JPH11354125A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-capacity lithium battery of which battery capacity is hardly reduced when charge and discharge are repeated. SOLUTION: In this lithium secondary battery wherein a sheet-like positive electrode 1 faces to a sheet-like negative electrode 2 interposing a separator 3, at least one sheet-like electrode out of the sheet-like positive electrode 1 and the sheet-like negative electrode 2 is composed by forming a film containing an active material and a binder on at least one surface of a conductive base, and an acrylic acid at least not forming a metal salt with a metal such as sodium is used for the binder. The binder is preferably formed from a polyacrylic acid not forming a metal acid such as a sodium acid and at least one kind selected from a group comprised of polyethylene glycol, polyethylene oxide, polypropylene glycol and polypropylene oxide.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery, and more particularly, to a high-capacity lithium secondary battery that has a small decrease in battery capacity when charging and discharging are repeated.

[0002]

2. Description of the Related Art In general, a sheet prepared by adding a binder or a solvent to a positive electrode active material, dispersing and stirring, coating a conductive substrate, and drying to form a coating film containing the positive electrode active material and the like. -Like positive electrode, and a sheet in which a coating prepared by adding a binder and a solvent to the negative electrode active material, dispersing and stirring the same, coated on a conductive substrate, and dried to form a coating containing the negative electrode active material and the like A lithium secondary battery manufactured by encapsulating an electrode body with a laminated structure in which a negative electrode in the form of a separator is interposed via a separator together with an organic solvent-based electrolytic solution in a battery case has an energy density per unit capacity and unit. It has the feature of high energy density per weight.

[0003] As a binder used for a sheet-like electrode such as the above-mentioned sheet-like positive electrode and sheet-like negative electrode, an electrolyte solution is used so that the electrode coating structure is not broken during operation of the battery. Polyvinylidene fluoride-based polymers containing vinylidene fluoride as the main component monomer are preferred because they are required to have both the property of being difficult to dissolve and the solvent solubility required for preparing the paint for forming an electrode coating film. Has been used as

However, the above-mentioned polyvinylidene fluoride-based polymer, which is a fluororesin, has a weak adhesive force to a metal foil generally used as a conductive substrate,
The electrode coating film formed using the polyvinylidene fluoride-based polymer as a binder has a low adhesive strength to a conductive substrate, and therefore, the battery manufactured using the electrode having the coating film has repeated charge and discharge. In this case, there has been a problem that electrical contact between the electrode coating film and the conductive substrate deteriorates, whereby the utilization rate of the electrode active material decreases and the battery capacity decreases.

[0005]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems of the conventional lithium secondary battery, suppresses a decrease in battery capacity when charging and discharging are repeated, and provides a high capacity lithium secondary battery. It is intended to provide a secondary battery.

[0006]

Means for Solving the Problems The present invention has been made as a result of conducting various studies to achieve the above object, and at least one sheet-like electrode has at least one surface on a conductive substrate. In a lithium secondary battery in which a sheet-shaped positive electrode and a sheet-shaped negative electrode made of a coating film containing an active material and a binder are formed with a separator interposed therebetween, at least as the binder, at least a metal such as sodium is used. By using polyacrylic acid that does not form a metal salt (hereinafter, this “polyacrylic acid that does not form a metal salt” is represented by “non-metal salt polyacrylic acid”), An object of the present invention is to provide a high-capacity lithium secondary battery with a small decrease in battery capacity.

That is, in the present invention, the binder of the electrode coating film is composed of a non-metal salt polyacrylic acid, and the non-metal salt polyacrylic acid has a chemical formula (-COOH) in a repeating unit constituting the polymer. Since the binder has a carboxyl group represented by the formula, the binder has a larger adhesive force with the conductive substrate than the electrode coating film composed of the polyvinylidene fluoride-based polymer alone, and therefore, the electrode in the case where charge and discharge are repeated. Deterioration of electric contact between the coating film and the conductive substrate is suppressed, and a decrease in battery capacity due to the deterioration of the electric contact can be suppressed.

