in the target negative electrode active material for a lithium-ion secondary battery of the present invention, since the charging ratio of the raw materials is almost the same as the elemental ratio of silicon and carbon in the target negative electrode active material for a lithium-ion secondary battery of the present invention. Specifically, the content of the silicon-containing material is preferably 20 to 45 mass%, more preferably 25 to 40 mass%, of the total amount of the raw material mixture taken as 100 mass%. Furthermore, the content of the carbon material is preferably 50 to 80 mass%, more preferably 55 to 75 mass%, of the total amount of the raw material mixture taken as 100 mass%.

[0060] In the present invention, the amount of energy input in the mechanochemical treatment is preferably 10 to 110 kWh / kg of raw material mixture, more preferably 25 to 80 kWh / kg of raw material mixture, from the viewpoint of facilitating the production of the layered disordered silicon carbide of the present invention.

[0061] The energy input for mechanochemical treatment is based on the following literature: Burgio, N., Lasonna, A., Magini, M., Martelii, S. and Padella, F., Il Nuovo Cimento, Vol. 13, pp. 459-476 (1991). It is calculated using the formula shown below.

[0062] In mechanochemical treatment, when the raw material mixture is subjected to rotation and revolution, a powerful combined centrifugal force can be applied, causing convection and generating vortex currents due to the rotation. These flows are effectively combined to perform precise stirring, allowing the stacked irregular structure silicon carbide of the present invention to be produced efficiently.

[0063] In this case, assuming that a P-5 manufactured by Fritsche is used, the rotation / revolution ratio is 2, and the upper limit of the revolution speed is set to 400 rpm. Therefore, the rotation speed is not particularly limited, but from the viewpoint of easily obtaining the stacking irregular structure silicon carbide of the present invention, it is preferably 500 to 800 rpm, more preferably 600 to 700 rpm. Furthermore, the revolution speed is not particularly limited, but from the viewpoint of easily obtaining the stacking irregular structure silicon carbide of the present invention, it is preferably 250 to 400 rpm, more preferably 300 to 350 rpm.

[0064] The temperature during the mechanochemical treatment is not particularly limited and can be appropriately adjusted from the viewpoint of facilitating the production of the negative electrode active material for a lithium ion secondary battery of the present invention, and can be, for example, room temperature.

[0065] The duration of the mechanochemical treatment is not particularly limited, and can be any duration until the desired negative electrode active material for a lithium ion secondary battery of the present invention is obtained. For example, the mechanochemical treatment can be carried out for 6 to 24 hours (particularly 10 to 18 hours). Note that the faster the rotation speed, the shorter the reaction time tends to be, so it is preferable to adjust the rotation speed appropriately. Furthermore, this mechanochemical treatment can be carried out in multiple steps with breaks in between, as necessary.

[0066] When the mechanochemical treatment is repeated multiple times, the above conditions can be applied to each mechanochemical treatment step.

[0067] When carrying out the above-described mechanochemical treatment, specifically, mixing and grinding can be carried out using a mechanical grinding device such as a ball mill, planetary ball mill, bead mill, rod mill, vibration mill, disk mill, hammer mill, jet mill, surface modification / grinding device, or high-pressure gas pulverizer. [Example]

[0068] The present invention will be specifically described based on examples, but the present invention is not limited to these examples.

[0069] The raw material powders used were silicon (Silicon; manufactured by Kojundo Chemical Laboratory Co., Ltd.; average particle size 45 μm, 99.99%), natural graphite (manufactured by Shanghai Shanshan New Materials Co., Ltd.; average particle size 17 μm, ash 0.02%), acetylene black (Li-100 manufactured by Denka Co., Ltd.; average particle size 35 nm), and carbon black (#8500F manufactured by Tokai Carbon Co., Ltd.; average particle size 14 nm).

[0070] [Example 1] Mixing ratio The raw material mixture ratio was 3.5136 g of sample and 5.4 g of distilled water, and the sample was weighed to contain 28.3 mass% silicon, 47.2 mass% natural graphite, 9.4 mass% acetylene black, 7.5 mass% polyacrylic acid (PAA), and 7.5 mass% carboxymethyl cellulose (CMC).

