JP2013254746A - Carbon material for lithium ion secondary battery negative electrode, and negative electrode mixture for lithium ion secondary battery and lithium ion secondary battery, each using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a carbon material used for a negative electrode of a lithium ion secondary battery, having high capacity, excellent in cycle characteristic, having a high level of adhesiveness between a negative electrode agent and a current collector, and capable of maintaining rapid charge and discharge characteristics even after a number of cycles; and a negative electrode mixture for the lithium ion secondary battery and the lithium ion secondary battery, each using the same.SOLUTION: A carbon material used for a negative electrode of a lithium ion secondary battery has an average grain diameter of 10-50 μm, a 30-tap density of 0.5 g/cmor more, and a specific surface area of 7 m/g or less; the product of the 30-tap density and the specific surface area is 30000/cm or more and 60000/cm or less. An interlayer distance d (002) of graphite crystal determined by X-ray diffraction measurement is 3.400 Å or less. Further, there are provided a lithium ion secondary battery negative electrode and a lithium ion secondary battery, each using the carbon material.

Description

  The present invention relates to a carbon material for a negative electrode of a lithium ion secondary battery, a negative electrode mixture for a lithium ion secondary battery using the same, and a lithium ion secondary battery. More specifically, the high capacity suitable for applications such as portable equipment, electric vehicles and electric storage, excellent rapid charge / discharge characteristics and cycleability, high adhesion between the negative electrode agent and the current collector, and after repeated cycles The present invention also relates to a carbon material for a negative electrode of a lithium ion secondary battery that maintains excellent rapid charge / discharge characteristics, a negative electrode mixture for a lithium ion secondary battery and a lithium ion secondary battery using the same.

Lithium secondary batteries were initially investigated as negative electrodes because of their excellent voltage and capacity, but their cycle characteristics were poor due to the growth of needle crystals (dendrites). On the other hand, if a carbon-based material is used for the negative electrode and an oxide such as LiCoO ₂ is used for the positive electrode and lithium ions are intercalated / deintercalated between these electrodes, the problem is solved and the cycle characteristics are improved. As can be seen, first, amorphous carbon that is not easily affected by the type of electrolyte is used as a carbon material for the negative electrode, and a lithium ion secondary battery using the carbon is known (for example, see Patent Document 1). Thereafter, an electrolyte solution having no problem even in combination with graphite was found, and a graphite material having a higher capacity density was used as a carbon material instead of amorphous carbon (for example, see Patent Document 2).

  Natural graphite was used as the graphite material, but its shape is mainly scale-like by grinding. In order to use carbon powder as a negative electrode, it is mixed with an organic binder and a solvent or water, applied to a current collector, dried with a solvent or water, molded, and used as a negative electrode. There is a strong tendency to orient so that the flat surface (basal surface) faces the surface of the current collector. Lithium ion intercalation / deintercalation occurs not on the basal plane but mainly on the edge plane, so that scaly graphite oriented in the plane direction of the aggregate has a problem of low rapid charge / discharge characteristics.

  As a method for solving this problem, artificial graphite obtained by firing coke, pitch, or an organic material at 2000 ° C. or higher is used as a carbon material. For example, artificial graphite is a carbon particle assembled or bonded so that the orientation plane of the original flat particle is non-parallel in the surface and cross section of the carbon particle in the carbon particle completed using a plurality of flat particles, Due to its excellent characteristics, it is widely adopted in lithium ion secondary batteries (see, for example, Patent Document 3).

  However, in these artificial graphites, graphite crystals have been developed to increase the charge / discharge capacity. Therefore, when the number of cycles is repeated, the graphite particles expand due to lithium ion intercalation / deintercalation. At present, there is a problem that peeling occurs between the current collectors and the rapid charge / discharge characteristics after the number of cycles is deteriorated.

Japanese Patent Publication No. 2865777 Japanese Examined Patent Publication No. 62-23433 Japanese Patent No. 3213575

  The present invention overcomes the above-mentioned problems of the prior art. Specifically, it has a high capacity and excellent cycleability, and has a high adhesion between the negative electrode agent and the current collector, so that rapid charge / discharge even after repeated cycles. An object of the present invention is to provide a carbon material used for a negative electrode of a lithium ion secondary battery capable of maintaining the characteristics.

As a result of intensive studies to solve the above-mentioned problems, the present inventors can increase the discharge capacity by setting the carbon material to a specific average particle diameter and a specific interlayer distance d (002) of a specific graphite crystal, When the number of cycles is repeated in a lithium ion secondary battery, the adhesive problem between the two, which causes separation between the negative electrode agent and the current collector, has a strong correlation with the product of the tap density and specific surface area of the carbon material. I found out.
That is, the present invention is as follows.
In the present invention, (1) the average particle size is 10 to 50 μm, the 30-time tap density is 0.5 g / cm ³ or more, the specific surface area is 7 m ² / g or less, and the product of the 30-time tap density and the specific surface area is 30000 / More specifically, the present invention relates to a carbon material for a negative electrode of a lithium ion secondary battery, which has a distance of d (002) between graphite crystals of 3.400 mm or less, which is from cm to 60000 / cm.

