JPH1017694A - Microporous polyethylene membrane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a microporous polyethylene membrane having excellent processability and productivity and also having such a high heat resistance that the safety of a battery can be assured even under sever conditions by making the membrane so that it may have a specified heat distortion behavior. SOLUTION: This membrane has a distortion-hardening elongation viscosity, a gel fraction of below 1% and a mean pore diameter of 0.001-0.1μm. The polyethylene used is desirably a high-density polyethylene comprising a crystalline polymer based on ethylene and may be a blend thereof with at most 30% polyolefin such as polypropylene, medium-density polyethylene, linear low-density polyethylene, low-density polyethylene, or EPR. The weight-average molecular weight of the polyethylene is desirably in the range of 200,000-1,000,000. This membrane can be desirably used as a battery separator.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a microporous polyethylene membrane suitable for a battery separator.

[0002]

2. Description of the Related Art In recent years, as represented by lithium ion batteries, batteries have been increasing in capacity. Along with this, importance has been placed on safety issues that occur during abnormalities such as short circuits. Polyethylene microporous membranes are used as separators for these high capacity batteries, especially lithium ion batteries. Polyethylene microporous membranes are used in addition to general properties such as mechanical strength and permeability,
This is because, when the inside of the battery is overheated, the separator melts to form a film covering the electrodes, thereby interrupting the current and thereby exhibiting a "fuse effect" of ensuring the safety of the battery.

In the case of a polyethylene microporous membrane, the temperature at which the fuse effect is exhibited, that is, the fuse temperature is generally about 13 ° C.
It is known that the temperature is 0 to 150 ° C., and even if the inside of the battery is overheated for some reason, the current is cut off when the fuse temperature is reached, and the battery reaction stops. However, a sufficient fuse effect may not be exhibited when the temperature rises rapidly. This is because the separator is stretched and damaged by the contraction stress generated when the separator melts and the pressure between the electrodes remaining after the melting, and the positive and negative electrodes are short-circuited (short-circuited). In order to impart high heat resistance to the separator so as to ensure the safety of the separator, a crosslinked polyethylene microporous membrane has been used.

[0004] However, the conventional techniques for cross-linking a microporous polyolefin membrane all have a problem that they involve a gel component and are difficult to process such as stretching, and lack in production efficiency. For example, JP-A-1-167344 discloses a method for producing a crosslinked polyolefin microporous membrane using a crosslinking agent. However, the polyolefin microporous membrane according to the publication contains a large amount of gel and is difficult to process such as stretching. A high-strength film cannot be obtained.

Japanese Patent Application Laid-Open Nos. 56-73856 and 3-59947 disclose cross-linking methods for microporous polyolefin membranes by ionizing radiation. In addition, processing with high energy is necessary, and there is a problem that the polyolefin generates heat during irradiation and the microporous film is melted or shrunk. Will be needed.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a microporous polyethylene membrane which is excellent in workability and productivity and has high heat resistance so that the safety of a battery can be ensured even under severe conditions. It is in.

[0007]

As a result of intensive studies to solve the above-mentioned problems, a microporous polyethylene film having a specific thermal deformation behavior has a gel fraction of 1 compared with a microporous film which does not show the same. %, It has been found that it has high heat resistance and is also excellent in workability and productivity, and has accomplished the present invention. That is, the first aspect of the present invention is a microporous polyethylene membrane having strain hardening property in elongational viscosity measurement and having a gel fraction of less than 1%, and preferably has an average pore size of 0 by a permeation method. A microporous polyethylene membrane in the range of 0.001 to 0.1 μm. A second aspect of the present invention is a battery separator using the above microporous polyethylene membrane. Further, a third aspect of the present invention is a battery using the second battery separator.

Hereinafter, the present invention will be described in detail. First, the microporous polyethylene membrane of the present invention will be described. It is not clear why a microporous polyethylene membrane having a gel fraction of less than 1% having a strain hardening property shows high heat resistance, but it is more difficult to conduct an overcharge test and a higher temperature than a normal microporous polyethylene membrane having no strain hardening property. It has become possible to greatly improve the heat resistance represented by the film rupture test. Further, the method of imparting strain hardening properties is simple without impairing the processability and productivity of the conventional film.

The elongational viscosity is a material constant that greatly affects the melt tension during elongational deformation.
It can be easily measured using a Toyo Seiki Merten Rheometer, and is usually expressed as a function of strain rate and time. The elongational viscosity of a melted ordinary gel-free polyethylene microporous membrane increases when stretched at a constant strain rate from a relatively fixed point, as shown in Figure 2, up to a certain time that depends on the strain rate And then tend to decrease rapidly as breakage approaches. This form of fracture is referred to as ductile fracture.

