JPH11111297A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery having excellent battery characteristic such as a high capacity, high charge and discharge efficiency, excellent cycle lifetime, flatness of charging voltage and excellent battery characteristics such as quick charge and discharge cycle. SOLUTION: In a lithium secondary battery provided with a positive electrode 4, a negative electrode 6 including the carbonaceous material for storing and discharging lithium ion, and the non-aqueous electrolyte, as the carbonaceous material, power having a surface space between the (002) surfaces of the graphite structure detected by the X-ray diffraction exists in a range at 0.335-0.336 nm, and having a peak strength ratio (P101 /P100 ) of the (101) diffraction peak P101 and the (100) diffraction peak P100 more than 2.2 and a ratio of a length La of a crystal in an a-axial direction and a length Lc of a crystal in a c-axial direction (La/Lc) less than 1.5 is used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery having an improved negative electrode structure.

[0002]

2. Description of the Related Art In recent years, non-aqueous electrolyte batteries using lithium as a negative electrode active material have attracted attention as high energy density batteries, and manganese dioxide (MnO ₂ ) has been used as a positive electrode active material.
Fluorocarbon [(CF ₂ ) _n ], thionyl chloride (SOCl
Primary batteries using ₂ ) and the like are already widely used as power supplies for calculators, watches, and as backup batteries for memories.

Further, in recent years, with the reduction in size and weight of various electronic devices such as VTRs and communication devices, the demand for secondary batteries having a high energy density as a power source for these devices has increased, and lithium secondary batteries using lithium as a negative electrode active material have been required. Battery research is being actively conducted.

A lithium secondary battery uses lithium as a negative electrode and propylene carbonate (PC), 1,2-dimethoxyethane (DME), γ-butyrolactone (γ-
BL), a non-aqueous electrolyte in which a lithium salt such as LiClO ₄ , LiBF ₄ , or LiAsF ₆ is dissolved in a non-aqueous solvent such as tetrahydrofuran (THF) or a lithium ion conductive solid electrolyte. TiS ₂ ,
The use of compounds that undergo a topochemical reaction with lithium, such as MoS ₂ , V ₂ O ₅ , V ₆ O ₁₃ , and MnO ₂ , has been studied.

[0005] However, the above-mentioned lithium secondary battery has not yet been put to practical use. The main reason for this is that the charge / discharge efficiency is low and the number of times that charge / discharge can be performed (cycle life) is short. It is considered that this is largely due to the deterioration of lithium due to the reaction between the lithium of the negative electrode and the non-aqueous electrolyte. That is, lithium dissolved in the non-aqueous electrolyte as lithium ions at the time of discharge reacts with the solvent at the time of deposition at the time of charging, and its surface is partially inactivated. Therefore, when charge and discharge are repeated, lithium precipitates in a dendritic (dendritic) or small spherical shape, and further, phenomena such as detachment of lithium from the current collector occur.

[0006] Therefore, by using a carbonaceous material that absorbs and releases lithium, for example, coke, a resin fired body, carbon fiber, and pyrolysis gas phase carbon, as the negative electrode incorporated in the lithium secondary battery, lithium and lithium can be removed. It has been proposed to improve the reaction with a non-aqueous electrolyte and the deterioration of the negative electrode characteristics due to the precipitation of dendrites.

[0007] The negative electrode containing the carbonaceous material has a structure (graphite structure) in which hexagonal mesh layers mainly composed of carbon atoms are stacked in the carbonaceous material, and lithium ions are present in a portion between the layers. It is considered that charging and discharging are performed by going in and out. For this reason, it is necessary to use a carbonaceous material having a somewhat developed graphite structure for the negative electrode of the lithium secondary battery. However, when a carbonaceous material obtained by pulverizing a graphitized giant crystal is used as a negative electrode in a non-aqueous electrolyte, the non-aqueous electrolyte is decomposed, and as a result, the capacity and charge / discharge efficiency of the battery are reduced. In particular, when the battery is operated at a high current density, the capacity, the charge / discharge efficiency, and the voltage at the time of discharge significantly decrease. Further, as the charge / discharge cycle progresses, the crystal structure or microstructure of the carbonaceous material is broken, the ability to insert and extract lithium is deteriorated, and the cycle life is shortened.

In addition, since the powder of the graphitized material is in the form of flakes, the c of graphite crystallites into which lithium ions are inserted is c.
Since the area where the surface in the axial direction is exposed to the electrolyte becomes smaller, there is a problem that the capacity is rapidly reduced in a high-rate charge / discharge cycle. For this reason, although improvement has been made by adding carbon black or the like, there is a problem that the negative electrode packing density is reduced. As a result, a high-capacity lithium secondary battery could not be realized with the conventional graphite.

Further, even in the case of graphitized carbon fibers, the non-aqueous electrolyte solution is decomposed when powdered, and the performance as a negative electrode is greatly reduced as in the case of using powder of a giant crystal. Had a point.

On the other hand, in the case of carbonized materials such as coke and carbon fiber having a low degree of graphitization, although the decomposition of the solvent can be suppressed to some extent, the capacity and the charge / discharge efficiency are low, and the overvoltage of the charge / discharge is large, and the discharge voltage of the battery is low. Have low flatness and low cycle life.

Conventionally disclosed in JP-A-62-268058, JP-A-2-82466, JP-A-4-61747, JP-A-4-115458, JP-A-4-184862, JP-A-4-190557 and the like. As described above, it has been proposed to control the degree of graphitization of various carbonized materials and graphitized materials and to optimize the parameters of the graphite structure. However, a negative electrode having sufficient characteristics has not been obtained. Also, JP-A-4-79
No. 170 and JP-A-4-82172 disclose a carbon fiber used as a negative electrode, but the performance of a negative electrode using a carbonaceous material obtained by pulverizing the carbon fiber has a problem.

[0012]

SUMMARY OF THE INVENTION An object of the present invention is to provide a lithium secondary battery having high capacity and excellent battery characteristics such as charge / discharge efficiency, cycle life, flatness of discharge voltage, and rapid charge / discharge cycle. Things.

[0013]

A lithium secondary battery according to the present invention is a lithium secondary battery comprising a positive electrode, a negative electrode containing a carbonaceous substance that occludes and releases lithium ions, and a non-aqueous electrolyte. The carbonaceous material is at least one powder selected from graphite and graphitized coke,
And mesophase pitch-based carbon fibers.

The mesophase pitch-based carbon fiber preferably has a crystallite orientation in a cross section perpendicular to the fiber length direction of a radial, lamellar or Brooks-Taylor type.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a lithium secondary battery (for example, a cylindrical lithium secondary battery) according to the present invention will be described in detail with reference to FIG. A cylindrical container 1 with a bottom has an insulator 2 disposed at the bottom. The electrode group 3 is housed in the container 1. The electrode group 3 is formed by laminating a positive electrode 4, a separator 5, and a negative electrode 6 in this order on the negative electrode 6.
Is spirally wound so that is located outside.

The container 1 contains an electrolytic solution. The insulating paper 7 having a central portion opened is placed above the electrode group 3 in the container 1. The insulating sealing plate 8
The sealing plate 8 is disposed at the upper opening of the container 1 and caulked in the vicinity of the upper opening inward.
Is fixed to the container 1 in a liquid-tight manner. The positive terminal 9 is
The insulating sealing plate 8 is fitted at the center. Positive lead
One end of the cathode 10 is connected to the positive electrode 4, and the other end is connected to the positive electrode terminal 9.
Connected to each other. The negative electrode 6 is connected to the container 1 as a negative terminal via a negative lead (not shown).

The container 1 is made of, for example, stainless steel. The positive electrode 4 is manufactured by suspending a conductive agent and a binder in a suitable solvent in a positive electrode active material, applying the suspension to a current collector, and drying to form a thin plate.

As the positive electrode active material, various oxides,
For example, manganese dioxide, lithium manganese composite oxide,
Examples include lithium-containing nickel oxide, lithium-containing cobalt compound, lithium-containing nickel-cobalt oxide, lithium-containing vanadium oxide, and chalcogen compounds such as titanium disulfide and molybdenum disulfide. Among them, lithium cobalt oxide (LiCoO)
₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMn ₂ O ₄ , LiMnO ₂ )
Is preferable because a high voltage can be obtained.

