JP6771900B2 - Method for manufacturing negative electrode material for lithium secondary battery, method for improving lithium secondary battery, and its charge / discharge characteristics and cycle characteristics - Google Patents

Description

The present invention relates to a method for producing a negative electrode material for a lithium secondary battery, a lithium secondary battery provided with the negative electrode material obtained by this manufacturing method, and a method for improving charge / discharge characteristics and cycle characteristics of the lithium secondary battery.

As electronic devices become smaller, thinner, and lighter, lithium secondary batteries that can be charged with high energy density and can be discharged with high efficiency are attracting attention as batteries for power supply of electronic devices and backup batteries for electronic devices. Are collecting. In addition, since lithium has little impact on the environment and is highly safe, lithium secondary batteries have been developed as power sources for electric vehicles and as distributed power storage batteries.

In a typical conventional lithium secondary battery, graphite is used as the negative electrode active material, and charging and discharging are repeated by inserting (intercalating) and desorbing (deintercalating) lithium into the carbon material in the ionic state. ing. When graphite is used, the composition by charging becomes LiC ₆ , and the theoretical charge / discharge capacity is 372 Ah / kg. As various portable electronic devices are used as they are today, and as they become smaller and higher in performance, materials with even higher discharge capacities are required as negative electrode materials. As a means for increasing the discharge capacity, it has been proposed to use a negative electrode material for a lithium secondary battery obtained by combining a material having a high discharge capacity such as silicon and graphite (for example, Patent Document 1 and the like). reference).

Japanese Unexamined Patent Publication No. 2003-223892

According to the negative electrode material for a lithium secondary battery disclosed in Patent Document 1, it is possible to obtain a lithium secondary battery having excellent charge / discharge cycle characteristics and capable of maintaining a high charge / discharge capacity for a long period of time. However, in recent years, in addition to ensuring the charge / discharge cycle characteristics of lithium secondary batteries and the charge / discharge capacity over a long period of time, there has been a demand for negative electrode materials for lithium secondary batteries that can increase the capacity retention rate after the cycle. There is. Although the negative electrode material for a lithium secondary battery disclosed in Patent Document 1 can certainly impart excellent cycle characteristics and high charge / discharge capacity to a lithium secondary battery, it has a capacity retention rate after a plurality of cycles. From the perspective of improvement, there was room for improvement.

The present invention has been made in view of the above, and is a lithium secondary battery having excellent charge / discharge cycle characteristics, capable of maintaining a high charge / discharge capacity for a long period of time, and maintaining a high capacity retention rate after the cycle. An object of the present invention is to provide a method for manufacturing a negative electrode material for a lithium secondary battery, which is useful for manufacturing. Another object of the present invention is to provide a lithium secondary battery provided with a negative electrode material for a lithium secondary battery obtained by this manufacturing method, and a method for improving the charge / discharge characteristics and cycle characteristics of the lithium secondary battery.

As a result of diligent research to achieve the above object, the present inventor has prepared a graphite-free mixture by mixing a silicon-containing powder and a binder, and found that the above object can be achieved by firing this mixture. We have found and completed the present invention.

