JPH06223821A - Manufacture of negative electrode for lithium secondary battery - Google Patents

Abstract

PURPOSE:To manufacture a lithium secondary battery negative electrode body of graphite type which exert excellent charging/discharging performance through combining with an electrolyte particularly containing ethylene carbonate. CONSTITUTION:A film-form molding of 200mum thick or less made from aromatic polyimide is baked and turned into graphite at a temp, over 2300 deg.C in a non- oxidative atmosphere. The graphite film obtained is used as it is as the body of negative electrode or fine crushed into the mean particle size 100mum followed by molding together with components of organic binder material to yield a body of negative electrode.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a graphite-based negative electrode body for carrying lithium as an active material in a lithium secondary battery.

[0002]

2. Description of the Related Art In recent years, lithium secondary batteries with high energy density have been attracting attention as batteries for power sources or electric power storage of small electronic devices. However, since metallic lithium is used as the negative electrode, there is a drawback that the cycle life is short due to dendrite formation during charging. Further, the use of metallic lithium has a problem in terms of safety.

As a battery composition for solving such a problem, many attempts have been made to carry out a negative electrode by supporting lithium, which is a negative electrode active material, on a certain kind of carbon material, and the characteristics of the supported carbon material are targeted. Many proposals have been made (JP-A-62-90863, JP-A-62-193463, JP-A-63-236259, JP-A-64-2258, JP-A-64-2258).
1-274360, JP2-44644, JP2-6685
No. 6, JP-A-2-230660, JP-A-3-93162, etc.). However, this type of secondary battery using a carbon-based doped base material generally has a lower energy density than a negative electrode using metallic lithium, and the self-discharge characteristics are also deteriorated. For this reason, increasing the lithium doping amount, facilitating reversible doping / undoping cycle, and at the same time obtaining a stable doped body have become indispensable subjects in the development of the battery of the type described above. In order to solve these problems, most of the technologies are dominated by those in which the distance between graphite crystal planes of the carbonaceous negative electrode body doped with metallic lithium is the main subject of regulation.

By the way, when a general propylene carbonate-based electrolytic solution is used, it is known that the carbon material constituting the negative electrode body shows a larger charge / discharge capacity when it is not graphitized too much. However, in a carbon negative electrode body in which graphitization has not progressed, the flatness of the potential (the characteristic that the potential gradually changes with charging and discharging) is diminished, and this characteristic deterioration is an obstacle to practical use depending on the application. .

On the other hand, recently, it has been reported that a carbon material, such as natural graphite, which has undergone sufficient graphitization promotes a smooth Li insertion / desorption reaction only when an electrolytic solution containing ethylene carbonate is used. Therefore, it is expected to be used as a negative electrode body of a lithium secondary battery. Due to the high melting point and high viscosity of ethylene carbonate, the range of use is restricted, but in this system, 6C + Li ⁺ + e ⁻ → Li
When LiC ₆ is synthesized by the C ₆ formula, a charge / discharge capacity corresponding to a theoretical capacity of 372 mAh / g is obtained, and excellent flatness of potential is exhibited.

[0006]

However, natural graphite, which exhibits the most excellent properties when combined with an ethylene carbonate-based electrolytic solution, is a natural product and therefore has low quality stability and is suitable as a raw material for precision industrial products. The artificial graphite product, which is poor in properties and has the same properties as that of the product, requires a complicated operation and is difficult to mass-produce, which is extremely expensive.

An object of the present invention is to provide a method for industrially producing a novel graphite-based negative electrode for a lithium secondary battery that exhibits excellent charge / discharge capacity and potential flatness in combination with an ethylene carbonate-based electrolytic solution. It's about to be tried.

[0008]

In order to achieve the above object, a method for producing a negative electrode for a lithium secondary battery according to the present invention comprises a step of forming a film-like molded article of aromatic polyimide having a thickness of 200 μm or less in a non-oxidizing atmosphere. The constitutional feature is that it is calcined and graphitized at a temperature of 2300 ° C. or higher.

In general, various kinds of aromatic polyimides are commercially available depending on the composition of the acid component and the amine component, but there is no particular limitation on the composition for the purpose of the present invention, and the imide bond is formed in a straight chain. All of the included polymers are targets for carbon source materials. However, as the shape of the aromatic polyimide, a film-shaped molded body having a thickness of 200 μm or less is selected. Usually when forming a thin sheet with a resin material, the molecules are oriented in the molding method, but especially in the case of an aromatic polyimide that can be molded as an extremely thin film, a film with excellent molecular orientation can be formed, and this property makes it possible to perform subsequent firing. It is a requirement for smooth graphitization in the process. In this case, if the film shape of the aromatic polyimide film has a film thickness of more than 200 μm, the degree of molecular orientation becomes low, and it becomes difficult to smoothly obtain a high degree of graphite crystal structure.

