Negative electrode active material for lithium-ion secondary battery and method for manufacturing negative electrode active material for lithium-ion secondary battery

Abstract

This negative electrode active material for a lithium ion secondary battery contains a layered, irregularly structured silicon carbide and an amorphous carbon material. The layered, irregularly structured silicon carbide has a content of 21.6–60% by mass relative to a total mass of 100% by mass, and is dispersed in the amorphous carbon material so as to have a conductive path with the amorphous carbon material. The negative electrode active material for a lithium ion secondary battery is a negative electrode active material that can be used to manufacture a lithium ion secondary battery with excellent capacity and cycle characteristics. If silicon is included in the negative electrode active material, and if the layered, irregularly structured silicon carbide and the silicon are preferably dispersed in the amorphous carbon material so as to have a conductive path with the amorphous carbon material, the amorphous carbon material absorbs expansion and contraction of the silicon due to reaction with lithium, allowing the cycle characteristics to be particularly improved.

Description

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

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

[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 not yet been 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, when lithium ions inserted into and extracted from the negative electrode react with silicon to form a compound during charging, the volume of the silicon expands by about 3 to 4 times, and when the lithium ions dissociate from the compound during discharge, the volume of the silicon decreases to about 1 / 4 to 1 / 3, and the contact between the active material and the electrode is not maintained during the charge / discharge cycle, resulting in a significant decrease in cycle characteristics. For example, even after just five cycles, the capacity drops to about 35%. For this reason, SiO (Si + SiO), which has little volume change, is used as the negative electrode active material. ２ ) and cubic β-SiC have been studied as active materials. For example, the latter has been reported in various papers (see, for example, Non-Patent Documents 1 to 8), but all of them use highly crystalline cubic β-SiC, and because of poor reproducibility and unsuitability for mass production, a decisive active material has not yet been determined.

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

[001] 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).

[0004] RSC Advances, 2013, 3, 15028.Mater. Res. Soc. Symp. Proc. Vol. 1678, 2014 Materials Research Society.Solid State Ionics, 263 (2014) 23-26.Materials. Front. Chem. Vol. 6 (2018) 166.ACS Appl. Energy Mater. 2020, 3, 12613-12626.New J. Chem., 2021, 45, 19105-19117.Nanomaterials 2022, 12, 659.J. Mater. Chem. A, 2022, 10, 5230-5243.J. Am. Chem. Soc., 98, 50-56 (2015).

[0005] However, according to the research of the present inventors, even if an attempt is made to use stacked disordered silicon carbide prepared with a silicon:carbon ratio of 1:1 (molar ratio) as a negative electrode active material as described 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 having 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.

[0007] As a result of extensive research, the present inventors have discovered that in a negative electrode active material for a lithium ion secondary battery containing stacking disordered silicon carbide and an amorphous carbon material, silicon carbide formed at the Si-C interface migrates to an electrically conductive excess carbon matrix, which can reversibly absorb and release lithium ions as the battery is charged and discharged, thereby enabling the production of a lithium ion secondary battery with excellent capacity and cycle characteristics. Such a negative electrode active material for a lithium ion secondary battery can be easily obtained by producing stacking disordered silicon carbide under carbon-rich conditions. Furthermore, when the stacking disordered structure silicon carbide and silicon are dispersed in an amorphous carbon material so as to form a conductive path between the silicon carbide and the amorphous carbon material, while adjusting the amount of silicon to a constant level, the silicon carbide formed at the Si-C interface migrates to the conductive excess carbon matrix, which can reversibly absorb and release lithium ions as the lithium ions are charged and discharged. Furthermore, the presence of silicon in the amorphous carbon material prevents the negative electrode active material for lithium ion secondary batteries from being destroyed by the expansion and contraction of silicon, further improving capacity and cycle characteristics. Based on these findings, further research was conducted and the present invention was completed. Specifically, the present invention encompasses the following features:

[0008] Item 1. A negative electrode active material for a lithium ion secondary battery, comprising stacking irregular structure silicon carbide and an amorphous carbon material, wherein the content of the stacking irregular structure silicon carbide is 21.6 to 60 mass % relative to 100 mass % of the total amount, and the stacking irregular structure silicon carbide is dispersed in the amorphous carbon material so as to form a conductive path between the stacking irregular structure silicon carbide and the amorphous carbon material.

[0009] Item 2. The negative electrode active material for a lithium ion secondary battery according to Item 1, 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 %, based on a total amount of 100 mass %.

[0010] Item 3. The negative electrode active material for a lithium ion secondary battery according to Item 1, further containing silicon, wherein the content of the silicon is 5.0 to 28.0 mass % and the content of the stacked irregular structure silicon carbide is 21.6 to 57.0 mass % relative to 100 mass % of the total amount.

[0011] Item 4. The negative electrode active material for a lithium ion secondary battery according to item 3, wherein the content of the amorphous carbon material is 28.8 to 66.5 mass % relative to 100 mass % of the total amount.

[0012] Item 5. The negative electrode active material for a lithium ion secondary battery according to Item 3 or 4, wherein the stacked disordered structure silicon carbide and the silicon are 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.

[0013] Item 6. The negative electrode active material for a lithium ion secondary battery according to any one of Items 1 to 5, wherein the stacked irregular structure silicon carbide has an average particle size of 5 to 20 nm.

[0014] Item 7. The negative electrode active material for a lithium ion secondary battery according to any one of Items 1 to 6, wherein the average particle diameter of the non-crystalline carbon material is 5 to 25 nm.

[0015] Item 8. The negative electrode active material for a lithium ion secondary battery according to any one of Items 1 to 7, wherein the stacking disordered structure 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.

[0016] Item 9. 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 8.

[0017] Item 10. A lithium ion secondary battery comprising the negative electrode for a lithium ion secondary battery according to Item 9.

[0018] Item 11. A method for producing a negative electrode active material for a lithium ion secondary battery according to any one of Items 1 to 10, comprising a step of subjecting a raw material mixture containing a silicon-containing material and a carbon material to a mechanochemical treatment.

[0019] Item 12. The manufacturing method according to Item 11, 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 100 mass% of the total amount of the raw material mixture.

[0020] Item 13. The manufacturing method according to Item 11, comprising a step of subjecting the raw material mixture to a mechanochemical treatment until the silicon content is 5.0 to 28.0 mass% and the stacking disordered structure silicon carbide content is 21.6 to 57.0 mass%, with the total amount being 100 mass%.

[0021] 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.

[0022]

[0033] Figure 1 shows an X-ray diffraction pattern of the negative electrode active material obtained in Example 1. Figure 2 shows an X-ray diffraction spectrum of stacked disordered silicon carbide (SD-SiC). Figure 3 shows an X-ray diffraction pattern of highly crystalline cubic β-SiC. Figure 4 shows an X-ray diffraction spectrum of amorphous carbon. Figure 5 shows an X-ray diffraction pattern of the negative electrode active material obtained in Example 3. Figure 6 shows an X-ray diffraction pattern of the negative electrode active material obtained in Example 4. Figure 7 shows a high-resolution transmission electron microscope (TEM) image of the negative electrode active material obtained by reacting graphite and silicon with a pulverization time of 4 hours. Figure 8 shows a high-resolution transmission electron microscope (TEM) image of the negative electrode active material obtained in Example 4. Figure 9 shows a photograph of SD-SiC after synthesis, observed with a high-resolution TEM, published in J. Am. Chem. Soc., 98, 50-56 (2015). The results are shown for a lithium ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 1, where the charge and 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 discharge capacity for each cycle was calculated relative to the weight of silicon. The results are shown for a lithium ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 2, where the charge and 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 discharge capacity for each cycle was calculated relative to the weight of silicon. 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 results for each rate are shown. The charge and discharge capacities were calculated relative to the weight of silicon. 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, followed by charging and discharging at 50 mA / g (0.012 C). The charge and discharge capacities were calculated relative to the weight of silicon.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, followed by charging and discharging at 50 mA / g (0.012 C). The charge-discharge capacity was calculated relative to the weight of silicon carbide. A lithium-ion secondary battery (half cell) manufactured using the negative electrode obtained in Comparative Example 2 was charged at a constant current (CC) to 5.0 mV, followed by constant-current, low-voltage charging (CCCV charging) in which charging was continued at a constant voltage (CV) down to 0.1 C, and then discharged to 2.0 V. The charge-discharge capacity was calculated relative to the weight of silicon carbide. An X-ray diffraction pattern of the negative electrode active material obtained in Example 5 is shown. An X-ray diffraction pattern of the negative electrode active material obtained in Example 6 is shown. Test Example 6 shows the charge-discharge curves for a lithium-ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 5, where the charge-discharge current 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 eleventh cycles. The charge-discharge rates were calculated based on the total weight of the resulting stacking disordered SiC and the unreacted Si and an equimolar amount of C. The discharge capacity for each cycle was calculated based on the total weight of SiC and Si, the contents of which were calculated using the calculation method shown in (1-2) "Method for measuring the content." Test Example 6 shows the charge-discharge curves for a lithium-ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 6, where the charge-discharge current 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 eleventh cycles. The charge-discharge rates were calculated based on the total weight of the resulting stacking disordered structure SiC and the unreacted Si and an equimolar amount of C. The discharge capacity for each cycle was calculated based on the total weight of SiC and Si, the contents of which were calculated using the calculation method shown in (1-2) "Method for measuring the content."Test Example 6 shows the change in discharge capacity when a lithium ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 5 was 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 the second to eleventh cycles. The charge and discharge rates were calculated based on the total weight of the resulting stacking disordered structure SiC, unreacted Si, and an equimolar amount of C. The discharge capacity for each cycle was calculated based on the total weight of SiC and Si determined by the calculation method shown in (1-2) "Method for determining the content." Test Example 6 shows the change in discharge capacity when a lithium ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 6 was 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 the second to eleventh cycles. The charge and discharge rates were calculated based on the total weight of the resulting stacking disordered SiC and the unreacted Si and an equimolar amount of C. The discharge capacity for each cycle was calculated based on the total weight of SiC and Si determined by the calculation method shown in (1-2) "Method for determining the content." Test Example 7 shows the charge-discharge curves for a lithium-ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 5, where the charge-discharge current was 100 mA / g Si-C (Si / C = 1 / 1 in mol%) for the first to fifth cycles and 1000 mA / g Si-C (Si / C = 1 / 1 in mol%) for the sixth to tenth cycles. The charge-discharge rates were calculated based on the total weight of the resulting stacking disordered SiC and the unreacted Si and an equimolar amount of C. The discharge capacity for each cycle was calculated based on the total weight of SiC and Si determined by the calculation method shown in (1-2) "Method for determining the content." Test Example 7 shows the change in discharge capacity when a lithium ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 5 was charged and discharged at 100 mA / g Si—C (Si / C=1 / 1 in mol%) for the first to fifth cycles and at 1000 mA / g Si—C (Si / C=1 / 1 in mol%) for the sixth to tenth cycles.The charge / discharge rates were calculated relative to the total weight of the resulting stacking disordered SiC, unreacted Si, and equimolar C. The discharge capacity for each cycle was calculated relative to the total weight of SiC and Si calculated using the calculation method shown in (1-2) "Method for measuring the content." The charge / discharge curves for a lithium-ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 7 are shown for the case where the first cycle was 100 mA / g Si-C (Si / C = 1 / 1 in mol%) and the second to fifth cycles were 1000 mA / g Si-C (Si / C = 1 / 1 in mol%). The charge / discharge rates were calculated relative to the total weight of the resulting stacking disordered SiC, unreacted Si, and equimolar C. The discharge capacity for each cycle was calculated relative to the total weight of SiC and Si calculated using the calculation method shown in (1-2) "Method for measuring the content." This figure shows the change in discharge capacity when a lithium ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 7 was 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 the second to fifth cycles. The charge and discharge rates were calculated based on the total weight of the resulting stacking disordered structure SiC and the unreacted Si and an equimolar amount of C. The discharge capacity for each cycle was calculated based on the total weight of SiC and Si determined by the calculation method shown in (1-2) "Method for determining the content."

