JP2000203817A - Composite carbon particle, its production, negative pole material, negative pole for lithium secondary battery or cell and lithium secondary battery or cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a composite carbon particle having a high charge and discharge capacity and suitable as a negative pole material for a lithium secondary battery or cell improved in problems such as deterioration of slurry characteristics, orientation of the particle in a collector planar direction, deterioration of cycle characteristics and an irreversible increase in capacity when using a conventional expanded graphite particle as the negative pole material for the lithium secondary battery or cell, to provide a method for producing the composite carbon particle, to obtain the negative pole material and to provide both the negative pole for the lithium secondary battery or cell and the lithium secondary battery or cell. SOLUTION: This composite carbon particle contains an expanded graphite part and an amorphous carbon part. The method for producing the composite carbon particle comprises dissolving an organic compound in a solvent, mixing and dispersing an expanded graphite particle in the resultant solution, removing the solvent, drying the resultant mixture and then carbonizing the organic compound. The negative pole material contains the above composite carbon particle or the composite carbon particle obtained by the above method for production. The negative pole for the lithium secondary battery or cell uses the negative pole material. The lithium secondary battery or cell has the negative pole for the lithium secondary battery or cell.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery, a negative electrode thereof, a negative electrode material, a composite carbon particle suitable for the negative electrode material and a method for producing the same, and particularly to a lithium secondary battery having excellent charge / discharge capacity and cycle characteristics. The present invention relates to a battery, a negative electrode thereof, a negative electrode material, composite carbon particles suitable for the negative electrode material, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for small, lightweight, and high energy density secondary batteries for portable equipment, electric vehicles, and power storage. In response to such demands, lithium secondary batteries have attracted attention especially as batteries having high voltage and high energy density.

As a negative electrode material of a lithium secondary battery, lithium metal, amorphous carbon particles, and highly graphitized carbon particles are used. Metal lithium can achieve a high charge-discharge capacity, but due to its high reactivity, it reacts with the solvent in the electrolytic solution with the progress of the charge-discharge cycle, the capacity decreases, and dendritic metal lithium is easily generated, There is a problem that a short circuit is likely to occur through the separator provided between the positive electrode and the negative electrode.

[0004] Amorphous carbonaceous materials are characterized by low reactivity with an electrolytic solution and difficulty in producing dendritic metal lithium, but generally have a low charge / discharge capacity and a low true density. It has a drawback of low charge / discharge capacity, and has not achieved a secondary battery with a high energy density. On the other hand, highly graphitized carbon particles have a higher charge / discharge capacity than amorphous carbon particles, have lower reactivity with an electrolytic solution than metal lithium, and are less likely to form dendritic metal lithium. Due to its characteristics, it has been actively studied in recent years as a negative electrode material.

As highly graphitized carbon particles, natural graphite particles are subjected to an acid treatment to form an intercalation compound, which is heated and expanded to apply expanded graphite to a negative electrode material. Expanded graphite is generally used to insert graphite particles between graphite crystal layers by contacting graphite particles with highly developed graphite crystals, such as natural graphite, with a concentrated sulfuric acid solution containing an oxidizing agent such as concentrated nitric acid or hydrogen peroxide, or It is manufactured by electrochemically inserting sulfuric acid between graphite crystal layers, then rapidly heating to release the sulfuric acid inserted between the layers, and expanding at this time,
The expansion rate during this expansion process may reach several hundred times.
It has been reported that a high charge / discharge capacity exceeding the theoretical capacity of graphite (372 mAh / g) can be obtained when the obtained expanded graphite is pulverized and residual sulfate groups are removed by heat treatment or the like and used as a negative electrode material. I have.

The expanded graphite pulverized for the negative electrode material is
According to the manufacturing method, the shape becomes flaky, the bulk density is low, and the material has a high specific surface area. For this reason, when expanded graphite is used as a negative electrode material, the following problems occur. That is, the viscosity of the slurry made of expanded graphite particles, binder, solvent, and the like used for electrode production increases, and workability deteriorates. When the electrode is produced by applying the slurry to the current collector, drying, and pressing, the flakes are formed. Particles are oriented at high density in the direction of the current collector surface, and as a result, the permeability of the electrolytic solution into the negative electrode layer is deteriorated, and it takes a long time to inject the electrolytic solution. It has been clarified that there is a problem that the particles easily peel due to the strain in the thickness direction caused by the repetition of occlusion / release, and the cycle characteristics are deteriorated, and the irreversible capacity at the time of the first charge / discharge is large.

