JP3536607B2 - Porous polymer film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a porous polymer film useful as a separator for a battery or a diaphragm for an electrolytic capacitor.

[0002]

2. Description of the Related Art Hitherto, polyolefin-based porous films have been frequently used as separators for batteries and diaphragms for electrolytic capacitors. In particular, with the advancement of technology in recent years, a high-precision and high-performance separator has been required for a lithium battery or the like.

Although a separator is interposed in a battery to prevent a short circuit between the positive and negative electrodes, a non-aqueous electrolyte battery such as a lithium battery having a high energy density, a high electromotive force and low self-discharge, especially a lithium secondary battery has been recently used. Batteries have been developed and put into practical use. As the negative electrode of the lithium battery, for example, metal lithium,
Alloys of lithium and other metals, carbon materials having the ability to adsorb or occlude lithium ions such as carbon and graphite, and the ability to occlude by intercalation, conductive polymer materials doped with lithium ions, and the like are known. As the positive electrode, for example, fluorinated graphite represented by (CF _x ) _n , MnO ₂ , V ₂ O ₅ , CuO, Ag ₂ Cr
Metal oxides such as O ₄ , TiO ₂ , LiCoO ₄ and LiMn ₂ O ₄ , sulfides and chlorides are known.

As a non-aqueous electrolyte, ethylene carbonate is used.
, Propylene carbonate, γ-butyrolactone,
Acetonitrile, 1,2-dimethoxyethane, tetrahi
LiPF for organic solvents such as drofuran₆, LiBF_Four, L
iCLO_Four, LiCF_ThreeSO _ThreeDissolved electrolytes such as
Is used. But lithium is particularly reactive
If an abnormal current flows due to an external short circuit or incorrect connection,
If the battery temperature rises significantly, a fire or other accident may occur.
Or cause thermal damage to equipment incorporating it.
I have a mind. To avoid such dangers,
Various porous films such as
Has been proposed.

A single-layer porous film of a thermoplastic resin such as polyethylene or polypropylene (Japanese Patent Publication No. 46-401)
No. 19, Japanese Patent Publication No. 55-32531, Japanese Patent Publication No. 5
9-37292, JP-A-60-23954, JP-A-2-75151, U.S. Pat.
No. 538). Porous films made of a polyethylene mixture having a different molecular weight or a mixture of polyethylene and polypropylene (JP-A-2-21559, JP-A-5-33130)
No. 6). Porous films using a thermoplastic resin or a non-woven fabric as a support (JP-A-3-245457, JP-A-1-25883)
58 gazette). Laminated porous films in which a plurality of porous films of thermoplastic resins of different materials are laminated (JP-A-62-10857, JP-A-63-308866, JP-A-2-77)
No. 108, Japanese Patent Laid-Open No. 5-13062, Japanese Patent Publication No. 3
JP-A-65776, JP-A-6-55629, JP-A-6-20671, JP-A-7-307146, and the like. The above porous film is generally prepared by stretching a non-stretched film by stretching, or extracting a filler, a plasticizer, etc. with a solvent from an unstretched film containing an extractable filler and a plasticizer. It is manufactured by an extraction method in which uniaxial or biaxial stretching is performed before or after extraction as necessary.

[0006] The basic concept of using a single-layer or laminated porous film as a separator is to prevent short-circuiting between both electrodes.
While maintaining the battery voltage, etc., when the internal temperature of the battery rises above a predetermined temperature due to abnormal current, etc., the porous film is made nonporous, in other words, the pores are closed, and ions flow between the two electrodes. An object of the present invention is to increase the electric resistance so as not to cause the battery function to stop, prevent a danger such as ignition due to an excessive temperature rise, and ensure safety. The function of preventing danger due to excessive temperature rise is a very important function as a battery separator, and is generally called non-porous or shutdown (abbreviated as SD).

[0007] In a battery separator, if the non-porous temperature is too low, the flow of ions is prevented by a slight rise in temperature, so that there is a problem in practicality. There is a problem in terms of safety because there is a risk of causing ignition. Generally, it is recognized that the non-porous temperature is 110 to 160 ° C, preferably 120 to 150 ° C. In a battery using a porous film as the separator, if the temperature inside the battery rises above the heat-resistant temperature of the porous film, the film is melted and broken, the non-porous state is lost, and ions are returned again. It starts to flow and causes a further rise in temperature. Therefore, it is required that the battery separator has an appropriate non-porous temperature, has a high heat-resistant temperature, and is reliably non-porous in the non-porous temperature range. In addition, in addition to the properties related to non-porosity, the battery separator is required to have a low electric resistance, a high mechanical strength such as a tensile elastic modulus, and a small variation in thickness unevenness and electric resistance. You.

[0008]

Various types of porous films have been proposed as described above, but there is a problem in the reliability of SD, which is an extremely important function as a battery separator. In a single-layer porous film, the temperature range between the non-porous temperature and the heat-resistant temperature is narrow,
There is a problem that achieving is difficult. In addition, a laminated porous film in which a plurality of porous films of thermoplastic resin of different materials are laminated and laminated is combined with two types of porous films having different melting points to provide an appropriate non-porous temperature and high heat resistance. Can bring temperature, SD
Is more reliable than a single-layer porous film. However, even in this case, the porous film on the lower melting point, which becomes nonporous at a lower temperature, is surely made nonporous near the melting point, and is completely nonporous near the melting point of the porous film on the higher melting point. It was difficult to keep the state. Therefore, development of a separator that can be made nonporous reliably is desired.

[0009]

SUMMARY OF THE INVENTION The present invention provides a stretched porous polymer film at room temperature in an electrolytic solution.
Elasticity in a non-porous temperature range that is 100 times or more the resistance value of the porous polymer film is not less than 10 ⁴ dyne / cm ² , and the porous polymer film is made non-porous by substantial shrinkage in the non-porous temperature range, The present invention relates to a porous polymer film having a shrinkage ratio in a nonporous temperature range of 5% or more.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In general, when a polymer material is heated to a temperature close to its melting point, even if the crystal part is partially or completely melted, the polymer material does not suddenly show a fluidized state like a low molecular weight substance, but is called a so-called rubbery state. There is an area. Therefore, in the nonporous temperature range generally near the melting point, the porous polymer film does not easily deform or flow, and it is difficult to achieve complete nonporous. The porous polymer film used in the present invention is a stretched porous film, and a film having a relatively high elastic modulus is used in consideration of film strength, shape retention during melting, and the like. Generally, a material having a density of 10 ⁴ dyne / cm ² or more in a non-porous temperature range is used. If the elastic modulus is too high, the film becomes brittle, so that the elastic modulus is 10 ¹¹ d in the non-porous temperature range.
yne / cm ² or less is preferably used.

In the present invention, the porous polymer film has an elastic modulus in a nonporous temperature range of 10 ⁴ dyne /
if cm ² or more ones are used, large film strength, relatively high viscosity, is excellent in shape retention and the like at the time of melting, yet complete by substantially shrink-free hole temperature range Non-porous can be achieved. In particular, the action of the shrinking force has the effect of shrinking the porous polymer film that is not easily flowable, and complete nonporosity can be achieved. When the elastic modulus in the non-porous temperature range is excessively high, the moldability is inferior. Therefore, a porous polymer film having an elastic modulus in the non-porous temperature range of 10 ¹¹ dyne / cm ² or less is preferable. .

Therefore, when the porous polymer film of the present invention is used as a battery separator, the porous polymer film is reliably made nonporous at the time of occurrence of an abnormality and has excellent shape retention during melting. It is possible to stably maintain the nonporous state up to the region.

[0013] The shrinkage required for nonporosity is usually at least several percent at the nonporous temperature, preferably at least 5%. A larger shrinkage ratio is preferable in that the larger the shrinkage force, the larger the shrinkage force. However, if the shrinkage ratio is too large, the dimensional change becomes too large. As a method for obtaining such a shrinkage ratio, for example, in the extraction method, monoaxial or biaxial stretching is performed at an appropriate stretching ratio before or after extraction (Japanese Patent Application Laid-Open No. 60-1985).
In the stretching method, a known method such as stretching at an appropriate stretching ratio (Japanese Patent Publication No. 55-32531) is used. The shrinkage ratio is in a proportional relationship with the stretching ratio, and when the stretching ratio is increased, the shrinkage ratio can be increased,
The adjustment of the shrinkage in the above range can be easily achieved by adjusting the stretching ratio. The stretching method is superior to the extraction method in that the porosity and the provision of the shrinkage ratio can be simultaneously performed.

It is preferable that the main contraction direction is the longitudinal direction of the porous polymer film (the length direction of the porous polymer film). Here, the longitudinal direction refers to the direction in which the porous polymer film is wound, and for example, when used in a battery separator, is the direction in which it is wound with the electrodes. If the shrinkage in the horizontal direction (the width direction of the film) is larger than the shrinkage in the vertical direction, the shrinkage undesirably exposes the electrode plate. It is preferable that the resin does not contract in the horizontal direction but expands slightly, or even if contracted, has a smaller shrinkage in the vertical direction. In the case of expanding in the horizontal direction, the shrinkage ratio is preferably 0 to -5% (-means expansion) in the nonporous temperature range, and in the case of contracting in the horizontal direction, contraction in the vertical and horizontal directions is preferable. The ratio (vertical shrinkage / lateral shrinkage) is 2 or more, more preferably 4 or more.

The non-porous temperature in the present invention is defined as the resistance of the porous polymer film at room temperature in an electrolyte at room temperature.
The temperature is at least 00 times, and the measuring method and the resistance measuring device are as follows. Measurement method: Electrolyte propylene carbonate 150 ml Dimethoxyethane 150 ml Lithium perchlorate 31.92 g Electrode area 1 cm ^{2 A} porous polymer film sample was immersed in an electrolyte for 5 minutes, then set between electrodes, and placed in an oven at 2 ° C./min. The resistance value was measured while increasing the temperature at a speed. Resistance measuring device: LCR HiTester (manufactured by Hioki Electric Co., Ltd.) Measurement frequency 1 kHz

Further, the method for measuring the elastic modulus in the present invention is carried out by the following method. Measurement method of elastic modulus Measurement device: Dynamic Spectrometer RDS2
(Manufactured by Rheometrics) Measurement conditions: torsional mode dynamic measurement Frequency 10 rad / sec

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

Example 1 Using a T-die having a discharge width of 1000 mm and a discharge lip opening of 2 mm, a number average molecular weight of 70,000 and a weight average molecular weight of 48 were used.
0000, a melt index of 3, and a melting point of 166 ° C. polypropylene (Ube Polypro F10 manufactured by Ube Industries, Ltd.)
3EA) was melt extruded at 200 ° C. The discharge film was guided to a cooling roll at 90 ° C., cooled by blowing cool air at 25 ° C., and then taken out at 40 m / min.
The film thickness of the obtained unstretched polypropylene film is 11.
It was 5 μm. This unstretched polypropylene film is placed in a 3-inch diameter paper tube by 350 mm to heat-treat it.
After being wound in a state of being wound by 0 m and placed in a hot-air circulation oven (PS-222 manufactured by Tabai Seisakusho) maintained at 120 ° C. for 24 hours, it was taken out of the oven and cooled to room temperature.
The birefringence of the heat-treated unstretched polypropylene film is 20.1 × 10 ⁻³ (17.0 × 10 ⁻³ before heat treatment), 10
The elastic recovery at 0% elongation is 90.5% (75.
3%). In addition, the elastic recovery rate (%) at 100% elongation was determined by the following equation (1).

[0019]

(Equation 1)

Discharge width 1000 mm, discharge lip opening 2 m
Using a T die of m, a high-density polyethylene having a density of 0.964, a melt index of 0.33, and a melting point of 132 ° C. (manufactured by Mitsui Petrochemical Co., Ltd., Hyzex 5202B)
It was melt extruded at 173 ° C. The discharge film was guided to a cooling roll of 115 ° C., cooled by blowing cool air of 25 ° C., and then taken out at 20 m / min. The thickness of the obtained unstretched polyethylene film was 8 μm. This unstretched polyethylene film is heated at 95 ° C. in a state of being wound around a 3 inch diameter paper tube at 3,500 m in order to heat-treat the unstretched polyethylene film.
Hot air circulation type oven (PS-made by Tabai Seisakusho)
222 type) and left for 24 hours, then removed from the oven and cooled to room temperature. The birefringence of the heat-treated unstretched polyethylene film is 40.5 × 10 ⁻³ (36.0 × 10 ⁻³ before heat treatment), and the elastic recovery at 50% elongation is:
72.5% (42.0% before heat treatment). In addition, 5
The elastic recovery rate (%) at 0% elongation was determined by the following equation (2).

[0021]

(Equation 2)

Next, a three-layer laminated film having a sandwich structure in which both outer layers were polypropylene and the inner layer was polyethylene was manufactured as follows. The heat-treated unstretched polypropylene film and unstretched polyethylene film were unwound at a winding speed of 4.0 m / min, respectively, from three sets of raw roll tunds.
Thermocompression bonding was performed at 34 ° C. and a linear pressure of 1.8 kg / cm. The speed at this time was 4.0 m / min, and the unwinding tension was 3 kg for the polypropylene film and 0.9 kg for the polyethylene film.
kg.

The three-layer laminated film was stretched at a low temperature of 20% between nip rolls maintained at 35 ° C. At this time, the distance between the rolls was 350 mm, and the roll speed on the supply side was 1.
It was 6 m / min. It is then led into a hot-air circulation oven heated to 126 ° C, stretched at high temperature between rolls using a difference in roll peripheral speed until the total stretch amount reaches 180%, and then stretched by rolls heated to 126 ° C. The amount was relaxed by 17% and heat set for 25 seconds to continuously obtain a laminated porous film.

Table 1 shows the measurement results of the film thickness, porosity, maximum pore size, Gurley value and tensile strength of the obtained laminated porous film. Further, among the constituent films of the laminated porous film, the elastic modulus of the porous polyethylene on the low melting point side was 6.7 × 10 ⁸ dyne / cm ² in a non-porous temperature range of 135 ° C.

The above evaluation method was performed as follows. Porosity and maximum pore diameter The porosity and the maximum pore diameter were determined from the maximum value of a pore distribution curve measured by a mercury porosimeter (manufactured by Yuasa Ionic). Specifically, a 30 mm × 300 mm strip sample piece was collected and placed in a cell, and the porosity and the maximum pore diameter were determined from the amount of mercury and the pressure relative to the pore diameter. Gurley value Measured according to JIS P8117. A B-type Gurley densometer (manufactured by Toyo Seiki Co., Ltd.) was used as a measuring device. The sample piece is clamped in a circular hole having a diameter of 28.6 mm and an area of 645 mm ² . With the inner cylinder weight of 567 g, the air in the cylinder is allowed to pass through the test hole to the outside of the cylinder. The time required for 100 cc of air to pass was measured and defined as the air permeability (Gurley value). Tensile strength was measured according to ASTM D-822.

[0026]

[Table 1]

The change in the resistance value of the porous polymer film was determined as the resistance value ratio at each temperature when the value at 25 ° C. was set to 1. Table 2 shows the results. The shrinkage in the table is
The porous polymer film (250 mm × 250 mm) was treated in an oven at 135 ° C. for 1 hour, and the dimensions of the film before and after the treatment were measured and determined from the area. Table 2
In the middle, “−” in the column of the lateral shrinkage means expansion.

[0028]

[Table 2]

Example 2 Three layers of a porous polymer film having a sandwich structure of both outer layers of polypropylene and inner layer of polyethylene obtained in Example 1 were used, in which 0.2 mmφ holes were formed in a grid pattern at 5 mm intervals. In the same manner as in Example 1, the change in the resistance value of the porous polymer film was determined as the resistance value ratio at each temperature when the value at 25 ° C. was set to 1. Table 2 shows the results.

COMPARATIVE EXAMPLE 1 In the same manner as in Example 1, a laminated film in which each unstretched film was laminated in the manufacturing process of Example 1 but having the same holes as in Example 2 in a lattice pattern was used. The change in the resistance value of the film was determined as a resistance value ratio at each temperature when the value at 25 ° C. was set to 1. Table 2 shows the results.

[0031]

The porous polymer film of the present invention has high film strength, relatively high viscosity, excellent shape retention during melting, and complete nonporosity in a nonporous temperature range. Can be achieved. Therefore, when used as a battery separator, it can be substantially shrunk in a non-porous temperature range, and even if there is a defect such as a pinhole, the porous polymer film is reliably non-porous when an abnormality occurs. Be converted to In particular, a porous polymer film that substantially shrinks at a non-porous temperature can have a function of shrinking a porous polymer film in a state where it does not easily flow due to the action of a shrinking force, thereby achieving complete non-porous film. .

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI C08L 23:04 C08L 23:04

Claims (4)

(57) [Claims] 1. A stretched porous polymer film that is at least 100 times the resistance value at room temperature in an electrolytic solution.
Become modulus at no Anaka temperature range ¹⁰ 4 dyne / cm
² or more, wherein the porous polymer film is made nonporous by substantial shrinkage in a nonporous temperature range, and the shrinkage ratio in the nonporous temperature range is 5% or more. . 2. An elastic modulus in a non-porous temperature range of 10 ⁴ dyn.
The porous polymer film according to claim 1, wherein the film thickness is not less than e / cm ^{2 and not} more than 10 ¹¹ dyne / cm ² . 3. A shrinkage ratio in a non-porous temperature range of 5% or more and 90% or less.
% Or less. 4. The porous polymer film according to claim 1, wherein the shrinking direction in the non-porous temperature range is the stretching direction of the porous polymer film.