[0023] In this specification, the term "containing" is a concept that encompasses all of "comprise," "consist essentially of," and "consist only of."

[0024] In addition, in this specification, when a numerical range is expressed as A to B, it means A or more and B or less.

[0025] 1. Negative Electrode Active Material for Lithium-Ion Secondary Battery The negative electrode active material for a lithium-ion secondary battery of the present invention contains stacking irregular structure silicon carbide and an amorphous carbon material, the content of the stacking irregular structure silicon carbide being 21.6 to 60 mass % relative to the total amount being 100 mass %, and the stacking irregular structure silicon carbide is dispersed in the amorphous carbon material so as to form a conductive path between the stacking irregular structure silicon carbide and the amorphous carbon material.

[0026] The negative electrode active material for a lithium ion secondary battery of the present invention is: (First embodiment) A negative electrode active material for a lithium ion secondary battery containing stacking irregular structure silicon carbide and an amorphous carbon material, wherein, with a total amount taken as 100 mass %, the content of the stacking irregular structure silicon carbide is 30 to 60 mass % and the content of the amorphous carbon material is 40 to 70 mass %, and the stacking irregular structure silicon carbide is dispersed in the amorphous carbon material so as to have a conductive path between the stacking irregular structure silicon carbide and the amorphous carbon material. (Second embodiment) A negative electrode active material for a lithium ion secondary battery containing stacking irregular structure silicon carbide, an amorphous carbon material, and silicon, wherein, with a total amount taken as 100 mass%, the content of the silicon is 5.0 to 28.0 mass%, the content of the stacking irregular structure silicon carbide is 21.6 to 57.0 mass%, and the stacking irregular structure silicon carbide is dispersed in the amorphous carbon material so as to have a conductive path between the stacking irregular structure silicon carbide and the amorphous carbon material.

[0027] (1-1) First Embodiment The negative electrode active material for a lithium ion secondary battery according to the first embodiment 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, and the content of the stacked irregular structure silicon carbide is preferably 30 to 60 mass % and the content of the amorphous carbon material is preferably 40 to 70 mass % relative to the total amount of 100 mass %. In this case, the negative electrode active material for a lithium ion secondary battery according to the first embodiment of the present invention is intended to be substantially free of (particularly, completely free of) free silicon.

[0028] In stacked irregular silicon carbide, the closest packed layers of silicon are stacked irregularly, and carbon atoms are inserted into every other tetrahedral site (half of the tetrahedral sites), and the other half of the tetrahedral sites are vacant. In stacked irregular silicon carbide, all of the octahedral sites, the same number as the number of silicon, are vacant. When this stacked irregular silicon carbide active material is used as the negative electrode of a lithium ion secondary battery, Li ＋ Ions are inserted into all of these empty half tetrahedral sites and empty octahedral sites, receiving electrons from the counter electrode and occupying the vacant sites as lithium atoms. This is well known for CuSn and InSb, which have a zinc blende structure where close-packed layers are regularly stacked. For example, in the case of CuSn, the chemical formula is Li ２ The structure becomes CuSn, which is similar to a Heusler compound, and the expansion and contraction of the active material during charge and discharge is significantly suppressed (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, the chemical formula is Li ２ SiCx(SD)(Li ２Based on the concepts presented in the two papers mentioned above, the theoretical discharge capacity is calculated to be 1336 mAh / g, which is almost the same as the discharge capacity of the stacked disordered silicon carbide obtained through experiments. Furthermore, it is expected that expansion and contraction during charging and discharging will be suppressed, resulting in good cycle characteristics.

[0029] Stacked irregular silicon carbide The stacked irregular silicon carbide contained in the negative electrode active material for a lithium ion secondary battery according to the first embodiment 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.

[0030] The stacking disordered structure silicon carbide contained in the negative electrode active material for a lithium ion secondary battery according to the first embodiment of the present invention can have broad peaks at 2θ=36.0°, 60.0°, and 72.0°, which are attributed to silicon carbide, within an allowable range of ±0.5° in X-ray diffraction measurement using CuKα radiation.

[0031] 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 in the first embodiment 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° of 2.0° or more, more preferably 2.5 to 10.0°, within a tolerance of ±0.5° in X-ray diffraction measurement using CuKα rays.

[0032] 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.

[0033] In the negative electrode active material for a lithium ion secondary battery according to the first embodiment of the present invention, it is preferable that small silicon carbide particles are dispersed in an amorphous carbon material in order to facilitate the occlusion 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 using a high-resolution transmission electron microscope.

[0034] In the negative electrode active material for a lithium ion secondary battery according to the first embodiment 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% by mass, and more preferably 35 to 55% by mass, where the total amount of the negative electrode active material for a lithium ion secondary battery according to the present invention is taken as 100% by mass.

[0035] Amorphous Carbon Material In the negative electrode active material for a lithium ion secondary battery according to the first embodiment of the present invention, the inclusion of an amorphous carbon material improves the conductivity and enables the flow of electricity.

[0036] The amorphous carbon material contained in the negative electrode active material for a lithium ion secondary battery in the first embodiment of the present invention can have a broad peak at 2θ = 22.5° and, if necessary, 42.0°, which is due to the amorphous carbon material, within an allowable range of ±0.5° in X-ray diffraction measurement using CuKα rays.

[0037] In terms of charge / discharge capacity, cycle characteristics, and the like, the amorphous carbon material contained in the negative electrode active material for a lithium ion secondary battery in the first embodiment 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, and more preferably 3.0 to 10.0°, within a tolerance of ±0.5°, in X-ray diffraction measurement using CuKα radiation.

[0038] 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.

[0039] In the negative electrode active material for a lithium ion secondary battery according to the first embodiment 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 using a high-resolution transmission electron microscope.

[0040] In the negative electrode active material for a lithium ion secondary battery according to the first embodiment of the present invention, the content of the amorphous carbon material is not particularly limited, but from the viewpoints of easily improving electrical conductivity, charge / discharge capacity, cycle characteristics, and the like, the content is preferably 40 to 70 mass %, and more preferably 45 to 65 mass %, relative to the total amount of the negative electrode active material for a lithium ion secondary battery being 100 mass %.

[0041] Negative Electrode Active Material for Lithium-Ion Secondary Battery The negative electrode active material for a lithium-ion secondary battery in the first embodiment of the present invention contains stacked disordered structure silicon carbide and an amorphous carbon material, as described above.

[0042] In the negative electrode active material for a lithium ion secondary battery of the present invention, the stacking disordered structure silicon carbide formed at the Si-C interface migrates to the conductive excess carbon matrix phase, and this can reversibly absorb and release lithium ions as the lithium ions are charged and discharged. Therefore, the stacking disordered structure silicon carbide is dispersed in the amorphous carbon material, preferably without agglomeration, so as to have a conductive path between the silicon carbide and the amorphous carbon material.

[0043] In addition to the above-described stacked irregular silicon carbide and amorphous carbon material, the negative electrode active material for lithium ion secondary batteries in the first embodiment of the present invention may contain, as long as the effects of the present invention are not impaired, a third component such as a silicon-containing raw material (silicon nitride, silicon oxide, etc.), a crystalline carbon material (graphite, etc.), crystalline silicon carbide (cubic β-SiC), or 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 lithium ion secondary batteries in the first embodiment of the present invention. When these third components are contained, they are preferably dispersed in the amorphous carbon material without agglomeration so as to form a conductive path between the amorphous carbon material and the third component.

