JP6401117B2 - Method for producing negative electrode material for lithium ion secondary battery - Google Patents

Description

  The present invention relates to a method for producing composite particles for a negative electrode material for a lithium ion secondary battery comprising scaly graphite particles, calcined carbon, and metal particles that can be alloyed with lithium.

Lithium ion secondary batteries have excellent characteristics such as high voltage and high energy density compared to other secondary batteries, and are therefore widely used as power sources for electronic devices. In recent years, electronic devices have been reduced in size and performance, and there has been an increasing demand for further increase in energy density of lithium ion secondary batteries.
Currently, lithium ion secondary batteries generally use LiCoO _{2 for} the positive electrode and graphite for the negative electrode. However, although the graphite negative electrode is excellent in reversibility of charge and discharge, its discharge capacity has already reached a value close to the theoretical value 372 mAh / g corresponding to the intercalation compound LiC ₆ in order to achieve further higher energy density. Needs to develop a negative electrode material having a larger discharge capacity than graphite.
Although metallic lithium has the highest discharge capacity as a negative electrode material, there is a problem that lithium is deposited in a dendritic state during charging, the negative electrode is deteriorated, and the charge / discharge cycle is shortened. In addition, lithium deposited in a dendrite shape may penetrate the separator and reach the positive electrode, causing a short circuit.
Therefore, metallic materials that form an alloy with lithium have been studied as negative electrode materials that replace metallic lithium. These alloy negative electrodes have discharge capacities far surpassing that of graphite, although they do not reach metal lithium. However, the active material is pulverized and peeled off due to volume expansion accompanying alloying, and the practical cycle characteristics have not yet been obtained.

  In order to improve the drawbacks of the above-described alloy negative electrode, a composite of a metal material and one or both of a graphitic material and a carbonaceous material has been studied. When roughly classified, a metal material, a graphite material, and a carbonaceous material precursor are mixed and then heat-treated (Patent Document 1) (2) A metal material is coated with a carbonaceous layer using a CVD method (Patent Document 2) (3 ) (2) using mechanical alloying together (Patent Document 3). However, in any of the above (1) to (3), there is no description about voids inside the particles. In these methods, since it is not possible to provide a gap for relaxing the expansion of the metallic material during charging, problems such as powdering and peeling of the active material have not been solved.

  In Patent Document 4, lithium and an alloy having an average particle diameter of 2 to 5 μm and an average particle diameter of 1/2 or less of the average particle diameter of the graphite particles on the surface of the graphite particles having an aspect ratio of 3 or less. After the mechanochemical treatment is applied to the metal particles that can be converted, the mechanochemically treated product is granulated with a resin or the like (for example, spray-dried), and the granulated product is impregnated with a carbonaceous precursor. Describes a method for producing metal-graphitic particles, which is heat-treated to produce metal-graphitic particles. However, in this method, the metal particles are only adhered on the graphite, and the adhesion is not sufficient. For this reason, there is a problem in that the metal particles fall off and segregate from the graphite during the granulation operation, and the voids cannot be appropriately disposed around the metal particles.

JP 2002-231225 A JP 2002-151066 A Japanese Patent Laid-Open No. 2002-216751 JP 2006-294476 A

  The present invention has been made in view of the situation as described above, and can be used as a negative electrode material for a lithium ion secondary battery, which can sufficiently relax expansion of a metallic material during charging, and has a high discharge exceeding the theoretical capacity of graphite. It is an object of the present invention to provide a method for producing a material exhibiting capacity and excellent initial charge / discharge efficiency and cycle characteristics.

In order to solve the above-mentioned problems, the present invention is a method for producing a spherical composite comprising scaly graphite particles, calcined carbon, and metal particles that can be alloyed with lithium, the carbonaceous precursor on the scaly graphite particles A step of fixing metal particles through a body , a step of dispersing the particles in a binder solution and performing spray drying to form spherical particles, and heat-treating the spherical particles to form the carbonaceous precursor Provided is a method for producing a negative electrode material for a lithium ion secondary battery, comprising a step of using a body and a binder as calcined carbon.

