JPH08180869A - Secondary battery electrode - Google Patents

Abstract

PURPOSE: To provide a secondary battery electrode with high charging and discharging capacity which is easy to mold by constituting the electrode by a carbonaceous material obtained by setting a carbon body having specified orientation and crystallite thickness to a specified length. CONSTITUTION: A carbon body having an orientation P of 70<=P<=85% and a crystallite thickness Lc by X-ray diffraction of 13<=Lc<=20Åis crushed to provide a carbonaceous material having an average length not more than 5mm, and this material is used to provide a secondary battery electrode. As the carbon body, carbon fiber obtained by baking an organic material such as polyacrylonitrile is preferred. This electrode is used as a secondary battery negative electrode, and as a positive electrode material at that time, a transition metal oxide, for example, LiCoO2 or LiNiO2 is preferably used.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an electrode using a carbonaceous material obtained from a carbon body having a low degree of orientation and crystallinity, and a secondary battery using the electrode.

[0002]

2. Description of the Related Art In recent years, with the widespread use of portable devices such as video cameras and notebook computers, demand for small and high capacity secondary batteries has increased. Most of the secondary batteries currently used are nickel-cadmium batteries using an alkaline electrolyte, but the battery voltage is low at about 1.2 V, and it is difficult to improve the energy density. Therefore, high-energy secondary batteries have been studied using lithium metal, which is the base metal, as the negative electrode.

However, in a secondary battery in which lithium metal is used for the negative electrode, there is a risk that lithium will grow into dendrites due to repeated charging and discharging, causing a short circuit and igniting. In addition, since highly active metallic lithium is used, it is inherently dangerous, and there are many problems in using it for consumer use. In recent years, a lithium ion secondary battery using various carbonaceous materials has been devised as a device that solves such a safety problem and enables high energy peculiar to a lithium electrode. This method utilizes that the carbonaceous material is doped with lithium ions during charging and has the same potential as metallic lithium, so that it can be used for the negative electrode instead of metallic lithium. Further, during discharge, the doped lithium ions are dedoped from the negative electrode. When such a carbonaceous material doped with lithium ions is used as the negative electrode, there is no problem of dendrite generation, and since there is no metallic lithium, there is a feature that it is also excellent in safety, Currently, research and development are actively carried out.

Secondary batteries using an electrode utilizing the above-mentioned carbonaceous material doped with lithium ions are disclosed in JP-A-57-208079 and JP-A-58-9317.
6, JP-A-58-192266, JP-A-6-
No. 2-90863, Japanese Patent Laid-Open No. 62-122066, Japanese Patent Laid-Open No. 2-66856 and the like are known.

Such a carbonaceous material is generally in the form of powder, and is used for an electrode as a sheet-shaped molded body to which a polymer binder such as Teflon or vinylidene fluoride is added. On the other hand, as a secondary battery using carbon fibers or a carbon fiber structure as an electrode as a carbonaceous material, instead of powder, JP-A-60-36315 and JP-A-60-54 are known.
181, JP 62-103991 A, JP 62-154564 A, JP 63-58763 A.
Japanese Laid-Open Patent Publication No. 2-82466 and the like are known.

[0006]

However, the technique disclosed in Japanese Patent Laid-Open No. 60-36315 has a problem that the discharge capacity is insufficient.

The present invention is intended to solve the drawbacks of the prior art, and an object thereof is to provide an electrode for a secondary battery, which has a high charge / discharge capacity and is easy to mold.

[0008]

The present invention has the following constitution in order to solve the above problems.

"Orientation degree P is 70% ≤P≤85%, and
An electrode for a secondary battery made of a carbonaceous material having an average length of 5 mm or less obtained from a carbon body having a crystallite thickness Lc by X-ray diffraction of 13 Å ≦ Lc ≦ 20 Å. The intercalation (or doping) of carbon materials has been studied for a long time, and much knowledge has been accumulated. Conventionally, carbon materials that can be intercalated have a graphitization degree (crystallinity). It has been considered that it is limited to high quality items. However, in recent years, it has been found that even an amorphous carbon material having a small Lc, such as a fired body of an organic material, can be intercalated. Further, since a high-performance secondary battery utilizing intercalation of such a fired organic material is realized, interest in amorphous carbon materials is increasing. Rechargeable batteries that utilize intercalation of lithium into a fired organic material can overcome the safety problems of lithium batteries and become high-capacity secondary batteries, which is a feature of lithium batteries. Is being watched as.

