JP2017088437A - Method for producing graphite-covered silicon composite body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production method on an industrial scale capable of obtaining a graphite-covered silicon composite body reduced in the variation of battery properties, having high capacity and excellent in cycle properties, also capable of mass production and suppressed in cost.SOLUTION: Provided is a method for producing a graphite-covered silicon composite body comprising: a mixing step (I-1) where (A) particles selected from silicon particles and silicon oxide particles represented by general formula SiOx and (B) a polymer material are mixed to produce a mixture; and a firing step (II-1) where the obtained mixture is fired in an inert atmosphere or in a vacuum atmosphere to produce a fired matter, and further comprising: a mixing step where the fired mater obtained in the (I-2) and (B) a polymer material are mixed to produce a mixture of the fired matter and the polymer material; and a fire step (II-2) where a mixture of the obtained fired matter and the polymer material is fired in an inert atmosphere or in a vacuum atmosphere to produce a fired matter, or the (I-2) and (II-2) steps are repeated for a plurality of times.SELECTED DRAWING: None

Description

  The present invention provides a graphite coating suitable as a negative electrode material for a nonaqueous electrolyte secondary battery having high capacity and excellent cycle characteristics when used as a negative electrode active material for a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery. The present invention relates to a method for producing a silicon composite.

In recent years, with the remarkable development of portable electronic devices, communication devices, etc., there is a strong demand for non-aqueous electrolyte secondary batteries with high energy density from the viewpoints of economy and downsizing and weight reduction of devices. Conventionally, as a measure for increasing the capacity of this type of secondary battery, for example, a method of using an oxide such as V, Si, B, Zr, Sn, or a composite oxide thereof as a negative electrode material (Patent Document 1: Japanese Patent Laid-Open No. Hei 5) -174818, Patent Document 2: JP-A-6-60867, etc., a method of applying a molten and quenched metal oxide as a negative electrode material (Patent Document 3: JP-A-10-294112), oxidation to a negative electrode material A method using silicon (Patent Document 4: Japanese Patent No. 2999741), a method using Si ₂ N ₂ O and Ge ₂ N ₂ O as a negative electrode material (Patent Document 5: Japanese Patent Laid-Open No. 11-102705), and a negative electrode material A method of using a composite particle in which silicon and silicon oxide are dispersed in a carbonaceous material and a material having a carbonaceous material coating layer that covers the entire surface of the particle (Patent Document 6: JP-A-2006-92969), etc. It is known. Further, for the purpose of imparting conductivity to the negative electrode material, a method of carbonizing SiO with graphite and then carbonizing (Patent Document 7: Japanese Patent Laid-Open No. 2000-243396), a carbon layer is formed on the surface of silicon particles by chemical vapor deposition. (Patent Document 8: Japanese Patent Laid-Open No. 2000-215887), a method of coating the surface of silicon oxide particles with a carbon layer by chemical vapor deposition (Patent Document 9: Japanese Patent Laid-Open No. 2002-42806), and the like. .

JP-A-5-174818 JP-A-6-60867 JP 10-294112 A Japanese Patent No. 2999741 JP-A-11-102705 JP 2006-92969 A JP 2000-243396 A JP 2000-215887 A JP 2002-42806 A

  In the above conventional method, although the charge / discharge capacity is increased and the energy density is increased, the required characteristics of the market are insufficient, the conductivity imparting process is complicated, the productivity is inferior, and the cost is increased. There is a problem, which is not always satisfactory, and further improvement in energy density has been desired. In particular, in Japanese Patent No. 2999741, silicon oxide is used as a negative electrode material for a lithium ion secondary battery to obtain a high-capacity electrode. However, since silicon oxide has very low electronic conductivity, it simply coexists with a conductive assistant. However, it is not sufficient to do so, and techniques for increasing the electron conductivity are being studied. Regarding the technology for imparting conductivity to a conventional negative electrode material, in Japanese Patent Application Laid-Open No. 2000-243396, since it is a solid-solid fusion, a uniform carbon film is not formed and the conductivity is insufficient. There is a problem, and in the method of Japanese Patent Application Laid-Open No. 2000-215887, although a uniform carbon film can be formed, since Si is used as a negative electrode material, expansion / contraction at the time of adsorption / desorption of lithium ions occurs. In order to prevent this because it is too large and cannot endure practically as a result, and the cycle performance is reduced, it is necessary to provide a limit on the amount of charge, and in the method of JP-A-2002-42806, Although improvement in cycle performance is confirmed while maintaining a high capacity, the process is complicated, so that it is not suitable for mass production, and there is a problem that costs increase.