[0071] Powder Synthesis Using a high-energy ball mill (Fritsch P5; SiN pot: 250 mL; SiN balls: diameter 10 mm), 2.50 g of silicon, 4.1807 g of natural graphite, and 5.0137 g of acetylene black were added to the pot, and mechanochemical treatment was carried out at an orbital speed of 300 rpm, a rotation speed of 600 rpm, a milling time of 24 hours, and a ball weight:powder weight ratio of 40:1, to obtain a negative electrode active material for a lithium ion secondary battery of Example 1.

[0072] Negative electrode fabrication The stirring and defoaming conditions for producing the negative electrode were as follows: stirring was performed at a rotation speed of 800 rpm and a revolution speed of 2000 rpm for 60 seconds, and defoaming was performed at a rotation speed of 60 rpm and a revolution speed of 2200 rpm for 30 seconds.

[0073] First, 0.264 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, 2.9803 g of the negative electrode active material for a lithium ion secondary battery of Example 1 obtained above was added to the aqueous binder solution, mixed with a spatula, and 2 mL of distilled water was added in portions while checking the mixture, and the mixture was stirred and degassed twice.

[0074] Further, 0.2693 g of carboxymethyl cellulose (CMC) was added to the obtained aqueous solution, and after stirring and degassing, 1 mL of distilled water was added, and stirring and degassing were carried out twice to obtain a negative electrode mixture.

[0075] Thereafter, the obtained negative electrode mixture was applied to a copper foil with a doctor blade to a thickness of 175 μm, and vacuum dried at 80° C. for 12 hours to obtain a negative electrode for a lithium ion secondary battery of Example 1.

[0076] [Example 2] Mixing ratio The raw materials were mixed in a ratio of 3.5076 g of sample to 6.6 g of distilled water, and the sample was weighed to contain 28.0 mass % silicon, 57.0 mass % carbon black, 7.5 mass % polyacrylic acid (PAA), and 7.5 mass % carboxymethyl cellulose (CMC).

[0077] Powder Synthesis Using a high-energy ball mill (Fritsch P5; SiN pot: 250 mL; SiN balls: diameter 10 mm), 2.50 g of silicon and 5.0137 g of carbon black were added to the pot, and mechanochemical treatment was carried out under the conditions of a revolution speed of 300 rpm, a rotation speed of 600 rpm, a milling time of 24 hours, and a ball weight:powder weight ratio of 40:1, to obtain a negative electrode active material for a lithium-ion secondary battery of Example 2.

[0078] Negative electrode fabrication The stirring and defoaming conditions for producing the negative electrode were as follows: stirring was performed at a rotation speed of 800 rpm and a revolution speed of 2000 rpm for 60 seconds, and defoaming was performed at a rotation speed of 60 rpm and a revolution speed of 2200 rpm for 30 seconds.

[0079] First, 0.264 g of polyacrylic acid (PAA) and 2.0 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, 2.9777 g of the negative electrode active material for a lithium ion secondary battery of Example 2 obtained above was added to the aqueous binder solution, mixed with a spatula, and 2.2 mL of distilled water was added in portions while checking the mixture, and the mixture was stirred and degassed twice.

[0080] Further, 0.2659 g of carboxymethyl cellulose (CMC) was added to the obtained aqueous solution, and after stirring and degassing, 2.4 mL of distilled water was added in portions, and stirring and degassing were carried out twice to obtain a negative electrode mixture.

[0081] Thereafter, the obtained negative electrode mixture was applied to a copper foil with a doctor blade to a thickness of 175 μm, and vacuum dried at 80° C. for 12 hours to obtain a negative electrode for a lithium ion secondary battery of Example 2.

[0082] [Example 3] Mixing ratio The raw materials were mixed in a ratio of 3.5066 g of sample to 6.6 g of distilled water, and the sample was weighed to contain 35.7 mass% silicon, 39.3 mass% natural graphite, 10.0 mass% acetylene black, 7.5 mass% polyacrylic acid (PAA), and 7.5 mass% carboxymethyl cellulose (CMC).

[0083] Powder Synthesis Using a high-energy ball mill (Fritsch P5; SiN pot: 250 mL; SiN balls: diameter 10 mm), 3.1564 g of silicon, 3.4675 g of natural graphite, and 0.8840 g of acetylene black were added to the pot, and mechanochemical treatment was carried out at an orbital speed of 300 rpm, a rotation speed of 600 rpm, a milling time of 24 hours, and a ball weight:powder weight ratio of 40:1, to obtain a negative electrode active material for a lithium ion secondary battery of Example 3.

