JPH03245457A - Fine porous membrane for separator of non-aqueous electrolyte battery - Google Patents

Abstract

PURPOSE:To enhance safety and reliability by forming at least one fine porous membrane of a bridged polyethylene raw material for a support body of a separator consisting of two or more layered fine porous membranes made of a thermoplastic resin. CONSTITUTION:In a layer of two or more fine porous membranes, at least one of the fine porous membranes is formed of a bridged polyethylene raw material while at least one of the membranes is formed of a thermoplastic resin such as a polyethylene resin. In this constitution, since a melting point of the polyethylene resin is lower than that of an active material of a negative electrode, conversion to non-porosity is commenced at a temperature lower than the melting point of the active material to increase electric resistance and suppress temperature rise inside a battery. Consequently, even when the temperature is raised inside the battery due to an external short circuit and the like, the non-porosity state is maintained and chemical reaction inside the battery is certainly suppressed so as to enhance safety and reliability.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a separator for batteries, and more particularly to a microporous membrane for separators for non-aqueous electrolyte batteries.

[Prior art]

Various types of battery separators are known, and for example, a polypropylene or polyethylene film with micropores has been proposed, as seen in Japanese Patent Laid-Open No. 60-23954. For the purpose of increasing
A separator has been proposed in which a microporous membrane made of a low melting point resin having a melting point of 120 DEG C. or lower is laminated on one side of a porous support material, as seen in JP-A-1-258358.

In general, a battery with high energy density that is compact, has a high electromotive force, and can draw a large current for a long period of time is desired. In order to increase the amount of active material so that it can be extracted for a long period of time, a battery has been developed in which the active material of the negative electrode and the active material of the positive electrode are wrapped in seaweed with a separator interposed between them.

However, in non-aqueous electrolyte batteries that use light metal elements such as lithium as the active material of the negative electrode, external short circuits can cause the temperature of the battery to rise, and the decomposition of the electrolyte can generate gas, causing battery fires and explosions. Because of the risk of It is stated that a porous membrane is used as a separator.

In addition, in JP-A-1-258358, in order to compensate for the shortcomings of non-woven fabrics and to improve safety, a separator is disclosed in which a microporous membrane with a low melting point, that is, a microporous membrane made of polyethylene or polypropylene, and a non-woven fabric are laminated together. is proposed.

[Problem to be solved by the invention]

However, as disclosed in Japanese Patent Application Laid-Open No. 60-23954, separators made of nonwoven fabric with large pores have a safety problem as batteries. Although this idea seems to be valid, there was no mention of reliability related to safety, and there were doubts as to whether it could be used as a highly reliable battery separator.

Furthermore, as disclosed in JP-A-1-258358, there are problems in using nonwoven fabric as a separator, and even if a microporous membrane made of a low-melting point material is used, it cannot be used as a separator at high temperatures. There are questions about gender.

[Means to solve the problem]

In order to achieve the above object, the present invention is a microporous membrane having the following configuration.

That is, the present invention provides a separator for a non-aqueous electrolyte battery that is formed by bonding together two or more microporous membranes made of thermoplastic resin, in which at least one of the microporous membranes is made of a crosslinked polyethylene material as a support. It is characterized by

The material for the microporous membrane of the present invention may be any thermoplastic resin, preferably polyolefin resin from the viewpoint of safety, oxidation resistance, and organic solvent resistance, and more preferably polypropylene or polyethylene.

The pores of the microporous membrane are naturally in communication, and the average pore diameter is 0.01 μm to 3 μm, preferably 0.02 μm.
m to 1.0 μm, and more preferably 0.1 μm to 1.0 μm in consideration of air permeability and electrolyte permeability.

In addition, the maximum pore diameter must be so small that it can close when heated and melted, and must not cause internal short circuits, and the maximum pore diameter is 5 μm or less, preferably 1.0 μm or less. .

Preferably, at least one microporous membrane has a sponge structure. The sponge structure in the present invention refers to a structure in which a plurality of pores are present in any cross section of the microporous membrane.

Here, pores refer to voids and are areas without resin.

The presence of multiple holes means that the resin is interposed between the holes and the holes are connected to each other. However, it is not necessary that all holes are connected;
There is no problem as long as the air permeability and porosity satisfy the conditions disclosed in the present invention.