  The present invention has been made in view of the above circumstances, and provides a graphite-coated silicon composite suitable for a negative electrode material for a non-aqueous electrolyte secondary battery that has little variation in battery characteristics, high capacity, and excellent cycle characteristics, and is mass-produced. An object of the present invention is to provide an industrial-scale manufacturing method that can be manufactured at low cost.

  As a result of intensive studies to achieve the above object, the present inventors have repeatedly expected that the charge / discharge capacity is several times that of the graphite-based one, which is currently the mainstream, and is repeated. Attention was focused on silicon-based materials, which have a major bottleneck in performance degradation due to charging and discharging. And the remarkable battery characteristic improvement was seen by coat | covering the surface of a silicon-type substance with a graphite film. However, the conventional graphite coating treatment is complicated and difficult to produce on an industrial scale. A detailed study was conducted on a graphite coating treatment method capable of withstanding industrial scale production. As a result, it was confirmed that mass production was possible by using a polymer material as a graphite source. However, the simple mixing and firing as in the prior art has a problem that it is difficult to form a uniform film and the battery characteristics tend to vary. Therefore, it has been found that the polymer film can be uniformly coated by mixing and baking the polymer material a plurality of times, and as a result, the battery characteristics are stabilized, and the present invention has been completed.

Accordingly, the present invention provides the following method for producing a graphite-coated silicon composite.
[1]. (I-1) (A) silicon particles, silicon oxide particles represented by the general formula SiOx (0.5 ≦ x <1.5), particles having a fine structure in which silicon fine particles are dispersed in a silicon-based compound, And a mixing step of mixing particles selected from these mixtures and (B) a polymer material to produce a mixture,
(II-1) The obtained mixture is fired in an inert atmosphere or in a vacuum atmosphere, and includes a firing step for producing a fired product. Further, (I-2) the fired product obtained below and (B) A mixing step of mixing the polymer material to produce a mixture of the fired product and the polymer material; and (II-2) the obtained mixture of the fired product and the polymer material in an inert atmosphere or a vacuum atmosphere. A method for producing a graphite-coated silicon composite comprising a firing step of firing in a fired product to produce a fired product, or repeating the steps (I-2) and (II-2) a plurality of times.
[2]. (A) The method for producing a graphite-coated silicon composite according to [1], wherein the particles have an average particle diameter of 0.1 to 30 μm and a BET specific surface area of 0.1 to 30 m ² / g.
[3]. (B) The method for producing a graphite-coated silicon composite according to [1] or [2], wherein the polymer material is a polymer selected from an aromatic group-containing thermoplastic polymer and a polyolefin-based thermoplastic polymer.
[4]. (I) A mixing process mixes (B) the solution which melt | dissolved the polymeric material in the organic solvent, and (A) particle | grains or a baked material, It is any one of [1]-[3] characterized by the above-mentioned. A method for producing a graphite-coated silicon composite.
[5]. (II) The method for producing a graphite-coated silicon composite according to any one of [1] to [3], wherein a firing temperature in the firing step is 600 to 1,200 ° C.
[6]. The average particle diameter of the graphite-coated silicon composite is 0.1 to 30 μm, the BET specific surface area is 0.1 to 30 m ² / g, and the graphite coverage is 0.5 to 40% by mass. A method for producing a graphite-coated silicon composite according to any one of the above.
[7]. The method for producing a graphite-coated silicon composite according to any one of [1] to [6], wherein the graphite-coated silicon composite is used for a non-aqueous electrolyte secondary battery negative electrode material.

