JPH0359947A - Separator for battery and battery using the separator - Google Patents

Abstract

PURPOSE:To make the title separator highly preventive against over heating by cross-linking at least one of laminated polyethylene-based porous films and making at least another one of the film not cross-linked. CONSTITUTION:At least one of not cross-linked polyethylene-based porous film 1 and at least one of cross-linked polyethylene-based porous film 2 are laminated and molded. The cross-linked film 2 is obtained from a non cross-linked film 1 by chemical cross-linking or chemical cross-linking by radioactive ray radiation. The gel ratio after cross-linking is normaly >=15%, preferably 30-90%, in terms of shape retaining property at the time of exposure of the film to high temperature. In this case, when the inner temperature of the battery rises owing to abnormal current, the not cross-linked polyethylene-based porous film 1 is melted and fluidized firstly and since the viscosity of the melt is low, the fine pores of the cross-linked polyethylene porous film are filled and sealed quickly. In this way, ion transfer between both a cathode and an anode is inhibited and thus abnormal current is shut and excessive temperature rise is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a battery (for example, a non-aqueous electrolyte battery) and a separator incorporated into the battery to isolate positive and negative electrodes and prevent short circuits.

(Prior Art) Non-aqueous electrolyte batteries using lithium or the like as a negative electrode have high energy density and little self-discharge, and have recently attracted attention as batteries for large currents.

This non-aqueous electrolyte battery has a low electrolyte conductivity.

In order to extract a large current, it is necessary to increase the surface area of both the positive and negative electrodes, and for this reason, we have adopted a spiral electrode in which a nonwoven fabric made of polyethylene or polypropylene is interposed as a separator between the positive and negative electrodes, and these are wound in a spiral shape. are doing.

By the way, in a battery such as a spiral type battery that has a structure in which a large current flows, an abnormal current flows due to an external short circuit caused by improper use such as forced discharge.

As a result, the internal temperature may rise rapidly.

If such an internal temperature rises, if a nonwoven fabric separator is used, the separator will soften or melt in the bath, shorting the positive and negative electrodes, causing the temperature to rise further, and eventually causing a serious accident such as fire or explosion. There was a risk that it would lead to

In order to avoid such a danger, it has been proposed to use a crosslinked polyethylene porous film as a cenocrator (Japanese Patent Laid-Open No. 205048/1983).

Since the crosslinked polyethylene porous film has countless micropores, it has good ionic tetraelectricity.

In this respect, it is suitable as a separator.

This separator made of cross-linked polyenalene porous film softens or becomes tIrM! when the internal temperature of the cell increases. block the micropores, thereby blocking the movement of ions between the positive and negative electrodes, thereby blocking the current;
The purpose is to prevent excessive temperature rise.

This separator is similar to the nonwoven fabric separator in that it softens or melts when the internal temperature of the battery rises, but because it is crosslinked, it has a high viscosity when softened or melts, and has poor shape retention. This has the advantage that short circuits between the positive and negative electrodes are less likely to occur when a nonwoven fabric separator is used.

(Problems to be Solved by the Invention) However, a separator made of a crosslinked polyethylene porous film has a high melt viscosity and poor fluidity, so the speed at which the micropores close is slow.

It takes time to cut off the current, and there are difficulties in preventing excessive temperature rises.
Improvement in this point was necessary.

(Means for solving problems) The inventors of the present invention have conducted extensive studies in view of the current situation.

The inventors discovered that a separator made by laminating crosslinked and uncrosslinked polyethylene porous films has the following characteristics 11) to (3), and completed the present invention.

(1) When the internal temperature of the battery rises due to abnormal current, the uncrosslinked polyethylene porous film first melts and flows, since its melt viscosity is low.

Capable of quickly filling and closing the micropores of a crosslinked polyethylene porous film, blocking ion movement between the positive and negative electrodes, and interrupting abnormal current.

(2) Crosslinked polyethylene porous film.

It has a high viscosity when softened or melted in a bath, has excellent shape retention, and is unlikely to cause short circuits between the positive and negative poles.

(3) Above +11. +21 prevents excessive temperature rise, so there is little risk of fire or explosion.

The battery separator according to the present invention has at least two polyethylene porous films laminated together, at least one of the laminated porous films is cross-linked, and at least one is unfinished. It is characterized by being crosslinked.