That is, the present invention relates to the following method for manufacturing a negative electrode material for a lithium secondary battery, a lithium secondary battery, and a method for improving its charge / discharge characteristics and cycle characteristics.
Item 1. Including the step of calcining a mixture of a silicon-containing powder and a binder to obtain a calcined product.
A method for producing a negative electrode material for a lithium secondary battery, wherein the fired product contains particles having a size of 10 μm or less in a volume ratio of 15% or less.
Item 2. Item 2. The production method according to Item 1, wherein the average particle size of the silicon-containing powder is 0.1 to 2 μm.
Item 3. Item 2. The production method according to Item 1 or 2, wherein the silicon-containing powder contains a powder of simple substance silicon.
Item 4. The production method according to any one of Items 1 to 3 above, wherein the binder contains a carbonizable material.
Item 5. The production method according to any one of Items 1 to 4, wherein the binder comprises at least one selected from the group consisting of pitch and tar.
Item 6. The production method according to any one of Items 1 to 5, wherein the mixture contains 10 to 200 parts by weight of a binder with respect to 100 parts by weight of a powder containing silicon.
Item 7. The lithium secondary is configured to include at least a negative electrode formed by using the negative electrode material for a lithium secondary battery obtained by the production method according to the above item 1, a positive electrode, and a non-aqueous electrolyte. battery.
Item 8. A mixture was prepared by mixing 10 to 200 parts by weight of a carbonizable binder in an aromatic hydrocarbon solvent with respect to 100 parts by weight of a simple substance of silicon having an average particle diameter of 0.1 to 2 μm.
By calcining the mixture, a calcined product containing particles having a size of 10 μm or less in a volume ratio of 15% or less was obtained.
A method for improving the charge / discharge characteristics and cycle characteristics of a lithium secondary battery by applying the fired product to a negative electrode material for a lithium secondary battery.

According to the method for producing a negative electrode material for a secondary battery according to the present invention, the step of firing a mixture of a silicon-containing powder and a binder is included. As a result, a negative electrode material for secondary batteries that has excellent charge / discharge cycle characteristics, can maintain a high charge / discharge capacity for a long period of time, and can maintain a high capacity retention rate after the cycle, and is useful for manufacturing lithium secondary batteries. Can be manufactured.

Since the lithium secondary battery according to the present invention includes the negative electrode material for the secondary battery obtained by the above manufacturing method, it has excellent charge / discharge cycle characteristics, can maintain a high charge / discharge capacity for a long period of time, and after the cycle. The capacity retention rate of the battery is also kept high.

Hereinafter, embodiments of the present invention will be described in detail.

First, a method for manufacturing a negative electrode material for a lithium secondary battery will be described. The method for producing a negative electrode material for a lithium secondary battery includes a step of firing a mixture of a silicon-containing powder and a binder to obtain a fired product (hereinafter abbreviated as "firing step").

The silicon-containing powder is composed of a material containing a silicon element, and the type thereof is not particularly limited.

Examples of the material containing a silicon element include elemental silicon (Si), silicon oxide (SiO and SiO ₂ ), silicide and the like. Examples of silicides include silicon nitride, silicon carbide, silicon boride, TiSi ₂ , ZrSi ₂ , VSi ₂ , CrSi ₂ , MoSi ₂ , WSi ₂ , and CoSi. The materials containing the silicon elements listed above can be used alone or in any combination of two or more. The silicon-containing powder may contain oxygen, in which case, for example, the inside of the particles may be composed of silicon elements. When the silicon-containing powder contains oxygen, the weight ratio of silicon to oxygen can be 90:10 to 50:50. The material containing a preferable silicon element is silicon alone or silicon oxide, but it is particularly preferable to contain silicon alone. The material containing the element silicon may be composed of only silicon alone.

The material containing a silicon element may be composed of an amorphous material, or may be composed of a crystal containing a single crystal or a polycrystal.

The average particle size of the silicon-containing powder is not particularly limited, but is preferably 0.1 to 5 μm, for example. When the average particle size is in this range, the obtained negative electrode material for a lithium secondary battery can easily impart a high charge / discharge capacity and excellent cycle characteristics to the lithium secondary battery. A more preferable average particle size is 0.1 to 2 μm, a more preferable average particle size is 0.1 to 1.5 μm, and a particularly preferable average particle size is about 0.1 to 1 μm. The average particle size referred to here means a value measured by a dynamic light scattering method.

As the silicon-containing powder, the above-mentioned material containing a silicon element can be obtained by a known pulverizing means such as a wet method or a dry method. For example, a powder containing silicon can be prepared by preparing a coarse silicon material such as silicon having a size of about 10 to 100 μm and pulverizing the material using “Bead Mill LMZ” manufactured by Ashizawa Finetech Co., Ltd.

