JPH076752A - Electrode, method for manufacturing the same, and secondary battery using the electrode - Google Patents

Abstract

PURPOSE:To increase discharge capacity and cycle life by uniformly dispersing a binder used for an electrode in an electrode material, press molding, and heat treating. CONSTITUTION:To uniformly disperse a binder in an electrode material, an active material, a conductor, and the binder are blended to prepare the electrode material, and the electrode material is placed on a current collector to prepare an electrode and dried. The electrode is press molded to stick the electrode material to the current collector, then heat treated. As the active material, a lithium composite oxide selected from LixCoO2 (0<x<=1.0), LixNiO2 (0<x<=1.0), and LixCoyNi1-yO2 (0<x<=1.0, 0<y<=1.0) is used. The discharge capacity and especially cycle life are increased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode, a method for manufacturing the 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, there has been proposed a secondary battery in which various carbonaceous materials are used for the negative electrode and used by doping and dedoping with lithium ions. Further, such various carbonaceous materials can be used as a positive electrode after being doped with anions. Secondary batteries using electrodes made by doping lithium ions or anions into the above carbonaceous materials are disclosed in JP-A-57-208079 and JP-A-58.
-93176, JP-A-58-192266, JP-A-62-90863, and JP-A-62-12.
Japanese Laid-Open Patent Publication No. 2066 and Japanese Laid-Open Patent Publication No. 3-66856 are known.

Such carbonaceous material may be used in any form such as powder, carbon fiber or carbon fiber structure.

Further, recently, in order to meet the demand for higher energy density, a battery voltage of around 4V has appeared and has attracted attention. Higher battery voltage has been pursued by searching for and developing an active material exhibiting a high potential in the positive electrode, and inorganic compounds such as transition metal oxides and transition metal chalcogens containing alkali metals are known. Among them, Li _x C
oO ₂ (0 <x ≦ 1.0), Lix NiO ₂ (0 <x ≦
1.0) and Li _x Co _y Ni _1-y O ₂ (0 <x ≦
1.0, 0 <y ≦ 1.0) and the like are the most promising in terms of high potential, stability, and long life.

[0006]

However, even when these lithium composite oxides are used for the positive electrode active material,
By repeating the charge / discharge cycle, deterioration (reduction) of the discharge capacity is recognized, and this is a practical problem as a secondary battery in which charge / discharge is required to be repeated several hundred times.

[0007]

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

"(1) An electrode characterized in that a binder constituting a battery electrode is uniformly dispersed in an electrode material.

(2) (1) A step of preparing an electrode material by mixing an electrode active material, a conductive agent and a binder, and (2) a step of forming the electrode material on a current collector to prepare an electrode. , (3) a step of drying the electrode, (4) a step of press-molding the electrode and bringing an electrode material into close contact with a current collector, and (5) a step of heat-treating the electrode. 2. The method for producing the electrode according to 1 above.

(3) A secondary battery using the electrode described in (1) above. The electrode of the present invention is not particularly limited in what kind of battery such as a primary battery, a secondary battery and a fuel cell is used. Among them, it is preferably used as a positive electrode or a negative electrode of a secondary battery. As a particularly preferable secondary battery, a secondary battery using a non-aqueous electrolytic solution containing an alkali metal salt as described above can be mentioned. Therefore, a lithium secondary battery will be taken as an example and described in detail below with specific examples.

The inventors of the present invention have made extensive studies on the cause of the deterioration of the discharge capacity of the lithium secondary battery due to the charge / discharge cycle. As a result, the distribution of the binder in the electrode material has a great influence on the deterioration of the discharge capacity. I found a thing. That is, in the case of a lithium secondary battery, polyvinylidene fluoride (PVD
Fluorine-based resins such as F) and polytetrafluoroethylene are often used as a binder, and are usually dissolved in an organic solvent such as N-methylpyrrodoline to form a slurry for a current collector, such as a bar coater or It is applied to a uniform thickness with a doctor knife. However, due to factors such as insufficient mixing and separation during drying, the binder in the dried electrode agent is not always uniformly dispersed. The term "uniform" as used herein means morphological observation by SEM, and there is no portion where the binder is unevenly distributed at least larger than the particle size of the electrode active material or the conductive agent on the surface and cross section of the electrode, and / or 5 to It refers to a state in which there is no portion where about 10 or more electrode active materials or conductive agent particles are not in contact with the binder. There are problems that the discharge capacity is reduced due to overvoltage due to uneven distribution of the binder, and the discharge capacity is deteriorated with charge / discharge cycles due to insufficient binding force due to insufficient binder.

