JPH05283062A - Negative electrode for lithium secondary battery and lithium secondary battery using same electrode - Google Patents

Abstract

PURPOSE:To prevent the deterioration of capability due to a drop in electrical conductivity by constituting a negative electrode of a carbon material comprising a stacked composite member of carbon powder and a carbon film. CONSTITUTION:A negative electrode is constituted of the stack of a film layer 1 and a carbon powder layer 2. The film layer 1 is made of a carbon material allowing the doping an dedoping of alkaline metal, such as a film made of coal and oil mesophase pitch, and can be used as a host for intercalation reaction. The carbon power layer 2 is made of a carbon material allowing the doping and dedoping of alkaline metal or the like, such as thermally decomposed carbon, activated carbon and organic polymer compound, and can be used as a host for intercalation reaction. According to this construction, the deterioration of capability due to a drop in electrical conductivity can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery and a negative electrode for a lithium secondary battery, and more specifically, by compounding carbon powder with another carbon material, only the carbon powder is used as the negative electrode in the negative electrode structure. A lithium secondary battery having improved discharge capacity, power density, and cycle characteristics by improving the performance deterioration due to the decrease in electrical conductivity found in the conventional type used for the element, and for such a lithium secondary battery Regarding the negative electrode.

[0002]

2. Description of the Related Art A so-called lithium secondary battery in which lithium is used as a negative electrode active material, metal chalcogenide and a metal oxide are used as a positive electrode active material, and various salts are dissolved in an aprotic organic solvent is used as an electrolytic solution. Batteries are attracting attention as a type of high energy density secondary battery and are being actively researched.

However, in the conventional lithium battery,
Lithium as a negative electrode active material is often used as a simple substance such as a foil, and has a drawback that the cycle life of charge / discharge is short because dendritic lithium is deposited and both electrodes are short-circuited during repeated charge / discharge.

Therefore, a method has been proposed in which a fusible alloy containing aluminum, lead, cadmium, and indium is used to deposit lithium as an alloy during charging and to dissolve lithium from the alloy during discharging [US Pat. ,
002, 492 (1977)]. However, although such a method can prevent the deposition of dendritic lithium,
Energy density decreases.

Further, various attempts have been made to support lithium on a carbon material for the purpose of improving discharge capacity. For example, attempts have been made to use various fibrous or powdery carbon materials [Toshiba Battery Co., Ltd. and Mitsubishi Petrochemical Co., Ltd. joint application JP-A-63-11].
No. 4056 (1988), Japanese Patent Application Laid-Open No. 62-268056 (1987) filed by Mitsubishi Gas Chemical Co., Inc.].

[0006]

Among the various carbons described above, the conventional lithium secondary battery using only carbon powder as a carbon material as a negative electrode constituent element has a performance deterioration due to a decrease in electric conductivity. In particular, the problem of power density is remarkable, and improvement has been strongly demanded. Therefore, an object of the present invention is to solve these problems.

[0007]

In view of the above circumstances, the present inventors have focused their attention on compounding carbon powder with other carbon materials, and have tried various carbon materials to be compounded. Surprisingly, they have found that the above problems can be solved by using a carbon film, and completed the present invention.

That is, the present invention provides a negative electrode for a lithium secondary battery, which is made of a carbon material obtained by laminating and compositing carbon powder and a carbon film. Hereinafter, the negative electrode for a lithium secondary battery of the present invention will be described with reference to the drawings. FIG. 1 is a sectional view schematically showing the negative electrode for a lithium secondary battery of the present invention. Referring to FIG. 1, the lithium secondary battery of the present invention has a laminated structure including a film layer (1) and a carbon powder layer (2). As the carbon material used in this way, the object of the present invention is achieved for the first time by combining a carbon film with carbon powder and forming a laminated structure.

First, a carbon film is used for the film layer (1). The carbon film may be any carbon material capable of being doped / dedoped with an alkali metal or the like and used as a host for the intercalation reaction. For example, a coal-based or petroleum-based mesophase pitch is used as a raw material. A film can be used, and in this case, there is no problem even if it contains some isotropic pitch. The thickness of one carbon film is generally within the range of 10 to 40 μm.