The type of the binder is not particularly limited, but it is preferably composed of an organic compound which is a material that can be carbonized by firing (a material to be carbonized). Specific examples of the binder include asphalt substances such as tar and pitch, resins and the like. In particular, the binder preferably contains at least one selected from the group consisting of pitch and tar. The biogenic material may be derived from petroleum or coal. Further, the asphalt material may be isotropic or anisotropic, and examples thereof include isotropic pitch and anisotropic pitch.

In the mixture of the silicon-containing powder and the binder, the mixing ratio of the silicon-containing powder and the binder is not particularly limited. For example, the binder can be 10 to 350 parts by weight with respect to 100 parts by weight of the silicon-containing powder. When the mixing ratio is in the above range, the binding force between the silicon-containing powders is further enhanced, so that the cycle characteristics and the discharge capacity of the lithium secondary battery can be easily improved. The binder may be 10 to 200 parts by weight, preferably 10 to 150 parts by weight, more preferably 20 to 100 parts by weight, and 20 to 100 parts by weight, based on 100 parts by weight of the silicon-containing powder. It is particularly preferable to use 80 parts by weight.

The above mixture can be prepared by preparing a powder containing a predetermined amount of silicon and a binder and mixing them. In performing this mixing, a solvent capable of dissolving a part or all of the pitch and tars may be used in combination. Examples of such a solvent include aromatic hydrocarbon solvents such as benzene, toluene, xylene and quinoline. The solvent may be a mixed solvent containing two or more kinds.

When a solvent is used in combination as described above, it becomes easy for the solvent-soluble component to uniformly coat the surface of the silicon-containing powder in the solvent. This makes it easier to suppress the agglomeration of silicon-containing powders.

In preparing the mixture in combination with the solvent, for example, the silicon-containing powder and the binder may be dispersed in the solvent, then the solvent may be removed, and the obtained solid content may be pulverized.

The mixture may contain additives and the like as long as the effects of the present invention are not impaired.

In the firing step, the mixture is fired by heat-treating the mixture at a predetermined temperature. When the mixture is prepared by using the solvent in combination as described above, the pulverized product obtained by the subsequent pulverization may be calcined, or the mixture before pulverization may be calcined. Further, before firing, oxidation treatment may be performed in advance at around 300 ° C. for about 1 hour.

The firing temperature is not particularly limited and can be selected from the range of about 700 to 1500 ° C., and is usually about 800 to 1200 ° C., preferably about 900 to 1100 ° C. Firing can usually be carried out in an atmosphere of an inert gas, for example, an atmosphere of nitrogen, helium, argon or the like.

The mixture may be prepared in the firing step or in another step in advance.

By going through the above firing step, a fired product can be obtained. This fired product has, for example, a structure in which a material containing a silicon element is bonded with a carbon material. More specifically, it has a dispersed structure in which a material containing a silicon element is dispersed or scattered in a carbon material, that is, a so-called carbon material matrix, and the carbon material (carbon material matrix) and the material containing the silicon element are composited. It is in a state. Therefore, the fired product can also be called a composite carbon material.

The fired product (composite carbon material) can usually be used in powder granules (so-called powder granular carbon material). That is, the fired product (composite carbon material) can be in the form of particles. In this case, the average particle size of the particles constituting the fired product can usually be 5 to 40 μm. The average particle size referred to here refers to a value measured by the laser diffraction / scattering method. The average particle size is preferably 5 to 30 μm, more preferably 10 to 30 μm. When the average particle size is 40 μm or less, the electrode does not become too thick, so that the thickness is likely to be reduced. Further, when the average particle size is 5 μm or more, the layer of the carbon material covering the surface of the composite carbon material does not become too thin, so that expansion and contraction during charging and discharging are easily suppressed, and excellent cycle characteristics can be obtained. Easy to get rid of.

