JPH0821375B2 - Negative electrode for lithium secondary battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to a negative electrode material for a non-aqueous electrolyte lithium secondary battery. More specifically, the present invention relates to a negative electrode material suitable for a high-performance non-aqueous electrolyte lithium secondary battery that requires long life and high energy density.

2. Description of the Related Art Generally, a battery using an alkali metal as a negative electrode active material has many advantages such as high energy density, light weight and small size, and excellent long-term storage stability because it uses a non-aqueous electrolyte solution. It has been put to practical use as a primary battery and is widely used.

However, when this negative electrode active material is used as a negative electrode of a secondary battery, a new problem not found in the primary battery occurs. That is, the secondary battery using lithium as a negative electrode has problems such as short cycle life of charge / discharge and low discharge efficiency. This is because the lithium deposited on the negative electrode during charging forms so-called dendrites (dendritic crystals).

The following methods have been studied as a solution to the problems in converting lithium into a secondary battery. That is, improving the electrolytic solution, using an alloy containing lithium in the negative electrode, using a conductive polymer in the negative electrode, and using a carbon material in the negative electrode.

To improve the electrolyte solution, compound organic solvents, add a small amount of solvent (for example, electrochemistry and industrial physical chemistry, 57 , 523 (1
989)) is being considered. These methods rely on the fact that the deposition state of lithium during charging changes depending on the organic solvent, but at present, no electrolytic solution that can completely eliminate the generation of dendrites has been found.

As an alloy containing lithium, for example, a Li-Al alloy, a wood alloy, etc. have been studied (National Tech. Report,
Vol.32, No.5 (1986)). By using these alloys for the negative electrode, the precipitated lithium is alloyed and taken into the electrode, so that the growth of dendrite can be eliminated in principle. However, since the electrode expands and contracts due to the change in the crystal structure of the alloy depending on the lithium concentration, it is not possible to charge and discharge the lithium concentration largely, and the diffusion of lithium into the alloy is rate-determining. Therefore, there is a problem that the current density cannot be increased.

In addition, the conductive polymer applies lithium doping / dedoping to the negative electrode reaction. Since lithium is doped into the polymer without depositing, dendrite generation can be eliminated in principle (solid physics, 17 , 793 (198
2)). However, the potential of the lithium-doped polymer is 1.2 V (lithium standard), which is significantly higher than that of lithium, and the lithium doping amount cannot be increased due to deterioration of the charge / discharge cycle life. The problem remains.

As described above, the three types of means do not always succeed in solving the drawbacks of the negative electrode. A method that uses a carbon material as a negative electrode active material has been drawing attention in recent years as a new alternative to these methods. By using a carbon material for the negative electrode, lithium is taken in between the carbon layers during charging (intercalation reaction) and forms a so-called graphite intercalation compound. Therefore, the generation of dendrites can be eliminated in principle.

Further, the negative electrode using the carbon material is a lithium-containing alloy,
Compared with the case of using a conductive polymer, it is chemically stable against lithium metal, theoretically it is possible to incorporate lithium up to C ₆ Li, and the light weight of the carbon material,
Combined with good conductivity, it has the advantage of being able to increase the energy density of the negative electrode material. Furthermore, because of the intrinsic reversibility of the reaction, which is a characteristic of the intercalation reaction,
It has a long life with repeated charge and discharge and is expected as a negative electrode for secondary batteries.

As a carbon material suitable for a negative electrode of a lithium secondary battery having the above characteristics, the inventors of the present application have proposed an X-ray diffraction method,
In addition, it has been clarified that carbon fibers whose raw material has a pitch defined by Raman spectroscopy show excellent performance, and a patent application has already been made (see Japanese Patent Laid-Open No. 2-82466). However, when this pitch-based carbon fiber is used as the negative electrode and charging and discharging are repeated, there is a tendency that the discharge capacity tends to decrease with the cycle, and it was necessary to improve the cycle stability of the capacity.

The present invention has been made in view of the current situation as described above,
An object of the present invention is to provide a pitch-based carbon fiber having a high stability of discharge capacity against repeated charging / discharging and suitable for a negative electrode for a non-aqueous electrolyte-based lithium secondary battery.