[0084] Negative electrode fabrication The stirring and defoaming conditions for producing the negative electrode were as follows: stirring was performed at a rotation speed of 800 rpm and a revolution speed of 2000 rpm for 60 seconds, and defoaming was performed at a rotation speed of 60 rpm and a revolution speed of 2200 rpm for 30 seconds.

[0085] First, 0.263 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, 2.9790 g of the negative electrode active material for a lithium ion secondary battery of Example 3 obtained above was added to the aqueous binder solution, mixed with a spatula, and 2 mL of distilled water was added in portions while checking the mixture, and the mixture was stirred and degassed twice.

[0086] Further, 0.2646 g of carboxymethyl cellulose (CMC) was added to the obtained aqueous solution, and after stirring and degassing, 2.2 mL of distilled water was added, and stirring and degassing were carried out twice to obtain a negative electrode mixture.

[0087] Thereafter, the obtained negative electrode mixture was applied to a copper foil with a doctor blade to a thickness of 150 μm, and vacuum dried at 80° C. for 12 hours to obtain a negative electrode for a lithium ion secondary battery of Example 3.

[0088] [Example 4] Mixing ratio The raw material mixture ratio was 3.5028 g of sample and 6.4 g of distilled water, and the sample was weighed to contain 28.3 mass% silicon, 51.2 mass% natural graphite, 10.0 mass% acetylene black, 7.5 mass% polyacrylic acid (PAA), and 7.5 mass% carboxymethyl cellulose (CMC).

[0089] Powder Synthesis Using a high-energy ball mill (Fritsch P5; SiN pot: 300 mL; SiN balls: diameter 10 mm), 2.1049 g of silicon, 4.5214 g of natural graphite, and 0.8848 g of acetylene black were added to the pot, and mechanochemical treatment was carried out at an orbital speed of 300 rpm, a rotation speed of 600 rpm, a milling time of 24 hours, and a ball weight:powder weight ratio of 40:1, to obtain a negative electrode active material for a lithium ion secondary battery of Example 4.

[0090] Negative electrode fabrication The stirring and defoaming conditions for producing the negative electrode were as follows: stirring was performed at a rotation speed of 800 rpm and a revolution speed of 2000 rpm for 60 seconds, and defoaming was performed at a rotation speed of 60 rpm and a revolution speed of 2200 rpm for 30 seconds.

[0091] First, 0.264 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, 2.9803 g of the negative electrode active material for a lithium ion secondary battery of Example 3 obtained above was added to the aqueous binder solution, mixed with a spatula, and 2 mL of distilled water was added in portions while checking the mixture, and the mixture was stirred and degassed twice.

[0092] Further, 0.2693 g of carboxymethyl cellulose (CMC) was added to the obtained aqueous solution, and after stirring and degassing, 1 mL of distilled water was added, and stirring and degassing were carried out twice to obtain a negative electrode mixture.

[0093] Thereafter, the obtained negative electrode mixture was applied to a copper foil with a doctor blade to a thickness of 175 μm, and vacuum dried at 80° C. for 12 hours to obtain a negative electrode for a lithium ion secondary battery of Example 4.

[0094] [Comparative Example 1] Using a high-energy ball mill (Fritsch P5; Si3N4 pot: 250 mL; Si3N4 balls: diameter 10 mm), 2.50 g of silicon was added to the pot, and mechanochemical treatment was carried out at an orbital speed of 300 rpm, a rotation speed of 600 rpm, a milling time of 24 hours, and a ball weight:powder weight ratio of 40:1, to obtain silicon of Comparative Example 1. Furthermore, a negative electrode for a lithium-ion secondary battery of Comparative Example 1 was produced in the same manner as in Examples 1 and 2.

[0095] [Test Example 1: X-ray diffraction measurement] X-ray diffraction measurements were carried out using CuKα radiation as the X-ray source in the range of 2θ=10 to 80°.

