JPH11354127A - 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 a mixture containing at least a cellulose polymer and at least one kind selected from a group comprised of polyethylene glycol, polyethylene oxide, polypropylene glycol and polypropylene oxide is used for the binder.

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 laminated electrode body in which a negative electrode in the form of a separator is opposed via a separator, together with an organic solvent-based electrolytic solution,
Lithium rechargeable batteries made by enclosing them in a battery case
It is characterized by high energy density per unit capacity and high energy density per unit weight.

The binder used for the sheet-like electrode such as the above-mentioned sheet-like positive electrode and sheet-like negative electrode is dissolved in an electrolyte so that the electrode coating structure is not broken during the operation of the battery. Since it is required to have both the characteristics that are difficult to perform and the solvent solubility required for preparing the paint for forming the electrode coating film, polyvinylidene fluoride-based polymer containing vinylidene fluoride as the main component monomer Thus, this polymer is referred to as “polyvinylidene fluoride-based polymer”).

However, since the polyvinylidene fluoride-based polymer has a weak adhesive force to a metal foil generally used as a conductive substrate, an electrode formed using the polyvinylidene fluoride-based polymer as a binder is used. The coating film has a weak adhesive force to the conductive substrate, and therefore, the battery produced using the electrode having the above coating film has an electric charge between the electrode coating film and the conductive substrate when charging and discharging are repeated. However, there has been a problem that the electrical contact 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 opposed to each other with a separator interposed therebetween, at least as the binder, at least a cellulose-based polymer and polyethylene glycol ,
By using together with at least one selected from the group consisting of polyethylene oxide, polypropylene glycol and polypropylene oxide, a high-capacity lithium battery with a small decrease in battery capacity when charging and discharging are repeated is provided. .

That is, in the present invention, the binder of the electrode coating film is composed of a cellulose polymer and at least one selected from the group consisting of polyethylene glycol, polyethylene oxide, polypropylene glycol and polypropylene oxide. Has a carboxyl group (—COOH), an ether group (—O—), a hydroxyl group (—OH group), and the like. Therefore, compared with the case where a polyvinylidene fluoride polymer is used alone as the binder, The adhesive force between the coating film and the conductive substrate is large, and therefore, the deterioration of the electrical contact between the electrode coating film and the conductive substrate when charging and discharging are repeated is suppressed. A decrease in battery capacity can be suppressed. On the other hand, such a cellulosic polymer alone has poor toughness of the coating film. Therefore, when the sheet-like electrode is wound with a small radius of curvature, the coating film may be cracked or peeled off. Therefore, in the present invention, the above problem has been solved by using at least one selected from the group consisting of polyethylene glycol, polyethylene oxide, polypropylene glycol and polypropylene oxide together with the above-mentioned cellulosic polymer.

In the present invention, the use of at least one selected from the group consisting of polyethylene glycol, polyethylene oxide, polypropylene glycol and polypropylene oxide together with the cellulosic polymer makes it possible to prevent cracking and peeling of the coating film. The reasons for this are not always clear at the moment,
Presumably, they act as plasticizers by being added into the rigid cellulosic polymer, thereby increasing the toughness of the coating.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the mixing ratio of the cellulosic polymer to at least one selected from the group consisting of polyethylene glycol, polyethylene oxide, polypropylene glycol and polypropylene oxide is such that the cellulosic polymer is 20 to 95%. It is preferable that at least one selected from the group consisting of polyethylene glycol, polyethylene oxide, polypropylene glycol and polypropylene oxide is 5 to 80% by weight, particularly 20 to 70% by weight. . By setting the mixing ratio of the cellulosic polymer to 20% by weight or more, the adhesive force between the electrode coating and the conductive substrate is increased, and the electricity between the electrode coating and the conductive substrate when charge and discharge are repeated is repeated. Deterioration of contact can be suppressed, and a decrease in battery capacity can be reduced. Further, by setting the mixing ratio of at least one selected from the group consisting of polyethylene glycol, polyethylene oxide, polypropylene glycol and polypropylene oxide to 5% by weight or more, the sheet-like electrode has a small curvature having a winding radius of 2 mm or less. Even when wound with a radius, it is possible to suppress the occurrence of cracks in the coating film and the peeling of the coating film from the conductive substrate.