[0007]

DISCLOSURE OF THE INVENTION The present invention provides a high charge / discharge capacity, and deteriorates slurry characteristics when conventional expanded graphite particles are used as a negative electrode material of a lithium secondary battery.
An object of the present invention is to provide composite carbon particles suitable as a negative electrode material for a lithium secondary battery, in which problems such as orientation of particles in the direction of a current collector surface, deterioration of cycle characteristics, and increase in irreversible capacity have been improved.

Further, the present invention provides a method for producing a negative electrode material for a lithium secondary battery, which has a high charge / discharge capacity and deteriorates the slurry characteristics, the orientation of the particles in the current collector surface direction, and the like. An object of the present invention is to provide a method for easily producing composite carbon particles suitable as a negative electrode material for a lithium secondary battery, in which problems such as deterioration of cycle characteristics and increase of irreversible capacity are improved.

In addition, the present invention provides a high charge / discharge capacity and the use of the conventional expanded graphite particles, the deterioration of slurry characteristics, the orientation of particles in the direction of the current collector surface, the deterioration of cycle characteristics,
An object of the present invention is to provide a negative electrode material in which problems such as an increase in irreversible capacity are improved.

Further, the present invention improves the problems such as the orientation of particles in the direction of the current collector surface, the deterioration of cycle characteristics, and the increase of irreversible capacity when the conventional expanded graphite particles have a high charge / discharge capacity. To provide a negative electrode for a rechargeable lithium battery. The present invention also provides a lithium secondary battery having high charge / discharge capacity, excellent cycle characteristics, and low irreversible capacity.

[0011]

SUMMARY OF THE INVENTION The present invention relates to a composite carbon particle containing an expanded graphite portion and an amorphous carbon portion. Further, the present invention relates to the composite carbon particle having a structure in which a plurality of expanded graphite particles coated with amorphous carbon are aggregated. The present invention also relates to the composite carbon particles having an average particle diameter of 2 to 60 μm. Further, in the present invention, the ratio of the expanded graphite portion is 50 to 9 with respect to the weight of the composite carbon particles.
8% by weight of the composite carbon particles. The present invention also relates to the composite carbon particles having a specific surface area of 1 to 50 m ² / g and a tap density defined by JIS 1469 of 0.15 to 0.4 g / cm ³ .

The present invention also provides a composite carbon particle characterized in that expanded graphite particles are mixed and dispersed in a solution in which an organic compound is dissolved in a solvent, the solvent is removed and dried, and then the organic compound is carbonized. A method for producing the same. The present invention also provides
The present invention relates to a method for producing the composite carbon particles, wherein the expanded graphite particles used have an impurity content of 2000 ppm or less.

[0013] The present invention also relates to a negative electrode material comprising the composite carbon particles according to any of the above or the composite carbon particles obtained by the above-mentioned production method. The present invention also relates to a negative electrode for a lithium secondary battery using the above negative electrode material. The present invention also relates to a lithium secondary battery having the above-described negative electrode for a lithium secondary battery.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The composite carbon particles of the present invention contain an expanded graphite portion and an amorphous portion. Here, the term “composite” means that not only a simple mixture of expanded graphite particles and carbonaceous particles but also an expanded graphite portion and an amorphous carbon portion are integrally contained in one particle. Here, the expanded graphite is obtained by mixing graphite particles with an acid to form an intercalation compound, and heating and expanding the intercalation compound. Here, the graphite particles used are preferably those having a high degree of graphitization, and natural graphite is preferred. Examples of the treating agent used for the acid treatment include concentrated nitric acid, hydrogen peroxide, chromic anhydride, potassium chlorate, concentrated sulfuric acid containing an oxidizing agent such as potassium perchlorate, fuming nitric acid, and the like. Then, an intercalation compound in which nitric acid is inserted between the graphite crystal layers is generated. There is no particular limitation on the mixing ratio of each component and the processing time. For example, sulfuric acid (concentration of 2
100 to 250 parts by weight of nitric acid (those having a concentration of 30 to 70%) and 100 to 250 parts by weight of
It can be manufactured by treating for about 0 minutes to 2 hours.

Then, the graphite intercalation compound obtained by the acid treatment is expanded by heating. Heating temperature is 700-120
A range of 0 ° C. is preferred. The heating rate is not particularly limited, and is 1
A method of increasing the temperature at a rate of not less than ° C./hour, a method of rapidly heating by putting the mixture in a furnace maintained at a high temperature, and the like can be adopted. As the heating atmosphere, an air atmosphere, an inert atmosphere, a vacuum atmosphere, or the like can be employed.

