JPH0831404A - Electrode and secondary battery using the same - Google Patents

Abstract

(57) [Abstract] [Purpose] To suppress low Coulombic efficiency and decrease in discharge capacity after charging and discharging one day after charging and discharging in an electrode using a carbon material. [Structure] An electrode whose active material is a carbon material having fluorine atoms on its surface. According to the present invention, an electrode having excellent charge / discharge characteristics and a secondary battery using the same can be obtained by using a carbon material having a fluorine atom on the surface as an active material.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode 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, a lithium secondary battery using lithium metal for the negative electrode has been studied.

However, in a secondary battery in which lithium metal is used as the negative electrode, lithium tends to grow into dendrites due to repeated charging / discharging, resulting in short circuits and shortened life. Therefore, a secondary battery has been proposed in which various carbon materials are used for the negative electrode and used by doping and dedoping with lithium ions. Secondary batteries using electrodes made by doping lithium ions or anions into the above carbon materials are disclosed in JP-A-57-208079, JP-A-58-93176, and JP-A-58-192266. Japanese Laid-Open Patent Publication No. 62-90863, Japanese Laid-Open Patent Publication No. 62-1
JP-A 22066 and JP-A-3-66856 are known.

As a form of such a carbon material, powder, fiber and the like have been tried, but particularly carbon fiber has recently attracted attention.

[0005]

However, various functional groups remain on the surface of the carbon material, depending on the raw material and the firing temperature, which adversely affects the doping and dedoping of lithium ions. Was a problem.

[0006] For example, a secondary battery using a carbon material as an electrode has a problem that the ratio of the amount of discharged electricity to the amount of electricity required for initial charging after the battery is manufactured (Coulomb efficiency) is small. This is due to the fact that there are lithium ions that are taken into the carbon material and not dedoped, but it can also be mentioned as a cause of the amount of electricity due to the electrochemical reaction due to the surface functional groups of the carbon material.

Particularly, in a lithium ion secondary battery in which a carbonaceous material is used as a negative electrode and the carbon negative electrode is doped with lithium ions or dedoped, the low Coulombic efficiency is a serious problem. In this lithium ion secondary battery, the lithium ions doped into the carbon negative electrode are supplied from the positive electrode via the electrolytic solution. If the Coulomb efficiency at the first time is low, it is necessary to add an extra positive electrode by the amount of extra lithium not used after the second time.
For this reason, the volume or weight of the battery is increased, and the energy density is disadvantageous.

Also, when a non-aqueous electrolyte containing a fluorine-containing electrolyte such as LiBF ₄ or LiPF ₆ is used, the surface functional groups promote the decomposition of anions, and if left for 1 day after charging / discharging, LiF is left on the electrode surface. Is generated, which becomes an obstacle to doping / dedoping of lithium ions, and there is a problem that the discharge capacity is reduced.

The present invention is intended to eliminate the drawbacks of the prior art, has the characteristics of a carbon material, and
It is an object to provide an electrode and a secondary battery having excellent charge / discharge characteristics.

[0010]

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

"An electrode characterized by using a carbon material having a fluorine atom on the surface as an active material." In the electrode of the present invention, the fluorine atom is present on the surface of the carbon material as described above. That is, the raw material of the carbon material, the manufacturing method, and other characteristics are not particularly limited. Further, other constituent elements such as the positive electrode of the secondary battery using this electrode and the electrolytic solution are not particularly limited. The fluorine element may be present only on the surface, or may be present inside the carbon material.

In the present invention, the fluorine compound existing on the surface of the carbon material is not particularly limited.
For example, any state such as a state in which the carbon material surface is bound to carbon atoms or a state in which the carbon material surface is covered with a compound containing a fluorine atom is possible. When a fluorine atom is bonded to the surface of the carbon material, it is possible to chemically treat the carbon material with a compound containing a fluorine atom. Four
Plasma treatment with lower fluorocarbons such as fluorinated methane (trade name Freon), immersion in a solution of hydrofluoric acid, tetrafluoroboric acid, hexafluorophosphoric acid, or fluorine gas treatment at high temperature For example, the surface of the carbon material can be fluorinated. Further, it is possible to make the carbonaceous material have a fluorine atom present on the surface thereof by immersing the carbonaceous material in a surfactant containing a fluorine atom, a silane coupling agent, or the like.

As described above, a carbon material having a fluorine atom on the surface thereof can be obtained by various methods. To make the fluorine atom uniformly and continuously exist on the surface of the carbon material, It is preferably in the form of fibers.

In the present invention, the fluorine compound can be provided on the surface of the carbon material by various methods as described above. The fluorine compound coated on the surface of the carbon material is
The coating state can be confirmed by various analysis methods. When chemically bonded to the surface functional group of the carbon material, it is detected as a carbonized fluoride or a carbon fluoride by X-ray photoelectron spectroscopy (XPS or ESCA). In addition, secondary ion mass spectrometry (SIMS)
It can also be analyzed by reflection infrared spectroscopy. Further, in the case where the fluorine compound is not chemically bonded to the surface of the carbon material, the fluorine compound can be confirmed by infrared spectroscopy, gas chromatography, mass spectrometry or the like after solvent extraction. In the present invention, it is sufficient if the fluorine atom is present on the surface, but particularly when analyzed using, for example, X-ray photoelectron spectroscopy, the ratio of the number of fluorine atoms to the number of carbon atoms is 0.001 or more, It is preferably 0.5 or less from the viewpoint of improving charge / discharge characteristics.

The carbon material used for the electrode of the present invention can be used without any particular limitation on the raw material and the manufacturing method. Raw materials include coke and pitch such as petroleum and coal, plants such as wood, low molecular weight organic compounds such as natural gas and lower hydrocarbons, polyacrylonitrile, polyvinyl alcohol, polyvinyl chloride, polyamide, polyimide, phenol resin and furfuryl. Examples include synthetic polymers such as alcohol resins, which are subjected to flameproofing treatment depending on the raw material and application, and then carbonized or graphitized by firing at 700 to 3000 ° C. to obtain a carbon material. The properties of the carbon material include density, crystal thickness (Lc),
Examples include the crystal plane spacing (d ₀₀₂ ), electric resistance, strength, elastic modulus, etc., which should be appropriately determined according to the intended electrode characteristics of the secondary battery and are not particularly limited. Among these carbon materials, PAN-based carbon fiber obtained from polyacrylonitrile (PAN), pitch-based carbon fiber obtained from pitch of coal or petroleum, cellulose-based carbon fiber obtained from cellulose, gas of low molecular weight organic substance The obtained vapor grown carbon fiber and the like are preferably used. In particular, PAN-based carbon fiber, especially "Torayca" T series or "Torayca" manufactured by Toray Industries, Inc., in that lithium ion doping is good and the effect of the fluorine compound coated on the surface can be exhibited.
PAN-based carbon fibers such as M series and pitch-based carbon fibers obtained by firing mesophase pitch coke are more preferably used. Further, in any of the carbon materials, the effect of providing the fluorine compound of the present invention is great in the case of a carbon material having a low firing temperature.

When the fibers are used as the carbon material in the electrode as described above, it is a preferable electrode form to dispose the carbon fibers uniaxially or to form a fabric-like or felt-like structure. Examples of fabric-like or felt-like structures include woven fabrics, knitted fabrics, braids, laces, nets, felts, papers, nonwoven fabrics, and mats. Among these, from the viewpoint of the properties of the carbon fibers and the electrode characteristics, those arranged uniaxially, such as along the winding direction or in the vertical direction, and woven fabrics, felts, mats and the like are preferably used.

When the electrode of the present invention is used for the negative electrode, it is possible to use a metal as a current collector in order to enhance the current collecting effect. The metal current collector is not particularly limited in its form such as a foil shape or a fibrous shape and a connection mode with the carbon material.

The diameter of the carbon fiber when the carbon fiber is used for the electrode of the present invention should be determined so that each form can be easily adopted, but preferably the carbon fiber having a diameter of 0.01 to 1000 μm is used, and 0.1 ˜10 μm is more preferred.
It is also preferable to use several kinds of carbon fibers having different diameters.

