JPH10134796A - Battery electrode and secondary battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a secondary battery with its superior cycle characteristics. SOLUTION: In an electrode using a carbon body in which a Li-ion is stored and released from in charging and discharging, an inter-planar distance on a face (002) of the carbon body during discharging is 0.365nm or less, an expansion/contraction percentage of an electrode thickness associated with charging/discharging of the electrode consisting of the carbon body is 8.0% or less, and a discharge capacity per carbon body is 300mAh/g or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode for a battery using a carbon body that stores and releases lithium, and a secondary battery using the same.

[0002]

2. Description of the Related Art A secondary battery using lithium intercalation in a carbon body can overcome the safety problem of a secondary battery using metallic lithium and is a high capacity secondary battery. It is attracting attention as a new high-performance battery.

As a carbon body used in such a secondary battery, a crystalline carbon body having a graphite layered structure developed and having a small average interplanar distance, and a crystalline carbon body having a small layered structure and having an average interplanar distance having a small average plane distance have been developed. It can be broadly divided into broad amorphous carbon bodies.

When a crystalline carbon body is used, the carbon body repeatedly undergoes expansion and contraction during charging and discharging, which causes a problem that the electrode is deteriorated and the cycle characteristics are poor.

On the other hand, in recent years, (0)
It has been found that intercalation is possible even in an amorphous carbon body having a large interplanar distance (d) between planes 02), and a high-performance secondary battery utilizing such intercalation into an organic fired body has been developed. Has been realized. However, when an amorphous carbon body is used, although the expansion and contraction of the carbon body is suppressed and the cycle characteristics are good, since the face-to-face distance is generally large, the density is generally small and the filling amount in the battery can is reduced. There is a problem.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned drawbacks of the prior art, and it is an object of the present invention to provide a secondary battery using a carbon body having a small expansion / shrinkage ratio and a small distance between planes. An object is to provide an electrode for a battery.

[0007]

SUMMARY OF THE INVENTION The present invention has the following arrangement to solve the above-mentioned problems.

(1) In an electrode using a carbon body in which Li ions are inserted and extracted during charging and discharging, the distance between the (002) planes of the carbon body during discharge is 0.365 nm or less, and An electrode for a battery, wherein a rate of expansion and contraction of an electrode thickness due to charge and discharge of the body electrode is 8.0% or less.

(2) A secondary battery using the electrode according to (1).

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In general, an electrode having a large expansion and contraction rate due to charge and discharge has a large cycle deterioration, and it is necessary to put the electrode in a battery can in consideration of the expansion and contraction rate. . In addition, a carbon body having a small expansion and contraction rate generally has a large inter-plane distance of the (002) plane and a small density, and thus has a problem that the packing density in a battery is reduced.

In the present invention, it has been found that by using a carbon body having an appropriate degree of crystallinity, a capacity per volume is high, and a reduction in expansion / shrinkage rate and a reduction in irreversible capacity can be achieved.

In the present invention, the change in expansion / shrinkage rate due to charging / discharging means that the thickness of an electrode is measured in an inert gas in a charged state and in a discharged state, and the thickness of the current collector is subtracted from the electrode thickness. It can be obtained by determining the rate of change in thickness.

In the present invention, a carbon body having a distance (d) between planes (002) of 0.365 nm or less is used as the carbon body. d can be obtained from the (002) diffraction line peak position using the following equation after performing absorption correction, polarization correction, and correction for the atomic scattering factor.

D = λ / 2 sin θ where λ; wavelength of X-ray (0.154 in case of CuKα ray)
nm), θ; Bragg angle. D in the present invention is a value before charging or at the end of discharging. In addition, the value of d is slightly different between the case of measuring the carbon fiber that has not been pulverized and the powdered carbon fiber that has been subjected to the pulverization treatment. However, d in the present invention refers to the X-ray of the powdered carbon fiber. This is a value obtained from the diffraction result.

[0015] The carbonaceous material in the present invention is not particularly limited, and a material obtained by baking an organic substance is generally used, for example, polyacrylonitrile (PA).
N), fired pitch such as coal or petroleum, cellulose, polyvinyl alcohol, lignin, polyvinyl chloride, polyamide, polyimide, phenol resin, and furfuryl alcohol. Further, a carbon body obtained from a copolymer with the above organic substance, for example, a copolymer of acrylonitrile with styrenes and maleimides is also preferably used. Further, the form of the carbon body may be any of powder, fiber, and the like. Among these carbon bodies, a carbon body satisfying the characteristics is appropriately selected according to the characteristics of the electrode and the battery. When used for a negative electrode of a secondary battery using a non-aqueous electrolyte containing an alkali metal salt in the carbon body, a PAN-based carbon fiber, a PAN copolymer-based carbon fiber,
Pitch-based carbon fibers and the like are preferably used.

