JPH1074502A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the breakage of a separator arising when a battery is compressed and avoid internal short circuit by using a separator formed by stacking a first separator having the specified porosity and a second separator also having the specified porosity. SOLUTION: In a stacked electrode body formed by stacking a belt-shaped negative electrode and a belt-shaped positive electrode through a separator, a laminated separator prepared by stacking a first separator having a porosity of 30% or more but less than 50% and a second separator having a porosity of 50% or more but less than 90% is used. When a battery is compressed by the outside force, the second separator having high porosity is compressed at first, the compression force is relaxed, and the compression force is hardly applied to the first separator. The complete break down of the first separator is prevented, the insulation between the positive electrode and the negative electrode is kept, and internal short circuit can be avoided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery having a laminated electrode assembly, and more particularly to an improvement in a separator used in the above-mentioned laminated electrode assembly.

[0002]

2. Description of the Related Art In recent years, remarkable progress in electronic technology has enabled electronic devices to become smaller and lighter one after another. Along with this, there is an increasing demand for batteries as mobile power sources to be smaller and lighter and have higher energy densities.

Conventionally, aqueous secondary batteries such as lead batteries and nickel-cadmium batteries have been mainly used as secondary batteries for general use. However, although these aqueous secondary batteries have excellent cycle characteristics, they cannot be said to be sufficiently satisfactory in terms of battery weight and energy density.

On the other hand, recently, research and development of a non-aqueous electrolyte secondary battery using lithium or a lithium alloy for a negative electrode have been actively conducted. This nonaqueous electrolyte secondary battery has excellent features of high energy density, low self-discharge, and light weight.

However, when lithium is used for the negative electrode, the lithium grows in a dendritic manner on the negative electrode during charging as the charge / discharge cycle progresses, and finally reaches the positive electrode to cause an internal short circuit. There is a high possibility that it will be reached. Further, when a lithium alloy is used for the negative electrode, the lithium alloy of the negative electrode becomes fine particles with the charge / discharge cycle. In any case, practical application is difficult.

Therefore, a non-aqueous electrolyte secondary battery using a carbon material for the negative electrode has been proposed. This non-aqueous electrolyte secondary battery utilizes the doping / dedoping of lithium between carbon layers or fine pores of a carbon material in a battery reaction. Even when a charge / discharge cycle proceeds, dendrite remains on a negative electrode. No phenomenon such as the precipitation of lithium in the form is observed, and it has a high energy density, is lightweight, and has excellent charge / discharge cycle characteristics.

As the electrode structure of such a non-aqueous electrolyte secondary battery, a laminated electrode structure is generally adopted because the electrode area can be widened. The laminated electrode body has a band-shaped negative electrode in which a negative electrode mixture containing a negative electrode active material is applied to both surfaces of a current collector, and a positive electrode mixture containing a positive electrode active material is applied to both surfaces of a current collector. A band-shaped positive electrode is laminated via a separator, and the laminated body is spirally wound.

[0008]

Incidentally, a microporous polymer film made of polypropylene, polyethylene or the like is used as a separator of the laminated electrode assembly. In this microporous polymer film, the thickness and the porosity are set in consideration of the permeability of the electrolyte salt while ensuring the insulation between the negative electrode and the positive electrode.
Usually, the porosity is about 30 to 50%.

However, such a separator having such a thickness and porosity has a problem that when the battery is compressed by some external force, the battery is easily broken, and the possibility of causing an internal short circuit is high.

Accordingly, the present invention has been proposed in view of such a conventional situation. When the battery is compressed, the separator is prevented from being broken and the internal short circuit is avoided. It is intended to provide a secondary battery.

[0011]

In order to achieve the above-mentioned object, a non-aqueous electrolyte secondary battery of the present invention comprises a laminated electrode body in which a strip-shaped negative electrode and a strip-shaped positive electrode are stacked via a separator. In the non-aqueous electrolyte secondary battery, the separator is obtained by laminating a first separator having a porosity of 30% or more and less than 50% and a second separator having a porosity of 50% or more and 90% or less. It is characterized by becoming.

In the case of a laminated separator comprising a first separator having a porosity of 30% or more and less than 50% and a second separator having a porosity of 50% or more and 90% or less, when the battery is compressed by an external force, First, since the second separator having a high porosity is compressed first, it is relaxed by the compression, and almost no compressive force is applied to the first separator.
Therefore, the first separator does not break completely, the insulation between the negative electrode and the positive electrode is maintained, and an internal short circuit is avoided.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the nonaqueous electrolyte secondary battery of the present invention will be described.

The non-aqueous electrolyte secondary battery to which the present invention is applied has a laminated electrode body in which a strip-shaped negative electrode and a strip-shaped positive electrode are stacked with a separator interposed therebetween.

In the present invention, as a separator of the laminated electrode assembly, a first separator having a porosity of 30% or more and less than 50% and a second separator having a porosity of 50% or more and 90% or less are laminated. Is used.

