JPH0294356A - Polyethylene microporous film for lithium battery separator - Google Patents

Abstract

PURPOSE:To obtain good assemble capability and low electric resistance by making of high density polyethylene having a specified weight-average molecular weight and a specified ratio of weight-average molecular weight to number-average molecular weight, and specifying the thickness, porosity, and electric resistance in an organic electrolyte. CONSTITUTION:A polyethylene microporous film 1 consists of high density polyethylene whose weight average molecular weight is 400,000-2,000,000 and the ratio of weight- average molecular weight to number-average/molecular weight (Mw/Mn) is 25 or less, and in addition its thickness is 20-50mum, porosity is 50-80%, and electric resistance in an organic electrolyte is 0.5-1.5OMEGAcm<2>/piece. If the film thickness is thicker than 50mum, the occupied ratio by the separator is increased and a battery having compact size and high energy density is difficult to obtain. If the film thickness is thinner than 20mum, the generating ratio of internal short circuit is increased. The porosity less than 50% decreases ion permeability, and that more than 80% increases the generation ratio of internal short circuit. Electric resistance more than 1.5OMEGAcm<2>/piece decreases the battery performance and that less than 0.5OMEGAcm<2>/piece increases trouble generation ratio in assembly process.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a polyethylene microporous membrane for lithium battery separators.

<Prior Art and its Problems> In recent years, with the spread of small electronic devices, there are extremely high expectations for smaller, lighter weight, and higher output batteries that serve as power sources for these devices. In order to make batteries smaller, lighter, and higher in output, it is necessary and extremely important to make the positive and negative electrodes smaller and lighter, as well as to make the separator thinner.

In particular, in the case of thinly wound lithium batteries, the electrode area is large and a large amount of separators are used, so the thickness has a large effect on the miniaturization of the battery, making it possible to make it smaller, lighter, and higher output. Therefore, there is an urgent need to develop thinner separators.

However, the thinner the separator is, the more likely it is to break, break through, or suffer from internal short circuits caused by particles such as manganese dioxide used in the positive electrode, so the separator must also have sufficient mechanical strength. There must be. Particularly when the separator is used in thinly wound lithium primary batteries, the demand for which has been increasing in recent years, mechanical properties, especially rolling resistance in the thickness direction of the membrane, are becoming a major problem in the assembly process, particularly in the winding process.

Conventionally, glass fiber separators, polypropylene nonwoven fabrics, etc. have been known as battery separators, but when these are used in thinly wound batteries, the separators end up occupying most of the volume of the battery due to their thickness. The amount of active material is reduced, and excellent battery characteristics cannot be obtained, and it is also impossible to reduce the size and weight. Furthermore, batteries constructed using these separators also have safety problems.

In other words, when the battery is short-circuited externally, even if the internal temperature of the battery rises due to Joule heat caused by the short-circuit current, the separator has fundamentally large pores (several microns), so the movement of ions cannot be prevented and the current decreases. If the battery continues to flow, the temperature inside the battery will rise and eventually gas will be generated due to the decomposition of the electrolyte, creating a risk of the battery catching fire or exploding.

On the other hand, polyolefin microporous membranes are known as thin separators. For example, as disclosed in Japanese Patent Publication No. 46-4011, a microporous membrane obtained by solution-stretching a polyolefin resin is an excellent membrane from the viewpoint of thinness, and because of its microporosity, it can be used for the above-mentioned batteries. It is safe, but has a small porosity of 30-45%.
When used as a battery separator, it has poor electrolyte impregnation properties and increases the internal resistance of the battery, so it does not have satisfactory performance as a battery separator.

In addition, in the case of polyolefin resin, inorganic fine powder, and organic liquid, the microporous membrane obtained by extracting and removing the organic liquid and inorganic fine powder from the film-formed body has a large porosity and is therefore used as a battery separator. It has good impregnability with electrolyte and low internal resistance of batteries, giving it excellent performance as a battery separator. However, improvements in the separator are desired from the viewpoint of mechanical properties.

That is, as disclosed in JP-A No. 58-59072, for example, the mechanical strength of the separator is not sufficiently high, so problems such as the occurrence of defective products remain in the battery assembly process. There is.

In particular, when the separator is used as a separator for a thinly wound battery, internal short circuits caused by tearing or tearing during the winding process or by being pressed by particles such as manganese dioxide of the positive electrode are serious problems.