The film-shaped molded product made of the aromatic polyimide having the above-mentioned properties is held in a non-oxidizing atmosphere such as nitrogen or argon and placed in a heating furnace for firing graphitization. In the treatment, the aromatic polyimide film-shaped molded product is sandwiched in advance by a graphite plate having a smooth surface, and charged into a furnace with an appropriate pressure applied. It is effective for increasing the orientation of hexagonal graphite network effective for dope. It is preferable to apply a pressing pressure of at least 20 g / cm ^{2 to} the sandwiched graphite plates. The treatment temperature for the firing graphitization must be set to 2300 ° C. or higher. At a temperature below 2300 ° C., graphitization does not proceed sufficiently, and a high level charge / discharge capacity of lithium cannot be obtained.

The carbonaceous material obtained in the above process is a graphite film having appropriate flexibility, and the carbon spacing d (00
2) is about 0.335 nm, and the crystallite size Lc (002) in the c-axis direction shows an excellent graphitization degree exceeding 100 nm. Therefore, it can be directly applied as a carbon negative electrode body for a lithium secondary battery. In this case, since the electrode is a 100% carbonaceous electrode containing no binder component, the packing density of the negative electrode active material inside the battery is large, and the negative electrode body has a high energy density.

Another method for producing a negative electrode for a lithium secondary battery according to the present invention is the above graphite film having an average particle size of 100 μm.
The constituent requirement is that the particles are finely pulverized to m or less and molded together with the organic binder component.

The fine pulverization process of the graphite film needs to be adjusted so that the average particle size is 100 μm or less.
If it exceeds 100 μm, smooth doping / undoping of lithium cannot be achieved. As the binder, for example, tetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF),
Resins such as polyethylene (PE) and polypropylene (PP) are used and mixed with finely pulverized graphite powder and then pressure-molded, or by coating and molding on a metal conductor to form a negative electrode body of a lithium secondary battery. To do.

To construct a lithium secondary battery using the graphite negative electrode according to the present invention, ethylene carbonate alone or an organic solvent containing ethylene carbonate is selectively used by adding an appropriate amount of electrolyte as an electrolytic solution. .
The electrolyte to be added is L, which is usually used for batteries.
It is a lithium salt such as iClO ₄ , LiBF ₄ , and LiPF ₆ . The lithium secondary battery is composed of a positive electrode body and a separator in addition to the negative electrode body and the electrolytic solution, and is formed into a paper type, a button type, a cylindrical type or the like. The positive electrode is preferably formed by molding a transition metal chalcogen compound, and the transition metal is molybdenum,
Titanium, vanadium, cobalt, nickel, manganese, and the like, and chalcogen compounds such as oxides, sulfides, and selenides are used. The separator is not particularly limited in terms of material, and an ordinary porous film formed of a synthetic resin non-woven fabric can be used.

[0015]

The negative electrode body of a lithium secondary battery manufactured according to the present invention is a film-shaped graphite molded body obtained by firing and graphitizing an aromatic polyimide, and is a novel carbon that has not been noticed as a negative electrode body in the prior art. It is a quality material. Since the graphite-based negative electrode body is formed by calcining and graphitizing a film-shaped molded body made of an aromatic polyimide and having a thickness of 200 μm or less at a temperature of 2300 ° C. or higher as a carbon source material, it has an extremely smoothly structured graphite crystal structure. Will be converted. This graphitized structure exhibits particularly suitable battery performance in a system combined with an ethylene carbonate-based electrolytic solution, and exhibits a high level of charge / discharge capacity and potential smoothness.

Unlike the other carbonaceous films, the above graphite film is dense and has appropriate flexibility, so that it can be easily incorporated into a battery. The negative electrode body of the embodiment in which the graphite film is finely pulverized and then molded together with the organic binder has more manufacturing steps than the negative electrode body of 100% carbonaceous material having the graphite film itself as an electrode, and has a lithium packing density. However, it becomes inferior, but by finely pulverizing the graphite film, the diffusion distance of lithium taken in between the graphite layers is shortened, and there is an advantage that the negative electrode performance is improved in that the number of lithium insertion / removal ports is increased. Therefore, it is possible to provide advantageous battery performance depending on the purpose.

[0017]

EXAMPLES Examples of the present invention will be described below in comparison with comparative examples.

Example 1 A film-like molded body having a thickness of 75 μm and made of an aromatic polyimide having the chemical structural formula shown in Chemical formula 1 was sandwiched between graphite plates, and 60
It was placed in a firing furnace maintained in a nitrogen atmosphere with a pressing pressure of g / cm ² applied, and subjected to firing graphitization at a temperature of 2700 ° C.

[0019]

[Chemical 1]

The graphitization property of the obtained graphite film was measured by X-ray diffraction based on the Gakushin method. The carbon spacing d (002) was 0.3355 nm, and the crystallite size Lc (in the c-axis direction was Lc ( 002) was 100 nm or more.