The obtained expanded graphite is preferably ground. In this pulverization, the expanded graphite may be pulverized as it is, but it is better to pulverize the expanded graphite once compacted into an arbitrary shape such as a sheet, block, ring, etc. It is preferable because it is more efficient than doing. The pulverization can be performed by a known mechanical pulverization method. In addition, as a material of the composite carbon particles, a pulverized product obtained by directly pulverizing expanded graphite and a pulverized product obtained by pulverizing a compression molded product of expanded graphite may be used.

The expanded graphite particles used as the material of the composite carbon particles preferably have a volume average particle diameter of 0.1 to 60 μm. When the average particle size is less than 0.1 μm, the specific surface area of the obtained composite carbon particles becomes large, and problems such as an increase in irreversible capacity and a decrease in workability in electrode production tend to occur. On the other hand, when the average particle diameter exceeds 60 μm, the particle diameter of the composite material obtained with carbon is excessively large, and irregularities are easily generated on the electrode surface, which is not preferable. The average particle diameter is preferably set to 0.5 to 20 μm. The volume average particle diameter can be measured by, for example, a particle diameter measuring device using laser light scattering.

The expanded graphite particles used have an impurity content of 20.
It is preferably at most 00 ppm. Impurity content is 2
If it exceeds 000 ppm, the irreversible capacity tends to increase when used as a negative electrode material for a lithium secondary battery. As a method for realizing this impurity content, high-purity treatment such as heating at a high temperature in a known non-oxidizing atmosphere, high-temperature heat treatment in a halogen-containing atmosphere, or wet purification using hydrofluoric acid or the like is used. Processing can be adopted. This high purity treatment
It may be performed before or after the pulverization of the expanded graphite particles. The impurity content is measured as an ash content when the expanded graphite particles are incinerated. The expanded graphite particles used herein preferably have a d002 of 0.336 nm or less from the viewpoint of increasing the charge / discharge capacity of the composite carbon particles.

The amorphous carbon referred to in the present invention is also referred to as amorphous carbon, which is carbon having a low degree of crystal growth, and is called high graphitized carbon. Low graphitized carbonized product (d002 is large, L
c and La are small) and non-graphitizable carbon which does not become highly graphitizable carbon even when subjected to high temperature treatment.

The composite carbon particles of the present invention contain an expanded graphite portion and an amorphous carbon portion, and the production method thereof is not particularly limited, but the expanded graphite particles are added to a solution in which an organic compound is dissolved in a solvent. After mixing and dispersing, removing the solvent and drying to form an expanded graphite particle / organic compound composite, the method of carbonizing the organic compound is simply, fine expanded graphite particles and amorphous carbon. Is preferable because it is possible to obtain uniform composite particles of According to this method, composite carbon particles having a structure in which a plurality of expanded graphite particles coated with amorphous carbon are aggregated can be obtained.

The organic compound used in the present invention is not particularly limited, and various resins, various pitches such as petroleum pitch, coal pitch, synthetic pitch, tars such as coal tar, polyvinyl chloride and the like can be used. Among them, various pitches such as petroleum pitch, coal pitch, and synthetic pitch are preferable.

The ratio of these organic compounds to be composited with the expanded graphite particles is such that the ratio of the expanded graphite particles in the composite material of the obtained expanded graphite particles and the amorphous carbon formed by carbonization of the organic compound is 50 to 98. It is preferable to set the weight ratio so as to be 2% by weight (that is, the ratio of amorphous carbon is 2 to 50% by weight). The ratio of the expanded graphite portion in the composite carbon particles is 50% by weight.
If the amount is less than the above, the discharge capacity tends to decrease. If the amount exceeds 98% by weight, the effect of complexing with carbon becomes insufficient. This ratio can be determined from the following equation based on the weight loss when the expanded graphite particles / organic compound composite is heated to carbonize the organic compound.