[0044] In the negative electrode active material for lithium ion secondary batteries according to the first embodiment 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 possessed by the negative electrode active material for lithium ion secondary batteries according to the first embodiment of the present invention is a material containing silicon, it does not expand or contract during charging and discharging to the same extent as silicon. Furthermore, in the negative electrode active material for lithium ion secondary batteries according to the first embodiment 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 according to the first embodiment of the present invention, a lithium ion secondary battery having 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.

[0045] (1-2) Second Embodiment A negative electrode active material for a lithium ion secondary battery according to a second embodiment of the present invention is a negative electrode active material for a lithium ion secondary battery containing stacking irregular structure silicon carbide, an amorphous carbon material, and silicon, wherein, taking the total amount as 100 mass%, the content of the silicon is 5.0 to 28.0 mass%, the content of the stacking irregular structure silicon carbide is 21.6 to 57.0 mass%, and it is preferable that the stacking irregular structure silicon carbide is dispersed in the amorphous carbon material so as to have a conductive path between the stacking irregular structure silicon carbide and the amorphous carbon material.

[0046] In stacked irregular silicon carbide, the closest packed layers of silicon are stacked irregularly, and carbon atoms are inserted into every other tetrahedral site (half of the tetrahedral sites), and the other half of the tetrahedral sites are vacant. In stacked irregular silicon carbide, all of the octahedral sites, the same number as the number of silicon, are vacant. When this stacked irregular silicon carbide active material is used as the negative electrode of a lithium ion secondary battery, Li ＋ Ions are inserted into all of these empty half tetrahedral sites and empty octahedral sites, receiving electrons from the counter electrode and occupying the vacant sites as lithium atoms. This is well known for CuSn and InSb, which have a zinc blende structure where close-packed layers are regularly stacked. For example, in the case of CuSn, the chemical formula is Li ２ The structure becomes CuSn, which is similar to a Heusler compound, and the expansion and contraction of the active material during charge and discharge is significantly suppressed (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, the chemical formula is Li ２ SiCx(SD)(Li ２Based on the concepts presented in the two papers mentioned above, the theoretical discharge capacity is calculated to be 1336 mAh / g, which is almost the same as the discharge capacity of the stacked disordered silicon carbide obtained through experiments. Furthermore, it is expected that expansion and contraction during charging and discharging will be suppressed, resulting in good cycle characteristics.

[0047] 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 stacked irregular structure silicon carbide and unreacted silicon act as active materials. Carbon and silicon react to each other, and a structure can be expected in which stacked irregular structure silicon carbide, which reacts at the interface between carbon and silicon, and unreacted silicon are dispersed in carbon aggregates that act as a conductive material, so as to ensure a good conductive path. Silicon is Li ＋ It reacts with ions to form Li ４．４ When Si is formed, the theoretical discharge capacity is large at 4198 mAh / g, but the silicon expands 3 to 4 times and contracts when it releases lithium ions, which can lead to the loss of conductive paths and the release of active material from the coating material, deteriorating the cycle characteristics of the lithium-ion secondary battery. However, according to the second embodiment of the present invention, when silicon and stacked irregular structure silicon carbide are produced in the amorphous carbon material, it is expected that the amorphous carbon material will absorb the expansion and contraction of silicon due to the reaction with lithium.

[0048] Stacked irregular structure silicon carbide The stacked irregular structure silicon carbide contained in the negative electrode active material for a lithium ion secondary battery according to the second embodiment 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.

[0049] The stacking disordered structure silicon carbide contained in the negative electrode active material for a lithium ion secondary battery according to the second embodiment of the present invention can have broad peaks at 2θ=36.0°, 60.0°, and 72.0°, which are attributed to silicon carbide, within an allowable range of ±0.5° in X-ray diffraction measurement using CuKα radiation.

[0050] 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 in the second embodiment 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° of 2.0° or more, more preferably 2.5 to 10.0°, within a tolerance of ±0.5° in X-ray diffraction measurement using CuKα radiation.

[0051] 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.

[0052] In the negative electrode active material for a lithium ion secondary battery according to the second embodiment of the present invention, it is preferable that small silicon carbide particles are dispersed in an amorphous carbon material in order to facilitate the occlusion and desorption of lithium ions, and therefore it is preferable that the average particle size of the stacking irregular structure silicon carbide is small. Therefore, the average particle size of the stacking irregular structure silicon carbide is preferably 5 to 20 nm, more preferably 5 to 10 nm. The average particle size of the stacking irregular structure silicon carbide is measured by observation using a high-resolution transmission electron microscope.

[0053] In the negative electrode active material for a lithium ion secondary battery according to the second embodiment 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 21.6 to 57.0 mass %, and more preferably 28.5 to 46.2 mass %, where the total amount of the negative electrode active material for a lithium ion secondary battery according to the second embodiment of the present invention is taken as 100 mass %.

[0054] Amorphous Carbon Material In the negative electrode active material for a lithium ion secondary battery according to the second embodiment of the present invention, the inclusion of an amorphous carbon material improves the conductivity and enables the flow of electricity.

[0055] The amorphous carbon material contained in the negative electrode active material for a lithium ion secondary battery according to the second embodiment of the present invention can have a broad peak at 2θ=22.5° and, if necessary, 42.0°, which is dependent on the amorphous carbon material, within an allowable range of ±0.5° in X-ray diffraction measurement using CuKα radiation.

[0056] In terms of charge / discharge capacity, cycle characteristics, and the like, the amorphous carbon material contained in the negative electrode active material for a lithium ion secondary battery according to the second embodiment 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, and more preferably 3.0 to 10.0°, within a tolerance of ±0.5°, in X-ray diffraction measurement using CuKα radiation.

[0057] 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.

[0058] In the negative electrode active material for a lithium-ion secondary battery according to the second embodiment 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 using a high-resolution transmission electron microscope.

[0059] In the negative electrode active material for a lithium ion secondary battery according to the second embodiment of the present invention, the content of the amorphous carbon material is not particularly limited, but from the viewpoints of easily improving conductivity, charge / discharge capacity, cycle characteristics, and the like, the content is preferably 28.8 to 66.5 mass %, and more preferably 38.0 to 53.9 mass %, relative to the total amount of the negative electrode active material for a lithium ion secondary battery taken as 100 mass %.

[0060] Silicon In the negative electrode active material for a lithium ion secondary battery according to the second embodiment of the present invention, the capacity can be improved by containing an amorphous carbon material. In addition, since silicon is dispersed in the amorphous carbon material so as to have a conductive path between the silicon and the amorphous carbon material, expansion and contraction of silicon can be suppressed. This makes it possible to suppress destruction of the negative electrode active material for a lithium ion secondary battery due to expansion and contraction of silicon, and also improves cycle characteristics.

[0061] The silicon contained in the negative electrode active material for a lithium ion secondary battery according to the second embodiment of the present invention can have peaks at 2θ=28.0°, 47.0°, and 56.0° due to silicon, within an allowable range of ±0.5°, in X-ray diffraction measurement using CuKα radiation.

[0062] The shape of the silicon is not particularly limited, and any shape such as powder, plate, granule, sphere, fiber, or lump can be used, but the silicon is usually spherical.

[0063] In the negative electrode active material for a lithium ion secondary battery according to the second embodiment of the present invention, small silicon particles are preferably dispersed in the amorphous carbon material, which facilitates improving capacity and cycle characteristics by suppressing the expansion and contraction of silicon. Therefore, the average silicon crystallite diameter is preferably small. Therefore, the average silicon crystallite diameter is preferably 5 to 100 nm, more preferably 6 to 50 nm. The average silicon crystallite diameter is measured by high-resolution transmission electron microscopy.

[0064] In the negative electrode active material for a lithium ion secondary battery according to the second embodiment of the present invention, the content of silicon is preferably 5.0 to 28.0% by mass, and more preferably 10.0 to 20.0% by mass, based on the total amount of the negative electrode active material for a lithium ion secondary battery of the present invention being 100% by mass, from the viewpoint that capacity can be easily improved and expansion and contraction of silicon can be easily suppressed by the amorphous carbon material, thereby easily improving cycle characteristics. Therefore, it is preferable that small silicon particles are dispersed in the amorphous carbon material, preferably without agglomeration, so as to have a conductive path between the small silicon particles and the amorphous carbon material.

[0065] In the second embodiment of the present invention, the content of each component is evaluated by examining an electrode fabricated by coating a current collector copper foil 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 carboxymethyl cellulose (CMC) as a binder.

[0066] In this way, when the conductive carbon and active material account for 85% by mass and the conductive material and active material are equal in amount, silicon is weighed and synthesized so that 100% reacts with carbon to produce stacked irregular silicon carbide. Typically, when the reaction is carried out in a high-energy ball mill for 24 hours, all of the mixed silicon is converted to stacked irregular silicon carbide. In this case, 85% by mass of the material coated on the current collecting copper foil is silicon and carbon, half of which is stacked irregular silicon carbide and the rest is carbon.

[0067] Here, carbon may also intercalate and deintercalate lithium ions. However, unlike graphite, which has a layered structure, the theoretical charge / discharge capacity of an amorphous carbon material cannot be calculated. Furthermore, the amount of lithium ions that carbon intercalates and deintercalates varies depending on the pulverization conditions of the mechanochemical treatment, etc., and is difficult to specify. Therefore, for the sake of convenience, calculations in this specification will be made on the assumption that carbon does not intercalate and deintercalate lithium ions, that is, that only silicon carbide and silicon intercalate and deintercalate lithium ions.

