JPH06333559A - Nonaqueous lithium ion secondary battery - Google Patents

Abstract

PURPOSE:To enhance discharge capacity by forming a negative electrode by a composition in which 1-30wt.% of carbon black is contained in a graphitized gas phase developing carbon fiber having a carbon lattice spacing of less than 0.345nm. CONSTITUTION:A negative electrode is formed by a composition in which 1-30wt.% of carbon black is contained in a graphitized gas phase developing carbon fiber having a carbon lattice spacing d002 of less than 0.345nm, preferably, less than 0.338nm. As the carbon black to be used, acetylene black is suitably used, but thermal black and furnace black may be also used. The mixing ratio of carbon black in the gas phase developing carbon fiber is 2-12wt.% when electrolyte mainly consists of LiPF6, and 4-20wt.% when it mainly consists of LiClO4. A part of the gas phase developing carbon fiber is substituted by carbon black, whereby a discharge capacity value exceeding the theoretical charge capacity of the gas phase developing carbon fiber alone can be provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement of a non-aqueous lithium ion secondary battery, and particularly to provide a high performance secondary battery having a larger capacity than ever before.

[0002]

2. Description of the Related Art In recent years, the demand for high-performance secondary batteries has increased with the development of various types of electronic equipment. Among them, lithium secondary batteries have been attracting attention because of their high capacity, but since metal lithium was used for the electrodes, it was recognized that there was a problem with safety, and now carbon, especially graphite, is used as the negative electrode. The number of researches on non-aqueous lithium-ion secondary batteries, which utilize the ability to form an intercalation compound of carbon and lithium ions, is increasing, and some of them are in the stage of practical application. However, this electrode also has a drawback that the capacity becomes small because the lithium concentration in the intercalation compound of lithium and carbon is considerably lower than the lithium concentration in metallic lithium. In the carbon-lithium intercalation compound, the ratio of the number of carbon atoms to the number of lithium atoms was 6 to 1 even in the first stage, which contained the largest amount of lithium, and the theoretical chargeable electric capacity of the electrode at that time was 372 mA. It is Hr / g, and many efforts have been made to develop a battery having a discharge capacity of this theoretical value, but this target value has not yet been achieved. According to one theory, since it is only the graphite crystal part of carbon that forms the intercalation compound at the above ratio, the charging capacity should be corrected by the degree of graphitization. The present invention not only achieves this target value, but also provides a high-capacity secondary battery that exceeds the target value.

[0003]

In the secondary battery of the present invention, the carbon lattice spacing d002 is 0.345 nm or less, preferably 0.
It is characterized in that the negative electrode is formed from a composition containing 1 to 30% by weight of carbon black in graphitized vapor-grown carbon fiber of 338 nm or less. Vapor grown carbon fibers are known. This is, for example, by injecting a mixed gas of ferrocene, benzene, and hydrogen into a furnace heated to about 1000 ° C. to obtain fibers in a suspended state, a method of forming ultrafine iron particles on a ceramic substrate, and flowing a hydrogen stream. Put it in the furnace
A substrate method for obtaining fibers grown from a substrate by injecting benzene is known. It is also well known that the carbon lattice planes of these vapor phase carbon fibers are arranged in an annual ring shape around the fiber axis. These vapor-grown carbon fibers may be used as they are as long as graphitization has progressed, but they are usually graphitized in an inert gas at 2000 ° C. or higher to set the lattice spacing d002 to 0.345 nm or less. . d00
When 2 is 0.338 or less, the electric capacity is further increased. Further, when the fibers are crushed and cut, it is preferable to carry out after the graphitization, not before graphitization, because the effect of the present invention is large. The fiber diameter is preferably 1.0 to 3.0 μm, the length is 3.0 to 200.0 μm, and the aspect ratio is 3.0 to
100.0 is preferable.