Moreover, the uniformity of the shape and diameter of the pores does not matter. For example, the shape and size of the pores may vary in the thickness direction of the microporous membrane.

Preferably, it is a microporous membrane having a homogeneous three-dimensional network structure.

Regarding the film thickness, the thinner it is, the better, but an appropriate film thickness that does not cause internal short circuits is required, and is 10 μm to 5 μm.
0 um, preferably 15 μm to 40 μm.

Air permeability is 60 seconds/1 from the viewpoint of air permeability and electrical resistance.
00 cc~280 seconds/100 cc, preferably 2
00 seconds/100 cc or less.

As is well known, since the separator is wound between the positive electrode and the negative electrode, it must have strength to prevent defects such as tearing during winding. For example, about 30 kg/cm" is sufficient.

The most important configuration of the present invention is the use of a microporous membrane made of crosslinked polyethylene material as a support, and this configuration greatly contributes to increasing the reliability of the separator.

In other words, when the internal temperature of a non-aqueous electrolyte battery increases due to an external short circuit, etc., and reaches the melting point of the microporous membrane material or a temperature higher than the melting point, the microporous membrane becomes non-porous and the internal temperature of the battery increases. Suppresses chemical reactions and suppresses temperature rise inside non-aqueous electrolyte batteries. Furthermore, the temperature inside the non-aqueous electrolyte battery does not drop suddenly, but tends to drop gradually. This means that because the microporous membrane is kept at a temperature close to its melting point for a long time and is held in a sandwiched state, it may not become porous, or even if it becomes nonporous, defects may occur in the microporous membrane within a short period of time. , the chemical reaction will start again, the internal temperature of the battery will rise again, and the electrolyte will gasify, potentially leading to ignition and explosion. Conventional separators cannot be said to have sufficient reliability. However, if a microporous membrane made of cross-linked polyethylene material is used as a support as disclosed in the present invention, it will be safe and reliable without any defects for a long period of time even at temperatures above the melting point. It is possible to build a compact battery with high performance.

The reason is not clear, but with a separator made of only one microporous membrane made of cross-linked polyethylene material, the degree of non-porosity is low even when the temperature reaches the melting point of the polyethylene material, and the safety of non-aqueous electrolyte batteries is compromised. cannot fully fulfill its role as a separator.

However, as disclosed in the present invention, by using a microporous membrane made of a crosslinked polyethylene material as a support, reliability regarding safety can be improved.

The characteristics of the microporous membrane for the support made of crosslinked polyethylene are preferably similar to those of the microporous membrane described above, but are not particularly limited.

In the superposition of two or more microporous membranes, at least one microporous membrane is a microporous membrane made of a crosslinked polyethylene material, and at least one microporous membrane is a microporous membrane made of a thermoplastic resin. Although it is a porous membrane, polyethylene resin is most preferable as the thermoplastic resin.

This means that the melting point of polyethylene resin is 45°C higher than 181°C, which is the melting point of the active material of the negative electrode, such as lithium metal.
Since porosity starts to become non-porous at a low temperature and increases the electrical resistance inside the battery, the temperature rise inside the battery can be suppressed from a lower temperature, so the possibility of reaching the melting point of lithium is low, improving the safety and reliability of the battery. You can increase your sexuality.

In the present invention, a microporous membrane is manufactured by mixing a thermoplastic resin with a resin solvent, a plasticizer, an inorganic fine powder, etc., molding, extracting, drying, and further stretching.

For example, the mixed composition of thermoplastic resin, inorganic fine powder, and organic liquid is 5 to 70% by volume, 10 to 55% by volume, and 2% by volume, respectively.
0 to 75% by volume, mixed with a normal mixer such as a Henschel mixer, then kneaded with a melt kneading device such as an extrusion wire, and the obtained kneaded product is extruded to a size of 50 μm.
Mold to a thickness of ~450 μm. Furthermore, the organic liquid is extracted from the molded product using an organic liquid solvent, and then the inorganic fine powder is extracted using an inorganic fine powder extraction solvent to obtain a porous membrane.