[0023]

(Equation 1)

The solvent used in preparing the solution of the organic compound is not particularly limited as long as it can dissolve the organic compound to be used. When pitch or tar is used as the organic compound, quinoline, pyridine, toluene, Benzene, tetrahydrofuran, creosote oil and the like can be used. When polyvinyl chloride is used, tetrahydrofuran, cyclohexanone, nitrobenzene and the like can be used. When there is an undissolved portion in the solution, it is preferable to remove the undissolved portion by means such as filtration in order to produce uniform composite carbon particles. There is no particular limitation on the mixing ratio of the organic compound and the solvent, but when the concentration is high, it is difficult to uniformly disperse the expanded graphite particles with high viscosity, and as a result, uniform expanded graphite particles / amorphous carbon composite particles are obtained. Therefore, the amount of the organic compound is preferably set to 2 to 60% by weight.

After forming a composite of expanded graphite particles and an organic compound, the organic compound is carbonized to form composite carbon particles of expanded graphite particles and amorphous carbon. There is a method of heating a mixture of expanded graphite particles to carbonize an organic compound. Prior to this, it is preferable that the composite of the expanded graphite particles and the organic compound is once crushed. A pulverizer such as a cutter mill or a pin mill can be used for the pulverization.

As the atmosphere used for the carbonization, an inert atmosphere, a vacuum atmosphere, or an atmosphere containing low oxygen can be employed. The maximum temperature during carbonization is preferably set to 800 to 2000 ° C. If the temperature is lower than 800 ° C., the irreversible capacity of the amorphous carbon portion in the obtained composite carbon particles tends to increase, and as a result, the irreversible capacity of the entire composite tends to increase. Even if the temperature is increased, the effect of reducing the irreversible capacity of the carbon portion is small, and the irreversible capacity may increase due to decomposition of the electrolyte solution due to the progress of graphitization. It is preferable that the rate of temperature rise to the maximum temperature is 2 ° C / hour to 100 ° C / hour. Heating rates of less than 2 ° C./hour are not effective enough and tend to cause an increase in process time,
If the heating rate exceeds 100 ° C./hour, foaming during the carbonization of the organic compound and an increase in the specific surface area of the obtained composite carbon particles tend to occur.

When pitches and tars are used as the organic compound, the organic compound can be made infusible before heating and carbonizing the expanded graphite / organic compound composite. By this infusibilization step, fusion of particles in the carbonization step can be suppressed. As the infusibilization method, a known method such as that employed in the production of pitch-based carbon fibers can be used, for example, a dry method of contacting with an oxidizing agent (air, oxygen, NO ₂ , chlorine, bromine, etc.). , An aqueous solution of nitric acid, an aqueous solution of chlorine, an aqueous solution of sulfuric acid, an aqueous solution of hydrogen peroxide and the like, and a method combining these methods. The dry method of contacting with an oxidizing agent is performed at 200 to 300 ° C. for 0.1 to 10 hours,
Preferably, it is brought into contact with an oxidizing gas. 1 for wet method
It is preferable to contact with various aqueous solutions at a temperature of 0 to 90 ° C. for 0.1 to 10 hours. After the infusibilization treatment, pulverization and classification treatment may be further performed as necessary.

The complex obtained by the carbonization is crushed and classified, if necessary. A pulverizer such as a cutter mill or a pin mill can be used for pulverization, and a wind-type or mechanical classifier can be used for classification. The volume average particle diameter of the finally obtained composite carbon particles is preferably 2 to 60 μm when used as a negative electrode material for a lithium secondary battery. If the volume average particle diameter is less than 2 μm, the specific surface area tends to be large, and the workability at the time of manufacturing the electrode and the safety of the battery tend to be reduced.
If it exceeds, unevenness is likely to be formed on the electrode surface during electrode fabrication.

The specific surface area of the obtained composite carbon particles is from 1 to
The tap density is preferably 50 m ² / g, and the tap density is 0.1
It is preferably 5 to 0.4 g / cm ³ . Here, the specific surface area is a value measured according to the BET method using a nitrogen gas at a liquid nitrogen boiling point. The composite carbon particles having a specific surface area of less than 1 m ² / g tend to have a small discharge capacity. On the other hand, in the case of composite carbon particles having a specific surface area of more than 50 m ² / g, the irreversible capacity tends to increase, and the workability at the time of manufacturing the electrode tends to deteriorate.

Here, the tap density is measured by a method specified in JIS1469. This is 0.1
If it is less than 5 g / cm ³ , the irreversible capacity tends to increase,
In addition, the workability at the time of manufacturing the electrode tends to be poor, while the workability at the time of 0.1.
If it exceeds 4 g / cm ³ , the discharge capacity tends to decrease.

The composite carbon particles thus obtained are:
It can be used as a negative electrode material of a lithium secondary battery. The negative electrode material of the present invention can be used as a negative electrode molded body for a lithium secondary battery, for example, as follows. For example, it can be kneaded with an organic polymer binder to form a paste and formed into a shape such as a sheet.