[0008]

Further, when the binder is composed of the above-mentioned non-metal salt polyacrylic acid and at least one selected from the group consisting of polyethylene glycol, polyethylene oxide, polypropylene glycol and polypropylene oxide, the above-mentioned polyethylene glycol, At least one selected from the group consisting of ethylene oxide, polypropylene glycol and polypropylene oxide acts as a plasticizer on the non-metal salt polyacrylic acid, and improves the flexibility of the electrode coating film. Even when wound with a small radius of curvature (winding radius), cracks in the electrode coating film and peeling of the electrode coating film from the conductive substrate are suppressed.

As described above, when the binder is composed of the non-metal salt polyacrylic acid and at least one selected from the group consisting of polyethylene glycol, polyethylene oxide, polypropylene glycol and polypropylene oxide, the mixing ratio of both is as follows: 10% by weight or more, especially 20 to 98% by weight of non-metal salt polyacrylic acid
In, polyethylene glycol, polyethylene oxide,
90% by weight of at least one selected from the group consisting of polypropylene glycol and polypropylene oxide
Hereinafter, it is particularly preferable that the content be 2 to 80% by weight. When the ratio of the non-metal salt polyacrylic acid is less than 10% by weight, the adhesion of the electrode coating to the conductive substrate is reduced, and the charge and discharge between the electrode coating and the conductive substrate is reduced when charging and discharging are repeated. The electrical contact of the battery may be deteriorated and the battery capacity may be reduced.

If the molecular weight of the polyethylene glycol is small, the polyethylene glycol may be melted or softened during use of the battery in a high-temperature environment, thereby deteriorating the electrode function. And a solvent such as an organic solvent may be difficult to dissolve, and the coating composition may be difficult to prepare.
00000 is preferred.

In the present invention, the content of the binder in the electrode coating film is 0.2 to 20% by weight, particularly 0.1 to 20% by weight.
It is preferably from 5 to 10% by weight. When the content of the binder is less than the above range, the mechanical strength of the electrode coating film is insufficient, the electrode coating film may be peeled from the conductive substrate, and when the content of the binder is more than the above range, In addition, there is a possibility that the active material in the electrode coating film decreases and the battery capacity decreases.

The binder containing at least the non-metal salt polyacrylic acid in the present invention may be used for any of the positive electrode and the negative electrode, and may be used for both the positive electrode and the negative electrode.

In the present invention, examples of the positive electrode active material include:
For example, lithium nickel oxide, lithium cobalt oxide
Material, lithium manganese oxide (these are usually LiN
iO _Two, LiCoO_Two: LiMn_TwoO_FourIs represented by
Li: Ni ratio, Li: Co ratio, Li: Mn
Ratio often deviates from the stoichiometric composition).
Titanium-containing composite metal oxides may be used alone or in combination of two or more.
Compound, or their solid solution
You.

In preparing the positive electrode, an electron conduction aid such as flaky graphite and carbon black can be added to the positive electrode active material, if necessary.

The positive electrode contains, for example, the above-mentioned positive electrode active material,
If necessary, a paint containing an electron conduction aid such as flaky graphite and carbon black, and further containing a binder is applied on the conductive substrate, and dried, so that at least one surface of the conductive substrate has at least a positive electrode active material. It is produced through a step of forming a coating film containing a substance and a binder. In preparing the paint, the binder may be dissolved in a solvent in advance to form a solution, and the binder solution may be mixed with solid particles such as the positive electrode active material to prepare the paint.

In the present invention, as the negative electrode active material, for example, lithium metal or a lithium-containing compound is used. As the lithium-containing compound, there are a lithium alloy and others. Examples of the lithium alloy include lithium-aluminum, lithium-
Examples include alloys of lithium and other metals such as lead, lithium-bismuth, lithium-indium, lithium-gallium, and lithium-indium-gallium. Examples of the lithium-containing compound other than the lithium alloy include a carbon material having a turbostratic structure, graphite, and the like. Some of these do not contain lithium at the time of manufacture, but when they act as a negative electrode active material, they become lithium-containing by chemical means, electrochemical means, and the like.