However, there are still many unclear points about the mechanism of intercalation into an amorphous carbon material, and a guideline for searching for a high-performance carbon material for a secondary battery has not been established yet. At present, we are developing carbon materials through repeated trial and error. Currently, the search for high-performance carbon materials is directed toward amorphous carbon materials and toward crystalline carbon materials. As a result of diligent studies on the high-capacity carbon material, the present inventors have found that a carbon body having an appropriate degree of orientation and a crystallite thickness Lc is excellent as the high-capacity carbon material. That is, the degree of orientation P is 70% ≦ P ≦ 85%,
Moreover, a carbon body having a crystallite thickness Lc by X-ray diffraction of 13 Å ≦ Lc ≦ 20 Å is used as an electrode.

Generally, a fibrous material has directionality because it is oriented in the fiber direction. Naturally, the carbon body which is a fired body of such fibers also has an orientation, but in this case having an orientation indicates that the carbon layer surfaces are aligned in a substantially constant direction. When a carbon body is used as a material for the active electrode, if the orientation is too high, structural anisotropy is strong and intercalation easily occurs depending on the direction, so that the capacity does not increase. . Therefore, the degree of orientation P is 70% ≦ P ≦ 85%.

On the other hand, if the crystallinity is too high, the mobility of ions in the carbon body is low, which causes a problem that the discharge capacity at the time of high current discharge is lowered. On the contrary, a carbon body having a too low degree of crystallinity is not suitable as a carbon body electrode because carbonization is not sufficient. Therefore, the crystallite thickness Lc by X-ray diffraction is 13 Å ≦ Lc ≦ 20 Å,
More preferably, a carbon body with 13 Å ≦ Lc ≦ 16 Å is used.

Here, the orientation degree P is an index showing how much the carbon layer surface in the carbon body is oriented with respect to the fiber axis direction, and can be measured by the following method.

When the carbon fiber axis is arranged on the fiber sample stand so as to be vertical and irradiated with X-rays (Cu, Kα) from a right angle direction, a strong diffraction of (002) occurs in the vicinity of the diffraction angle 2θ = 26 ° in the horizontal plane. A line appears. Next, while rotating the carbon fiber in the plane perpendicular to the incident X-ray, the diffraction angle 2θ = 26 in the horizontal plane.
The rotation angle dependence of the diffraction intensity is measured at a position near °.
The degree of orientation P is calculated from the following equation, where the half-value width obtained from the angle dependence of this intensity is the angle H.

P = {(180−H) / 180} × 100 (%) (1) Further, the thickness Lc of the crystallite is the value of the full width at half maximum of the plane index (002) peak obtained by X-ray diffraction. From the below
It was calculated using the Scherrer formula.

L _C (002) = Kλ / β ₀ cos θ _B (2) where L _C (002): average size in the direction perpendicular to the (002) plane of the microcrystal, K: 1.0, λ : Wavelength of X-ray (Cu
In the case of Kα, 1.54), β ₀ = (β _E ² −β _I ² )
^1/2 , β _E : apparent half width (measured value), β _I : correction value, θ _B : Bragg angle.

The carbon body constituting the battery electrode according to the present invention will be described in detail below with reference to specific examples.

The carbon body in the present invention is not particularly limited, and generally, a carbon material obtained by firing an organic material is used. Specifically, polyacrylonitrile (PA
N), PAN-based carbon bodies, pitch-based carbon bodies obtained from pitch such as coal or petroleum, cellulose-based carbon bodies obtained from cellulose, vapor-grown carbon bodies obtained from gas of low molecular weight organic matter, and the like. However, in addition thereto, a carbon body obtained by firing polyvinyl alcohol, lignin, polyvinyl chloride, polyamide, polyimide, phenol resin, furfuryl alcohol, or the like may be used. Among these carbon bodies, depending on the characteristics of the electrode and battery in which the carbon body is used, it is necessary to appropriately select the carbon body that satisfies the characteristics.

Among the above carbon bodies, when used as a negative electrode of a secondary battery using a non-aqueous electrolyte containing an alkali metal salt, PAN-based carbon bodies, pitch-based carbon bodies and vapor-grown carbon bodies are preferable. Particularly, the PAN-based carbon body is preferably used in that the doping of alkali metal ions, particularly lithium ions is good.

In the invention of the present application, the form of the carbon body is not particularly limited, such as a particulate form or a fibrous form. However, in the case of a fibrous form, a method for producing a PAN-based carbon fiber is disclosed in JP-B-37. -4405, Japanese Patent Publication No. 44-2
No. 1175, Japanese Patent Publication No. 47-24185, Japanese Patent Publication No. 51-
6244 and many other known methods.
However, most of them are methods of manufacturing carbon fiber as a reinforcing material, and therefore, a technique of increasing orientation in a yarn making and firing step is adopted. That is, as a method for achieving high strength, a method by constant length treatment, further controlling the shrinkage against free shrinkage, a method of substantially suppressing orientation relaxation, such as a method of carbon fiber having a high orientation Manufacturing methods are known. However, since it is important that the carbon fiber in the present invention has a low degree of orientation and crystallinity as described above, it is necessary to improve the known technique to some extent. As a method for producing a carbon fiber having low orientation and amorphousness, first, in the raw yarn, the main component is polyacrylonitrile, and a component such as methyl acrylate or methyl methacrylate that easily causes orientation relaxation by heating. The method using the copolymer composition polymer of is preferably used. Further, in the firing step, carbon fiber having low orientation and non-crystallinity can be produced by carrying out under low tension and further under no tension.

More specifically, in the method disclosed in Japanese Examined Patent Publication No. 51-24603, the draw ratio in the flameproofing step is 0.78, and the draw ratio in the carbonization step is 0.92.
By adopting the improved technology of shrinking twice,
It is possible to manufacture a carbon fiber having an orientation degree of 78% and a crystallite size (Lc) of 15 angstroms, which is extremely low with respect to an orientation degree of 81% before improvement and a crystallite size (Lc) of 17 angstroms. . Also in pitch-based carbon fibers and carbon fibers starting from other organic polymers, low orientation carbon fibers can be produced by the same technical idea.

In the present invention, the average length obtained from the carbon body having the above-mentioned suitable degree of orientation and crystal thickness is 5 mm or less, preferably 1 mm or less, more preferably 100 μm.
The following carbonaceous materials are used. This carbonaceous material is used as an electrode by shaping it into a sheet from a slurry containing a binder and a solvent, but if the average length exceeds 5 mm, the coatability deteriorates. As a method for cutting or crushing the carbon body to an average length of 5 mm or less, various fine pulverizers can be used. In the present invention, the average length is obtained by measuring the length of 20 or more carbonaceous materials in the orientation direction by observing with a microscope such as SEM.

When used as carbon fiber, its diameter is
It should be determined so that each form can be easily adopted, but carbon fibers having a diameter of 1 to 1000 μm are preferably used, and 1 to 20 μm is more preferable. It is also preferable to use several kinds of carbon fibers having different diameters.

The electrode composed of a carbon body of the present invention is
It can be used as an active electrode of various batteries, and what kind of battery such as a primary battery or a secondary battery is used is not particularly limited. Among these, it is preferably used as a negative electrode of a secondary battery. Particularly preferable secondary batteries include secondary batteries using a non-aqueous electrolyte solution containing an alkali metal salt such as lithium perchlorate, lithium borofluoride, and phosphorus hexafluoride / lithium.

When the electrode of the present invention is used in a non-aqueous electrolyte secondary battery containing an alkali metal salt, the cation or anion doping of the carbon body is utilized, and the cation-doped carbon body is used as the negative electrode. A carbon body doped with anions will be used for the positive electrode. These should be able to be used for the positive electrode or the negative electrode depending on various characteristics of the carbon body, but it is not always necessary to use both electrodes as the electrodes of the present invention, and an electrode composed of the carbon body of the present invention for the negative electrode,
It is also a preferred embodiment to use an electrode containing no carbon body as a positive electrode.

