JPH07326357A - Electrode material - Google Patents

Abstract

PURPOSE:To provide an electrode without any deterioration of the function of a carbon material, even if the oil absorption thereof is large, and further provide an electrode having a large size. CONSTITUTION:An organic high molecular compound having large oil absorption is baked and a carbon material of reduced ring aromatic and order-less lamination structure is thereby prepared. Then, the same organic high molecular compound is further added to the material and hot pressed, thereby forming an electrode material. Also, the electrode material is obtainable by adding the fine fiber of polytetrafluoroethylene and a binder soluble in other solvents than electrolytic liquid to a carbon material similar to the above.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode material for a lithium secondary battery, and more particularly to an electrode material for a lithium secondary battery suitable when a carbonaceous material having a large oil absorption is used.

[0002]

2. Description of the Related Art In recent years, miniaturization of electronic devices has progressed, and along with this demand for higher energy density of batteries, various non-aqueous electrolyte batteries have been proposed. For example, as a negative electrode for a non-aqueous electrolyte battery, it has been conventionally proposed to use a carbon negative electrode that adsorbs and desorbs lithium and that utilizes a carbon intercalation compound of lithium that can be electrochemically facilitated. There are various kinds of such carbon negative electrodes, for example, a carbon material obtained by firing crystalline cellulose (Japanese Patent Laid-Open No. 176963/1993), a coal pitch or a petroleum pitch graphitized (special Kaihei 2-8
No. 2466), a highly graphitized material treated at a high temperature of over 2,000 ° C., and the like are used.

As a method of molding such a carbon material into an electrode, an organic binder such as polyvinylidene difluoride is dissolved in an organic solvent such as N-methyl-pyrrolidone to prepare an electrode active material, that is, a carbon-based material. Known methods include kneading to obtain a slurry, applying it to a collector such as a copper foil by a doctor blade method, and drying it, or compacting an electrode active material and an organic polymer as a binder. ing.

However, even with such a negative electrode, sufficient cycle stability is not obtained in charging / discharging at a high current density, and depending on the type of carbon, almost no charging / discharging or theoretical capacity (charging) can be performed. In many cases, the capacity is extremely low compared to the LiC ₆ state (assuming the maximum capacity). Further, even if the initial capacity is relatively large, it deteriorates due to repeated charging and discharging, and the capacity drops sharply, and a conventional carbon negative electrode has not provided a negative electrode with satisfactory performance.

Therefore, the present invention aims to obtain an excellent alkali metal-occluding carbon-based material which has a large amount of alkali metal absorbed, has no structural change in the compound that adsorbs and desorbs the alkali metal, and has a large rate of absorption-desorption reaction. It has been found that a carbon-based material having a disordered laminated structure in which the laminated structure is not developed is very excellent as an electrode, and a patent application has already been filed (Japanese Patent Application No. 5-202860,
Japanese Patent Application No. 5-278884, etc.). However, such a carbon-based material has a problem that it has a large oil absorption amount and is difficult to be molded into an electrode by a conventional electrode molding method.

[0006]

In the slurry method, which is one of the electrode molding methods, when a carbon-based powder having a large oil absorption is used to mold an electrode, a large amount of solvent is required to form a slurry. To do. Therefore, when the solvent is dried, the volume shrinkage is large and the electrode is easily cracked or broken, and there is a problem that a particularly large electrode cannot be molded. On the other hand, in the case of molding the electrode by compaction, if the organic polymer compound before carbonization is compacted or a binder is added and compacted and then heating for carbonization is performed, the resin is regularly formed during compaction. Since the resulting carbon-based material has a developed laminated structure, a carbon-based material having a disordered laminated structure cannot be obtained.

The present inventors have found that a carbon-based material having a disordered laminated structure, in which the laminated structure has not yet been developed, is very excellent as an electrode. Is not applicable. The present invention has been made against the background of such conventional technical problems, and an object thereof is to obtain an electrode using a carbon-based powder having a large amount of oil absorption, and an electrode having a larger size, thereby providing a high capacity. It is an object of the present invention to obtain an electrode material for a secondary battery, which has excellent cycle stability and can be used for high output (high current density) charging / discharging.