  According to the manufacturing method of the present invention, as a non-aqueous electrolyte secondary battery negative electrode material having high capacity and excellent cycle characteristics by being used as a negative electrode active material of a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery. A suitable graphite-coated silicon composite is obtained. Since this manufacturing method is simple, mass production is possible and cost reduction can be achieved.

Hereinafter, each process is demonstrated in detail about the manufacturing method of this invention.
In the present invention, step (I) refers to step (I-1) and step (I-2), and step (II) refers to step (II-1) and step (II-2). .

(I-1) silicon particles, silicon oxide particles represented by the general formula SiOx (0.5 ≦ x <1.5), particles having a fine structure in which silicon fine particles are dispersed in a silicon-based compound, and these Mixing step of mixing particles selected from a mixture and (B) a polymer material to produce a mixture

  In the present invention, silicon oxide is a general term for amorphous silicon oxide obtained by cooling and precipitating silicon monoxide gas generated by heating a mixture of silicon dioxide and metal silicon. In the invention, the general formula SiOx (0.5 ≦ x <1.5) is used. x is preferably 1.0 ≦ x <1.3, and more preferably 1.0 ≦ x ≦ 1.2.

  Particles with a fine structure in which silicon fine particles are dispersed in silicon-based compounds are disproportionated by heat treatment using silicon oxide particles represented by the general formula SiOx (0.5 ≦ x <1.5) as a starting material. It can be obtained by reacting. The size of the silicon fine particles is preferably 1 to 500 nm. As for the size of the fine particles, a diffraction peak attributed to Si (111) centered around 2θ = 28.4 ° was observed in the X-ray diffraction (Cu-Kα) using copper as the counter-cathode. This is the particle diameter of the silicon crystal determined by the Scherrer equation based on the broadening of the diffraction lines. Moreover, about a silicon type compound, an inactive thing is preferable and a silicon dioxide is preferable at the point of the ease of manufacture.

(A) 0.1-30 micrometers is preferable and the average particle diameter of particle | grains has more preferable 0.3-25 micrometers. Further, BET specific surface area is preferably 0.1~30m ² / g, more preferably 0.1~25m ² / g, more preferably 0.2~20m ² / g. (A) If the average particle diameter and BET specific surface area of the particles are outside the above ranges, a graphite-coated silicon composite having a desired average particle diameter and BET specific surface area may not be obtained. The average particle diameter is the weight average particle diameter in the particle size distribution measurement by the laser beam diffraction method, and the BET specific surface area is a value measured by the BET one-point method evaluated by the N ₂ gas adsorption amount.

(B) Polymer material The graphite-coated silicon composite is obtained by coating the surface of the particle (A) with a graphite film using the polymer material (B) as a carbon source. Examples of the polymer material include polystyrene, xylene resin, biphenyl resin, naphthylene resin, anthracene resin, poly-1-vinylnaphthalene, poly-3-vinylpyrene, polyalkylfluorene-based thermoplastic polymer, polyethylene, polypropylene, Polyolefin thermoplastic polymers such as polybutene, polypentene, polyhexane, polyoctene, polynonene, polydecene, etc., thermoplastic resins such as condensed polynuclear aromatic resins, furan resins, phenol resins, polytriazine resins, polypyridazine resins, polypyridine resins, polypiperidines Nitrogen-containing resins such as resins, polytriazole resins, polypyrazole resins, polypyrrolidene resins, etc. are used, and these may be used alone or in combination of two or more. It can be. Among these, aromatic group-containing thermoplastic polymers and polyolefin-based thermoplastic polymers are preferable from the viewpoint of the graphite coating effect. Among these, polystyrene and polyethylene are preferably used because of cost and availability.