The carbon black used in the present invention is not particularly limited. It is preferable that the electric conductivity is as high as possible and the particle size is small. In that sense, acetylene black is suitable, but thermal black, furnace black, channel black and the like can also be used. Ketjen black whose surface is graphitized can also be used.
Some carbon blacks have a chain structure in which particles are aggregated, but they can be used favorably. The mixing ratio of carbon black in the vapor grown carbon fiber is 1 to 30% by weight. That is, 70 to 9 of total carbon
9% is vapor grown carbon fiber and 1 to 30% is carbon black. Of course, it can be used as an electrode outside this range, but within this range, the discharge capacity is larger than the theoretical value of the vapor grown carbon fiber. A more preferable range depends on the electrolyte used, but it is preferable to use 2 to 12% by weight of carbon black when the electrolyte is mainly LiPF ₆ and ₄ to 20% by weight when the electrolyte is mainly LiClO _4. . The weight ratio here indicates the ratio in the total carbon, and the electrode material includes a binder as well.

In the manufacture of electrodes, the above mixture was used
% Or less and a resin as a binder is mixed and molded. As the resin, a thermoplastic resin having no reactive group such as polyvinylidene fluoride, fluorinated polyethylenes, polyethylene, polypropylene and nylon is used. For granular resin, mix the three with a high shear agitator such as a kneader or an automatic mortar.If a solvent is used, mix the resin solution with a high shear agitator and use nickel mesh as a current collector. Apply or apply on, and if necessary, lightly hot press to finish. At this time, press pressure is 50 kg / c
It is also possible to make the pressure higher than m. Before assembling the battery, it is preferable to dry it sufficiently to remove water and solvent. It is preferable that the amount of resin and other additives is as small as possible within the range where the purpose can be achieved.

As the positive electrode of the battery, various known materials for non-aqueous lithium ion secondary batteries can be used. M
_{nO 2, Li X Mn 2 O} 4, Li X Co Y Mn 2-Y O
₄ , α-V ₂ O ₅ , TiS ₂ , Li _X CoO _{2 and the} like can be preferably used by a known method. LiClO ₄ , LiPF ₆ , LiAsO ₆ , LiB as the electrolyte in the battery
F _4, and the like trifluoropropyl meta Ruch lithium propionic acid. The concentration of the electrolyte in the solvent is 0.2 to 5.
0 mol. The range of / liter is preferred. The effect of adding carbon black is particularly great when LiPF ₆ is used as the electrolyte.

As the solvent, propylene carbonate, diethylene carbonate, ethylene carbonate, acetonitrile, methyl acetate, tetrahydrofuran, γ-butyrolactone, dimethoxyethane, dioxolane, 2-
Methyldioxolane, 4-methyldioxolane, methyl formate and the like can be used alone or in combination. For vapor-grown carbon fibers having a high degree of graphitization, the use of propylene carbonate alone is not preferable because the decomposition of the solvent may occur and the Coulomb efficiency may be reduced. As is known, it is not preferable that the solution contains oxygen and water, and it is necessary to purify the solution as much as possible.

A known method can be directly used for assembling the battery. Although the effects of the present invention have been described in the examples by the so-called three-electrode method, they can be assembled into a button type battery, a coin type battery, a cylindrical battery of single to single size cells, or a stacked type battery by a known method. Since no gas is generated by charging and discharging, a secondary battery having a high energy density can be easily obtained as long as air and moisture are completely sealed.

[0009]