In the present invention, carbon fibers are preferably used as described above, and more preferably, carbon bodies obtained by pulverizing these carbon fibers are used. As such a carbon body, an average length of 1 mm or less, more preferably 50 μm or less is used. If the fiber length exceeds 1 mm, the coating properties may be insufficient when the slurry is formed to form a sheet-like electrode, and when the electrode is used, a short circuit between the positive and negative electrodes is likely to occur. Tend.

The average length of the fiber can be determined, for example, by measuring the length in the fiber direction of 20 or more carbon bodies by microscopic observation such as SEM. Various pulverizers can be used to cut or pulverize the carbon fiber to 1 mm or less.

When carbon fibers are used in the present invention, the irreversible capacity increases when the concentration of oxygen atoms on the surface of the carbon fibers is high. This is considered to be due to a side reaction on the carbon fiber surface. For this reason, it is desirable that the ratio of oxygen atoms to carbon atoms on the carbon fiber surface be 6% or less.

In the present invention, the ratio of oxygen atoms to carbon atoms near the surface can be determined by X-ray electron spectroscopy. More specifically, a sample is irradiated with, for example, magnesium Kα ray as an X-ray source, and photoelectrons emitted from the sample surface are energy-divided and detected by an analyzer. The binding energy of bound electrons in a substance is obtained as a spectrum, and information on constituent elements near the surface can be obtained from the energy value of atomic orbitals.

In the electrode using the carbon body of the present invention, a metal can be used as the current collector to enhance the current collecting effect. The metal current collector is not particularly limited, such as a foil shape, a fiber shape, and a mesh shape.
When a foil-like metal current collector is used, a sheet-like electrode is produced by applying a slurry to a metal foil. It is also desirable to add a conductive powder such as a carbon powder and a metal powder as a conductive material to the sheet-shaped electrode to further enhance the current collecting effect.

The bulk density of the carbon body in the electrode using the carbon body of the present invention is 0.8 g / cm ³ or more and 1.6 g / cm ^3.
cm ³ or less is desirable. If less than 0.8 g / cm ³ ,
When the battery capacity per volume decreases, and exceeds 1.6 g / cm ³ , the electrolyte does not easily permeate, and the expansion coefficient tends to increase. Such bulk density can be preferably adjusted by pressure shaping.

The electrode composed of the carbon body of the present invention can be used as an active electrode of various batteries, and there is no particular limitation on what kind of battery such as a primary battery or a secondary battery is used. Among them, it is preferably used for a negative electrode of a secondary battery. Particularly preferred secondary batteries are lithium perchlorate,
A secondary battery using a non-aqueous electrolyte containing an alkali metal salt such as lithium borofluoride and phosphorus / lithium hexafluoride can be given.

When the electrode of the present invention is used for a negative electrode, examples of the positive electrode active material include artificial or natural graphite powder, inorganic compounds such as carbon fluoride and metal oxides, and organic polymer compounds. Specifically, inorganic compounds such as transition metal oxides and transition metal chalcogens containing alkali metals, conjugated polymers such as polyparaphenylene, polyphenylenevinylene, polyaniline, polypyrrole, and polythiophene, and polymers having a disulfide bond are usually used. The positive electrode used in the secondary battery described above can be mentioned. Among these, in the case of a secondary battery using a non-aqueous electrolyte containing a lithium salt, a transition metal oxide or a transition metal chalcogen such as cobalt, manganese, molybdenum, vanadium, chromium, iron, copper, and titanium is used. It is preferably used.
Particularly, Li _x CoO ₂ (0 <x ≦ 1.0), Li _x NiO
₂ (0 <x ≦ 1.0) and Li _x Co _Y Ni _1-Y O ₂
(0 <x ≦ 1.0, 0 <y ≦ 1.0), etc.
It is preferable in terms of stability and long life.

