JP2001035468A - Polyolefin porous membrane on which inorganic thin film is formed and method for producing the same - Google Patents

Abstract

(57) [Problem] To provide a shutdown temperature range of a polyolefin porous membrane having a narrow shutdown temperature range, without remarkably changing the characteristics as a porous membrane. SOLUTION: A porous film formed by forming an inorganic thin film on at least one surface of a porous film made of a heat-meltable polyolefin by using a vacuum film forming method.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a porous polyolefin membrane exhibiting a wide shutdown temperature range and a method for producing the same, as a separator of a battery, particularly a non-aqueous electrolyte secondary battery for which improvement in safety is required.

[0002]

2. Description of the Related Art In recent years, with the development of electronic portable devices, batteries with high energy density and high electromotive force have been developed. Among them, from the viewpoint of high electromotive force, non-aqueous electrolyte batteries, particularly lithium ion secondary batteries, have been vigorously researched and developed. It is pointed out that one of the problems of such non-aqueous electrolyte batteries is the danger of using a flammable organic solvent. If both electrodes of the battery are short-circuited or cause a decomposition reaction of the battery contents, etc., the battery contents may squirt or
Or explode. As countermeasures against such problems, mounting of a safety valve, provision of a shutdown function by a separator containing a fusible component, and the like are given as current measures.

[0003] However, safety valves are not an essential safeguard against short circuits, they only mitigate sudden pressure increases inside the battery.

On the other hand, the separator has a shutdown function by using a porous membrane made of a heat-meltable material by using a separator, and when the temperature inside the battery reaches a certain temperature due to a short circuit or the like, the porous material is melted by heat. By closing the holes in the membrane, the ionic conductivity is suppressed, and the battery reaction that causes heat generation is suppressed. Such a separator is disclosed in Japanese Patent Publication No. 2642206,
JP-A-6-212006, JP-A-8-13864
Patent Document 3 discloses a porous film of a polyolefin-based polymer material. However, when such a heat-fusible material is used, even if the shutdown function works due to a rise in heat, the film itself melts with a further rise in temperature, isolating both electrodes of the battery, which is the original function as a separator. Function is lost. This is a phenomenon called meltdown, which is not preferable as battery characteristics. As a solution to such a problem, these patents disclose a technique of setting the meltdown temperature to a higher temperature by specifying the polyolefin composition and the film forming method, but there is also a technique that does not necessarily have sufficient characteristics. It can not be said.

As another method for widening the range of the shutdown temperature, Japanese Patent Publication No. 4-1692, Japanese Patent Application Laid-Open No. 60-52, Japanese Patent Application Laid-Open No. 61-232560,
JP-A-3-291848, JP-A-10-6453
As disclosed in Japanese Unexamined Patent Publication (Kokai) No. H10-209, there is also a technique in which a heat-fusible material is laminated on a porous film or a nonwoven fabric substrate, modified by coating, or the like. However, these manufacturing methods may be complicated, and a method having sufficient insulating properties at the time of shutdown is not always obtained.

[0006] In addition, Japanese Patent Application Laid-Open No. 9-12575 discloses a technique in which the surface of a polyethylene porous membrane is irradiated with radiation to extend a shutdown temperature range and improve mechanical strength. The temperature is below about 170 ° C. Further, since polyethylene modified by radiation treatment usually shows a crosslinked structure, although the tensile elasticity is improved, the strength at break may tend to be reduced, so that a so-called brittle film is easily formed.

[0007]

The property of melting at a certain temperature to block the pores of the porous film and maintaining the continuous film state at a high temperature without melting as such is a characteristic of the material. The properties are inconsistent with each other, and it is difficult to produce such a separator by an easy means. We conducted a sincere study on such issues, and found that by laminating an inorganic thin film on a heat-meltable polyolefin porous film using a vacuum film forming method, the shutdown temperature of the polyolefin porous film, which originally had a narrow shutdown temperature range, was It turns out that the range can be greatly expanded.

As a similar technique, Japanese Patent Application Laid-Open No. 1-304933 discloses a technique in which a polysiloxane is applied to a polyolefin porous membrane. The purpose of this is to reduce the internal short-circuit failure rate of the battery by improving the mechanical strength of the separator. However,
The technical scope of this publication mainly includes an electrolytic capacitor and an electric double layer capacitor, and its shutdown characteristics have not been studied. Rather, when a polyolefin porous membrane that is easy to shut down is used as a substrate, the pores of the porous membrane may be blocked by the prolonged curing time of the polysiloxane, and good ionic conductivity of the separator may be obtained. There are problems that hinder it.