On the other hand, the elongational viscosity of the melted polyethylene microporous membrane of the present invention, when elongated under substantially the same conditions as shown in FIG. Shows a tendency to increase linearly or more even when it approaches, and to break at a stretch. This form of fracture is called elastic fracture. This property is indicative of strain hardening. The elongational viscosity is described in detail, for example, in Kiyoto Koyama; Journal of Rheological Society of Japan, 19, 174 (1991). The gel fraction is a value evaluated by a measuring method based on ASTM D2765. The gel fraction according to the invention is 1%
Is less than. Having a gel content of 1% or more is not preferred because processing such as stretching becomes difficult or productivity is poor.

The heat resistance of the microporous membrane of the present invention as a separator is finally evaluated by an accelerated test such as an overcharge test, an external short-circuit test, and a heating test when a battery is assembled using the same. The present inventors have studied in detail the film rupture behavior after melting, and have found that the evaluation results of these accelerated tests strongly correlate with the film rupture time in silicon oil at 160 ° C.

That is, the microporous polyethylene membrane of the present invention had a membrane rupture time of at least 20 seconds in silicone oil at 160 ° C., and the membrane passed all the above acceleration tests. On the other hand, all the conventional microporous polyethylene membranes failed in one or more accelerated tests, but the breakage time was 20 seconds or less, which corresponded well to the results of the accelerated test. That is, the polyethylene microporous membrane of the present invention
It is characterized by the rupture time in silicone oil at ℃.

As described above, the microporous polyethylene membrane of the present invention has high heat resistance. However, with respect to general physical properties other than heat resistance, an air permeability of not more than 2000 seconds in terms of 25 μm and a breaking strength of 50 μm or less are obtained.
It is 0 kg / cm ² or more, and has a performance exceeding that of a conventional microporous polyethylene membrane not only in heat resistance but also in mechanical strength and permeability. The polyethylene used in the present invention is preferably a high-density polyethylene which is a crystalline polymer mainly composed of ethylene, and further contains 30% or less of a polyolefin such as polypropylene, medium-density polyethylene, linear low-density polyethylene, low-density polyethylene, and EPR. May be blended.

The weight average molecular weight of polyethylene ranges from 100,000 to 4,000,000, preferably from 200,000 to 1,000,000, and more preferably from 200,000 to 700,000. Molecular weight 10
If it is smaller than 10,000, it tends to break during stretching, and if it is larger than 4,000,000, it becomes difficult to produce a hot solution, which is not preferable. Further, the weight average molecular weight may be adjusted to a preferable range by means such as blending of polyethylene having different molecular weights or multi-stage polymerization. The film thickness is in the range of 1 to 200 μm, preferably 10 to 50 μm. If the film thickness is less than 1 μm, the mechanical strength is not sufficient, and if it exceeds 200 μm, there is a problem in reducing the size and weight of the battery.

The porosity is 20 to 80%, preferably 30 to 80%.
When the porosity is less than 20%, the permeability is not sufficient, and when the porosity exceeds 80%, sufficient mechanical strength cannot be obtained. The average pore size is 0.001 to 0.1 μm, preferably 0.005 to 0.5 μm, and more preferably 0.
0.01 to 0.03 μm, and the average pore size is 0.00
If it is less than 1 μm, the transparency is not sufficient, and if it exceeds 0.1 μm, current interruption due to the fuse effect is delayed, and in addition, a short circuit due to precipitated dendrites or collapsed electrode active material is not preferable.

Next, the method for producing the microporous polyethylene membrane of the present invention will be described. An ordinary method for producing a microporous polyethylene membrane comprises the following three steps: a film formation step, a stretching step, and an extraction step. << film formation step >> The polymer gel, which is an intermediate product of the present invention, dissolves polyethylene in a plasticizer at a melting point or higher to form a hot solution,
It is produced by cooling it below the crystallization temperature. The plasticizer as referred to herein is an organic compound capable of forming a uniform solution with polyethylene at a temperature equal to or lower than the boiling point, and specifically, decalin, xylene, dioctyl phthalate, dibutyl phthalate, stearyl alcohol,
Oleyl alcohol, decyl alcohol, nonyl alcohol, diphenyl ether, n-decane, n-dodecane, paraffin oil and the like are mentioned. Of these, paraffin oil and dioctyl phthalate are preferred. Although the ratio of the plasticizer is not particularly limited, it is 20% to 90%, preferably 5%.
It is in the range of 0% to 70%. If it is less than 20%, it is difficult to obtain an appropriate porosity, and if it exceeds 90%, the viscosity is reduced and continuous molding becomes difficult.