  (B) The polymer material can also be mixed as a powder, and a solution previously dissolved in an organic solvent such as toluene, acetone, hexane, xylene, ethanol, etc. can be mixed with (A) particles, but more uniform mixing Therefore, it is preferable to mix with a solution. The mixing method is not particularly limited, and examples thereof include a ball mill, a stirring mixer, a mortar, and the like, but a ball mill is simple and more preferably used.

(II-1) Firing step in which the obtained mixture is fired in an inert atmosphere or vacuum atmosphere to produce a fired product. As the inert gas, argon or nitrogen is generally used, and the gas is vented or sealed. However, in order to prevent an increase in pressure and to discharge a by-product gas out of the system, it is preferable to perform the firing while the gas is flowing in / in. In addition, when the flow rate of the inert gas is excessive, the hydrocarbon-based gas is discharged out of the system and there is a possibility that the graphite coating processing speed is lowered, so that it is preferably the minimum necessary.

  The firing temperature is preferably 600 to 1,200 ° C, and more preferably 700 to 1,100 ° C. If the firing temperature is lower than 600 ° C., it takes a long time for the graphite coating treatment, which may reduce the productivity. Conversely, when the treatment temperature exceeds 1,200 ° C., when the silicon oxide represented by the general formula SiOx (0.5 ≦ x <1.5) is coated with graphite, the disproportionation reaction proceeds too much, When this graphite-coated silicon composite is used as a negative electrode material for a non-aqueous electrolyte secondary battery, cycle characteristics are deteriorated. Although baking time is suitably selected, 0.2 to 15 hours are preferable and 0.5 to 10 hours are more preferable.

  The baking apparatus may be a reactor having a heating mechanism in an inert gas atmosphere or a vacuum atmosphere, and is not particularly limited, and can be processed by a continuous method or a batch method, specifically, a fluidized bed reactor. A rotary furnace, a ring furnace, a vertical moving bed reaction furnace, a tunnel furnace, a batch furnace, a rotary kiln and the like can be appropriately selected according to the purpose.

(I-2) Mixing step of mixing the obtained fired product and (B) the polymer material to produce a mixture of the fired product and the polymer material In the present invention, it is obtained in the step (I) above. The fired product is further mixed with (B) a polymer material to produce a mixture of the fired product and the polymer material. About the suitable component of (B) component, a mixing method, etc., it is the same as that of the description of the said (I-1) process, (B) The polymeric material can also be mixed as a powder and the solution previously melt | dissolved in the organic solvent (A) can be mixed with the particles, and the mixing method is not particularly limited, and examples thereof include a ball mill, a stirring mixer, and a mortar, but a ball mill is simple and more preferably used. (B) Other conditions such as the polymer material and the mixing method may be the same as or different from the above (I-1).

(II-2) A firing step of firing a mixture of the obtained fired product and the polymer material in an inert atmosphere or a vacuum atmosphere to produce a fired product. Conditions such as a firing temperature in the firing step are the above ( Although it can select like the process of I-2), it may be the same as that of said (I-2), or may differ. The firing temperature is preferably 600 to 1,200 ° C, and more preferably 700 to 1,100 ° C. Although baking time is suitably selected, 0.2 to 15 hours are preferable and 0.5 to 10 hours are more preferable.

  In the present invention, the steps (I-2) and (II-2) are included, and the steps (I-2) and (II-2) may be repeated a plurality of times. In this way, the graphite film can be uniformly coated by repeatedly mixing and baking the polymer material a plurality of times. The number of steps (I-2) and (II-2) is not particularly limited and is appropriately selected depending on the mixing ratio of the polymer material and the firing temperature, but preferably 1 to 4 times (including step (I) 2 to 5 times), 1 time is more preferable. When the number of steps (I-2) and (II-2) exceeds 4, it is difficult to achieve mass production and cost reduction, which are the original purposes.