In the present invention, examples of the cellulosic polymer which is one of the constituent components of the binder include carboxymethylcellulose, methylcellulose, ethylcellulose, hydroxypropylcellulose, hydroxyethylmethylcellulose, and hydroxypropylmethylcellulose.

In the present invention, the other component constituting the binder, polyethylene glycol, is
If the molecular weight is small, it may be melted or softened during use of the battery in a high-temperature environment, thereby deteriorating the electrode function.If the molecular weight is too large, it will be difficult to dissolve in water or an organic solvent, etc. Since the preparation may be difficult, a number average molecular weight of about 2,000 to 2,000,000 is preferable.

In the present invention, the content of the binder in the electrode coating film is preferably 0.2 to 20% by weight, particularly preferably 0.5 to 10% by weight. By setting the content of the binder to 0.2% by weight or more, the mechanical strength of the electrode coating film can be improved, and the electrode coating film can be prevented from peeling from the conductive substrate.
By setting the content of the binder to 20% by weight or less, the amount of active material in the electrode coating film can be increased and the battery capacity can be improved. In addition, the characteristics of the present invention are not impaired even if other binders are used in combination.
However, in that case, it is preferable that other binders be contained in 30% by weight or less, particularly 20% by weight or less of all binders.

The binder comprising the cellulosic polymer of the present invention and at least one selected from the group consisting of polyethylene glycol, polyethylene oxide, polypropylene glycol and polypropylene oxide is as follows:
Although it may be used for any of the positive electrode and the negative electrode, in order to reduce the occurrence of cracks and peeling of the electrode coating from the conductive substrate during winding by matching the toughness of each electrode, the positive electrode,
It is preferable to use it for both of the negative electrodes.

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 the preparation of the paint, the binder is previously dissolved in a solvent such as water or an organic solvent to prepare a solution, and then mixed with the solid particles such as the positive electrode active material to prepare the paint. Good.

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 them do not contain lithium at the time of manufacture, but when they act as a negative electrode active material, they are in a state containing lithium by chemical means or electrochemical means.

For the negative electrode, for example, a paint is prepared by adding an electron conduction aid such as flake graphite and carbon black to the above negative electrode active material, if necessary, further adding a binder and a solvent, and mixing. Then, the paint is applied on a conductive substrate and dried 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 such as water or an organic solvent in advance and mixed with a negative electrode active material as a binder solution to prepare the paint.

In the present invention, the above-mentioned cellulosic polymer and polyethylene glycol, polyethylene oxide,
As a solvent for a paint used for forming a coating film using at least one selected from the group consisting of polypropylene glycol and polypropylene oxide as a binder, a cellulose-based polymer and polyethylene glycol, polyethylene oxide, polypropylene glycol and polypropylene oxide may be used. It is preferable to use a solvent that dissolves at least one selected from the group consisting of, for example, such a solvent,
Water, N-methylpyrrolidone, dimethylacetamide, dimethylformamide, tetrahydrofuran and the like can be mentioned, and they can be used alone or in combination of two or more.

Examples of a method of applying the above-mentioned paint to the conductive substrate include an extrusion coater, a reverse roller, a doctor blade, and an applicator.
Various coating methods 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 metallic conductive material such as aluminum, stainless steel, titanium, or 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-dimethyl
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
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 has a laminated structure such as a spiral electrode body manufactured by spirally winding a sheet-shaped positive electrode and a sheet-shaped negative electrode manufactured as described above with a separator interposed therebetween. Is manufactured by inserting the electrode body into a nickel-plated iron or stainless steel battery case, injecting an electrolyte, and sealing the battery. Further, the battery usually incorporates an explosion-proof mechanism for discharging the gas generated inside the battery to the outside of the battery when the pressure has risen to a certain pressure, thereby preventing the battery from bursting under high pressure.

[0025]

Next, an embodiment of the present invention will be described.
However, the present invention is not limited to only these examples. In the examples below,% indicating the concentration and the like is% by weight.

Example 1 (1) Production of Positive Electrode First, lithium cobalt oxide as a positive electrode active material,
Flake graphite as an electron conduction aid, hydroxypropylcellulose and polyethylene glycol (number-average molecular weight 50
000) and water as a solvent having the following composition were prepared.