Japanese Patent Application Laid-Open No. 10-172531 discloses a technique of forming an inorganic thin film on a surface of a pore portion of a polyolefin porous film by spraying or dipping a sol liquid. The purpose here is also to reduce the internal short circuit rate by increasing the mechanical strength. Therefore, improvement of the shutdown temperature range has not been considered. Further, in this case, since the inner surface of the pores is substantially entirely covered with the inorganic thin film, when the film thickness of the inorganic thin film is increased, the heat resistance of the polyolefin porous film is improved, and particularly, the shutdown start temperature is expected to increase. . As a result, the temperature inside the battery becomes too high, which is not advantageous in terms of safety. In the examples of this publication, a wider shutdown temperature range is shown as compared with Comparative Example 1, but this is a technique achieved by mixing polypropylene and polyethylene having different shutdown temperature ranges. The range does not mean that the shutdown temperature range has expanded.

An object of the present invention is to provide a polyolefin porous membrane having a narrow shutdown temperature range, which has a narrow shutdown temperature range, without significantly changing the characteristics of the porous membrane.

Another object of the present invention is to provide a porous membrane having excellent shutdown characteristics, which is useful as a battery separator.

[0012]

According to the present invention, an inorganic thin film is formed on at least one surface of a porous film made of a hot-melt polyolefin, and substantially formed on the inner surface of a pore. This is achieved by a polyolefin porous membrane on which an inorganic thin film is formed. This makes it possible to provide a porous film having a wide shutdown temperature range required for nonaqueous electrolyte batteries and the like.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The porous polyolefin film of the present invention on which an inorganic thin film is formed has an inorganic thin film formed on at least one surface of a porous film made of a hot-melt polyolefin.

Here, the heat-meltable polyolefin is an aliphatic hydrocarbon-based polymer that remarkably melts at a certain temperature, and any polyolefin exhibiting such properties is
Any of linear, branched, and cross-linked structures may be used. As more preferable polyolefin, polyethylene, polypropylene or a copolymer or a mixture thereof is selected in view of remarkable melting property and chemical stability.

As will be described later, the polyolefin porous membrane on which the inorganic thin film of the present invention is formed melts at 110.degree. C., preferably 120.degree. C. or higher, and is particularly useful as a battery separator due to such characteristics. . Therefore, the molecular weight of the polyolefin in the present invention,
What is necessary is just to select within the range which satisfies the above-mentioned melting characteristics, in consideration of the handleability, moldability, mechanical characteristics, etc. of the porous film. For example, when using polyethylene as the polyolefin,
The weight average molecular weight is preferably in the range of 15,000 to 10,000,000. 1
If it is lower than 55,000, the melting temperature may be lower than 110 ° C. On the other hand, if the amount is more than 10 million, the formability of the porous film is deteriorated, and the speed at which shutdown is started (melting speed) becomes extremely slow, and the shutdown start temperature becomes substantially high. The melting temperature of the polyolefin is 150 ° C. (for polyethylene).
The following is preferred.

The heat-meltable polyolefin porous film of the present invention can be formed into a porous film by using the above-mentioned heat-meltable polyolefin by various methods. The production method is not particularly limited. For example, the 7th Polymer Material Forum (1998) Abstract 1BIL09
And the like, roughly describe two types, namely, a stretching opening method and a phase separation method. In the stretch opening method, heat treatment is applied to a continuous film of polyolefin, microcrystals of the polyolefin are grown inside the continuous film, and the film is stretched in that state to form voids held by polyolefin fine fibers between the crystals. is there. In the phase separation method (wet method), a polyolefin is put into a solution state, a phase separation film or a fine particle dispersion film structure is formed by a solution casting method, and a phase separation agent or fine particles are eluted and stretched before or after this. To form a porous film. Such a porous membrane may be further stretched and used.

[0017] A porous film produced using such a material has a myriad of fine pores having an average diameter of about 10 nm to 10 µm formed in the film thickness direction, so that a solution or gas can easily pass through the film. Refers to a film that can move to

As such a porous membrane, for example, 25 μm
A m-thick film having a basis weight of 8 to 17 g / m ² can be preferably used.

The thickness of the porous film is 5 to 100.
μm is preferred. If the film thickness is less than 5 μm, the mechanical strength of the film is not sufficient, and even if a polyolefin porous film having a high molecular weight and a high degree of stretching is used, in actual handling, holes are easily opened and the film is broken. This causes problems such as easy internal short-circuiting when assembled in a battery. On the other hand, when the film thickness is greater than 100 μm, the two electrodes of the battery are too far apart, the internal resistance of the battery increases, the charge / discharge speed decreases, and the electric capacity of the battery per unit volume decreases. Becomes The thickness of the porous film is more preferably from 10 to 50 μm.

In the present invention, an inorganic thin film is formed on at least one surface of the porous film made of the heat-meltable polyolefin, and is not substantially formed on the inner surface of the pore. In other words, the polyolefin porous film on which the inorganic thin film of the present invention is formed is a porous film made of a heat-meltable polyolefin, and the inorganic thin film is formed on at least one macroscopic film surface excluding the inside of the pores. I have. Here, the macroscopic film surface excluding the inside of the pores is different from the pore portion surface described in JP-A-10-172531, and is a macroscopic surface of a porous film assumed as a uniform continuous film. That is. And not all surfaces inside the holes are covered with the inorganic thin film.

However, more finely, an inorganic thin film may exist on the surface of the pores very close to the surface of the porous film. In the thickness direction of the polyolefin porous film, most of the inorganic thin film is distributed on the surface, rapidly decreases in the thickness direction, and hardly exists inside the film (inside of the pores). Even if present, it is not essentially thick enough to significantly alter the mechanical and temperature properties inside the pores.

As described above, the macroscopic surface is covered with the inorganic thin film, but since the inorganic thin film does not substantially exist on the surface inside the pores of the porous film, the inorganic thin film is dissolved at the melting temperature of the polyolefin, The original shutdown start temperature of the porous film before formation can be maintained.

The term "at least one surface" means that an inorganic thin film may be formed on only one side of the porous film, or may be formed on both surfaces. Usually easy to manufacture,
It is better to form an inorganic thin film on one side only from the viewpoint of economy, but in this case, depending on the thickness of the inorganic thin film, the inorganic thin film-formed porous film is greatly curled due to the in-plane stress of the inorganic thin film, and the handleability is reduced. May be.

On the other hand, when the inorganic thin film is formed on both surfaces, such curling is suppressed. However, since an inorganic thin film is formed on each side, productivity and economy are inferior.

The thickness of such an inorganic thin film is 10 nm.
It is preferably at least 200 nm. When the thickness is smaller than 10 nm, the effect of improving the shutdown performance cannot be remarkably observed. On the other hand, when the thickness is larger than 200 nm, the flexibility of the inorganic thin film itself becomes poor, and the inorganic thin film is likely to be cracked or peeled off after formation, which is not preferable. The more preferable range of this film thickness is 20 nm or more and 100 nm or less.

As a main component of such an inorganic thin film, it is possible to use an electrically conductive material to improve the current collection of the electrodes independently of both electrodes. It is preferable to use an inorganic oxide having a surface resistance of 1 TΩ or more from the viewpoint of suppressing internal short circuit of the battery.

That is, an insulating inorganic oxide is preferable.
In order to electrically isolate both electrodes of the battery, a component having good electronic conductivity through a porous membrane but not having electronic conductivity is generally not used as a component of a separator. And various inorganic thin films are mentioned as the composition, but an inorganic oxide is preferable from a viewpoint of insulating property, economical efficiency, and productivity. Examples of such an inorganic oxide include silicon oxide, magnesium oxide, aluminum oxide, calcium oxide, titanium oxide, zinc oxide, and tin oxide. A single composition may be selected from these, or a composite oxide having a plurality of compositions selected from these may be used.

Among such oxides, silicon oxide, aluminum oxide and magnesium oxide are particularly preferred from the viewpoint of economy and productivity.

The method of forming the inorganic thin film is described in JP-A-1-304933 and JP-A-10-17.
By using a vacuum film forming method instead of a method of forming a film by performing coating and heat treatment as described in Japanese Patent No. 2531, an inorganic thin film is present substantially only on a macroscopic surface of a porous film. Therefore, it is easy to control that the pores are not substantially formed on the inner surface, and the effect of improving the shutdown characteristics can be more simply and easily exhibited.