In such a laminated separator, when the battery is compressed by an external force, the second separator having a high porosity is first used.
Of the first separator is compressed first, so that it is alleviated, and almost no compressive force is applied to the first separator. Therefore, the first separator does not break completely, the insulation between the negative electrode and the positive electrode is maintained, and an internal short circuit is avoided.

In the laminated separator, as the first separator, a microporous polymer film having a porosity of 30% or more and less than 50%, which has been conventionally used as a separator for a non-aqueous electrolyte secondary battery, is used. Is done. As a material, polypropylene or polyethylene is generally used.

On the other hand, as the second separator, a material obtained by impregnating a resin with a nonwoven fabric made of synthetic pulp and inorganic short fibers such as alumina is used. The nonwoven fabric can be manufactured by a conventionally known method, for example, a round net machine or a long net machine. The porosity of the obtained nonwoven fabric is controlled in a desired range by impregnating the resin with an appropriate amount.

As in the case of the single-layer separator, the total thickness of the laminated separator is selected within a range that does not impair the electrode packing density while sufficiently achieving insulation between the negative electrode and the positive electrode. Is appropriate.

On the other hand, the thickness of each of the first separator and the second separator is larger than the thickness of the first separator from the viewpoint of reliably preventing the breakage of the first separator when the battery is compressed. Is desirably thicker.

In the present invention, the separator as described above is used. As the negative electrode and the positive electrode, any of those usually used in this type of non-aqueous electrolyte secondary battery can be used.

For example, as the negative electrode, lithium metal,
A lithium alloy is used as the strip-shaped metal plate.

In addition, a carbonaceous material capable of storing lithium can be used as the negative electrode active material. Examples of carbonaceous materials include pyrolytic carbons, cokes (pitch coke, needle coke, petroleum coke, etc.), graphites,
Glassy carbons, organic polymer compound fired bodies (furan resin etc. fired at appropriate temperature and carbonized), carbon fiber,
Activated carbon etc. are mentioned. In order to form a strip-shaped negative electrode using these carbonaceous materials as an active material, a powder of the carbonaceous material is dispersed in a solvent together with a binder to prepare a negative electrode mixture slurry, and the negative electrode mixture slurry is used as a current collector. After coating and drying on both sides of, it is molded.

As the positive electrode active material, a lithium-containing compound, for example, a general formula Li _X MO ₂ (where M represents one or more transition metals, preferably at least one of Mn, Co and Ni, and x represents 0 0.05 ≦ x ≦ 1.10) is used. In particular, LiCoO ₂ and LiNiO ₂ can increase the battery voltage and are advantageous for improving the energy density. To form a belt-shaped positive electrode using these transition metal composite oxides as an active material, a powder of this oxide is dispersed in a solvent together with a binder and a conductive agent to prepare a positive electrode mixture slurry. Is applied to both sides of the current collector, dried, and then molded.

As the non-aqueous electrolyte, a solution in which a lithium salt is dissolved in an organic solvent is used. As an organic solvent,
Propylene carbonate, ethylene carbonate, 1,
2-dimethoxyethane, 1,2-diethoxyethane, γ
-Butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolan, 4-methyl-1,3-dioxolan, sulfolane, methylsulfolane, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate and the like can be used. .

The supporting electrolyte is LiCl
O ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiB (C
_{6 H 5) 4, CH 3} SO 3 Li, CF 3 SO 3 Li, LiN
(CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiCl,
LiBr and the like.

[0027]

Embodiments of the present invention will be described below based on experimental results.

Example 1 First, a negative electrode was manufactured as follows.

A petroleum pitch was used as a starting material, and was baked to obtain coarse-grained pitch coke. This coarse-grained pitch coke was pulverized into a powder having an average particle diameter of 20 μm. Then, this pitch coke powder was baked at a temperature of 1000 ° C. in an inert gas to remove impurities to produce a coke material powder.

The coke material powder thus produced was used as a negative electrode active material carrier, 90 parts by weight of the coke material powder, and polyvinylidene fluoride (PVdF) as a binder.
The mixture was mixed with 10 parts by weight to prepare a negative electrode mixture, which was dispersed in N-methylpyrrolidone as a solvent to obtain a slurry. Subsequently, the negative electrode mixture slurry is applied to a 10 μm-thick strip-shaped copper foil serving as a negative electrode current collector, dried, and then compression-molded by a roller press to form a strip-shaped negative electrode (thickness: 190 μm).
m, width: 55.6 mm, length: 551.5 mm).

Subsequently, a positive electrode was produced as follows.

0.5 mol of lithium carbonate and cobalt carbonate 1
The moles were mixed and calcined at 900 ° C. for 5 hours in air to produce LiCoO ₂ .