【Example】

(Example 1) 0.05 g of polyvinylidene fluoride (PVDF; KF # 1000 manufactured by Kureha Chemical Co., Ltd.) was accurately weighed and placed in an agate mortar, and 1-methyl-2-pyrrolidone (NMP) 0. 5 cc was weighed out with a syringe and added to dissolve completely. Precisely weighed 0.05 g of acetylene black (Denka Black ^R manufactured by Denki Kagaku Kogyo Co., Ltd.), added it to the mortar and kneaded it sufficiently to form a paste. Separately, 28
Graphitized at _{00 ℃ (d 002 = 0.336nm)} , then ground flat diameter 2.1 .mu.m, and mixed well further added to the paste product weighed exactly 0.9g an average length of about 20μm . On the other hand, the above mixed paste was applied in a range of 10 × 10 mm on a 10 × 40 mm nickel mesh thoroughly washed with acetone, and vacuum dried at 100 ° C. for 2 hours. Using the electrode thus obtained as a working electrode, a 3-electrode cell was assembled using a Luggin tube in a glove box under argon gas from which oxygen and water were sufficiently removed. 10 × 40 × 2 mm metallic lithium was used for the counter electrode and the reference electrode, and the electrolyte solution had a concentration of 1 mol. / Liter in LiP
F ₆ as ethylene carbonate and diethyl carbonate 1
It was dissolved in a 1: 1 mixed solvent and used. Each electrode is connected to a charging / discharging device and left to stand until the voltage becomes constant, and then the potential difference between the working electrode and the reference electrode is 0 to 2.5 V, and the current density is 25 m.
Charge and discharge were repeated at AHr / g. The results are shown in Table 1.

[Table 1]

(Comparative Example 1) The battery characteristics were measured in the same manner as in Example 1, except that PVDF was 0.1 g and NMP was 0.5 cc, and acetylene black was not used, and a three-electrode cell was assembled. The results are shown in Table 2.

[Table 2]

Comparative Example 2 PVDF in Example 1
0.33g, acetylene black 0.66g, NMP
The battery characteristics were measured in the same manner except that the carbon fiber was 0.25 cc and no carbon fiber was used.
The results are shown in Table 3.

[Table 3]

(Example 2) Charge and discharge of 0 to 2.5 V were repeated in exactly the same manner as in Example 1 except that the ratio of acetylene black and graphitized vapor grown carbon fiber was changed. FIG. 1 shows the relationship between the discharge capacity at the third cycle in which the discharge capacity is stable and the acetylene black ratio. Here, the theoretical charge capacity 1 is the theoretical charge capacity when it is assumed that all the carbon is graphite, and the theoretical charge capacity 2 is the theoretical charge capacity when the charge capacity of acetylene black is 0 in the above. The theoretical discharge capacity 2 decreases as the amount of acetylene black increases, but the measured discharge capacity exceeds the theoretical charge capacity 2 for acetylene black 2 to 13%, and exceeds the theoretical charge capacity 1 for 2 to 12%. You can see that it is turning.

(Example 3) In Example 1, the electrolyte was changed to L
Charge / discharge of 0 to 2.5 V was repeated in exactly the same manner except that iClO ₄ was changed and the ratio of acetylene black and graphitized vapor grown carbon fiber was changed. The relationship between the discharge capacity at the third cycle and the acetylene black ratio is shown in FIG. From this, it is clear that a discharge capacity exceeding each theoretical charge capacity can be obtained with acetylene black of 4% or more.

[0014]

[Effect] As is clear from the examples and comparative examples, the discharge capacity of the vapor-grown carbon fiber is slightly high when carbon components are used alone, and the discharge capacity of the carbon black is very low. It was found that by replacing with carbon black, the discharge capacity does not decrease in a weighted average, but rather increases, and a discharge capacity value that exceeds the theoretical charge capacity of vapor grown carbon fiber alone is obtained. .

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a discharge capacity and an acetylene black ratio in Example 2.

FIG. 2 is a diagram showing a relationship between a discharge capacity and an acetylene black ratio in Example 3.

Claims (4)

[Claims] 1. A carbon lattice spacing d002 is 0.345 nm.
A non-aqueous lithium ion secondary battery characterized in that a negative electrode is formed from the following composition containing 1 to 30% by weight of carbon black in graphitized vapor-grown carbon fiber. 2. The non-aqueous lithium ion secondary battery according to claim 1, wherein the carbon black is acetylene black. 3. The non-aqueous lithium ion secondary battery according to claim 1, wherein the carbon black is contained in an amount of 2 to 12% by weight, and the electrolyte mainly comprises LiPF ₆ . 4. The non-aqueous lithium ion secondary battery according to claim 1, wherein carbon black is contained in an amount of 4 to 20% by weight, and the electrolyte mainly comprises LiClO ₄ .