<Means and effects for solving the problems> The present invention has been made in order to provide a polyethylene microporous membrane for battery separators that does not have the above problems and has good assembly workability and low electrical resistance. .

That is, the present invention is made of high-density polyethylene having a weight average molecular weight of 400,000 to 2,000,000 and a weight average molecular weight/number average molecular weight (Hw/Mn) ratio of 25 or less, a thickness of 20 to 50 μm, and a pore size. This is a polyethylene microporous membrane for a lithium battery separator, which has an electric resistance of 50 to 80% and an electrical resistance in an organic electrolyte of 0.5 to 1.5 Ωci/sheet.

The thickness of the microporous membrane in the present invention is 20 to 50μ. A separator of 20 to 38 microns is more preferable for a separator for small, lightweight batteries and high-output spiral-wound batteries. Film thickness is 5
If it is thicker than 0μ, the separator occupies at least 10% of the internal volume of the battery, which hinders miniaturization and higher energy storage of the battery. Moreover, if the film thickness becomes thinner than 20 μm, the probability of occurrence of internal short circuits due to tearing and pressing becomes high.

The porosity as referred to in the present invention is expressed by the following formula.

Porosity = pore volume / porous membrane volume x 100 pore volume = water content - bone dry weight The porosity of the microporous membrane of the present invention must be 50 to 80%, preferably 60 to 80%. .

If the porosity is less than 50%, a membrane with excellent permeability such as ion permeability cannot be obtained. In particular, when the microporous membrane of the present invention is used as a separator in a lithium battery, the porosity has a large effect on the internal resistance of the battery due to the low ionic conductivity of the electrolyte, and if the porosity is small, the characteristics This will give you a very bad battery. Furthermore, when the porosity is 80% or more, the occurrence of problems such as puncture, tearing, and internal short circuits due to pressing becomes uniform.

The electrical resistance of the microporous membrane of the present invention in an organic electrolyte is 0.5
It needs to be ~1.5ΩcrA/sheet.

If the electrical resistance is 1.5 ΩcrA/sheet or more, a battery with excellent battery characteristics cannot be obtained. Also, the electrical resistance is 0.5Ωci/
If it is less than 500 ml, a battery with excellent characteristics can be obtained, but from the viewpoint of the membrane, the membrane is thin, has a high porosity, and has a large pore diameter, which inevitably increases the chance of trouble occurring during the assembly process. The organic electrolyte herein refers to a mixture of propylene carbonate and 1,2-dimethoxyethane in which lithium perchlorate is dissolved.

The weight average molecular weight of the polyethylene microporous membrane of the present invention is 4
It needs to be between 0 and 2 million.

Those with a weight average molecular weight of less than 400,000 have excellent porosity, but the occurrence of problems such as tearing, tearing, and internal short circuits due to pressing due to insufficient mechanical strength is not reduced. Furthermore, if the weight average molecular weight is greater than 2,000,000, the fluidity is poor and it is difficult to form a thin film, and the pore size is also small, less than 0.05 μm, and the porosity is reduced, resulting in only poor battery performance.

The ratio of weight average molecular weight to number average molecular weight (Miy/Mn) of the polyethylene microporous membrane of the present invention needs to be 25 or less, preferably 15-5.

The number average molecular weight of polyethylene mainly affects the mechanical properties of the resulting porous membrane, and the weight average molecular weight mainly affects the average pore diameter and porosity of the porous membrane.

When polyethylene with a Mw/Mn ratio of 25 or more is used, the microporous membrane obtained has low elongation and becomes a floppy membrane.

The film produced using this high-density polyethylene with a weight average molecular weight of 400,000 to 2 million and h/Mn of 25 or less, and subjected to the stretching process described below, is surprisingly superior to conventional polyethylene microporous films. The incidence of trouble during battery assembly was around 10% when used, but this has been drastically reduced to less than 1%.

In the present invention, polyethylene is a crystalline homopolymer of ethylene, or ethylene and 10 mol% or less of propylene, 1-butene, 4-methyl-1-
Examples include copolymers with pentene and 1-hexene.

Furthermore, there is no problem in mixing polyethylenes having different molecular weights to have a weight average molecular weight of 400,000 to 2,000,000.

In addition, the above polyethylene may contain antioxidants and
Various additives such as ultraviolet absorbers, lubricants, and anti-blocking agents can be added within the range that does not impair the purpose of the present invention.