Examples of the organic polymer binder include polyethylene, polypropylene, polyethylene terephthalate, aromatic polyamide, aromatic polyimide, cellulose, polyvinylidene fluoride, polytetrafluoroethylene, and copolymerized fluoropolymer containing tetrafluoroethylene. Polymer materials, rubbery polymer materials such as styrene / butadiene rubber, isoprene rubber, butadiene rubber, ethylene / propylene rubber, and soft polymer materials such as ethylene / vinyl acetate copolymer and propylene / α-olefin copolymer , Such as a system in which a lithium salt or an alkali metal salt mainly composed of lithium is combined with an organic polymer material such as polyethylene oxide, polypropylene oxide, polyepichlorohydrin, polyphazene, polyvinylidene fluoride, polyacrylonitrile, etc. A conductive polymer material can be used.

In addition to these organic polymer binders, carboxymethylcellulose, sodium polyacrylate, other acrylic polymers and the like may be added as a viscosity modifier. The mixing ratio of the organic polymer binder to the negative electrode material of the present invention is preferably 0.1 to 30 parts by weight, more preferably 0.5 to 2 parts by weight, based on 100 parts by weight of the negative electrode material.
0 parts by weight is more preferable, and 1 to 15 parts by weight is further preferable.

The negative electrode material of the present invention can be mixed with the above-mentioned organic polymer binder and then molded as it is into a shape of an electrode by a method such as roll molding or compression molding to produce a molded negative electrode. Further, a mixture of the powder of the negative electrode material of the present invention and the above organic polymer binder may be dispersed in a solvent to form a slurry, which may be applied to a metal current collector or the like. Roller copper foil, electrolytic copper foil, punching copper foil,
Nickel foil or the like is used. The shape of the electrode molded body can be arbitrarily set, such as a sheet shape or a pellet shape.

The solvent is not particularly limited, and N-
Examples thereof include methyl-2-pyrrolidone, dimethylformamide, and isopropanol, and the amount is not particularly limited.
A battery is assembled using the negative electrode obtained as described above. Prior to or during the assembly, lithium metal as an active material can be supported on the negative electrode molded body.
Thereby, the irreversible capacity at the time of the first charging can be significantly reduced. Examples of the supporting method include a chemical method, a physical method, and an electrochemical method. For example, a method of immersing a negative electrode molded body in a lithium ion-containing electrolytic solution, and electrically impregnating the negative electrode with metallic lithium, and a method of preparing a negative electrode molded body Sometimes, a method of mixing metallic lithium powder, a method of electrically contacting metallic lithium and the molded negative electrode, and the like are available.

The lithium secondary manufactured as described above
Battery negative electrode is placed facing the positive electrode with the separator interposed
Thus, a lithium secondary battery is formed. As a positive electrode material
Is not particularly limited, for example, vanadium oxide, vanadium
Disulfide, molybdenum oxide, molybdenum sulfide,
Manganese oxide, manganese sulfide, chromium oxide, titanium
Oxides, titanium sulfides, their composite oxides,
Metal chalcogen compounds such as chlorides, lithium cobalt oxidation
(LiCoO_Two), Lithium nickel oxide (LiN
iO_Two), Lithium manganese oxide (LiMn)_TwoO _Four, L
iMnO_Three), Lithium nickel cobalt oxide (Li_x
Ni_yCo_(1-y)O_Two) And other complex oxides
Added elemental elements (Al, Fe, Mn, Mg, Co, etc.)
A composite oxide or the like can be used. Also, polyanily
And conductive polymers such as polypyrrole.
Wear.

As the electrolytic solution, a solution in which a lithium salt serving as an electrolyte is dissolved in a non-aqueous solvent is used. As the _{electrolyte, LiClO 4, LiPF 6, LiAsF} 6, LiB
Lithium metal salts such as F ₄ , LiSO ₃ CF ₃ , and LiN (SO ₂ CF ₃ ) ₂ , and tetraalkylammonium salts can be used. The concentration of the lithium salt is preferably 0.2 to 2 mol / l, more preferably 0.3 to 1.9 mol / l.
Liters.