The vacuum film forming method referred to here is a method in which a porous film of a heat-fusible polyolefin is arranged in a vacuum and an inorganic thin film composition is grown on the surface thereof. As such a method, a vacuum evaporation method, a sputtering method, and a CVD (chemical vapor deposition) method are preferable from the viewpoint of productivity.

In laminating such an inorganic thin film, a known surface pretreatment such as a corona discharge treatment or a sputter etching treatment may be performed from the viewpoint of adhesion.

By the way, usually, in an environment where a battery is used,
For example, 100 on the dashboard inside the car in midsummer
It is said to rise to about ° C. If the shutdown start temperature is lower than 100 ° C., even if the battery is operating normally, the separator may shut down, the ionic conductivity inside the battery may be reduced, and the battery may not operate. From such a viewpoint, the melting temperature of the polyolefin porous membrane that can be used here is preferably 110 ° C. or higher as described above.

In the vacuum film forming method, an inorganic thin film source is usually formed on a porous film while being heated in a vacuum.
If the porous film having such a melting temperature characteristic is formed directly on the surface of the porous film by the vacuum film forming method, the porous film may be easily melted by such heat. This phenomenon becomes remarkable as the thickness of the formed inorganic thin film increases.

Therefore, as described above, the formation of the inorganic thin film having a thickness necessary for effectively exhibiting the shutdown performance requires the melting temperature Tm of the porous film to be 30 ° C.
Tc = Tm−30 ° C. or less (Tr = Tr)
It is preferable to control the surface temperature of the porous membrane so that ≦ Tc). This temperature also means that the temperature at the time of forming the inorganic thin film having the highest surface temperature is similarly controlled.

Usually, a polyolefin-based porous film is made highly porous by stretching at a high degree, or the film strength is improved. Therefore, when Tr is at a temperature higher than Tc, even if the porous film itself does not melt, large heat shrinkage occurs, and the dimensions, ionic conductivity, mechanical properties, and thermal properties of the original porous film itself May be greatly changed.

In order to maintain the dimensions, ionic conductivity, and mechanical strength of the porous film before forming the inorganic thin film, it is particularly important to control the surface temperature of the porous film when forming the inorganic thin film.

As described above, the present invention provides a battery, particularly a safety device, by forming an inorganic thin film on at least one surface of a porous film made of a heat-meltable polyolefin, preferably by using a vacuum film forming method. A porous membrane having a wide shutdown temperature range can be provided as a separator for a nonaqueous electrolyte secondary battery for which improvement is desired.

[0038]

EXAMPLES Various evaluations described in the following examples and comparative examples were performed in the following manner. When forming the inorganic thin film on the polyolefin porous film, a temperature indication label was attached to a part of the porous film in advance, and the substrate temperature was controlled so that the surface temperature of the porous film was in the range of the above conditions. .

The polyolefin porous membrane on which the inorganic thin film of the present invention has been formed is characterized in that the thickness of the inorganic thin film is determined by the thickness of the inorganic thin film in addition to the basic properties of the separator such as film thickness, ionic conductivity, mechanical strength and shutdown temperature characteristics. It was evaluated by X-ray measurement.

Evaluations of ionic conductivity and mechanical strength were performed to confirm that the characteristics did not significantly change before and after the formation of the inorganic thin film.

The ionic conductivity was calculated by sufficiently impregnating a porous membrane prepared with an organic electrolyte solution and obtaining a resistance value at this time.

The mechanical strength was evaluated by obtaining a tensile modulus and a breaking strength by performing a tensile test, and evaluating a piercing strength by a piercing test.

The thickness of the porous film was measured in μm using a known micrometer.

The average pore diameter of the porous membrane is determined according to JIS K38.
The measurement was performed with an automatic pore measuring device manufactured by Porous Materials Co., Ltd., which was measured by the bubble point method described in No. 32.

The ionic conductivity was measured as follows. First, an organic electrolyte was prepared by dissolving 1 mol / l of lithium tetrafluoroborate in an organic solvent in which propylene carbonate and ethylene carbonate were mixed in an equal weight ratio.

On the other hand, the produced porous membrane was subjected to a surfactant treatment in advance, and was treated so as to be sufficiently wetted with the prepared organic electrolyte solution. The surfactant used was a nonionic surfactant (polyoxyethylene cetyl ether). A methanol solution containing 3% by weight of this surfactant was used as a treatment liquid, and the porous membrane to be evaluated was immersed in this treatment liquid. ,
Next, excess liquid was wiped off and dried to give an affinity for the electrolytic solution.