That is, the present invention provides the following.
(1) A method for producing a spherical composite comprising scaly graphite particles, calcined carbon, and metal particles that can be alloyed with lithium, 1) a step of mixing the metal particles and the scaly graphite particles, 2 ) A step of adding a carbonaceous precursor to the mixture and then performing a heat treatment to form primary particles in which metal particles are immobilized on scaly graphite particles. 3) Dispersing the primary particles in a binder solution. A step of performing spray drying to form spherical secondary particles, and 4) heat-treating the secondary particles in a temperature range of 700 ° C. or higher and 1200 ° C. or lower, and firing the carbonaceous precursor and binder described above. The manufacturing method of the negative electrode material for lithium ion secondary batteries which has the process made into carbon.
(2) The method for producing a negative electrode material for a lithium ion secondary battery according to (1), wherein a mechanochemical treatment is performed as the step of mixing the metal particles and the scaly graphite particles.
(3) The method for producing a negative electrode material for a lithium ion secondary battery according to (1) or (2), wherein mechanical pulverization is performed as the step of mixing the metal particles and the scaly graphite particles.
(4) In the step of mixing the metal particles and the flaky graphite particles, a graphitic carbon fiber is further added. The negative electrode material for a lithium secondary battery according to any one of (1) to (3), Production method.
(5) The lithium secondary battery according to any one of (1) to (3), wherein a graphite carbon fiber is further added in the step of dispersing the primary particles in the binder solution. Manufacturing method for negative electrode material.
(6) The negative electrode material for a lithium ion secondary battery according to any one of (1) to (5), wherein a carbonaceous precursor is further adhered to the secondary particles and then the heat treatment is performed. Production method.

  The composite obtained by the production method of the present invention, when used as a negative electrode material for a lithium ion secondary battery, sufficiently relaxes the expansion of metal particles during charging and prevents the metal particles from peeling off from the graphite particles. It exhibits either high discharge capacity exceeding the theoretical capacity of graphite, excellent initial charge / discharge efficiency, or cycle characteristics.

1 is an electron micrograph showing the appearance of the composite obtained in Example 1. FIG. FIG. 2 is an electron micrograph showing the cross section of the composite obtained in Example 1, and an EDX mapping image showing the Si element measured by energy dispersive X-ray spectroscopy in the same field of view. FIG. 3 is an electron micrograph showing a cross section of the composite obtained in Comparative Example 1, and an EDX mapping image showing Si elements measured by energy dispersive X-ray spectroscopy existing in the same field of view. FIG. 4 is a cross-sectional view of an evaluation battery for evaluating the battery characteristics of the negative electrode of the present invention.

Below, the content of the manufacturing method of the negative electrode material for lithium secondary batteries by this invention is explained in full detail.
(Flaky graphite particles)
The scaly graphite particles used in the present invention are not particularly limited as long as they can occlude and release lithium ions. A part or all of which is made of graphite, for example, natural graphite, and artificial graphite obtained by finally heat-treating tar and pitch at 1500 ° C. or higher. Specifically, mesophase fired bodies obtained by heat-condensing petroleum-based and coal-based tar pitches called carbonitizable carbon materials and subjected to polycondensation, and cokes are graphite at 1500 ° C. or higher, preferably 2800-3300 ° C. It can be obtained by the chemical treatment.
The average particle diameter of the flaky graphite particles is preferably in the range of 0.1 μm to 20 μm, and more preferably in the range of 0.3 μm to 10 μm. In the present invention, the average particle size is a particle size (D ₅₀ ) at which the cumulative frequency of the laser diffraction particle size distribution meter is 50% in terms of volume distribution.
The ratio of the scale-like graphite particles is preferably 98 to 60% by mass with respect to the total amount of the composite particles. More preferably, it is 95-60 mass%.