[0008]

The first aspect of the present invention is to provide an electrode comprising hot-pressing a carbon-based material obtained by firing an organic polymer compound and adding the organic polymer compound. It provides the material.

The electrode material of the present invention is very effective when a carbon material having a large oil absorption amount is used as the electrode. This is because, as described in the section of the problem, a carbon-based material having a large oil absorption amount cannot form an electrode material by the conventional technique. As such a carbon-based material having a large oil absorption amount, a carbon-based material obtained by firing an organic polymer compound and having a condensed aromatic structure and having a disordered laminated structure is used. Can be mentioned. Here, the disordered laminated structure refers to a structure in which the condensed aromatic ring planes are completely disordered or have no more than four, preferably more than three, orientations. What has a developed laminated structure, not a disordered laminated structure, intercalates only alkali ions between layers, and does not occlude covalently bonding alkali metals or alkali ions between carbon-carbon atoms,
Not preferable.

Examples of the organic polymer compound include compounds obtained by introducing a factor that inhibits lamination into an organic polymer which is essentially a graphitizable material. The organic compound which is essentially a graphitizing material may be any as long as it has an aromatic structure, for example, polyphenylene, polyphenylene vinylene, polyphenylene xylene, polystyrene,
Examples include novolac resins. Examples of the factor that inhibits the laminated structure include a bent structure (o-, m-position structure), a branched structure, and a crosslinked structure. In addition, these organic compounds may include organic compounds having 5 members or 7 members. The carbon-based material used in the present invention is not so high in temperature as these organic polymer compounds, and has a low limit at which electrical conductivity occurs, usually 300 to 1,000 ° C.
It may be fired at.

The carbon-based material used in the present invention has a hydrogen / carbon atomic ratio (H / C) of 0.05 to 0.6, preferably 0.15 to 0.6. If it is less than 0.05, the graphite structure will develop, and the crystal structure will be destroyed by the expansion and contraction of the crystal during the lithium doping / de-doping reaction during charge / discharge, and the cycle stability will decrease, while if it exceeds 0.6, The discharge capacity is significantly reduced.

Next, the method for producing such a carbon-based material applied in the present invention will be specifically described. First, the carbon-based material used in the present invention is an organic compound having a condensed aromatic structure, that is, an aromatic compound having a main chain in which a factor that inhibits the development of a laminated structure (bending, branching, crosslinking, etc.) is introduced. The organic polymer compound is usually contained in an inert gas such as argon, helium, or nitrogen, or a reducing gas such as hydrogen at a temperature of 300 to 1,000 ° C., preferably 600 to 800 ° C. It is obtained by heat treatment for a time, preferably 0 to 1 hour.

As a specific heat treatment method, in the thermal analysis, any heating rate may be used up to the weight reduction start temperature of the organic polymer compound as a raw material, and the temperature decrease rate from the weight reduction start temperature to the heat treatment temperature may be 5 ° C. / A method of raising the temperature at a temperature rising rate of time to 200 ° C./hour, preferably 20 ° C./hour to 100 ° C./hour can be mentioned. Here, the heat treatment temperature refers to a temperature at which a weight reduction of 70 to 95% by weight is shown with respect to the weight reduced until the weight does not decrease in thermal analysis.

The heat-treated product thus obtained is usually a powder or a solid, and an excellent electrode material can be obtained by mechanically pulverizing this carbonaceous material. When a negative electrode is manufactured using this electrode material, the particle size of the electrode material is not necessarily limited, but a high performance negative electrode can be manufactured by using a material having an average particle size of 5 μm or less.

The electrode material of the present invention is obtained by adding the organic polymer compound, which is a starting material before firing used for obtaining the carbon-based material, to the carbon-based material obtained by firing as described above. Obtained by hot pressing. The organic polymer compound may be different from the starting material. The amount of the organic polymer compound is 3 to 100 parts by weight of the carbonaceous material.
The amount is preferably 50 parts by weight, more preferably 10 to 30 parts by weight. Hot press is 300-1,000 ℃
Then, it is preferable to raise the temperature at a temperature rise rate similar to that at the time of the carbonization at a temperature not exceeding the carbonization temperature and to carry out at a pressure of 10 to 500 kg / cm ² .