Such a polymer gel is formed into a sheet having a thickness of several tens μm to several mm. The polymer gel in this state is called a raw material, and a process of manufacturing the raw material is called a film forming process. In. Although there is no particular limitation on a film forming method, for example, a high-density polyethylene powder and a plasticizer are supplied to an extruder, melt-kneaded at a temperature of about 200 ° C., and then cast from a usual hanger coat die onto a cooling roll. In this way, a film can be continuously formed.

<Stretching Step> Next, the raw film is stretched in at least one axial direction to form a stretched film. The stretching method is not particularly limited, but a tenter method, a roll method, a rolling method, or the like can be used. Of these, simultaneous 2 by the tenter method
Axial stretching is preferred. The stretching temperature is from room temperature to the melting point of the polymer gel, preferably 80 to 130 ° C, more preferably 10 to 130 ° C.
The range is from 0 to 125 ° C. Stretching ratio is 4 ~
It is 400 times, preferably 8 to 200 times, more preferably 16 to 100 times. If the stretching ratio is less than 4 times, the strength as a separator is insufficient, and if it exceeds 400 times, not only stretching is difficult but also adverse effects such as a decrease in porosity are likely to occur.

<< Extraction Step >> Next, a plasticizer is extracted and removed from the stretched film to obtain a microporous film. Although there is no particular limitation on the extraction method, when paraffin oil or dioctyl phthalate is used, it can be removed by extracting with an organic solvent such as methylene chloride or methyl ethyl ketone (MEK) and then heating and drying at a fuse temperature or lower. it can. When a low boiling point compound such as decalin is used as the plasticizer, it can be removed by heating and drying at a fuse temperature or lower. In either case, it is preferable to restrain the film in order to prevent a decrease in physical properties due to contraction of the film. In order to impart the strain hardening property, it is preferable to perform an organic peroxide treatment in the film forming step or a treatment with ionizing radiation after any of the steps.

<< Organic peroxide treatment >> A predetermined amount of an organic peroxide is added to polyethylene or a plasticizer, and the mixture is melt-kneaded under conditions such that the peroxide is not substantially decomposed. After adjusting the hot solution, the temperature of the hot solution was raised to a temperature equal to or higher than the decomposition temperature of the organic peroxide, and then a raw material subjected to a peroxide treatment was prepared by cooling the hot solution to a temperature lower than the crystallization temperature of polyethylene. Further, through a stretching step and an extraction step, a polyethylene microporous membrane having strain hardening properties can be produced.

Here, the term "peroxide does not substantially decompose" means that the active oxygen content of the peroxide is reduced to 1/2 by the time the polyethylene, the plasticizer and the organic peroxide form a uniform heat solution.
Refers to a state that does not decrease below, for example, when melt kneading takes 10 minutes, the peroxide is substantially not decomposed by melt kneading at a temperature not more than the half life of the peroxide becomes 10 minutes. A uniform hot solution can be prepared. The half-life is a time required for decomposing a 0.1 mol / l benzene solution of an organic oxide at a predetermined temperature until the amount of active oxygen becomes 1/2.

The term "organic peroxide" used herein means 150
Peroxyketal, dialkylperoxide, peroxyester, etc. having a half-life at 1 ° C. of 1 minute or more, such as α, α′-bis (t-butylperoxy) diisopropylbenzene, dicumylperoxide, ,
5-dimethyl-2,5-bis (t-butylperoxy)
Hexane, t-butylcumylperoxide, di-t-
Butylperoxide, 2,5-dimethyl-2,5-bis (t-butylperoxy) hexyne-3 and the like. The ratio of the organic peroxide is not particularly limited.
001% to 1%, preferably 0.01% to 0.5%
Range. If it is less than 0.001%, sufficient heat resistance cannot be obtained, and if it exceeds 1%, a gel component insoluble in the plasticizer is generated, and it becomes difficult to process into a uniform film.

Further, a polyfunctional monomer may be added at a ratio of 1% or less. As polyfunctional monomers,
For example, divinylbenzene, diallyl phthalate, triallyl cyanurate, triallyl isocyanurate and the like can be used. For example, a high-density polyethylene powder and a plasticizer in which an organic peroxide is dissolved are supplied to an extruder, and the mixture is melt-kneaded at a temperature equal to or higher than the melting point of polyethylene and equal to or lower than a half-life of the organic oxide of 10 minutes. The solution can be continuously formed by casting the solution from a normal hanger coat die heated to a temperature at which the half life of the organic oxide is 10 seconds or more onto a cooling roll.