The average particle diameter D50 of the flaky graphite is preferably 6 μm or less, and more preferably in the range of 1 μm to ₅ μm. If it is less than 1 μm, the influence of the exposure of the edge surface of graphite is large, and the charge / discharge efficiency is lowered. When it exceeds 6 μm, the void formed by overlapping the flake graphite non-parallelly becomes too large. About a shape, it is preferable that average flatness (Ly / t) is 2 or more, and it is more preferable that it is 5-40. Here, the average flatness means the ratio of the short axis length Ly to the thickness t of one particle of scaly graphite (Ly / t), and 100 scaly graphite particles are observed with a scanning electron microscope. Calculated as a simple average value of the measured flatness of each particle. When the average flatness is within this range, the formation of voids when graphite overlaps in a non-equilibrium state is appropriate and is not too small, and the voids do not become too large when overlapping in a non-equilibrium state.

(Metal particles that can be alloyed with lithium)
Examples of metals that can be alloyed with lithium include metals such as Al, Pb, Zn, Sn, Bi, In, Mg, Ga, Cd, Ag, Si, B, Au, Pt, Pd, Sb, Ge, and Ni. Preferably, Si and Sn are used. Further, the metal particles may be an alloy of two or more of the above metals, and the alloy may further contain other elements in addition to the above metal. Part of the metal may form an oxide, nitride, or carbide, and it is particularly preferable that at least a part of the oxide is included.
The average particle diameter of the metal particles is preferably 10 μm or less, more preferably 5 μm or less, and particularly preferably 1 μm or less. When the average particle diameter of the metal particles exceeds 10 μm, the effect of improving the cycle characteristics may be reduced.
There are no particular restrictions on the shape of the metal particles. Any of a granular shape, a spherical shape, a plate shape, a scale shape, a needle shape, a thread shape and the like may be used.
The ratio of the metal particles is preferably 1% by mass or more and 20% by mass or less, and particularly preferably 2% by mass or more and 20% by mass or less with respect to the total amount of the composite particles. When the amount of the metal or metal compound is less than 1% by mass, the effect of improving the capacity may be small, and when it is more than 20% by mass, the effect of improving the cycle characteristics may be small.

(Process of mixing scaly graphite particles and metal particles)
The present invention includes a step of mixing metal particles that can be alloyed with lithium and scaly graphite particles. The mixing step of the present invention is a step of mixing by mechanical force such as a kneader. A general mixer or a kneader can be used for mixing the metal particles and the graphite particles. Instead of mixing, mechanochemical treatment or mechanical pulverization treatment may be performed, or these may be combined.
Here, the mechanochemical treatment refers to a treatment in which a compression force and a shear force are simultaneously applied to the graphite particles and the metal particles, and the adhesion between the graphite particles and the metal particles can be enhanced by the compression force and the shear force. As long as the mechanochemical treatment apparatus is an apparatus that can simultaneously apply a compressive force and a shearing force to graphite particles and metal particles, the type and structure of the apparatus are not particularly limited. For example, hybridization systems (manufactured by Nara Machinery Co., Ltd.), mechano-micro systems (manufactured by Nara Machinery Co., Ltd.), mechano-fusion systems (Hosokawa Micron Corp.), dry attritors (Nippon Coke Industrial Co., Ltd.), etc. Can be used.
In the mechanical pulverization treatment, the metal particles and the graphite particles can be more uniformly dispersed by simultaneously performing pulverization and mixing. A general pulverizer can be used for the mechanical pulverization treatment, and either a wet method or a dry method can be applied, but a wet method is more preferable.

Graphite carbon fibers may be added when mixing the metal particles and the scaly graphite particles. The graphitic carbon fiber is not particularly limited as long as it is fibrous graphite having conductivity. Preferred shapes are an average fiber diameter of 10 to 1000 nm and an average fiber length of 1 to 20 μm, and examples include carbon nanotubes, carbon nanofibers, and vapor grown carbon fibers. The graphitic carbon fiber may be either dry powder or a dispersion liquid dispersed in an appropriate solvent, or a dispersion liquid added with a dispersant.
The ratio of the graphitic carbon fiber is preferably 0.5% by mass or more and 5% by mass or less, and more preferably 1% by mass or more and 3% by mass or less with respect to the entire final composite. When the graphitic carbon fiber is less than 0.5% by mass, the effect of improving the cycle characteristics may be reduced, and when it exceeds 5% by mass, the initial efficiency may be decreased.