For the negative electrode, for example, if necessary, an electron conduction aid such as flaky graphite or carbon black is added to the negative electrode active material, a binder and a solvent are added and mixed to prepare a coating material. It is produced through a step of applying a coating material on a conductive substrate and drying it to form a coating film containing at least a negative electrode active material and a binder on at least one surface of the conductive substrate. In preparing the paint, the binder may be dissolved in a solvent in advance to form a solution, and the binder solution may be mixed with a negative electrode active material or the like to prepare the paint.

In the present invention, as a solvent for a paint used for forming an electrode coating film, a non-metal salt polyacrylic acid is dissolved, or a non-metal salt polyacrylic acid and polyethylene glycol, polyethylene oxide, polypropylene glycol are used. And a solvent that dissolves at least one selected from the group consisting of polypropylene oxide. Examples of such a solvent include water, N-methylpyrrolidone, cimethylacetamide, and tetrahydrofuran. And
They can be used alone or in combination of two or more.

In the present invention, as a method of applying the above-mentioned paint to the conductive substrate, various application methods such as an extrusion coater, a reverse roller, a doctor blade, an applicator and the like can be adopted.

In the present invention, as a conductive substrate of an electrode such as a positive electrode and a negative electrode, for example, a metal conductive material such as aluminum, stainless steel, titanium, and copper is processed into a mesh, a punched metal, a foam metal, or a plate. A foil or the like is used.

As the electrolytic solution, for example, 1,2-dimethoate
Xiethane, 1,2-diethoxyethane, propylene
-Carbonate, ethylene carbonate, γ-butyrolact
, Tetrahydrofuran, 1,3-dioxolan, die
Chill carbonate, dimethyl carbonate, ethyl methyl
Single or mixed solvent of two or more such as carbonate
For example, LiCF_ThreeSO_Three, LiC_FourF₉SO_Three,
LiClO_Four, LiPF ₆, LiBF_FourSuch as electrolyte
Organic solvent system prepared alone or by dissolving two or more
An electrolyte is used.

As the separator, for example, a thickness of 10 to 10
A microporous polyethylene film or a microporous polypropylene film having a pore size of 50 μm and a porosity of 30 to 70% is preferably used.

The battery is, for example, spirally wound with a separator interposed between the sheet-shaped positive electrode and the sheet-shaped negative electrode produced as described above, and is laminated in a spiral wound structure. Alternatively, an electrode body having a laminated structure obtained by folding and laminating is inserted into a battery case made of nickel-plated iron or stainless steel, injected with an electrolytic solution, and sealed. Further, the battery usually incorporates an explosion-proof mechanism for discharging gas generated inside the battery to the outside of the battery when the pressure has increased to a certain pressure, thereby preventing the battery from bursting under high pressure.

[0024]

Next, an embodiment of the present invention will be described.
However, the present invention is not limited to only these examples.

Example 1 (1) Preparation of Positive Electrode First, lithium cobalt oxide, flaky graphite as an electron conduction aid, nonmetal salt polyacrylic acid constituting a binder and polyethylene glycol (average molecular weight 5000
00) and a coating for forming a positive electrode coating film containing water as a solvent in the following composition.

Composition of paint for forming positive electrode coating film Lithium cobalt oxide 91 parts by weight Flake graphite 6 parts by weight Nonmetal salt polyacrylic acid 2.4 parts by weight Polyethylene glycol 0.6 parts by weight Water 104 parts by weight

The preparation of the paint was carried out as follows.
First, a non-metal salt polyacrylic acid and polyethylene glycol were dissolved in water to prepare a binder solution. Next,
To this binder solution, lithium cobalt oxide as a positive electrode active material and flaky graphite as an electron conduction aid were added and mixed to prepare a paint.