In addition, the present invention provides (2) a carbon material for a negative electrode of a lithium ion secondary battery, which is a mixed powder of two or more types of carbon particles having different average particle diameters, 30 times tap density and specific surface area,
At least carbon particles A, the carbon particles A are carbon particles in which a plurality of flat particles are assembled or bonded, and the orientation surfaces of the flat particles are not on the surface or cross section of the carbon particles combined or bonded. It is related with the carbon material for lithium ion secondary battery negative electrodes of the said (1) description which is parallel and whose aspect ratio is 1.2-5.
Moreover, this invention relates to the carbon material for lithium ion secondary battery negative electrodes of said (2) whose ratio with respect to the whole carbon material (3) said carbon particle A is 15-65 mass%.

Moreover, this invention contains the carbon material and organic binder as described in any one of (4) said (1)-(3), and the density is 1.5-1.9 g / cm. ^{The present invention} relates to a negative electrode mixture for lithium ion secondary battery ³ .
The present invention also relates to a lithium ion secondary battery having (5) the negative electrode for a lithium ion secondary battery of (4) and a positive electrode containing a lithium compound.

  According to the present invention, high capacity, excellent cycleability, high adhesion between the negative electrode agent and the current collector, and suitable for a negative electrode for a lithium ion secondary battery that maintains rapid charge / discharge characteristics even after repeated cycles. Carbon material can be provided.

It is a figure which shows the relationship between the ratio of 30 times tap density and a specific surface area, and adhesive force. The SEM photograph of the surface and cross section of the carbon particle A is shown. It is the schematic which shows an example of the lithium secondary battery of this invention.

The carbon material for a lithium ion secondary battery negative electrode in the present invention has an average particle size of 10 to 50 μm, a 30-time tap density of 0.5 g / cm ³ or more, a specific surface area of 7 m ² / g or less, and a 30-time tap density and ratio. The product with the surface area is 30000 / cm or more and 60000 / cm or less, and the interlayer distance d (002) of the graphite crystal obtained by X-ray diffraction measurement is 3.400 mm or less.

The 30-time tap density of the carbon material for a lithium ion secondary battery negative electrode in the present invention is preferably 0.5 g / cm ³ or more. If the 30-time tap density of the carbon material is less than 0.5 g / cm ³ , when used for the negative electrode of a lithium ion secondary battery, more binder tends to be required, and as a result, the lithium ion secondary The energy density of the battery tends to decrease. Although there is no restriction | limiting in particular in the upper limit of a 30 times tap density, Usually, it shall be 1.2 g / cm < ³ > or less. Tapping density is measured 30 times. The sample volume is divided by the sample volume after dropping the measuring cylinder 30 times from the height of 5 cm after plugging the measuring sample about 100 cm ³ into a measuring cylinder of 100 cm ³ capacity. Is calculated.
In order to set the 30-time tap density of the carbon material to 0.5 g / cm ³ or more, a method of adjusting the particle diameter using a pulverizer or a sieve can be used.

In addition, the specific surface area of the carbon material for a lithium ion secondary battery negative electrode in the present invention is preferably 7 m ² / g or less, more preferably 6 m ² / g or less, and further preferably ² to 6 m ² / g. is there. When the specific surface area exceeds 7 m ² / g, the irreversible capacity at the first cycle of the obtained lithium ion secondary battery tends to increase and the energy density of the battery tends to decrease. When the specific surface area is smaller than 2 m ² / g, the cycle characteristics of the lithium ion secondary battery tend to be lowered. The specific surface area of the carbon material can be measured by a known method such as the BET method (BET 5-point method by nitrogen gas adsorption).
In order to set the specific surface area of the carbon material to 7 m ² / g or less, a method of mechanically adjusting the particle diameter by surface modification treatment, pulverization or the like can be mentioned. For example, when the particle size is decreased, the specific surface area tends to increase, and when the particle size is increased, the specific surface area tends to decrease.

The product of the 30-time tap density and the specific surface area of the carbon material for a lithium ion secondary battery negative electrode in the present invention is preferably 30000 / cm or more and 60000 / cm or less, and preferably 40000 / cm or more and 60000 / cm or less. More preferred. The problem that peeling occurs between the negative electrode agent and the current collector when the number of cycles is repeated in a lithium ion secondary battery can be attributed to the low adhesiveness between the two. Therefore, the relationship between the adhesion between the negative electrode agent and the current collector and the carbon material was examined as follows.
First, several types of carbon materials having a product of 30 times tap density and specific surface area of 20000 to 70000 were prepared. 1 part by mass of SBR (manufactured by Nippon Zeon Co., Ltd., BM-400B), 1 part by mass of CMC (manufactured by Daicel Chemical Industries, Ltd., CMC2200) and 103 parts by mass of water with respect to 98 parts by mass of each carbon material Each of the slurries of the ratio was prepared.