Means for Solving the Problems As a result of extensive studies on the structure of the pitch-based carbon fiber in order to solve the above-mentioned problems, the present inventors have found that a graphite crystallite having an appropriate size and a non-circular structure surrounding it. It has been found that the composite structure, which is an aggregate of crystalline portions, is essentially important for the cycle stability of lithium secondary batteries. Furthermore, the purpose of defining this composite structure is to use, in addition to the index by the X-ray diffraction method that was conventionally used for defining carbon materials for batteries, the orientation coefficient that defines the size of crystallites and the carbon by TEM photographs. Two indicators of the size of the graphite crystallite in the fiber cross-section direction,
The present invention has been found out to be the most suitable for the regulation of carbon fiber.

That is, the present invention relates to a lithium secondary battery using an organic electrolytic solution in which a lithium salt is dissolved in an organic solvent, and relates to a negative electrode characterized by using carbon fiber defined below as an active material. That is, the lattice spacing (d ₀₀₂ ) which is a crystallite parameter by the X-ray diffraction method is 0.338 nm or more and 0.343 nm or more.
Below, the crystallite size (Lc) in the c-axis direction is 10 nm to 30 nm.
A carbon fiber having an orientation coefficient (FWHM) of 7 ° or more and 20 ° or less, and an average length in the fiber cross-section direction of graphite crystallites elongated in the fiber axis direction of 20 nm or more and 100 nm or less is there.

The method of expressing the above-mentioned various physical properties used in the definition of carbon fiber will be described below: (1) X-ray diffraction method CuKα is an X-ray source, high-purity silicon is used as a standard substance, and The 002 diffraction pattern is measured, and the lattice plane spacing d ₀₀₂ and the average length Lc of the graphite crystallite in the c-axis direction are calculated from the peak position and the half width. The calculation method is
For example, it is described on pages 733 to 742 of "Carbon Fiber" (Modern Editing Co., Ltd., published in March 1986).

(2) Orientation coefficient of carbon fiber by X-ray diffraction With a detector that irradiates X-rays perpendicularly to the plane containing the straight carbon fiber bundle and sets the transmitted diffracted light in the direction in which the 002 plane diffraction becomes maximum. To detect. When the diffracted light is measured by rotating the fiber bundle in a plane perpendicular to the incident X-ray with the X-ray diffraction measurement system fixed under these conditions, the diffraction pattern becomes a periodic function of the fiber rotation angle of 180 ° One peak is shown for each. The orientation coefficient is the full width at half maximum of this peak (FWHM)
Is defined as

(3) Average length in the cross-sectional direction of graphite crystallites extending in the fiber axis direction Measured by a transmission electron microscope (TEM) as described below.

After embedding the carbon fibers in the resin, a thin piece is obtained using, for example, a diamond cutter. At this time, the surface of the thin piece is parallel to the carbon fiber axis. When the 002 dark-field image of the thin piece is observed using a transmission electron microscope, lattice stripes of white bands and black bands parallel to the fiber axis are observed. The average length l in the cross-sectional direction of the meso-face region extending in the fiber axis direction from the average interval Δ of the lattice fringes and the magnification A of the image is calculated by the following formula: l = Δ / A or less, The negative electrode using the pitch-based carbon fiber of the invention will be described in detail.

The raw material pitch for spinning used in the present invention is not limited as long as it is formed of molecular species that are easily oriented and gives an optically anisotropic pitch, that is, a mesophase pitch, and various mesophase pitches are used. Face pitch can be used. Examples of the carbonaceous raw material for obtaining these mesophase pitches include coal-based coal tar pitch,
Examples include heavy petroleum oil and pitch. The pitch for spinning in the present invention is basically desirable to be graphitizable, and a pitch containing a mesophase content of 70% or more, preferably 90% or more, and optimally 95% or more is suitable. is there.