Further, if necessary, the film is stretched to a predetermined thickness using a -axis or two-wheel stretching machine to adjust the film thickness.

The support is obtained by selecting polyethylene resin as the thermoplastic resin, producing a microporous membrane in the same manner as described above, and subjecting the obtained microporous membrane to T-ray treatment.

Further, as a method for overlapping two sheets, there are a method of simply overlapping two sheets and winding them into a roll, and a method of overlapping them and then slightly crushing them with two rolls and winding them into a roll.

[Effect]

According to the above configuration, if the internal temperature of the battery rises due to an external short circuit, reaches a temperature close to the melting point, and continues to be maintained at that temperature, or even if the temperature gently decreases, defects may occur in the film. Since no porosity occurs, that is, a non-porous state is maintained, chemical reactions inside the battery are reliably suppressed, resulting in a highly safe and highly reliable battery.

〔Example〕

EXAMPLES Hereinafter, the present invention will be explained with reference to examples, but the present invention is not limited to the examples. Note that the measurement method and evaluation method are summarized below.

(1) Dial gauge manufactured by Ozaki Seisakusho Co., Ltd. (product name, PE
ACOCK Nα25).

(2) Average pore diameter Determined by a half-dry method in accordance with ASTM F-316-70.

(3) Maximum pore diameter Calculated from the bubble point in ethanol in accordance with ASTM E-128-61.

(4) Air permeability Compliant with JIS-P-8117.

(5) Non-porous The degree of nonporous formation was defined by the following formula.

Degree of nonporousity = (A/air permeability at room temperature) A: A 6 cm x 6 cm sample was fixed at its four corners to prevent deformation, and placed in a gear oven set at a predetermined temperature for 30 minutes.
After being left for 0 minutes, the sample was immediately removed from the gear oven and cooled in air or water. This refers to the case where the ventilation amount is 25 cc or less 10 minutes after the start of air temperature measurement.

Example 19% by weight of finely divided silicic acid and 47% by weight of dioctyl phthalate
were mixed with a Henschel mixer, and polyethylene resin (manufactured by Asahi Kasei Corporation, 5UNFINESH-80) was added to this.
0) 34% by weight was added and mixed again using the Henschel mixer.

The mixture was molded into a film with a thickness of 95 μm using a film manufacturing device equipped with a 30 m/mφ twin screw extruder and a T-die with a width of 450 mZm.

The formed membrane was formed by LL, 1 in 1-trichloroethane.
The sample was immersed for 0 minutes to extract dioctyl phthalate and then dried, and further immersed in 25% caustic soda at 60°C for 60 minutes to extract finely divided silicic acid and then dried.

Further, the microporous membrane was stretched to a film thickness of 34 μm using a roll stretching machine heated to 114'C.
Heat treatment was performed for 5 seconds in an atmosphere of °C.

The obtained membrane had a sponge structure, and its properties are shown in Table 1.

The microporous membrane obtained here was placed under the 5M radOT line for 20 minutes.
It was left to stand for a minute to crosslink. Subsequently, the non-crosslinked microporous membrane and the crosslinked microporous membrane were superimposed.

Next, the degree of non-porosity of the two superimposed microporous membranes was measured. The results are shown in Table 2.

Comparative Example 1 Comparative example 1 was carried out in the same manner as in the example except that two non-crosslinked microporous membranes were used. The results are shown in Table 2.

Comparative Example 2 The same procedure as in Example was carried out except that one crosslinked microporous membrane was used. The results are shown in Table 2.

〔Effect of the invention〕

According to the present invention, the temperature range in which the degree of non-porosity is high is wide, that is, the temperature range in which the chemical reaction inside the battery can be suppressed is wide, and the chemical reaction can be suppressed stably for a long time, so the safety of the battery is improved. A battery that is not only expensive but also highly reliable can be obtained.

It is also suitable as a packaging material for sterilization.

No. table No. 2 table

Claims (1)

[Claims] 1. A separator for non-aqueous electrolyte batteries consisting of two or more microporous membranes made of thermoplastic resin stacked together, characterized in that at least one of the microporous membranes as a support is made of a crosslinked polyethylene material. Microporous membrane for separators in non-aqueous electrolyte batteries.