When an electrode containing no carbon body is used for the positive electrode, artificial or natural graphite powder, fluorinated carbon, an inorganic compound such as metal or metal oxide, or an organic polymer compound can be used. In a positive electrode using an inorganic compound such as a metal or a metal oxide, charge-discharge reaction occurs by utilizing cation doping and dedoping. In the case of an organic polymer compound, a charge / discharge reaction occurs due to anion doping and undoping. Examples of the metal include transition metal oxides containing alkali metals, transition metal chalcogens, and the like. Further, as the organic polymer, conjugated polymers such as polyacetylene, polyparaphenylene, polyphenylene vinylene, polyaniline, polypyrrole, and polythiophene, cross-linked polymers having a disulfide bond, thionyl chloride and the like are used. Among these, in the case of a secondary battery using a non-aqueous electrolytic solution containing a lithium salt, transition metal oxides and transition metal chalcogens such as cobalt, manganese, molybdenum, vanadium, chromium, iron, copper and titanium are It is preferably used.

The electrolytic solution of the secondary battery using the electrode of the present invention is not particularly limited, and a conventional electrolytic solution is used, and examples thereof include an acid or alkaline aqueous solution or a non-aqueous solvent. Among them, as the electrolytic solution of the secondary battery composed of the above-mentioned non-aqueous electrolytic solution containing an alkali metal salt, propylene carbonate, ethylene carbonate, γ-butyrolactone, N-methylpyrrolidone, acetonitrile, N, N-dimethylformamide, Dimethyl sulfoxide, tetrahydrofuran, 1,3-dioxolane,
Methyl formate, sulfolane, oxazolidone, thionyl chloride, 1,2-dimethoxyethane, dimethyl carbonate, diethylene carbonate, and derivatives and mixtures thereof are preferably used. As the electrolyte contained in the electrolytic solution, alkali metal, particularly lithium halide, perchlorate, thiocyanate, borofluoride, phosphorus fluoride, arsenic fluoride, aluminum fluoride, trifluoromethyl sulfate, etc. Is preferably used.

The secondary battery using the electrode of the present invention is lightweight, has a high capacity and has a high energy density, and can be used for portable applications such as video cameras, personal computers, word processors, radio-cassettes, and mobile phones. It can be widely and suitably used as a small electronic device.

[0029]

EXAMPLES Specific embodiments of the present invention will be described below with reference to examples, but the present invention is not limited thereto.

Example 1 Acrylic fiber having a (400) orientation degree of PAN of 92% was flame-proofed in air at 200 ° C. to 250 ° C. under no stress, and then fired in nitrogen under no stress for 1 minute at 1100 ° C. By doing so, a carbon fiber was produced. Degree of orientation of the carbon fiber,
It was measured by wide-angle X-ray diffraction (counter method). The degree of orientation of the carbon fiber obtained from the formula (1) was 77.0%. Further, the crystallite thickness Lc obtained from the equation (2) is 1
It was 4.3 angstroms.

Next, the carbon fiber is crushed to obtain an average length of 30.
A μm milled fiber was prepared, 10 parts by weight of polyvinylidene fluoride as a binder was added thereto, and 1-methyl-2-pyrrolidone was used as a solvent to prepare a slurry. After applying this slurry to a copper foil, it was dried and pressed to form an electrode, and the charge was evaluated. The electrolytic solution was evaluated by a three-electrode cell using propylene carbonate containing 1 M LiBF ₄ and a metallic lithium foil for the counter electrode and the reference electrode. The current density per weight of carbonaceous material is 40 mA / g at a constant current of 0 V (vs. Li
⁺ / Li). The discharge capacity of the carbonaceous material electrode determined from the amount of charge discharged after charging was 400 mAh / g.

Example 2 A carbon fiber was produced in the same manner as in Example 1, except that the firing temperature and time were 1200 ° C. and 5 minutes.

The degree of orientation of the carbon fibers was measured by wide-angle X-ray diffraction (counter method). The degree of orientation of the carbon fiber obtained from the formula (1) was 78.0%. Further, the crystallite thickness Lc calculated from the equation (2) was 14.8 angstrom.

Next, an electrode was prepared in the same manner as in Example 1 using the above carbon fiber, and the charge was evaluated. The electrolyte is 1M
Evaluation was carried out in a three-electrode cell in which propylene carbonate containing LiBF ₄ and metallic lithium foil were used for the counter electrode and the reference electrode. The current density per weight of carbonaceous material was 40 mA / g constant current, and the battery was charged to 0 V (vs. Li ⁺ / Li). The discharge capacity of the carbon fiber electrode calculated from the amount of charge discharged after charging is 38
It was 0 mAh / g.

Example 3 Acrylic fibers were subjected to flameproofing treatment in the same manner as in Example 1, and then fired at about 1200 ° C. for 5 minutes in a nitrogen atmosphere under tension.