  The mixing ratio of (A) particles to (B) polymer material is preferably 20 to 500 parts by mass of (B) polymer material with respect to 100 parts by mass of particles (A) as the total amount of all steps. Preferably it is 30-400 mass parts. The mixing amount of each step (I) with respect to the particles (A) or the baked product is appropriately selected from ± 30 parts by mass obtained by dividing the amount of all the steps by the number of steps (I). The mixing amount per process is preferably 5 to 300 parts by mass of (B) polymer material, more preferably 7 to 250 parts by mass with respect to 100 parts by mass of (A) particles or fired product. (B) If the polymer material is too small, the graphite coating amount may decrease. On the other hand, if the polymer material is too large, the graphite coating amount will increase, and the charge / discharge capacity will be increased when used as a non-aqueous electrolyte secondary battery negative electrode material. May decrease.

[Graphite-coated silicon composite]
The graphite-coated silicon composite obtained by the above production method is fine in that silicon particles, silicon oxide particles represented by the general formula SiOx (0.5 ≦ x <1.5), and silicon fine particles are dispersed in a silicon-based compound. The particle surface selected from particles having a simple structure and a mixture thereof is graphite-coated using the (B) polymer material as a graphite source. The particles having a fine structure in which silicon particles, silicon oxide particles, and silicon fine particles are dispersed in a silicon-based compound are as described in the (A) particles. When silicon oxide particles represented by the general formula SiOx (0.5 ≦ x <1.5) are used as raw materials, disproportionation reaction proceeds by sintering, and silicon fine particles are dispersed in the silicon-based compound. In some cases, the surface of the particles having a fine structure becomes a graphite-coated silicon composite coated with graphite using (B) a polymer material as a graphite source.

  The graphite coverage (%: ratio relative to the graphite-coated silicon composite) of the graphite-coated silicon composite is preferably 0.5 to 40% by mass, and more preferably 2 to 30% by mass. If the graphite coating amount is less than 0.5% by mass, it is insufficient in terms of formation of a conductive film, and there is a possibility that sufficient conductivity cannot be maintained. As a result, when a negative electrode material for a nonaqueous electrolyte secondary battery is obtained. Cycle performance may be reduced. On the other hand, even if the graphite coating amount exceeds 40% by mass, the effect is not improved, and the proportion of graphite in the negative electrode material increases, and the graphite-coated silicon composite is used as a negative electrode material for non-aqueous electrolyte secondary batteries. When used, the charge / discharge capacity decreases. In addition, the graphite coating amount in each time of mixing and firing is not particularly limited, and it is uniform by setting the total graphite coating amount to about ± 30% of the graphite coating amount obtained by dividing each time of mixing and firing. Graphite coating treatment is possible.

The graphite-coated silicon composite is a particle, and the average particle diameter is preferably 0.1 to 30 μm, and more preferably 0.3 to 25 μm. If the average particle size is smaller than 0.1 μm, the purity decreases due to the effect of surface oxidation, and when used as a non-aqueous electrolyte secondary battery negative electrode material, the charge / discharge capacity decreases, the bulk density decreases, and the unit volume The charge / discharge capacity per unit may be reduced. On the other hand, if it is larger than 30 μm, the amount of graphite deposited in the graphite coating treatment decreases, and as a result, when used as a nonaqueous electrolyte secondary battery negative electrode material, the cycle performance may be lowered. Further, BET specific surface area is preferably 0.1~30m ² / g, more preferably 0.1~25m ² / g, more preferably 0.2~20m ² / g. When the BET specific surface area is less than 0.1 m ² / g, the surface activity becomes small, and as a result, when a non-aqueous electrolyte secondary battery negative electrode material is used, the charge / discharge capacity may be reduced. On the contrary, if the BET specific surface area exceeds 30 m ² / g, the amount of the binder at the time of producing the electrode increases, and the capacity as an electrode may be reduced.