Examples of the conductive agent include acetylene black, carbon black, graphite and the like. Examples of the binder include polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVD).
E), ethylene-propylene-diene copolymer (EPD)
M), styrene-butadiene rubber (SBR) and the like can be used.

The mixing ratio of the positive electrode active material, the conductive agent and the binder is 80 to 95% by weight of the positive electrode active material, 3 to 2% of the conductive agent.
It is preferable that the content be in the range of 0% by weight and 2 to 7% by weight of the binder.

As the current collector, for example, aluminum foil, stainless steel foil, nickel foil or the like can be used. As the separator 5, for example, a synthetic resin nonwoven fabric, a polyethylene porous film, a polypropylene porous film, or the like can be used.

The negative electrode 6 contains a carbonaceous material described in the following (1) to (4). However, La, d ₀₀₂ , Lc and intensity ratio (P ₁₀₁ / P ₁₀₀ ) for determining carbonaceous matter
The measurement and definition of are as follows.

(A) All data measured by the X-ray diffraction method used CuKα as an X-ray source and high-purity silicon as a standard substance. La, d ₀₀₂ , and Lc were determined from the position of each diffraction peak and the half width. As a calculation method, a half-width midpoint method was used.

(B) The length La of the crystallite in the a-axis direction and the length Lc of the crystallite in the c-axis direction are values when K, which is the form factor of Scherrer's equation, is 0.89. (C) by X-ray diffraction (101) and the diffraction peak P ₁₀₁ (100) intensity ratio of the diffraction peak P ₁₀₀ (P ₁₀₁ / P
₁₀₀ ) is obtained from the height ratio of those peaks.

(1) Carbonaceous matter The carbonaceous matter has a graphite structure of (00) by X-ray diffraction.
2) The plane spacing d ₀₀₂ is in the range of 0.335 to 0.336 nm, and the (101) diffraction peaks P ₁₀₁ and (100)
The peak intensity ratio (P ₁₀₁ / P ₁₀₀ ) of the diffraction peak P ₁₀₀ exceeds 2.2, and the ratio of the length La of the crystallite in the a-axis direction to the length Lc of the crystallite in the c-axis direction (La / Lc) Is less than 1.5.

The carbonaceous material has an average plane distance d ₀₀₂ of the (002) plane obtained by the X-ray diffraction method of 0.335-0.
336 nm. In such a carbonaceous material, the number of sites into which lithium ions can be inserted increases, and the capacity increases.
However, in the case of a carbonaceous material having an average plane spacing outside the above range, the capacity is reduced, and the flatness of the voltage at the time of discharging the battery is reduced.

The carbonaceous material having the strength ratio (P ₁₀₁ / P ₁₀₀ ) exceeding 2.2 has a very developed graphite structure. Such a graphite structure has little deviation, twist, and angle between the stacked hexagonal mesh layers. Since this has the characteristics of conventional natural graphite, the capacity is reduced under high-rate charge / discharge conditions. For this reason, by setting the ratio (La / Lc) of the length La of the crystallite in the a-axis direction to the length Lc of the crystallite in the C-axis direction by the X-ray diffraction method to be less than 1.5, the high-rate A graphitized material which could avoid a decrease in capacity under charge and discharge conditions could be obtained. That is, by making the La shorter and the Lc longer than conventional graphite, the lithium ion insertion / desorption reaction can be carried out smoothly, and it becomes possible to overcome the problems of the layer structure, A high capacity can be obtained under the following charge / discharge conditions.

The carbonaceous material preferably has a crystallite length La in the a-axis direction of the graphite structure of 20 to 100 nm, more preferably 40 to 80 nm, based on the diffraction peak of the (110) plane obtained by X-axis diffraction. desirable. Further, the shape of the a-axis surface in the graphite structure of the carbonaceous material is preferably rectangular.

The carbonaceous material whose La is within the above range is as follows:
Since the graphite structure is moderately developed, and the length of the crystallite in the a-axis direction is moderate, lithium ions are easily diffused between the layers of the hexagonal mesh layer, and sites into and out of which lithium ions enter and exit are formed. It shows the property that more lithium ions can be inserted and extracted.

The carbonaceous material whose La exceeds 100 nm becomes a giant crystal, making it difficult for lithium ions to diffuse between the layers of the hexagonal mesh layer, making it difficult to secure the amount of lithium ions absorbed and released. In addition, the surface is active with respect to a non-aqueous solvent, and the solvent is liable to undergo reductive decomposition. On the other hand, the carbonaceous material having a La of less than 20 nm contains a large amount of carbonaceous material having an undeveloped graphite structure in the carbonaceous material, so that lithium ions have less reversible occlusion / release between layers of the hexagonal mesh layer. There is a risk of becoming.

The carbonaceous material preferably has a crystallite size Lc in the c-axis direction obtained by an X-ray diffraction method of 15 nm or more, more preferably 20 to 100 nm. The carbonaceous material having Lc in such a range exhibits a property that a graphite structure is appropriately developed and a large amount of lithium ions is inserted and released reversibly.

As a measure of the ratio between the graphite structure and the turbostratic structure constituting the carbonaceous material, an argon laser (wavelength 51
(4.5 nm) as a light source. In the Raman spectrum measured for the carbonaceous material, there are a peak derived from a turbostratic structure appearing around 1360 cm ^-1 and a peak derived from a graphite structure appearing around 1580 cm ^-1 . The peak intensity ratio, i.e. the argon laser Raman spectrum peak intensity 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 in (wavelength 514.5 nm) (R2) (R
It is preferable to use a carbonaceous material having a ratio (R1 / R2) of 1) of 0.2 or less.

The carbonaceous material preferably has a true density, which is a measure of the degree of graphitization, of at least 2.20 g / cm ³ . The carbonaceous material has a particle size distribution of 90% by volume or more in a range of 1 to 100 μm, and an average particle size of 1 to 80 μm.
m is preferable. In addition, N ₂ gas adsorption BET
The specific surface area by the method is preferably from 0.1 to 40 m ² / g. The carbonaceous material having such a particle size distribution and specific surface area can improve the packing density of the negative electrode, and at the same time, reduce the content of the carbonaceous material or amorphous carbon having a graphite crystal structure that is active against a nonaqueous solvent, The reductive decomposition of the non-aqueous solvent can be suppressed. Further, the carbonaceous material containing fine particles having a particle size of 0.5 μm or less in a particle size distribution of 5% by volume or less further reduces the carbonaceous material having a broken graphite crystal structure or amorphous carbon, and effectively reduces and decomposes a nonaqueous solvent. It becomes possible to suppress. Conversely, the particle size distribution and N ₂
If the specific surface area of the gas adsorption by the BET method becomes too large, the content of amorphous carbonaceous material and the content of carbonaceous material with broken graphite crystal structure increase, so that the solvent tends to undergo reductive decomposition, and the packing density of the negative electrode further increases. Decrease.

The carbonaceous material has a sulfur content of 1000 pp
m or less (including 0 ppm). Such a carbonaceous material having a low sulfur content increases the amount of insertion and extraction of lithium ions and reduces the reductive decomposition of the non-aqueous solvent.

In a carbonaceous material having a developed graphite structure to a certain extent, the sulfur content is set to 1000 ppm or less (0 ppm).
), The graphite structure has fewer defects,
As a result, it is considered that the graphite structure is less likely to collapse, and that the amount of occluded and released lithium ions increases.
Therefore, the capacity, charge / discharge efficiency, and cycle life of the lithium secondary battery can be improved. In addition, the decomposition of the non-aqueous solvent and the inhibition of the electrode reaction due to the reaction of the non-aqueous solvent and lithium ions with sulfur or a sulfur compound are considered to be reduced, and the charge and discharge efficiency and the cycle life can be improved. That is, when sulfur in the carbonaceous material reacts with lithium ions, a stable group such as LiS—
Make compounds such as It is considered that the lithium does not contribute to the reversible storage / release reaction. In addition, the resulting compound becomes an obstacle between the hexagonal mesh layers, and lithium ions,
It is thought to prevent smooth insertion. These factors reduce charge / discharge efficiency and cycle life.

It is preferable that oxygen, nitrogen, silicon, or metal elements such as Fe and Ni as other impurities in the carbonaceous material be as small as possible. Specifically, the oxygen content is 500 ppm or less, and the nitrogen content is 10 ppm.
The metal elements such as Fe and Ni are preferably at most 50 ppm, respectively. If these impurity elements exceed the above range, lithium ions existing between carbon layers may react with the impurity elements and be consumed. However, it is allowed that aluminum, boron, phosphorus, calcium, tin and the like are contained.

An example of the method for producing the carbonaceous material will be described below. First, short fibers having a fiber length of 200 to 300 μm are spun from a mesophase pitch as a raw material by a melt blow method, and then are infused and carbonized to such an extent that they can be pulverized. This heat treatment for carbonization is desirably performed at 600 to 2000 ° C, preferably 800 to 1500 ° C. The distance d ₀₀₂ of the (002) plane of the carbonized mesophase pitch-based carbon fiber by X-ray diffraction method is 0.344 nm or more;
More preferably, it is desirable to be 0.357 nm or more. Such a carbon fiber has a microstructure suitable for shortening the fiber length, and is therefore suitable for pulverization. On the other hand, when the carbon fiber having the interplanar spacing d ₀₀₂ of less than 0.344 nm is pulverized, longitudinal cracks occur, making it difficult to shorten the fiber length. Subsequently, the carbon fiber subjected to the carbonization and pulverization treatment is heated to 2000 ° C. or more, more preferably 2500 ° C.
The above-mentioned carbonaceous material is manufactured by graphitizing at ~ 3200 ° C. At this time, the pulverization and baking steps are extremely important. When the pulverization is performed for an appropriate time (within 20 hours) using a ball mill or a jet mill during the pulverization, a spherical shape is obtained.
It is preferable to graphitize particles, blocks, and columns. In addition, it is necessary to prevent the crystal structure of the graphitized material from having a rhombohedral system in 30% or more. This is because the crushing of the hexagonal graphite structure by grinding
This is because a rhombohedral structure is formed, but it is preferable that the volume is reduced to 0 to 30% by volume by appropriately pulverizing. If it exceeds this range, the decomposition of the electrolyte solution will occur and the performance of the negative electrode will decrease.

The La of the carbonaceous material produced by the above method
And the crystal structure can be controlled by defining the pulverization conditions (pulverization time, atmosphere, pulverization power).

When the carbonaceous material is produced by the above-mentioned method, it is desirable that the particle size distribution and specific surface area of the obtained carbonaceous material have the above-mentioned values. The carbonaceous material can be obtained by graphitizing coke or bulk mesophase obtained from low-sulfur petroleum pitch, coal tar or the like at 2500 to 3200 ° C., in addition to those obtained by the above-described method.

In the carbonaceous material, the ratio between the rhombohedral system and the hexagonal system can be defined so that the ratio of the (101) diffraction peak by X-ray diffraction is 0.6 or less. A carbonaceous material having a rhombohedral structure having such a ratio can increase the negative electrode charge / discharge efficiency.

(2) Carbonaceous matter This carbonaceous matter has a graphite structure of (00) by X-ray diffraction.
2) The diffraction peak is 0.340 nm or less and 0.344 to
This powder appears in the range of 0.370 nm.

The carbonaceous material includes a graphitized material having giant graphite crystals (having a thickness La in the a-axis direction of 20 nm or more) having a (002) diffraction peak in the range of 0.340 nm or less.
02) Since it is made of a small (Lc: 6 nm or less) carbonized graphite crystal having a diffraction peak in the range of 0.344 to 0.370 nm, it is possible to maintain high capacity with little cycle deterioration even in rapid charging. Become. The latter (00
2) A carbonized material having a diffraction peak in the range of 0.356 to 0.370 is more preferable.

Conventionally, the (002) diffraction peak is 0.3
In the negative electrode composed of the above-mentioned graphitized material in the range of 40 nm or less, the cycle deterioration is large and the capacity is small in rapid charging. The (002) diffraction peak is 0.344 to 0.37.
The carbonaceous material in the range of 0 nm has a true density of 1.70.
Since it is about 2.15 / cm ^3, it is smaller than the above-mentioned graphitized material, and there is a problem that the capacity per unit volume (mAh / cm ³ ) of the negative electrode becomes smaller.

In order to solve such problems, the carbonaceous material constituting the negative electrode of the present invention has both the graphitized material and the carbonized material having the above-mentioned characteristics, whereby the rapid charging characteristics and the capacity are greatly improved. It became possible. This is because the (002) diffraction peak is mainly 0.344 to 0.370 n at the time of rapid charging at a constant current ( ² mA / cm ² or more).
Lithium ions are selectively inserted into graphite crystals in the range of m. At this time, the diffusion rate of lithium ions is (00
2) Rapid charging is possible because the diffraction peak is much faster than that of a graphitized material having a diffraction peak of 0.340 nm or less. When rapid charging further proceeds, the charging end voltage is reached, and when switching to constant voltage charging at that voltage, the (002) diffraction peak is mainly 0.1.
Lithium ions are selectively inserted into graphite crystals of 340 nm or less at a moderate rate. By such a selective insertion reaction of lithium ions, rapid charging becomes possible and high capacity can be maintained.

As the carbonaceous material, for example, the (002) diffraction peak of the graphite structure by the X-ray diffraction method is 0.340 n.
m or less and 0.344 to 0.370 nm, or a multiphase graphitized substance having a (002) diffraction peak of 0.34
Graphite having a diameter of 0 nm or less and (002) diffraction peak of 0.3
A mixture of carbonized materials in the range of 44 nm to 0.370 nm can be mentioned.

Examples of the multi-phase graphitized material include polyvinylidene chloride, phenol resin, furfural resin, charcoal, anthracite, sugar, pitch, etc. under normal pressure or high pressure.
It is obtained by performing non-uniform graphitization at a temperature of 500 to 3000 ° C. to form multi-phase graphitization. By performing the graphitization non-uniformly in this way, the graphitized (002) diffraction peak has a phase of 0.340 nm or less and a low graphitized (0
02) A diffraction peak having a phase of 0.344 nm to 0.370 nm can coexist.

Examples of the graphitized material as one component of the mixture include powders of graphite, anisotropic pitch-based carbon fiber, spherical carbon, pyrolytic vapor grown carbon, and coke. The graphitized material has a specific surface area of 0.5 to 10
Graphite in the range of m ² / g, powder of mesophase pitch-based carbon fiber, and mesophase spherules are more preferable.

Examples of the carbonized material as the other component of the mixture include coke containing low sulfur (2000 ppm or less) and low nitrogen (1000 ppm or less), high-purity anisotropic pitch-based carbon fiber, spherical carbon, heat Powders such as a decomposition vapor-grown carbon body and a resin fired body can be given. The carbonized material is a carbon fiber powder, a high-purity mesophase pitch-based carbon fiber powder, a mesophase spheroid, a low sulfur-containing powder having a specific surface area of ² to 30 m ² / g in the range of synthetic pitch (such as naphthalene). Is more preferred. Since any of the carbonized materials has a firing temperature in the range of 600 to 1200 ° C., it is important to reduce impurities such as sulfur and transmetal (iron, nickel) in the carbonized material. For this reason, it is necessary to select a raw material such as petroleum pitch, mesophase pitch, and coal tar that contain less of the impurities.

(3) Carbonaceous Material The carbonaceous material is a graphitized material mainly having a block-like, fibrous or spherical shape, and has a graphite structure having a (002) plane spacing d ₀₀₂ of 0. 335-0.337
nm, and the (101) diffraction peaks P ₁₀₁ and (1
00) Peak intensity ratio of diffraction peak P ₁₀₀ (P ₁₀₁ / P
₁₀₀ ) has a property exceeding 2.2.