Composition of paint for forming positive electrode coating film Lithium cobalt oxide 91 parts by weight Flake graphite 6 parts by weight Hydroxypropyl cellulose 2.4 parts by weight Polyethylene glycol 0.6 parts by weight Water 71 parts by weight

The preparation of the paint was carried out as follows.
First, a binder solution is prepared by dissolving hydroxypropylcellulose and polyethylene glycol in water, and lithium cobalt oxide of the positive electrode active material and flaky graphite as an electron conduction aid are added to the binder solution, and mixed. A paint was prepared.

The obtained paint is used as a conductive substrate having a thickness of 2
It was applied on a 0 μm aluminum foil using an applicator, and dried at 100 to 120 ° C. to form a coating film containing a positive electrode active material, a binder, and the like. Similarly, the above-mentioned paint was applied to the back surface side of the aluminum 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 positive electrode having a coating film thickness of 75 μm on one side.

(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 hydroxypropyl cellulose and polyethylene glycol as those used for the positive electrode coating film forming paint were used. Was prepared at the following ratio to prepare a coating for forming a negative electrode coating film.

Composition of paint for forming negative electrode coating film 95 parts by weight of artificial graphite 4 parts by weight of hydroxypropyl cellulose 1 part by weight of polyethylene glycol 105 parts by weight of water

The thickness of the obtained paint as a conductive substrate is 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, this electrode body was roll-pressed to produce a sheet-shaped negative electrode having a coating thickness of 70 μm on one surface. In addition,
The coating density of the positive electrode and the negative electrode was adjusted so that the weight ratio of both active materials was 2: 1.

(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 315 mm, and the sheet-shaped negative electrode was cut into a width of 41 mm × length 345
mm. Then, an aluminum lead was ultrasonically welded to the positive electrode, and a nickel lead was applied to the negative electrode, at a portion where the metal foil was exposed by removing a part of the coating film at one end of each electrode. And 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, and has a minimum winding radius of 1 mm.
To form a spiral electrode body by spirally winding so that
The spiral electrode body was inserted into a battery case made of stainless steel.

Then, the tip of the lead body on the negative electrode side 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 electrolyte solution is injected in a dry atmosphere, sealed, and sealed with a cylindrical AA lithium secondary battery shown in FIG. 1 (outer diameter: 14 mm, height: 50 mm). Was prepared.

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. Before inserting the spirally wound electrode body, a polypropylene insulator 6 is arranged at the bottom of the battery case 5. 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.
Is connected to the positive electrode 1, an insulator 14 is arranged on the upper part of the spiral 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 the spiral electrode body including the positive electrode 1, the negative electrode 2, and the separator 3, the electrolyte 4, and the upper part of the spiral electrode body The insulator 14 and the like are housed in the battery case 5, and after the housing, an annular groove having a bottom protruding inward is formed in the vicinity of the opening end of the battery case 5. An annular gasket 12 having a sealing plate 7, an insulating packing 10, and an explosion-proof valve 9 inserted into the opening of the battery case 5 is provided.
, And the terminal plate 8 is inserted from above, and the portion of the battery case 5 beyond the groove is fastened inward, whereby the opening of the battery case 5 is sealed. However,
In assembling the battery as described above, it is preferable to connect the negative electrode 2 and the battery case 5 with the lead 15 and connect the positive electrode 1 and the sealing plate 7 with the lead 13 in advance, as described above. In the battery assembled as described above, the thin portion 7a of the sealing plate 7 and the protruding portion 9a of the explosion-proof valve 9 come into contact at the welded portion 11, and the peripheral portion of the explosion-proof valve 9 and the peripheral portion of the terminal plate 8 And the positive electrode 1 and the sealing plate 7 are connected by the lead 13 on the positive electrode side, so that the positive electrode 1 and the terminal plate 8 are connected to the lead 13, the sealing plate 7, the explosion-proof valve 9, and their welded parts 11. As a result, an electrical connection is obtained and functions normally 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-walled 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. The explosion-proof valve 9 is deformed, and the protrusion 9a of the explosion-proof valve 9 is deformed.
After the welded portion 11 of the sealing plate 7 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 torn, and gas is discharged from the gas discharge hole 8a of the terminal plate 8 to the outside of the battery. It is designed to be able to prevent the rupture.