[Nonaqueous electrolyte secondary battery negative electrode material]
The graphite-coated silicon composite is suitable as a negative electrode active material for a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery, and is used for a negative electrode material for a non-aqueous electrolyte secondary battery. When producing a negative electrode using the nonaqueous electrolyte secondary battery negative electrode material, a conductive agent such as graphite can be added to the nonaqueous electrolyte secondary battery negative electrode material. Also in this case, the kind of the conductive agent is not particularly limited, and any electronic conductive material that does not cause decomposition or alteration in the constituted battery may be used. Specifically, Al, Ti, Fe, Ni, Cu, Metal powder and metal fiber such as Zn, Ag, Sn, Si, or natural graphite, artificial graphite, various coke powders, mesophase carbon, vapor-grown carbon fiber, pitch-based carbon fiber, PAN-based carbon fiber, various resin firing Graphite such as a body can be used.

[Negative electrode]
Examples of the method for preparing the negative electrode (molded body) include the following methods. A paste-like mixture is prepared by kneading a graphite-coated silicon composite, a conductive agent if necessary, and other additives such as a binder with a solvent such as N-methylpyrrolidone or water. Is applied to the sheet of the current collector. In this case, as the current collector, any material that is usually used as a negative electrode current collector, such as a copper foil or a nickel foil, can be used without any particular limitation on thickness and surface treatment. In addition, the shaping | molding method which shape | molds a mixture into a sheet form is not specifically limited, A well-known method can be used.

[Nonaqueous electrolyte secondary battery]
Nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries are characterized by the use of the above graphite-coated silicon composite, and other materials such as positive electrodes, negative electrodes, electrolytes, separators, and battery shapes are known. There is no particular limitation. For example, as the positive electrode active material, LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , V ₂ O ₅ , MnO ₂ , TiS ₂ , MoS _{2 and} other transition metal oxides, lithium, chalcogen compounds, and the like are used. As the electrolyte, for example, a non-aqueous solution containing a lithium salt such as lithium hexafluorophosphate and lithium perchlorate is used. As the non-aqueous solvent, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethoxyethane, γ-butyrolactone, One type or a combination of two or more types such as 2-methyltetrahydrofuran is used. Various other non-aqueous electrolytes and solid electrolytes can also be used.

  EXAMPLES Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated concretely, this invention is not restrict | limited to the following Example.

[Example 1]
50 g of silicon oxide particles represented by the general formula SiO _x (x = 1.02) having an average particle diameter of 5 μm were placed in a solution of 25 g of polystyrene in 25 g of toluene and mixed for 1 hour in a ball mill. A polystyrene mixture was made. Next, the entire amount of the mixture was placed in a φ120 annular furnace, and baked at 1,000 ° C. for 1 hour while flowing argon (Ar) gas at 0.2 L / min. After the firing, the temperature was lowered to obtain a black powder. The obtained black powder had a graphite coverage of 2.5% by mass. Next, the obtained black powder was mixed with polystyrene in the same manner as described above. That is, the obtained black powder was put into a solution of 25 g of polystyrene in 25 g of toluene and mixed for 1 hour by a ball mill to prepare a black powder / polystyrene mixture. Thereafter, the same baking treatment as described above was performed. The black powder finally obtained was a graphite-coated silicon composite having an average particle diameter of 5.0 μm, a BET specific surface area of 4.7 m ² / g, and a graphite coverage of 4.5% by mass.

Battery Evaluation Next, a battery evaluation using the obtained graphite-coated silicon composite as a negative electrode active material was performed by the following method.
First, 10% by mass of polyimide was added to the obtained graphite-coated silicon composite, and further N-methylpyrrolidone was added to form a slurry. This slurry was applied to a copper foil having a thickness of 20 μm, dried at 80 ° C. for 1 hour, The electrode was pressure-formed by a roller press, and this electrode was vacuum-dried at 350 ° C. for 1 hour, and then punched out to 2 cm ² to obtain a negative electrode.