Further, the fired product contains particles having a size of 10 μm or less in a volume ratio of 15% or less. By setting the volume ratio of the particles constituting the fired product to 10 μm or less to 15% or less, the secondary battery formed of the obtained negative electrode material for the lithium secondary battery has reduced cycle characteristics. Is unlikely to occur. The method for adjusting the particle size distribution as described above is not particularly limited, but for example, the particle size distribution of the fired product can be desired by using a crushing classifier that combines a crusher such as a commercially available jet mill and a classifier. Can be adjusted to the range of.

The aspect ratio of the particles constituting the fired product (ratio of the major axis to the minor axis of the particles) is, for example, 1 to 10, preferably 1 to 6, and more preferably 1 to 3.

The fired product produced as described above can be used as a negative electrode material for a lithium secondary battery. Since the negative electrode material for a lithium secondary battery is formed of a fired product in which a carbon material and a powder containing a silicon element are compounded as described above, the moving speed of lithium ions is high. Therefore, if the negative electrode material for a lithium secondary battery is applied as a negative electrode material for a lithium secondary battery, the discharge capacity does not easily decrease even after repeated charging and discharging, and a lithium secondary battery having excellent cycle characteristics can be manufactured. Is possible.

Further, when the negative electrode material for a lithium secondary battery is applied as a negative electrode material for a lithium secondary battery, a high capacity retention rate is ensured even after a plurality of charge / discharge cycles are performed on the lithium secondary battery. It becomes possible. Therefore, the negative electrode material for a lithium secondary battery manufactured as described above is also excellent in that the life of the lithium secondary battery can be improved as compared with the conventional case.

The negative electrode material for a lithium secondary battery can be used as a constituent material of the negative electrode for a lithium secondary battery, and the negative electrode for a lithium secondary battery can be formed by a conventional method. For example, a negative electrode for a lithium secondary battery can be formed by a method of molding a mixture containing a negative electrode material for a lithium secondary battery, a binder for forming a negative electrode, and the like. Specifically, a paste containing a negative electrode material for a lithium secondary battery, an organic solvent, a binder, etc. is prepared, and this paste is applied to a negative electrode current collector by a coating means such as a doctor blade to form a lithium ion having an arbitrary shape. A negative electrode for a next battery can be formed. In forming the negative electrode for a lithium secondary battery, it may be combined with a terminal if necessary.

The negative electrode current collector is not particularly limited, and a known current collector, for example, a conductor such as copper can be used. As the organic solvent, a solvent capable of dissolving or dispersing a binder for forming a negative electrode is usually used, and an organic solvent such as N-methylpyrrolidone is exemplified. The amount of the organic solvent used is not particularly limited as long as it is in the form of a paste, and can be, for example, 60 to 150 parts by weight, preferably 80 to 100 parts by weight, based on 100 parts by weight of the negative electrode material for a lithium secondary battery. It is a department.

Examples of the binder for forming the negative electrode include a fluorine-containing resin such as polyvinylidene fluoride and polytetrafluoroethylene. The amount of the binder used for forming the negative electrode (in the case of a dispersion liquid, the amount used in terms of solid content) is not particularly limited, and the lower limit thereof is usually 3 parts by weight or more, preferably 3 parts by weight or more, based on 100 parts by weight of the negative electrode material. Is 5 parts by weight or more. The upper limit of the amount of the binder used for forming the negative electrode is usually 20 parts by weight or less (for example, 15 parts by weight or less), preferably 10 parts by weight or less, based on 100 parts by weight of the negative electrode material. More specifically, the amount of the binder used is, for example, 3 to 20 parts by weight, preferably 5 to 15 parts by weight, and more preferably 5 to 10 parts by weight with respect to 100 parts by weight of the negative electrode material in terms of solid content. .. The method for preparing the paste is not particularly limited, and examples thereof include a method of mixing a mixed solution or dispersion of a binder for forming a negative electrode and an organic solvent and a negative electrode material for a lithium secondary battery.