In order to obtain a pitch-based carbon fiber suitable for a lithium secondary battery negative electrode having high cycle stability against repeated charging / discharging, the present inventors have made detailed studies on a carbon fiber production process and an evaluation method. As a result, the essential point in the structure of the carbon fiber is to suppress the development of the graphite crystal structure by subdividing the graphitizable mesophase pitch in the manufacturing process such as the spinning process. It has been found. A fiber having a finely divided mesophase as an internal structure is composed of a region in which a graphite crystal structure is developed (hereinafter referred to as a graphite crystallite) and an amorphous portion that connects these graphite crystallites to each other. Is. That is, it has been found that the composite structure composed of the graphite crystallites and the amorphous portion connecting them is the most suitable structure for the negative electrode for the lithium secondary battery.

The reason why the carbon fiber having the composite structure of the crystallite and the amorphous portion in the lithium secondary battery negative electrode, which is claimed by the present invention, has excellent characteristics as a lithium secondary battery negative electrode active material will be described below. .

During charging, lithium is inserted into the negative electrode carbon fiber from the electrolytic solution. And the lithium in the carbon fiber is
It is selectively inserted in a graphite crystallite with a developed energetically stable crystal structure. The intercalation of the carbon layer surface that constitutes the graphite crystallite is expanded by the insertion of lithium, and at the same time the graphite crystallite expands, but this expansion is because the amorphous part surrounding the crystallite elastically absorbs it. , It does not lead to the destruction of the macroscopic structure of carbon fiber.

During discharge, the lithium in the carbon fiber is released into the electrolytic solution. When lithium is released from the layers, the layers shrink, but the amorphous part surrounding the graphite crystallites elastically restores the crystallites to their original state. It is not accompanied by.

That is, the insertion reaction of lithium into the graphite crystallite causes expansion of the graphite crystallite, which is an irreversible destruction of the crystal structure, but the carbon fiber claimed by the present invention has The amorphous portion is surrounded, and the amorphous portion elastically absorbs the expansion and contraction of the graphite crystallite, thereby exhibiting reversibility. In addition, in the lithium release reaction from the graphite crystallite, the expanded graphite crystallite contracts to the original state, but this contraction causes the elasticity of the amorphous part to destroy the structure of the macro carbon fiber. I can't reach it.

That is, the conditions of the carbon fiber suitable for stable repeated charging and discharging are the following two points. One is the presence of a developed portion of the graphite structure, which is the field of reaction, and the other is the presence of an amorphous portion that elastically absorbs expansion and contraction.

As described above, the important point in the present invention is to suppress the development of graphite crystallites, that is, by finely dividing the mesophase pitch, the crystallites and the amorphous portion surrounding them are forcibly forced into the carbon fiber. Is to introduce a complex structure.

The physical properties of the carbon fiber having such a structure are expressed as follows; (1) In the crystallite parameters by the X-ray diffraction method, the lattice spacing (d ₀₀₂ ) is 0.338 nm or more and 0.343 nm or less, Also, the crystallite size (Lc) in the c-axis direction is 10 nm or more and 30 nm or less.

(2) The orientation coefficient (FWHM) in the X-ray diffraction method is 7 ° or more and 20 ° or less.

(3) The average length in the cross-sectional direction of the graphite crystallite extending in the fiber axis direction is 20 nm or more and 100 nm or less.

Among the above conditions, (1) defines the range suitable for the negative electrode in the average degree of graphitization of carbon fiber, and (2) corresponds to the length of the graphite crystallite extending in the fiber axis direction. . That is,
If the length of the graphite crystallite is long, the graphite crystallite tends to be aligned parallel to the fiber axis, so that the crystallite orientation develops and the orientation angle decreases. (3) defines the size of the graphite crystallite in the cross-sectional direction. (2), (3)
Is a regulation regarding the size of the graphite crystallite. Thus, focusing on the internal structure of the carbon fiber, we found that the essence of the optimum structure for the negative electrode of the Li secondary battery was a composite structure consisting of a graphite crystallite and an amorphous part, and d by X-ray diffraction The greatest feature of the present invention is that the composite structure can be defined by ₀₀₂ , Lc, the orientation angle of the graphite crystallites, and the size of the graphite crystallites in the fiber cross-sectional direction. By using the carbon fiber defined in this way, it is possible to provide a stable negative electrode against repeated charge and discharge.

The carbon fiber defined in the present invention is preferably produced by the following methods alone or in combination.