The average pore diameter of the microporous membrane of the present invention is in the range of 0.05 to 0.5μ and has a narrow pore diameter distribution.

As one of the manufacturing methods for obtaining the microporous membrane of the present invention,
As disclosed in Japanese Patent No. 37292, after forming a film by mixing a polyethylene resin with an inorganic fine powder such as silica and an organic liquid such as mineral oil or dioctyl phthalate, the inorganic fine powder and organic liquid are extracted and removed, and then stretched. One example is a method in which this is carried out under specific conditions.

The organic liquid and the inorganic fine powder are allowed to remain in the microporous membrane after the extraction of the organic liquid and the inorganic fine powder to the extent that they do not impair the performance of the membrane. The residual amount of the organic liquid in the microporous membrane is 3% by volume or less, preferably 2% by volume or less, and the residual amount of the inorganic fine powder in the microporous membrane is 3% by volume or less, preferably 2% by volume. It is as follows. That is, (1) Use a raw film containing 20 to 50% by volume of polyethylene resin based on the total amount of polyethylene resin, inorganic fine powder, and organic liquid. ■When stretching after extracting and removing inorganic fine powder and organic liquid, the stretching ratio is 1.
Stretch it so that it becomes 5 times or more.

Under the above conditions, a microporous polyethylene membrane satisfying the conditions of the present invention is obtained.

A specific example of the stretching ratio is 1.5 to 5.0 in uniaxial stretching.
The range is preferably 2.0 times to 3.5 times, more preferably 2.0 times to 3.5 times.
It's double.

The microporous membrane obtained by this method has excellent mechanical properties and high porosity, so it can be used as a separator for lithium batteries, especially high-output thinly wound lithium batteries that have been rapidly growing in recent years. The occurrence of trouble in the winding process, which was a major problem in the battery assembly process, has been reduced, improving yield efficiency, and in terms of battery performance as a product, it has become possible to be smaller and lighter, lower internal resistance, and increase output. Extremely useful. When this separator is used in a lithium battery, the negative electrode Li, the positive electrode (:gO, M
No troubles such as breakage were observed in button-type batteries made of n0z or the like. Furthermore, when this separator was used in a spiral-wound battery consisting of a negative electrode LifI3 and a positive electrode Mud2, very favorable results were obtained from the viewpoints of a significant reduction in the occurrence of trouble in the winding process, a reduction in size and weight, and furthermore, an increase in output.

In general, it is thought that the thinner the separator film thickness and the higher the porosity, the higher the probability of troubles caused by puncturing, tearing, etc. The number of short circuits caused by these factors has decreased significantly. Therefore, the microporous membrane for lithium battery separators of the present invention is extremely useful in efficiently providing small, lightweight, and high-output batteries.

〔Example〕

The present invention will be explained in more detail below using Examples and Comparative Examples.

The test method in the examples is as follows.

(1) Film thickness: Read with a dial gauge (minimum scale: 1μ).

(2) Maximum pore diameter! Compliant with ASTM E-128-61.

Bubble point in ethanol Calculated from

(3) Porosity: (pore volume/porous membrane volume) xlOO
Pore volume Pore volume Weight Water content Weight (4) Air permeability: Based on JIS-P-8117.

(5) Electrical resistance 2 Model used: Ando Electric LCR
Meter AG-4311 type assembly - As shown in Figure 1 (6) Weight average molecular weight (Mw): F'fh ÷ station (
7) Viscosity average molecular weight UV): Measured using a solvent (decalin) at a measurement temperature of 135°C [η] = 6.2
angO formula) % formula % Column: Tosoh G700S-G Solvent: Trichlorobenzene measurement temperature: Measured at 135°C.

Example 1 13% by volume of finely divided silicic acid and 61.5% by volume of dioctyl phthalate were mixed in a Henschel mixer, 25.5% by volume of a polyethylene resin having a weight average molecular weight of 600,000 and Mw/Mn = 15 was added thereto, and the mixture was mixed again with a Henschel mixer. Mixed with a mixer.

The mixture was kneaded into pellets using a 30 m/m φ twin screw extruder. The pellets were molded into a film with a thickness of 100 μm using a film manufacturing apparatus equipped with a 30 ml φ twin screw extruder equipped with a T-die having a width of 450 m/m. The formed membrane is 1
, 1.1-) Soaked in dichloroethane for 5 minutes, DoP
was extracted and dried, and further immersed in 20% caustic soda at 70°C for 30 minutes to extract fine powder silicic acid and then dried.