The negative electrode material for a lithium secondary battery obtained by the above-mentioned manufacturing method and a conductive material (including a carbonaceous material and a conductive carbon material) may be used in combination to manufacture a negative electrode.

The proportion of the conductive material used is not particularly limited, but can be about 1 to 10% by weight, preferably about 1 to 5% by weight, based on the total amount of the negative electrode material for the lithium secondary battery and the conductive material.

By using a conductive material in combination, the conductivity as an electrode can be improved, and the discharge capacity and cycle characteristics of the lithium secondary battery can also be improved. As such a conductive material, for example, carbon black such as acetylene black, thermal black, furnace black and the like can be used. Only one type of conductive material may be used, or two or more types may be used in combination. The conductive material may be mixed with, for example, a paste containing a negative electrode material for a lithium secondary battery and a solvent. The amount of the paste applied to the negative electrode current collector is not particularly limited, but is usually 5 to 15 mg / cm ² , preferably 7 to 13 mg / cm ² .

A lithium secondary battery can be manufactured by using the above-mentioned negative electrode for a lithium secondary battery (hereinafter, may be simply abbreviated as “negative electrode”).

The lithium secondary battery is configured to include at least a negative electrode, a positive electrode, and a non-aqueous electrolyte. That is, the lithium secondary battery includes at least a negative electrode formed of the above-mentioned negative electrode material for a lithium secondary battery, a positive electrode, and a non-aqueous electrolyte as constituent elements. As described above, the negative electrode material for a lithium secondary battery is formed by containing a fired product of a mixture of a silicon-containing powder and a binder, and the fired product has a volume ratio of 15% or less of particles having a size of 10 μm or less. Included in. The lithium secondary battery can be manufactured by a conventional method using a negative electrode, a positive electrode, an electrolytic solution, a separator and the like.

The positive electrode is preferably a material capable of occluding and releasing lithium. A known positive electrode can be used for such a positive electrode, but for example, a material composed of a positive electrode current collector, a positive electrode active material, a conductive agent, or the like can be used. As the positive electrode current collector, for example, aluminum and the like can be exemplified. Examples of the positive electrode active material include lithium composite oxides such as LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ . Examples of the conductive agent include conductive carbon black such as acetylene black.

The electrolytic solution is not particularly limited, and a known material can be used. For example, a non-aqueous lithium secondary battery can be produced by using a solution in which an electrolyte is dissolved in an organic solvent as the electrolytic solution. As the electrolyte, for _{example, LiPF 6, LiClO 4, LiBF} 4, LiClF 4, LiAsF 6, LiSbF 6, LiAlO 4, LiAlCl 4, LiCl, be mentioned lithium salt that produces a solvated hard anions such LiI it can. Examples of the organic solvent include carbonates such as propylene carbonate, ethylene carbonate and diethyl carbonate, lactones such as γ-butylolactone, chain ethers such as 1,2-dimethoxyethane, dimethyl ether and diethyl ether, tetrahydrofuran and 2-. Cyclic ethers such as methyl tetrahydrofuran, dioxolane and 4-methyldioxolane, sulfolanes such as sulfolane, sulfoxides such as dimethyl sulfoxide, nitriles such as acetonitrile, propionitrile and benzonitrile, N, N-dimethylformamide, N, Aprotic solvents can be exemplified, such as amides such as N-dimethylacetamide and polyoxyalkylene glycols such as diethylene glycol. The organic solvent may be used alone or as a mixed solvent of two or more kinds.

The separator is not particularly limited, and examples thereof include known separators, for example, a polyolephine-based porous membrane such as a porous polypropylene non-woven fabric and a porous polyethylene non-woven fabric.