[0012] The inventors of the present invention, in order to solve such a problem,
As a result of further study, it was found that an electrode in which the binder is uniformly dispersed can be obtained by pressure-forming with a uniaxial press or roller press and then heat-treating. It is considered that this is because the binder is uniformly blended with the electrode active material, the conductive agent and the current collector by heat treatment after pressurization. The heat treatment temperature may be higher than the temperature at which the binder has fluidity, preferably the melting point of the binder or higher, and more preferably 20 to 50 ° C. higher than the melting point. If the heat treatment temperature is too low, it is difficult to obtain effective fluidity, and if it is too high, the resin used as the binder may deteriorate or deteriorate, or the binder may accumulate at the interface between the current collector and the electrode material. There is a risk that Further, the heat treatment time is also suitable for the same reason as the heat treatment temperature described above. For example, in the case of PVDF described above, if the melting point is about 180 ° C., 200 to 220 ° C. and 10 to 20 minutes are preferable. Regarding the heat treatment temperature and the heat treatment temperature, the higher the heat treatment temperature, the shorter the heat treatment time tends to be.

That is, the electrode manufacturing method described above is summarized as follows.

(1) A step of preparing an electrode material by mixing an electrode active material, a conductive agent and a binder. (2) A step of forming an electrode by forming the electrode material on a current collector.

(3) A step of drying the electrode.

(4) A step of pressure-molding the above electrode and bringing the electrode material into close contact with the current collector.

(5) A step of increasing the binding force by heat-treating the above-mentioned electrode to uniformly apply the binding agent to the electrode active material, the conductive agent and the current collector.

The positive electrode used in the present invention includes inorganic compounds such as transition metal oxides and transition metal chalcogens containing an alkali metal, polyacetylene, polyparaphenylene,
Examples of the positive electrode used in ordinary secondary batteries include conjugated polymers such as polyphenylene vinylene, polyaniline, polypyrrole, and polythiophene, crosslinked polymers having a disulfide bond, and thionyl chloride.
Among these, in the case of a secondary battery using a non-aqueous electrolyte containing a lithium salt, cobalt, manganese, molybdenum,
Transition metal oxides and transition metal chalcogens such as vanadium, chromium, iron, copper and titanium are preferably used. In particular, as described above, Li _x CoO ₂ (0 <x ≦ 1.0), L
i _x NiO ₂ (0 <x ≦ 1.0) and Li _x Co _y N
i _{1 -y} O ₂ (0 <x ≦ 1.0, 0 <y ≦ 1.0) and the like are the most promising from the viewpoint of high potential, stability, and long life.

The carbonaceous material used in the present invention includes:
The material is not particularly limited, and a material obtained by firing an organic material is generally used. When the electron conductivity of the carbonaceous material is sufficiently high for the purpose of collecting electricity, it is not necessary to add a conductive agent. The same applies to the case of carbon fiber, specifically, 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, Examples thereof include vapor grown carbon fibers obtained from a gas of a low molecular weight organic substance. In addition, carbon fibers obtained by firing polyvinyl alcohol, lignin, polyvinyl chloride, polyamide, polyimide, phenol resin, furfuryl alcohol, etc. But it doesn't matter. Among these carbon fibers, depending on the characteristics of the electrode and battery in which the carbon fibers are used, it is necessary to appropriately select the carbon fibers that satisfy the characteristics.

Among the above carbon fibers, when used as a negative electrode of a secondary battery using a non-aqueous electrolyte containing an alkali metal salt, PAN-based carbon fibers, pitch-based carbon fibers and vapor-grown carbon fibers are preferable. In particular, in terms of good doping with alkali metal ions, especially lithium ions, P
AN-based carbon fibers and pitch-based carbon fibers are preferable, and among them, PAN-based carbon fibers such as "Torayca" T series or "Torayca" M series manufactured by Toray Industries, Inc. and mesophase pitch coke are obtained by firing. Pitch-based carbon fibers are more preferably used.