The thus prepared LiCoO ₂ is used as a positive electrode active material, 91 parts by weight of this LiCoO ₂ , 6 parts by weight of graphite as a conductive agent, and 3 parts by weight of polyvinylidene fluoride as a binder are mixed to form a positive electrode mixture. It was prepared and dispersed in N-methylpyrrolidone to form a slurry. continue,
This positive electrode mixture slurry is coated with a 20 μm thick positive electrode current collector.
After applying and drying both sides of the aluminum foil strip
A belt-shaped positive electrode (thickness: 160 μm, width: 53.6 mm, length: 52) is formed by compression molding using a roller press.
(3.5 mm).

Next, as the first separator, a porosity of 4
0% polypropylene porous film (thickness: 25
μm) and a second separator was produced as follows.

That is, in order to fabricate the second separator, short alumina fibers and polyolefin-based synthetic pulp are mixed for 7 minutes.
5% by weight: 25% by weight and put into the pulper,
Prepare these 3% slurry dispersions. Then, the slurry is stored in a chest and supplied to a paper machine for a while. The paper machine is composed of a mesh part, a press part and a drying part. The slurry is first dewatered in the mesh part to form a wet paper. This wet paper is mechanically squeezed and dewatered in a press section, and then heated and dried in a drying section to produce a nonwoven fabric.

[0036] The nonwoven fabric thus produced is mixed with N containing 10 parts by weight of polyvinylidene fluoride (PVDF).
-Methylpyrrolidone solution impregnated and dried. Then, the porosity is reduced to 80% by cutting to a predetermined size.
Second separator (thickness: 25 μm, width: 58.1 m)
m, length: 555 mm).

Next, the second separator and the previously prepared first separator are laminated to form a laminated separator, and the band-shaped negative electrode, the band-shaped positive electrode, and the laminated separator are replaced with the negative electrode, the laminated separator, the positive electrode, and the laminated separator. The layers were stacked in the order of separators. Then, this laminate was wound many times along the length direction with the negative electrode inside, and the final end of the outermost peripheral separator was fixed with a tape to produce a spiral electrode body.

The spiral electrode body manufactured as described above is
The battery was housed in a nickel-plated iron battery can, and insulators were arranged on both upper and lower surfaces of the electrode body. Subsequently, in order to collect current from the negative electrode and the positive electrode, an aluminum positive electrode lead was led out from the positive electrode current collector and welded to the battery lid, and a nickel negative electrode lead was drawn out from the negative electrode current collector and put into a battery can. Welded.

Then, Li was added to a mixed solvent of propylene carbonate and diethyl carbonate in an equal amount in a battery can.
5.0 g of a non-aqueous electrolyte in which PF ₆ was dissolved at a concentration of 1 mol / l was injected, and impregnated in the spiral electrode body.

Subsequently, the battery lid is fixed by caulking the battery can through an insulating sealing gasket coated on the surface with asphalt, and the airtightness in the battery is maintained to form a cylindrical type having a diameter of 18 mm and a height of 65 mm. A non-aqueous electrolyte secondary battery was manufactured.

Comparative Example 1 A non-aqueous electrolyte secondary solution was prepared in the same manner as in Example 1, except that a polypropylene porous film having a porosity of 40% (thickness: 50 μm) was used instead of the laminated separator. A battery was manufactured.

With respect to the battery manufactured as described above,
The battery was compressed to ／ of the battery diameter using a round bar, and the voltage change during the compression process was examined. The result is shown in FIG.

Referring to FIG. 1, first, in the case of the first embodiment using the laminated separator, it can be seen that the battery does not show a sudden change in the battery voltage even when compressed, and prevents an internal short circuit. I understand. On the other hand, in the battery of Comparative Example 1 in which the porous film having a low porosity was used alone for the separator, the battery voltage was rapidly reduced immediately after compression, and an internal short circuit was induced.

[0044]

As is apparent from the above description, in the nonaqueous electrolyte secondary battery of the present invention, the first separator having a porosity of 30 to 50% and the first separator having a porosity of 50 to 90%. Since the laminated separator obtained by laminating the second separator is used, the separator does not break even when compressed by an external force,
Internal short circuit can be prevented. Therefore, the present invention can greatly contribute to improving the reliability of the battery.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing a change over time of a battery voltage due to compression.

Claims (5)

[Claims] 1. A non-aqueous electrolyte secondary battery including a laminated electrode body in which a strip-shaped negative electrode and a strip-shaped positive electrode are stacked with a separator interposed therebetween, wherein the separator has a porosity of 30% or more and less than 50%. A first separator having a porosity of 50% or more and 90% or less;
A non-aqueous electrolyte secondary battery comprising: 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the thickness of the second separator is larger than the thickness of the first separator. 3. The separator has a thickness of 10 to 100 μm.
The non-aqueous electrolyte secondary battery according to claim 1, wherein 4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the first separator is made of a microporous film made of polypropylene or polyethylene. 5. The non-aqueous electrolyte secondary battery according to claim 1, wherein the second separator is made of a nonwoven fabric.