[0068] Considering an electrode containing m (g) of stacked disordered silicon carbide, the theoretical discharge capacity is 1336 mAh / g, so the discharge capacity can be calculated as follows: m×1336 mAh / g=Di (mAh), where Di is the initial discharge capacity.

[0069] When m = 0.0030 g, Di = 4.008 mAh. However, if the time required to synthesize stacking irregular silicon carbide (the grinding time in the mechanochemical treatment) is extended to 12 hours, the measured discharge capacity increases significantly to 6.564 mAh. This is thought to be due to the presence of unreacted silicon, which has a large theoretical discharge capacity (4198 mAh / g). If the weight fraction of the resulting stacking irregular silicon carbide is x and the total moles of silicon and carbon that did not form the stacking irregular silicon carbide are (1-x), the weight fraction and weight of each active material (stacking irregular silicon carbide and silicon), i.e., the content, can be calculated from the following formula: m × 1336 mAh / g × x + m × 4198 mAh / g × (1 - x) × 28.08 / 40.09 = 6.564 mAh When m = 0.0030 g, x = 0.4689, and (1 - x) = 0.5311 Therefore, the weight of the stacking disordered structure silicon carbide is 0.0030 × 0.4689 (g), and the weight of silicon is 0.0030 × 0.5311 × 28.08 / 40.09 (g).

[0070] Negative Electrode Active Material for Lithium Ion Secondary Battery The negative electrode active material for a lithium ion secondary battery according to the second embodiment of the present invention contains stacked disordered structure silicon carbide, an amorphous carbon material, and silicon, as described above.

[0071] In the negative electrode active material for a lithium ion secondary battery according to the second embodiment of the present invention, the stacking disordered structure 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, the stacking disordered structure silicon carbide is dispersed in the amorphous carbon material, preferably without agglomeration, so as to have a conductive path between the silicon carbide and the amorphous carbon material.

[0072] Furthermore, in the negative electrode active material for a lithium ion secondary battery according to the second embodiment of the present invention, silicon is covered with an excess carbon matrix phase, which absorbs the expansion and contraction of silicon and prevents destruction of the negative electrode active material for a lithium ion secondary battery according to the second embodiment of the present invention. Therefore, silicon is also preferably dispersed in the amorphous carbon material, preferably without agglomeration, so as to have a conductive path between the silicon and the amorphous carbon material.

[0073] In the negative electrode active material for lithium ion secondary batteries according to the second embodiment of the present invention, in addition to the above-described stacked irregular silicon carbide, amorphous carbon material, and silicon, third components such as raw silicon-containing materials (silicon nitride, silicon oxide, etc.), crystalline carbon materials (graphite, etc.), crystalline silicon carbide (cubic β-SiC), and metal components (iron, etc.) unavoidable during synthesis, or metal-nonmetal compounds, may be contained, as long as the effects of the present invention are not impaired. The content of these third components may be 0 to 10% by mass, particularly 0.01 to 5% by mass, with the total amount of the negative electrode active material for lithium ion secondary batteries according to the present invention being 100% by mass.

[0074] In the negative electrode active material for lithium ion secondary batteries according to the second embodiment of the present invention, the stacking irregular 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 silicon carbide contained in the negative electrode active material for lithium ion secondary batteries according to the second embodiment of the present invention is a material containing silicon, it does not expand or contract during charging and discharging to the same extent as silicon. Furthermore, in the negative electrode active material for lithium ion secondary batteries according to the second embodiment of the present invention, the stacking irregular silicon carbide and silicon are dispersed in the amorphous carbon material, preferably without agglomeration, so as to form a conductive path between the stacking irregular silicon carbide and the amorphous carbon material. This allows the amorphous carbon material adjacent to the stacking irregular silicon carbide and silicon to follow the expansion and contraction of the stacking irregular silicon carbide and a small amount of silicon. Therefore, by using the negative electrode active material for lithium ion secondary batteries according to the second embodiment of the present invention, a lithium ion secondary battery 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.

[0075] 2. Negative electrode for lithium ion secondary battery The negative electrode for lithium ion secondary battery of the present invention contains the negative electrode active material for lithium ion secondary battery of the present invention. More specifically, the negative electrode for lithium ion secondary battery of the present invention can include a negative electrode active material layer containing the negative electrode active material for lithium ion secondary battery of the present invention.

[0076] The negative electrode active material layer can be composed solely of the negative electrode active material for a lithium ion secondary battery 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 also be included. In particular, when the content of the stacked irregular silicon carbide in the negative electrode active material for a lithium ion secondary battery of the present invention is high, it is particularly effective to use a conductive agent. These conductive agents can be used alone or in combination of two or more.

[0077] In addition, 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 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.

[0078] The content of the negative electrode for a 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 viewpoints of charge / discharge capacity, cycle characteristics, etc. Furthermore, the content of the binder, thickener, or dispersant can be 5 to 30% by mass, particularly 10 to 20% by mass, based on 100% by mass of the total amount of the negative electrode active material layer, from the viewpoints of charge / discharge capacity, cycle characteristics, etc.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 3. Lithium-ion secondary battery 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.

[0085] The positive electrode may be any electrode capable of supplying lithium ions to the negative electrode, and any known positive electrode may be used.

[0086] 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.

[0087] The positive electrode active material used to form the positive electrode is usually a material capable of absorbing and releasing lithium ions. For example, α-NaFeO ２ Examples of suitable lithium transition metal composite oxides include those having a α-type crystal structure, those having a spinel-type crystal structure, polyanion compounds, chalcogen compounds, and sulfur. ２ As the lithium transition metal composite oxide having a crystalline structure, for example, Li[Li ｘ１ Ni γ１ Mn β１ Co(1-x1-γ1-β1)]O ２ (0≦x1<0.5, 0≦γ1≦1, 0≦β1≦1, 0≦γ1+β1≦1), Li[Li ｘ２ Ni γ２ Co β２ Al(1-x2-γ2-β2)]O ２ (0≦x2<0.5, 0≦γ2≦1, 0≦β2≦1, 0≦γ2+β2≦1). Examples of lithium transition metal oxides having a spinel crystal structure include Li ｘ３ Mn ２ O ４ (0.9≦x3<1.5), Li ｘ４ Ni γ４ Mn （２－γ４） O ４ (0.9≦x4<1.5, 0≦γ4≦2) and the like. Examples of polyanion compounds include LiFePO ４ , LiMnPO ４ , LiNiPO ４ , LiCoPO ４ , Li ３ V ２ (P.O. ４ ) ３ , Li ２ MnSiO ４ , Li ２ CoPO ４Examples of the positive electrode active material include titanium disulfide, molybdenum disulfide, and molybdenum dioxide. The 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.

[0088] 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.

[0089] 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 a nonwoven fabric or the like to prevent short-circuiting between the positive electrode and the negative electrode.

[0090] 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.

[0091] 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.

[0092] 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 includes, but is not particularly limited to, a step of subjecting a raw material mixture containing a silicon-containing material and a carbon material to a mechanochemical treatment.

[0093] For example, when producing the negative electrode active material for a lithium ion secondary battery according to the first embodiment of the present invention, the total amount of the raw material mixture is 100 mass %, and the content of the silicon-containing material can be 20 to 45 mass %, and the content of the carbon material can be 50 to 80 mass %.

[0094] Furthermore, when producing the negative electrode active material for a lithium ion secondary battery according to the second embodiment of the present invention, a raw material mixture containing a silicon-containing material and a carbon material can be subjected to mechanochemical treatment until the content of the silicon reaches 5.0 to 28.0 mass % and the content of the stacking disordered structure silicon carbide reaches 21.6 to 57.0 mass %, with the total amount taken as 100 mass %.

[0095] The silicon-containing material used as the raw material is not particularly limited, and in addition to silicon, silicon-containing compounds and the like can be used. These silicon-containing materials can be used alone or in combination of two or more. Furthermore, since the silicon-containing materials are 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 materials can usually be used.

[0096] The carbon material used as a 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.

[0097] 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.

[0098] 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.

[0099] Regarding the mixing ratio of raw materials, when producing a negative electrode active material for a lithium ion secondary battery according to the first embodiment of the present invention, that is, when producing a negative electrode active material for a lithium ion secondary battery that is substantially free of free silicon (particularly, completely free of free silicon), the charging ratio of the raw materials almost directly reflects the ratio of each element in the product, and can be the same ratio as the elemental ratio of silicon and carbon in the desired negative electrode active material for a lithium ion secondary battery according to the first embodiment of the present invention. Specifically, the content of the silicon-containing material is preferably 20 to 45% by mass, more preferably 25 to 40% by mass, of the total amount of the raw material mixture taken as 100% by mass. Furthermore, the content of the carbon material is preferably 50 to 80% by mass, more preferably 55 to 75% by mass, of the total amount of the raw material mixture taken as 100% by mass.

[0100] On the other hand, when producing a negative electrode active material for a lithium ion secondary battery according to the second embodiment of the present invention, that is, when producing a negative electrode active material for a lithium ion secondary battery intentionally containing free silicon (silicon), the mixing ratio of the raw materials is, as described below, in the production method of the present invention, the grinding time in the mechanochemical treatment is shortened to produce stacked irregular structure silicon carbide, and the raw material silicon is intentionally left remaining and dispersed in the amorphous carbon material so as to form a conductive path between the amorphous carbon material and the silicon. However, even in this case, the charging ratio of the raw materials almost directly reflects the ratio of each element in the product, and therefore the same ratio of silicon and carbon as the desired negative electrode active material for a lithium ion secondary battery according to the second embodiment of the present invention can be achieved. Specifically, the content of the silicon-containing material is preferably 20 to 60% by mass, more preferably 25 to 50% by mass, of the total amount of the raw material mixture taken as 100% by mass. Furthermore, the content of the carbon material is preferably 40 to 80% by mass, more preferably 55 to 75% by mass, of the total amount of the raw material mixture taken as 100% by mass.