Here, the organic polymer compound has thermoplasticity and works as a binder for the carbonaceous material.
The electrode obtained using a conventional polyethylene binder or the like has no strength and a large one cannot be obtained, but the electrode using the organic polymer compound of the present invention as a binder has a large strength and a large electrode is formed. It is possible to Further, when the organic polymer compound is used, the volume change due to charge / discharge of the binder portion is smaller than that of other binders, and the electrode is not cracked even when charge / discharge is repeated. Further, heating during hot pressing has the same effect as firing and forms a carbon-based material, so that there is also an advantage that the binder portion also participates in charging and discharging, and the discharge capacity per electrode weight increases. In addition, the carbonaceous material once fired does not have regularity even when pressure is applied.

Next, a second aspect of the present invention is to provide another electrode material suitable for a carbon-based material having a large oil absorption amount. That is, a second aspect of the present invention is an electrode material comprising a carbon-based material and a fine fiber and a binder soluble in a solvent other than the electrolyte solution.

In such an electrode material, the carbon-based material described above is preferable as the carbon-based material. The fine fiber preferably has a wire diameter of 100 to 10,000Å, more preferably 200 to 300Å, and a length of 1 to 10.
The thickness is 1,000 μm, more preferably 10 to 100 μm. Examples of such fine fibers include fluororesin, aliphatic polyamide (nylon), glass, natural cellulose,
Examples include fine fibers made of polyester, wholly aromatic polyamide, carbon, etc., but are not limited thereto. As this fine fiber, it is stable against charge and discharge,
Any material may be used as long as it does not dissolve in the electrolyte and is stable, but preferred is a fluororesin, particularly polytetrafluoroethylene. Further, the fine fibers are preferably those that do not dissolve in an organic solvent. The amount of the fine fibers added is preferably 0.1 to 5 parts by weight, more preferably 0.6 to 2.4 parts by weight, based on 100 parts by weight of the carbon-based material. If the amount is less than 0.1 parts by weight, there is no effect. On the other hand, if the amount exceeds 5 parts by weight, the amount of the solvent for making a slurry increases due to the addition of fine fibers, which may cause cracking during drying. The energy density per hit decreases.

If such a fine fiber is dispersed in the electrode, it has a reinforcing effect even in a small amount. By using this, even when a carbon-based material having a large oil absorption is used, a doctor can be used in the same manner as in the conventional method. The electrode can be easily formed by the blade method. In the present invention, a binder that is soluble in a solvent other than the electrolyte solution is used. This is because the binder needs to be uniformly dispersed when the slurry is formed.

As the binder soluble in a solvent other than the electrolyte solution, both organic and inorganic binders can be used. As the organic binder, 10 μm of ethylene-acrylic acid (salt) copolymer, acrylic polymer, vinyl polymer, styrene-butadiene rubber, etc.
A dispersion in an aqueous dispersion medium containing particles of m or less, a fluororesin such as polytetrafluoroethylene or polyvinylidene fluoride, polyvinyl chloride, carboxymethyl cellulose,
Many binders such as styrene-butadiene rubber can be used. Further, as the inorganic binder,
A silicon-based binder such as silicon glass can be used, but in this case as well, heat treatment at a temperature exceeding the melting point is necessary in order to exert the performance as a binder.

The amount of the binder is preferably 3 to 30 parts by weight with respect to 100 parts by weight of the carbonaceous material. In addition, examples of the solvent that dissolves the binder include water and N-methyl-2-pyrrolidone. As a method of manufacturing such an electrode, a slurry method which has been conventionally used can be mentioned. For example, it can be obtained by adding fine fibers, a binder, and other necessary components to a carbon-based material, making a slurry using a dispersion medium, and applying the slurry. As the dispersion medium, a solvent that dissolves the binder is used. The coating method may be any method, but the doctor blade method is preferable, and the blade gap is 0.3 to 0.
Electrodes obtained as 7 mm are preferred.