After sufficiently penetrating such a porous membrane into the previously prepared organic electrolytic solution, it is sandwiched between disc-shaped stainless steel electrodes having a radius of 1 cm, and complexed between 100 kHz and 1 Hz with a Solartron 1260 type impedance analyzer. The impedance was measured, and the measured data curve on the high frequency side was extrapolated to the real axis to obtain the actually measured resistance value.
Then, the electrical conductivity per unit area was calculated from the reciprocal of this measured value, and this was defined as ionic conductivity. Tensile test is J
The test was performed at a rate of 10 mm / min in an environment of 25 ° C. using an Instron type tensile tester with reference to IS standard K7127. The measurement sample was cut out to have a length of 150 mm and a width of 10 mm, and a tensile test was performed at intervals of 100 mm. The measured values were expressed as tensile modulus and breaking strength.

The piercing test was carried out on page 49 by Adachi in "Summary of Lithium Ion Battery Separators" in the 28th Semi-Conference "Expected Lithium Secondary Battery" Abstracts. Was evaluated using a handy type compression tester manufactured by Kato Tech Co., Ltd.

The shutdown temperature characteristic is described in Japanese Patent No. 2642.
According to the method of measuring the film resistance described in Japanese Patent No. 206,
The measurement was performed using a measuring device. The measurement conditions were the same as in the above-described ion conductivity evaluation, by dissolving lithium tetrafluoroborate in an organic electrolyte (an organic solvent obtained by mixing propylene carbonate and ethylene carbonate in an equal weight ratio) to a concentration of 1 mol / liter. Organic electrolyte solution), an AC frequency of 1 kHz, an AC amplitude of 100 mV,
The measurement was performed at a heating rate of 2 ° C./min. When the temperature is raised, the impedance value rises during the
The temperature at the time when the resistance becomes 0 Ω square cm is the shutdown start temperature Ts, and the temperature at the time when the impedance value is reduced to 1 kΩ square cm after the impedance value has increased from 1 kΩ square cm due to the shutdown is the meltdown temperature, that is, The shutdown end temperature Te was set.
When the temperature reaches 190 ° C., the impedance value becomes 1 k.
In the case of Ω square cm or more, the shutdown end temperature Te was 190 ° C. or more.

The melting temperature of the polyolefin porous membrane is DS
C (differential thermal analysis). In the measurement of the melting temperature using DSC, the endothermic peak temperature measured at 100 ° C. or higher when the temperature is measured from room temperature at a heating rate of 20 ° C./min by using a known DSC measuring device is determined. The melting temperature was used.

The measurement of the thickness of the inorganic thin film by the fluorescent X-ray measurement was performed as follows. An inorganic thin film was previously formed on the surface of a 100 μm polyethylene terephthalate film under various conditions. First, for each inorganic thin film, the fluorescent X
The line peak intensity was measured. Next, the measured thickness of the inorganic thin film was physically determined from the cross-sectional observation of the film with a known scanning electron microscope. Thereby, a correspondence diagram of physical film thickness-fluorescent X-ray peak intensity was created as a calibration curve. For the inorganic thin film on the porous film according to the present invention, the fluorescent X-ray peak intensity was similarly obtained, and the film thickness was calculated from the prepared calibration curve.

The formation of the inorganic thin film described in each of the following examples and comparative examples was performed in the following manner. An inorganic thin film made of silicon oxide was formed on one surface of the porous film as follows. The porous film was set in a take-up type magnetron sputtering apparatus and evacuated to 0.02 mTorr. At this time, the target has a length of 150 mm, a width of 390 mm,
A boron-doped silicon target with a thickness of 5 mm was used. Thereafter, 50 sccm of an argon / oxygen mixed gas (argon: oxygen = 70: 30 by volume ratio) was introduced to reduce the pressure to 0.6 m.
After maintaining the pressure at Torr, the temperature of the main roll was controlled to 10 ° C. Then, sputtering was performed with a discharge power set to 2.4 kW as a film formation condition.
At this time, the thickness of the silicon oxide film formed by changing the moving speed of the porous film was changed.

With respect to the surface resistance of the formed inorganic thin film-formed porous film, a silicon oxide film was formed by sputtering on a 100 μm polyethylene terephthalate film under the same conditions, and a continuous film was formed. Keithley insulation resistance testers 6157A and 800
The measurement was carried out in a 25 ° C. 0% RH environment using a type 9 resistivity chamber, which was substituted. As a result, it was confirmed that the thickness was 30 TΩ or more in the film thickness range of this example.

Further, this porous film (sometimes referred to as an inorganic thin film-formed porous film) is subjected to freeze-fracture, and the distribution of silicon amount in the film thickness direction is measured by a known energy dispersive X-ray analyzer. did.

[Comparative Example 1] A porous film made of ultrahigh molecular weight polyethylene having a weight average molecular weight of 1.2 million (thickness: 25 μm, basis weight: 11 g / m ² , average pore diameter: 0) .32 μmφ). Each of the above properties was evaluated without forming an inorganic thin film on the surface of the porous film. Table 1 shows the evaluation results. FIG. 1 shows an impedance-temperature curve when the shutdown characteristics were evaluated. This indicates that this porous membrane has a narrow shutdown temperature range like a conventional polyolefin porous membrane.

Example 1 A silicon oxide film was formed on one surface of the porous polyolefin film of Comparative Example 1 by a sputtering method at a moving speed of 1.5 m / min. The maximum temperature on the surface of the porous film during film formation was about 45 ° C. The inorganic thin film-formed porous film thus produced did not show any particular change in appearance. The thickness of the inorganic thin film on the surface of the porous film of the heat-fusible polyolefin was 15 nm.
Table 1 shows the results of evaluating the characteristics of the inorganic thin film-formed porous film.
Shown in FIG. 2 shows an impedance-temperature curve when the shutdown characteristics were evaluated. Compared with Comparative Example 1, the shutdown start temperature Ts did not change, and the shutdown end temperature Te increased by 3 ° C. Than this,
It was confirmed that the shutdown temperature region was wider than in Comparative Example 1.

[Example 2] In Example 1, a silicon oxide film was formed at a moving speed of the porous film of 0.75 m / min. At this time, the maximum temperature of the porous membrane surface was about 65 ° C. The inorganic thin film-formed porous film thus produced did not show any particular change in appearance. The thickness of the inorganic thin film is 38
nm. Table 1 shows the results of evaluating the characteristics of the inorganic thin film-formed porous film. FIG. 3 shows an impedance-temperature curve when the shutdown characteristics were evaluated.
The shutdown end temperature was as high as 170 ° C. or higher. This indicates that this inorganic thin film-formed porous film has a wide shutdown temperature range while having good porous film characteristics.

[Example 3] In Example 1, a silicon oxide film was formed at a moving speed of the porous film of 0.5 m / min. At this time, the maximum temperature of the porous membrane surface was about 85 ° C. The inorganic thin film-formed porous film thus produced did not show any particular change in appearance. The thickness of the inorganic thin film is 54
nm. For this sample, the distribution of silicon in the film thickness direction was examined. No silicon was detected at least at a depth of at least 9 μm in the thickness direction of the porous film as the depth of the silicon oxide. This indicates that the silicon oxide film is present only in the vicinity of the surface of the heat-fusible polyolefin porous film, and is hardly formed on the inner surface of the pore. Table 1 shows the results of evaluating the characteristics of the inorganic thin film-formed porous film. FIG. 4 shows an impedance-temperature curve when the shutdown characteristics were evaluated. The shutdown end temperature was as high as 170 ° C. or higher.
This indicates that this inorganic thin film-formed porous film has a wide shutdown temperature range while having good porous film characteristics.

Comparative Example 2 A porous film made of high-molecular-weight polyethylene having a weight-average molecular weight of 650,000 (film thickness: 24 μm, basis weight: 15 g / m ² , average pore size: 0,000) was used as a heat-meltable polyolefin porous film. 058 μmφ). Each of the above properties was evaluated without forming an inorganic thin film on the surface of the porous film. Table 1 shows the evaluation results. FIG. 5 shows an impedance-temperature curve when the shutdown characteristics were evaluated. This indicates that this porous membrane has a narrow shutdown temperature range like a conventional polyolefin porous membrane.

Example 4 Using the polyolefin porous membrane of Comparative Example 2, the moving speed of this porous membrane was 0.75 m /
A silicon oxide film was formed as a part. At this time, the maximum temperature of the porous membrane surface was 58 ° C. The inorganic thin film-formed porous film thus produced did not show any particular change in appearance.
Table 1 shows the results of evaluating the characteristics of the inorganic thin film-formed porous film.
Shown in FIG. 6 shows an impedance-temperature curve when the shutdown characteristics were evaluated. The shutdown end temperature was as high as 170 ° C. or higher. Than this,
It was shown that this inorganic thin film-formed porous film has a wide shutdown temperature range while having good porous film characteristics.

[0061]

[Table 1]

Ts: shutdown start temperature Te: shutdown end temperature

[0063]

According to the present invention, the tensile elastic modulus and the piercing strength tend to be improved without impairing the characteristics of the original porous membrane, and the shutdown end temperature is raised without increasing the shutdown start temperature. Thereby, the shutdown temperature range can be significantly expanded. As a result, a short-circuit between the two electrodes due to the meltdown of the separator, which was observed when a porous membrane having a narrow shutdown temperature range was used as a separator, as has been pointed out particularly in the use of non-aqueous secondary batteries in the past, has been observed. The risk of heat generation can be greatly reduced.

[Brief description of the drawings]

FIG. 1 shows a shutdown characteristic of a porous film of Comparative Example 1.

FIG. 2 shows the shutdown characteristics of the inorganic thin film-formed porous film of Example 1.

FIG. 3 shows a shutdown characteristic of the inorganic thin film-formed porous film of Example 2.

FIG. 4 shows a shutdown characteristic of the inorganic thin film-formed porous film of Example 3.

FIG. 5 shows a shutdown characteristic of a porous film of Comparative Example 2.

FIG. 6 shows a shutdown characteristic of the porous film of Example 4.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Takahiro 2-1 Hinode-cho, Iwakuni-shi, Yamaguchi F-term in the Teijin Iwakuni Research Center (Reference)

Claims (11)

[Claims] 1. An inorganic thin film characterized in that an inorganic thin film is formed on at least one surface of a porous film made of a heat-meltable polyolefin, and is not substantially formed on the inner surface of pores. Is formed on the porous polyolefin membrane. 2. The inorganic thin film has a thickness of 10 nm or more and 200 n.
The polyolefin porous membrane according to claim 1, wherein m is equal to or less than m. 3. The inorganic thin film has a surface resistance of 1
Characterized by being composed of an inorganic oxide having TΩ or more,
The polyolefin porous membrane according to claim 1. 4. The method according to claim 1, wherein the inorganic oxide is at least one inorganic oxide selected from the group consisting of silicon oxide, aluminum oxide, magnesium oxide, calcium oxide, titanium oxide, zinc oxide and tin oxide. The polyolefin porous membrane according to claim 3. 5. The hot-melt polyolefin has a melting temperature of 1
The polyolefin porous membrane according to any one of claims 1 to 4, which is at least 10 ° C. 6. The polyolefin porous membrane according to claim 1, wherein the inorganic thin film is formed by a vacuum film forming method. 7. An organic solvent in which propylene carbonate and ethylene carbonate are mixed in an equal weight ratio is impregnated with an organic electrolyte solution in which lithium tetrafluoroborate is mixed at a concentration of 1 mol / liter and the temperature is increased. In the process, after the impedance value rises and once exceeds 1 kΩ square cm,
The polyolefin porous membrane according to any one of claims 1 to 6, wherein the temperature (shutdown end temperature) at which the impedance value decreases to 1 kΩ square cm (shutdown termination temperature) is 170 ° C or higher. 8. A polyolefin, wherein an inorganic thin film is formed on at least one surface of a porous film made of a heat-meltable polyolefin by a vacuum film forming method, and is not substantially formed on the inner surface of the pores. A method for producing a porous membrane. 9. The method according to claim 8, wherein the vacuum film forming method is a vacuum deposition method, a sputtering method or a CVD method.
The method for producing a polyolefin porous membrane according to the above. 10. When forming an inorganic thin film, a melting temperature of 1
9. The method according to claim 8, wherein a porous film made of a heat-meltable polyolefin having a temperature of 10 ° C. or more is used, and the surface temperature of the porous film is formed at (melting temperature of the porous film−30) ° C. or less. Or a method for producing a polyolefin porous membrane according to item 9. 11. A separator for electrode separation using the polyolefin porous membrane on which the inorganic thin film according to claim 1 is formed.