On the other hand, the carbon powder used in the carbon powder layer (2) may be any carbon material that can be doped or dedoped with an alkali metal or the like and can be used as a host for the intercalation reaction. For example,
Pulverized products such as pyrolytic carbons, activated carbon, fired bodies of organic polymer compounds, glassy carbon, graphites and cokes can be used. The particle size of the carbon powder is not particularly limited, but from the viewpoint of dispersibility and moldability, it is 0.1 to 100.
The range of μm is preferred.

The negative electrode for a lithium secondary battery of the present invention comprises a plurality of film layers and carbon powder layers using such materials, which are alternately laminated. The body is cut into the desired shape and size for use.

Next, a method of manufacturing the laminated structure will be described. Several manufacturing methods are possible, but as the first example,
While the carbon film after molding and before being made infusible is maintained at a temperature at which its surface can maintain its adhesiveness, it is made infusible by passing it through a jet bed of carbon powder and adhering the carbon powder to its surface. Several to several tens of the obtained carbon powder-attached films are laminated and hot pressed to obtain a sheet in which film layers and carbon powder layers are alternately laminated. Alternatively, a carbon film that has undergone a carbonization / graphitization process is coated with a mixture of carbon powder and an appropriate amount of a binder, and these are laminated and then press-molded with a press machine to form a carbon film and a carbon powder layer ( A sheet in which + binders are alternately laminated is obtained.

An example of the method for producing the carbon film itself that can be used in the present invention will be described here for reference. In the first step, the melted pitch is extruded from a slit-shaped nozzle to give a film-like or sheet-like shape. The apparatus for such a process is the extrusion apparatus shown in FIGS.

First, referring to FIG. 1, a molten pitch is supplied to an apparatus (3), and a slit type nozzle (6) (see FIG. 4).
The sheet-shaped pitch (4) is extruded in a sheet shape from the sheet, and then the sheet-shaped pitch (4) is pulled by the winding device (5) and wound up. The device (3) is devised so that it can be wound up in the form of a film or a sheet.

That is, in the device (3), toward the vicinity of both axial end faces of the sheet-like pitch (4) and in a direction symmetrical to an imaginary plane P perpendicular to the slit length direction passing through the slit center point C, At least two airflow outlets (7) are provided for blowing an airflow having component forces of an outward component and a downward component.

By arranging a plurality of air flow outlets in this way, before the sheet-like pitch (4) which is being drawn out from the slit type nozzle (6) and being pulled by the winding device (5) is sufficiently solidified. In other words, at the time when it is still directly below the slit type nozzle (6), the sheet-like pitch (4)
The neck down phenomenon is suppressed and a wide tape-shaped pitch film (8) is obtained by blowing an air current in the vicinity of both ends in the width direction of the sheet to exert a component force for expanding the sheet-shaped pitch (4) in the width direction. Be done.

The blowing gas is preferably blown at a velocity of about 50 to 100 m / sec (velocity at the outlet of the airflow outlet), and the amount of the air blown is 0.1 per airflow outlet.
It is preferably about 4 to 0.5 liter / minute. Air, nitrogen, gas combustion waste gas, or the like can be used as the type of sprayed gas. The temperature is usually 200 to 400 at which the sheet-like pitch (4) can be stretched by winding.
C., preferably 250 to 350.degree. The airflow outlets (7) are provided at least at two places near each end so that the airflow can be blown from both sides near the ends of the sheet-like pitch (4), and at least at a total of four places. preferable.

In the next step, the pitch film (usually 12 to 50 μm in thickness and 2 to 45 mm in width) obtained in the previous step is infusibilized according to a conventional method used in the production of carbon fibers. This infusibilizing step is intended to maintain a film or sheet shape in the subsequent carbonization and graphitization.

The infusibilization is performed, for example, in an air atmosphere at 280
It can be carried out at a temperature in the range of ˜340 ° C. The time required for infusibilization can be appropriately selected according to the thickness of the sheet-like pitch, and the thinner the time, the shorter the time required for completion.

The firing of the infusible substance as the final step is as follows.
It can be performed at a temperature according to the desired electrical resistance of the carbon material film. That is, as the firing temperature becomes higher, the degree of carbonization and the degree of graphitization become higher, and carbonization with a low electric resistance is performed in an inert atmosphere (an atmosphere such as nitrogen, carbon dioxide, or argon) in the range of 1000 to 2000 ° C. It can be done at temperature. Further, graphitization of the infusibilized product can be performed at a temperature in the range of 2000 to 3000 ° C. in an atmosphere such as argon.