[0101] 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 irregular structure silicon carbide of the present invention.

[0102] The energy input for the mechanochemical treatment is calculated using the formula published in the following literature: Burgio, N., Lasonna, A., Magini, M., Martelii, S. and Padella, F., Il Nuovo Cimento, Vol. 13, pp. 459-476 (1991).

[0103] 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.

[0104] 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 facilitating the production of the stacking irregular structure silicon carbide of the present invention, it is preferably 500 to 800 rpm, and more preferably 600 to 700 rpm. Furthermore, the revolution speed is not particularly limited, but from the viewpoint of facilitating the production of the stacking irregular structure silicon carbide of the present invention, it is preferably 250 to 400 rpm, and more preferably 300 to 350 rpm.

[0105] 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.

[0106] The duration of the mechanochemical treatment is not particularly limited when producing a negative electrode active material for a lithium ion secondary battery according to the first embodiment of the present invention, i.e., when producing a negative electrode active material for a lithium ion secondary battery that is substantially free of (particularly, completely free of) free silicon. The mechanochemical treatment can be performed for any length of time until the desired negative electrode active material for a lithium ion secondary battery according to the first embodiment of the present invention is obtained. For example, the mechanochemical treatment can be performed for 15 to 48 hours (particularly, 20 to 36 hours). In a previous report ( J. Am. Chem. Soc., 98, 50-56 (2015)), a 24-hour reaction was performed at a molar ratio of 1:1 to completely react Si and C to synthesize irregularly stacked silicon carbide. Therefore, when performing the reaction under the same conditions, the milling time (treatment time) of the mechanochemical treatment is preferably approximately the same, i.e., 18 to 48 hours, particularly 20 to 36 hours.

[0107] In the case of producing a negative electrode active material for a lithium ion secondary battery according to the second embodiment of the present invention, that is, in the case of producing a negative electrode active material for a lithium ion secondary battery intentionally containing free silicon (silicon), the silicon-containing material and the carbon material are intentionally reacted to produce stacked irregular structure silicon carbide, and the raw material silicon is intentionally left to remain and dispersed in the amorphous carbon material so as to form a conductive path between them and the amorphous carbon material. Therefore, the grinding time (treatment time) of the mechanochemical treatment can be intentionally short, for example, the mechanochemical treatment can be carried out for a treatment time of 3 to 20 hours (particularly 5 to 18 hours). In a previous report (J. Am. Chem. Soc., 98, 50-56 (2015)), a 24-hour reaction was required to synthesize stacked disordered silicon carbide at a molar ratio of 1:1, where Si and C react completely. Therefore, when carrying out the reaction under the same conditions, a shorter grinding time (treatment time) for the mechanochemical treatment, 6 to 18 hours, and especially 8 to 15 hours, is preferred. Furthermore, this mechanochemical treatment can also be carried out in multiple steps, with breaks in between, if necessary.

[0108] Since the reaction time tends to be shorter as the rotation speed increases, it is preferable to adjust the rotation speed appropriately. Furthermore, this mechanochemical treatment can be carried out in multiple steps with breaks in between, if necessary.

[0109] When the mechanochemical treatment is repeated multiple times, the above conditions can be applied to each mechanochemical treatment step.

[0110] 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.

[0111]

[0033] Note that even when producing the negative electrode active material for a lithium ion secondary battery according to the second embodiment of the present invention, that is, when attempting to produce a negative electrode active material for a lithium ion secondary battery intentionally containing free silicon (silicon), unlike the production method of the present invention described above, when a material in which stacked irregular structure silicon carbide obtained by a long-term mechanochemical treatment is dispersed in an amorphous carbon material so as to have a conductive path between the amorphous carbon material and the material is mixed with silicon, the silicon cannot be dispersed in the amorphous carbon material so as to have a conductive path between the amorphous carbon material and the material. As a result, the expansion and contraction of the silicon cannot be suppressed, destruction of the negative electrode active material for a lithium ion secondary battery due to the expansion and contraction of the silicon cannot be suppressed, and the cycle characteristics cannot be improved.

[0112] The present invention will be specifically described based on examples, but the present invention is not limited to these examples.

[0113] 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).

[0114] (1) Examples 1 to 4 and Comparative Examples 1 and 2: First Embodiment [Example 1] Mixing ratio The mixing ratio of the raw materials was 3.5136 g of sample and 5.4 g of distilled water, and the sample was weighed out to have 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).

[0115] Powder synthesis: High-energy ball mill (Fritsch P5; Si ３ N ４ Pot: 250 mL; Si ３ N ４Using a stainless steel ball (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 performed under the conditions of a revolution speed of 300 rpm, a rotation speed of 600 rpm, a grinding 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.

[0116] Negative Electrode Fabrication The stirring and defoaming conditions for fabricating the negative electrode were: stirring at a rotation speed of 800 rpm and a revolution speed of 2000 rpm for 60 seconds; and defoaming at a rotation speed of 60 rpm and a revolution speed of 2200 rpm for 30 seconds.

[0117] 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, and the mixture was mixed with a spatula. While checking the mixture, 2 mL of distilled water was added in portions, and the mixture was stirred and degassed twice.

[0118] 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.

[0119] 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.

[0120] [Example 2] Mixing ratio The mixing ratio of the raw materials was 3.5076 g of sample and 6.6 g of distilled water, and the sample was weighed to have 28.0 mass % silicon, 57.0 mass % carbon black, 7.5 mass % polyacrylic acid (PAA), and 7.5 mass % carboxymethyl cellulose (CMC).

[0121] Powder synthesis: High-energy ball mill (Fritsch P5; Si ３ N ４ Pot: 250 mL; Si ３ N ４Using a stainless steel ball (diameter: 10 mm), 2.50 g of silicon and 5.0137 g of carbon black were added to the pot, and mechanochemical treatment was performed under the conditions of a revolution speed of 300 rpm, a rotation speed of 600 rpm, a grinding 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.

[0122] Negative Electrode Fabrication The stirring and defoaming conditions for fabricating the negative electrode were: stirring at a rotation speed of 800 rpm and a revolution speed of 2000 rpm for 60 seconds; and defoaming at a rotation speed of 60 rpm and a revolution speed of 2200 rpm for 30 seconds.

[0123] 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.

[0124] 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.

[0125] 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.

[0126] [Example 3] Mixing ratio The mixing ratio of the raw materials was 3.5066 g of sample and 6.6 g of distilled water, and the sample was weighed out to have a composition of 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).

[0127] Powder synthesis: High-energy ball mill (Fritsch P5; Si ３ N ４ Pot: 250 mL; Si ３ N ４A pot was used, and 3.1564 g of silicon, 3.4675 g of natural graphite, and 0.8840 g of acetylene black were added thereto using a stainless steel ball (diameter: 10 mm). Mechanochemical treatment was then carried out at a revolution speed of 300 rpm, a rotation speed of 600 rpm, a pulverization 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.

[0128] Negative Electrode Fabrication The stirring and defoaming conditions for fabricating the negative electrode were: stirring at a rotation speed of 800 rpm and a revolution speed of 2000 rpm for 60 seconds; and defoaming at a rotation speed of 60 rpm and a revolution speed of 2200 rpm for 30 seconds.

[0129] 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.

[0130] 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.

[0131] 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.

[0132] [Example 4] Mixing ratio The mixing ratio of the raw materials was 3.5028 g of sample and 6.4 g of distilled water, and the sample was weighed out to have a composition of 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).

[0133] Powder synthesis: High-energy ball mill (Fritsch P5; Si ３ N ４ Pot: 300 mL; Si ３ N ４A pot was used, and 2.1049 g of silicon, 4.5214 g of natural graphite, and 0.8848 g of acetylene black were added thereto using aluminum balls (diameter: 10 mm). Mechanochemical treatment was then performed at a revolution speed of 300 rpm, a rotation speed of 600 rpm, a pulverization 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.

[0134] Negative Electrode Fabrication The stirring and defoaming conditions for fabricating the negative electrode were: stirring at a rotation speed of 800 rpm and a revolution speed of 2000 rpm for 60 seconds; and defoaming at a rotation speed of 60 rpm and a revolution speed of 2200 rpm for 30 seconds.

[0135] 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.

[0136] 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.

[0137] 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.

[0138] Comparative Example 1 High-energy ball mill (Fritsch P5; Si ３ N ４ Pot: 250 mL; Si ３ N ４ Using a stainless steel ball (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 crushing 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.

[0139] [Comparative Example 2] Stacked irregular structure silicon carbide obtained by the method described in J. Am. Chem. Soc., 98, 50-56 (2015) was used to make 70% by mass of the total coating amount, and 15% by mass of acetylene black, 7.5% by mass of polyacrylic acid (PAA), and 7.5% by mass of carboxymethyl cellulose (CMC) were added to make 100% by mass. The slurry was prepared using a stirring and degassing machine, and a Cu foil current collector was coated with this slurry to produce a negative electrode. In other words, carbon and SD-SiC were simply mixed.

[0140] Test Example 1: X-ray diffraction measurement (part 1) Using CuKα rays as an X-ray source, X-ray diffraction measurement was carried out in the range of 2θ=10 to 80°.

[0141] 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, as 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 a lithium ion secondary battery 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)).