The carbonaceous material has an average interplanar spacing d ₀₀₂ of the (002) plane obtained by the X-ray diffraction method of 0.335-0.
337 nm. In such a carbonaceous material, the number of sites into which lithium ions can be inserted increases, and the capacity increases.
However, in the case of a carbonaceous material having an average plane spacing outside the above range, the capacity is reduced, and the flatness of the voltage at the time of discharging the battery is reduced.

The carbonaceous material whose strength ratio (P ₁₀₁ / P ₁₀₀ ) exceeds 2.2 has a very developed graphite structure. Such a graphite structure has little deviation, twist, and angle between the stacked hexagonal mesh layers. Since it has the characteristics of conventional natural graphite, the capacity may be reduced under high-rate charge / discharge conditions. Therefore, the X
It is preferable that the ratio (La / Lc) of the length La of the crystallite in the a-axis direction to the length Lc of the crystallite in the C-axis direction is less than 1.5 by the line diffraction method. By defining La / Lc in this manner, a graphitized material can be obtained in which the capacity is not reduced under the high-rate charge / discharge conditions. That is,
By making the La shorter and the Lc longer than conventional graphite, lithium ion insertion / desorption reactions can be performed smoothly, and the problems of the layer structure can be overcome, and the high rate can be satisfied. High capacity can be obtained under discharge conditions.

The carbonaceous material has a block shape,
It has a fibrous or spherical shape. Here, the block shape is a granular shape other than the flake shape, and the ratio of the major axis to the minor axis (major axis / minor axis) is 5
Or less, more preferably 2 or less. The fibrous form is a ratio of fiber length to fiber diameter (fiber length / fiber diameter).
Is in the range of 0.5 to 10, and the orientation of crystallites in a cross section perpendicular to the fiber length direction is preferably radial, lamellar, or Brooks-Taylor type. The sphere is a sphere having a ratio of major axis to minor axis (major axis / minor axis) in the range of 1 to 2, and preferably has a crystallite orientation of radial, lamellar or Brooks-Tailer type in its cross section.
Such a block-like, fibrous or spherical carbonaceous material can increase the exposed area in the c-axis direction, so that a high-capacity secondary battery can be obtained under high-rate charge / discharge conditions. . In particular, the carbonaceous material having a fibrous or spherical shape in which the crystallite orientation is radial, lamellar or Brooks-Taylor type, can further increase the exposed area in the c-axis direction, and the lithium ion insertion / desorption reaction can be prevented. It can be performed smoothly.

The carbonaceous material preferably has a crystallite length La of 20 to 100 nm, more preferably 40 to 80 nm, in the a-axis direction of the graphite structure according to the diffraction peak of the (110) plane obtained by the X-ray diffraction method. Is desirable. Further, the shape of the a-axis surface in the graphite structure of the carbonaceous material is preferably rectangular.

The carbonaceous material whose La is within the above range is as follows:
Since the graphite structure is moderately developed, and the length of the crystallite in the a-axis direction is moderate, lithium ions are easily diffused between the layers of the hexagonal mesh layer, and sites into and out of which lithium ions enter and exit are formed. It shows the property that more lithium ions can be inserted and extracted.

The carbonaceous material whose La exceeds 100 nm becomes a giant crystal, making it difficult for lithium ions to diffuse between the layers of the hexagonal mesh layer, making it difficult to secure the amount of lithium ions absorbed and released. In addition, the surface is active with respect to a non-aqueous solvent, and the solvent is liable to undergo reductive decomposition. On the other hand, the carbonaceous material having a La of less than 20 nm contains a large amount of carbonaceous material having an undeveloped graphite structure in the carbonaceous material, so that lithium ions have less reversible occlusion / release between layers of the hexagonal mesh layer. There is a risk of becoming.

The carbonaceous material preferably has a crystallite size Lc in the c-axis direction obtained by an X-ray diffraction method of 15 nm or more, more preferably 25 to 200 nm. The carbonaceous material having Lc in such a range exhibits a property that a graphite structure is appropriately developed and a large amount of lithium ions is inserted and released reversibly.

As a measure of the ratio between the graphite structure and the turbostratic structure constituting the carbonaceous material, an argon laser (wavelength 51
(4.5 nm) as a light source. In the Raman spectrum measured for the carbonaceous material, there are a peak derived from a turbostratic structure appearing around 1360 cm ^-1 and a peak derived from a graphite structure appearing around 1580 cm ^-1 . The peak intensity ratio, i.e. the argon laser Raman spectrum peak intensity 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 in (wavelength 514.5 nm) (R2) (R
It is preferable to use a carbonaceous material having a ratio (R1 / R2) of 1) of 0.2 or less.

The carbonaceous material preferably has a true density, which is a measure of the degree of graphitization, of at least 2.20 g / cm ³ . The carbonaceous material has a particle size distribution of 90% by volume or more in a range of 1 to 100 μm, and an average particle size of 1 to 80 μm.
m is preferable. In addition, N ₂ gas adsorption BET
The specific surface area by the method is preferably 0.1 to 10 m ² / g. The carbonaceous material having such a particle size distribution and specific surface area can improve the packing density of the negative electrode, and at the same time, reduce the content of the carbonaceous material or amorphous carbon having a graphite crystal structure that is active against a nonaqueous solvent, The reductive decomposition of the non-aqueous solvent can be suppressed. Further, the carbonaceous material containing fine particles having a particle size of 0.5 μm or less in a particle size distribution of 5% by volume or less further reduces the carbonaceous material having a broken graphite crystal structure or amorphous carbon, and effectively reduces and decomposes a nonaqueous solvent. It becomes possible to suppress. Conversely, the particle size distribution and N ₂
If the specific surface area of the gas adsorption by the BET method becomes too large, the content of amorphous carbonaceous material and the content of carbonaceous material with broken graphite crystal structure increase, so that the solvent tends to undergo reductive decomposition, and the packing density of the negative electrode further increases. Decrease.

The carbonaceous material has a sulfur content of 1000 pp
m or less (including 0 ppm). Such a carbonaceous material having a low sulfur content increases the amount of insertion and extraction of lithium ions and reduces the reductive decomposition of the non-aqueous solvent.

In a carbonaceous material having a developed graphite structure to a certain extent, the sulfur content is reduced to 1000 ppm or less (0 ppm).
), The graphite structure has fewer defects,
As a result, it is considered that the graphite structure is less likely to collapse, and that the amount of occluded and released lithium ions increases.
Therefore, the capacity, charge / discharge efficiency, and cycle life of the lithium secondary battery can be improved. In addition, the decomposition of the non-aqueous solvent and the inhibition of the electrode reaction due to the reaction of the non-aqueous solvent and lithium ions with sulfur or a sulfur compound are considered to be reduced, and the charge and discharge efficiency and the cycle life can be improved. That is, when sulfur in the carbonaceous material reacts with lithium ions, a stable group such as LiS—
Make compounds such as It is considered that the lithium does not contribute to the reversible storage / release reaction. In addition, the resulting compound becomes an obstacle between the hexagonal mesh layers, and lithium ions,
It is thought to prevent smooth insertion. These factors reduce charge / discharge efficiency and cycle life.

It is preferable that oxygen, nitrogen, silicon, or metal elements such as Fe and Ni as other impurities in the carbonaceous material be as small as possible. Specifically, the oxygen content is 500 ppm or less, and the nitrogen content is 10 ppm.
The metal elements such as Fe and Ni are preferably at most 50 ppm, respectively. If these impurity elements exceed the above range, lithium ions existing between carbon layers may react with the impurity elements and be consumed. However, it is allowed that aluminum, boron, phosphorus, calcium, tin and the like are contained.

An example of the method for producing the carbonaceous material will be described below. First, short fibers having a fiber length of 200 to 300 μm are spun from a mesophase pitch as a raw material by a melt blow method, and then are infused and carbonized to such an extent that they can be pulverized. The condition of the infusibilization at this time greatly affects the graphitization described later, and it is desirable that the infusibilization be performed slowly. The heat treatment for carbonization is performed at 600 to 2000 ° C., preferably 800 to 15 ° C.
It is desirable to carry out at 00 ° C. It is desirable that the plane distance d ₀₀₂ of the (002) plane of the carbonized mesophase pitch-based carbon fiber determined by X-ray diffraction is 0.344 nm or more, more preferably 0.357 nm or more. Such a carbon fiber has a microstructure suitable for shortening the fiber length, and is therefore suitable for pulverization. On the other hand, when the carbon fiber having the interplanar spacing d ₀₀₂ of less than 0.344 nm is pulverized, longitudinal cracks occur, making it difficult to shorten the fiber length. Subsequently, the carbonized and pulverized carbon fibers are subjected to 2,000 ° C. or more, more preferably 2500 to 3200.
The above-mentioned carbonaceous material is produced by graphitization at a temperature of ° C. At this time, the crushing and firing steps are extremely important,
At the time of pulverization, it is necessary to graphitize a spherical, block-like, or fibrous material by performing pulverization for an appropriate period of time (within 20 hours) using a ball mill or a jet mill.
Further, a mixture containing at least two types of graphitized products of a spherical shape, a block shape and a fibrous shape may be used. In addition, it is necessary to prevent the crystal structure of the graphitized material from having a rhombohedral system in 30% or more. This is because the hexagonal graphite structure is destroyed by the pulverization, and a rhombohedral structure is formed. However, it is preferable that the pulverization is appropriately reduced to 0 to 30% by volume. If it exceeds this range, the decomposition of the electrolyte solution will occur and the performance of the negative electrode will decrease.

The La of the carbonaceous material produced by the above method
And the crystal structure can be controlled by defining the pulverization conditions (pulverization time, atmosphere, pulverization power).

When the carbonaceous material is produced by the above-described method, it is desirable that the particle size distribution and the specific surface area of the obtained carbonaceous material have the above-mentioned values. The carbonaceous material may be, for example, coke, mesophase spherules or bulk mesophase obtained from low-sulfur petroleum pitch, coal tar or the like as a raw material in addition to those obtained by the above-described method.
It can be obtained by graphitizing at 500 to 3200 ° C.

In the carbonaceous material, the ratio between the rhombohedral system and the hexagonal system can be defined so that the ratio of the (101) diffraction peak by X-ray diffraction is 0.6 or less. A carbonaceous material having a rhombohedral structure having such a ratio can increase the negative electrode charge / discharge efficiency.

(4) Carbonaceous matter This carbonaceous matter has a graphite structure of (00)
2) It consists of a graphitized substance having a diffraction peak of 0.3370 nm or less and a graphitized substance having a range of 0.3370 nm or more and 0.340 nm or less.

The carbonaceous material has a large graphite crystal (a) having a (002) diffraction peak in the range of 0.3370 nm or less.
(Graphite) having an axial thickness La of 40 nm or more) and a (002) diffraction peak exceeding 0.3370 nm.
Relatively small graphite crystals in the range of 340 nm or less (La
Is 40 nm or less), and it is possible to maintain high capacity with little cycle deterioration even in rapid charging. The latter graphitized product preferably has a (002) diffraction peak in the range of 0.3370 to 0.3380 nm.

(002) Diffraction peak: 0.3370 nm
In the following range of the negative electrode made of the graphitized material, the cycle deterioration is large and the capacity is small in the rapid charging. Further, the (002) diffraction peak exceeds 0.3370 nm,
A negative electrode made of a graphitized material in the range of 340 nm has a problem that the capacity is 200 to 280 mAh / cm ³ , which is smaller than that of the graphitized material.

In order to solve such a problem, the carbonaceous material constituting the negative electrode of the present invention can be provided with the two types of graphitized materials having the above characteristics, whereby the rapid charging characteristics and the capacity can be significantly improved. It has become possible. This is mainly due to (00) during rapid charging at constant current ( ² mA / cm ² or more).
2) The diffraction peak exceeds 0.3370 nm and 0.340
Lithium ions are selectively inserted into graphite crystals in the range of nm or less. At this time, the diffusion rate of lithium ions is much faster than that of the graphitized substance having a (002) diffraction peak of 0.3370 nm or less, so that rapid charging is possible. When the rapid charging further proceeds, the charging end voltage is reached, and when the voltage is switched to the constant voltage charging, lithium ions are selectively inserted into graphite crystals having a (002) diffraction peak of 0.3370 nm or less selectively at a moderate rate. I will be. By such a selective insertion reaction of lithium ions, rapid charging becomes possible and high capacity can be maintained.

As the carbonaceous material, for example, the (002) diffraction peak of the graphite structure by the X-ray diffraction method is 0.3370.
nm and less than 0.3370 nm and 0.3
A multi-phase graphitized material comprising a graphitized material in a range of 40 nm or less,
Alternatively, a mixture of a graphitized substance having a (002) diffraction peak of 0.3370 nm or less and a graphitized substance having a (002) diffraction peak of more than 0.3370 nm and 0.340 nm or less can be given.

Examples of the multi-phase graphitized product include polyvinylidene chloride, phenol resin, furfural resin, charcoal, anthracite, sugar, pitch, etc. under normal pressure or high pressure.
It is obtained by performing non-uniform graphitization at a temperature of 500 to 3000 ° C. to form multi-phase graphitization. By performing graphitization non-uniformly in this manner, the graphitized (002) diffraction peak has a phase of 0.3370 nm or less and the low graphitized (002) diffraction peak exceeds 0.3370 nm.
A phase coexisting in the range of 340 nm or less can be obtained.

One of the components of the mixture (002)
Examples of the graphitized material having a diffraction peak of 0.3370 nm or less include powders of graphite, anisotropic pitch-based carbon fibers graphitized at 2300 to 3200 ° C., spherical carbon, pyrolytic vapor grown carbon, and coke. . The graphitized material has a specific surface area of 2 to 10 m ² / g and an average particle size of 1 to 10.
Graphite in the range of 30 μm, powder of mesophase pitch-based carbon fiber, and mesophase spherules are more preferred.

The other component of the mixture (002)
Diffraction peak exceeds 0.3370 nm, 0.340 nm
Graphitized materials in the following ranges include, for example, low sulfur content (2
Powders such as coke having a low nitrogen content (1000 ppm or less), high-purity anisotropic pitch-based carbon fiber, spherical carbon, pyrolytic vapor-grown carbon body, and resin fired body. The graphitized material has a specific surface area of 1 to 10
More preferred are carbon fiber powder, high-purity mesophase pitch-based carbon fiber powder, mesophase spherules, and low sulfur-containing coke having a synthetic pitch (such as naphthalene) in the range of m ² / g. Each of the graphitized products can be obtained by firing the raw material at a temperature in the range of 2300 to 3000 ° C.

The (002) diffraction peak was 0.3370.
The mixing weight ratio of the graphitized material having a diameter of not more than 5 nm is 5 to 30% by weight
Is preferable. The mixing weight ratio is 30% by weight
If it exceeds, the rate characteristics may be degraded. On the other hand, if the mixed weight ratio is less than 5% by weight, the battery capacity may be reduced.

The negative electrode 6 containing the carbonaceous material described in the above (1) to (4) is specifically manufactured by the following method. The binder is suspended in an appropriate solvent in the carbonaceous material, and this suspension is applied to a current collector and dried to form a thin plate, thereby producing the positive electrode.

Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR),
Carboxymethyl cellulose (CMC) or the like can be used.

The mixing ratio of the carbonaceous material and the binder is as follows:
It is preferable that the carbon material is in the range of 90 to 98% by weight and the binder is in the range of 2 to 10% by weight. In particular, the carbonaceous material preferably ranges from 5 to ²⁰ mg / cm ^{2 when} the negative electrode 6 is manufactured.

As the current collector, for example, a copper foil, a stainless steel foil, a nickel foil or the like can be used. The non-aqueous electrolyte contained in the container 1 is prepared by dissolving an electrolyte in a non-aqueous solvent.