By the above steps, the thickness is usually 10 to 4
A carbon material film having a width of 0 μm and a width of about 1.5 to 45 mm is obtained. This can be cut into an appropriate size and used in the negative electrode of the present invention.

Next, in the negative electrode for a lithium secondary battery of the present invention, the laminated body may have various shapes such as a sheet shape, a rod shape, a plate shape, and a film shape, depending on the final use and the shape of the battery. Can be As for size, the thickness is 0.05-1m
The width m is in the range of 1.5 to 150 mm, and the length is not particularly limited, and can be appropriately cut into a desired length. With such a form, the negative electrode for a lithium secondary battery of the present invention can be obtained.

The negative electrode for a secondary battery according to the present invention is an electrolyte such as propylene carbonate, ethylene carbonate, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane, 4-methyldioxolane, sulfolane, acetonitrile, etc.
A lithium secondary battery can be assembled by a conventional method in combination with a positive electrode such as V ₂ O ₅ or Mn ₂ O. The present invention also provides such a lithium secondary battery, and can be suitably used as a power source for portable electronic devices and the like, backup of various memories and solars, and also as an electric vehicle, a battery for power storage, and the like. it can.

Next, the effect of using the above-mentioned composite of carbon powder and carbon film will be described. First, by combining carbon powder and carbon film, (1) the conductivity is significantly improved and the charge / discharge reaction speed is improved as compared with the case where carbon powder is used alone, and the metallic property that has been conventionally required. A current collector is unnecessary, and weight reduction can be achieved. (2) A bulky structure is formed as compared with the case where a carbon film is used alone, and pores are formed that facilitate diffusion of an electrolytic solution or the like into a negative electrode.
Due to such actions, the charge / discharge reaction is facilitated, and as a result, the output density is improved.

When a carbon material having a layered structure in the crystallite is used for an electrode of a secondary battery, chemical species enter and leave the layers during charge / discharge, and the c-axis of the crystallite is accompanied with it. Expansion / contraction in the direction occurs. When the expansion and contraction are repeated, the strain increases in the direction parallel to the electrode surface, and eventually the electrode breaks down. In the carbon material of the hybrid structure in the present invention, the crystallites whose a-axis direction is oriented parallel to the electrode surface are used, so that the expansion / contraction thereof occurs in the direction perpendicular to the electrode surface. The electrode is less likely to be broken and the life can be extended.

Further, as factors that greatly affect the battery characteristics, electrical conductivity and thermal conductivity can be mentioned. Since the π-electrons of carbon move in the conjugated system of the hexagonal network structure of carbon atoms, the disorder of the orientation of the carbon crystal causes a decrease in electrical conductivity and an increase in Joule heat. In the carbon material of the hybrid structure used in the present invention, the a-axis of the crystallite on the carbon film side is oriented parallel to the electrode surface, so that anisotropy with respect to electric / thermal conductivity is remarkably exhibited. .. As a result, the battery electrode can be provided with both the electrical conductivity in the direction of increasing the current collecting effect and the thermal conductivity in the direction of facilitating the movement and heat dissipation of Joule heat.

By compositing a carbon powder and a carbon film with a material capable of occluding lithium ions, the advantages of both can be utilized complementarily, and at the same time, the lithium ions occluded per negative electrode body. , The discharge capacity can be increased.

[0028]

EXAMPLES The present invention will be described in more detail with reference to the following examples. [Production of carbon film] Softening point (Mettler method) = 100
℃, quinoline insoluble matter (QI component) = 0.2%, benzene insoluble matter (BI component) = 30% Coal tar pitch 2
A double amount of hydrogenated anthracene oil was added, the mixture was heated at 430 ° C. for 60 minutes, and the hydrogenated anthracene oil was removed at 300 ° C. under reduced pressure to obtain reduced pitch. Then, nitrogen gas was introduced into this reduced pitch to remove low molecular weight components,
Thermal polymerization at 0 ° C. for 5 hours, softening point (Mettler method) = 26
A mesophase pitch for extrusion was obtained at 2.4 ° C., QI component = 50%, BI component = 98%, and mesophase content of 90% or more. The pitch is melted by using the device shown in FIGS. 2 to 4, and the slit type nozzle (6) is used.
The sheet-like pitch-based film was obtained by extruding from the sheet and winding it while blowing air. Next, the pitch-based film was heated to 300 ° C. in air at a temperature rising rate of 3 ° C./min, and then held at the same temperature for 2 hours for infusibilization treatment. Then, the tape-shaped pitch type | system | group carbon film was obtained by baking at 1000 degreeC in argon. Table 1 shows specific production conditions and properties of the obtained tape-shaped pitch-based carbon film.