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 alkali aqueous solution and a non-aqueous solvent. Among these, propylene carbonate (PC), ethylene carbonate (EC), γ-butyrolactone (BL), N-methylpyrrolidone ( MP), acetonitrile (AN), N, N-
Dimethylformamide, dimethylsulfoxide, tetrahydrofuran (THF), 1,3-dioxolane, methyl formate, sulfolane (SL), oxazolidone, thionyl chloride, 1,2-dimethoxyethane (DME), diethylene carbonate (DEC), dimethyl carbonate (DMC), derivatives and mixtures thereof, and the like are preferably used.

The electrolyte contained in the electrolyte may be an alkali metal, especially lithium halide, perchlorate, thiocyanate, borofluoride, phosphorus fluoride, arsenic fluoride, aluminum fluoride, trifluoromethyl. Sulfates and the like are preferably used. The secondary battery using the electrode of the present invention is widely used in portable electronic devices such as a video camera, a personal computer, a word processor, a radio-cassette, a mobile phone, etc. by utilizing the features of light weight, high capacity, and high energy density. Available.

[0026]

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 A carbon fiber ("Torayca" T-300 manufactured by Toray Industries, Inc.) was pulverized using a pulverizer to obtain a powdery carbon fiber having an average length of 30 μm.
Next, the powdered carbon fiber was heat-treated at 1400 ° C. for 4 hours under vacuum. Wide-angle X-ray diffraction of the carbon body (counter method)
The inter-plane distance of the (002) plane obtained from the result is 0.352 n
m. The ratio of oxygen atoms to carbon atoms near the surface was 3%.

Next, the above-mentioned powdered carbon fiber was used as a negative electrode active material, acetylene black was used as a conductive agent, and polyvinylidene fluoride was used as a binder. N-methylpyrrolidone was added to the negative electrode mixture having a weight ratio of the negative electrode active material: conductive agent: binder of 80: 5: 15, kneaded and slurried, and the slurry was applied in a copper foil form. After drying, it was pressed with a roller and pressed against a copper foil to produce a negative electrode. In the negative electrode, the bulk density of the carbon fibers was 1.2 g / cm ³ . A mixed solution (1 molar concentration) of propylene carbonate and dimethyl carbonate containing lithium phosphate hexafluoride was used as the electrolytic solution, and metallic lithium foil was used as the counter electrode and the reference electrode. The discharge capacity is a constant current with a current density of 61.5 mA / g per carbon fiber weight.
V (vs. Li ⁺ / Li) and then 307 mA
/ G at a constant current of 1.5 V (vs. Li ⁺ / Li). The discharge capacity of the carbon fiber electrode was 315 mAh / g. The irreversible capacity in the first charge / discharge was 83 mAh / g. The expansion / contraction rate at this time was 1.4%. The capacity retention at the 100th cycle was 95%.

Example 2 91.5 mol% of acrylonitrile, 5 mol% of styrene,
A copolymer fiber consisting of 2 mol% of N-phenylmaleimide and 1.5 mol% of itaconic acid was heated at 180 to 250 ° C.
, And then baked at 1300 ° C. for 5 minutes to produce carbon fibers. The carbon fiber was pulverized using a pulverizer to obtain a powdery carbon fiber having an average length of 40 μm. Next, the powdered carbon fiber was crushed under vacuum for 3 hours at 1000
Heat treated at ℃. The inter-plane distance of the (002) plane obtained from the result of wide-angle X-ray diffraction (counter method) of the carbon body was 0.35.
It was 5 nm. The ratio of oxygen atoms to carbon atoms near the surface was 4%. An electrode was produced using the powdered carbon fiber in the same manner as in Example 1 (the bulk density of the carbon fiber in the negative electrode was 1.0 g / cm ³ ), and the electrode was evaluated. The discharge capacity at this time was 391
mAh / g, irreversible capacity in initial charge / discharge is 130 m
Ah / g. The expansion / contraction rate at this time was 1.8%. The capacity retention at the 100th cycle was 93%.

Comparative Example 1 An electrode was prepared in the same manner as in Example 1 using a spherical crystalline carbon body heat-treated at 2800 ° C. (The bulk density of carbon fibers in the negative electrode was 1.7 g / cm ^3. ), And the electrode was evaluated. The discharge capacity at this time is 302
mAh / g, irreversible capacity at first charge / discharge is 48 mA
h / g. The expansion / contraction rate at this time was 8.9%. The capacity retention at the 100th cycle was 85%. The inter-plane distance of the (002) plane of the carbon body determined from the result of wide-angle X-ray diffraction (counter method) was 0.338 nm.