The polyethylene porous film used in the present invention is:

It may be made of polyethylene of normal molecular weight, or it may have a relatively large molecular weight.

Even if it is made from polyethylene called "ultra-high molecular weight polyethylene," it is also made from a mixture of polyethylene whose molecular weight is 1 to 7 and ultra-high molecular weight polyethylene (hereinafter abbreviated as UHPE). It may be something.

These polyethylenes are commercially available in various molecular weights and are therefore easily available. for example.

UHPEu High Six Million (Mitsui Petrochemical Industries, Ltd., 1
41) and Hostalen GUR (manufactured by Hoechst) are commercially available.

Although the present invention uses polyethylene after making it porous, there are various known methods for producing polyethylene porous films. Public bulletin, equity 1185
No. 3-18553, Japanese Unexamined Patent Application Publication No. 62-212463, etc.) and Fu et al. fLfc may be used.

The physical properties of this porous polyethylene film (r1) vary depending on various settings, but the thickness is usually about 10 to 50 microns, the pore diameter is about 0.1 to 10 microns, and the porosity is about 20 to 80%.

The present invention is a laminate of at least one uncrosslinked polyethylene porous film (hereinafter abbreviated as uncrosslinked film) and at least one crosslinked polyethylene porous film (hereinafter abbreviated as crosslinked film), If the laminate contains one uncrosslinked film and one crosslinked film.

The product MI structure is not particularly limited.

Figures 1 to 3 show examples of the laminated structure of the battery separator Ic according to the present invention, in which 1 is an uncrosslinked film, and 2 is a crosslinked film laminated on the uncrosslinked film I. It is.

Although the separators shown on these sails have a two-layer or 03M structure, they can also have a multi-layer structure of four or more layers.

However, as the number of layers increases, the separator becomes thicker.
As the electrical resistance increases, the VcII pond becomes larger, so these points need to be taken into consideration.

As described above, the present invention is used as a complete crosslinked film as a component, and this crosslinked film can be obtained by chemically crosslinking or crosslinking an uncrosslinked film by irradiation with radiation (electron beam, etc.). The degree of crosslinking is determined from the point of view of shape retention when exposed to high temperatures.

Gel fraction is usually about 15% or more! 〈 shall be 30 to 90%. When crosslinking is carried out by electron beam irradiation, it is usually about 1 to
Irradiation is carried out at 100 Mrad.

Although the battery separator of the present invention has a structure in which an uncrosslinked film and a crosslinked film are laminated, it is preferable to avoid using an adhesive during lamination to ensure ionic conductivity.

According to the research of the present inventor, lamination of an uncrosslinked film and a crosslinked film is possible by laminating two films in a specific temperature range below the mA of these films (if the melting points of both films are different, the melting point of the low point film). It has been found that this can be achieved by applying pressure using V.

In this temperature range, if the melting point is x (℃), then [, (X-
40) to (x −10) ) 'C, and the caloric pressure condition is approximately 0.01 to 1 Okg/CIIL (
i pressure) theal.

When working under such conditions, the two films can be tightly integrated without causing melting of the uncrosslinked film and the crosslinked film. According to this method, the film does not melt, so
Porous films can be laminated together without the inconvenience of clogging the micropores of the films. However, the strong deterioration of the lamination between films was not so severe, and it was necessary to prevent peeling.
) K should also be handled with care after manufacture.

The separator of the present invention can be used in a state where it is interposed between a positive electrode and a negative electrode to assemble a battery in the same way as conventional separators.

In this case, the positive electrode, negative electrode, battery case,! There is no special requirement for the material of the solution solution or the arrangement structure of these components, and they may be the same as conventional ones9.
It may be as shown in the publication.

However, inside the battery, the crosslinked film and uncrosslinked film are not necessarily required to be integrated by lamination, and even if the crosslinked film and uncrosslinked film are simply overlapped. good.

(Example) Hereinafter, the present invention will be explained in more detail with reference to Examples.

Example 1 UHPE with an average molecular weight of 111 million (melting point 136°G) 1
00 parts by weight, average molecule [200,000, density 0.955
Using a composition containing 33% polyethylene (melting point 135°C), the thickness was 25μ, the pore size was 1μ, and the porosity was 60.
% uncrosslinked film.