(1) Ultrafine Fiber The finer the raw material pitch, the more the growth of crystallites in the cross-sectional direction of the carbon fiber can be suppressed, and at the same time, the orientation of crystallites can be disturbed. And, due to the effects of both, the average degree of graphitization of the carbon fibers by X-ray diffraction can be suppressed. Specific physical properties of carbon fiber are not determined only by the diameter of carbon fiber, but the properties of raw material pitch,
The diameter of the carbon fiber suitable for the present invention is 8 μm, although it varies depending on the pitch viscosity during spinning, the shape of the spinning nozzle, and the like.
It is the following.

(2) Subdivision of mesophase by mesh filter In the spinning process, a net (mesh filter) is installed in advance in the flow path of the pitch until the raw material pitch is discharged from the spinning nozzle, and the pitch is subdivided according to the eyes of the filter. To do so. The carbon fiber spun in this way is
In the cross section, the mesophase is subdivided according to the mesh filter, so that the growth of graphite crystallites can be suppressed and the orientation of crystallites can also be suppressed. And, due to the effects of both, the average degree of graphitization of the carbon fibers by X-ray diffraction can be suppressed.

(3) Fragmentation of mesophase by stirring In the spinning process, a stirrer is installed in advance in the pitch flow path until the raw material pitch is discharged from the spinning nozzle, and the pitch is subdivided by the rotation of the stirrer. . The carbon fiber spun in this manner has a cross-sectional direction, and
Since the mesophase is subdivided in the fiber axis direction, the development of graphite crystallites is suppressed and the crystallite orientation is also suppressed. And, due to the effects of both, the average degree of graphitization of the carbon fibers by X-ray diffraction can be suppressed.

The graphitization treatment of carbon fibers is carried out in an inert atmosphere, and the temperature thereof is 2000 ° C or higher, preferably 2600 ° C or higher. The stability of the carbon fiber to the charge / discharge cycle basically requires a developed carbon layer surface, and thus graphitization treatment at a somewhat high temperature is desirable.

The present invention relates to a negative electrode for a lithium secondary battery, which is characterized by using a carbon fiber having a structure suitable for the reaction of inserting and releasing lithium as described above, and is an electrode using the carbon fiber of the present invention. If so, it does not limit the shape of the electrode.

As the electrolyte of the lithium secondary battery in the present invention, lithium perchlorate (LiClO ₄ ) and lithium borofluoride (LiBF
_4), lithium hexafluoro antimonate (LiSbF _6), six lithium chloride antimonate (LiSbCl _6), hexafluoride Hisosan lithium (LiAsF _6), lithium hexafluoro phosphate (LiP
F ₆ ), lithium trifluoromethyl sulfonate (LiCF ₃ S
Lithium salts such as O ₃ ) are preferably used. The lithium salt used as the electrolyte is basically required to have stability of the anion in the electrochemical reaction. Therefore, as long as this condition is satisfied, the lithium salt is particularly limited to the above lithium salt. is not.

The negative electrode using the carbon fiber provided by the present invention,
It is put into practical use by being incorporated in a lithium secondary battery as a negative electrode by a usual method. That is, as the positive electrode, for example,
A transition metal chalcogen compound, a conjugated polymer compound, activated carbon, or the like may be used. A synthetic fiber non-woven fabric, a woven fabric, a glass fiber non-woven fabric, a woven fabric, or the like can be used as a separator between the negative electrode and the positive electrode. The negative electrode, the positive electrode, and the separator are immersed in an electrolytic solution or the separator is impregnated with the electrolytic solution to form a battery.

The organic solvent used in the electrolytic solution of the lithium secondary battery in the present invention may be one capable of dissolving a lithium salt, but is preferably an aprotic organic solvent, has a large dielectric constant and is stable against redox. Is high, so-called
It is desired to have characteristics such as a wide electrochemical potential window and low viscosity.

Specifically, propylene carbonate, ethylene carbonate, sulfolane, γ-butyrolactone, 1,2-dimethoxyethane,
Tetrahydrofuran, 2 methyl-tetrahydrofuran,
A solvent such as acetonitrile or dimethyl sulfoxide, or a mixed solvent thereof can be used. The concentration of the electrolytic solution depends on the type of solvent and electrolyte and the electrode material, but is 0.
The range of 1 to 10 mol / liter is preferable.