When the carbon fiber is used as an electrode, it may have any form, but a preferred form is to arrange it in a uniaxial direction, or to form a fabric-like or felt-like structure. Examples of the fabric-like or felt-like structure include woven fabrics, knitted fabrics, braids, laces, nets, felts, papers, non-woven fabrics, mats, and the like. Felt and the like are preferred.

The electrolytic solution of the secondary battery using the electrode of the present invention is not particularly limited, and a conventional electrolytic solution may be used, and examples thereof include an acid or alkaline aqueous solution or a non-aqueous solvent. Among these, 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, diethylene carbonate, derivatives and mixtures of these are preferably used. The electrolyte contained in the electrolytic solution is an alkali metal,
Particularly, lithium halides, perchlorates, thiocyanates, borofluorides, phosphorous fluorides, arsenic fluorides, aluminum fluorides, trifluoromethylsulfates and the like are preferably used.

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.

[0024]

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 Commercially available lithium carbonate (Li ₂ CO ₃ ) and basic cobalt carbonate (2CoCO ₂ .3Co (OH) ₂ were used in a molar ratio of Li.
/ Co = 1/1, weighed with a zirconia ball mill (we use ethanol as a grinding solvent), and then 90
LiCoO ₂ was synthesized by heat treatment at 0 ° C. for 20 hours. This was crushed with the above ball mill to obtain LiCoO ₂ powder. Artificial graphite was added to this powder as a conductive agent, and 10 parts by weight and 3 parts by weight of polyvinylidene fluoride (hereinafter abbreviated as PVDF) as a binder were added, and the viscosity was adjusted with a solvent to form a paste. This is applied to an aluminum foil having a thickness of 20 μm whose surface has been rubbed with # 1000 emery paper beforehand, dried, and roller-pressed, and further 200
Heat treatment for 15 minutes at ℃, width of electrode part 10mm, length 2
A 0 mm LiCoO ₂ electrode was prepared. Morphological observation by an SEM was performed on the electrode prepared in the same manner. As a result, it was found that the binder was LiC on the surface and cross section of the electrode.
There was no part that was unevenly distributed larger than the particle size of oO ₂ or artificial graphite, and there was no part where 8 or more LiCoO ₂ or artificial graphite particles were not in contact with the binder.

Next, the discharge capacity of the electrode was evaluated. The electrolyte is propylene carbonate containing 1M LiPF ₆ ,
Evaluation was carried out using a three-electrode cell in which metallic lithium foil was used for the counter electrode and the reference electrode. The current density per LiCoO ₂ is 50
It was charged to 4.3 V (vs. Li ⁺ / Li) at a constant current of mA / g. The charge amount at this time was 156 mAh / g. After charging, the discharge capacity of the LiCoO ₂ electrode was 150 mAh / g, which was calculated from the amount of charge discharged to 3.0 V (vs. Li ⁺ / Li) at the same current density as charging. Further, when the charge and discharge cycle was repeated and the 50th discharge capacity and the 1st discharge capacity were compared, the discharge capacity retention rate represented by the following equation was 92%.

Discharge capacity retention rate (%) = (50th discharge capacity / first discharge capacity) × 100 Comparative Example 1 Same as Example 1 except that heat treatment of the electrode was not performed.
An electrode was prepared in the same manner as in Example 1 using iCoO ₂ ,
Charge / discharge evaluation was performed. The charging amount at this time is 154 mAh /
g, the discharge capacity was 148 mAh / g, but the discharge capacity retention rate was 82%. Further, when the morphology of the electrode was observed by SEM in the same manner as in Example 1, there was a portion on the surface of the electrode in which the binder was unevenly distributed larger than the particle diameters of LiCoO ₂ and artificial graphite, and There was a portion where at least 15 or more of LiCoO ₂ and artificial graphite particles were not in contact with the binder.

Example 2 A LiNiO ₂ electrode was prepared in the same manner as in Example 1 except that basic nickel carbonate (NiCO ₃ .2Ni (OH) ₂ .4H ₂ O) was used instead of basic cobalt carbonate. Then, charge and discharge evaluation was performed in the same manner as in Example 1 except that the charge potential was 4.2 V (vs. Li ⁺ / Li). At this time, the charge amount was 147 mAh / g, the discharge capacity was 144 mAh / g,
The discharge capacity retention rate was 91%. Further, when the morphology of the electrode was observed by the SEM in the same manner as in Example 1, it was found that the binder was LiN on the surface and cross section of the electrode.
There was no portion that was unevenly distributed larger than the particle size of iO ₂ or artificial graphite, and there was no portion where 10 or more LiNiO ₂ or artificial graphite particles were not in contact with the binder.

Comparative Example 2 The same LiNi as in Example 2 except that no heat treatment was performed.
An electrode was prepared using O ₂ in the same manner as in Example 2, and charge / discharge evaluation was performed. At this time, the charge amount was 145 mAh / g and the discharge capacity was 142 mAh / g, but the discharge capacity retention rate was 78%. Also, in the same manner as in Comparative Example 1,
When the morphology of the electrode was observed by SEM, the surface of the electrode had a portion where the binder was unevenly distributed larger than the particle diameters of LiNiO ₂ and artificial graphite, and at least 20 or more of LiNiO ₂ and artificial graphite were found. There were parts where the particles did not come into contact with the binder.

Example 3 The electrode active material raw materials used in Examples 1 and 2 were weighed and mixed to obtain Li _1.0 (Co _0.5 Ni _0.5 ) O _{2 in} terms of oxide. An electrode was prepared in the same manner as in 2, and the same heat treatment was performed. Charge / discharge evaluation was performed in the same manner as in Example 1. The charging amount at this time is 156 mAh / g
The discharge capacity was 149 mAh / g, and the discharge capacity retention rate was 92%. Further, as in the first embodiment, S
When the morphology of the electrode was observed by EM, it was found that the binder was Li _1.0 (Co _0.5 N
i _0.5 ) O ₂ and there is no portion unevenly distributed larger than the particle size of artificial graphite, and eight or more Li _1.0 (Co _0.5 Ni)
_0.5 ) There was no part where O ₂ or artificial graphite did not come into contact with the binder.

Comparative Example 3 Li _1.0 as in Example 3 except that no heat treatment was performed.
An electrode was prepared in the same manner as in Example 3 using (Co _0.5 Ni _0.5 ) O ₂ , and the charge and discharge was evaluated. At this time, the charge amount was 148 mAh / g and the discharge capacity was 144 mAh / g, but the discharge capacity retention rate was 79%. Also,
When the morphology of the electrode was observed by SEM in the same manner as in Comparative Example 1, it was found that the binder was Li
_1.0 (Co _0.5 Ni _0.5 ) O ₂ or artificial graphite has a portion unevenly distributed larger than the particle diameter, and at least 15 or more Li _1.0 (Co _0.5 Ni _0.5 ) O ₂ or artificial graphite particles are used as the binder. There was a part that was not in contact.

Example 4 90 parts by weight and 10 parts by weight of commercially available pitch coke and PVDF as a binder were added, and the viscosity was adjusted with a solvent to form a paste. This is applied to a copper foil having a thickness of 20 μm whose surface has been rubbed with # 1000 emery paper in advance, dried, and roller-pressed, and further 200 ° C.
Heat treatment for 15 minutes at a width of 10 mm and a length of 20
A mm electrode was prepared.

Next, the discharge capacity of the electrode thus manufactured was evaluated. The electrolytic solution was evaluated by a three-electrode type cell using propylene carbonate containing 1M LiPF ₆ and metallic lithium foil for the counter electrode and the reference electrode. The current density per pitch coke is 0.0 mA at a constant current of 40 mA / g.
It was charged to V (vs. Li ⁺ / Li). The charge amount at this time is 300
It was mAh / g. After charging, the discharge capacity of the electrode was 220 mAh / g, which was calculated from the amount of charge discharged to 1.5 V (vs. Li ⁺ / Li) at the same current density as charging, and the discharge capacity retention rate was 88%. Also, in the same manner as in Example 1,
When the electrode morphology was observed by SEM, there was no portion where the binder was unevenly distributed over the pitch coke particle size on the surface and cross section of the electrode, and 5 or more pitch coke particles were formed as the binder. There was no part that was not in contact with.

Comparative Example 4 A pitch coke electrode was produced in the same manner as in Example 4 except that heat treatment was not performed. The charge and discharge of the electrode was evaluated in the same manner as in Example 4. After charging, the discharge capacity when discharged at a constant current of 40 mA / g was 210 mAh / g per weight of pitch coke, but the discharge capacity retention rate was
It was 72%. Further, in the same manner as in Comparative Example 1, SEM
When the morphological observation of the electrode was carried out, there was no portion on the surface and cross section of the electrode where 10 or more pitch coke particles were not in contact with the binder, but on the surface of the electrode, the binder was pitch coke. There was a portion unevenly distributed that was larger than the particle diameter of.