Example 3 The electrode evaluation of the carbon fiber powder was performed in the same manner as in Example 1 except that the heat treatment of the carbon fiber powder was not performed. The inter-plane distance of the (002) plane of the carbon body is 0.353 nm,
The ratio of oxygen atoms to carbon atoms near the surface was 12%. The bulk density of the carbon fibers in the negative electrode is 1.
It was 2 g / cm ³ .

In this case, the discharge capacity is 400 mAh /
g, the irreversible capacity in the first charge / discharge is 220 mAh / g
And the retention at the 100th cycle was 70%.

The expansion and contraction rate at this time was 2%.

Example 4 Commercially available lithium carbonate (Li ₂ CO ₃ ) and basic cobalt carbonate (2CoCO ₃ .3Co (OH) ₂ ) were weighed so that the molar ratio Li / Co = 1/1, and ball milled. And heat-treated at 900 ° C. for 20 hours to obtain LiCoO ₂ . This is pulverized by a ball mill, and acetylene black is used as a conductive material, polyvinylidene fluoride (PVdF) is used as a binder, N-methylpyrrolidone is used as a solvent, and LiCoO ₂ / acetylene black / PV is used in a weight ratio.
A positive electrode slurry was prepared by mixing so that dF = 91/4/5, and this slurry was applied on an aluminum foil, dried and pressed to obtain a positive electrode.

The same negative electrode as in Example 1 was used. The expansion and contraction rate of the negative electrode sheet was 1.4%, and the inter-plane distance of the (002) plane of the carbon body was 0.352 nm. The negative electrode was overlapped with the positive electrode prepared above via a separator made of a porous polypropylene film (Celgard # 2500, manufactured by Daicel Chemical Industries, Ltd.) to produce an AA secondary battery. Using a mixed solution of propylene carbonate and dimethyl carbonate containing 1 molar concentration of lithium hexafluoride as an electrolyte, charge / discharge evaluation of the secondary battery prepared above was performed. Charging was performed at a constant current of 400 mA to 4.3 V, and discharging was performed at 80 mA to 2.75 V. At this time, the discharge capacity of the secondary battery was 500 mAh. The capacity retention at the 100th cycle was 95%.

Comparative Example 2 AA secondary batteries were prepared in the same manner as in Example 4 except that the negative electrode sheet of Comparative Example 1 was used as the negative electrode sheet, and the battery performance was evaluated at a charging potential of 4.1 V. The expansion and contraction rate of the negative electrode sheet was 8.9%, and the inter-plane distance of the (002) plane of the carbon body was 0.338 nm.

At this time, the discharge capacity of the secondary battery was 650.
mAh. The capacity retention at the 100th cycle was 70%.

Comparative Example 3 An AA secondary battery was prepared in the same manner as in Example 4 except that a negative electrode sheet produced using an amorphous carbon powder having a (002) plane-to-plane distance of 0.370 nm was used. 3. Prepare and charge potential
Battery performance was evaluated at 3V. The bulk density of the carbon fibers in the negative electrode was 1.0 g / cm ³ . The expansion and contraction rate of the negative electrode sheet was 2.3%. At this time, the discharge capacity was 400 mAh. The capacity retention at the 100th cycle was 90%.

[0039]

According to the present invention, a secondary battery having excellent cycle characteristics can be provided.

Claims (8)

[Claims] In an electrode using a carbon body into which Li ions are inserted and discharged during charging and discharging, (0)
02) An electrode for a battery, wherein the distance between planes is 0.365 nm or less, and the rate of expansion and contraction of the electrode thickness of the electrode made of the carbon body during charging and discharging is 8.0% or less. . 2. The discharge capacity per carbon body is 300 mAh.
/ G or more. 3. The inter-plane distance of the (002) plane of the carbon body is 0.
The battery electrode according to claim 1, wherein the thickness is 350 nm or more. 4. The battery electrode according to claim 1, wherein said carbon body is a carbon fiber. 5. The battery electrode according to claim 1, wherein the ratio of oxygen atoms to carbon atoms in the vicinity of the surface of the carbon body is 6% or less. 6. The battery electrode according to claim 4, wherein the carbon body is a short fiber. 7. The battery electrode according to claim 1, wherein the expansion / contraction rate is 3.0% or less. 8. A secondary battery using the battery electrode according to any one of claims 1 to 6.