Examples The following examples further illustrate the present invention.

Example 1 A pitch fiber having a diameter of 6.2 μm was spun from a pitch having a mesophase content of 95% as a raw material, and graphitized at 2800 ° C. to obtain a carbon fiber. This carbon fiber is 20mm ~ 30m long
A negative electrode was prepared by bundling nickel wire having a diameter of 0.1 mm with m and weight of 10 mg. The above carbon fiber has a diameter after graphitization,
It was about 4.5 μm.

The crystal parameters of this carbon fiber by the X-ray diffraction method are
d ₀₀₂ = 0.3391 nm and Lc = 21.1 nm. The orientation coefficient is 9.
The size of the graphite crystallite in the cross-sectional direction of the fiber at 2 °, TEM photograph was about 45 nm.

For measurement, a metal lithium sheet of about 0.1 g was crimped onto a nickel net as the counter electrode, and a small piece of metal lithium sheet was connected to a nickel wire as with the counter electrode.
A triode cell using a solution of F ₆ dissolved in propylene carbonate at a concentration of 1.5 mol / liter as an electrolytic solution was installed in a closed glass container using argon gas as an atmosphere gas. The potential change of the negative electrode when charging and discharging at constant current was repeated was measured with the reference electrode as a reference.

A constant current charge / discharge cycle test was performed. Both charging current and discharging current were 0.3 mA. Charging, that is, the lithium insertion reaction ends when the potential of the test electrode with respect to the reference electrode is 0 V and becomes the same potential as the lithium metal, and after an open state for 30 minutes, discharge, that is, the lithium release reaction. I switched. The discharge reaction was terminated when the potential of the test electrode with respect to the reference electrode became 1 V, and after the open state for 30 minutes, the battery was charged again.

The natural potential of carbon fiber was about 3V. As a result of charging and discharging under these conditions, the discharge efficiency is about 80% in the first cycle, but 96% or more 99% after the second cycle.
It was stable below%. The change in the discharge curve due to repeated charging and discharging was small, and the discharge capacity was stable at 190 mAhr / g.

Example 2 Under the same pitch raw material and spinning conditions as in Example 1, pitch fibers were spun into a diameter of 4 μm and graphitized at 2800 ° C. to obtain carbon fibers. The diameter of the carbon fiber after graphitization was about 3 μm.

The crystal parameters of this carbon fiber by the X-ray diffraction method are
d ₀₀₂ = 0.3401 nm and Lc = 17.7 nm. The orientation coefficient is 1
The size of the graphite crystallite in the fiber cross-sectional direction at 1.3 ° by a TEM photograph was about 32 nm.

As a result of testing under the same conditions as in Example 1, the discharge efficiency in the first cycle was 86%, and the discharge efficiency in the second and subsequent cycles was 96% or more and 100% or less. The discharge capacity was 210 mAhr / g, and the cycle was stable without a decrease in capacity.

FIG. 1 shows the cycle change of the discharge capacity of the carbon fiber used as the negative electrode in Example 1 and Example 2.

Example 3 Using a raw material pitch having a mesophase content of 96%, 400
Spinning was performed using a mesh filter, and pitch fibers were made to have a diameter of 8.5 μm, and then heat treated at 2800 ° C. The diameter of the carbon fiber after heat treatment was about 6.4 μm.

The index of graphitization degree of this carbon fiber by X-ray diffraction method is
d ₀₀₂ = 0.3395 nm and Lc = 22.1 nm. The orientation coefficient is 9.
It was 2 °. The size of the graphite crystallite in the cross-sectional direction of the fiber was about 40 nm on the TEM photograph.

Using a solution of LiClO ₄ dissolved in propylene carbonate at a concentration of 1.5 mol / liter as the electrolytic solution, the other conditions were the same as in Example 1, and the results were tested. As a result, stable charge / discharge was repeated at 170 mAhr / g. It was

Example 4 The same pitch as in Example 3 was used as a raw material, and a 3000-mesh mesh filter was used to obtain pitch fibers having a diameter of 4 μm
A carbon fiber obtained by heat-treating the spun fiber at 2800 ° C. was tested as a negative electrode. Diameter of carbon fiber after heat treatment is about 3μm
Met.