As the non-aqueous solvent, a non-aqueous solvent known as a solvent for a lithium secondary battery can be used, and is not particularly limited. Ethylene carbonate (EC) has a melting point lower than that of ethylene carbonate and a donor. It is preferable to use a non-aqueous solvent mainly composed of a mixed solvent with one or more non-aqueous solvents having a number of 18 or less (hereinafter, referred to as a second solvent). Such a non-aqueous solvent is advantageous in that it is stable with respect to the carbonaceous material having a graphite structure that constitutes the negative electrode, hardly causes reductive decomposition or oxidative decomposition of the electrolytic solution, and has high conductivity.

The non-aqueous electrolyte containing only ethylene carbonate has the advantage of being hardly reductively decomposed to the graphitized carbonaceous material, but has a high melting point (39 ° C. to 40 ° C.).
℃) Because of high viscosity, the conductivity is small and not suitable for a secondary battery operated at room temperature. The second solvent mixed with ethylene carbonate has a lower viscosity than the ethylene carbonate in the mixed solvent to improve conductivity. Further, by using the second solvent having a donor number of 18 or less (provided that the number of donors of ethylene carbonate is 16.4), the ethylene carbonate is not easily solvated selectively with lithium ions, and the graphite structure is developed. It is considered that the reduction reaction of the second solvent with respect to the carbonaceous material is suppressed. Further, by setting the number of donors of the second solvent to 18 or less, the oxidative decomposition potential becomes 4 with respect to the lithium electrode.
V or more, and has an advantage that a high-voltage lithium secondary battery can be realized.

Examples of the second type of solvent include dimethyl carbonate (DMC) and diethyl carbonate (D
EC), propylene carbonate (PC), γ-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA), toluene, xylene or methyl acetate (MA). These second solvents are
They can be used alone or in the form of a mixture of two or more. In particular, the second type solvent more preferably has a donor number of 16.5 or less.

The viscosity of the second solvent is 2 at 25 ° C.
It is preferably 8 mp or less. The blending amount of the ethylene carbonate in the mixed solvent is 10 to 10 by volume.
Preferably it is 80%. If you deviate from this range,
There is a possibility that charge and discharge efficiency may decrease due to a decrease in conductivity or decomposition of the solvent. A more preferable blending amount of the ethylene carbonate is 20 to 75% by volume ratio. By increasing the amount of ethylene carbonate in the non-aqueous solvent to 20% by volume or more, solvation of ethylene carbonate with lithium ions is facilitated, so that the effect of suppressing the decomposition of the solvent can be improved.

A more preferable composition of the mixed solvent is EC
And DMC, EC and PC and DMC, EC and DEC, EC and PC and DEC, EC and γ-BL and DEC,
It is preferable that the volume ratio of DMC or DEC is 60% or less. By setting the DEC ratio to 60% or less, more preferably 35% or less, the flash point of the mixed solvent can be increased and the safety can be improved. However, from the viewpoint of further lowering the viscosity, ethers such as diethoxyethane may be added in an amount of 30% by volume or less.

The main impurities present in the mixed solvent (non-aqueous solvent) include water and organic peroxides (eg, glycols, alcohols, carboxylic acids). It is considered that each of the impurities forms an insulating film on the surface of the graphitized material and increases the interfacial resistance of the electrode. Therefore, there is a possibility that the cycle life and capacity may be reduced. In addition, self-discharge during high-temperature (60 ° C. or higher) storage may increase. For this reason, it is preferable that the impurities be reduced as much as possible in the electrolytic solution containing the non-aqueous solvent. Specifically, the water content is 50 pp
m or less, and the amount of the organic peroxide is preferably 1000 ppm or less.

Examples of the electrolyte contained in the non-aqueous electrolyte include lithium perchlorate (LiClO ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium borofluoride (LiBF ₄ ), and lithium arsenide hexafluoride (LiBF ₄ ). LiAsF
₆ ), lithium trifluorometasulfonate (LiCF
₃ SO ₃ ) and lithium bis (trifluoromethylsulfonylimide) [LiN (CF ₃ SO ₂ ) ₂ ]. Among them, LiPE ₆ , LiB
It is preferable to use F ₄ and LiN (CF ₃ SO ₂ ) ₂ . In particular, when LiN (CF ₃ SO ₂ ) ₂ is used, the reaction with the positive electrode active material at a high temperature (for example, 60 ° C.) is small, and excellent charge / discharge cycle characteristics can be obtained at a high temperature. Further, it has an advantage that it is stable with respect to the carbonaceous material and can improve cycle life. The amount of the electrolyte dissolved in the non-aqueous solvent is desirably 0.5 to 2.0 mol / 1.

[0086]

An embodiment of the present invention will be described below in detail with reference to FIG. Example 1 First, lithium cobalt oxide (Li _x CoO ₂ (0.
8 ≦ x ≦ 1)) 91% by weight of powder was added to 3.5% by weight of acetylene black, 3.5% by weight of graphite and 2% by weight of ethylene propylene diene monomer powder and mixed with toluene, and mixed together to form an aluminum foil (30 μm). After coating on the electric body, pressing was performed to produce a positive electrode.

Further, after coke made from low sulfur (content of 8000 ppm or less) petroleum pitch was carbonized at 1000 ° C. in an argon atmosphere, 90% by volume with an average particle diameter of 40 μm and a particle diameter of 1 to 80 μm was present. Then, after appropriately pulverizing so that particles having a particle size of 0.5 μm or less were reduced (5% or less), it was graphitized at 3000 ° C. under vacuum to produce a carbonaceous material.

The obtained carbonaceous material is graphitized carbon powder having an average particle size of 40 μm, and has a particle size distribution of 90 to 90 μm.
% Or more, and the particle size distribution of the particles having a particle size of 0.5 μm or less was 0% by volume. The specific surface area by the N ₂ gas adsorption BET method was 3 m ² / g. The shape of the powder was a block. X-ray diffraction intensity ratio (P ₁₀₁ / P
₁₀₀ ) was 3.6. d ₀₀₂ is 0.3354
nm, Lc is 43 nm, La is 43 nm, La / Lc
Was 1. The ratio between the rhombohedral and the hexagonal (101) diffraction peak intensities by X-ray diffraction was 0.4. The content of sulfur in the carbonaceous material was 100 ppm or less. In addition, the oxygen content was 100 ppm or less, the nitrogen content was 100 ppm or less, and Fe and Ni were each 1 ppm.

Next, 96.7% by weight of the carbonaceous material was mixed with 2.2% by weight of styrene-butadiene rubber and 1.1% by weight of carboxymethylcellulose, and the mixture was applied to a copper foil as a current collector and dried. To produce a negative electrode.

After the positive electrode, the separator made of a porous film made of polyethylene, and the negative electrode were respectively laminated in this order, they were spirally wound so that the negative electrode was located outside, thereby producing an electrode group.

Further, lithium hexafluorophosphate (LiP)
F ₆ ) with ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC)
In a mixed solvent (mixed volume ratio of 40:30:30)
A non-aqueous electrolyte was prepared by dissolving at a molar ratio of 1: 1.

The above-mentioned electrode group and the above-mentioned electrolytic solution were housed in stainless steel bottomed cylindrical containers, respectively, to assemble the above-mentioned cylindrical lithium secondary battery shown in FIG. Example 2 First, bulk mesophase pitch having anisotropy obtained from petroleum pitch was carbonized at 1000 ° C. in an argon atmosphere, and then 9 μm with an average particle size of 15 μm and a particle size of 1 to 80 μm.
The powder was appropriately ground so that 0% by volume was present and particles having a particle size of 0.5 μm or less were reduced (5% or less).
Thereafter, a carbonaceous material was produced by heat treatment at 3000 ° C. under vacuum to graphitize.