In the present invention, it is preferable to use, as the positive electrode, a molded product of a mixture containing at least a powdered active material and a binder. The active material is not particularly limited. For example, transition metal oxides such as lithium cobalt oxide, lithium nickel oxide, lithium manganate, lithium niobate, lithium vanadate, molybdenum sulfide, transition metal chalcogens such as titanium sulfide, or a mixture thereof, or mercaptothiadiazole or the like. Disulfide compound, polyalkylene oxide, polyalkylene sulfide, polyaniline,
Examples thereof include heteropolymers such as polythiophene and polypyrrole, and conjugated polymer compounds such as polyacetylene, polydiacetylene, polyparaphenylene, and polyphenylenevinylene. The above-mentioned substances capable of occluding and releasing lithium ions or anions can be used as the positive electrode active material without limitation, and their oxidation potential is preferably 2.5 V or more with respect to lithium.
The particle size of the positive electrode active material powder is 0.1 to 100 μm, preferably 1 to 50 μm.

It is also preferable to add a conductive agent to the positive electrode used in the present invention in addition to the above-mentioned active material in order to improve electron conductivity. As such a conductive agent, carbonaceous materials, carbon materials such as artificial or natural graphite and acetylene black, and shapes such as powder and fiber are not particularly limited. The particle size of these conductive agents in the case of powder is preferably 0.1 to 100 μm,
More preferably, it is 1 to 50 μm.

It is also preferable to add a binder to the active material and the conductive agent in the positive electrode used in the present invention in order to improve the moldability. The binder is not particularly limited in addition to polymer compounds such as polytetrafluoroethylene, polyvinylidene fluoride, polyacrylonitrile, polyimide, polyethylene, polypropylene, epoxy resin and phenol resin. These binders are used as a powder mixed with an active material or a conductive agent, and are also dissolved in a solvent or dispersed as an emulsion to be used as a slurry with the active material or the conductive agent. It is not limited.

In the present invention, it is preferable that the active material, the conductive agent and the binder are mixed or dispersed, and a current collector is used to conduct electricity from the positive electrode to the terminal. As such a current collector, metals such as aluminum, titanium, platinum, and nickel can be used in the form of foil, mesh, lath, etc., but they are not particularly limited. Also, as a method of contacting the positive electrode with the current collector, the powder mixture containing the positive electrode active material is directly pressure-bonded to the current collector, the slurry containing the positive electrode active material is applied to the current collector, and the solvent is dried and then pressure bonded The manufacturing method is not particularly limited.

The non-aqueous electrolyte used in the secondary battery of the present invention is not particularly limited, and a conventional one can be used. For example, cyclic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, cyclic esters such as γ-butyrolactone, tetramethylsulfolane, N-methylpyrrolidone, dimethylformamide, dimethylacetamide, dimethylsulfoxide, hexamethylsulfoamide. , Pyridine and their derivatives, chain ethers such as dimethoxyethane, ethoxymethoxyethane and diethoxyethane, cyclic ethers such as tetrahydrofuran, dioxolane and dioxane, chain carbonates such as dimethyl carbonate and diethyl carbonate and their derivatives. Is used, but is not particularly limited.

The solvent of the non-aqueous electrolyte used in the present invention is
In addition to the above solvent, addition of up to 5% by volume as a trace component is a preferred embodiment. The additives used in this case can include various organic compounds or inorganic compounds.

The electrolyte contained in the non-aqueous electrolyte used in the present invention can be used without particular limitation, and examples thereof include LiClO ₄ , LiBF ₄ , LiPF ₆ , and LiPF ₆ .
_{CF 3 SO 3, LiAsF 6,} LiSCN, LiI, and the like LiAlO _4. In particular, in the case of an electrolyte containing fluorine, the effect of the carbon material whose surface is coated with the fluorine compound of the present invention is exhibited.

The secondary battery using the electrode of the present invention can be used as a video camera, a personal computer, a word processor, a radio-cassette, by utilizing the features of light weight, high capacity and high energy density.
It is widely applicable to portable small electronic devices such as mobile phones.

[0027]

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 (1) Surface Modification of Carbon Fiber Negative Electrode Commercially available PAN fiber (“Torayca” T-300, Toray Industries, Inc.)
Manufactured by the company) and then CF _{4 is used} as a reaction gas.
Was introduced at 100 ml / min, and plasma treatment was performed at an output of 300 W for 10 minutes to obtain a carbon fiber whose surface was fluorinated. When this carbon fiber surface is analyzed by ESCA,
A carbon atom bonded to a fluorine atom was confirmed at 289 eV, 291 eV, and 293 eV, and a fluorine atom was confirmed at 689 eV, and the number of fluorine atoms relative to the number of carbon atoms was 0.4.