The negative electrode body thus obtained can be used as a negative electrode for a lithium battery by supporting lithium or an alkali metal mainly containing lithium thereon. As a method of supporting, a lithium foil is contacted for thermal diffusion, or lithium is electrochemically doped in a lithium salt solution, or it is immersed in molten lithium to diffuse lithium in carbon, which has been conventionally performed. Any method is available. INDUSTRIAL APPLICABILITY The electrode material of the present invention can be widely used as a negative electrode of a lithium battery, and should be used in combination with various positive electrodes such as manganese dioxide and oxides such as vanadium pentoxide and positive electrodes using organic polymers such as polypyrrole. You can

The non-aqueous electrolyte used in the battery using the electrode material of the present invention is chemically stable with respect to the positive electrode material and the negative electrode material, and lithium ions are electrochemically reacted with the positive electrode active material. Any non-aqueous substance that can be transferred to do so can be used, in particular, a compound consisting of a combination of a cation and an anion, Li ⁺ as a cation, and PF as an example of an anion.
₆ ^-, AsF ₆ ^-, SbF ₆ ^- halide anion such Va group element as, ^I -, I ₃ ^-, Br ^-, Cl ^- halogen anion, ClO _4, such as ^- perchlorate anion such as, _{^{HF 2 -, CF 3 SO 3}} -, SCN - can be exemplified compounds having an anion such as, but not necessarily limited to these anions. Such cation, specific examples of the electrolyte with _{anion, LiPF 6, LiAsF 6, LiSbF} 6, LiB
F ₄ , LiClO ₄ , LiI, LiBr, LiCl, L
iAlCl ₄ , LiHF ₂ , LiSCN, LiCF ₃ S
O _{3 and the} like can be mentioned. Among these, especially LiP
F ₆ , LiAsF ₆ , LiSbF ₆ , LiBF ₄ , Li
ClO ₄ and LiCF ₃ SO ₃ are preferred.

The non-aqueous electrolyte is usually used in a state of being dissolved in a solvent. In this case, the solvent is not particularly limited, but a solvent having a relatively large polarity is preferably used. Specifically, glymes such as propylene carbonate, ethylene carbonate, butylene carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane, dioxane, dimethoxyethane, diethylene glycol dimethyl ether, lactones such as γ-butyrolactone, and phosphoric acid esters such as triethyl phosphate. , Boric acid esters such as triethyl borate, sulfur compounds such as sulfolane and dimethyl sulfoxide, nitriles such as acetonitrile, amides such as dimethylformamide and dimethylacetamide, dimethyl sulfate, nitromethane, nitrobenzene, dichloroethane and the like, or two or more. Mention may be made of mixtures of more than one species. Of these, especially propylene carbonate, ethylene carbonate, butylene carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, dioxane, dimethoxyethane, dioxolane and γ-.
One or a mixture of two or more selected from butyrolactone is suitable.

Further, as the non-aqueous electrolyte, the non-aqueous electrolyte is impregnated with a polymer such as polyethylene oxide, polypropylene oxide, an isocyanate cross-linked product of polyethylene oxide, or a phosphazene polymer having an ethylene oxide oligomer as a side chain. It is also possible to use an organic solid electrolyte, an inorganic ion derivative such as Li ₃ N or LiBCl ₄ , or an inorganic solid electrolyte such as lithium glass such as Li ₄ SiO ₄ or Li ₃ BO ₃ .

The lithium secondary battery using the electrode material of the present invention will be described in more detail with reference to the drawings. That is, as shown in FIG. 7, the lithium secondary battery using the electrode material of the present invention as the negative electrode has a separator 30 having fine holes inside the button-shaped positive electrode case 10 in which the opening 10 a is sealed with the negative electrode cover plate 20. A positive electrode 50 in which the positive electrode current collector 40 is arranged on the positive electrode case 10 side in the divided positive electrode side space.
On the other hand, the negative electrode 70 in which the negative electrode current collector 60 is arranged on the negative electrode cover plate 20 side is stored in the negative electrode side space.

As the separator 30, a non-woven fabric, a woven fabric or a knitted fabric, which is porous and capable of passing or containing an electrolytic solution, such as polytetrafluoroethylene, polypropylene or polyethylene, is used. can do. Further, as the positive electrode material used for the positive electrode 50, burned particles such as lithium-containing vanadium pentoxide and lithium-containing manganese dioxide can be used. Reference numeral 80 denotes a polyethylene insulating packing that is provided around the inner peripheral surface of the positive electrode case 10 to insulate and support the negative electrode cover plate 20.