(Step of immobilizing metal particles on scaly graphite particles)
The present invention includes a step of adding a carbonaceous precursor to the mixture and then performing a heat treatment to obtain primary particles in which metal particles are immobilized on scaly graphite particles. By immobilizing the metal particles on the graphite particles, it is possible to prevent the metal particles from dropping and segregating in the granulation operation in the next step.
The carbonaceous precursor is not particularly limited as long as it is a material that is easily carbonized by heat treatment, and examples thereof include tar pitches and / or resins. Specifically, tar pitches include coal tar, tar light oil, tar medium oil, tar heavy oil, naphthalene oil, anthracene oil, coal tar pitch, pitch oil, mesophase pitch, oxidized cross-linked petroleum pitch, heavy oil, and the like. . Examples of resins include polyvinyl alcohol, polyacrylic acid, polyvinyl chloride, polyvinylidene chloride, thermoplastic resins such as vinyl halide resins such as chlorinated polyvinyl chloride, phenol resins, furan resins, furfuryl alcohol resins, and cellulose resins. And thermosetting resins such as polyacrylonitrile, polyamideimide resin, and polyamide resin. A carbonaceous material can be obtained by heat-treating these carbonaceous precursors at a temperature described below.
The proportion of the carbonaceous precursor is preferably 0.5% by mass or more and 5% by mass or less, and more preferably 1% by mass or more and 3% by mass or less with respect to the mixture. When the carbonaceous precursor is less than 0.5% by mass, immobilization of the metal particles may be insufficient, and when it is 5% by mass or more, the discharge capacity may be reduced.
By heat-treating the mixture of graphite particles, metal particles and carbonaceous precursor in a non-oxidizing atmosphere, primary particles in which the metal particles are immobilized on the scaly graphite particles can be obtained. The heat treatment temperature is preferably 300 ° C. or higher and 700 ° C. or lower. The non-oxidizing atmosphere means, for example, an inert gas atmosphere such as nitrogen or argon having an oxygen concentration of 1000 ppm or less; a reducing gas atmosphere containing a reducing gas such as hydrogen or carbon monoxide; . The reason why baking is performed in a non-oxidizing atmosphere is to prevent oxidation.

(Spray drying granulation process)
The present invention includes a step of dispersing the primary particles in a binder solution and performing a spray drying process to obtain spherical secondary particles.
The binder may be any binder as long as it dissolves in a suitable solvent, and those exemplified as the carbonaceous precursor can be used in the same manner. The addition amount of the binder as a raw material is preferably 1 to 30% by mass with respect to 10% by mass of the primary particles. More preferably, it is 1-15 mass%. Any of an aqueous solution, an alcohol solution, an organic solvent solution, and the like may be used as the binder solution. A solution obtained by adding a surfactant, polyvinyl alcohol or the like as a viscosity adjusting agent to water is preferable.
Graphite carbon fibers may be added to the dispersion of primary particles and binder solution. The graphitic carbon fibers that can be used and the proportions thereof are the same as described above. Further, some of the primary particles may be replaced with graphite particles.
The spray drying treatment may be any method as long as the dispersion of the primary particles and the binder solution is sprayed together with an air stream and the solvent is instantaneously dried with hot air. A spray type, a disk type, a two-fluid nozzle type Etc. can be used. Due to the surface tension of the dispersion, the dried particles form spherical secondary particles. At this time, by adjusting the solid content ratio of the dispersion and the air flow so that bubbles do not intervene in the droplets of the spray, it is not a complete hollow structure, but a structure in which scaly graphite particles existed inside Can be formed.
For example, the solid content ratio of the dispersion is preferably 5 to 25% by mass in the total amount, the inlet temperature of the spray dryer is 150 to 250 ° C., and the nozzle air amount is preferably 20 to 100 liter / min.
In the spray drying treatment, the particle size can be adjusted to an arbitrary particle size by adjusting the solid content ratio of the stock solution and the air flow, and a step of finally pulverizing and adjusting the particle size is unnecessary. In addition, since graphite particles are used as the main raw material, graphitization is not necessary, and sufficient capacity can be developed as a negative electrode material for a lithium ion secondary battery only by firing.

(Baking process)
The present invention includes a step of heat-treating the secondary particles in a temperature range of 700 ° C. or more and 1200 ° C. or less in an inert atmosphere, and using the carbonaceous precursor and the binder as calcined carbon. When the firing temperature is lower than 700 ° C. or higher than 1200 ° C., the capacity may decrease. As the inert atmosphere, N ₂ , Ar, He, a vacuum atmosphere, or the like, or a mixture thereof can be used.
Prior to the firing treatment, different types of graphite materials, carbonaceous or graphite fibers, carbon materials such as amorphous hard carbon, organic materials, inorganic materials, and metal materials may be attached, embedded, and combined. Baking by dipping the spray dried product in a solution of carbonaceous precursor before processing, it may be attached a carbonaceous precursor to the spray dried product. This can reduce the reactivity (charge / discharge loss) by strengthening the granulated structure and coating. The preferable adhesion amount (the amount before firing) of the carbonaceous precursor is preferably 1 to 30% by mass with respect to 100% by mass of the scaly graphite particles. More preferably, it is 1-15 mass%.
Proportion of baked formed carbon 1-15% by weight, based on the total composite particles, preferably from 3 to 12% by weight.

  EXAMPLES Next, although an Example demonstrates this invention concretely, this invention is not limited to these Examples. In the following Examples and Comparative Examples, as shown in FIG. 4, a current collector (negative electrode) 7b having a negative electrode mixture 2 having the negative electrode material of the present invention attached to at least a part of its surface and a counter electrode comprising a lithium foil A button type secondary battery for single electrode evaluation composed of (positive electrode) 4 was prepared and evaluated. An actual battery can be produced according to a known method based on the concept of the present invention.

Example 1
[Production of negative electrode material]
A scaly graphite particle having an average particle diameter of 5 μm and an average flatness of 20 and silicon particles having an average particle diameter of 0.15 μm were mixed with a biaxial kneader, and further, an oil-in-tar solution of tar with a coal tar pitch was added and kneaded. Next, the kneaded product was heat-treated at 500 ° C. in a nitrogen atmosphere to obtain primary particles in which silicon particles were immobilized on scaly graphite particles. The obtained primary particles were dispersed in an aqueous polyacrylic acid solution and spray-dried with a spray dryer to obtain spherical secondary particles. Finally, the obtained secondary particles were heat-treated at 1000 ° C. in a nitrogen atmosphere to obtain a composite which is a spherical final product composed of scaly graphite particles, calcined carbon, and silicon particles. The blending amount of each material was adjusted so that the respective abundance ratios in the composite were as shown in Table 1. From the SEM image shown in FIG. 1, it was found that the composite was spherical. Moreover, it was found from the SEM image and EDX-Si mapping image of the cross section shown in FIG. 2 that the silicon particles were uniformly dispersed inside the composite.

[Preparation of negative electrode mixture paste]
Next, a negative electrode was produced using the negative electrode material. First, 96 parts by mass of the negative electrode material composed of the composite, 2 parts by mass of carboxymethyl cellulose as a binder, and 2 parts by mass of styrene-butadiene rubber were placed in water and stirred to prepare a negative electrode mixture paste. Next, the negative electrode mixture layer applied on the copper foil was pressed by a hand press. Further, the copper foil and the negative electrode mixture layer were punched into a columnar shape having a diameter of 15.5 mm to produce a working electrode (negative electrode) having a negative electrode mixture layer adhered to the copper foil. The density of the negative electrode mixture layer was 1.4 g / cm ³ .

[Production of counter electrode (positive electrode)]
Next, a button type secondary battery for single electrode evaluation was produced using the negative electrode. As the positive electrode, a current collector made of nickel net and an electrode plate made of lithium metal foil in close contact with the current collector were used.

[Electrolyte, separator]
The electrolytic solution was prepared by dissolving LiPF ₆ at a concentration of 1 mol / L in a mixed solvent of 33% by volume of ethylene carbonate and 67% by volume of methyl ethyl carbonate to prepare a nonaqueous electrolytic solution. The obtained nonaqueous electrolytic solution was impregnated into a 20 μm-thick polypropylene porous body as a separator to produce a separator impregnated with the electrolytic solution. In addition, about a real battery, it can produce according to a well-known method based on the concept of this invention.

[Configuration of evaluation battery]
FIG. 4 shows a button type secondary battery as a configuration of the evaluation battery.
The exterior cup 1 and the exterior can 3 were sealed by interposing an insulating gasket 6 at the peripheral portion thereof and caulking both peripheral portions. A copper current collector 7 a made of nickel net, a cylindrical counter electrode (positive electrode) 4 made of lithium foil, a separator 5 impregnated with an electrolytic solution, and a negative electrode mixture 2 are attached to the inside of the outer can 3 in that order. A battery system in which current collectors 7b made of foil are laminated.
In the evaluation battery, the separator 5 impregnated with the electrolytic solution was sandwiched between the current collector 7b and the working electrode (negative electrode) made of the negative electrode mixture 2, and the counter electrode 4 in close contact with the current collector 7a. The current collector 7b is accommodated in the exterior cup 1, the counter electrode 4 is accommodated in the exterior can 3, the exterior cup 1 and the exterior can 3 are combined, and an insulating gasket is provided at the peripheral edge between the exterior cup 1 and the exterior can 3. 6 was interposed, and both peripheral portions were caulked and sealed.
About the evaluation battery produced by the above, the following charging / discharging test was done at the temperature of 25 degreeC, and the initial stage charging / discharging efficiency, the charge expansion coefficient, and the cycle characteristic were calculated. The results are shown in Table 2.

[Initial charge / discharge efficiency]
After constant current charging of 0.9 mA until the circuit voltage reached 0 mV, switching to constant voltage charging was performed when the circuit voltage reached 0 mV, and charging was continued until the current value reached 20 μA. The charging capacity per unit mass (unit: mAh / g) was determined from the amount of electricity applied during that time. Then, it rested for 120 minutes. Next, constant current discharge was performed until the circuit voltage reached 1.5 V at a current value of 0.9 mA, and the discharge capacity per unit mass (unit: mAh / g) was determined from the amount of electricity supplied during this period. The initial charge / discharge efficiency was calculated according to the following formula.
Initial charge / discharge efficiency (%) = (discharge capacity / charge capacity) × 100
In this test, the process of occluding lithium ions in the negative electrode material was charged, and the process of detaching from the negative electrode material was discharged.

[Charge expansion rate]
After constant current charging of 0.9 mA until the circuit voltage reached 0 mV, switching to constant voltage charging was continued until the current value reached 20 μA. The evaluation battery was disassembled in a charged state, the negative electrode was washed with ethyl methyl carbonate under an argon atmosphere, and the thickness was measured with a micrometer. From the thickness of the negative electrode before and after charging and the thickness of the copper foil (15 μm), the charge expansion coefficient of the negative electrode active material was calculated by the following formula.
Charge expansion coefficient (%) = ((Thickness of negative electrode after charging−Thickness of negative electrode before charging) / (Thickness of negative electrode before charging−Thickness of copper foil)) × 100

[Cycle characteristics]
An evaluation battery different from the evaluation battery that evaluated the discharge capacity per mass, the rapid charge rate, and the rapid discharge rate was produced and evaluated as follows.
After performing 4.0 mA constant current charging until the circuit voltage reached 0 mV, switching to constant voltage charging was continued until the current value reached 20 μA, and then rested for 120 minutes. Next, constant current discharge was performed at a current value of 4.0 mA until the circuit voltage reached 1.5V. The charge / discharge was repeated 20 times, and the cycle characteristics were calculated from the obtained discharge capacity per mass using the following formula.
Cycle characteristics (%) = (discharge capacity in 20th cycle / discharge capacity in 1st cycle) × 100

(Example 2)
In Example 1, the obtained secondary particles were impregnated with coal tar pitch, and then heat-treated at 1000 ° C. to form a composite that is a spherical final product composed of scaly graphite particles, calcined carbon, and silicon particles. Got. Except for this, a composite was produced in the same manner as in Example 1, and an evaluation test of battery characteristics was performed.

(Example 3)
In the step of mixing the scaly graphite particles and the silicon particles in Example 2, graphite carbon fiber is further added (hereinafter abbreviated as fiber addition in the table), mixed with a biaxial kneader, and further, tar tar oil in coal tar pitch. The solution was added and kneaded. Next, a composite was produced from the kneaded product in the same manner as in Example 2, and an evaluation test of battery characteristics was performed.

Example 4
For the spray drying step in Example 2, prepared in advance a graphite carbon fiber dispersed in an aqueous solution of polyacrylic acid, and here, primary particles in which silicon particles are immobilized on scaly graphite particles are added, A dispersion was obtained. Except for this, a composite was produced in the same manner as in Example 2, and an evaluation test of battery characteristics was performed.

(Example 5)
A scaly graphite particle having an average particle size of 5 μm, silicon particles having an average particle size of 0.15 μm, and isopropyl alcohol as a solvent were placed in a bead mill and subjected to wet pulverization. The dispersion of the mixture and the water-soluble phenol resin were added to a biaxial kneader and heated to 100 ° C. while kneading to remove the solvent. Next, the kneaded product was heat-treated at 500 ° C. to obtain primary particles in which silicon particles were fixed on scaly graphite particles. Except for this, a composite of the final product was produced in the same manner as in Example 2, and a battery characteristic evaluation test was performed.

(Example 6)
The scaly graphite particles and silicon particles in Example 5 were subjected to wet pulverization treatment, and further added with graphitic carbon fiber to perform wet pulverization treatment. A composite was produced from the obtained wet pulverized product in the same manner as in Example 5, and an evaluation test of battery characteristics was performed.

(Example 7)
According to Example 5, spray drying was performed to obtain secondary particles. After adding and mixing coal tar pitch and graphitic carbon fibers to the obtained secondary particles, heat treatment is performed at 1000 ° C. to form a spherical final product composed of scaly graphite particles, calcined carbon, graphitic carbon fibers and silicon particles. A composite as a product was manufactured, and an evaluation test of battery characteristics was performed.

(Example 8)
About the process which mixes the scaly graphite particle | grains and silicon particle in Example 2, it replaced with the biaxial kneader and performed the mechanochemical process using the mechano fusion system (made by Hosokawa Micron Corporation). Thereafter, the mixture and a tar oil solution of coal tar pitch were added to a biaxial kneader and kneaded. Next, the kneaded product was heat-treated at 500 ° C. to obtain primary particles in which silicon particles were fixed on scaly graphite particles. Except for this, a composite was produced in the same manner as in Example 2, and an evaluation test of battery characteristics was performed.

Example 9
In the step of mixing the scaly graphite particles and silicon particles in Example 2, a graphitic carbon fiber was further added, and mechanochemical treatment was performed using a mechanofusion system. Except for this, a composite was produced in the same manner as in Example 2, and an evaluation test of battery characteristics was performed.

(Example 10)
According to Example 8, spray drying was performed to obtain secondary particles. After adding and mixing coal tar pitch and graphitic carbon fibers to the obtained secondary particles, heat treatment is performed at 1000 ° C. to form a spherical final product composed of scaly graphite particles, calcined carbon, graphitic carbon fibers and silicon particles. A composite as a product was manufactured, and an evaluation test of battery characteristics was performed.

(Comparative Example 1)
The scaly graphite particles having an average particle diameter of 5 μm and silicon particles having an average particle diameter of 0.15 μm were dispersed in an aqueous polyacrylic acid solution and spray-dried with a spray drying apparatus to obtain spherical particles. Next, using a planetary mixer, the spherical particles were impregnated with coal tar pitch, and then heat treated at 1000 ° C. to obtain a composite. Except for this, the battery characteristics were evaluated in the same manner as in Example 1.
From the SEM image and EDX-Si mapping image of the cross section shown in FIG. 3, it was found that silicon particles were segregated near the surface of the composite in the composite.