(2) Evaluation 10 mg of the carbon fiber surface-modified with the fluorine compound prepared in the above (1) was sandwiched between stainless steel nets, the counter electrode and the reference electrode were metallic lithium, and the electrolyte solution was propylene carbonate / dimethoxy containing 1 M LiBF _4. Ethane (volume ratio 50: 5
0), 0V (vs. Li ⁺ / Li) up to 8 at 1mA
After constant-potential charging for 1.5 hours, 0.4mA at 1.5V (Li ⁺
/ Li) was discharged with a constant current. The charge capacity at this time is 5
The discharge capacity was 10 mAh / g, the discharge capacity was 340 mAh / g, and the initial capacity loss, which was the difference between the charge capacity and the discharge capacity, was 170 mAh / g.

Comparative Example 1 When the same evaluation was performed using the carbon fiber which was not subjected to the plasma treatment in Example 1, the initial capacity loss was 23.
It was very large at 0 mAh / g.

Example 2 (1) Surface Modification of Carbon Fiber Negative Electrode A commercially available carbon fiber similar to that used in Example 1 was subjected to a desizing treatment, then immersed in an aqueous hydrofluoric acid solution overnight, thoroughly washed and dried.

(2) Evaluation When 10 mg of the carbon fiber surface-modified with the fluorine compound prepared in (1) above was used and evaluated in the same manner as in Example 1, the initial capacity loss was 190 mAh / g. Example 3 (1) Preparation of positive electrode 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 in a ball mill, and heat-treated at 900 ° C. for 20 hours to obtain LiCoO ₂ . This is crushed with a ball mill, artificial graphite is used as the conductive material, and Teflon (PTF) is used as the binding material.
E) with LiCoO ₂ / artificial graphite / PTFE = 80/15 /
The mixture was mixed so as to be 5 and pressure-molded together with the nickel mesh of the collecting electrode to obtain 30 mg of a positive electrode. This positive electrode material had a diameter of 1.6 cm and a thickness of 50 μm.

(2) Surface Modification of Carbon Fiber Negative Electrode The same carbon fiber as in Example 1 was subjected to desizing treatment, CF ₄ was introduced as a reaction gas at 100 ml / min, and plasma treatment was performed at an output of 300 W for 10 minutes. The carbon fiber whose surface was fluorinated was obtained.

(3) Preparation of Secondary Battery 10 mg of the carbon fiber surface-modified with the fluorine compound prepared in (1) above was uniaxially arranged and placed on the nickel mesh of the current collector as the negative electrode. And A porous polypropylene film (Celgard # 2500, manufactured by Daicel Chemical Industries, Ltd.) was used as a separator,
The positive electrode prepared in (1) above was overlaid to prepare a coin-type secondary battery. The electrolytic solution was propylene carbonate / dimethoxyethane containing 1M LiBF ₄ (volume ratio 50
/ 50) was used.

(4) Evaluation This secondary battery was charged at a constant potential up to 4.1 V at 1 mA for 8 hours and then discharged at a constant current up to 2.5 V at 0.4 mA. At this time, the charge capacity was 5.1 mAh, the discharge capacity was 3.4 mAh, and the initial capacity loss was 1.7 mAh.

[0036]

According to the present invention, an electrode having excellent charge / discharge characteristics and a secondary battery using the same can be obtained by using carbon fiber whose surface is coated with a fluorine compound.

Claims (10)

[Claims] 1. An electrode comprising a carbon material having a fluorine atom on its surface as an active material. 2. The electrode according to claim 1, wherein the carbon material is carbon fiber. 3. The electrode according to claim 2, wherein the carbon fiber is a fired product of polyacrylonitrile. 4. An electrode comprising a carbon material treated with a plasma containing a fluorine-containing gas as an active material. 5. The electrode according to claim 1, wherein the concentration of the fluorine atom on the surface is higher than the concentration on the inside. 6. A secondary battery using the electrode according to claim 1. 7. The secondary battery according to claim 6, wherein a nonaqueous electrolytic solution is used. 8. The secondary battery according to claim 7, wherein the non-aqueous electrolytic solution uses a lithium salt as an electrolyte. 9. The secondary battery according to claim 6, wherein the electrolyte of the non-aqueous electrolytic solution is a lithium salt containing a fluorine atom. 10. The secondary battery according to claim 6, wherein a lithium composite oxide is used for the positive electrode.