The degree of orientation of the carbon fibers was measured by wide-angle X-ray diffraction (counter method). The degree of orientation of the carbon fiber obtained from the formula (1) was 80.5%. Further, the crystallite thickness Lc calculated from the equation (2) was 16.0 angstrom.

Next, an electrode was prepared in the same manner as in Example 1 using the above carbon fiber, and the charge was evaluated. The electrolyte is 1ML
Evaluation was carried out in a three-electrode cell using propylene carbonate containing iBF ₄ , and metallic lithium foil for the counter electrode and the reference electrode. The current density per weight of carbonaceous material was 40 mA / g constant current, and the battery was charged to 0 V (vs. Li ⁺ / Li). The discharge capacity of the carbonaceous material calculated from the amount of charge discharged after charging is 370 mAh
It was / g.

Comparative Example 1 A carbon fiber was prepared in the same manner as in Example 1, except that the firing temperature and time were set to 1700 ° C. for 5 minutes.

The degree of orientation of the carbon fibers was measured by wide-angle X-ray diffraction (counter method). The degree of orientation of the carbon fiber obtained from the formula (1) was 82.0%. Further, the crystallite thickness Lc calculated from the equation (2) was 21.0 angstrom.

Next, an electrode was prepared in the same manner as in Example 1 using the above carbon fiber, and the charge was evaluated. The electrolyte is 1MLi
Evaluation was carried out in a three-electrode cell in which propylene carbonate containing BF ₄ and metallic lithium foil were used for the counter electrode and the reference electrode. The current density per weight of carbonaceous material was 40 mA / g constant current, and the battery was charged to 0 V (vs. Li ⁺ / Li). The discharge capacity of the carbon fiber electrode calculated from the amount of charge discharged after charging is 220 m.
It was Ah / g.

Example 4 Commercially available lithium carbonate (Li ₂ CO ₃ ) and basic cobalt carbonate
(2CoCO ₃ .3Co (OH) ₂ ) was weighed so that the molar ratio was Li / Co = 1/1, mixed with a ball mill, and heat-treated at 900 ° C. for 20 hours to obtain LiCoO ₂ . This was crushed with a ball mill, and artificial graphite was used as a conductive material, polyvinylidene fluoride (PVdF) was used as a binder, and N-methylpyrrolidone was used as a solvent. The weight ratio of LiCoO ₂ / artificial graphite / PVdF = 80/15/5 To prepare a positive electrode slurry, which was applied on an aluminum foil, dried and pressed to obtain a positive electrode.

The electrode made of the carbonaceous material prepared in Example 1 was used as a negative electrode, and the positive electrode prepared above was put through a separator made of a porous polypropylene film (Celguard # 2500, manufactured by Daicel Chemical Industries, Ltd.). A secondary battery was produced by stacking them. As the electrolyte, propylene carbonate containing 1M LiBF ₄ was used.

The charging evaluation of the secondary battery produced above was performed. The current density per weight of carbonaceous material was 40 mA / g, and the battery was charged to 4.3 V at a constant current. The discharge capacity of the secondary battery obtained from the amount of charge discharged after charging was 370 mAh / g based on the weight of the carbonaceous material used in the battery.

Comparative Example 2 A secondary battery was manufactured in the same manner as in Example 4, using the carbon fiber electrode manufactured in Comparative Example 1 as the negative electrode. The weight of the carbonaceous material used in this battery was 190 mAh / g.

[0045]

According to the present invention, a battery having a carbon body as an electrode and having a high discharge capacity can be provided.

Claims (5)

[Claims] 1. An orientation degree P of 70% ≦ P ≦ 85% and X
The crystallite thickness Lc by line diffraction is 13 Å ≦
An electrode for a secondary battery, which is made of a carbonaceous material having an average length of 5 mm or less, which is obtained from a carbon body having Lc ≦ 20 Å. 2. The electrode for a secondary battery according to claim 1, wherein the carbon body is carbon fiber. 3. The secondary battery electrode according to claim 1, which is used as a negative electrode. 4. The electrode for a secondary battery according to claim 1, wherein a transition metal oxide is used as the positive electrode material. 5. The electrode for a secondary battery according to claim 4, wherein the transition metal oxide is LiCoO ₂ or LiNiO ₂ .