(Comparative Example 2)
The scaly graphite particles having an average particle diameter of 5 μm and silicon particles having an average particle diameter of 0.15 μm were mixed with a biaxial kneader, and further, a tar-in-oil solution of coal tar pitch was added and kneaded. Next, the kneaded product was heat-treated at 500 ° C. to obtain primary particles in which silicon particles were fixed on scaly graphite particles. A polyacrylic acid aqueous solution was added to the obtained primary particles and kneaded, followed by heat treatment at 1000 ° C. to obtain a composite. Except for this, the battery characteristics were evaluated in the same manner as in Example 1.

(Comparative Example 3)
A mechanochemical treatment was performed on flaky graphite particles having an average particle diameter of 5 μm and silicon particles having an average particle diameter of 0.15 μm using a mechanofusion system (manufactured by Hosokawa Micron Corporation). The mechanochemically treated product was dispersed in an aqueous polyacrylic acid solution and spray-dried with a spray dryer to obtain spherical particles. Next, using a planetary mixer, the spherical particles were impregnated with coal tar pitch, and then heat treated at 1000 ° C. to obtain a composite. Except for this, the battery characteristics were evaluated in the same manner as in Example 1.

  The above evaluation results are shown in Tables 1 to 3. From Examples 1-10, it turns out that the lithium ion secondary battery using the negative electrode material obtained by the manufacturing method of this invention has the high discharge capacity exceeding the theoretical capacity | capacitance of graphite. Moreover, from the comparison of Examples 1 to 10 and Comparative Examples 1 and 2, by using the negative electrode material obtained by the production method of the present invention, the initial charge and discharge efficiency is high, the expansion rate can be relaxed, and the cycle characteristics are excellent. I understand that it will be something. Furthermore, it was found that when mixing using wet pulverization and mechanochemical treatment were performed, the expansion coefficient could be further relaxed in this order and the cycle characteristics would be further improved.

  As a negative electrode material for a lithium ion secondary battery, the present invention can sufficiently relax expansion of a metal material during charging, and has a high discharge capacity exceeding the theoretical capacity of graphite, and excellent initial charge / discharge efficiency and cycle characteristics. A method for manufacturing a material is provided. Therefore, the lithium ion secondary battery using the negative electrode material according to the present invention satisfies the recent demand for higher energy density of the battery and is effective for downsizing and higher performance of the equipment to be mounted. The negative electrode material according to the present invention can be used for high-performance lithium ion secondary batteries ranging from small to large, taking advantage of the characteristics.

DESCRIPTION OF SYMBOLS 1 Exterior cup 2 Negative electrode mixture 3 Exterior can 4 Counter electrode 5 Separator 6 Insulation gasket 7a, 7b Current collector

Claims (6)

A method for producing a spherical or substantially spherical composite comprising scaly graphite particles, calcined carbon, and metal particles that can be alloyed with lithium,
1) a step of mixing the metal particles and the scaly graphite particles;
2) A step of adding a carbonaceous precursor to the mixture and then performing a heat treatment to obtain primary particles in which metal particles are immobilized on scaly graphite particles,
3) A step of dispersing the primary particles in a binder solution and performing spray drying to form spherical secondary particles; and
4) A method for producing a negative electrode material for a lithium ion secondary battery, comprising a step of heat-treating the secondary particles in a temperature range of 700 ° C. or more and 1200 ° C. or less and using the carbonaceous precursor and the binder as calcined carbon. .   The method for producing a negative electrode material for a lithium ion secondary battery according to claim 1, wherein the step of mixing the metal particles and the scaly graphite particles is a mechanochemical treatment.   The method for producing a negative electrode material for a lithium ion secondary battery according to claim 1 or 2, wherein the step of mixing the metal particles and the flaky graphite particles is a mechanical pulverization treatment.   4. The method for producing a negative electrode material for a lithium secondary battery according to claim 1, further comprising adding a graphitic carbon fiber in the step of mixing the metal particles and the scaly graphite particles. 5. .   4. The production of a negative electrode material for a lithium secondary battery according to claim 1, wherein a graphite carbon fiber is further added in the step of dispersing the primary particles in the binder solution. 5. Method. The method for producing a negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 5, wherein the heat treatment is performed after further adhering a carbonaceous precursor to the secondary particles.