[0028]

The electrode material of the present invention is obtained by hot pressing an organic polymer compound to a carbon-based material, not a slurry method, as one method for forming a carbon-based material having a large oil absorption into an electrode. It is a thing. Here, the organic polymer compound has thermoplasticity and functions as a binder. The use of an organic polymer compound compared to other binders makes it possible to obtain electrodes with greater strength, so it is possible to obtain electrodes of a larger size, and the volume change due to charging / discharging of the binder part is small, and charging / discharging is repeated. However, the electrode does not break. Further, by heating during hot pressing, the same effect as firing is obtained, and the organic polymer compound is fired to become a carbon material, so that there is also an advantage that the binder portion also participates in charging and discharging. Further, the carbon-based material does not have a developed laminated structure.

Further, another electrode material is obtained by adding fine fibers and molding by a slurry method.
Such fine fibers have a reinforcing effect when dispersed in the electrode, and even if a carbon-based material having a large oil absorption amount is used by using this, the doctor blade method can be easily used in the same manner as the conventional method. Even large size electrodes can be molded.

[0030]

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. Example 1 Poly (p-phenylene) synthesized by the Kovacic method was fired at 700 ° C. under a nitrogen stream to obtain a carbon-based material. The obtained carbon-based material was in powder form. 20% by weight of poly (p-phenylene) synthesized by the Cobatic method was added to this carbon powder, and the mixture was mixed with a ball mill at 3%.
Mix for 0 minutes. This is put into a dusting jig made of isotropic graphite, and the temperature is raised from room temperature to 500 ° C. in 2 hours and from 500 ° C. to 700 ° C. in 5 hours with a pressure of 150 kg / cm ² being applied, The temperature was maintained at 700 ° C. for 2 hours, the pressure was released, and the mixture was naturally cooled to room temperature to obtain a carbon molded body. This molded body was processed into 40 × 40 × 0.6 mm to obtain an electrode. This electrode was evaluated under the following conditions.

Lithium metal is used as the counter electrode and the reference electrode,
EC (ethylene carbonate) and DME as electrolyte
In a solvent of (dimethyl ether) (volume ratio = 1: 1), L
Using iPF ₆ dissolved at a concentration of 1 mol / l,
Charge / discharge current density was 1.6 mA / cm ² , 300 mA / g, and charging / discharging was repeated under the condition of discharge end potential of 3V.
The results are shown in Fig. 1.

Comparative Example 1 Poly (p-phenylene) synthesized by the Kovactic method was fired at 700 ° C. in a nitrogen stream to obtain a carbon-based material. The obtained carbon-based material was in powder form. Add 30% by weight of polyethylene binder to this carbon powder,
The powder was compacted at a pressure of 0 kg / cm ² . With polyethylene binder, the strength of the electrode is low, so φ10 × 0.3m
It was difficult to mold an electrode larger than an electrode of about m. φ
The electrode molded to 10 × 0.3 mm was evaluated in the same manner as in Example 1. The discharge capacity of the carbon powder is shown in FIG.

Comparative Example 2 A carbonaceous material was obtained by firing poly (p-phenylene) synthesized by the Cobatic method at 700 ° C. in a nitrogen stream. The obtained carbon-based material was in powder form. Add 30% by weight of polyethylene binder to this carbon powder,
Powder compacted at a pressure of 0 kg / cm ² , φ10 x 0.3 m
m electrodes were obtained and evaluated in the same manner as in Example 1. The discharge capacity per electrode weight is shown in FIG. From FIG. 1, it is clear that the electrode material of the present invention can be formed into an electrode without impairing the performance of the carbon-based powder, and the binder component also functions as an active material.

Example 2 100 parts by weight of poly (p-phenylene) baked at 700 ° C., 10 parts by weight of polyvinylidene difluoride (manufactured by Aldrich), Teflon 30J (manufactured by Mitsui Fluorochemicals, fluororesin fine fiber) ) 0.6 parts by weight and 400 parts by weight of N-methyl-2-pyrrolidone were added and mixed by a ball mill for 2 hours to obtain a slurry. This was cast on a copper foil with a doctor blade device (DW-150 manufactured by Tsugawa Seiki Co., Ltd.) to obtain a film. This was cut into a predetermined size to obtain a negative electrode. Charge and discharge were repeated in the same manner as in Example 1. The results are shown in Figure 2.