This slurry was applied to one side of a rolled copper foil having a thickness of 11 μm and then dried at 120 ° C. for 1 hour. After drying, the electrode was pressure-formed by a roll press to a thickness of 86 μm. The coating amount of the negative electrode agent per unit area was 12 mg / cm ² , the negative electrode agent density was 1.6 g / cm ³ , and a test piece for measuring adhesiveness was cut into a size of 10 mm in width and 55 mm in length. A test in which the test piece is bent 90 ° inside the copper foil near the center in the length direction, one negative electrode surface is fixed with double-sided tape, the other end is connected to a stress detector, and the test piece is pulled at a predetermined speed Was performed with each carbon material, and the stress at which peeling occurred between the negative electrode agent and the current collector was defined as the adhesive strength. As a result, as shown in FIG. 1, it can be seen that the product of the 30-time tap density and the specific surface area of the carbon material for the negative electrode of the lithium ion secondary battery has a very strong correlation (correlation coefficient: 0.889) with the adhesiveness. It was issued. When the product of the 30-time tap density and the specific surface area is smaller than 30000 / cm, the adhesiveness between the negative electrode agent and the current collector tends to be small and peeling tends to occur. When the product of the 30 times tap density and the specific surface area is larger than 60000 / cm, the adhesiveness is excellent, but the specific surface area is substantially more than 7 m ² / g. It was found that the irreversible capacity in the first cycle of the secondary battery tends to increase and the energy density of the battery tends to decrease, which is not preferable.
In order for the product of the 30-time tap density and the specific surface area of the carbon material to be 30000 / cm or more and 60000 / cm or less, an appropriate 30-time tap density and an appropriate specific surface area value are set so that the product is within the range. What is necessary is just to select the carbon material to have suitably.

The carbon material for a lithium ion secondary battery negative electrode in the present invention may be a mixed powder of two or more carbon particles having different average particle diameter, 30-time tap density, and specific surface area, at least one of which is It is preferable that it is the carbon particle A mentioned later.
The carbon particles A are carbon particles in which a plurality of flat particles are assembled or bonded, and the orientation planes of the flat particles are non-parallel in the surface and cross section of the carbon particles bonded or bonded, Carbon particles having an aspect ratio of 1.2 to 5 are meant.
Since the carbon particles A have an aspect ratio of 1.2 to 5 and tend to be difficult to be oriented in the surface direction of the current collector, the carbon material including the carbon material A is also difficult to be oriented. It becomes easy to discharge and quick charge / discharge characteristics are improved.

When using the carbon material containing the said carbon particle A, it is preferable that the said carbon particle A is 15-65 mass% with respect to the whole carbon material for lithium ion secondary battery negative electrodes, and is 20-65 mass%. Is more preferable. If it is less than 15% by mass, the rapid charge / discharge characteristics tend to deteriorate. When the mass exceeds 65 mass%, generally, the aggregated or bonded carbon particles tend to have a product of a 30-times tap density and a specific surface area, so that the 30-times tap density of the entire carbon material for a lithium ion secondary battery negative electrode The product with the specific surface area also becomes small, and the adhesion between the negative electrode material using the carbon material having the carbon particles A and the current collector tends to decrease.
When using the carbon material containing the carbon particles A, the 30-times tap density of the carbon particles A and other carbon particles is not particularly limited as long as the carbon material is 0.5 g / cm ³ or more.
When the carbon material A is used, there is no particular limitation as long as the average particle size of the carbon particles A and other carbon particles is 10 to 50 μm in the entire carbon material.
When the carbon material A is used, the specific surface area of the carbon particles A and other carbon particles is not particularly limited as long as the total carbon material is 7 m ² / g or less.
Moreover, when using the carbon material containing the said carbon particle A, there will be no restriction | limiting in particular if the product of the 30 times tap density and specific surface area of the said carbon particle A and other carbon particles will be 30000-60000 / cm.

  The carbon particles A are produced, for example, by adding 1 to 50% by mass of a graphitization catalyst to a mixture of graphitizable aggregate or graphite and a carbonizable binder, mixing, firing, and pulverizing. it can. The manufacturing method will be described later.

As the aggregate capable of graphitization, for example, coke powder and resin carbide are used, but other graphitizable powder materials may be used. Preferable materials include easily graphitized coke powder such as needle coke. Moreover, as said graphite, natural graphite powder and artificial graphite powder can be utilized, and if it is a powder form, there will be no restriction | limiting in particular.
The average particle size of graphitizable aggregate or graphite mixed with the binder is preferably smaller than the average particle size of the completed carbon material A, and more preferably 2/3 or less.

  Examples of the carbonizable binder include tar, pitch, thermosetting resin, and thermoplastic resin. A preferable blending amount of the binder is 5 to 80 parts by mass, more preferably 10 to 80 parts by mass, and further preferably 15 to 80 parts by mass with respect to 100 parts by mass of the graphitizable aggregate or graphite. . When the blending amount of the binder is less than 5 parts by mass or exceeds 80 parts by mass, the aspect ratio and specific surface area of the produced carbon particles A tend to be too large. The aspect ratio will be described later.