In addition, this uncrosslinked film was irradiated with an electron beam of 30 Mrad to form a crosslinked film with a gel fraction of 40% (melting point: 136°C).
).

Next, the uncrosslinked film and the crosslinked film were stacked one by one at a temperature of 100°C and a pressure of 0.3 kg/(X (linear pressure)
The separators were laminated using a laminator under the following conditions to obtain a separator having the same structure as shown in FIG.

Note that the above gel fraction is calculated by first weighing the sample (weight W
1) Pour it into a flask and add xylene (approximately 5 Wl) to it.
000 times), heated at a temperature of 145°C for about 5 hours to dissolve the uncrosslinked components, and passed through a wire mesh of 80 mesh into a 61
Separated, the gel portion on the wire mesh was dried at a temperature of 120° C. for 2 hours, and the weight fi (W2) was weighed and calculated using the following formula (1).

2 Gel fraction (%) = -xloo... (1) Wl Example 2 UHPE with an average molecular weight of 1 million is used, and the thickness is 25μ.

Pore diameter 1μm1. An uncrosslinked film with a porosity of 60% is obtained.

It was a cabinet. This uncrosslinked film K 30 Mrad is irradiated with an electron beam to obtain a crosslinked film (IL-) with a gel fraction of 40%.

Next, the uncrosslinked film and the crosslinked film were stacked one by one, and both films were laminated under the same conditions as in Example 1 to obtain a separator.

Example 3 An uncrosslinked polyethylene film (average molecular weight: 200,000) having a thickness of 25 μm, a pore diameter of 1 μm, and a porosity of 55% is prepared.

This uncrosslinked film was irradiated with electrons M of 30 Mrad.

A crosslinked film (melting point: 135° C.) with a gel fraction of 40% is obtained.

Next, the uncrosslinked film and the crosslinked film were stacked one by one, and both films were laminated under the same conditions as in Example 1 to obtain a separator.

Comparative Example 1 A separator made of a commercially available polyethylene nonwoven fabric with a thickness of 100 μm.

Comparative Example 2 A separator consisting only of the crosslinked film used in Example 1.

Comparative Example 3 A separator consisting only of the crosslinked film used in Example 2.

Comparative Example 4 A separator consisting only of the crosslinked film used in Example 3.

A non-aqueous electrolyte battery (adopting a spiral electrode) was produced using the separators of Example P and Comparative Example.

A short circuit test (five batteries each using each separator was tested) for safety evaluation was conducted, and the results obtained are shown in FIGS. 4 to 6.

Comparative example 1 in Figure 4 and Figure 5 takes 3Kk.
'' mark indicates that all five batteries caught fire. Although not shown in Figure 1, in Comparative Example 2 in Figure 4, 14vA out of the five batteries ignited, and in Comparative Example 4 in Figure 6, two out of the five batteries ignited at a temperature of about 200°C. Therefore,
Comparative Example 2 shows data for four batteries, and Comparative Example 4 shows data for three batteries.

From FIGS. 4 to 6, the separator of the present invention and the battery using this separator do not cause any ignition phenomenon even if the temperature rapidly increases after a short circuit, and the temperature of the casing wall (maximum It can be seen that the temperature reached is also low. This is because the uncrosslinked film f# melts, closes the pores of the crosslinked film, and cuts off the current.

(Effects of the Invention) The battery separator of the present invention is configured as described above and p
Since the crosslinked film and uncrosslinked film are laminated, the pores in the crosslinked film can be reliably closed by the melted portion of the uncrosslinked film, and it has excellent resistance to excessive temperature rise.
Batteries incorporating this separator have excellent safety.

[Brief explanation of drawings]

1 to 3 are front views showing examples of the battery separator according to the present invention, and FIGS. 4 to 6 are graphs showing the characteristics of the battery according to the present invention.

Claims (3)

[Claims] (1) At least two polyethylene porous films are laminated, at least one of the laminated porous films is crosslinked, and at least one
A battery separator characterized in that each sheet is uncrosslinked. (2) The battery separator according to claim 1, wherein at least one of the porous films is made of ultra-high molecular weight polyethylene. (3) It has a positive electrode, a negative electrode, and a separator interposed between these two electrodes, the separator is made of at least two polyethylene porous films, at least one of these porous films is crosslinked, and at least A battery characterized in that one sheet is uncrosslinked.