[0029]

[Table 1]

[Preparation of carbon powder] A needle coke pulverized to have a central particle size of 20 μm was mixed with a dispersion type PTFE (concentration 10%) as a binder at a weight ratio of 90:10 to form a paste.

[Production of Negative Electrode Body] A carbon powder paste was applied onto the above carbon film using a coating machine and dried. Ten pieces of the obtained carbon film with a paste were laminated and pressed at 260 ° C. for 10 minutes to obtain a plate-shaped negative electrode body. Using the obtained negative electrode body as a working electrode, lithium metal was used for the counter electrode and the reference electrode, and lithium was stored in the negative electrode body until the potential became 0V. The conditions (electrolyte solution, current density, etc.) were the same as the conditions for measuring battery characteristics described later.

[Preparation of Battery] As shown in the sectional view of FIG. 5, in addition to the negative electrode body (9) obtained above, a positive electrode body (1
Electrolytic manganese dioxide is used as 0), propylene carbonate in which LiClO ₄ is dissolved at a concentration of 1 mol / l is used as an electrolytic solution, polypropylene nonwoven fabric is used as a separator (11), and further a case (12), a sealing plate (13) and an insulating material. Using the packing (14), a lithium secondary battery was prepared by a conventional method.

[Measurement of Battery Characteristics] The output density, discharge capacity and cycle characteristics of the lithium secondary battery obtained above were measured. The measurement was usually performed under constant current charge / discharge of 50 mA / g (negative electrode carbon standard). The discharge capacity was the capacity until the battery voltage dropped to 2.0V. The cycle characteristics were evaluated by the number of cycles until the discharge capacity decreased to 90% of the initial discharge capacity. The output density was obtained from the current-potential curve obtained by changing the discharge current density.
As a control, a lithium secondary battery prepared in the same manner except that the above carbon powder was used alone as the carbon material for the negative electrode was also measured under the same conditions. The results are shown in Table 2.

[0034]

[Table 2]

As shown in Table 2, the lithium secondary battery of the present invention was superior to the control lithium secondary battery in discharge capacity, cycle characteristics,
It can be seen that the power density is excellent in both respects.

[0036]

EFFECTS OF THE INVENTION According to the present invention, the performance deterioration due to the decrease in electric conductivity, which is observed in the conventional type using only carbon powder as the carbon material for the negative electrode, is improved, and the weight is reduced, and the unit volume (weight) A lithium secondary battery having improved capacity, power density and cycle characteristics, and a negative electrode therefor are provided.

[Brief description of drawings]

FIG. 1 is a cross-sectional view schematically showing a negative electrode for a lithium secondary battery of the present invention.

FIG. 2 is a perspective view showing an outline of an extrusion apparatus for producing a carbon film.

FIG. 3 is a front view of the extrusion device shown in FIG.

FIG. 4 is a schematic diagram showing the nozzle and the air flow outlet of the extrusion device shown in FIG. 2 with particular emphasis.

FIG. 5 is a cross-sectional view of a lithium secondary battery of the present invention created in an example.

[Explanation of symbols]

1: film layer, 2: carbon powder layer, 3: extrusion device, 4:
Sheet-like pitch, 5: winding device, 6: slit type nozzle, 7: air flow ejection port, 8: tape-like pitch film, 9: negative electrode, 10: positive electrode, 11: separator, 12:
Case, 13: sealing plate, 14: insulating packing

Claims (2)

[Claims] 1. A negative electrode for a lithium secondary battery, comprising a carbon material obtained by laminating and compositing carbon powder and a carbon film. 2. A lithium secondary battery comprising the negative electrode according to claim 1 as a battery constituent element.