<< Electron Beam Treatment >> A strain hardening property can also be imparted by performing a treatment with ionizing radiation after any of the above-mentioned conventional methods for producing a polyethylene microporous membrane.
In particular, treatment after extraction with an electron beam is preferred. The dose for performing the electron beam treatment is 0.1 to 10 Mrad, preferably 1 to 5 Mrad. If the dose is too small, the improvement in heat resistance is insufficient, and if the dose is too large, the polyethylene microporous film is heated by the energy of the electron beam, and the film is melted or shrunk. As described above, the strain hardening property can be easily provided without greatly changing the ordinary production method and productivity.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail using embodiments. The test method shown in the examples is as follows. (1) Film thickness dial gauge (Ozaki Seisakusho: PEACK No.
25). (2) Porosity A 20 cm square sample was taken, and the porosity was calculated from the volume and weight using the following equation. Porosity (%) = [volume (cm ³ ) -weight (g) /0.9]
5) / volume (cm ³ )] × 100

(3) When a 0.05 wt% aqueous solution of pullulan (manufactured by Showa Denko) is circulated under a differential pressure of 0.5 kg / cm ² , the concentration of pullulan contained in the filtrate is determined by the differential refractive index. It was determined from the measurement.
The average pore diameter d (μm) was calculated from the molecular weight M of pullulan at which the rejection became 50% and the intrinsic viscosity [η] of the aqueous solution using the following equation. [Η] M = 2.1 × 10 ²¹ ((d / 2) ² ) ^3/2 (4) Gel fraction 1 in boiling para-xylene based on ASTM D2765
The ratio of the residual mass after the extraction to the mass of the sample before the extraction was determined from the following equation based on the weight change when the soluble matter was extracted for 2 hours. Gel fraction (%) = residual mass (g) / sample mass (g) × 1
00

(5) Puncture strength Using a KES-G5 handy compression tester manufactured by Kato Tech, the radius of curvature of the needle tip is 0.5 mm, and the piercing speed is 2 m.
A piercing test was performed under the conditions of m / sec, and the maximum piercing load was defined as the piercing strength (g). Also, by multiplying the piercing strength by the film thickness (μm) / 25 (μm),
The stab strength was calculated as μ conversion. (6) Air permeability Measured with a Gurley air permeability meter according to JIS P-8117. Further, the air permeability was multiplied by the film thickness (μm) / 25 (μm) to obtain a 25 μ conversion air permeability.

(7) Extensional Viscosity After immersing the microporous membrane in silicon oil at 150 ° C. to relax the orientation, the melt extensional flow measurement device (Toyo Seiki Co., Ltd .: Melten Rheometer) was used. Strain rate 0.
The elongational viscosity at 1 / sec was measured. The presence or absence of strain hardening was determined by the form of fracture. For example, when a conventional microporous polyethylene membrane is stretched, neck-in occurs in the middle of the sample, and the elongational viscosity rapidly decreases from a certain time and breaks (ductile breakage), whereas the strain-hardened polyethylene microporous membrane breaks down. The elongational viscosity of the porous membrane continues to rise and breaks (elastic break). (8) Membrane rupture test A polyethylene microporous membrane was sandwiched between two stainless steel washers having an inner diameter of 13 mm and an outer diameter of 25 mm, and four peripheral points were clipped, and then silicone oil at 160 ° C (KF-96 manufactured by Shin-Etsu Chemical Co., Ltd.) -10CS), and the film was broken by visual observation within 20 seconds, and x was not broken.

(9) Overcharge test LiCoO ₂ was used as a positive electrode active material, graphite and acetylene black were used as conductive agents, and fluororubber was used as a binder. LiCoO ₂ : graphite: acetylene black: fluororubber = 88: 7.5 : 2.5: 2 mixture at a weight ratio of dimethylformamide paste A
(1) A sheet coated and dried on a foil is used as a positive electrode, a mixture obtained by mixing needle coke: fluororubber at a weight ratio of 95: 5 as a dimethylformamide paste is applied on a Cu foil and dried as a negative electrode, and propylene carbonate is used as an electrolyte. And butyrolactone mixed solvent (volume ratio = 1:
Using a liquid prepared by adjusting lithium borofluoride at a concentration of 1.0 M in 1), a lithium ion battery was manufactured. This battery was charged at 4.2 V for 5 hours, and then overcharged at a constant current. The internal temperature of the battery rises due to overcharging, and the current is cut off when the temperature reaches the fuse temperature. In addition,
Since this test is an acceleration test, the safety device such as a PTC element mounted on an actual battery was removed.