[0096] The X-ray diffraction spectrum of the negative electrode active material for lithium-ion secondary batteries obtained in Example 1 is shown in Figure 1. From Figure 1, broad peaks at 2θ = 36.0°, 60.0°, and 72.0° attributed to stacking disordered silicon carbide (SD-SiC) and a broad peak at 2θ = 22.5° attributed to amorphous carbon were observed. This indicates that the negative electrode active material obtained in Example 1 contains stacking disordered silicon carbide (SD-SiC) and amorphous carbon. For reference, the X-ray diffraction spectra of stacking disordered silicon carbide (SD-SiC) and highly crystalline cubic β-SiC, which have been previously reported (J. Am. Chem. Soc., 98, 50-56 (2015)), are shown in Figures 2 and 3, and the X-ray diffraction spectrum of amorphous carbon is shown in Figure 4. From these, it can be understood that the silicon carbide contained in the negative electrode active material for lithium ion secondary batteries obtained in Example 1 is not highly crystalline cubic β-SiC, but is stacked disordered silicon carbide (SD-SiC) as previously reported (J. Am. Chem. Soc., 98, 50-56 (2015)).

[0097] 5 and 6 also show the X-ray diffraction spectra of the negative electrode active materials for lithium ion secondary batteries obtained in Examples 3 and 4. From these results, although there are differences in peak intensities, broad peaks at 2θ=36.0°, 60.0°, and 72.0° attributed to stacking disordered silicon carbide (SD-SiC) and a broad peak at 2θ=22.5° attributed to amorphous carbon were obtained, and it can be seen that the negative electrode active materials obtained in Examples 3 and 4 contain stacking disordered silicon carbide (SD-SiC) and amorphous carbon.

[0098] In addition, the content of stacking disordered silicon carbide (SD-SiC) is estimated to be 47% by mass and the content of amorphous carbon is estimated to be 53% by mass in the negative electrode active material obtained in Example 1. Similarly, the content of stacking disordered silicon carbide (SD-SiC) is estimated to be 47% by mass and the content of amorphous carbon is estimated to be 53% by mass in the negative electrode active material obtained in Example 2, the content of stacking disordered silicon carbide (SD-SiC) is estimated to be 60% by mass and the content of amorphous carbon is estimated to be 40% by mass in the negative electrode active material obtained in Example 3, and the content of stacking disordered silicon carbide (SD-SiC) is estimated to be 40% by mass and the content of amorphous carbon is estimated to be 60% by mass in the negative electrode active material obtained in Example 4.

[0099] [Test Example 2: Electron Microscope Observation] To investigate the size of the stacking disordered SiC formed at the interface between amorphous carbon and silicon, an equimolar mixture of carbon and silicon was milled in a high-energy ball mill for 4 hours to obtain a negative electrode active material for lithium-ion secondary batteries. The results are shown in Figure 7.

[0100] As a result, it can be seen that silicon carbide with an average particle size of about 5 to 10 nm is generated at the interface between the black silicon and the white amorphous carbon. Taking this into consideration in addition to the results of Test Example 1, it can be seen that the silicon carbide generated 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)). From this, it can be understood that the reaction between silicon and carbon produces stacked disordered silicon carbide (SD-SiC) (average particle size: approximately 5 to 10 nm), with amorphous carbon material (average particle size: approximately 10 to 20 nm) surrounding it, and that the stacked disordered silicon carbide (SD-SiC) (average particle size: approximately 5 to 10 nm) is dispersed in the amorphous carbon material (average particle size: approximately 10 to 20 nm) without agglomeration, so as to form a conductive path between it and the amorphous carbon material.

[0101] [Manufacturing example: Manufacturing of lithium-ion secondary batteries (half cells)] As the negative electrode, the negative electrodes obtained in Examples 1 to 3 and Comparative Example 1 were used.

[0102] In addition, lithium metal was used as the positive electrode.

[0103] The electrolyte used was a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a ratio of 50 / 50 (v / v) as the solvent and 1 mol / L of lithium hexafluorophosphate (LiPF6) as the salt. This electrolyte was impregnated into a porous polypropylene film, which served as a separator.

[0104] A lithium ion secondary battery was fabricated using the above negative electrode, positive electrode, electrolyte, and separator.

[0105] [Test Example 3: Charge / Discharge Measurement] Charge and discharge measurements were performed using a two-electrode cell with a potentio / galvanostat analyzer ECstat-302. The cell was temperature-controlled in a thermostatic chamber at 20°C.