In addition to the negative electrode including the negative electrode material of the present invention, the positive electrode, and the electrolytic solution, the lithium secondary battery may further include, for example, a gasket, a sealing plate, a case, etc., which are usually used in the art.

The shape of the lithium secondary battery is not particularly limited, but may be any shape such as a cylindrical type, a square type, and a button type.

The above-mentioned lithium secondary battery includes a negative electrode formed by using a negative electrode material for a lithium secondary battery obtained by the above-mentioned manufacturing method. Therefore, this lithium secondary battery has excellent charge / discharge cycle characteristics, can maintain a high charge / discharge capacity for a long period of time, and has a high capacity retention rate after the cycle. Therefore, the lithium secondary battery as described above can be used as a distributed, portable battery as a power source or an auxiliary power source for electronic devices, electric devices, automobiles, electric power storage, and the like.

As described above, by applying the negative electrode material for a lithium secondary battery obtained by the above-mentioned manufacturing method to the negative electrode material for a secondary battery, it is possible to improve the charge / discharge characteristics and the cycle characteristics of the lithium secondary battery. .. Therefore, it can be said that using such a negative electrode material for a lithium secondary battery is an effective method for improving the performance of the lithium secondary battery.

In particular, the negative electrode material for a lithium secondary battery is a mixture of 100 parts by weight of silicon having an average particle size of 0.1 to 2 μm and 20 to 80 parts by weight of a carbonizable binder mixed in an aromatic hydrocarbon solvent. Is preferably a calcined product obtained by preparing and calcining this mixture. By using such a negative electrode material for a lithium secondary battery, it is possible to provide a lithium secondary battery having remarkably excellent performance.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the aspects of these Examples.

(Example 1)
25 g of silicon powder (325 mesh manufactured by Aldrich, maximum particle diameter 43 μm) was pulverized for 5 hours by a planetary ball mill containing 100 g of stainless steel balls. The average particle size of the crushed silicon powder was 0.93 μm. To the pulverized product thus obtained, 20 g of an isotropic pitch having a softening point of 280 ° C. and 100 g of benzene as a solvent were added, and the mixture was stirred for 2 hours. Then, the solvent was removed using an evaporator. The obtained mixture of silicon and pitch was pulverized in an agate mortar and then air-oxidized in an electric furnace at 300 ° C. for 1 hour. Then, it was calcined in nitrogen gas at 1000 ° C. for 1 hour, and then pulverized to obtain a negative electrode material for a lithium secondary battery. The average particle size of the obtained negative electrode material for a lithium secondary battery was 2.9 μm. The average particle size was measured using "MT3300EXII" manufactured by Microtrac Bell.

The obtained negative electrode material for a lithium secondary battery and an N-methylpyrrolidone (NMP) solution of polyvinylidene fluoride (PVDF) as a binder were mixed to prepare a paste. An electrode (negative electrode) was prepared by applying this paste on a copper foil at a coating amount of 10 mg / cm ² . The ratio of the binder (PVDF) was 8 parts by weight with respect to 100 parts by weight of the obtained negative electrode material for the lithium secondary battery.

Using the electrode prepared as described above, metallic lithium as the counter electrode, and 1M-LiPF _6- EC / DMC (volume ratio 1: 1) as the electrolytic solution, a bipolar sealed cell was assembled and a charge / discharge test was performed. In the charge / discharge test, charging / discharging was performed for 7 cycles in a voltage range of 0 to 2.0 V with a constant current of 0.5 mA / cm ² .

As a result, the initial discharge capacity was 2200 Ah / kg, and the capacity retention rate after the cycle was 85%.

(Example 2)
25 g of silicon powder (325 mesh manufactured by Aldrich, maximum particle diameter 43 μm) was ground in an ethanol solvent for 5 hours using a bead mill manufactured by Ashizawa Finetech Co., Ltd. The average particle size of the crushed silicon powder was 0.7 μm. Using this pulverized product, a negative electrode material for a lithium secondary battery was prepared under the same conditions as in Example 1, a bipolar sealed cell was assembled, and a charge / discharge test was performed.