The index of graphitization degree of this carbon fiber by X-ray diffraction method is
d ₀₀₂ = 3.413A, was Lc = 133A,. The orientation coefficient is 17.2
It was °. The size of the graphite crystallite in the cross-sectional direction of the fiber was about 25 nm on the TEM photograph.

As a result of a charge / discharge test conducted under the same conditions as in Example 1, the battery was stably cycled at a discharge capacity of 160 mAhr / g.

FIG. 2 shows the cycle changes of the discharge capacities of Example 3 and Example 4.

Example 5 A raw material pitch having a mesophase content of 96% was placed in a spinning machine equipped with a stirring bar, and spun with a pitch fiber to a diameter of about 8 μm while rotating the stirring bar at 30 rpm. Then, it heat-processed at 2800 degreeC and obtained the carbon fiber. The diameter of the carbon fiber was about 6 μm.

The index of graphitization degree of this carbon fiber by X-ray diffraction method is
It was d ₀₀₂ = 0.3396 nm and Lc = 20.8 nm. The orientation coefficient is
It was 11.2 °. The size of the graphite crystallite in the cross-sectional direction of the fiber was about 30 nm on the TEM photograph.

As a result of a charge / discharge test conducted under the same conditions as in Example 1, a stable cycle behavior was exhibited at a discharge capacity of 180 capacity mAhr / g.

Comparative Example 1 A carbon fiber was prepared by using pitch having a mesophase content of 96% as a raw material, spinning the pitch fiber to a diameter of 15 μm, and then heat-treating the carbon fiber at 2900 ° C. The diameter of this carbon fiber is approximately 11.4.
μm.

The index of graphitization degree by X-ray diffraction method is d ₀₀₂ = 0.3379n
m, Lc = 39.7 nm. The orientation coefficient was 5.6 °. The size of the graphite crystallite in the cross-sectional direction of the fiber was about 160 nm according to the TEM photograph.

As a result of the same test as in Example 1, the initial capacity was 15
It was 0 nAhr / g, but its capacity decreased almost in proportion to the number of cycles when the charge and discharge cycles were repeated. FIG. 2 shows the cycle change of the discharge capacity in combination with Example 3 and Example 4.

EFFECTS OF THE INVENTION According to the present invention, it is possible to provide a carbon material having a large discharge capacity and a high stability of the discharge capacity against repeated charge and discharge, which is suitable for a negative electrode for a non-aqueous electrolyte lithium secondary battery. And

[Brief description of drawings]

FIG. 1 shows the cycle changes of the discharge capacities of Example 1 and Example 2. FIG. 2 shows Example 3 and Example 4.
And the cycle change of the discharge capacity of Comparative Example 1.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kimito Suzuki, 1618 Ida, Nakahara-ku, Kawasaki-shi, Kanagawa Kanagawa, Ltd. 1st Technical Research Laboratory, Nippon Steel Co., Ltd. (72) Masaki Sato, Ida, Nakahara-ku, Kawasaki, Kanagawa 1618 Nippon Steel & Co., Ltd. First Technology Research Institute (72) Inventor Kenichi Fujimoto Ida, Nakahara-ku, Kawasaki-shi, Kanagawa 1618, Nippon Steel & Co., Ltd. Technical Research Center (56) References 2-82466 (JP, A) JP-A 1-292753 (JP, A) JP-A-62-165857 (JP, A) Inagaki Michio 6 persons, "Carbon" 1979, No 99, PP. 130-137.

Claims (1)

[Claims] 1. A carbon fiber made of mesophase pitch as a raw material, which has a lattice spacing (d ₀₀₂ ) which is a crystallite parameter according to an X-ray diffraction method of 0.338 nm or more and 0.343 nm or less and a crystallite in the c-axis direction. The size (Lc) is 10 nm or more and 30 nm or less, the orientation coefficient (FWHM) is 7 ° or more and 20 ° or less, and the average length in the fiber cross-section direction of the graphite crystallites elongated in the fiber axis direction is A negative electrode for a lithium secondary battery, which is characterized by using a carbon fiber of 20 nm or more and 100 nm or less.