The obtained carbonaceous material is a graphitized carbon powder having an average particle size of 15 μm, and has a particle size distribution of 90 to 90 μm.
The distribution of particles having a volume percentage of not less than 0.5 μm was 1% by volume. The specific surface area was 7 m ² / g as determined by the N ₂ gas adsorption BET method. The shape of the powder was spherical. The value of the intensity ratio by X-ray diffraction (P ₁₀₁ / P ₁₀₀₎ was 2.6. d ₀₀₂ is 0.3357nm, Lc is 45nm, La in 58nm, La / Lc was 1.29. The ratio of the (101) diffraction peak intensities of the rhombohedral system and the hexagonal system was 0.5. Further, the content of sulfur in the carbonaceous material was 100 ppm or less. In addition, the oxygen content is 100 ppm or less, and the nitrogen content is 100 p.
pm or less, Fe and Ni were each 1 ppm.

Using the carbonaceous material, a negative electrode was produced in the same manner as in Example 1. A cylindrical lithium secondary battery shown in FIG. 1 described above was assembled in the same manner as in Example 1 except that such a negative electrode was used.

Example 3 Powder A of mesophase pitch-based carbon fiber carbonized at 900 ° C. had an average fiber length of 30 μm, an average fiber diameter of 7 μm, and N
₂ Gas adsorption BET specific surface area 5 m ² / g, sulfur content 1600 ppm, nitrogen content 200 ppm, Fe 5 ppm, Ni 3 ppm, (002) diffraction peak of graphite structure by X-ray diffraction 0.361 nm, Lc Is 1.
4 nm and La were 3.9 nm.

The powder B of mesophase pitch-based carbon fiber graphitized at 3100 ° C. has an average fiber length of 40 μm, an average fiber diameter of 12 μm, a specific surface area of 3.8 m ² / g by N ₂ gas adsorption BET method, and a sulfur content of 100 ppm or less. At a nitrogen content of 100 ppm or less, the (002) diffraction peak of the graphite structure by X-ray diffraction was 0.3365 nm, and Lc was 37.
nm and La were 67 nm, and P ₁₀₁ / P ₁₀₀ was 2.3.

The powder A and the powder B were mixed in a weight ratio of 1: 2.
A negative electrode was produced in the same manner as in Example 1 by using the carbonaceous material obtained by mixing the carbonaceous materials described above. A cylindrical lithium secondary battery shown in FIG. 1 described above was assembled in the same manner as in Example 1 except that such a negative electrode was used.

Example 4 Powder A of mesophase pitch-based carbon fiber carbonized at 1000 ° C. had an average fiber length of 30 μm, an average fiber diameter of 7 μm,
N ₂ gas adsorption BET method according to a specific surface area of 5 m ² / g, sulfur content 1500 ppm, nitrogen content 100 ppm, Fe
Was 5 ppm and Ni was 3 ppm. The (002) diffraction peak of the graphite structure by X-ray diffraction was 0.360 nm, Lc was 1.48 nm, and La was 3.4 nm.

The artificial graphite powder B obtained by graphitizing coke at 3000 ° C. has an average particle diameter of 40 μm, a specific surface area of 3.3 m ² / g by an N ₂ gas adsorption BET method, and a sulfur content of 100 p.
pm or less, nitrogen content 100 ppm or less, Fe and Ni are each 1 ppm, and the graphite structure (002)
The diffraction peak is 0.3354 nm, Lc is 45 nm, La
Was 60 nm.

The powder A and the powder B were mixed at a weight ratio of 1: 1.
A negative electrode was produced in the same manner as in Example 1 using the carbonaceous material obtained by mixing the above. A cylindrical lithium secondary battery shown in FIG. 1 described above was assembled in the same manner as in Example 1 except that such a negative electrode was used.

Comparative Example 1 Natural graphite was pulverized to an average particle size of 6 μm to obtain a graphite powder. The graphite powder had a particle size distribution of 1 to 80 μm in particle size of 85% by volume, and the distribution of particles having a particle size of 0.5 μm or less was 10% by volume. N ₂ gas adsorption B
The specific surface area by the ET method was 10 m ² / g. The value of the intensity ratio by X-ray diffraction (P ₁₀₁ / P ₁₀₀₎ was 2.1. d ₀₀₂ is 0.3357 nm, Lc is 33 nm, L
a was 60 nm. La / Lc is 1.82.
The ratio of the (101) diffraction peak intensities of the rhombohedral system and the hexagonal system was 0.85.

Using the graphite powder, a negative electrode was produced in the same manner as in Example 1. A cylindrical lithium secondary battery shown in FIG. 1 described above was assembled in the same manner as in Example 1 except that such a negative electrode was used.

Comparative Example 2 Example 1 using only the artificial graphite powder B of Example 4 above
A negative electrode was produced in the same manner as in the above. A cylindrical lithium secondary battery shown in FIG. 1 described above was assembled in the same manner as in Example 1 except that such a negative electrode was used.

Comparative Example 3 Powder A of Mesophase Pitch-Based Carbon Fiber of Example 3
A negative electrode was produced in the same manner as in Example 1 using only the same. A cylindrical lithium secondary battery shown in FIG. 1 described above was assembled in the same manner as in Example 1 except that such a negative electrode was used.

The obtained Examples 1 to 4 and Comparative Examples 1 to 3
The lithium secondary battery was charged at a charging current of 400 mA for 3 hours to 4.2 V, and repeatedly charged and discharged at a high rate current of 1 A to 2.7 V, and the discharge capacity and cycle life of each battery were measured. did. The result is shown in FIG.

As is apparent from FIG. 2, the lithium secondary batteries of Examples 1 to 4 were different from the batteries of Comparative Examples 1 to 3.
It can be seen that the capacity is high and the cycle life is significantly improved even at high rate discharge.

On the other hand, LiN (CF ₃ S
When the same evaluation was performed using O ₂ ) ₂ , it was confirmed that the cycle life was longer than in Examples 1 to 4. Example 5 First, a lithium cobalt oxide (Li _x CoO ₂ (0.
8 ≦ x ≦ 1)) 91% by weight of powder was added to 3.5% by weight of acetylene black, 3.5% by weight of graphite and 2% by weight of ethylene propylene diene monomer powder and mixed with toluene, and mixed together to form an aluminum foil (30 μm). After coating on the electric body, pressing was performed to produce a positive electrode.

After carbonizing coke from low sulfur (content of 8000 ppm or less) petroleum pitch as a raw material at 1000 ° C. in an argon atmosphere, an average particle size of 20 μm, a particle size of 1 to 80 μm, and 90 vol% are present. And then pulverized in a block shape so that particles having a particle size of 0.5 μm or less (5% or less) are reduced.
A carbonaceous material was produced by graphitization at 0 ° C.

The obtained carbonaceous material is a block-like graphitized carbon powder having an average particle size of 20 μm, and has a particle size distribution of 1 to 8 μm.
90% by volume or more was present at 0 μm, and the particle size distribution of the particles having a particle size of 0.5 μm or less was 0% by volume. The specific surface area by the N ₂ gas adsorption BET method was 3 m ² / g. The shape of the powder was granular. Intensity ratio by X-ray diffraction (P
₁₀₁ / value of P ₁₀₀₎ was 3.6. d ₀₀₂ is 0.
3354 nm, Lc is 43 nm, La is 43 nm, L
a / Lc was 1. The ratio of the (101) diffraction peak intensity of the rhombohedral system to the hexagonal system by X-ray diffraction is 0.4
Met. The sulfur content in the carbonaceous material is 100 p.
pm or less. In addition, the oxygen content is 100pp
m, the content of nitrogen was 100 ppm or less, and Fe and Ni were each 1 ppm.

Next, 96.7% by weight of the carbonaceous material was mixed with 2.2% by weight of styrene-butadiene rubber and 1.1% by weight of carboxymethylcellulose, and the mixture was applied to a copper foil as a current collector and dried. To produce a negative electrode.

After laminating the positive electrode, the separator made of a porous film made of polyethylene, and the negative electrode in this order, they were spirally wound so that the negative electrode was located on the outside, thereby producing an electrode group.

Further, lithium hexafluorophosphate (LiP)
F ₆ ) with ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC)
In a mixed solvent (mixed volume ratio of 40:30:30)
A non-aqueous electrolyte was prepared by dissolving at a molar ratio of 1: 1.