[0142] 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 were 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.

[0143] In addition, in the negative electrode active material obtained in Example 1, the content of stacking irregular structure silicon carbide (SD-SiC) is estimated to be 47% by mass, and the content of amorphous carbon is estimated to be 53% by mass. Similarly, in the negative electrode active material obtained in Example 2, the content of stacking irregular structure 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 3, the content of stacking irregular structure 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 4, the content of stacking irregular structure silicon carbide (SD-SiC) is estimated to be 40% by mass, and the content of amorphous carbon is estimated to be 60% by mass.

[0144] Test Example 2: Electron Microscope Observation (Part 1) In order to investigate the size of the stacking disordered structure 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 a lithium ion secondary battery. The results are shown in FIG. 7.

[0145] 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 stacked irregular structure silicon carbide (SD-SiC) (average particle size: approximately 5 to 10 nm) is produced by the reaction between silicon and carbon, and that amorphous carbon material (average particle size: approximately 10 to 20 nm) is present around it, and that the stacked irregular structure 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.

[0146] Furthermore, taking the example of Example 4 of the present application, where the stacking irregular structure silicon carbide and the amorphous carbon material are estimated to be 40 mass % and 60 mass %, respectively, the high-resolution TEM image is as shown in Figure 8 below. In the TEM image of Figure 8, the scale bar in (a) is 20 nm, (b) is an enlarged view of the illustrated portion in (a) with a scale bar of 10 nm, and (c) is an enlarged view of the illustrated portion in (b) with a scale bar of 10 nm. From this, it can be seen that the stacking irregular structure silicon carbide is covered with the amorphous carbon material, and the stacking irregular structure silicon carbide has a structure in which it is dispersed in the amorphous carbon material so as to have a conductive path between it and the amorphous carbon material.

[0147] On the other hand, as described in the introduction of Journal of Nanoparticle Research (2007) 9:797-806, "Dispersion of nano-silicon carbide (SiC) powder in aqueous suspensions," DOI 10.1007 / s11051-006-9121-6, dispersing nano-sized particles into primary particles in aqueous and non-aqueous solutions is known to be thermodynamically unstable due to the high surface energy of finely dispersed systems. This paper also describes the difficulty of dispersing SiC nanoparticles without special treatment. Therefore, when both SiC and carbon are nanoparticles, it is not easy to achieve a homogeneous dispersion. Using conventional methods, it is difficult to disperse stacked disordered silicon carbide so that it has a conductive path between itself and the amorphous carbon material. Figure 9 shows a high-resolution TEM photograph of synthesized SD-SiC, published in J. Am. Chem. Soc., 98, 50-56 (2015). Because particles of about 30 to 100 nm are agglomerated and tightly bonded, it is extremely difficult to break them down into primary particles, as shown in J. Am. Chem. Soc., 98, 50-56 (2015).

[0148] It can be seen from FIG. 9 that when stacking irregular structure silicon carbide is produced and then mixed with a carbonaceous material, the stacking irregular structure silicon carbide aggregates and does not disperse, making it difficult to disperse the stacking irregular structure silicon carbide in the carbonaceous material.

[0149] In contrast, according to the present invention, as described above, it is possible to adopt a structure in which 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.

[0150] [Production Example 1: Production of Lithium Ion Secondary Battery (Half Cell) (Part 1)] The negative electrodes obtained in Examples 1 to 3 and Comparative Examples 1 and 2 were used as the negative electrodes.

[0151] In addition, lithium metal was used as the positive electrode.

[0152] The electrolyte used was a solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) at a ratio of EC / DEC = 50 / 50 (v / v), and a salt of 1 mol / L lithium hexafluorophosphate (LiPF ６ The electrolyte solution was impregnated into a porous polypropylene film, which was a separator.

[0153] A lithium ion secondary battery was fabricated using the above negative electrode, positive electrode, electrolyte, and separator.

[0154] Test Example 3: Charge / Discharge Measurement (Part 1) Charge / 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.

[0155] In a lithium ion secondary battery (half cell) manufactured using the negative electrode obtained in Examples 1 and 2, the results of charging and discharging were shown in Figures 10 and 11, where the first to third cycles were 420 mA / g (0.1 C), the fourth to eighth cycles were 2100 mA / g (0.5 C), and the ninth to eleventh cycles were 420 mA / g (0.1 C). Figures 10 and 11 show the results of rate characteristics when the discharge capacity in 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 lithium ion secondary batteries according to the first embodiment of the present invention results in excellent rate characteristics.

[0156] 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 only maintained at 56.6% at the third cycle and 35.4% at the fifth cycle. Considering that a discharge capacity of approximately 92.3% was maintained at the third cycle in Example 1 and that the initial discharge capacity in Example 2 was unusually high, a discharge capacity of nearly 100% was maintained, it can be seen that the use of the negative electrode active material for a lithium ion secondary battery according to the first embodiment of the present invention also results in excellent cycle characteristics.

[0157] In addition, for a lithium-ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 2, the first to third cycles were charged and discharged at 420 mA / g (0.1C), the fourth to eighth cycles were charged and discharged at 2100 mA / g (0.5C), and the ninth to eleventh cycles were charged and discharged at 420 mA / g (0.1C). The results of the initial charge and discharge capacity at each rate are shown in Figure 12. As a result, the discharge capacity at the first cycle (420 mAh / g; 0.1C) was 1200 mAh / g, the discharge capacity at the fourth cycle (2100 mAh / g; 0.5C) was 160 mAh / g, and the discharge capacity at the ninth cycle (420 mAh / g; 0.1C) was 940 mAh / g. Figure 12 shows the results when the charge and discharge capacity was calculated relative to the weight of silicon.

[0158] 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 13 and 14 . Figure 13 shows the results calculated relative to the weight of silicon, and Figure 14 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 that is 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, and the cycle characteristics are particularly excellent (the discharge capacity at the third cycle is 106.7% of that at the first cycle). When a lithium ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 3 was similarly measured, the discharge capacity at the third cycle after charge / discharge tests at various current values ​​for evaluating the rate characteristics described above was 96.2% of that at the first cycle, demonstrating excellent charge / discharge characteristics. Furthermore, when a lithium ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 4 was similarly measured, the discharge capacity at the first cycle after charge / discharge tests at various current values ​​for evaluating the rate characteristics described above was 1370 mAh / g, demonstrating excellent charge / discharge characteristics similar to those of Example 1.

[0159] Next, a lithium ion secondary battery (half cell) manufactured using the negative electrode obtained in Comparative Example 2 was charged at a constant current (CC) to 5.0 mV, and then subjected to constant current low voltage charging (CCCV charging) in which charging was continued at a constant voltage (CV) to 0.1 C, and then discharged to 2.0 V. The results are shown in FIG.

[0160] As shown in FIG. 15 , even if stacking irregular structure silicon carbide is used, if stacking irregular structure silicon carbide is first produced and then mixed with a carbonaceous material, the stacking irregular structure silicon carbide will aggregate and not disperse, making it difficult to disperse the stacking irregular structure silicon carbide in the carbonaceous material. As a result, the initial charge capacity, including the formation of an SEI (solid electrolyte interface), was only 89 mAh / g-SiC, and the initial discharge capacity was only 48 mAh / g-SiC.

[0161] (2) Examples 5 to 7 and Comparative Examples 3 to 4: Second Embodiment [Example 5: 12 hours] Mixing ratio The mixing ratio of the raw materials was 3.5028 g of sample and 5.4 g of distilled water, and the sample was weighed to be 29.8 mass % silicon, 45.2 mass % natural graphite, 10.0 mass % acetylene black, 7.5 mass % polyacrylic acid (PAA), and 7.5 mass % carboxymethyl cellulose (CMC).

[0162] Powder synthesis: High-energy ball mill (Fritsch P5; Si ３ N ４ Pot: 250 mL; Si ３ N ４ A pot was used, and 2.6267 g of silicon, 3.9907 g of natural graphite, and 0.8830 g of acetylene black were added thereto using aluminum balls (diameter: 10 mm). Mechanochemical treatment was then performed under the conditions of a revolution speed of 300 rpm, a rotation speed of 600 rpm, a grinding time of 12 hours, and a ball weight:powder weight ratio of 40:1, to obtain a negative electrode active material for a lithium ion secondary battery of Example 5.

[0163] Negative Electrode Fabrication The stirring and defoaming conditions for fabricating the negative electrode were: stirring at a rotation speed of 800 rpm and a revolution speed of 2000 rpm for 60 seconds; and defoaming at a rotation speed of 60 rpm and a revolution speed of 2200 rpm for 30 seconds.

[0164] 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 negative electrode active material for a lithium ion secondary battery of Example 5 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.

[0165] Further, 0.2633 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.

[0166] 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 5.

[0167] [Example 6: 10 hours] Mixing ratio The mixing ratio of the raw materials was 3.5009 g of sample and 5.4 g of distilled water, and the sample was weighed out to be 29.8 mass % silicon, 45.2 mass % natural graphite, 10.0 mass % acetylene black, 7.5 mass % polyacrylic acid (PAA), and 7.5 mass % carboxymethyl cellulose (CMC).

[0168] Powder synthesis: High-energy ball mill (Fritsch P5; Si ３ N ４ Pot: 250 mL; Si ３ N ４ Using a stainless steel ball (diameter: 10 mm), 2.6261 g of silicon, 3.9911 g of natural graphite, and 0.8835 g of acetylene black were added to the pot, and mechanochemical treatment was performed under the conditions of a revolution speed of 300 rpm, a rotation speed of 600 rpm, a grinding time of 10 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 6.