Example 3 An electrode was prepared and evaluated in the same manner as in Example 2 except that the amount of Teflon 30J was 1.2 parts by weight. The results are shown in Fig. 3.

Example 4 An electrode was prepared and evaluated in the same manner as in Example 2 except that the amount of Teflon 30J was 1.8 parts by weight. The result is shown in Figure 4.
Shown in.

Example 5 An electrode was prepared and evaluated in the same manner as in Example 2 except that the amount of Teflon 30J was changed to 2.4 parts by weight. The results are shown in Figure 5.
Shown in.

Reference Example 30% by weight of a polyethylene binder was added to the powder of the fired poly (p-phenylene), and the powder was compacted at 1 t / cm ² ,
It was used as the negative electrode. The evaluation was performed in the same manner as in Example 2. Results are shown in FIG. However, the polyethylene binder is stable with respect to the electrolytic solution, but its adhesive strength is low, so that it is limited to make a molded body of about φ20, and a large electrode could not be molded. The results of FIG. 6 show the characteristics of only the carbon-based powder. Examples 2 to 5 show results equivalent to those of this reference example, and it can be seen that the addition of fine fibers has no adverse effect and shows the same stability as the carbon-based powder.

Example 6 A slurry was obtained in the same manner as in Example 1 except that the addition amount of fine fibers, that is, Teflon 30J was set to a predetermined amount in Table 1 [% by weight relative to the powder of the poly (p-phenylene) fired product], and a slurry was obtained. A film was formed with a blade having a blade plate with a predetermined width shown in Table 1, and the film formability was examined. Table 1 shows the amount of fine fibers added when the plate gap was changed and the state of the film after drying. As the amount of fine fibers added increases up to 1.8% by weight, cracks are less likely to occur even in a thick film. When the amount was 2.4% by weight, the slurry viscosity increased due to the addition of fine fibers, and mixing and casting became impossible unless the solvent was further increased. Therefore, it is understood that the limit of the thickness at which cracks enter is thinned by further adding the solvent.

[0040]

[Table 1]

◯: The film shape is retained after drying. Δ: The film does not become a film due to surface tension during casting. X: Cracks occur when dried.

[0042]

With the electrode material of the present invention, an electrode can be obtained without impairing the performance of the carbon-based material even if the carbon-based material has a large oil absorption amount, and an electrode having a larger size can be obtained. Therefore, by using this electrode material, it is possible to obtain a secondary battery having high capacity, excellent cycle stability, and capable of withstanding charge / discharge with a high current density.

[Brief description of drawings]

FIG. 1 is a diagram showing evaluation results in Example 1 and Comparative Examples 1 and 2.

FIG. 2 is a diagram showing an evaluation result in Example 2.

FIG. 3 is a diagram showing an evaluation result in Example 3.

FIG. 4 is a diagram showing an evaluation result in Example 4.

FIG. 5 is a diagram showing an evaluation result in Example 5.

FIG. 6 is a diagram showing an evaluation result in a reference example.

FIG. 7 is a front view including a partial cross-sectional view of a lithium secondary battery using the carbon-based material of the present invention as a negative electrode.

[Explanation of symbols]

 30 Separator 50 Positive electrode 70 Negative electrode

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Naohiko Oki 1-4-1 Chuo, Wako, Saitama Stock Company Honda R & D Co., Ltd.

Claims (5)

[Claims] 1. An electrode material comprising a carbon-based material obtained by firing an organic polymer compound, and adding the organic polymer compound thereto and hot pressing. 2. An electrode material comprising a carbon-based material and a fine fiber and a binder soluble in a solvent other than an electrolyte solution. 3. The electrode material according to claim 2, wherein the fine fibers are polytetrafluoroethylene. 4. The electrode material according to claim 3, wherein 0.6 to 2.4 parts by weight of polytetrafluoroethylene is added to 100 parts by weight of the carbonaceous material. 5. The electrode material according to claim 2, which is obtained by molding by a doctor blade method with a blade gap of 0.3 to 0.7 mm.