Example 5 The positive electrode (positive electrode active material 30 mg) prepared in Example 1 was
A commercially available PAN-based carbon fiber ("Torayca" T-300, manufactured by Toray Industries, Inc.) 1 strand (3K: 3000 pieces) 7 mg was used as a negative electrode, and a porous polypropylene film (Celgard # 2500, manufactured by Daicel Chemical Co., Ltd.) was prepared. A secondary battery was produced by stacking the secondary batteries via a separator. 1 electrolyte
Propylene carbonate containing M lithium perchlorate was used.

Using the secondary battery thus manufactured, the battery was charged to 4.3 V at a constant current of 40 mA / g of current density per weight of carbon fiber. After charging, the discharge capacity when discharged at a constant current of 40 mA / g was 320 mAh / g based on the weight of the carbon fiber of the negative electrode, and the discharge capacity retention rate was 89%. Further, as in the first embodiment, S
When the morphology of the positive electrode was observed by EM, there was no portion where the binder was unevenly distributed over the particle size of LiCoO ₂ or artificial graphite on the surface and cross section of the positive electrode, and
There was no portion where at least one LiCoO ₂ or artificial graphite particle was not in contact with the binder.

Comparative Example 5 A secondary battery was prepared in the same manner as in Example 5, except that the electrode prepared in Comparative Example 1 was used as the positive electrode. The charge / discharge evaluation of the secondary battery was performed in the same manner as in Example 4. 40 mA / g after charging
The discharge capacity when discharged at a constant current of was 310 mAh / g based on the weight of the carbon fiber of the negative electrode, but the discharge capacity retention rate was 76%. Further, when the morphology of the electrode was observed by SEM in the same manner as in Comparative Example 1, there was a portion on the surface of the electrode in which the binder was unevenly distributed larger than the particle size of LiCoO ₂ or artificial graphite, and There was a portion where at least 15 or more of LiCoO ₂ and artificial graphite particles were not in contact with the binder.

[0038]

According to the present invention, it becomes possible to manufacture a high performance secondary battery having excellent discharge capacity characteristics, particularly cycle characteristics.

Claims (14)

[Claims] 1. An electrode characterized in that a binder constituting a battery electrode is uniformly dispersed in an electrode material. 2. The electrode according to claim 1, wherein the electrode active material is a lithium composite oxide. 3. The lithium composite oxide is Li _x CoO ₂
(0 <x ≦ 1.0), Li _x NiO ₂ (0 <x ≦ 1.0) and Li _x Co _y
3. The electrode according to claim 2, wherein the electrode is selected from Ni _1-y O ₂ (0 <x ≦ 1.0, 0 <y ≦ 1.0). 4. The electrode according to claim 1, wherein the electrode active material is a carbonaceous material. 5. A step of preparing an electrode material by mixing (1) an electrode active material, a conductive agent and a binder, and (2) a step of forming the electrode material on a current collector to prepare an electrode. (3) a step of drying the electrode, (4) a step of press-molding the electrode to bring the electrode material into close contact with a current collector, and (5) a step of heat-treating the electrode. The method for manufacturing the electrode according to claim 1. 6. The method for producing an electrode according to claim 5, wherein the temperature for heat-treating the electrode is higher than the melting point of the binder. 7. The method for producing an electrode according to claim 5, wherein the electrode active material is a lithium composite oxide. 8. The lithium composite oxide is Li _x CoO ₂
(0 <x ≦ 1.0), Li _x NiO ₂ (0 <x ≦ 1.0) and Li _x Co _y
The method for producing an electrode according to claim 5, wherein the electrode is selected from Ni _1-y O ₂ (0 <x ≦ 1.0, 0 <y ≦ 1.0). 9. The method for producing an electrode according to claim 5, wherein the electrode active material is a carbonaceous material. 10. A secondary battery comprising the electrode according to claim 1. 11. A secondary battery using the electrode according to claim 2. 12. A secondary battery using the electrode according to claim 3. 13. The secondary battery according to claim 10, wherein a carbonaceous material is used as the negative electrode active material. 14. The secondary battery according to claim 13, wherein the carbonaceous material is carbon fiber.