As the non-aqueous solvent, propylene carbonate, ethylene carbonate, butylene carbonate,
Vinylene carbonate, cyclic esters such as γ-butyrolactone, chain esters such as diethyl carbonate,
Ketones such as methyl ethyl ketone, 1,2-dimethoxyethane, dioxolan, tetrahydrofuran, 1,2-
Ethers such as dimethyltetrahydrofuran and crown ether can be used. In addition, a solid electrolyte in which the above salts are mixed with polyethylene oxide, polyphosphazene, polyaziridine, polyacrylonitrile, polyethylene sulfide, or the like, or a derivative, a mixture, or a complex thereof can also be used. In this case, the solid electrolyte can also serve as a separator, and the separator is not required.

As the separator for separating the negative electrode and the positive electrode and holding the electrolytic solution, a microporous film of polyethylene, polypropylene, a composite of polypropylene / polypropylene, a composite of polypropylene / fluororesin, a nonwoven fabric, or the like can be used. .

[0040]

Embodiments will be described below. Example 1 A powdered product of expanded graphite was added to a tetrahydrofuran solution of coal tar pitch (coal tar pitch: 11 parts by weight, tetrahydrofuran: 300 parts by weight) from which undissolved components were removed by filtration, mixed, and refluxed at the boiling point for 1 hour. While stirring and mixing. The pulverized expanded graphite used was obtained by pulverizing an expanded graphite sheet using a pin mill, and then crushing the expanded graphite sheet in an atmosphere containing chlorine.
It was manufactured by heating at 00 ° C. for 1 hour to perform a high-purification treatment. The average particle diameter of the expanded graphite pulverized product was 12 μm, the content of impurities was 20 ppm, and d002 was 0.3355 nm.

Next, tetrahydrofuran was removed by using a rotary evaporator, and the resultant was dried under vacuum at 100 ° C. for 3 hours. The resulting composite comprising the pulverized expanded graphite and coal tar pitch was pulverized with a cutter mill to 200 mesh or less. The obtained composite powder was heated to 250 ° C. at a rate of 3 ° C./min in the air and kept for 1 hour. Then, the temperature is raised to 1300 ° C. at a rate of 20 ° C./hour in a nitrogen gas stream, and the temperature is maintained for 1 hour to expand the amorphous carbon coated with the amorphous carbon. A graphite composite was obtained. This is crushed with a cutter mill and 200mesh
It was as follows.

The carbon content derived from coal tar pitch in the composite determined from the weight loss upon heating was 6.2% by weight, and the content of expanded graphite was 93.8% by weight. The specific surface area measured by the BET method using nitrogen is 17
m ² / g, and the tap density was 0.20 g / cm ³ . The average particle size was 15 μm.

Example 2 A composite powder was prepared in the same manner as in Example 1 except that the amount of coal tar pitch was changed to 27 parts by weight. The content of the expanded graphite in the obtained composite was 86.6% by weight.
Specific surface area and tap density are 15m ^{2 /} g and 0.25 respectively.
g / cm ³ . The average particle size was 18 μm.

Example 3 A composite powder was prepared in the same manner as in Example 1 except that the blending amount of the coal tar pitch was changed to 78 parts by weight. The content of the expanded graphite in the obtained composite was 79.0% by weight.
Specific surface area and tap density are 10m ^{2 /} g and 0.31, respectively.
g / cm ³ . The average particle size was 21 μm.

Example 4 A composite powder was prepared in the same manner as in Example 1 except that the amount of coal tar pitch was changed to 130 parts by weight. The content of the expanded graphite in the obtained composite powder was 57.5% by weight. The specific surface area and tap density are 8 m ² / g and 0.
It was 35 g / cm ³ . The average particle size was 25 μm.

Comparative Example 1 The pulverized expanded graphite used in Examples 1 to 4 was used as it was. Specific surface area is 36m ² / g, tap density is 0.11g / cm ³
Met. The average particle size was 12 μm.

Example 5 A composite powder comprising pulverized expanded graphite and carbon was prepared in the same manner as in Example 1 except that the blending amount of coal tar pitch was changed to 4 parts by weight. The content of the expanded graphite in the obtained composite powder was 98.5% by weight. Specific surface area and tap density are 25m ^{2 /} g and 0.13g / cm, respectively.
^{Was 3} . The average particle size was 16 μm.

Example 6 A composite powder was prepared in the same manner as in Example 1 except that the blending amount of coal tar pitch was changed to 270 parts by weight. The content of expanded graphite in the obtained composite powder is 4
0.3% by weight. The specific surface area and tap density were 25 m ^{2 /} g and 0.13 g / cm ³ , respectively. The average particle size was 30 μm.