Example 1 40 parts of high-density polyethylene having a weight-average molecular weight of 250,000, 60 parts of paraffin oil (P350P, manufactured by Matsumura Petroleum Institute, Ltd.), and dicumyl peroxide (1)
0.2 part of a batch-type melt kneader (manufactured by Toyo Seiki Co., Ltd .: about 10 minutes at 50 ° C. and about 7 seconds at 200 ° C.)
5 at 150 ° C and 50 rpm using Labo Plastmill
Kneaded for minutes. The obtained kneaded material was molded by a 200 ° C. superheated press, heat-treated for 10 minutes, and then cooled by a water-cooled press to obtain a 1000 μm thick raw material. Simultaneously 2
Using an axial stretching machine (manufactured by Toyo Seiki Co., Ltd.) at 6 × 6 times 120 °
Then, paraffin oil was extracted and removed with methylene chloride. Table 1 shows the physical properties of the obtained microporous polyethylene membrane.

Example 2 A microporous polyethylene membrane was prepared in the same manner as in Example 1 except that 0.8 parts of dicumyl peroxide was used. Table 1 shows the physical properties of the obtained microporous polyethylene membrane. Comparative Example 1 A microporous polyethylene membrane was prepared in the same manner as in Example 1 without adding an organic peroxide. Table 1 shows the physical properties of the obtained microporous polyethylene membrane. (Comparative Example 2) An attempt was made to produce a microporous polyethylene membrane in the same manner as in Example 1 except that the addition of the organic peroxide was changed to 6 parts. However, the stretching stress was large, the membrane was broken, and a predetermined magnification was obtained. Could not be processed.

[0032]

[Table 1]

Example 3 40 parts of high-density polyethylene having a weight average molecular weight of 250,000, 60 parts of paraffin oil (Matsumura Petroleum Institute: P350P), and dicumyl peroxide 0.1 part.
Four parts were kneaded at 150 ° C. using a 35 mm twin-screw extruder, cast from a hanger coat die adjusted to 200 ° C. with a lip interval of 1400 μm on a cooling roll adjusted to 30 ° C., and a 1400 μm thick raw material was cast. It was anti. The raw fabric was stretched 7 × 7 times at 120 ° C. using a simultaneous biaxial stretching machine, and then paraffin oil was extracted and removed with methylene chloride. Table 2 shows the physical properties of the obtained polyethylene microporous membrane.

Example 4 40 parts of high-density polyethylene having a weight-average molecular weight of 250,000 and 60 parts of paraffin oil (Matsumura Petroleum Research Institute: P350P) were kneaded at 200 ° C. using a 35 mm twin-screw extruder. It was cast from a 1400 μm hanger coat die on a cooling roll adjusted to 30 ° C. to obtain a 1400 μm thick raw material. This web was stretched 7 × 7 times using a simultaneous biaxial stretching machine, and then paraffin oil was extracted and removed with methylene chloride. Thereafter, the extraction membrane was irradiated with 3 Mrad of electron beam under a nitrogen atmosphere having an oxygen concentration of 50 ppm or less. The acceleration voltage was 150 kV. Table 2 shows the physical properties of the obtained polyethylene microporous membrane. (Comparative Example 3) A microporous polyethylene membrane was prepared in the same manner as in Example 2 without adding an organic peroxide. Table 2 shows the physical properties of the obtained polyethylene microporous membrane.

[0035]

[Table 2]

[0036]

Since the microporous polyethylene membrane of the present invention has high heat resistance, the stability of the fuse state is improved especially when used as a battery separator, and the recovery of the current due to the rupture of the membrane is prevented beforehand, thereby improving the battery performance. It is possible to further enhance safety.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the time (second) and the elongational viscosity (poise) of the microporous polyethylene film having strain hardening properties described in Examples 1 and 2.

FIG. 2 is a diagram showing the relationship between time and elongational viscosity of a polyethylene microporous membrane having no strain hardening property described in Comparative Example 1.

Claims (3)

[Claims] 1. It has a strain hardening elongational viscosity and a gel fraction of 1.
% And an average pore diameter of 0.001 to 0.1 μm. 2. A battery separator using the microporous polyethylene membrane according to claim 1. 3. A battery using the battery separator according to claim 2.