Then, the above graphite film was directly used as a sample electrode of the negative electrode, metallic lithium was used as the counter electrode and the reference electrode, and ethylene carbonate in which 1 mol / l of LiPF ₆ was dissolved in the electrolytic solution was used as the electrolytic solution to prepare a single cell. Then, the performance as a negative electrode was evaluated by the constant current charge / discharge test method. The constant current charge / discharge test method uses a charge / discharge end potential of 0 V vs. Li / L.
i ⁺ (when charging), 1.5 V vs. Li / Li ⁺ (when discharging)
And the current density was set to 30 mA / g.

The resulting charge / discharge curve is shown in FIG. Figure 1
From this, it was confirmed that the flatness of the potential was excellent in both charging and discharging. The charge / discharge capacity at this time is shown in Table 1 in comparison with the negative electrode production conditions and the graphitization property.

Example 2 A negative electrode body of a graphite film was produced under the same conditions as in Example 1 except that a film-like molded body having a thickness of 185 μm and made of an aromatic polyimide having the chemical structural formula shown in Chemical formula 1 was used as a carbon source material. did. When this negative electrode body was used to evaluate the battery performance in the same manner as in Example 1, the charge / discharge curve showed good flatness of the potential as shown in FIG. The charge / discharge capacity was as shown in Table 1.

[0024]

[Chemical 1]

Example 3 A negative electrode body of a graphite film was manufactured under the same conditions except that the temperature of the firing graphitization treatment in Example 1 was changed to 2350 ° C. When the battery performance was evaluated using this negative electrode body in the same manner as in Example 1, the charge / discharge curve was as shown in FIG.
As shown in, the flatness of the potential was good. The charge / discharge capacity was as shown in Table 1.

Comparative Example 1 A negative electrode body of a graphite film was manufactured under the same conditions except that the temperature of the firing graphitization treatment in Example 1 was changed to 2200 ° C. Using the obtained negative electrode body, the battery performance was evaluated in the same manner as in Example 1. The charge / discharge curve in this example is shown in FIG.
As shown in, the flatness of the potential was extremely poor.
The charge / discharge capacity was as shown in Table 1.

Comparative Example 2 A negative electrode body of a graphite film was manufactured under the same conditions except that the aromatic polyimide used in Example 2 was changed to a film-like molded body having a thickness of 220 μm. As a result of evaluating the battery performance using this negative electrode body in the same manner as in Example 1, the charge / discharge curve was inferior in the flatness of the potential as shown in FIG. The charge / discharge capacity was as shown in Table 1.

[0028]

[Table 1]

From the results shown in Table 1, the charge and discharge capacities in the case of using the negative electrode bodies according to the respective examples are significantly increased as compared with the comparative example products which deviate from the requirements of the present invention, and it is confirmed that the energy density is high. Was given.

Example 4 The graphite film produced in Example 1 was pulverized in a mortar to obtain a powder having an average particle size of 90 μm. To 90 parts by weight of this graphite powder, commercially available tetrafluoroethylene (PTFE) powder 10
Parts by weight were mixed and sufficiently kneaded, and then molded into a sheet having a thickness of 0.1 mm by roll forming to prepare a negative electrode body. The battery performance of this negative electrode was evaluated in the same manner as in Example 1.
As a result, it shows an excellent charge-discharge curve as shown in FIG.
The charge / discharge capacity at that time was 370 mAh / g, which was higher than that in Example 1.

[0031]

As described above, according to the present invention, a thin film-shaped molded product made of an aromatic polyimide is subjected to firing graphitization, or the graphite film obtained in the above step is finely pulverized and molded with an organic binder component. By doing so, it is possible to obtain a graphite-based negative electrode body for a lithium secondary battery, which exhibits a high level of discharge and charge capacity and excellent flatness of potential by combining an electrolytic solution containing ethylene carbonate. Moreover, the quality of the raw material of the negative electrode body is stable, and the manufacturing process is relatively simple, so that a high-performance negative electrode body can be industrially produced at low cost.

[Brief description of drawings]

FIG. 1 is a graph showing a charge / discharge curve of a unit cell according to Example 1.

FIG. 2 is a graph showing a charge / discharge curve of a unit cell according to Example 2.

FIG. 3 is a graph showing a charge / discharge curve of a unit cell according to Example 3.

FIG. 4 is a graph showing a charge / discharge curve of a single cell according to Comparative Example 1.

5 is a graph showing a charge / discharge curve of a single cell according to Comparative Example 2. FIG.

FIG. 6 is a graph showing a charge / discharge curve of a unit cell according to Example 4.

[Chemical 2]

Claims (2)

[Claims] 1. A thickness of 200 μm made of aromatic polyimide.
230 m or less of a film-shaped molded product in a non-oxidizing atmosphere
A method for producing a negative electrode for a lithium secondary battery, which comprises firing and graphitizing at a temperature of 0 ° C. or higher. 2. A method for producing a negative electrode for a lithium secondary battery, characterized in that the graphite film obtained in claim 1 is finely pulverized to have an average particle size of 100 μm or less and molded together with an organic binder component.