As a result, the initial discharge capacity was 2300 Ah / kg, and the capacity retention rate after the cycle was 90%.

(Example 3)
30 g of silicon powder (manufactured by High Purity Chemical Co., Ltd., 45 μm or less) was pulverized for 10 hours by a planetary ball mill containing 100 g of stainless steel balls. The average particle size of the crushed silicon powder was 0.5 μm. To the pulverized product thus obtained, 100 g of an isotropic pitch at a softening point of 280 ° C. and 100 g of benzene as a solvent were added, and the mixture was stirred for 2 hours. Then, the solvent was removed using an evaporator. The obtained mixture of silicon and pitch was pulverized in an agate mortar and then air-oxidized in an electric furnace at 300 ° C. for 1 hour. Then, it was calcined in nitrogen gas at 1000 ° C. for 1 hour, then pulverized and sieved with a 53 μm mesh to obtain a negative electrode material for a lithium secondary battery under 53 μm.

The obtained negative electrode material for a lithium secondary battery and an N-methylpyrrolidone (NMP) solution of polyvinylidene fluoride (PVDF) as a binder were mixed to prepare a paste. An electrode (negative electrode) was prepared by applying this paste on a copper foil at a coating amount of 10 mg / cm ² . The ratio of the binder (PVDF) was 8 parts by weight with respect to 100 parts by weight of the obtained negative electrode material for the lithium secondary battery.

Using the electrode prepared as described above, metallic lithium as the counter electrode, and 1M-LiPF _6- EC / DMC (volume ratio 1: 1) as the electrolytic solution, a bipolar sealed cell was assembled and a charge / discharge test was performed. In the charge / discharge test, charging / discharging was performed for 7 cycles in a voltage range of 0 to 2.0 V with a constant current of 0.5 mA / cm ² .

As a result, the initial discharge capacity was 1200 Ah / kg. The capacity retention rate after 20 cycles was 95%.

(Example 4)
30 g of silicon powder (manufactured by High Purity Chemical Co., Ltd., 45 μm or less) was pulverized for 10 hours by a planetary ball mill containing 100 g of stainless steel balls. The average particle size of the crushed silicon powder was 0.5 μm. To the pulverized product thus obtained, 100 g of an isotropic pitch at a softening point of 280 ° C. and 100 g of benzene as a solvent were added, and the mixture was stirred for 2 hours. Then, the solvent was removed using an evaporator. The obtained mixture of silicon and pitch is crushed in an agate mortar, then fired in nitrogen gas at 1000 ° C. for 1 hour, and then crushed and crushed using a jet mill SJ-100 manufactured by Nisshin Engineering Co., Ltd. Classified. When the particle size distribution and the average particle size of the obtained fired product were measured by the same measuring machine as in Example 1, the abundance ratio of the particles of 10 μm or less was 12% by volume, and the average particle size was 17.8 μm (17.8 μm). D ₁₀ = 2.9 μm, D ₅₀ = 26.1 μm, D ₉₀ = 48.1 μm).

A charge / discharge test was performed in the same manner as in Example 1 using the obtained negative electrode material for a lithium secondary battery. As a result, the initial discharge capacity was 1200 Ah / kg, and the capacity retention rate after 20 cycles was 97%.

(Comparative Example 1)
A negative electrode material for a lithium secondary battery was prepared in the same manner as in Example 1 except that the pitch was not used. A bipolar sealed cell was assembled using this negative electrode material for a lithium secondary battery, and a charge / discharge test was performed. As a result, the initial discharge capacity was 1200 Ah / kg, and the capacity retention rate after the cycle was 5%. It was.