  As the graphitization catalyst, for example, metals such as iron, nickel, titanium, silicon and boron, or carbides and oxides of these metals can be used. Of these, silicon and boron carbides or oxides are preferred. The addition amount of these graphitization catalysts is preferably 1 to 50% by mass, more preferably 3 to 40% by mass, and further preferably 5 to 35% by mass with respect to the total amount of carbon particles A to be obtained. If the addition amount of the graphitization catalyst is less than 1% by mass, the crystallinity of the carbon particles A tends to be low, and if it exceeds 50% by mass, it becomes difficult to uniformly mix with other materials, and the quality of the carbon particles A is improved. There is a tendency that variations easily occur.

  There are no particular limitations on the method of mixing the graphitizable aggregate or graphite and binder to obtain the carbon particles A, and a mixture is obtained using a kneader or the like, but may be mixed at a temperature above the softening point of the binder. preferable. Specifically, when the binder is pitch or tar, 50 to 300 ° C is preferable, and when the binder is a thermosetting resin, 20 to 120 ° C is preferable.

  Next, the mixture is fired and graphitized. In addition, you may grind | pulverize and shape | mold the said mixture before this baking process, and you may pre-fire at the temperature of about 700-1300 degreeC after the shaping | molding. The graphitization is preferably performed under conditions where the mixture does not easily oxidize. Examples thereof include a method in which the baking atmosphere is nitrogen, argon, or vacuum. The graphitization temperature is preferably 2200 ° C. or more, more preferably 2500 ° C. or more, and further preferably 2800 to 3200 ° C. When the graphitization temperature is less than 2200 ° C., the graphite crystals are not sufficiently developed, and the charge / discharge capacity tends to be low. When the graphitization temperature is higher than 3200 ° C., the tendency of the graphite to sublimate becomes strong.

  The pulverization of the graphite after firing for obtaining the carbon particles A is not particularly limited as long as the average particle size, 30-time tap density, and specific surface area can be adjusted within the scope of the present invention. Known apparatuses such as a mill, a pin mill and a hammer mill can be used.

  The carbon particle A in the present invention is a carbon particle in which a plurality of flat particles are assembled or bonded as described above, and the orientation plane of the flat particles is not on the surface and cross section of the collected or bonded carbon particles. Carbon particles that are aggregated or bonded so as to be parallel and have an aspect ratio of 1.2 to 5 are preferable.

  In the present invention, flat particles are particles having a major axis and a minor axis, and are not completely spherical. For example, those having a shape such as a scale shape, a scale shape, or a part of a lump shape are included. Specifically, artificial graphite or scaly graphite obtained by graphitizing the pulverized coke powder can be used, but other powder materials may be used as the flat particles as long as they are flat primary particles. In addition, the flat particle | grains in this invention say the particle | grain unit recognized that the carbon particle A is comprised when the carbon particle A is observed with a scanning electron microscope (SEM) etc., for example.

In the carbon particles A, the orientation planes of the plurality of flat particles are non-parallel in the surface and cross section of the carbon particles A. The flat surfaces in the shape of the respective flat particles, in other words, the planes closest to the flat are oriented. A surface refers to a state in which a plurality of flat particles are gathered without aligning their respective orientation surfaces in a certain direction.
Confirmation of “a state in which a plurality of flat particles are gathered without aligning their orientation planes in a certain direction” is performed by observing a cross section of the particles with a scanning electron microscope (SEM) photograph. An example of the surface of the carbon particle A and a cross-sectional SEM photograph of the carbon particle A is shown in FIG.

In the carbon particles A, the flat particles are aggregated or bonded, and the bonding means that the particles are carbonized by carbonizing a binder such as tar, pitch, thermosetting resin, and thermoplastic resin. , Refers to a chemically bonded state. The term “aggregate” refers to a state in which the particles are not chemically bonded but the shape of the aggregate is maintained due to the shape or the like. From the viewpoint of mechanical strength, those bonded are preferable.
In one carbon particle A, the number of flat particles aggregated or bonded is preferably 3 or more. The size of each flat particle is preferably 1 to 100 μm in terms of average particle diameter, and preferably 2/3 or less of the average particle diameter of the carbon particles in which they are aggregated or bonded.

  When the carbon particles A are used as a carbon material for the negative electrode, the carbon material is difficult to be oriented in the surface direction of the current collector, the wettability with the electrolytic solution is improved, and lithium ions are easily occluded / released into the negative electrode graphite. Therefore, the rapid charge / discharge characteristics and cycle characteristics of the obtained lithium ion secondary battery can be improved.

  In addition, the carbon particles A having an aspect ratio of more than 5 tend to be easily oriented in the surface direction of the current collector, and have a tendency to hardly occlude / release lithium ions. When the aspect ratio is less than 1.2, the contact area between the carbon particles decreases, and the conductivity tends to decrease. A particularly preferred aspect ratio is 1.3 to 3.