  Here, in order to evaluate the charge / discharge characteristics of the obtained negative electrode, a lithium foil was used as a counter electrode, and lithium hexafluorophosphate was mixed with ethylene carbonate and diethyl carbonate in 1/1 (volume ratio) as a non-aqueous electrolyte. A lithium ion secondary battery for evaluation using a non-aqueous electrolyte solution dissolved in a liquid at a concentration of 1 mol / L and using a polyethylene microporous film having a thickness of 30 μm as a separator was produced.

The prepared lithium ion secondary battery was allowed to stand at room temperature overnight, and then charged with a secondary battery charge / discharge tester (manufactured by Nagano Co., Ltd.) until the test cell voltage reached 0 V at 0.5 mA / cm ² . Charging was performed at a constant current, and after reaching 0V, charging was performed by decreasing the current so as to keep the cell voltage at 0V. Then, charging was terminated when the current value fell below 40 μA / cm ² . Discharging was performed at a constant current of 0.5 mA / cm ² , and discharging was terminated when the cell voltage exceeded 2.0 V, and the discharge capacity was determined.

  The above charge / discharge test was repeated, and a charge / discharge test after 50 cycles of the lithium ion secondary battery for evaluation was performed. As a result, the initial charge capacity 1,850 mAh / g, the initial discharge capacity 1,500 mAh / g, the initial charge / discharge efficiency 81.1%, the 50th cycle discharge capacity 1,350 mAh / g, the cycle retention 90 after 50 cycles It was confirmed that the lithium-ion secondary battery had a high capacity of 0.0% and excellent initial charge / discharge efficiency and cycleability.

[Example 2]
The graphite coating treatment was performed under the same conditions as in Example 1 except that the firing temperature was 650 ° C. and the firing time was 5 hours.
The graphite coating amount of the black powder obtained by the first mixing / firing process (I) was 2.3% by mass. The finally obtained black powder was a graphite-coated silicon composite having an average particle size of 5.1 μm, a BET specific surface area of 9.8 m ² / g, and a graphite coverage of 4.1% by mass.

  As a result of battery evaluation of this graphite-coated silicon composite by the same method as in Example 1, the initial charge capacity was 1,920 mAh / g, the initial discharge capacity was 1,520 mAh / g, the initial charge / discharge efficiency was 79.2%, 50 It was confirmed that the lithium ion secondary battery had a high discharge capacity of 1,350 mAh / g at the cycle, a cycle retention of 88.8% after 50 cycles, and excellent initial charge / discharge efficiency and cycleability. It was.

[Example 3]
The graphite coating treatment was performed under the same conditions as in Example 1 except that a solution obtained by dissolving 80 g of polystyrene in 80 g of toluene was used.

The graphite coating amount of the black powder obtained by the first mixing / firing treatment (I) was 3.5%. The finally obtained black powder was a graphite-coated silicon composite having an average particle size of 5.1 μm, a BET specific surface area of 7.3 m ² / g, and a graphite coverage of 6.2% by mass.

  As a result of battery evaluation of this graphite-coated silicon composite by the same method as in Example 1, the initial charge capacity 1,830 mAh / g, the initial discharge capacity 1,480 mAh / g, the initial charge / discharge efficiency 80.9%, 50 It was confirmed that the lithium-ion secondary battery had a high discharge capacity of 1,330 mAh / g at the cycle, a cycle retention of 89.9% after 50 cycles, and excellent initial charge / discharge efficiency and cycleability. It was.

[Example 4]
A graphite coating treatment was performed under the same conditions as in Example 1 except that a solution of 5 g of polystyrene in 25 g of toluene was used, and the mixing and baking treatment was repeated 5 times (I: 1 time, II: 4 times).
The graphite coating amount of the black powder obtained by the mixing / firing treatment in the first (I) is 1.2% by mass, 2.1% by mass in the second (II), and 3. in the third (II). 1% by mass and 4.0% by mass in the fourth time (II), and the black powder finally obtained in the fifth time (II) has an average particle size of 5.2 μm and a BET specific surface area of 5.3 m ² / g. A graphite-coated silicon composite having a graphite coverage of 4.8% by mass.