(Comparative Example 2)
25 g of silicon powder (325 mesh manufactured by Aldrich, maximum particle diameter 43 μm) was pulverized for 5 hours by a planetary ball mill containing 100 g of stainless steel balls. The average particle size of the crushed silicon powder was 0.93 μm. To this pulverized product, 5 g of artificial graphite (“SFG-6” manufactured by TimcalAG) and 10 g of pitch were added and mixed with a planetary ball mill for another 1 hour. The obtained composite particles were calcined in nitrogen gas at 1000 ° C. for 1 hour and crushed to obtain a negative electrode material. The average particle size of the obtained negative electrode material was 2.9 μm.

As a result, the initial discharge capacity was 521 Ah / kg. Further, the discharge capacity increased to 533 Ah / kg in the third cycle, but then the discharge capacity began to decrease again, and the discharge capacity after 7 cycles decreased to 416 Ah / kg.

(Comparative Example 3)
30 g of silicon powder (manufactured by High Purity Chemical Co., Ltd., 45 μm or less) was pulverized for 10 hours by a planetary ball mill containing 100 g of stainless steel balls. The average particle size of the crushed silicon powder was 0.5 μm. To the pulverized product thus obtained, 100 g of an isotropic pitch at a softening point of 280 ° C. and 100 g of benzene as a solvent were added, and the mixture was stirred for 2 hours. Then, the solvent was removed using an evaporator. The obtained mixture of silicon and pitch is crushed in an agate mortar, then fired in nitrogen gas at 1000 ° C. for 1 hour, and then crushed and classified using a crusher "MX1200XTM" manufactured by Osaka Chemical Co., Ltd. went. When the particle size distribution and the average particle size of the obtained fired product were measured by the same measuring machine as in Example 1, the abundance ratio of the particles of 10 μm or less was 65% by volume, and the average particle size was 3.8 μm ( D ₁₀ = 0.6 μm, D ₅₀ = 3.5 μm, D ₉₀ = 24.6 μm).

A charge / discharge test was performed in the same manner as in Example 1 using the obtained negative electrode material for a lithium secondary battery. As a result, the initial discharge capacity was 1070 Ah / kg, and the capacity retention rate after 20 cycles was 10%.

From the above results, it can be seen that the negative electrode materials for lithium secondary batteries obtained in Examples 1 to 4 have excellent cycle characteristics. On the other hand, since the negative electrode material of Comparative Example 2 was produced containing graphite, the initial discharge capacity was also significantly different from that of the example, and the discharge capacity after the cycle was also reduced. Since the initial capacity of silicon alone is about 4200 Ah / kg, that of graphite is only 372 Ah / kg, it is considered that the discharge capacity is reduced in Comparative Example 2 in which artificial graphite is mixed.

Claims (3)

A step of calcining a mixture of a silicon-containing powder and a binder containing a carbonizable material (however, graphite is not included) to obtain a calcined product having a structure in which a material containing a silicon element is bonded with a carbon material. Including
The average particle size of the silicon-containing powder is 0.1 to 2 μm.
The silicon-containing powder contains the powder of simple substance silicon,
The fired product contains particles having a size of 10 μm or less in a volume ratio of 15% or less.
A method for producing a negative electrode material for a lithium secondary battery, wherein the mixture contains 10 to 200 parts by weight of a binder with respect to 100 parts by weight of a powder containing silicon. The production method according to claim 1, wherein the binder comprises at least one selected from the group consisting of pitch and tar. A mixture (however, graphite is not included) is prepared by mixing 10 to 200 parts by weight of a carbonizable binder in an aromatic hydrocarbon solvent with respect to 100 parts by weight of a simple substance of silicon having an average particle size of 0.1 to 2 μm. And
Said mixture by calcining, 10 [mu] m contained in particles with a volume ratio of 15% or less is less, and to obtain a fired product that have a structure material containing silicon element is bonded with a carbon material,
A method for improving the charge / discharge characteristics and cycle characteristics of a lithium secondary battery by applying the fired product to a negative electrode material for a lithium secondary battery.