The aspect ratio is represented by a / b, where a is the length in the major axis direction of the carbon particles A and b is the length in the minor axis direction. The aspect ratio in the present invention is obtained by enlarging the carbon particles A with an electron microscope, arbitrarily selecting 20 carbon particles A, measuring a / b, and taking the average value.
Here, when the major axis and the minor axis of the carbon particle A are determined, the carbon particle A is enlarged by a scanning electron microscope (SEM), and the carbon particle A is observed from various directions, and the three-dimensional of the carbon particle A is observed. In consideration of these characteristics, the length of the carbon particles A in the major axis direction is determined as a, and the length in the minor axis direction is determined as b.
For example, when the carbon particle A is approximately spherical, such as spherical, spherical, or massive, the longest part of the carbon particle A projected in the two-dimensional field of view in the SEM image is the major axis. Let a be the longest axis perpendicular to the major axis be the minor axis b.
Further, when the carbon particle A is thin, flat, and has a thickness direction such as a scale shape, a plate shape, or a block shape, the short axis b is the thickness of the particle. Further, in the case of carbon particles A such as rod-like or needle-like, the major axis a is the length of the carbon particles, and the minor axis b is the thickness of the rod-like (or needle-like) carbon particles. Further, for example, when the shape of the carbon particle A is changed by applying mechanical force or the like, the carbon particle is observed from various directions, and the carbon particle A is approximated by considering the three-dimensional characteristics of the carbon particle. After determining the shape of the particle A, the values of the major axis a and the major axis b are determined as described above.
By the manufacturing method, the aspect ratio of the carbon particles A is in the range of 1.2 to 5.

  The average particle diameter of the carbon material for a lithium ion secondary battery negative electrode in the present invention is preferably 10 to 50 μm, and more preferably 10 to 40 μm. When preparing a negative electrode for a lithium ion secondary battery, first, a carbon material is mixed with an organic binder and a solvent or water to prepare a slurry. If the average particle size is too small, the viscosity of the slurry is reduced. It tends to be high and handling becomes inconvenient. On the other hand, if the average particle size exceeds 50 μm, the surface of the negative electrode of the lithium ion secondary battery to be produced tends to be uneven, such as streaking. In the present invention, the average particle size of the carbon material is determined as 50% D measured with a laser diffraction particle size distribution meter. In addition, what is necessary is just to obtain the particle size of a desired magnitude | size using a grinder and a sieve, in order to adjust the average particle diameter of a carbon material.

In the carbon material for a negative electrode of a lithium ion secondary battery in the present invention, the interlayer distance d (002) of the graphite crystal is preferably 3.400 mm or less, more preferably 3.380 mm or less, and 3.370 mm or less. More preferably it is. When the interlayer distance d (002) of the graphite crystal exceeds 3.400 mm, the discharge capacity tends to be small.
The measurement of d (002) of the carbon material of the present invention can be obtained by wide angle X-ray diffraction measurement by the Gakushin method. Specifically, the carbon material is irradiated with X-rays (CuKα-rays), and the diffraction profile obtained by measuring the diffraction lines with a goniometer shows that the carbon d (002) plane appears at a diffraction angle 2θ = 24 to 26 °. Calculate from the corresponding diffraction peak using the Bragg equation.
In order to set the interlayer distance d (002) to 3.400 mm or less, the crystallinity of the carbon particles and graphite particles used may be increased. In order to increase the crystallinity, a method in which heat treatment is performed at a high temperature of 2200 ° C. or higher can be used.
The crystallite size Lc (002) in the C-axis direction of the carbon material crystal is preferably 500 liters or more from the viewpoint of increasing the discharge capacity, more preferably 700 liters or more, and even more preferably 1000 liters or more. In order to set the crystallite size Lc (002) to 500 mm or more, the crystallinity of the carbon material to be used may be increased.
The true specific gravity of the carbon material of the present invention is preferably 2.1 or more, more preferably 2.2 or more. The true specific gravity of the carbon material of the present invention can be determined by a butanol substitution method. In order to set the true specific gravity to 2.1 or higher, the crystallinity of the carbon material to be used may be increased.
The carbon material for the negative electrode of the lithium ion secondary battery of the present invention is preferably high in crystallinity in terms of discharge capacity. When the crystallinity is low, Lc (002) and true specific gravity tend to be low.

  In the present invention, when two or more kinds of carbon particles having different average particle diameter, 30 times tap density and specific surface area are used as the carbon material, examples of the carbon particles used in addition to the carbon material A include artificial graphite, Graphite particles such as scaly graphite and scaly graphite are used. Preferably, spheroidized natural graphite obtained by processing natural graphite into a spherical shape is used.

The carbon material for a negative electrode of a lithium ion secondary battery in the present invention is mixed with an organic binder and a solvent or water, applied to a current collector, dried on the solvent or water, and pressure-molded to form a lithium secondary battery. The negative electrode mixture, which is a mixture of a carbon material and an organic binder, preferably has a density of 1.5 to 1.9 g / cm ³ . By setting the density of the negative electrode mixture to 1.5 g / cm ³ or more, the energy density per volume of the lithium ion secondary battery obtained using this negative electrode can be increased. On the other hand, when it exceeds 1.9 g / cm ³ , not only does the pouring property of the electrolytic solution when producing a lithium ion secondary battery worsen, but the rapid charge / discharge characteristics and cycle characteristics of the produced lithium ion secondary battery are also reduced. It tends to decrease.
The density of the negative electrode mixture can be calculated from the measured values of the mass and volume of the molded mixture layer (negative electrode layer).
The density of the negative electrode mixture is appropriately adjusted by applying a mixture of a carbon material and an organic binder to the current collector and pressurizing the solvent after drying, the clearance of an apparatus such as a roll press, etc. be able to.

  Examples of the organic binder include polymer compounds such as polyethylene, polypropylene, ethylene propylene rubber, butadiene rubber, styrene butadiene rubber, carboxymethyl cellulose, polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, and polyacrylonitrile. The mixing ratio of the carbon material and the organic binder is preferably 0.5 to 20 parts by mass of the organic binder with respect to 100 parts by mass of the carbon material.

  The solvent used for mixing the carbon material and the organic binder is not particularly limited, and N-methylpyrrolidone, dimethylacetamide, dimethylformamide, γ-butyrolactone and the like are used.

  The carbon material is kneaded with an organic binder and a solvent or water to adjust the viscosity, and then applied to a current collector, dried, and pressure-molded to form a negative electrode. As the current collector, for example, a foil or mesh of nickel, copper or the like can be used.

In order to obtain the lithium ion secondary battery of the present invention using the obtained negative electrode, for example, the positive electrode containing a lithium compound is placed opposite to the separator, and an electrolytic solution is injected. The positive electrode containing a lithium compound, for _example, can be used alone or as a mixture of _{LiNiO 2, LiCoO 2, LiMn 2} O 4 or the like. The positive electrode can form a positive electrode layer on the current collector surface in the same manner as the negative electrode.
Further, the electrolytic solution is, for example, a lithium salt such as LiClO ₄ , LiPF ₄ , LiAsF, LiBF ₄ , LiSO ₃ CF ₄ dissolved in, for example, ethylene carbonate, diethyl carbonate, dimethoxyethane, dimethyl carbonate, methyl ethyl carbonate tetrahydrofuran, etc. Can be used. Also, a solid or gel so-called polymer electrolyte can be used in place of the electrolytic solution.

  As the separator, for example, a nonwoven fabric, a cloth, a microporous film, or a combination thereof having a polyolefin as a main component such as polyethylene or polypropylene can be used. In addition, when it is set as the structure where the positive electrode and negative electrode of the lithium ion secondary battery to produce are not directly contacted, it is not necessary to use a separator.

  FIG. 3 shows a partially sectional front view of one embodiment of the cylindrical lithium ion secondary battery of the present invention. A thin plate-like positive electrode 1 and a similarly processed negative electrode 2 are stacked with a separator 3 such as a polyethylene microporous membrane, and this is inserted into a battery can 7 made of metal or the like. It is sealed. The positive electrode 1 is bonded to the positive electrode lid 6 via the positive electrode tab 4, and the negative electrode 2 is bonded to the battery bottom via the negative electrode tab 5. The positive electrode lid 6 is fixed to the battery can 7 with a gasket 8.

Hereinafter, the present invention will be described with reference to examples.
Example 1
Coke powder 50 parts by mass as flat particles having an average particle size of 12 μm, pitch 15 parts by mass and coal tar 20 parts by mass, and silicon carbide 10 parts by mass as a graphitization catalyst at 230 ° C. Kneaded for 1 hour. Next, the mixture was pulverized to an average particle size of 25 μm, and the pulverized product was put into a mold and press-molded to obtain a rectangular parallelepiped. This molded body was heat treated at 1000 ° C., and further heat treated at 3000 ° C. to obtain a graphite molded body. This graphite molded body was made into carbon particles A1 by pulverization. Table 1 shows the measurement results of the average particle diameter, 30-time tap density, specific surface area, and aspect ratio of the carbon particles A1.
Confirmation that the individual flat particles are non-parallel on the surface and cross section of the carbon particle A1 was performed using a scanning electron microscope (SEM) photograph.
The average particle size was determined with a laser diffraction particle size analyzer. The specific surface area was determined by the BET 5-point method by nitrogen gas adsorption. The aspect ratio was calculated by enlarging the obtained carbon particles A1 with an electron microscope, arbitrarily selecting and measuring 20 particles.
Carbon particles A1 and graphite particles a obtained by processing flaky natural graphite into a spherical shape (average particle size: 19.4 μm, 30-time tap density: 0.84 g / cm ² , specific surface area: 7.6 m ² / g, aspect ratio : 1.8, d002: 3.359) at a mass ratio of 6/4 to obtain a carbon material 1. Table 1 shows the measurement results of the average particle diameter, 30-time tap density, and specific surface area of graphite particles a obtained by processing natural graphite into a spherical shape.
Table 2 shows the average particle size, 30-time tap density, specific surface area, measurement result of d (002), and the product of the 30-time tap density and the specific surface area of the carbon material 1.

Next, a lithium ion secondary battery was produced using the obtained carbon material 1. The lithium ion secondary battery of the present invention shown in FIG. 3 was produced as follows.
Using 88% by mass of LiCoO ₂ as the positive electrode active material, 7% by mass of flaky natural graphite having an average particle diameter of 1 μm as the conductive agent, and 5% by mass of polyvinylidene fluoride (PVDF) as the binder, N-methyl-2-pyrrolidone was added and mixed to prepare a slurry of the positive electrode agent. The positive electrode agent was applied to both surfaces of an aluminum foil having a thickness of 20 μm, and then dried at 120 ° C. for 1 hour. After drying, the electrode was pressure-formed by a roll press to a thickness of 181 μm. The coating amount of the positive electrode agent per unit area was 29 mg / cm ² , and the positive electrode was produced by cutting it into a size of 55 mm in width and 600 mm in length. However, the positive electrode agent was not applied to the 10 mm long portions at both ends of the positive electrode, and the aluminum foil was exposed, and a positive electrode tab was welded to this one.
On the other hand, the negative electrode for evaluation was produced using the carbon material 1 which mixed carbon particle A1 and the graphite particle a which made natural graphite spherical with the mass ratio 6/4 as a negative electrode active material. First, 98 parts by mass of carbon material 1, 1 part by mass of SBR (manufactured by Zeon Corporation, BM-400B), 1 part by mass of CMC (manufactured by Daicel Chemical Industries, Ltd., CMC2200) and a viscosity modifier A slurry having a ratio of 103 parts by mass of water was prepared. This slurry was applied to both sides of a rolled copper foil having a thickness of 11 μm, and then dried at 120 ° C. for 1 hour. After drying, the electrode was pressure-formed by a roll press to a thickness of 152 μm. The coating amount of the negative electrode agent mixture per unit area was 12 mg / cm ² , the negative electrode mixture density was 1.7 g / cm ² , and the negative electrode was produced by cutting into a size of 55 mm in width and 600 mm in length. Similar to the positive electrode, the negative electrode agent was not applied to the 10 mm long portions at both ends of the negative electrode, and the copper foil was exposed, and a negative electrode tab was welded to this one.

As the separator, a microporous membrane made of polyethylene having a thickness of 25 μm and a width of 58 mm was used. The positive electrode, the separator, the negative electrode, and the separator were stacked in this order, and this was wound to form an electrode group. This was inserted into an 18650 cylindrical battery can, and the negative electrode tab was welded to the bottom of the can to provide a throttle for caulking the positive electrode lid. After injecting into the battery can an electrolytic solution in which 1 mol / liter of lithium hexafluorophosphate was dissolved in a mixed solvent of ethylene carbonate and dimethyl carbonate having a volume ratio of 1: 3, the positive electrode tab was welded to the positive electrode lid, A positive electrode lid was caulked to prepare a battery.
<Measurement of charge / discharge characteristics>
(1) Initial charge / discharge efficiency Using the above battery, charge / discharge was repeated with a charge / discharge current of 850 mA and a charge / discharge end voltage of 4.2 V and 2.8 V, respectively. The initial charge / discharge efficiency was examined from the charge capacity and discharge capacity of the first cycle.
(2) Second cycle discharge capacity and 300th cycle discharge capacity retention rate Using the above battery, charge / discharge was repeated with a charge / discharge current of 850 mA and a charge / discharge end voltage of 4.2 V and 2.8 V, respectively. The discharge capacity at the second cycle and the discharge capacity maintenance rate at the 300th cycle when this was taken as 100% were examined.
(3) Discharge capacity maintenance rate at 2550 mA The rapid charge / discharge performance was evaluated by increasing the charge / discharge current from 850 mA to 2550 mA and examining the discharge maintenance rate when the discharge capacity at the second cycle was 100%.
(4) The discharge capacity maintenance rate at 2550 mA after 300 cycles The charge / discharge current was increased from 850 mA to 2550 mA after 300 cycles, and the number of cycles was repeated by evaluating the discharge maintenance rate when the discharge capacity at the 300th cycle was 100%. After that, the rapid charge / discharge characteristics were examined. The results are shown in Table 2.

(Example 2)
The carbon material 2 was obtained in the same manner as in Example 1 except that the mass ratio of the carbon particles A1 of the carbon material 1 in Example 1 to the graphite particles a in which the scaly natural graphite was made spherical was changed to 3/7. A secondary battery was produced and evaluated. Table 2 shows the measurement results of the average particle size, 30-time tap density, specific surface area, and d (002) of the carbon material 2, the product of the specific surface area and the 30-time tap density, and the evaluation results of the lithium ion secondary battery.

(Example 3)
Carbon material 1 in Example 1 was obtained by converting carbon particles A1 and scaly natural graphite into spherical shapes (b) (average particle size: 22.1 μm, 30 times tap density: 0.97 g / cm ² , specific surface area: 5 .1 m ² / g, aspect ratio: 1.6, d002: 3.361) The lithium ion secondary battery was fabricated in the same manner as in Example 1 except that the carbon material 3 was obtained with a mass ratio of 3/7. Evaluation was performed. Table 1 shows the measurement results of the average particle diameter, 30-time tap density, and specific surface area of graphite particles b obtained by sphering natural graphite. Table 2 shows the average particle diameter, 30-time tap density, specific surface area, d (002) measurement result of carbon material 3, the product of 30-time tap density and specific surface area, and the evaluation results of the lithium ion secondary battery.

Example 4
50 parts by mass of coke powder having an average particle diameter of 14 μm as an aggregate capable of graphitization, 15 parts by mass of pitch as a binder and 20 parts by mass of coal tar, and 7 parts by mass of silicon carbide as a graphitization catalyst are obtained at 230 ° C. Kneaded for hours. Next, the mixture was pulverized to an average particle size of 25 μm, and the pulverized product was put into a mold and press-molded to obtain a rectangular parallelepiped. This molded body was heat treated at 1000 ° C., and further heat treated at 3000 ° C. to obtain a graphite molded body. The graphite molded body was made into carbon particles A2 by pulverization. Table 1 shows the measurement results of the average particle diameter, 30-time tap density, specific surface area, and aspect ratio of the carbon particles A2.
Carbon material 4 was obtained by mixing carbon particles A2 and graphite particles a obtained by processing flaky natural graphite into a spherical shape at a mass ratio of 5/5. Table 2 shows the average particle size, 30-time tap density, specific surface area, measurement result of d (002), and the product of the 30-time tap density and the specific surface area of the carbon material 4.
The production and evaluation of the lithium ion secondary battery were performed in the same manner as in Example 1. The evaluation results are shown in Table 2.

(Example 5)
Lithium ion 2 was obtained in the same manner as in Example 1 except that the carbon material 1 in Example 1 was obtained by changing the mass ratio of the graphite particles a obtained by processing the carbon particles A2 and the scaly natural graphite into a spherical shape to 2/8. A secondary battery was produced and evaluated. Table 2 shows the average particle size, 30-time tap density, specific surface area, d (002) measurement result of carbon material 5, the product of 30-time tap density and specific surface area, and the evaluation results of the lithium ion secondary battery.

(Comparative Example 1)
A lithium ion secondary battery was prepared and evaluated in the same manner as in Example 1 except that the carbon particles A1 were used instead of the carbon material 1 in Example 1. Table 2 shows the evaluation results of the lithium ion secondary battery.

(Comparative Example 2)
A lithium ion secondary battery was prepared and evaluated in the same manner as in Example 1 except that graphite particles a obtained by processing flaky natural graphite into a spherical shape instead of the carbon material 1 in Example 1 were used. Table 2 shows the evaluation results of the lithium ion secondary battery.

(Comparative Example 3)
The carbon material 1 in Example 1 was obtained by converting the carbon particles A1 and flaky natural graphite into spherical graphite particles c (average particle size: 19.5 μm, 30 times tap density: 0.78 g / cm ² , specific surface area: 5. 0m ² / g, aspect ratio: 2.0, d002: 3372) The mass ratio was 3/7, and a carbon material 6 was obtained in the same manner as in Example 1 except that a lithium ion secondary battery was produced and evaluated. It was. Table 1 shows the measurement results of the average particle diameter, 30-time tap density, and specific surface area of the graphite particles c obtained by spheroidizing natural graphite. Table 2 shows the average particle size, 30-time tap density, specific surface area, measurement result of d (002), the product of the 30-time tap density and the specific surface area, and the evaluation results of the lithium ion secondary battery.

As shown in Table 2, the carbon material of the present invention has a high capacity, excellent rapid charge / discharge characteristics and cycleability, and high charge / discharge even after the negative electrode agent and the current collector have high adhesiveness and the number of cycles is repeated. It is suitable for a negative electrode for a lithium ion secondary battery that maintains the characteristics.

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Separator 4 Positive electrode tab 5 Negative electrode tab 6 Positive electrode cover 7 Battery can 8 Gasket

Claims (5)

The average particle size is 10 to 50 μm, the 30-time tap density is 0.5 g / cm ³ or more, the specific surface area is 7 m ² / g or less, and the product of the 30-time tap density and the specific surface area is 30000 / cm or more and 60000 / cm or less. A carbon material for a negative electrode of a lithium ion secondary battery, wherein an interlayer distance d (002) of graphite crystals determined by X-ray diffraction measurement is 3.400 mm or less. The carbon material for the negative electrode of the lithium ion secondary battery is a mixed powder of two or more kinds of carbon particles having different average particle diameters, 30 times tap density and specific surface area,
At least carbon particles A are included, and the carbon particles A are carbon particles in which a plurality of flat particles are assembled or bonded, and the orientation planes of the flat particles are the surfaces and cross sections of the carbon particles in which the flat particles are combined or bonded. The carbon material for a lithium ion secondary battery negative electrode according to claim 1, wherein the carbon material is non-parallel and has an aspect ratio of 1.2 to 5.   The said carbon particle A is a carbon material for lithium ion secondary battery negative electrodes of Claim 2 whose ratio with respect to the whole carbon material is 15-65 mass%. A negative electrode for a lithium ion secondary battery comprising the carbon material according to any one of claims 1 to 3 and an organic binder and having a density of 1.5 to 1.9 g / cm ^3. Mixture.   The lithium ion secondary battery which has a negative electrode for lithium ion secondary batteries of Claim 4, and the positive electrode containing a lithium compound.