  As a result of battery evaluation of this graphite-coated silicon composite by the same method as in Example 1, the initial charge capacity 1,850 mAh / g, the initial discharge capacity 1,500 mAh / g, the initial charge / discharge efficiency 81.1%, 50 It was confirmed that the lithium ion secondary battery had a high discharge capacity of 1,370 mAh / g at the cycle, a cycle retention of 91.3% after 50 cycles, and excellent initial charge / discharge efficiency and cycleability. It was.

[Comparative Example 1]
50 g of silicon oxide powder represented by the general formula SiO _x (x = 1.02) having an average particle diameter of 5 μm is placed in a solution of 120 g of polystyrene in 120 g of toluene and mixed for a time with a ball mill to obtain a silicon oxide / polystyrene mixture. Was made. Next, the entire amount of the mixture was charged into a φ120 annular furnace, and calcination was performed at 1,000 ° C. for 1 hour while flowing Ar gas in an amount of 0.2 L / min. The resulting black powder was a graphite-coated silicon composite having an average particle size of 5.1 μm, a BET specific surface area of 5.3 m ² / g, and a graphite coverage of 4.7% by mass.

  As a result of battery evaluation of this graphite-coated silicon composite by the same method as in Example 1, the initial charge capacity 1,840 mAh / g, the initial discharge capacity 1,480 mAh / g, the initial charge / discharge efficiency 80.4%, 50 The discharge capacity at the cycle was 1,280 mAh / g, the cycle retention after 50 cycles was 86.5%, and it was confirmed that the lithium ion secondary battery was inferior in cycle performance as compared with the Examples.

  The conditions and results of Examples and Comparative Examples are shown in the following table.

Claims (7)

(I-1) (A) silicon particles, silicon oxide particles represented by the general formula SiOx (0.5 ≦ x <1.5), particles having a fine structure in which silicon fine particles are dispersed in a silicon-based compound, And a mixing step of mixing particles selected from these mixtures and (B) a polymer material to produce a mixture,
(II-1) The obtained mixture is fired in an inert atmosphere or in a vacuum atmosphere, and includes a firing step for producing a fired product. Further, (I-2) the fired product obtained below and (B) A mixing step of mixing the polymer material to produce a mixture of the fired product and the polymer material; and (II-2) the obtained mixture of the fired product and the polymer material in an inert atmosphere or a vacuum atmosphere. A method for producing a graphite-coated silicon composite comprising a firing step of firing in a fired product to produce a fired product, or repeating the steps (I-2) and (II-2) a plurality of times. The method for producing a graphite-coated silicon composite according to claim 1, wherein (A) the particles have an average particle diameter of 0.1 to 30 µm and a BET specific surface area of 0.1 to 30 m ² / g.   (B) The method for producing a graphite-coated silicon composite according to claim 1 or 2, wherein the polymer material is a polymer selected from an aromatic group-containing thermoplastic polymer and a polyolefin-based thermoplastic polymer.   (I) A mixing process mixes the solution which melt | dissolved (B) polymeric material in the organic solvent, and (A) particle | grains or a baked material, The any one of Claims 1-3 characterized by the above-mentioned. A method for producing a graphite-coated silicon composite.   (II) The method for producing a graphite-coated silicon composite according to any one of claims 1 to 3, wherein a firing temperature in the firing step is 600 to 1,200 ° C. The graphite-coated silicon composite has an average particle size of 0.1 to 30 µm, a BET specific surface area of 0.1 to 30 m ² / g, and a graphite coverage of 0.5 to 40% by mass. A process for producing a graphite-coated silicon composite according to item 1.   The manufacturing method according to any one of claims 1 to 6, wherein the graphite-coated silicon composite is used for a negative electrode for a nonaqueous electrolyte secondary battery.