Next, the porous membrane was stretched 2.7 times in the vertical direction using a roll stretching machine heated to 115°C, and then stretched at 120°C.
Heat treatment was performed for 10 seconds in an atmosphere of °C. The properties of the obtained film are shown in Table 1.

Next, a separator for lithium batteries using this microporous membrane was subjected to a general automatic winding machine for capacitors together with lithium negative electrode and manganese dioxide positive electrode, and the results are shown in Table 1.

Further, using this separator, a thinly wound battery as shown in FIG. 2 was prepared.

Table 1 shows the number of pulse discharges at 20°C for this battery.
Shown below.

Example 2 The same procedure as in Example 1 was conducted except that polyethylene with a weight average molecular weight of 400,000 and Mw/Mn=20 was used and stretched 3 times in the longitudinal direction. The results are shown in Table 1. Example 3 Example 1 except that polyethylene with a weight average molecular weight of 1.2 million and Mw/Mn = 15, liquid paraffin was used in place of dioctyl phthalate, and stretched 2.3 times in the longitudinal direction. I did the same thing. The results are shown in Table-1.

Comparative Example 1 Example 1 except that polyethylene with a weight average molecular weight of 200,000 and Mi4/Mn=13 was used and stretched 4 times in the vertical direction.
I did the same thing. The results are shown in Table-1.

Comparative Example 2 The same procedure as in Example 1 was carried out, except that polyethylene with a weight average molecular weight of 200,000 and Mw/Mn = 13 was used, the polyethylene amount was 38.4% by volume, and the polyethylene was stretched 4.5 times in the longitudinal direction. The results are shown in Table-1.

Comparative Example 3 The same procedure as in Example 1 was carried out except that the film was stretched twice in the vertical direction and twice in the horizontal direction. The results are shown in Table 1.

Comparative Example 4 Polyethylene with a weight average molecular weight of 200,000 and law/Mn = 13 was used, except that it was stretched 2.5 times in the vertical direction.
The same procedure as in Example 1 was carried out. The results are shown in Table-1.

Comparative Example 5 The same procedure as in Example 1 was conducted except that polyethylene with a weight average molecular weight of 1.2 million and Mw/Mn = 14 was used and stretched twice in the longitudinal direction. The results are shown in Table-1.

Example 4 The same procedure as in Example 1 was carried out, except that polyethylene with a weight average molecular weight of 1.8 million and Mw/Mn = 20, liquid paraffin was used instead of dioctyl phthalate, and the film was stretched 2.0 times in the longitudinal direction. The results are shown in Table-1.

Comparative Example 6 The same procedure as in Example 1 was conducted except that polyethylene with a weight average molecular weight of 600,000 and Mw/Mn = 30 was used and stretched 2.7 times in the longitudinal direction. The results are shown in Table-1.

〔Effect of the invention〕

According to the present invention, it is made of polyethylene with a weight average molecular weight of 400,000 to 2,000,000, has a thickness of 20 to 50 μm, and has a porosity of 50 μm.
-80% and electrical resistance in organic electrolyte is 0.5-1
．． By using a polyethylene microporous membrane with a resistance of 5 Ωd/sheet for lithium battery separators, it is possible to improve the yield rate in the assembly process, and in terms of battery performance, it is possible to achieve smaller size, lighter weight, and higher output. .

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of the assembly for measuring the electrical resistance of the membrane of the present invention, and FIG. 2 is a half-sectional view of a spiral-wound battery using the membrane produced in Example 1. 1 is a positive electrode, 2 is a microporous membrane, 3 is a negative electrode, 4 is an insulating plate, 5 is a negative electrode lead, 6 is a positive electrode lead, and 7 is a gasket. Patent applicant: Asahi Kasei Industries, Ltd.

Claims (1)

[Claims] Made of high-density polyethylene with a weight average molecular weight of 400,000 to 2,000,000 and a weight average molecular weight/number average molecular weight (■_w/■_n) ratio of 25 or less, with a thickness of 20 to 50μ,
Polyethylene microporous membrane for lithium battery separator having a porosity of 50 to 80% and an electrical resistance in an organic electrolyte of 0.5 to 1.5 Ωcm^2/sheet