Example 2 In the composition of the coating material for forming a positive electrode coating film of Example 1, 2.4 parts by weight of hydroxypropyl cellulose was changed to 1.5 parts by weight, and 0.6 parts by weight of polyethylene glycol was changed to 1.5 parts by weight. Parts, water 71 parts by weight was changed to 61 parts by weight, 4 parts by weight of hydroxypropyl cellulose in the composition of the coating material for forming a negative electrode coating was changed to 2 parts by weight, and 1 part by weight of polyethylene glycol was 2 parts by weight. To 105 parts by weight of water
Except that the parts were changed to parts by weight, an AA lithium secondary battery was produced in the same manner as in Example 1.

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 hydroxypropyl cellulose and 0.6 parts by weight of polyethylene glycol were used. Parts by weight of polyvinylidene fluoride, 4 parts by weight of water, 71 parts by weight of water, 40 parts by weight of N-methylpyrrolidone, 90 parts by weight of 95 parts by weight of artificial graphite in the composition of the coating for forming a negative electrode coating film. And replaced with 4 parts by weight of hydroxypropylcellulose and 1 part by weight of polyethylene glycol, polyvinylidene fluoride 10
AA type lithium secondary battery was produced in the same manner as in Example 1 except that 80 parts by weight of N-methylpyrrolidone was used instead of 105 parts by weight of water.

Comparative Example 2 In the composition of the coating material for forming a positive electrode coating film of Example 1, 2.4 parts by weight of hydroxypropyl cellulose was changed to 3 parts by weight, and 0.6 parts by weight of polyethylene glycol was changed to 0 parts by weight. AA type was prepared in the same manner as in Example 2, except that 4 parts by weight of hydroxypropylcellulose in the composition of the paint 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. A lithium secondary battery was manufactured.

The battery of Comparative Example 2 will be described in detail in comparison with the battery of Example 1. In Comparative Example 2, the coating composition of the positive electrode (water of the solvent was removed from the composition of the coating for forming the positive electrode coating). ), The content of the binder was 3 as in Example 1.
Although it is in parts by weight, without using polyethylene glycol, increase the amount of hydroxypropyl cellulose,
The content of the binder in the coating composition of the negative electrode (the composition obtained by removing the solvent water from the composition of the coating for forming a negative electrode coating) was 4 parts by weight as in Example 1, but without using polyethylene glycol. The amount of hydroxypropylcellulose is increased.

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

Method for measuring battery capacity: When the charge / discharge current is 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 cycles of the batteries of Comparative Example 2 and Comparative Examples 1 and 2 was determined. The results are shown in Table 1 and FIG. 2 as the battery capacity (%).

[0048]

[Table 1]

[0049]

[Table 2]

The reduction in the battery capacity of the batteries of Examples 1 and 2 shown in Table 1 and FIG. 2 and the reduction in the battery capacity of the batteries of Comparative Examples 1 and 2 and the results of the winding test shown in Table 2 are evident. Thus, the batteries in Examples 1 and 2 using hydroxypropylcellulose and polyethylene glycol as binders did not cause cracks or peeling in the electrode coating film at the center of the winding, and the battery capacity when charging and discharging were repeated And the battery capacity at the first charge / discharge was large and high.

On the other hand, the battery of Comparative Example 1 using the vinylidene fluoride-based polymer alone as the binder and the battery of Comparative Example 2 using the cellulose-based polymer alone were used as the binder. Cracks and peeling occurred, and the battery capacity was significantly reduced when charge and discharge were repeated.

In the examples, batteries using polyethylene glycol were evaluated, but similar effects were confirmed with other polyethylene oxides, polypropylene glycols, and polypropylene oxides.

[0053]

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 graph showing changes in battery capacity (%) when charging and discharging of the batteries of Examples 1 and 2 and Comparative Examples 1 and 2 are repeated.

[Explanation of symbols]

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

Claims (1)

[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. A coating film containing at least one active material and a binder is formed on at least one surface of the substrate, and the binder is at least selected from the group consisting of a cellulose-based polymer and polyethylene glycol, polyethylene oxide, polypropylene glycol and polypropylene oxide. A rechargeable lithium battery comprising at least one selected from the group consisting of: