JP3414729B1 - Lithium ion secondary battery - Google Patents

Abstract

A lithium ion secondary battery having excellent cycle characteristics is provided. SOLUTION: A sheet-like positive electrode in which a coating containing at least a positive electrode active material is formed on both surfaces of a current collector, and a coating containing at least a negative electrode active material are formed on both surfaces of a current collector. A lithium ion secondary battery having a spiral electrode body produced by winding a sheet-shaped negative electrode with a separator interposed therebetween, wherein a current collector is exposed at a winding center side of at least one of the positive electrode and the negative electrode. And a polymer film having a width of 50% or more of the width of the current collector is attached to the exposed portion of the current collector. As the polymer film, a polyimide film, a polypropylene film or the like is preferable, and the thickness thereof is 10 to 300 μm,
The length is preferably 5 to 100 μm, and is preferably longer than the circumference of the innermost circumference of the spiral electrode body.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium ion secondary battery, and more particularly to improvement of the structure of its electrode body.

[0002]

2. Description of the Related Art In a lithium-ion secondary battery, in order to increase the capacity, a sheet-shaped positive electrode and a metal sheet are formed by forming a coating film containing at least a positive electrode active material on both sides of a collector made of metal foil or the like. A spirally wound electrode body formed by spirally winding a sheet-shaped negative electrode formed by forming a coating film containing at least a negative electrode active material on both sides of a collector made of foil or the like with a separator interposed therebetween. It has a structure in which it is inserted into a battery can of a shape and sealed.

When the spirally wound electrode body is manufactured, the current collector is exposed at the center of winding, or the electrode is left as it is, that is, the active material-containing coating film is adhered to both surfaces of the current collector. It is wound in a state.

However, in the prismatic battery, the spiral electrode body must be inserted into a prismatic battery can, depending on the dimension ratio of the length to the width, so that it is composed of a straight portion and a semicircular portion. There is a problem that the shape is close to an ellipse and the radius of curvature of the semicircle becomes small especially at the center of winding, so that the electrode cracks and the cycle characteristics deteriorate.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems in the prior art and to provide a lithium ion secondary battery having excellent cycle characteristics.

[0006]

According to the present invention, a sheet-like positive electrode formed by forming a coating film containing at least a positive electrode active material on both sides of a current collector, and at least a negative electrode active material on both sides of the current collector. In a lithium-ion secondary battery having a spirally wound electrode body formed by spirally winding a sheet-shaped negative electrode formed with a coating film containing a separator, at least one of the positive electrode and the negative electrode The above object is achieved by providing a portion where the current collector is exposed on the winding center side and sticking a polymer film having a width of 50% or more of the width of the current collector to the exposed portion of the current collector. It was done.

That is, when the specific polymer film is attached to the end portion of the current collector on the winding center side as described above, the winding center portion becomes elastic due to the polymer film. If you bend it at the part where the polymer film is attached, the radius of curvature at the beginning of winding will increase and it will not be crushed, so
It is possible to prevent the electrode from cracking and prevent the deterioration of the cycle characteristics due to the electrode crack.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION In producing a spiral electrode body,
The length of the polymer film attached to the current collector is preferably longer than the circumferential length of the innermost peripheral portion of the spirally wound electrode body, that is, the length of one round of the innermost circumferential portion.

As the polymer film, for example, a film of a polymer having resistance to an electrolytic solution solvent such as a polyimide film or a polypropylene film can be used. An adhesive having resistance to an electrolytic solution solvent such as an acrylic adhesive can be used as the adhesive when the polymer film is attached to the current collector.

The thickness of the polymer film is 10
~ 300 μm is preferable, and the thickness of the polymer film is 1
If the thickness is less than 0 μm, the elasticity of the polymer film is not sufficient, and the radius of curvature when bent is not large enough to prevent the electrode from cracking. Therefore, the effect of sticking the polymer film decreases, and When the thickness of the polymer film is thicker than 300 μm, the proportion of the polymer film in the internal volume of the battery increases, the filling amount of the active material decreases, and the battery capacity may decrease.

The length of the polymer film is preferably longer than the innermost circumference of the spirally wound electrode body, but numerically, it is usually 5 in consideration of the size of the battery and the like.
It is preferable to select from the range of 100 mm. The width of the polymer film needs to be 50% or more of the width of the current collector of the electrode. This means that when the width of the polymer film becomes smaller than 50% of the width of the current collector, the center of winding of the spiral electrode body Since it is not possible to impart sufficient elasticity to the part, therefore,
This is because the effect of preventing the electrode from cracking is reduced, and it is preferable that the width of the polymer film is approximately the same as the width of the current collector.

The electrode on the side where the polymer film is attached to the current collector may be either a positive electrode, a negative electrode, or both.

In the present invention, as the positive electrode active material, for example, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide (these are usually represented by LiCoO ₂ , LiNiO ₂ , LiMnO _2, etc., respectively, The ratio of Li and Ni, the ratio of Li and Co,
Li-Mn ratio is often deviated from the stoichiometric composition) and the like, and lithium-containing composite metal oxides are used alone or as a mixture of two or more kinds, or as a solid solution thereof. However, the positive electrode active material is not limited to these and may be another material.

In producing the positive electrode, the positive electrode active material may be added to the positive electrode active material, if necessary, with an electron conduction aid such as phosphorous graphite, acetylene black, carbon black, or the like, such as polyvinylidene fluoride, polytetrafluoroethylene, or ethylene. Propylene diene rubber (EPD
Binders such as M) can be added.

The positive electrode contains, for example, the above positive electrode active material and, if necessary, an electron conduction aid and a binder, and is dissolved or dispersed in a solvent to prepare a coating material, which is applied to a current collector and dried. Then, it is manufactured through a step of forming a coating film containing at least a positive electrode active material on the current collector. However, the method for producing the positive electrode is not limited to the above method, and other methods may be adopted.

In the present invention, as the negative electrode active material, for example, a lithium-containing compound is used, and as the lithium-containing compound, for example, a carbon material having a turbostratic structure, graphite, tungsten oxide, lithium iron composite oxide is used. The thing etc. are mentioned. Some of these do not contain lithium at the time of manufacture, but when they act as a negative electrode active material, they are in a state of containing lithium by chemical means, electrochemical means, or the like.

The negative electrode may be formed by adding, for example, phosphorous graphite, acetylene black, or the like to the above negative electrode active material, if necessary.
An electron conduction aid such as carbon black is added, further, a binder such as polyvinylidene fluoride, polytetrafluoroethylene, ethylene propylene diene rubber, etc. is added, and a solvent is further added and mixed to prepare a paint. It is manufactured by applying a current collector, drying and forming a coating film. However, the method for producing the negative electrode is not limited to the above method, and another method may be adopted.

The amount ratio of the active material in the positive electrode and the negative electrode is
Depends on the type of positive electrode active material and negative electrode active material
However, as a positive electrode active material, lithium cobalt oxide (LiC
oO ₂) And graphite as the negative electrode active material,
Positive electrode active material / negative electrode active material = 1.8 to 2.8 (weight ratio)
Preferably there is.

In the present invention, various coating methods such as an extrusion coater, a reverse roller, a doctor blade and the like can be adopted as a coating method for coating the above current collector on the current collector.

In the present invention, examples of the current collectors for the positive and negative electrodes include metal nets such as aluminum, stainless steel, titanium and copper, punched metal, expanded metal, foam metal and foil. Is used.

Examples of the electrolytic solution include 1,2-dimethoxyethane, 1,2-diethoxyethane, propylene carbonate, ethylene carbonate, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolane, diethylene carbonate, dimethyl carbonate and ethyl. In a single solvent or a mixed solvent of two or more kinds such as methyl carbonate, for example, LiCF ₃ SO ₃ , LiC ₄ F ₉ S
An organic electrolyte solution in which one or more electrolytes such as O ₃ , LiClO ₄ , LiPF ₆ , and LiBF ₄ are dissolved is used.

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

The battery may be manufactured, for example, by using a spirally wound electrode body or the like manufactured by spirally winding a sheet-shaped positive electrode and a sheet-shaped negative electrode, which are manufactured as described above, with a separator interposed between them. It is manufactured by inserting it into a battery can made of plated iron or stainless steel and sealing it.
In addition, in the above-mentioned battery, normally, the gas generated inside the battery is discharged to the outside of the battery when it rises to a certain constant pressure,
An explosion-proof mechanism is incorporated to prevent the battery from bursting under high pressure.

The present invention was developed especially for the purpose of being applied to a prismatic battery, but may be applied to a cylindrical battery.

[0025]

EXAMPLES Next, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to only those examples.

Example 1 (1) Preparation of Positive Electrode Lithium cobalt oxide (usually Li
A positive electrode was prepared by using CoO ₂ , but the ratio of Li to Co is often slightly deviated from the stoichiometric composition.

A coating material for forming a coating film was prepared by using 91 parts by weight of the above lithium cobalt oxide, 6 parts by weight of phosphorus-like graphite as an electron conduction aid, and 3 parts by weight of polyvinylidene fluoride as a binder, N-methylpyrrolidone. Was prepared by mixing in the presence of

In the preparation of the coating material for forming the coating film, first, vinylidene fluoride is dissolved in N-methylpyrrolidone, and lithium nioxide as a positive electrode active material and a flaky shape as an electron conduction aid are added to the solution. Add graphite and N-methylpyrrolidone to make viscosity about 10,000 mP
It was performed by adjusting to a.

Then, this paint was placed on an aluminum foil having a thickness of 20 μm to be used as a current collector, the paint was rubbed off with an applicator provided with a constant slit (gap), and then dried. Similarly, the above coating material was applied to the back surface of this aluminum foil, and vacuum dried at 100 ° C. This electrode body was pressed to produce a sheet-shaped positive electrode having a thickness of about 170 μm. However, in applying the coating material, the coating material was not applied to a portion 36 mm long from one end portion of the aluminum foil serving as a current collector, and the aluminum foil was exposed.

Then, the above sheet-shaped positive electrode is cut so that a coating film forming portion has a rectangular shape of 270 mm × 54 mm, and at the time of winding, a current collector (aluminum) that comes into contact with a position 32 mm from the end on the winding center side. An aluminum tab is welded to the exposed part of the foil, and a polyimide film having a thickness of 50 μm and a length of 15 mm is formed on the exposed part of the current collector (aluminum foil) at the end on the winding center side of the sheet-shaped positive electrode. It was pasted with an acrylic adhesive.

Here, the sheet-shaped positive electrode will be described with reference to FIG. FIG. 1 schematically shows an end portion on the winding center side of the sheet-shaped positive electrode, 1a is a collector of the sheet-shaped positive electrode 1, and 1b is formed on the collector 1a. In the active material-containing coating film, the portion where the active material-containing coating film 1b is formed corresponds to the so-called main body of the sheet-shaped positive electrode 1, and the current collector 1a shown in FIG. It corresponds to the exposed part of the current collector.

The current collector 1a is made of an aluminum foil, and as described above, the current collector 1a shown in FIG. 1 corresponds to the exposed portion of the current collector in the sheet-shaped positive electrode 1 and has a length of 36 mm. is there.

A tab 1c made of aluminum is attached by welding at a position 32 mm from the end on the winding center side of the current collector 1a, and the end on the winding center side has a thickness of 50 μm and a long length. A 15 mm thick polyimide film is attached as the polymer film 1d. In addition,
The position of 32 mm from the end on the winding center side of the current collector 1a is the position measured at the center of the tab 1c. And in this Example 1, as shown in FIG. 1, the width of the polymer film 1d occupies about 90% of the width of the collector 1a.

As will be described later, the sheet-shaped positive electrode and the sheet-shaped negative electrode are spirally wound with a separator interposed therebetween to prepare a spirally wound electrode body, and the spirally wound electrode body is inserted into a rectangular battery can. In order to obtain a shape close to an elliptical diameter so that the spiral electrode body can be pressed, the spiral electrode body is pressed, but the bending point is the portion where the polymer film 1d is attached.

In the present embodiment, the inner circumference of the spirally wound electrode body is about 14 mm, and the polymer film 1
Since the length of d is 15 mm, the length of the polymer film 1d is 1 mm longer than the circumference of the innermost circumference of the spirally wound electrode body.

Reference numeral 2 in FIG. 1 denotes a sheet-like negative electrode, which will be described later, and schematically shows the end portion on the winding center side.

(2) Preparation of Negative Electrode Using artificial graphite (synthesized at 2800 ° C.) as the negative electrode active material and polyvinylidene fluoride as the binder,
A paint containing them in the following proportions was prepared. Artificial graphite (synthesized at 2800 ° C) 90 parts by weight Polyvinylidene fluoride 10 parts by weight

The coating film-forming coating material was prepared by first dissolving polyvinylidene fluoride in N-methylpyrrolidone, adding artificial graphite to the solution, and mixing them.

Then, the coating material prepared as described above is applied on both sides of a copper foil having a thickness of 10 μm to be used as a current collector in the same manner as in the case of the positive electrode and dried to form an active material-containing coating film, Then, it roll-pressed and produced the sheet-shaped negative electrode with a total thickness of 160 micrometers. The weight ratio of the positive electrode and the negative electrode active material was set to 2: 1. A nickel tab was welded to the exposed portion of the current collector (copper foil) at one end of the sheet-shaped negative electrode.

(3) Preparation of Electrolyte Solution 1 LiPF ₆ was added to a mixed solvent of ethylene carbonate and ethyl methyl carbonate (mixing ratio 1: 3 by volume).
Mol / l was dissolved to prepare an organic solvent-based electrolytic solution.

(4) Assembly of Square Battery Using the above-mentioned sheet electrode and electrolyte as main constituent materials, a prismatic battery (size: 17 mm × 10 mm × 65 m)
m) was assembled.

First, for the positive electrode, the side where the tape is attached is the winding center side, and for the negative electrode, the side where the nickel tabs are welded is the winding end side, and there is a thickness between these positive and negative electrodes. A spirally wound electrode body with a separator made of a microporous polyethylene film having a porosity of 25% and a porosity of 40% interposed therebetween to form a spirally wound electrode body, and the spirally wound electrode body is pressed to form a shape close to an ellipse. After that, it was inserted into a battery can made of stainless steel. The battery can has a cross section of 17 mm x 10 m.
It is composed of a rectangular cylindrical container of m, and the spirally-wound electrode body is formed into a substantially circular shape once, and is close to an elliptical shape so that it can be pressed as described above and inserted into a battery can having a rectangular cross section. It has a shape.

Then, the extension part of the nickel tab of the negative electrode is welded to the upper side surface of the battery can, the insulating plate is inserted in the upper part of the battery can, and the extension part of the aluminum tab of the positive electrode and the positive electrode terminal of the battery lid are connected. After welding, the battery lid is inserted into the opening of the battery can, the outer periphery of the battery lid and the open end of the battery can are laser-welded, and then the electrolyte solution previously provided in the body of the battery lid. Inject 2 ml of the above electrolyte from the inlet,
After that, a stainless steel plug was inserted into the electrolyte injection port, and the periphery of the plug was welded and sealed to the body portion of the battery lid to produce a prismatic lithium ion secondary battery having the structure shown in FIG.

The battery shown in FIG. 2 will be described below. 1 is the positive electrode and 2 is the negative electrode. However, Figure 2
However, in order to avoid complication, an aluminum foil, a copper foil, or the like as a current collector used in manufacturing the positive electrode 1 and the negative electrode 2 is not shown. And 3 is a separator.

In FIG. 2, the innermost peripheral portion is the positive electrode 1, but the innermost peripheral portion of the positive electrode 1 is another portion (that is, FIG. 2).
In the figure, the reason why it is shown thinner than the third and fifth laps from the center is that the active material-containing coating film is not formed on the innermost peripheral part. FIG. 3 is a schematic cross-sectional view of only the positive electrode 1 at the innermost peripheral portion of the spiral electrode body (that is, the spiral electrode body including the positive electrode 1, the negative electrode 2, and the separator 3) of the battery shown in FIG. However, the innermost peripheral portion of the positive electrode 1 has a shape close to an ellipse composed of a substantially semicircular portion and a substantially linear portion, and in the embodiment shown in this embodiment, the current collector 1a is The length of the polymer film 1d attached to is longer than the innermost circumference of the spirally wound electrode body, that is, the innermost circumference of the positive electrode 1.

Reference numeral 4 denotes a stainless steel battery can, and the battery can 4 also serves as a negative electrode terminal. An insulator 5 made of a polytetrafluoroethylene sheet is arranged on the bottom of the battery can 4, and an insulator 6 made of a polytetrafluoroethylene sheet is also arranged on the inner peripheral part of the battery can 4. The spiral electrode body including the negative electrode 2 and the separator 3 is housed in the battery can 4.

Reference numeral 7 is a battery lid, and the main body of the battery lid 7 is formed by attaching a positive electrode terminal 10 to a stainless steel body portion 8 via an insulating packing 9 made of polypropylene. The outer peripheral portion of 8 is joined to the open end of the battery can 4 by welding.

The positive electrode terminal 10 is formed of aluminum (or an aluminum alloy may be used), the bottom of which is crimped, and a lead on the positive electrode side which is an extension of the aluminum tab 1c (see FIG. 1) of the positive electrode 1 is attached to the bottom. One end of the body 11 is welded. The lead body 12 on the negative electrode side, which is an extension of the nickel tab of the negative electrode 2, is located on the winding end side of the spiral electrode body, and one end thereof is welded to the inner peripheral portion of the upper portion of the battery can 4.