[0106] In the lithium ion secondary battery (half cell) manufactured using the negative electrode obtained in Examples 1 and 2, the charge and discharge currents were set to 420 mA / g (0.1 C) for the first to third cycles, 2100 mA / g (0.5 C) for the fourth to eighth cycles, and 420 mA / g (0.1 C) for the ninth to eleventh cycles. The results are shown in Figures 8 and 9. Figures 8 and 9 show the rate characteristics when the discharge capacity for each cycle was calculated relative to the weight of silicon. From these results, it can be seen that the use of the negative electrode active material for a lithium ion secondary battery of the present invention results in excellent rate characteristics.

[0107] In contrast, in the lithium ion secondary battery (half cell) manufactured using the negative electrode obtained in Comparative Example 1, when charged and discharged at 420 mA / g (0.1 C), the discharge capacity was maintained at only 56.6% at the third cycle and 35.4% at the fifth cycle. In Example 1, a discharge capacity of approximately 92.3% was maintained at the third cycle, and in Example 2, considering that the initial discharge capacity was unusually high, a discharge capacity of nearly 100% was maintained. Considering this, it can be seen that the use of the negative electrode active material for a lithium ion secondary battery of the present invention also results in excellent cycle characteristics.

[0108] In addition, for a lithium-ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 2, the charge / discharge rates were 420 mA / g (0.1 C) for the first to third cycles, 2100 mA / g (0.5 C) for the fourth to eighth cycles, and 420 mA / g (0.1 C) for the ninth to eleventh cycles. The initial charge / discharge capacity at each rate is shown in Figure 10. As a result, it can be seen that the discharge capacity at the first cycle (420 mAh / g; 0.1 C) was 1200 mAh / g, the discharge capacity at the fourth cycle (2100 mAh / g; 0.5 C) was 160 mAh / g, and the discharge capacity at the ninth cycle (420 mAh / g; 0.1 C) was 940 mAh / g. Figure 10 shows the results when the charge / discharge capacity was calculated relative to the weight of silicon.

[0109] Next, a lithium-ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 1 was charged and discharged at 420 mA / g (0.1 C) for the first to third cycles, 2100 mA / g (0.5 C) for the fourth to eighth cycles, and 420 mA / g (0.1 C) for the ninth to eleventh cycles, as described above. The results are shown in Figures 11 and 12. Figure 11 shows the results calculated relative to the weight of silicon, and Figure 12 shows the results calculated relative to the weight of silicon carbide. From these results, it can be seen that the use of the negative electrode active material for a lithium-ion secondary battery of the present invention provides a charge / discharge capacity approximately three times larger than the initial discharge capacity of graphite, even after charge / discharge tests at various current values ​​to evaluate the rate characteristics described above. The discharge capacity can be maintained during subsequent charge / discharge cycles, resulting in particularly excellent cycle characteristics (the discharge capacity at the third cycle is 106.7% of that at the first cycle). A similar measurement was also performed on a lithium ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 3. In charge and discharge tests at various current values ​​for evaluating the rate characteristics described above, the discharge capacity at the third cycle was 96.2% of that at the first cycle, demonstrating excellent charge and discharge characteristics. Furthermore, a similar measurement was also performed on a lithium ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 4. In charge and discharge tests at various current values ​​for evaluating the rate characteristics described above, the discharge capacity at the first cycle was 1370 mAh / g, demonstrating excellent charge and discharge characteristics similar to those of Example 1.

Claims

1. A negative electrode active material for a lithium ion secondary battery containing stacked disordered silicon carbide and an amorphous carbon material, a content of the stacked irregular structure silicon carbide being 30 to 60 mass % and a content of the amorphous carbon material being 40 to 70 mass % relative to a total amount of 100 mass %;

2. 2. The negative electrode active material for a lithium ion secondary battery according to claim 1, wherein the stacked disordered silicon carbide is dispersed in the amorphous carbon material so as to have a conductive path between the stacked disordered silicon carbide and the amorphous carbon material.

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

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

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

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

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

8. A method for producing a negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 5, A step of subjecting a raw material mixture containing a silicon-containing material and a carbon material to mechanochemical treatment. Equipped with the total amount of the raw material mixture is taken as 100% by mass, and the content of the silicon-containing material is 20 to 45% by mass, and the content of the carbon material is 50 to 80% by mass.

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