Then, the obtained paint is applied on an aluminum foil having a thickness of 20 μm as a conductive substrate using an applicator, dried at 100 to 120 ° C., and coated with a positive electrode active material, a binder and the like. Was formed. Similarly, apply the above paint on the back side of the aluminum foil,
For 8 hours to form a coating film. Then, the obtained electrode body was roll-pressed, and the coating thickness on one side was 75 μm.
m of the sheet-shaped positive electrode was produced.

(2) Preparation of Negative Electrode As a negative electrode active material, artificial graphite (synthesized at 2800 ° C.) was used, and as a binder, the same non-metal salt polyacrylic acid and polyethylene glycol as those used in the positive electrode coating film forming paint were used. A negative electrode coating film-forming paint containing them and water in the following proportions was prepared.

Composition of paint for forming negative electrode coating film Artificial graphite 95 parts by weight Nonmetal salt polyacrylic acid 4 parts by weight Polyethylene glycol 1 part by weight Water 165 parts by weight

The obtained paint is used as a conductive substrate in a thickness of 1
0 μm copper foil is applied using an applicator,
It dried at -120 degreeC, and formed the coating film containing a negative electrode active material and a binder. Similarly, the paint was applied to the back side of the copper foil and vacuum dried at 100 ° C. for 8 hours to form a coating film. Then, the obtained electrode body was roll-pressed to produce a sheet-shaped negative electrode having a coating film thickness of 73 μm on one side.
The weight ratio of the active materials of the positive electrode and the negative electrode was 2.1: 1.
The coating density was adjusted so that

(3) Preparation of Electrolyte Solution 1 mol / l of LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate and ethyl methyl carbonate at a volume ratio of 1: 1 to prepare an organic solvent-based electrolyte solution.

(4) Assembly of cylindrical battery The above-mentioned sheet-shaped positive electrode was cut into a strip having a width of 39 mm × length of 315 mm, and the sheet-shaped negative electrode was cut into a width of 41 mm × length of 34.
It was cut into 5 mm strips. Then, an aluminum lead body was ultrasonically welded to the sheet-like positive electrode on the part where the metal foil was exposed by peeling off a part of the coating film at one end of each electrode, and the sheet-like negative electrode was formed. A nickel-made lead body is resistance-welded, a strip separator made of a microporous polyethylene film having a thickness of 25 μm and a porosity of 50% is interposed between the sheet-shaped positive electrode and the sheet-shaped negative electrode, The electrode body having a spirally wound structure was manufactured by spirally winding so as to have a radius of 1 mm, and the spirally wound electrode body was inserted into a battery case made of stainless steel.

Then, the tip of the negative electrode-side lead body is welded to the bottom of the battery case by penetrating the insulator, and the insulator is inserted into the opening of the battery case to form a groove. The lead body on the positive electrode side was welded. Then, the can body into which the electrodes and the like manufactured through these steps are filled is 6
After vacuum drying at 0 ° C. for 10 hours, 2 ml of an electrolytic solution was injected in a dry atmosphere, sealed, and a cylindrical AA lithium secondary battery (outer diameter: 14 mm, height: 50 mm) shown in FIG. 1 was obtained. Produced.

Referring to the battery shown in FIG. 1, reference numeral 1 denotes the positive electrode and 2 denotes the negative electrode. However, FIG. 1 does not show a metal foil or the like as a conductive substrate used in manufacturing the positive electrode 1 or the negative electrode 2 in order to avoid complication. The positive electrode 1 and the negative electrode 2 are spirally wound via a separator 3 and housed in a battery case 5 together with an electrolyte 4 as an electrode body having a spirally wound structure.

The battery case 5 is made of stainless steel and also serves as a negative electrode terminal. A polypropylene insulator 6 is arranged at the bottom of the battery case 5 before inserting the spirally wound electrode body. Have been. The sealing plate 7 is made of aluminum and has a disk shape. A thin portion 7a is provided in the center of the sealing plate 7, and the internal pressure of the battery is controlled around the thin portion 7a by an explosion-proof valve 9.
A hole is provided as a pressure inlet 7b for acting on the pressure. The projection 9a of the explosion-proof valve 9 is welded to the upper surface of the thin portion 7a to form a welded portion 11.
Note that the thin portion 7a provided on the sealing plate 7 and the protruding portion 9a of the explosion-proof valve 9 are provided so as to be easily understood in the drawings.
Only the cut surface is shown, and the contour line behind the cut surface is not shown. Also, the welded portion 11 between the thin portion 7a of the sealing plate 7 and the protruding portion 9a of the explosion-proof valve 9 is shown in an exaggerated state in order to facilitate understanding on the drawing.

The terminal plate 8 is made of rolled steel, has a nickel-plated surface, and has a hat-like shape with a peripheral edge formed in a flange shape. The terminal plate 8 is provided with a gas discharge hole 8a. . The explosion-proof valve 9 is made of aluminum and has a disk shape.
At the center thereof, a projecting portion 9a having a tip portion is provided on the power generation element side (the lower side in FIG. 1), and the lower surface of the projecting portion 9a is, as described above, the upper surface of the thin portion 7a of the sealing plate 7. To form a welded portion 11. The insulating packing 10 is made of polypropylene and has an annular shape.
The explosion-proof valve 9 is arranged on the upper part of the peripheral part of the, and insulates the sealing plate 7 from the explosion-proof valve 9,
The gap between the two is sealed so that the electrolyte does not leak from between the two. The annular gasket 12 is made of polypropylene, and the lead body 13 is made of aluminum.
And the positive electrode 1 are connected. An insulator 14 is disposed on the top of the spirally wound electrode body, and the negative electrode 2 and the bottom of the battery case 5 are connected by a nickel lead 15.

As described above, the insulator 6 is disposed at the bottom of the battery case 5, and has a spirally wound electrode body including the positive electrode 1, the negative electrode 2 and the separator 3, the electrolytic solution 4, and the spirally wound electrode body. The insulator 14 and the like at the top of the electrode structure having a round structure are accommodated in the battery case 5, and after these are accommodated, an annular groove having a bottom portion projecting inward is formed in the vicinity of the open end of the battery case 5. . Then, the sealing plate 7, the insulating packing 10, and the annular gasket 12 into which the explosion-proof valve 9 is inserted are inserted into the opening of the battery case 5, and the terminal plate 8 is further inserted therefrom. Is tightened inward to seal the opening of the battery case 5. However, in assembling the battery as described above, as described above, the negative electrode 2 and the battery case 5 are connected in advance with the lead 15, and the positive electrode 1 and the sealing plate 7 are connected with the lead 13.
It is preferable to connect them.

In the battery assembled as described above, the thin portion 7a of the sealing plate 7 and the projection 9a of the explosion-proof valve 9 are provided.
Contact at the welded portion 11, the peripheral portion of the explosion-proof valve 9 and the peripheral portion of the terminal plate 8 come into contact, and the positive electrode 1 and the sealing plate 7 are connected by the lead 13 on the positive electrode side. And terminal plate 8
The electrical connection is obtained by the lead body 13, the sealing plate 7, the explosion-proof valve 9 and the welded portion 11 thereof, and the lead body normally functions as an electric circuit.

When an abnormal situation occurs in the battery and gas is generated inside the battery and the internal pressure of the battery rises, the internal pressure rises and the central part of the explosion-proof valve 9 moves in the direction of the internal pressure (FIG. 1).
Then, it is deformed in the upper direction)
The shearing force acts on the thin portion 7a integrated at 1 and the thin portion 7a is broken or the projection 9a of the explosion-proof valve 9 is formed.
And the thin portion 7a of the sealing plate 7 is peeled off, whereby the electrical connection between the positive electrode 1 and the terminal plate 8 is lost and the current is cut off. As a result, the battery reaction does not proceed, so that even during overcharge or short circuit, the battery temperature rise and internal pressure rise due to the charging current and short circuit current do not progress further, so that ignition and rupture of the battery can be prevented. Designed.

The explosion-proof valve 9 is provided with a thin portion 9b. For example, when charging proceeds extremely and power generation elements such as an electrolyte and an active material are decomposed and a large amount of gas is generated. After the explosion-proof valve 9 is deformed and the welding portion 11 between the projection 9a of the explosion-proof valve 9 and the thin portion 7a of the sealing plate 7 is peeled off,
The thin portion 9b provided on the explosion-proof valve 9 is designed to be opened so that gas is discharged from the gas discharge holes 8a of the terminal plate 8 to the outside of the battery to prevent the battery from being ruptured.

Embodiment 2 In the step (4) of assembling the cylindrical battery of the embodiment 1, instead of forming the spirally wound electrode body by spirally winding so that the minimum winding radius is 1 mm. Then, an AA lithium secondary battery was manufactured in the same manner as in Example 1, except that an electrode body having a spirally wound structure was manufactured by spirally winding so that the minimum winding radius was 2 mm.

Example 3 In the composition of the coating material for forming a positive electrode coating film of Example 2, 2.4 parts by weight of the nonmetal salt polyacrylic acid was changed to 3 parts by weight, and 0.6 part by weight of polyethylene glycol was changed to 0 part by weight. Change to water 1
04 parts by weight was changed to 117 parts by weight, 4 parts by weight of the nonmetal salt polyacrylic acid in the composition of the coating for forming a negative electrode coating film was changed to 5 parts by weight, and 1 part by weight of polyethylene glycol was changed to 0 parts by weight. , Changing 165 parts by weight of water to 195 parts by weight,
Example 1 was repeated except that an electrode body having a spirally wound structure was manufactured by spirally winding so that the minimum winding radius was 4 mm.
In the same manner as in the above, an AA lithium secondary battery was produced.

COMPARATIVE EXAMPLE 1 In the composition of the coating material for forming a positive electrode coating film of Example 1, 91 parts by weight of lithium cobalt oxide was changed to 90 parts by weight, and 2.4 parts by weight of a nonmetal salt polyacrylic acid and polyethylene glycol 0 Using 4 parts by weight of polyvinylidene fluoride instead of 0.6 parts by weight, 40 parts by weight of N-methylpyrrolidone instead of 104 parts by weight of water, and 95 parts by weight of artificial graphite in the composition of the negative electrode coating film forming paint 90 parts by weight, 4 parts by weight of non-metal salt polyacrylic acid and 10 parts by weight of polyvinylidene fluoride instead of 1 part by weight of polyethylene glycol, and N-methylpyrrolidone 80 instead of 165 parts by weight of water.
Except that the parts by weight were used, an AA lithium secondary battery was produced in the same manner as in Example 1.

Comparative Example 2 In the composition of the coating material for forming a positive electrode coating film of Example 2, 91 parts by weight of lithium cobalt oxide was changed to 90 parts by weight, and 2.4 parts by weight of a nonmetal salt polyacrylic acid and polyethylene glycol 0 were added. Using 4 parts by weight of polyvinylidene fluoride instead of 0.6 parts by weight, 40 parts by weight of N-methylpyrrolidone instead of 104 parts by weight of water, and 95 parts by weight of artificial graphite in the composition of the negative electrode coating film forming paint 90 parts by weight, 4 parts by weight of non-metal salt polyacrylic acid and 10 parts by weight of polyvinylidene fluoride instead of 1 part by weight of polyethylene glycol, and N-methylpyrrolidone 80 instead of 165 parts by weight of water.
Except that the parts by weight were used, an AA lithium secondary battery was produced in the same manner as in Example 2.

Comparative Example 3 In the composition of the coating material for forming a positive electrode coating film in Example 3, 91 parts by weight of lithium cobalt oxide was changed to 90 parts by weight, and polyvinylidene fluoride was replaced with 3 parts by weight of the nonmetal salt polyacrylic acid. Ride 4 parts by weight, N-methylpyrrolidone 40 parts by weight instead of water 117 parts by weight, artificial graphite 95 parts by weight in the composition of the negative electrode coating film forming paint was changed to 90 parts by weight,
Other than using 10 parts by weight of polyvinylidene fluoride instead of 5 parts by weight of non-metal salt polyacrylic acid and 80 parts by weight of N-methylpyrrolidone instead of 195 parts by weight of water,
In the same manner as in Example 3, an AA lithium secondary battery was manufactured.

With respect to the batteries of Examples 1 to 3 and Comparative Examples 1 to 3 produced as described above, the change in the battery capacity when charging and discharging were repeated was measured. The results are shown in Table 1 and FIG. Moreover, the produced Examples 1-3 and Comparative Examples 1-3
Each of the batteries was disassembled, and the presence or absence of cracking of the coating film or peeling of the coating film on the inner peripheral side of the wound sheet electrode was examined. Table 2 shows the results.
The method for measuring the battery capacity is as follows.

Method for measuring battery capacity: When the charge / discharge current was indicated by C, charge / discharge was performed with 600 mA as 1C.
Charging was performed at a constant voltage of 4.1 V with a 1 C current limiting circuit, and discharging was performed until the voltage between the electrodes of the battery dropped to 2.75 V. Then, for each battery, the first charge, the 100th charge, the 200th charge, the 300th charge,
The discharge capacity at the 00th and 500th times was measured, and the discharge capacity at the first charge / discharge of the battery of Example 1 was set to 100%, and the ratio of the discharge capacity to the first charge / discharge battery capacity of the other batteries and Examples 1 to 5 The ratio of the discharge capacity at the 100th, 200th, 300th, 400th, and 500th charge / discharge of the batteries of Comparative Example 3 and Comparative Examples 1 to 3 was determined. The results are shown in Table 1 and FIG. 2 as the battery capacity (%).

[0049]

[Table 1]

[0050]

[Table 2]

As is clear from the comparison between the decrease in the battery capacity of the batteries of Examples 1 to 3 and the decrease in the battery capacity of the batteries of Comparative Examples 1 to 3 shown in Table 1 and FIG. The batteries of Examples 1 to 3 containing polyacrylic acid showed a small decrease in battery capacity when charging and discharging were repeated, and had a large battery capacity in the first charging and discharging, and were high in capacity.

On the other hand, the batteries of Comparative Examples 1 to 3 using the vinylidene fluoride-based polymer as the binder showed a large decrease in the battery capacity when charging and discharging were repeated.

In particular, Examples 1 to 3 in which non-metal salt polyacrylic acid and polyethylene glycol were used in combination as binders
The battery of No. 2 can be wound with a small winding radius of 2 mm or less when the minimum winding radius of the spirally wound electrode structure is manufactured.
As shown in Table 2, no cracks or peeling occurred in the coating film.

On the other hand, in the batteries of Comparative Examples 1 and 2 using only a vinylidene fluoride-based polymer as the binder, the winding radius at the time of manufacturing the spirally wound electrode body was almost the same as that of Examples 1 and 2. , As shown in Table 2,
Cracks and peeling occurred in the coating film.

[0055]

As described above, according to the present invention,
A high-capacity lithium secondary battery with a small decrease in battery capacity when charging and discharging are repeated was provided.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view schematically showing one example of a lithium secondary battery of the present invention.

FIG. 2 is a diagram showing charge / discharge cycle characteristics of batteries of Examples 1 to 3 and batteries of Comparative Examples 1 to 3.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Separator 4 Electrolyte 5 Battery case

Claims (2)

[Claims] In a lithium secondary battery in which a sheet-shaped positive electrode and a sheet-shaped negative electrode are opposed to each other with a separator interposed therebetween, at least one of the sheet-shaped positive electrode and the sheet-shaped negative electrode is electrically conductive. Consisting of a coating film containing at least one active material and a binder formed on at least one surface of the substrate, wherein the binder is
A lithium secondary battery comprising at least polyacrylic acid which does not form a metal salt with a metal such as sodium. 2. The binder comprises at least polyacrylic acid not forming a metal salt such as sodium and at least one selected from the group consisting of polyethylene glycol, polyethylene oxide, polypropylene glycol and polypropylene oxide. 2. The lithium secondary battery according to 1.