An insulating plate 13 made of polypropylene is fitted in the positive electrode terminal 10 and fixed to the positive electrode terminal 10 by caulking the bottom of the positive electrode terminal 10. In addition, an explosion-proof vent 14 is provided in the body portion 8 of the battery lid 7, and when the pressure inside the battery rises abnormally, the vent 14 is cleaved and gas inside the battery is discharged to the outside of the battery. It is designed so that it can be discharged and damage to the battery under high pressure can be prevented. Although the detailed structure of the explosion-proof vent 14 is omitted in FIG. 2 due to space limitations, the explosion-proof vent 14 provided in the battery of this embodiment is
An annular thin wall portion having a diameter of about 7 mm is formed on the body portion 8, and when the pressure inside the battery rises abnormally, the thin wall portion is cleaved first and the gas inside the battery is discharged to the outside of the battery. It is designed to prevent damage under high pressure. However, the explosion-proof vent 14 may have another configuration. Reference numeral 15 denotes a polypropylene insulating plate, which is inserted in the upper portion of the battery can 4 and insulates between the body portion 8 of the battery lid 7 and the positive electrode 1 of the spiral electrode body. . The lead body 11 on the positive electrode side is the insulating plate 1.
After passing through the slit of 5, twist about 90 °,
Its tip is welded to the positive electrode terminal 10.

Although not shown in FIG. 2,
An electrolytic solution is injected into this battery, and the electrolytic solution is injected into the battery through an electrolytic solution injection port provided in the body portion 8 of the battery lid 7. After the injection of the electrolytic solution, the electrolytic solution injection port is introduced. The electrolyte injection port is not shown in FIG. 2 because a stainless steel plug is inserted into the plug and the plug is sealed by welding.

Example 2 A polymer film having a thickness of 10 was attached to a current collector.
A battery was produced in the same manner as in Example 1 except that a polyimide film having a length of 0 μm and a length of 15 mm was used.

Example 3 As a polymer film to be attached to a current collector, a thickness of 25 μm
A battery was produced in the same manner as in Example 1 except that a polyimide film having a length of m and a length of 15 mm was used.

Comparative Example 1 The polymer film was not attached, and the exposed portion (length 19 mm) of the current collector for attaching the polymer film was not provided. A battery was produced in the same manner as in Example 1 except that the thickness was set to 17 mm.

The batteries of Examples 1 to 3 and Comparative Example 1 produced as described above were subjected to the following charge and discharge test,
The capacity was measured.

That is, when the charging / discharging current is represented by C, the charging / discharging was performed with 700 mA as 1C. Charge 1
A current limiting circuit for C was provided and the battery was discharged at a constant voltage of 4.1 V, and discharging was performed until the voltage between electrodes of the battery dropped to 2.75 V. Table 1 shows the capacities after 1 cycle, 10 cycles, 100 cycles and 300 cycles.

[0056]

[Table 1]

As shown in Table 1, Examples 1 to 3 were larger in capacity when the number of cycles was increased and were excellent in cycle characteristics, as compared with Comparative Example 1.

[0058]

As described above, the present invention can provide a lithium ion secondary battery having excellent cycle characteristics.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a state before winding of a winding center side portion of a spiral electrode body according to the present invention.

FIG. 2 is a vertical sectional view schematically showing an example of the lithium ion secondary battery of the present invention.

FIG. 3 is a transverse cross-sectional view schematically showing an example of the innermost peripheral portion of the spiral electrode body in the lithium ion secondary battery of the present invention.

[Explanation of symbols]

1 positive electrode 1a Current collector 1b Active material-containing coating film 1d polymer film 2 Negative electrode 3 separator

Front page continued (58) Fields surveyed (Int.Cl. ⁷ , DB name) H01M 10/40 H01M 4/00-4/62 H01M 2/20-2/34 H01M 10/04

Claims (5)

(57) [Claims] 1. A sheet-shaped positive electrode having a coating film containing at least a positive electrode active material formed on both surfaces of a current collector, and a coating film containing at least a negative electrode active material on both surfaces of the current collector. A lithium-ion secondary battery having a spirally wound electrode body produced by winding a sheet-shaped negative electrode with a separator interposed between the positive electrode and the negative electrode, the current collector being exposed on the winding center side of at least one of the positive electrode and the negative electrode. A lithium ion secondary battery, wherein a portion is provided, and a polymer film having a width of 50% or more of the width of the current collector is attached to an exposed portion of the current collector. 2. The lithium ion secondary battery according to claim 1, wherein the polymer film is a polyimide film or a polypropylene film. 3. The polymer film has a thickness of 10 to 30.
It is 0 micrometer, The lithium ion secondary battery of Claim 1 or 2 characterized by the above-mentioned. 4. The length of the polymer film is 5 to 100.
mm, The lithium ion secondary battery according to claim 1. 5. The length of the polymer film is longer than the circumference of the innermost circumference of the spiral electrode body.
The lithium ion secondary battery according to any one of 1 to 3.