Example 7 A composite powder composed of pulverized expanded graphite and carbon was prepared in the same manner as in Example 1 except that the pulverized expanded graphite was changed to a non-purified one. The impurity content of the pulverized expanded graphite product used was 2800 ppm, and d002 was 0.1%.
It was 3357 nm. The content of the expanded graphite in the obtained composite powder was 86.5% by weight. The specific surface area and tap density were 19 m ^{2 /} g and 0.19 g / cm ³ , respectively.
The average particle size was 15 μm.

[Measurement of Charge / Discharge Capacity] Using the obtained composite powder as a negative electrode material, 90% by weight of N-methyl-2-
1% polyvinylidene fluoride dissolved in pyrrolidone
0% by weight was added and kneaded to prepare a slurry. This slurry was applied to a rolled copper foil having a thickness of 10 μm and dried to obtain a negative electrode. The prepared sample electrode was charged and discharged at a constant current by a three-terminal method, and evaluated as a negative electrode of a lithium ion secondary battery.

FIG. 1 is a schematic diagram of the lithium ion secondary battery used in this experiment. As shown in FIG.
A solution obtained by dissolving LiPF ₆ in an equal volume mixed solvent of ethylene carbonate and dimethyl carbonate so as to have a concentration of 1 mol / liter was added as an electrolytic solution 2 to the sample electrode (negative electrode) 3, separator 4 and counter electrode (positive electrode). 5) were stacked and arranged, and the reference electrode 6 was suspended from above to produce a lithium ion secondary battery. Metal lithium was used for the counter electrode and the reference electrode, and a polyethylene microporous membrane was used for the separator. 1.5 V at a constant current of 0.28 mA / cm ²
(V vs Li / Li ⁺ ), and 0V (V v
s Li / Li ⁺ ). Table 1 shows the measured values of the charge / discharge capacity and the irreversible capacity during the initial charge / discharge.

[0052]

[Table 1]

(Preparation of Lithium Ion Secondary Battery) FIG. 2 shows a partial cross-sectional front view of an example of a cylindrical lithium ion secondary battery. In FIG. 2, 7 is a positive electrode, 8 is a negative electrode, 9 is a separator, 10 is a positive electrode tab, 11 is a negative electrode tab, 12 is a positive electrode lid, 13 is a battery can, and 14 is a gasket. FIG.
The lithium ion secondary battery shown in was manufactured as follows.

(Preparation of Positive Electrode) LiCo as a positive electrode active material
An average particle size of 1 μm as a conductive agent was added to 288 parts by weight of O ₂ .
m-scale natural graphite (7 parts by weight) and polyvinylidene fluoride (5 parts by weight) as a binder were added thereto, and N-methyl-2-
Pyrrolidone was added and mixed to prepare a slurry of the positive electrode mixture. Next, this positive electrode mixture was applied as a positive electrode current collector to an aluminum foil (thickness: 25 μm) on both sides by a doctor blade method, dried, and then pressure-formed by a roller press. This was cut out into a size having a width of 40 mm and a length of 285 mm to produce a positive electrode 10. However, the portion of the positive electrode 10 having a length of 10 mm at both ends is not coated with the positive electrode mixture,
The aluminum foil is exposed.
3 was pressed by ultrasonic bonding.

(Preparation of Negative Electrode) The composite terminal particles of the sample and polyvinylidene fluoride as a binder were mixed at a weight ratio of 90:10, and this was dispersed in a solvent (N-methyl-2-pyrrolidone). After the slurry was formed, both surfaces of a copper foil (thickness: 10 μm) as a negative electrode current collector were applied by a doctor blade method, dried, and then the electrodes were pressure-formed by a roller press. This was cut out to a size of 40 mm in width and 290 mm in length to produce a negative electrode. In the same manner as the positive electrode, the negative electrode was pressed by ultrasonic bonding to a negative electrode tab at one end of a portion having a length of 10 mm where the negative electrode mixture was not applied.

(Preparation of electrolyte solution) LiPF was added to an equal volume mixed solvent of ethylene carbonate and dimethyl carbonate.
₆ was dissolved at 1 mol / liter to prepare an electrolytic solution.

(Preparation of Battery) The positive electrode, a separator made of a porous film made of polyethylene (thickness 25 μm, width 44 mm), and the negative electrode were laminated in this order, and then spirally wound so that the negative electrode was positioned outside. To form an electrode group. The electrode group was housed in a stainless steel battery can, the negative electrode tab was welded to the bottom of the can, and a throttle portion for caulking the positive electrode lid was provided. Thereafter, the electrolyte is poured into a battery can,
The positive electrode tab was welded to the positive electrode lid, and the positive electrode lid was crimped to produce a cylindrical lithium ion secondary battery. Charge / discharge current is 200
mA, the charge / discharge cycle characteristics of each battery were measured. Table 2 shows the results.

[0058]

[Table 2]

[0059]

The composite carbon particles of the present invention have a high charge / discharge capacity and, when conventional expanded graphite particles are used as a negative electrode material of a lithium secondary battery, deteriorated slurry characteristics and reduced the current collector surface direction. It is suitable as a negative electrode material for a lithium secondary battery in which the problems such as particle orientation, cycle characteristics deterioration, and irreversible capacity increase have been improved.

According to the method for producing composite carbon particles of the present invention,
When the conventional expanded graphite particles have a high charge / discharge capacity and are used as a negative electrode material of a lithium secondary battery, the slurry characteristics deteriorate, the orientation of the particles in the current collector surface direction, the cycle characteristics deteriorate, and the irreversible capacity increases. Thus, composite carbon particles suitable as a negative electrode material for a lithium secondary battery in which the problem described above has been improved can be easily produced.

The negative electrode material of the present invention has a high charge / discharge capacity and, when conventional expanded graphite particles are used, deteriorated slurry characteristics, particle orientation in the current collector surface direction, deteriorated cycle characteristics, and irreversible capacity. Issues such as increase have been improved. The negative electrode for a lithium secondary battery of the present invention has a high charge / discharge capacity, and in the case of using conventional expanded graphite particles, orientation of particles in the current collector surface direction, deterioration of cycle characteristics, increase in irreversible capacity, and the like. The task has been improved. The lithium secondary battery of the present invention has high charge / discharge capacity, excellent cycle characteristics, and low irreversible capacity.

[Brief description of the drawings]

FIG. 1 is a schematic view of a lithium secondary battery used for measuring the discharge capacity of graphite particles alone.

FIG. 2 is a partial cross-sectional front view of a cylindrical lithium secondary battery.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Glass cell 2 Electrolyte 3 Sample electrode 4 Separator 5 Counter electrode 6 Reference electrode 7 Positive electrode 8 Negative electrode 9 Separator 10 Positive electrode tab 11 Negative electrode tab 12 Positive electrode cover 13 Battery can 14 Gasket

Continued on the front page (72) Inventor Takeshi Takeshi 3-3-1 Ayukawacho, Hitachi City, Hitachi, Ibaraki Prefecture Hitachi Chemical Co., Ltd. Yamazaki Plant F-term (reference) BA03 BB01 BC01 BC05 BD02 BD05 BD06 5H014 AA01 BB01 BB03 BB06 CC01 CC07 EE08 HH01 HH06 HH08 5H029 AJ03 AJ05 AK02 AK03 AK05 AL06 AM03 AM04 AM05 AM07 BJ02 BJ14 CJ02 CJ08 CJ12 HJ12 DJJ12 DJ14

Claims (10)

[Claims] 1. Composite carbon particles comprising an expanded graphite portion and an amorphous carbon portion. 2. The composite carbon particles according to claim 1, wherein the expanded graphite particles coated with the amorphous carbon have a plurality of aggregated structures. 3. The composite carbon particle according to claim 1, wherein the average particle diameter is 2 to 60 μm. 4. The composite carbon particle according to claim 1, wherein the proportion of the expanded graphite portion is 50 to 98% by weight based on the weight of the composite carbon particle. 5. Specific surface area of 1 to 50 m ² / g, JIS146
5. The composite carbon particle according to claim 1, wherein the tap density specified in 9 is 0.15 to 0.4 g / cm ³ . 6. A method for producing composite carbon particles, comprising mixing and dispersing expanded graphite particles in a solution in which an organic compound is dissolved in a solvent, removing the solvent, drying, and then carbonizing the organic compound. . 7. The expanded graphite particles used have an impurity content of 2%.
The method for producing composite carbon particles according to claim 6, wherein the content is 000 ppm or less. 8. A negative electrode material comprising the composite carbon particles according to claim 1 or the composite carbon particles obtained by the production method according to claim 6 or 7. 9. A negative electrode for a lithium secondary battery using the negative electrode material according to claim 8. 10. A lithium secondary battery comprising the negative electrode for a lithium secondary battery according to claim 9.