The above-mentioned electrode group and the above-mentioned electrolytic solution were respectively housed in a stainless steel bottomed cylindrical container to assemble the above-mentioned cylindrical lithium secondary battery shown in FIG. Example 6 First, a mesophase pitch having anisotropy obtained from a petroleum pitch was spun, and carbon fibers obtained by infusibilization were carbonized at 1000 ° C. in an argon atmosphere. 90% by volume is present at 1 to 80 μm, and particles having a particle size of 0.5 μm or less are reduced (5%
Below). Then, it was heat-treated at a temperature of 3100 ° C. in an argon atmosphere to be graphitized, thereby producing a fibrous carbonaceous material. The carbonaceous material is
The orientation in the cross section perpendicular to the length direction of the fiber was radial.

The carbonaceous material obtained had an average fiber diameter of 15 μm.
And a particle size distribution of 9 to 1 to 80 μm.
0% by volume or more was present, and the distribution of particles having a particle size of 0.5 μm or less was 1% by volume. The specific surface area was 4 m ² / g as determined by the N ₂ gas adsorption BET method. The shape of the powder was a fiber having a length-to-diameter ratio (length / diameter) of 3. The value of the intensity ratio by X-ray diffraction (P ₁₀₁ / P ₁₀₀₎ was 2.6. d ₀₀₂ is 0.3357 nm, Lc is 45 nm, La
Was 58 nm and La / Lc was 1.29. In addition,
The ratio of the (101) diffraction peak intensity between the rhombohedral system and the hexagonal system was 0.5. Further, the content of sulfur in the carbonaceous material was 100 ppm or less. Other, oxygen content 10
0 ppm or less, nitrogen content is 100 ppm or less, F
e and Ni were each 1 ppm.

Using the carbonaceous material, a negative electrode was produced in the same manner as in Example 5. A cylindrical lithium secondary battery shown in FIG. 1 described above was assembled in the same manner as in Example 5 except that this negative electrode was used.

Example 7 Powder A of mesophase pitch-based carbon fiber graphitized at 3000 ° C. had an average fiber length of 40 μm, an average fiber diameter of 7 μm,
Specific surface area by N ₂ gas adsorption BET method 3 m ² / g, sulfur content 1600 ppm, nitrogen content 200 ppm, Fe
Was 5 ppm and Ni was 3 ppm. The (002) diffraction peak of the graphite structure by X-ray diffraction was 0.3375 nm, Lc was 30 nm, and La was 60 nm.

The petroleum coke powder B obtained by graphitization and pulverization at 3100 ° C. has an average particle size of 6 μm.
It is a flaky powder having a specific surface area of 9 m ² / g by the N ₂ gas adsorption BET method, a sulfur content of 100 ppm or less, a nitrogen content of 100 ppm or less, and a graphite structure (0
02) Diffraction peak: 0.3358 nm, Lc: 60 n
m, La is 120nm, P ₁₀₁ / P ₁₀₀ was 2.5.

The powder A and the powder B were mixed at a weight ratio of 4: 1.
A negative electrode was prepared in the same manner as in Example 5 using the carbonaceous material obtained by mixing the above. A cylindrical lithium secondary battery shown in FIG. 1 described above was assembled in the same manner as in Example 5 except that this negative electrode was used.

Example 8 Mesophase pitch microspheres graphitized at 3000 ° C. are obtained by oxidizing and removing a surface layer having an average particle diameter of 20 μm and a thickness of 1 to 5 μm from the surface to expose a lamella layer. Also,
The mesophase pitch microspheres have a specific surface area of 5 m ² / g and a sulfur content of 200 pp by an N ₂ gas adsorption BET method.
m, nitrogen content 100 ppm, Fe 5 ppm, Ni
Is 3 ppm, the (002) diffraction peak of the graphite structure by X-ray diffraction is 0.3365 nm, Lc is 60 nm, La is 100 nm, and the intensity ratio by X-ray diffraction (P ₁₀₁ / P ₁₀₀ )
Is a 2.3 spherical graphite material.

A negative electrode was produced in the same manner as in Example 5 using the above graphitized material (carbonaceous material). A cylindrical lithium secondary battery shown in FIG. 1 described above was assembled in the same manner as in Example 5 except that this negative electrode was used.

Comparative Example 4 Natural graphite was ground to an average particle size of 6 μm to obtain flaky graphite powder. This graphite powder has a particle size distribution of 1 to 8
There are 85% by volume of particles having a particle size of 0 μm and a particle size of 0.
The distribution of particles having a size of 5 μm or less was 5% by volume. The specific surface area by N ₂ gas adsorption BET method was 10 m ² / g.
The value of the intensity ratio (P ₁₀₁ / P ₁₀₀ ) determined by X-ray diffraction is 3.
It was 6. d ₀₀₂ is 0.3357 nm, Lc is 60 n
m and La were 120 nm. La / Lc is 1.82
It is. The ratio between the (101) diffraction peak intensities of the rhombohedral system and the hexagonal system was 0.85.

A negative electrode was produced in the same manner as in Example 5 using the above graphite powder. A cylindrical lithium secondary battery shown in FIG. 1 described above was assembled in the same manner as in Example 5 except that this negative electrode was used.

Comparative Example 5 A negative electrode was produced in the same manner as in Example 5 using only the graphitized petroleum coke powder B of Example 7 described above. 1 except that this negative electrode was used.
Was assembled.

Comparative Example 6 Mesophase pitch-based carbon fiber powder A of Example 7
A negative electrode was produced in the same manner as in Example 5 using only the above. A cylindrical lithium secondary battery shown in FIG. 1 described above was assembled in the same manner as in Example 5 except that this negative electrode was used.

The obtained Examples 5 to 8 and Comparative Examples 4 to 6
The lithium secondary battery was charged at a charging current of 400 mA for 3 hours to 4.2 V, and repeatedly charged and discharged at a high rate current of 1 A to 2.7 V, and the discharge capacity and cycle life of each battery were measured. did. The result is shown in FIG.

As is clear from FIG. 3, the lithium secondary batteries of Examples 5 to 8 are different from the batteries of Comparative Examples 4 and 6 in that:
It can be seen that the capacity is high and the cycle life is significantly improved even at high rate discharge.

On the other hand, LiN (CF ₃ S
When the same evaluation was performed using O ₂ ) ₂ , it was confirmed that the cycle life was longer than in Examples 5 to 8. In the above embodiment, an example in which the present invention is applied to a cylindrical lithium secondary battery is described. However, the present invention can be similarly applied to a prismatic lithium secondary battery. Further, the electrode group housed in the battery container is not limited to the spiral shape, but may be a form in which a plurality of positive electrodes, separators, and negative electrodes are stacked in this order.

[0128]

As described above in detail, according to the present invention, it is possible to provide a lithium secondary battery capable of maintaining a high voltage, a high cycle life, and a high voltage over a long period of use. .

[Brief description of the drawings]

FIG. 1 is a partial sectional view showing a cylindrical lithium secondary battery according to the present invention.

FIG. 2 is a characteristic diagram showing a relationship between a charge / discharge cycle and a discharge capacity in the lithium secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 3.

FIG. 3 is a characteristic diagram showing a relationship between a charge / discharge cycle and a discharge capacity in the lithium secondary batteries of Examples 5 to 8 and Comparative Examples 4 and 6.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... container, 3 ... electrode group, 4 ... positive electrode, 6 ... negative electrode, 8 ... sealing plate.

Claims (2)

[Claims] 1. A lithium secondary battery comprising a positive electrode, a negative electrode containing a carbonaceous material that occludes and releases lithium ions, and a non-aqueous electrolyte, wherein the carbonaceous material is selected from graphite and graphitized coke. A lithium secondary battery comprising at least one kind of powder and mesophase pitch-based carbon fibers. 2. The mesophase pitch-based carbon fiber,
2. The lithium secondary battery according to claim 1, wherein the orientation of crystallites in a cross section perpendicular to the fiber length direction is a radial, lamellar or Brooks-Taylor type.