[0169] Negative Electrode Fabrication The stirring and defoaming conditions for fabricating the negative electrode were: stirring at a rotation speed of 800 rpm and a revolution speed of 2000 rpm for 60 seconds; and defoaming at a rotation speed of 60 rpm and a revolution speed of 2200 rpm for 30 seconds.

[0170] 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 negative electrode active material for a lithium ion secondary battery of Example 6 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.

[0171] Further, 0.2631 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.

[0172] 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 6.

[0173] Test Example 4: X-ray diffraction measurement (part 2) Using CuKα rays as an X-ray source, X-ray diffraction measurement was carried out in the range of 2θ=20 to 80°.

[0174] 16 to 17 show the X-ray diffraction spectra of the negative electrode active materials for lithium ion secondary batteries obtained in Examples 5 and 6. From Figures 16 to 17, broad peaks at 2θ = 36.0°, 60.0°, and 72.0° attributed to stacking disordered silicon carbide (SD-SiC), a broad peak at 2θ = 22.5° attributed to amorphous carbon, and peaks at 2θ = 28.0°, 47.0°, and 56.0° attributed to silicon were observed, and it can be seen that the negative electrode active materials obtained in Examples 5 and 6 contain stacking disordered silicon carbide (SD-SiC), amorphous carbon, and silicon. For reference, the X-ray diffraction spectra of stacking disordered silicon carbide (SD-SiC) and highly crystalline cubic β-SiC, which are described in a previous report (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 seen that the silicon carbide contained in the negative electrode active materials for lithium ion secondary batteries obtained in Examples 5 and 6 is not highly crystalline cubic β-SiC, but is stacking disordered silicon carbide (SD-SiC) as described in a previous report (J. Am. Chem. Soc., 98, 50-56 (2015)).

[0175] Test Example 5: Electron Microscope Observation (Part 2) In order 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 a lithium ion secondary battery. The results are shown in FIG. 7.

[0176] 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 4, 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 irregular silicon carbide (SD-SiC) (average particle size: about 5-10 nm), while silicon remains, with amorphous carbon material (average particle size: about 10-20 nm) surrounding it, and that the stacked irregular silicon carbide (SD-SiC) (average particle size: about 5-10 nm) and silicon are dispersed in the amorphous carbon material (average particle size: about 10-20 nm) without agglomeration so as to form a conductive path between them. Note that the silicon becomes smaller as the reaction time increases, and when the reaction time was about 10-12 hours as in Examples 5-6, the average crystallite size was about 8 nm.

[0177] [Production Example 2: Production of Lithium Ion Secondary Battery (Half Cell) (Part 2)] The negative electrodes obtained in Examples 5 and 6 were used as the negative electrodes.

[0178] In addition, lithium metal was used as the positive electrode.

[0179] The electrolyte used was a solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) at a ratio of EC / DEC = 50 / 50 (v / v), and a salt of 1 mol / L lithium hexafluorophosphate (LiPF ６ The electrolyte solution was impregnated into a porous polypropylene film, which was a separator.

[0180] A lithium ion secondary battery was fabricated using the above negative electrode, positive electrode, electrolyte, and separator.

[0181] Test Example 6: Charge / Discharge Measurement (Part 2) Charge / 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.

[0182] For lithium-ion secondary batteries (half cells) manufactured using the negative electrodes obtained in Examples 5 and 6, the charge-discharge curves and discharge capacity profiles are shown in Figures 18 to 21. The first cycle was charged at 100 mA / g Si-C (Si / C = 1 / 1 in mol%), and the second to eleventh cycles were charged at 1000 mA / g Si-C (Si / C = 1 / 1 in mol%). The subsequent charge-discharge rates were calculated based on the total weight of the resulting stacking disordered SiC and the equimolar amount of C relative to the unreacted Si. In Figures 18 to 21, the discharge capacity for each cycle was calculated based on the total weight of SiC and Si determined by the calculation method shown in "Method for Determining the Content" in (1-2). From these results, it can be seen that the use of the negative electrode active material for lithium-ion secondary batteries of the present invention results in excellent capacity, cycle characteristics, and rate characteristics.

[0183] Based on these results, the content ratios of carbon, silicon carbide, and silicon in the negative electrode active materials for lithium ion secondary batteries of Examples 5 and 6 were evaluated by the method described in the "Method for measuring content" in (1-2) above.

[0184] Here, carbon may also intercalate and deintercalate lithium ions. However, unlike graphite, which has a layered structure, the theoretical charge / discharge capacity of an amorphous carbon material cannot be calculated. Furthermore, the amount of lithium ions that carbon intercalates and deintercalates varies depending on the pulverization conditions of the mechanochemical treatment, etc., and is difficult to specify. Therefore, for the sake of convenience, calculations in this specification will be made on the assumption that carbon does not intercalate and deintercalate lithium ions, that is, that only silicon carbide and silicon intercalate and deintercalate lithium ions.

[0185] Example 5 (Derivation of SiC:(Si+C)) (0.003455x×1336)+(0.003455(1−x)×28.08 / 40.09×4198)=6.667 Since x=0.6299, SiC:(Si+C)=0.6299:0.3701 (weight ratio).

[0186] Here, x is the weight fraction of SiC, and 1-x means the weight fraction of Si+C. 6.667 mAh is the measured value, and 0.003455 g is the combined weight of SiC, unreacted Si, and an equimolar amount of C. The negative electrode layer (coated on the current collector) is 85% by mass of C+SiC, half of which (50% by mass) is C and half of which is SiC. Therefore, the raw materials are mixed so that when raw material C and raw material Si completely react, C and Si-C (equimolar) will each be 50% by mass.

[0187] (Calculation of specific capacity at first cycle) 6.667 / (0.003455x+0.003455(1-x)×28.08 / 40.09)=D D=2170 mAh / g.

[0188] (Calculation of C:SiC:Si) The weight of SiC is 0.003455g x 0.6299 (weight fraction of SiC). The weight of Si (unreacted Si) is 0.003455g x 0.3701 (weight fraction of Si + C) x 28.08 / 40.09. The weight of C (unreacted C that did not become SiC + 50 mass% excess C) is 0.003455g x 0.3701 (weight fraction of Si + C) x 12.01 / 40.09. In addition, since the raw materials (Si powder and C powder) were originally mixed to achieve 50 mass% C and 50 mass% SiC, 0.003455g of excess C was added.

[0189] From these results, the weight of SiC:weight of Si:weight of C is calculated by dividing the whole by 0.003455, as follows: 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.

[0190] From the above, C:SiC:Si=55.5 mass %:31.5 mass %:13.0 mass %.

[0191] Example 6 (Derivation of SiC:(Si+C)) (0.004426x×1336)+(0.004426(1−x)×28.08 / 40.09×4198)=10.33 Since x=0.3784, SiC:(Si+C)=0.3784:0.6216 (weight ratio).

[0192] Here, x is the weight fraction of SiC, and 1-x means the weight fraction of Si+C. 10.33 mAh is the measured value, and 0.004426 g is the combined weight of SiC, unreacted Si, and the equimolar weight of C. The negative electrode layer (coated on the current collector) is 85 mass% C+SiC, half of which (50 mass%) is C and SiC, respectively. Therefore, the raw materials are mixed so that when raw material C and raw material Si completely react, C and Si-C (equimolar) will each be 50 mass%.

[0193] (Calculation of specific capacity at first cycle) 10.33 / (0.004426x+0.004426(1-x)×28.08 / 40.09)=D D=2867 mAh / g.

[0194] (Calculation of C:SiC:Si) The weight of SiC is 0.004426g x 0.3784 (weight fraction of SiC). The weight of Si (unreacted Si) is 0.004426g x 0.6216 (weight fraction of Si + C) x 28.08 / 40.09. The weight of C (unreacted C that did not become SiC + 50 mass% excess C) is 0.004426g x 0.6216 (weight fraction of Si + C) x 12.01 / 40.09. In addition, since the raw materials (Si powder and C powder) were originally mixed to achieve 50 mass% C and 50 mass% SiC, 0.004426g of excess C was added.

[0195] From these results, the weight of SiC: weight of Si: weight of C is calculated by dividing the whole by 0.004426, as follows: 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.

[0196] From the above, C:SiC:Si=59.0 mass %:18.8 mass %:22.2 mass %.

[0197] Test Example 7: Charge / Discharge Measurement (Part 3) Charge / 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.

[0198] Figures 22 and 23 show the charge-discharge curves and discharge capacity curves for a lithium-ion secondary battery (half cell) manufactured using the negative electrode obtained in Example 5. The charge-discharge current was 100 mA / g Si-C (Si / C = 1 / 1 in mol%) for the first to fifth cycles and 1000 mA / g Si-C (Si / C = 1 / 1 in mol%) for the sixth to tenth cycles. As described above, the charge-discharge rate was calculated relative to the total weight of the resulting stacking disordered SiC and the unreacted Si and an equimolar amount of C. In Figures 22 and 23, the discharge capacity for each cycle was calculated relative to the total weight of SiC and Si, calculated using the calculation method shown in (1-2) "Method for Determining the Content." 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 capacity, cycle characteristics, and rate characteristics.

[0199] Based on these results, the content ratios of carbon, silicon carbide, and silicon in the negative electrode active material for a lithium ion secondary battery of Example 5 were evaluated by the method described in the "Method for measuring content" in (1-2) above.

[0200] Carbon may also intercalate and deintercalate lithium ions. However, unlike graphite, which has a layered structure, the theoretical charge / discharge capacity of an amorphous carbon material cannot be calculated. Furthermore, the amount of lithium ions that carbon intercalates and deintercalates varies depending on the pulverization conditions of the mechanochemical treatment, etc., and is difficult to specify. Therefore, for the sake of convenience, calculations in this specification will be made on the assumption that carbon does not intercalate and deintercalate lithium ions, that is, that only silicon carbide and silicon intercalate and deintercalate lithium ions.

[0201] (Derivation of SiC:(Si+C)) (0.003455x×1336)+(0.003455(1−x)×28.08 / 40.09×4198)=6.564 Since x=0.6485, SiC:(Si+C)=0.6485:0.3515 (weight ratio).

[0202] Here, x is the weight fraction of SiC, and 1-x means the weight fraction of Si+C. 6.667 mAh is the measured value, and 0.003455 g is the combined weight of SiC, unreacted Si, and an equimolar amount of C. The negative electrode layer (coated on the current collector) is 85% by mass of C+SiC, half of which (50% by mass) is C and half of which is SiC. Therefore, the raw materials are mixed so that when raw material C and raw material Si completely react, C and Si-C (equimolar) will each be 50% by mass.

[0203] (Calculation of specific capacity at first cycle) 6.564 / (0.003455x+0.003455(1-x)×28.08 / 40.09)=D D=2123 mAh / g.

[0204] (Calculation of C:SiC:Si) The weight of SiC is 0.003455g x 0.6485 (weight fraction of SiC). The weight of Si (unreacted Si) is 0.003455g x 0.3515 (weight fraction of Si + C) x 28.08 / 40.09. The weight of C (unreacted C that did not become SiC + 50 mass% excess C) is 0.003455g x 0.3515 (weight fraction of Si + C) x 12.01 / 40.09. In addition, since the raw materials (Si powder and C powder) were originally mixed to achieve 50 mass% C and 50 mass% SiC, 0.003455g of excess C was added.

[0205] From these results, the weight of SiC:weight of Si:weight of C is calculated by dividing the whole by 0.003455, as follows: 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.

[0206] From the above, C:SiC:Si=55.3 mass %:32.4 mass %:12.3 mass %.

[0207] From these results, the content ratio calculated was equivalent to the content ratio calculated from the results of Test Example 6, confirming the reliability of the method for calculating the content ratio.

[0208] [Comparative Example 3] A mixture of 60% by mass of carbon and 40% by mass of stacked disordered silicon carbide (SD-SiC) was used as an active material to make 80% by mass of the total electrode coating material, and 5% by mass of pulverized nano-Si with an average crystallite size of approximately 8 nm was added. 7.5% by mass of polyacrylic acid (PAA) and 7.5% by mass of carboxymethyl cellulose (CMC) were added to make a total of 100% by mass. The slurry was prepared using a stirring and degassing machine and coated on a Cu foil current collector to produce a 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 so as to have a conductive path between them. In this charge-discharge measurement, at a charge-discharge rate of 100 mA / g, the first discharge capacity after charging was below the theoretical charge-discharge capacity of SD-SiC of 1,336 mAh / g, and the second discharge capacity was even lower, falling to around 990 to 1,080 mAh / g, indicating that no significant effect of adding Si was observed.

[0209] [Comparative Example 4] As in Examples 5 and 6, a mixture of 50% by weight of carbon and 50% by weight of silicon carbide active material was used to prepare a total coating amount of 85% by weight. This mixture was then mixed with 7.5% by weight of polyacrylic acid (PAA) and 7.5% by weight of carboxymethyl cellulose (CMC) and adjusted using a stirrer / deaerator. A slurry was prepared by coating a Cu foil current collector with this slurry to prepare a negative electrode. This powder was synthesized under the powder synthesis conditions of Examples 5 and 6, but the mechanochemical treatment time under the milling conditions was reduced to 5 hours. X-ray diffraction results showed that at 5 hours, most of the diffraction peaks were Si. Since the diffraction peaks of stacked disordered silicon carbide (SD-SiC) were hidden by the diffraction peaks of Si, it can be seen that almost no SD-SiC was produced. Using this electrode, charge and discharge were performed at 100 mA / g Si-C (Si / C = 1 / 1 in mol%) initially, as in the charge and discharge experiment of Test Example 6. Subsequently, the second to tenth charge and discharge cycles were performed at a rate of 1000 mA / g. As a result, the discharge capacity was not maintained and steadily decreased.

[0210] [Example 7] As in Examples 5 and 6, a mixture of 50% by mass of carbon and 50% by mass of silicon carbide active material was used to prepare a total coating amount of 85% by mass. This mixture was then mixed with 7.5% by mass of polyacrylic acid (PAA) and 7.5% by mass of carboxymethyl cellulose (CMC) to prepare a slurry using a stirrer / deaerator. A negative electrode was then prepared by coating a Cu foil current collector with this slurry. The powder used was synthesized under the powder synthesis conditions of Examples 1 and 2, except that the mechanochemical treatment time under the pulverization conditions was reduced to 8 hours. As a result, as in Examples 5 and 6, it was confirmed that SD-SiC and silicon were dispersed in the amorphous carbon material so as to form a conductive path between the amorphous carbon material and the SD-SiC and silicon. Using this electrode, as in the charge-discharge experiment of Test Example 6, the initial charge-discharge was performed at 100 mA / g ~ Si-C (Si / C = 1 / 1 in mol%), followed by the second to fifth charge-discharge cycles at 1000 mA / g ~ Si-C (Si / C = 1 / 1 in mol%). As a result, as shown in Figure 24, the initial discharge capacity after the initial charge at 100 mA / g ~ Si-C (Si / C = 1 / 1 in mol%) was 3422 mAh / g. At a charge-discharge rate of 1000 mA / g from the second cycle onwards, the discharge capacity decreased slightly from 2760 mAh / g (second cycle) to 2544 mAh / g (fifth cycle), but still maintained a high discharge capacity. This is shown in Figure 25.

[0211] At this time, the weight fraction of the stacking disordered structure type SiC calculated from the initial discharge capacity of 100 mA / g using the calculation method of "Method for measuring content" in (1-2) was 0.2065, and the total 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 was the same as before):

[0212] (Derivation of SiC:(Si+C)) (0.004636x×1336)+(0.004636(1−x)×28.08 / 40.09×4198)=12.10 Since x=0.2065, SiC:(Si+C)=0.2065:0.7935 (weight ratio).

[0213] Here, x is the weight fraction of SiC, and 1-x means the weight fraction of Si+C. 12.10 mAh is the measured value, and 0.004636 g is the combined weight of SiC, unreacted Si, and an equimolar amount of C. The negative electrode layer (coated on the current collector) is 85% by mass of C+SiC, half of which (50% by mass) is C and SiC, respectively. Therefore, the raw materials are mixed so that when raw material C and raw material Si completely react, C and Si-C (equimolar) will each be 50% by mass.

[0214] (Calculation of specific capacity at first cycle) 12.10 / (0.004636x+0.004636(1-x)×28.08 / 40.09)=D D=3422 mAh / g.

[0215] (Calculation of C:SiC:Si) The weight of SiC is 0.004636g x 0.2065 (weight fraction of SiC). The weight of Si (unreacted Si) is 0.004636g x 0.7935 (weight fraction of Si + C) x 28.08 / 40.09. The weight of C (unreacted C that did not become SiC + 50 mass% excess C) is 0.004636g x 0.7935 (weight fraction of Si + C) x 12.01 / 40.09. In addition, since the raw materials (Si powder and C powder) were originally mixed to achieve 50 mass% C and 50 mass% SiC, 0.004636g of excess C was added.

[0216] From these results, the weight of SiC: weight of Si: weight of C is calculated by dividing the whole by 0.004636, as follows: 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.

[0217] From the above, C:SiC:Si=61.9 mass %:10.3 mass %:27.8 mass %.

Claims

1. A negative electrode active material for a lithium ion secondary battery, comprising stacking irregular structure silicon carbide and an amorphous carbon material, wherein the content of the stacking irregular structure silicon carbide is 21.6 to 60 mass % of the total amount taken as 100 mass %, and the stacking irregular structure silicon carbide is dispersed in the amorphous carbon material so as to form a conductive path between the stacking irregular structure silicon carbide and the amorphous carbon material.

2. The negative electrode active material for a lithium ion secondary battery according to claim 1, 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 %.

3. The negative electrode active material for a lithium ion secondary battery according to claim 1, further comprising silicon, wherein the silicon content is 5.0 to 28.0 mass% and the stacked irregular structure silicon carbide content is 21.6 to 57.0 mass% based on a total amount of 100 mass%.

4. The negative electrode active material for a lithium ion secondary battery according to claim 3, wherein the content of the amorphous carbon material is 28.8 to 66.5 mass % relative to the total mass of 100 mass %.

5. The negative electrode active material for a lithium ion secondary battery according to claim 3 or 4, wherein the stacked disordered structure silicon carbide and the silicon are dispersed in the amorphous carbon material so as to have a conductive path between them and the amorphous carbon material.

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

7. The negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 6, wherein the average particle size of the non-crystalline carbon material is 5 to 25 nm.

8. The negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 7, 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 of ±0.5° in X-ray diffraction measurement using CuKα radiation.

9. 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 8.

10. A lithium ion secondary battery containing the negative electrode for lithium ion secondary batteries according to claim 9.

11. A method for producing a negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 10, comprising a step of subjecting a raw material mixture containing a silicon-containing material and a carbon material to mechanochemical treatment.

12. The manufacturing method according to claim 11, 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 %.

13. The manufacturing method according to claim 11, further comprising a step of subjecting the raw material mixture to mechanochemical treatment until the silicon content is 5.0 to 28.0 mass % and the stacked irregular structure silicon carbide content is 21.6 to 57.0 mass %, with the total amount being 100 mass %.

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